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

Abstract

Methane (CH4) is the primary component of natural gas and a powerful greenhouse gas. CH4 can be utilized as an important raw material to produce value‐added chemicals via catalytic partial oxidation, oxidative/non‐oxidative coupling, steam reforming, dry reforming, etc. Redox potential is a key thermodynamic quantity in these processes while currently, only standard reduction potentials at 25°C and 1 atm are available. Herein, this is the first time to report the temperature (0–1000°C), pressure (1–100 atm), and adsorption‐dependent redox potentials of 13 CH4 oxidation reactions to generate methyl radical, methanol, methyl hydroperoxide, formaldehyde, formic acid, carbon monoxide, carbon dioxide, ethane, ethylene, ethanol, acetaldehyde, acetic acid, and benzene. It was found that redox potentials of all states (gaseous, aqueous, and adsorption states) decrease with temperature at an accelerated rate but show different responses to pressure change. Namely, while gas‐phase and aqueous‐phase redox potentials increase and decrease with pressure at a gradually declined rate, respectively, adsorption‐state redox potentials are insensitive to pressure. Most importantly, the significant differences up to 1.92 V under varied conditions reveal the necessity of applying operando redox potentials for CH4 oxidation reactions that are enabled by this report as an enriched database for broad applications.

Photocatalytic Conversion of Methane to Ethanol at a Three-Phase Interface with Concentration-Matched Hydroxyl and Methyl Radicals

“…Fe­(III)@ACN exhibits a strong signal of DMPO–CH 3 after 5 min (Figure b), consistent with its highest activity for CH 4 oxidation to C 2 H 5 OH. Considering that the valence band position of C 3 N 4 does not meet the potential requirements of CH 4 / • CH 3 , , it is inferred that • CH 3 comes from the oxidation of CH 4 by • OH rather than photogenerated holes. The EPR spectrum of Fe­(II)@ACN after 5 min of illumination shows a remarkably high • OH concentration without any • CH 3 (Figure c).…”

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

“…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][28][29] followed by single-point energy calculation at CCSD(T)/cc-pVTZ level. 32 and SMD continuum solvation model 33 were used and the solvation energy was calculated at M052X/6-31G(D) level.…”

“…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 .…”