Kinetics Of Methanol Oxidation Research Articles

Investigation of a direct methanol fuel cell based on a composite Nafion®-silica electrolyte for high temperature operation

Operation of a liquid-fed Direct Methanol Fuel Cell (DMFC) working at 145°C was demonstrated by using a composite membrane made of Nafion® ionomer and silica. The enhanced humidification conditions of the membrane–electrode (M&E) assembly allowed taking advantage of the high operation temperature with consequent speeding up of the methanol oxidation kinetics. A peak power density of 240 mW/cm−2 for an oxygen-fed cell was reached; the measured methanol cross-over rate was 4×10−6 moles min−1 cm−2. A physico-chemical characterization of the cell components is also reported.

Hydrothermal oxidation of model molecules and industrial wastes

AbstractThe environmental regulation evolution and the increasing wastewater disposal costs led to a new concept for the complete destruction of toxic substances and sludges. Hydrothermal oxidation of wastes was developed as an alternative technique in order to limit the risks of secondary pollution.After a brief presentation of previous works dealing with hydrothermal oxidation and supercritical water properties, results concerning two model molecules: methanol and glucose are presented. The oxidation kinetics of methanol is modeled and by‐products of glucose characterized.Results concerning industrial wastes such as paper mill sludges, mechanical wastewaters and cutting oils are examined. The influence of various parameters controlling the trans‐formation such as pressure, temperature and residence time are investigated.

Area determination in fractal surfaces of Pt and Pt–Ru catalysts for methanol oxidation

The surface areas of Pt and Pt–Ru alloys have been estimated by a fractal technique. Fractal measurements were made using a scanning tunneling microscope (STM), which measures the surface corrugations. This STM-based fractal technique is offered as an alternative to gas adsorption technique for area estimation. The usefulness of the fractal approach to area calculation in the Pt–Ru alloy, where the hydrogen adsorption technique cannot be used, is demonstrated using the kinetics of methanol oxidation.

Methanol Oxidation on Palladium Compared to Rhodium at Ambient Pressures as Probed by Surface-Enhanced Raman and Mass Spectroscopies

The influences of surface speciation upon the catalytic oxidation kinetics of methanol on palladium under ambient-pressure flow-reactor conditions was examined by surface-enhanced Raman spectroscopy (SERS) combined with mass spectrometry (MS) and compared with corresponding data on rhodium. The former technique provides uniquely sensitive surface vibrational information under real-time (≈1 s)in situconditions, by utilizing ultrathin catalyst films electrodeposited onto an inert SERS-active gold substrate. These transition-metal surfaces exhibit sufficiently robust SERS activity to enable temperature-dependent spectral measurements over the range 25–500°C. Parallel kinetic measurements undertaken with MS show the occurrence of methanol decomposition (to CO and H2) in the absence of O2on both Pd and Rh. While the presence of a molar deficiency of O2yields methanol oxidation (to form CO2and H2) in addition to methanol decomposition on Rh, only the latter occurred (at slower rates than Rh) on Pd. These dissimilar reaction selectivities are consistent with the absence of surface vibrational features on the latter surface and the observed presence of adsorbed CO on the former. The behavior can be rationalized by the paucity of adsorbed atomic oxygen, O(ad), on Pd compared with Rh arising from the greater ability of the latter to dissociatively chemisorb O2. Both catalysts induced exhaustive methanol oxidation (yielding CO2and H2O) in a heavily O2-rich reactant mixture, although Pd again yielded less facile reaction kinetics. In addition, a significant catalyst deactivation occurred upon heating Pd in this reactant mixture, which was entirely absent on Rh. The corresponding temperature-dependent SER spectra indicate the formation of palladium oxide (PdO) by 350°C, which was retained entirely upon subsequent cooling. While an oxide (Rh2O3) was also seen by SERS to form on Rh by 350°C under these conditions, this species was removed upon subsequent cooling. Transient SERS measurements following sudden exposure of such oxidized surfaces to a methanol gas stream revealed that PdO was entirely unreactive toward methanol even at 350°C, while, in contrast, Rh2O3was removed entirely within ca. 5 s. This remarkable difference in oxide reactivity, which accounts for the Pd catalyst deactivation, was deduced to be due primarily to the inability of methanol to yield a suitable adsorbed “oxygen scavenger” by dissociative chemisorption on Pd. The possible involvement of a methoxy intermediate in the reaction on Pd under O2-rich conditions, as suggested by the SERS data, is also discussed.

