pubs.acs.org/Langmuir © 2010 American Chemical Society
Deactivation Mechanisms of Ni-Based Tar Reforming Catalysts As Monitored by X-ray Absorption Spectroscopy† Matthew M. Yung*,‡ and John N. Kuhn*,§ ‡
National Bioenergy Center, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, and §Department of Chemical & Biomedical Engineering, University of South Florida, 4202 East Fowler Avenue, ENB 118, Tampa, Florida 33620 Received April 25, 2010. Revised Manuscript Received June 11, 2010 Deactivation mechanisms of alumina-supported, Ni-based catalysts for tar reforming in biomass-derived syngas were evaluated using extended X-ray absorption fine structure (EXAFS) spectroscopy. Catalysts were characterized before and after catalytic reaction cycles and regeneration procedures, which included oxidation by a mixture of steam and air, and reduction in hydrogen. Qualitative analysis of the EXAFS spectra revealed that oxidation of a portion of the Ni in the catalysts to form an oxide phase and/or a sulfide phase were likely scenarios that led to catalyst deactivation with time-on-stream and with increased reaction cycles. Deactivation through carbon deposition, phosphorus poisoning, or changes in particle size were deemed as unlikely causes. Quantitative analysis of the EXAFS spectra indicated sulfur poisoning occurred with time-on-stream, and the contaminating species could not be completely removed during the regeneration protocols. The results also verified that Ni-containing oxide phases (most likely a spinel also containing Mg and Al) formed and contributed to the deactivation. This study validates the need for developing catalyst systems that will protect Ni from sulfur poisoning and oxide formation at elevated reaction and regeneration temperatures.
1. Introduction The U.S. is dependent upon petroleum to produce its transportation fuels and currently consumes 140 billion gallons/year (bgy) of gasoline, 43 bgy of diesel in on-road applications, and 25 bgy of jet fuel.1 In the U.S., ethanol produced from the fermentation of starches from corn grain (i.e., “corn ethanol”) provides for nearly 10 bgy of fuel that can displace gasoline consumption,and it is estimated that the limit for U.S. corn ethanol production is around 15 bgy without unacceptable and lasting impacts on corndependent food products.2 Due to the dwindling supply of readily accessible oil reserves, alternative feedstocks to produce transportation fuels are being sought. A study from Oak Ridge National Laboratory3 estimates that using cellulosic biomass to produce ethanol and fuels has the potential to displace as much as 30% of the U.S.’s current gasoline consumption. Whereas the impact of first generation biofuels (i.e., corn ethanol) on food and feed prices has been debated, an overwhelming consensus is that advanced biofuels (utilizing the cellulosic portion of biomass) will greatly lessen any effect on food and feed prices. By using nonfood resources, advanced biofuels avoid direct competition with food and feed supplies, with the only likely impact on food and feed prices stemming from land-use competition. Each route to convert cellulosic biomass into liquid fuels consists of several catalytic processes.4 The thermochemical † Part of the Molecular Surface Chemistry and Its Applications special issue. *To whom correspondence should be addressed. E-mail: matthew_yung@ nrel.gov (M.M.Y.);
[email protected] (J.N.K.).
(1) Department of Energy, 2010, http://www.eia.doe.gov/. (2) Foust, T.; Yung, M. M. ME Today, 2010, http://www.asme.org/NewsPublicPolicy/Newsletters/METoday/Articles/Advanced_Biofuels_Transition.cfm. (3) Perlack, R. D.; Wright, L. L.; Turhollow, A. F.; Graham, R. L.; Stokes, B. J.; Erback, D. C. DOE/GO-102005-2135: Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-ton Annual Supply; Oak Ridge National Laboratory: Oak Ridge, TN, 2005; http://www.asme.org/NewsPublicPolicy/Newsletters/METoday/Articles/Advanced_Biofuels_Transition.cfm. (4) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044–4098.
