Development of a kinetic model for the oxidation of methane over Pd/Al2O3 at dry and wet conditions

The kinetics of the oxidation of methane over a commercial 0.5% Pd on γ-Al₂O₃ catalyst has been studied in a lab-scale fixed-bed reactor, the effect of temperature, and methane, oxygen and water partial pressures being investigated, in a range of interest for environmental applications. Different Eley–Rideal, Langmuir–Hinshelwood and Mars–van Krevelen models were fitted to the experimental results, the best fitting being obtained for a Mars–van Krevelen model that considers slow desorption of the reaction products. The model parameters obtained both from differential and integral treatment of the experimental data are in good agreement with each other. A modification of the proposed model, taking into account that water is adsorbed over oxidised sites, is also able to model the inhibition produced by steam.     

Understanding the activation mechanism induced by NOx on the performances of VOx/TiO2 based catalysts in the total oxidation of chlorinated VOCs

A previous investigation of the chlorobenzene combustion activity of VO_x/TiO₂, VO_x–WO_x/TiO₂ and VO_x–MoO_x/TiO₂ catalysts in the presence of NO pointed out the activation effect of NO. The suggested three-step mechanism based on catalytic performances data only was: (1) chlorobenzene is oxidized on the surface of the VO_x phase (as described by Mars–van Krevelen), (2) NO gets oxidized to NO₂, mainly on WO_x and MoO_x, and (3) the in situ produced NO₂ assists O₂ in the reoxidation of the VO_x phase thus speeding up the oxidation step of the Mars–van Krevelen mechanism. The latter effect macroscopically corresponds to the observed increase of chlorobenzene conversion. This contribution aims at validating this hypothetical mechanism by pointing out the favourable occurrence of an oxidation of NO to NO₂ on the WO_x and MoO_x phases and by pointing out the higher efficiency of NO₂ than O₂ to reoxidize the reduced VO_x sites. In addition, the present contribution clearly demonstrates that, in the absence of NO, the chlorobenzene total oxidation occurred following the Mars–van Krevelen mechanism. Moreover, a thorough characterization of the oxidation state of the vanadium proving that the improvement of the catalyst activity brought by the simultaneous presence of NO and O₂ is linked to the stronger reoxidation of the VO_x active phase. Furthermore, plotting all the catalytic activity data versus the mean vanadium oxidation level clearly depicts, for the first time, the strong dependence between them. Under a mean vanadium oxidation level of 4.82 the catalyst is inactive while above 4.87 the activity is stabilized at a high level of conversion independent of the vanadium oxidation level.     

Solid electrolyte membrane reactor for controlled partial oxidation of hydrocarbons: Model and experimental validation

A mathematical model of a solid electrolyte membrane reactor is presented which accounts for the prevailing physical phenomena of the electrochemical partial oxidation of n-butane to maleic anhydride. From an analysis of characteristic dimensionless numbers it was concluded that the reactor behaviour can be described by a one-dimensional pseudo-homogeneous approach with respect to the anodic gas channel and a one-plus-one-dimensional electrochemical model. Beside mass and charge transport processes, electrochemical charge transfer reactions as well as heterogeneously catalysed oxidation reactions are considered. As kinetic model a modified Mars–van Krevelen approach is suggested. Experimental results of oxygen pumping and butane oxidation experiments were used to determine kinetic parameters and to validate the model.     

Catalytic combustion of volatile organic compounds on gold/iron oxide catalysts

Catalytic oxidation of 2-propanol, methanol, ethanol, acetone and toluene was investigated on coprecipitated Au/Fe₂O₃ catalysts in the presence of excess of oxygen. Gold catalysts have been found to be very active in the oxidation of tested volatile organic compounds (VOCs). The high activity of these systems has been related to the capacity of highly dispersed gold to weaken the Fe–O bond thus increasing the mobility of the lattice oxygen which is involved in the VOCs oxidation probably through a Mars–van Krevelen reaction mechanism.     

