Who Is Doing the Job? Unraveling the Role of Ga₂O₃ in Methanol Steam Reforming on Pd₂Ga/Ga₂O₃

A systematic study of the nature, stability, and dynamics of surface species present under methanol steam reforming (MSR) conditions over Pd/Ga₂O₃ and Pd₂Ga/Ga₂O₃ was performed by combining steady state and concentration modulation FTIR spectroscopy. This powerful combination allowed us to obtain novel mechanistic insights into the selective pathway leading to the formation of H₂ and CO₂ and thus to contribute to the understanding of the remarkably different catalytic properties of Pd/Ga₂O₃ and Pd₂Ga/Ga₂O₃. Strongly enhanced formation of adsorbed formates at low temperatures was detected on Pd₂Ga/Ga₂O₃. We ascribe the facilitated formation of these species to the presence of reactive oxygen sites in the Ga₂O₃ surface, which are formed during high-temperature reduction and formation of the intermetallic compound Pd₂Ga. While the stability of involved formates is high under reaction conditions of methanol decomposition (i.e., in the absence of H₂O), the entire adsorption system behaves more dynamically in the presence of water. We propose that the introduction of H₂O into the system converts stable bridging- and bidentate formates into more reactive, monodentate species. These react either with adsorbed methoxy to methyl formate (MFO) in the absence of water or with OH groups supplied by H₂O to CO₂ and H₂. The reaction with OH is faster, leading to a smaller concentration of intermediate monodentate formate under MSR conditions. MFO is easily decomposed into CO and CH₃OH and therefore, it is unlikely to be an intermediate in the selective MSR reaction to CO₂ and H₂. While the formation of intermetallic particles by high-temperature reduction is a prerequisite to achieving high MSR selectivity, our results suggest that the reaction sequence predominantly proceeds on the Ga₂O₃ surface, that is modified by the high temperature reduction and the formation of Pd₂Ga, and is only promoted by the presence of the intermetallic particles.