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conversion route that will be addressed in this paper consists of biomass gasification to create a syngas stream, which can then undergo catalytic conditioning to produce syngas suitable for fuel synthesis.5-10 A major challenge, however, preventing commercialization of this technology is the high cost of catalysts due to deactivation, causing short lifetimes.8 The catalysts examined in this paper have been reported to deactivate and reaction cycle.11 As outlined in Scheme S1 in the Supporting Information, several processes, including sintering, degradation, poisoning, coking, and solid state reactions, can contribute to catalyst deactivation. The goal of this work is to use advanced characterization to test and further refine our deactivation model for these Ni-based catalyst systems. Whereas deactivation and stability characteristics have been linked to loss of the active metallic Ni phase through solid-state reactions (formation of NiAl2O411 or other Ni-containing mixed oxide species12-14), the role of other contaminants has not yet been assessed. It is proposed that contaminants, such as sulfur, which varies widely with biomass source,15 are also important. Since these contaminants would be (5) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Biomass Bioenergy 2003, 24, 125. (6) Dayton, D. U.S. DOE NREL Rep. 2002, NREL/TP-510-32815, 1. (7) Abu El-Rub, Z.; Bramer, E. A.; Brem, G. Ind. Eng. Chem. Res. 2004, 43, 6911. (8) Yung, M. M.; Jablonski, W. S.; Magrini-Bair, K. A. Energy Fuels 2009, 23, 1874–1887. (9) Sutton, D.; Kelleher, B.; Ross, J. R. H. Fuel Process. Technol. 2001, 73, 155. (10) Kuhn, J. N.; Zhao, Z.; Felix, L. G.; Slimane, R. B.; Choi, C. W.; Ozkan, U. S. Appl. Catal., B 2008, 81, 14–26. (11) Yung, M. M.; Magrini-Bair, K. A.; Parent, Y. O.; Carpenter, D. L.; Feik, C. J.; Gaston, K. R.; Pomeroy, M. D.; Phillips, S. D. Catal. Lett. 2010, 134, 242– 249. (12) Kuhn, J. N.; Zhao, Z.; Senefeld-Naber, A.; Felix, L. G.; Slimane, R. B.; Choi, C. W.; Ozkan, U. S. Appl. Catal., A 2008, 341, 43–49. (13) Zhao, Z.; Kuhn, J. N.; Felix, L. G.; Slimane, R. B.; Choi, C. W.; Ozkan, U. S. Ind. Eng. Chem. Res. 2008, 47, 717–723. (14) Zhao, Z.; Lakshminarayanan, N.; Kuhn, J. N.; Senefeld-Naber, A.; Felix, L. G.; Slimane, R. B.; Choi, C. W.; Ozkan, U. S. Appl. Catal., A 2009, 363, 64–72. (15) Cheah, S.; Carpenter, D. L.; Magrini-Bair, K. A. Energy Fuels 2009, 23, 5291–5307.
Published on Web 06/29/2010
DOI: 10.1021/la1016593
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Scheme 1. Protocol for Reaction Experiments and Catalyst Nomenclature
expected to influence the active Ni phase, we performed extended X-ray absorption fine structure (EXAFS) spectroscopy on the samples as a function of deactivation and regeneration cycle. In between each catalytic cycle, the sample was oxidized in steam and air to remove contaminants and then reduced in hydrogen to obtain the metallic Ni phase.
2. Experimental Section 2.1. Catalyst Synthesis and Kinetic Experiments. Singlesolution incipient wetness impregnation of Ni, Mg, and K nitrate precursors was used to add 6.1 wt % Ni, 2.4 wt % Mg, and 3.9 wt % K to the support. The AD90 support had a surface area of 0.75 m2/g with an average particle size of 140 μm. Catalytic properties for hydrocarbon reforming were measured using a molecular beam mass spectrometer (MBMS), gas chromatography, and nondispersive infrared spectroscopy. The feed was a raw syngas created from gasification of white oak, with the average composition shown in Table S1 in the Supporting Information. The oxidized form of the catalyst underwent activation through H2 reduction to create a reduced-form catalyst before it was exposed to reaction gases to create a postreaction sample. The catalyst was kept online until the methane conversion fell below the technical target for the experiment (50%), which led to times-on-stream ranging from 1 to 4 h. The postreaction catalyst was then regenerated using steam and air. This process was repeated for 10 reaction cycles, with catalyst samples collected at each stage in the process, as depicted in Scheme 1. Additional details about catalyst preparation, the reaction system, and reaction conditions have been previously reported11 and are given in the Supporting Information.
