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.
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.
A high-speed electrothermography approach is applied to investigate the mechanism and kinetics for nanostructured Al/Ni foils. Application of the Kolmogorov−Johnson−Mehl−Avrami and adiabatic thermal explosion models reveal that the activation energy for nucleation appears to be much higher than that for the reaction. It is shown that formation of intermetallic nuclei is the limiting step that defines the ignition characteristics of the foils at temperatures below 500 K, while the process is reaction-limited at higher temperatures. Nucleation is also shown to play an important role during rapid (∼10 m/s) propagation of the combustion (reaction) wave along the Al/Ni foils. These findings suggest new approaches for controlling the ignition and combustion processes for nanostructured reactive materials.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.