Langmuir-Hinshelwood Rate Equation Research Articles

Kinetics of Photocatalyzed Reactions: Five Lessons Learned.

Elucidation of kinetics of photocatalyzed chemical mechanisms occurring at interfaces (gas-solid, liquid-solid) has been challenging. We summarize here five lessons learned over five decades.1. An assumed reaction network leads to a single kinetic model, but a common model, the Langmuir–Hinshelwood rate equation, r = kcat K C/ [1 +KC], arises from multiple mechanisms, hence models alone do not reveal unique mechanisms.2. The Langmuir–Hinshelwood model parameter kcat represents the slow step at a catalyst surface, and in thermal catalysis, depends upon the reactant structure. However, early photocatalysis work with light chlorinated hydrocarbons in aqueous solutions showed a single kcat value, independent of reactant structure.3. The dependence of the Langmuir-Hinshelwood parameters, kcat and K, upon intensity indicates that a pseudo-steady state approach is more fundamental than the presumed equilibrated adsorption of the LH model.4. Dyes and phenols are commonly studied, and claimed as first order reactions, despite often exhibiting rate constants which diminish with increasing contaminant concentration. We show that such studies are the result of intrinsic zero order data plotted on a semilog graph, and involve zero order rate limitation by reactant saturation, electron transfer to O2, oxygen mass transfer, or light supply.5. The apparent kinetics for contaminant removal from photocatalytic self-cleaning surfaces depends upon multiple circumstances, including the geometry of reactant deposit, catalyst porosity, and reactant light absorption. A single decision table suffices to indicate the apparent reaction order, n, to assume when fitting photocatalytic kinetic data from self-cleaning surfaces to a power law rate form, rate = k Cn.

Morphological tunable three-dimensional flower-like zinc oxides with high photoactivity for targeted environmental Remediation: Degradation of emerging micropollutant and radicals trapping experiments

Abstract Zinc oxide (ZnO) with a high potentiality for tunable morphology is gaining immense research interests for targeted environmental remediation of emerging micropollutants in source waters. Various three-dimensional (3D) flower-like nanostructures of ZnO were synthesized by following a facile and low-temperature hydrothermal synthesis approach, using Zn(CH3COO)2∙2H2O as the starting precursor reagent, in the presence of different bases at varying concentrations as the structure-directing reagents. BET studies indicated that the ZnO-flowers 1 were more superior than ZnO-flowers 2 and 3, as it possessed the highest specific surface area (10.83 m2/g), pore volume (0.054 cm3/g) and pore size (23.52 nm). Subsequently, the photoactivities of ZnO-flowers on the degradation of emerging salicylic acid micropollutant under UV-A illumination were examined. Batch studies suggested that the ZnO-flowers were photoactive and photocatalysed optimally under neutral pH condition that are aligned with the cost-effective and practical implementation for water/wastewater treatment. ZnO-flowers 1 demonstrated the highest photoactivity according to its reaction rate constant, where the kinetics data was well-fitted using the pseudo-first order Langmuir-Hinshelwood rate equation. This finding was well correlated to the BET studies, where the high photoactivity in ZnO-flowers 1 could be ascribed to its exceptional surface morphology that maximises the surface contact area with salicylic acid during photodegradation. Finally, the radicals trapping experiments were carried out in order to validate the role of primary photogenerated holes (h+) and various secondary reactive oxygen species (ROS) in the photodegradation of salicylic acid using ZnO-flowers photocatalysts. It was confirmed that the active species of photogenerated h+, •OH, and •O2− were responsible for the photodegradation process. The plausible mechanistic pathways for the reaction between scavenging reagents and the reactive transitory species to form by-products and intermediates were also proposed.

