Synthesis, characterisation and performance evaluation of spinel-derived Ni/Al2O3 catalysts for various methane reforming reactions

1 2 3 4 5 Zouhair Boukha, Cristina Jiménez-González, 6 Beatriz de Rivas, Juan Ramón González-Velasco, 7 Jose Ignacio Gutiérrez-Ortiz and Rubén López-Fonseca* 8 Chemical Technologies for Environmental Sustainability Group, 9 Department of Chemical Engineering, Faculty of Science and Technology, 10 University of The Basque Country UPV/EHU, 11 P.O. Box 644, E-48080 Bilbao, Spain. 12 13 14 15 *Corresponding author:16 Phone: +34-94-6015985 17 Fax: +34-94-6015963 18 E-mail address: [email protected]19 20 SYNTHESIS, CHARACTERIZATION AND PERFORMANCE EVALUATION OF SPINEL-DERIVED Ni/Al2O3 CATALYSTS FOR VARIOUS METHANE REFORMING REACTIONS This is the accepted manuscript of the article that appeared in final form in Applied Catalysis B: Environmental 158/159 : 190-201 (2014), which has been published in final form at https://doi.org/10.1016/j.apcatb.2014.04.014. © 2014 Elsevier under CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) 2 Abstract 21 The catalytic performance of bulk nickel aluminate catalysts synthesised by co-22 precipitation (NiAl2O4-CP) and co-dissolution (NiAl2O4-D) was examined for various 23 methane reforming reactions, namely partial oxidation, steam reforming and oxidative 24 steam reforming. The calcined and reduced spinels were thoroughly characterised by a 25 wide number of analytical techniques including WDXRF, N2 physisorption, XRD, UV–26 visible–NIR DRS, XPS, TEM, H2-TPR and NH3-TPD. The characterisation results of 27 the calcined samples at 850 ºC showed that nickel aluminate phase was mainly obtained 28 on the NiAl2O4-CP near-surface together with a small NiO excess. By contrast, a large 29 amount of NiO phase was formed on NiAl2O4-D and deposited on the spinel surface. 30 The reduction at 850 ºC of the two samples produced monodispersed Ni particles 31 (10.6 nm) in the NiAl2O4-CP while larger metallic nickel (17.7 nm) was deposited on 32 the NiAl2O4-D. In the three investigated reforming reactions the NiAl2O4-CP catalyst 33 proved to be highly active, stable and resistant toward carbon deposition. Moreover, the 34 performances of the NiAl2O4-CP catalyst appeared to be generally comparable with that 35 of a commercial 1%Rh/Al2O3 catalyst. The difference in catalytic behaviour between 36 the two nickel aluminates was related to the Ni dispersion, its particle size and its 37 capacity to minimize the acid character of the alumina by covering a large part of its 38 surface. 39 40 41 42 43 Keywords: Nickel aluminate, Co-precipitation, Co-dissolution, Ni Crystallite size, 44 Methane reforming 45 46 3 1. Introduction 47 Methane steam reforming (SRM) is the most used technology for producing synthesis 48 gas (CO and H2) from natural gas [1-5]. Moreover, this technology is a preferable 49 choice for producing clean and larger hydrogen yield. However, because of the high 50 cost of the endothermic character of SRM, the exothermic methane partial oxidation 51 (POM) appears as an alternative as it offers some interesting advantages especially 52 when the posterior use of the syngas stream requires a suiTable H2/CO ratio [6-8]. 53 Currently industrial facilities are equipped with systems that offer the possibility of 54 using POM and/or SRM technologies. The combination of the POM and SRM strategies 55 called oxidative steam reforming (OSRM) has the advantage of controlling the process 56 heat and the distribution of the products by adjusting the O/C molar ratio [9-10]. 57 Ni-based catalysts are widely used for methane reforming reactions due to their high 58 activity and their competitive costs compared to noble metals [11-13]. Alumina 59 supported Ni systems have particularly received considerable attention because of their 60 practical applications for a variety of reforming reactions [14-18]. Nevertheless, despite 61 exhibiting advantageous catalytic properties, alumina lacks an adequate thermal stability 62 which provokes the sintering of supported metals. Furthermore, the use of alumina as 63 support material has the drawback of rapid deactivation of the catalyst due to carbon 64 deposition on its active acid surface [19-20]. For this reason, the interest of recent 65 investigations has been focused on the preparation of formulations leading to a high 66 dispersion of Ni species which can cover alumina surface, thus minimising the negative 67 effect of its acid character. In this sense, the effect of the addition of some alkali and 68 alkali earth metals, used as chemical promoters, on the activity and stability of 69 Ni/alumina catalysts has been extensively investigated. Typically these additives help to 70 4 avoid carbon deposition and enhance their stability but at the expense of a reduction of 71 their activity due to the blocking of the more reactive Ni sites [21,22]. 72 Within the same objective (the need to improve stability of Ni-based catalysts) nickel 73 aluminate synthesis is one of the routes proposed in the literature for the design of a 74 precursor capable to produce, after its reduction, metallic Ni presenting strong 75 interaction with alumina [14,15,23-25]. For so long, Ross et al. [25] had evidenced the 76 participation of the reduced surface nickel aluminate sites in the steam reforming of 77 methane over Ni/alumina catalysts. Since then, a number of studies dealing with the 78 preparation of nickel aluminate as precursor of active Ni metallic particles for methane 79 reforming reactions are available in the literature. A great majority of these works have 80 been devoted to the investigation of the influence of preparation methods on the 81 stability and the activity of the final formed Ni structures in reforming reactions 82 [14,15,24,26,27]. As stated in various references, it is however complicated to 83 synthesise pure or stoichiometric nickel aluminate by means of conventional methods of 84 synthesis. This is because of the formation of NiO as an excess even when heating at 85 very high temperatures (>1300 ºC) [28-30]. Therefore, many works have examined the 86 impact of this particularity on the catalysts performance [15,16,24,27,31]. 87 In our previous study, we compared two series of Ni/Al2O3 catalysts with similar metal 88 content prepared by co-impregnation, using solutions containing a mixture of nickel 89 acetate and aluminium nitrate to obtain a stoichiometric NiAl2O4 supported on alumina, 90 and simple impregnation of nickel on alumina [15]. We demonstrated that, in methane 91 steam reforming, the co-impregnated catalyst was more active and we attributed this 92 behaviour to its higher Ni–support interaction. We also noticed that obtaining the active 93 metallic Ni, with small particle size, was controlled by the NiO/NiAl2O4 ratio initially 94 present in the calcined samples. Achouri et al. [16] also studied the influence of the 95 5 preparation method on the catalytic activity in diesel steam reforming of two alumina-96 supported nickel aluminate catalysts prepared by wet-impregnation and co-precipitation. 97 They concluded that the impregnated catalyst exhibited a higher catalytic performance 98 and stability over time than the co-precipitated counterpart. This different behaviour 99 was explained by the higher carbon deposition tendency shown by the co-precipitated 100 sample. 101 In this paper the influence of the physico-chemical characteristics of two bulk nickel 102 aluminate catalysts, prepared by co-precipitation and co-dissolution methods, on their 103 catalytic behaviour in POM, SRM and OSRM reactions is investigated. To reach this 104 target, an extensive characterisation has been carried out by a wide number on analytical 105 techniques including BET measurements, XRD, XPS, UV–visible–NIR DRS, TEM and 106 H2-TPR. Likewise, TPD of NH3, a well-known probe molecule for acid sites, has been 107 used to characterise the surface chemistry of the investigated samples. A special 108 attention has been paid to the determination of the different nickel species formed after 109 high-temperature calcination and subsequent high-temperature reduction. In order to 110 examine and compare their catalytic efficiency the two prepared catalysts were 111 evaluated in partial oxidation, steam reforming and oxidative steam reforming of 112 methane. Interesting conclusions are drawn from the correlation between the 113 distribution of Ni active species of the two catalysts and their performance in these three 114 reforming reactions. 115 116 2. Experimental 117 2.1. Catalysts preparation 118 Bulk nickel aluminate samples were synthesised by co-precipitation (CP) and co-119 dissolution (D) methods. The sample named NiAl2O4-D was synthesised using two 120 6 aqueous solutions of Ni(CH3-COO)2·4H2O and Al(NO3)3·9H2O. Then they were mixed 121 (leading to a mixture with the desired 1:2 Ni/Al molar ratio), and evaporated on a hot 122 plate (150 ºC). In the case of NiAl2O4-CP, aqueous ammonia was added to the mixed 123 aqueous solutions to adjust the final suitable pH (=8). For comparative purposes NiO 124 was also prepared by simple calcination in air of Ni(CH3-COO)2·4H2O. All the samples 125 were dried at 110 ºC overnight and then calcined at 850 ºC in static air for 4 h at a 126 heating rate of 10 ºC min-1. Finally, pellets of bulk (nickel aluminate or nickel oxide) 127 samples were prepared by a process of compressing the powders into flakes in a 128 hydraulic press (Specac), crushing and sieving (0.3-0.5 mm). 129 130 2.2. Catalyst characterisation 131 The catalysts were characterised by N2 physisorption at -196 ºC, wavelength dispersive 132 X-ray fluorescence (WDXRF), X-ray diffraction (XRD), ultraviolet-visible-near 133 infrared diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy 134 (XPS), temperature programmed reduction with hydrogen (H2-TPR) and temperature 135 programmed desorption of NH3 (NH3-TPD). The experimental details of each analytical 136 technique are described elsewhere [14,15]. Additionally, the morphology and particle 137 size distribution of the nickel particles was examined by transmission electron 138 microscopy (TEM). Prior to analysis, the samples were dispersed in absolute ethanol 139 ultrasonically for 30 min, and 10 cm3 of each sample were then placed on flexible film 140 (Parafilm® M). Glow-discharged carbon-coated copper grids were inverted onto the 141 droplets of each sample. After incubation for 1 min at room temperature, the grids were 142 manually blotted with filter paper air-dried. Digitally recorded 2D images of each 143 solution were taken at room temperature at a nominal magnification of 80000 on a 144 Jeol JEM-1230 transmission electron microscope, with a LaB6 filament as the source of 145 7 electrons and operated at 100 kV. Digital images were recorded on an Orius SC1000 146 cooled slow-scan CCD camera, 4008×2672 pixels (GATAN), obtaining a final pixel 147 size of 0.85 Å pixel-1. The particle size distribution was obtained from the measurement 148 of at least 300 particles using ImageJ software, and the average diameter was calculated 149 by dM =di·ni/ni, where ni is the number of the particles of diameter di. 150 On the other hand, the amount of carbonaceous deposits on the used catalyst was 151 determined by dynamic thermogravimetry using a Setaram Setsys Evolution apparatus 152 under atmospheric pressure coupled to a Pfeiffer Prisma mass spectrometer (TPO-MS). 