Structure Sensitivity of Palladium-Catalyzed Liquid-Phase Alcohol Oxidation. A Combined in situ ATR-IR and Selective Site Blocking Study

Abstract

Knowledge of the structure of active sites is a prerequisite for the rational design of solid catalysts. In situ attenuated total reflection infrared (ATR-IR) spectroscopy combined with site selective blocking by CO and isotope labeling allowed us to discriminate the different sites involved in the liquid-phase oxidation of benzyl alcohol on Pd/Al2O3. The main reaction, the oxidative dehydrogenation of the alcohol to the corresponding aldehyde, showed only little dependence on structure and occurred on all exposed Pd faces, whereas the undesired product decarbonylation occurred preferentially on hollow sites on Pd (111) planes. Similar ATR-IR studies on a corresponding Bi-promoted Pd/Al2O3 catalyst showed virtually no adsorbed CO during dehydrogenation indicating that the sites on Pd (111) planes are blocked by selective deposition of Bi. Thus the improved selectivity of the promoted Pd-Bi/Al2O3 used industrially can be traced to the selective blocking of active decarbonylation sites on Pd(111) by Bi.