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In Situ XAS Study of Synergic Effects on Ni Co/ZrO2 Methane Reforming Catalysts Victor M. Gonzalez-delaCruz, Rosa Pere~niguez, 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. Americo Vespucio, 49, 41092 Seville, Spain 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 750 °C. 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.
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 reactions3 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 r 2011 American Chemical Society
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 past 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 the activity and, especially, the stability in the DRM reaction. Important synergetic effects have been found affecting the catalytic performances, dispersion, reducibility and oxidation states of metals under reaction conditions.
2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. Four 26 wt % Ni/ZrO2 (ZNi26), 26 wt % Co/ZrO2 (ZCo26), 13 wt % Ni 13 wt % Co/ZrO2 (ZNiCo26-11), and 8.7 wt % Ni 17.4 wt % Co/ZrO2 (ZNiCo26-12) were prepared by a one-step incipient wetness impregnation of a monoclinic zirconia support with nickel and/or Received: September 23, 2011 Revised: November 21, 2011 Published: December 16, 2011 2919
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Table 1. Crystalline Size of Crystalline Phases Detected in the 26 wt % (Ni and/or Co)/ZrO2 Systems after Calcination and Reduction Treatmentsa crystallite size (nm) sample
ZrO2 NiO Co3O4 NiCo2O4
ZNi26
20.3
48.0
ZNiCo26 11
19.1
18.0
19.7
ZNiCo26 12
20.1
19.5
24.8
ZCo26
20.1
reduced samples (nm) Ni0/Co0 59
44.5
35 46 70
a
Values were calculated from XRD results by applying the Scherrer equation.
Figure 1. XRD patterns of 26 wt % (Ni + Co)/ZrO2 systems. Some nickel and cobalt oxides are included as references.
cobalt nitrate solution of appropriate concentration (Ni(NO3)2 3 6H2O and Co(NO3)2 3 6H2O from Aldrich). Zirconia support was synthesized via forced hydrolysis27 using an aqueous solution of ZrO(NO3)2 3 xH2O (from Aldrich). The powder was calcined in air at 850 °C for 3 h, a temperature higher than the reaction temperature (750 °C), in order to prevent any change on the support under reaction temperatures. 2.2. 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 = 2.18°), with a Bragg Brentano configuration, using Cu Kα (λ = 1.5418 Å). General patterns diagrams were collected in the range 2θ = 20 80°, with a step of 0.05° 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 55°, with a step of 0.03° during 100 s each point. 2.3. Temperature Programmed Reduction (TPR). TPR experiments were done from room temperature up to 700 oC, with a heating rate of 10 °C/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 H2-reduction. All the experimental conditions were chosen to ensure that no peak coalescence occurs.28 2.4. 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 750 °C during 1 h. 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 750 at 1 °C/ min rate, held at 750 °C for 12 h, and finally cooled 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. 2.5. X-ray Absorption Spectroscopy (XAS). X-ray absorption spectra were recorded at the BM25 beamline (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 800 °C 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 750 °C. 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 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 26 wt % Ni/ZrO2 (ZNi26), 26 wt % Co/ZrO2
(ZCo26), 13 wt % Ni 13 wt % Co/ZrO2 (ZNiCo26-11), and 8.7 wt % Ni -17.4 wt % Co/ZrO2 (ZNiCo26-12) samples were 2920
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Figure 3. TPR profiles of 26 wt % (Ni + Co)/ZrO2 systems. Some nickel and cobalt oxides are included as references.
Figure 2. Detailed XRD patterns of 26 wt % (Ni + Co)/ZrO2 systems: (a) oxidized samples and (b) reduced samples.
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 37° ((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 36.9° in the cobalt monometallic system to about 36.7° in the Ni Co bimetallic one. The smaller 2-theta value is consistent with the larger value of the NiCo2O4 lattice constant32 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. 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 45°, 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 2921
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The Journal of Physical Chemistry C 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 12 h of reaction at 750 °C (a loss of 36% of the initial activity), both bimetallic catalysts have a much less drop, from 90% to 78% (13% less) for the ZNiCo11 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 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 cobalt is an active metal for C H activation and in fact some theoretical studies have shown that metals of 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 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
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of nickel systems,35,36 this effect could be used to explain the differences in the catalytic performances. However, other effects related to 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 750 °C.
Figure 5. In situ XANES spectra of the 26 wt % Ni/ZrO2 sample submitted to reducing treatments. All of the spectra have been collected in situ at the indicated temperatures in contact with the gas phase.
Figure 4. Evolution of methane conversion (a) and H2/CO selectivity (b) with time of stream for the dry reforming of methane at 750 °C using the 26 wt % (Ni + Co)/ZrO2 systems. Catalyst = 0.04 g. GHSV = 1.5 105 L/kg h. BET surface area = 19 ( 2 m2/g. 2922
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Figure 7. In situ XANES spectra of the 26 wt % Ni/ZrO2 sample submitted to dry reforming reaction treatments. All of the spectra have been collected in situ at the indicated temperatures in contact with the gas phase.
Figure 6. EXAFS oscillations (a) and its Fourier transform functions (b) (without phase correction) obtained for the 26 wt % Ni/ZrO2 sample submitted to reducing treatments. All of the spectra have been collected in situ at the indicated temperatures in contact with the gas phase.
