LIQUID-PHASE OXIDATION OF 1-METHOXY-2-PROPANOL WITH AIR .2. STRUCTURE AND CHEMICAL-PROPERTIES OF LEAD-PROMOTED PALLADIUM CATALYSTS

The structural and chemical properties of lead-promoted palladium on carbon catalysts used for the oxidation of 1-methoxy-2-propanol (MP) to 1-methoxy-2-propanone have been studied by means of STEM, XRD, XPS, AAS, electrochemical polarization and catalytic measurements. During catalyst prehydrogenation 75-95% of Pb2+ promoter is reduced to Pb0. Lead is located on the palladium surface either as bulk metal or ad-atom species (Pb(ad)), and on the carbon support as bulk metal forming big needles and polygonal crystals. During MP oxidation bulk lead is partially oxidized to Pb2+ and redissolves in a strongly alkaline medium. Pb(ad) on the palladium surface remains virtually unaltered. The lead coverage of the palladium increases with increasing promoter/catalyst ratio. This slightly enhances the rate of MP oxidation and suppresses the activity for C = C double bond hydrogenation. The role of lead ad-atoms is attributed to a geometric (matrix) effect, which suppresses the formation of by-products and thereby catalyst deactivation. The findings indicate that hydrogen adsorption on the catalyst surface during alcohol oxidation is unlikely to be an elementary step of the reaction mechanism.