Nickel Oxide Reduction by Hydrogen: Kinetics and Structural Transformations

Abstract

We studied the reduction kinetics of bulk NiO crystals by hydrogen and the corresponding structural transformations in the temperature range of 543–1593 K. A new experimental approach allows us to arrest and quench the reaction at different stages with millisecond time resolution. Two distinctive temperature intervals are found where the reaction kinetics and product microstructures are different. At relatively low temperatures, 543–773 K, the kinetic curves have a sigmoidal shape with long induction times (up to 2000 s) and result in incomplete conversion. Low-temperature reduction forms a complex polycrystalline Ni/NiO porous structure with characteristic pore size on the order of 100 nm. No induction period was observed for the high-temperature conditions (1173–1593 K), and full reduction of NiO to Ni is achieved within seconds. An extremely fine porous metal structure, with pore size under 10 nm, forms during high-temperature reduction by a novel crystal growth mechanism. This consists of the epitaxial-like transformation of micrometer-sized NiO single crystals into single-crystalline Ni without any crystallographic changes, including shape, size, or crystal orientation. The Avrami nucleation model accurately describes the reaction kinetics in both temperature regimes. However, the structural transformations during reduction in both nanolevel and atomic level are very complex, and the mechanism relies on both nucleation and the critical diffusion length for outward diffusion of water molecules.

Exsolution of Nickel Nanoparticles from Mixed‐Valence Metal Oxides: A Quantitative Evaluation by Magnetic Measurements

“…The doping also explains why a weak ferromagnetic component is observed in all compositions, even after calcination at 1000 °C (Figure ). It is known that the addition of dopants in the lanthanum chromite, like the Sr and Ni, can create weak ferromagnetic behavior due to distortion of the magnetic arrangement and creation of oxygen vacancies . However, the MxH curves of samples reduced at 750 °C (Figure ) show no significant differences from the as‐prepared samples, indicating that the reducing treatment at 750 °C did not provide the necessary driving force for the exsolution process.…”

Research into specifics of recycling the scale of nickel­molybdenum containing precision alloys by the method of hydrogen reduction

“…A study into reduction of NiO at temperatures 543-1593 K was performed by authors of paper [10]. It was determined that at 543-773 K the products of reaction comprised a mixture of Ni and NiO.…”

“…It follows from an analysis of the scientific literature that there are significant results reported in studies into hydrogen reduction processes of Ni and Mo on the example of separately taken oxides [7][8][9][10], as well as non-alloyed iron-containing oxide technogenic waste [11]. However, the mechanism of the course of the processes during hydrogen reduction of complex alloyed oxide technogenic raw materials, including the scale of nickel-molybdenum containing precision alloys, was not investigated sufficiently.…”

Investigation of CVD multilayered graphene as negative electrode for lithium-ion batteries

“…Using similar environment even at lower temperatures proves effective in reducing the oxide. [42,43] Another effect of heating at 1000˚C is the enlargement of the crystal grain size in the metallic nickel. For a series of samples at 700˚C with a deposition time of 60 minutes, the reducing step at 1000˚C was removed from the heating protocol.…”