The metal-support interaction in catalysis is of relevance both for academic studies and for industrial applications, and theoretical concepts have contributed to understanding the basic principles behind this interaction. [1] In most cases, however, the metal-support interaction is only described in terms of catalytic behavior and its nature often remains debatable. In recent years interest in gold catalysts for various applications in organic and inorganic chemistry [2] has increased and, our and other research groups have investigated the liquid-phase oxidation of polyol, [3][4][5][6][7] aminoalcohols, [8] and glucose [9] to carboxylates, and the gas-phase oxidation of alcohols to the corresponding carbonyl derivatives [10] using metal particles supported on different materials. In liquid-phase applications, carbon was found to be the support of choice, and in the case of ethane-1,2-diol oxidation, by comparing different commercial carbons, a tentative hypothesis of metal-support interactions, connected to the density of phenolic groups at the carbon surface, was formulated.[11] However, the synergism between gold particles and carbon was not demonstrated and this point remained unresolved.Although gold colloids have widely been employed to prepare supported gold catalysts, no report of particles derived from colloidal dispersion being used as catalysts has appeared. We have now found that, under controlled conditions, water-dispersed gold sol exhibits a surprising activity when used as "naked particles", that is, in the absence of common protectors as polyvinylalcohol (PVA), polyvinylpyrrolidone (PVP), tetrahydroxymethylphosphonium chloride (THPC). As a model reaction, we have investigated the aerobic oxidation of glucose to gluconate which occurs under mild conditions.As shown in the conversion-time plot (Figure 1), naked gold particles having a mean diameter of 3.6 nm behave as an active catalyst allowing 21 % glucose conversion in the first 200 s.These particles are produced as a colloidal sol by reducing HAuCl 4 in the presence of a large excess of glucose acting either as reagent or protector. From the initial rate, a specific molar activity of 18 043 mol gluconate [mol Au] À1 h À1 (calculated with respect to the total gold) can be derived. Under similar conditions, Cu, Ag, Pd, and Pt colloidal particles of similar dimension (3-5 nm) were scarcely active. During the catalytic test, gold coagulated into larger particles owing to the formation of sodium gluconate that, as is common with other electrolytes, promoted sol coagulation leaving a colorless, inactive solution after about 400 s. The growth of gold crystallites during the reaction has been followed by X-ray diffraction (XRD) analysis at various time intervals, after sol immobilization on carbon (Figure 2).Gold particles are also poisoned by sulfur compounds, such as sulfides and sulfites, and inhibited by protecting molecules, such as polyvinyl alcohol. Although the short life of the gold sol does not allow its use as a practical catalyst, its acti...
