ZrO2 Acting as a Redox Catalyst

Abstract

Surface defects are discussed and reviewed with regards to the use of ZrO2 in applications involving interactions with CO, H2, CH4, CO2, water and hydrocarbons. Studies of catalytic partial oxidation of methane reveal that part of the surface lattice oxygen in terraces can be removed by methane at high temperatures (e.g. 900 °C). The reaction proceeds via a surface confined redox mechanism. The studies presented here also highlight that defects play a decisive role in the water–gas-shift reaction, since the reaction is likely carried out via OH groups present at defect sites, which are regenerated by dissociating water. Hydroxyl chemistry on ZrO2 is briefly reviewed related to the studies presented. Finally, new density functional theory calculations were conducted to find out how H2S interacts with ZrO2 surface (defect sites), in order to explain enhancement of activity in naphthalene and ammonia oxidation by H2S. Molecularly adsorbed H2S as well as terminal SH species (produced by dissociation of H2S) cannot be responsible for enhanced reactivity of surface oxygen. In contrast, multi-coordinated SH induced a relatively weak increase of the reactivity of neighboring OH groups according to thermodynamic calculations. Probably, the right active site responsible for the observed H2S-induced enhancement of oxidation activity on ZrO2 is yet to be discovered.