In Situ XAS Study of Synergic Effects on Ni–Co/ZrO2 Methane Reforming Catalysts

Subscriber access provided by Red de Bibliotecas del CSIC The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Article In Situ XAS Study of Synergic Effects on Ni-Co/ZrO2 Methane Reforming Catalysts Victor Manuel Gonzalez de la Cruz, Rosa Pereñiguez, Fatima Ternero, Juan Pedro Holgado, and Alfonso Caballero J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp2092048 • Publication Date (Web): 16 Dec 2011 Downloaded from http://pubs.acs.org on January 12, 2012 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. 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Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. 1 In situ XAS study of synergic effects on Ni-Co/ZrO2 methane reforming catalysts Victor M. Gonzalez-delaCruz, Rosa Pereñiguez, Fatima Ternero, Juan P. Holgado and Alfonso Caballero* Instituto de Ciencia de Materiales de Sevilla (CSIC-University of Seville) and Departamento de Quimica Inorganica, University of Seville. Avda. Américo Vespucio, 49. 41092. Seville, Spain. cab[email protected] Abstract Four different mono and bimetallic Ni-Co/ZrO2 catalysts have been studied by means of in situ XAS, X-ray diffraction, TPR and measurements of the catalytic activity in the dry reforming reaction of methane (DRM). Even though the cobalt monometallic system has no activity for the methane reforming reaction, both bimetallic catalysts (with 1:1 and 1:2 Ni:Co ratio, respectively), showed a better activity and stability than the nickel monometallic system. The XRD data indicate that a mixed cobalt-nickel spinel is formed by calcination of the precursor solids, leading to the formation of an alloy of both metals after reduction in hydrogen. In situ XAS experiments showed a much better resistance of metals in the bimetallic systems to be oxidized under reaction conditions at temperatures until 750ºC. After these results, we proposed the formation in the bimetallic systems of a more reducible nickel-cobalt alloy phase, which remains completely metallic in contact with the CO2/CH4 reaction mixture at any temperature. The presence of adjacent nickel and cobalt sites seems to avoid the deactivation of cobalt in the DRM reaction. In the case of cobalt sites, the presence of adjacent nickel atoms seems to prevent the deposition of carbon over the cobalt sites, now showing its higher activity in the dry reforming reaction. Simultaneously, this higher activity of the cobalt sites in the bimetallic system produces more hydrogen as a product, maintaining the nickel atoms completely reduced under reaction conditions. This synergic effect accounts for the better performance of the bimetallic systems and points at both, the oxidation state of nickel particles under reaction conditions and the carbon deposition processes, as important factors responsible for differences in catalytic activities and stabilities in this hydrocarbon reaction. Page 1 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 Keywords: Nickel catalysts, in situ XAS, Nickel cobalt alloy, Methane reforming. 1. Introduction The recent discoveries of new unconventional natural gas fields has increased the interest for the reforming reactions of methane, actually one of the most important industrial reactions [1,2]. Ni-based catalysts are the more performing and economical catalytic systems for this kind of hydrocarbon reforming reactions [3-5], and in particular for the steam reforming of methane, nowadays one of the main source for hydrogen production [6]. Beside this reaction, the dry reforming of methane (DRM), using carbon dioxide as oxidant, has been lately extensively studied as an alternative, even though it has been pointed out as impractical for commercial hydrogen generation [7]. In spite of that, there is a great potential in the application of CO2 reforming of methane in environmental areas, such as the elimination of CO2 emissions from natural gas deposits or the utilization of these two greenhouse gases for obtaining synthesis gas (CO/H2) [8,9] which, in different proportions, is extensively used in industrial processes as methanol synthesis, hydrogenations or Fischer–Tropsch reactions [10]. Both noble (Rh, Ru, Pt, and Pd) and non-noble metals (Fe, Co, and Ni) have been tested for the DRM reaction [11-12], although nickel has been considered the best replacement for noble metals mainly due to its high catalytic performance and low cost. However, these kinds of Ni-based materials are deactivated in DRM reaction conditions, mainly due to the deposition of carbonaceous residues[3,13]., which hinders their use for long term applications in industrial practices. As stated in many previous papers, factors as dispersion or the interaction of the nickel metallic phase with the support or other metals strongly determine the catalytic performances of these catalysts [14-19]. In the last few years, a numbers of publications have shown as the combination of nickel and cobalt improves the catalytic performance in different hydrocarbon reactions as the dry and steam reforming of methane and others [20-26]. In this work, we have prepared some Ni and Co mono and bimetallic catalytic systems, studying in situ how their interactions affect to the activity and, especially the stability in the DRM reaction. Important synergetic effects have been found affecting the Page 2 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 catalytic performances, dispersion, reducibility and oxidation states of metals under reaction conditions. 