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. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 667–676, 2011

Kinetics of nonisothermal reduction and carburization of WO3─NiO nano‐composite powders by CO─CO2

“…The reduction of metal oxides progresses through nucleation of metal on the particle surface . For reduction by CO, the process begins with a chemical reaction of CO and metal oxides on the surface, which is followed by diffusion of CO toward the particle's core and outward diffusion of CO 2 to the surface.…”

“…For example, the reported activation energy is from approximately 0 to 200 kJ/mol, with a value of 28.7 kJ/mol for the reduction of nanosized NiO (20 nm) by H 2 . Seyd‐Hassan and Li have reported changes in the activation energy as the reaction progresses, due to the change in the diffusion and rearrangement of the atoms in the structure of NiO─Ni.…”

Thermochemical hydrogen production from water using reducible oxide materials: a critical review

“…5b in Ref. [47]). The E a for first reduction remains almost unchanged (indicating single rate-limiting step) throughout the whole reduction process, but for second and third reduction steps the E a profiles continuously increased till the complete reduction of NiO.…”

“…5a in Ref. [47]). The profiles for 20 and 24 nm are almost similar and very much resemble to that of 55 nm NiO supported on SiO 2 substrate.…”

“…There are not many reports available on high-temperature reduction of metal oxides in an inert atmosphere as in the case of solar thermal reduction, but there are plenty of studies available in the literature on reduction of metal oxides using H 2 or CO as reducing agents. For example, Syed-Hassan and Chun-Zhu [47] have studied the particle size effect on reduction of NiO in H 2 atmosphere. The reduction profiles for NiO particles of size 20 and 24 nm are very different from that of particle size of 3.3 nm (please refer Fig.…”