2.2. XAFS at Advanced Photon Source (Argonne National Laboratory). X-ray absorption fine structure (XAFS) spectroscopy was performed at DuPont-Northwestern-Dow (DND) Collaborative Access Team (CAT) beamline 5-BM-D (BM = bending magnet, http://www.dnd.aps.anl.gov/) at the Advanced Photon Source, Argonne National Laboratory using the procedures, including the Athena software package,16,17 as described in the Supporting Information.
3. Results 3.1. Catalyst Activity Measurements. The methane conversion during the syngas conditioning experiments was used as the metric for catalytic activity, as nearly complete conversions of benzene, ethylene, and heavy tars were typically observed during each reaction cycle. Characterization and analysis of these catalyst samples by X-ray diffraction, temperature-programmed reduction, and postreaction temperature-programmed oxidation have been previously discussed11 and are herein summarized. The methane conversions at t = 3 min and t = 60 min, during each of the 10 reaction cycles, are shown in Figure 1. For each reaction cycle, a decrease in activity with time-on-stream (i.e., XCH4,t=3 min > XCH4,t=60 min) was observed. The trend of decreasing initial (16) Ravel, B.; Newville, M. J. Synchrotron Radiat. 2005, 12, 537–541. (17) Newville, M. J. Synchrotron Radiat. 2001, 8, 322–324.
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Figure 1. Methane conversion at t3min and t60min during each reaction cycle during syngas conditioning reactions.
(XCH4,t=3 min) methane conversion during sequential reaction cycles has been attributed to the formation of , and subsequent inability to reduce, inactive phases such as NiAl2O4 and (Ni, Mg)O. Sintering, attrition, and coke formation were not determined to be the major modes of deactivation. Poisoning by trace contaminants such as sulfur, phosphorus, and halides found in biomass was not excluded. For a similar catalyst, time-on-stream studies18 were conducted to determine a kinetic model of catalyst deactivation and determine the amount of residual activity that the catalyst has once it reaches a pseudoequilibrium conversion level. For sequential reaction cycles, the catalyst was able to be partially regenerated, though it did not return to its initial activity level (i.e., XCH4,i > XCH4,iþ1). The decreased activity within each reaction cycle is expected to be by H2S poisoning of Ni active sites responsible for methane activation. However, poisoning by other contaminants is also possible. The remaining portion of this contribution will focus on the use of EXAFS to monitor the nature of Ni species at various stages in the catalytic processes outlined in this section. 3.2. Development of EXAFS Models. Due to the complex nature of the reaction feed, simple, qualitative models of EXAFS spectra are evaluated using physical and chemical insight from previous results for these catalysts and the available literature. The Fourier transform of the Ni K edge EXAFS spectra for the reduced and postreaction catalysts through 1 and 9 reaction cycles are shown in Figure 2. All of these spectra show generally good agreement with the Ni foil reference material (see Figure S1 in the Supporting Information and ref 19 as examples). These results agreed with X-ray diffraction (XRD) patterns11 for these samples, which revealed that a metallic Ni phase was the dominant Nicontaining crystalline phase in these samples. Consequently, the first model employed for quantitative analysis will be a simple model using metallic Ni. The similar peak intensities between the spectra in Figure 2 indicate that there is not a major change in the Ni-Ni coordination number as a function of reaction cycle. This finding signified no major change in particle size with the regeneration protocol and agreed with stable Ni crystallite size as calculated from XRD line broadening. However, several subtle differences including an apparent peak shift are observed through comparison of the spectra in Figure 2. We will now outline possibilities that could cause such a change in the measured interatomic distances. (18) Bain, R. L.; Dayton, D. C.; Carpenter, D. L.; Czernik, S. R.; Feik, C. J.; French, R. J.; Magrini-Blair, K. A.; Phillips, S. D. Ind. Eng. Chem. Res. 2005, 44, 7945. (19) Ferrandon, M.; Kropf, A. J.; Krause, T. Appl. Catal., A 2010, 379, 121–128.
