Thermal decomposition study of silica-supported nickel catalyst synthesized by the ammonia method

Silica-supported nickel catalyst synthesized by the ammonia method and their precursors treated and untreated with ammonia solution were studied by temperature-programmed reduction (TPR), scanning electron microscopy (SEM), and simultaneous thermogravimetry, differential scanning calorimetry and differential thermal analysis (TGA–DSC–DTA) carried out in nitrogen atmosphere (100 ml/min) at different heating rates (5, 10, 15 and 20 K/min). Temperature effect on the catalyst structural composition was studied by X-ray diffraction (XRD). The results indicated that synthesized catalyst shows mainly a higher temperature metal phase (973 K), constituted by a superposition of bi-dimensional sheets similar to nickel phyllosilicate, resulting from the strong metal–support interaction (SMSI) between nickel aqua ammine complex and silica surface. At high temperature, such a compound degrades to stable silica-supported nickel catalyst. Activation energies and kinetic parameters for each decomposition process were calculated by Kissinger's method. Enthalpy values were determined from differential scanning calorimetric profiles. The activity tests of such a catalyst to methane steam reforming indicated that deactivation process caused by carbon deposition was less pronounced than deactivation of a similar nickel content catalyst prepared by impregnation.

Comparison of TGA–DSC profiles of commercial nickel nitrate hexahydrate (left side) and nickel nitrate hexahydrate treated with ammonia solution (right side) lighted the effect of ammonia solution addition on commercial nickel salt. In fact, five steps of TGA decomposition pattern were observed on nickel nitrate hexahydrate and only three steps for the treated salt. Calculated molecular weight of treated nickel salt suggested that aqua ammine nickel ions were formed. DSC profiles of samples indicated a great structural rearrangement on the aqua ammine nickel ion before the last decomposition step. The first step of the chemistry involved in the silica-supported nickel catalyst preparation by the ammonia method was illustrated by comparison of such TGA–DSC profiles. Figure optionsDownload as PowerPoint slide