Non-isothermal reduction of silica-supported nickel catalyst precursors in hydrogen atmosphere: a kinetic study and statistical interpretation

Abstract

A series of silica-supported nickel catalyst precursors was synthesized with different SiO2/Ni mole ratios (0.20, 0.80 and 1.15). Non-isothermal reduction of Ni catalyst precursors was investigated by temperature-programmed reduction at four different heating rates (2, 5, 10 and 20 A degrees C min(-1)), in a hydrogen atmosphere. Kinetic parameters (E (a), A) were determined using Friedman isoconversional method. It was found that for all mole ratios, apparent activation energy is practically constant in conversion range of alpha = 30-70 %. In considered conversion range, the following values of apparent activation energy were found: E (a) = 129.5 kJ mol(-1) (SiO2/Ni = 0.20), E (a) = 133.8 kJ mol(-1) (SiO2/Ni = 0.80) and E (a) = 125.0 kJ mol(-1) (SiO2/Ni = 1.15). Using two special functions (y(alpha) and z(alpha)), the kinetic model was determined. It was established that reduction of Ni catalyst precursors with different SiO2/Ni mole ratios is a complex process and can be described by two-parameter estak-Berggren (SB) autocatalytic model. Based on established values of SB parameters for each mole ratio, the possible mechanism was discussed. It was found that for all investigated ratios, the Weibull distribution function fits very well the experimental data, in the wide range of conversions (alpha = 5-95 %). Based on obtained values of Weibull shape parameter (theta), it was found that experimentally evaluated density distribution functions of the apparent activation energies can be approximated by the unbalanced peaked normal distribution.