(433i) Deposition and Structure of MoO3 Clusters on Anatase TiO2(101)

Oxide clusters supported on metal oxide substrates are important as heterogeneous catalysts[1, 2] due to their potential tunability: producing variable cluster-support interactions with a range of chemically- and photochemically-active structures.[3] Here, the behavior of MoO₃ clusters deposited on a well-defined anatase TiO2(101) surface has been explored via scanning tunneling microscopy (STM), X-ray Photoelectron Spectroscopy (XPS), and density functional theory (DFT) calculations including ab initio molecular dynamics (AIMD) simulations. From this work, we are able to deduce the constraints on the MoO₃ cluster’s structure, distribution, and redox properties as a function of cluster coverage and temperature, which, now known, will indicate the reactive properties one can expect from this material.

Monodispersed MoO₃ clusters are deposited by the sublimation of MoO₃ powder at 1350 K over the anatase TiO₂(101) surface. Since the stable gas phase structure of MoO₃ is a trimer in cyclic configuration, i.e. (MoO₃)₃,[1, 4] it is assumed that MoO₃ deposits as this structure. After deposition, the STM images of the lowest concentration of MoO₃ show no preference for step edges, but three-unit clustering is observed. The adsorbed clusters appear as bright protrusions (see Figure 1a), with an apparent cluster height of approximately 1.5 Å. Annealing to 450 K results in a better-ordering of the overlayer, but further annealing to 650 K leads to three-dimensional clusters. The XPS results indicate that the Mo(3d_5/2) binding energy in as-deposited (MoO₃)₃ is characteristic of Mo⁶⁺, and the oxidation state of Mo remains (+6) upon heating to 500 K, consistent with DFT results suggesting that the O of MoO₃ monomers are very tightly bound.

AIMD simulation of the most stable adsorption configuration of terrace-bound (MoO₃)₃ reveals that this structure quickly decomposes into monomeric units of MoO₃. This decomposition is shown to be primarily entropically driven. As can be seen in Figure 1(a-b), STM imaging clearly shows that these monomeric units preferentially remain in one of four as-decomposed trimeric orientations—the three-unit clusters seen in STM—elucidated via DFT to be isoenergetic structures labeled A-D in Figure 1. DFT calculations show isolated monomers to be energetically (and naturally, entropically) preferred over these trimeric orientations. However, activation barriers for monomer MoO₃ diffusion are calculated to be significant. The along-row diffusion barrier is shown to be thermally surmountable while the across-row diffusion barrier is prohibitively high. This effectively constrains MoO₃ monomers to remain in the A-D trimeric orientations provided high temperatures are not accessed, at which point they are constrained to a single anatase TiO₂(101) row, similar to that found for adsorbed H.[5] As such, this system may offer great promise as an ideal platform for reactivity studies on well-defined supported model transition-metal oxide catalysts.

References:

1. Rousseau, R., et al., Chemical Society Reviews, 2014. 43(22): p. 7664-7680.
2. Bondarchuk, O., et al., Angewandte Chemie International Edition, 2006. 45(29): p. 4786-4789.
3. Tang, X., et al., Journal of Physics: Conference Series, 2013. 438: p. 012005.
4. Berkowitz, J., M.G. Inghram, and W.A. Chupka, The Journal of Chemical Physics, 1957. 26(4): p. 842-846.
5. Doudin, N., et al., ACS Catalysis, 2019. 9(9): p. 7876-7887.