Photocatalytic degradation kinetics of di- and tri-substituted phenolic compounds in aqueous solution by TiO2/UV

In this study, photocatalytic degradation of 2,4,6-trimethylphenol (TMP), 2,4,6-trichlorophenol (TCP), 2,4,6-tribromophenol (TBP), 2,4-dimethylphenol (DMP), 2,4-dichlorophenol (DCP) and 2,4-dibromophenol (DBP) has been studied by TiO₂/UV. Although degraded phenolic compound concentration increased by increasing initial concentration photocatalytic decomposition rates of di- and tri-substituted phenols at 0.1–0.5 mM initial concentrations decreased when the initial concentration increased. The fastest degradation observed for TCP and the slowest for TMP. Photodegradation kinetics of the compounds has been explained in terms of Langmuir–Hinshelwood kinetics model. Degradation rate constants have been observed to be extremely depended on electronegativity of the substituents on phenolic ring. Degradation rate constant and adsorption equilibrium constant of TCP were calculated as k 0.0083 mM min⁻¹ and K 9.03 mM⁻¹. For TBP and TMP the values of k and K were obtained as 0.0040 mM min⁻¹, 19.20 mM⁻¹, and 0.0017 mM min⁻¹, 51.68 mM⁻¹, respectively. Degradation rate constant of DBP was similar as DCP (0.0029 mM min⁻¹ for DBP and 0.0031 mM min⁻¹ for DCP) whereas adsorption equilibrium constants differed (48.40 mM⁻¹ for DBP and 30.52 mM⁻¹ for DCP). K and k of DMP found as 83.68 mM⁻¹ and 0.0019 mM min⁻¹, respectively. The adsorption equilibrium constants in the dark were ranged between 1.11 and 3.28 mM⁻¹ which are lower than those obtained in kinetics. Adsorption constants have inversely proportion with degradation rate constants for all phenolic compounds studied.     

H4SiW12O40-catalyzed oxidation of nitrobenzene in supercritical water: kinetic and mechanistic aspects

The catalytic effect of a heteropolyacid, H₄SiW₁₂O_40, on nitrobenzene (20 and 30 μM) oxidation in supercritical water was investigated. A capillary flow-through reactor was operated at varying temperatures (T=400–500 °C; P=30.7 MPa) and H₄SiW₁₂O₄₀ concentrations (3.5–34.8 μM) in an attempt to establish global power-law rate expressions for homogenous H₄SiW₁₂O₄₀-catalyzed and uncatalyzed supercritical water oxidation. Oxidation pathways and reaction mechanisms were further examined via primary oxidation product identification and the addition of various hydroxyl radical scavengers (2-propanol, acetone, acetone-d₆, bromide and iodide) to the reaction medium. Under our experimental conditions, nitrobenzene degradation rates were significantly enhanced in the presence of H₄SiW₁₂O₄₀. The major differences in temperature dependence observed between catalyzed and uncatalyzed nitrobenzene oxidation kinetics strongly suggest that the reaction path of H₄SiW₁₂O₄₀-catalyzed supercritical water oxidation (average activation E_a=218 kJ/mol; k=0.015–0.806 s⁻¹ energy for T=440–500 °C; E_a=134 kJ/mol for the temperature range T=470–490 °C) apparently differs from that of uncatalyzed supercritical water oxidation (E_a=212 kJ/mol; k=0.37–6.6 μM s⁻¹). Similar primary oxidation products (i.e. phenol and 2-, 3-, and 4-nitrophenol) were identified for both treatment systems. H₄SiW₁₂O₄₀-catalyzed homogenous nitrobenzene oxidation kinetics was not sensitive to the presence of OH√ scavengers.     

