Surface Reaction Kinetics of the Oxidation and Reforming of CH4 over Rh/Al2O3 Catalysts

An 48‐step sur face reaction mechanism with thermodynamic consistent kinetic data is presented for the catalytic conversion of the gaseous chemical system H₂/O₂/H₂O/CO/CO₂/CH₄ over Rh/Al₂O₃ catalysts. Total and partial oxidation as well as steam reforming and dry reforming of methane over Rh catalysts is studied experimentally and numerically at varying temperature and composition. The results are used to extend the kinetic schemes we developed for H₂ oxidation, CO oxidation kinetics, and the water‐gas‐shift reactions in former studies. Aside from the experiments in a stagnation‐flow reactor presented here, we modeled a number of experiments from the literature to test the newly established kinetic scheme.     

Experimental measurements and kinetic modeling of CO/H2/O2/NOx conversion at high pressure

This paper presents results from lean CO/H₂/O₂/NO_x oxidation experiments conducted at 20–100 bar and 600–900 K. The experiments were carried out in a new high‐pressure laminar flow reactor designed to conduct well‐defined experimental investigations of homogeneous gas phase chemistry at pressures and temperatures up to 100 bar and 925 K. The results have been interpreted in terms of an updated detailed chemical kinetic model, designed to operate also at high pressures. The model, describing H₂/O₂, CO/CO₂, and NO_x chemistry, is developed from a critical review of data for individual elementary reactions, with supplementary rate constants determined from ab initio CBS‐QB3 calculations. New or updated rate constants are proposed for important reactions, including OH + HO₂ ? H₂O + O₂, CO + OH ? [HOCO] ? CO₂ + H, HOCO + OH ? CO + H₂O₂, NO₂ + H₂ ? HNO₂ + H, NO₂ + HO₂ ? HONO/HNO₂ + O₂, and HNO₂(+M) ? HONO(+M). Further validation of the model performance is obtained through comparisons with flow reactor experiments from the literature on the chemical systems H₂/O₂, H₂/O₂/NO₂, and CO/H₂O/O₂ at 780–1100 K and 1–10 bar. Moreover, introduction of the reaction CO + H₂O₂ → HOCO + OH into the model yields an improved prediction, but no final resolution, to the recently debated syngas ignition delay problem compared to previous kinetic models. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 454–480, 2008     

The reaction kinetics of dimethyl ether. I: High‐temperature pyrolysis and oxidation in flow reactors

Dimethyl ether reaction kinetics at high temperature were studied in two different flow reactors under highly dilute conditions. Pyrolysis of dimethyl ether was studied in a variable‐pressure flow reactor at 2.5 atm and 1118 K. Studies were also conducted in an atmospheric pressure flow reactor at about 1085 K. These experiments included trace‐oxygen‐assisted pyrolysis, as well as full oxidation experiments, with the equivalence ratio (ϕ) varying from 0.32 ≤ ϕ ≤ 3.4. On‐line, continuous, extractive sampling in conjunction with Fourier Transform Infra‐Red, Non‐Dispersive Infra‐Red (for CO and CO₂) and electrochemical (for O₂) analyses were performed to quantify species at specific locations along the axis of the turbulent flow reactors. Species concentrations were correlated against residence time in the reactor and species evolution profiles were compared to the predictions of a previously published detailed kinetic mechanism. Some changes were made to the model in order to improve agreement with the present experimental data. However, the revised model continues to reproduce previously reported high‐temperature jet‐stirred reactor and shock tube results. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet: 32: 713–740, 2000     

Experimental measurements and kinetic modeling of CH4/O2 and CH4/C2H6/O2 conversion at high pressure

A detailed chemical kinetic model for homogeneous combustion of the light hydrocarbon fuels CH₄ and C₂H₆ in the intermediate temperature range roughly 500–1100 K, and pressures up to 100 bar has been developed and validated experimentally. Rate constants have been obtained from critical evaluation of data for individual elementary reactions reported in the literature with particular emphasis on the conditions relevant to the present work. The experiments, involving CH₄/O₂ and CH₄/C₂H₆/O₂ mixtures diluted in N₂, have been carried out in a high‐pressure flow reactor at 600–900 K, 50–100 bar, and reaction stoichiometries ranging from very lean to fuel‐rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Finally, the mechanism was extended with a number of reactions important at high temperature and tested against data from shock tubes, laminar flames, and flow reactors. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 778–807, 2008     

On the Phase Shifts in the Dynamics of the H2O2–Na2S2O3–H2SO4–CuSO4 Oscillator Revealed by Comparison of Potentiometric Responses of Different Indicator Electrodes

