Surface reactivity ofV2O5(001): Effects of vacancies, protonation, hydroxylation, and chlorination

Abstract

Using density-functional theory we analyze the thermodynamic stability of partially reduced, protonated, hydroxylated, and chlorinated ${\mathrm{V}}_{2}$O${}_{5}$(001) surfaces under flue gas conditions. These surfaces are characterized geometrically through surface relaxation calculations and electronically through charge distribution and density-of-states analysis to understand the change in surface reactivity under different pressure and temperature conditions, with a primary focus on coal-fired flue gas conditions. The stoichiometric surface is found to be the most favorable termination under flue gas conditions, but at low oxygen partial pressures (i.e., ultra-high-vacuum conditions) and elevated temperatures, the partially reduced ${\mathrm{V}}_{2}$O${}_{5}$(001) surfaces with one or two vanadyl oxygen vacancies are found to be stable. A surface semiconductor-to-metal transformation takes place with the addition of oxygen vacancies indicated by a decrease in the band gap. The protonation of the ${\mathrm{V}}_{2}$O${}_{5}$(001) surface only takes place at low oxygen partial pressures where the main source or sink of hydrogen atoms comes from ${\mathrm{H}}_{2}$. The study of the thermodynamic stability of protonated surfaces and surfaces with dissociated water with both H-- and OH-- groups indicated that these surfaces are not stable under flue gas conditions. Chlorinated surfaces were not stable under the flue gas and the coverage conditions tested. Larger HCl concentrations or smaller coverages may lead to stable chlorinated structures; however, the small coverages required to accurately represent the chlorine flue gas concentrations would require much larger unit-cell sizes that would be too computationally expensive. From this work it is evident that the stoichiometric surface of ${\mathrm{V}}_{2}$O${}_{5}$ is the most stable under flue gas conditions, and likely reactivity corresponding to NO${}_{x}$ reduction, surface chlorination, and mercury oxidation stems from support effects on the vanadia catalyst, which influences the vanadium oxidation state and subsequent surface reactivity.