Research Of Nickel Foam Structured Catalysts For CO₂Hydrogenation

Carbon dioxide, primarily generated by combusting fossil fuel, is themost important and abundant greenhouse gas which badly affects oursurvival environment. Pressed by the imperative to avoid furtherirreversible damage to the environment resulting from greenhouse effect,many scientists are focusing on the possibility of “CO₂recycle” all overthe world. Among the viable strategies of running “CO₂recycling” toreduce carbon dioxide emission, the use of CO₂as reagent seems veryattractive for producing valuable chemicals.Catalyst research is a key step to achieve the cyclic utilization ofcarbon dioxide. Synthesis of methanol from CO₂hydrogenation is a hotspot in CO₂chemistry which is always focused on by many scientists. Todate, most of the methanol catalysts from CO₂hydrogenation reported areCu-baesed catalysts （especially Cu-ZnO-based and Cu-ZnO-ZrO₂-based）;noble metals, like Pd-baesd catalysts, have high catalytic activity andselectivity in synthesis of methanol from CO₂hydrogenation. However,besides the catalytic activity and selectivity, energy consumption must betaken into consideration in industry. Thus, how to achieve a high activity and selectivity catalyst with low energy consumption is a significanttechnology in this area.Another forefront, carbon nanofibres （CNFs）, as a novel nanocarbonmaterial, have been drawing increasing attention in science area recently.CNFs are nano-sized filamentous carbon materials with a hollowconstruction similar to that of graphite fibres. CNFs possess excellentphysical and chemical properties, such as considerable length-to-diameteraspect ratio, high surface area, high mechanical strength, and highelectrical conductivity, thus they are expected to be promising materialsin a wide variety of technological fields. Numerous transition metals,including Ni, Co, Fe, Cu and their alloys, are the most commonly usedcatalysts for the growth of CNFs. hese metals are almost used asnano-sized particles, on which CNFs can be synthesized catalytically bydecomposing hydrocarbons or CO. Few reports have referred that CNFsare prepared via CO₂hydrogenation, thus it has theory significance andrealistic values to discuss the synthesis of CNFs from CO₂hydrogenation.Structured catalyst is a type of catalyst which has exhibited apromising application due to their perfect specific surface area, thermalconductivity, mechanical behavior and low pressure drop. These excellentproperties are in favor of the mass transfer and the heat transfer inchemical industry, leading to higher thermal efficiency, less energy lossand smaller reactor volume. This work has designed two type of structured catalysts for preparing methanol and CNFs from CO₂hydrogenation, respectively. The research result as follows:1. Research on synthesis of methanol from CO₂hydrogenationCu-ZnO/ZrO₂/Al₂O₃/SN catalyst was prepared via wash-coating andwet impregnation methods. The result indicated that viscosity of theAl₂O₃layer coated on the framework of nickel foam and adhesionintensity of catalyst powder coated on the Al₂O₃layer were best whileadding4wt%polyvinyl alcohol （PVA） into the Al₂O₃layer. The structuredcatalysts remained good framework structure by this preparation.Calcined temperature and metal molar ratio had effect on the catalyticacivity. When the calcined temperature was634K, Cu/Zn/Zr molar ratiowas3:2:1, the catalyst had higher activity. Highest space-time yield (STY,0.071g_CH3OH·g_cat^-1·h^-1) and the selectivity to methanol (S_CH3OH,28.0%)were achieved in optimized conditions as follows: the reactiontemperature was513K, the reaction pressure was3.0MPa, and the GHSVwas5600mL h^-1 g_cat^-1.2. Research on synthesis of CNFs from CO₂hydrogenationCNFs were prepared by catalytic hydrogenation of CO₂over thenickel foam at normal pressure. Pieces of nickel foam were heated at973K for1h in air. The CNFs were synthesized by feeding a mixed gas flow（CO₂:H₂=1:3, molar ratio） with a rate of30mL/min. The catalyst played a best catalytic performance at773K, with a STY of0.01g_carbong_cat^-1·h^-1.The reaction products were characterized by SEM, HRTEM, EDS andXRD. The SEM and HRTEM images of the products indicate that theCNFs products show multi-direction morphology with a diameter lessthan200nm. The CNFs are attached to the nickel foam. Themicrostructures of CNFs belong to fishbone type.