Manipulation Of Product Selectivity In Representative Organic Chemicals And CO₂ Catalytic Conversion

Energy crisis and environmental polluttion are two major problems faced in the 21st century.Converting solid waste like biomass,water pollutant and CO2 into useful chemical product or fuels will not only solve the increasingly serious environmental pollution,but also ease energy crisis and achieve sustainable development.To achieve product selectivity is the key issue of waste catalytic conversion.In this thesis,waste catalytic chmical conversion mechanism and product selectivity were explored.Catalytic conversion of wastes and pollutants was achieved by means of electrochemistry,catalyst structure,surface modification,and reaction molecule morphology.Manipulation of product selectivity in catalytic waste conversion could improve convesion efficiency and product selectivity,and thus promote the recycling of waste and pollutants.Main contents and results of this thesis are as follows:1.Density functional theory study of furfural electrochemical oxidation on the Pt(111)surface.Electrooxidation of furfural may allow for tunability of product selectivity by varying the electrode potential.We have applied density functional theory(DFT)to investigate the electrocatalytic oxidation mechanism on the Pt(111)surface.The potential-dependent reaction free energy profiles for furfural electrocatalytic oxidation to succinic acid,maleic acid,and maleic anhydride are reported.Comparing a number of possible furfural oxidation paths,electro-oxidation is found to preferentially proceed through furoic acid to succinic acid via 2(3H)-furanone as an intermediate and to maleic acid and maleic anhydride via 2(5H)-furanone as an intermediate.The rate of these processes is likely limited by the C-OH formation to form furoic acid.Selectivity between succinic acid and maleic acid products may be tuned by varying the electrode potential.Succinic acid is more likely to be formed at intermediate overpotentials once an appreciable oxidation rate is attained,with maleic acid selectivity driven at larger overpotentials.These results broaden our fundamental understanding into electrocatalytic oxidation of furfural in upgrading renewable biomass derivatives and provide guideline for tuning product selectivity.2.Mechanistic roles of catalyst surface coating in nitrobenzene selective reduction:A first-principles study.Adsorbed organic modifiers can alter selectivity of metal catalysts by modifying reactant,intermediate,or product adsorption affinities and configurations.Herein,we show how alkylamine self-assembled monolayers with varying surface densities can be used to tune selectivity to desired hydrogenation products of nitrobenzene(NB)reduction on a Pt(111)catalyst.Nitrobenzne is a toxic environmental pollutant with deleterious health effects,and its selective conversion to valuable chemicals can both convert this pollutant and improve catalytic process efficiency.DFT calculations demonstrate that the selectivity of NB reduction to phenylhydroxylamine(PHA)is achieved by controlling the surface crowding,with specific sites exposed for the selective reduction of NB on the Pt(111)surface through the selection of alkylamine modifier surface density.Surface crowding forces NB and subsequent reaction intermediates to bind with their long axis vertical to the Pt(111)surface,increasing the selectivity to the desired product,PHA.This surface crowding serves both to enhance selectivity and provide insight into the reaction mechanism of NB reduction.3.Selective reduction of nitrobenzene to azoxy compounds.Aromatic azo compounds are widely used in production of dyes,food additives and pharmaceutical products.Direct catalytic reduction of nitroaromatic compounds to corresponding azoaromatic compounds will be more controllable,efficient and environmentally friendly.In this study,Au nanomaterials were used as a model catalyst to expolre thermodynamic and kinetic properties of NB reduction process and the role of active surface H*in the reduction process of NB.DFT calculation results show that active surface H*on Au(111)surface can enhance the catalytic activity of NB reduction process to AN and azobenzene products.Coupling of nitrobenzene and hydroxyaniline to produce azobenzene was the most thermodynamically stable process among all the coupling process on Au(111)surface.Nitrobenzene tends to be reduced to azocompound on Au(111)surface mainly because potential energy surfaces of NB molecule and reaction intermediates in NB reduction process on Au(111)surface are flat.This could make NB molecule and reaction intermediates more favorable to move on Au(111)surface and lead to the coupling of two aromatic rings.4.Catalytic CO2 reduction to valuable chemicals using NiFe-based nanoclusters:A first-principle theoretical evaluation.Converting CO2 into valuable chemicals and fuels is one of the most practical routes for reducing CO2 emissions while fossil fuels continue to dominate the energy sector.Nobel-metal-free NiFe bimetal nanoparticles have shown the good catalytic activity in CO2 conversion.Herein we theoretically evaluated the catalytic performance and possible mechanisms of NiFe-based nanoclusters for hydrogenating CO2 to form formic acid and CO through bicarbonate by using a periodic and self-consistent DFT simulation.The theoretical results illustrated that the NiFe nanoclusters could have a good catalytic activity and selectivity for HCO3-reduction to formic acid and the possible pathway is that HCO3-preferred to react with adsorbed H atoms of H2 on NiFe alloy nanoclusters through the carbon atom site.Moreover,the NiFe alloy nanoclusters with Fe atom exposed on the surface of Ni cluster showed the better performance with a lower energy barrier compared to that with Fe doped in the corner of Ni cluster.However,the generation of CO from HCO3-reduction was shown to be neither thermodynamically nor kinetically favorable on NiFe alloy nanoclusters.Additionally,the simulation results also suggested that it was thermodynamically unfavorable for a further hydrogenated reduction of formic acid to formaldehyde on NiFe alloy nanoclusters themselves as well as supported on graphene.In summary,a molecular-level insight of CO2 reduction to valuable products on NiFe nanoclusters was offered in this study,which may provide some useful information for guiding the design of NiFe-based catalytic materials for efficient CO2 conversion to useful fuels.