Interface Regulation And CO(Preferential) Oxidation Performance Of 3d Transition Metal Composites

In the heterogeneous catalytic reaction,there are mainly the following steps:adsorption and activation of reactants,contact and reaction of activated molecules,and desorption of the product.The above reactions mainly occur at the interface of the catalyst.Therefore,the pathway,activity and selectivity of the reaction are controlled by the geometric configuration and the coordination environment of the interface.In addition,interfaces can alter the physical and chemical features of supported catalysts,which will further affect the adsorption and activation of the reactant,reaction intermediates,and desorption of the product etc.Thus,tailoring interface is being pursued to develop advanced catalysts for superior catalytic performance.CO（preferential）oxidation reaction,a model reaction to purify automobile exhaust and hydrogen,is expected to solve the energy depletion and environmental pollution simultaneously.3d transition metal composites have attracted considerable attention in CO（preferential）oxidation by virtue of adjustable electron density,flexible chemical states,abundant reserves and low price.However,the catalytic activity of these composites at low temperature is poor and the operation temperature window is narrow.The poor performance mainly originates from three primary factors:（i）the low concentration of active sites for CO adsorption on the surface;（ii）the impeded activation of oxygen;（iii）the too weak/strong adsorption strength of CO.These greatly limit its application in industry.Based on above considerations,this thesis focuses on the interfacial regulation of3d transition metal composites.Detailed investigation shows that in CO（preferential）oxidation,the performances of these composites can be greatly improved by optimizing the active sites,oxygen species and electronic structures at the interface.Moreover,the interface regulation is helpful to understand the complex surface catalytic process and the reaction mechanism of heterogeneous catalytic reactions.The main results obtained are as follows:1.Effect of graphene and the formation sequence of supports on the interface structure of CeO₂-Cu O_x-RGO composites.An interfacial regulating of CeO₂-Cu O_x-RGO composites was initiated through adding graphene and adjusting the addition sequence of components during the process of support formation.Results show that RGO is favorable for the generation of Cu⁺and the dispersion of copper-cerium species in the as-prepared catalysts,and the addition sequence of precursors directly controls the interface.In regulation of synthesizing for CeO₂-Cu O_x-RGO composites,the best catalytic performance was achieved when copper precursor was combined with graphene firstly to form CeO₂/Cu O_x-RGO catalyst.Its CO conversion reaches 100%at 140℃,and its operating window is140-220℃.The excellent catalytic performance of the sample is attributed to the high content of active species Cu⁺enriched on the surface,highly dispersed copper oxide clusters subjected to a strong interaction with ceria,and synergistic interactions between Cu-Ce mixed oxides and RGO,which all promoting the adsorption and activation of CO.2.Synthesis of CuCoO₂/CeO₂ catalysts with strong coupling interface.Layered p-type CuCoO₂ with cubic n-type CeO₂ was combined to construct a strong coupling interface.Taking advantage of unique layered structure of CuCoO₂ and excellent oxygen storage of CeO₂,interfacial enrichment and doping of Cuⁿ⁺species in CeO₂-CuCoO₂ catalysts were achieved via charge imbalance between Ce³⁺and Ce⁴⁺in the synthesis process.Through systematically regulating the content of CeO₂,the amount of enriched Cuⁿ⁺species and doping can be effectively controlled.Moreover,results of DRIFTS spectra suggest that intermediates are closely related to the amount of enriched Cuⁿ⁺species and doping.As the amount of enriched Cuⁿ⁺species increases,intermediates will undergo the following changes:carbonate/bicarbonate→formate→formate/carbonate.Through regulating the content of CeO₂,we obtained the excellent catalyst,70%CeO₂-CuCoO₂,with optimal structures and compositions of the interface.The sample with a number of oxygen vacancies and more active Cu O_x at the surface has shown a strikingly high catalytic oxidation activity（i.e.CO conversion more than 95.0%at 120-240 ℃）.3.Synthesis of Co₃O₄-CuCoO₂ composites with nanomesh structure.An interface-enhanced Co₃O₄-CuCoO₂ nanomesh was synthesized by a hydrothermal process using aluminum powder as a sacrificial agent.The synthesized nanomesh possess high-density nanopores,enabling a large number of CO adsorption sites Co³⁺exposed to the surface.Meanwhile,a large number of defects are generated in the etching process of aluminum.The defects promote electron transfer from O^2-to Co³⁺/Co²⁺and weakened bonding strength of Co-O bond at surfaces,thus optimizing the oxygen activation and redox ability of Co₃O₄.When tested as a catalyst for CO-PROX,this nanomesh with a lot of CO adsorption sites and an optimized oxygen activation,exhibits a strikingly high catalytic oxidation activity at low-temperature.Among Co-Cu-Al samples,the Co-Cu-Al-4-1-10 sample exhibits the best catalytic performance,which could realize 100%CO conversion at 100 ℃,showing a broad operation window up to 200 ℃.4.Synthesis of highly dispersed nanoalloy catalysts with specific interface.A series of highly dispersed nanoalloy catalysts with tailored interface was successfully fabricated by a topological transformation of Pt-loaded layered-double-hydroxide nanosheets.The morphology,chemical composition,and surface chemistry of the catalysts were precisely modified by adjusting the reduction temperature of LDH precursors from 200 to 650 ℃.When the reduction temperature is above 400 ℃,Pt⁰nanoclusters supported on amorphous Al₂O₃ will migrate and doping in Ni Fe alloy to form Pt Ni Fe single atom alloy.Modifying transition metals with Pt can effectively change the d-band center of transition metals,thus optimizing the adsorption strength of CO.Moreover,introducing monodispersed Pt atoms in Ni Fe alloys improves the oxygen activation and mobility,thereby improving its catalytic performance at the low temperature.When the topological transition temperature is 600 ℃,its catalytic performance is best,which could realize above 99%CO conversion at 100-400 ℃ in CO oxidation reaction.