Computational investigation of M-1/W6S8 (M=Fe, Ru, and Os) single-atom catalysts for CO2 hydrogenation

CO2 hydrogenation to produce useful chemicals (such as CO, CH4, CH3OH, C2H4 and C2H6) plays a pivotal role in future energy conversion and storage, in which catalysts lie at the heart. We first performed density functional theory calculations to investigate the mechanism for the CO2 hydrogenation to CO through the reverse water-gas shift (RWGS) reaction over M-1/W6S8 (M=Fe, Ru, and Os) single-atom catalysts (SACs). The results showed that mechanism C (formic acid mechanism) on the Fe-1/W6S8 single-atom catalyst is the most suitable pathway for RWGS with 30.1kcal/mol rate-determining energy barrier. Additionally, on the basis of the energetic span model (ESM) and d-band center position, it is demonstrated that Fe-1/W6S8 is effective catalysts for the reaction. Afterward, we chose the most higher catalytic activity catalysts Fe-1/W6S8 for CO hydrogenation to CH4CH3OH, C2H4 and C2H6. CH4* is formed via CO*-> COH*-> HCOH*-> CH*-> CH2*-> CH3*-> CH4*, the effective barrier for CH4* formation is 24.8kcal/mol. CH3OH* is formed via CO*-> COH*-> HCOH*-> H2COH*-> CH3OH*, the effective barrier for CH3OH* formation is 26.0kcal/mol. On Fe-1/W6S8, the CH* species is the most favorable monomeric CHx* species for production of C2H4 and C2H6, whose formation goes through a path of CO*-> COH*-> HCOH*-> CH*. Once CH* is produced, it will be more selective to C2H4 via C-C coupling of CH*, rather than its hydrogenation to CH4 due to the higher hydrogenation barrier of CH2 species relative to the barriers for CH*+CH* coupling and subsequent conversion to C2H4. Ultimately, C2H6 can be produced from further hydrogenation of C2H4 with moderate barriers. The present insights are helpful for the design and optimization of highly efficient Fe-1/W6S8 SACs used in C2H4 and C2H6 formation from CO2 hydrogenation.