Methanol adsorption/decomposition

TPRS experiments for this reaction step. The two hydrogens released in the methanol adsorption and the surface methoxy decomposition steps are eventually converted to water. Spectroscopic details about the formation of water are presently not available, but the formation of water most likely proceeds via the condensation of two surface hydroxyl groups. The reduced surface vanadia site is readily reoxidized back to vanadium (+5) by gas phase oxygen as shown by in situ Raman measurements.42... 

Methanol adsorption and decomposition on noble metals have been the subject of many surface-analytical investigations (e.g., References 94,171,320,350,378, 478 94). CH3OH dehydrogenation on palladium catalysts could be a valuable source of synthesis gas or hydrogen, but unfortunately catalyst deactivation by carbon deposits (coking) seriously limits this process (495-498). In this respect, the probability of O H vs. C O bond scission is important, the first path resulting in CO and H2, and the second in carbon or carbonaceous species (CH x = 0-3), CH4, and H2O (see scheme in Fig. 49 details are discussed below). 

As discussed in a previous section, metal oxides represent an important class of materials exhibiting a broad range of properties from insulators to semiconductors and conductors and have found applications as diverse as electronics, cosmetics and catalysts. Metal oxides have been widely used in many valuable heterogeneous catalytic reactions. Typical metal oxide-catalyzed reactions, including alkane oxidation, biodiesel production, methanol adsorption and decomposition, destructive adsorption of chlorocarbons and warfare agents, olefin metathesis and the Claisen-Schmidt condensation will be briefly discussed as examples of metal oxide-catalyzed reactions. 

The bifunctional mechanism is explained in terms of the independent function of atoms of different metals methanol adsorption and decomposition takes place on Pt, while the alternative metal atoms provide preferred sites to bind OH. Several metals, such as... 

Conversely, the gas/solid surfaces present essential differences regarding methanol adsorption [69]. There are three pathways in UHV, a simple decomposition via a methoxonium (CHsO-ads) intermediate, an SNl pathway via a methoxoniumcation ([CHsOHa]" ), and an SN2 pathway via a methoxonium intermediate. 

In general, to balance the decomposition of methanol to CO and further oxidation to CO2 requires the consideration of some key factors. In Gasteiger s model [5], it is assumed that methanol oxidation proceeds only via adsorbed CO, and that the dissociative adsorption of methanol is the rate-determining step for methanol oxidation on PtRu electrodes. The electronic effect of Ru on methanol adsorption was not considered. In fact, at low potentials, a low Ru content can promote methanol decomposition to form adsorbed CO, and the oxidation of methanol adsorbates could also control the oxidation of methanol on PtRu electrodes. 

Hydrogen Liquefaction. Hydrogen can be produced from caustic—chlorine electrolytic cells, by decomposition of ammonia or methanol, or by steam—methane reforming. Hydrogen recovered by these methods must be further purified prior to Hquefaction. This is generally achieved by utilizing pressure swing adsorption methods whereby impurities are adsorbed on a soHd adsorbent. 

The influence of the presence of sulfur adatoms on the adsorption and decomposition of methanol and other alcohols on metal surfaces is in general twofold. It involves reduction of the adsorption rate and the adsorptive capacity of the surface as well as significant modification of the decomposition reaction path. For example, on Ni(100) methanol is adsorbed dissociatively at temperatures as low as -100K and decomposes to CO and hydrogen at temperatures higher than 300 K. As shown in Fig. 2.38 preadsorption of sulfur on Ni(100) inhibits the complete decomposition of adsorbed methanol and favors the production of HCHO in a narrow range of sulfur coverage (between 0.2 and 0.5). 

Acid-base reactivity is an important property of oxide catalysts, and its control is of interest in surface chemistry as well as being of importance in industrial applications. The exposed cations and anions on oxide surfaces have long been described as acid-base pairs. The polar planes of ZnO showed dissociative adsorption and subsequent decomposition of methanol and formic acid related with their surface acid-base properties[3]. Further examples related to the topic of acid-base properties have been accumulated to date[ 1,4-6]. 

Dasler, W. et al., Ind. Eng. Chem. (Anal. Ed.), 1946,18, 52 Like other monofunctional ethers but more so because of the four susceptible hydrogen atoms, dioxane exposed to air is susceptible to autoxidation with formation of peroxides which may be hazardous if distillation (causing concentration) is attempted. Because it is water-miscible, treatment by shaking with aqueous reducants (iron(II) sulfate, sodium sulfide, etc.) is impracticable. Peroxides may be removed, however, under anhydrous conditions by passing dioxane (or any other ether) down a column of activated alumina. The peroxides (and any water) are removed by adsorption onto the alumina, which must then be washed with methanol or water to remove them before the column material is discarded [1], The heat of decomposition of dioxane has been determined (130-200°C) as 0.165 kJ/g. 

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... 

Li, C Domen, K Maruya K Onishi, T. Spectroscopic identification of adsorbed species derived from adsorption and decomposition of formic acid, methanol, and formaldehyde on cerium oxide, J. Catal, 1990, Volume 125, Issue 2, 445-455. 

The adsorption and reaction of methanol on metal surfaces has been widely studied (18-34). Methanol has C-0, C-H, and 0-H bonds, serving as one of the simplest systems for the selective activation of chemical bonds. The methoxyl (CH30(a)) species has been considered as an intermediate of the methanol decomposition. On many transition metal surfaces, adsorbed methanol molecules are usually decomposed to H2 and CO, although Ag and Cu are used as catalysts for the conversion of methanol to formaldehyde. The adsorption and reaction of alcohol molecules on Mo surfaces has been studied on the (100) (4) and (110) (35) surfaces. Alcohol molecules are decomposed effectively also on these surfaces. 

The desorption peak of 16 amu around 200 K is due to methane, which is produced in the titanium sublimation pump during methanol exposure, and adsorbed on the sample holder. The main desorption products are H2 around 400 K and CO above 800 K. The H2 desorption peak is much larger than that observed upon the H2 adsorption up to saturation on the clean surface. The H2 desorption peaks seem to consist of three components two relatively small components at 350-400 K and around 500 K and a sharp peak at 410 K. For the HD desorption trace, only the peak at 350-400 K is seen. These results suggests that the methanol decomposition reaction on the clean Mo(l 12) surface proceeds as follows. 

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