The stoichiometry of hydrogen reduced zirconia and its influence on catalytic activity Part 1: Volumetric and conductivity studies

Abstract

There is still little known about the origin, quantity and reactivity of defect species in reduced zirconia. Reduction of zirconia with flowing dry hydrogen leads to the adsorption of hydrogen and to the formation of oxygen vacancies. The number of vacancies increases with increasing treatment temperature, with increasing hydrogen flow rate and with increasing treatment time. The presence of water vapour in the reducing hydrogen causes the number of oxygen vacancies to decrease, presumably due to an equilibrium shift according to the equation: Zr⁴⁺ + O²⁻ + H₂  →  H₂O + V_O + Zr³⁺ + e⁻. The oxygen uptake of the reduced zirconia occurs in two steps, resulting in a low temperature peak (LTP) at about 500 K and in a high temperature peak (HTP) at about 773 K in the temperature-programmed oxidation (TPO). At reduction below 800 K the oxygen vacancies represented by the LTP are most likely located on the zirconia surface; their increase with temperature corresponds to a reaction energy of 20 kJ mol⁻¹ for the reaction given above. When zirconia is reduced above 823 K the effect of temperature increases, characterized by an activation energy of about 120 kJ mol⁻¹. Because of the high activation energy the corresponding vacancies are, most likely, located in the bulk of zirconia. Parallel to the formation of vacancies the electric conductivity increases proving that the electrons formed are most likely delocalized in the conduction band of the bulk phase. The oxygen vacancies corresponding to the HTP probably represent defects caused by the iron content of the zirconia. These vacancies are created at lower temperatures but need stronger conditions for reoxidation. Furthermore, the degree of reduction influences the catalytic activity for the hydrodeoxygenation of propan-2-ol.