H₂ Activation on Pristine and Substitutional ZnO(101̅0) and Cr₂O₃(001) Surfaces by Density Functional Theory Calculations

As a major component of syngas (a mixture of hydrogen and carbon monoxide), activation of hydrogen molecules on metal oxides plays a vital role in various hydrogenation reactions and synthesis of valuable chemicals. Herein, we present a density functional theory investigation of H 2 activation and subsequent dehydration over pristine and substituted ZnO(101̅0) and Cr 2 O 3 (001) surfaces. An ab initio thermodynamics study shows that Cr 1 substitution on ZnO(101̅0) and Zn 1 substitution on Cr 2 O 3 (001) are stable in a wide range of temperatures and pressures. The binding strength of H to the low-valent Zn cations is the weakest one, followed by the high-valent Cr cation and the lattice oxygen coordinated to Cr as wells as to Zn. Heterolytic dissociation is found to be preferential than the homolytic one. The pristine ZnO(101̅0) is active for H 2 dissociation, and the presence of Cr 1 increases the formation energy of oxygen vacancies and lowers the reactivity to H 2 dissociation. Although Cr 2 O 3 (001) is less active for H 2 dissociation, the presence of Zn 1 lowers the formation energy of oxygen vacancies and promotes the surface reactivity even higher than that of pristine ZnO(101̅0). The dissociated H species turn to recombine and form H 2 rather than reacting with the lattice oxygen to form H 2 O. The present work provides a deeper insight into H 2 activation over pristine and heterometal-atom-modified oxide surfaces and a rationale for the design of active catalysts for syngas conversion.