Study On Preparation And Property Of Cu Based Catalysts For CO₂ Hydrogenation

As the world economy grows more and more energy is required.Fossil fuels are used for propulsion,electrical power generation,and heating.The combustion of fossil fuels is by far the largest contributor to man-made anthropogenic CO2 emissions.CO2 is the primary greenhouse gas.The resource recycling of CO2 can not only mitigate the global warming,but also help alleviate the energy crisis.Cu based catalyst has been the focus of attention because of its many advantages in CO2 hydrogenation to methanol.At present,it is still not clear about the active site and deactivation mechanism of Cu based catalyst,and its catalytic performance needs to be further improved.The performance of Cu based catalyst is easily influenced by the additive and morphology.In this paper,different methods were used to prepare various Cu based catalysts such as doped with SrCO3 or Mn and Cu nanowire catalyst for CO2 hydrogenation.The relationship between the properties of the catalysts and composition,phase and morphology was successfully obtained,and the deactivation mechanism of catalyst was discussed in detail.The preparation process of Cu nanowire and anodic aluminum oxide(AAO)template was improved.Hemispherical Cu nanowire array with special structure and properties was found.Pure Cu nanowires,which have been one of international frontier research hotspots,were used for the first time in CO2 hydrogenation.The main research contents and results are as follows.SrCO3 as a carrier was added into CuZnAl catalyst prepared by hydrothermal method.The prepared CuZnAlSr catalyst was mixed with HZSM-5 to form composite catalyst by mechanical mixed method,and the properties of the composite catalysts were tested.The results showed that the optimal content of SrCO3 is about 6%.The conversion of CO2 reaches 30.3%,which is higher than about 15%on conventional methanol synthesis catalyst,and the overall yield of methanol and dimethyl ether(DME)is 27.8%.The addition of SrCO3 enhances the activity of the catalyst through providing a tridimensional frame and electron transfer bridge.Cu/Mn oxide catalysts for CO2 hydrogenation were prepared by coprecipitation and temperature-programmed calcination method.For the catalyst with Cu accounted for 40%of the total mass of Cu and Mn,almost all of Cu in the precursor reacted with Mn to produce Cu1.5Mn1.5O4 spinel phase.Thus,the catalyst has the largest specific area and the highest catalytic capacity,and the conversion of CO2 is also highest,reaching 16.3%.The spinel structure is beneficial for improving catalysis activity.The Cu/Mn oxide catalysts are more suitable to promote reverse water gas shift(RWGS)reaction.Melting method was used in the preparation of Cu/Mn catalysts for CO2 hydrogenation.The catalysts are alloy solid solution(ss)with higher alloying degree.With the increase of Mn content,the distribution of Cu in the alloy is more dispersive.The lattice constant increases with the increase of Mn content.For the catalyst with Cu accounted for 40%of the total mass of the alloy,the lattice constant is the largest,and the selectivity for methanol is the highest,about 35.8%.The selectivity of the alloyed catalyst for methanol increased significantly.The higher methanol selectivity of the catalyst may be attributed to more defects on its surface.Alloy generally has excellent resistance especial high temperature resistance to corrosion,and it has potential application in synthesis of low carbon alcohols from CO2 hydrogenation.Aluminum substrate was placed outside the oxalic acid electrolyte,and AAO template was prepared by aluminium anode oxidation.Both channel diameter of AAO template and electrolyte temperature are affected by anodic voltage and cooling mode.A mixed solution of ethanol and concentrated hydrochloric acid with the volume ratio of 2-3 was used to dissolve the aluminum substrate,the time for the dissolution of aluminum substrate is about 30 min,the AAO template is not damaged,and solid and liquid are easy to separate.The almost all metal ions in the solution are aluminum ions without heavy metal ions,which is convenient for recycling with less environmental pollution.Cu nanowires were prepared using commercial double pass AAO template by electroless chemical deposition method.Cu nanotubes,Cu nanowires or nano-porous film formed respectively by controlling deposition time.The Cu nanowires are mainly in metal state and are of polycrystalline structure.The sensitizing solution diluted with 95%ethanol is more stable than that with water,but it is not easy to form nanowires,comparing the sensitizing solution with water in about the same deposition time.Under magnetic stirring,Cu nanowires were prepared using commercial double pass AAO template by electrochemical deposition method.A layer of metal(Au)film serving as a conductive layer is sputtered onto one side of the template before electrodeposition.The electrolyte temperature of 30?50 ? and the deposition time of 25 min are suitable for preparation of Cu nano wires.The Cu nanowires are mainly in metal state and are polycrystalline structure.The sintering resistance of Cu nanowires is significantly better than that of Cu nanoparticles in inert atmosphere.Under stirring by moving cathode,Cu nanowires were prepared using AAO template with aluminum substrate as cathode by electrochemical deposition method.Besides Cu nanowires inside template,hemispherical Cu nanowire arrays were also produced on the surface of template.The Cu nanowires are mainly in metal state,the average diameter is about 80 nm,the specific surface area is about 14.8 m2/g,and the average grain size of Cu is about 26 nm.Electrodeposition time affects the morphology of samples.The hemispherical Cu nanowire array with special structure and property has not been reported in previous literatures,and it may have an important value in the field of high resistance of nano copper to corrosion.Cu nano wire sample was prepared under stirring by moving cathode in less than 30 min of electrochemical deposition time,as catalyst,and were used for CO2 hydrogenation.The performance and structural development of the catalyst were investigated.The results showed that Cu nanowires have high catalytic stability and sintering resistance.The changes of grain sizes,surface morphologies and catalytic performance of Cu nanowire sample indicated that the migration of Cu atoms on the surface of Cu nanowires occurred during CO2 hydrogenation process.Continuous migration of Cu atoms and continuous sintering of Cu grains can lead to the blockage of gas channels.The blockage of gas channel and the sintering of Cu grains can lead to deactivation of the catalyst.The shape of catalytic performance curve indicated that the main reason for the deactivation of the catalyst is the blockage of gas channels.