The dynamics of the H₂O₂–Na₂S₂O₃–H₂SO₄–CuSO₄ homogeneous pH oscillator was studied in the flow reactor potentiometrically using different sensors: platinum electrode, Cu(II) ion‐selective electrode (Cu‐ISE), and pH‐electrode. It was found that for the flow rates close to two bifurcation values, between which the oscillations exist, there is a detectable phase shift between the response of the Cu‐ISE and other electrodes, while it practically vanishes for the intermediate flow rates. To explain both the oscillations of the Cu‐ISE potential and the relevant phase shift, the system's dynamics was studied both experimentally and numerically. The literature kinetic mechanism of the pH oscillator was extended for the dynamics of the copper(II) and copper(I) species in the form of thiosulfate complexes, and kinetic parameters of the redox equilibria, ensuring the oscillations, were estimated. It was found that the phase shift at the relatively low flow rates occurs due to limited efficiency of the supply of CuSO₄ catalyst, as the species of lowest concentration, to the reactor, and therefore it can be minimized either by increasing the flow rate of all reactants or, alternatively, by enhancing the model concentration of CuSO₄ in the feeding stream, for its fixed flow rate. This work is one more proof that it is useful to monitor the dynamics of the homogeneous oscillatory systems with more than one electrode, if the experimental potential–time courses are to be explained in terms of an appropriate kinetic mechanism.     

Experimental and kinetic modeling study of the effect of NO and SO2 on the oxidation of CO?H2 mixtures

The effect of NO and SO₂ on the oxidation of a CO? H₂ mixture was studied in a jet‐stirred reactor at atmospheric pressure and for various equivalence ratios (0.1, 1, and 2) and initial concentrations of NO and SO₂ (0–5000 ppm). The experiments were performed at fixed residence time and variable temperature ranging from 800 to 1400 K. Additional experiments were conducted in a laminar flow reactor on the effect of SO₂ on CO? H₂ oxidation in the same temperature range for stoichiometric and reducing conditions. It was demonstrated that in fuel‐lean conditions, the addition of NO increases the oxidation of the CO? H₂ mixture below 1000 K and has no significant effect at higher temperatures, whereas the addition of SO₂ has a small inhibiting effect. Under stoichiometric and fuel‐rich conditions, both NO and SO₂ inhibit the oxidation of the CO? H₂ mixture. The results show that a CO? H₂ mixture has a limited NO reduction potential in the investigated temperature range and rule out a significant conversion of HNO to NH through reactions like HNO + CO ?? NH + CO₂ or HNO + H₂ ?? NH + H₂O. The chain terminating effect of SO₂ under stoichiometric and reducing conditions was found to be much more pronounced than previously reported under flow reactor conditions and the present results support a high rate constant for the H + SO₂ + M ?? HOSO + M reaction. The reactor experiments were used to validate a comprehensive kinetic reaction mechanism also used to simulate the reduction of NO by natural gas blends and pure C₁ to C₄ hydrocarbons. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 564–575, 2003     

Gold‐Nanoparticle‐Loaded Carbonate‐Modified Titanium(IV) Oxide Surface: Visible‐Light‐Driven Formation of Hydrogen Peroxide from Oxygen

Gold nanoparticle‐loaded rutile TiO₂ with a bimodal size distribution around 10.6 nm and 2.3 nm (BM‐Au/TiO₂) was prepared by the deposition precipitation and chemical reduction (DP‐CR) technique. Visible‐light irradiation (λ>430 nm) of the BM‐Au/TiO₂ plasmonic photocatalyst yields 35 μm H₂O₂ in aerated pure water at irradiation time (t_p)=1 h, and the H₂O₂ concentration increases to 640±60 μm by the addition of 4 % HCOOH as a sacrificing electron donor. Further, a carbonate‐modified surface BM‐Au/TiO₂ (BM‐Au/TiO₂‐CO₃^2?) generates a millimolar level of H₂O₂ at t_p=1 h with a quantum efficiency (Φ) of 5.4 % at λ=530 nm under the same conditions. The recycle experiments confirmed the stable performance of BM‐Au/TiO₂.     

