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

Abstract

The hydrogenation of CO 2 was investigated over a wide range of reaction conditions, using two Pd/γ-Al 2 O 3 catalysts with different Pd loadings (5% and 0.5%) and dispersions (∼11% and ∼100%, respectively). Turnover rates for CO and CH 4 formation were both higher over 5% Pd/Al 2 O 3 with a larger average Pd particle size than those over 0.5% Pd/Al 2 O 3 with a smaller average particle size. The selectivity to methane (22–40%) on 5% Pd/Al 2 O 3 was higher by a factor of 2–3 than that on 0.5% Pd/Al 2 O 3 . The drastically different rate expressions and apparent energies of activation for CO and CH 4 formation led us to conclude that reverse water gas shift and CO 2 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 CO 2 and H 2 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 CO 2, H 2, and CO 2 + H 2 . The DRIFTS and MS results also demonstrate that no direct dissociation of CO 2 takes place over the two catalysts and that CO 2 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: CO 2 activation on the oxide support and H 2 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 CH 4 formation. 5% Pd/Al 2 O 3 contains a larger fraction of terrace sites favorable for forming these more multibound and stable CO species than 0.5% Pd/Al 2 O 3 . Consequently, we propose that the difference in the formation rate and selectivity to CH 4 on different Pd particle sizes stems from the different concentrations of the reactive intermediate for the methanation pathway on the Pd surface.