Preparation of alumina supported gold catalysts: Gold complexes genesis, identification and speciation by mass spectrometry

All of the key stages in the synthesis of alumina supported gold catalysts are presented here in detail: from the evolution of a gold precursor—HAuCl4 in aqueous solution, to its interaction with alumina. The gold precursor hydrolyses with the production of both neutral and charged species of type: [AuCl3−x(H2O)(OH)x]0 and [AuCl4−x(OH)x]− along with protons to maintain an electrical balance. This evolution depends on salt concentration, pH value and temperature. The [AuCl4]− species is the only one easy to quantify by UV–vis spectroscopy. Mass spectrometry is able to quantify gold species in the solution as a function of various parameters prior to and following addition of the support. This method allows determining the nature and amount of species grafted on the surface of support. The increase in temperature generates an evolution towards species, which are charged and more hydrolysed. The optimal conditions for quick and successful grafting are determined to be gold salt concentration 10−4 mol L−1 and temperature of gold solution 70 °C. Under these conditions the gold complexes present in the solution, namely [AuCl3(OH)]− and trans-[AuCl2(OH)2]−, provoke the formation of monodentate surface species over alumina, while cis-[AuCl2(OH)2]− and [AuCl(OH)3]− allow grafting in the bidentate mode. We will report how this preparation method leads to a quantitative gold deposit. The resulted catalysts are active in the reaction of CO oxidation.