Synergy effect on a suspended mixture of ceria and activated carbon for the photocatalytic degradation of phenol

Ceria, prepared by a precipitation method, was used as a new photocatalyst under UV light irradiation. The as-prepared sample was characterized by XRD and SEM. Results showed that the ceria was cubic fluorite structure and sized in nanometer range. The photocatalytic degradation of phenol was investigated in the presence of a suspended mixture of ceria and activated carbon. A synergy effect was observed with an enhancement of the apparent rate constant by a factor of 5.6 times. The apparent quantum yield of the ceria-AC system was also increased 2.9 times. The activated carbon with strong adsorption activity provided sites for the adsorption of phenol. Then, the adsorbed phenol would migrate continuously to the surface of ceria particles. Some phenol still remained adsorbing on the catalyst when no traces of phenol were detected in the solution. This adsorbed phenol could be degraded by maintaining UV-irradiation.     

Correct use of the Langmuir–Hinshelwood equation for proving the absence of a synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon

This study is a critical approach to the widespread use of the first order form of the Langmuir–Hinshelwood (LH) equation for analyzing kinetics in heterogeneous photocatalytic processes. The different kinetic protocols analyzed have been applied to the results, published in the literature, of the photocatalytic degradation of phenol in an aqueous solution by a physical mixture of TiO₂ particles and activated carbon (AC), the impact of which has been enormous over the last decade. It is commonly accepted that there is a strong synergy in this mixture due to the transfer of phenol from the activated carbon particles to TiO₂. However, we found in this study that the apparent synergy between activated carbon and TiO₂ particles arises from the erroneous use of the first order form of the LH equation. When applying the extended form of the LH equation, that includes the inhibitory effect of the phenol concentration, AC/TiO₂ synergy should be disregarded. In this physical mixture the activated carbon merely alleviates the inhibitory effect of the phenol concentration by decreasing its initial value.     

Development of a dispersion process for carbon nanotubes in ceramic matrix by heterocoagulation

The surface properties of carbon nanotubes have been changed by heat treatment with ammonia to yield a basic surface or with N₂ to yield an acidic surface without destroying the integrity of the nanotubes. The dispersion state of CNTs could be much improved by the adsorption of a cationic dispersant of poly(ethylenimine) (PEI) or an ionic dispersant of sodium dodecyl sulfate (SDS). Through the electrostatic interaction between components, CNTs could be successfully coated with particles of titania or alumina by colloidal heterocoagulation. A titania coating on 1.5wt% CNTs, shows the same photocatalytic properties in phenol degradation and has advantages in separation and reuse.     

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.     

Synthesizing and Comparing the Photocatalytic Properties of High Surface Area Rutile and Anatase Titania Nanoparticles

Rutile titania nanocrystalline particles with high specific surface areas were directly prepared by thermal hydrolysis of titanium tetrachloride aqueous solution. The as-prepared rutile titania powder was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller surface area analysis, and Fourier transform Raman and IR spectroscopies. Neither anatase nor amorphous titania could be detected in this titania powder by XRD, Raman spectroscopy, and high-resolution TEM. In the phenol degradation reaction, the rutile titania powder with an initial crystalline size of 7 nm was found to have higher photocatalytic activity than that of anatase titania with the same specific surface area. The rutile titania powders calcined at 300° and 450°C also showed a relatively high photocatalytic property. The high activity of the as-prepared rutile titania was attributed to the abundance of hydroxy groups in the powder, as was proven by thermogravimetric analysis data, which provided more active sites for the degradation reaction.     

