Enhanced Methanol Selectivity in CO2 Hydrogenation by Decoration of K on MoS2 Catalyst
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 CO2 hydrogenation 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.
CO2 hydrogenationMethanolCH4Selectivity tuningK-MoS2
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