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 (ν_CO at 2150 cm⁻¹ and ν_NO 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.