153 The mass loss and the sample temperature were continuously recorded by a 154 computerised data acquisition system. Previously, the samples (20 mg) were dried from 155 room temperature to 150 ºC. Then, the temperature was increased from 150 to 850 ºC at 156 a constant heating rate of 5 ºC min-1. The oxidant stream was 5%O2/He (50 cm3 min-1) 157 flowing downwards onto the cylindrical sample holder. 158 159 2.3. Catalytic tests 160 The three methane reforming reactions were studied in a bench-scale fixed-bed reactor 161 operated at atmospheric pressure. The reactor was made of stainless steel with an 162 internal diameter of 9 mm and a height of 305 mm. Prior to the reaction the catalyst 163 (0.125 g) was diluted with inert quartz (0.875 g, 1-1.25 mm). The catalyst bed was 164 maintained in the reactor on a quartz wool plug. The temperature was measured by a 165 thermocouple placed between the particles of the catalyst. Before the reaction the 166 NiAl2O4-D and NiAl2O4-CP catalysts were reduced in situ with a mixture of 5%H2/N2 167 at 850 °C for 2 h. Likewise, a 1%Rh/Al2O3 (Alfa Aesar, 132 m2 g−1) commercial 168 catalyst was reduced at 700 ºC and its activity was used for comparative purposes in the 169 three methane reforming reactions. 170 8 Three different feed gas mixtures balanced with N2 (38400 cm3 CH4 g−1 h−1) were used 171 in each reforming reaction as follows: 172 i) 10% CH4 and 5% O2 in POM reaction. 173 ii) 10% CH4 and 30% H2O in SRM reaction. 174 iii) 10% CH4, 30% H2O and 5% O2 in OSRM reaction. 175 The runs were sequentially carried out by increasing and decreasing the reaction 176 temperature (450 ºC-550 ºC-650 ºC-550 ºC-450 ºC) with an accumulated time online of 177 about 63 h. Catalytic activity, product yields and stability were recorded during 12.5 h 178 at each reaction temperature. Feed and effluent streams were analysed online by a 179 MicroGC (Agilent 3000) equipped with a TCD detector. Two columns, Molecular Sieve 180 5A and Plot U, were used in a series/bypass arrangement for the complete separation of 181 H2, N2, O2, CH4, CO and CO2. A cold trap at the outlet of the reactor was used to 182 condense out any water from the product gas stream. On basis of the molar flow at the 183 inlet and outlet of the reactor, conversion and product yields were calculated, according 184 to the following equations: 185         2 4 4 ,% 100   out out in F CO F CO X CH F CH (1) 186     2 2 4 ()2  out in FH YH F CH (2) 187     4 ()out in F CO Y CO F CH (3) 188     2 2 4 ()out in F CO Y CO F CH (4) 189 The thermodynamic data were calculated via the HSC Chemistry software package by 190 the GIBBS program using the so-called Gibbs Energy Minimization Method. For these 191 calculations, only enthalpy, entropy and heat capacity data for all prevailing compounds 192 9 were needed. The software calculated the amounts of products at equilibrium under 193 isothermal and isobaric conditions. The substances to be taken into account in the 194 calculations, the amount of reactants, the potentially sTable phases as well as the 195 temperature of raw species were specified as input. In addition to solid carbon, the 196 following substances in the gas phase were considered: CH4, O2, N2, CO, CO2, H2 and 197 H2O. Calculations were performed in the 450-650 ºC temperature range at atmospheric 198 pressure. Hence, the GIBBS program found the most sTable phase combination and 199 determined the phase composition where the Gibbs energy for the system reached its 200 minimum at constant pressure and temperature. 201 202 3. Results and discussions 203 3.1 Characterisation of the samples 204 3.1.1. N2-physorption (BET measurements) 205 N2 adsorption at -196 ºC on NiAl2O4-D and NiAl2O4-CP showed that the isotherms (not 206 shown) were characteristic of mesoporous solids of type IV according to the IUPAC 207 classification. In the case of NiAl2O4-D the nitrogen desorption gave rise to a hysteresis 208 loop, H2 type, which was characteristic of disordered porous materials. Table 1 lists the 209 data obtained from the analysis of the textural properties of the calcined and reduced 210 samples. The prepared NiAl2O4-CP spinel (76 m2 g-1) had a higher surface area than 211 NiAl2O4-D (55 m2 g-1). This difference probably resulted from the preparation method 212 and the corresponding proportion of NiO formed in each sample, which will be 213 discussed in the XRD, H2-TPR and XPS sections. Fig. 1 shows the pore sizes 214 distribution for the NiAl2O4-D and NiAl2O4-CP catalysts. The pore size distribution 215 trace for the NiAl2O4-D sample exhibited one maximum at 7 nm in the low mesoporous 216 range (<10 nm) whereas the NiAl2O4-CP samples exhibited a peak centred at 20 nm. 217 16 Fig. 5 368 The difference in Ni/Al atomic ratio determined by XPS between NiAl2O4-CP and 369 NiAl2O4-D also supported our conclusion about the structural composition of near-370 surface. According to Table 2, the Ni/Al atomic ratio of NiAl2O4-CP (0.6) was smaller 371 than that determined for NiAl2O4-D (1), suggesting that the surface of the co-372 precipitated sample was comparatively deficient in nickel. Thus, in view of the 373 stoichiometry of the pure spinel (Ni/Al=0.5), it could be concluded that nickel 374 aluminate phase was mainly obtained on the NiAl2O4-CP near-surface together with a 375 small NiO excess. A larger amount of NiO phase was however formed on NiAl2O4-D. 376 3.1.6. Transmission electron microscopy (TEM) 377 NiAl2O4-CP and NiAl2O4-D samples, reduced at 850 °C, were investigated by 378 transmission electron microscopy as well. Fig. 6(a) and 6(b) show the TEM 379 micrographs and size distribution diagrams (obtained from the measurement of about 380 300 particles). The micrographs of both samples showed no significant differences in 381 morphology. In both cases spherical particles of nickel were observed. Nevertheless, it 382 was visualised that nickel was homogeneously dispersed in NiAl2O4-CP while it was 383 much more heterogeneous on the NiAl2O4-D catalyst. The particle size distribution in 384 the co-precipitated sample presented a unimodal shape with an average size of 10.6 nm 385 (dispersion of 9.5%). By contrast a broad distribution ranging from 10 to 50 nm was 386 obtained for the NiAl2O4-D catalyst with a corresponding average size of 17.7 nm 387 (dispersion of 4%). By correlating the Ni particle size distributions with the results of 388 H2-TPR, XRD, XPS and UV-visible-NIR DRS it could be reasonably assumed that the 389 smaller mono-dispersed particles present on the NiAl2O4-CP corresponded to particles 390 resulted from the reduction of nickel aluminate phase whereas the range of larger 391 particles deposited on the NiAl2O4-D was originated by the reduction of the free NiO. 392 17 As stated previously, the NiAl2O4-D catalyst contained two Ni species as a mixture of 393 free NiO and nickel aluminate phase while NiAl2O4-CP was composed mainly by nickel 394 aluminate together with a relatively small fraction of highly dispersed NiO (as proved 395 by H2-TPR). Furthermore, we thought that the difference between NiAl2O4-D and 396 NiAl2O4-CP probably resulted from the textural properties of each catalyst as well. The 397 NiAl2O4-D average pore size was smaller (7.5 nm) compared to its Ni particles average 398 size, suggesting that nickel was mainly deposited on the external surface. However, the 399 NiAl2O4-CP average pore size was around 15 nm and this might limit the growth of the 400 Ni crystallites during the reduction. 401 Fig. 6 402 3.1.7. Temperature programmed desorption of NH3 (NH3-TPD) 403 The surface acidity of the two catalysts was characterised by means of temperature 404 programmed desorption, using ammonia as probe molecule, followed by dynamic 405 thermogravimetry and coupled to a mass spectrometer [15]. The overall acidity of the 406 samples (calcined and reduced spinel catalysts) was quantified from the net weight gain 407 during the adsorption step at 100 ºC followed by the removal of physically bound 408 ammonia from the surface with flowing helium. 409 The deconvolution of the NH3-TPD thermograms of the calcined NiAl2O4-D and 410 NiAl2O4-CP samples exhibited two bands corresponding to two types of acid sites 411 (Fig. 7). The first one consisted of a peak located at low temperatures (<220 ºC) 412 accompanied by a second much more intense feature at relatively high temperatures 413 (250-370 ºC), the latter being probably due to a fraction of strong acid sites present at 414 the surface of the catalysts. It must be pointed out that, in the case of the two reduced 415 samples, the MS analysis of the exit stream revealed the presence of trace amounts of 416 N2 and H2 related to the decomposition of the probe molecule on the reduced sites. 417 18 The obtained quantitative results were summarised in Table 1. It was found that the 418 surface density of NH3 adsorbed on the calcined NiAl2O4-CP, 3.82 mol NH3 m-2, was 419 smaller than that determined for the calcined NiAl2O4-D, 4.95 mol NH3 m-2. The 420 difference could be related to the structural composition of each catalyst. As 421 demonstrated by XPS analysis the NiAl2O4-D catalyst contained alumina phase in its 422 near-surface which could improve its surface acid properties. It should be pointed out 423 that the H2 reduction at 850 ºC of the two catalysts appeared to increase both the amount 424 and the thermal stability of the adsorbed NH3 by shifting the desorption peaks towards 425 higher temperatures. The acid surface density for reduced NiAl2O4-CP was estimated to 426 reach 4.38 mol NH3 m-2 in comparison with 5.7 mol NH3 m-2 for NiAl2O4-D 427 (Table 1). These results might be related to the formation of alumina instead of nickel 428 aluminate structure in the two reduced samples, as proved by XRD analysis. In addition, 429 the presence of homogeneous and highly dispersed nickel in the reduced NiAl2O4-CP 430 catalyst, as confirmed by TEM analysis, could explain the occurrence of a small fraction 431 of surface acid sites in comparison with NiAl2O4-D. By contrast, the low Ni dispersion 432 (4%) on the latter led to a large uncovered acid support surface. 