Photochemical behaviour of activated carbons under UV irradiation

A series of activated carbons were used to investigate the photochemical behaviour of carbons under UV light as catalysts in the photo-oxidation of phenol in the absence of semiconductor additives. Conventional photocatalytic tests showed an improved photo-oxidation in the presence of activated carbons, beyond the so-called synergistic effect reported in the literature for carbon/titania composites. A novel approach based on UV irradiation of carbons pre-loaded with phenol was used to demonstrate the anomalous photochemical response of carbon materials towards phenol degradation. Analysis of the catalytic reaction from a different standpoint (inside the carbonaceous matrix) demonstrated the catalytic activity of certain carbon materials for phenol photodegradation, without considering photolytic breakdown and adsorption kinetics. The pseudo photochemical quantum yield of several activated carbons was higher than that of photolysis under the same conditions; the nature of the degradation intermediates was also modified in the presence of the carbon materials. Moreover, the degradation of the adsorbed fraction retained inside the pore structure of the carbons has been demonstrated. Our results suggest the occurrence of carbon–photon interactions which could be propagated through the graphene sheets of the materials, and could reach the adsorbed molecules inside the pores.     

Photocatalytic degradation of methanol on titania and titania–silica aerogels prepared by non-alkoxide sol–gel route

Titania and titania–silica aerogels were prepared by alkoxide or non-alkoxide sol–gel route and subsequent supercritical drying with carbon dioxide at low temperature. The resulting aerogels having high surface area and mesoporosity were used as photocatalysts for gas phase methanol degradation reaction. Photocatalytic degradation reactions were carried out on titania and titania–silica aerogels, and commercial Degussa P-25 titania. The photocatalytic activities of titania and titania–silica aerogels were higher than that of the P-25. While the conversion of methanol degradation over the P-25 catalyst was only 50–60%, that for the titania aerogel was observed to be above 98% due to the higher specific surface area and the well developed mesoporous structure. In spite of lower titania contents, much higher surface area and high dispersion of titania of titania–silica aerogel gave rise to the high photocatalytic activity in comparison to those of titania aerogels. Moreover, titania–silica aerogel was also used for the photodegradation and adsorption hybrid system. It was observed that the high removal efficiency for methanol was caused by the combination of higher catalytic activity and adsorption capacity.     

Photocatalytic Degradation of Phenol with Fe-Titania Catalysts

Iron titanate (FeTiO₃) obtained from rich titanium and iron mineral presented some absorption of UV light and photocatalytic activity for the degradation of phenol in aqueous solution. However, most of the reactant is converted to carboxylic acids that remain in the reaction mixture because natural ilmenite do not produce enough HO^? radicals to completely mineralize the original reactant at the same rate as pure titania (Degussa P25). The XRD analysis of Fe-titania catalysts prepared by simple mixing of FeTiO₃ and TiO₂ showed that the structure of TiO₂ is not modified when small amounts of FeTiO₃, in the order of 10% weight, are well mixed with pure titania. Therefore, the photocatalytic activity of these catalysts is not affected by the presence of FeTiO₃. But, heavy ilmenite particles induce the separation of the catalysts particles after the photocatalytic degradation processes.     

Surface nano-aggregation and photocatalytic activity of TiO2 on H-type activated carbons

The main objective of this work is to detect any associative or synergistic effects between TiO₂ and activated carbon in 4-chlorophenol photodegradation. Different activated carbons (AC) were prepared from Tabebuia pentaphyla wood by means of physical activation with CO₂ or by extensive carbonization under N₂ flow at temperatures from 450 °C up to 1000 °C during 1 h. Results indicate a clear correlation between photocatalytic activities of titania with texture and surface chemistry of AC. Kinetic results of 4-chlorophenol photodegradation indicate that for most of mixed TiO₂ and AC solids a synergistic effect between both solids is observed. Surface nano-aggregation of TiO₂ on AC was observed by scanning electronic microscopy and dispersion of TiO₂ nanoparticles was improved as a function of more basic surface pH of AC. In conclusion, an increase of electronic density in carbon support clearly introduces an enhancement in titania's photoactivity for 4CP photodegradation. This beneficial effect indicates that it is possible to obtain clean water in a much shorter period of time by employing some selected AC in conjunction with TiO₂.     

