Exploiting Phase Transitions in Catalysis: Adsorption of CO on doped VO 2 -Polymorphs.

VO ₂ is well known for its low-temperature metal-insulator transition between two phases with tetragonal rutile and monoclinic structure. The adsorption of CO on the two polymorphs of Mo-doped VO ₂ is calculated to investigate the effect of a substrate phase change on the adsorption energy. The system is investigated theoretically at density-functional theory level using a hybrid functional with London dispersion correction. We establish a computational protocol applicable for the study of physisorption on open-shell transition metal oxides. The main task is to control the spin state of open-shell slab models used to model adsorption of closed-shell molecules in order to obtain numerically stable adsorption energies and to reduce spin contamination within the broken-symmetry unrestricted Kohn-Sham approximation. Applying this procedure, it is possible to identify the most stable adsorption positions of CO on both phases of VO ₂ . CO adsorbs vertically with the C atom on a surface V atom in the monoclinic phase with an adsorption energy of -56 kJ/mol. The same adsorption position has an adsorption energy of only -46 kJ/mol on the rutile phase. Similar differences were obtained with multireference methods using an embedded cluster model. This effect may inspire experimental strategies exploiting the rutile ↔ ${ \leftrightarrow }$ monoclinic VO ₂ phase transition in catalytic processes where CO is formed as product or as an intermediate.
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