Converting CO2 to liquid fuel on MoS2 vacancies

Catalytic materials design for upgrading CO2 into value-added fuels and chemicals at low temperature is attractive to reduce carbon pollution and dependence on fossil fuels. In the current issue of Nature Catalysis, Deng, Wang, and colleagues developed a vacancy-rich MoS2 catalyst for hydrogenation of CO2 into methanol with high catalytic activity, excellent selectivity, and industrial-relevant stability. Furthermore, an integration of in situ spectroscopic characterizations and theoretical calculations confirmed the key role of in-plane sulfur vacancies to this hydrogenation catalysis. Catalytic materials design for upgrading CO2 into value-added fuels and chemicals at low temperature is attractive to reduce carbon pollution and dependence on fossil fuels. In the current issue of Nature Catalysis, Deng, Wang, and colleagues developed a vacancy-rich MoS2 catalyst for hydrogenation of CO2 into methanol with high catalytic activity, excellent selectivity, and industrial-relevant stability. Furthermore, an integration of in situ spectroscopic characterizations and theoretical calculations confirmed the key role of in-plane sulfur vacancies to this hydrogenation catalysis. Transformation of atmospheric carbon dioxide (CO2) into hydrocarbon compounds provides an attractive and promising path to address current environmental and energy crisis.1Fan L., Xia C., Yang F., Wang J., Wang H., Lu Y. Strategies in catalysts and electrolyzer design for electrochemical CO2 reduction toward C2+ products. Sci. Adv. 6:eaay3111.Google Scholar As one of the most significant C1 products from CO2 updating, methanol holds a vital role in chemistry and industry, which could be utilized as fundamental solvents, fuels, and feedstocks.2Bai S.-T. De Smet G. Liao Y. Sun R. Zhou C. Beller M. Maes B.U.W. Sels B.F. Homogeneous and heterogeneous catalysts for hydrogenation of CO2 to methanol under mild conditions.Chem. Soc. Rev. 2021; 50: 4259-4298Crossref PubMed Google Scholar The development of highly active, selective, and stable catalysts for converting CO2 into methanol is extremely beneficial and desired in low-cost, energy-saving, and eco-friendly industrial fields.3Graciani J. Mudiyanselage K. Xu F. Baber A.E. Evans J. Senanayake S.D. Stacchiola D.J. Liu P. Hrbek J. Fernández Sanz J. Rodriguez J.A. Catalysis. Highly active copper-ceria and copper-ceria-titania catalysts for methanol synthesis from CO2.Science. 2014; 345: 546-550Crossref PubMed Scopus (878) Google Scholar Commonly used metal oxide catalysts require high temperatures (more than 300°C) to maintain high catalytic activity, which escalates energy loss and inevitable reverse water-gas shift (RWGS) side reactions for the carbon monoxide (CO) formation.4Behrens M. Studt F. Kasatkin I. Kühl S. Hävecker M. Abild-Pedersen F. Zander S. Girgsdies F. Kurr P. Kniep B.-L. et al.The active site of methanol synthesis over Cu/ZnO/Al2O3 industrial catalysts.Science. 2012; 336: 893-897Crossref PubMed Scopus (1602) Google Scholar Adding transition metals onto these metal oxides could present an improved activity at a lower temperature but a sacrificed methanol selectivity, due to the superfluous conversion of CO2 into methane, intensified RWGS reaction, and thermodynamically favorable hydrogenation of CO.5Rui N. Wang Z. Sun K. Ye J. Ge Q. Liu C.-j. CO2 hydrogenation to methanol over Pd/In2O3: Effects of Pd and oxygen vacancy.Appl. Catal. B. 2017; 218: 488-497Crossref Scopus (269) Google Scholar Here, it is a challenge to find a catalyst that could possess both efficient productivity and high selectivity for methanol synthesis from CO2 reduction. In the current issue of Nature Catalysis,6Hu J. Yu L. Deng J. Wang Y. Cheng K. Ma C. Zhang Q. Wen W. Yu S. Pan Y. et al.Sulfur vacancy-rich MoS2 as a catalyst for the hydrogenation of CO2 to methanol.Nat. Catal. 2021; 4: 242-250Crossref Scopus (71) Google Scholar Deng, Wang, and co-workers introduced an efficient hydrogenation of CO2 into methanol at low temperature (even ambient temperature, albeit with a low conversion rate) over a H2-pre-treated few-layered MoS2 nanosheet (FL-MoS2) catalyst, achieving a methanol selectivity of up to 94.3% at a CO2 conversion of 12.5% over a continuous reaction duration of more than 3,000 h without sacrificing catalytic performance. Noteworthily, the used FL-MoS2 catalyst after 3,000 h only had a slight layer aggregation but barely changed the crystallographic structure, which were confirmed by high-resolution transmission electron microscopy (HRTEM) and powder X-ray diffraction (XRD), respectively. This robustness of FL-MoS2 apparently increases the application potential in industry. In order to identify the real active sites of MoS2 and the reaction mechanism, the authors used a plethora of impressive in situ spectroscopic and microscopic characterizations, together with density functional theory (DFT) calculations. Initially, the authors found that H2 pre-treated FL-MoS2 catalyst dramatically increased its catalytic performance compared with that of the fresh catalyst. In the meantime, a diminished S/Mo peak area ratio after H2 pre-treatment (Figure 1A) was certified by high-sensitivity low-energy ion scattering spectroscopy (HS-LEIS) and the signals of sulfur-containing gases were detected by in situ synchrotron-based VUV photoionization mass spectrometry (SVUV-PIMS), suggestive of the sulfur