K-Cu/?-Mo₂C Catalysts Catalyze The Reverse Water Vapor Shift Reaction With High Selectivity

Today,rapid economic development has led to an increase in CO₂ emissions,which has been considered as one of the critical reasons for climate change and ocean acidification.Consequently,capturing CO₂ and converting it into fuels and commodity chemicals has attracted numerous attentions.One attractive route is converting CO₂ into CO via CO₂ hydrogenation or the reverse water gas shift reaction(RWGS),which can be used as feedstock in the Fischer-Tropsch process,a well known route that has been used in industry to produce chemicals and synthetic fuels from syngas(CO+H₂)for many decades.In this work,we have developed a series of novel Cu/?-Mo₂C and K-Cu/?-Mo₂C catalysts for CO₂ conversion via RWGS reaction.The relationship between the reaction performance and physicochemical properties of these catalysts was discussed.The main findings are summarized as follows:(1)Different experiments were designed for the exploration of the Mo₂C carbonization process by using(NH₄)6MO7O₂4·4H₂O as a precursor.The results showed that MoO3 was converted to Mo₂C by Mo in the transition state,which has been proved a non-topological transition process.Based on it,the preparation methods of Cu/?-Mo₂C and K-Cu/?-Mo₂C catalysts with different Cu loadings were also investigated.(2)The physical and chemical properties of the prepared Cu/?-Mo₂C and K-Cu/?-Mo₂C catalysts were investigated using N₂ adsorption-desorption,transmission electron microscopy(TEM),scanning electron microscope(SEM),hydrogen temperature-programmed desorption(H₂-TPD),X-ray diffraction(XRD),and X-ray photoelectron spectroscopy(XPS).The results showed that the addition of alkali metal K could promote the dispersion of the active phase and decrease the particle size of Cu.Besides,K can promote the electron transfer between Cu and Mo,which facilitates the CO₂ adsorption on the catalysts surface.However,too much K can result in the appearance of metal Mo in the catalyst.(3)The catalytic performances of Cu/?-Mo₂C and K-Cu/?-Mo₂C catalysts for the RWGS reaction was evaluated in the temperature window 350?600 ?.The effects of the adding amounts of Cu and K,reaction temperature and H₂/CO₂ ratio for RWGS were investigated.The results showed that K significantly promotes the performance RWGS reaction.Under the condition of 600?,H₂/CO₂ and WHSV=84000 mL·g-1·h-1,the 1%K-5%Cu/?-Mo₂C resulted in 54%conversion and 100%CO selectivity,which outperforms ?-Mo₂C and Cu/?-Mo₂C catalysts.(4)The RWGS reaction mechanism on ?-Mo₂C,Cu/?-Mo₂C,and K-Cu/?-Mo₂C catalysts was investigated.The results showed that the RWGS reaction on the Mo₂C and 5%Cu-Mo₂C catalyst followed the redox mechanism.After the addition of alkali metal,the reaction on the l%K-5%Cu/?-Mo₂C catalyst not only followed the redox mechanism but also the decomposition mechanism of formate.The interaction of K and Cu promoted the generation of adsorbed CO₂ intermediate.In fact,the selectivity improvement is mainly associated to K as promoter,which resulted in an increase of Cu⁺ and Moo on the K-5%Cu/?-Mo₂C catalyst.