Combined TiO2/SiO2 mesoporous photocatalysts with location and phase controllable TiO2 nanoparticles

Combined TiO₂/SiO₂ mesoporous materials were prepared by deposition of TiO₂ nanoparticles synthesised via the acid-catalysed sol–gel method. In the first synthesis step a titania solution is prepared, by dissolving titaniumtetraisopropoxide in nitric acid. The influences of the initial titaniumtetraisopropoxide concentration and the temperature of dissolving on the final structural properties were investigated. In the second step of the synthesis, the titania nanoparticles were deposited on a silica support. Here, the influence of the temperature during deposition was studied. The depositions were carried out on two different mesoporous silica supports, SBA-15 and MCF, leading to substantial differences in the catalytic and structural properties. The samples were analysed with N₂-sorption, X-ray diffraction (XRD), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM) to obtain structural information, determining the amount of titania, the crystal phase and the location of the titania particles on the mesoporous material (inside or outside the mesoporous channels). The structural differences of the support strongly determine the location of the nanoparticles and the subsequent photocatalytic activity towards the degradation of rhodamine 6G in aqueous solution under UV irradiation.     

Direct synthesis of nano titania on highly-ordered mesoporous SBA-15 framework for enhancing adsorption and photocatalytic activity

This paper reports the synthesis of TiO₂-containing mesoporous catalysts for effectively enhancing the adsorption and photocatalytic activity. The factors that affect the photocatalytic activity of catalyst composites, including types of silica support, TiO₂ content, calcination temperature, and catalyst mass, were examined in this study. The samples were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and surface area analysis. The experimental results showed that incorporating TiO₂ nanoparticles into silica gel or SBA-15 frameworks could enhance the photodegradation rate more effectively than pure TiO₂. The TiO₂/SBA-15 sample displayed much higher adsorption and photocatalytic activity levels than did TiO₂/silica-gel. The pore volume and pore size of TiO₂/SBA-15 were as high as 1.317 cm³/g and 7.51 nm, respectively, which exceeded those of TiO₂/silica-gel (0.437 cm³/g and 3.68 nm, respectively). The rate constants of photocatalysis were determined. The photodegradation rate of the catalyst increased with decreasing TiO₂ content and increasing calcination temperature. The proposed method of preparing mesoporous photocatalysts is simple and suitable for mass production.     

Preparation of freestanding and crack-free titania–silica aerogels and their performance for gas phase,photocatalytic oxidation of VOCs

Freestanding and crack-free titania–silica aerogels with high titanium content (i.e., Ti/Si = 1) were successfully prepared by adjusting the hydrolysis of the two alkoxide precursors to a comparable rate during the sol–gel processing. Two titania–silica aerogels were prepared by ethanol and CO₂ supercritical drying methods. Well-dispersed, nanometer-sized anatase crystal domains (ca. 10 nm) were crystallized by high temperature, ethanol supercritical drying. The crystalline domains were solidly anchored to the aerogel network by Ti–O–Si bonds. Titania–silica aerogels prepared by CO₂ supercritical drying method were devoid of TiO₂ crystals. A molecular-level mixing was achieved and anatase TiO₂ was only crystallized with difficulty by high temperature calcination (1073 K). Both aerogels were mesoporous and displayed similar open pore structure that is readily accessible to reactant molecules. However, only the titania–silica aerogel with anatase TiO₂ prepared by ethanol supercritical drying was active for the gas phase, photocatalytic oxidation of volatile organic compounds (i.e., isopropanol and trichloroethylene). Catalysts prepared from Degussa P25 TiO₂ displayed lower activity under similar reaction conditions.     

Synthesis of TiO2 nanoparticles on mesoporous aluminosilicate Al-SBA-15 in supercritical CO2 for photocatalytic decolorization of methylene blue

In the present study, titania nanoparticles were first constructed on mesoporous aluminosilicate Al-SBA-15 in supercritical carbon dioxide (Sc-CO₂) and the resultant samples were characterized by a combination of various techniques, such as X-ray diffraction (XRD), nitrogen physisorption, ²⁷Al MAS NMR, UV–vis diffuse reflectance spectroscopy, and transmission electron microscopy (TEM). It was identified that the Al species incorporated samples retained structures similar to that of the parent SBA-15. In addition, the content of titania loading varied with reaction temperature and time in Sc-CO₂. As-synthesized TiO₂/Al-SBA-15 samples were evaluated in terms of photocatalytic decolorization of methylene blue in aqueous solutions. It was observed that all TiO₂/Al-SBA-15 samples showed satisfactory decolorization efficiency that was much higher than those of TiO₂/SBA-15 and commercial TiO₂ under identical conditions, which could be mainly attributed to the effective adsorption capability, resulting from the extension of specific surface area after substitution of Si species with Al species.     