Predicting MSR Rates in SOFCs Using Experimental Data and CFD Methods

Fuel cells provide a way to convert gas to electricity with little formation of nitrous oxides, sulphurous oxides and particulate matter, due to the absence of high peak temperatures and pressures, and the low sulphur tolerance. When natural gas is used, solid oxide fuel cells (SOFCs) achieve the highest electrical efficiencies. This is due to ability of direct internal reforming (DIR), which enables the use of steam and waste heat produced by the electrochemical reaction to convert hydrocarbons to a hydrogen-rich gas. In addition, the heat consumption by the endothermic reforming reaction reduces the cooling requirement with cathode air, and hence the parasitic power consumption by the air blower. However, practical constraints limit the degree of DIR. At low temperatures, for example, slow kinetics limit the extent of the reforming reaction, while rapid kinetics at high temperatures may induce high temperature gradients, causing thermal quenching of the stack. Due to these issues, the fuel is usually partially pre-reformed. Resolving the complex and coupled flow, species and temperature distributions, as well as the reaction rates of the reforming, water gas shift and electrochemical reaction, provides insight in the electrochemical performance of the SOFC, as well as the risk of carbon depositing, nickel oxidation and the induced thermal stresses. Computational fluid dynamics (CFD) can be used for this purpose, but require appropriate intrinsic kinetic rate expressions to quantify the methane reforming rate and accurately predict the species and temperature distributions in the SOFC. Although methane steam reforming is an established industrial process, SOFC anodes are usually developed to catalyse the electrochemical oxidation of hydrogen rather than reform methane, which may change the reforming mechanism compared to dedicated reformers. The complicated fundamental chemistry in SOFC anodes is poorly understood, which is reflected in a large number of kinetic expression, derived for various operating conditions and anodes. In this study, the two most common rate expressions, one of the power law (PL) and one of the Langmuir-Hinshelwood (LH) type, are parameterised using methane conversion data, obtained in a previous study during single cell experiments on Ni-GDC anodes. Both the PL and LH rate equation have four fitted parameters, of which two are the rate constant and the activation energy. In addition, reaction orders of methane and steam are fitted for the PL, while an oxygen adsorption isotherm is fitted for the LH rate equation. The kinetic parameters are fitted by minimisation of the coefficient of variation of the rate constant, where SOFC anode compartment is assumed to be a one-dimensional ideal plug flow reactor. A CFD model is developed of a single channel of the single cell test setup, to compare methane conversion rates with the experimental data, and quantify the difference between the distributions of species, temperature and reaction rates predicted by the PL and LH rate equation. The CFD model is used to solve the mass, momentum, energy, species and charge balances. User defined functions are added to resolve the current distribution, cell voltage and implement the kinetic expressions for methane steam reforming rates. The various experimental conditions are simulated with the CFD model, evaluating different inlet compositions of methane and steam, cell temperatures between 973 and 1023K, and current densities between 0 and 1000 A/m2. The methane conversions calculated with the single channel CFD model are then compared to those obtained in the single cell experiment. Although the CFD model with the LH type rate equation achieves a better fit, both the PL and LH equation show statistically good agreement with the experimental conversions. However, the results reveal that the spatial variation of the reforming reaction rate is quite different for the two rate equations. The LH kinetics predicts relatively high reaction rates at the inlet section, which results in temperature gradients up to 50% higher compared to the PL equation. In addition, the PL fails to predict the enhancement of the reforming rate by the electrochemical reaction due to the apparent negative reaction order of steam, while this effect is captured correctly by the LH kinetics. The method proposed in this paper can be used to predict the electrochemical performance and thermal gradients in various SOFC operating with high degrees of internal reforming. This knowledge allows to minimise or eliminate the need of pre-reforming, and ultimately provide ways to alter SOFC construction such that complete internal reforming is possible without imposing adverse condition on the stack. Figure 1

K2CO3-catalyzed steam gasification of ash-free coal char in a pressurized and vertically blown reactor. Influence of pressure on gasification rate and gas composition

