Design,preparation Of Catalytic PEO Coatings With High Porosity And Their In Situ Growth In Microchannels

Catalytic science and technology is closely related to human life.The development and application of advanced catalytic materials and processes are of great significance to the sustainable development of human civilization.Plasma electrolytic oxidation?PEO?is a simple and environmentally friendly technology for preparing oxide film with improved surface performances in terms of strong adherence and good corrosion resistance.Moreover,a tailored PEO film can be obtained by adjusting reaction parameters.In recent years,PEO technique is attracting increasing interest for its use in catalysis and is thought to be a potential approach for fabricating catalyst for microreactors.However,the actual application of such film in catalysis is limited due to its low porosity,and the use of PEO coating in microchannel reactor is rarely reported.Therefore,the present work mainly focuses on the design and preparation of catalytic PEO film with high porosity.Besides,the in-situ growth of such coatings in microchannels is also studied.The following research has been carried out:?1?For the purpose of producing PEO film with high porosity,the idea that ?keeping the coating thickness at a low value,maintaining the discharge sparks at a ?high density,small size,low energy‘ state? was proposed.Firstly,the PEO treatment was conducted in an alkaline silicate electrolyte.The influence of electrolyte composition on film formation process was investigated.By designing and optimizing the reaction conditions,PEO films with high porosity were formed and the corresponding formation mechanism was proposed.Secondly,a high porous PEO coating was achieved in alkaline aluminate solutions by optimizing the reaction conditions,i.e.electrolyte and electrical parameters.The structure,composition and catalytic performance of the obtained film were analyzed.The results showed that: 1)Na2SiO3 and NaAlO2 can cause strong passivation of substrate,which contributes to the film formation process.On the contrary,KOH provides fast dissolution of substrate and oxide film,which would suppress the growth of PEO film.2)The dissolution effect caused by KOH was dominant during the PEO treatment.The discharge sparks were large in quantity and small in size,resulting in a thin and porous PEO film,surface of which was completely covered by nanoparticles.3)The coating porosity increased with increasing time,and the film treated for 60 min exhibited the highest porosity?33.3±1.7% for alkaline silicate solution and 30.5±1.3% for alkaline aluminate solution?.The film treated for 120 min showed an outer dendritic and inner porous structure.4)The high porous PEO film was formed by the long term modification by tiny sparks and the dissolution effect caused by electrolyte.The film formation mechanism involves a dynamic balance theory that contains gas liberation,coating formation and coating dissolution process.5)The nanoparticles had similar shapes and sizes and were in well-dispersion,and its formation process can be described as follows:the nanoparticles were formed from the insoluble species,which were first incorporated into the film during the discharge process and then exposed on the surface because of the dissolution of alumina.6)In alkaline aluminate solution,the PEO film treated for 120 min exhibited the best catalytic performance that 78.2% of methyl orange was removed within 10 h.?2?A composite photocatalytic film containing ZnO/Al₂O₃ was prepared by coupling PEO with electrodeposition technology.The morphology,composition,catalytic performance and formation process of film were studied.The results that: 1)the PEO carrier film showed high porosity and mainly consisted of Al₂O₃.The sheet-like ZnO crystal can be uniformly loaded on the surface of the PEO carrier film using electrodeposition technique.The size and thickness of ZnO crystal increased with increasing electrolyte concentration,and prolonging the duration was helpful to improve the ZnO content of the composite film.2)The sample deposited for 90 min showed the highest photocatalytic activity that 86.3% of the methyl orange was removed within 6 h.Furthermore,the specimen treated for 60 min exhibited the best comprehensive performance due to its good catalytic activity?86.3% of the methyl orange was removed within 6 h?and short preparation time.?3?The feasibility of the in-situ fabrication of high porous catalytic PEO film within microchannels was investigated.The surface morphologies of the PEO films formed inside and outside microchannel were comparatively studied,and the influence of microchannel geometry on coating formation process was studied.The results showed that: 1)PEO coatings with high porosity are able to grow in microchannels.The sample treated for 60 min showed a high porosity of 32.4±0.5% and was evenly covered with nanoparticles.2)The influence of microchannel geometry on the formation of PEO film is mainly reflected in two aspects,namely ?shielding effect" and "mass transfer effect".The shielding effect would weaken the electric field in microchannel,resulting in discharge sparks with low energy.Hence,the PEO film inside microchannels showed lower pore size and higher porosity.The mass transfer effect would weaken the mass transfer process inside microchannels and reduce the erosion strength of electrolyte on coaitng surface.It would help the insoluble particles accumulate on coating surface and be incorporated into the film with the aid of discharge sparks.