Time-Resolved in Situ ATR Spectroscopy of 2-Propanol Oxidation over Pd/Al₂O₃: Evidence for 2-Propoxide Intermediate

In situ attenuated total reflection infrared spectroscopy in a flow-through cell combined with online UV−vis spectroscopy was used to investigate the oxidation of 2-propanol over Pd/Al₂O₃ catalyst. The state of the catalyst was driven fast between reduced and oxidized by admitting alternately dissolved hydrogen and oxygen, and the response of the catalytic solid−liquid interface was followed in time. Besides the oxidation product acetone and the water that forms, when hydrogen and oxygen are simultaneously adsorbed on the catalyst surface, an additional species was observed with a characteristic band at ∼1065 cm-1. On the basis of the transient character of the adsorbate and density functional theory calculations, we assign this species to adsorbed 2-propoxide. Its observation indicates that the second dehydrogenation step is rate limiting in an oxidative dehydrogenation mechanism. The results furthermore show that adsorbed hydrogen and oxygen limit the dissociative adsorption of 2-propanol and that 2-propoxide can be hydrogenated back to the reactant in the presence of adsorbed hydrogen.