Synthesis Of Transition Metal Catalysts With Promising Application In Catalytic Hydrogenation Of Carbon Dioxide

Unrestrained fossil fuel combustion is ascribed to be the main cause of increased concentration of CO2 in the atmospheric.Recently,a strong correlation has been developed linking the increased CO2 concentration with adverse climate change,such as global warming.As a C1 resource,CO2 utilization has received considerable attention,in which hydrogenation of CO2 is an efficient approach to reduce CO2 emission and protect environment.Highly efficient catalysts are necessary due to the inertia of CO2 and the limitation of reaction kinetics.To date,most research has focused on the CO2 hydrogenation over noble metal and Ni catalysts.However,noble metal catalysts still face difficulty in large-scale application for their high cost,despite the excellent hydrogenation performance.On the other hand,the Ni-based materials will deactivate due to metal particles sintering and carbon deposition.Thus,the development of transition metal catalysts with high catalytic activity and cycle stability is urgently needed.Herein,several catalysts with low cost are synthesized,and the catalytic properties of the catalysts are studied.The contents of this paper are as follows:We report the use of UiO-66 metal-organic frameworks to anchor ultrasmall Ni nanoparticles(NPs),thus avoiding the sintering of Ni NPs.Transmission electron microscope images and EDX mapping images show that Ni NPs with an average size of 2 nm are highly dispersed in UiO-66(Ni@UiO-66).Moreover,the obtained catalyst with an optimal Ni loading of 20 wt%displays an outstanding activity(57.6%of CO2 conversion)and high selectivity(100%)for methane in long-term stability tests up to 100 h at a reaction temperature as low as 300 ?.Compared with Ni/Zr02 and Ni/SiO2,the good dispersion of ultrasmall Ni NPs and low activation energies(Ea=68.9 kJ/mol)facilitate a high catalytic activity,which makes Ni@UiO-66 a promising catalyst for CO2 methanation.A flower-like CoAl-LDH has been utilized as a precursor to obtain aluminium oxide supported Co4N/Al2O3 and Co/Al2O3 catalysts by reducing in hydrogen and ammonia atmosphere,respectively.The catalytic hydrogenation performance of carbon dioxide was evaluated in a fix-bed reactor.The catalysts were characterized by nitrogen adsorption and desorption,SEM,TEM and XPS techniques.Results showed that the Co4N/Al2O3 catalysts composed of small particles had higher activity than Co/Al2O3 at low temperature.The conversion of CO2 was 68%under the 24000 mL h-1 g-1 GHSV at 280 ?.More importantly,a promising selectivity to methane(even as high as 100%)was obtained at a temperature range of 160-360 ?,which was much better than that of Co/Al2O3.There was no obvious decrease of CO2 catalytic conversion and product selectivity during the stability test over 100 h.With the introduction of nitrogen,the catalytic activity and stability of the catalyst can be greatly enhanced.In addition,with the help of Al2O3 support,the structure could be maintained during the reaction process,thus,avoiding the sintering and agglomeration of the metal particles.This work provides an important strategy for catalytic methanation of CO2 at low temperature.Carbon supported Co/C and Co4N/C nanocatalysts were prepared by using ZIF-67 as precursor and the performance of CO2 hydrogenation were investigated under mild conditions.The carbon coating on the surface of catalysts formed during pyrolysis could protect active metal from sintering ang agglomeration.The catalytic results showed that the as-prepared Co/C nanomaterials exhibit excellent catalytic activity and selectivity of CH4 than commercial metallic Co powder.Benefit from the large specific surface area,the stable structure and the introduction of N,catalyst of Co4N/C exhibited excellent performance in hydrogenation of carbon dioxide to methanol.It is mainly attributed to the fact that the Co4N on the surface of the nanomaterial could combine with dissociated H and be reconstructed into Co4NHx,which greatly promoted the combination of CO2 molecules to form HCOO?.This work has a broad application prospect in the conversion of carbon dioxide to valuable chemicals.