Conditions of Formation of Copper Phyllosilicates in Silica-Supported Copper Catalysts Prepared by Selective Adsorption

It is shown that, during the preparation of Cu/SiO2 samples by selective adsorption of copper tetraammine complex [Cu(NH3)4(H2O)2]2+ on silica, two types of supported CuII species form: grafted CuII ions and copper phyllosilicate. The formation of grafted CuII ions results from electrostatic adsorption of CuII ammine complex cations in solution onto the surface of silica particles negatively charged due to the high solution pH. Grafting of CuII ions occurs during the subsequent drying step. The formation of copper phyllosilicate takes place in solution. It results from a reaction between silicic acid arising from silica dissolution and [Cu(OH)2(H2O)4]0 complex in solution. Both CuII species lead to small metal particles after reduction. Several preparation parameters were investigated in this paper: the metal precursor concentration, the pH of the precursor solution, the solution/silica contact time, and the addition of ammonium nitrate to the precursor solution. The amount of copper phyllosilicate depends on the pH of the precursor solution; it is maximum at pH 9, that is, at the pH at which the concentration of [Cu(OH)2(H2O)4]0 complex is maximum according to calculation of CuII speciation. The amount of copper phyllosilicate increases with the solution/silica contact time because its formation is a kinetically limited process. It was found that the addition of ammonium nitrate to the precursor solution prevents the formation of copper phyllosilicate. This can be explained by calculations of the CuII speciation in solution. The formation of CuII ammine complexes is favored in the presence of NH4 + so that the concentration of [Cu(OH)2(H2O)4]0 complex strongly decreases and its maximum of concentration shifts to pH lower than the pH’s of preparation.