Photocatalytic activity of titania modified mesoporous silica for pollution control

A series of titania modified mesoporous silicates with variable Si/Ti ratios were prepared using titanium tetrabutoxide by impregnation method. The samples were characterized by different analytical techniques such as XRD, TEM, FT-IR, low temperature N₂ sorption, and UV–Vis diffused reflectance spectroscopy. The TiO₂ supported on mesoporous silica was found to be in the anatase form. Crystallite size calculated using Scherrer’s formula was found to be in the range 18–20 nm. However, the TiO₂ particle diameter estimated by TEM was 5 nm. UV–Vis spectra showed a blue-shift of the absorption edge for all the samples. The BET surface area decreased with TiO₂ loading. Photodegradation of basic dye-like methylene blue (MB) was studied on these titania modified mesoporous silica using UV irradiation. Volatile organic pollutants like phenol and toluene can also be photocatalytically degraded using these titania modified mesoporous silicates.     

Enhanced photocatalytic activity of TiO2/SiO2 by the influence of Cu-doping under reducing calcination atmosphere

Cu-doped titania photocatalyst supported on silica beads (Cu-TiO₂/SiO₂) were prepared under different Cu-ion concentration and under different calcination atmosphere. The properties and performance of Cu-TiO₂/SiO₂ were compared with undoped TiO₂/SiO₂ photocatalyst. The effect of Cu-doping and calcination atmosphere on photocatalytic degradation of phenol under both black light and visible light irradiation were investigated, where in both cases the degradation rate of phenol by Cu-doped catalyst prepared under reducing calcination atmosphere was found to be higher than the undoped catalyst or Cu-doped catalyst prepared under air atmosphere. This may be attributed to increase in visible light absorption and lengthening of photogenerated electron–hole pair recombination time. The photocatalytic beads were characterized by X-ray diffraction (XRD), UV-Vis absorption spectroscopy and SEM/EDAX analysis.     

Replication Route Synthesis of Mesoporous Titanium–Cobalt Oxides and Their Photocatalytic Activity in the Degradation of Methyl Orange

Mesoporous Ti–Co oxides were synthesized via a replication route, using a 3-D wormlike mesoporous silica as template and tetra-tert-butyl orthotitanate (TBOT) and Co(NO₃)₂ as source materials. The prepared materials were characterized by X-ray diffraction (XRD), N₂-physisorption, TEM, EDS, and UV/Vis-DRS and found to possess a spherical morphology and a 3-D wormhole-like mesoporous structure, with the average pore size between 4.5 and 16.0 nm. The pore walls consisted mainly of a cobalt-incorporated anatase phase. The Co³⁺ ions were generated in the replicated mesoporous Co–Ti oxides, via the transfer of electrons from Co²⁺ to Ti⁴⁺ ions. The formation of cobalt-incorporated anatase phase and Co³⁺ ions were both favored by larger Co/Ti atomic ratios and by relatively low calcination temperatures. The specific surface area decreased and the mesopore sizes increased, with increasing Co/Ti atomic ratio or calcination temperature. The average crystal size of the anatase phase decreased with increasing Co/Ti atomic ratio but increased with increasing calcination temperature. The photocatalytic activity of the replicated mesoporous Co–Ti oxides in the degradation of methyl orange dye was investigated. It was observed that the photocatalytic activity increased with increasing Co/Ti atomic ratio and exhibited a maximum with increasing calcination temperature. With the exception of those prepared at too high calcination temperatures, the replicated mesoporous Co–Ti oxides were much more active than the pure titania. It is concluded that, in addition to a higher diffusion, the cobalt-containing anatase, as the active phase, and the Co³⁺ ions, as the active sites, are responsible for the high photocatalytic activity of the replicated mesoporous Co–Ti oxide.     