Enhanced photocatalytic decomposition of 4-nonylphenol by surface-organografted TiO2: a combination of molecular selective adsorption and photocatalysis

n-Octyl-grafted TiO₂ (C8-TiO₂) was prepared as a model of a photocatalyst with high molecular adsorption selectivity, and its photocatalytic activity for the decomposition of dilute 4-nonylphenol (an endocrine disrupter, ca. 2 ppm) in water was investigated. The catalyst was highly active in the presence of concentrated phenol (1000 ppm), and the 4-nonylphenol was decomposed in 180 min, while pristine TiO₂ (P-25) under the same conditions showed much lower activity. The high C8-TiO₂ activity was ascribed to the molecular selective adsorption of the organic molecules on the alkyl-grafted hydrophobic surface. Infrared spectra showed that the grafted alkyl groups were gradually decomposed under photoirradiation. The model catalyst demonstrated that molecular selective adsorption is important for removal of low-concentration contaminants in the presence of other, more concentrated compounds.     

纳米复合结构二氧化钛薄膜的制备及其光降解水中若丹明B的能力

用双氧水低温氧化金属钛片的方法制备了TiO2纳米棒阵列,并以该阵列为基体,用浸渍渗透溶胶-凝胶技术制备了TiO2纳米颗粒嵌入TiO2纳米棒阵列基体的复合结构薄膜.用X射线衍射、场发射扫描电镜及光致发光光谱研究制得薄膜的结构和发光性能.结果表明:纳米棒阵列为金红石与锐钛矿的混晶结构,而溶胶-凝胶获得的TiO2为纯锐钛矿结构.在复合构造薄膜中TiO2纳米颗粒嵌入到纳米棒阵列间隙中,其光生电子-空穴对的空间分离效果得到明显改善.用制得的薄膜进行光催化降解水中若丹明B的实验结果显示:复合结构薄膜的光催化效率高于相应的纳米棒阵列,其表观反应常数是相同质量的溶胶-凝胶法制备薄膜的3倍,这是因为第二相TiO2纳米颗粒嵌入TiO2纳米棒阵列中促进了光生电子-空穴对的空间分离,从而提高了复合结构薄膜的光催化活性.     

Size dependent photocatalytic activity of hydrothermally crystallized titania nanoparticles on poorly adsorbing phenol in absence and presence of fluoride ion

The photocatalytic degradation of phenol has been performed by adopting nanosized titanium dioxide, prepared with the sol–gel method, both in presence and absence of fluoride ions. Several catalyst treatments, that is hydrothermal heat treatment and calcination, have been applied in order to increase the crystallinity of the particles. A close relationship was found between the rate of phenol disappearance and the particle size, with an efficiency that becomes maximum when the combination of large particle size (7.8 nm) and surficial sites covered by fluoride is fulfilled. Intermediates profiles have been also evaluated, in order to verify if the surficial process occurring in the diverse materials still remains the same. It is accomplished in the case of the fluorinated titania, where both the rate of disappearance and the intermediates formation closely resemble those seen on Degussa P25, while a different formation ratio between catechol and hydroquinone was observed in the case of naked titania.     

Catalytic wet air oxidation of phenol using active carbon: performance of discontinuous and continuous reactors

Catalytic wet air oxidation (CWAO) of an aqueous phenol solution using active carbon (AC) as catalytic material was compared for a slurry and trickle bed reactor. Semi‐batchwise experiments were carried out in a slurry reactor in the absence of external and internal mass transfer. Trickle‐bed runs were conducted under the same conditions of temperature and pressure. Experimental results from the slurry reactor study showed that the phenol removal rate significantly increased with temperature and phenol concentration, whereas partial oxygen pressure had little effect. Thus, at conditions of 160 °C and 0.71 MPa of oxygen partial pressure, almost complete phenol elimination was achieved within 2 h for an initial phenol concentration of 2.5 g dm^?3. Under the same conditions of temperature and pressure, the slurry reactor performed at much higher initial rates with respect to phenol removal than the trickle bed reactor, both for a fresh active carbon and an aged active carbon, previously used for 50 h in the trickle bed reactor, but mineralisation was found to be much lower in the slurry reactor. Mass transfer limitations, ineffective catalyst wetting or preferential flow in the trickle bed alone cannot explain the drastic difference in the phenol removal rate. It is likely that the slurry system also greatly favours the formation of condensation polymers followed by their irreversible adsorption onto the AC surface, thereby progressively preventing the phenol molecules to be oxidised. Thus, the application of this type of reactor in CWAO has to be seriously questioned when aiming at complete mineralisation of phenol. Furthermore, any kinetic study of phenol oxidation conducted in a batch slurry reactor may not be useful for the design and scale‐up of a continuous trickle bed reactor. © 2001 Society of Chemical Industry     

