Stabilization of Iron-Based Catalysts against Oxidation: An In Situ

Apr 11, 2017 - Abstract. Abstract Image. Stabilization of base metals at low oxidation state is of vital importance in their application as heterogene...
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Stabilization of Iron-Based Catalysts against Oxidation: An In Situ Ambient-Pressure X‑ray Photoelectron Spectroscopy (AP-XPS) Study Yongchun Hong,†,‡ Shiran Zhang,§ Franklin Feng Tao,§ and Yong Wang*,†,‡ †

The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99164, United States ‡ Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, United States § Department of Chemical & Petroleum Engineering and Department of Chemistry, University of Kansas, Lawrence, Kansas 66047, United States S Supporting Information *

ABSTRACT: Stabilization of base metals at low oxidation state is of vital importance in their application as heterogeneous catalysts, especially in the presence of water. In this work, addition of a small amount of Pd on Fe surface was found to provide a remarkable enhancement of its stability in hydrodeoxygenation (HDO) of m-cresol. The deactivation of Fe catalyst and the effect of Pd on the stability of Fe were uncovered with an in situ ambientpressure X-ray photoelectron spectroscopy (XPS). The deactivation is attributed to oxidation of the catalytic active metallic Fe during reaction, while Pd addition limits the steady-state coverage of oxidized Fe on the surface.

KEYWORDS: AP-XPS, bimetallic catalyst, Pd−Fe, hydrodeoxygenation, m-cresol, bio-oil upgrading

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of the high oxygenate and water content in biomass feedstock, high production volume, and low profit margin of targeted products, the ideal catalyst candidate for HDO should be stable in the presence of water (a coproduct in HDO and major component in biomass feedstock) and less costly but highly selective in C−O bond cleavage (minimizing hydrogen consumption by avoiding aromatic ring saturation). In our recent reports, Fe-based catalysts were found to be promising in HDO of lignin-derived phenolics, such as guaiacol and m-cresol.8,9 Fe-based catalysts showed a superior selectivity toward aromatic hydrocarbons, achieving ∼90% selectivity toward formation of benzene, toluene, and xylene (BTX).8 More interestingly, the addition of a small amount of Pd to Fe catalysts significantly improved the activity of Fe without altering its selectivity.8,9 The high selectivity toward aromatic hydrocarbons over Fe catalysts was proposed to result from favored direct C−O bond cleavage of phenolic compounds on the Fe surface,10 while the enhanced activity by Pd addition was attributed to a combination of Pd-promoted H2 activation8 and Pd-facilitated formation of water.11 Despite the superior selectivity, Fe catalysts suffer from rapid deactivation during HDO of lignin-derived compounds, especially in the presence of water. Dufour et al. suggested

e-based catalysts are widely used in the production of bulk chemicals and fuels such as ammonia1 and synthetic fuels.2 The abundance and low cost of Fe catalysts make them more economically feasible than other metal catalysts, especially noble-metal catalysts. However, the poor chemical stability of Fe in the presence of water, compared with noble metals, has hindered its application as a catalyst. For example, the oxidation of active Fe species to inactive Fe3O4 phase has been proposed to be one major reason for deactivation of Fe-based catalysts in Fischer−Tropsch synthesis.3 It is therefore of great interest to improve Fe’s stability under reaction conditions without sacrificing its catalytic performance, in terms of activity and selectivity. In this work, hydrodeoxygenation (HDO) of m-cresol, which is a model reaction for lignin-derived bio-oil upgrading, was chosen as a probe reaction to study the deactivation behavior of Fe and to understand the role of Pd in the stabilization of Fe catalysts during HDO reaction. Lignin is becoming more and more attractive as feedstock for biofuel and chemical production, because of its vast availability and nonedible nature.4 In a typical lignin conversion process, lignin is first decomposed into smaller molecules via procedures such as fast pyrolysis.5 The resultant liquid products (bio-oils) cannot be directly utilized as gasoline, diesel, or jet fuel, because of their high oxygenate content (up to 50 wt %).6 HDO is one of the approaches to convert oxygenates to hydrocarbons by removing oxygen in the form of water under a H2 atmosphere.7 Because © 2017 American Chemical Society

