Methanol oxidation over

On M0O3, methanol (CH3OH or MeOH) chemisorbs at a low temperature of 100°C, which suggests that some defects (or dangling bonds) are necessary for chemisorption. In situ ETEM experiments in methanol show the formation of misfit defects at 100°C (and surface domains) accommodating the shape 

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GP 4] [R 11] For methanol oxidation over sputtered silver catalyst, conversion is higher when using micro channels of smaller diameter (8.5 vol.-% methanol balance oxygen 510 °C 4—27 ms sHghfly > 1 atm) [72]. For two channels of the same width, but different depths (70 pm, 130 pm), concentration differences of nearly 10% at the same residence time were detected, all other parameters being equal. 

Islam M, Basnayake R, Korzeniewski C. 2007. A study of formaldehyde formation during methanol oxidation over PtRu hulk alloys and nanometer scale catalyst. J Electroanal Chem599 31-40. 

The apparent activation energies for methanol oxidation over the series of support vanadia catalysts is a composite of the activation energy and the heats of adsorption ... 

The above studies with methanol oxidation over the different supported vanadia catalysts demonstrated the dramatic effect of the specific oxide support on the reaction TOF, approximately four orders of magnitude for methanol oxidation. Several other oxidation reactions over the same series of supported vanadia catalysts were also investigated to determine if the effect of the oxide support is a general phenomenon, and the results are listed in Table 2. 

The presence of solution can dramatically affect dissociative chemisorption. In the vapor phase, most metal-catalyzed reactions are homolyticlike, whereby the intermediates that form are stabilized by interactions with the surface. Protic solvents, on the other hand, can more effectively stabilize charge-separated states and therefore aid in heterolytic activation routes. Heterolytic paths can lead to the formation of surface anions and cations that migrate into solution. This is directly relevant to methanol oxidation over PtRu in the methanol fuel cell. The metal-catalyzed route in the vapor phase would involve the dissociation of methanol into methoxy or hydroxy methyl and hydrogen surface intermediates. Subsequent dehydrogenation eventually leads to formation of CO and hydrogen. In the presence of an aqueous media, however, methanol will more likely decompose heterolytically into hydroxy methyl (—1) and intermediates. 

Scheme 3 Reaction pathways of methanol oxidation over early transition metal oxides.

The surface science studies of methanol oxidation over metals were carried out under conditions where no (y) oxygen was present on the surfaces. A temperature-programmed reaction experiment was performed [49] in order to... 

Figure 4. Oxidation of alcohols over catalysts in loose powder form at 230°C as a function of reaction time (a) methanol oxidation over 4% MoOj/TiO jphysical mixture, (b) methanol oxidation over 4% VjOj/TiOj physical mixture, (c) ethanol oxidation over 4% MoOj/TiOj physical mixture and (d) 2-butanol oxidation over 4% MoOjATiOj physical mixture.

Figure 5 In-situ XPS data in oxygen Is region of methanol oxidation over polycrystalline copper. Total pressure 0.6 mbar, reaction temperature 673 K, flow-through mode. Gas composition as methanol oxygen molar ratio A—1 2, B—3 1,C=6 1...

Methanol Oxidation Over Supported Vanadium Oxide Catalysts ... 

Steady state kinetics of methanol oxidation over the supported vanadia catalysts... 

Turnover frequencies (TOFs) and activation energies for methanol oxidation over supported vanadia catalysts at monolayer coverages of surface vanadia species... 

The TPRS experiments also provided additional insights into the kinetics of methanol oxidation over the supported vanadia catalysts. The TPRS experiments only provide kinetics about the surface reaction steps since the adsorption events precede the initiation of the transient temperature ramp. The TPRS peak temperatures for the production of HjCO and CO from the different vanadia catalysts are presented in Table 4. 