433 Fig. 7 434 435 3.2. Catalytic activity 436 3.2.1 Partial oxidation of methane (POM) 437 The POM reaction was carried out on the activated (calcination followed by reduction) 438 NiAl2O4-CP, NiAl2O4-D and 1%Rh/Al2O3 (used as reference) catalysts, at 450, 550 and 439 650 ºC, by sequentially increasing the reaction temperature with 100 °C intervals. The 440 inverse sequence was conducted by decreasing the reaction temperature (from 650 to 441 550 and 450 ºC) in order to probe the stability of the catalysts. Fig. 8 shows the obtained 442 19 results in terms of CH4 conversion and H2, CO and CO2 yields. The corresponding 443 equilibrium data are included as well (calculated via the HSC Chemistry software 444 package by the GIBBS program using the so-called Gibbs Energy Minimization 445 Method). The reported data evidenced that the NiAl2O4-CP catalyst was typically more 446 active than the NiAl2O4-D. Furthermore, over the co-precipitated sample methane 447 conversion as well as H2, CO and CO2 yields were fairly stable during 12.5 h on stream 448 (Fig. 9). At 450 ºC the conversion on this sample was around 25% and it increased to 449 reach about 36% at 550 ºC and 58% at 650 ºC. Likewise, the increase in the reaction 450 temperature promoted the H2 yield, which attained 0.56. Carbon monoxide production 451 also increased up to 0.44 at 650 ºC. It is noteworthy that at 650 ºC both conversion and 452 yields achieved with NiAl2O4-CP were close to the values obtained with the commercial 453 1%Rh/Al2O3 catalyst. As the performance of the noble metal-based catalyst is 454 considered as reference in the POM reaction [40-43], these catalytic features evidenced 455 the potential of NiAl2O4-CP as a promising alternative catalyst. By contrast, the activity 456 appeared to decrease with time on stream in the case of NiAl2O4-D catalyst. It was 457 thought that this difference in the catalytic activity between the two reduced spinel 458 catalysts was related to the Ni particle size. Indeed, our TEM results showed that on the 459 NiAl2O4-CP catalyst metallic nickel was deposited with a smaller size (10.6 nm) 460 compared to the NiAl2O4-D (17.7 nm). 461 As shown in Fig. 9, methane conversion at 450 ºC gradually decreased from 29% at the 462 beginning of the reaction to 25% after 12.5 h on stream. Likewise, H2, CO and CO2 463 yields were not stable with time on line. Indeed, a slight increase in activity followed by 464 a stable plateau was observed when the reaction temperature decreased which could be 465 a result of an excess of carbon formation (Fig. 9). 466 20 Table 3 lists the H2/CO and CO/CO2 ratios calculated for each reaction temperature on 467 the tested catalysts. Note that, on the two tested nickel catalysts, the CO/CO2 ratio 468 seemed to increase with reaction temperature. For instance, at 450 ºC the CO/CO2 ratio 469 on the NiAl2O4-CP was around 0.1 and it increased to reach about 3 at 650 ºC. At low 470 CO/CO2 values (corresponding to relatively low temperatures) CO disproportionation 471 and/or methane total oxidation reaction may occur producing CO2 and carbon and/or 472 H2O. On the other hand, the H2/CO ratio was in all the cases higher than 2 (ranging 473 between 2.2 and 2.5). It is widely accepted that the encapsulation of the metal particles 474 by deposited carbon does not occur if H2/CO or H2O/hydrocarbon ratios are sufficiently 475 high [32]. Accordingly, our results showed, by comparing the NiAl2O4-CP and 476 NiAl2O4-D catalysts, that the catalyst with the highest H2/CO ratio (NiAl2O4-CP) did 477 not suffer apparent deactivation. 478 Fig. 8/Fig. 9/Table 3 479 Thermogravimetric (not shown) and TPO-MS analyses (Fig. 10) performed in order to 480 determine the amount of carbon accumulated on the used catalysts in POM resulted in 481 deposited carbon masses of 2.4 wt.% on NiAl2O4-CP (close to 2 wt% observed for the 482 commercial rhodium catalyst) and 46.5 wt.% on NiAl2O4-D (Table 3). Both TPO traces 483 consisted of a CO2 production major peak at approximately 670 ºC (Fig. 10). Thus, the 484 same type of carbonaceous species was present on the two catalysts. It could be 485 concluded, therefore, that the deactivation of NiAl2O4-D catalyst might mainly occur as 486 a result of the substantial formation of coke. This was in agreement with XRD analysis 487 of the spent catalysts which confirmed the formation of carbon (graphite) on the two 488 catalysts. Specially, it was observed that the principal peak (26.4º) was much more 489 intense in the NiAl2O4-D diffractogram than that of NiAl2O4-CP catalyst (Fig. 2). On 490 the other hand, the Ni (200) (2θ = 51.6°) diffraction line broadening was used to 491 21 estimate, by Scherrer equation, the evolution of metallic Ni crystallite size after catalytic 492 test (Table 1). It was found that the growth of Ni0 crystallites during POM reaction on 493 NiAl2O4-D for extended periods of time was significant (size estimated to be around 494 23 nm in the reduced sample while it was around 35 nm after catalytic test). In 495 agreement with a previous study which reported that, in the methane reforming 496 reactions, the larger Ni crystallites on the Ni catalyst favoured the formation of graphitic 497 carbon [44], we could conclude that the same phenomenon occurred in the case of our 498 NiAl2O4-D catalyst. As stated previously, the reduction of the Ni species deposited on 499 this sample produced large metallic Ni which was less resistant to sintering and coke 500 formation. 501 Fig. 10 502 The XRD diffractograms of the spent nickel aluminate catalysts also evidenced the 503 presence of NiO phase (Fig. 2). The NiO characteristic peaks were much more intense 504 on the NiAl2O4-CP diffractogram suggesting that the oxidation of Ni species during the 505 POM reaction was influenced by their interaction with the support. Accordingly, it 506 seemed that the oxidation of the smaller Ni particles, during POM reaction, might be 507 easier. Indeed, our H2-TPR and TEM studies indicated that more than two distinct 508 metallic nickel populations were present on NiAl2O4-D in contrast with the NiAl2O4-CP 509 catalyst with a narrower and more homogeneous size distribution (size = 10.6 nm). In 510 their study on the mechanism for POM reaction, Jin et al. [45] claimed that during the 511 CH4/O2 reaction over Ni/Al2O3 catalysts, Ni0 was first oxidised to NiO, and the latter 512 was reduced again by CH4 during the transient process. Since the reduction of NiO by 513 CH4 is endothermic and as the last step of our catalytic experiments occurred at 450 ºC 514 (relatively low temperature) the presence of the characteristic peaks of NiO in our XRD 515 diffractograms of the used samples could be justified. Furthermore, the presence of the 516 22 NiO could explain the low CO/CO2 ratios obtained at low temperatures suggesting that 517 this gave rise to total oxidation instead of reforming reaction. 518 On the other hand, as described in previous works the carbon deposition may deactivate 519 the catalyst either by covering of the active sites and/or by pore blocking [32,44-46]. In 520 our case, it was found that, after catalytic tests, the pore size distribution of NiAl2O4-D 521 catalyst shifted towards higher values (8.1 nm to 10.7 nm) suggesting the blocking of 522 the pores with small sizes by carbon deposition (Table 1). In addition, it should be noted 523 that carbon deposition on the NiAl2O4-CP and NiAl2O4-D catalysts appeared to 524 significantly increase the surface area (Table 1). Thus, on NiAl2O4-D the specific 525 surface area increased from 48 to 57 m2 g-1 whereas over NiAl2O4-CP catalyst it 526 increased from 55 to 64 m2 g-1, suggesting that this might be due to the similar porous 527 nature of the carbon deposited on the two catalysts [46]. 528 On the other hand, the chemical properties of their near surface could have an effect on 529 the catalytic activity and stability of NiAl2O4-D and NiAl2O4-CP catalysts. Generally, 530 acidity is considered to induce a negative impact on methane reforming behaviour by 531 catalysing the coke formation [32]. In agreement with our NH3-TPD studies, the surface 532 of NiAl2O4-D bears more acid sites than that of NiAl2O4-CP catalyst. This difference in 533 the acid character could explain the markedly larger formation of coke on the surface of 534 the NiAl2O4-D catalyst in comparison with NiAl2O4-CP. 535 In addition, the inversion degree of the formed spinel was another factor to take into 536 consideration in order to explain the different performances of NiAl2O4-D and NiAl2O4-537 CP in POM reaction. In their study on the methane dry reforming reaction over nickel 538 aluminate catalysts Kathiraser et al. [26] concluded that the inverse NiAl2O4 spinel 539 structure positively affected the catalytic activity compared to the normal spinel. Our 540 characterization results showed that NiAl2O4-CP tended to be in the inverse 541 23 coordination while the NiAl2O4-D was close to the normal spinel phase. The observed 542 divergence between NiAl2O4-D and NiAl2O4-CP might influence the catalytic 543 behaviour of these catalysts as well. 544 3.2.2. Steam reforming of methane (SRM) 545 Fig. 8 show CH4 conversion and H2, CO and CO2 yields in the SRM reaction for the 546 examined reduced catalysts as function of temperature. In all cases methane conversion 547 as well as H2, CO and CO2 yields were stable with time on stream. Moreover, the 548 NiAl2O4-CP and NiAl2O4-D catalysts were markedly more active than 549 1%Rh/Al2O3 (80% and 53% vs. 46% at 650 °C, respectively). The comparison of the 550 performance of the NiAl2O4-D catalyst in POM and SRM reactions showed that 551 replacing oxygen with steam resulted in poorer methane conversion and CO yield while 552 it improved the H2 production. By contrast, when compared to its performance in POM 553 reaction, an increase in activity as well as CO and H2 yields were observed over 554 NiA2O4-CP catalyst in SRM reaction, especially at 550 ºC and 650 ºC. Furthermore, 555 among the three catalysts the best methane conversion and the largest H2, CO and CO2 556 yields were achieved with NiAl2O4-CP at the three reaction temperatures. Thus, at 557 450 ºC the NiAl2O4-D and 1%Rh/Al2O3 catalysts exhibited a poor performance as their 558 activity did not exceed 10% (the NiAl2O4-CP catalyst gave a conversion of 21% at this 559 temperature). It should be noted that the CO production, at 450 ºC, was very low or 560 negligible over the three tested catalysts. However, considerable yields of CO2 (0.18 561 over NiAl2O4-CP and 0.07 over NiAl2O4-D) and H2 (0.5 over NiAl2O4-CP and 0.22 562 over NiAl2O4-D) were obtained suggesting the main occurrence of water gas shift 563 reaction. On the effect of the SRM reaction temperature on the catalytic performance of 564 the Ni catalysts, we noted that higher temperatures improved conversion which rapidly 565 increased to reach about 80% and 53% over NiAl2O4-CP and NiAl2O4-D respectively 566 24 (at 650 ºC). Likewise, irrespective of the used catalyst both H2 and CO yields increased 567 as the SRM reaction temperature increased. This trend was more pronounced on the 568 NiAl2O4-CP catalyst which led to CO (0.47 at 650 ºC) and H2 (1.6 at 650 ºC) yields and 569 a H2/CO ratio close to the thermodynamic equilibrium. 