Fabrication And Characterization Of Single-layer MoS₂/Graphene Based Composites And Their Application For The Catalytic Hydrogenation Of Carbonyl Sulfide

Integrated gasification combined cycle?IGCC?is an effective technology to solve the problem such as environmental pollution and energy waste which exist in the coal traditional utilizing way.However,sulfur compounds such as hydrogen sulfide?H₂S?and carbonyl sulfide?COS?were inevitably mixed in the process of coal conversion.These sulfur compounds may affect the ecology and power generation equipment.Due to the stability and neutral nature,COS is much more difficult to be directly removed.Hydrodesulfurization?HDS?is an effective way for the removal of COS because hydrogen can act as both reducing agent and hydrogenation agent without additional gas source.COS could be transformed into H₂ S which was easy to remove.Traditionally,the mixed oxides of Ni,Co and Mo supported on ?-Al2O3 are used as catalysts for the COS hydrogenation.However,there are several defects for these traditional catalysts which limit their application in industry.For example,the oxides catalysts need presulfurization treatment to transform into activated sulfides and the HDS reaction conducts only at high temperature?>320 oC?.Thus,developing the new support materials and modified active components for HDS catalysts of COS is an effective way to improve the catalytic activity.As a typical transition-metal dichalcogenide,molybdenum disulfide?MoS₂?with laminate structure exhibits excellent catalytic activity and has been widely applied in various hydrogenation reactions.Especially,single-layer MoS₂ may possess more edge-defect active sites which can effectively enhance the catalytic activity.Graphene as the new carbon material has many superior features such as extending of two-dimensional plane,the larger specific surface area,high electrical conductivity and thermal stability,etc.Graphene nanoribbons as a kind of one-dimension derivative of graphene sheets may exhibit much more edge-defect sites.The increased defects of supports are beneficial to the active component defects.Hence,aimed at the current situation of traditional HDS catalysts,the study focused on the design and preparation of single-layer MoS₂/graphene based composites in order to construct a new efficient HDS catalyst for COS conversion at low temperatures.The dependencies of the catalytic performance on MoS₂ layer structures and the property of supports have been investigated and a series of novel results are described as follows.First of all,highly active monolayer MoS₂/reduced graphene oxide?ML-MoS₂/RGO?catalyst has been fabricated with the assistance of cetyltrimethylammonium bromide?CTAB?via a solution route followed by annealing at elevated temperature,using graphene oxide?GO?and ammonium tetrathiomolybdate?ATTM?as the precursors and hydrazine hydrate as the reducing agent.By changing the kinds of precursors,few-layer MoS₂/reduced graphene oxide?FL-MoS₂/RGO?,bulk MoS₂?B-MoS₂?and RGO can be synthesized.Layer-depended catalytic performance of MoS₂ active component has been demonstrated in the HDS reaction of COS.The ML-MoS₂/RGO composites show superior catalytic activity compared with few-layer FL-MoS₂/RGO catalyst,while the activity of bulk MoS₂ with multilayer structure and RGO itself are very poor.Combined with the characterization of XRD,Raman,HRTEM,etc and the investigation of catalytic reaction kinetics,it has been clarified that the high efficient activity of ML-MoS₂/RGO within low temperature range?180-260 oC?should be ascribed to more exposure of MoS₂ active sites on monolayer MoS₂ derived from high dispersity of monolayer MoS₂ nanoparticles on RGO sheets.Secondly,on the basis of the above study,the modification of ML-MoS₂/graphene catalyst using transition metal doped into the single-layer MoS₂ has been investigated so as to further improve the hydrogenation performance for COS.Cobalt?Co?or nickel?Ni?single component doped Co?Ni?MoS₂/graphene sheet hybrids were synthesized by using Co?NO3?2·6H₂O or Ni?NO3?2·6H₂O as precursors.Owing to the dopping of Co/Ni,more sulphur vacancy could be exposed in single-layer MoS₂,therefore the activity of the hybrid catalysts for the COS hydrogenation was further improved,and the catalytic activity of Co doping was significantly higher than that of Ni doping.Furthermore,in view of the interesting phenomenon that the HDS activity depended on the controllable MoS₂ layers which loaded on the graphene,the contrast research based on graphene nanoribbons?GNRs?as the support material has been carried out.Benefited from the edge-effect of GNRs with both one-dimension and two-dimension structure,single layer MoS₂ supported on GNRs has more advantage in exposure active sites than graphene sheets as a carrier.The results for HDS research confirmed that the catalytic activity of MoS₂/GNRs catalysts were also depended on the layer numbers of active component,MoS₂.In addition,compared with the traditional HDS catalysts loaded on ?-Al2O3,novelty structure of SL-MoS₂/GNRs greatly reduced the reaction temperature of COS hydrogenation.This work offers a feasible strategy for the synthesis of single-layer MoS₂ supported on GNRs for efficient HDS application.