Photocatalytic gaseous carbon dioxide reduction for valuable fuel production

Abstract

Most of the previous photocatalytic carbon dioxide (CO₂) reduction reactions proceeded in water using protons as a mediator. In contrast to conventional CO₂ reduction reactions in water, dry reforming of methane (DRM) reaction is attractive because it is uphill and can directly convert gaseous CO₂ with methane (two major greenhouse gasses) into valuable syngas (CH₄ + CO₂ → 2CO + 2H₂). Previous researches on DRM were mainly studied in the field of thermal catalysis, and it requires high operating temperature and causes deactivation by carbon precipitation so-called coking. The present paper reports an efficient photocatalytic DRM (Photo-DRM) using metal oxide-based semiconductors, such as rhodium loaded strontium titanate (Rh/SrTiO₃). The developed photocatalysts can drive DRM reaction over 50 % H₂ production yield under light irradiation even at low-temperature conditions for long-term. Generated amounts of CO and H₂ are twice as those of CH₄ and CO₂ consumption, suggesting the stoichiometric DRM reaction without any side reactions like coking. The mechanism of Photo-DRM is comprehensively studied by various analyses, including surface temperature measurement, action spectrum, electron spin resonance, isotope trace experiment, and gas-phase photoelectrochemical studies. Based on these analyses, photogenerated electron–hole pairs are the dominant active species for CO₂ reduction and CH₄ oxidation rather than the photo-thermal effect. Interestingly, the lattice oxygen ions (O²⁻) play an important role in Photo-DRM, where O²⁻ ions act as a mediator to link CH₄ oxidation and CO₂ reduction for producing H₂ and CO equally. By optimizing the conductivity of O²⁻ ions and band structure in semiconductors like CeO₂ and TaON, the Photo-DRM activities are greatly improved. The present mechanism using O²⁻ ions is different from the reported conventional photocatalysts, which use protons as a mediator. The concept of this study is not limited to the DRM reaction but is applicable to various other gas-phase reactions.