The K2CO3-catalyzed steam gasification of ash-free coal char was conducted in a pressurized and vertically blown reactor which was also characterized as a differential reactor. At a typical condition (total pressure, 0.5MPa; steam partial pressure, 0.15MPa; temperature, 750°C; catalyst loading, 10%; superficial gas velocity, 55cms−1), the gasification proceeded so fast that it was completed within 5min. The dependence of either instantaneous gasification rate or average gasification rate on pressure was found to follow the Langmuir-Hinshelwood rate equation and the nth order rate equation, but the parameters for either rate equation varied with the carbon conversion. For a given coal char with a fixed catalyst loading, an overall kinetic model with the constancy of parameters is proposed to predict the gasification rate as a function of steam partial pressure, temperature and carbon conversion. While the elevation of steam partial pressure promoted the methanation mainly via the gas-char reaction, the yield of CH4 was negligible. The gas composition was not in the equilibrium of water gas shift reaction.

Phenomenologically based kinetics of ODH of ethane to ethylene using lattice oxygen of VOx/Al2O3–ZrO2 catalyst

This study addresses the kinetic modeling of ethane oxidative dehydrogenation (ODH) over a 10%wt VOx catalyst supported on Al2O3–ZrO2 (1:1) in the CREC Riser Simulator Reactor. The catalyst was synthesized using impregnation by a soaking method. Following this, the catalyst was characterized using BET, XRF, TPR, NH3-TPD and Raman Spectroscopy. The ODH catalyst was studied in a fluidized CREC Riser Simulator at a temperature range of 525–600°C and with 20–50s contact time. Good ethylene selectivity of up to 82% at 8.5% ethane conversion was attained. A kinetics model was established using a Langmuir–Hinshelwood rate equation. Catalyst activity (availability of lattice oxygen) was first modeled using TPR results. This model was then incorporated with the developed L–H model for the series-parallel reaction network. Nonlinear regression was employed to estimate kinetic parameters and activation energies with their respective confidence intervals. The proposed kinetics for ODH, satisfactorily predicted ethane conversion and selectivity at the selected operating conditions.

Visible-solar-light-driven photo-reduction and removal of cadmium ion with Eosin Y-sensitized TiO2 in aqueous solution of triethanolamine

Abstract Cadmium is toxic to humans, animals, and plants, and show persistence because of its non-degradability. Photo-reduction of cadmium ion (Cd (II)) was studied with Eosin Y-sensitized TiO2 photocatalyst for the reduction/removal of Cd (II) in an aqueous medium under visible-solar light. Triethanolamine (TEOA) was used as a sacrificial electron donor. The band-gap of Eosin Y-sensitized TiO2 photocatalyst was reduced by ∼1 eV, and visible light activity was observed for Cd (II) photo-reduction. Parametric studies were performed for visible-solar light intensity, solution pH, and initial concentrations of metal ion, dye, photocatalyst, and an electron donor. Complete photo-reduction of Cd (II) was achieved in 3 h at a pH of 7.0 and a light intensity of 100 mW cm−2. The photo-reduction rate of Cd (II) in TEOA solution followed Langmuir-Hinshelwood rate equation. The highest apparent quantum yield (∼0.3%) for Cd (II) photo-reduction was observed at the lower visible light intensity with illuminated Eosin Y-sensitized TiO2.

Kinetic investigations of the steam reforming of methanol over a Pt/In2O3/Al2O3 catalyst in microchannels

A kinetic study of methanol steam reforming over bimetallic Pt/In2O3/Al2O3 catalyst was carried out. The kinetic measurements were performed in a microstructured monolithic reactor with an external recycle free of temperature and concentration gradients. By the help of residence time distribution measurements it could be verified that the reactor showed the behaviour of an ideal continuous stirred tank reactor (CSTR). The absence of external and internal concentration gradients could be proven by corresponding experiments and theoretical diagnostic criteria.The kinetic measurements performed by variation of the reactant inlet partial pressures revealed that in the temperature range from 310°C to 355°C the molar rate of methanol consumption mainly depends on the methanol partial pressure, especially at higher temperatures, whereas there is only minor dependence on the water partial pressure. Carbon dioxide has no inhibiting effect, whereas hydrogen showed a weak inhibiting effect.Two power laws and three Langmuir-Hinshelwood rate equations were created for the modelling of the kinetic data. Power laws could not be fitted to the measured values. Therefore the uses of Langmuir-Hinshelwood rate laws with temperature dependent sorption constants are inevitable for the modelling. The model discrimination revealed that the rate law derived from a mechanism, which assumes the dehydrogenation of an adsorbed methoxy-species as rate determining step, described the measured kinetic data second best. Optimum agreement between observed and predicted molar rates of methanol consumption was obtained when applying a Langmuir-Hinshelwood rate law assuming dissociative methanol and molecular water adsorption on the catalyst surface. Dissociative adsorption of water and methanol at the same active site may be excluded. This leads to a better evaluation of the models that assume molecular water adsorption at the same site where methanol ties or the ones with no participation of water in the rate determining step (RDS) under discrimination.

