Green Chemical Engineering (Dec 2020)
Mechanism investigation and catalyst screening of high-temperature reverse water gas shift reaction
Abstract
Reverse water gas shift (RWGS) catalysis, a prominent technology for converting CO2 to CO, is emerging to meet the growing demand of global environment. However, the fundamental understanding of the reaction mechanism is hindered by the complex nature of the reaction. Herein, microkinetic modeling of RWGS on different metals (i.e., Co, Ru, Fe, Ni, Cu, Rh, Pd, and Pt) was performed based on the DFT results to provide the mechanistic insights and achieve the catalyst screening. Adsorption energies of the carbon-based species and the oxygen-based species can be correlated to the adsorption energy of carbon and oxygen, respectively. Moreover, oxygen adsorption energy is an excellent descriptor for the barrier of CO2 and CO direct dissociation and the difference in reaction barrier between CO2 (or CO) dissociation and hydrogenation. The reaction mechanism varies on various metals. Direct CO2 dissociation is the dominating route on Co, Fe, Ru, Rh, Cu, and Ni, while it competes with the COOH-mediated path on Pt and Pd surface. The eights metals can be divided into two groups based on the degree of rate control analysis for CO production, where CO–O bond cleavage is rate relevant on Pt, Pd, and Cu, and OH–H binding is rate-controlling on Co, Fe, Ru, Ni, and Rh. Both CO-direct dissociation and hydrogen-assisted route to CH4 contribute to the methane formation on Co, Fe, Pt, Pd, Ru, and Rh, despite the significant barrier difference between the two routes. Besides, the specific rate-relevant transition states and intermediates are suggested for methane formation, and thus, the selectivity can be tuned by adjusting the energy. The descriptor (C- and O- formation energy) based microkinetic modeling proposed that the activity trend is Rh~Ni > Pt~Pd > Cu > Co > Ru > Fe, where Fe, Co, Ru, and Ni tends to be oxidized. The predicted activity trend is well consistent with those obtained experimentally. The interpolation concept of adsorption energy was used to identify bimetallic materials for highly active catalysts for RWGS.
