CO Disproportionation Research Articles

Infrared study of the reaction of H2+ CO on a Ru/SiO2 catalyst

The amount and dynamic behaviour of adsorbed hydrocarbon species formed during the reaction of H2+ CO on Ru/SiO2 were measured by the in situ infrared spectroscopic technique using Fourier-transform methods; the average chain length (the CH2/CH3 ratio), and the dependences of the rate of accumulation of such hydrocarbons on temperature and partial pressures of H2 and CO gases were investigated, together with their reactivities under various reaction conditions, such as hydrogenation. These adsorbed hydrocarbons grow in a linear form on a limited part of the Ru surface, whereas most of the rest of the surface is covered by adsorbed CO. The adsorbed hydrocarbons are readily dehydrogenated to form carbon and hydrogen on evacuation, are also hydrogenated to form mainly CH4 with some higher hydrocarbons, and exchange their hydrogen with molecular hydrogen. Their reactivities, however, were markedly retarded by adsorbed CO when CO was present in the gas phase.Infrared observations demonstrated that small amounts of carbon deposited by the CO disproportionation reaction or of accumulated hydrocarbon cuts down the amount of weakly held adsorbed CO markedly, which suggests that the sites for the weakly held CO play an important role in carbon deposition, which is followed by its hydrogenation to form reaction products.

The thermal decomposition of Plutonium (IV) oxalate hexahydrate

The thermal decomposition of Pu(C2O4)2 · 6H2O has been studied in both argon and oxygen using a combination of thermogravimetry and infrared spectroscopy. Decomposition in an inert atmosphere involves reduction of the cation to the trivalent state and its subsequent reoxidation to form PuO2. In an oxidizing atmosphere, with unrestricted access of oxygen, reduction of the cation does not take place and decomposition to PuO2 is through the oxycarbonate. The reduction of Pu(IV) appears to take place by a carbon monoxide catalyzed mechanism and the presence of carbon in the PuO2 decomposition product is attributed to the disproportionation of CO.

On the mechanism of CO and CO 2 methanation over Ru/molecular-sieve catalyst

Disproportionation of CO and reduction of CO 2 on Ru/molecular-sieve catalyst and the subsequent hydrogenation of the species formed has been studied in the temperature range 400 to 575 K using a through-flow microcatalytic reactor. The results indicate that CO disproportionates on the catalyst to give “active” carbon and CO 2; the active carbon thus formed reacts with H 2 to give CH 4. Unlike CO, CO 2 becomes chemisorbed on the catalyst and is subsequently reduced by H 2 to give CH 4 through the intermediate formation of active carbon. The time and temperature dependence of the reactivity of the carbon has been studied in detail and a mechanism of catalytic methanation of CO and CO 2 through the formation of active carbon intermediate has been proposed.

Studies of CO desorption and reaction with H 2 on alumina-supported Ru

The temperature-programmed desorption (TPD) of CO and temperature-programmed reaction (TPR) of chemisorbed CO in H 2 have been studied on a Ru Al 2O 3 catalyst. Adsorption at 303 K occurs molecularly and leads to the observation of two peaks in the TPD spectrum. The activation energies associated with these peaks are 27 and 37 kcal/mole. Upon heating above 415 K CO disproportionation takes place, forming CO 2 and carbon. The carbon deposited on the catalyst surface readily reacts with H 2 at 303 K to produce methane and ethane. These products are also formed during TPR of chemisorbed CO. The mechanism of CO disproportionation, the effect of surface carbon on the bonding of chemisorbed CO, and the reactions of chemisorbed CO with H 2 are discussed.

Temperature programmed desorption of carbon monoxide adsorbed on supported platinum catalysts

