Redox kinetics of nickel oxide foils: Structural evolution and rate-limiting steps

• Published in: Combustion and flame Vol. 207; pp. 71 - 88
• Main Authors: Zhao, Zhenlong, Uddi, Mruthunjaya, Ghoniem, Ahmed F.
• Format: Journal Article
• Language: English
• Published: New York Elsevier Inc 01.09.2019; Elsevier BV
• Subjects: Attenuation; Chemical-looping redox processes; Clusters; Conversion; Decay rate; Dependence; Diffusion rate; Evolution; Grain structure; Ion diffusion; Mathematical models; Metal foils; Nickel; Nickel oxide; Nickel oxides; Nucleation; Organic chemistry; Oxidation; Oxidation kinetics; Oxidation rate; Oxygen; Reaction kinetics; Reduction; Reduction kinetics; Surface kinetics; Thickness; Oxidation kinetics; Reduction kinetics; Nickel oxide; Chemical-looping redox processes
• DOI: 10.1016/j.combustflame.2019.05.012
• ISSN: 0010-2180

We examine the redox activity of nickel/nickel oxide foils. Thin nickel foils (2.5 µm, 10 µm, and 100 µm) were subjected to redox conditions in a fixed-bed reactor within 800–1000 °C, and samples were examined using SEM at different stages of conversion. We identify some key features or the process that are used to guide model development: (1) Oxidation starts with the nucleation of oxide grains, followed by their rapid growth leading to overlap and the attenuation of fast diffusion pathways. (2) Reduction is impacted by the evolution of macro-pores which facilitate gas-oxide contact, and dense metallic clusters that shield the reactants. (3) The transitions between stages during conversion and the characteristics of grains, pores and clusters depend on the sample thickness. Guided by these observations, analytical models are formulated. Oxidation is modeled as a nucleation nucleation-growth process while reduction is characterized by an adsorption-surface reaction process. We extract the corresponding reaction parameters by training the model using the measurements and show that the model formulation captures the controlling mechanisms of the conversion. The oxidation rate is controlled by the rapid decay of oxygen transport across the products layer, and the dependency of the oxides grain structure on the sample thickness contributes to the rate of decay. The reduction rate is largely controlled by the accessibility of lattice oxygen to surface kinetics, and mostly by slow ionic diffusion. While using different foil thicknesses is useful in extracting the kinetics, if used in rapid redox processes applicable to chemical-looping applications, oxygen carriers (foils or other forms) with characteristic active metal thickness of ∼1 µm are recommended.