Abstract:

Selective hydrogenation of CO₂ to methanol with renewable H₂ is a promising approach to effectively utilize the anthropogenic greenhouse gas CO₂ in response to the growing environmental and energy challenges. Recently, MoS₂ has gained attention as an attractive catalyst for CO₂ hydrogenation due to its tunable S vacancy sites. However, its catalytic reactivity towards methanol production is still unsatisfactory because the general edge S vacancy site tends to favor CH₄ formation. Herein, we report that the alkali K decorated MoS₂ catalyst enables a dramatically enhancement in selective hydrogenation of CO₂ to methanol, in contrast to the pristine MoS₂ nanosheets that produce mainly CH₄. We incorporated the K promoter into MoS₂ using a simple physical mixture method, and we found that the loading of K has a crucial impact on the catalytic performance. The K-MoS₂ catalyst with an appropriate K loading of 0.5 wt.% (mass fraction) delivers an optimized methanol selectivity of 81% and a methanol space time yield of 3.6 mmol·g^-1·h^-1 at mild reaction conditions of 220 ℃ and 5 MPa, which greatly outperforms the bare MoS₂. Higher K loading would lead CO as the dominating product, while lower K loading is insufficient to tune the selectivity. Detailed characterization techniques, including X-ray diffraction (XRD), Raman, H₂-temperature programmed reduction (TPR), electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), CO-diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and H₂-D₂-temperature programmed surface reaction (TPSR), reveal that K atoms tend to occupy the edge sites on MoS₂ and serve as electron donators, which enhance the density of states at the Fermi surface and the basicity of the edge active sites, while preventing H₂ dissociation on the edge S vacancy. The reaction mechanism, as studied by CO₂-temperature programmed desorption (TPD) and CO₂ + H₂ DRIFTS, suggests a reverse water-gas shift route for CO₂hydrogenation to methanol. The increased basicity at the edge active site has therefore facilitates CO₂ adsorption and lowers the activation barrier for CO₂ dissociation to CO. It also restrains the methanation activity of intermediate CO and directs the reaction path toward CO hydrogenation to methanol. However, the excessive inhibition of H₂ dissociation at higher K loading levels causes the facile desorption of CO, resulting in high CO selectivity. These results highlight the appearing effect of K promoter on modulating the edge active sites of MoS₂ to favor methanol formation over CH₄, and provide a simple yet effective strategy for tuning the structure and catalytic performance of MoS₂. This extends the application of MoS₂-based catalysts in methanol synthesis via CO₂ hydrogenation.

Key words: CO₂ hydrogenation, Methanol, CH₄, Selectivity tuning, K-MoS₂