Absorption with chemical reaction: development of a new relation for the Danckwerts model

An approximate solution for absorption with irreversible second-order chemical reaction by the Danckwerts model is derived and discussed. This corresponds to the well known relation of van Krevelen and Hoftijzer for the film model. The derivation consists of developing a ‘bridging relation’ between concentrations of reactants and using this relation to obtain an explicit approximate expression for the enhancement factor. The resulting expression is more convenient than previous approximations, and it agrees as well or better with available numerical results. The method can be extended to other forms of chemical kinetics.     

Phenol oxidation over CuO/Al2O3 in supercritical water

Phenol was oxidized in supercritical water at 380–450°C and 219–300 atm, using CuO/Al₂O₃ as a catalyst in a packed-bed flow reactor. The CuO catalyst has the desired effects of accelerating the phenol disappearance and CO₂ formation rates relative to non-catalytic supercritical water oxidation (SCWO). It also simultaneously reduced the yield of undesired phenol dimers at a given phenol conversion. The rates of phenol disappearance and CO₂ formation are sensitive to the phenol and O₂ concentrations, but insensitive to the water density. A dual-site Langmuir–Hinshelwood–Hougen–Watson rate law used previously for catalytic SCWO of phenol over other transition metal oxides and the Mars–van Krevelen rate law can correlate the catalytic kinetics for phenol disappearance over CuO. The supported CuO catalyst exhibited a higher activity, on a mass of catalyst basis, for phenol disappearance and CO₂ formation than did bulk MnO₂ or bulk TiO₂. The CuO catalyst had the lowest activity, however, when expressed on the basis of fresh catalyst surface area. The CuO catalyst exhibited some initial deactivation, but otherwise maintained its activity throughout 100 h of continuous use. Both Cu and Al were detected in the reactor effluent, however, which indicates the dissolution or erosion of the catalyst at reaction conditions.     

Kinetics of the selective catalytic reduction of NO by NH3 on a Cu-faujasite catalyst

The kinetics of the selective catalytic reduction (SCR) of NO by NH₃ in the presence of O₂ has been studied on a 5.5% Cu-faujasite (Cu-FAU) catalyst. Cu-FAU was composed of cationic and oxocationic Cu species. The SCR was studied in a gas phase-flowing reactor operating at atmospheric pressure. The reaction conditions explored were: 458<T_R<513 K, 2503 (ppm) < 4000, 12 (%) < 4. The kinetic orders were 0.8–1 with respect to NO, 0.5–1 with respect to O₂, and essentially 0 with respect to NH₃. Based on these kinetic partial orders of reactions and elementary chemistry, a wide variety of mechanisms were explored, and different rate laws were derived. The best fit between the measured and calculated rates for the SCR of NO by NH₃ was obtained with a rate law derived from a redox Mars and van Krevelen mechanism. The catalytic cycle is described by a sequence of three reactions: (i) Cu^I is oxidized by O₂ to “Cu^II-oxo”, (ii) “Cu^II-oxo” reacts with NO to yield “Cu^II-N_xO_y”, and (iii) finally “Cu^II-N_xO_y” is reduced by NH₃ to give N₂, H₂O, and the regeneration of Cu^I (closing of the catalytic cycle). The rate constants of the three steps have been determined at 458, 483, and 513 K. It is shown that Cu^I or “Cu^II-oxo” species constitute the rate-determining active center.     

