Hydrogen reduction and metal-support interaction in a metastable metaloxide system: Pd on rhombohedral In2O3

Abstract

Structural and chemical consequences of reactive metal-support interaction (RMSI) effects occurring under hydrogen reduction in a metastable metal-oxide system have been exemplified for small PdO particles loaded onto rhombohedral In2O3 (PdO/rh-In2O3) using synchrotron-based in situ X-ray diffraction experiments at temperatures up to 500 °C. rh-In2O3 is a meta-stable In2O3 polymorph that is prone to gas-phase dependent phase transformation to cubic In2O3 (c-In2O3). Cross-influence of metal-support interaction and phase transformation can therefore be expected in similar temperature regimes. To separate both effects, comparable experiments have also been conducted on pure rh-In2O3. Phase transformation of pure rh-In2O3 to cubic In2O3 in hydrogen occurs between 415 °C and 450 °C. On Pd/rh-In2O3, a sequence of PdO reduction to Pd metal, followed by PdH0.706 hydride formation and subsequently, InPd and In3Pd2 intermetallic compound formation have been observed between 30 °C and 500 °C. After the intermetallic compound formation is finished at around 400 °C, the phase transformation to c-In2O3 sets in at exactly the same temperature as on pure rh-In2O3 and extends over the same temperature range. This proves that the phase transformation of rh-In2O3 to c-In2O3 is not influenced by the reduction and the intermetallic compound formation. In contrast to Pd on c-In2O3, In-Pd compound formation from rh-In2O3 occurs at much lower temperatures (230 °C vs. 300 °C), despite finally approaching the same compound stoichiometries (InPd and In3Pd2). This points to a high structural stability of reduced rh-In2O3/stability of oxygen vacancies (compared to c-In2O3) as well as to facilitated diffusion of reduced In(-O) species at low temperatures.