Kinetics of Fischer–Tropsch synthesis on titania-supported cobalt

Rate data have been obtained for CO hydrogenation on a well-characterized 11.7% Co/TiO₂ catalyst in a differential fixed bed reactor at 20 atm, 180–240°C, and 5% conversion over a range of reactant partial pressures. The resulting kinetic parameters can be used to model precisely and accurately the kinetics of this reaction within this range of conditions. Turnover frequencies and rate constants determined from this study are in very good to excellent agreement with those obtained in previous studies of other cobalt catalysts, when the data are normalized to the same conditions of temperature and partial pressures of the reactants. Based on this comparison CO conversion and the partial pressure of product water apparently have little effect on specific rate per catalytic site. The data of this study are fitted fairly well by a simple power law expression of the form −r_CO=kP_H2^0.74P_CO^−0.24, where k=5.1×10⁻³ s⁻¹ at 200°C, P=10 atm, and H₂/CO=2/1; however, they are best fitted by a simple Langmuir–Hinshelwood (LH) rate form −r_CO=aP_H2^0.74P_CO/(1+bP_CO)² similar to that proposed by Yates and Satterfield.     

Modelling formation and growth of H2SO4-H2O aerosols: Uncertainty analysis and experimental evaluation

An aerosol dynamics model, AERO2, is presented, which describes the formation of H₂SO₄-H₂O aerosol in a smog chamber. The model is used to analyse how the uncertainties on four input parameters are propagated through an aerosol dynamics model. The input parameters are: the rate of the reaction between SO₂ and OH (k₁), the ratio between the nucleation rate used in AERO2 and that derived from classical nucleation theory (t_n), the H₂SO₄ mass accommodation coefficient () and a measure of the turbulence intensity in the reactor (k_e). Uncertainties for these parameters are taken from the literature. One of the results of the analysis is that AERO2 and aerosol dynamics models in general can only predict upper bounds for the total number (N_tot) and total volume (V_tot) concentrations of the particles. The uncertainties on N_tot and V_tot are mainly due to the uncertainties on k₁, and t_n. An uncertainty factor of 20–100 still remains when the uncertainty on k₁, is reduced to ±5%. Aerosol measurements from three smog chamber experiments have therefore been used, in an attempt to reduce the uncertainty on k₁ and t_n. Values for k₁ are obtained in the reduced range 7.8 × 10⁻¹³ to 1.0 × 10⁻¹² cm³ s⁻¹, which is within the range found in the literature. For t_n, values in the range 10⁴–10⁷ are obtained, which is close to the upper bound of the range in literature. These values for t_n are in marked contrast with a recent set of experiments on nucleation in H₂SO₄-H₂O mixtures, which suggests a value for t_n of at most 10⁻⁵.     

Kinetics and equilibrium swelling of gelatine gels

The swelling features of gelatine gels in water (good solvent) were studied as a function of thermodynamic conditions of sol—gel transition and ripening. It is shown that the degree of equilibrium swelling Q_e varies with the volume fraction of the polymer in a casting solution φ_o in accordance with the prediction of the classic theory: Q_e φ_o^−0.4. Q_c, as a function of the gelation temperature T_g, the ripening time t_r and φ_o, can be rescaled and described by the single empirical equation: Q_e T_g^−x t_r^−yφ_o^−0.4, where x = 0.1, y = 0.15 for wet gels and X = −0.5, y = 0.04 for dried gels. The kinetics of macroscopic swelling is described by the equation of Peters and Candau, with values of collective diffusion coefficients being in good agreement with values obtained by other workers via photon correlation spectroscopy.     

Kinetics of the water–gas shift reaction on Pt catalysts supported on alumina and ceria

We report the kinetic parameters for the water–gas shift (WGS) reaction on Pt catalysts supported on ceria and alumina under fuel reformer conditions for fuel cell applications (6.8% CO, 8.5% CO₂, 22% H₂O, 37.3% H₂, and 25.4% Ar) at a total pressure of 1 atm and in the temperature range of 180–345 °C. When ceria was used as a support, the turnover rate (TOR) for WGS was 30 times that on alumina supported Pt catalysts. The overall WGS reaction rate (r) on Pt/alumina catalysts as a function of the forward rate (r_f) was found to be: r = r_f(1 − β), where r_f = k_f[CO]^0.1[H₂O]^1.0[CO₂]^−0.1[H₂]^−0.5, k_f is the forward rate constant, β = ([CO₂][H₂])/(K_eq[CO][H₂O]) is the approach to equilibrium, and K_eq is the equilibrium constant for the WGS reaction. The negative apparent reaction orders indicate inhibition of the forward rate by CO₂ and H₂. The surface is saturated with CO on Pt under reaction conditions as confirmed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The small positive apparent reaction order for CO, in concert with the negative order for H₂ and the high CO coverage is explained by a decrease in the heat of adsorption as the CO coverage increases. Kinetic models based on redox-type mechanisms can explain the observed reaction kinetics and can qualitatively predict the changes in CO coverage observed in the DRIFTS study.     

