On the nature of Pdⁿ⁺surface carbonyl and nitrosyl complexes formed on Pd-promoted tungstated zirconia catalyst

Abstract

Adsorption of CO on Pd/WO3-ZrO2 sample at 85 K results in formation of carbonyls typical of the support as well as of Pd3+-CO species (2212 cm−1). The latter are masked by the strong Zr4+-CO band. Even at 90 K the Pd3+ sites are reduced to Pd2+ (the respective carbonyls absorbing at 2184 cm−1) and, at higher temperatures, to Pd+ (carbonyl bands at 2145 and 2139 cm−1) and Pd0. Adsorption of NO on a CO precovered sample results in formation of Pd2+-NO complexes (1850 cm−1) that are stable towards evacuation. Subsequent CO adsorption leads to the appearance of mixed Pd2+(CO)(NO) complexes (νCO at 2150 cm−1 and νNO at 1803 cm−1). Decrease of the CO equilibrium pressure and evacuation result in decomposition of these species to CO and Pd2+-NO nitrosyls. The existence of some mixed complexes with the participation of Pd3+ cations is also proposed.