PREPARATION OF PROMOTED PLATINUM CATALYSTS OF DESIGNED GEOMETRY AND THE ROLE OF PROMOTERS IN THE LIQUID-PHASE OXIDATION OF 1-METHOXY-2-PROPANOL

Alumina-supported or unsupported M/Pt-type catalysts were prepared by consecutive reduction of Bi, Pb, Sn, Ru, Au, or Ag modifiers (M) onto Pt particles. Structural and chemical properties of the bimetallics were studied bx electron microscopy combined with energy dispersive X-ray analysis and an electrochemical (cyclic voltammetric) polarization method. Preferential deposition of promoter metal submonolayers on Pt was obsersved at moderate surface coverages (θM < 0.5-0.8). Some bulk metal crystallite formation as “bridges” between small Pt particles covered partially with promoter was also observed on alumina-supported Bi/Pt and Pb/Pt catalysts. Measurement of the electrochemical potential of the catalyst slurry during the oxidation of l-methoxy-2-propanol to methoxyacetone and the cyclic voltammetric polarization of the bimetallic catalysts revealed that the catalysts are in an oxidized state during reaction. The following order of promoting influence was observed: Bi > Pb ∼ Sn > Au ∼ Ru. Two major effects of promoters ale suggested: (i) they suppress the initial irreversible adsorption of the reactant alcohol on Pt which results in self-poisoning, and (ii) they form new active centers that adsorb the oxidixing species (OH) better than Pt. A formal rate equation is suggested (r = · θorg · θOH) which explains the optimum in promoter/platinum ratio. The different influences of the promoters are explained by their hydrogen and oxygen sorption characteristics and by the surface geometry of the bimetallic catalysts. © 1993 Academic Press, Inc.