2. Experimental Catalysts preparation Four 26wt%Ni/ZrO2 (ZNi26), 26wt%Co/ZrO2 (ZCo26), 13wt%Ni-13wt%Co/ZrO2 (ZNiCo26-11) and 8.7wt%Ni-17.4wt%Co/ZrO2 (ZNiCo26-12) were prepared by a onestep incipient wetness impregnation of a monoclinic zirconia support with nickel and/or cobalt nitrate solution of appropriate concentration (Ni(NO3)2·6H2O and Co(NO3)2·6H2O from Aldrich). Zirconia support was synthesized via forced hydrolysis [27] using an aqueous solution of ZrO(NO3)2·xH2O (from Aldrich). The powder was calcined in air at 850ºC during 3 hours, a temperature higher than the reaction temperature (750ºC), in order to prevent any change on the support under reaction temperatures. X-ray diffraction (XRD) X-ray diffractograms were recorded in a Panalytical XPert PRO device, equipped with a X’Celerator Detector (active range of 2θ = 2.18o), with a Bragg-Brentano configuration, using CuKα (λ =1.5418 Å). General patterns diagrams were collected in the range 2θ = 20-80o, with a step of 0.05o and an acquisition time of 80 s for each point. To calculate the mean size of the crystalline particles of Nickel and nickel oxide by Scherrer formula, a spherical particle shape was assumed and X-ray diffractograms were also collected in the range of 2θ = 35-55o, with a step of 0.03o during 100s each point. Temperature Programmed Reduction (TPR) TPR experiments were done from room temperature up to 700 ◦C, with a heating rate of 10ºC/min. A thermal conductivity detector (TCD), previously calibrated using CuO, and a mass spectrometer in line with the TCD, calibrated with reference mixtures, were used to detect variations of reducing agent concentration, and possible subproducts formation. A H2/Ar mixture (5% H2, 50 ml/min flow) was used for H2reduction. All the experimental conditions were chosen to assure that no peak coalescence occurs [28]. Page 3 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 Catalytic activity tests Reaction was carried out in a fixed-bed tubular reactor described elsewhere [29], using 40 mg of catalysts between two pompons of quartz wool. Before reaction, samples were reduced with a 5% H2/Ar mixture at 750ºC during 1 hour. The CH4 and CO2 reactants were mixed at a ratio of 1 diluted in He (10:10:80 in volume). Samples were heated under a gas flow of 100 ml/min from room temperature up to 750ºC at 1ºC/min rate, held at 750ºC during 12 h, and finally cooled down to room temperature in the same reaction mixture. Reactives and products were analyzed by means of a gas chromatograph (Varian, CP-3800) equipped with a thermal conductivity detector (TCD) and a Porapak Q and a molecular sieve packed columns. X-ray absorption spectroscopy (XAS) X-ray absorption spectra were recorded at the BM25 beam line (SPLINE) of the ESRF synchrotron (Grenoble, France). The spectra were acquired in transmission mode, using self-supported wafers of the Ni and/or Co/ZrO2 samples, in a modified commercial infrared cell (Specac) able to work up to 800ºC under controlled atmosphere [29]. XAS spectra were collected in situ in contact with hydrogen or the reactive gas flow during the treatments of the samples at selected temperatures from room temperature to 750ºC. In all cases the self-supported pellets were prepared using the optimum weight to maximize the signal-to-noise ratio in the ionization chambers (log I0/I1 ≈ 1). Mass flow controllers were used for dosing the gases to the cell. The composition of the gas mixtures were similar to that previously used in the catalytic activity measurements. For energy calibration, a standard Ni or Co foil was introduced after the second ionization chamber and measured simultaneously. Typical XAS spectra were recorded from 8200 to 9100 eV for Ni K-edge and from 7500 to 8700 eV for Co K-edge, with a variable step energy value, with a minimum 0.5 eV step across the XANES region. Once extracted from the XAS spectra, the EXAFS oscillations were Fourier transformed in the range 2–12.5 Å−1. Spectra were analyzed using the software package IFEFFIT [30]. The theoretical paths for Ni–Ni, Ni–O, Co-Co and Co-O species used for fitting the first Page 4 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 5 coordination shell of the experimental data were generated using the ARTEMIS program and the FEFF 7.0 program [31]. The coordination number, interatomic distance, Debye–Waller factor and inner potential correction were used as variable parameters for the fitting procedures. Reference spectra for metallic Ni, Co, NiO and CoO were recorded using standard reference samples. 