Tuning the Properties of Molybdenum Oxide on Al₂O₃/NiAl(110): Metal versus Oxide Deposition

To investigate how the properties of model mixed metal oxide catalysts can be influenced by the choice of evaporating species during physical vapor deposition, we have compared MoO_x on Al₂O₃/NiAl(110) prepared via an oxidic and a metallic precursor. In the former case, MoO_x was prepared via direct deposition of MoO_x, while in the latter case, metallic Mo was deposited in an O₂ background. The structure of the resulting catalysts was compared to that of metallic Mo deposited on Al₂O₃/NiAl(110) in the absence of O₂. For directly deposited MoO_x, we observe predominantly point defect nucleation and high particle densities. In contrast, when MoO_x is prepared by deposition of metallic Mo in 5 × 10^-7 mbar O₂, we find lower particle densities and preferential nucleation at step edges and domain boundaries, thus reflecting the particle dispersion of metallic Mo. This suggests that the Mo atoms are oxidized typically only after having attached to a stable Mo or MoO_x nucleus. We interpret our findings in terms of the interaction between the deposited material and the support, which is stronger for MoO_x than for Mo. These results demonstrate that the choice of evaporating material crucially influences the catalyst structure and is therefore a useful parameter in tuning the properties of model mixed oxide catalysts.