Oxidation of polymethylated benzenes promoted by.OH radicals

OH radicals, produced in aqueous acid media by the electrogenerated system Fe(II)/H₂O₂, promote the oxidation of polymethylated benzenes. Under our experimental conditions only one -CH₃ group was oxidized to -CHO. A reduction of aldehyde to alcohol by^.OOH radicals was shown to occur when H₂O₂ was produced in excess over Fe(II). In hydrochloric acid or in perchloric acid with Cl^– ions added, a remarkable increase of the yields was observed. The results are discussed on the basis of the kinetic parameters of the reactions involved in the process.     

Synergistic effect of TiO2 and iron oxide supported on fluorocarbon films. Part 1: Effect of preparation parameters on photocatalytic degradation of organic pollutant at neutral pH

Iron oxide and TiO₂ were immobilized on modified polyvinyl fluoride films in a sequential process. Synergic effects of iron oxide and TiO₂ on the polymer film were observed during the heterogeneous degradation of hydroquinone (HQ) in the presence of H₂O₂ at pH close to neutrality and under simulated solar irradiation. Within the degradation period, little iron leaching (<0.5 mg/L) was observed.The surface of commercial polyvinyl fluoride (PVF) film was modified by TiO₂ under light inducing oxygen group (C–OH, CO, COOH) formation on the film surface. During this treatment, TiO₂ nanoparticles simultaneously bind to the film, leading to PVF^f–TiO₂. The possible mechanistic pathway for the TiO₂ deposition and the nature of the polymer–TiO₂ interaction are discussed. Furthermore PVF and PVF^f–TiO₂ were immersed in an aqueous solution for the deposition of iron oxide layer by hydrolysis of FeCl₃, leading to PVF–Fe oxide and to PVF^f–TiO₂–Fe oxide respectively.HQ degradation and mineralization mediated by PVF^f–TiO₂, PVF–Fe oxide and PVF^f–TiO₂–Fe oxide were investigated under different conditions. Remarkable synergistic effects were observed for PVF^f–TiO₂–Fe oxide possibly due to Fe(II) regeneration, accelerated by electron transfer from TiO₂ to the iron oxide under light.     

On the kinetic mechanism of the reaction of NH2 with O2 in O-, H-, and N-containing flames. 1. Kinetic parameters of the NH2+O2=NHO+OH reaction

The reaction pathways for NH₂+O₂Products are considered on the basis of experimental data on the ignition of an NH₃/O₂/Ar mixture in reflected shock waves (p=1–10 atm, T=900–2160 K) and on the NH₃/O₂/Ar flame structure (p=35 torr, T=1050–2600 K) using a multistage kinetic mechanism. The rate constants of the NH₂+O₂=HNO+OH reaction, obtained from a comparison of experimental and calculated data, are reported (k=3·10¹¹ exp (–15,000/RT) cm³/(mole·s) at T1500–2160 L and k=3·10⁹ cm³/(mole·s) at T900–1400 K).Novosibirsk. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 1, pp. 60–65, January–February, 1994.     

The effect of benzotriazole on mass transfer in the corrosion of a copper rotating disk electrode

The effect of benzotriazole, BTA, on mass transfer in dissolution-corrosion of the copper rotating disk electrode in 0.02 M Fe(III)–0.5 M H₂SO₄ has been studied by means of atomic absorption spectrometry. The mass transfer coefficient, K, was determined from the slope of ln(C ₀/C)_Fe(III) vs. time plots. In the absence of BTA the corrosion process can be described by the correlation Sh = KR/D = 4.47Re ^0.5. The difference in values between Sh and Sh _Levich, and the change in slope in the Arrenhius plot points to mixed control for the cathodic process Fe³⁺ + 1e^– Fe ²⁺ and charge transfer control for the anodic process, Cu Cu²⁺ + 2e. The average activation energies were 7.7 kJ mol^–1 and 19.5 kJ mol^–1 at (500–1500) and (2000–3000) rpm, respectively. At low concentration of BTA the inhibiting action of BTA increases with concentration and with rotation speed. For [BTA] 5 × 10^–3 M, the K value, 10^–4 cm s^–1, remains constant and is independent of rotation rate. The morphology of the copper rotating disk after corrosion in the absence and presence of BTA was examined using scanning electron microscopy (SEM).     