96/05671 Methanol and hydrogen oxidation kinetics in water in supercritical states

96/05671 Methanol and hydrogen oxidation kinetics in water in supercritical states

Inhibition of Methanol Oxidation by Water Vapor—Effect on Measured Kinetics and Relevance to the Mechanism

The kinetics of methanol oxidation to formaldehyde was studied over an iron molybdenum oxide catalyst in a continuous flow reactor with external recycling at temperatures of 200–300°C. The kinetics of the reaction were well described by a power law rate expression of the formr=kPCH3OHxPO2yPH20z, wherex=0.94±0.06,y=0.10±0.05, andz=−0.45±0.07. The measured activation energy was 98±6 kJ/mol. When product inhibition by water vapor is not taken into account in such a power law kinetic rate expression, the apparent reaction orders in methanol and oxygen,x′andy′, and the activation energyE′are all lower than their true values:x′=(1−δ)x,y′=(1−δ)y, andE′=(1−δ)E, where δ=−z/(1−z). Methanol chemisorbs dissociatively to form methoxy and hydroxyl groups, and the rate-determining step is the decomposition of the methoxy intermediate. Product inhibition occurs through kinetic coupling, whereby water vapor chemisorbs dissociatively to form hydroxyl groups, which serve to reduce the steady state concentration of methoxy groups on the catalyst surface by reacting with them to reform methanol.

Methanol and hydrogen oxidation kinetics in water at supercritical states

The oxidation kinetics of methanol and hydrogen in a supercritical water medium were investigated. Numerical analyses were performed using an isobaric, plug-flow reactor model coupled with the Chemkin Real-Gas software package to handle real-gas thermodynamic effects and elementary kinetics. In the present work, select unimolecular reactions that involve thermal decomposition have been corrected for the high-pressure conditions found in supercritical reactors. Predictions of fuel destruction rates obtained using an elementary reaction mechanism for hydrogen oxidation under isobaric, isothermal supercritical conditions ( P > 22.1 MPa, and T > 374 C) were verified by comparison with previous experimental results obtained in a laboratory-scale supercritical water reactor. The H 2O 2 model is a subset of a proposed elementary reaction mechanism for methanol oxidation which was also verified by comparing the present model predictions of the kinetic rate calculations with previous measurements. The mechanism was validated over a temperature range of 726 to 873 K, a fuel concentration range of 0.001 to 0.004 mole/L, and at a pressure of 246 bar. To facilitate future computational fluid dynamic (CFD) modeling efforts in SCWO, a two-step reduced reaction mechanism was constructed to simulate the oxidation process of methanol in supercritical water and H 2O. The reduced model reflects the sequential oxidation of methanol into carbon monoxide, and eventually into final products consisting primarily of carbon dioxide and H 2O. The calculations of the two-step reduced mechanism matched the elementary reaction model well with respect to major species concentration profiles.

Kinetics of methanol oxidation on carbon-supported Pt and Pt + Ru catalysts

The kinetics of methanol electro-oxidation on carbon-supported particulate Pt and Pt + Ru catalysts is studied on PTFE-bonded electrodes made of these materials. The electrochemical characterization is carried out both galvanostatically and potentiostatically in sulphuric acid, and special attention is paid to the effects of low methanol concentrations in the range from 5.0 mM to 2.0 M. Furthermore, the effects of reaction temperature, catalyst loading and electrolyte concentration are discussed. A steady-state adsorption model which explains the observed polarization behaviour is presented. The model highlights the subtle balance between the initial methanol adsorption-dehydrogenation and the subsequent oxidative removal steps as well as the important roles of water and surface intermediates in the oxidation reaction. Rough estimates are given for the rate constants and activation energies of the individual reaction steps.

TAP reactor study of the partial oxidation of methanol to formaldehyde using an industrial Fe-Cr-Mo oxide catalyst

The mechanism and kinetics of methanol oxidation to formaldehyde on an industrial Fe-Cr-Mo catalyst have been investigated using the TAP (temporal analysis of products) reactor. The Mars-van Krevelen redox mechanism of partial oxidation was confirmed. By kinetic isotope experiments, the abstraction of hydrogen from the adsorbed methoxy intermediate was found to be the slow step on the catalyst surface. Water was observed to readsorb strongly on the catalyst, thus explaining the reduced rate of reaction that has been found at high methanol conversions. Weaker readsorption of formaldehyde was observed. The kinetics of the reaction were studied on a fully oxidized and a partially reduced catalyst. On the fully oxidized catalyst, the activation energy for formaldehyde formation was estimated to be 87 kJmol/. At 360°C the average surface residence time of the adsorbed species in the conversion of methanol to formaldehyde was approximately 40 ms, meaning that the surface reaction rate constant for the main reaction is 25 s −1. The surface residence time was significantly longer on the partially reduced catalyst, indicating a much slower surface reaction rate constant and emphasizing the importance of oxidation state on the rate of formaldehyde formation.

