Mass transfer measurements and modeling in a microchannel photocatalytic reactor

The main objective of this paper was to evaluate the influence of mass transfer on the photocatalytic efficiency at a low flow rate in the order of several mL per hour. Several continuous flow microchannel reactors have been used to study the degradation of salicylic acid (SA) taken as a model pollutant. The photocatalytic degradation of salicylic acid, under UV illumination of 1.5 mW cm⁻², was assessed from the outlet concentration measured by liquid chromatography HPLC. It was shown that the degradation of SA by UV was limited by mass transfer. Numerical simulations have allowed establishing a relationship of the Sherwood number valuable for all the microchannel geometries. Computational fluid dynamics with Comsol Multiphysics is useful for predicting the degradation yield for a given geometry of the microreactor. The best representation of the experimental data is obtained by introducing a kinetic law taking into account mass transfer limitation.     

Comparison of the efficiency of immobilized and suspended systems in photocatalytic degradation

The photocatalytic degradation of formic acid in suspended and immobilized systems, with and without oxygen addition, are compared. In the immobilized system, oxygen addition to the reactor appeared to increase the efficiency, not only because oxygen acts as an efficient electron scavenger, but also due to increased mass transfer in this two-phase reactor. This immobilized system had an efficiency comparable to that of the suspended system. The addition of oxygen to the immobilized system appeared to increase the quantum yield with a factor 4, whereas the addition of oxygen to the suspended system hardly had any effect.     

Light-transmitting AlON carrier with enhanced photocatalytic degradation performance

A skeleton structure AlON (aluminum oxynitride) carrier with light transmission performance was prepared by carbothermal reduction and nitridation (CRN). The AlON carrier can disperse nano active components and enhance its photocatalytic performance. The microstructure of the TiO₂/AlON composite photocatalyst shows that the dispersion of the agglomerated TiO₂ is achieved by the physical adsorption mechanism, the embedding mechanism and the single particle dispersion mechanism. The morphology of the BiVO₄/AlON and g-C₃N₄/AlON composite photocatalysts further proves the universality of the AlON carrier in the dispersion of nano photocatalyst. The photocatalytic performance of the nano active component was enhanced by combining with AlON carrier. After 240 min of visible light irradiation, the degradation rate of MB increased by 7.4% (BiVO₄/AlON) and 8.1% (g-C₃N₄/AlON), respectively. High light transmittance was the main reason for AlON carrier to improve the photocatalytic performance of loaded active components. In addition, AlON carrier type composite photocatalyst can be reused for many times because it has physicochemical stability and can be recovered easily.     

Role of Cu in the Cu-TiO2 photocatalytic degradation of dihydroxybenzenes

The photocatalytic degradation of three dihydroxybenzenes (hydroquinone, catechol and resorcinol) by TiO₂ and Cu-TiO₂ has been studied. Degradation rates depend on the molecule catalyst surface interaction type. FTIR studies have shown that Cu presence modifies the organics interactions with the catalysts surface. Therefore, catalysts have shown different types of behaviour with each compound.

Resorcinol interaction with Cu-TiO₂ may produce a hydroxyphenolate thus reducing the number of activating centres in the aromatic ring. However, catechol interaction provides a catecholate bidentate which virtually deactivates the aromatic ring, hence reducing its degradation by OH radicals attacks. Hydroquinone interaction with Cu-TiO₂ occurs by means of a hydrogen bond, while with TiO₂ this interaction is established through the formation of a hydroxyphenolate monodentate. This justifies the improved catalytic behaviour of Cu-TiO₂.

Studies in presence of H₂O₂ have shown the existence of similar degradation mechanisms, but degradation rates have been notably higher.     

Physicochemical properties and photocatalytic activities of TiO2-films prepared by sol–gel methods

The physicochemical properties, the photocatalytic activities in aqueous solution and the adhesion properties of supported TiO₂ films prepared by different sol–gel methods have been studied. The thickness, the TiO₂ loading and the photocatalytic activities are influenced by the nature of the stabilising agent. By contrast, the nature of the organic titanium precursor, as well as the solvent and the absence of stabilising agents are determining for the resulting photocatalytic activities. Titania-sol generated by non-controlled hydrolysis of titanium isopropoxide was used to determine the influence (i) of number of coating, (ii) of the calcination temperature and (iii) of the nature of support on the photocatalytic activity under direct and backside irradiation. The higher the coating number, the thicker the TiO₂-film and the higher the photocatalytic efficiency. the optimum calcination temperature was found to be 400 °C. Migration of cationic species into TiO₂-films and the decrease of thickness at higher temperatures lead to the decline of activity.     