Gasification kinetics during entrained flow gasification – Part II: Intrinsic char reaction rate and surface area development

The reaction behaviour of two different coals is analysed in a pressurised entrained flow reactor (PiTER) and a pressurised thermogravimetric analyser (PRETA). The entrained flow reactor is used to produce char samples under conditions relevant to larger scale entrained flow gasifiers, i.e. at temperatures up to 1600°C and pressures up to 2.5MPa. The conversion rate of these char samples is measured under defined conditions in the PRETA using CO2 and H2O atmospheres.Furthermore, the development of char surface area during devolatilisation and char gasification is analysed in the PiTER. The initial surface area after devolatilisation is dependent on pressure. In the later stages of conversion a significant loss of surface area is detected. The decrease in surface area is attributed to a melting of mineral matter as the PiTER is operated above the ash melting temperature of the coals. The development of the mass specific surface area during gasification cannot be described by the Random Pore Model probably because of the ash melting, but by an empirical correlation.The surface area data are combined with the thermogravimetric analysis to derive the intrinsic reaction rates at the char surface. The intrinsic rate is modelled by nth order and Langmuir–Hinshelwood rate equations. The temperature influence on the char–CO2 reaction can be described by an activation energy of 200kJ/mol, and on the char–H2O reaction by an activation energy of 212kJ/mol. The activation energy is found to be independent of the char preparation conditions, whereas the pre-exponential factor significantly decreases for a higher heat treatment severity of the char sample.The model equations can be used to predict the intrinsic gasification behaviour in the absence of mass transport limitations.

High-Pressure Char Gasification Kinetics: CO Inhibition of the C–CO2 Reaction

Results of high-pressure (up to 3.0 MPa) CO-inhibited char–CO2 reaction rate measurements are presented and analyzed using a Langmuir–Hinshelwood (LH) kinetic model commonly used to describe gasification kinetics at atmospheric pressure. Despite previous success by the authors and others in using this reaction scheme to describe high-pressure kinetics of uninhibited gasification reactions, this work shows that it is unable to describe well CO-inhibited reaction kinetics at (relatively) high partial pressures of CO and CO2. Measurements of the char surface area suggest significant impact of CO and CO2 partial pressures on the development of the char surface area during the reaction (and, therefore, most likely on the term describing the number of active sites on the char surface in the LH rate equation). This term is considered to be a constant in the standard LH formulation; therefore, an alternative LH approach to describe the kinetics is presented, whereby rates are described using the LH equation on a ...

Continuous production of biodiesel in a packed-bed reactor using shell–core structural Ca(C 3H 7O 3) 2/CaCO 3 catalyst

The continuous production of biodiesel was studied by using a steady-state packed-bed reactor. The shell–core Ca(C 3H 7O 3) 2/CaCO 3 solid-base catalyst was prepared with a mechanical strong core of CaCO 3 for continuous transesterification of soybean oil in a packed-bed reactor. Alcohol–oil ratio, retention time and reaction temperature were evaluated to obtain optimum reaction conditions. The yield of fatty acid methyl esters (FAME, i.e. biodiesel) achieved 95% at the reaction temperature 60 °C, alcohol–oil molar ratio of 30:1 and retention time of 168 min. The reusability of catalyst was checked up to 5 cycles and found negligible decrease in the catalyst activity. Water in the oil can significantly decrease the yield due to the deactivation of Ca(C 3H 7O 3) 2 and hydrolysis of FAME. The transesterification of soybean oil, canola oil and sunflower oil also was compared with model compound, triolein, using powder Ca(C 3H 7O 3) 2 in the batch reactor. Although these oils contained different triglyceride mixtures, their FAME yields were comparable. A Langmuir–Hinshelwood rate equation was established for the transesterification of soybean oil with methanol. Regression of experimental data indicated that the transesterification was an endothermic reaction with the enthalpy change of 23,504 J/mol and the activation energy was 42,096 J/mol.

