|Abstract or Summary
- The corrosion and degradation of Ni and its alloys in supercritical carbon dioxide (Sc-CO2) is an emerging issue for material selection and implementation of Sc-CO2 thermal cycles in industry. In this study, we apply density functional theory (DFT) to study the initial steps of carburization of the Ni (111) surface and two types of Ni-Cr alloy surface in a CO2 environment. The overall carburization reaction, CO2 Cads + O2 can be broken into four elementary steps, CO2 activation, two surface reactions, CO2 dissociation to CO and O followed by dissociation of CO to O and C. In this study, we concentrate on the thermodynamics of the two surface reactions and determine their activation barriers. We determine the energy of the activate state of CO2 and its dissociated products to compare the thermodynamics of the different elementary reactions steps. On Ni (111), activated CO2 is most stable on a hollow site with adsorption energy of 0.24 eV, employing linear CO2 as reference state, indicating that it is thermodynamically unfavorable. The coadsorption of CO and O, which are products of the first step of CO2 dissociation, are most stable on fcc site and hcp site respectively. The interactions between the two adsorbates are slightly attractive. This initial CO2 dissociation step is exothermic by 0.53 eV with a kinetic barrier of 0.61 eV. The further dissociation of CO to C and O on Ni (111) is endothermic by more than 2 eV and has large activation barrier, 3.38 eV. The results of these two elementary steps of CO2 dissociation are in good agreement with previous literature. The high barriers makes this reaction path unlikely to be the main
reaction path for the carburization of Ni surfaces but co-adsorbates and surface defects are likely to play a major role in this process. The CO2 dissociation step is also studied on two Ni based alloy surfaces, Ni8.33Cr1.67 and Ni5Cr5. On Ni8.33Cr1.67, activated CO2 is most stable on a bridge site with the energy of 0.18 eV, compared to a linear CO2 in the gas phase, indicating that the interaction between CO2 and alloy surface is thermodynamically unfavorable but more favorable then on Ni. The co-adsorption of CO and O, the products of the initial dissociation step, is most stable when two adsorbates stay on ontop site and hcp site, respectively. The interaction between the two adsorbates are still slightly attractive similar to the Ni (111) surface. The activation barriers for the two elementary steps of CO2 dissociation are 1.44 eV and 1.90 eV, respectively. Both of these steps are exothermic with the reaction energy of 1.00 eV and 0.46 eV, respectively. The higher activation barrier for the first step compared to pure Ni surface indicates that Ni-Cr has a better corrosion resistance in Sc-CO2 environment. A Ni5Cr5 alloy is studied to model Cr surface enrichment, on this surface activated CO2 is most stable on fcc site with the adsorption energy of -0.76 eV, relative to linear CO2 in gas phase, indicating that the interaction is thermodynamically favorable. In general, the interactions between CO2 and the surface increase with Cr content. The co-adsorption of CO and O is most stable on bridge and hcp site with attractive interaction between the two adsorbates. The activation barrier for the first elementary step of CO2 on Ni5Cr5 is 0.91 eV which is higher than pure Ni surface. The step is exothermic with the energy of 0.68 eV. Comparison of CO2 dissociation on different Ni and NixCry surfaces shows that the stability of CO2,ads increases with Cr content and the initial dissociation step of CO2 to CO + O has lowest barrier on the Ni surface of the surfaces studied suggesting a more rapid CO2 dissociation on the Ni surface.