570 On the other hand, as expected, the activity of the three catalysts was not accompanied 571 by the carbon deposition (Table 3) which might be explained by adding water to the 572 feed with a high H2O/CH4 ratio (around 3). The observed H2/CO ratio was in all the 573 cases higher than 6. As no significant carbon deposition was detected (Table 3), one 574 could conclude that this was the reason for the stability of the NiAl2O4-D and NiAl2O4-575 CP tested catalysts. Note that, for both NiAl2O4-D and NiAl2O4-CP catalysts, no 576 significant loss of their surface area was observed. XRD patterns of the NiAl2O4-D and 577 NiAl2O4-CP catalysts recorded after the tests did not show any noticeable difference 578 when compared with that of the freshly reduced samples suggesting that they did not 579 undergo any noticeable alteration of their crystalline structures during the reaction 580 (Fig. 2). However, an appreciable increase of Ni particle size was noticed in the case of 581 NiAl2O4-D. Since this did not affect its catalytic stability the Ni particles growth was 582 supposed to rapidly occur at the start of the reaction. Note that, while the stable catalytic 583 behaviour could be eventually masked by mass transfer limitations at 650 ºC, the fact is 584 that a good stability was also noticed at lower temperatures (450 and 550 ºC). In 585 principle, the contribution of mass transfer limitations to the observed catalytic 586 performance could be considered negligible under these conditions. In sum, it seems 587 that the experimentally reforming behaviour of the NiAl2O4-D actually was that of a 588 reduced spinel catalyst with a crystallite size of 47 nm (instead of 23 nm). This 589 behaviour could be related with the nature of the Ni species deposited and their 590 interactions with the carrier. Our characterisation results showed that the difference 591 25 between the two NiAl2O4-CP and NiAl2O4-D catalysts was related to the fact that the 592 reduction of the Ni species produced monodispersed fixed nickel on the first one 593 whereas it produced larger particles of free nickel on the second one. On the effect of 594 this distribution we could conclude, then, that the growth of Ni particles in SRM 595 reaction, observed exclusively on NiAl2O4-D catalyst, concerned only the free deposited 596 metallic Ni. 597 3.2.3. Oxidative steam reforming of methane (OSRM) 598 Finally the catalytic performance in OSRM reaction was also studied and the results are 599 collected in Fig. 8. At the three investigated reaction temperatures the best methane 600 conversion and the largest H2 and CO yields, over the Ni catalysts, were achieved with 601 the NiAl2O4-CP catalyst. Moreover, compared to its behaviour in POM reaction, a 602 significant improvement of the catalytic activity could be noted; especially at 450 ºC 603 and 550 ºC reaction temperatures where it was even more active than the 1%Rh/Al2O3 604 catalyst. By contrast, the NiAl2O4-D catalyst clearly gave the lowest methane 605 conversion. Furthermore, the BET surface area of this sample decreased from 48 to 606 37 m2 g-1 after reaction. Nevertheless, the water addition to the feed significantly 607 improved the catalytic stability of NiAl2O4-D with respect to the POM reaction. Indeed, 608 the literature reported that the excess of water or oxygen to the feed cleans the metallic 609 surface and improve the stability of the catalyst [47]. Accordingly, our estimation of 610 deposited coke, by thermogravimetric and TPO-MS analysis, was less than 1%wt. on 611 both NiAl2O4-D and NiAl2O4-CP catalysts. On the other hand, the combination of POM 612 and SRM gas mixtures seemed to increase the CO2 yield, which attained 0.41 on the 613 NiAl2O4-CP catalyst at 650 ºC. Moreover, CO/CO2 ratio did not exceed 0.5, which 614 could be explained by a high combustion activity and a low reforming activity [10]. 615 32 [43] T. Bruno, A. Beretta, G. Groppi, M. Roderi, P. Forzatti, Catal. Today 99 (2005) 762 89-98. 763 [44] Z.L. Zhang, X. E. Verykios, Catal. Today 21 (1994) 589-595. 764 [45] R. Jin, Y. Chen, W. Li, W. Cui, Y. Ji, C. Yu, Y. Jiang, Appl. Catal A 201 765 (2000) 71-80. 766 [46] S.Y. Foo, C.K. Cheng, T. Nguyen, E.M. Kennedy, B.Z. Dlugogorski, A.A. 767 Adesina, Catal. Commun. 26 (2012) 183-188. 768 [47] S.M. Lima, A.M. Silva, L.O.O. Costa, U.M. Graham, G. Jacobs, B.H. Davis, 769 L.V. Mattos, F. B. Noronha, J. Catal. 268 (2009) 268-281. 770 771 33 CAPTIONS FOR TABLES AND FIGURES 772 773 Table 1 Characterisation data of the synthesised nickel aluminate catalysts (calcined, reduced and used in POM, SRM and OSRM reactions). Table 2 Results from H2-TPR and XPS studies of the calcined NiAl2O4-D and NiAl2O4-CP samples. Table 3 Values for H2/CO and CO/CO2 ratios and coke content in POM, SRM and OSRM reactions over NiAl2O4-D and NiAl2O4-CP catalysts. Fig. 1 Mesoporous size distribution for (a) calcined and (b) reduced NiAl2O4-D and NiAl2O4-CP catalysts. Fig. 2 XRD patterns of NiAl2O4-D and NiAl2O4-CP catalysts: (a) calcined, (b) reduced, and tested in (c) POM, (d) SRM and (e) OSRM reactions. Fig. 3 H2-TPR profiles of NiAl2O4-D and NiAl2O4-CP catalysts. Fig. 4 UV–vis–NIR spectra of NiAl2O4-D and NiAl2O4-CP catalysts. Fig. 5 XPS spectra of Ni 2p3/2 region (A) and O 1s region (B) of NiAl2O4-D and NiAl2O4-CP catalysts. Fig. 6 TEM images and Ni particle size distribution of reduced NiAl2O4-D and NiAl2O4-CP catalysts. Fig. 7 NH3-TPD patterns of (a) calcined and (b) reduced NiAl2O4-D and NiAl2O4CP catalysts. Fig. 8 Methane conversion and H2, CO and CO2 yields over reduced NiAl2O4-CP and NiAl2O4-D catalysts versus reaction temperature in POM, SRM and OSRM reactions. Reactions conditions: 38400 cm3 CH4 g-1 h-1; W=0.125 g. Gas mixtures: POM: 10%CH4/5%O2/N2, SRM: 10%CH4/30%H2O/N2 and OSRM: 10%CH4/30%H2O/5%O2/N2. Data corresponding to the commercial 1%Rh/Al2O3 catalyst and the thermodynamic equilibrium were also included. Fig. 9 Evolution of CH4 conversion and H2 and CO yield over the reduced NiAl2O4D and NiAl2O4-CP catalysts with time on stream in POM reaction at 450, 550 and 650 C. Reactions conditions: 38400 cm3 CH4 g-1 h-1; W=0.125 g. Gas mixture: 10%CH4/5%O2/N2. Fig. 10 TPO-MS analysis over spent NiAl2O4-D and NiAl2O4-CP catalysts after POM reaction. 774 34 775 Samples Ni(1), wt.% SBET, m2 g1 Vp, cm3 g1 dp, nm Ni0 size, nm(2) Ni0 size, nm(3) mol(4) NH3 g1 mol(4) NH3 m2 Alumina 133 0.55 2.4 - -   NiAl2O4-D Calcined 33 55 0.14 7.5 - - 272 4.95 Reduced 48 0.12 8.1 23 17.7 274 5.70 POM 57 0.12 10.7 35 - - - SRM 47 0.16 11.2 47 - - - OSRM 37 0.11 9.4 29 - - - NiAl2O4-CP Calcined 33 76 0.35 15.4 - - 291 3.82 Reduced 55 0.32 19.2 11 10.6 240 4.38 POM 64 0.34 18.4 5 - - - SRM 52 0.33 20.3 11 - - - OSRM 52 0.32 17.7 7 - - - (1) Determined by WDXRF. (2) Ni0 crystallites size determined by XRD. (3) Ni0 crystallites size determined by TEM. (4) Total acidity determined by NH3-TPD. 776 Table 1 777 35 778 Catalyst H2-TPR XPS Theoretical H2 uptake mmol g-1 Experimental H2 uptake mmol g-1 Relative amount of reducible Ni species, % Ni/Al Ni 2p3/2 O 1s    Peak, eV Ni, % in the sites Satellite, eV Peak, eV NiAl2O4-D 5.7 5.8 33 18 49 1 854.1(1) 856.8(2) 61.7 38.3 861.3 528.5 (a) 531.1 (b) 532.6 (c) NiAl2O4-CP 5.7 5.6 9 47 44 0.6 855.7 (2) 100 861.8 530.6 (b) reductionpeak of NiO excess  and (***) reduction peaks of Ni2+ in nickel aluminate (1) Ni2+ as NiO and (2) Ni2+ as NiAl2O4. (a) O2− as NiO, (b) O2− as NiAl2O4 and (c) O2− as Al2O3 779 Table 2 780 781 782 783 784 785 786 36 Reaction Catalyst H2/CO CO/CO2 Coke, %(1) 450 550 650 450 550 650 - POM Equilibrium 21.3 6.0 2.7 0.19 1.06 5.86 - NiAl2O4-D 5.5 3.5 2.2 0.24 0.57 3.00 46.5 NiAl2O4-CP 12.7 4.4 2.5 0.10 0.60 3.00 2.5 1%Rh/Al2O3 2.5 2.8 2.4 0.74 1.29 3.46 2 SRM Equilibrium 44.9 11.9 6.7 0.10 0.45 1.09 - NiAl2O4-D 118.6 19.2 8.5 0.05 0.34 1.03 <1 NiAl2O4-CP 68.5 13.3 6.9 0.08 0.51 1.42 <1 1%Rh/Al2O3 19.7 12.6 8.2 0.31 0.56 1.06 <1 OSRM Equilibrium 36.3 12.1 7.3 0.07 0.28 0.56 - NiAl2O4-D 43.1 20.1 10.3 0.05 0.16 0.39 <1 NiAl2O4-CP 55.9 20.1 9.4 0.04 0.18 0.50 <1 1%Rh/Al2O3 8.9 10.8 7.8 0.14 0.31 0.65 <1 Reactions conditions: 38400 cm3 CH4 g-1 h-1; W=0.125 g. Gas mixtures: POM: 10%CH4/5%O2/N2, SRM: 10%CH4/30%H2O/N2 and OSRM: 10%CH4/30%H2O/5%O2/N2. 787 (1) Deposited carbon for used catalyst determined by TPO and TG analyses 788 789 790 Table 3791 37 792 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 NiAl2O4-CP NiAl2O4-D (a) Vads, cm3 g-1 (a) (b) (b) Pore diameter, nm 793 794 Fig. 1 795 796 797 798 38 10 20 30 40 50 60 70 80 (220) (440) Relative intensity, a. u. NiO Niº NiAl2O4 -Al2O3 NiAl2O4-D (c) (e) (d) (b) (a) C graphite Angle, 2 10 20 30 40 50 60 70 80 (e) (d) (c) (b) (a) C graphite NiAl2O4-CP Relative intensity, a. u. Angle, 2 799 Fig. 2 800 801 39 802 H2Consumption, mmol g-1 Temperature, ºC Time, min 200 400 600 800 20 30 Isothermal hold NiO Isothermal hold NiAl2O4-CP Isothermal hold -NiO -NiO -NiO (NiAl2O4) NiAl2O4-D H2Consumption, mmol g-1 Temperature, ºC Time, min 200 400 600 800 20 30 Isothermal hold NiO Isothermal hold NiAl2O4-CP Isothermal hold -NiO -NiO -NiO (NiAl2O4) NiAl2O4-D 803 804 Fig. 3 805 806 40 807 300 600 900 1200 1500 NiO NiAl2O4-D NiAl2O4-CP Ni2+ in Td 3 d-d transition of Ni2+ in Oh Ni2+ in Oh MLCT Ni2+ in Td d-d transitions of Ni2+ in unsupported NiO 1 d-d transition of Ni2+ in Oh Absorbance, a. u. Wavelenght, nm 808 809 Fig. 4 810 811 41 812 875 870 865 860 855 850 845 Intensity, a.u. Binding energy, eV Intensity, a.u. 540 535 530 525 520 Binding energy, eV NiAl2O4 NiO Al2O3 NiAl2O4-D NiAl2O4 NiO Sat (Ni 2p3/2) (A) (B) NiAl2O4-CP NiAl2O4-D NiAl2O4-CP 875 870 865 860 855 850 845 Intensity, a.u. Binding energy, eV Intensity, a.u. 540 535 530 525 520 Binding energy, eV NiAl2O4 NiO Al2O3 NiAl2O4-D NiAl2O4 NiO Sat (Ni 2p3/2) (A) (B) NiAl2O4-CP NiAl2O4-D NiAl2O4-CP 813 814 Fig. 5 815 2 Abstract 21 The catalytic performance of bulk nickel aluminate catalysts synthesised by co-22 precipitation (NiAl2O4-CP) and co-dissolution (NiAl2O4-D) was examined for various 23 methane reforming reactions, namely partial oxidation, steam reforming and oxidative 24 steam reforming. The calcined and reduced spinels were thoroughly characterised by a 25 wide number of analytical techniques including N2 physisorption, XRD, UV–visible–26 NIR DRS, XPS, TEM, H2-TPR and NH3-TPD. The characterisation results of the 27 calcined samples at 850 ºC showed that nickel aluminate phase was mainly obtained on 28 the NiAl2O4-CP near-surface together with a small NiO excess. By contrast, a large 29 amount of NiO phase was formed on NiAl2O4-D and deposited on the spinel surface. 