Kinetic modeling of the total oxidation of propane over CuO-CeO2/γ-Al2O3

The total oxidation of propane is studied by means of steady-state experiments over a CuO–CeO2/γ-Al2O3 catalyst at propane inlet partial pressures of 0.2–1.0kPa, oxygen inlet partial pressures of 2.7–27.6kPa and temperatures from 543 to 648K. The inlet ratio of oxygen to propane is varied between 6.1 and 28.0 molO2,0 molC3H8,0−1. Water and carbon dioxide inlet partial pressures up to 12.4kPa are investigated. The complete experimental dataset was regressed with a priori possible Langmuir–Hinshelwood (LH), Eley–Rideal (ER) and Mars–van Krevelen (MVK) rate equations. A LH rate equation, the rate-determining step being the surface reaction between one adsorbed propane species and one dissociatively adsorbed oxygen species, allows the best description of the data.The surface reaction activation energy amounts to 74.4±4.6kJmol−1. All adsorption steps in the selected reaction mechanism are quasi-equilibrated. The values of the preexponential factors of the adsorption equilibrium coefficients correspond to rather mobile surface species. Molecular water adsorption is followed by a dissociation of the adsorbed water by interaction with a dissociatively adsorbed oxygen species to form two hydroxyl species.

Photocatalytic activity of Ag-substituted and impregnated nano-TiO 2

Ag-substituted (Ag sub) and Ag-impregnated (Ag imp), anatase phase nano-TiO 2 have been synthesized by solution combustion technique and reduction technique, respectively. The catalysts were characterized extensively by powder XRD, TEM, XPS, FT-Raman, UV absorption, FT-IR, TGA, photoluminescence, BET surface area and isoelectric pH measurements. These catalysts were used for the photodegradation of dyes and for the selective photooxidation of cyclohexane to cyclohexanone. The photoactivities of the combustion-synthesized catalysts were compared with those of commercial Degussa P 25 (DP 25) TiO 2, and Ag-impregnated DP 25 (Ag DP). For the photocatalytic degradation of dyes, unsubstituted combustion-synthesized TiO 2 (CS TiO 2) exhibited the highest activity, followed by 1% Ag imp and 1% Ag sub. For the photoconversion of cyclohexane, the total conversion of cyclohexane and the selectivity of cyclohexanone followed the order: 1% Ag sub > DP 25 > CS TiO 2 > 1% Ag imp > 1% Ag DP. The kinetics of the photodegradation of dyes and of the photooxidation of cyclohexane were modeled using Langmuir–Hinshelwood rate equation and a free radical mechanism, respectively, and the rate coefficients were determined. The difference in activity values of the catalysts observed for these two reactions and the detailed characterization of these catalysts are described in this study.

Kinetic modelling of the catalytic hydrogenation of CO 2-expanded α-methylstyrene

Kinetic modelling of the catalytic hydrogenation of CO 2-expanded α-methylstyrene is reported. Langmuir–Hinshelwood rate equations are evaluated for the correlation of kinetic rate data obtained in an earlier study in which the reaction was carried out at 323 K and 7–13 MPa using a Pd/C catalyst. The Peng-Robinson equation of state is used for the estimation of concentration terms that appear in the rate equations. Overall, the phase equilibria and kinetic modelling provide evidence that the rate-enhancing effect of CO 2 is influenced by two competing factors: solvent strength and reactant concentration. The presence of CO 2 modifies the solvent strength of the liquid phase, leading to more favourable adsorption equilibrium constants for the surface reaction. However, CO 2 also acts as a diluent, an effect that leads to reduced reaction rates. The optimum level of CO 2 for rate enhancement depends on the operating pressure.