The molecular characterization studies demonstrated that the oxidized surface vanadia species in the different supported vanadia catalysts possess essentially the same molecular structures predominantly consisting of isolated and polymerized sur ice VO4 species with the same ratio of polymerized to isolated species at comparable surface vanadia coverages. The surface vanadia species even became reduced to comparable extents during methanol oxidation for all the supported vanadia catalysts. The terminal V=0 bond lengths for the different supported vanadia catalysts were 0 essentially identical, see Table 1, and the minor variations in the V=0 bonds did not correlate with Ae methanol oxidation TOFs, (see Tables 1 and 2). Thus, there are no significant molecular structural differences among the surface vanadia species on the different oxide supports to account for the dramatic variation in the TOFs during methanol oxidation over the supported vanadia catalysts. 

The kinetics of methanol oxidation over metal oxide catalysts were elegantly derived by Holstein and Machiels [16], The kinetic analysis demonstrated that the dissociative adsorption of water must be included to obtain an accurate kinetic model. The reaction mechanism can be represented by three kinetic steps equilibrated dissociative adsorption of methanol to a surface methoxy and surface hydroxyl (represented by K,), equilibrated dissociative adsorption of water to two surface hydroxyls (represented by K ), and the irreversible hydrogen abstraction of the surface methoxy intermediate to the formaldehyde product and a surface hydroxyl (the rate determining step, represented by kj). For the case of a fully oxidized surface, the following kinetic expression was derived ... 

In the above kinetic expression, the Arrhenius rate constant, 1, is modified by the two adsorption equilibrium constants of methanol and water during the steady state kinetic studies. During the transient TPRS experiments, however, only the Arrhenius rate constant, kj, is measured since there is no vapor phase methanol or water to be equilibrated. The similar TPRS results for the oxidation of the surface methoxy intermediate to formaldehyde over the different supported vanadia catalysts reveal that all the catalysts possess the same kj. Consequently, the dramatic differences in the steady state TOFs during methanol oxidation over the different supported vanadia catalysts must be associated with the methanol and water equilibrium adsorption constants. Both methanol and water can be viewed as weak acids that will donate a proton to a basic surface site, but methanol is more strongly adsorbed than water on oxide surfeces, K, > [16]. Furthermore, the methanol oxidation TOFs... 

Pre-treated polyciystalline copper foil catalysts show rate oscillations under conditions of the partial methanol oxidation over a wide range of the methanol to O2 flow and temperature parameters. The gas phase composition was monitored by means of a calibrated mass spectrometer that was coupled with in situ Raman spectrometiy and video recordings. The gas phase oscillations are partly linked with visible oscillations of the copper oxidation state of the surface, depending on the o gen to methanol ratio and the temperature. 

In the surface science based studies on the methanol oxidation over single crystalline Cu(llO), Wachs and Madix [1, 2] have shown that the active state of the copper surface for the methanol oxidation was a partially oxidized copper surface exhibiting nucleophilic oxygen ad-atoms. The ojQ gen activated the surface for methanol adsorption and removed hydrogen released by water formation on the surface via a low-energy reaction pathway. 

A kinetic study of methanol oxidation over stoichiometric iron molybdate catalyst was performed in a fixed-bed integral reactor showing kinetic influences of reaction products. In the temperature range of 548-618 K it was not possible to fit the fomoation rate data to a single power rate law. Dimethyl ether formation presents only a second order dependence with respect to methanol. CO formation seems to be inhibited by water and formaldehyde and rate data fit well to the power rate law ... 

Several reaction schemes have been proposed to explain tee formation of all byproducts during methanol oxidation over Mo-Fe catalysts. Edwards et al [5] and Machiels [6] have suggested reactional mechanisms for tee formation of formaldehyde and by-products. However intermediate speeies proposed in such mechanisms have not been identift by any spectroscopic or other techniques. More recently Busca [7] on basis of infrared studies of surface intermediates species has proposed a rake-type mechanism for methanol oxidation over oxide catalysts. This mechanism account for the formation of formaldehyde and byproducts. 

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