Electrosorption and inhibition properties of p-norborn2-yl phenolate ions at the mercury/solution interface

The electrosorption properties of p-norborn-2-yl phenolate ions in alkaline solutions were investigated by ac polarographic and electrocapillary measurements.

Two adsorption regions were found. At low bulk surfactant concentrations the adsorption at the positively charged electrode (−0.2 E −0.6 V) is predominant while at higher surfactant concentrations the adsorption at the negatively charged electrode (−0.6 E −1.0 V) is more pronounced. At E = −0.40 V the adsorption parameters were determined (a ≈ 2; ΔG°_A = −32.5 ± 1 kJ mol⁻¹. Between −0.6 E −1.0 V one potential of maximum adsorption for all concentrations does not exist and therefore the adsorption parameters could not be calculated.

At E = −0.40 V progressive two-dimensional nucleation with a nucleation order of 3 was observed which corresponds well with the high attraction constant.

The electrode reaction S₂O²⁻₈ + 2e⁻ → 2 SO²⁻₄ is inhibited by norborn-2-yl phenolate ions in the potential range −0.2 E −0.6 V. In the second potential range of capacity decrease the electrode process is much less retarded. At E = −0.40 V, in a similar manner as described for neutral molecules, a linear dependence of the log k_s (k_s apparent rate constant) on ln c_A and π (π = surface film pressure), respectively, has been found.     

Dilute solution properties of carboxymethylchitins in high ionic-strength solvent

The hydrodynamic characteristics in aqueous solution at ionic strength I=0.2  of carboxymethylchitins of different degrees of chemical substitution have been determined. Experimental values varied over the following ranges: the translational diffusion coefficient (at 25.0°C), 1.1<10⁷×D<2.9 cm² s⁻¹; the sedimentation coefficient, 2.4<s<5.0 S; the Gralen coefficient (sedimentation concentration-dependence parameter), 130<k_s<680 mL g⁻¹; the intrinsic viscosity, 130<[η]<550 mL g⁻¹. Combination of s with D using the Svedberg equation yielded ‘sedimentation–diffusion' molecular weights in the range 40 000<M<240 000 g mol⁻¹. The corresponding Mark–Houwink–Kuhn–Sakurada (MHKS) relationships between the molecular weight and s, D and [η] were: [η]=5.58×10⁻³ M^0.94; D=1.87×10⁻⁴ M^−0.60; s=4.10×10⁻¹⁵ M^0.39. The equilibrium rigidity and hydrodynamic diameter of the carboxymethylchitin polymer chain is also investigated on the basis of wormlike coil theory without excluded volume effects. The significance of the Gralen k_s values for these substances is discussed.     

Cl含量对钙基吸收剂微观结构以及动力学性能的影响

通过浸渍法向分析纯CaCO₃中添加Cl,在双固定床反应器系统和热重分析仪上研究了其对钙基吸收剂循环捕集CO₂性能的影响,利用离子反应模型对添加Cl后吸收剂化学反应控制阶段进行动力学分析。结果显示：Cl对钙基吸收剂循环捕集CO₂性能具有不利影响。当Cl/Ca摩尔比大于0.25%后,随Cl/Ca摩尔比增加,化学反应控制阶段反应速率和持续时间均减小,导致在该阶段最终碳酸化转化率降低。对添加Cl前后吸收剂孔隙分布特性进行分析发现,添加Cl导致煅烧后吸收剂烧结加剧,比表面积降低,10~120nm范围内孔分布减少,导致CO₂在吸收剂内部扩散阻力增加,同时能与CO₂反应的CaO量减少,这是导致吸收剂化学反应控制阶段碳酸化反应速度较慢、最终碳酸化转化率较低的主要原因。鉴于Cl的不利影响,在选择钙基材料作为CO₂吸收剂或合成高活性复合吸收剂时,应避免吸收剂中Cl含量过高。     

