Kinetics and Mechanism of Nickel Oxide Reduction by Methane

Abstract

Nickel oxide reduction by methane is of particular importance in catalysis, extractive metallurgy, and clean power generation technologies. Despite extensive investigations of the NiO + CH 4 reaction, many questions remain about its kinetics and molecular and structural transformation mechanisms. This work reports the reduction kinetics of bulk polycrystalline NiO by CH 4 using a new calorimetric method. The method permits rapid, controllable heating of the NiO/Ni wires and continuous data (electrical power, the electrical resistivity of the wire, and temperature) acquisition with a frequency of 10 kHz. The method also allows arresting and preservation of the structure of the sample by programmed termination of electric power in thin specimens. This approach, coupled with ex situ electron microscopy, allows determination of the reaction rate and tracking of the ongoing structural transformations. The mechanism of NiO reduction by methane is suggested based on direct correlations between reaction kinetics and the observed microstructure transformations. The kinetic data treatment revealed that the nucleation−growth model describes the reaction kinetics. Ex situ microscopy observations of reacted specimens showed that the first nuclei rapidly accelerate the process. Pore formation occurs at the nucleation sites, and the porous structure then quickly grows into the bulk NiO. The second type of reduced Ni microstructure, compact layer, can be observed only on the outer surface of the wire. Based on kinetic data and microstructural observations, a mechanism of reduction was suggested according to which porous microstructure forms predominantly by NiO reduction with carbon. The compact layer observed on the outer surface of the wire forms through the diffusion-controlled hydrogen reduction.

Solid‐State Reoxidation Kinetics of A/B‐Site Substituted LaMnO3 During Solar Thermochemical CO2 Conversion

“…NGMs assume the formation of a product nuclei, followed by subsequent growth, with the reaction rate proportional to the number of this newly formed nuclei. [ 12–14,25–27 ] Growth of the nuclei and formation of new nuclei will take place simultaneously until they overlap causing nucleation. Following the nucleation event, the grains grow throughout the surface until the transformation is completed.…”

“…SCM assumes that the reaction occurs on the external surface of the particle, undergoing gradual changes during the course of the reaction. [ 12–14,25–27 ] Here, the surface chemical reaction is considered as rate limiting. The 2D variant assumes a contracting cylinder mechanism as expressed in Equation (4) and 3D variant assumes a contracting sphere mechanism as expressed in Equation (5)…”

Solid-State Redox Kinetics of CeO2 in Two-Step Solar CH4 Partial Oxidation and Thermochemical CO2 Conversion

“…The mechanism of solid-state reduction kinetics of CeO 2 -Fe 2 O 3 systems was found to vary depending on the amount of Fe 2 O 3 [28]. LaFeO 3 and NiO oxygen carriers were found to follow a 2D nucleation and nuclei growth model for solid-state reduction in the presence of CH 4 [33,37]. Similar studies incorporating perovskites indicated that the solid-state mechanism for reduction and re-oxidation relied on the composition, temperature, and extent of conversion [31,34].…”

“…This is because during the first reduction step, apart from producing oxygen vacancies in the solid oxygen carrier, a phase change is generally not involved. Different models have been used to describe the conversion profiles of such noncatalytic gas-solid thermochemical reactions [32][33][34][35][36][37]. The mechanism of solid-state reduction kinetics of CeO 2 -Fe 2 O 3 systems was found to vary depending on the amount of Fe 2 O 3 [28].…”

Relationship between reaction and diffusion during ultrafine NiO reduction toward agglomeration fluidization

“…Unfortunately, the Ni‐based oxygen carrier used in CLC processes has suffered from severe mechanical degradation (particle attrition, agglomeration), aging and deactivation, which in turn will inevitably lead to a decrease in the performance of the process and significant efficiency loss during operating time 7,8 . For this reason, the NiO reduction with hydrogen and the subsequent agglomeration behavior are of practical importance in determining the structure of the oxygen carrier in CLC process 5,9‐11 …”