Kinetics of methane combustion over CVD-made cobalt oxide catalysts

The present investigation provides the required kinetic parameters to evaluate and to predict the rate of the catalytic combustion of methane over cobalt oxide. For this purpose, monolithic cordierites with low specific surface area were uniformly coated with cobalt oxide thin films of controlled thickness using the chemical vapor deposition (CVD) process. The obtained catalysts were tested in the catalytic combustion of methane in oxygen-deficient and -rich conditions. Catalysts with loadings above 0.46 wt.% are active starting at a temperature of 250 °C and completely convert methane to CO₂ below 550 °C where the conversion rate reaches 35 μmol (CH₄)/g_cat s. The involvement of the bulk-oxide-ions in the catalytic reaction was supported by the constant value of the normalized reaction rate to the weight of deposited cobalt oxide. The experimental data fit well to the Mars–Van Krevelen redox model and can be approximated with a power rate law in oxygen-rich mixtures. The resulting activation energies and frequency factors allow the identification of the rate-limiting step and accurately reproduce the effect of the temperature and partial pressure of the reactants on the specific reaction rate.     

Gas phase photocatalysis and liquid phase photocatalysis: Interdependence and influence of substrate concentration and photon flow on degradation reaction kinetics

The photocatalytic degradation of metolachlor in water and butyric acid in air on coated TiO₂ has been investigated to study the interdependency on the degradation rate of the UV photon flux and the concentration of the organic compound. Experimental results clearly showed that the kinetic change of order with respect to light intensity depended on the value of the concentration of organic compound. The fitting of experimental results with the Langmuir–Hinshelwood (L-H) model emphasized that the corresponding “apparent adsorption constant”, K_R, could not be considered as an equilibrium constant in the dark since the value of K_R varied with regard to the photon flow. The assumption of a pseudo-steady-state for the concentration of hydroxyl radicals associated either with L-H model (under certain conditions) or Eley–Rideal model explains consistently the dependence of the apparent kinetic parameter, k_obs, and K_R with the light intensity. A rate expression taking into account the interdependency of the degradation rate on the UV photon flux and the initial concentration of the organic compound is proposed and validated for a liquid and a gas phase reaction. The constants of this correlation are independent of the initial concentration and light intensity.     

Oxidation of carbonaceous matter—II: X-ray diffraction and transmission electron microscopy

Products of various elemental composition ( ) at 0.45 and ) at < 0.45) were oxidized under an air flow at various temperatures (from 150 to 280°C) and for various times (from half an hour to one week). When plotted in a Van Krevelen diagram ( vs ), their elemental composition follows an oxidation path, the slope of which depends only on the original ( ) atomic ratio and on the oxidizing temperature. Oxidation reactions have an “apparent activation energy” of 20–40 kcal/mole. Cross-linking may be due either to ether bonding or to hydrogen bonding as indicated by IR spectrometry. The final product, named “oxychar”, has a constant composition ( ).     

Structure sensitivity of dimethylamine deep oxidation over Pt/Al2O3 catalysts

The deep oxidation of dimethylamine (DMA) was studied over Pt/Al₂O₃ catalysts with small (1 nm) and large (7.8–15.5 nm) Pt crystallite sizes. The turnover frequency (TOF) was higher for the large than for the small Pt crystallites, indicating that the reaction is structure sensitive. Two kinetic models were used to interpret the obtained results, i.e., the Mars van Krevelen and a mechanism based on the adsorption of oxygen and adsorption of dimethylamine on different active sites were employed. Both models showed that the activation energy for the oxygen chemisorption rate constant (k_o) decreased with increasing of Pt crystallite size and that the activation energy for the surface reaction rate constant (k_i) was independent of the Pt crystallite size. The structure sensitivity may be explained by differences in the reactivity of the oxygen adsorbed on these Pt crystallites.The Mars van Krevelen model fits the TOF values very well at concentrations of DMA higher than 1500 ppm, while in the lower concentrations region, the model under predicts the experimental data. The model based on the adsorption of oxygen and DMA on different active sites fits the experimental data quite well over the whole temperature and concentration range. The fitted values of the Henry adsorption constant are independent of the Pt crystallite size.     