Received: February 25, 2017 Revised: April 7, 2017 Published: April 11, 2017 3639

DOI: 10.1021/acscatal.7b00636 ACS Catal. 2017, 7, 3639−3643

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ACS Catalysis

catalytic behavior of Fe was eliminated in order to establish the relationship between catalyst deactivation and intrinsic characteristics of Fe. Stability tests of Pd−Fe and Fe (fresh and regenerated) catalysts in HDO of m-cresol were carried out at similar initial m-cresol conversion levels (8%−10%) obtained by adjusting the gas hourly space velocity (GHSV). Pd−Fe catalysts showed better activity, compared with Fe catalysts, as similar m-cresol conversion was achieved over Pd−Fe catalyst at a higher GHSV. As shown in Figure 2a, fresh Fe catalyst suffered from a

that carbon deposition is a major cause for the deactivation of Fe catalyst during HDO reaction, while the exposure of reduced Fe catalyst to H2O/H2 steam (H2O:H2 molar ratio = 1:5) at 400 °C also significantly deactivates Fe catalysts with the formation of inactive Fe oxide.12 Carbon deposition during HDO reaction is significantly dependent on the nature of the support13 and feedstock,14 while the water-induced oxidation is dependent on the redox properties of the Fe surface.15 In our previous report, Pd was found to facilitate the reduction of Fe oxide, as evidenced by temperatureprogrammed reduction (TPR) results and DFT calculations.11 In this work, we further studied the role of Pd in the stabilization of Fe surface against oxidation, which leads to the improvement of Fe catalyst life during HDO reaction. Information regarding the dynamics of catalyst surface during reaction is essential to understanding of the deactivation mechanism of Fe-based catalysts, as well as the role of Pd in stabilizing the Fe catalyst. In situ ambient-pressure X-ray photoelectron spectrometry (AP-XPS) can be used to track the surface of catalyst under a reaction condition or during catalysis.16−20 In situ AP-XPS with an monochromatic Al Ka built in Tao group16 was used in this work to directly observe the surface structure of Fe-based catalysts during HDO reaction, with an aim to unravel the deactivation mechanism of Fe and the role of Pd on stability of Fe. Support-free Fe and Pd−Fe catalysts (Pd:Fe molar ratio = 1:130) were synthesized using methods described in our previous reports8,11 (see the Supporting Information for details). The structure of passivated Pd−Fe catalyst is shown in Figure 1. The high-contrast spots are Pd nanoparticles with

Figure 2. (a) Catalytic performance of Pd−Fe, Fe, and regenerated Fe catalysts in HDO of m-cresol at 300 °C (top, catalyst loading = 100 mg; pretreated/regenerated in 50% H2/N2 at 300 °C for 5 h; reaction gas = 0.45% m-cresol in 40% H2/N2; GHSV = 96 000 h−1 for Pd−Fe catalyst, GHSV = 48 000 h−1 for Fe catalysts); and (b) pseudo-firstorder deactivation kinetics during initial reaction stage (bottom, TOS ≤ 2 h).

