Controllable Fabrication Of Oxide Nanofibers And The Mechanism Research On Their Confining Effects For Stabilizing Supported Metal Against Sintering

Catalyst sintering,a main cause of the irreversible deactivation is a major concern and grand challenge in the general area of heterogeneous catalysis.The functionalization and performance optimization as well as the integration of structure and function of basic building blocks is extremely difficult and challenge for developing sinter-resistant catalysts.This dissertation designed and synthesized a series of electrospun mono-and dual-oxide nanofibers with unique structures as intriguing catalyst supports for precious metal nanoparticles.Due to the integrated physical and energetical confinements,including enhancing metal-support interactions,physical confining,constructing energy barriers and/or eliminating the difference of chemical potential between metal nanoparticles,the sinter resistance of metal/oxide nanofibers were highly improved.At the same time,with the help of advanced characterization methods,the mechanisms of the crystallization,grain growth,phase transition and sintering of both oxide nanofibers and the supported metal nanoparticles under thermal stress were systematically analyzed,offering new aspects for the design of sinter-resistant heterogeneous catalysts.The main content is as follows:(1)Without using complex coaxial electrospinning equipment or high-cost template,loofah-like?-Al₂O₃ hollow nanofibers were facilely prepared by single-needle electrospinning.Due to the pretend migration of Al(acac)₃ toward the surface of the nanofibers upon heating,a hollow and porous structure was readily obtained after calcination at 900?.The sintering resistance of 3 nm-Pt nanocrystals on the surface of Al₂O₃ nanofibers,as well as their catalytic performance were carefully investigated.Based on the results,the wrinkle surface of?-Al₂O₃ nanofibers effectively prevents the Pt nanocrystals from contacting and agglomerating in the Brownian motion at high temperatures.Moreover,the strong interaction between Pt and?-Al₂O₃ can increase the adhesion energy for Pt nanocrystals.Therefore,the 3 nm-Pt nanocrystals maintain their small size and octahedral structure after being calcined at 500?.Distinguished from traditional thermally stable catalysts with core-shell structure,Pt@?-Al₂O₃ catalysts do not sacrifice metal active sites at the expense of the material.After calcination at 500?,Pt@?-Al₂O₃ catalyst exhibited 4 times higher catalytic activity than that of free Pt nanocrystals toward the reduction of p-nitrophenol.Besides,the sinter-resistant Pt@?-Al₂O₃ was endowed with a high catalytic activity in the CO oxidation(Ea=107.81±0.93 k J/mol),which is competitive to commercial Pt catalysts.(2)Al₂O₃ heterostructures were in situ and controllably constructed on the surface of TiO₂nanofibers,accurately and facilely adjusting the surface roughness of composite nanofibers and thus the surface wettability.Upon thermal stress,Al₂O₃ spontaneously migrates to the surface of the nanofibers,resulting in an‘island in the sea'configuration.In this case,the naturally formed grooves between adjacent Al₂O₃ nanoparticles induced intriguing capillary effect,and thus drawing water toward inside and making the surface more hydrophilic.When under water,the surface of Al₂O₃/TiO₂were fully and quickly occupied by water,so that oil droplets are significantly repelled.Therefore,the Al₂O₃/TiO₂ composite nanofiber membrane can achieve a water-oil separation efficiency of 97.7%under gravity driving and a 98%dye capture efficiency simultaneously.Moreover,the excellent mechanical property of Al₂O₃/TiO₂ composite nanofibers enables it to be promising candidate as catalyst support.(3)The grain growth and phase transition of the aforementioned Al₂O₃/TiO₂ nanofibers were systematically studied.The results showed that a small amount of thermally stable Al₂O₃ can help inhibit the growth and phase transition of TiO₂ nanograins during high-temperature calcination.From a thermodynamic point of view,amorphous Al₂O₃ reduced the grain boundary energy of TiO₂nanograins and therefore weakened the driving force for sintering of TiO₂ nanograins.From a dynamic point of view,each TiO₂ nanograin is surrounded by Al₂O₃,hindering the diffusion between adjacent TiO₂ nanograins.Therefore,the agglomeration path of TiO₂ nanograin is effectively blocked.Finally,the Al₂O₃/TiO₂ nanofibers maintain a small grain size of about 22.2 nm and 47.1%anatase phase at high temperatures up to 900?.This strategy can be extended to a wide range of nanomaterials for inhibiting the undesirable grain growth and the corresponding poor performances.The sinter-resistant mechanism of 3 nm-Pt nanocrystals supported on Al₂O₃/TiO₂ nanofibers was systematically investigated.The high proportion of anatase TiO₂ provides higher adhesion energy and the high fraction of grain boundaries offers rich and homogeneous loading sites for Pt nanoparticles.In addition,two different oxides contributed energy barriers for neighboring Pt nanoparticles and thus effectively prevented them from migration and agglomeration.Due to the synergistic confining effects,3 nm-Pt nanoparticles can be immobilized on the surface of Al₂O₃/TiO₂nanofibers at an ultra-close proximity of 4.56 nm upon 500?.In the soot oxidation which is a common gas-phase high temperature oxidation reaction for exhaust emission control,Pt@Al₂O₃/TiO₂can still maintain a small size of about 6.32 nm after the reaction and effectively decrease the ignition temperature for soot particles.(4)The rough surface of electrospun TiO₂ nanofibers was controllably modified with rutile nanorods,and thus its accessible surface was modified wi th a high proportion of rutile{111}crystal facets.The uniform TiO₂ facets provide a balanced adhesion energy for Pt nanoparticles,effectively weakening the driving force for sintering.As a result,3 nm-Pt nanoparticles with a high areal density of 0.98 mg/m² remain their ultra-small size at 500? in the air.The catalytic activity of Pt nanoparticles supported on the rutile nanorods modified TiO₂ nanofibers toward the reduction of p-nitrophenol is 3 times higher than that of Pt nanoparticles loaded on the surface of unmodified TiO₂nanofibers.Moreover,after involving the soot oxidation at high temperatures,the Pt nanoparticles on the rutile nanorods modified TiO₂ nanofibers maintain the size of 6.99 nm.(5)Ultra-small Pt nanocrystals(3 nm)were finely and controllably encapsulated on the surface of potassium titanium oxide covered rutile-TiO₂ nanorods(K-TiO₂),due to the driving force of reducing the total surface energy of the system upon heating.Thanks to the combination of the physical confinement by support and the strong metal-support interaction,partially encapsulated 3nm-Pt nanoparticles can be well preserved upon 500? even in a sintering-promote oxidative atmosphere.Moreover,the partially-encapsulated structure enriched the three-phase boundary for heterogeneous reactions,which involves solid reactant,catalyst,and gaseous reactant.In-situ HRTEM observations showed that the support suffered from a severe densification and structural collapse at first,upon 700?,causing the Pt nanoparticles to separate from the support.In this case,the free Pt nanoparticles inevitably suffered from severe sintering.This phenomenon reveals the important role of the thermal stability of the support on the sintering resistance of a catalyst system,and provides an important supplement for the catalyst sintering mechanism.