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

Abstract

It is shown that, during the preparation of Cu/SiO 2 samples by selective adsorption of copper tetraammine complex [Cu(NH 3 ) 4 (H 2 O) 2 ] 2+ on silica, two types of supported Cu II species form: grafted Cu II ions and copper phyllosilicate. The formation of grafted Cu II ions results from electrostatic adsorption of Cu II ammine complex cations in solution onto the surface of silica particles negatively charged due to the high solution pH. Grafting of Cu II 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 (H 2 O) 4 ] 0 complex in solution. Both Cu II 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 (H 2 O) 4 ] 0 complex is maximum according to calculation of Cu II 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 Cu II speciation in solution. The formation of Cu II ammine complexes is favored in the presence of NH 4 + so that the concentration of [Cu(OH) 2 (H 2 O) 4 ] 0 complex strongly decreases and its maximum of concentration shifts to pH lower than the pH's of preparation.

Comparison between a Water-Based and a Solvent-Based Impregnation Method towards Dispersed CuO/SBA-15 Catalysts: Texture, Structure and Catalytic Performance in Automotive Exhaust Gas Abatement

“…Finally, when comparing the average pore size of the materials, a significant increase can be recognized for the ADP sample. This seems to originate from the dissolution of the silica's outer layer in the pore, accomplished by the high solution pH (≈11) of the ADP method [27]. Despite the significant loss in surface area and pore size increase due to this dissolution under basic conditions, the pore shape of the ADP sample remains intact with a pore volume comparable to that of the MDD sample.…”

“…The peak at a lower binding energy (BE) is attributed to the CuO species distributed on the surface, while the peak at a higher BE can be due to the formation of copper hydroxyl groups or to the interaction with the neighboring Si-O bonds. Toupance et al reported the existence of two types of well-dispersed Cu 2+ species when using the ammonia-based method for metal impregnation, i.e., copper phyllosilicate and grafted Cu 2+ [27]. Copper phyllosilicate has a lamellar structure wherein the layers of SiO4 tetrahedra are sandwiched between discontinuous layers of CuO6 octahedra [27,32].…”

“…Copper phyllosilicate has a lamellar structure wherein the layers of SiO4 tetrahedra are sandwiched between discontinuous layers of CuO6 octahedra [27,32]. It is a result of the hetero-condensation reaction between the Cu(OH)2(H2O)4 and the silicic acid (Si(OH)4) formed by the silica dissolution at basic pH [11,27,33]. The latter phenomenon is in line with the support's pore enlargement observed in the ADP sample by the N2 physisorption (Table 1).…”

“…Toupance et al reported the existence of two types of well-dispersed Cu 2+ species when using the ammonia-based method for metal impregnation, i.e., copper phyllosilicate and grafted Cu 2+ [27]. Copper phyllosilicate has a lamellar structure wherein the layers of SiO 4 tetrahedra are sandwiched between discontinuous layers of CuO 6 octahedra [27,32]. It is a result of the hetero-condensation reaction between the Cu(OH) 2 (H 2 O) 4 and the silicic acid (Si(OH) 4 ) formed by the silica dissolution at basic pH [11,27,33].…”

“…Herein, the formed copper-ammonia complex replaces the metal acetylacetonate complex in its role as a steric hindrance provider. Furthermore, because of the ammonia addition, the pH of the solution (pH ≈ 10) rises far above the point of zero charge (PZC) of the support (≈2 for silica), creating a strong electrostatic complex-support interaction during the synthesis [26,27].…”