纳米二氧化钛光催化剂共掺杂的协同效应

综述了近年来共掺杂二氧化钛光催化剂的研究进展。对二氧化钛进行共掺杂,掺杂的元素产生协同效应,可以进一步提高二氧化钛的光催化降解效率。从促进二氧化钛可见光响应、抑制光生电子与空穴的复合、提高催化剂表面羟基含量以及其它协同效应等方面,阐述了共掺杂二氧化钛的协同作用机制,介绍了共掺杂二氧化钛协同作用的失效现象,并对共掺杂二氧化钛光催化剂未来的研究方向提出了建议。     

ZSM-5/TiO<Subscript>2</Subscript> Hybrid Photocatalysts: Influence of the Preparation Method and Synergistic Effect

In this work, the influence of the preparation method of ZSM-5/TiO₂ hybrids on the photocatalytic performance for removal of formaldehyde (HCHO) or trichloroethylene (C₂HCl₃) in gas phase was analyzed. For this purpose, two methods for the synthesis of the hybrids, the incipient wetness impregnation (I) and the mechanical mixing method (M), were selected. The photocatalysts were characterized by N₂ adsorption–desorption, TEM, UV–Vis spectroscopy, XRD and electrophoretic migration. Also, the adsorption ability of the individual materials and hybrids was analyzed. ZSM-5/TiO₂ hybrids showed higher photocatalytic activity than bare TiO₂, independently of the preparation method selected. Mechanical mixing is a simple and easily scalable method to prepare highly active photocatalyst with high amounts of titania. The internal diffusion processes of the reactants to the active sites could be improved due to the micro–mesoporous structure developed on these hybrids. Incipient wetness impregnation method leads to photocatalysts with higher photodegradation rates per active site. The hybrids synthetized by this method show TiO₂ nanoparticles homogeneously dispersed on the ZSM-5 phase. The fraction of TiO₂ exposed on the surface ca. 75 mol% was similar for materials prepared by both methods, explaining the similar adsorption and photocatalytic properties, independently of the TiO₂ content. The nature of the pollutant has an important role in the adsorption and photocatalytic properties of the composites. Finally, the effect of the incorporation of the zeolite in the photocatalytic system was analyzed. For this purpose, the influence of the zeolite and titania arrangement in the sample holder on the photodegradation rate was analyzed. Although the incorporation of the zeolite induces a positive effect on the photocatalytic performance, independently of the position on the sample holder, a clear synergistic effect when both phases were in intimate contact such as in the ZSM-5/TiO₂ hybrid was observed.     

Parameter effect on photocatalytic degradation of phenol using TiO2-P25/activated carbon (AC)

P25 powder embedded and TiO₂ immobilized on activated carbon (TiO₂-P25/AC) was prepared by P25 powder modified sol-gel and dip-coated method. The photocatalysts were characterized by XRD, BET, SEM and their photocatalytic activities were evaluated through phenol degradation in a fluidized bed photoreactor. The addition of P25 in the photocatalysts could significantly enhance the photocatalytic activity, and the optimum loading of P25 was 3 g L^?1. The operating parameter results indicated that the optimum pH for phenol degradation was 5.2; the effect of air flow rate gave an optimal value of 2 L min^?1; the increasing of UV light intensity led to an increase of degradation efficiency due to more photons absorbed on the surface of the photocatalyst. The kinetics of the phenol degradation fitted well with the Langmuir-Hinshelwood kinetics model. Finally, the photocatalytic ability of TiO₂-P25/AC was reduced only 10% after five cycles for phenol degradation.     