Temperature programmed desorption (TPD) measurements have been made for CO from Pt/aerosil ( d Pt ≈ 2.5 nm), Pt γ-Al 2O 3 ( d Pt, 2.4 nm) and Pt-Au/aerosil ( d Pt, 1.5–2.0 nm) catalysts. CO was adsorbed at room temperature, with TPD at 35 K min −1. On Pt/aerosil and Pt/γ-Al 2O 3 the TPD profile showed a weak maximum at 388–398 K, with the rest of the profile continuining with very substantial strength out to about 800 K. Increasing the gold content caused an enhancement of the peak at around 390 K relative to the rest of the profile. Adsorbed CO was recovered quantitatively by 860–870 K, while on Pt/aerosil a second adsorption/TPD sequence without intervening catalyst clean-up gave an identical profile, confirming negligible dissociative chemisorption of CO on Pt under these conditions. A small amount of CO 2 was desorbed along with the CO (CO 2 ≈ 6–7% of CO on Pt/ and Pt-Au/aerosil; CO 2 ≈ 12% of CO on Pt γ-Al 2O 3 ). Evidence is given for believing that the CO 2 originated from a catalysed reaction between CO and traces of residual H 2O (the latter from the support) rather than from CO disproportionation. Monolayer uptake measurements with CO and H 2 gave values for the ratio CO( ads)/H( ads) : Pt/aerosil, 0.75; Pt γ-Al 2O 3 , 0.84; Pt76-Au24/aerosil, 0.98; Pt15-Au85/ aerosil, 1.3.1. Reasons for the effect of Au on CO adsorption on Pt-Au are discussed.

Coadsorption of carbon monoxide and hydrogen on a Ni(111) surface: Influence of the “surface carbide”

Vibrational electron energy loss spectroscopy (HRELS) combined with LEED, work function changes and thermodesorption measurements have been used to study H 2 CO interactions on a Ni(111) surface. Coadsorption of H 2 and CO on a Ni(111) surface, maintained at 140°C and then cooled down to room temperature, under the reactional mixture, leads to the appearance of three surface species: (a) a weakly bonded CO thermally desorbed around 75°C and exhibiting a ν-C0 frequency loss peak near 221 meV; (b) an oxyhydrocarbonated compound characterized by vibrations at 362, 178 and 168 meV and by simultaneous desorption of H 2 and CO at 180°C; (c) a hydrocarbonated species which dehydrogenates at 280°C and exhibits vibrations at 376 and 95 meV. Since disproportionation of CO occurs at 140°C and leaves the so called “surface carbide” behind, it is possible that this carbide plays an important role in the formation of the observed surface species. In fact, coadsorption of CO and H 2, at room temperature on a carburized Ni(111) surface, allows the appearance of the weakly bonded CO in addition to the oxyhydrocarbonated species. Nevertheless, the third species (hydrocarbonated) is then not observed. Tentative identification of the different surface complexes is developed by reference to their vibrational spectra.

Hydrogenation of catalytic carbons obtained by CO disproportionation or CH 4 decomposition on nickel

The hydrogenation of catalytic carbons has been studied in the temperature range of their deposition (300–700°C) by CO disproportionation or by CH 4 decomposition on nickel powders. When obtained under non carbiding conditions, the catalytic carbons are very reactive between 350 and 600°C where uncatalysed carbons are inert. The reactivity does not depend on the temperature of deposition and on preliminary heat treatment, but depends on the degree of gasification. This reactivity is imputed to the quality of the metal carbon interface which allows a good deposition-gasification reversibility. When deposition occurs under carbiding conditions, both deposition and subsequent hydrogenation are poisoned by the carbon formed by thermal decomposition of the carbide.

Reactions of CO and CO 2 on [formula omitted] above 373 K as studied by infrared spectroscopic and magnetic methods

Reactions of CO and CO 2 on Ni SiO 2 catalysts have been studied in the 373–700 K range by ir, magnetic (saturation magnetization), and volumetric measurements. At room temperature, CO is adsorbed as a mixture of the linear form, NiCO ads, and the bridged form, Ni 2CO ads. Upon heating the system beyond 400 K, these forms are irreversibly transformed into new CO adspecies bonded to four Ni atoms, Ni 4CO ads, with a vco frequency at about 1830 cm −1. This multibonded species is in equilibrium with gaseous CO 2 according to the reaction 2 CO CO 2 + C; the remaining carbon atom is reversibly dissolved into the bulk in an interstitial position. This CO disproportionation is assumed to occur via the CO rupture of Ni 4CO ads with formation of a surface carbon atom bonded to three nickel atoms Ni 3C surf and an oxygen atom bonded to one nickel atom. CO 2 is dissociated on pure nickel into CO, adsorbed as Ni 4CO ads, and adsorbed oxygen atoms.