3. Results and discussion 3.1. Physical and chemical characterization of catalytic systems The 26wt%Ni/ZrO2 (ZNi26), 26wt%Co/ZrO2 (ZCo26), 13wt%Ni-13wt%Co/ZrO2 (ZNiCo26-11) and 8.7wt%Ni-17.4wt%Co/ZrO2 (ZNiCo26-12) samples were characterized by means of X-Ray diffraction (XRD) and Temperature Programmed Reduction (TPR). Figure 1 includes the XRD diagrams obtained for the calcined samples, included those of zirconium oxide and some nickel and cobalt oxides as references. As shown in Table 1, in all samples the zirconia presents monoclinic phase crystallites of about 20 nm in diameter, as estimated by applying the Scherrer formula. The monometallic ZNi26 and ZCo26 samples present peaks characteristic for NiO and Co3O4, respectively, both with sizes around 45 nm. In the case of both bimetallic systems, two different oxides are detected: NiO (ca. 19 nm in diameter), and NiCo2O4 (20-25 nm). Although it is hard to differentiate between cobalt spinel and nickel-cobalt spinel, they can be distinguish by the shift in the position of the peak near 37o ((311) of spinel structure; (111) of NiO structure). As shown in Figure 2a, this peak has a lower position in the bimetallic sample, shifting from about 36.9o in the cobalt monometallic system to about 36.7o in the Ni-Co bimetallic one. The smaller 2-theta value is consistent with the larger value of the NiCo2O4 lattice constant [32], and unambiguously identified the presence of this ternary phase. After these results, and without ruling out the presence of amorphous phases, the presence of an oxidized bimetallic phase in the calcined state is clear. In fact, the cobalt is forming part of a spinel phase with nickel, which according to the stoichiometry of the catalysts, could be the majority in the ZNiCo26-12 catalyst. It is also worth noting that, in agreement with the findings of other authors [20,26], the presence of both metals, Ni and Co, has improved the dispersion of the oxidized phases on the zirconia surface, as the crystallite sizes are about half of that on the monometallic samples. Page 5 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 6 The TPR profile obtained for the different catalytic systems also show differences depending on its metallic composition. As shown in Figure 3, both monometallic systems present a main peak at lower temperature than the massive nickel and cobalt oxides, which must be related to the dispersed state of these phases onto the support surface. As previously found by others [20], these reduction temperatures are even lower in the bimetallic samples, which can be again related to the higher dispersion state (lower crystallite sizes) of the bimetallic oxides. After the XRD diagram obtained for the reduced sample (Figure 2b), a bimetallic phase is formed. As shown, the bimetallic samples present the diffraction peak at 44-45o, corresponding to (111) peak of the cubic Ni and/or Co metallic phase, in an intermediate the position, confirming that an alloy is formed after reduction. In these reduced samples, the crystallites size estimated by applying the Scherrer formula shows that the bimetallic particles have again lower sizes than the monometallic ones (Table 1). 3.2. Catalytic studies The four catalytic systems were tested in the dry reforming of methane (DRM) reaction. As shown in Figure 4, the methane conversions are very different depending on the catalyst. First of all, the ZCo26 monometallic sample has very low activity, while the ZNi26 monometallic system present an initial activity about half of the bimetallic ones. The two bimetallic catalysts have a similar activity, with the ZNiCo26-11 having a slightly higher value for the methane conversion. Besides, there is also a remarkable difference in the stability of these systems, being evident the better behavior of both bimetallic systems. So, while the activity of the nickel monometalllic system decreases from about a 50% initial conversion to 32% after 12h of reaction at 750ºC (a loss of 36% of the initial activity), both bimetallic catalysts have a much less drop, from 90% to 78% (13% less) for the ZNiCo-11 sample. This improvement in the catalytic performance is especially relevant as the cobalt monometallic catalyst is completely inactive for this reaction. Also shown in the figure is the H2/CO selectivity ratio, which has values around 1 in the bimetallic systems (0.85 in the nickel monometallic system), showing that no water gas shift reaction occurs as a consequence of the presence of cobalt. Although a relatively low activity for cobalt systems has been previously Page 6 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 7 observed by other authors [20,33], a number of considerations must be taken in account to explain the inactivity of our cobalt monometallic catalyst. First of all, according to Wei and Iglesia [14], the C-H bond activation is the only kinetically relevant step in the reforming reaction of methane. As the cobalt is an active metal for C-H activation, and in fact some theoretical studies have shown that metals of the group 9 (as cobalt) are expected to be a little more active than later transition metals [34], the low activity detected for our cobalt system must be caused by a quick