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 750 °C. The figure depicts spectra for the original sample, heated under hydrogen at 600 and 750 °C and finally cooled to RT, respectively. The initial spectrum corresponds to a well formed nickel oxide phase.16 As expected, the reduction treatment in hydrogen at 600 °C generates a spectrum characteristic of a completely reduced metallic nickel,16 which remains virtually unchanged by increasing the temperature up to 750 °C and after cooling 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 750 °C), 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 oxidant37 (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), whereas at 600 °C, 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 750 °C, when the XANES spectrum is characteristic of a well reduced nickel phase. By cooling 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 evidence of net oxidation of nickel. The fitting analysis accomplished for the Ni Ni first coordination shell shows a Ni Ni coordination number of 12 and a Debye Waller factor increasing at 600 and 750 °C, which is again liable for the lower intensity of the FT main peak as the temperature increases. Therefore, the observed changes must be related just with electronic effects caused by the surface adsorption of reactant on the metallic particles, without causing remarkable structural changes by insertion of oxygen atoms into the structure. 2923
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Figure 8. EXAFS oscillations (a) and its Fourier transform functions (b) (without phase correction) obtained for the 26 wt % Ni/ZrO2 sample submitted to dry reforming reaction treatments. All of the spectra have been collected in situ at the indicated temperatures in contact with the gas phase.
The redox behavior of cobalt in the monometallic Co/ZrO2 system (ZCo26) is pretty much similar to that observed for nickel. As shown in Figure 9a, the initial spectrum corresponds to a cobalt oxide phase,39 whereas it is completely reduced after hydrogen treatment at 600 and 750 °C. Although less noticeable, the changes in XANES spectra during the reforming reaction are also similar to that described for the ZNi26 system (Figure 9b). In this case the oxidation and reduction processes produce changes in the intensity of the Co K-edge XANES feature at 7720 eV. As regardless of the treatment, the EXAFS and the FTs detect the presence of just metallic cobalt with a C.N. of 12 (not shown), once again the changes in the XANES feature must be provoked by the CO2 adsorption on the surface of the metallic particles, without further oxidation of cobalt particles. This relatively high resistance of cobalt atoms to be oxidized could be related to the deactivation process observed in reaction conditions, with the carbon deposits protecting the surface of the metallic particles. A parallel in situ XAS study has been accomplished with the two bimetallic Ni Co/ZrO2 (ZNiCo26-11 and ZNiCo26-12)
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Figure 9. In situ XANES spectra of the 26 wt % Co/ZrO2 sample submitted to reducing (a) and dry reforming reaction (b) treatments. All of the spectra have been collected in situ at the indicated temperatures in contact with the gas phase.
catalysts, both at the Ni K-edge and Co K-edge regions. As similar results have been obtained for both samples, only those corresponding to ZNiCo26-12 will be presented. The Ni K-edge and Co K-edge XANES spectra obtained at RT after reduction at 750 °C and under reaction conditions at 600 and 750 °C are included in Figure 10. These spectra clearly show that both metals remain completely reduced after any tried treatment. As expected, the EXAFS spectra (not shown for simplicity) also indicate that both metals remain completely reduced and with a C. N. of 12. The comparison of the XANES spectra obtained under reaction conditions with those of both monometallic samples (ZNi26 and ZCo26) unambiguously shows that, as observed previously for a similar Ni Co in an ex situ experiment,33 the metals in the bimetallic catalytic systems are more resistant to oxidation under reaction conditions. This behavior is even more significant when considering the smaller particle size of nickel and cobalt phases in the bimetallic systems (Table 1), which in principle should increase the oxidation of the metallic particles. 2924
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Simultaneously, this higher activity of the cobalt sites in the bimetallic system produces more hydrogen as a product, which can explain that the nickel atoms are now completely reduced under reaction conditions, as shown in the XANES spectra of Figure 10.
4. CONCLUSIONS We have investigated the catalytic performance of some Ni Co/ZrO2 systems. Our findings show that, even though the monometallic cobalt system is inactive in the dry reforming reaction of methane, the Ni Co bimetallic systems have a much better activity and selectivity in this reaction. The improved performance of the bimetallic catalysts must be related with the formation of smaller sized bimetallic oxides in the calcined samples. The study by XRD, TPR and in situ XAS has shown that the adjacent Ni and Co sites in the bimetallic phase hinder the deactivation of the cobalt single sites and the partial oxidation of both metals under reaction conditions. Both effects play a main role in the improved catalytic performance of the bimetallic systems. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the Ministry of Education and Science of Spain and Junta de Andalucía for financial support (Projects ENE200767926-C02-01 and P07-FQM-02520), the ESRF facility, and the BM25 Spline beamline staff for their experimental support. ’ REFERENCES
Figure 10. In situ XANES spectra at the (a) Ni K-edge and (b) Co K-edge of the 8.7 wt % Ni 17.4 wt % Co/ZrO2 sample submitted to dry reforming reaction treatments. All of the spectra have been collected in situ at the indicated temperatures in contact with the gas phase.
So, the bimetallic systems are more resistant to oxidation by CO2, irrespective of the temperature, showing a synergic effect between nickel and cobalt. Considering the previous results by XRD and TPR, the formation of Ni Co bimetallic particles seems to protect each other from the surface oxidation under reaction conditions, which appears as a main factor affecting the catalytic performance, activity, and selectivity, of nickel in the dry reforming of methane. According to the discussion presented in section 3.2, where it was concluded that cobalt sites seem to be more active than nickel although they are much more quickly deactivated, the results obtained in this work can be explained considering a synergic effect between adjacent nickel and cobalt sites. As the activation of C H bonds of methane needs just a single site to occur,14,40 the reforming reaction of methane can take place simultaneously on nickel or cobalt metallic sites. In the case of cobalt sites, the presence of adjacent nickel atoms can contribute to prevent the formation of carbon deposits over the cobalt sites, now showing its higher activity in the dry reforming reaction.
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