A Study of Carbon Dioxide Catalytic Activation for its Conversion to Value-added Products

To mitigate carbon dioxide (CO2) emission, carbon capture and utilization seems to make more sense than carbon storage. Catalytic conversion plays key roles in the efforts to make the conversion of CO2 into commodity chemicals or products feasible not only technically but also economically. Due to its thermodynamic stability, CO2 conversion needs energy input as well as the participation of second molecules or reactants, for example, CH4 in CO2 reforming to produce syngas, or H2 in CO2 hydrogenation for methanol synthesis. It has been found that CO2 and the second molecules are activated on different catalytic sites on a catalyst surface. This work studies the catalytic reaction mechanism of CO2 reforming of methane (CRM) and CO2 hydrogenation, focusing on the contiguity of the CO2 activation sites and the activation sites for the second molecules, i.e., the orientation, distribution, and interaction of the two kinds of catalytic sites. In CRM reaction, the Ni monometallic and NiM2 (M2=Co, Mn, Cu, and Fe) bimetallic catalysts supported by MgO-spinel prepared by the co-precipitation method were studied. It was observed that M2 affected metallic particle sizes and slightly affected the basicity of the catalysts. During the reaction, the initial TOF of CO2 and CH4 based on the number of metallic sites had a good correlation with the average metallic particle size. The deactivation behavior of the catalysts was explained by a pushing-pulling theory proposed in this study. A stable or suitable carbon-resistant catalyst should let the CO2 species on the basic sites can fully oxidize the carbon species formed from CH4 dissociation on the metallic sites. This study also revealed the incomplete dissociation of CH4 on the metallic sites, which may be the reason leading to lower H2/CO instead of reverse water-gas shift reaction. In CO2 hydrogenation reaction to synthesize methanol, the ZnO over-coated Cu/SiO2 catalysts prepared by the combination of strong electronic absorption (SEA) and the atomic layer deposition (ALD) to form catalysts with Cu nanoparticles surrounded by ZnO with different uniformity were studied. The catalyst activity was correlated with the number of metallic sites. The catalyst containing 5 wt% Cu over-coated with a single atomic layer of ZnO exhibited higher methanol selectivity. This catalyst has comparatively more metallic sites (Cu particles with uniform distribution) and basic sites (with uniform ZnO layer) formation, and the good contiguity between them both with their physical location and chemical interaction, which provided necessary synergy for the CO2 activation and hydrogenation to form methanol. In conclusion, the mechanism of CO2 catalytic activation was analyzed based on the CRM and CO2 hydrogenation reaction, especially on the aspect of contiguity of metallic sites and basic sites. For CO2 utilization, (a) enough and strong basic sites should be created on the catalysts for CO2 activation; (b) the properties of metallic sites can lead to different products formation (CO or coke formation in CRM, methanol or CO formation in CO2 hydrogenation reaction); (c) good contiguity of basic and metallic sites on the catalysts is necessary for these reactions to allow the activation of the reactants on these two catalytic sites so that they can reach and react with each other, further improving the stability and activity of the catalyst. These conclusions can be instrumental in developing effective catalysts for CO2 utilization in future