Deoxygenation of benzoic acid on metal oxides: 1. The selective pathway to benzaldehyde

The mechanism of the selective deoxygenation of benzoic acid to benzaldehyde was studied on ZnO and ZrO₂. The results show conclusively that the reaction proceeds as a reverse type of Mars and van Krevelen mechanism consisting of two steps: hydrogen activates the oxide by reduction resulting in the formation of oxygen vacancies. Subsequent re-oxidation of these vacancy sites by benzoic acid yields benzaldehyde. Inhibition of the deoxygenation reaction can be achieved by addition of suitable polar compounds with a high affinity for the oxygen vacancy sites such as carbon dioxide or water. Differences in the catalytic activity and selectivity of ZnO and ZrO₂ can be attributed to differences in hydrogen activation, redox properties and extent of benzoic acid coverage.     

In situ analysis of metal-oxide systems used for selective oxidation catalysis: how essential is chemical complexity?

The mode of operation of selective oxidation reactions is described by a series of chemical rules defining the catalyst and some reaction intermediates. In contrast to catalytic processes over metallic elements, little is known, however, about the atomistic details of selective oxidation. In particular, the participation of the subsurface region of the catalyst in the kinetically relevant elementary steps (Mars–van Krevelen mechanism) is not positively verified. Using in situ X-ray absorption techniques to study binary and ternary molybdenum oxides the present contribution shows that it is possible to tackle some of the problems in selective oxidation by direct experimental observation. The modification of the Mo–O local bonding interaction upon thermal reduction of MoO₃to MoO_3-x is illustrated. This was also found for mixed Mo–V oxides in which the chemical state of the vanadium seemed unaffected by the reaction but the surface Mo:V ratio varied substantially with the gas phase composition. It is further shown that the solid-state phase transformation between reduced and oxidised forms of molybdenum oxides occur so rapidly, that possibly relevant suboxide cannot be identified by ex situ phase analysis. Observation of the time-law of redox transformations showed that lattice oxygen is only available for selective oxidation if the associated solid-state transformation occurs in the kinetic regime of reaction control and not in that of diffusion control.     

Liquid‐Phase Aerobic Oxidation of Benzyl Alcohol Catalyzed by Pt/ZrO2

The oxidation of benzyl alcohol by molecular oxygen in the liquid phase and catalyzed by Pt/ZrO2 using n‐heptane as the solvent was studied. Pt/ZrO₂ was very active and 100 % selective for benzyl alcohol conversion to benzaldehyde. The catalyst can be separated by filtration and reused. No leaching of Pt or Zr into the solution was observed. Typical batch reactor kinetic data were obtained and fitted to the Langmuir‐Hinshelwood, Eley‐Rideal and Mars‐van Krevelen models of heterogeneously catalyzed reactions. The Langmuir‐Hinshelwood model was found to give a better fit. The rate‐determining step was proposed to involve direct interaction of an adsorbed oxidizing species with the adsorbed reactant or an intermediate product of the reactant. H₂O₂ was also proposed to be an intermediate product. n‐Heptane was found to be an appropriate solvent in this reaction system.     

Kinetics of o-Xylene oxidation over sintered vanadium pentoxide in a spinning catalyst basket reactor

Kinetics of vapor phase oxidation of o-xylene has been studied over sintered and compacted vanadium pentoxide in a continuous stirred tank catalytic reactor in the temperature range of 450–517°C at atmospheric pressure. The major product obtained is phthalic anhydride. The other products are maleic anhydride and carbon dioxide. The reaction rate data are well represented (with average absolute deviation less than 6%) by the following expression derived by applying the steady-state oxidation-reduction model of Mars and van Krevelen to a parallel reaction scheme and assuming first order with respect to both o-xylene and oxygen: Significantly, the activation energies for all three postulated reactions with rate constants k₁, k₂, and k₃ turn out to be identical having a value of 14.8 kcal/mole, which may be taken to imply that there is only one rate-influencing reaction step for all the products and not three as assumed in deriving this equation.     