Figure 1. Representative AC-HAADF-STEM images for passivated Pd−Fe sample (reduced in 50% H2/N2 at 300 °C for 2 h and passivated in 1% O2/N2 at room temperature for 12 h).

rapid deactivation (from 10% to 7%) at the initial stage (TOS ≤ 2 h) and a slow deactivation (from 7% to 5%) after TOS = 2 h. Although treatment of deactivated Fe catalyst by H2 reduction at 300 °C partially regenerated its activity (8% initial conversion), the regenerated catalyst also suffered from a rapid deactivation (8% to 6%) at the initial stage (TOS ≤ 2 h). On the other hand, the Pd−Fe catalyst showed a much slower deactivation (from 9.5% to 9% in 10 h), compared with fresh and regenerated Fe catalysts. Note that the selectivity for

sizes smaller than 2 nm while the underlying low contrast matrix is Fe-based substrate with sizes witin the range of 20−30 nm. This is consistent with our previous reports that the surface of reduced Pd−Fe catalysts is enriched in Pd, as confirmed by pseudo-in situ XPS results and density functional theory (DFT) calculations.8,9 Since no support was used to prepare Fe and Pd−Fe catalysts, the contribution of catalyst supports to the 3640

DOI: 10.1021/acscatal.7b00636 ACS Catal. 2017, 7, 3639−3643

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ACS Catalysis targeted hydrocarbon product, toluene, was >85% for all catalysts tested under the conditions studied. For a more quantitative analysis, the deactivation rate constants were calculated based on a pseudo-first-order kinetics for Pd−Fe and Fe catalysts during initial stage (TOS ≤ 2 h) of HDO reaction. In Figure 2b, the natural logarithms of relative rates (rate divided by initial rate) are plotted against the TOS and the deactivation rate constants (k) are calculated based on following equation, assuming a rate at TOS = 0.25 h as an initial rate (r0): ⎛r⎞ ln⎜ ⎟ = −k(t − 0.25) ⎝ r0 ⎠

(1)

The deactivation rate constants for fresh and regenerated Fe catalysts are 0.22 h−1 and 0.13 h−1, respectively. However, the deactivation rate constant of Pd−Fe catalyst is negligible (k = 0.01 h−1). To probe the surface evolution of Fe and Pd−Fe catalysts during HDO reaction, XPS features in Fe 2p region were monitored for both catalysts under reaction conditions (0.05 mbar m-cresol in 0.45 mbar H2 at 300 °C, both catalysts were pretreated in 0.2 mbar H2 at 450 °C). The reduced Pd−Fe and Fe catalysts featured with an almost fully reduced Fe surface, evidenced by the distinct Fe 2p3/2 peaks centered at ∼707 eV, which is attributed to metallic Fe (Figure 3, black curves).21 As

Figure 4. In situ AP-XPS spectra in Fe 2p region for spent Fe catalyst during regeneration under 0.2 mbar H2 at 300 °C; the status of catalysts are annotated on the spectra.

regeneration (reduction in 0.2 mbar H2 at 300 °C). A slow recovery of metallic Fe is evidenced by the increasing intensity of the peak centered at 707 eV and the simultaneous decrease in the intensity of the shoulder peak centered at 710 eV. Full reduction of Fe was not achieved after 6 h reduction under 0.2 mbar H2 during the AP-XPS study, probably due to a low partial pressure of H2, compared with the regeneration condition (400 mbar H2). In our previous report, it was found that reduced Fe catalyst can be reoxidized by H2O/H2 gas treatment (H2O/H2 molar ratio = 4.6:100) at 300 °C, while the Fe in Pd−Fe cannot be oxidized under the same conditions, as suggested by the temperature-programmed reduction (TPR) results of treated samples.8 Carbon deposition has long been regarded as a major reason for catalyst deactivation during bio-oil upgrading,13,14 especially for the catalyst supported on acidic support such as alumina.13 Based on our in situ AP-XPS measurements of C/Fe ratios on Pd−Fe and Fe during HDO reaction, as well as the reduction (regeneration) of spent Fe catalysts (Table 1), the deactivation observed during the reaction over Fe catalyst in this study cannot be attributed to the accumulation of carbon species. Specifically, the C:Fe ratio on Pd−Fe during reaction (C/Fe =

Figure 3. In situ AP-XPS spectra in the Fe 2p region for Fe (left) and Pd−Fe (right) catalysts during HDO of m-cresol HDO at 300 °C (pretreated in 0.2 mbar H2 at 450 °C for 1 h; reaction gas = 0.05 mbar m-cresol in 0.45 mbar H2); the status of catalysts are annotated on the spectra.

