Effects of Crystallite Size on the Kinetics and Mechanism of NiO Reduction with H2

Abstract

A kinetic investigation on the reduction of nanometer-sized nickel oxide particles (supported and unsupported) with hydrogen was carried out. The reduction behavior was found to be related to the average size of NiO crystallites. When the NiO crystallite size was less than about 20 nm, the dissociation of H2 was the key rate-limiting factor and remained unchanged almost throughout the reduction process. However, when the NiO crystallite size was >20 nm, its reduction kinetics changed with conversion, showing two kinetic compensation effects at lower and higher NiO conversion levels. It is believed that the movement and rearrangement of H atoms inside such bulk Ni/NiO solid could be an important aspect of the reduction kinetics, especially at higher (50%–90%) NiO conversion levels.