removal from fresh FL-MoS2 catalyst. Furthermore, X-ray photoelectron spectroscopy (XPS) and in situ techniques such as X-ray absorption near-edge structure (XANES) characterization and electron paramagnetic resonance (EPR) spectrum verified the formation of S vacancies and the exposure of coordinatively unsaturated Mo atoms by H2 reduction. Therefore, S vacancies over FL-MoS2 catalyst from H2 treatment were found to be associated with the high production rate of methanol. Through the concentration investigation on S vacancies of MoS2, in situ EPR spectroscopy and in situ NO pulse adsorption experiments unveiled that higher density of S vacancies contributes to higher activity. Additionally, the used FL-MoS2 bearing more S vacancies than reduced catalyst, as reflected by in situ EPR and XPS characterization, was in good agreement with the observation of an introduction period (Figure 1A). H2 pre-treatment could produce in-plane and edge S vacancies because of the potential removal of S from the basal plane and edges of the MoS2 lattice. In order to clarify the effect of two different S vacancies, the team performed a series of comparable investigation between FL-MoS2 and a nanoporous MoS2 (NP-MoS2) catalyst bearing more abundant edges. Based on the measurements of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) during O2 adsorption process, together with DFT calculations and 31P magic-angle-spinning NMR (MAS NMR) spectroscopy, the type of in-plane S vacancy was found to be the real active site over FL-MoS2 for hydrogenation of CO2 to methanol. Remarkably, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) clearly displayed the existence of in-plane S vacancies in the FL-MoS2 catalyst (Figure 1B). To have a better understanding of the hydrogenation mechanism of CO2 into methanol, the authors performed detailed DFT calculations, showing that both double S vacancy (double-Sv) and triple S vacancy (triple-Sv) are active sites either in the basal plane or at the edge, along with the dissatisfied absorption of CO2 in single S vacancy (Figure 1C). Both in the basal plane and at the Mo edge, CO2 can be directly decomposed to CO∗ and O∗ with a low energy barrier, which was proven by in situ measurements DRIFTS and XANES spectrum, independently. However, the prevailing hydrogenation mechanisms3Graciani J. Mudiyanselage K. Xu F. Baber A.E. Evans J. Senanayake S.D. Stacchiola D.J. Liu P. Hrbek J. Fernández Sanz J. Rodriguez J.A. Catalysis. Highly active copper-ceria and copper-ceria-titania catalysts for methanol synthesis from CO2.Science. 2014; 345: 546-550Crossref PubMed Scopus (878) Google Scholar,7Wang J. Li G. Li Z. Tang C. Feng Z. An H. Liu H. Liu T. Li C. A highly selective and stable ZnO-ZrO2 solid solution catalyst for CO2 hydrogenation to methanol.Sci. Adv. 2017; 3: e1701290Crossref PubMed Scopus (412) Google Scholar,8Larmier K. Liao W.-C. Tada S. Lam E. Verel R. Bansode A. et al.CO2-to-methanol hydrogenation on zirconia-supported copper nanoparticles: Reaction intermediates and the role of the metal–support interface.Angew. Chem. Int. Ed. 2017; 56: 2318-2323Crossref PubMed Scopus (294) Google Scholar of CO2 to COOH∗ or HCOO∗ intermediates do not work at the in-plane Sv sites, because of the unfavorable energy barrier. Although the formation of HCOO∗ at Mo-edge sites is energetically favorable, following dissociation or hydrogenation of HCOO∗ is depressed due to the higher energy barrier. In comparison to the stronger CO∗ desorption process from either in-plane or Mo-edge Sv sites, CHO∗ can be generated easily via hydrogenation of CO∗. Because of lower energy barrier than competitive C–O bond dissociating into CH2∗ and CH3∗ intermediates, methanol was selectively synthesized through CH2O∗ and CH3O∗ intermediates at both in-plane and Mo-edge Sv sites, which was confirmed by the observed signals of desorbed HCHO and CH3O∗ surface species from in situ spectroscopic techniques including SVUV-PIMS and DRIFTS. During the desorption and dissociation process of CH3OH∗, DFT calculations described converse reactivities between the Mo-edge and in-plane active sites, and as a result Mo-edge sites prefer energetically favorable dissociation into CH3∗ and OH∗ and in-plane Sv sites are more likely to provide methanol formation from the desorption of CH3OH∗. Surprisingly, subsequent methanol decomposition experiments into CH4 and the application of accompanied micro-kinetics modeling further verified these computational details. In this work, unusual rich in-plane S vacancies rather than the MoS2-edge sites were proven the key factor to accomplish the highly active, selective, and stable methanol synthesis via a direct energetically favorable dissociation from CO2 into CO∗ intermediate. This extraordinary finding points out an attractive and promising direction to modify various heterogenous catalysts in consideration of the vacancy. Higher density of in-plane S vacancies results in higher performance of the catalyst, which has been demonstrated by multiple in situ spectroscopic and computational details. Therefore, the issue of how to make catalysts possess the highest level of active S vacancies would be the next step to be explored. When the density of the active vacancies reaches the maximum, the catalysts theoretically hold a better performance, potentially making the methanol synthesis at a lower temperature under lower CO2 concentration.