Table 1. Surface Carbon:Fe Ratio for Pd−Fe and Fe Catalysts during Reaction and Regeneration

shown in the left panel of Figure 3, the relative intensity of a shoulder peak centered at ∼710 eV, corresponding to oxidized Fe species,22 increased (referenced to a peak at 707 eV) during the reaction over Fe catalyst, indicating that the surface of pure Fe catalyst was gradually oxidized during the HDO reaction. Distinctly different from Fe catalyst, there is no oxidized Fe species formed on the Pd−Fe catalyst in the right panel of Figure 3. The absence of the peak at 710 eV on Pd−Fe catalyst during catalysis suggests that the Fe on the Pd−Fe catalyst surface remained metallic during the HDO reaction. The evaluation of surface Fe species was also monitored during the regeneration process. Figure 4 shows the XPS features in Fe 2p region for a spent Fe catalyst during

catalyst Pd−Fe

Fe spent Fe

C/Fe ratio 0.2, fresh

3.4, after 1 h of reactiona

3.7, after 2 h of reaction

3.7, after 3 h of reaction

0.2, fresh

0.3, after 1 h of reactiona

0.5, after 2 h of reaction

0.6, after 4 h of reaction

0.6, spentb

0.6, after 1 h of regenc

0.6, after 3 h of regen

0.5, after 6 h of regen

a

Surface C:Fe ratio during reaction at TOS = 1 h. bSurface C:Fe ratio for spent catalyst (TOS = 4 h). cSurface C:Fe ratio during regeneration at TOS = 1 h. 3641

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metallic Fe surface against oxidation, and thus preserve the active phase for HDO reaction. In our previous report, the reduction of Fe oxide surface was facilitated by addition of Pd, as evidenced by a shift of TPR peak to lower temperature.11 The DFT calculation results also suggested that the interaction between Pd and reduced Fe oxide surface stabilizes the Fe-based substrate through formation of Pd−Fe alloy, preferentially via donation of electrons from Pd to the reduced surface, as indicated by the upshift of Pd 3d peaks in Pd/Fe2O3 catalyst, compared with those in the Pd/C reference in the pseudo-in situ XPS study.8,11 The upshift of Pd 3d peaks was also confirmed by AP-XPS (see Figure 5). The Pd 3d5/2 peaks were centered at 336 eV for both

3.7 at TOS = 2 h) is 1 order of magnitude higher than that on Fe catalyst during reaction (C/Fe = 0.5 at TOS = 2 h), while the deactivation of Pd−Fe catalyst was negligible (Figure 2). In addition, carbon species on Fe cannot be removed under the regeneration condition as the C:Fe ratio over spent Fe catalyst remains at the same level during the regeneration. Therefore, based on the above analysis, carbon deposition is not considered to be a major cause of observed deactivation of Fe catalyst during HDO reaction under the conditions studied. Recently, a direct C−O bond cleavage mechanism was found to be the most plausible for HDO of phenol on Fe(110) surface, compared with other mechanisms, including partial ring saturation and tautomerization.10 For the HDO of m-cresol, the surface Fe−OH species could be generated from a direct C−O bond cleavage or H2O dissociation, as illustrated in Scheme 1. Scheme 1. Evaluation of Surface Species during m-Cresol HDO over Fe Catalyst

