(172b) Periodic Trends in the Morphology, Charge Distribution, and Energetics of Oxygen Vacancies on Doped MoO3 (010)

Authors: 
Choksi, T. S., Stanford University
Greeley, J., Purdue University
Reducible oxides like MoO3 are widely utilized as partial oxidation catalysts in the chemical industry, among more recent applications such as materials in energy storage, as well as plasmonic and electrochromic devices [1]-[2]. Their reducible nature engenders morphological transformations which directly influences surface catalysis [3]. Recent ab initio studies have elucidated redox characteristics at a fundamental level and have ultimately exploited these features to design materials possessing improved catalytic functionalities by applying strategies like substitutional doping, nanostructure functionalization, and strain engineering [4]-[5].

Herein, we explore periodic trends on redox properties and reactivity of doped MoO3 (010) by employing methanol oxidation as a probe reaction. Considered dopants include 1st, 2nd, and 3rd row transition metals from groups 4 through 8 of the periodic table, placed at different proximities to the oxygen defect. We begin by formulating a DFT-derived microkinetic model on pristine MoO3 that describes the interplay between catalytic chemistry and redox processes on the (010) basal planes existing during the early stages of the reaction [6]. Building on our model, we then elucidate periodic trends in oxygen vacancy formation, a key descriptor for redox chemistries. On pristine MoO3 (010), such vacancies are characterized by two reduced Mo+5 centers along with an anisotropic charge density distribution and local distortions in Mo – O bonds. XPS investigations by Kim et al. report similar observations, corroborating our analysis [2]. Dopant induced charge transfer (both short and long range) systematically influences the local geometric distortions and the accompanying anisotropic charge densities at oxygen defects. For instance, low valent dopants (LVDs) transform two Mo+5 atoms into a single Mo+4 center, facilitating vacancy formation while attenuating the extent of anisotropy. Periodic trends reveal that the degree of attenuation and its consequence on vacancy formation, can be controllably varied as a function of the oxidation state and ionic radius of the dopant. This intriguing trend obtained using BEEF-vdW and PBE+U, is verified through geometry optimized calculations using the hybrid HSE06 functional. We also highlight the effect of charge compensating the doped systems on vacancy formation. Finally, we examine the influence of dopants on thermodynamic and kinetic barriers of methanol oxidation in the context of the microkinetic model. LVDs tend to weaken oxygen adsorption reducing vacancy re-oxidation, while simultaneously lowering dehydrogenation barriers, thus, promoting methanol oxidation in comparison to pristine MoO3. Through this model system, we demonstrate that the complex interplay between local structure and charge distributions presents an additional strategy to rationally engineer reducible oxides.

References

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[2] Kim. H. S., Cook J. B., Lin H., Ko J. S., Tolbert S. H., Ozolins V., Dunn B., Nat. Mater., 2017, 16, 454.

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[5] Gopal C. B., Garćia-Melchor M., Lee C. S., Shi Y., Shavorskiy A., Monti M., Guan Z., Sinclair R., Bluhm H., Vojvodic A., Chueh W. C., Nat. Commun., 2017, 8, 15360.

[6] Choksi T., and Greeley J. ACS Catal., 2016, 6, 7260.