Naphthalene hydrogenation over a NiMo/γ-Al 2O 3 catalyst: Experimental study and kinetic modelling

A kinetic study of the liquid phase naphthalene hydrogenation over a commercial and sulphided NiMo/γ-Al 2O 3 was performed using experimental data obtained in a Robinson–Mahoney reactor at 523–583 K and 2.0–4.0 MPa. The effect of H 2S on the hydrogenation rate was investigated varying its partial pressure from 0.09 to 0.89 MPa. Under these conditions, an exponential decrease in the hydrogenation rate was observed as the H 2S partial pressure increased. The naphthalene conversion ranged from 6 to 89% with tetralin as the main hydrogenation product with selectivities of at least 89%. Reaction networks were formulated based on stepwise hydrogenation mechanisms assuming that both hydrogen and hydrogen sulphide could be dissociated either homolytically or heterolytically on both coordinatively unsaturated metal ion sites (CUS) and sulphur anions (SA) of the sulphided catalyst. Langmuir–Hinshelwood rate equations were derived and discriminated through model regression to experimental data. Two models best describe the experimental data according to a statistical analysis and the physical meaning of the parameter estimates. These models correspond to the third hydrogen addition chemisorbed on CUS and supplied from either homolytic or heterolytic hydrogen dissociation, as rate-determining step. The calculated catalyst surface species concentrations indicate that hydrogen and sulphydril groups are the most abundant species under the investigated operating conditions while the hydrocarbons surface concentration is almost negligible.

Photocatalytic oxidation of gaseous trichloroethylene by UV/TiO2 process

The objective of this study was to investigate the reaction behavior of the photocatalytic oxidation of gaseous trichloroethylene (TCE) using titanium dioxide at room temperature. The experiments were carried out under various humidity levels and oxygen contents of carrier gas in an annular photoreactor. Experimental results indicated that the factors affecting the photocatalysis of TCE by UV/TiO2 process are carrier gases and humidity. It was found that increasing the relative humidity in inlet gas flow under low humidity could improve the decomposition of trichloroethylene and the mineralization of organic intermediates at an irradiation UV light intensity of 2.82 W m-2 by UV/TiO2 process. The photocatalytic kinetics of gaseous trichloroethylene can be described by the Langmuir-Hinshelwood rate equation.

Heterogeneous photocatalytic kinetics: beyond the adsorption/desorption equilibrium concept

Rigorous kinetic analysis is presented for a range of photocatalytic mechanisms based on the steady-state approximation, while typically photochemical kinetics is discussed within the framework of Langmuir-Hinshelwood rate equations, assuming a slow rate-determining step and adsorption/desorption equilibrium. Performed comparison with experimental data reveals additional features inconsistent with adsorption/desorption equilibrium concept.

BIOMASS GASIFICATION KINETICS: INFLUENCES OF PRESSURE AND CHAR STRUCTURE

ABSTRACT In this study, the gasification kinetics of chars from different biomass species have been investigated within the temperature range 800–950°C, CO2 concentrations of 10 to 100% v/v, and pressures between 1 and 20 bar using thermogravimetric analysis (TGA). The quartz wool matrix (QWM) method was used to simulate circulating fluidized-bed (CFB) conditions. The reactivity results obtained by the QWM method were compared with those extracted using TGA data. Kinetic data from TGA analyses was found to be compatible for predicting CFB gasification reactivity. Effects of pyrolysis pressure on the chemical structure of char and char conversion reactivities were also investigated. Pressure has been found to have no effect on reactivity during char conversion while having a dramatic effect on chemical and physical structure during the pyrolysis process. The Langmuir–Hinshelwood rate equation represents the pressurized radiata pine char gasification kinetics well. The Langmuir–Hinshelwood expression for studied conditions can be described as Kinetic data found in this study are in good agreement with the literature.

