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/Ga 2 O 3 and Pd 2 Ga/Ga 2 O 3 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 H 2 and CO 2 and thus to contribute to the understanding of the remarkably different catalytic properties of Pd/Ga 2 O 3 and Pd 2 Ga/Ga 2 O 3 . Strongly enhanced formation of adsorbed formates at low temperatures was detected on Pd 2 Ga/Ga 2 O 3 . We ascribe the facilitated formation of these species to the presence of reactive oxygen sites in the Ga 2 O 3 surface, which are formed during high-temperature reduction and formation of the intermetallic compound Pd 2 Ga. While the stability of involved formates is high under reaction conditions of methanol decomposition (i.e., in the absence of H 2 O), the entire adsorption system behaves more dynamically in the presence of water. We propose that the introduction of H 2 O 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 H 2 O to CO 2 and H 2 . 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 CH 3 OH and therefore, it is unlikely to be an intermediate in the selective MSR reaction to CO 2 and H 2 . 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 Ga 2 O 3 surface, that is modified by the high temperature reduction and the formation of Pd 2 Ga, and is only promoted by the presence of the intermetallic particles.