Mechanism of CO2 Hydrogenation on Pd/Al2O3 Catalysts: Kinetics and Transient DRIFTS-MS Studies

详细信息    查看全文

• 作者：Xiang Wang ; Hui Shi ; Ja Hun Kwak ; J谩nos Szanyi
• 刊名：ACS Catalysis
• 出版年：2015
• 出版时间：November 6, 2015
• 年：2015
• 卷：5
• 期：11
• 页码：6337-6349
• 全文大小：769K
• ISSN：2155-5435

文摘

The hydrogenation of CO₂ was investigated over a wide range of reaction conditions, using two Pd/纬-Al₂O₃ catalysts with different Pd loadings (5% and 0.5%) and dispersions (鈭?1% and 鈭?00%, respectively). Turnover rates for CO and CH₄ formation were both higher over 5% Pd/Al₂O₃ with a larger average Pd particle size than those over 0.5% Pd/Al₂O₃ with a smaller average particle size. The selectivity to methane (22鈥?0%) on 5% Pd/Al₂O₃ was higher by a factor of 2鈥? than that on 0.5% Pd/Al₂O₃. The drastically different rate expressions and apparent energies of activation for CO and CH₄ formation led us to conclude that reverse water gas shift and CO₂ methanation do not share the same rate-limiting step on Pd and that the two pathways are probably catalyzed at different surface sites. Measured reaction orders in CO₂ and H₂ pressures were similar over the two catalysts, suggesting that the reaction mechanism for each pathway does not change with particle size. In accordance, the DRIFTS results reveal that the prevalent surface species and their evolution patterns are comparable on the two catalysts during transient and steady-state experiments, switching feed gases among CO₂, H₂, and CO₂ + H₂. The DRIFTS and MS results also demonstrate that no direct dissociation of CO₂ takes place over the two catalysts and that CO₂ has to first react with surface hydroxyls on the oxide support. The thus-formed bicarbonates react with dissociatively adsorbed hydrogen on Pd particles to produce adsorbed formate species (bifunctional catalyst: CO₂ activation on the oxide support and H₂ dissociation on the metal particles). Formates near the Pd particles (most likely at the metal/oxide interface) can react rapidly with adsorbed H to produce CO, which then adsorbs on the metallic Pd particles. Two types of Pd sites are identified: one has a weak interaction with CO, which easily desorbs into gas phase at reaction temperatures, whereas the other interacts more strongly with CO, which is mainly in multibound forms and remains stable in He flow at high temperatures, but is reactive toward adsorbed H atoms on Pd leading eventually to CH₄ formation. 5% Pd/Al₂O₃ contains a larger fraction of terrace sites favorable for forming these more multibound and stable CO species than 0.5% Pd/Al₂O₃. Consequently, we propose that the difference in the formation rate and selectivity to CH₄ on different Pd particle sizes stems from the different concentrations of the reactive intermediate for the methanation pathway on the Pd surface.