Pd/C catalysts prepared by controlled adsorption of Pd(II) species on SX PLUS carbon in the aqueous phase

Pd/C catalysts were prepared on SX PLUS activated carbon by a controlled adsorption method in which the interactions between the Pd precursor and the carbonaceous support in aqueous solution were optimized. This strategy involved (i) determining the surface charge state of the carbon support as a function of the pH, (ii) establishing adsorption curves for Pd on C to identify the pH windows where maximum adsorption occurs, and (iii) determining the distribution of Pd(II) species in solution from the stability constants of the various compounds involving the ligands H2O, OH-, and/or Cl-. It was found that the charge of metal species present in solution, which depends on the pH, influences markedly the adsorption of Pd on carbon. The actual catalysts were then prepared by keeping the pH of the impregnating Pd solution within the optimal adsorption range throughout all synthetic steps, followed by chemical or thermal activation. This optimized preparation method led to highly efficient catalysts for the selective oxidation of glyoxal into glyoxalic acid, with activities superior to that of comparable monometallic materials (prepared by other methods or commercial manufacturing) and equivalent to that of bimetallic (promoted) catalysts. The Pd/C catalysts were characterized by XPS, SEM, TEM, and CO chemisorption, and it was found that the catalytic activity could be correlated to high Pd dispersion in the form of very small particles homogeneously dispersed on the support and to an important degree of Pd reduction.