Hydrothermal synthesis of titanium dioxide using acidic peptizing agents and their photocatalytic activity

We have prepared TiO₂ nanoparticles by the hydrolysis of titanium tetraisopropoxide (TTIP) using HNO₃ as a peptizing agent in the hydrothermal method. The physical properties of nanosized TiO₂ have been investigated by TEM, XRD and FT-IR. The photocatalytic degradation of orange II has been studied by using a batch reactor in the presence of UV light. When the molar ratio of HNO₃/TTIP was 1.0, the rutile phase appeared on the titania and the photocatalytic activity decreased with an increase of HNO₃ concentration. The crystallite size of the anatase phase increased from 6.6 to 24.2 nm as the calcination temperature increased from 300 °C to 600 °C. The highest activity on the photocatalytic decomposition of orange II was obtained with titania particles dried at 105 °C without a calcination and the photocatalytic activity decreased with increasing the calcination temperature. In addition, the titania particles prepared at 180 °C showed the highest activity on the photocatalytic decomposition of orange II. This paper was prepared at the 2004 Korea/Japan/Taiwan Chemical Engineering Conference held at Busan, Korea between November 3 and 4, 2004.     

Photocatalytic evolution of hydrogen over nanocrystalline mesoporous titania prepared by surfactant-assisted templating sol–gel process

Nanocrystalline mesoporous titania was synthesized via a combined sol–gel process with surfactant-assisted templating route and evaluated for photocatalytic activity of hydrogen evolution from an aqueous methanol solution. In this proposed method, applied surfactant template molecules advantageously behaved as both mesopore-forming and gel formation-assisting agent. The activity of the mesoporous titania thermally treated under appropriate conditions, i.e., at calcination temperature of 600 °C for 4 h, was considerably higher than that of commercial titania powders, Ishihara ST-01 and Degussa P-25. It is clearly revealed that introducing mesoporous framework into TiO₂ by this synthetic system provided much better photocatalytic performance.     

Trypsin immobilization on mesoporous silica with or without thiol functionalization

The effects of pore size, structure, and surface functionalization of mesoporous silica on the catalytic activity of the supported enzyme, trypsin, were investigated. For this purpose, SBA-15 with 1-dimensional pore arrangement and cubic Ia3d mesoporous silica with 3-dimensional pores were prepared and tested as a support. Materials with varying pore diameters in the range 5–10 nm were synthesized using a non-ionic block copolymer by controlling the synthesis temperature. Thiol-group was introduced to the porous materials via siloxypropane tethering either by post synthesis grafting or by direct synthesis. These materials were characterized using XRD, SEM, TEM, N₂ adsorption, and elemental analysis. Trypsin-supported on the solids prepared was active and stable for hydrolysis of N-α-benzoyl-DL-arginine-4-nitroanilide (BAPNA). Without applying thiol-functionalization, cubic Ia3d mesoporous silica with ca. 5.4 nm average pore diameter was found to be superior to SBA-15 for trypsin immobilization and showed a better catalytic performance. However, enzyme immobilized on the 5% thiol-functionalized SBA-15 prepared by directly synthesis was found to be the most promising and was also found recyclable.     

Role of Preparation Techniques in the Activity of Au/TiO2 Nanostructures Stabilised on SiO2: CO and Preferential CO Oxidation

Au colloid and titania in different sequence or Au-oxide nano ensembles preformed in hydrosol were deposited on inert amorphous silica or mesoporous SBA-15. It was compared with gold colloid adsorbed on TiO₂ or silica support. The formation of the Au/TiO₂ interface is discussed in terms of surface charges. Preferential CO oxidation in the presence of hydrogen (PROX) has been correlated with the CO oxidation activity and structural properties, perimeter and the influence of the TiO₂ morphology on the catalytic activity has been demonstrated.     

Mesoporous TiO2/SBA-15, and Cu/TiO2/SBA-15 Composite Photocatalysts for Photoreduction of CO2 to Methanol

A series of mesoporous TiO₂/SBA-15, Cu/TiO₂ and Cu/TiO₂/SBA-15 composite photocatalysts were prepared by sol–gel synthesis for photoreduction of CO₂ with H₂O to methanol. It was found that optimum amount of titanium loading of TiO₂/SBA-15 was 45 wt% which exhibited higher photoreduction activity than pure TiO₂. An addition of copper on TiO₂ or TiO₂/SBA-15 catalyst as cocatalyst was found to enhance the catalytic activity because copper serves as an electron trapper and prohibits the recombination of hole and electron.     

Synthesis of hierarchical mesoporous titania with interwoven networks by eggshell membrane directed sol–gel technique

Hierarchical mesoporous titania with interwoven networks was successfully prepared through a surface sol–gel process followed by a calcination treatment and using eggshell membrane (ESM) as the biotemplate. The biotemplating synthesis was systematically investigated by controlling calcination temperature (550–800 °C), heating rate (1–35 °C/min), impregnant pH value (1–3), and so on. Different from traditional immersion techniques, the nucleation, the growth, and the assembly of mesoporous TiO₂ in our work depended more on some reactions involving ESM biomacromolecules. As-prepared ESM-morphic TiO₂ was composed of intersectant fibers assembled by 6 nm nanocrystallites at 3D with hierarchical pores from 2 nm up to 8 μm.     