Chemical and electrochemical considerations on the removal process of hexavalent chromium from aqueous media

Two methods were used to remove Cr(VI) from industrial wastewater. Although both are based in the same general reaction: 3Fe(II)_(aq) + Cr(VI)_(aq) ; 3Fe(III)_(aq) + Cr(III)_(aq) the way in which the required amount of Fe(II) is added to the wastewater is different for each method. In the chemical method, Fe(II)_(aq) is supplied by dissolving FeSO₄ · 7(H₂O)_(s) into the wastewater, while in the electrochemical process Fe(II)_(aq) ions are formed directly in solution by anodic dissolution of an steel electrode. After this reduction process, the resulting Cr(III)_(aq) and Fe(III)_(aq) ions are precipitated as insoluble hydroxide species, in both cases, changing the pH (i.e., adding Ca(OH)_2(s)). Based on the chemical and thermodynamic characteristics of the systems Cr(VI)–Cr(III)–H₂O–e^– and Fe(III)–Fe(II)–H₂O–e^– both processes were optimized. However we show that the electrochemical option, apart from providing a better form of control, generates significantly less sludge as compared with the chemical process. Furthermore, it is also shown that sludge ageing promotes the formation of soluble polynuclear species of Cr(III). Therefore, it is recommended to separate the chromium and iron-bearing phases once they are formed. We propose the optimum hydraulic conditions for the continuous reduction of Cr(VI) present in the aqueous media treated in a plug-flow reactor.     

Direct partial oxidation of methane over ZSM-5 catalyst: Zn-ZSM-5 catalyst studies

Extended studies on Zn-ZSM-5 catalyst for the production of liquid hydrocarbons in the direct partial oxidation (DPO) of CH₄ with O₂ are reported. Previously, it was reported that metal-containing ZSM-5 catalysts could produce C₅₊ hydrocarbons from pure CH₄/O₂ feeds without feed additives. Zn-ZSM-5 produced the highest C₅₊ yields of the catalysts tested. This work shows that the method of introducing Zn onto the catalyst, ion-exchange versus impregnation, does not significantly alter C₅₊ yields if low Zn content is maintained ( 0.4–0.5 wt%). Liquid hydrocarbon yields in this system doubled after 8 h on stream while overall C₂₊ yields increased by over 300%. Mechanistic implications of these findings are discussed. Finally, processing a natural gas feed over Zn-ZSM-5 gave higher C₅₊ yields over CH₄ feed but these yields were not improved over previously published results using HZSM-5.     

Measurement of the hydroxyl radical formation from H2O2, NO3−, and Fe(III) using a continuous flow injection analysis

Production of hydroxyl radical (OH) is of significant concern in engineered and natural environment. A simple in situ method was developed to measure OH formation in UV/H₂O₂, UV/Fe(III), and UV/NO₃^? systems using trapping of OH by benzoic acid (BA) and measuring fluorescence signals from hydroxylated products of BA. Method development included characterization of OH trapping mechanism and measurement of quantum yields (Φ_OH) for OH. The distribution of OHBA isomers was in the order of o-OHBA > p-OHBA > m-OHBA, although it changed with the H₂O₂ concentration and light intensity. This supports that OH attacks dominantly on the benzene rings. The quantum yields for OH formation in the UV/H₂O₂ process were 1.02 and 0.59 at 254 and 313 nm, which were in good agreement with the literature values, confirming that the method is suitable for the measurement of OH production from UV/H₂O₂ processes. Using the continuous flow method developed, quantum yields for OH in UV/H₂O₂, UV/Fe(III), and UV/NO₃^? systems were measured varying the initial concentration of OH precursors. The Φ_OH values increased with increasing concentrations of H₂O₂, Fe(III), and NO₃^? and approached constant values as the concentration increased. The Φ_OH values were 0.009 for H₂O₂ at 365 nm, showing that OH production is not negligible at such high wavelength. The Φ_OH values during the photolysis of Fe(OH)²⁺ (pH 3.0) and Fe(OH)₂⁺ (pH 6.0) at 254 nm were 0.34 and 0.037, respectively. The Φ_OH values for NO₃^? approached a constant value of 0.045 at 254 nm at the initial concentration of 10 mM.     

