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.
Highlights
This review presents an overview of research performed at Aalto University and the University of Twente on studies on ZrO2 catalyzed oxidation over the past 10 years
Reactivity of surface oxygen on ZrO2 is connected to the formation of OH groups, which will always be present on the surface under conditions relevant to catalysis
Based on density functional theory (DFT) model calculations presented in the study by Kouva et al it was indicated that both carbonates and hydroxyl groups preferably form on low-coordinated sites
Summary
This review presents an overview of research performed at Aalto University and the University of Twente on studies on ZrO2 catalyzed oxidation over the past 10 years. Reactivity of surface oxygen on ZrO2 is connected to the formation of OH groups, which will always be present on the surface under conditions relevant to catalysis. Despite the fact that doping will have an important influence on the properties of ZrO2, we do not make systematic distinction because generally the properties discussed here are not influenced qualitatively. The goal of this short review is to combine information available on ZrO2 catalyzed reactions (CPOM, WGS and tar decomposition) in which redox cycles at the surface of ZrO2 are involved, focusing on the role of specific surface sites including OH groups. Connected to this we will present new results on DFT calculations in order to explain the improved performance of ZrO2 in tar decomposition in the presence of H2S
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