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Based on previous XRD results,11 which inferred the formation of a Ni-containing spinel phase with increasing reaction cycle for the reduced form catalysts, the second scenario to examine would be the formation of an oxide phase. Due to the similar interatomic distances, which will be outlined below, for NiAl2O4, NiO, and mixed oxide phases of Ni and Mg in either of those phases, no attempt to differentiate between these phases is made at this time. For NiO, phase uncorrected peaks were recently reported19 at 1.62 and 2.54 A˚, which agreed well with previous results for NiO.20,21 As shown in Supporting Information Figure S1, the EXAFS spectrum for the fresh catalyst (oxidized (1)) was in good agreement with the results for NiO just mentioned. These interatomic distances also correspond well to (Ni,Mg)O22 and NiAl2O4.19,23 With an oxygen backscatter at a shorter distance than the Ni backscatter in metallic Ni, and the various possible metal atoms (Ni, Mg, and/or Al) involved with the backscatter pair at the longer distance, it is plausible that spectrum changes are caused by the formation of a Ni-containing oxide phase. A third potential option for deactivation is coking. Although Ni-based catalysts commonly suffer deactivation associated to coking, it was not identified as a major factor in the deactivation processes for these samples.11 The high temperatures used for regeneration imply that a Ni-C phase would likely be unstable. For example, coke deposits on Ni-based catalysts were removed under oxidizing and reducing conditions below 650 and 850 °C, respectively.24 Similar findings under oxidizing conditions were reported for Ni3C.25 Thus, the similar trend for the reduced and postreaction catalysts before and after several reaction cycles indicated Ni-C species were not responsible. Moreover, using Ni3C as an example, the interatomic distances of this phase do not agree with the changes observed in Figure 2.25 Finally, the presence of sulfur (which contains many detrimental catalytic effects described elsewhere in this work) can alter a Ni catalyst to
prevent coking.26,27 Consequently, a Ni-C model will not be evaluated during this study. The final source of deactivation considered is that caused by trace contaminants such as species containing sulfur or phosphorus. Sulfur is a known catalyst poison for metals, especially Ni,28 and NiP catalysts have been shown29-31 to have unique catalytic properties. Various Ni-S phases such as millerite (NiS) and heazlewoodite (Ni3S2) have similar interatomic distances to each other with Ni-S distances near 2.3 A˚ and Ni-Ni distances near 2.5 A˚ (both values are true distances, after a phase correction has been applied).32 The proximity between these distances and the Ni-Ni interatomic distance of metallic Ni (Ni-Ni spacing of 2.49 A˚) means that the combination of these phases may be able to account for an apparent shift toward smaller interatomic distances and peak broadening observed in Figure 2 as the catalyst was used for more reaction cycles. Moreover, an EXAFS study16 showed nearly identical spectra, including shifts and broadening, when comparing several Ni and NiS reference compounds (both supported and unsupported samples for each species) to the results shown in Supporting Information Figure S1 for the current reference materials, and in Figure 2 for the current catalysts. Alternatively, both the Ni-P and Ni-Ni bond lengths in Ni-P phases29,30 and in Ni-S-P phases31 are longer than the Ni-Ni bond length in metallic Ni, leaving it unlikely that a Ni-P phase is being formed. This logic leaves a Ni-S phase as the most plausible explanation for poisoning of Ni by reaction feed contaminants. For the reasons outlined in this section, three models including (i) metallic Ni (model 1, Ni), (ii) metallic Ni and oxide phase (model 2, Ni and oxide), and (iii) metallic Ni and NiS (model 3, Ni and NiS) will be quantitatively explored as potential models. 