Experimental and Kinetic Modeling Study of C2H2 Oxidation at High Pressure

A detailed chemical kinetic model for oxidation of acetylene at intermediate temperatures and high pressure has been developed and evaluated experimentally. The rate coefficients for the reactions of C₂H₂ with HO₂ and O₂ were investigated, based on the recent analysis of the potential energy diagram for C₂H₃ + O₂ by Goldsmith et al. and on new ab initio calculations, respectively. The C₂H₂ + HO₂ reaction involves nine pressure‐ and temperature‐dependent product channels, with formation of triplet CHCHO being dominant under most conditions. The barrier to reaction for C₂H₂ + O₂ was found to be more than 50 kcal mol^?1 and predictions of the initiation temperature were not sensitive to this reaction. Experiments were conducted with C₂H₂/O₂ mixtures highly diluted in N₂ in a high‐pressure flow reactor at 600–900 K and 60 bar, varying the reaction stoichiometry from very lean to fuel‐rich conditions. Model predictions were generally in satisfactory agreement with the experimental data. Under the investigated conditions, the oxidation pathways for C₂H₂ are more complex than those prevailing at higher temperatures and lower pressures. Acetylene is mostly consumed by recombination with H to form vinyl (reducing conditions) or with OH to form a CHCHOH adduct (stoichiometric to lean conditions). Both C₂H₃ and CHCHOH then react primarily with O₂. The CHCHOH + O₂ reaction leads to formation of significant amounts of glyoxal (OCHCHO) and formic acid (HOCHO), and the oxidation chemistry of these intermediates is important for the overall reaction.     

Oxidative Dissolution of Silver Nanoparticles by Dioxygen: A Kinetic and Mechanistic Study

We have recently reported a kinetic and mechanistic study on oxidative dissolution of silver nanoparticles (AgNPs) by H₂O₂. In the present study, the parameters that govern the dissolution of AgNPs by O₂ were revealed by using UV/Vis spectrophotometry. Under the same reaction conditions (Tris‐HOAc, pH 8.5, I=0.1 M at 25 °C) the apparent dissolution rate (k_app) of AgNPs (10±2.8 nm) by O₂ is about 100‐fold slower than that of H₂O₂. The reaction rate is first‐order with respect to [Ag⁰], [O₂], and [Tris]_T, and inverse first‐order with respect to [Ag⁺] (where [Ag⁰]=total concentration of Ag metal and [Tris]_T=total concentration of Tris). The rate constant is dependent on the size of AgNPs. No free superoxide (O₂⁻) and hydroxyl radical (⋅OH) were detected by trapping experiments. On the basis of kinetic and trapping experiments, an amine‐activated pathway for the oxidation of AgNPs by O₂ is proposed.     

Mechanism of the chain process forming H2 in the photooxidation of formaldehyde

A mechanism for the formation in a chain of H₂, CO, and HCOOH in the photooxidation of formaldehyde is proposed. This mechanism is initiated by the addition of HO₂ to formaldehyde. Hydrogen atoms are produced by the thermal dissociation of the HOCH₂O radical: HOCH₂O → H + HCOOH; ΔH = + 3.2 kcal/mol [5]. Photolysis of CH₂O? O₂? NO mixtures and product analysis were carried out in conjunction with kinetic simulation yielding an estimate for the activation energy of the dissociation reaction : E₅ = 14.9 ± 1.0 kcal/mol. Previous observations of this chain process are considered in view of this mechanism.     

Enhanced visible photocatalytic degradation of diclofen over N-doped TiO2 assisted with H2O2: A kinetic and pathway study

Increasing discharge and inadequate removal of pharmaceutical compounds pose significant concerns over global aquatic systems and human health. The accomplishment of affordable and safe water requires a stringent elimination of these micropollutants. This study evaluated the performance of Visible/N-doped TiO₂ and Visible/N-doped TiO₂/H₂O₂ processes using a submerged photocatalytic membrane reactor (SMPR) with suspended N-doped TiO₂ to address the removal of diclofenac (DCF). The kinetic and pathway of photodegradation of DCF were of particular interest in this study. The initial DCF concentrations upon the experiments were also examined using a wide range of 5–50 mg/L and 20–100 mg L⁻¹ for Vis/N-doped TiO₂, and Vis/N-doped TiO₂/H₂O₂ process, respectively. The results indicated that higher initial concentration reduces the efficiency of the process, but one with H₂O₂ demonstrated an enhanced performance. The experimental data were found to fit well a pseudo-first-order kinetic model. Our findings demonstrated the analogous pathways of DCF for both processes. The Vis/N-doped TiO₂/H₂O₂ process tends to hasten the degradation rate as evidenced by the disappearance of some DCF byproducts at a similar irradiation period as compared to the other. The study provided useful information of the degradation rate and the potential formation of DCF intermediates upon the hybrid photocatalytic systems, therefore being of importance for scaling-up as well as evaluating potential detoxification of DCF upon the novel photocatalytic system.     