Decolorization of organic dyes on Pilkington Activ™ photocatalytic glass

The air–solid photocatalytic degradation of organic dye films Acid Blue 9 (AB9) and Reactive Black 5 (RBk5) is studied on Pilkington Activ™ glass. The Activ™ glass comprises of a colorless TiO₂ layer deposited on clear glass. The Activ™ glass is characterized using atomic force microscopy (AFM) and X-ray diffraction (XRD). Using AFM, the TiO₂ average agglomerate particle size is 95 nm, with an apparent TiO₂ thickness of 12 nm. The XRD results indicate the anatase phase of TiO₂, with a calculated crystallite size of 18 nm.

Dyes AB9 and RBk5 are deposited in a liquid film and dried on the Activ™ glass to test for photodecolorization in air, using eight UVA blacklight-blue fluorescent lamps with an average UVA irradiance of 1.4 mW/cm². A novel horizontal coat method is used for dye deposition, minimizing the amount of solution used while forming a fairly uniform dye layer. About 35–75 monolayers of dye are placed on the Activ™ glass, with a covered area of 7–10 cm². Dye degradation is observed visually and via UV–vis spectroscopy.

The kinetics of photodecolorization satisfactorily fit a two-step series reaction model, indicating that the dye degrades to a single colored intermediate compound before reaching its final colorless product(s). Each reaction step follows a simple irreversible first-order reaction rate form. The average k₁ is 0.017 and 0.021 min⁻¹ for AB9 and RBk5, respectively, and the corresponding average k₂ is 2.0 × 10⁻³ and 1.5 × 10⁻³ min⁻¹. Variable light intensity experiments reveal a p = 0.44 ± 0.02 exponent dependency of initial decolorization rate on the UV irradiance. Solar experiments are conducted outdoors with an average temperature, water vapor density, and UVA irradiance of 30.8 °C, 6.4 g water/m³ dry air, and 1.5 mW/cm², respectively. For AB9, the average solar k₁ is 0.041 min⁻¹ and k₂ is 5.7 × 10⁻³ min⁻¹.     

Kinetics of the water–gas shift reaction over nanostructured copper–ceria catalysts

Water–gas shift reaction was studied over two nanostructured Cu_xCe_1−xO_2−y catalysts: a Cu_0.1Ce_0.9O_2−y catalyst prepared by a sol–gel method and a Cu_0.2Ce_0.8O_2−y catalyst prepared by co-precipitation method. A commercial low temperature water–gas shift CuO–ZnO–Al₂O₃ catalyst was used as reference. The kinetics was studied in a plug flow micro reactor at an atmospheric pressure in the temperature interval between 298 and 673 K at two different space velocities: 5.000 and 30.000 h⁻¹, respectively. Experimentally estimated activation energy, E_af, of the forward water–gas shift reaction at CO/H₂O = 1/3 was 51 kJ/mol over the Cu_0.1Ce_0.9O_2−y, 34 kJ/mol over the Cu_0.2Ce_0.8O_2−y and 47 kJ/mol over the CuO–ZnO–Al₂O₃ catalyst. A simple rate expression approximating the water–gas shift process as a single reversible surface reaction was used to fit the experimental data in order to evaluate the rate constants of the forward and backward reactions and of the activation energy for the backward reaction.     