3.3. Quantitative Analyses of EXAFS Results. The three qualitative models developed in the previous section will now be quantitatively assessed as described in the Supporting Information. Artemis and its underlying codes16,33 were used for the fittings. In addition to metallic Ni, structures of the following phases such as NiO,19-22 (Ni,Mg)O,22 NiAl2O4,19,23 (Ni, Mg)Al2O4 (all have a similar Ni-O spacing), and NiS32 were considered. 3.3.1. Metallic Ni Model (Model 1, Ni). The simplest model for these samples is a metallic Ni model, and it works reasonably well. As shown in Supporting Information Figures S2 and S3 for the reduced (1) and reduced (9) catalysts, respectively, fits are displayed for the magnitude (A and B), real parts (C and D), and imaginary parts (E and F) of the Fourier transformed EXAFS function with k-weights of 1 (A-C) and 3 (D-F). These figures show the Ni-Ni model fits the freshly reduced catalyst (reduced (1)) better than it does for the recycled catalyst (reduced (9)). This comparison of the fit quality was also established quantitatively as demonstrated in Table 1. With the exception of the first to second cycle, the R-factor increased with reaction cycle, indicating worse fits for both the reduced and postreaction series. The first cycle may be an exception because the sample may not have been completely reduced (also note the lowest Ni-Ni coordination
(20) Clause, O.; Bonneviot, L.; Che, M. J. Catal. 1992, 138, 195–205. (21) Clause, O.; Kermarec, M.; Bonneviot, L.; Villain, F.; Che, M. J. Am. Chem. Soc. 1992, 114, 4709–4717. (22) Yoshida, T.; Tanaka, T.; Yoshida, H.; Funabiki, T.; Yoshida, S. J. Phys. Chem. B 1996, 100, 2302–2309. (23) Kelly, S. D.; Yang, N.; Mickelson, G. E.; Greenlay, N.; Karapetrova, E.; Sinkler, W.; Bare, S. R. J. Catal. 2009, 263, 16–33. (24) Natesakhawat, S.; Watson, R. B.; Wang, X.; Ozkan, U. S. J. Catal. 2005, 234, 496. (25) Struis, R. P. W. J.; Bachelin, D.; Ludwig, C.; Wokaun, A. J. Phys. Chem. C 2009, 113, 2443–2451.
(26) Rostrup-Nielsen, J. R. J. Catal. 1984, 85, 31. (27) Kuhn, J. N.; Lakshminarayanan, N.; Ozkan, U. S. J. Mol. Catal. A: Chem. 2008, 282, 9–21. (28) Rostrup-Nielsen, J. R.; Sehested, J. Adv. Catal. 2002, 47, 65. (29) Lee, Y.-K.; Oyama, S. T. J. Catal. 2006, 239, 376–289. (30) Oyama, S. T.; Lee, Y.-K. J. Catal. 2008, 258, 393–400. (31) Loboue, H.; Guillot-Deudon, C.; A.F., P.; Lafond, A.; Rebours, B.; C., P.; Cseri, T.; Berhault, G.; Geantet, G. Catal. Today 2008, 130, 63–68. (32) Gibbs, G. V.; Downs, R. T.; Prewitt, C. T.; Rosso, K. M.; Ross, N. L.; Cox, D. F. J. Phys. Chem. B 2005, 109, 21788–21795. (33) Rehr, J. J.; Albers, R. C. Rev. Mod. Phys. 2000, 72, 621–654.
Figure 2. EXAFS spectra (Fourier transform of EXAFS function) at the Ni K edge of reduced and postreaction catalysts through various amounts of reaction cycles (see Scheme 1 for details).
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Table 1. Parameters Extracted from Processed EXAFS Spectra after Reduction and after Tar Reforming Using a Metallic Ni Model (Model 1)a catalyst
model
absorber-backscatter pair
R (A˚)
CN
σ2 (A˚2)
ΔE0 (eV)
R factor (%)
reduced (1) 1, Ni Ni-Ni 8.5 2.48 0.006 5.2 1.5 reduced (2) 1, Ni Ni-Ni 10.5 2.48 0.006 4.0 1.3 reduced (4) 1, Ni Ni-Ni 10.5 2.48 0.006 -8.9 2.1 reduced (6) 1, Ni Ni-Ni 9.5 2.47 0.006 -9.4 3.0 reduced (9) 1, Ni Ni-Ni 9.5 2.47 0.007 -9.9 4.7 postreaction (1) 1, Ni Ni-Ni 10.5 2.47 0.006 4.1 1.4 postreaction (2) 1, Ni Ni-Ni 10.5 2.48 0.006 4.7 1.3 postreaction (4) 1, Ni Ni-Ni 10.5 2.48 0.006 4.7 1.6 postreaction (6) 1, Ni Ni-Ni 9.5 2.48 0.006 -7.1 2.6 postreaction (8) 1, Ni Ni-Ni 9.5 2.48 0.006 -8.8 3.4 postreaction (9) 1, Ni Ni-Ni 9.5 2.48 0.007 -9.2 3.8 a CN is the coordination number, R is the interatomic distance between absorber and backscatter pair, σ2 is the mean-square displacement in the distribution of interatomic distances, ΔE0 is the inner potential correction, and the R factor is a measure of the goodness of fit.