Redox reactions of metal carbonyls. I. Kinetics and mechanism of disproportionation of Co 2(CO) 8 with piperidine

The disproportionation reaction of Co 2(CO) 8 by nucleophilic attack of piperidine has been investigated at different temperatures in n-heptane with the stopped-flow technique. The reaction is given by a series of successive and/or competitive pathways with different kinetic dependence on the ligand concentration. The experimental findings suggest that a rapid coordination of the ligand (L) occurs until the Co 2(CO) 7L 3 species is formed; then the adduct dissociates into ionic fragments. The mechanism and the kinetic parameters are discussed with reference to available findings on Co 2(CO) 8.

Redox reactions of metal carbonyls.: II. Kinetic studies on the disproportionation of Co 2(CO) 8 induced by N-bases

The kinetics of the disproportionation reaction of Co 2(CO) 8 with triethylamine, benzylamine, cyclohexylamine and diethylamine were investigated employing stopped-flow technique. The kinetic behaviour is suggestive of two competing mechanisms, an amine-independent path involving probably a ratedetermining CO dissociation, followed by amine uptake, and an associative mechanism characterized by two or three consecutive attacks of the amine on the substrate, followed by a first-order decomposition of the final intermediate. The reaction rates are sensitive more to the steric characteristics of the entering ligand than to its basicity. Attention is drawn to correlate the reaction mechanism and the different structures of Co 2(CO) 8 in solution.

Methanation of carbon monoxide on nickel and nickel-copper alloys

Methanation of CO and CO 2, disproportionation of CO, and hydrogenation of carbon deposited on the surface of catalysts have been studied on Ni and Ni-Cu alloy films at low pressures, at temperatures of 250–350 °C. Under these reaction conditions, two steps of the overall mechanism have been identified; namely, dissociation of CO and hydrogenation of deposited carbon by adsorbed hydrogen. CO can be dissociated only on places where carbon atoms can be bound to several Ni atoms simultaneously; therefore addition of Cu to Ni strongly reduces the rate of methanation.

Carbon from carbon monoxide disproportionation on nickel and iron catalysts: Morphological studies and possible growth mechanisms

Carbon was prepared by disproportionation of CO; Ni(CO) 4 or Fe(CO) 5 was fed to the CO stream before it entered the decomposition furnace. Electron microscopy reveals differences in the carbons obtained: adding Fe(CO) 5 resulted in the usual fibrilar growth whilst Ni(CO) 4 gave rise to spherical shells and sausage-like forms which are hollow inside. X-ray powder diffraction shows that the carbon obtained using Ni(CO) 4 is not as well organised as the carbon obtained using Fe(CO) 5. Possible formation mechanisms are discussed; very likely, the carbon shells obtained using Ni(CO) 4 are precipitated from solution in nickel whereas carbon obtained in the presence of iron is catalytically recrystallised by an iron carbide. A mechanism for the formation of helical fibres is suggested.

Disproportionation of CO: II. Over cobalt and nickel single crystals

The catalytic disproportionation of CO in the presence of Ni and Co single crystals has been studied in the temperature range 400–800 °C. For both Ni and Co, at the lower temperatures, the formation of an intermediate carbide phase, prior to the formation of carbon which was present mainly in the form of filaments, was shown to be a principal factor. Furthermore, such intermediate carbides often grew in the form of single crystal hexagonal and pseudohexagonal shaped plates. At higher temperatures, a transient carbide phase was predicted to explain the morphology of the finally appearing graphite plates. This graphite displayed physical characteristics typical of a highly crystalline material and was comparable to the graphite formed over heated metals during the pyrolysis of hydrocarbons. A new form of cobalt carbide, β-Co 3C, is reported. This has the hexagonal DO 24-type structure with lattice parameters a = 5.26 A ̊ and c = 8.41 A ̊ .

Thin Pyrolytic Graphite Films for Electron Microscope Substrates

A routine procedure for growing very thin graphite substrate films has been developed. The films are grown pyrolytically in an ultra-high vacuum chamber by exposing (111) epitaxial nickel films to carbon monoxide gas. The nickel serves as a catalyst for the disproportionation of CO through the reaction 2C0 → C + CO2. The nickel catalyst is prepared by evaporation onto artificial mica at 400°C and annealing for 1/2 hour at 600°C in vacuum. Exposure of the annealed nickel to 1 torr CO for 3 hours at 500°C results in the growth of very thin continuous graphite films. The graphite is stripped from its nickel substrate in acid and mounted on holey formvar support films for use as specimen substrates.The graphite films, self-supporting over formvar holes up to five microns in diameter, have been studied by bright and dark field electron microscopy, by electron diffraction, and have been shadowed to reveal their topography and thickness. The films consist of individual crystallites typically a micron across with their basal planes parallel to the surface but oriented in different, apparently random directions about the normal to the basal plane.