Kinetic effects of SDS on the photocatalytic degradation of 2,4,5-trichlorophenol

The effect of sodium dodecyl sulphate (SDS) on the photocatalytic degradation of 2,4,5-trichlorophenol (2,4,5-TCP) in aqueous TiO₂ dispersions under irradiation with simulated AM1 solar light was investigated. A peculiar kinetic behaviour was observed, with increasing inhibition of the substrate degradation as a function of SDS concentration below the critical micellar concentration (cmc), followed by a subsequent faster degradation around this concentration range and a further degradation rate decrease above the cmc. The role of surfactant adsorption onto the suspended semiconductor and the disaggregation of TiO₂ microparticles seems to be crucial. The complete degradation of the investigated substrate, still achievable in the presence of SDS without relevant changes in the reaction mechanism, was confirmed suggesting the possible application of photocatalysis for the treatment of surfactant-containing wastes.     

Effect of dissolved oxygen concentration and light intensity on photocatalytic degradation of phenol

Catalyst loading is an important parameter that needs to be optimized in the operation of photocatalytic slurry reactors. In this study on photocatalytic degradation of phenol, the optimum catalyst loading was found to depend mainly on the dissolved oxygen (DO) concentration in the aqueous solution, especially at higher irradiation intensities. The estimated Langmuir-Hinshelwood (L-H) kinetics constants were found to vary with light intensity and dissolved oxygen concentration. The intermediate products of photocatalytic oxidation were identified.     

Effects of radiation absorption and catalyst concentration on the photocatalytic degradation of pollutants

This study is dealing with the reaction rate dependence on catalyst concentration and light absorption in photocatalytic processes. Models relating the reaction rate to the absorbed radiation by the catalyst (titania in suspension) are proposed. To apply these models, the system is divided into layers and each layer is divided into cells, assuming that there is only one particle (agglomerate) of catalyst in each cell. The extensive reaction rate can be calculated as the sum of the reaction rate in each cell, being this proportional to the light absorbed by each particle. Two different models are proposed for light propagation through the reaction medium (exponential and probabilistic model). The extinction coefficients have been estimated by using transmittance measurements related to sedimentation rates changing according to catalyst concentration and pH. The integration of these models, taking into account the expression of the reaction rate, allows to obtain equations that can explain the trends observed in the photocatalytic treatment of Cr(VI) and phenol, by using TiO₂ (Degussa-P25) in suspension.     

Design consideration of photocatalytic oxidation reactors using TiO2-coated foam nickels for degrading indoor gaseous formaldehyde

In the design process of the photocatalytic oxidation (PCO) reactor using TiO₂-coated foam nickels, the optimum of catalyst film thickness, light intensity and flow velocity were considered. A model was developed to study the effect of catalyst film thickness on photocatalytic degradation of formaldehyde by a TiO₂-coated foam nickel at continuous flow mode. In this model, external mass transfer and internal molecule diffusion-reaction were considered. A first-order kinetics equation was used to account for the photocatalytic reaction. Two exponential equations were employed to describe the distribution of light intensities in foam nickels and catalyst films, respectively. Validated with experimental data, the model can be used to predict the optimal thickness of catalyst films. A method for determining appropriate light intensities was proposed and discussed. The appropriate light intensity can be obtained by giving a margin, regarded as an excess coefficient, to the light intensity calculated based on the assumption of complete use of excited electron–hole pairs. The excess coefficient needs to be determined experimentally. In addition, the optimal flow velocity of PCO reactors could be consistent with the required one by changing the windward area of foam nickels. Based on the theoretical analyses, a novel PCO reactor containing 15 parallel-connected cells was designed. Each reaction cell was composed of an UV lamp and a TiO₂-coated tubular foam nickel. The performance of the reactor was tested by degrading gaseous formaldehyde at an indoor concentration level. The results showed that the reactor had low pressure loss and good degradation capability.     

Study of the synergic effect of sulphate pre-treatment and platinisation on the highly improved photocatalytic activity of TiO2

An important improvement of the photocatalytic activity of sol–gel prepared TiO₂ has been achieved by sulphate pre-treatment, calcination at high temperature and further platinisation of the samples.