Characterizations of TiO2/SiO2/Ni–Cu–Zn Ferrite Composite for Magnetic Photocatalysts

The TiO₂/SiO₂/Ni–Cu–Zn ferrite composite for magnetic photocatalysts with high photocatalytic activity is successfully prepared in this study. The composite are composed of spherical or elliptical Ni–Cu–Zn ferrite nanoparticles about 20–60 nm as magnetic cores, silica as barrier layers with thickness of 15 nm between the magnetic cores and titania shells with thickness approximately 1.5 nm. Photodegradation examination of TiO₂/SiO₂/ Ni–Cu–Zn ferrite composite was carried out in methylene blue (MB) solutions illuminated under a Xe arc lamp with 35 W and color temperature of 6000 K. The results indicated that about 47.1% of MB molecules adsorbed on the TiO₂/SiO₂/Ni–Cu–Zn ferrite composite within 30 min mixing due to it higher pore volume of 0.034 cm³/g, and after 6 h Xe lamp irradiation, 83.9% of MB 16.1% was photodegraded. Compared with the TiO₂ /Ni–Cu–Zn ferrite composite, the TiO₂/SiO₂/Ni–Cu–Zn ferrite composite with silica barrier layer prohibited the photodissolution and enhanced the photocatalytic ability. The magnetic photocatalyst shows high photocatalytic efficiency that the apparent first‐order rate constant k_obs is 0.18427 h^?1, and good magnetic property that the saturation magnetization (M_s) of is 37.45 emu/g, suggesting the magnetic photocatalyst can be easily recovered by the application of an external magnetic field.     

Synthesis and characterization of carbon-doped titania as a visible-light-sensitive photocatalyst

The synthesis and use of carbon-doped TiO₂ particles in photocatalysis under visible light are demonstrated. The carbon-doped titania with its mesoporous structure was prepared by chemical modification and characterized by several techniques including X-ray diffraction, transmission electron spectroscopy (TEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance spectra (EPR), and diffuse reflectance UV-Vis. absorption spectra, with emphasis on the effect of carbon as a doping compound to the titania. Based on EPR data, the photocatalytic activity by visible light can be ascribed to the trapping of electrons at interior sites of the carbon-doped titania between the valence and conduction bands in the titania band structure, and is able to activated by visible light of a wavelength of up to 550 nm. The photocatalytic activity of the carbon-doped TiO₂ nanoparticles was evaluated by examining the decomposition of phenol by irradiation with artificial solar light (>420 nm) and the results were compared with those using Degussa P25, a commercially available titania nanomaterial.     

Trypsin immobilization on mesoporous silica with or without thiol functionalization

The effects of pore size, structure, and surface functionalization of mesoporous silica on the catalytic activity of the supported enzyme, trypsin, were investigated. For this purpose, SBA-15 with 1-dimensional pore arrangement and cubic Ia3d mesoporous silica with 3-dimensional pores were prepared and tested as a support. Materials with varying pore diameters in the range 5–10 nm were synthesized using a non-ionic block copolymer by controlling the synthesis temperature. Thiol-group was introduced to the porous materials via siloxypropane tethering either by post synthesis grafting or by direct synthesis. These materials were characterized using XRD, SEM, TEM, N₂ adsorption, and elemental analysis. Trypsin-supported on the solids prepared was active and stable for hydrolysis of N-α-benzoyl-DL-arginine-4-nitroanilide (BAPNA). Without applying thiol-functionalization, cubic Ia3d mesoporous silica with ca. 5.4 nm average pore diameter was found to be superior to SBA-15 for trypsin immobilization and showed a better catalytic performance. However, enzyme immobilized on the 5% thiol-functionalized SBA-15 prepared by directly synthesis was found to be the most promising and was also found recyclable.     

