Reduction of Hydrogenated ZrO₂ Nanoparticles by Water Desorption

Reduction of zirconia by water desorption from a hydrogenated surface is the topic of this study. The focus is on the role of nanostructuring the oxide reducibility measured by the cost of formation of oxygen vacancies by water desorption. We have performed density functional theory calculations using the Perdew-Burke-Ernzerhof + U approach and including dispersion forces on the adsorption, dissociation, diffusion of hydrogen on the ZrO₂ (101) surface and on Zr₁₆O₃₂, Zr₄₀O₈₀, and Zr₈₀O₁₆₀ nanoparticles (NPs). The process involves the formation of a precursor state via diffusion of hydrogen on the surface of zirconia. The results show that O vacancy formation via H₂O desorption is more convenient than via direct O₂ desorption. The formation of an O_sH₂ surface precursor state to water desorption is the rate-determining step. This step is highly unfavorable on the ZrO₂ (101) surface both thermodynamically and kinetically. On the contrary, on zirconia NPs, characterized by the presence of low coordinated ions, water desorption becomes accessible such that even at temperatures close to 450 K the reaction becomes exergonic. The study shows the role of nanostructuring on the chemical and electronic properties of an oxide.