Promoting effect of Sn on supported Ni catalyst during steam reforming of glycerol

Promoting effect of Sn on supported Ni catalyst during steam reforming of glycerol L. F. Bobadilla,†* F. Romero-Sarria, M. A. Centeno and J. A. Odriozola Departamento de Química Inorgánica e Instituto de Ciencia de Materiales de Sevilla (ICMSE). Centro mixto CSIC - Universidad de Sevilla, 49 Av. Américo Vespucio, 41092 Sevilla (Spain) † Current address: Instituto de Tecnología Química (ITQ). Centro mixto CSIC- Universidad Politécnica de Valencia, s/n Av. de los naranjos, 46022 Valencia (Spain) * Corresponding author: [email protected] Abstract: The promoting effect of Sn on the catalytic performance of supported Ni catalyst in the reaction of glycerol steam reforming was studied. The physico-chemical properties of the prepared samples were investigated by X-ray fluorescence (XRF), BET surface area, in situ X-ray diffraction (XRD), laser Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and temperature programmed oxidation (TPO) techniques. The characterization results of the samples after reduction treatment (in the same conditions than the activation before catalytic activity measurements) revealed the formation of Ni-Sn alloy. The Sn-doped catalyst exhibited a high activity and it was demonstrated that the Sn addition increase the catalyst stability and durability by decreasing the coke deposition. Keywords: Hydrogen production; Sn addition; Coke deposition 1. Introduction During the last decades important efforts are ongoing to reduce fossil fuel dependency and greenhouse gas emissions. Among the different possibilities to find a viable option, the use of hydrogen as an energy carrier is considered to be an interesting alternative for the future and could have a crucial role in reducing atmospheric pollution. Biomass is considered as one of the most attractive renewable source for hydrogen production and numerous studies are being directed toward the development of novel technologies to produce hydrogen from biomass [1-4]. Steam reforming of glycerol is an important approach for hydrogen production from biomass. Glycerol is the main by-product generated in the biodiesel production. Biodiesel is one renewable biofuel obtained by catalytic trans-esterification of triglycerides with methanol [5]. About 10 wt.% of glycerol can be produced during the conversion of vegetable oils into biodiesel. The use of glycerol for hydrogen generation is a very advantageous option since its use would decrease the price of biodiesel making it more competitive [6]. The overall reaction of glycerol steam reforming is given by the following equation: C3H8O3 + 3H2O → 3CO2 + 7H2 (1) which can be expressed as a combination of glycerol decomposition (2) and the watergas shift reaction (3): C3H8O3 → 3CO + 4H2 (2) CO + H2O ↔ CO2 + H2 (3) Theoretically, a maximum of 7 moles of H2 per mol of glycerol can be produced, although this ratio depends on the reaction conditions such as temperature, pressure and steam-to-glycerol steam ratio. Although the glycerol steam reforming process is very attractive and it could be developed on an industrial-scale, it has some challenges that must to be overcome in order to accomplish its effective commercialization. For example, the process is an endothermic reaction and requires high temperatures increasing the operation costs. Besides, the catalyst deactivation by coke deposition is also an issue since it affects hydrogen yield and long term operation. With the aim of overcoming these challenges, the development of active, stable and inexpensive catalytic materials is mandatory. Catalysts containing group 8-10 metals such as Ni [7-12], Co [13-16], Pt [17-21], Ru [22-24], Rh [25], Pd [26] or Ir [27] on different oxides have been largely investigated as active catalysts for glycerol steam reforming. Ni-supported is one of the most promising active metals for such an application because of its high activity, low cost and wide availability. However, Ni-based catalysts suffer deactivation by coke deposition on the catalyst surface that block active sites and favour side reactions. Deactivation of nickelbased catalysts by sintering of nickel crystallites is another important drawback. Promoting nickel catalysts with a second metal has been proven to be one of the most promising approach