Catalysis Today 61 (2000) 255–262 Stability of bimetallic Bi–Pd and Pb–Pd carbon-supported catalysts during their use in glyoxal oxidation F. Alardin a , B. Delmon b , P. Ruiz b , M. Devillers a,∗ a Unité de Chimie des Matériaux Inorganiques et Organiques, Université Catholique de Louvain, Place Louis Pasteur 1, B-1348 Louvain-la-Neuve, Belgium b Unité de Catalyse et Chimie des Matériaux Divisés, Université Catholique de Louvain, Place Croix du Sud 2/17, B-1348 Louvain-La-Neuve, Belgium Abstract The incorporation of Bi or Pb as promoting elements in Pd-based carbon-supported catalysts drastically increases the catalytic activity in the selective 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 these catalysts. Dissolution tests in the presence of the individual constituents of the reaction medium (glyoxal, glyoxylate, glycolate, oxalate) were carried out in air or nitrogen to identify the factors responsible for Pb or Bi leaching. Pb- or Bi-promoted Pd/C catalysts were prepared by thermal degradation 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. Monometallic Bi/C and Pb/C catalysts were found to lose smaller amounts of promoting agent than the bimetallic M–Pd/C (M=Bi, Pb) catalysts. Losses are more pronounced from Pb–Pd/C catalysts than from their Bi-based partners. Both glyoxal and glyoxylate seem to be among the main factors responsible for the promoter losses in relation to their complexing properties. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Pd/C catalysts; Glyoxal oxidation; Bismuth; Lead 1. Introduction Bismuth and lead are well known for displaying attractive properties as promoting elements in no- ble metal-based catalysts for the selective oxidation of alcohols or aldehydes by molecular oxygen in aqueous solution [1]. Carbon-supported bimetallic Bi–Pd/C catalysts were particularly studied for the selective oxidation of glucose into gluconic acid [2]. The incorporation of bismuth is assumed to increase ∗ Corresponding author. Tel: +32-10-472-827; fax: +32-10-472-330. E-mail address: devillers@inan.ucl.ac.be (M. Devillers). drastically the catalytic activity of monometallic Pd/C catalysts by preventing deactivation of the catalyst in the presence of oxygen. However, the actual origin of the promoting role of these heavy post-transition ele- ments on the catalytic activity of Pd- or Pt-based cata- lysts remains a debated matter. The interest of similar bimetallic Pb- or Bi-promoted Pd/C catalysts for the selective oxidation of glyoxal into glyoxylic acid has also been reported [3,4]. As illustrated in Scheme 1, this process remains a real challenge for two main reasons: (i) selectivity towards glyoxylic acid is essen- tially limited by the further oxidation to oxalic acid, and (ii) glycolic acid is a second by-product resulting from a competitive Cannizzaro reaction occurring in 0920-5861/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII:S0920-5861(00)00377-1