PdO _x /Pd at Work in a Model Three-Way Catalyst for Methane Abatement Monitored by Operando XANES

The oxidation state of palladium in a model Pd/ACZ three-way catalyst was monitored by synchronous XANES and mass spectrometry during two consecutive heating (to 850 °C) and cooling (to 100 °C) cycles under stoichiometric conditions simulating exhaust after treatment of a natural gas engine. During heating in the first cycle, PdO reduction occurred around 500 °C and the initial fully oxidized state of Pd was never recovered upon heating and cooling cycles. A mixed Pd²⁺/Pd oxidation state was at work in the second cycle. Hence, the operando XANES study reveals that the PdO _x /Pd pair exists in a working catalyst but is less active than the catalyst in its initial state of fully oxidized palladium. It is also evident from XANES spectra that ceria–zirconia promotes re-oxidation of metallic Pd, thus reasonably sustaining catalytic activity after exposure to high temperatures.