Table 2. Parameters Extracted from Processed EXAFS Spectra before and after Tar Reforming Using a Ni and Oxide Model (Model 2)a catalyst
model
absorber-backscatter pair
CN
reduced (1)
2, Ni and oxide
2.2 8.5
reduced (2)
2, Ni and oxide
reduced (4)
2, Ni and oxide
reduced (6)
2, Ni and oxide
reduced (9)
2, Ni and oxide
Ni-O Ni-Ni Ni-O Ni-Ni Ni-O Ni-Ni Ni-O Ni-Ni Ni-O Ni-Ni
postreaction (1)
2, Ni and oxide
0.9 10.5 1.5 10.5 1.8 9.5
R (A˚)
σ2 (A˚2)
2.08 0.040 2.48 0.006 no plausible fit existed 2.07 2.48 2.07 2.47 2.06 2.47
0.020 0.006 0.028 0.007 0.025 0.007
ΔE0 (eV)
R factor (%)
2.5 4.4
1.1
-7.6 -9.0 -9.4 -9.7 -8.6 -9.9
1.9 2.4 3.2
Ni-O no plausible fit existed Ni-Ni postreaction (2) 2, Ni and oxide Ni-O no plausible fit existed Ni-Ni postreaction (4) 2, Ni and oxide Ni-O no plausible fit existed Ni-Ni postreaction (6) 2, Ni and oxide Ni-O no plausible fit existed Ni-Ni postreaction (8) 2, Ni and oxide Ni-O 1.3 2.07 0.019 -5.8 2.4 Ni-Ni 10.5 2.47 0.007 -8.9 postreaction (9) 2, Ni and oxide Ni-O 1.5 2.07 0.019 -5.8 2.5 Ni-Ni 10.5 2.48 0.007 -9.4 a CN is the coordination number, R is the interatomic distance between absorber and backscatter pair, σ2 is the mean-square displacement in the distribution of interatomic distances, ΔE0 is the inner potential correction, and the R factor is a measure of the goodness of fit.
number of all samples and the reducing conditions of the reaction could further reduce the catalyst). These increasingly poor fits with increased reaction cycle indicate a change in at least a portion of the Ni from metallic to a new phase. 3.3.2. Metallic Ni and an Oxide Phase Model (Model 2, Ni and Oxide). Since previous results proposed the formation of a Ni-O phase simultaneous to the metallic Ni phase, this system is the next evaluated. As shown in Supporting Information Figures S4 and S5 for the reduced (1) and reduced (9) catalysts, respectively, fits are displayed for the magnitude (A and B), real parts (C and D), and imaginary parts (E and F) of the Fourier transformed EXAFS function with k-weights of 1 (A-C) and 3 (D-F). The fit parameters using model 2 are presented in Table 2. The reasonable fit of the reduced (1) catalyst implied that the catalyst was not initially fully reduced and became reduced during the first reaction cycle. Other than this freshly reduced catalyst, the second model did not fit the EXAFS spectra well until more than four cycles were performed. Once in this regime, the coordination number of the oxygen to Ni increased with reaction cycle. This trend, in combination with the result that this model, on a whole, fit better with the reduced series than the postreaction series, implied that a portion of the oxide phases was reduced during the H2 reduction prior to the reaction, which is consistent with XRD results. Additionally, the reducing conditions of the 16592 DOI: 10.1021/la1016593
