OXIDATION-STATE OF LEAD PROMOTOR ON A PALLADIUM CARBON CATALYST BEFORE AND DURING THE OXIDATION OF ALCOHOLS

The oxidation state and the location of lead on a promoted commercial palladium/carbon catalyst were studied. The catalyst was promoted by lead acetate without exposure to air before the liquid phase oxidation of 1-methoxy-2-propanol. The hydrogen sorption-desorption characteristics and the rate of hydrogen evolution on promoted catalysts were. studied by electrochemical polarization methods. These studies in conjunction with STEM and AAS measurements revealed that after catalyst prehydrogenation (before the oxidation reaction) lead is deposited on both palladium and carbon surfaces. On palladium lead was found to exist as bulk metal and as lead ad-atom. Part of Pb2+ discharged and deposited on the carbon support, forming bulk metal species. A fraction of bulk lead is oxidized to Pb2+ during alcohol oxidation and it may dissolve from the catalyst surface depending on its solubility in the reaction medium. The high stability towards oxidation and dissolution of lead ad-atoms on the surface of palladium appears to be an important factor for their behavior as promotors in the selective oxidation of alcohols. The studies revealed the structural differences of the catalysts resulting from reduction with hydrogen or the reactant alcohol.