Removal of dibenzothiophene in diesel oil by oxidation over a promoted activated carbon catalyst

Oxidative removal of dibenzothiophene (DBT) in n-octane solution by H₂O₂ on a promoted activated carbon (AC) catalyst was studied. DBT adsorption and catalytic behaviors on AC were examined. Effects of pH in aqueous phase, amounts of AC and formic acid (HCOOH) for promotion as well as initial molar H₂O₂/S ratio were investigated. Experimental results led to conclusion that DBT was readily oxidized by H₂O₂ over an AC catalyst promoted by HCOOH. Suitable amount of AC can improve the activity of H₂O₂ resulting in a deeper extent of sulfur removal. A 100% conversion of DBT in an octane solution by H₂O₂ oxidation was attained on the HCOOH-H₂O₂/AC catalyst at 80°C for a reaction time of 30 min.     

Photodegradation of EDTA using Fenton’s reagent: a pilot-scale study

The presence of ethylenediaminetetraacetic acid (EDTA) in decontamination wastes can cause complexation of cations resulting in interferences in their removal by various treatment processes, for example chemical precipitation, ion exchange, etc., and can negatively affect the quality of the final form of the waste. Advanced oxidation processes using ozone, H₂O₂, ultrasonics (US), ultraviolet (UV) light, Fenton’s reagent (Fe(II) + H₂O₂), alone or in combination, are regarded as possible methods of clean and ecologically safe remedial treatment for the degradation of organics. In this study, the development of a column photoreactor (15 L) and a shallow-tank photoreactor (100 L) was carried out at the Centralised Waste Management Facility, Kalpakkam, India. Pilot-scale (batch) studies of the photocatalytic degradation of EDTA (20,000 mg/L) using UV + Fenton’s reagent in these reactor geometries were attempted. The effect of the power of the UV radiation on the kinetics of photodegradation of EDTA (20,000 mg/L) was studied using the column photoreactor. The shallow-tank reactor was used to study the photodegradation of EDTA (20,000 mg/L) using UV radiation, visible radiation, and sunlight. The successful use of sunlight as a source of energy and its greater effectiveness than UV radiation in the treatment of EDTA are presented.     

Rate determination of the catalytic oxidation of methane and propane in an electrochemical solid-electrolyte reactor

The possibility of using a solid-electrolyte reactor in kinetic studies of the catalytic oxidations of hydrocarbon with molecular oxygen was investigated. A theoretical analysis of processes in a catalytically asymmetric gas-diffusion cell in N₂ + O₂ + CH₄ and N₂ + O₂ + C₃H₈ gas mixtures was performed. Analytical expressions are presented for calculating the oxygen, methane, and propane concentrations and the methane and propane oxidation rates in the inner space of the cell from the emf of the latter. The potentiometric response was studied experimentally after the addition of methane and propane in the gas mixture in a reactor with silver electrodes and samples with applied catalytic materials. The concentrations of the components in the inner space of the reactor and the oxidation rates of hydrocarbons were calculated from the experimental data.     

Powering Artificial Enzymatic Cascades with Electrical Energy

We have developed a scalable platform that employs electrolysis for an in vitro synthetic enzymatic cascade in a continuous flow reactor. Both H₂ and O₂ were produced by electrolysis and transferred through a gas‐permeable membrane into the flow system. The membrane enabled the separation of the electrolyte from the biocatalysts in the flow system, where H₂ and O₂ served as electron mediators for the biocatalysts. We demonstrate the production of methylated N‐heterocycles from diamines with up to 99 % product formation as well as excellent regioselective labeling with stable isotopes. Our platform can be applied for a broad panel of oxidoreductases to exploit electrical energy for the synthesis of fine chemicals.     

Identification of adsorption forms by ir spectroscopy for formaldehyde and formic acid on K3PMo12O40

Interaction of HCOOH and H₂CO with K₃PMo₁₂O₄₀ has been studied by IR spectroscopy. HCOOH adsorbed mainly in molecular form due to hydrogen bonds with surface oxygen ions. Two forms of adsorbed H₂CO were observed depending on the pressure. At low pressures it adsorbed on Br?nsted acid sites with the formation of hydrogen-bonded complexes. Dioxymethylene groups and a small amount of polyoxymethylene groups were formed at higher pressures. In contrast to oxide catalysts, formates were formed only in small amounts in the absence of oxygen. This can be explained by the formation of new surface adsorption sites Mo ions-due to a partial reduction of K₃PMo₁₂O₄₀ during its interaction with HCOOH and H₂CO.     

A new optical fiber reactor for the photocatalytic degradation of gaseous organic contaminants

The main objective of this study was to develop a simple, energy-efficient photoreactor for operating at room temperature. In this work, the design of a new gas-phase optical fiber photoreactor (OFP) was introduced which operated under various parameters, such as the UV light intensity and the initial concentration for the photocatalytic decomposition of acetone. Experimental results indicated that increasing the UV light intensity or decreasing the initial concentrations of acetone by a UV/TiO₂ process would result in improving the decomposition and mineralization efficiencies. The apparent quantum yield of the novel optical fiber reactor is about 2 to 3 times greater than that of the traditional annular reactor.     

Intrinsic kinetic study on the oxidation of 6-pentyl-1,2,3,4-tetrahydroanthacene-9,10-diol

A new working solution consisting of 2-pentylanthraquinone (PAQ) and 6-pentyl-1,2,3,4-tetrahydroantraquinone (4HPAQ) was hydrogenated and then oxidized by O₂ to produce H₂O₂. The oxidation reaction was conducted in a well-stirred batch reactor at 30~50^oC and 0.10~0.20 MPa. By measuring the concentrations of generated H₂O₂ by iodometry, the intrinsic kinetics for the gas-liquid oxidation of 6-pentyl-1,2,3,4-tetrahydroanthacene- 9,10-diol (4HPAQH₂, the only hydrogenated product in the hydrogenated working solution) with molecular oxygen were studied. An exponential model was employed to describe the reaction rate and the kinetic parameters were obtained. The results show that the reaction rate is 0.7 and 1.4 order in the concentration of 4HPAQH₂ and oxygen pressure respectively, and the activation energy for oxidation is 41.3 kJ/mol. This revised version was published online in June 2006 with corrections to the Cover Date.     

On the plasma chemistry of the C/H system relevant to diamond deposition processes

The chemistry of hydrogen-rich hydrocarbon-hydrogen mixtures is of primary interest for the understanding of the low-pressure synthesis of diamond. We per formed experiments under well-defined conditions like temperature, pressure, initial gas composition, etc. The gas composition at the end of a flow reactor was analyzed by a calibrated mass spectrometer and compared to results obtained from the Chemkin computer code. Residence thne in the reactor as well as other process parameters were similar to those of diamond-growing PA CVD processes performed earlier with the same experimental set-rip. Modeling and experiment under isothermal conditions show quantitative agreement. We realized time-resolved mass .spectrometry by means of a helium-flushed gas sampling probe. There is evidence that the commonly used reaction kinetic data for the dissociation C₂H₆ (+ M) 2CH,(+M) gives 2 too small C₂H₄ concentrations for hydrogen-rich conditions. This could be attributed to the poorly known third-body efficiencies of the H₂ molecules compared to Ar or C₂H₆ from which kinetic data are commonly derived.     

Water splitting in low-temperature ac plasmas at atmospheric pressure

Plasma-induced water splitting at atmospheric pressure has been studied with a novel fan-type Pt reactor and several tubular-type reactors: an all-quartz reactor, a glass reactor, and three metal reactors with Pt. Ni, and Fe as electrodes. Reaction products have been analyzed by using GC (gas chromatography) and Q-MS (quadrupole mass spectrometry). Optical emission spectroscopic studies of the process have been carried out by employing a CCD (charge-coupled device) detector. Water splitting with tubular quartz and glass reactors is probably non-catalytic. However, a heterogeneous catalytic function of surface of metal electrodes has been observed. The variation of hydrogen yield (Y_H) and energy efficiency (E_e) with operational parameters such as input voltages (U_in), flow rates of carrier gas (F_He), and concentrations of water (C_W) has been examined. Plasma-induced water splitting can be described with a kinetic equation of-dC_w/dt = kC_W ^0.2. The rate constants at 3.25 kV are 2.8 × 10⁻⁴, 3.5 × 10⁻³, and 3.4 × 10⁻² mol^0.8L^−0.8 min⁻¹ for tubular glass reactor, a tubular Pt reactor, and a fan-type Pt reactor, respectively. A CSTR (continuous-stirred tank reactor) and PFR (piston-flow reactor) model have been applied to a fan-type reactor and tubular reactor, respectively. A mechanism on the basis of optical emission spectroscopic data has been obtained comprising the energy transfer from excited carrier gas species to water molecules, which split via radicals of HO^·, O^·, and H^· to form H₂ and O₂. The fan-type Pt reactors exhibit highest activity and energy efficiency among the reactors tested. Higher yields of hydrogen are achieved at higher input voltages, low flow rates, and low concentrations of water (Y_H = 78 % at U_in of 3.75 kV, F_He of 20 mL/min, and C_W of 0.86 %). The energy efficiency exhibits an opposite trend (E_e = 6.1 % at U_in of 1.25 kV, F_He of 60 mL/min and C_W of 3.1 %).