to obtain more stable and optimal catalysts [28]. Bimetallic catalysts comprising Sn as a Ni promoter have been proved to outperform Ni monometallic catalyst in steam reforming processes [29]. The coke deposition can be markedly reduced by using Sn-doped catalysts in the steam reforming reaction to generate hydrogen from hydrocarbons. Sn alloyed with nickel prevents the formation of nickel atom ensembles, which are the responsible of the coke formation, and avoids the diffusion of carbon to form larger coke agglomerates [30]. Therefore, NiSn-based catalysts have the potential to decrease the catalyst deactivation caused by coking maintaining its high specific activity. Sn-doped Ni catalysts have been reported for aqueous phase reforming [31, 32] and methane steam reforming [33] where the formation NixSny alloys play a key role to inhibit coke deposition. Pengpanich et al. [34] reported a clear example of this positive effect in the partial oxidation of isooctane. These authors found that the addition of small amounts of tin decreased by more than 50% the formation of carbon deposits without changes in the conversion. This enhancement was ascribed to the ability of Sn to reduce the growth of carbon filament by retarding carbon solubility in the Ni particles [28]. Moreover, Saadi et al. [35] have demonstrated, using density functional theory (DFT) calculations, the ability of Ni-Sn to inhibit graphite formation during steam reforming reactions. They demonstrated that the presence of Sn increases the C-C bond formation barrier. We have investigated previously the effect of the nature of the support (acidity, basicity and redox properties) in the catalytic performance of Sn-doped Ni catalysts during the steam reforming of alcohols [36-38]. The objective of the present work is to investigate the effect of tin on the catalytic performance of Ni-supported catalysts in terms of activity, selectivity and durability in the steam reforming of glycerol. 2. Experimental 2.1. Catalysts preparation The alumina support was obtained by ball milling spherical alumina pellets (SASOL, 1.78 mm diameter), by using a PM4 Retsch instrument particle sizes in 7 – 8 μm range were achieved. A monometallic Ni-based catalyst was prepared by impregnating the alumina with an aqueous solution of all the inorganic precursors with the desired concentrations of cerium (III) nitrate hexahydrate (Sigma-Aldrich), magnesium nitrate hexahydrate (Sigma-Aldrich), and nickel (II) nitrate hexahydrate (Sigma-Aldrich). Then, the sample was dried overnight at 100ºC and calcined at 700ºC for 12 h in flowing air using a heating rate of 10 C min-1 to obtain 20 wt.%Ni, 12 wt.%CeO2, 8 wt.%MgO, and 60 wt.%Al2O3. The bimetallic NiSn catalyst was prepared following the same procedure but substituting a fraction of the nickel salt by the adequate amount of anhydrous tin chloride (Fluka) in order to obtain 17 wt.%Ni, 3 wt.%Sn, 12 wt.%CeO2, 8 wt.%MgO, and 60 wt.%Al2O3. The resulting catalysts were labelled as Ni/CeMgAl and Ni- Sn/CeMgAl, respectively. 2.2. Materials characterization The chemical composition of the samples was determined by X-ray fluorescence (XRF) spectroscopy in a Panalytical AXIOS PW4400 sequential spectrometer with Rh tube as source of radiation. The analysis were carried out onto pressed wafers containing 6 wt.% of wax. The textural properties (BET surface, pore size and pore volume) were studied by nitrogen physical adsorption-desorption isotherms at 77 K with Micromeritics ASAP 2010 equipment. Before each measurement, the samples were outgassed for 2 h at 150ºC in vacuum to remove the adsorbed impurities. X-ray diffraction (XRD) analysis was performed on a Siemens D-500 diffractometer using Cu Kα radiation (40 mA, 40 kV) and a position-sensitive detector using a step size of 0.05º and a step time of 1 s. The reducibility of the catalysts was investigated by in situ XRD analysis using a high temperature camera Anton Paar HTK 1200 coupled with an X´Pert Pro Philips diffractometer. The system was equipped with X´Celerator detector with a step of 0.05º and an equivalent time acquisition of 30 s. The diffractograms were recorded in the 25 – 900ºC temperature range under a flow of 100 mL min-1 (5% H2 in Ar). Surface characterization was carried out by X-ray photoelectron microscopy (XPS) on a Leybold-Heraeus