deactivation process, due for instance to the deposition of carbon and/or to the surface oxidation of the cobalt particles [33]. So, the simultaneous presence of cobalt and nickel in the bimetallic system must modify their physical-chemical state, improving the activity and stability of the bimetallic active sites. As shown above, the results obtained by XRD and TPR show that the presence of cobalt induces the formation of smaller (Table 1) and more reducible metallic particles (Figure 3). As the particle size is an important factor determining the catalytic performance of nickel systems [35,36], this effect could be used to explain the differences in the catalytic performances. However, other effects related with the catalyst behavior under reaction conditions cannot be discarded as responsible for these changes in performance. To shed light on the chemical and physical state of metals under real reaction conditions, an in situ XAS study has been accomplished. 3.3. In situ XAS under reaction conditions Both metals, nickel and cobalt, have been studied in situ by X-ray Absorption Spectroscopy at the Ni and/or Co K-edges. The spectra were collected in contact with the gases, hydrogen or the DRM gas mixture, between room temperature (RT) and 750ºC. Figure 5 shows the sequence of in situ XANES spectra obtained for the monometallic Ni/ZrO2 catalyst (ZNi26) during the reduction treatment in hydrogen up to 750ºC. The figure depicts spectra for the original sample, heated under hydrogen at 600 and 750ºC and finally cooled down to RT, respectively. The initial spectrum corresponds to a well formed nickel oxide phase [16]. As expected, the reduction treatment in hydrogen at 600ºC generates a spectrum characteristic of a completely reduced metallic nickel [16], which remains virtually unchanged by increasing the temperature Page 7 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 8 up to 750ºC and after cooling down to room temperature. The EXAFS spectra (Figure 6a) and the Fourier Transforms (FTs) of these spectra (Figure 6b) show a similar behavior, appearing after reduction a main peak centered at 2.4 Å, characteristic of metallic nickel (without phase shift correction) [29]. In all cases, the fitting analysis of the first peak in the FT yields a coordination number (C.N.) of 12 and Ni-Ni bond lengths of 2.48 Å, characteristic of metallic nickel. It is worthy of note that some of these XAS spectra are collected at high temperature (600 or 750ºC), and a higher Debye-Waller factor is obtained in these conditions. The clearer consequence of this effect is the decrease in the intensity of the FT´s peaks of the spectra collected at high temperature (Figure 6b), even though the C.N. does not change after the reduction treatments. So, differences in the intensity of this peak with temperature must be related with thermal disorder of nickel particles at high temperatures, as previously shown by us in a similar catalytic system [16]. More interesting is the behavior of the metallic phase of nickel under reaction conditions. As shown in the XANES spectra of Figure 7, under dry reforming reaction conditions the redox behavior of the nickel phase in the Ni/ZrO2 system is pretty complex. The simultaneous presence in the reaction mixture of a mild oxidant [37] (CO2) and a reductant (CH4) produces changes in the electron density of the metallic nickel particles. These changes are responsible for the fluctuations observed in the intensity of the XANES feature at 8347 eV (white line) [38]. So, by treatment under CO2/CH4 at RT, the metal remains completely reduced (no changes in the intensity), while at 600ºC, the intensity of the peak at 8347 eV increases, indicating that the nickel atoms undergo a partial oxidation. This effect is reverted by increasing the temperature at 750ºC, when the XANES spectrum is characteristic of a well reduced nickel phase. By cooling down the sample under the reaction mixture, the oxidation state of nickel is again increased, reaching the 8347 eV feature its maximum intensity at RT. So, these results show the competing effect of CO2 and methane reducing or oxidizing the nickel as a function of the reaction temperature. In spite of that, the EXAFS spectra and FTs obtained under these reaction conditions (Figure 8) show only Ni-Ni distances, indicating that the nickel always remains metallic, with no evidences of net oxidation of nickel. The fitting analysis accomplished for the Ni-Ni first coordination Page 8 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 15 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 16 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 17 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 18 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 19 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 20 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 21 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 22 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 23 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 24 of 25 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60