Abstract
In situ X-ray photoelectron spectroscopy (in situ XPS) was used to study the structural and catalytic properties of Pd-In near-surface intermetallic phases in correlation with previously studied PdZn and PdGa.Room temperature deposition of ∼4 monolayer equivalents (MLEs) of In metal on Pd foil and subsequent annealing to 453 K in vacuum yields a ∼1:1 Pd/In near-surface multilayer intermetallic phase. This Pd(1)In(1) phase exhibits a similar "Cu-like" electronic structure and indium depth distribution as its methanol steam reforming (MSR)-selective multilayer Pd(1)Zn(1) counterpart.Catalytic characterization of the multilayer Pd(1)In(1) phase in MSR yielded a CO(2)-selectivity of almost 100% between 493 and 550 K. In contrast to previously studied In(2)O(3)-supported PdIn nanoparticles and pure In(2)O(3), intermediate formaldehyde is only partially converted to CO(2) using this Pd(1)In(1) phase. Strongly correlated with PdZn, on an In-diluted PdIn intermetallic phase with "Pd-like" electronic structure, prepared by thermal annealing at 623 K, methanol steam reforming is suppressed and enhanced CO formation via full methanol dehydrogenation is observed.To achieve CO(2)-TOF values on the isolated Pd(1)In(1) intermetallic phase as high as on supported PdIn/In(2)O(3), at least 593 K reaction temperature is required. A bimetal-oxide synergism, with both bimetallic and oxide synergistically contributing to the observed catalytic activity and selectivity, manifests itself by accelerated formaldehyde-to-CO(2) conversion at markedly lowered temperatures as compared to separate oxide and bimetal. Combination of suppression of full methanol dehydrogenation to CO on Pd(1)In(1) inhibited inverse water-gas-shift reaction on In(2)O(3) and fast water activation/conversion of formaldehyde is the key to the low-temperature activity and high CO(2)-selectivity of the supported catalyst.