30 The reduction at 850 ºC of the two samples produced monodispersed Ni particles (10.6 31 nm) in the NiAl2O4-CP while larger metallic nickel (17.7 nm) was deposited on the 32 NiAl2O4-D. In the three investigated reforming reactions the NiAl2O4-CP catalyst 33 proved to be highly active, stable and resistant toward carbon deposition. Moreover, the 34 performances of the NiAl2O4-CP catalyst appeared to be generally comparable with that 35 of a commercial 1%Rh/Al2O3 catalyst. The difference in catalytic behaviour between 36 the two nickel aluminates was related to the Ni dispersion, its particle size and its 37 capacity to minimize the acid character of the alumina by covering a large part of its 38 surface. 39 40 41 42 43 Keywords: Nickel aluminate, Co-precipitation, Co-dissolution, Ni Crystallite size, 44 Methane reforming 45 46 3 1. Introduction 47 Methane steam reforming (SRM) is the most used technology for producing synthesis 48 gas (CO and H2) from natural gas [1-5]. Moreover, this technology is a preferable 49 choice for producing clean and larger hydrogen yield. However, because of the high 50 cost of the endothermic character of SRM, the exothermic methane partial oxidation 51 (POM) appears as an alternative as it offers some interesting advantages especially 52 when the posterior use of the syngas stream requires a suitable H2/CO ratio [6-8]. 53 Currently industrial facilities are equipped with systems that offer the possibility of 54 using POM and/or SRM technologies. The combination of the POM and SRM strategies 55 called oxidative steam reforming (OSRM) has the advantage of controlling the process 56 heat and the distribution of the products by adjusting the O/C molar ratio [9-10]. 57 Ni-based catalysts are widely used for methane reforming reactions due to their high 58 activity and their competitive costs compared to noble metals [11-13]. Alumina 59 supported Ni systems have particularly received considerable attention because of their 60 practical applications for a variety of reforming reactions [14-18]. Nevertheless, despite 61 exhibiting advantageous catalytic properties, alumina lacks an adequate thermal stability 62 which provokes the sintering of supported metals. Furthermore, the use of alumina as 63 support material has the drawback of rapid deactivation of the catalyst due to carbon 64 deposition on its active acid surface [19-20]. For this reason, the interest of recent 65 investigations has been focused on the preparation of formulations leading to a high 66 dispersion of Ni species which can cover alumina surface, thus minimising the negative 67 effect of its acid character. In this sense, the effect of the addition of some alkali and 68 alkali earth metals, used as chemical promoters, on the activity and stability of 69 Ni/alumina catalysts has been extensively investigated. Typically these additives help to 70 4 avoid carbon deposition and enhance their stability but at the expense of a reduction of 71 their activity due to the blocking of the more reactive Ni sites [21,22]. 72 Within the same objective (the need to improve stability of Ni-based catalysts) nickel 73 aluminate synthesis is one of the routes proposed in the literature for the design of a 74 precursor capable to produce, after its reduction, metallic Ni presenting strong 75 interaction with alumina [14,15,23-25]. For so long, Ross et al. [25] had evidenced the 76 participation of the reduced surface nickel aluminate sites in the steam reforming of 77 methane over Ni/alumina catalysts. Since then, a number of studies dealing with the 78 preparation of nickel aluminate as precursor of active Ni metallic particles for methane 79 reforming reactions are available in the literature. A great majority of these works have 80 been devoted to the investigation of the influence of preparation methods on the 81 stability and the activity of the final formed Ni structures in reforming reactions 82 [14,15,24,26,27]. As stated in various references, it is however complicated to 83 synthesise pure or stoichiometric nickel aluminate by means of conventional methods of 84 synthesis. This is because of the formation of NiO as an excess even when heating at 85 very high temperatures (>1300 ºC) [28-30]. Therefore, many works have examined the 86 impact of this particularity on the catalysts performance [15,16,24,27,31]. 87 In our previous study, we compared two series of Ni/Al2O3 catalysts with similar metal 88 content prepared by co-impregnation, using solutions containing a mixture of nickel 89 acetate and aluminium nitrate to obtain a stoichiometric NiAl2O4 supported on alumina, 90 and simple impregnation of nickel on alumina [15]. We demonstrated that, in methane 91 steam reforming, the co-impregnated catalyst was more active and we attributed this 92 behaviour to its higher Ni–support interaction. We also noticed that obtaining the active 93 metallic Ni, with small particle size, was controlled by the NiO/NiAl2O4 ratio initially 94 present in the calcined samples. Achouri et al. [16] also studied the influence of the 95 5 preparation method on the catalytic activity in diesel steam reforming of two alumina-96 supported nickel aluminate catalysts prepared by wet-impregnation and co-precipitation. 97 They concluded that the impregnated catalyst exhibited a higher catalytic performance 98 and stability over time than the co-precipitated counterpart. This different behaviour 99 was explained by the higher carbon deposition tendency shown by the co-precipitated 100 sample. 101 In this paper the influence of the physico-chemical characteristics of two bulk nickel 102 aluminate catalysts, prepared by co-precipitation and co-dissolution methods, on their 103 catalytic behaviour in POM, SRM and OSRM reactions is investigated. To reach this 104 target, an extensive characterisation has been carried out by a wide number on analytical 105 techniques including BET measurements, XRD, XPS, UV–visible–NIR DRS, TEM and 106 H2-TPR. Likewise, TPD of NH3, a well-known probe molecule for acid sites, has been 107 used to characterise the surface chemistry of the investigated samples. A special 108 attention has been paid to the determination of the different nickel species formed after 109 high-temperature calcination and subsequent high-temperature reduction. In order to 110 examine and compare their catalytic efficiency the two prepared catalysts were 111 evaluated in partial oxidation, steam reforming and oxidative steam reforming of 112 methane. Interesting conclusions are drawn from the correlation between the 113 distribution of Ni active species of the two catalysts and their performance in these three 114 reforming reactions. 115 116 2. Experimental 117 2.1. Catalysts preparation 118 Bulk nickel aluminate samples were synthesised by co-precipitation (CP) and co-119 dissolution (D) methods. The sample named NiAl2O4-D was synthesised using two 120 6 aqueous solutions of Ni(CH3-COO)2·4H2O and Al(NO3)3·9H2O. Then they were mixed 121 (leading to a mixture with the desired 1:2 Ni/Al molar ratio), and evaporated on a hot 122 plate (150 ºC). In the case of NiAl2O4-CP, aqueous ammonia was added to the mixed 123 aqueous solutions to adjust the final suitable pH (=8). For comparative purposes NiO 124 was also prepared by simple calcination in air of Ni(CH3-COO)2·4H2O. All the samples 125 were dried at 110 ºC overnight and then calcined at 850 ºC in static air for 4 h at a 126 heating rate of 10 ºC min-1. Finally, pellets of bulk (nickel aluminate or nickel oxide) 127 samples were prepared by a process of compressing the powders into flakes in a 128 hydraulic press (Specac), crushing and sieving (0.3-0.5 mm). 129 130 2.2. Catalyst characterisation 131 The catalysts were characterised by N2 physisorption at -196 ºC, wavelength dispersive 132 X-ray fluorescence (WDXRF), X-ray diffraction (XRD), ultraviolet-visible-near 133 infrared diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy 134 (XPS), temperature programmed reduction with hydrogen (H2-TPR) and temperature 135 programmed desorption of NH3 (NH3-TPD). The experimental details of each analytical 136 technique are described elsewhere [14,15]. Additionally, the morphology and particle 137 size distribution of the nickel particles was examined by transmission electron 138 microscopy (TEM). Prior to analysis, the samples were dispersed in absolute ethanol 139 ultrasonically for 30 min, and 10 cm3 of each sample were then placed on flexible film 140 (Parafilm® M). Glow-discharged carbon-coated copper grids were inverted onto the 141 droplets of each sample. After incubation for 1 min at room temperature, the grids were 142 manually blotted with filter paper air-dried. Digitally recorded 2D images of each 143 solution were taken at room temperature at a nominal magnification of 80000 on a Jeol 144 JEM-1230 transmission electron microscope, with a LaB6 filament as the source of 145 7 electrons and operated at 100 kV. Digital images were recorded on an Orius SC1000 146 cooled slow-scan CCD camera, 4008×2672 pixels (GATAN), obtaining a final pixel 147 size of 0.85 Å pixel-1. The particle size distribution was obtained from the measurement 148 of at least 300 particles using ImageJ software, and the average diameter was calculated 149 by dM =di·ni/ni, where ni is the number of the particles of diameter di. 150 On the other hand, the amount of carbonaceous deposits on the used catalyst was 151 determined by dynamic thermogravimetry using a Setaram Setsys Evolution apparatus 152 under atmospheric pressure coupled to a Pfeiffer Prisma mass spectrometer (TPO-MS). 153 The mass loss and the sample temperature were continuously recorded by a 154 computerised data acquisition system. Previously, the samples (20 mg) were dried from 155 room temperature to 150 ºC. Then, the temperature was increased from 150 to 850 ºC at 156 a constant heating rate of 5 ºC min-1. The oxidant stream was 5%O2/He (50 cm3 min-1) 157 flowing downwards onto the cylindrical sample holder. 158 159 2.3. Catalytic tests 160 The three methane reforming reactions were studied in a bench-scale fixed-bed reactor 161 operated at atmospheric pressure. The reactor was made of stainless steel with an 162 internal diameter of 9 mm and a height of 305 mm. Prior to the reaction the catalyst 163 (0.125 g) was diluted with inert quartz (0.875 g, 1-1.25 mm). The catalyst bed was 164 maintained in the reactor on a quartz wool plug. The temperature was measured by a 165 thermocouple placed between the particles of the catalyst. Before the reaction the 166 NiAl2O4-D and NiAl2O4-CP catalysts were reduced in situ with a mixture of 167 5%H2/N2 at 850 °C for 2 h. Likewise, a 1%Rh/Al2O3 (Alfa Aesar, 132 m2 g−1) 168 commercial catalyst was reduced at 700 ºC and its activity was used for comparative 169 purposes in the three methane reforming reactions. 