| Gargantuan-scale utilisation of fossil fuels by human beings since the Industrial Revolution has finally escalated the concentration of atomospheric CO2 on earth from a pre-industrial 280 ppm to nowadays 408 ppm.The soaring CO2 in the atomosphere is believed to be the main cause of global warming,ocean acidification and frequent occurence of extreme weather.To dissipate these deteriorating impacts on global ecological environment and human society,one effective way is to use catalyst to efficiently transform CO2 into methanol,which is a basic feed stock chemical and can also be used directly as fuel.Among various catalysts,Cu-ZnO based catalysts have been the most studied ones due to the considerable abundance of Cu and Zn in Earth's crust as well as their high activity toward CO2 hydrogenation to methanol.In the work of this thesis,we have adopted three different ways,as alternatives to the traditional co-precipitation method,to construct Cu-ZnO catalysts for catalytic CO2 hydrogenation to methanol and we have also studied the structure-activity relationship for each kind of catalysts.In Chapter 2,we have used a diethylene gycol-therma-reduction method to synthesize Cu-ZnO catalyst and delicately modified the surface Cu with Pd.As the molar ratio of Pd to Cu is increasing from 0 to 4%,the methanol yield is first increasing and then decreasing,showing a vocalno-shaped relationship with Pd loading.When Pd loading is 1%molar to Cu,the catalyst shows the best performance with a methanol yield 2.5 times that of undoped Cu-ZnO catalyst and a methanol turnover frequency(TOF)3.5 times that of the latter.We found that Pd doping can bring down the activation energy of methanol synthesis from 59 kJ mol-1 for Cu-ZnO to 31 kJ mol-1 for 1%Pd doped catalyst.Through chemisorption characterization,we confirm the positive hydrogen spillover effect brought by Pd to interfacial Cu and also the negative effect of surface Cu blockage.Thus the above volcano-shaped relationship is an interplay between the two effects.In Chapter 3,we infiltrated 3.5,7 and 14 wt%ZnO nanoparticles to the vicinity of 4 nm Cu,which was synthesized by the reduction of copper silicate,to construct a series of Cu-ZnO catalysts.Catalytic data shows that the gradual introduction of ZnO to the SiO2 loaded Cu catalyst has brought down the activation energy of methanol synthesis from 72 kJ mol-1 for Cu-SiO2 to 33 kJ mol-1 for 14 wt%ZnO modified catalyst and in the meantime increased the activation energy of water gas shift reaction(RWGS)from 61 kJ mol-1 for Cu-SiO2 to 103 kJ mol-1 for 14 wt%ZnO modified catalyst.The methanol TOF over the ZnO modified catalyst can be 2-3 times higher than that over pure Cu catalyst.We also found through surface temperature programmed reduction(TPR)that the oxidized surface copper on ZnO modified catalyst is more difficult to be reduced that that on Cu-SiO2,causing the reduction peak to shift towards higher temperature zone.In Chapter 4,we first encapsulate copper species into ZIF-8,a porous metal organic framework material with high surface area,and then pyrolyzed the precursor in air to prepare Cu-Zno catalyst.When copper is in the form of copper nitrate,the structure of ZIF-8 in the yielded precursor will be destructed and thus the final catalyst possessed Cu and ZnO particles with a large diameter,leading to medium catalytic activity of methanol formation.Then we tried to load metallic Cu nanoparticles in/onto ZIF-8 as thus preserved the ZIF-8 structure in the precursor.In the subsequent mild calcination process,the Cu loading ZIF-8 was transformed into porous Cu-ZnO catalyst with relatively high surface area.The activity of the best catalyst reached 934 gmethanol kgcataiyst h-1,higher than the state-of-the-art commercial Cu-ZnO-Al2O3.With various characterization techniques,especially chemisorption methods,we found that the key to the high activity is the abundant Cu-ZnO interface in the catalysts formed by the stacking of sub-5 nm ZnO over relatively larger Cu nanoparticles. |
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