Oxidative coupling of methane over BaF2-TiO2 catalysts

The addition of F^– to Ba-Ti mixed oxide catalysts significantly improves the catalytic performances for the oxidative coupling of methane (MOC), which can achieve high C₂ yields at wide feed composition range and high GHSV. The effect is particularly marked for the BaF_2– TiO₂ catalysts containing more than 50 mol% BaF₂. The C₂ yield of 17% and the C₂ selectivity of > 60% were achieved over these catalysts at 700 ° C. After being on stream for 31 h, the 50 mol% BaF₂-TiO₂ catalysts showed only a 1–1.5% decrease in the C₂ yields. Results obtained by XRD show that various Ba-Ti oxyfluoride phases were formed due to the substitution of F^– to O^2–.     

Degradation of the fluoroquinolone enrofloxacin by electrochemical advanced oxidation processes based on hydrogen peroxide electrogeneration

Solutions of the veterinary fluoroquinolone antibiotic enrofloxacin in 0.05 M Na₂SO₄ of pH 3.0 have been comparatively degraded by electrochemical advanced oxidation processes such as anodic oxidation with electrogenerated H₂O₂ (AO-H₂O₂), electro-Fenton (EF), photoelectro-Fenton (PEF) and solar photoelectro-Fenton (SPEF) at constant current density. The study has been performed using an undivided stirred tank reactor of 100 ml and a batch recirculation flow plant of 2.5 l with an undivided filter-press cell coupled to a solar photoreactor, both equipped with a Pt or boron-doped diamond (BDD) anode and a carbon-polytetrafluoroethylene gas diffusion cathode to generate H₂O₂ from O₂ reduction. In EF, PEF and SPEF, hydroxyl radical (OH) is formed from Fenton's reaction between added catalytic Fe²⁺ and generated H₂O₂. Almost total decontamination of enrofloxacin solutions is achieved in the stirred tank reactor by SPEF with BDD. The use of the batch recirculation flow plant showed that this process is the most efficient and can be viable for industrial application, becoming more economic and yielding higher mineralization degree with raising antibiotic content. This is feasible because organics are quickly oxidized with OH formed from Fenton's reaction and at BDD from water oxidation, combined with the fast photolysis of complexes of Fe(III) with generated carboxylic acids under solar irradiation. The lower intensity of UVA irradiation used in PEF with BDD causes a slower degradation. EF with BDD is less efficient since OH cannot destroy the most persistent Fe(III)-oxalate and Fe(III)-oxamate complexes. AO-H₂O₂ with BDD yields the poorest mineralization because pollutants are only removed with OH generated at BDD. All procedures are less potent using Pt as anode due to the lower production of OH at its surface. Enrofloxacin decay always follows a pseudo first-order reaction. Its primary aromatic by-products and short intermediates including polyols, ketones, carboxylic acids and N-derivatives are detected by GC-MS and chromatographic techniques. The evolution of F⁻, NO₃⁻ and NH₄⁺ ions released to the medium during each process is also determined.     

On the Possibility of Forming Complexes with the Participation of O2– Ions in Melts and Glasses of the Na2O–SiO2–Cu2O System

The structural role of copper ions in melts (glasses) of the Na₂O–SiO₂–Cu₂O–CuO system is analyzed in the framework of the acid–base concept with due regard for the geometric (the radius ratio for Cu²⁽¹⁾⁺ and O^2– ions) and energy (the mean enthalpies of the Cu²⁽¹⁾⁺–O bonds) factors. It is demonstrated that copper ions in the structure fulfill the function of modifier cations. In these melts, the Cu¹⁺–Cu²⁺ redox equilibrium can be described without regard for the formation of [Cu²⁽¹⁾⁺O_4/2]^2(3)– ionic complexes (which could be incorporated into the structure of silicon–oxygen anions) and [Cu²⁺O _b/k ]^{2 – b/k} polyhedra providing the interaction between Cu²⁺ ions and anions. The influence of the formation of these polyhedra on the redox equilibrium is considered within the formalism of chemical thermodynamics. The composition dependence of the oxygen ion exponent pO is measured by an electromotive force (emf) technique. The ratio between the numbers of copper atoms with different valences is determined by chemical analysis. The experimental data obtained are in agreement with the theoretical inferences.     