170 8 Three different feed gas mixtures balanced with N2 (38400 cm3 CH4 g−1 h−1) were used 171 in each reforming reaction as follows: 172 i) 10% CH4 and 5% O2 in POM reaction. 173 ii) 10% CH4 and 30% H2O in SRM reaction. 174 iii) 10% CH4, 30% H2O and 5% O2 in OSRM reaction. 175 The runs were sequentially carried out by increasing and decreasing the reaction 176 temperature (450 ºC-550 ºC-650 ºC-550 ºC-450 ºC) with an accumulated time online of 177 about 63 h. Catalytic activity, product yields and stability were recorded during 12.5 h 178 at each reaction temperature. Feed and effluent streams were analysed online by a 179 MicroGC (Agilent 3000) equipped with a TCD detector. Two columns, Molecular Sieve 180 5A and Plot U, were used in a series/bypass arrangement for the complete separation of 181 H2, N2, O2, CH4, CO and CO2. A cold trap at the outlet of the reactor was used to 182 condense out any water from the product gas stream. On basis of the molar flow at the 183 inlet and outlet of the reactor, conversion and product yields were calculated, according 184 to the following equations: 185         2 4 4 ,% 100   out out in F CO F CO X CH F CH (1) 186     2 2 4 ()2  out in FH YH F CH (2) 187     4 ()out in F CO Y CO F CH (3) 188     2 2 4 ()out in F CO Y CO F CH (4) 189 190 191 9 3. Results and discussions 192 3.1 Characterisation of the samples 193 3.1.1. N2-physorption (BET measurements) 194 N2 adsorption at -196 ºC on NiAl2O4-D and NiAl2O4-CP showed that the isotherms (not 195 shown) were characteristic of mesoporous solids of type IV according to the IUPAC 196 classification. In the case of NiAl2O4-D the nitrogen desorption gave rise to a hysteresis 197 loop, H2 type, which was characteristic of disordered porous materials. Table 1 lists the 198 data obtained from the analysis of the textural properties of the calcined and reduced 199 samples. The prepared NiAl2O4-CP spinel (76 m2 g-1) had a higher surface area than 200 NiAl2O4-D (55 m2 g-1). This difference probably resulted from the preparation method 201 and the corresponding proportion of NiO formed in each sample. Fig.1 shows the pore 202 sizes distribution for the NiAl2O4-D and NiAl2O4-CP catalysts. The pore size 203 distribution trace for the NiAl2O4-D sample exhibited one maximum at 7 nm in the low 204 mesoporous range (<10 nm) whereas the NiAl2O4-CP samples exhibited a peak centred 205 at 20 nm. After reduction at high temperature (850 ºC) both samples decreased their 206 total surface area (48 m2 g−1 for NiAl2O4–D and 55 m2 g−1 for NiAl2O4-CP); however it 207 did not influence considerably their pore size distribution. 208 3.1.2. X-ray diffraction (XRD) 209 The general formula of stoichiometric nickel aluminates (spinel structure) is NiAl2O4. It 210 crystallizes in the cubic system and belongs to Fd-3m space group [32-36]. Typically, 211 the framework of the “normal” spinel structures consists of an ensemble of tetrahedral 212 and octahedral coordination occupied by bivalent (Ni2+) and trivalent (Al3+) cations, 213 respectively [32,33]. However, this distribution can change when the Ni2+ partially 214 adopts the octahedral site while the tetrahedral site hosts the Al3+ together with Ni2+ 215 10 ions. This structural flexibility generates a family of compounds of inverse spinel 216 structure described as Ni1-xAlx[NixAl2-x]O4 (0 < x < 1) [32-36]. 217 The NiAl2O4-D and NiAl2O4-CP samples were characterized by means of X-ray powder 218 diffraction in order to investigate their structural properties. Fig. 2 compares the 219 diffractograms of the catalysts before and after reduction at 850 ºC. The patterns of the 220 calcined NiAl2O4-D and NiAl2O4-CP revealed the formation of the spinel structure 221 (JCPDS 78-1601). Furthermore, in the case of NiAl2O4-D catalyst additional peaks 222 associated with NiO structure, at 43.5° and 63.1°, were observed whereas the 223 diffractogram of NiAl2O4-CP showed no lines of NiO. A more careful analysis of the 224 pattern of NiAl2O4-D evidenced that the intensity ratio of the peaks corresponding to 225 (220) and (440) reticular planes differed from that of NiAl2O4-CP. This difference could 226 be explained by the cation distribution in the octahedral and tetrahedral sites affecting 227 the inversion degree of the spinel structure. The diffraction lines associated with the 228 (220) plane are related to the tetrahedrally-coordinated cations while the diffraction 229 signals belonging to the (440) plane are attributed to both tetrahedrally and 230 octahedrally-coordinated cations [34-36]. From our experimental data I(220)/I(440) 231 ratios were estimated. For the NiAl2O4-CP catalyst this ratio was 0.22, whereas it 232 increased up to 0.33 for the NiAl2O4-D. The increase in I(220)/I(440) ratio was also 233 observed by Wang et al. [35] in XRD patterns of various inverse spinel structures. This 234 feature was assigned to the increasing occupancy of the heavier cations on the 235 tetrahedral sites. We might accordingly conclude that on the NiAl2O4-D catalyst nickel 236 was preferentially hosted in tetrahedral sites of the inverse spinel whereas nickel tended 237 to occupy the octahedral coordination in the case of NiAl2O4-CP sample prepared by 238 co-precipitation. 239 11 Fig. 2 also includes the X-ray diffraction patterns of the bulk nickel aluminates reduced 240 at 850 °C. A comparison of the patterns before and after reduction confirmed that the 241 transformation of Ni2+ ions of the spinel framework into metallic Ni (JCPDS 89-7128) 242 was complete. Likewise, the formation of alumina structure (JCPDS 79-1558) instead of 243 the characteristic peaks attributed to spinel structure was noted. However, in the case of 244 the NiAl2O4-D sample, diffraction peaks with a low intensity attributed to NiO were 245 also observed. This could be reasonably explained by the surface room temperature 246 passivation of the catalyst [31]. Table 1 reports the metallic Ni crystallite size, 247 calculated by Scherrer equation, by using Ni (200) (2θ = 51.6°) diffraction line 248 broadening. The average size of Ni particles was found to be around 11 nm on the 249 reduced NiAl2O4-CP while larger particles with an average size of about 23 nm were 250 detected for the reduced NiAl2O4-D. 251 In sum, the obtained XRD results indicated that the preparation method of nickel 252 aluminate had an important effect on the excess of NiO and the Ni2+ ions distribution, 253 between tetrahedral and octahedral sites in the spinel structure. Moreover, the reduction 254 of the NiAl2O4-CP sample prepared by co-precipitation method produced smaller Ni 255 particles in comparison with the NiAl2O4-D catalyst. 256 3.1.3. Temperature programmed reduction (H2-TPR) 257 The TPR experiments were performed in order to determine the different Ni reducible 258 species present in the two prepared spinel catalysts (Fig. 3). It was observed that the 259 shape of the TPR traces depended on the preparation method. The thermogram of the 260 NiAl2O4-D sample displayed a typical reduction spectrum of a mixture of two Ni 261 species consisting of free NiO (-peak at 400 ºC) and nickel aluminates structures (-262 peak at 600 ºC and -peak at 800 ºC) [14-15]. In the case of NiAl2O4-CP catalyst it was 263 noted that the characteristic reduction peak attributed to bulk NiO (at 400 ºC) was 264 18 temperature promoted the H2 yield, which attained 0.56. Carbon monoxide production 415 also increased up to 0.44 at 650 ºC. It is noteworthy that at 650 ºC both conversion and 416 yields achieved with NiAl2O4-CP were close to the values obtained with the commercial 417 1%Rh/Al2O3 catalyst. As the performance of the noble metal-based catalyst is 418 considered as reference in the POM reaction [39-42], these catalytic features evidenced 419 the potential of NiAl2O4-CP as a promising alternative catalyst. By contrast, the activity 420 appeared to decrease with time on stream in the case of NiAl2O4-D catalyst. As shown 421 in Fig. 9, methane conversion at 450 ºC gradually decreased from 29% at the beginning 422 of the reaction to 25% after 12.5 h on stream. Likewise, H2, CO and CO2 yields were 423 not stable with time on line. Indeed, a slight increase in activity followed by a stable 424 plateau was observed when the reaction temperature decreased suggesting that an 425 activation of the catalyst occurred with time-on-stream (Fig. 9). 426 Table 3 lists the H2/CO and CO/CO2 ratios calculated for each reaction temperature on 427 the tested catalysts. Note that, on the two tested nickel catalysts, the CO/CO2 ratio 428 seemed to increase with reaction temperature. For instance, at 450 ºC the CO/CO2 ratio 429 on the NiAl2O4-CP was around 0.1 and it increased to reach about 3 at 650 ºC. At low 430 CO/CO2 values (corresponding to relatively low temperatures) CO disproportionation 431 reaction may occur producing CO2 and carbon. On the other hand, the H2/CO ratio was 432 in all the cases higher than 2 (ranging between 2.2 and 2.5). It is widely accepted that 433 the encapsulation of the metal particles by deposited carbon does not occur if H2/CO or 434 H2O/hydrocarbon ratios are sufficiently high [43]. Accordingly, our results showed, by 435 comparing the NiAl2O4-CP and NiAl2O4-D catalysts, that the catalyst with the highest 436 H2/CO ratio (NiAl2O4-CP) did not suffer apparent deactivation. 437 Thermogravimetric (not shown) and TPO-MS analyses (Fig. 10) performed in order to 438 determine the amount of carbon accumulated on the used catalysts in POM resulted in 439 19 deposited carbon masses of 2.4 wt.% on NiAl2O4-CP (close to 2 wt% observed for the 440 commercial rhodium catalyst) and 46.5 wt.% on NiAl2O4-D (Table 3). Both TPO traces 441 consisted of a CO2 production major peak at approximately 670 ºC (Fig. 10). Thus, the 442 same type of carbonaceous species was present on the two catalysts. It could be 443 concluded, therefore, that the deactivation of NiAl2O4-D catalyst might mainly occur as 444 a result of the substantial formation of coke. This was in agreement with XRD analysis 445 of the spent catalysts which confirmed the formation of carbon (graphite) on the two 446 catalysts. Specially, it was observed that the principal peak (26.4º) was much more 447 intense in the NiAl2O4-D diffractogram than that of NiAl2O4-CP catalyst (Fig. 2). On 448 the other hand, the Ni (200) (2θ = 51.6°) diffraction line broadening was used to 449 estimate, by Scherrer equation, the evolution of metallic Ni crystallite size after catalytic 450 test (Table 1). It was found that the growth of Ni0 crystallites during POM reaction on 451 NiAl2O4-D for extended periods of time was significant (size estimated to be around 452 23 nm in the reduced sample while it was around 35 nm after catalytic test). In 453 agreement with a previous study which reported that, in the methane reforming 454 reactions, the larger Ni crystallites on the Ni catalyst favoured the formation of graphitic 455 carbon [44], we could conclude that the same phenomenon occurred in the case of our 456 NiAl2O4-D catalyst. As stated previously, the reduction of the Ni species deposited on 457 this sample produced large metallic Ni which was less resistant to sintering and coke 458 formation. 