Mixture Effects and Methanol Oxidation Kinetics over a Palladium Monolith Catalyst

Methanol oxidation has been investigated using a simulated methanol fueled vehicle exhaust and a commercial palladium monolith catalyst. The presence of water and CO led to severe inhibition of the methanol conversion at any temperature; propylene addition had no effect on the conversion profile. The CO inhibition is described as competitive adsorption of the CO on palladium oxidation sites. The methanol conversion was described through a Langmuir-Hinshelwood model. At low temperature, and under reducing conditions, the methanol conversion displayed a local maximum. This maximum is attributable to a shift in the oxidation state of the palladium catalyst within the temperature range of the methanol oxidation reaction. Temperature programmed oxidation and differential scanning calorimetry reveal that the catalyst changes from a relatively reduced surface at low temperature to a less active, relatively oxidized surface as the temperature is increased.

Electrochemical Oxidation of Methanol at Chemically Prepared Platinum‐Ruthenium Alloy Electrodes

Chemically prepared Pt‐Ru alloy catalysts were investigated to study the effects of temperature, Pt loading, and heat‐treatment on the kinetics of methanol oxidation in acid electrolyte. The maximum current density obtained for the Pt‐Ru catalyst, dispersed on Vulcan XC‐72R carbon, was about 600 mA/cm2 at 60°C and a Pt loading of 2.05 mg/cm2. The maximum catalytic shift due to the Ru modification of Pt was 200 mV at a current density of 160 mA/cm2. The activation energy for methanol oxidation is similar for both Pt and Pt‐Ru. The Ru composition of the electrode stabilized at higher levels when the electrode was heat‐treated. The oxidation current per mg of Pt decreased with an increase in the Pt loading. A phenomenological analysis of the catalytic shift for methanol oxidation at Pt‐Ru electrodes favored the bifunctional catalyst model over models based on geometric and electronic factors.

ChemInform Abstract: Kinetics and Reaction Pathways of Methanol Oxidation on Platinum

Methanol oxidation kinetics were measured on Pt wires in a flow reactor at pressures between 30 and 130 Pa. The kinetics were measured as a function of oxygen-to-methanol equivalence ratio phi and wire temperature. In methanol-lean feeds (phi 1), CO, H/sub 2/, H/sub 2/CO, CO/sub 2/, and H/sub 2/O were observed. Experiments with /sup 18/O/sub 2/ showed that the principal methanol oxidation pathway does not involve C-O bond dissociation. However, the /sup 18/O/sub 2/ experiments, together with other features of the methanol oxidation data, also provided evidence for a minor oxidation pathway (accounting for less than 1% of the product CO/sub 2/) which proceeds through a carbon intermediate. A mathematical model is presented which describes the principal CH/sub 3/OH oxidation pathway as a series reaction involving adsorbed H/sub 2/CO and CO intermediates. Consistent with experimental results, the model predicts that inhibition by adsorbed CO should be weaker for CH/sub 3/OH and H/sub 2/CO oxidation than for CO oxidation. 34 references, 10 figures, 2 tables.

Diffusion of oxygen in alginate gels related to the kinetics of methanol oxidation by immobilized Hansenula polymorpha cells

In the yeast Hansenula polymorpha an oxygen-requiring enzyme, alcohol oxidase, catalyzes the conversion of methanol into formaldehyde. After growth on methanol cells of the organism were harvested and entrapped in barium-alginate gels. The diffusion of oxygen towards these cells is seriously hindered by the polymer network. This caused significant changes in the kinetics of methanol oxidation by the immobilized cells as compared to that observed with cells free in suspension. The apparent K S(O2) of the immobilized cells was dependent upon the density of the cells in the alginate beads and the bead radius. Using a well-known theoretical model, originally developed to describe the kinetics of oxygen diffusion in mold pellets, the diffusion coefficient for oxygen in barium-alginate gels was estimated. This coefficient was only 25% of that in water. The model also allowed an adequate simulation of the observed kinetics of methanol oxidation by the immobilized cells.

Reaction-deactivation kinetics of methanol oxidation over a silica-supported iron oxide-molybdenum oxide (Fe2O3-MoO3) catalyst

Reaction-deactivation kinetics of methanol oxidation over a silica-supported iron oxide-molybdenum oxide (Fe2O3-MoO3) catalyst

State and action of the tin atoms in platinum-tin catalysts for methanol fuel cells

The behavior of platinum-tin electrodes in cyclic voltammetry in pure acid and the kinetics of methanol oxidation on these electrodes have been studied. By using the cyclic voltammetry technique it was possible to demonstrate that the tin atoms are present in the zero-valent state and hence are not acting as a cocatalyst via a redox couple between their oxidic forms. It is furthermore shown that the zero-valent tin atoms influence the adsorption properties of the platinum atoms. The enhancement in methanol oxidation rate on platinum-tin electrodes is explained in terms of a “ligand” effect.