The presence of sulphuric acid clearly stabilised TiO₂ surface area against sintering, maintaining at the same time anatase phase until higher calcination temperatures than in non-sulphated samples. Platinisation of the samples with different nominal amounts of platinum (from 0.5 to 2.5 wt%) was performed and the influence of sulphate treatment on the dispersion and deposit size of platinum on the TiO₂ surface was studied.

Characterisation results and photocatalytic activity of these catalysts were compared with those of unmodified TiO₂. Simultaneously sulphated and platinised TiO₂ samples were highly active for phenol degradation, used as model reaction for the photocatalytic studies, having higher activities than only platinised or only sulphated samples. The activity of these samples were several orders of magnitude higher than that of the commercial TiO₂ Degussa P25 (platinised or unmodified) as well, with independence of the nominal amount of platinum of the samples.

A wide characterisation of the samples was performed and correlations between characterisation results and activity properties are reported.     

焦化废水中挥发酚光催化降解去除影响因素的研究

笔者以紫外光为光源,对焦化废水中挥发酚进行了光催化降解去除的实验研究,系统研究了催化剂用量、焦化废水pH值、温度、搅拌强度等因素对去除焦化废水中挥发酚的影响。结果表明,随着催化剂用量和pH值的增加,挥发酚的去除率先提高后降低;挥发酚的去除率首先随着温度和搅拌强度的增加而提高,当达到一定值后基本不再变化。光催化去除挥发酚过程符合一级动力学反应规律。并且随着挥发酚初始浓度的增加,表观速率常数降低。向废水中曝气可提高挥发酚去除率,但曝气量存在最佳范围。     

Computation and measurements of mass transfer and dispersion coefficients in fluidized beds

Conventional design of circulating fluidized beds requires the knowledge of dispersion and mass transfer coefficients, expressed in dimensionless forms as Sherwood numbers. However, these are known to vary by five or more orders of magnitude. Furthermore, the Sherwood numbers for fine particles reported in the literature are several orders of magnitude lower than the Sherwood number of two for diffusion to a single particle. We have shown that by replacing the particle diameter in the conventional Sherwood number with cluster or bubble diameter, the modified Sherwood number is again of the order of two.We have also shown that the kinetic theory based computational fluid dynamics codes correctly compute the dispersion and mass transfer coefficients. Hence, the kinetic theory based computational fluid dynamics codes can be used for fluidized bed reactor design without any such inputs.     

TIO2-photocatalyzed degradation of phenol and ortho-substituted phenolic compounds

The photocatalytic degradation of phenol, guaiacol, 2-chlorophenol and catechol in aqueous suspensions of TiO₂ under different experimental conditions has been investigated. The photodegradation of the different organics follows a Langmuir–Hinshelwood kinetics, showing rate constants that decrease in the order: guaiacol>2-chlorophenolphenol>catechol. A similar trend is also observed, except for catechol, for the stability of the σ-complexes that may be formed between the aromatic ring and the OH√ radical. From different analytical techniques (HPLC, GC/MS and HPLC/MS), various hydroxylated intermediate compounds have been reported and different mechanisms of degradation of the starting compounds have been proposed. From experiments performed using aqueous solutions containing the four organics, it is observed that the competitive Langmuir–Hinshelwood kinetics for the degradation of each one of the phenolic compounds is obeyed.     

A route for the study on mass transfer enhancement by adding particles in liquid phase

This work presents a route for the study on the absorption performance of gas into liquid under the condition of adding particles in a stirred constant temperature reactor. Two evaluated systems, hydrogen–water and hydrogen–methanol, with the addition of activated carbon particles (ACP) were carried out, respectively. The results showed that the addition of ACP into the water can enhance the mass transfer between hydrogen and water, the enhancement factor increases rapidly with the increase of the ACP content, and then tends to be unchanged. However, for the hydrogen–methanol system, ACP has little effect on the mass transfer performance. In addition, a gas–liquid mass transfer model considering the effect of solid particle enhancement was established based on the shuttle effect and two-film model. Results indicated that the predicted value agreed well with the experimental value in both hydrogen–methanol–ACP and hydrogen–water–ACP systems.     

Performance characteristics of a packing with boundary layer turbulizers. III. Liquid film controlled mass transfer

Liquid film controlled mass transfer in a novel design of a honeycomb packing (Turbo-Pack) with turbulizers in the region of the boundary layer has been studied. For comparison, packings without turbulizers have also been investigated. Dimensionless equations for evaluation of mass transfer are proposed and their constants have been analyzed statistically. It is shown that the presence of turbulizers leads to an increase of the mass-transfer coefficient by up to 55% and to a decrease of up to 4.5-times in the packing pressure drop per mass transfer unit. A comparison over the most effective packings known from the literature shows that, as far as its low pressure drop per mass transfer unit is concerned, the novel packing is superior to all other packings.     