459 The XRD diffractograms of the spent nickel aluminate catalysts also evidenced the 460 presence of NiO phase (Fig. 2). The NiO characteristic peaks were much more intense 461 on the NiAl2O4-CP diffractogram suggesting that the oxidation of Ni species during the 462 POM reaction was influenced by their interaction with the support. Accordingly, it 463 seemed that the oxidation of the smaller Ni particles, during POM reaction, might be 464 20 easier. Indeed, our H2-TPR and TEM studies indicated that more than two distinct 465 metallic nickel populations were present on NiAl2O4-D in contrast with the NiAl2O4-CP 466 catalyst with a narrower and more homogeneous size distribution (size = 10.6 nm). In 467 their study on the mechanism for POM reaction, Jin et al. [45] claimed that during the 468 CH4/O2 reaction over Ni/Al2O3 catalysts, Ni0 was first oxidised to NiO, and the latter 469 was reduced again by CH4 during the transient process. Since the reduction of NiO by 470 CH4 is endothermic and as the last step of our catalytic experiments occurred at 450 ºC 471 (relatively low temperature) the presence of the characteristic peaks of NiO in our XRD 472 diffractograms of the used samples could be justified. 473 On the other hand, as described in previous works the carbon deposition may deactivate 474 the catalyst either by covering of the active sites and/or by pore blocking [43-46]. In our 475 case, it was found that, after catalytic tests, the pore size distribution of NiAl2O4-D 476 catalyst shifted towards higher values (8.1 nm to 10.7 nm) suggesting the blocking of 477 the pores with small sizes by carbon deposition (Table 1). In addition, it should be noted 478 that carbon deposition on the NiAl2O4-CP and NiAl2O4-D catalysts appeared to 479 significantly increase the surface area (Table 1). Thus, on NiAl2O4-D the specific 480 surface area increased from 48 to 57 m2 g-1 whereas over NiAl2O4-CP catalyst it 481 increased from 55 to 64 m2 g-1, suggesting that this might be due to the similar porous 482 nature of the carbon deposited on the two catalysts [46]. 483 On the other hand, the chemical properties of their near surface could have an effect on 484 the catalytic activity and stability of NiAl2O4-D and NiAl2O4-CP catalysts. Generally, 485 acidity is considered to induce a negative impact on methane reforming behaviour by 486 catalysing the coke formation [43]. In agreement with our NH3-TPD studies, the surface 487 of NiAl2O4-D bears more acid sites than that of NiAl2O4-CP catalyst. This difference in 488 21 the acid character could explain the markedly larger formation of coke on the surface of 489 the NiAl2O4-D catalyst in comparison with NiAl2O4-CP. 490 In addition, the inversion degree of the formed spinel was another factor to take into 491 consideration in order to explain the different performances of NiAl2O4-D and NiAl2O4-492 CP in POM reaction. In their study on the methane dry reforming reaction over nickel 493 aluminate catalysts Kathiraser et al. [26] concluded that the inverse NiAl2O4 spinel 494 structure positively affected the catalytic activity compared to the normal spinel. Our 495 characterization results showed that NiAl2O4-CP tended to be in the inverse 496 coordination while the NiAl2O4-D was close to the normal spinel phase. The observed 497 divergence between NiAl2O4-D and NiAl2O4-CP might influence the catalytic 498 behaviour of these catalysts as well. 499 3.2.2. Steam reforming of methane (SRM) 500 Fig. 8 show CH4 conversion and H2, CO and CO2 yields in the SRM reaction for the 501 examined reduced catalysts as function of temperature. In all cases methane conversion 502 as well as H2, CO and CO2 yields were stable with time on stream. Moreover, the 503 NiAl2O4-CP and NiAl2O4-D catalysts were markedly more active than 504 1%Rh/Al2O3 (80% and 53% vs. 46% at 650 °C, respectively). The comparison of the 505 performance of the NiAl2O4-D catalyst in POM and SRM reactions showed that 506 replacing oxygen with steam resulted in poorer methane conversion and CO yield while 507 it improved the H2 production. By contrast, when compared to its performance in POM 508 reaction, an increase in activity as well as CO and H2 yields were observed over 509 NiA2O4-CP catalyst in SRM reaction, especially at 550 ºC and 650 ºC. Furthermore, 510 among the three catalysts the best methane conversion and the largest H2, CO and CO2 511 yields were achieved with NiAl2O4-CP at the three reaction temperatures. Thus, at 512 450 ºC the NiAl2O4-D and 1%Rh/Al2O3 catalysts exhibited a poor performance as their 513 22 activity did not exceed 10% (the NiAl2O4-CP catalyst gave a conversion of 21% at this 514 temperature). It should be noted that the CO production, at 450 ºC, was very low or 515 negligible over the three tested catalysts. However, considerable yields of CO2 (0.18 516 over NiAl2O4-CP and 0.07 over NiAl2O4-D) and H2 (0.5 over NiAl2O4-CP and 0.22 517 over NiAl2O4-D) were obtained suggesting the main occurrence of water gas shift 518 reaction. On the effect of the SRM reaction temperature on the catalytic performance of 519 the Ni catalysts, we noted that higher temperatures improved conversion which rapidly 520 increased to reach about 80% and 53% over NiAl2O4-CP and NiAl2O4-D respectively 521 (at 650 ºC). Likewise, irrespective of the used catalyst both H2 and CO yields increased 522 as the SRM reaction temperature increased. This trend was more pronounced on the 523 NiAl2O4-CP catalyst which led to CO (0.47 at 650 ºC) and H2 (1.6 at 650 ºC) yields and 524 a H2/CO ratio close to the thermodynamic equilibrium. 525 On the other hand, as expected, the activity of the three catalysts was not accompanied 526 by the carbon deposition (Table 3) which might be explained by adding water to the 527 feed with a high H2O/CH4 ratio (around 3). The observed H2/CO ratio was in all the 528 cases higher than 6. As no significant carbon deposition was detected (Table 3), one 529 could conclude that this was the reason for the stability of the NiAl2O4-D and NiAl2O4-530 CP tested catalysts. Note that, for both NiAl2O4-D and NiAl2O4-CP catalysts, no 531 significant loss of their surface area was observed. XRD patterns of the NiAl2O4-D and 532 NiAl2O4-CP catalysts recorded after the tests did not show any noticeable difference 533 when compared with that of the freshly reduced samples suggesting that they did not 534 undergo any noticeable alteration of their crystalline structures during the reaction 535 (Fig. 2). However, an appreciable increase of Ni particle size was noticed in the case of 536 NiAl2O4-D. Since this did not affect its catalytic stability the Ni particles growth was 537 supposed to rapidly occur at the start of the reaction. This behaviour could be related 538 23 with the nature of the Ni species deposed and their interactions with the carrier. Our 539 characterisation results showed that the difference between the two NiAl2O4-CP and 540 NiAl2O4-D catalysts was related to the fact that the reduction of the Ni species produced 541 monodispersed fixed nickel on the first one whereas it produced larger particles of free 542 nickel on the second one. On the effect of this distribution we could conclude, then, that 543 the growth of Ni particles in SRM reaction, observed exclusively on NiAl2O4-D 544 catalyst, concerned only the free deposited metallic Ni. 545 3.2.3. Oxidative steam reforming of methane (OSRM) 546 Finally the catalytic performance in OSRM reaction was also studied and the results are 547 collected in Fig. 8. At the three investigated reaction temperatures the best methane 548 conversion and the largest H2 and CO yields, over the Ni catalysts, were achieved with 549 the NiAl2O4-CP catalyst. Moreover, compared to its behaviour in POM reaction, a 550 significant improvement of the catalytic activity could be noted; especially at 450 ºC 551 and 550 ºC reaction temperatures where it was even more active than the 1%Rh/Al2O3 552 catalyst. By contrast, the NiAl2O4-D catalyst clearly gave the lowest methane 553 conversion. Furthermore, the BET surface area of this sample decreased from 48 to 554 37 m2 g-1 after reaction. Nevertheless, the water addition to the feed significantly 555 improved the catalytic stability of NiAl2O4-D with respect to the POM reaction. Indeed, 556 the literature reported that the excess of water or oxygen to the feed cleans the metallic 557 surface and improve the stability of the catalyst [47]. Accordingly, our estimation of 558 deposited coke, by thermogravimetric and TPO-MS analysis, was less than 1%wt. on 559 both NiAl2O4-D and NiAl2O4-CP catalysts. On the other hand, the combination of POM 560 and SRM gas mixtures seemed to increase the CO2 yield, which attained 0.41 on the 561 NiAl2O4-CP catalyst at 650 ºC. Moreover, CO/CO2 ratio did not exceed 0.5, which 562 could be explained by a high combustion activity and a low reforming activity [10]. 563 24 Fig. 2 also shows the XRD patterns of the NiAl2O4-D and NiAl2O4-CP catalysts used in 564 OSRM reaction. The absence of 26.4º diffraction peak indicated that, on both catalysts, 565 no graphite carbon deposition occurred during the tests. By contrast, changes in the Ni 566 crystallite size were noted. For the NiAl2O4-D catalyst the Ni particle size increased 567 from 22 up to 29 nm, whereas for the NiAl2O4-CP it decreased from 11 to 7 nm 568 (Table 1). Similar changes were observed on the XRD patterns of the latter in POM 569 reaction suggesting that a re-distribution of Ni active phases might have occurred which 570 favoured its activity and stability. However, on the NiAl2O4-D catalyst this re-571 distribution (increased Ni particle size) provoked the decay of its activity. The XRD 572 diffractograms of the used NiAl2O4-D and NiAl2O4-CP catalysts in OSRM reaction also 573 showed the presence of intense diffraction peaks attributed to the NiO phase (Fig. 2). A 574 similar behaviour was reported by Yoshida et al. [10] in their study of OSR reaction 575 over Ni/α-Al2O3 and they explained it by the oxidation of the Ni species in the presence 576 of gas-phase oxygen. The formation of these Ni oxidised species could explain the high 577 activity of our Ni catalysts for methane combustion. 