Hydrogen-related defects in titanium dioxide at the interface to palladium

Abstract

A metal oxide support and a catalytically active metal are the two main ingredients for complex catalysts used in heterogeneous catalysis. The gas environment can change the catalyst during the reaction, modifying its structural and electronic properties. Here, we use monochromated electron energy loss spectroscopy (EELS) to reveal hydrogen-pressure-dependent changes of the electronic structure at the Pd/rutile-TiO$_2$ interface in an environmental transmission electron microscope (ETEM). Hydrogen-induced changes are observed in rutile-TiO$_2$ within $2$~nm from the interface at $10$~Pa of hydrogen pressure, in the Ti $L_{3,2}$ EEL spectra. Lower pressures such as $1$~Pa show no changes in the EEL spectra. We attribute the observed changes in the EEL spectra to hydrogen-induced defects accumulating in the vicinity of the interface. Based on DFT calculations, we developed a thermodynamic multistate defect (TMD) model of the interface and the bulk of the rutile-TiO$_2$. This TMD model predicts high concentrations of positively charged defects accumulating at the interface. The presence of the Schottky barrier stabilizes these defects by significantly lowering their formation energy. Our findings provide important new insights into catalytic processes taking place at metal/metal oxide interfaces in hydrogen gas environments.