The Investigation Of Cu-Based Catalysts For Electrochemical CO₂ Reduction To CH₄

Since the industrial revolution,the world’s economic development and human life have been extremely dependent on the fossil fuels,which has led to the emission of large amount of CO₂ and the growing problem of the greenhouse effect.Therefore,it is urgent to adjust the energy structure and develop a low-carbon and environmentally friendly life style.Meanwhile,feasible methods should be adopted to fix the CO₂in the atmosphere or emission from daily production and life,so as to achieve the goal of energy saving and emission reduction.Using renewable power systems such as wind and light energy,electrochemical reduction of CO₂（CO₂RR）into high value-added products（such as CO,CH₄ and C₂H₄,etc.）can not only deal with the utilization of sustainable energy,but also convert CO₂ into industrial chemicals and fuels so as to build carbon-neutral process.However,the stable molecular structure of CO₂ results in a slow reduction kinetics,so it is extremely important to develop efficient and stable catalysts for CO₂ conversion.So far,the catalysts reported to catalyze the deep electrochemical reduction（>2e^-）of CO₂ to produce hydrocarbons and C₂₊oxygenated compounds are mainly Cu-based materials,because the adsorption energy of*CO（the key intermediate in the deep reduction of CO₂RR）over Cu is moderate and locating near the optimal value based on Sabatier’s rule.That makes it possible of*CO hydrogenation and coupling to produce the reduction products beyond 2e^-such as methane（CH₄）,ethylene（C₂H₄）,ethanol（C₂H₅OH）,etc.However,Cu-based catalysts commonly undergoes 2～18e^-reduction processes accompanied with complicated and diverse reaction pathways and intermediates,resulting in a low selectivity of the single product of CO₂RR,which is not conducive to the product separation and purification.Therefore,it is urgent to develop modified Cu-based catalysts to promote the faraday efficiency of the single CO₂RR product.In this dissertation,we take the advantages of a well-defined Cu-N₄single-site structure and strong adsorption sites at the Cu/CeO₂ interface to modulate Cu-based catalysts for electrochemical CO₂reduction to CH₄ with highly selective conversion.The reasons for their high CH₄ selectivity are also discussed.The main research of this dissertation is as follows:1.The catalysts with Cu-N_x active sites have shown promising catalytic activity in electrochemical CO₂ reduction reaction（CO₂RR）.However,it remains elusive to systematically modulate the CO₂RR energetics and activities of Cu-N_x sites.It is highly urgent to establish a quantified correlation between CO₂RR performance and the molecular structure,while specific descriptor is demanded.Herein,substituted copper phthalocyanines with various electron-withdrawing/donating groups（Cu Pc-R,R=-NH₂,-OH,-H,-Cl,COOH and-Cl₄）are prepared applied to CO₂RR to CH₄.Notably,a disparity of 0.28 V for the Cu^2+/1+redox potential of Cu-N₄ center is observed by changing various substituents of Cu Pc-R catalysts,where the increased redox potential is associated with larger Hammett constants（i.e.,greater electron-withdrawing）of substituents which contributes to decreased electron density of Cu-N₄ centers.Moreover,the CO₂RR-to-CH₄activity of the CNT loaded Cu Pc-R catalysts,labeled as Cu Pc-R/CNT,is basically enhanced constantly as the substituents change from the electron-donating effect to the strong electron-withdrawing effect.As a consequence,an approximately linear relationship is observed between CH₄ production activity and the Hammett constants of the functional groups,in which Cu Pc-Cl₄/CNT with the strongest electron-withdrawing substituent shows the best CO₂RR-to-CH₄ performance with maximum faradic efficiency(FE_CH4)of 73.7%and partial current density(j _CH4)of-147.4 m A/cm² at the total current density of-200 m A/cm².This versatile strategy can be further applied to systemically construct M-N₄ based catalysts for CO₂RR and beyond.2.Cu has unique advantages that can catalyze the electrochemical reduction of CO₂to hydrocarbons and oxygenates.However,the complex and diverse CO₂RR products over Cu is not conducive to subsequent separation and purification.Herein,we design a series of reversed-phase catalysts that are Cu-loaded CeO₂（Cu-CeO₂-x）to generate highly active Cu/CeO₂ interfacial sites.The interfacial sites can enhance the adsorption and activation of CO₂and intermediates such as*CO and thus improve the selectivity of deep-reduction products.Meanwhile*CHO is verified to be the common key intermediate for the production of C₂₊and CH₄ of CO₂RR over these catalysts,while OC*CHO was another key intermediate for CO₂RR to C₂₊products.We speculate that Cu/CeO₂ interfacial sites have strong adsorption and activation ability for intermediates*CO which is prone to generate*CHO intermediates,while pure Cu sites have relatively weak adsorption ability.The strongly adsorbed*CHO over moderate amount of Cu/CeO₂interfacial sites tends to couple with the weakly adsorbed*CO over adjacent Cu sites with enough amount to produce OC*CHO and promote C₂₊production,while more Cu/CeO₂interfacial sites accompanied with decreased pure Cu sites significantly lower the coupling probability of*CHO and*CO.The strong activation ability of Cu/CeO₂interfacial sites strongly leads to multiple hydrogenations of*CHO to produce CH₄.As a consequence,Cu-CeO₂-5%exhibits the best C₂₊selectivity with a maximum FE_C2+of62.6%and a relative j _C2+of-250.4 m A/cm²,while Cu-CeO₂-20%has the best CH₄selectivity with a maximum FE_CH4of 51.3%and a corresponding j _CH4of-153.9 m A/cm².3.The nanoconfinement effect of porous catalysts can significantly improve the enrichment and conversion efficiency of gas-phase catalytic reactants and intermediates.Here,a series of Cu-based catalysts（Cu/m CeO₂-x,x=1%,3%,5%,wt%）supported on mesoporous CeO₂microspheres(S_BET=142.4 m²/g)are synthesized and applied to electrochemical CO₂ reduction.The results show that Cu is relatively evenly distributed on the surface and channels of mesoporous CeO₂（m CeO₂）accompanied with the generation of Cu/CeO₂ interface sites.The Cu/m CeO₂-x catalysts with abundantly open pores can significantly enhance the adsorption capacity of CO₂,thus promoting the enrichment of CO₂ near the Cu active sites.As a result,the HER side reaction is inhibited and CO₂RR selectivity is significantly enhanced.Finally,Cu/m CeO₂-3%(S_BET=127.4m²/g)with moderate Cu loading shows the best CH₄ selectively with a maximum FE_CH4of 71.8%,and relative j _CH4 of-215.4 m A/cm² as well as a maximum j _CH4 of-308.2m A/cm².The Cu-based catalyst loaded on mesoporous C microspheres with same Cu content is further synthesized to make the comparison(Cu/m C-3%,S_BET=263.4 m²/g).Even though Cu/m C-3%has significantly higher specific surface area,the chemisorption strength of CO₂ at m CeO₂and the interface of Cu/CeO₂is much stronger than the physical adsorption effect over Cu/m C-3%.Therefore,the CO₂ adsorption capacity of Cu/m CeO₂-3%is much stronger than that of Cu/m C-3%.As a result,Cu/m CeO₂-3%shows much higher CH₄ selectivity than Cu/m C-3%.