Surface chemistry and reactivity of Pd/BaCeO3∙2ZrO2 catalyst upon sulphur hydrothermal treatment for the total oxidation of methane

Abstract

Starting from metal nitrates and glycine, 2% Pd/BaCeO3∙2ZrO2 catalyst was prepared by solution combustion synthesis and tested towards the total oxidation of methane. The catalyst underwent heavy sulphur-hydrothermal treatment at 800°C up to 450h. The catalyst was fully characterized (XRD, BET, SEM, O2-TPD, FTIR analyses and catalytic activity via CH4-TPC) every 150h. With ageing, catalytic activity tests demonstrated that the catalyst was heavily poisoned after 150h; then it recovered the catalytic activity after 300h, with a resulting performance better than the one reached in the fresh status. At the end of the sulphur-hydrothermal treatment, after 450h, the catalyst resulted heavily poisoned again. On the fresh catalyst surface, IR analysis of CO adsorption evidenced the formation of highly dispersed Pd metal clusters and Pd ions. After 300h of sulphur hydrothermal ageing, the increased catalytic activity towards methane combustion was probably supported by the segregation of ZrO2 and CeO2. Moreover, subsurface and bulk sulphate formation was detected with ageing and Pd metal species were not anymore available for CO coordination, probably because hindered by sulphate deposits. Prevailing ageing mechanisms resulted in the oxidation of the surface Pd metal particles and surface-bulk sulphates formation, the latter destroying the starting crystallographic structure, leading thus to the final catalytic activity decay.