Kinetic modeling and transient DRIFTS–MS studies of CO₂ methanation over Ru/Al₂O₃ catalysts

Journal Article · Tue Nov 01 00:00:00 EDT 2016 · Journal of Catalysis

DOI:https://doi.org/10.1016/j.jcat.2016.02.001· OSTI ID:1339875

Wang, Xiang; Hong, Yongchun ; Shi, Hui; Szanyi, János

CO₂ methanation was investigated on 5% and 0.5% Ru/Al₂O₃ catalysts (Ru dispersions: ~18% and ~40%, respectively) by steady-state kinetic measurements and transient DRIFTS–MS. Methanation rates were higher over 5% Ru/Al₂O₃ than over 0.5% Ru/Al₂O₃. The measured activation energies, however, were lower on 0.5% Ru/Al₂O₃ than on 5% Ru/Al₂O₃. Transient DRIFTS–MS results demonstrated that direct CO₂ dissociation was negligible over Ru. CO₂ has to first react with surface hydroxyls on Al₂O₃ to form bicarbonates, which, in turn, react with adsorbed H on Ru to produce adsorbed formate species. Formates, most likely at the metal/oxide interface, can react rapidly with adsorbed H forming adsorbed CO, only a portion of which is reactive toward adsorbed H, ultimately leading to CH4 formation. The measured kinetics are fully consistent with a Langmuir–Hinshelwood type mechanism in which the H-assisted dissociation of the reactive CO* is the rate-determining step (RDS). The similar empirical rate expressions (r_CH4 = kP$$0.1\atop{CO2}$$P$$0.3-0.5\atop{H2}$$) and DRIFTS–MS results on the two catalysts under both transient and steady-state conditions suggest that the mechanism for CO₂ methanation does not change with Ru particle size under the studied experimental conditions. Kinetic modeling results further indicate that the intrinsic activation barrier for the RDS is slightly lower on 0.5% Ru/Al₂O₃ than on 5% Ru/Al₂O₃. Due to the presence of unreactive adsorbed CO under reaction conditions, the larger fraction of such surface sites that bind CO too strongly on 0.5% Ru/Al₂O₃ than on 5% Ru/Al₂O₃, as revealed by FTIR measurements, is regarded as the main reason for the lower rates for CO₂ methanation on 0.5% Ru/Al₂O₃. The catalyst preparation and catalytic measurements were supported by a Laboratory Directed Research and Development (LDRD) project. The authors gratefully acknowledge the financial support of this work by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1339875
• Report Number(s): PNNL-SA-113614; KC0302010
• Journal Information: Journal of Catalysis, Vol. 343, Issue C; ISSN 0021-9517
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY