Characterization and photocatalytic activity of transition-metal-supported surface bond-conjugated TiO2/SiO2

TM/TiO₂/SiO₂ photocatalysts were prepared by the photodeposition method using transition metal salts (TM=Fe³⁺, Co²⁺, Ni²⁺ and Cu²⁺) as precursors and the surface bond-conjugated TiO₂/SiO₂ as supporter in N₂ atmosphere, and were characterized by XRD, XPS, UV-Vis diffuse reflection and zeta-potential. Their photocatalytic activities were evaluated using reactive brilliant red K-2G (K-2G) and cationic blue X-GRL (CBX) showing different adsorption behavior on the oxides. Fe, Cu supported TiO₂/SiO₂ can efficiently extend the light absorption to the visible region. XPS analysis verified that the introduction of transition metal lead to the changes of the electronic environmental of Ti cations and the zeta-potential of oxides. As a result, K-2G has higher adsorption on the modified TiO₂/SiO₂ than that on the baked one, while the adsorption of CBX has a little change on the both oxides. At the same time, for the photodegradation of K-2G, Fe³⁺, Co²⁺, Ni²⁺-modified catalysts show that their photoactivities are 3.3–2.2 times higher than the bare one. On the contrast, all transition-metal-supported catalysts have no significant activity improvement except that Fe/TiO₂/SiO₂ shows 1.68 times higher activity for the photodegradation of CBX. The results indicate that the photoactivity could be increased in photodegradation of dyes by changing the performances of adsorption to dyes and absorption to light of photocatalyst.     

Optimized photodegradation of Reactive Blue 19 on TiO2 and ZnO suspensions

The photocatalytic discoloration of Reactive Blue 19 (RB-19), a textile anthraquinone dye, were investigated in aqueous suspensions containing TiO₂ or ZnO as catalysts. The reactions can be mathematically described as a function of parameters pH, amount of catalyst and dye concentration being modeled by the use of response surfaces methodology. Optimized values for the concentration of the catalyst and the pH for each reaction systems were determined. ZnO showed greater degradation activity than TiO₂ when the reaction was carried out in a 4 l circulating reactor under optimized conditions. The figure-of-merit electric energy per order (EE/O) allows to determine that the electrical energy cost of reducing the color by one order of magnitude was three times higher for TiO₂. In both reaction systems, the acute toxicity evolved from zero to around 50 toxicity units in the initial stages of irradiation, depleting to minimal values after 30 min of reaction.     

Relationship concerning the nature and concentration of Fe(III) species on the surface of TiO2 particles and photocatalytic activity of the catalyst

There have been contradictory results concerning the effect of doping TiO₂ with Fe(III) upon its photocatalytic activity. We believe that this is due to the method used in adding the Fe(III) to TiO₂. This paper addresses this issue by using a precise adsorption process for loading stable aqueous suspensions (sols) of TiO₂ doped with Fe(III). In this manner, we control the speciation of Fe(III) on the surface of TiO₂ and subsequently use these loaded sols to produce stable thin-film photocatalysts. Adsorption isotherms for these systems at different pH (2.0 and 2.5) values showed that both pH and the concentration of Fe(III) in solution influence the adsorption density of Fe(III), the stability of the resulting sols, and the speciation (degree of polymerization) of Fe(III) ions on the surface of the TiO₂ nano-particles. UV-Vis spectroscopy on selected systems was the technique employed to confirm these results. The pore structure of the resulting gels (xerogels) prepared from these sols, as well as thermal stability of porous materials prepared by firing the xerogels have been determined by measuring N₂ adsorption as a function of firing temperature. Thermal analysis (TG–DTA) studies on xerogels obtained from some of these systems showed that the presence of Fe(III) species on the TiO₂ retards the conversion of anatase into rutile and therefore increases the thermal stability of the gel microstructure. Lastly, porous films, resulting from the casting of the sols on glass rings and subsequent firing, were used in photocatalytic studies. In the presence of UV light, these supported films were capable of degrading ethanol in the gas phase. While photocatalytic activity for Fe(III) loaded TiO₂ films is less than for TiO₂ films without Fe(III), the speciation of Fe(III) on the surface directly influences the behavior of the intermediate—acetaldehyde—formed in this reaction. Since we have only evaluated the effect of iron speciation on the degradation of ethanol, other target species may be influenced either advantageously or deleteriously depending upon the state of Fe(III) on the surface of TiO₂ photocatalysts.     

Kinetics of dye decolorization in an air–solid system

The photocatalytic decolorization of adsorbed organic dyes (Acid Blue 9, Acid Orange 7, Reactive Black 5 and Reactive Blue 19) in air was examined, applicable to self-cleaning surfaces and catalyst characterization. Dye-coated Degussa P25 titanium dioxide (TiO₂) and dye-coated photo-inert aluminum oxide (Al₂O₃) particles, both of sub-monolayer initial dye coverage, were illuminated with 1.3 mW cm⁻² of near-UV light. Visual evidence of color removal is reported with photographic images. Two methods, Indirect and Direct Analysis, were employed to quantitatively examine the decolorization kinetics of dyes using UV–visible transmission and diffuse reflectance spectroscopy, respectively. A decrease in dye concentration with time was observed with near-UV illumination of dye-coated TiO₂ powders for all dyes. Dyes did not photodegrade significantly on photo-inert Al₂O₃.

UV–visible spectroscopy data was used to model the kinetics of the photocatalytic degradation. Two first-order reactions in series provided the most convincing rate form for the photodegradation of dyes adsorbed to TiO₂, with a first step the conversion of colored dye to colored intermediate, and the second the conversion to colorless product(s). The first rate constant was of similar magnitude for all dyes, averaging k₁ = 0.13 min⁻¹. Similarly, for the second, k₂ = 0.0014 min⁻¹.     

Photocatalytic degradation of short-chain organic diacids

The heterogeneous photocatalytic oxidation of fumaric, maleic and oxalic acids over TiO₂ has been investigated. For aqueous suspensions at pH lower than the point of zero charge (pzc) of TiO₂, the photocatalytic degradation of the three studied diacids follows the Langmuir–Hinshelwood kinetic model, with the rate constant of the process decreasing in the order oxalic acid>maleic acidfumaric acid. At these low pH media, the adsorption of the organic diacids onto TiO₂ particles is a key feature for their degradation, which is initiated by a photo-Kolbe process. For fumaric and maleic acids, a cis–trans isomerisation induced by the interaction between adsorbed molecule and semiconductor surface occurs. At pH’s higher than the pzc of TiO₂ the rate of oxalic acid oxidation decreases noticeably, while fumaric and maleic acids are both efficiently degraded in homogeneous phase by reacting with OH√ radicals photochemically generated on the TiO₂ surface, giving rise to a significant increment of both isomers degradation rate with increasing pH. At these pH’s higher than the pzc of the TiO₂, the three studied diacids show a very low degree of adsorption onto the semiconductor surface and no evidence of cis–trans isomerisation for both maleic and fumaric acids is detected. In accordance with the observed pH effects on degradation rate and over detected intermediates, a different mineralisation pathway is proposed as function of initial pH.     

Surface-enhanced Raman study of electrochemical and photocatalytic degradation of the azo dye Janus Green B

The photocatalytic degradation of Janus Green B azo dye over silver modified titanium dioxide films was investigated by surface-enhanced Raman spectroscopy (SERS). An optimized SERS-active substrate was employed to study the photodegradation reaction of Janus Green B. Considering that photocatalytic degradation processes of organic molecules adsorbed on TiO₂ might involve either their oxidation or reduction reaction, the vibrational spectroelectrochemical study of the dye was also performed, in order to clarify the transformations involved in initial steps of its photochemical decomposition. In order to understand the changes in Raman spectra of Janus Green B after photodegradation and/or electrochemical processes, a vibrational assignment of the main Raman active modes of the dye was carried out, based on a detailed resonance Raman profile. Products formed by electrochemical and photochemical degradation processes were compared. The obtained results revealed that the first steps of the degradation process of Janus Green B involve a reductive mechanism.     

碳酸银光催化性能研究

银系光催化剂制备方法简单且带隙较窄,在可见光照射下具有良好的光催化效果,因此受到人们的广泛关注。以罗丹明B为降解物,在模拟可见光条件下利用碳酸银进行光催化降解实验,比较同等条件下碳酸银和二氧化钛的光催化性能,探究了碳酸银在不同pH、降解物质量浓度和催化剂投加量条件下的光催化活性,并利用总有机碳（TOC）分析矿化程度。通过X射线衍射仪（XRD）、激光拉曼光谱仪（RRS）、傅里叶变换红外光谱仪（FT-IR）和紫外吸收光谱仪（UV-Vis）对碳酸银进行表征并分析其晶相组成。结果表明,在同等条件下碳酸银对罗丹明B的光催化降解速率远大于二氧化钛,并且在溶液pH小于7、碳酸银投加量为 1 g/L条件下催化剂对质量浓度为5 mg/L的罗丹明B染料的光催化降解效果最好,紫外灯照射100 min后罗丹明B染料降解率达到98%以上,最终矿化率达到66%。     

Relationship between the formation of surface species and catalyst deactivation during the gas-phase photocatalytic oxidation of toluene

Various partial oxidation products were identified on the surface of TiO₂ and an 8% SiO₂–TiO₂ binary catalyst used for the photocatalytic oxidation of gas-phase toluene. Using in situ FTIR spectroscopy, benzaldehyde and benzoic acid were identified on the surface of the deactivated photocatalysts. Additional GC/MS analysis of methanol-extracted surface species confirmed the presence of benzaldehyde and benzoic acid and detected small concentrations of benzyl alcohol. Apparently, benzaldehyde is the main partial oxidation product that is further oxidized to benzoic acid. Benzoic acid is strongly adsorbed on the surface of the catalyst. There seems to be a correlation between the accumulation of benzoic acid on the surface and catalyst deactivation. The presence of gas-phase water in the reactive mixture seems to retard the formation of benzoic acid.

The SiO₂–TiO₂ photocatalyst is more active and appears to deactivate slower than TiO₂. This binary oxide is photocatalytically active even in the absence of gas-phase oxygen. It also seems to have a higher toluene adsorption capacity than TiO₂. The acidity of the different oxides was examined using FTIR spectroscopy of adsorbed pyridine. The results indicate that no pure metal oxide displays Brønsted acidity but when SiO₂ is cofumed with TiO₂, Brønsted acidity of intermediate strength is generated. The generation of new surface sites may be responsible for the increased activity. The mechanism of this promotion effect is not clearly understood and further studies are required to elucidate it.     

Photocatalytic oxidation of nitrogen oxide over titania–zeolite composite catalyst to remove nitrogen oxides in the atmosphere

The photooxidation of NO with oxygen over Hycom TiO₂ and zeolite (A and Y form zeolite: TiO₂-AZ and TiO₂-YZ) composite catalysts was studied to remove NO_x in the atmosphere. The photocatalytic oxidation activity of the titania in the composite catalyst in a proportion of AZ:TiO₂=3:7 is about three times larger than that in the bare titania. The adsorption behaviors of NO and NO₂ for the bare titania sample obey Langmuir adsorption equations of NO and NO₂, respectively. In the titania–zeolite composite catalysts, the adsorption data indicate the increase in the amount of NO adsorption on the TiO₂ phase and the decrease in the amount of NO₂ adsorption, compared with the bare titania. The acceleration of NO photooxidation rate, resulting from the increase in the amount of NO adsorbed and the decrease in the amount of NO₂ adsorbed, thus occurs on the TiO₂ phase. IR spectra, when irradiating the catalysts with UV, showed the immediate formation of nitrate and NO₂ species on the catalyst. The results lead to the conclusion that the zeolites promote the photocatalytic oxidation of NO over the titania.     

Role of the support on the activity of silica-supported TiO2 photocatalysts: Structure of the TiO2/SBA-15 photocatalysts

Immobilization of TiO₂ on silica materials has been commonly proposed in order to make easier the separation of the catalyst after the photocatalytic reactions in aqueous systems. The main drawback of the supported photocatalysts is that they usually show lower activities in comparison with powdered TiO₂ materials. The aim of this work is to elucidate the structure of some silica-supported TiO₂ photocatalysts recently developed as well as to evaluate the role that the porous structure of the support can play in the observed photocatalytic activities. In comparison with the use of an amorphous silica support, the use of the mesostructured silica SBA-15 produces an ordered structure in which TiO₂ crystals of similar sizes, independently of titania loading, are located inside the mesoporous channels of the support. The photocatalytic treatment of several cyanide-containing compounds is analyzed and the results are explained in terms of the structure of every catalyst. Depending on the model compound, the characteristic structure of the TiO₂/SBA-15 materials allows increasing up to eight times the activity achieved by the Degussa P25 TiO₂. The main conclusion of this work is the strong influence of the textural properties of the support on the catalytic activity of immobilized TiO₂ photocatalysts.     

TiO2 coating types influencing the role of water vapor on the photocatalytic oxidation of methyl ethyl ketone in the gas phase

Four TiO₂-based materials, named A, B, C and D, are used to investigate the influence of water vapor on the gas–solid adsorption and heterogeneous photocatalytic oxidation of gaseous methyl ethyl ketone (MEK). Two of the photocatalysts (A and B) are constituted of powdered TiO₂ deposited onto two different supports (ordinary glass and non-woven cellulose fibers). The other ones (C and D) are composed of a thin film of TiO₂ coated on glass substrates. The effect of water vapor on MEK initial conversion rates is studied for the four photocatalytic materials using the Langmuir–Hinshelwood model at the initial time. On the concentrations range where the model hypotheses are verified, adsorption constants K and kinetics constants k are calculated for experiments under both dry and humid atmosphere. When the relative humidity is increased, the evolution of these constants shows that water vapor acts differently depending on the form of deposited TiO₂ (powder and film).     

Photocatalysis by polyoxometallates and TiO2: A comparative study

Polyoxometallates (POMs) as a homogeneous photocatalyst and TiO₂ as a heterogeneous photocatalyst seem to exhibit overall similar photocatalytic behavior. Both systems cause photodecomposition of a variety of organic pollutants via the formation and decay of several similar intermediates formed by OH addition (hydroxylation), dehalogenation, deamination, decarboxylation, etc. The final degradation products, for most organic substrates for both systems are CO₂, H₂O and inorganic anions. The similarity of behavior has been attributed to the formation of the common powerful oxidizing reagent, OH radical, from the reaction of the excited catalyst and water molecules.

On the other hand, lately, various laboratories have pointed out differences in reactivity and degradation mechanism between the two photocatalysts. The results are interesting and to a great extent contradictory.

This study compares the photodegradation of four substrates with diversified structures, namely, atrazine, fenitrothion, 4-chlorophenol (4-ClPh), and 2,4-dichlorophenoxyacetic acid (2,4-D) by both PW₁₂O₄₀³⁻ and TiO₂ and how their photodegradation is affected by the presence of strong OH radical scavengers, i.e., Br⁻ and isopropyl alcohol (i-prOH).

The results provide substantial evidence that the literature data on the apparent photooxidation mechanism of these two categories of photocatalysts is circumstantial, depending on substrate and the mode of investigation. Overall, though, the action of OH radicals relative to h⁺ appears to be more pronounced with PW₁₂O₄₀³⁻ than TiO₂.

With respect to thermal (dark) reaction of photoreduced catalysts, both systems can deliver their electrons to a variety of oxidants including metal ions. The advantages of POM relative to TiO₂ relates to the selective reduction precipitation of metal ions and to their unique ability to form metal nanoparticles in which POM serve both as reducing reagents and stabilizers.     

Photocatalytic oxidation of methyl parathion over TiO2 and ZnO suspensions

The photocatalytic degradation of methyl parathion in aqueous solutions, using two different photocatalysts (TiO₂ and ZnO) has been investigated. The degradation of methyl parathion follows first order kinetics according to the Langmuir–Hinshelwood model. Complete degradation is achieved within 45 or 150 min when treated with illuminated TiO₂ or ZnO, respectively. It was observed that the initial rate increases linearly with an increase of the amount of catalyst up to a level where it reaches a plateau corresponding to the optimum of light absorption. The addition of an oxidant (K₂S₂O₈) leads to an increase in the rate of photooxidation. Moreover, illuminated TiO₂ suspensions were proved to be more effective in mineralizing the insecticide compared to ZnO suspensions. Measurements of phosphate, sulfate and nitrate ions gave valuable information about how this process is achieved. Addition of the oxidant enhances mineralization for both photocatalytic systems. Up to eight by-products were identified by GC–MS technique during the photocatalytic degradation of the insecticide that proceeds via oxidation, hydroxylation, dealkylation and hydrolysis of the ester group reaction pathways. Finally, the toxicity of the treated solution was reduced only in the presence of TiO₂, while ZnO suspensions appear to increase the toxicity due to photo-dissolution of ZnO releasing zinc in the treated solution.     

Influence of operating parameters on photocatalytic degradation of phenol in UV/TiO2 process

The use of hydrogen peroxide (H₂O₂) for improved photocatalytic degradation of phenol in aqueous suspension of commercial TiO₂ powders (Degussa P-25) was investigated. Photodegradation was compared using direct photolysis (UV alone), H₂O₂/UV, TiO₂/UV, and H₂O₂/TiO₂/UV processes in a batch reactor with high-pressure mercury lamp irradiation. The effects of operating parameters such as catalyst dosage, light intensity, pH of the solution, the initial phenol, and H₂O₂ concentrations on photodegradation process were examined. It was shown that photodegradation using H₂O₂/TiO₂/UV process was much more effective than using either H₂O₂/UV or TiO₂/UV process. The effect of the initial phenol concentration on TOC removal was also studied, demonstrating that more than 8 h was required to completely mineralize phenol into water and carbon dioxide. For all the four oxidation processes studied, photodegradation followed the first-order kinetics. The apparent rate constants with 400-W UV ranged from 5.0 × 10⁻⁴ min⁻¹ by direct photolysis to 1.4 × 10⁻² min⁻¹ using H₂O₂/TiO₂/UV process. The role of H₂O₂ on such enhanced photodegradation of phenol in aqueous solution was finally discussed.     

Degradation of neonicotinoid insecticides by different advanced oxidation processes and studying the effect of ozone on TiO2 photocatalysis

Although some studies concerning the effect of pH and ozone dosage on TiO₂ photocatalysis (O₂/TiO₂/UV) have already been published, no complete investigation and explanation of the effects of both parameters on photocatalytic ozonation (O₃/TiO₂/UV) have been carried out. Aqueous solutions of neonicotinoid insecticides (thiacloprid and imidacloprid) were chosen as a degradation medium, since they exhibit a high threat for aquatic systems and it is of great importance to find an effective method for their elimination from the environment. In preliminary stability tests, thiacloprid showed higher photo- and chemical stability compared to imidacloprid, therefore its degradation was studied in detail. To assess the suitability of various treatments for degradation and mineralization of thiacloprid in water at different pH values and ozone dosages, we applied ozonation (O₃) and three different photochemical advanced oxidation processes, namely ozonation, coupled with UV radiation (O₃/UV), O₃/TiO₂/UV and O₂/TiO₂/UV. Light source emitting mainly in UVA range was applied in all three processes. The photocatalytic ozonation (O₃/TiO₂/UV) was found to be the most efficient process irrespective of pH. The synergistic effect of ozone and TiO₂ photocatalysis was noticed at acidic and neutral pH, but the synergism was lost at basic pH, probably due to faster self-decompositon of ozone under alkaline conditions. At acidic pH, also the oxidation of chloride anions to chlorate(V) was noticed in O₃/TiO₂/UV and in O₃/UV processes. By plotting the disappearance rate constants of thiacloprid degradation in O₃/TiO₂/UV and O₃/UV systems as a function of the flow rate of ozone, the synergistic effect of ozone was undoubtedly proven. The slope of the linear fit in case of O₃/TiO₂/UV process was considerably steeper than in case of O₃/UV, which would not happen in absence of synergistic effect. The linearity in O₃/TiO₂/UV system was lost only at very high flow rates of ozone.     

A chemical etching route to controllable fabrication of TiO2 hollow nanospheres for enhancing their photocatalytic activity

The TiO₂ hollow nanospheres with diameters of about 230 nm were prepared by a simple and controllable route based on hydrolysis of Ti(OBu)₄ on the surfaces of the Cu₂O solid nanospheres followed by inward etching of the Cu₂O nanospheres. The as-prepared samples were characterized by X-ray diffraction, transmission electron microscopy and scanning electron microscopy. The further post-heat treatment led to the high crystallization of the TiO₂ hollow nanospheres. The photocatalytic performances of these samples were evaluated for the photodegradation of rhodamine B (RhB) under UV-light irradiation. The as-prepared TiO₂ hollow nanospheres showed higher photocatalytic activity than the CuO and the CuO/TiO₂ hollow nanospheres. Effects of temperature and time for post-heat treatment of TiO₂ as well as initial RhB concentrations on the RhB photodegradation have also been studied. The results show that the TiO₂ hollow nanospheres have the good reusability as photocatalysts and are promising in waste water treatment.     

Pathways of solar light-induced photocatalytic degradation of azo dyes in aqueous TiO2 suspensions

The photocatalytic degradation of aqueous solutions of Acid Orange 7 in TiO₂ suspensions has been investigated with the use of a solar light simulating source. The photoreaction was followed by monitoring the degradation of the dye and the formation of intermediates and final products, as functions of time of irradiation, both in solution and on the photocatalyst surface. It has been found that the dye adsorbs on TiO₂ and undergoes a series of oxidation steps, which lead to decolorization and formation of a number of intermediates, mainly aromatic and aliphatic acids. These molecules are further oxidized toward compounds of progressively lower molecular weight and, eventually, to CO₂ and inorganic ions, such as sulfate, nitrate and ammonium ions. A TiO₂-mediated photodegradation mechanism for Acid Orange 7 is proposed on the basis of quantitative and qualitative detection of intermediate compounds.     

Photocatalytic activation of TiO2 under visible light using Acid Red 44

The activation of TiO₂ photocatalyst for photocatalysis under the visible light using Acid Red 44 (C₁₀H₇N=NC₁₀H₃(SO₃Na)₂OH) is described. Adjustment of the pH enhanced the photocatalytic activation of TiO₂ in the presence of visible light. This confirms that the adsorption of a dye on TiO₂ surface is an important factor in dye-photosensitization. The differences in the photocatalytic activation mechanism under visible irradiated conditions with that of UV irradiated condition are proposed. The dye-sensitized photocatalysis under visible light was applied to the decomposition of phenol, is a toxic chemical used in industry and frequently discharged into water.     

Immobilization of highly active titanium(IV) oxide particles A novel strategy of preparation of transparent photocatalytic coatings

Titanium(IV) oxide (TiO₂) powders synthesized by HyCOM (hydrothermal crystallization in organic media) method, which had been proved to exhibit ultra-high photocatalytic activity in several reaction systems, were used as starting material for fabrication of transparent TiO₂ thin films. HyCOM-TiO₂ powders were dispersed in aqueous sol of nitric acid to yield a TiO₂ sol stable for more than 90 days. Transparent TiO₂ thin films were successfully produced by dip-coating from the TiO₂ sol and used for photocatalytic decomposition of malachite green (MG) in an aqueous sol under aerated conditions. These films exhibited much higher rate of MG decomposition compared with those prepared from a commercially available TiO₂ sol developed for photocatalytic use (Ishihara STS-01), indicating that the excellent photocatalytic activity of original HyCOM-TiO₂ particles was preserved after immobilization on glass substrates by the present method.     

不同无机酸对聚吡咯/TiO2复合物的吸附性能影响

考察了不同无机酸对PPy/TiO₂复合物吸附性能的影响。首先分别在HNO₃、H₂SO₄和H₃PO₄体系中合成了聚吡咯(PPy)/TiO₂复合物(分别简写为N-PPy/TiO₂、S-PPy/TiO₂和P-PPy/TiO₂)。以红外光谱、热重分析、Zeta电位、比表面积分析和扫描电镜等测试方法对复合物的物理化学性能进行了表征。接着以几种合成的复合对阴离子染料酸性红G和阳离子染料亚甲基蓝进行吸附研究,发现无机酸对合成的复合物的吸附性有较大的影响。几种复合物均可在30min内达到吸附平衡,并且可以重复使用6次以上吸附量没有明显的降低。其中S-PPy/TiO₂的吸附性能最好,其对ARG和MB的最大吸附量分别达到218.34mg/g和314.68mg/g。复合物对染料的吸附过程符合准二级动力学模型和Langmuir吸附等温式。对S-PPy/TiO₂进行吸附热力学研究表明,其对染料的吸附过程为熵增的自发过程。