Active sites of the cobalt catalysts controlled by surface silanols of silicalite-1 in CO2 hydrogenation to ethanol

Abstract

The soaring emissions of CO2 have resulted in an extreme climate crisis worldwide. One of the promising solutions is to hydrogenate the CO2 into ethanol. However, a significant challenge lies in the low activity or ethanol selectivity of the catalysts, hindering its industrialization. For the Co-based catalysts, regulation of the active cobalt valance state is crucial for boosting ethanol synthesis by CO hydrogenation. In this work, silicalite-1 (S-1) was used as support to control cobalt sites, which was synthesized by a novel grinding synthesis method. The S-1 can form Si-O-Co chemical bonds with cobalt particles by grafting cobalt atoms onto isolated silanols located on the external surface of S-1. The strong metal-support interactions originating from the Si-O-Co bonds stabilized the coexistence of Co0 and CoO (CoOx sites), preventing complete reduction to Co0 during reduction process. The S-1-supported catalysts with CoOx sites showed superior catalytic performance compared to other catalysts with only Co0 sites. The S-1-supported catalysts with Co loading of 5 wt% exhibited an ethanol selectivity of 27 % at 250 °C and 2 MPa, corresponding to a space–time yield (STY) of 0.83 mmolethanol·gcat−1·h−1. In situ DRIFTS and DFT calculation results demonstrated that Co0 sites facilitated the formation of CHx* species, whereas CoO sites favored the HCOO* species. The CHx* species on Co0 sites tended to be further hydrogenated to CH4, whereas the CHx* species on CoOx sites could be coupled with the HCOO* species, inhibiting the formation of CH4 and enhancing ethanol selectivity. Our work provides an approach to prepare highly efficient catalysts with controlled cobalt sites and deepens an insight into the roles of Co0 and CoO sites in CO2 hydrogenation to ethanol.