Surface Modification Of Metal Oxides For Efficient Co₂ Electrochemical Reduction And N₂ Electrochemical Reduction

Sustainable development is the main issue for the human society's survival and prosperity in the 21st century.The exploration of high-activity catalytic materials and the innovation of environmentally-friendly energy conversion technologies are strongly demanded to meet the energy future.The electrochemical catalytic process can convert the electrical energy generated by renewable green energy resources?nuclear,solar,wind,and tidal?into chemical energy.The electrocatalytic process is also preffered for its facile operation,controllable reaction and mild condition.Electrochemical reduction reaction is an important component of the aforementioned electrochemical catalytic conversion process,including CO2 reduction reaction and N2 reduction reaction etc.At present,searching alternative catalysts with abundant resources,high efficiencies,low cost and good performances to replace the commonly used precious metal-based materials has become an irreversible trend in the industrialization of the electrocatalytic process.Transition metal oxides?TMOs?are widely used in industrial catalysts,owning to their low cost,environmental friendliness and desirable catalytic activity.However,compared with other transition metal composites such as nitrides,chalcogenides,carbides,phosphides,borides and silicides,TMOs are relatively less used in electrocatalytic reactions due to their lower electrical conductivity and poor chemical stability under acidic conditions.Hybridizing the transition metal oxides with conductive materials?carbon-based material,conductive polymer,inorganic material,etc.?can facilitate their electron-transfer kinetics.More efforts such as manipulation of morphology,size,composition,and crystal plane are adopted to further optimize their electrocatalytic activity.This dissertation mainly involves the following two sections:?1?PdTe_CeOx/C was prepared by ultrasonic-assisted wet chemical method,and a series of structural characterization and catalytic performance measurements were carried out.Due to the poor catalytic activity of bare ceria,metal doping and modification method was carried out.By controlling the Pd-CeO2 interface and doping with Te atoms,the process of electrochemical CO2 reduction to CO can be greatly enhanced.A very high mass activity of 92 mA mgPd-1?at-1.0 V vs.RHE?and a TOF value of 0.86 s-1 for CO formation were achieved.The Pd catalysts comprising ceria exhibited more positive onset potentials than the Pd catalysts in the absence of CeO2,enabling ECR to CO at-0.6 V.And the interface of CeO2 and Pd can significantly enhance the adsorption of CO2.The modified Pd catalyst also afforded an unprecedented CO faradaic efficiency of more than 84%at a low Pd loading?3 wt%?,which is 2.4-fold higher than the maximum FE of the unmodified palladium catalyst of similar size.Control experiments suggested that CeO2-based catalysts have better kinetics for ECR.DFT calculations revealed that CeO2 played an important role in promoting the CO formation thermodynamics by stabilizing*COOH and destabilizing*CO,and Te further enhanced the CO formation as well as suppressed competitive HER.?2?TiO2 with different oxygen deficiency contents were prepared by controlling of calcination temperatures under Ar gas,and the existence of oxygen defects was verified by a series of characterization methods?EXAFS,XPS,TPD,Raman,ESR,FTIR and TGA?.Specifically,the percentage of Vo in surface O species for the catalysts treated at 700,800,and 900? was calculated to be 18.4,32.2,and 25.6%,the total VO concentrations were determined to be 1.33%,5.5%and 4%,respectively.Electrochemical results showed that an oxygen vacancy-rich TiO2 can produce an NH3 yiled rate of 3.04 ?g h-1 mg-1 at-0.12 V?vs.RHE?,being 2.4,1.4 and 1.2 times higher than that of pristine TiO2,TiO2?Vo?₇00 and TiO2?Vo?₉00 samples.The TiO2?Vo?₈00 also exhibited a higher NH3 FE of up to 6.5%at-0.12 V,in contrast to 2.4,4.1,and 4.8%for pristine TiO2,TiO2?Vo?₇00,and TiO2?Vo?₉00,respectively.The electrochemical catalytic activity for NH3 formation can be summarized in the order of TiO2?Vo?₈00>TiO2?Vo?₉00>TiO2?Vo?₇00>pristine TiO2,which is perfectly consistent with the trend in oxygen vacancy concentration.The DFT calculation showed that the presence of oxygen deficiency sites can activate the*NNH formation?*N2+?H++e-??*NNH?and lower the free energy change for NRR considerably,as well as facilitate nitrogen chemical adsorption and activation of the N=N triple bond,favoring NRR.