Thermal aging of Pd/Pt/Rh automotive catalysts under a cycled oxidizing–reducing environment

Mono- and multi-metallic (bi- and tri-) Pt, Pd and Rh supported on cerium-promoted alumina (La Roche, SAS-1/16) catalysts were tested for activity as TWC, both fresh [G.C. Koltsakis, and A.M. Stamatelos, Progr. Energy Combust. Sci. 23 (1997) 1] and after accelerated aging. Aging consisted of a treatment at 900°C for 5 h during which an oxidizing (2.5% O₂, 10% H₂O, in N₂) and a reducing (5.0% CO, 10% H₂O, in N₂) feedstream were cycled at 0.017 Hz through the catalyst. Activity tests were carried out by increasing temperature from 100 to 600°C at 3°C min⁻¹, while two oxidizing and reducing (±0.5 A/F) feedstreams were alternately (1 Hz) fed through the reactor at 125 000 h⁻¹ (STP). Conversion was continuously analyzed. Light-off temperature, T₅₀, conversion at 500°C (normal running temperature), X₅₀₀, and the stoichiometric window (A/F from 14.13 to 15.13) for stationary feedstreams, were determined.     

Theoretical investigation on cyclic reciprocal derivative chronopotentiometry: Kinetic study of totally irreversible electrode processes

Theoretical expressions and mathematical analysis in cyclic reciprocal derivative chronopotentiometry (CRDCP) are presented for totally irreversible electrode processes corresponding to the application of symmetrical and unsymmetrical programmed currents. For two successive unsymmetrical programmed currents, the effect of the currents ratio b (b = |I₂(t)/I₁(t)|) on the (dt/dE)–E curves is discussed. The electrochemical behavior of totally irreversible electrode processes has been studied corresponding to the application of the unique unsymmetrical programmed Φ^m(I₀) proposed recently. CRDCP characteristic parameters obtained for totally irreversible electrode processes are different from those of reversible electrode processes. Therefore, a comparison of CRDCP between both mechanisms is presented. Based on the mathematical derivation, alternative methods for kinetic measuring are described. It is prospected that CRDCP is convenient and applicable for studying the reversibility of the electrode processes in form of CRDCP characteristic parameters.     

Dynamics of subsurface oxygen formation in catalytic water formation on a Rh(1 1 1) surface — experiment and simulation

The catalytic O₂+H₂ reaction on Rh(1 1 1) has been investigated in the 10⁻⁶–10⁻⁵ mbar range using photoelectron emission microscopy as spatially resolving method. While the reaction without pretreatment of the sample displays simple bistable behavior, we find that after extended pre-oxidation of the sample (p_O2=2×10⁻⁴ mbar, T=770 K, t_OX>24 h), low work function (LWF) areas develop dynamically in the collision of reaction fronts. The LWF areas have been assigned to subsurface oxygen. We present a simple three-variable model which reproduces the formation of LWF areas in the collision of reaction fronts.     

Catalyzing a cycloaddition with molecularly imprinted polymers obtained via immobilized templates

Polymeric catalysts to be applied in the Diels–Alder cycloaddition of hexachlorocyclopentadiene and maleic acid have been prepared via molecular imprinting with template molecules immobilized on silica particles. These enzyme mimicking polymers exhibit specific catalytic effects compared to non-imprinted control polymers or polymer-free solutions. It could be demonstrated that the activity of the molecularly imprinted material rises when increasing the temperature. By this means, the reduction of the activation energy (as expected for catalysts) from 63 to 55 kJ mol⁻¹ could be observed. Furthermore, the reaction was characterized based on the Michaelis–Menten model. For the diene compound a Michaelis constant of K_M=5.8 mmol l⁻¹ and an effective reaction rate of r_max,eff=0.4 μmol l⁻¹ s⁻¹, leading to a reaction rate constant k_eff=1.1×10⁻³ s⁻¹, were determined.     