The behaviour of a trickle tower electrolytic cell for the production of cobalt(III) acetate from cobalt(II) acetate

The production of Co(III) acetate from Co(II) acetate using a bipolar trickle tower of graphite Raschig rings was investigated. Space time yields up to 18 kg m^–3 h^–1 were obtained, which showed no improvement over those achievable in a conventional plate and frame cell. A mathematical model of the system indicated that the electrode reactions occurred almost entirely at the opposing annular surfaces between consecutive layers of Raschig rings and that the unexpectedly low performance of the device was most probably due to the unfavourable mass transport conditions which existed in the intervening gaps.Nomenclature a annular cross sectional area of one Raschig ring (m²) - b _C kinetic exponential constant for reduction of Co(III) (V^–1) - b _A kinetic exponential constant for oxidation of Co(II) (V^–1) - b _H kinetic exponential constant for hydrogen evolution (V^–1) - b ₀ kinetic exponential constant for oxygen evolution (V^–1) - [Co(II)] concentration of Co(II) (mol m^–3) - [Co(III)] concentration of Co(III) (mol m^–3) - F Faraday constant (96 487 C mol^–1) - f fraction of total flow by-passing the annular gap between adjacent Raschig rings in a vertical row - I current per vertical column of rings (A) - k _C rate constant for reduction of Co(III) (A m mol^–1) - k _A rate constant for oxidation of Co(II) (A m mol^–1) - k _H rate constant for hydrogen evolution (A m^–2) - k _O rate constant for oxygen evolution (A m^–2) - k _L mass transfer coefficient (m s^–1) - Q flow rate per vertical row of Raschig rings (m³s^–1) - v volume of annular gap between adjacent Raschig rings in a vertical row (m³) - V superficial velocity of electrolyte (m s^–1) - _A anodic potential (V) - _C cathodic potential (V)     

Exploration of molten hydroxide electrochemistry for thermal battery applications

The electrochemistry of molten LiOH–NaOH, LiOH–KOH, and NaOH–KOH was investigated using platinum, palladium, nickel, silver, aluminum and other electrodes. The fast kinetics of the Ag⁺/Ag electrode reaction suggests its use as a reference electrode in molten hydroxides. The key equilibrium reaction in each of these melts is 2 OH^– = H₂O + O^2– where H₂O is the Lux-Flood acid (oxide ion acceptor) and O^2– is the Lux–Flood base. This reaction dictates the minimum H₂O content attainable in the melt. Extensive heating at 500 °C simply converts more of the alkali metal hydroxide into the corresponding oxide, that is, Li₂O, Na₂O or K₂O. Thermodynamic calculations suggest that Li₂O acts as a Lux–Flood acid in molten NaOH–KOH via the dissolution reaction Li₂O_(s) + 2 OH^– = 2 LiO^– + H₂O whereas Na₂O acts as a Lux–Flood base, Na₂O_(s) = 2 Na⁺ + O^2–. The dominant limiting anodic reaction on platinum in all three melts is the oxidation of OH^– to yield oxygen, that is 2 OH^– 1/2 O₂ + H₂O + 2 e^–. The limiting cathodic reaction in these melts is the reduction of water in acidic melts ([H₂O] [O^2–]) and the reduction of Na⁺ or K⁺ in basic melts. The direct reduction of OH^– to hydrogen and O^2– is thermodynamically impossible in molten hydroxides. The electrostability window for thermal battery applications in molten hydroxides at 250–300 °C is 1.5 V in acidic melts and 2.5 V in basic melts. The use of aluminum substrates could possibly extend this window to 3 V or higher. Preliminary tests of the Li–Fe (LAN) anode in molten LiOH–KOH and NaOH–KOH show that this anode is not stable in these melts at acidic conditions. The presence of superoxide ions in these acidic melts likely contributes to this instability of lithium anodes. Thermal battery development using molten hydroxides will likely require less active anode materials such as Li–Al alloys or the use of more basic melts. It is well established that sodium metal is both soluble and stable in basic NaOH–KOH melts and has been used as a reference electrode for this system.     

Numerical models for reactions catalysed by homogeneous mediators: the case of Fenton's reagent

A numerical model has been developed to describe the behaviour of a batch reactor in which Fenton's reagent is used for hydroxylating aromatic hydrocarbons under conditions of electrochemical regeneration. The test reaction considered is the conversion of benzene into phenol. Comparison is made with previously published experimental results.Nomenclature A electrode area, m² - a ₁ parameter defined by Equation 21 - C _i concentration of species, i, in the bulk solution, mol m^–3 - c _i local concentration of species, i, in the diffusion layer, mol m^–3 - K _i effective mass-transfer coefficient, m s^–1 - k _j rate constant of reaction j - R _j rate of reaction j, mol m^–3 s^–1 - r _i rate of change of concentration of species i due to chemical reaction, mol m^–3 s^–1 - t time, s - V reactor volume, m³ - x distance from the cathode surface, m - x ^* maximum thickness of the diffusion layer, m - period of diffusion layer renewal, s Subscrpts 1 oxygen - 2 Fe³⁺ - 3 hydrogen peroxide - 4 Fe²⁺ - 5 benzene - 6 phenol - 7 biphenyl This paper was presented at the meeting on Electroorganic Process Engineering held in Perpignan, France, 19–20 September 1985.     

