Surface methanol

Methanol co-adsorbed with water displaced most of the water on the surface methanol co-adsorbed with oxygen formed surface methoxides stable to 625 K. Oxygen pretreatment of the surface did lead to the formation of a species assigned as formaldehyde, which Henderson proposed to be formed via a disproportionation reaction between two methoxides. Spectroscopic probes of the controlling intermediate were inconclusive [71,72]. 

Kinetic isotope effects using CH3OH and CH3OD show that the O-H bond is at least partially involved in the rate-limiting step. TPD experiments with pure Cu°, pure ZnO, and the catalyst Cu/ZnO showed that methanol can be activated by both ZnO and copper. On the ZnO surface, methanol can form intermediates, which in the presence of copper might react and desorb more easily probably via a reverse spillover process. The isotopic product distribution of H2, HD, D2,H20, HDO and D2O in the... 

Alcohol oxidation from Pt is one of the most studied in PCs because it is the basis of the anode reaction in direct methanol PCs. Methanol is oxidized in a six-electron reaction to COj, H and HjO on nanoscale Pt at around 0.4 V to 0.5 vs RHE. Methanol is oxidized more efficiently at lower potentials on PtRu alloys and related materials. The mechanism of the oxidation process on alloy catalysts and the form of the catalysts has been the subject of hundreds of papers and is beyond the scope of this paper. When alcohol is present on a catalyst in low concentrations as an impurity, it can be oxidized by exposing the PC to open-circuit conditions, or implementing various air starvation and transient operation methods discussed in the patent literature cited above. Methanol oxidation to COj and H completes its removal from the catalyst surface. Methanol poisoning at the cathode can also be a problem at DMFC cathodes, as methanol can easily cross over the Nafion. If the PC becomes severely contaminated, even open-circuit potentials might be too low to fully oxidize the methanol, requiring aggressive procedures to oxidize the methanol from the catalysts. 

A reaction mechanism is suggested which involves dissociative chemisorption of hydrogen and water in competition on one type of active sites and chemisorption of carbon dioxide on the other type. Chemisorption of carbon dioxide is so strong that it prevents chemisorption of carbon monoxide. Chemisorbed carbon dioxide and hydrogen are in equilibrium on the surface. Reverse shift takes place by dissociation of the reaction product into carbon monoxide and a chemisorbed hydroxyl-species. The shift reaction is taking place by reaction between carbon monoxide from the gas phase and hydroxyl-species on the surface. Methanol is formed by step-wise hydrogenation of chemisorbed carbon dioxide. 

An analysis of the kinetics of methanol decomposition after completion of DMTM [45] showed that the decomposition of methanol is particularly effectively promoted by a copper surface, the presence of which quickly leads to an almost complete disappearance of methanol from the products. Methanol is more stable with respect to the decomposition on a stainless steel surface however, at 450 °C and a residence time of 100 s, more than 50% of the methanol formed is lost in the presence of this surface. Methanol was practically stable in a Pyrex reactor, without any noticeable decomposition even at 500 °C (Fig. 6.7). 

Similarly, on Ru-modified Pt(lll) and Pt(332) surfaces, methanol decomposition to form COads is also enhanced at low potentials. For example, the coverage of methanol adsorbate formed at -hO.35 V on Pt(lll)/Ru and Pt(332)/Ru is much higher than on pure Pt(lll) and Pt(332) (Table 1). However, Sn modification of... 

Recently, we studied methanol electrooxidation on ordered intermetallic PtPb electrodes by quantitative DBMS and in situ Fourier transform infrared spectroscopy (FTIRS) [14]. It is very interesting and, to some extent, surprising that PtPb can help completely oxidize methanol to CO2, as indicated by the very high current efficiency (about 90%), in addition to the fact that no adsorbed CO was detected on the PtPb surface. Methanol electrooxidation on PtPb seems to have a different mechanism from that on Pt and PtRu methanol can be directly oxidized to CO2 without forming adsorbed CO. 

Catalytic gas-phase reactions play an important role in many bulk chemical processes, such as in the production of methanol, ammonia, sulfuric acid, and nitric acid. In most processes, the effective area of the catalyst is critically important. Since these reactions take place at surfaces through processes of adsorption and desorption, any alteration of surface area naturally causes a change in the rate of reaction. Industrial catalysts are usually supported on porous materials, since this results in a much larger active area per unit of reactor volume. 

Make a theoretical plot of surface tension versus composition according to Eq. III-53, and compare with experiment. (Calculate the equivalent spherical diameter for water and methanol molecules and take o as the average of these.)... 

The location and shape of the entire electrocapillary curve are affected if the general nature of the medium is changed. Fawcett and co-workers (see Ref. 126) have used nonaqueous media such as methanol, V-methylformamide, and propylene carbonate. In earlier studies, electrocapillaiy curves were obtained for O.OIA/ hydrochloric acid in mixed water-ethanol media of various compositions [117, 118]. The surface adsorption of methanol, obtained from... 