Direct evidence for purely silver ion conduction in CuI-doped silver oxysalt superionic systems: Combined electrolysis and EDS studies

Maiden attempt has been made for the direct estimation of the contributions of silver and copper ions to the ionic conductivity in superionic solids obtained in CuI-doped silver oxysalt systems. The application of the combined electrolysis and EDS techniques towards qualitative and quantitative analyses of the mobile ionic species in solid electrolyte systems having more than one possible mobile ion is reported. These studies confirmed that these electrolyte materials are purely Ag⁺ conducting up to 50 mol% CuI in xCuI–(100 − x)[2Ag₂O–0.7V₂O₅–0.3B₂O₃] and xCuI–(100 − x)[Ag₂O–0.7MoO₃–0.3WO₃] systems and small fraction of t_Cu+ exists above 60 mol% CuI. These solid electrolyte materials exhibited a high ionic transport numbers (t_i) of >0.985 and the t_i increases when two glass formers are used.     

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.     

Phase transition phenomena of the adsorbed 1-dodecanol monolayer at the air–water interface

Phase transition phenomenon of the 1-dodecanol monolayer at the air/water interface was studied by the dynamic γ(t) curves and the adsorption isotherm obtained by ellipsometry at 20 °C. The surface-concentration adsorption isotherm clearly showed three abrupt increases at bulk concentration C of 1.3 × 10⁻⁹, 2 × 10⁻⁹ and 3.7 × 10⁻⁹ mol/mL, respectively. The 1st and the 3rd transitions observed herein, that were typical 2D first-order transitions, were consistent with the gas to liquid expanded (G–LE) and the liquid expanded to liquid condensed (LE–LC) phase transitions observed in a previous tensiometry study. The 2nd transition that occurred at C = 2 × 10⁻⁹ mol/mL was not identified from any previous dynamic surface-tension profiles. Judging from the substantial increase in the film thickness of the transition, it was believed that the orientation change of the adsorbed molecule was involved in the LE phase. A LE_h and a LE_v phase, that denoted the “lie-down” and “stand-up” types of adsorption, respectively, was used to describe this transition and a cusp, instead of a constant surface-tension region, was observed in the dynamic γ(t) curves for this transition. This suggested that, since the surface tension varied during the transition process, the newly identified LE_h and LE_v transition might not be the typical first-order type of phase transition.     

Reduction of nitrite by sulfamic acid and sodium azide from aqueous solutions treated by gliding arc discharge

Nitrous and nitric acids form in aqueous solutions exposed to a gliding arc discharge burning in humid air. The anions interfere when the concentration of particular solutes such as pollutants must be determined. In particular they falsify the COD measurements and spectral investigations and thus the efficiency of the plasma treatment in pollutant abatement. The nitrite anions must be thus removed, which require specific reagents. The influence of parameters such as solution pH and [reducers]/[NO₂⁻] ratio on the reduction reaction was evaluated. The reduction of nitrite into N₂ either by sulfamic acid or sodium azide is a first-order pH-dependant reaction with regard to nitrite and reducers (k₁ = 2.93 × 10⁻¹ m³ kmol⁻¹ s⁻¹; k₂ = 6.21 × 10⁻¹ m³ kmol⁻¹ s⁻¹, respectively). Sodium azide is thus more reactive than sulfamic acid.     

Evaluation of carbon monoxide oxidation over CeO2/Co3O4 catalysts: Effect of ceria loading

Modification of cobaltic oxide (obtained from the reduction of high-valence cobalt oxide and assigned as R230, S_BET = 100 m² g⁻¹) with different loading of ceria was proceeded using the impregnation method (assigned as CeX/R230, X = 4, 12, 20, 35 and 50 wt%). The CeX/R230 catalysts were characterized by X-ray diffraction (XRD), nitrogen adsorption at −196 °C, temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). Their catalytic activities towards the CO oxidation were studied in a continuous flow micro-reactor. The results revealed that the optimal modification, i.e., Ce20/R230, can increase the surface area (S_BET = 109 m² g⁻¹) of cobaltic oxide, further weaken the bond strength of CoO and lower the activation of CO oxidation among CeX/R230 catalysts due to the combined effect of cobaltic oxide and ceria. The Ce20/R230 catalyst exhibited the best catalytic activity in CO oxidation with T₅₀ (temperature for 50% CO conversion) at 88 °C.