A new insight on the NO-CO reaction at the electronic level: Homogeneous, E-R and L-H mechanisms

Carbonaceous surface, as one of the major carriers in coal combustion, was found to exert great influence on nitric oxide with carbon monoxide (NO-CO) reactions. Although there have been some studies addressing the NO-CO reaction, the inherent mechanism still remains obscure. In this work, some updated mechanisms with details were proposed at the electronic level. Using density functional theory (DFT) calculations, the preferred pathways were identified with three channels consisting of homogeneous reaction, the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) heterogeneous reactions. Through the analyses of chemical bond and electronic transfer, the reasons for the difference in thermodynamic energy among the three mechanisms were discussed in detail. Results show that among these channels, the NO-CO reaction is more likely to occur along E-R mechanism, due to its lower energy barrier of rate-determining step. At the initial stage, there is a higher degree of electronic localization between NO molecules in E-R mechanism, leading to a lower energy barrier during the formation of NO dimer. Meanwhile, a large quantify of electrons floods into the N-N, N-O and O-O bonds of NO dimer in the homogeneous reaction, which certainly gets more difficult for the dissociation of O atoms in gas phase. Accordingly, the following stage of N 2 formation has a higher energy barrier than both E-R and L-H mechanisms. Compared to L-H mechanism, the E-R mechanism exhibits a lower degree of electronic localization between N 2 O and carbonaceous surface. Therefore, the interfacial interaction between them in E-R mechanism is weaker than that in L-H mechanism, so that it is easy to take N 2 out of carbonaceous surface in E-R mechanism. To sum up, the result deepens the knowledge about the NO-CO reaction, which will help to further develop the oxy-fuel combustion technology.