Disproportionation of CO: I. Over iron and silicon-iron single crystals

A detailed study of the catalytic disproportionation of CO over Fe and 3.5% SiFe single crystals in the range 550–800 °C has been made. For both Fe and SiFe it has been demonstrated that γ-Fe 2O 3 is the active catalytic species. It is considered that the defect type structure associated with this oxide aids the diffusion of Fe 3+ ions to the surface where they may facilitate the breakdown of CO molecules. The notable reduction in rate of reaction at temperatures above 550 °C was shown to be a direct result of the formation of α-Fe 2O 3 in which the diffusion of Fe 3+ ions is considerably less than in γ-Fe 2O 3. Electron microscopy and selected area diffraction studies of the highly fibrous carbonaceous products formed at various stages in the reaction revealed that at 550 °C the reaction proceeds via the breakdown of Fe 2C whiskers (predominately) and platelets. The overall reaction is discussed in terms of the conditions necessary for the formation of the particular Fe oxides and the growth and reactions of Fe carbides.

Studies of the Molten Carbonate Electrolyte Fuel Cell

Fuel cells with electrolyte contained in a porous matrix have been operated at 650°C on synthetic, “realistic” fuels containing combinations of H2, CO, , , and N2. A mixture of air and was used as the oxidant. Hydrogen reacted nearly reversibly when a completely oxidized state of the electrode was avoided. No reversible oxidation of CO was observed, and current‐time data confirmed that electrode oxidation occurred preferentially. H2 and CO were oxidized at oxidized electrodes with 0.6v activation polarization. Carbon deposition from the disproportionation of CO occurred even during the oxidation of H2. The disproportionation can be prevented by addition of or .

Thermal decomposition of europium(III) oxalate

It is shown that the first step in the thermal decomposition of europium(III) oxalate leads to reduction to the divalent state. In an oxidizing atmosphere the europium is reoxidized and continues decomposing gradually to Eu 2O 3 at 390°. In an atmosphere of CO 2, Eu(II) Ox is stabilized at 320°. In vacuum, Eu(II)CO 3 admixed with carbon (originating in disproportionation of CO) is obtained in the temperature range 360–420°. A unified mechanism for the decomposition of oxalates is proposed, in which a trace of carbon monoxide acts as catalytic agent in the reductive decomposition of oxalates of reducible metal ions.

The thermal decomposition of erbium and lutetium oxalates

The thermal decomposition behavior of erbium and lutetium oxalates was found to differ in many ways from that of the light rare earth group. On heating in vacuum at 300 deg C, the erbium and lutetium salts retained more than 25% of their water of crystallization. The last traces of water stabilizes the oxalates; decomposition in vacuum begins at 340 to 360 deg C, as with the light rare earth oxalates, but only after a long induction period. lnitially carbonates are formed, and the CO disproportionates only to a small extent, about 17% with erbium oxalate and about 6% with lutetium oxalate. The carbonates are unstable and decomposition to the oxides follows soon. This reaction has an unstabilizing effect on eventual oxalato-carbonate intermediates. The regularlties in the stabillty of the oxalates and in the disproportionation of CO, observed in the case of the light rare earths, are thus seen to extend also to the heavy rare earths. (P.C.H.)

The thermal decomposition of thorium oxalate

The thermal decomposition of thorium oxalate dihydrate has been studied in the temperature region 270–900°C in both air and nitrogen atmospheres. The water of hydration is readily removed at 270°. At 300° decomposition takes place with liberation of CO and CO 2, the quantity of CO being slightly greater than that of CO 2. The results are compatible with formation of a carbonate as an intermediary in the degradation process. Some free carbon is found which is probably formed by disproportionation of CO. The thoria prepared at low temperatures 300–500° from the oxalate has a small crystallite size and is hygroscopic.