Studies Of Rhodium-based Catalysts For Higher Alcohols Synthesis From Syngas

Conversion of syngas acquired by gasification of coal or biomass to higher alcohols(C2+ alcohols)is a sustainable method for direct production of valuable oxygenates.The most prominent catalysts are Rh-based catalysts,which provide a unique activity for oxygenates stemming from the moderate ability of Rh for CO dissociation.Regarding of reactivity of Rh based catalysts,both support and promoter have been demonstrated to play a crucial role.However,due to the complexity of the system and difficulty in structural and chemical characterization,direct atomic-scale observation of structural and chemical composition of metal-promoter interaction is still missing.In this thesis,we selected various nanocarbons as a support to anchor Rh,Rh-Fe,and Rh-Mn nanoparticles as CO hydrogenation catalysts for C2+ alcohols synthesis.A comprehensive chemical electron microscopy study of states of metal and promoters and atomic-scale insights on the metal-promoter.interaction were demonstrated.The key factors determining the catalytic performance such as support effect and catalyst structure under reaction condition were systemically investigated.The brief detail of our work will describe below.1.The surface modification of carbon nanotubes(CNTs)via hydrothermal carbonization(HTC)technique in the presence of glucose and urea was investigated.We found that the surface of CNTs is successfully coated by nitrogen containing hydrothermal carbon layers.The HTC functionlized CNTs(F-CNTs-004)enable depositing of smaller Rh nanoparticles.The catalysts were tested in H2/CO=2 at 553 K and 30 bar for a period of 20 h.3Rh/F-CNTs-004 shows an ethanol selectivity of 27.39%at a steady-state CO conversion of 19.14%,which is more than three times as much as on 3Rh/F-CNTs-016O(Xco= 5.55%,Sethanol=8.87%),corresponding to the smaller Rh particles on F-CNTs-004.The iron modified catalyst shows higher CO conversion 53%but lower C2+ alcohol selectivity probably due to weak interaction between Rh and Fe promoter,which is confirmed by high resolution transmission electron microscopy(HRTEM).Interestingly,a significant increase in activity to C2+alcohols was observed for the Mn doped catalyst.3RhlMn/F-CNTs-004 shows a significantly higher CO conversion of 67.47%and ethanol selectivity of 23.19%,and the space-time yield is up to 78.23 mol/(kg h).The increase activity of Mn doping has to be attributed most likely to the strong interaction between Rh and Mn as confirmed by energy-dispersive X-ray spectroscopy(EDX)mapping analysis.2.Two kinds of hydrothermal carbon spheres derived from glucose and urea were used to support Rh and Rh-Mn particles.HRTEM was used to investigate atomic-scale structure of Rh and chemical composition of metal-promoter of Rh-based catalysts.These types of catalysts were found to show a significantly selectivity for C2+ alcohols.As such,nitrogen doped carbon supported Rh(3Rh-GU)displays higher CO conversion of 6.84%and ethanol selectivity of 13.28%,which is higher than nitrogen-free carbon supported Rh(3Rh-G).It's presumably due to nitrogen doped carbon spheres play an important role in improving the CO conversion.Moreover,the addition of Mn into the Rh catalysts as promoter again resulted in better catalytic performance owing to the interaction between Rh and Mn.