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 I combination allowed us to obtain novel mechanistic insights into the selective pathway leading to the formation of H-2 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 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 CH3OH and therefore, it is unlikely to be an intermediate in the selective,MSR reaction to CO2 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 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.