From Oxide-Supported Palladium to Intermetallic Palladium Phases: Consequences for Methanol Steam Reforming

Abstract

Abstract

This Minireview summarizes the fundamental results of a comparative inverse-model versus real-model catalyst approach toward methanol steam reforming (MSR) on the highly CO2-selective H2-reduced states of supported Pd/ZnO, Pd/Ga2O3, and Pd/In2O3 catalysts. Our model approach was extended to the related Pd/GeO2 and Pd/SnO2 systems, which showed previously unknown MSR performance. This approach allowed us to determine salient CO2-selectivity-guiding structural and electronic effects on the molecular level, to establish a knowledge-based approach for the optimization of CO2 selectivity. Regarding the inverse-model catalysts, in situ X-ray photoelectron spectroscopy (in situ XPS) studies on near-surface intermetallic PdZn, PdGa, and PdIn phases (NSIP), as well as bulk Pd2Ga, under realistic MSR conditions were performed alongside catalytic testing. To highlight the importance of a specifically prepared bulk intermetallic[BOND]oxide interface, unsupported bulk intermetallic compounds of PdxGay were chosen as additional MSR model compounds, which allowed us to clearly deduce, for example, the water-activating role of the special Pd2Ga-β-Ga2O3 intermetallic[BOND]oxide interaction. The inverse-model studies were complemented by their related "real-model" experiments. Structure-activity and structure-selectivity correlations were performed on epitaxially ordered PdZn, Pd5Ga2, PdIn, Pd3Sn2, and Pd2Ge nanoparticles that were embedded in thin crystalline films of their respective oxides. The reductively activated "thin-film model catalysts" that were prepared by sequential Pd and oxide deposition onto NaCl(001) exhibited the required large bimetal[BOND]oxide interface and the highly epitaxial ordering that was required for (HR)TEM studies and for identification of the structural and catalytic (bi)metal[BOND]support interactions. To fully understand the bimetal[BOND]support interactions in the supported systems, our studies were extended to the MeOH- and formaldehyde-reforming properties of the clean supporting oxides. From a direct comparison of the "isolated" MSR performance of the purely bimetallic surfaces to that of the "isolated" oxide surfaces and of the "bimetal[BOND]oxide contact" systems, a pronounced "bimetal[BOND]oxide synergy" toward optimum CO2 activity/selectivity was most evident. Moreover, the system-specific mechanisms that led to undesired CO formation and to spoiling of the CO2 selectivity could be extracted.