Adsorption of hydrogen and oxygen and oxidation of methanol on ruthenium electrodes

Sorption of hydrogen and oxygen on ruthenium in 1N H 2SO 4 have been investigated by potentiodynamic and potentiostatic methods. It has been shown that adsorption and absorption of hydrogen and oxygen take place on a smooth ruthenium electrode. The solubility of hydrogen grows linearly with potential in the range 0·41–0·04 V, whereas the solubility of oxygen increases linearly with potential in the range 0·37–1·3 V. Adsorption of hydrogen on a smooth ruthenium electrode takes place in the potential range 0·4–0·0 V; on ruthenized ruthenium electrode the main portion of hydrogen is adsorbed at potentials 0·2–0·0 V. No maxima corresponding to specific types of chemisorbed oxygen exist on the potentiodynamic curves of oxygen adsorption. The oxygen adsorption rate on ruthenium is of the same order as on platinum. The kinetics of methanol oxidation on smooth and ruthenized ruthenium electrodes were investigated. The chemisorption of methanol was found to be the limiting step in both cases in the potential range 0·4–1·0 V; the chemisorption rate on smooth ruthenium exceeds the chemisorption rate on ruthenized electrode by two orders of magnitude. This results is consistent with the difference in the surface bond energies of adsorbed hydrogen for smooth and ruthenized ruthenium.

The kinetics of methanol oxidation on a supported silver catalyst

The oxidation of methanol to formaldehyde on silver catalysts was studied at about 420 °C and atmospheric pressure in a fixed-bed differential reactor. The reaction was zero order to oxygen except at very low pressures, slightly less than first order to methanol, and inhibited by the reaction products. The data can be fitted by the following equation, which is based on competitive adsorption of methanol and products on sites formed by reversible chemisorption of oxygen on the silver and reaction of adsorbed methanol with underlying oxygen atoms: r = (kKmPm1 + KmPm + KpPp) (KoPo121 + KoPo12).

Kinetics of the vapor-phase oxidation of methyl alcohol on manganese dioxide-molybdenum trioxide catalyst

The kinetics of methanol oxidation was studied over manganese dioxide-molybdenum trioxide catalyst in an integral flow reactor between 250 and 460 °C, at space velocities of 9.6 × 10 3 to 8.4 × 10 4 hr −1. The highest conversion of methanol and yield of formaldehyde under these conditions were 84% with a selectivity of almost 100% at 365 °C. The rate equation assuming a steady state involving a two-stage irreversible oxidation-reduction process, represented the data satisfactorily.

On the mechanism of CH 3OH oxidation to CH 2O over MoO 3Fe 2(MoO 4) 3 catalyst

The kinetics of methanol oxidation over MoO 3Fe 2(MoO 4) 3 catalyst were studied by means of a differential flow reactor and a pulse reactor in the temperature range 180–280 °C. The reaction rate does not depend on the partial pressures of the reactants, except for the lowest ones. Formaldehyde and, especially, water act as inhibitors of the reaction. A competitive adsorption was observed between methanol and water. The reaction rates measured in the pulse reactor were much higher than those measured in the steady-state flow reactor, while the reverse was found for the apparent activation energies. A mechanism is proposed which satisfactorily agrees with the experimental results. The rate-determining step seems to be the desorption of the products.

The behaviour of platinized-platinum and platinum-ruthenium electrodes in methanol solutions

Measurements of the curves showing the dependence of the shift in the Pt/Pt electrode potential, after the introduction of methanol, upon pH of the solution and the potential at the moment of introduction of methanol, lead to the conclusion that the potentials of a Pt/Pt electrode, in methanol solutions, are determined by the adsorbed hydrogen. The presence of adsorbed hydrogen has been established from the charging curves and the potentiostatic curves of a platinum electrode measured after its contact with methanol. The analysis of these curves shows, also, that under definite conditions a predominant chemisorption of particles with the composition HCO occurs on platinized platinum in methanol solutions. The kinetics of methanol oxidation under non-steady-state and steady-state conditions have been studied and the difference in the mechanisms of the process on the bare platinum surface and on that covered with a stationary layer of chemisorbed substance, shown. The influence of pH of the solution, the dependence of the rate upon the potential and the kinetic characteristics of electro-oxidation of the chemisorption products, are used as possible criteria for the elucidation of the mechanism of the process. A hypothesis has been advanced, that on a bare surface the rate of the process is determined by dehydrogenation of the original alcohol molecules and under steadystate conditions-by the oxidation of the products of their chemisorption. In acid solutions the latter process may proceed by way of interaction with OH radicals. Some possible schemes of the oxidation process in alkaline solutions have been discussed. Composite electrolytic deposits of platinum and ruthenium, and of palladium and ruthenium with a small percentage of ruthenium, have been found to be extremely catalytically active in the reaction of methanol electro-oxidation.