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 temperature
