Stability of bimetallic Bi-Pd and Pb-Pd carbon-supported catalysts during their use in glyoxal oxidation

The incorporation of Bi or Pb as promoting elements in Pd-based carbon-supp orted catalysts drastically increases the catalytic activity in the selecti ve oxidation of glyoxal into glyoxylic acid. Because partial dissolution of the promoter was clearly demonstrated by atomic absorption analysis of the reaction medium, experiments are performed to examine the stability of the se catalysts. Dissolution tests in the presence of the individual constitue nts of the reaction medium (glyoxal, glyoxylate, glycolate, oxalate) were c arried out in air or nitrogen to identify the factors responsible for Pb or Bi leaching. Pb- or Pi-promoted Pd/C catalysts were prepared by thermal de gradation of acetate-type precursors and characterized by X-ray diffraction and X-ray photoelectron spectroscopy before and after their use in glyoxal oxidation. Promoter leaching increases with the reaction time. Monometalli c Bi/C and Pb/C catalysts were found to lose smaller amounts of promoting a gent than the bimetallic M-Pd/C (M=Bi, Pb) catalysts. Losses are more prono unced from Pb-Pd/C catalysts than from their Bi-based partners. Both glyoxa l and glyoxylate seem to be among the main factors responsible for the prom oter losses in relation to their complexing properties. (C) 2000 Elsevier S cience B.V. All rights reserved.