Multistep Kinetics of the Thermal Decomposition of β-Nickel Hydroxide: Extraction of the Primary Reaction Steps and the Effect of Water Vapor Pressure

Abstract

The thermal decomposition of nickel hydroxide(Ni(OH)₂) was investigated using thermoanalytical techniques with specific focuses on the multistep kinetic behavior and the effect of the partial pressure of water vapor (p(H₂O)). The thermal decomposition process was modeled as a four-step kinetic process, comprising the dehydration of absorbed or included water, two-step primary reaction process yielding nickel oxide (NiO), and the evolution of the trapped water molecules as the crystal growth of NiO progressed. The kinetic characteristics of the individual reaction steps in the primary reaction process were revealed using advanced kinetic analysis methodologies for multistep reactions. A distinctive retardation effect of atmospheric water vapor pressure (p(H₂O)_ATM) was evidenced by systematically tracing the reaction process at varying p(H₂O)_ATM values. Combining the kinetic analysis methodologies for multistep reactions and universal kinetic description across different p(H₂O) values, the individual reaction steps in the primary reaction process were described as a function of temperature, degree of reaction, and p(H₂O)_ATM. This approach was further extended to incorporate the effect of the self-generated water vapor pressure, thereby enabling the universal kinetic description covering all kinetic data in a stream of dry and wet N₂ gases. The kinetic results indicated the initial reaction step in the primary reaction process as being regulative of the primary reaction process in the context of the physico-geometrical kinetic behavior and the effect of p(H₂O). The novel kinetic findings are expected to serve as the necessary information to refine the thermal processing of Ni(OH)₂, yielding NiO with the desired properties and morphologies.