Study On Preparation, Characteristics Of MWNTs And TiO₂ Composite Catalyst With Its Photocatalytic Redox Properties

Photocatalysis science is a rising cross-disciplinary science,which integrates the multi-disciplinarity of chemistry,physics,materials,environment and so on.From the development status quo,photocatalysis can be broadly divided into two major research fields:one is energy photocatalysis(photocatalytic hydrogen production from splitting water,that is photocatalytic hydrogen production from renewable resources) and the other is environmental photocatalysis(advanced oxidation technology in environmental pollution field).These two research fields are so closely related to the sustainable development of society and economy that they have broad application prospects and enormous social and economic benefits.Through a variety of means to improve the photocatalytic activity and increase the utilization of sunlight is the important step to make this technology come true.In recent years,some papers reported multi-wall carbon nanotubes(MWNTs) and TiO₂ composite catalyst has a good photocatalytic activity for oxidation organic compounds.MWNTs introduced into TiO₂ matrix by Sol-gel method will produce the synergy effect between MWNTs and TiO₂.The effect is of great benefit to the photoelectron transfer from conduction band of TiO₂ to MWNTs,which can elong the life-span of e^--h⁺ pair.In this paper,we supposed the synergy effect will be equally effective to photocatalytic hydrogen production from splitting water.In order to confirm that idea,we first investigated the photocatalytic activity of hydrogen production for MWNT-TiO₂ composite catalyst,which was prepared by same Sol-gel method as reported.It was found that doped MWNTs will stop phase transition and growing up of TiO₂;With increasing of MWNTs content in composite catalyst,their surface area are also increased;When the doped amount of MWNTs is 5%and co-load amount of metal Ni is 1%,under UV light irradiation,the photocatalytic activity reaches the maximum of 122μmol/h which is almost twice than the activity of raw TiO₂:Ni.Furthermore,the result that activities of MWNT-TiO₂:Ni series are generally increased by doped MWNTs shows there is a certain interaction between MWNTs and TiO₂ directly impacting on the photocatalytic reaction process of hydrogen production.So maximizing the joint between both should help to improve the photocatalytic activity.After determining the research interest,the paper further studied the photocatalytic activity of composite catalyst made by self-designed PCM-CVD method.The role of metal Ni in catalyst is not only as the growing point of MWNTs but also as a cocatalyst for hydrogen generation.The adoptive PCM can make Ni evenly dispersed in TiO₂ matrix and then reduced into fine granules on the surface by hydrogen.On next step,the way of mutli-growth of MWNTs was taken in order to reach our purpose for increasing the joints as more as possible.During each growth period,hydrogen was used to get rid of amorphous carbon by methanation.SEM and Raman results show MWNTs with the average diameter of ca.28 nm and good graphite structure can be grown on the surface of TiO₂ by modified CVD method. After second growth period of MWNTs,the photocatalytic activities of composite catalyst reached the maximum,2076μmol/h and 38.1μmol/h under UV light and visible light irradiation respectively.At the same time,the composite catalyst did not lost much activity after 20 h repeat test.Combined with a lot of means of XPS, UV-Vis DRS and PL,a systematical investigation was conducted to analyse the reason of high activity of MWNT-TiO₂ composite catalyst.The key contributions of grown MWNTs are to transfer photoelectron and increase the photo absorbance.Ti³⁺ appearance in the preparation process is good for improving photoactivity as well.On the basis of above study,this paper further attempts to explain the reaction process of hydrogen production according to kinetic regulation.When methanol acts as sacrifice reagent the activity of composite catalyst is higher than other alcohol's. The activity sequence of alcohols is methanol＞ethanol＞i-propanol = n-propanol＞butanol.Methanol dissociation adsorption on the catalyst surface is the first step of whole photoreaction.Along with the subsequent oxidation step it control the whole reaction speed.Furthermore,MWNTs growth speeds up the desorpion step of H from methanol dissociation adsorption,which greatly enhances the photocatalytic activity.In the second part of this paper,we took methyl orange(MO) as a model for azo dye and discussed about photocatalytic oxidation performance of MO by composite catalyst.Before the test,metal Ni should be rinsed out by diluted acid because it would snatch the life of free radicals so as to reduce the photocatalytic oxidation activity.Finally,the kinetics constants ka is 0.066 min-1 under UV light irradiation and 0.017 min^-1 under visible light irradiation repectively.Through investigating the effects of dissolved oxygen,CCl₄,t-butyl alcohol on the decolorization of MO,the mechanisms are proposed that the superoxides(·O₂^-) mediated oxidation pathways are dominant for cleavage of azo bond of MO under visible light irradiation,whereas both of·O₂ and·OH mediated oxidation pathways are dominant under UV light irradiation. In the end,charge transfer process on MWNT-TiO₂ composite catalyst and the implication to photocatalytic reaction pathway were discussed.