| CO2 emission issue is getting more and more sophisticated because it is not only an environmental problem but also becoming an economical and political one. Thus, reducing CO2 emission is becoming more and more urgent. It is generally accepted that the conversion of CO2 to fuels can reduce CO2 emission efficiently and, simultaneously, alleviate our dependence on fossil energy.Methanol, being a clean liquid fuel, could provideconvenient storage of energy for fuel cell applications,particularly in transportation and mobile devices.Whenused as a fuel, it is a cleaner energy compared with mostother sources. In addition, methanol is an important chemical feedstock. Thus CO2 hydrogenation to methanol has been recognized as one of the most interesting target product. This will not only alleviate environmental problems, but also pave way for clean energy development.It has been widely reported that the Cu catalysts supported on zirconia exhibit high catalytic activity for methanol synthesis from CO2 hydrogenation.Carbon material, having a strong hydrophobicity, is beneficial to the rapid desorption of the generated water, which can effectively reduce the inhibitory effect of water on the catalytic reaction.Carbon nanotubes(CNTs) have been to be as excellent catalyst supports based on their unique electronic properties and structure, such as high mechanical strength, thermol-chemical stability, field limitation, and well conductivity, etc.Besides, carbon surface chemistry also plays vital roles on the catalytic properties.The incorporation ofnitrogen can enhance the stability and sometimes also thecatalytic activity of carbon materials. As compared tonon-doped carbon materials, nitrogen-doped carbon has ahigher oxidation resistance.A series of Cu/Zr O2/CNTscatalystswere prepared for methanol synthesis from CO2 hydrogenation by a co-precipitation method. The effect of functional groups, nitrogen groups content and species over the surface of CNTs on the catalytic properties were investigated. The results were shown below.Firstly, CNTs with basic and acidic surface functional groups supported Cu-Zr O2 catalysts have been prepared by deposition-precipitation method and tested for CO2 hydrogenation to methanol. CNTs, functionalized with nitrogen containing groups, supported Cu-Zr O2 catalyst(CZ/CNT-3) demonstrate high intrinsic activity(turnover frequency 1.61x10-2 s-1) and comparably high space time yield(84.0 mg·g-1cat·h-1) with Cu loading level of only 10.3 wt% under the reaction conditions of 3.0 MPa, 260 °C, V(H2):V(CO2):V(N2) = 69:23:8, and GHSV = 3600 h-1. The existence of nitrogen containing functional groups enhance the dispersion of Cu oxides, promote their reduction, then results in small Cu crystal size, leading to high H2 and CO2 adsorption capability, finally high CO2 conversion, CH3 OH selectivity and CH3 OH yield. The existence of acidic oxygen containing functional groups is responsible for negative effects on Cu dispersion, resulting in low CO2 conversion, finally low CH3 OH yield.Secondly, CNTs with different content and types of N species supported Cu-Zr O2 catalysts have been prepared by deposition-precipitation method and tested for CO2 hydrogenation to methanol. CNTs with higher nitrogen content(2.96%), supported Cu-Zr O2 catalyst(CZ/CNT-4) demonstrate highercatalytic activity(CH3OH selectivity 75%, CH3 OH yield 8.64% and comparably high space time yield 102 mg·g-1cat·h-1) with Cu loading level of only 10.8 wt% under the reaction conditions of 3.0 MPa, 260 °C, V(H2):V(CO2):V(N2) = 69:23:8, and GHSV = 3600 h-1. The large content of nitrogen species enhance the dispersion of Cu oxides, promote their reduction, then results in small Cu crystal size, leading to high H2 adsorption capability, finally high CH3 OH selectivity and CH3 OH yield. |
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