Solar photoassisted anodic oxidation of carboxylic acids in presence of Fe using a boron-doped diamond electrode

This paper reports a comparative study on the anodic oxidation of 2.5 l of 50 mg l⁻¹ TOC of formic, oxalic, acetic, pyruvic or maleic acid in 0.1 M Na₂SO₄ solutions of pH 3.0 with and without 1.0 mM Fe³⁺ as catalyst in the dark or under solar irradiation. Experiments have been performed with a batch recirculation flow plant containing a one-compartment filter-press electrolytic reactor equipped with a 20 cm² boron-doped diamond (BDD) anode and a 20 cm² stainless steel cathode, and coupled to a solar photoreactor. This system gradually accumulates H₂O₂ from dimerization of hydroxyl radical (OH) formed at the anode surface from water oxidation. Carboxylic acids in direct anodic oxidation are mainly oxidized by direct charge transfer and/or OH produced on BDD, while their Fe(III) complexes formed in presence of Fe³⁺ can also react with OH produced from Fenton reaction between regenerated Fe²⁺ with electrosynthesized H₂O₂ and/or photo-Fenton reaction. Fast photolysis of Fe(III)-oxalate and Fe(III)-pyruvate complexes under the action of sunlight also takes place. Chemical and photochemical trials of the same solutions have been made to better clarify the role of the different catalysts. Solar photoassisted anodic oxidation in presence of Fe³⁺ strongly accelerates the removal of all carboxylic acids in comparison with direct anodic oxidation, except for acetic acid that is removed at similar rate in both cases. This novel electrochemical advanced oxidation process allows more rapid mineralization of formic, oxalic and maleic acids, without any significant effect on the conversion of acetic acid into CO₂. The synergistic action of Fe³⁺ and sunlight in anodic oxidation can then be useful for wastewater remediation when oxalic and formic acids are formed as ultimate carboxylic acids of organic pollutants, but its performance is expected to strongly decay in the case of generation of persistent acetic acid during the degradation process.     

Measurement of Temperature and Concentration of OH Radicals in Combustion of Hydrogen and Ethanol by the Laser‐Induced Fluorescence Technique

Diffusion combustion of ethanol and hydrogen and homogeneous combustion of hydrogen–oxygen mixtures are studied by the laserinduced fluorescence technique in the linear regime and with signal saturation. Data on the flame temperature and OH concentration are obtained. The burning temperature of 3090 K for a stoichiometric O₂–H₂ mixture is in agreement with the known value. It is shown that the maximum concentrations of radicals in the hydrogen–air and stoichiometric hydrogen–oxygen flames are close to each other (4.4· 10¹⁶ cm^-3).     

Selective oxidation of methane with air over silica catalysts

Partial oxidation of methane by oxygen to form formaldehyde, carbon oxides, and C₂ products (ethane and ethene) has been studied over silica catalyst supports (fumed Cabosil and Grace 636 silica gel) in the 630–780 °C temperature range under ambient pressure. The silica catalysts exhibit high space time yields (at low conversions) for methane partial oxidation to formaldehyde, and the C₂ hydrocarbons were found to be parallel products with formaldehyde. Short residence times enhanced both the C₂ hydrocarbons and formaldehyde selectivities over the carbon oxides even within the differential reactor regime at 780 °C. This suggests that the formaldehyde did not originate from methyl radicals, but rather from methoxy complexes formed upon the direct chemisorption of methane at the silica surface at high temperature. Very high formaldehyde space time yields (e.g., 812 g/kg cat h at the gas hourly space velocity = 560 000 (NTP)/kg cat h) could be obtained over the silica gel catalyst at 780 °C with a methane/air mixture of 1.5/1. These yields greatly surpass those reported for silicas earlier, as well as those over many other catalysts. Low CO₂ yields were observed under these reaction conditions, and the selectivities to formaldehyde and C₂ hydrocarbons were 28.0 and 38.8%, respectively, at a methane conversion of 0.7%. A reaction mechanism was proposed for the methane activation over the silica surface based on the present studies, which can explain the product distribution patterns (specifically the parallel formation of formaldehyde and C₂ hydrocarbons).     