Kinetics of propylene hydrogenation on nanostructured palladium clusters

Surfactant-stabilised palladium nanoclusters with an average diameter of about 7.3 nm (determined by XRD) are used as catalysts in propylene hydrogenation. Experiments performed in an isothermal batch reactor ( T = 308 K), with total pressures in the range 0.25–9.0 bar and with initial hydrogen molar ratios varying between 0.05 and 0.40, provide a further insight on the reaction kinetics. It is shown that a Langmuir–Hinshelwood rate equation well represents the reaction data, which mechanism involves competitive adsorption of the reagents, with dissociation of hydrogen on the catalyst surface, and where surface reaction is the limiting step. Nonlinear optimization of the initial rate data provided the kinetic parameters of the rate law ( k = 5.569 mol g Pd − 1 s −1; K H 2 = 3.799 × 10 − 2 bar −1 and K C 3 H 6 = 0.996 bar −1), which is validated by integration of the mass balance in the batch reactor.

Photocatalytic degradation of natural organic matter: Kinetic considerations and light intensity dependence

The current study was conducted to investigate the photocatalytic degradation kinetics of humic acid at different light intensities using commercial TiO2 powders. The pseudo first order kinetic model and Langmuir‐Hinshelwood (L‐H) rate equation in modified forms were used to compare the photocatalytic activities of TiO2 materials as a function of light intensity. Under constant irradiation conditions, the pseudo first order reaction rates as well as L‐H rates were found to be decreasing in the following trend; Degussa P‐25, Millennium PC‐500 and Millennium PC‐100. The pseudo first order rate constants showed the same decreasing trend as the pseudo first order reaction rates while L‐H rate constants exhibited a light intensity related change in the ordering of the photocatalysts. At the lowest light intensity, L‐H rate constants decreased as follows: Millennium PC‐500 > Millennium PC‐100 > Degussa P‐25. However, increasing the light intensity changed the order to; Millennium PC‐100 > Millennium PC‐500 > Degussa P‐25 revealing the significance of the L‐H adsorption constant. Under constant irradiation conditions, ionic strength dependent changes in the structure of humic acid did not alter degradation efficiency trend of the photocatalyst specimens and they were ordered such as; Degussa P‐25 > Millennium PC‐500 > Hombikat UV‐100 > Millennium PC‐100 > Merck. The results presented in this research also confirmed the effectiveness of Degussa P‐25 as a photocatalyst for the degradation of humic acid.

Hydrogenation of Cyclohexene over in Situ Fluorinated NiMoS Catalysts Supported on Alumina and Silica–Alumina

The effect of fluorine on the hydrogenation of cyclohexene over sulfided NiMo catalysts supported on Al2O3 and amorphous silica–alumina (ASA) was studied at 5.0 MPa and between 310 and 350°C. Fluorination was performed in situ after sulfidation of the catalyst. The hydrogenation of cyclohexene produced mainly cyclohexane over the Al2O3-supported catalysts. On the ASA-supported catalysts, large amounts of the isomerization products methylcyclopentene and methylcyclopentane were formed in addition to the hydrogenation product cyclohexane. At the applied partial pressure of cyclohexene between 16 and 100 kPa, the hydrogenation is between zero and first order in the partial pressure of cyclohexene on all catalysts. The Al2O3-supported catalysts showed higher conversions of cyclohexene than their ASA-supported counterparts. Fluorination mainly enhances the isomerization of cyclohexene to methylcyclopentene. Kinetic parameters were obtained by fitting the kinetic data with Langmuir–Hinshelwood rate equations in a wide range of conversions. The unchanged activation energy and heat of adsorption for the hydrogenation of cyclohexene to cyclohexane indicate that the active sites for hydrogenation are not influenced by in situ fluorination. The activation energy for the isomerization of cyclohexene remained constant after fluorination. In situ fluorination introduces more acid sites for the isomerization reaction over both the Al2O3- and ASA-supported catalysts.