Hydrogen Spillover from Pt, Pd, and Au Evaluated with MnOx Temperature-Programmed Reduction

Abstract

Hydrogen spillover occurs when H₂ adsorbs on an active metal and is then transferred to a reducible metal oxide. While spillover is involved in many catalytic reactions, fundamental studies of spillover have been limited by the paucity of experimental methods available to study it. Here, we develop an experimental temperature-programmed reduction (TPR) protocol to rapidly compare spillover from different metals (Pt, Pd, Au ) onto P25 titania, using MnO_x reduction probe reaction. X-ray photoelectron and UV–visible spectroscopies indicated reduction to MnO in the presence and absence of active metals, and quantitative H₂ consumption measurements were consistent across all samples. Observed MnO_x reduction temperatures, which decrease by shifting as much as 150 °C, depend strongly on the metal loading and confirm H₂ activation occurs at metal sites. Adding only 300 ppm Au caused the Kissinger apparent activation energy to decrease from 55 to <10 kJ/mol. Comparisons across metals and loadings indicate that MnO_x reduction is limited by H₂ activation only at very low metal loadings and temperatures. Gold induces the largest shift in MnO_x reduction, indicating that it is the most competent spillover metal under these conditions. At moderately higher loadings (≥1 site/100 nm²), Pt and Au become equivalent, signaling that MnO_x reduction is no longer limited by H₂ activation at higher metal loadings. Palladium is moderately slower, consistent with the generation of a PdH_x phase at low temperature. At higher active site loadings, MnO_x reduction depends primarily on the surface concentration of spillover H, providing a simple and useful approach for comparing catalysts.