Preparation of Gold Catalysts by Chlorine Exchange on Alumina and Ceria
Veronique PITCHON, Corinne PETIT, Renaud COUSIN, Svetlana IVANOVA, Veronique PITCHON, Corinne PETIT,
The chemistry of gold has known recent developments with the use of its compounds in biology and catalytic applications. High activity for various reactions is obtained with suitably prepared gold catalysts. Supported gold catalysts by different methods of preparation (coprecipitation, deposition - precipitation, impregnation and ion-exchange) are reported. The choice of supports is very important, some metal oxides are very effective but Al2O3 and SiO2 are less interesting. It is not obvious to reproduce the routes of synthesis published in the literature and often higher size of gold particles that the one claimed are obtained. In this work, the nature of gold particles and the interactions with the oxide support are correlated with the evolution of precursors during the solution steps. The coprecipitation and deposition - precipitation methods induce the increasing of pH gold chloride solution by addition of basic compounds. This parameter modifies the gold and support species. It is therefore of interest to investigate the properties of the Au (III) complex in solution especially from HAuCl4. Different parameters have been studied such as the concentration of salt, the temperature of the solution and the evolution of the pH as a function of time, with and without addition of the support (Al2O3, CeO2). The pH of a gold chloride solution is decreased during an increase of temperature to 345K. Cl- exchange on the square-planar [AuCl4]- complex by the aquation of the complex giving [AuCl3(H2O)] is complicated by the deprotonation of the aquo complex [AuCl3(OH]-. This step occurs during the increase of temperature and is observed for both tested concentrations (2.10-4 and 10-3 M). Atomic adsorption analysis shows that the gold deposition on the support (Al2O3 or CeO2) is complete 10 min after its addition at 345K. Chloride is not lost by heating treatment under He or oxygen atmospheres as shown by TGA. On Al2O3 an increase of pH is noted and aquo species are formed. These species do not interact with the gold complex and the interaction Au (III)- Al2O3 with hydroxide groups is not so strong that the one obtained at higher pH value. A model on Al2O3 at pH equal to 3 is proposed in the scheme 1.
The particle size calculated from the XRD peak of gold (111) is higher that 11 nm on Al2O3 and increases with the concentration of oxygen during the calcination at 300°C. A part of the catalyst is mixed with an ammonia solution in order to remove completely the Cl- ions. In this case, after calcination, the particle size of Au is too small to be detectable by XRD, showing that the chloride is responsible to the increasing of gold particles size. Value of the pH observed is constant during the impregnation on CeO2 indicated no evolution of the species on its surface. On CeO2 the particle size is smaller that on alumina prepared in the same condition and cannot be detected by XRD. A model of gold precursor on CeO2 at pH equal to 3 is proposed on the scheme 2.
The difference of the catalytic behavior is correlated with physical characterizations.
These considerations lead to the view that a successful catalyst should be prepared with a low concentration of gold salt interacting strongly with an oxide support that can itself inhibits the interaction of gold particles by the
Gold Catalysis, Gold Complex, Hydrocarbons oxidation, CO oxidation, Gold Catalysis, CO oxidation, Gold chemistry, catalysis, Hydrocarbons oxidation