reaction processes were able to further reduce the regenerated catalysts. Although a complete analysis is not a goal of the present work, interesting solid-state chemistry occurred during the regeneration protocols and reaction cycles. It is proposed that a Mgrich spinel forms during early cycles (i < 4) and then Ni exchanges with the Mg to form (Ni,Mg)Al2O4 during elevated cycle numbers. In the future, we plan to further develop this model by also characterizing the calcined samples as a function of reaction cycle. 3.3.3. Metallic Ni and NiS Model (Model 3, Ni and NiS). The final model involves sulfur poisoning of Ni. As shown in Supporting Information Figures S6 and S7 for the reduced (2) and postreaction (9) catalysts, respectively, fits are displayed for the magnitude (A and B), real parts (C and D), and imaginary parts (E and F) of the Fourier transformed EXAFS function with k-weights of 1 (A-C) and 3 (D-F). The fit parameters using model 3 are presented in Table 3. The model describes the spectra well for the reduced series of catalysts, with the exception of the first sample (reduced (1)). For this catalyst, it was not possible to fit the data with a reasonable reduction factor (s02). This inability to model the data agreed with the expected result, since this sample had not yet been exposed to a sulfur-containing feed. Generally, this model fit the spectra well. A few vagrancies did exist for the extracted fit parameters (absolute values of inner potential greater than 10 for three samples). Langmuir 2010, 26(21), 16589–16594
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Table 3. Parameters Extracted from Processed EXAFS Spectra before and after Tar Reforming Using a Ni and a Sulfide Model (Model 3)a catalyst
model
absorber-backscatter pair
reduced (1)
3, Ni and NiS
reduced (2)
3, Ni and NiS
reduced (4)
3, Ni and NiS
reduced (6)
3, Ni and NiS
reduced (9)
3, Ni and NiS
Ni-S Ni-Ni Ni-S Ni-Ni Ni-S Ni-Ni Ni-S Ni-Ni Ni-S Ni-Ni
CN
R (A˚)
σ2 (A˚2)
ΔE0 (eV)
R factor (%)
-2.7 2.9 11.6 0.1 11.2 0.8 2.8 -3.7
1.4
no plausible fit existed 1.4 10.5 1.1 9.5 1.0 10.5 0.7 11.5
2.39 2.47 2.34 2.52 2.34 2.53 2.32 2.50
0.020 0.007 0.001 0.008 0.001 0.010 0.001 0.012
1.9 2.6 3.3
Ni-S 1.3 2.36 0.023 -4.9 1.3 Ni-Ni 10.5 2.47 0.007 3.0 postreaction (2) 3, Ni and NiS Ni-S 1.4 2.35 0.037 -6.2 1.3 Ni-Ni 10.5 2.47 0.007 3.9 postreaction (4) 3, Ni and NiS Ni-S 0.7 2.36 0.084 -7.4 0.0 Ni-Ni 10.5 2.47 0.007 3.9 postreaction (6) 3, Ni and NiS Ni-S 0.6 2.16 0.011 -23.3 1.8 Ni-Ni 9.5 2.49 0.006 -5.1 postreaction (8) 3, Ni and NiS Ni-S 0.6 2.32 0.033 2.1 2.5 Ni-Ni 9.5 2.49 0.010 -4.5 postreaction (9) 3, Ni and NiS Ni-S 0.6 2.31 0.033 -1.9 2.6 Ni-Ni 9.5 2.49 0.009 -5.9 a CN is the coordination number, R is the interatomic distance between absorber and backscatter pair, σ2 is the mean-square displacement in the distribution of interatomic distances, ΔE0 is the inner potential correction, and the R factor is a measure of the goodness of fit. postreaction (1)
3, Ni and NiS
Despite the possible concern from values of the inner potentials, it is proposed that this model still holds physical relevance. This model is simplified out of necessity, and this simplification provides leeway with the fitting parameters. The reason for the simple model is that 14 independent points exist over the ΔR range and thus fewer shells than needed for simultaneous Ni-S (NiS or millerite contains three Ni-S bonds of difference distances in addition to a Ni-Ni bond32) and metallic Ni phases, meaning a total of four shells would be needed for a refined model.
