Preparation Of CuO Catalyst By Template-assisted Hydrothermal Synthesis And Its Electrochemical Reduction Of CO₂ To CO Performance

The excessive depletion of fossil fuels has resulted in the severe global warming and energy crisis.Electrochemical CO₂ reduction to value-added chemicals and fuels using renewable energy could be a win-win strategy to solve these issues.The strategy could achieve the recyle and utilization of C1 resource and the efficient storage of clean energy,which is critically important for future carbon netural society.Cu-based catalysts,as the only proven metal catalysts capable of efficient electroreduction of CO₂ to multiple products,have attracted extensive attention from scholars at home and abroad.However,Cu-based catalysts still face the challenges such as low CO₂ reduction activity and limited selectivity towards desired products.Therefore,it is vitally important to promote the electrochemical CO₂ reduction performance of Cu-based catalysts by improving their microstructures.In this thesis,CuO catalysts were prepared by template-assisted hydrothermal synthesis method.We aim to optimize the employed templates and calcination temperature in hydrothermal synthesis to regulate the microstructures of CuO catalysts,to improve their electrochemical CO₂ reduction(ECR)performance.A three-electrode electrocatalytic reaction system was employed to study the ECR performance of the CuO catalysts.The as-synthesized catalysts were characterized by different techniques.The effects of template and calcination temperature on the structure and ECR performance of the CuO catalysts were investigated.The possible deactivation mechanisms of the CuO catalysts in long-term CO₂reduction tests were discussed and reactivation methods for restoring the ECR poermance were proposed.The major advances and conclusions of this work are aummarized as follows:1.Effect of template on the structure and ECR performance of the CuO catalystsThe CuO catalysts were prepared by hydrothermal synthesis using PVP,SDS,F127and P123 as templates.Effects of template type and content on the structure and ECR performance of CuO catalysts were investigated.Results indicated that the surface morphology,average particle size,the concentration of oxygen-vacancy defects and ECR performance of CuO catalysts depended on the different templates.For CuO-PVP and CuO-SDS,the nanoparticles(NPs)were uniformly distributed on the surface,and they showed relatively smaller average particle sizes,which would endow the catalysts with more exposed active sites for ECR.Besides,the CuO-PVP and CuO-SDS catalysts also owned enriched oxygen-vacancy defects,which were favorable for CO₂ adsorption and activation over the nanostructured catalysts,and they therefore exhibited better ECR performance.The effect of template content on ECR performance was associated with the particle size effect.CuO-PVP-25 with template content of 25 wt.%exhibited the minimum average particle size of 29.53 nm,and the smaller particle size would offer more low-coordination defects for facilitated CO₂ adsorption and activation and enhanced ECR performance.2.Effect of calcination temperature on the structure and ECR of the CuO catalystsWe further investigated the effect of calcination temperature on the structure and ECR performance of the desired CuO-PVP-25 catalysts.Results indicated that calcination temperature significantly affected the surface morphology,particle size and the concentration of oxygen-vacancy defects.The CuO-PVP-25 catalysts calcined under different temperatures were capable of reducing CO₂ to form syngas with tunable CO/H₂ratios in the range of 1:2-2:1.Both the CO₂ reduction activity and CO selectivity increased first and then decreased with the increasing calcination temperature.Under the applied potential of-0.93 V vs.RHE,the desired CuO-PVP-25-400 catalyst exhibited high reduction activity and good CO selectivity with a high ECSA normalized CO partial current density of 1.44 m A/cm² and a great CO Faradaic efficience of 48.2%.The CuO-PVP-25-400 catalyst showed good surface morphology with spherical NPs uniformly distributed.In addition,the small particle size and abundant oxygen-vacancy defects promoted CO₂ adsorption and activation for enhanced CO₂reduction activity.3.Deactivation mechanisms of the CuO catalysts and reactivation methods for restoring the ECR poermanceThe stability of the desired CuO-PVP-25-400 catalyst in CO₂ reduction tests was investigated.Results indicated that the desired catalyst suffered reduced activity after 2 hours operation.The possible deactivation mechanisms were proposed as the following aspects.Fisrtly,the active CuO species would be reduced to Cu₂O and metallic Cu in CO₂ reduction tests.Secondly,the spent CuO catalyst showed inferior morphology with aggregates and blocks,and the increased pariticle size corresponded to lost active sites.Moreover,the adsorbed and activated CO₂ species would interact with the CuO surface to form surface carbonate species,and this would result in the coverage of the active sites,and the CO₂adsorption and activitaion processes would be hampered.Offsite oxidation method(thermal annealing)was proposed to reactivate the spent CuO catalyst to restore its ECR performance.Results indicated that thermal annealing worked satisfactorily in re-oxidizing the Cu₂O species in the spent catalyst to CuO active species,and this could be favorable for partially restoring the ECR performance of the spent CuO catalyst.