spectrometer (LHS-10/20), working with a constant pass energy of 50 eV and vacuum of 5 x 10-9 Torr. The system was equipped with an EA-200 MCD hemispherical electron analyser with a dual X-ray source working with Al Kα radiation (1486.6 eV) at 120 W and 30 mA. Binding energies were referenced to the spurious C (1s) signal at 284.6 eV and the atomic composition was estimated by the elemental sensitivity factor of each atom. Before each analysis, the sample was reduced in an external tube furnace for 1 h at 750ºC under a total flow of 100 mL min-1 (50% H2 in N2). Then, the sample was cooled down to room temperature and placed in the prechamber of the XPS spectrometer. The morphology of the catalysts was evaluated by SEM using a JEOL 5400 instrument equipped with an Energy Dispersive X-ray Spectrometer (OXFORD LINK TETRA 1128-231). Raman spectra were recorded in a dispersive Horiva Jobin Yvon LabRam HR800 Confocal Raman microscope with a green laser (532.05 nm) working at 5 mV power and using a 600 grooves/mm grating. The microscope used a 50x objective with confocal pinhole of 1000 μm. The Raman spectrometer was calibrated using a silicon wafer. The temperature-programmed oxidation (TPO) was carried out in a U-shaped reactor under a total flow of 50 mL min-1 (5% O2 in He) from room temperature to 900ºC with heating rate of 10 ºC min-1. The formed products were analysed by mass spectrometry (MS) in a Pfeiffer vacuum mass spectrometer. 2.3. Catalytic performance The glycerol steam reforming reaction was evaluated as a function of time for periods of 4, 12 and 24 h in isothermal conditions at 750ºC, with a steam-to-carbon molar ratio of 4 and 100000 mL g-1 h-1 space velocity. Prior to reaction, the samples were pressed, crushed, and sieved to achieve particles in the 100-200 μm range. In each run, 100 mg of sample was diluted with the same amount of crushed quartz sieved to the same particle size and placed into the reactor between two quartz wool plugs. The catalysts were activated under a total flow of 100 mL min-1 (50% H2 in N2) at 750ºC for 1 h. The experiments were carried out at atmospheric pressure in a commercial Microactivity Reference Reactor made by PiD Eng&Tech using a Hastelloy C-276 tubular reactor with 9 mm internal diameter. Gas products were analysed on-line using a microGC (Varian 4900) equipped with Porapak Q and Molecular Sieve – 5A columns. At the reactor outlet a Peltier liquid cooler was fitted allowing the analysis of condensable products by HPLC (Varian 356-LC) equipped with a refractive index detector and a Hi-Plex H column with milli-Q water as eluent. 3. Results and discussion 3.1. Catalysts characterization Table 1 shows the chemical compositions and textural properties (BET surface, pore size and pore volume) of the prepared catalysts. For comparison, the support was also measured. The experimental compositions of all samples were close to the nominal ones confirming the effectiveness of the impregnation method employed to prepare the catalysts. The parent support presents superior BET surface area, which decreases after Ni or Ni-Sn addition caused by the incorporation of metal particles into the pores of the support. Figure 1 shows the XRD patterns recorded for the prepared catalysts. The diffraction peaks at 2θ = 28.8º, 33.3º, 47.7º and 56.6º can be assigned to the (111), (200), (220) and (311) planes of the cubic fluorite phase characteristic of cerium oxide (JCPDS 34- 0394). It can also be observed the diffraction lines corresponding to the MgAl2O4 spinel phase (JCPDS 21-1152). As described in a previous work [38] the presence of nickel induces the transformation of the MgAl2O4 spinel structure into a continuous MgNi spinel layer although only a part of nickel partially substitute Mg in the MgAl2O4 phase forming a new NixMgyAl2O4 phase. Diffraction peaks characteristic of nickel oxide (JCPDS 71-1179) were observed at 37º and 43º. Therefore, it can be established that a fraction of Ni is well dispersed as NiO particles with moderate interaction with the support and another portion of Ni is incorporated within the non-stoichiometric spinellike phase NixMgyAl2O4 [8] with strong metal-support interaction. XRD peaks associated to Sn phases were not observed in the Ni-Sn/CeMgAl catalyst likely due to its high dispersion. In order to analyse the