Temperature, pressure, and adsorption‐dependent redox potentials: I. Processes of CO₂ reduction to value‐added compounds

Abstract

Catalytic conversion of carbon dioxide (CO 2 ) into value-added chemicals and fuels can not only mitigate global warming but also alleviate the energy crisis caused by fossil fuel depletion. Redox potential is a key thermodynamic quantity of CO 2 reduction reactions, while currently, only standard reduction potential (25°C, 1 atm, 1 M) is available for application. Herein, it is the first time to report the influence of temperature (0-1000°C), pressure (1-100 atm), and molecular adsorption on the redox potentials of a series of CO 2 reduction reactions to form carbon monoxide, formic acid, formaldehyde, methanol, methane, oxalic acid, acetic acid, acetaldehyde, ethanol, ethylene, and ethane.Although almost all gas-phase and aqueous-phase redox potentials decrease with the temperature at an increased rate and increase with pressure at a decreased rate, adsorption-state redox potentials are only sensitive to temperature and no smaller than gas-phase redox potentials. Significant differences of up to 2.35 V with regard to the standard reduction potential would directly affect the thermodynamics and kinetics of reactions. This report will serve as an enriched database of redox potentials for CO 2 reduction reactions that allows the use under actual experimental conditions.

Temperature, pressure, and adsorption‐dependent redox potentials: Ⅱ. Processes of CH4 oxidation to value‐added compounds

“…2 /CO can be found in our previous report 28. The combination of these redox potentials suggests theF I G U R E 4…”

“…At the anode, H 2 O may also be oxidized, which constitutes a competitive O 2 evolution reaction with the desirable CH 4 oxidation. The temperature and pressure‐dependent gas‐phase, adsorption‐state, and aqueous‐phase redox potentials of O 2 /H 2 O, H + /H 2 , and CO 2 /CO can be found in our previous report 28 . The combination of these redox potentials suggests the tunability of reaction thermodynamics and kinetics by varying temperatures and pressures, which further allows the control of product selectivity.…”

“…27 The calculation method is the same as that in our previous report. 28 Briefly, structure optimization and FANG AND HU | 763 frequency analysis were conducted at B3LYP/6-31G(D) level, [29][30][31][32][33] after which the single-point energy was calculated at CCSD(T)/cc-pVTZ level. 34,35 For liquid water and aqueous oxygenates, DFT-D3BJ dispersion correction 36 and SMD continuum solvation model 37 were employed in structure optimization and frequency analysis followed by calculating the solvation energy at M052X/6-31 G(D) level.…”

“…Calculations were carried out with Gaussian 16 package 27 . The calculation method is the same as that in our previous report 28 . Briefly, structure optimization and frequency analysis were conducted at B3LYP/6‐31G(D) level, 29–33 after which the single‐point energy was calculated at CCSD(T)/cc‐pVTZ level 34,35 .…”

Temperature, pressure, and adsorption dependent redox potentials: III. Processes of CO conversion to value‐added compounds

“…DFT and WFT calculations were performed with Gaussian 16 package 24 . The detailed calculation methods can be found in our previous reports 25,26 . In short, structure optimization and frequency analysis were carried out at B3LYP/6‐31G(D) level, 27–29 followed by single‐point energy calculation at CCSD(T)/cc‐pVTZ level 30,31 .…”