Methanol decomposition and oxidation

Borasio M, Rodriguez de la Euente O, Rupprechter G, Ereund H-J (2005) In situ studies of methanol decomposition and oxidation on Pd(lll) by PM-IRAS and XPS spectroscopy. J Phys Chem B Lett 109 17791... 

Fig. 15.12 Methanol oxidation on Pd-Al O (mean particle size 6 nm) and Pd(lll) [27, 50, 75]. (a) In situ steady-state PM-IRAS and SFG spectra, shown together with the CHjOH conversion as monitored by gas chromatography, (b) Comparison of Pd3d XP spectra obtained before upper) and after (lower) the methanol oxidation reaction indicated a partial oxidation of Pd nanoparticles during the reaction, whereas Pd(lll) remained metalhc adapted in part from [75] with permission. Copyright (2007) The PCCP Owner Societies, (c) PM-IRAS (p-s) surface vibrational spectra measured during CH OH decomposition on Pd(lll) at 300 K, with the various species indicated. The time-dependent evolution of CH O (as observed by PM-IRAS) and of CH (values deduced from XPS) upon methanol decomposition at -lO mbar suggests a correlation between the two species adapted in part from [27] with permission. Copyright (2005) American Chemical Society, (d) Suggested mechanism of CH OH decomposition and oxidation on Pd catalysts reprinted [50] with permission. Copyright (2007) Elsevier...

This example can be applied to a broad class of catalytic reactions but it is much more obvious for partial oxidation reactions where secondary reactions (total combustion) result in a dramatic decrease of selectivity. This is the case with methanol decomposition and methane conversion, where the intensification of gas-phase catalytic operations in micro- or nanochannels clearly appears. 

Iron-iron oxide catalysts have been repeatedly reported to lie unsatisfactory for methanol decomposition or oxidation, because of their activity in causing complete oxidation to carbon dioxide or decomposition to carbon if a deficiency of oxygen prevails. However, catalysts composed of iron and molybdenum oxide have been found to be very efficient for methanol oxidation.21 Such a mixed catalyst apparently combines the excellent directive power of molybdenum and the activity of iron. Molybdenum oxide deposited on small iron balls was shown to be 100 per cent efficient... 

In general, acetic acid production via acetaldehyde oxidation takes place continuously in a bubble column at 50-80 °C with pressures of 1-10 bar. The construction material of choice for the reactor is austenitic Cr-Ni-steel. The acetic acid product serves as process solvent and the concentration of acetaldehyde is kept at 3%. It is necessary to keep the temperature over 50 °C to obtain a sufficient peroxide decomposition and oxidation rate. To remove the heat of the exothermic reaction, the reaction mixture is circulated through an external heat exchanger. Accurate temperature control is important to decrease oxidative degradation of acetic acid to formic acid, CO2, and water. The reaction mixture is separated by several distillation units. The process yields are typically in the range of 90-97% and the purity of acetic acid is higher than 99%. Typical by-products are CO2, formic acid, methyl acetate, methanol, methyl formate, and formaldehyde. 

Methanol electrooxidation is a surface structure sensitive reaction [69-74]. Of the three basal planes of platinum, Pt(110) has been found to be the most active. Pt(lll) has been shown to be the least reactive toward methanol decomposition, and the rate of methanol oxidation increases with increasing step density on ft(l 11) surfaces. To determine the effect of steps of monocrystalline Pt surfaces on the product distribution during methanol oxidation, Pt(l 11) and Pt(332) single crystals... 

Ab initio methods allow the nature of active sites to be elucidated and the influence of supports or solvents on the catalytic kinetics to be predicted. Neurock and coworkers have successfully coupled theory with atomic-scale simulations and have tracked the molecular transformations that occur over different surfaces to assess their catalytic activity and selectivity [95-98]. Relevant examples are the Pt-catalyzed NO decomposition and methanol oxidation. In case of NO decomposition, density functional theory calculations and kinetic Monte Carlo simulations substantially helped to optimize the composition of the nanocatalyst by alloying Pt with Au and creating a specific structure of the PtgAu7 particles. In catalytic methanol decomposition the elementary pathways were identified... 

Without the direct pathway contribution, this equation may either yield an increasing or decreasing current transient, depending on the value of A ox/ dec- If this ratio is larger than 4, i.e., if methanol decomposition is slow compared with CO oxidation, then the current is predicted to increase with time. Experimentally, this simation has been observed for a low methanol concentration and an almost perfect Pt(l 11) electrode [Housmans and Koper, 2003], which both lead to a low methanol decomposition rate. Typically, however, current transients decrease with time, suggesting that the rate... 

Figure 12.13 Electrochemistry and kinetics of CO resulting from methanol decomposition on polycrystalline Pt with O.IM H2SO4 electrol3de and 0.1 M methanol, (a-d) Current, SFG amphtude, frequency, and width of adsorbed CO, scanning the potential in both directions as indicated with the solid hne and fiUed circles denoting the forward (anodic) scan and the dashed hne and unfilled circles denoting the back (cathodic) scan, (e-g) Starting at 0.6 V, where the adsorbed CO is rapidly electro-oxidized, the potential is suddenly jumped to 0.2 V. The reformation of the CO layer (CO chemisorption) due to methanol decomposition occurs in about 20 s. The adsorbed CO molecules are redshifted, and have a broader spectrum at shorter times, when the adlayer coverage is low.