Kinetic Study of CO2 Reforming of Propane over Ru/Al2O3

The rate of reaction of propane over a Ru/Al₂O₃ catalyst was determined as a function of the partial pressures of the reactants, C₃H₈ and CO₂ at 600 and 650°C. The order of the reaction was found to be fractional with respect to carbon dioxide, indicating its involvement in the rate-determining step of the reaction. The order of the reaction was zero in propane indicating the fast reaction of propane over the catalyst. The apparent activation energies for propane and the formation of hydrogen and carbon monoxide were investigated. Values for the formation of hydrogen and carbon monoxide indicated a decrease in the CO:H₂ ratio with an increase in temperature. Modelling of the kinetic data was inconclusive in the selection of a possible mechanism as good fits were observed for a Langmuir–Hinshelwood and a Mars–van Krevelen mechanism.     

Redox kinetics of benzene oxidation to maleic anhydride

Steady state kinetics of the oxidation reaction have been determined with the help of a gradientless semi-differential, fixed-bed reactor. The Mars and van Krevelen phenomenological model satisfactorily correlates the experimental data but a modification of the Langmuir-Hinshelwood model taking into account partial coverage of the catalyst surface with reaction intermediates is preferred. Transient kinetics have been studied with a new automated periodic-pulse reactor, directly connected to a gas chromatograph. The response of a catalyst (essentially V₂O₅–MoO₃) to reduction and oxidation has been investigated. The rate of bulk (lattice) oxygen utilization as well as the degree of carbon coverage are estimated by this technique. Selectivity is dependent on the oxidation state of the catalyst: high partial pressure of either benzene or oxyen and high temperatures are detrimental to selectivity.     

A kinetic study of the electrocatalytic oxidation of reduced glutathione at Prussian blue film-modified electrode using rotating-disc electrode voltammetry

In this work a thorough study of the oxidation of reduced glutathione (GSH) by electro-generated Berlin Green (BG) at Prussian blue (PB) film-modified glassy carbon electrode (GCE) was attempted by employing cyclic voltammetry (CV) and rotating-disc electrode (RDE) techniques. It has been shown that oxidation of GSH occurs at the potential coinciding with that of Fe^II(CN)₆ to Fe^III(CN)₆ transformation in the PB film, where no oxidation signal is observed at a bare GCE. The kinetics of catalytic reaction was investigated using a rotating-disc electrode voltammetry. The results obtained for various thicknesses of film and GSH concentrations are explained using the theory of electrocatalytic reactions at chemically modified electrodes (Andrieux–Saveant model) and it was concluded that the reaction has a “surface” reaction mechanism in which a few monolayers at film/solution side engaged in the catalytic process. However, the “surface” reaction tends to a saturation limit with increasing GSH concentration was observed and the behavior has been explained by using Michealis–Menten inner sphere kinetics. Tafel plots for various concentrations of GSH have been drawn and the slope values of 95–110 mV/decade indicate that the first electron transfer is not rate limiting process. The reaction order with respect to GSH and H⁺ were calculated as 0.6 and −0.4, respectively.     

Seawater-soluble pigments and their potential use in self-polishing antifouling paints: simulation-based screening tool

This work concerns the on-going development of efficient and environmentally friendly antifouling paints for biofouling control on large ocean-going ships. It is illustrated how a detailed mathematical model for a self-polishing antifouling paint exposed to seawater can be used as a product engineering tool to obtain a quick estimate of the paint behaviour that a given seawater-soluble pigment will provide. In the present context, “pigment” refers to relevant particulate solids of organic-, inorganic-, or biological nature. Simulations performed at 15 and 30 °C suggest that pigment solubility and seawater diffusivity of dissolved pigment species have a significant influence on the polishing and leaching behaviour of a typical self-polishing paint system. The pigment size distribution, on the other hand, only has a minor influence on the paint–seawater interaction. Simulations also indicate that only compounds which are effective against biofouling at very low seawater concentrations are useful as active antifouling paint ingredients. The need for model verification and exploration of practical issues, subsequent a given pigment has been found of interest, is discussed. The model approach is of relevance in the search for novel antifouling paints and for the development of accelerated test methods.