Computational Investigation of Catalytic Activity Modifications with an Electric Field

概要

Nowadays, it is widely recognized that all the countries of the world has severe energy issues such as depletion of fossil fuels, and it is essential that the activity improvement of catalytic technologies that is used in most energy conversion systems. Non-faradaic electrochemical modification of catalytic activity (NEMCA) by the impression of an external electric field has had an attractive attention as one of the methods to improve catalyst performance. NEMCA has been proposed firstly by Vayenas group in 1981 and has been discussed that oxygen anions that is forced electrochemically to adsorb on catalyst surface alter the catalyst electric property. Furthermore, it has been discussed that given electric field also changes the catalyst electric property directly. However, which effects are the main factor in the detailed NEMCA mechanism had not been revealed before. This argument points to a need for the theoretically detailed comparison and discussion between these possible phenomena as the detailed NEMCA mechanism.

The research purpose in this study is to reveal theoretically the detailed NEMCA mechanism by the comparison activation effects as mentioned above with computational science because it is difficult to divide down these effects with experimental approaches. As the target reaction system, CO2 methanation (CO2 + 4H2 → CH4 + 2H2O, Δ𝑟𝐻0 = −165kJ/mol) on Ni metal catalyst in Solid oxide electrolysis cell (SOEC) is used which has been reported that the reaction acceleration effect beyond the current value when electric voltage is applied and has been regarded as one of the effective methods for a carbon dioxide capture and storage (CCS). Rate determining steps (RDSs) in CO2 methanation and the related seven hydrocarbon species are utilized in density functional theory (DFT) calculation and the detailed kinetic simulation to discuss external electric field and oxygen co-adsorption effects towards kinetic energy changes.

This dissertation is roughly divided into five sections. In chapter 3, the sensitivity analysis and flow analysis simulations in CO2 methanation on Ni (211), (111) and (100) at the temperature range from 400 to 700 °C have been performed so as to identify detailed elementary steps behavior in CO2 methanation and their dependence on surface facets and temperature conditions. In chapter 4, the effect of direct electric field impressions on Ni (111) used the thin film condenser model has been calculated in order to discuss the direct change in a catalytic electron state with an electric field. In chapter 5, the effect of direct electric charge impressions on Ni (111) used the electric battery model with Effective screening medium (ESM) method has been calculated in order to confirm the direct change in a catalytic electron state with an electric field in more practical SOEC systems. In chapter 6 and 7, the effects of oxygen co-adsorption on Ni (111) and (211) used the co-adsorbed oxygen atoms surface model on flat and step sites have been calculated so as to discuss the spillover effect of a lattice oxygen toward the catalyst surface on flat and step sites, respectively.

The novelty and originality of this dissertation can be described as follows. Firstly, I have found RDSs in CO2 methanation are CHO*, CO2* dissociations and CH4 desorption on Ni (211) surface at all temperature ranges and that CH4 generation process depends on surface reactions on Ni (211) surface although all surfaces have the almost same mechanisms. Secondly, I found that a direct electric field enhances the stability of adsorbed species on the catalyst surface and accelerates all RDSs for the relation between electric fields and electric dipole moments, and the change amount of charge transference from Ni surface into adsorbed species with electric fields. Also, I have found that overall CO2 methanation accelerates in SOEC mode with the detailed kinetic simulation. In addition, these phenomena have been confirmed in more practical SOEC setup with the direct electric charge impressions. Thirdly, I have found that all intermediates adsorb on the surface less strongly and that CHO and CO2 dissociations decelerate and CH4 desorption accelerates with co-adsorbed oxygen atoms for steric barrier effects. However, I have found that overall CO2 methanation accelerates in SOEC mode using the detailed kinetic simulation. Also, I have found that steric barrier effect magnitude differs at adsorption sites on step sites (peculiarity of S4 site). Fourthly, compared with effects to the kinetic energy of direct electric field impressions and co-adsorbed oxygen atoms, I have found that the spillover effect of a lattice oxygen toward the catalyst surface may be larger than the direct change in the catalytic electron state with an electric field. Fifthly, I have found that detailed NEMCA mechanism can not be explained by the activation theory on the entire catalytic surface that has been discussed until now.

It is my expectation that the results from the study in this dissertation will provide fundamental understandings on the theoretical mechanism of NEMCA and I believe that this research will connect to develop more energetically-effective catalysis technology and help to overcome serious energy issues. These findings also hopefully direct the attention in the catalytic reaction research community to the application probability of NEMCA to catalytic technologies in the chemical industry.

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