Another study on the preparation of supported oxides illustrates how SIMS can be used to follow the decomposition of catalyst precursors during calcination. We discuss the formation of zirconium dioxide from zirconium ethoxide on a silica support [15], Zr02 is catalytically active for a number of reactions such as isosynthesis, methanol synthesis, and catalytic cracking, but is also of considerable interest as a barrier against diffusion of catalytically active metals such as rhodium or cobalt into alumina supports at elevated temperatures. 

As was previously mentioned, PtRu alloys exhibit improved performance over pure Pt alloys.117,118 This is primarily a result of the ability of Ru to dissociate H20 for reaction with CO adsorbed on Pt sites.115,116 That CO oxidation on pure Ru is unfavorable indicates that on the bimetallic surface, CO is oxidized only on the Pt sites.119 Thus, CO is oxidized on Pt sites adjacent to Ru sites, where water is activated.120,121 This is known as a bifunctional mechanism. In addition, the presence of Ru atoms reduces the adsorption energy of CO on neighboring Pt atoms, lowering the activation energy of CO oxidation.122 This effect is purely electronic and is less significant than the bifunctional effect of Ru.123 One significant limitation of PtRu is the weak adsorption of methanol on Ru, particularly at room temperature.117,124 The weak adsorption severely hinders methanol decomposition, which is evident in Fig. 7 by the drop in current density for PtRu electrodes with high Ru composition.125... 

Following the reports of the effect of nitrogen doping anatase, visible light photocatalysis has also been reported for SrTiOs (NO elimination), MOx-ZnO (where M = W, V or Fe for acetaldehyde decomposition), and TaON, 135,136 methanol oxidation ). 

The higher decomposition points were obtained when adrenochrome methyl and ethyl ethers were prepared by oxidation of the appropriate catecholamine in methanol with silver oxide. The BOlid aminochromes were then obtained as microcrystalline solids on addition of dry ether and cooling the resultant solution to — 80°. The slightly less pure products were obtained when the oxidation was carried out in acetonitrile. [R. A. Heacock and B. D. Scott, loc. cit. (footnote c )]. 

However, if the photochemical reaction is run in the presence of oxygen, then of course, the methyl radicals are oxidized, and one obtains instead methanol, formaldehyde, and their decomposition products. Now, if the vessel is pumped out after a photo-oxidation and once again a normal photolysis of acetone is run, the products in the first 10 or 15 minutes are still oxidation products rather than hydrocarbon products. It takes from 15 to 30 minutes to remove whatever it is that is attached to the wall before the normal photochemical decomposition of pure acetone products are produced. These results should remind us that oxidation system do produce species, some of which are not known or understood. 

Kwart and Kahn have found that benzenesulfonyl azide forms a complex with freshly reduced copper powder.189 190 This copper azide complex decomposes at a lower temperature than the pure sulfonyl azide. In refluxing methanol, benzene-sulfonamide (27) is isolated as the major product. In the presence of dimethyl sulfoxide, N-benzenesulfonyldimethyl-sulfoximine (28) is obtained in almost quantitative yield. In cyclohexene solution benzenesulfonamide (29), N-benzenesul-fonyl-7-azabicyclo[4.1.0]heptane (30), and 1-cyclohexenylben-zenesulfonamide (31) are isolated as the main reaction products. According to the authors, Schemes VII and VIII represent an acceptable interpretation of the experimental data.189 190 In pure alcohol, the decomposition should occur by two competitive reactions (Scheme VII) producing benzenesulfonamide together with a ketone and oxidized copper. These last two products have indeed been observed in the reaction mixture. In the presence of DMSO, it seems that a copper-nitrene intermediate is formed which is trapped by DMSO. In cyclohexene solution, the authors have observed that the aziridine (30) disappears from the product composition when DMSO is added. The yield of enamine 31, however, is... 

Spinel oxides with a general formula AB2O4 (i.e. the so-called normal spinels) are important materials in industrial catalysis. They are thermally stable and maintain enhanced and sustained activities for a variety of industrially important reactions including decomposition of nitrous oxide [1], oxidation and dehydrogenation of hydrocarbons [2], low temperature methanol synthesis [3], oxidation of carbon monoxide and hydrocarbon [4], and oxidative dehydrogenation of butanes [5]. A major problem in the applications of this class of compound as catalyst, however, lies in their usually low specific surface area [6]. 

The photochemistry of Eosin under both reductive and oxidative conditions has been studied by several groups [145-151], Photoreduction by amines such as tribenzylamine (R = CH2, R" = ) produces two leuco analogues, the dihydro derivative, and the cross-coupled product formed from the amine radical and the dye radical anion (2) [152], In addition, debromination of Eosin is reported during photobleaching with amines and phenols. The reader however is referred to the extensive studies of Rose Bengal dehalogenation by Paczkowski and Neckers [153]. Radiolysis of Eosin in methanol shows that debromination is a consequence of the photochemical decomposition of semireduced Eosin [154],... 

As in the case of normal supported catalysts, we tried with this inverse supported catalyst system to switch over from the thin-layer catalyst structure to the more conventional powder mixture with a grain size smaller than the boundary layer thickness. The reactant in these studies (27) was methanol and the reaction its decomposition or oxidation the catalyst was zinc oxide and the support silver. The particle size of the catalyst was 3 x 10-3 cm hence, not the entire particle in contact with silver can be considered as part of the boundary layer. However, a part of the catalyst particle surface will be close to the zone of contact with the metal. Table VI gives the activation energies and the start temperatures for both methanol reactions, irrespective of the exact composition of the products. 

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