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 α-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 .
The structure and chemical state of supported Pd nanoparticles in contact with H 2 in the aqueous phase have been explored by X-ray absorption spectroscopy to better understand their surface reactivity in polar condensed media. The Pd−Pd distances at substantial H 2 pressures indicate the presence of sorbed hydrogen and point to the presence of Pd hydrides, proving that such nanoparticles are hardly influenced by the presence of water. During the hydrogenation of the reactants (phenol, cyclohexanone, and cyclohexene), the Pd−Pd bond length decreased, indicating a drastically lower concentration of sorbed H compared to Pd in the absence of the reactants. This steady state concentration of sorbed hydrogen is established by all reactions involving H 2 , i.e., the sorption/desorption into the bulk, the adsorption at the surface, and the reaction with unsaturated reactants, but not by reaction with water. This demonstrates that neither the Pd particles nor the H/Pd ratio is influenced by water, but dynamically adapt to reaction conditions. Consistently, ab initio molecular dynamic simulations indicate that Pd−water interactions are relatively weak for Pd metal and that these interactions become even weaker, in the presence of H 2 and when hydrogen is incorporated into the metal particles.
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