Influence of activated carbon upon titania on aqueous photocatalytic consecutive runs of phenol photodegradation

The photocatalytic degradation of phenol was performed at room temperature in aqueous suspended mixtures of TiO₂ and activated carbon (AC). The main objective of the present work was to verify the potential of TiO₂/AC system in the photocatalytic degradation of phenol and the principal intermediate products after performing three consecutive runs. The phenol disappearance follows a first-order kinetics. Therefore, the apparent first-order rate constant of phenol and total organic carbon photodegradations were selected to evaluate the photoefficiency of the system. From the present results it can be concluded that there is a synergistic effect between both solids which is determined by the numbers of photocatalytic runs. From a practical point of view, TiO₂/AC is able to photomineralize phenol and total organic carbon for three and two, respectively, consecutive runs more efficiently than TiO₂ alone.     

Comparison of several titanium dioxides for the photocatalytic degradation of benzenesulfonic acids

The photocatalytic efficiency of several TiO₂ (namely Degussa P25 and Millennium PC50, PC100, PC105, PC500) used as suspensions are compared for the photocatalytic degradation of 3-nitrobenzenesulfonic acid (3-NBSA) and 2,5-anilinedisulfonic acid (2,5-ADSA). With 3-NBSA, P25 is clearly the most efficient and there is no apparent relationship between photocatalytic activity and specific surface area. This result is consistent with that obtained with phenol, but a contrast was noticed with 2,5-ADSA where PC500 is more efficient than P25 in spite of a higher adsorption of the latter. In the case of TiO₂ Millennium relative photonic efficiencies ζ_r are smaller with 3-NBSA than with 2,5-ADSA, that is consistent with the electron-withdrawing effect of nitro group. For the same reason the direct phototransformation in sunlight is much more rapid with of 2,5-ADSA than for 3-NBSA and 4-nitrotoluene-2-sulfonic acid.     

A new active sulfur-sensitive bismuth oxide on titania for photocatalyst

Bismuth oxide on titania catalyst was prepared and characterized, and its photocatalytic activitiy was tested. Also investigated was the possibility of a sulfur-sensitive photocatalytic property of bismuth oxide on titania catalyst. It showed high photocatalytic activity, especially in the presence of polysulfide as one of the sacrificial agents. The activity of the evolution of hydrogen from the solution containing polysulfide showed a maximum at 5 wt% of loading of bismuth oxide on titania. The main cause of its activity, which could be explained by the amounts of sulfur and of hydroxyl ion adsorbed on bismuth oxide, was inferred from the characterization of the surface state of bismuth oxide on titania, considering the extent of exposure of bismuth on the catalyst surface which could provide adsorption sites of sulfur in solution if treated by reduction and/or oxidation.     

Fabrication of titania thin film with composite nanostructure and its ability to photodegrade rhodamine B in water

A titania nanorod film was synthesized by direct oxidation of metallic Ti with hydrogen peroxide solution under a low temperature. Titania nanoparticles were then filled into the gaps among the nanorods through an infiltration sol-gel procedure to form a composite titania film with an ordered nanostructure. X-ray diffraction spectra indicate that the composite film was a mixture of anatase and rutile while the titania film obtained by only using a sol-gel procedure was pure anatase. Field emission scanning electron microscopy observations show that titania nanoparticles were embedded into the titania nanorod film. Photoluminescence spectra suggest the enhanced separation of electron and hole pairs for the obtained composite titania film over the corresponding titania nanorod film. The composite titania film exhibited improved ability to photodegrade rhodamine B in water compared with the titania nanorod film. The apparent photodegradation rate constant, fitting a pseudo-first-order, was 3 times of that obtained by the sol-gel derived titania film at the same weight. The improved photocatalytic activity for the composite titania film could be attributed to the enhanced separation of electron and hole pairs due to the embedding of the titania nanoparticles within the titania nanorods.