Room Temperature Photocatalytic Oxidation of Carbon Monoxide Over Pd/TiO2–SiO2 Catalysts

The photocatalytic oxidation of carbon monoxide over TiO₂–SiO₂ and Pd/TiO₂–SiO₂ catalysts was studied. The catalyst samples were synthesized by using sol–gel technique coupled with hydrothermal treatment and all samples were hydrothermally treated before calcination in air. The catalyst samples were characterized by XRD, BET and DRIFTS techniques. The photocatalytic activity of the samples was determined by using circulated batch photoreactor coupled with in line gas transmission FTIR cell charged with 2,000 ppm carbon monoxide in air initially over 0.5 g of catalyst sample under 33 W (254 nm) irradiation power. XRD and BET results confirmed the presence of anatase phase and the decrease on the crystallite size of TiO₂ with SiO₂ addition which yield higher surface area and better dispersion of TiO₂ over mesoporous SiO₂. DRIFTS results indicated the presence of surface hydroxyls coordinated to Ti⁴⁺ and Si–O–Ti sites. All samples containing 10–90 % TiO₂ over SiO₂ exhibited significant photo oxidation activity at room temperature. The photocatalytic oxidation rate of carbon monoxide is favored by SiO₂ addition due to high surface area, better dispersion of TiO₂ particles and higher surface defects. The addition of PdO improves the photocatalytic activity significantly and the synergy between the TiO₂ and PdO phases.     

Synthesis of mixed silica–titania by the sol–gel method using polyethylenimine: porosity and catalytic properties

By means of the sol–gel method, hybrid xerogels of silica, titania and silica–titania mixed networks of different composition with polyethylenimine (PEI) were formed. After removal of PEI followed by calcination, mesoporous oxides of high surface area with monomodal and narrow porosity distribution were obtained. The surface area of the mixed oxides decreases with increasing titania content, but pore size remains almost constant when a PEI:(SiO₂ + TiO₂) ratio of 1 is kept constant in the xerogel precursor. By doubling and tripling the proportion of PEI, the surface area increases by 15% and 30%, respectively, but pore size remains constant, indicating the formation of a larger number of PEI domains of similar size in the hybrid. Isopropanol decomposition reactions were carried out using the mixed oxides as catalysts, and it was found that the networks contain Lewis acid sites but lack Brønsted acid sites.     

Preparation and characterization of high-surface-area titanium dioxide by sol-gel process

One of the important ways to improve photocatalytic efficiency is to prepare catalyst with enhanced surface area. In this work, titanium dioxide (TiO₂) nanoparticles having enhanced surface area were synthesized under the interference of SiO₂. The mixed oxide, SiO₂-TiO₂ (10% mol% Si), was prepared by a sol-gel procedure using titanium tetra-n-butoxide as Ti-precursor. The commercial SiO₂ nanoparticles were added into the TiO₂ sols after hydrolysis. After condensation and calcination heat treatment, the SiO₂-TiO₂ nanoparticles were obtained. To achieve the purpose of obtaining the high-surface-area TiO₂, the SiO₂ was removed subsequently by aqueous NaOH solution. The TiO₂ products were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), electron spectroscopy for chemical analysis (ESCA), and by N₂ adsorption-desorption isotherm. A fine mesoporous structure was formed for as-prepared TiO₂ after calcination at 400^°C and the average pore diameter was about 7 nm. The porous TiO₂ products possess mixing phases of anatase and rutile. Phase transformation from anatase to rutile occurred when the samples were calcined. The phase transition temperature is sensitive to the silicon content. The particle size of ～43 nm remained constant upon calcinations from 500 to 700^°C. The specific surface area was increased up to 66% compared to regular TiO₂ samples that were prepared by the similar sol-gel procedure. The porous TiO₂ nanostructures exhibited enhanced photocatalytic performance to decompose methylene blue under UV irradiation.     

Photocatalytic degradation of iron–cyanocomplexes by TiO2 based catalysts

The removal of iron–cyanocomplexes in industrial effluents is a difficult process, due to the resistance of these compounds to conventional treatments for cyanide wastewater detoxification. The mechanism of both the homogeneous photolysis of these compounds and their heterogeneous photocatalytic oxidation with Degussa P25 TiO₂ and silica-supported TiO₂ photocatalysts has been investigated. The activities of the tested catalysts for complexed cyanide degradation were found to be different from those observed for free cyanide photo-oxidation. The best activity was found for the photocatalyst synthesized by supporting 20 wt.% of TiO₂ on SBA-15 silica as compared with the commercial catalyst Degussa P25 and the other supported catalysts tested. On the basis of detected intermediate species, a mechanism for iron–cyanocomplexes photodegradation is suggested. The influence of the textural properties of the support and titania loading on the process is discussed. The results point out that the high activity observed, when SBA-15 is used as support of TiO₂, seems to be related to the microporosity of the material acting as molecular sieve, which avoids the deactivation of the semiconductor. The porous structure of the SBA-15 material limits the access of the iron–cyanocomplexes to the TiO₂ particles whereas the free cyanides homogeneously released can reach the semiconductor surface, being subsequently oxidized to cyanate.