evolution and the changes in the crystalline phases produced under reductive conditions, in situ X-ray diffraction analyses were performed. Reduction was carried out in an atmosphere of hydrogen diluted in Ar. The obtained X- ray diffractograms as a function of temperature are shown in Fig. 2. In both cases, the structural modifications are only appreciable in the high temperature range (700-900 ºC). For the Ni/CeMgAl catalyst, the NixMgyAl2O4 and NiO phases are reduced to metallic nickel (JDPDS 45-1027), whereas the reduction of the NiSn/CeMgAl catalyst leads to the formation of NixSny alloys coexisting with metallic Ni particles. It is known that nickel can be completely alloyed with tin forming Ni3Sn, Ni3Sn2 and/or Ni3Sn4 intermetallic compounds depending of the Ni/Sn molar ratio [39]. In our case, the characteristic peaks of the Ni3Sn alloy (JCPDS 35-062) were evidenced although the presence of other NixSny intermetallic compounds cannot be ruled out. In both cases, metallic nickel was also present after reduction at 700 ºC. A further increase of the temperature to 900 ºC led to an increase of the Ni diffraction lines, which indicate a sintering of the nickel particles. Surface analysis measurements by XPS were performed over Ni/CeMgAl and Ni- Sn/CeMgAl catalysts to investigate the chemical surface properties and oxidation states of both metals. Prior to XPS analysis, fresh catalysts were reduced for 1 h at 750ºC under flowing hydrogen diluted in inert to get information about the chemical state of the activated catalysts. Ni 2p and Sn 3d XPS spectra are shown in Fig. 3. Both Ni/CeMgAl and Ni-Sn/CeMgAl catalysts exhibited two deconvoluted Ni2p3/2 core-level peaks with the corresponding satellite peaks positioned to ~6 eV, respectively; the peak at 856 eV can be assigned to nickel aluminate phase, while that the peak at 853 eV was attributed to metallic nickel [40, 41]. It was observed that the relative amount of nickel species incorporated in the non-stoichiometric spinel-like phase NixMgyAl2O4 is similar in both catalysts while that the relative quantity of metallic nickel is slightly superior in the Ni/CeMgAl catalysts. This observation is coherent with the chemical composition of the catalysts suggesting that the additional nickel in the Ni/CeMgAl catalyst is highly dispersed over the support as nickel oxide crystallites. From the XPS results, it is evident that only the crystalline nickel oxide was reduced and the nickel with strong interaction with the support requires higher temperatures. Both Ni/CeMgAl and Ni- Sn/CeMgAl catalysts exhibited identical amount of nickel aluminate species strongly bound to the support which were not reduced under our conditions at 750ºC. The reduction of Ni2+ ions incorporated non-stoichiometrically in tetrahedral or octahedral sites of spinel phases requires temperatures as high as 800-850 ºC [42]. The Sn 3d region spectra for the Ni-Sn/CeMgAl catalyst is shown in Fig. 3. The peak at 485 eV can be assigned to Sn0 species while that the higher binding energy Sn 3d5/2 peak at 487 eV is attributable to tin oxides (Sn2+ or Sn4+) [32]. The reduced species of tin is probably a Ni-Sn alloy. Discriminating between Sn2+ and Sn4+ with XPS is complicated, but the presence of this species indicates that the reduction of tin was not complete, and oxidized species are existent even after reduction at 750 ºC, in agreement with previous results on alumina-supported catalysts [43]. Alumina interacts strongly with the tin and hinders the redution of SnO [44]. Thererefore, XPS analysis of the Ni- Sn/CeMgAl after reduction at 750ºC indicates the presence of Ni and Sn separate phases. The ongoing reduction of Ni and Sn can be concurrent with the formation of a Ni-Sn alloy, however this alloys are indistinguishable by XPS from reduced Ni surfaces [32]. Consequently, it can be proposed that the Ni-Sn catalyst after activation may be represented by the formation of bimetallic particles with a central core of nickel surrounded by an outer layer rich in tin (Fig. 3). During the reduction phase, a part of Sn atoms migrate to the metallic nickel particles surface to form a Sn layer that is partially reduced forming a Ni-Sn alloy covering the surface of Ni particles. As can be observed in Table 2, the surface Ni/Sn determined by XPS was lower than the bulk one suggesting that Sn is located mainly in the surface layers of the catalyst. Similar observations were found by other authors in the literature [34, 45]. 