578 4. Conclusions 579 The behaviour of two bulk nickel aluminate (NiAl2O4-CP and NiAl2O4-D) has been 580 investigated in the partial oxidation, steam reforming and oxidative steam reforming of 581 methane. These catalysts have been prepared by co-precipitation and co-dissolution 582 methods, respectively, and characterised by N2 physisorption, XRD, UV–visible–NIR 583 DRS, XPS, TEM, H2-TPR and NH3-TPD. 584 XRD, H2-TPR, UV–visible spectroscopy and XPS analyses show that the two catalysts, 585 calcined at 850 ºC, contain nickel aluminate as a major phase together with NiO which 586 was considered as an excess. Moreover, it has been proved that on the NiAl2O4-D 587 catalyst nickel is preferentially hosted in tetrahedral sites of the inverse spinel whereas it 588 25 tends to occupy the octahedral coordination in the case of NiAl2O4-CP sample. As 589 showed by H2-TPR, the interaction of the resulted NiO excess with the nickel aluminate 590 seems to depend on the preparation method. Indeed, the reduction of this NiO excess on 591 the NiAl2O4-D occurs at lower temperatures compared to the NiAl2O4-CP suggesting 592 that on the latter NiO has a strong interaction with nickel aluminate. TEM results 593 indicate that on the NiAl2O4-CP catalyst the reduction of the Ni species at 850 ºC 594 produces homogeneous and monodispersed fixed nickel whereas heterogeneous and 595 larger metallic free nickel is detected on the NiAl2O4-D catalyst. In addition, this 596 distribution has a marked effect on the chemical properties of the reduced catalysts 597 since the surface of reduced NiAl2O4-D contains more acid sites than that of reduced 598 NiAl2O4-CP. 599 A clearly different behaviour in the reforming of methane has been observed over the 600 NiAl2O4-CP and NiAl2O4-D catalysts with same Ni composition but prepared by two 601 different methods. The results show that the reforming efficiency is highly dependent on 602 the type of interaction of the Ni active phase with the carrier. In the three tested 603 reforming reactions the NiAl2O4-CP catalyst, prepared by co-precipitation method, has 604 proved to be highly active and stable. However, as a result of the carbon deposition 605 found in POM reaction, methane conversion, H2, CO and CO2 yields are not stable with 606 time over the NiAl2O4-D catalyst. At low POM reaction temperatures (450 ºC and 607 550 ºC) CO disproportionation reaction occurs producing CO2 and carbon. On the other 608 hand, the H2/CO ratio has resulted in all the cases higher than 2. By using steam instead 609 of oxygen no significant carbon deposition has been detected on NiAl2O4-D catalyst, 610 but it leads to lower methane conversion and CO yields while it improves the H2 611 production. By contrast, it has been observed that this change of the composition of the 612 reaction gas mixture is beneficial in the case of the NiAl2O4-CP catalyst. On both nickel 613 26 aluminate catalysts the main occurrence of water gas shift reaction at low temperature is 614 evident. The combination of POM and SRM gas mixtures seems to increase the 615 CO2 yield and to decrease CO yields suggesting that there are high combustion activity 616 and low reforming activity. 617 Concerning the behaviour of active metallic Ni species laying on the catalysts, the most 618 important finding reported in this study is the remarkable stability of the reduced 619 NiAl2O4-CP catalyst, which maintains its Ni particle size during the reforming 620 reactions. However, the presence of free metallic Ni (with larger size) on the NiAl2O4-D 621 catalyst, which displayed an increase in its particle size, provokes a decay of its activity 622 and stability. 623 Acknowledgements 624 The authors wish to thank the financial support for this work provided by the Spanish 625 Science and Innovation Ministry (CTQ2010-16752), the Basque Government 626 (PRE_2013_2_453, IT657-13) and the University of The Basque Country (UFI 11/39). 627 Technical and human support from SGIker (XRD (A. Larrañaga), WDXRF (F.J. 628 Sangüesa), XPS (M.B. Sánchez) and UV-vis-DRS (L.J. Bartolomé)) and CIC bioGUNE 629 (D. Gil and S. Delgado) is also gratefully acknowledged. 630 5. References 631 [1] Y. Men, G. Kolb, R. Zapf, M. O’Connell, A. Ziogas, Methanol steam reforming 632 over bimetallic Pd-In/Al2O3 catalysts in a microstructured reactor, Appl. Catal. 633 A 380 (2010) 15-20. 634 [2] P. Engelhardt, M. Maximini, F. Beckmann, M. Brenner, Integrated fuel cell 635 APU based on a compact steam reformer for diesel and a PEMFC, Int. J. 636 Hydrogen Energy 37 (2012) 13470-13477. 637 27 [3] O. Pasdag, A. Kvasnicka, M. Steffen, A. Heinzel, Highly integrated steam 638 reforming fuel processor with condensing burner technology for maximised 639 electrical efficiency of CHP-PEMFC systems, Energy Procedia 28 (2012) 57-640 65. 641 [4] M. O’Connell, G. Kolb, K.P. Schelhaas, J. Schuerer, D. Tiemann, A. Ziogas, V. 642 Hessel, Development and evaluation of a microreactor for the reforming of 643 diesel fuel in the kW range, Int. J. Hydrogen Energ. 34 (2009) 6290-6303. 644 [5] J. Thormann, L. Maier, P. Pfeifer, U. Kunz, O. Deutschmann, K. Schubert, 645 Steam reforming of hexadecane over a Rh/CeO2 catalyst in microchannels: 646 experimental and numerical investigation, Int. J. Hydrogen Energ. 34 (2009) 647 5108-5120. 648 [6] V.A. Tsipouriari, Z. Zhang, X.E. Verykios, Catalytic partial oxidation of 649 methane to synthesis gas over Ni-based catalysts: I. Catalyst performance 650 characteristics, J. Catal. 179 (1998) 283-291. 651 [7] P. Ferreira-Aparicio, M.J. Benito, J.L. Sanz, New trends in reforming 652 technologies: from hydrogen industrial plants to multifuel microreformers, 653 Catal. Rev. Sci. Eng. 47 (2005) 491-588. 654 [8] Y. Song, H. Liu, S. Liu, D. He, Partial oxidation of methane to syngas over 655 Ni/Al2O3 catalysts prepared by a modified sol-gel method, Energ. Fuel 23 656 (2009) 1925-1930. 657 [9] U.S. Amjad, A. Vita, C. Galletti, L. Pino, S. Specchia, Comparative study on 658 steam and oxidative steam reforming of methane with noble metal catalysts, 659 Ind. Eng. Chem. Res. 52 (2013) 15428-15436. 660 34 776 Samples Ni(1), wt.% SBET, m2 g-1 Vp, cm3 g-1 dp, nm Ni0 size, nm(2) Ni0 size, nm(3) mol(4) NH3 g-1 mol(4) NH3 m-2 Alumina 133 0.55 2.4 - -   NiAl2O4-D Calcined 33 55 0.14 7.5 - - 272 4.95 Reduced 48 0.12 8.1 23 17.7 274 5.70 POM 57 0.12 10.7 35 - - - SRM 47 0.16 11.2 47 - - - OSRM 37 0.11 9.4 29 - - - NiAl2O4-CP Calcined 33 76 0.35 15.4 - - 291 3.82 Reduced 55 0.32 19.2 11 10.6 240 4.38 POM 64 0.34 18.4 5 - - - SRM 52 0.33 20.3 11 - - - OSRM 52 0.32 17.7 7 - - - (1) Determined by WDXRF. (2) Ni0 crystallites size determined by XRD. (3) Ni0 crystallites size determined by TEM. (4) Total acidity determined by NH3-TPD. 777 Table 1 778 35 779 Catalyst H2-TPR XPS Relative amount of reducible Ni species, % Ni/Al Ni 2p3/2 O 1s    Peak, eV Ni, % in the sites Satellite, eV Peak, eV NiAl2O4-D 33 18 49 1 854.1(1) 856.8(2) 61.7 38.3 861.3 528.5 (a) 531.1 (b) 532.6 (c) NiAl2O4-CP 9 47 44 0.6 855.7 (2) 100 861.8 530.6 (b) reductionpeak of NiO excess  and (***) reduction peaks of Ni2+ in nickel aluminate (1) Ni2+ as NiO and (2) Ni2+ as NiAl2O4. (a) O2− as NiO, (b) O2− as NiAl2O4 and (c) O2− as Al2O3 780 Table 2 781 782 783 784 785 786 787 36 Reaction Catalyst H2/CO CO/CO2 Coke, %(1) 450 550 650 450 550 650 - POM Equilibrium 21.3 6.0 2.7 0.19 1.06 5.86 - NiAl2O4-D 5.5 3.5 2.2 0.24 0.57 3.00 46.5 NiAl2O4-CP 12.7 4.4 2.5 0.10 0.60 3.00 2.5 1%Rh/Al2O3 2.5 2.8 2.4 0.74 1.29 3.46 2 SRM Equilibrium 44.9 11.9 6.7 0.10 0.45 1.09 - NiAl2O4-D 118.6 19.2 8.5 0.05 0.34 1.03 <1 NiAl2O4-CP 68.5 13.3 6.9 0.08 0.51 1.42 <1 1%Rh/Al2O3 19.7 12.6 8.2 0.31 0.56 1.06 <1 OSRM Equilibrium 36.3 12.1 7.3 0.07 0.28 0.56 - NiAl2O4-D 43.1 20.1 10.3 0.05 0.16 0.39 <1 NiAl2O4-CP 55.9 20.1 9.4 0.04 0.18 0.50 <1 1%Rh/Al2O3 8.9 10.8 7.8 0.14 0.31 0.65 <1 Reactions conditions: 38400 cm3 CH4 g-1 h-1; W=0.125 g. Gas mixtures: POM: 10%CH4/5%O2/N2, SRM: 10%CH4/30%H2O/N2 and OSRM: 10%CH4/30%H2O/5%O2/N2. 788 (1) Deposited carbon for used catalyst determined by TPO and TG analyses 789 790 791 Table 3792 37 793 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 NiAl2O4-CP NiAl2O4-D (a) Vads, cm3 g-1 (a) (b) (b) Pore diameter, nm 794 795 Fig. 1 796 797 798 799 38 10 20 30 40 50 60 70 80 (220) (440) Relative intensity, a. u. NiO Niº NiAl2O4 -Al2O3 NiAl2O4-D (c) (e) (d) (b) (a) C graphite Angle, 2 10 20 30 40 50 60 70 80 (e) (d) (c) (b) (a) C graphite NiAl2O4-CP Relative intensity, a. u. Angle, 2 800 Fig. 2 801 802 39 803 H2Consumption, mmol g-1 Temperature, ºC Time, min 200 400 600 800 20 30 Isothermal hold NiO Isothermal hold NiAl2O4-CP Isothermal hold -NiO -NiO -NiO (NiAl2O4) NiAl2O4-D H2Consumption, mmol g-1 Temperature, ºC Time, min 200 400 600 800 20 30 Isothermal hold NiO Isothermal hold NiAl2O4-CP Isothermal hold -NiO -NiO -NiO (NiAl2O4) NiAl2O4-D 804 805 Fig. 3 806 807 40 808 300 600 900 1200 1500 NiO NiAl2O4-D NiAl2O4-CP Ni2+ in Td 3 d-d transition of Ni2+ in Oh Ni2+ in Oh MLCT Ni2+ in Td d-d transitions of Ni2+ in unsupported NiO 1 d-d transition of Ni2+ in Oh Absorbance, a. u. Wavelenght, nm 809 810 Fig. 4 811 812 41 813 875 870 865 860 855 850 845 Intensity, a.u. Binding energy, eV Intensity, a.u. 540 535 530 525 520 Binding energy, eV NiAl2O4 NiO Al2O3 NiAl2O4-D NiAl2O4 NiO Sat (Ni 2p3/2) (A) (B) NiAl2O4-CP NiAl2O4-D NiAl2O4-CP 875 870 865 860 855 850 845 Intensity, a.u. Binding energy, eV Intensity, a.u. 540 535 530 525 520 Binding energy, eV NiAl2O4 NiO Al2O3 NiAl2O4-D NiAl2O4 NiO Sat (Ni 2p3/2) (A) (B) NiAl2O4-CP NiAl2O4-D NiAl2O4-CP 814 815 Fig. 5 816 42 010 20 30 0 2 4 6 8 Frequency, % Average size of Ni0, nm 0 20 40 60 80 100 Accumulated Frequency, % 20 nm NiAl2O4-CP 010 20 30 0 2 4 6 0 20 40 60 80 100 Frequency, % Average size of Ni0, nm Accumulated Frequency, % 20 nm NiAl2O4-D 010 20 30 0 2 4 6 8 Frequency, % Average size of Ni0, nm 0 20 40 60 80 100 Accumulated Frequency, % 010 20 30 0 2 4 6 8 Frequency, % Average size of Ni0, nm 0 20 40 60 80 100 Accumulated Frequency, % 20 nm NiAl2O4-CP 20 nm20 nm NiAl2O4-CP 010 20 30 0 2 4 6 0 20 40 60 80 100 Frequency, % Average size of Ni0, nm Accumulated Frequency, % 010 20 30 0 2 4 6 0 20 40 60 80 100 Frequency, % Average size of Ni0, nm Accumulated Frequency, % 20 nm NiAl2O4-D 20 nm20 nm NiAl2O4-D 817 Fig. 6818 43 100 200 300 400 500 600 700 800 -Al2O3 NiAl2O4-D (a) (a) (b) (b) 340 ºC 367 ºC 256 ºC 348 ºC 323 ºC Temperature, ºC dTG Inverted Signal, a.u. NiAl2O4-CP 100 200 300 400 500 600 700 800 -Al2O3 NiAl2O4-D (a) (a) (b) (b) 340 ºC 367 ºC 256 ºC 348 ºC 323 ºC Temperature, ºC dTG Inverted Signal, a.u. NiAl2O4-CP 819 820 Fig. 7 821