J. Phys. Chem. C 2008, 112, 8617–8622
8617
Bimetallic Gold/Palladium Catalysts: Correlation between Nanostructure and Synergistic Effects Di Wang,† Alberto Villa,‡,§ Francesca Porta,‡ Laura Prati,‡ and Dangsheng Su*,† Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, D-14195 Berlin, Germany, and Dipartimento di Chimica Inorganica Metallorganica e Analitica e ISTM, Centre of Excellence CIMAINA UniVersita` di Milano, Via Venezian 21, I-20133 Milano, Italy ReceiVed: January 28, 2008; ReVised Manuscript ReceiVed: March 19, 2008
Au nanoparticles are known to be a good catalyst or an effective promoter for a wide range of catalytic reactions. Bimetallic Au-Pd nanoparticles supported on activated carbon were synthesized following a twostep procedure: immobilization of Au sol onto activated carbon followed by immobilization of Pd(0). The catalysts showed superior activities compared to monometallic Pd or Au nanoparticles on the same support. A series of catalysts with Au:Pd ratios varying from 9.5:0.5 to 2:8 were prepared. These catalysts were characterized by TEM, HRTEM, EDX, and X-ray mapping techniques to obtain morphological information, particle size distributions, crystalline structure, and distribution of the two metals. Correlating with the result from catalytic tests of selective oxidation of glycerol to glyceric acid, we found that the surface configuration of Pd monomers isolated by Au atoms has a substantial effect on activity and stability. The Au:Pd ratio on the surface of the particles is the key parameter and can be finely tuned to achieve optimal catalytic performance. The segregation or inhomogeneity of Pd weakens the synergistic effect of the bimetallic catalyst. 1. Introduction The oxidation of alcohols to carbonyl compounds is an important task in fine chemical industry. Traditional methods for the oxidation of alcohols are based on the use of stoichiometric amounts of either inorganic or organic oxidants.1–3 These methods lead to environmental and economical problems due to disposal of large amounts of byproduct. An alternative more environmentally friendly method is to use molecular oxygen or hydrogen peroxide4,5 in the presence of a catalyst, the main byproduct being water. It is of particular interest in catalytic oxidation of some polyalcohols like sorbitol and glycerol since they represent important chemical intermediates that can be derived from biomass resources. In the case of heterogeneous catalysts, Pt/C and Pd/C have been reported to be active in the liquid phase oxidation of alcohols,6–10 but they suffer from severe deactivation due to overoxidation and poisoning by (by)-products.11 Recently, supported gold nanoparticles have attracted more and more interest due to their special activity in catalytic reactions such as low-temperature oxidation of CO, hydrochlorination of alkyne, liquid phase oxidation of alcohols and polyols, etc.12 The addition of Au to Pd or Pt catalysts in the liquid phase oxidation of polyols (sorbitol and glycerol) in the presence of O2 under mild conditions (
