Preparation of supported catalysts: A study of the effect of small amounts of silica on Ni/Al2O3 catalysts

Nickel catalysts supported on pure alumina (Puralox) and 5% silica-containing alumina (Siralox) have been prepared, characterized (XRD, UV-vis-NIR, IR, H2-TPR and FE-SEM) and tested. It is confirmed that small amounts of silica hinder the surface area loss of alumina upon calcination, allowing the retention of higher surface areas also when NiO is deposited on the support. Depending on the Ni loading, calcination temperature and on the presence/absence of silica, several species of Ni2+ form on the catalyst: highly dispersed ions, Ni-aluminate defective spinel species and NiO particles. The presence of 5% silica hinders the dispersion of Ni2+ ions and the formation of Ni aluminate phase, and favors the formation of the NiO phase. This is attributed to the competition of silica and nickel oxide for interaction on the most reactive surface sites of alumina. Silica shifts Ni species to less reactive sites where the Ni-alumina interaction is weaker. As result of this, the addition of silica to alumina supports gives rise to more easily reducible Ni catalysts, that become active in ethanol steam reforming at lower temperature.