Insights into Liquid-Phase Titration of Palladium Surfaces

Chemisorption of strongly bound adsorbents to catalyst surfaces has been utilized extensively for site titration in the gas phase; however, extension of these molecular surface interactions to the liquid phase is not straightforward. Here, metal surface titration by solvated aromatic organothiols and hydrocarbons was studied on silica-supported Pd nanoparticle catalysts (Pd/SiO2) during batch benzyl alcohol (BA) oxidation reactions. Competitive effects of reversible titrant adsorption, titrant stabilization in varying environments, and reactant accessibility to titrated surfaces are evaluated to determine the primary drivers of benzaldehyde formation rates (rBzH) under varying titrant concentrations, solvents, and Pd oxidation states. In neat BA solvent, rBzH remains nonzero even at dibenzothiophene (DBT) and fluorene (DBT analogue without sulfur) loadings that exceed total metal atoms by a factor of 6, thus suggesting reversible titrant adsorption–desorption that is corroborated by Fourier transform infrared spectroscopy. Further depression of rBzH at titrant/Pdtot = 100:1 is consistent with titrant adsorption–desorption that is quasi-equilibrated, as well as titrant binding energies and selective site titration that influence apparent activation barriers. Effects of titrant stabilization by solvent molecules are further realized in BA oxidation reactions in n-decane, in which titrant solvation in the bulk liquid is less favored relative to BA, yielding more favorable titrant adsorption efficiency. Overall, titration efficiency in the liquid phase is found to be the net result of titrant adsorption configuration, binding energy, and stabilization by solvent molecules in the bulk that influence the relative favorability of adsorption and desorption.