离子对于TiO_2光催化处理废水的影响

废水中一般会有离子存在,而离子对于TiO2光催化作用的影响是非常显著的。对于水体中常见的阳离子、阴离子、高浓度和多种离子共存的影响进行了归纳和分析,其中对于高浓度和多种离子共存的影响进行探讨是较少见报道的。此简述以期为相关研究的拓展提供参考。     

One step synthesis and characterization of copper doped sulfated titania and its enhanced photocatalytic activity in visible light by degradation of methyl orange

This paper reports on the synthesis of copper doped sulfated titania nano-crystalline powders with varying(2.0%–10.0%, by mass) by single step sol gel method. The synthesized photo catalyst has been characterized by employing various techniques like X-ray Diffraction(XRD), Ultraviolet–Visible Diffuse Reflection Spectroscopy(UV–Vis DRS), X-ray Photoelectron Spectroscopy(XPS), Scanning Electron Microscopy(SEM), Energy Dispersive Spectrometry(EDS), Fourier Transform Infrared Spectroscopic Studies(FT-IR), and Transmission Electron Microscopy(TEM). From the XRD and TEM results, all the samples were reported in anatase phase with reduction in particle size in the range of 7 to 12 nm. SEM indicated the change in morphology of the particles. The presence of copper in titania lattice was evidenced by XPS. From UV–Vis DRS and FT-IR studies indicated that prominent absorption shift is observed towards visible region(red shift), the entry of Cu2+into Ti O2 lattice as a substitutional dopant and SO42-ions were covalently bonded with Ti4 +on the surface of the copper doped titania respectively.The photocatalytic activity studies were investigated by considering methyl orange(MO) as dye pollutant in the presence of the visible light. The effect of various parameters like effect of dosage of the catalyst, dopant concentration, p H of the solution, and concentration of the dye was studied in detail.     

FTIR study of gas-phase alcohols photocatalytic degradation with TiO2 and AC-TiO2

In this paper the degradation of gaseous alcohols (methanol, ethanol, 1-propanol and 1-butanol) has been studied in a continuous reaction system. Methanol and ethanol, but not 1-propanol and 1-butanol, have caused the catalyst bare-TiO₂ to become deactivated. The catalyst containing activated carbon, AC-TiO₂, has not shown almost deactivation in any experiment.

To explain this behaviour variation, a thorough FTIR study concerning the interaction of gaseous alcohols with the catalyst surface and the effect of light on the process was performed. Thus, the interaction of the different catalysts with the selected alcohols, the formed species and the hydroxyl group evolution under irradiation was studied.     

Photocatalytic degradation of 4-nitroaniline using solar and artificial UV radiation

The photocatalytic degradation (PCD) of 4-nitroaniline was studied in the presence of TiO₂ suspensions in a batch and continuous annular reactor. Artificial and solar radiation was employed as sources of UV radiation. The effect of catalyst loading, pH, presence of anions and initial concentration on the rate of photocatalytic degradation was investigated. p-Aminophenol, p-benzoquinone and hydroquinone were identified as the intermediates during the degradation process. A kinetic expression for PCD of 4-NA is provided.     

Syntheses of TiO2 photocatalysts by the molten salts method: Application to the photocatalytic degradation of Prosulfuron

Titania photocatalysts were synthesised by the molten salts method by reaction of a titanium precursor with three different alkali metal nitrates (LiNO₃, NaNO₃, KNO₃). The titania powders obtained have been characterised using TEM, XRD, BET and UV-Vis absorption spectroscopy. Their photoactivities were evaluated by degrading a commercial sulfonylurea herbicide, Prosulfuron^®. It has been found that the rate constants increased in the following order: Li2 prepared in KNO₃, with a rate constant 1.4 times smaller than that of P25. The effect of calcination was also studied, especially for the sample prepared in NaNO₃ (TiO₂[Na]). An improvement in photocatalytic activity was observed when TiO₂[Na] was calcined in the region of 700–800 °C. Even though the photocatalytic activities obtained did not exceed or even equal to that of Degussa P25, nevertheless, this study constitutes the first attempt to synthesise titania photocatalysts by the molten salts method.