Impact of the oxygen defects and the hydrogen concentration on the surface of tetragonal and monoclinic ZrO₂ on the reduction rates of stearic acid on Ni/ZrO₂

The role of the specific physicochemical properties of ZrO₂ phases on Ni/ZrO₂ has been explored with respect to the reduction of stearic acid. Conversion on pure m-ZrO₂ is 1.3 times more active than on t-ZrO₂, whereas Ni/m-ZrO₂ is three times more active than Ni/t-ZrO₂. Although the hydrodeoxygenation of stearic acid can be catalyzed solely by Ni, the synergistic interaction between Ni and the ZrO₂ support causes the variations in the reaction rates. Adsorption of the carboxylic acid group on an oxygen vacancy of ZrO₂ and the abstraction of the a-hydrogen atom with the elimination of the oxygen atom to produce a ketene is the key to enhance the overall rate. The hydrogenated intermediate 1-octadecanol is in turn decarbonylated to heptadecane with identical rates on all catalysts. Decarbonylation of 1-octadecanol is concluded to be limited by the competitive adsorption of reactants and intermediate. The substantially higher adsorption of propionic acid demonstrated by IR spectroscopy and the higher reactivity to O₂ exchange reactions with the more active catalyst indicate that the higher concentration of active oxygen defects on m-ZrO₂ compared to t-ZrO₂ causes the higher activity of Ni/m-ZrO₂.