4. Discussion The goal of this work was to correlate the physiochemical properties occurring with time-on-stream and regeneration cycle to deactivation characteristics observed in Figure 1. In the previous section, we described the reaction data and the approach used for the analysis of the EXAFS spectra. Now, we explain physiochemical properties according to the structure-function relationships from our analysis. In particular, three regimes are observed with increasing cycle numbers. Initially, the prepared catalyst consists of oxide phases, primarily NiO as shown for the spectrum in Supporting Information Figure S1 for the fresh catalyst (oxidized (1)). After the initial reduction, most of the oxide phase becomes reduced to form metallic Ni. Based on the low Ni-Ni coordination number of the sample reduced (1) as compared to subsequent reduced samples and the good quality of fit for model 2, it is concluded that the oxide phase is not completely reduced. Because of the reducing nature of the reaction atmosphere and potentially slow reduction kinetics, the Ni-Ni coordination increases and the quality of fit using model 2 decreases for the first postreaction sample, indicating that the degree of Ni reduction increased. The second region consists of reaction cycles 2-4. In this regime, the quality of fit offered by model 1 continually decreases. Moreover, model 2 does not provide reasonable fits, especially for the postreaction catalysts. It is proposed that a portion of the Ni, presumably near the surface, becomes sulfided as evidenced by the decrease in the quality of the fit using model 1 and the reasonable fit of model 3 on the postreaction and reduced series of samples. Therefore, we infer that the sharp decrease of activity Langmuir 2010, 26(21), 16589–16594
with time-on-stream is caused by the formation of Ni-S species and the decrease of activity from cycle 1 to cycles 2 and 3 is caused formation of Ni-S moieties, which remain stable through the operational regeneration cycle. The third regime begins after reaction cycle 4. Many of the fit parameters (e.g., the inner potential) undergo changes, suggesting changes in the structures of the samples. As previously mentioned, the quality of the fits using model 1 continued to decrease and, for high reaction cycle values, exited the appropriate range to trust as reasonable fits. Moreover, model 2 became much improved as an accurate representation and indicated the presence of an oxide phase. Simultaneously, with the use of model 3, Ni-S species change as mainly indicated by a decrease in coordination number. These results suggested the steady deactivation with increasing cycle number in this region was caused by formation of oxide phases, which may occur simultaneously to transformation associated to the sulfur phase. Possible explanations are that (i) the formation of Ni-containing mixed oxides is kinetically slow, leading to several reduction and oxidation cycles being needed for reaching the stable phase or (ii) sulfur pries the nickel away from a stable cluster and, once free from a labile phase, can form an oxide phase. Additional work is being performed to refine the exact oxide and sulfide structures in these catalysts in hopes of pinpointing a more exact explanation of the deactivation phenomena.
5. Conclusions The structural features of alumina-supported Ni-based catalysts were characterized using EXAFS spectroscopy to assess potential deactivation mechanisms during tar reforming catalysis in biomass-derived syngas. Qualitative arguments based on spectra for standard materials and the literature ruled out deactivation by carbon formation, changes in particle size, and phosphorus poisoning. Sulfur poisoning and oxide formation were selected as possible scenarios in agreement with the EXAFS spectra. Qualitative analysis suggested that both sulfur poisoning and oxide formation occur and likely contribute to deactivation. Sulfur poisoning occurred with time-on-stream during the reaction and also could not be completely removed during the regeneration protocols after a few reaction cycles. With increasing DOI: 10.1021/la1016593
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Article
reaction cycle, the Ni phase became increasingly more difficult to reduce prior to the reaction and the amount of oxide phase present is lower after reaction than before due to the reducing nature of the reaction conditions. Future studies will contain an equally rigorous characterization of the phases present in the oxidized catalyst series in hopes of better understanding the solidstate chemistry during the reaction and regeneration cycles and additional spectroscopic identification of the Ni-S phase. Acknowledgment. The authors thank Gabor A. Somorjai for his guidance and leadership in many areas including surface science and catalysis. Funding for this work, provided by the U.S. Department of Energy’s Biomass Program Contract DEAC36-99-GO-10337 and from the University of South Florida,
16594 DOI: 10.1021/la1016593
Yung and Kuhn
is gratefully acknowledged. Portions of this work were performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by E.I. DuPont de Nemours & Co., The Dow Chemical Company and the State of Illinois. Use of the APS was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract Number DE-AC02-06CH11357. Assistance from the DND-CAT beamline scientists, especially Qing Ma, is greatly appreciated. Supporting Information Available: Additional experimental details and EXAFS results. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(21), 16589–16594