Wu M-C, Estrada C A, Corneille J S and Goodman D W 1996 Model surface studies of metal oxides adsorption of water and methanol on ultrathin MgO films on Mo(IOO) J. Chem. Phys. 96 3892... 

Size density surface (top left) space filling model (top right) potential map (bottom left) and tube model (bottom right) for methanol... 

The reaction occurs at essentially adiabatic conditions with a large temperature rise at the inlet surface of the catalyst. The predominant temperature control is thermal ballast in the form of excess methanol or steam, or both, which is in the feed. If a plant is to produce a product containing 50 to 55% formaldehyde and no more than 1.5% methanol, the amount of steam that can be added is limited, and both excess methanol and steam are needed as ballast. Recycled methanol requited for a 50—55% product is 0.25—0.50 parts per part of fresh methanol (76,77). 

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

Because the synthesis reactions are exothermic with a net decrease in molar volume, equiUbrium conversions of the carbon oxides to methanol by reactions 1 and 2 are favored by high pressure and low temperature, as shown for the indicated reformed natural gas composition in Figure 1. The mechanism of methanol synthesis on the copper—zinc—alumina catalyst was elucidated as recentiy as 1990 (7). For a pure H2—CO mixture, carbon monoxide is adsorbed on the copper surface where it is hydrogenated to methanol. When CO2 is added to the reacting mixture, the copper surface becomes partially covered by adsorbed oxygen by the reaction C02 CO + O (ads). This results in a change in mechanism where CO reacts with the adsorbed oxygen to form CO2, which becomes the primary source of carbon for methanol. 

A process based on a nickel catalyst, either supported or Raney type, is described ia Olin Mathieson patents (26,27). The reduction is carried out ia a continuous stirred tank reactor with a concentric filter element built iato the reactor so that the catalyst remains ia the reaction 2one. Methanol is used as a solvent. Reaction conditions are 2.4—3.5 MPa (350—500 psi), 120—140°C. Keeping the catalyst iaside the reactor iacreases catalyst lifetime by maintaining a hydrogen atmosphere on its surface at all times and minimises handling losses. Periodic cleaning of the filter element is required. 

Formaldehyde. Pure formaldehyde, CH2O, is a colorless, pungent smelling reactive gas (see Formaldehyde). The commercial product is handled either as soHd polymer, paraformaldehyde (13), or in aqueous or alcohoHc solutions. Marketed under the trade name Formcel, solutions in methanol, / -butanol, and isobutyl alcohol, made by Hoechst-Celanese, are widely used for making alcohol-modified urea and melamine resins for surface coatings and treating textiles. 

Several properties of the filler are important to the compounder (279). Properties that are frequentiy reported by fumed sihca manufacturers include the acidity of the filler, nitrogen adsorption, oil absorption, and particle size distribution (280,281). The adsorption techniques provide a measure of the surface area of the filler, whereas oil absorption is an indication of the stmcture of the filler (282). Measurement of the sdanol concentration is critical, and some techniques that are commonly used in the industry to estimate this parameter are the methyl red absorption and methanol wettabihty (273,274,277) tests. Other techniques include various spectroscopies, such as diffuse reflectance infrared spectroscopy (drift), inverse gas chromatography (igc), photoacoustic ir, nmr, Raman, and surface forces apparatus (277,283—290). 

There are two commercial solvent crystaUi2ation processes. The Emersol Process, patented in 1942 by Emery Industries, uses methanol as solvent and the Armour-Texaco Process, patented in 1948, uses acetone as solvent. The fatty acids to be separated are dissolved in the solvent and cooled, usually in a double-pipe chiller. Internal scrapers rotating at low rpm remove the crystals from the chilled surface. The slurry is then separated by means of a rotary vacuum filter. The filter cake is sprayed with cold solvent to remove free Hquid acids, and the solvents are removed by flash evaporation and steam stripping and recovered for reuse (10). 

In the presence of metallic copper, metallic silver, or a copper-silver alloy used in the form of gauze or as metal deposited on a low surface area inert support, methanol can be dehydrogenated to formaldehyde at 400—500°C. 

Only recently has a mechanism been proposed for the copper-cataly2ed reaction that is completely satisfactory (58). It had been known for many years that a small amount of carbon dioxide in the feed to the reactor is necessary for optimum yield, but most workers in the field beHeved that the main reaction in the formation of methanol was the hydrogenation of carbon monoxide. Now, convincing evidence has been assembled to indicate that methanol is actually formed with >99% selectivity by the reaction of dissociated, adsorbed hydrogen and carbon dioxide on the metallic copper surface in two steps ... 

Fig. 6. Surface area of coals as estimated from (o) neon sorption at 25°C and (x) methanol heat of wetting.

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