New Perspectives in Gold Catalysed Oxidation
Gold has received a growing interest as a catalyst for the selective oxidation of organic molecules using oxygen under mild conditions1. In the liquid phase application small gold particles (2.5-7 nm) deposited over different solid supports can be used but, like many other noble metals, their catalytic activity depends upon the nature of the support and the preparation metod2. In particular, gold on carbon is the preferred catalyst for the oxidation of the alcoholic 3,4 and the aldehydic groups and one of the best preparation method is the immobilisation of colloidal sols5.
In order to produce more efficient catalysts for the liquid phase oxidation and to derive new insights into the reaction mechanism, we have made a comparison between catalytic systems based on different metals. The first effort was to test the activity of naked metal particles and then to study the influence of various supporting material.
Therefore, we have prepared Au, Ag, Pd and Pt particles in form of colloidal sols (2-5 nm) by reducing the corresponding soluble salts in water solution with NaBH4 in the presence of polyvinylalcohol as a protecting agent and we have compared their catalytic activity in a model reaction, the oxidation of glucose (glucose:metal=3000) with O2 at 303 K, 101Kpa and pH 9.5. While the original sols were stable for several hours, during the oxidation test the small particles collapsed into larger ones leaving a colourless and almost inactive solution after 200-250 sec. However, as shown in fig .1, the kinetic of glucose oxidation until this time can be followed by titrating the formed gluconic acid at constant pH and the results show that gold particles are much more active than the other ones, reaching 21% conversion in 200 sec respect to a maximum of 4% conversion for Pd and lower values for the other metals. The observed activity of the colloidal gold particles, corresponding to a mean TOF value of 11340 h-1, is surprisingly high as compared with carbon supported gold particles reported in our previous studies showing TOF values around to 500 h-1(5). Although the short life of the gold sol avoids its practical use as a catalytic system, we have used its high initial activity as a reference for evaluating the role played by different compounds used as a support for the gold particles. In order to study this point, we have programmed a series of experiments in which carbon, alumina, silica and titania were separately added to the original gold sol 70 s after the start up of the glucose oxidation. The amount of the support was calculated in order to prepare “in situ” 1%w Au/support .
The results, shown in fig 2a and 2b, are consistent with the following comments:
a) The slope of the time/conversion curve of the gold sol catalyst is almost unaffected by the addition of silica, carbon, alumina and titania during the first 200s. This means that supported and unsupported gold particles are similarly efficient during this time and, therefore, that metal support interactions are negligible in terms of activity.
b) By adding carbon and titania the original activity of gold sol was prolonged for long time allowing the total conversion of glucose in the first case and more than 90% conversion in the second case after 2000s. By adding alumina a lower conversion (67%) was obtained whereas by adding silica only a small benefit (5%) was observed. Therefore, in the case here investigated the role of a good support, like carbon and titania, lies in stabilising the gold particles activity avoiding their aggregation during the catalytic test.
Glucose oxidation using different metal sols.
Effect of various supporting materials on gold activity during the glucose oxidation.
1 G.C.Bond and D.Thompson,Cat.Rev.-Sci.Eng., 1999, 41, 319.
2 C. Bianchi, F. Porta, L. Prati and M. Rossi, Top. Cat, 13 (200) 231-236
3 S.Biella, L.Prati and M.Rossi, Journal of Cataluysis, 206, 242-247 (2002)
4 S.Carrettin, P.McMorn, P.Johnston, K.Griffin and G.J.Hutchings, ChemComm, 2002, 696-697
5 S. Biella, G.L. Castiglioni, C.Fumagalli, L.Prati and M.Rossi, Catal. Today, 2002, 72, 43.
Gold, Oxidation, catalysis, Interaction, Support