Atomistic Simulation Studies of WO3 stabilized tetragonal Zirconia

Tungstated Zirconia (WO3-ZrO2) is a technologically important catalyst; however, there is still a considerable lack of understanding for the presence of different WO3 surface species found in this catalyst which are responsible for its high activity. We report here the application of atomistic simulation techniques based on interatomic potentials to explore the nature of WO3 species on WO3-ZrO2 systems. At first modelling of the pure structure of t-ZrO2 was investigated and the reliability of computed structures was found to agree with earlier theoretical and experimental studies. An investigation of all low miller index pure surfaces of t-ZrO2 revealed the highest stability in the following order {101} > {001} > {111} > {110} > {100}. The adsorption of WO3 at partial monolayer coverages of 20 % and 50 % were then investigated onto the surfaces of t-ZrO2.

Conclusions that can be drawn from this work are that the preference WO3 species for a particular surface depended on the type of the surface and temperature considered. The highest favourability to WO3 addition was detected on the {111} surface, where a 50 % monolayer coverage was found to be stable. The {100} and {110} surfaces were favourable to WO3 addition, although this favourability depended on the temperature and amounts of WO3 added. The surfaces {001} and {101} were not favourable to WO3 addition at low temperatures. The addition of WO3 resulted in stabilizing of surfaces which were otherwise unstable in t-ZrO2 such as the {111}, {110} and {100}. The study also confirms the formation of polymerized WO3 layers, dimers, monomers and Zr-WO3 linkages which were detected in earlier experiments. The nature of WO3 species were found to be surface specific, which also depended on the surface area and temperature. The knowledge obtained from this study could be used to design and optimize an efficient catalyst.