Insight Into The Microcosmic Oxidation/reduction Mechanisms Of CuO Oxygen Carrier

Chemical-looping combustion?CLC?,which allows a sequestration-ready CO₂ stream to be directly generated from an innovative two-step reaction,has been raised to be a promising CCS technology.As a kind of important oxygen carrier material,copper oxide is able to release and absorb gaseous O₂ cyclically at temperatures of interest for fossil fuels combustion?800–1000 ^oC?,and therefore is widely utilized in many chemical looping processes,especially in chemical looping with oxygen uncoupling?CLOU?.Understanding the oxidation/reduction processes of CuO oxygen carrier and the interaction between active components and supports at a microcosmic level is vital to the exploration of intrinsic reaction mechanisms and kinetics,as well as the rational design of high-performance Cu-based oxygen carriers.In this research,the detailed information about the oxidation of Cu₂O and Cu,which belongs to the two kinds of reduced product of CuO,was investigated by periodical Density Functional Theory?DFT?with GGA-PW91 firstly.In oxidation process,the interface reactions between OCs and air are exothermal with relatively small energy barriers.And the oxidation of Cu?111?surface would product a layer of CuO preferentially,instead of the direct formation of Cu₂O.Compared to the surface reaction,extra heat is consumed by ions diffusion in the oxidized region,and the diffusion processes exhibit significant energy barriers for both Cu₂O and Cu,which would be the rate-limiting step in the whole oxidation process.The inward diffusion of anions,especially,is very difficult to happen in kinetics,which means that the regeneration of copper oxide was mainly governed by the outward diffusion of cations.Then the DFT calculation was performed to gain insight into the interaction between two kinds of support with Cu₂O.Based on the interface models,strong interaction between CuAl₂O₄ and Cu₂O slab was observed,which will promote the crushing strength of OC particles and prevent the agglomeration of grain.Though the binding energy with ZrO₂ is higher than its surface energy,Cu₂O would still go through a slip or agglomeration because of the sparse bonds on the interface.The effect of supports on the OC performance in term of oxidation reactivity comes from two aspects,one is the distribution of electron,which leads to a weaker adsorption of O₂ on Cu₂O/CuAl₂O₄ surface and harder cations diffusion.Another aspect is the change of lattice configuration,which becomes the dominant influence exerted by ZrO₂.In Cu₂O/ZrO₂ model,the adsorption of O₂ is enhanced and the movement of atoms becomes more flexible.Finally,the DFT calculation was used to study the reaction between three kinds of gaseous molecule and CuO.The calculation results show that there is a strong reactivity in fuel molecules,which indicates a clear advantage in kinetics.Since there are significant energy barriers and heat adsorption,the reactivity of sulfur and nitrogen molecules would not be so good as that of fuel molecules.However,the affinity between S,N-contained molecules and surface metallic atoms are stronger,it may affect the adsorption of carbon-based molecules such as CO and light hydrocarbon.