Ozone generation via the electrolysis of fluoboric acid using glassy carbon anodes and air depolarized cathodes

An electrochemical ozone generation process was studied wherein glassy carbon anodes and air depolarized cathodes were used to produce ozone at concentrations much higher than those obtainable by conventional oxygen-fed corona discharge generators. A mathematical model of the build up of ozone concentration with time is presented and compared to experimental data. Products based on this technology show promise of decreased initial costs compared with corona discharge ozone generation; however, energy consumption per kg ozone is greater. Recent developments in the literature are reviewed.Nomenclature A electrode area (m²) - Ar ^* modified Archimedes number, d _b ³ g_G/² (1 — _G) - C _{O 3 (aq)} concentration of dissolved ozone (mol m^–3) - C _{O 3 i} concentration at interface (mol m^–3) - C _{O 3 1} concentration in bulk liquid (mol m^–3) - D diffusion coefficient (m² s^–1) - E electrode potential against reference (V) - F charge of one mole of electrons (96 485 C mol^–1) - g gravitational acceleration (9.806 65 m s^–2) - i current density (A m^–2) - i ₁ limiting current density (A m^–2) - I current (A) - j material flux per unit area (mol m^–2 s^–1) - k _obs observed rate constant (mol^–1 s^–1) - k _t thermal conductivity (J s^–1 K^–1) - L reactor/anode height (m) - N _{O 3} average rate of mass transfer (mol m^–2 s^–1) - Q heat flux (J s^–1) - r _i radius of anode interior (m) - r _a radius of anode exterior (m) - r _c radius of cathode (m) - R gas constant (8.314 J K^–1 mol^–1) - S _c Schmidt number, v/D - Sh Sherwood number, k _m d _b/D = i _L d _b/zFD[O₃] - t time (s) - T _i temperature of inner surface (K) - T _o temperature of outer surface (K) - U reactor terminal voltage (V) - electrolyte linear velocity (m s^–1) - V volume (m³) - V _{O 3} volume of ozone evolved (10^–6 m³ h^–1) - z _i number of Faradays per mole of reactant in the electrochemical reaction Greek symbols _G gas phase fraction in the electrolyte - (mean) Nernst diffusion layer thickness (m) - fractional current efficiency - overpotential (V) - electrolyte kinematic viscosity (m² s^–1) - electrolyte resistivity (V A^–1 m)     

Aspects of electrochemical behaviour of carbon steel in different Bayer plant solutions

Cyclic voltammetric and potentiodynamic studies were carried out on 300W carbon steel in Bayer plant solution, at 100 °C, with different alumina concentrations. Alumina behaves as an anodic inhibitor, shifting the critical passivation potentials positively and decreasing the critical passivation current with increasing concentration. Increase in alumina concentration promotes the formation of a uniform and less porous film. The pore resistance model describes the properties of the oxide films. Aluminium was found in all oxides formed, supporting the formation of a mixed oxide Fe_3–x Al _x O₄. Thermodynamic calculation of some equilibrium potentials was carried out using the Fe(OH)₃ ^– ion rather than HFeO₂ ^– ion. Moreover, the Al(OH)₄ ^– ion was considered instead of AlO₂ ^– ion in the oxidation process.     

Synthesis gas formation by direct oxidation of methane over Rh monoliths

The production of H₂ and CO by catalytic partial oxidation of CH₄ in air or O₂ at atmospheric pressure has been examined over Rh-coated monoliths at residence times between 10^–4 and 10^–2 s and compared to previously reported results for Pt-coated monoliths. Using O₂, selectivities for H₂ ( ) as high as 90% and CO selectivities (S _CO) of 96% can be obtained with Rh catalysts. With room temperature feeds using air, Rh catalysts give of about 70% compared to only about 40% for Pt catalysts. The optimal selectivities for either Pt or Rh can be improved by increasing the adiabatic reaction temperature by preheating the reactant gases or using O₂ instead of air. The superiority of Rh over Pt for H₂ generation can be explained by a methane pyrolysis surface reaction mechanism of oxidation at high temperatures on these noble metals. Because of the higher activation energy for OH formation on Rh (20 kcal/mol) than on Pt (2.5 kcal/mol), H adatoms are more likely to combine and desorb as H₂ than on Pt, on which the O+ H OH reaction is much faster.This research was partially supported by DOE under Grant No. DE-FG02-88ER13878-AO2.