Specifically, m-cresol dissociates on Fe sites, forming a quasitoluene species (Fe−C7H9) and a surface Fe−OH species (step 1 in Scheme 1). The quasi-toluene species further reacts with activated H atom to form toluene, a desired product (step 2 in Scheme 1), while the surface Fe−OH species can be further reduced by activated H atom to form water (step 3 in Scheme 1). The water formation step (step 3 in Scheme 1) is highly reversible, as suggested by theoretical calculations.10 Therefore, Fe−OH could be formed via H2O dissociation. At the same time, the Fe−OH species can also undergo dehydrogenation to form surface oxide species Fe−O (step 4 in Scheme 1), as observed during water dissociation on the Fe(100) surface.15 The activation barriers for hydrogenation of Fe−OH (step 3 in Scheme 1) and Fe−O (reverse of step 4 in Scheme 1) are significant, as calculated by Niemantsverdriet et al. (1.10 and 1.14 eV, respectively)23 and in our previous report (1.32 eV for step 3 in Scheme 1).10 Despite the origin of Fe−OH species, the deactivation of Fe catalyst during HDO of m-cresol is correlated with the accumulation of oxidized Fe species (Fe−OH and/or Fe−O), as suggested by AP-XPS studies (Figure 3). During the regeneration process, the oxidized Fe species is re-reduced to metallic form by H2 and, thus, the catalyst activity can be regenerated. However, the regenerated catalyst also suffers from deactivation due to the reoxidization of surface Fe species under HDO conditions. For the Pd−Fe catalyst, no oxidized surface Fe species was observed during HDO of m-cresol, as confirmed by AP-XPS studies (Figure 3). At the same time, only negligible deactivation was observed over Pd−Fe catalyst for m-cresol HDO, indicating that the addition of Pd can stabilize the

Figure 5. In situ AP-XPS spectra in Pd 3d region for Pd−Fe catalyst during HDO of m-cresol HDO at 300 °C (pretreated in 0.2 mbar H2 at 450 °C for 1 h; reaction gas = 0.05 mbar m-cresol in 0.45 mbar H2); the statuses of the catalysts are annotated on the spectra.

the reduced sample and the sample during reaction. The peak position for metallic pure Pd is centered at 335 eV.24 Compared to the Pd 3d5/2 of pure metallic Pd, Pd 3d5/2 is 1 eV higher, which shows electron transfer from Pd to Fe, as suggested in the literature.25 The fact that the Pd 3d5/2 peak positions remained constant suggests that the chemical environment of Pd atoms of the active surface of Pd−Fe catalyst remains unchanged during catalysis. To conclude, the cause of deactivation for Fe catalyst during HDO of m-cresol is unraveled by AP-XPS study. The deactivation is due to the formation of inactive oxidized surface Fe species under reaction conditions. The addition of a small amount of Pd enhances the stability of metallic Fe surface against oxidation, leading to significantly improved stability of Fe catalysts. This study illustrates a promising approach to stabilize base metals against oxidation in reactions when water molecules are present.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b00636. 3642

DOI: 10.1021/acscatal.7b00636 ACS Catal. 2017, 7, 3639−3643

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(19) Zhang, S. R.; Shan, J. J.; Zhu, Y.; Frenkel, A. I.; Patlolla, A.; Huang, W. X.; Yoon, S. J.; Wang, L.; Yoshida, H.; Takeda, S.; Tao, F. J. Am. Chem. Soc. 2013, 135, 8283−8293. (20) Zhu, Y.; Zhang, S. R.; Ye, Y. C.; Zhang, X. Q.; Wang, L.; Zhu, W.; Cheng, F.; Tao, F. ACS Catal. 2012, 2, 2403−2408. (21) Andersson, S. L. T.; Howe, R. F. J. Phys. Chem. 1989, 93, 4913− 4920. (22) Tan, B. J.; Klabunde, K. J.; Sherwood, P. M. A. Chem. Mater. 1990, 2, 186−191. (23) Govender, A.; Curulla Ferré, D.; Niemantsverdriet, J. W. ChemPhysChem 2012, 13, 1583−1590. (24) Jenks, C. J.; Chang, S. L.; Anderegg, J. W.; Thiel, P. A.; Lynch, D. W. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 6301−6306. (25) Wu, C.-T.; Yu, K. M. K.; Liao, F.; Young, N.; Nellist, P.; Dent, A.; Kroner, A.; Tsang, S. C. E. Nat. Commun. 2012, 3, 1050.

