ZrO₂ Acting as a Redox Catalyst

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.