Enhanced Photocatalytic Activity Of Microwave Treated TiO₂ Pillared Montmorillonite

Since the discovery of photoelectrochemical splitting of water onn-TiO2 electrodes by Fujishima and Honda at 1972, n-typesemiconductor heterogeneous photocatalytic technology has attractedmuch attention. TiO2, which is non-toxic and has good chemicalstability, has quite high photocatalytic activity in the degradation ofmost organic pollutants in water and in air, so a lot of work have beendone due to it's potential applications to degradate organic pollutantsin water with low concentration. Supported TiO2 has been one ofresearch focuses because it can be recovered easily.Montmorillonite intercalated by inorganic metal oxide particleshas attracted extensive interest as adsorbents, catalysts and catalystsupports due to montmorillonite's better cation exchange capacity.TiO2 pillared montmorillonite has been widely studied because it hasboth a two-dimensional porous network structure, larger specificsurface area and an advantage of being recovered easily duringphotodegradation organic pollutants. Such new methods as templatedsynthesis, hydrothermal synthesis, supercritical carbon oxide methodand microwave irradiation have been developed in recent years.Microwave dielectric heating is 'body heating' caused bydielectric loss of reactants in microwave field, which changesmicrowave energy to heat energy and is conducted in space aselectromagnetic wave. By microwave heating, we can acceleratechemical reaction, increase reaction rate, decrease reaction time andsave energy. Much research in synthesizing inorganic materials bymicrowave dielectric heating have been done, for example: ceramicmaterials calcined by microwave irradiation, zeolite and nanomaterialssynthesized by microwave irradiation.In this article, TiO2 pillared montmorillonite was synthesized bymicrowave irradiation. The best synthesis condition was determinedafter discussing the influences of H+/Ti, Ti/M, pre-stirred time,irradiation power, irradiation time and montmorillonite concentrationin detail, moreover, we discussed the samples about photodegradationmethyl orange performance in contrast with the sample synthesized bytraditional method. The photocatalytic performance of methyl blue,Rhodamine B and the reuse photocatalytic performance of methylorange were also observed.When the ratio of H+/Ti is 6, the crystalline size of anatase andrutile is 9.00nm and 8.51nm respectively;the ratio between anataseand rutile (ωA/ωB) is the largest: 8.06, which indicates anatase is themajor content;the surface area is the largest: 230 m2/g, which notespillared structure exists;when irradiated 50min under UV light, themethyl orange concentration drops by 70%, the photodegradationperformance of the sample is the best.When the ratio of Ti/M is 5, the fraction of rutile increases, TiO2is not enough to exchange with cation of montmorillonite, the sampleexhibits the worst photocatalytic performance;while the ratio of Ti/Mis 20, pure TiO2 is major in the sample and pillared structure exists less;when the ratio of Ti/M is 15, pillared structure exists well, thephotocatalytic performance is as well as the sample synthesized as theratio of Ti/M is 20, which indicates the ratio of Ti/M is 15 is suitable.When the pre-stirred time is 1h, TiO2 is well crystallized;thepillar structure exists well;the sample exhibits the best photocatalyticperformance;when the pre-stirred time increases (2h, 3h), the effect ofmicrowave irradiation in accelerating exchange rate and enhancingcrystalline decreases, so the photocatalytic performance get worse;when pre-stirred time decrease (0h), TiO2 sol particle is heterogeneousso as to decrease exchange rate in the microwave field.When microwave irradiation power is 70%, TiO2 is wellcrystallized while anatase is major, which enhance the photocatalyticperformance of the sample;pillared structure exists well while surfacearea is bigger (237m2/g), which also enhance the photocatalyticperformance of the sample;when irradiated 50min under UV light, themethyl orange concentration drops by 83%, the photodegradationperformance of the sample is the best.When microwave irradiation time is 20min or 30min, thecrystalline size of TiO2 becomes smaller, while the fraction of rutileincrease, pillared structure exists, but the photocatalytic performance isnot the best;when irradiation time is 5min, the effect of microwaveirradiation in accelerating exchange rate and enhancing crystallinedecreases, pure TiO2 is the major in the sample so as to thephotocatalytic performance is the best;when irradiation time is 10min,the crystalline sizes of TiO2 are the largest, while anatase is the major,pillared structure exists well, the photocatalytic performance is better,so the microwave irradiation time : 10min is suitable.When montmorillonite concentration changes (1wt%, 2wt%,5wt%), when the concentration increases, the crystalline sizes of TiO2become larger, while the fraction of rutile increases, surface areabecomes smaller, photocatalytic performance gets worse, which can beascribed to the worse swelling performance of montmorillonite so as todecrease cation exchange rate. To make sure that pillared structureexists well in the sample, the 1wt% montmorillonite concentration isadopted.In contrast with the sample synthesized by traditional method,TiO2 pillared montmorillonite synthesized by microwave irradiationhas well crystallized TiO2, while anatase is the major, and betterphotocatalytic performance of methyl orange under UV light. WhileTiO2 pillared montmorillonite synthesized by microwave irradiationexhibits better photocatalytic performance than per mount TiO2 fordegradation of methyl blue under solar light and degradation ofRhodamine B under UV light. When reused for degradation of methylorange under UV light, the performance decreases a little, however85% photocatalytic ability is retained and becomes stable.To sum up, TiO2 pillared montmorillonite was synthesized bymicrowave irradiation. In contrast with the sample prepared bytraditional method, TiO2 pillared montmorillonite exhibits excellentphotocatalytic degradation performance;the sample exhibits betterphotocatalytic performance than per mount TiO2 for degradation ofmethyl blue under solar light and degradation of Rhodamine B underUV light;When reused for degradation of methyl orange under UVlight, 85% photocatalytic ability is retained and becomes stable, whichis an advantage in potential application in practice.