Ordered mesoporous silica to study the preparation of Ni/Si2 ex nitrate catalysts: impregnation, drying, and thermal treatments

In this contribution, we investigated the preparation of Ni/SiO2 catalysts with aqueous [Ni(OH2)6](NO3)2 solutions via the impregnation and drying method using ordered mesoporous silica SBA-15 (mesopore diameter of 9 nm) as model support to study each step in the preparation: impregnation, drying, calcination, and reduction. After impregnation, not all the mesopores of SBA-15 appeared filled with precursor solution. Consecutive drying led to formation of 9 nm Ni3(NO3)2(OH)4 crystallites exclusively within the mesopores. During air calcination, severe sintering and redistribution took place, resulting in a low NiO dispersion, including large NiO crystals outside of the mesopores and rodlike NiO particles inside the mesopores. The degree of sintering depended on the concentration of Ni3(NO3)2(OH)4 decomposition products (NO2, N2O, O2 and H2O), and in particular NO2 and O2 were found to promote sintering and redistribution. Therefore, maintaining low concentrations of the latter components during the thermal nitrate decomposition is advocated, which was achieved by carrying out the treatment in the presence of H2. The latter treatment prevented formation of NO2/O2 as decomposition products, moderated the decomposition rate of Ni3(NO3)2(OH)4 into NiO as observed from in situ XRD experiments, and led to NiO particles of 3 nm on average at a loading of 20 wt % Ni/SiO2.