| Supported noble metal catalysts have a wide range of applications in the field of chemical industry,and their excellent catalytic performance not only comes from their unique electronic structures,but also from their carriers.Oxides were often used as supports to improve the dispersion of noble metal heterogeneous catalysts.Meanwhile,the electronic structure of noble metal can be affected by oxide due to the formation of the noble metal-oxide interface.The reaction conditions catalyzed by noble metals are often milder compared to other metals,and thus the metal-oxide interfacial effect is particularly prominent.As such,rationally engineering interfacial structure of oble metal catalysts on oxide supports is a very effective strategy for improving the activity and stability of catalysts.In this thesis,the interfacial effects among noble metals catalysts were studied.Based on the controlling of metal-oxide interfacial structure,we have constructed a series of supported noble metal catalysts,including atomically dispersed Pt/CeO2,Ir/CeO2 with subsurface doped interface,Pt/Fe2O3 with different faceted Fe2O3 as support,and a TiO2/Au interface.Catalytic oxidation reaction of CO had been chosen to evaluate these "interfaces",and the structure-activity relationship was studied.The main results and conclusion are summarized as follows:1)Atomically dispersed metal catalysts often exhibit a superior performance than nanoparticle catalysts in many catalysis processes.However,these so-called'singleatom' catalysts have a consistently low loading density(<0.5 wt%)on the support surface and easily aggregate at high temperatures,hindering their practical application.Herein,we demonstrate a facile surface engineering protocol using molecule-surface charge transfer adducts to fabricate highly stable noble metal catalysts with atomic dispersion,using a Pt/CeO2 catalyst as an example.The key of this approach is the generation of an adequate amount of Ce3+defective sites on the porous CeO2 surface through the adsorption of reductive ascorbic acid molecules and a subsequent surface charge transfer process.Subsequently,noble metal Pt atoms can be well-dispersedly anchored onto the generated Ce3+sites of porous CeO2 nanorods with a loading density of up to 1.0 wt%.The as-prepared highly dispersed Pt/CeO2 catalyst showed outstanding catalytic activity at near room temperature toward CO oxidation,with excellent stability over several days,which is far superior to the traditional impregnation-prepared catalysts,the activity(complete conversion at 90?)of which is severely decayed within a couple of hours.2)The competitive adsorption behavior of CO with H2 and O2 on noble metal catalysts usually leaded to a poor selectivity in PrOx reaction,especially under a high temperature region.To solve this problem,a simple modified co-deposition method was proposed to achieve the doping of highly dispersed Ir atom clusters on the superficial surface of porous CeO2 nanorods.Our results demonstrated that compared with the catalyst prepared by traditional impregnation method,the as-prepared Ir/CeO2 catalyst doped with Ir atoms in the superficial layer showed a higher activity and an amazing wide working temperature range,with keeping a CO conversion of>99.7%from 100 to 140?.Meanwhile,the catalyst exhibited a great stability in PrOx reaction,with the high conversion maintaining for more than 22 hours under 100? time on the stream test.Based on CO adsorption DRIFTS spectra,it was found that CO and O2 were competitively adsorbed on Ir-O or Ir-OH sites,resulting in a unique selectivity and high stability toward PrOx reaction of CO.3)The exposed crystal facets of oxide support have been proven to greatly affect the catalytic performance of noble metal catalyt,especially in the catalytic reaction under the reducing atmosphere.However,the mechanism behind the facet effect from the support is still unclear to date.Herein,the sintering processes of Pt nanoparticles on two high energy facets of hematite support({113} and {012})was systematically studied,with preferential oxidation of CO as a model reaction that is sensitive to the size of noble metal catalysts.With the increase of reaction time and reaction temperature,the size of Pt nanoparticles on the two Fe2O3 surfaces tended to increase,but the morphology change exhibited a completely different trend.The surface reconstruction of {012} facets leaded to the oriented attachment of Pt nanoparticles along the direction of[111],forming a rod-like morphology.By contract,the surface reconstruction of {113} facets formed small pits aound Pt nanoparticles,which enhanced the "Strong Metal Support Interaction" effect between Pt and Fe2O3.This effect inhibited the aggregation of Pt nanoparticles to some extent and prevented them from deactivation in the stability test.It can be concluded that the anisotropy between{012} and {113} facets of ?-Fe2O3 resulted in different surface reconstruction behaviors in reaction atmosphere,which strongly affected their catalysis performances of Pt/Fe2O3(especially the catalytic stability).4)Spherical Au nanoparticles exhibited a poor activity toward CO oxidation,even if their particle size had been reduced to<5 nm.Herein,a small amount of TiO2 was deposited on spherical Au nanoparticles with the size of about 3 nm to improve catalytic activity.According to TEM characterization,the deposited TiO2 was located between Au nanoparticles and P25,which makes Au nanoparticles bond to the support more firmly.After the Au-TiO2 interface was formed,the CO oxidation performance over the resulting catalyst was greatly enhanced,with the activity up to about 0.4 s-1.The excellent performance was not only attributed to the new interface between TiO2 and Au nanoparticles,but also it was related to its good space dispersion among P25 supports.According to the XPS analysis,we proposed that the metal-oxide interfacial effect mainly enhanced the ability for oxygen activation,rather than changing the electronic structure of Au nanoparticles. |
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