3.2. Activity measurements The catalyst performances were tested for the hydrogen production in the glycerol steam reforming. Figure 4 shows the specific activity for both Ni/CeMgAl and Ni- Sn/CeMgAl catalysts. It is noteworthy that the catalytic behaviour was different in both cases. The activity for Ni/CeMgAl catalyst was found to be higher than that of the Snpromoted one and, apparently, very stable for the first 12 hours. Once this period is reached the catalytic activity drastically decreases suggesting a fast deactivation process. The partial substitution of Ni by Sn in the NiSn/CeMgAl catalyst reduces the catalytic activity but notably enhances the stability of the catalyst. As can be observed, after an initial stabilization period for the first 4 hours, the catalytic activity remains stable. The observed activity loss upon tin addition might be due to the fact that the nickel active sites for the glycerol steam reforming are covered by Sn species as confirmed by XPS analysis. The Sn-enriched surface prevents the adsorption of the glycerol molecules on the metallic nickel surface and the subsequent reforming reactions. Similar observations were early reported in the literature for the NiSn/Ce0.75Zr0.25O2 [34] and PtSn/C [46] systems. The same gaseous products were detected in both catalysts (H2, CO, CO2, CH4 and C2H4, Figure 5), although with a different product distribution. Condensable products such as oxygenated compounds were not detected at this reaction temperature in good agreement with previous results [36]. Hydrogen was the main component in both catalysts and remained invariant during all time-on-stream. It can be noticed that during the first 12 hours, the Ni/CeMgAl catalyst produced the maximum hydrogen molar fraction possible (70%) according to the stoichiometry of the reaction (1). After 12 h of reaction changes in the molar composition were observed. From here, the production of CO2 decreases along with the increase of CO, CH4 and C2H4. Moreover, the change of selectivity appears when activity loss is observed. For the Ni-Sn/CeMgAl catalyst, an increase of CO and decrease of CO2 started from the beginning of the reaction while that CH4 and C2H4 were produced continuously. The differences observed in both cases would be associated to the catalytic processes involved in the glycerol decomposition process. The overall reaction of glycerol steam reforming (1) may be viewed as the combination of the glycerol decomposition (2) and the water gas shift (WGS) reaction (3). Furthermore, it may also be accompanied by other thermodynamically feasible reactions that include: 2C3H8O3 (g) + H2 (g) ↔ 3CH4 (g) + 3CO (g) + 3H2O (g) (4) CO (g) + 3H2 (g) ↔ CH4 (g) + H2O (g) (5) CO2 (g) + 4H2 (g) ↔ CH4 (g) + 2H2O (g) (6) CO2 (g) + CH4 (g) ↔ 2CO (g) + 2H2 (g) (7) C(s) + H2O (g) → CO (g) + H2 (g) (8) C(s) + CO2 (g) ↔ 2CO (g) (9) C(s) + 2H2 (g) ↔ CH4 (g) (10) The initial gas products distribution for Ni/CeMgAl catalyst indicates that the WGS reaction was predominant in the first 12 h. This is consistent with a production of hydrogen close to the equilibrium value. After 12 h of reaction methane and ethylene are produced. Araque et al. 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Surface composition obtained by XPS analysis of the reduced and spent catalysts at.% composition a Al Mg Ce O Ni Sn C Ni/Sn Ni/CeMgAl reduced 18.7 5.1 2.0 69.5 4.6 (27.5) - - Ni/CeMgAl spent - - 0.15 4.35 - - 95.5 Ni-Sn/CeMgAl reduced 17.8 6.1 2.0 69.2 3.7 (25.6) 1.1 (2.7) - 3.4 (9.4) Ni-Sn/CeMgAl spent 3.5 2.1 0.4 21.7 0.5 0.1 71.8 5.0 a Bulk values in parenthesis Fig. 1 XRD patterns for the prepared catalysts Fig. 2 In situ XRD patterns in reductive atmosphere for the prepared catalysts: (a) Ni/CeMgAl and (b) Ni-Sn/CeMgAl Fig. 3 XPS spectra in the Ni 2p and Sn 3d level and pictorial representation of reduced catalyst Fig. 4 Catalytic activity as a function of time for glycerol steam reforming at 750ºC for the prepared catalysts Fig. 5 Product distribution in glycerol steam reforming at 750ºC for the studied catalysts Fig. 6 SEM micrograph of the fresh and used catalysts, after 4 h and 24 h on stream in glycerol steam reforming at 750 ºC