Catalyst preparation, characterization and catalytic testing procedures, configuration of in situ ambient pressure XPS (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yongchun Hong: 0000-0002-8109-3282 Franklin Feng Tao: 0000-0002-4916-6509 Yong Wang: 0000-0002-8460-7410 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences (Nos. DE-FG02-05ER15712, FWP-47319, and DE-0014561). A portion of the research was performed at Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at PNNL.

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REFERENCES

(1) Schlögl, R. Angew. Chem., Int. Ed. 2003, 42, 2004−2008. (2) Dry, M. E. Catal. Today 2002, 71, 227−241. (3) de Smit, E.; Weckhuysen, B. M. Chem. Soc. Rev. 2008, 37, 2758− 2781. (4) Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. A.; Gilna, P.; Keller, M.; Langan, P.; Naskar, A. K.; Saddler, J. N.; Tschaplinski, T. J.; Tuskan, G. A.; Wyman, C. E. Science 2014, 344, 1246843. (5) Liu, C. J.; Wang, H. M.; Karim, A. M.; Sun, J. M.; Wang, Y. Chem. Soc. Rev. 2014, 43, 7594−7623. (6) Mortensen, P. M.; Grunwaldt, J. D.; Jensen, P. A.; Knudsen, K. G.; Jensen, A. D. Appl. Catal., A 2011, 407, 1−19. (7) Wang, H. M.; Male, J.; Wang, Y. ACS Catal. 2013, 3, 1047−1070. (8) Hong, Y. C.; Zhang, H.; Sun, J. M.; Ayman, K. M.; Hensley, A. J. R.; Gu, M.; Engelhard, M. H.; McEwen, J. S.; Wang, Y. ACS Catal. 2014, 4, 3335−3345. (9) Sun, J. M.; Karim, A. M.; Zhang, H.; Kovarik, L.; Li, X. H. S.; Hensley, A. J.; McEwen, J. S.; Wang, Y. J. Catal. 2013, 306, 47−57. (10) Hensley, A. J. R.; Wang, Y.; McEwen, J. S. ACS Catal. 2015, 5, 523−536. (11) Hensley, A. J. R.; Hong, Y. C.; Zhang, R. Q.; Zhang, H.; Sun, J. M.; Wang, Y.; McEwen, J. S. ACS Catal. 2014, 4, 3381−3392. (12) Olcese, R.; Bettahar, M. M.; Malaman, B.; Ghanbaja, J.; Tibavizco, L.; Petitjean, D.; Dufour, A. Appl. Catal., B 2013, 129, 528− 538. (13) Popov, A.; Kondratieva, E.; Goupil, J. M.; Mariey, L.; Bazin, P.; Gilson, J.-P.; Travert, A.; Maugé, F. J. Phys. Chem. C 2010, 114, 15661−15670. (14) Weber, R. S.; Olarte, M. V.; Wang, H. Energy Fuels 2015, 29, 273−277. (15) Hung, W.-H.; Schwartz, J.; Bernasek, S. L. Surf. Sci. 1991, 248, 332−342. (16) Nguyen, L.; Tao, F. Rev. Sci. Instrum. 2016, 87, 064101. (17) Zhang, S. R.; Nguyen, L.; Zhu, Y.; Zhan, S. H.; Tsung, C. K.; Tao, F. Acc. Chem. Res. 2013, 46, 1731−1739. (18) Zhang, S. R.; Nguyen, L.; Liang, J. X.; Shan, J. J.; Liu, J. Y.; Frenkel, A. I.; Patlolla, A.; Huang, W. X.; Li, J.; Tao, F. Nat. Commun. 2015, 6, 7938. 3643

DOI: 10.1021/acscatal.7b00636 ACS Catal. 2017, 7, 3639−3643