Effect of Calcination Temperature on the Efficiency of Ni/Al2O3 in the Hydrodechlorination Reaction

Abstract

The precursors of Ni/Al2O3 catalysts with different metal contents (2–10 wt % Ni) and calcination temperatures were obtained by wet impregnation. Their composition and physicochemical properties were determined by low-temperature nitrogen adsorption, atomic absorption spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction (TPR) with hydrogen. The formation of free and chemically bound forms of nickel was studied by TPR-H2. Calcination above 400°C leads to complete binding of nickel with Al2O3, forming nickel–aluminum spinel. According to TPR-H2, free NiO appears in the samples calcinated at 550°C only at 10% Ni. The stability of the reduced catalysts to oxidation in air and the effect of the temperature of treatment with hydrogen on the reduction of the nickel forms bonded with the support were determined. The XPS and TPR studies showed that the oxidation of reduced nickel in air led to a transition from Ni2+ forms tightly bound with support to weakly bound forms that are more easily reduced. The catalytic action in hydrodechlorination of chlorobenzene (CB HDC) of the samples that differed in the ratio of weakly and tightly bound Ni forms was considered. The spinel forms that are nearly inactive in chlorobenzene hydrodechlorination can be reduced with hydrogen to form more active Ni0 sites under the reaction conditions. The active sites obtained by the reduction of Ni2+ forms weakly bound to the support are stable under chlorobenzene hydrodechlorination conditions.