Kinetics of methanol

Methanol synthesis served as the model for the true mechanism. Stoichiometry, thermodynamics, physical properties, and industrial production rates were all taken from the methanol literature. Only the reaction mechanism and the kinetics of methanol synthesis were discarded. For the mechanism a four step scheme was assumed and from this the... 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

This results from the slow kinetics of methanol oxidation and oxygen reduction. An additional loss is due to the cell resistance (arising mainly... 

Kauranen, R, E. Skou, and J. Munk, Kinetics of methanol oxidation on carbon-supported Pt and Pt + Ru catalysts, J. Electroanal. Chem., 404, 1 (1996). 

For potentials higher than 0.5 V vs. RHE, the formation of adsorbed oxygen species at Ru as well as at Pt will block the catalytic surface, leading to a decrease in the methanol adsorption kinetics. Therefore, in a potential range higher than 0.5 V vs. RHE, the kinetics of methanol oxidation is optimized at a Ru-poor catalyst, because methanol adsorption is not blocked and because the presence of Ru provides the extra oxygen atom needed to complete the oxidation of adsorbed CO to CO2. 

Tadjeddine and co-workers have used SFG [Guyot-Sionnest and Tadjeddine, 1990 Eisenthal, 1992 Richmond, 2002 Vidal et al., 2002, 2004, 2005] to study the adsorbed CO produced from a variety of solution species, including methanol [Vidal et al., 2002, 2005]. With BB-SFG, we studied the electrochemical kinetics of methanol chemisorption as surface CO, as shown in Fig. 12.13. We used apolycrystal-line Pt electrode and 0.1 M H2SO4 electrolyte with 0.1 M methanol. Figure 12.13a-d characterize the potential-dependent SFG spectra obtained under the voltammetric... 

High area platinum showed different voltammetric features from smooth platinum for methanol oxidation and provided slightly higher sustained current density. These results provided evidences that the morphology of platinum affects the mechanisms and the kinetics of methanol oxidation. 

Robb, D. A. and P. Harriott. 1974. The kinetics of methanol oxidation on a supported silver catalyst. J. Catal. 35 176-183. 

The Kinetics of Methanol Carbonylation Over RhX, RhY and IrY zeolites Carbonylation of methanol proceeds readily at atmospheric pressure under mild temperature conditions 150°-180°C. This reaction ZCH OH + CO - CH COOCH + HjO produces mainly methyl acetate and water. Acetic acid was detected at high conversions and high temperatures. Traces of dimethyl ether could also form. In most cases the selectivity to methyl acetate was at least 90% in presence of the iodide promotor. 

Reprinted from Chemical Engineering Science, 44, A.N.R. Bos et. al., The kinetics of methanol synthesis on a copper catalysts an experimental study, 2435-2449,1989, with kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK. 

A study of the kinetics of methanol synthesis at differential conversion was performed in order to eliminate the influence of the products. The only products observed were methanol, CO and water. Data was obtained as initial activity measurements to eliminate the interference from catalyst deactivation in comparisons of the activities of different catalysts. The initial rate of deactivation in the microflow reactor at differential conditions was < 1%/hour for all catalysts. Each data point reported in Figure 2 and Table 2 is the initial activity from a unique experiment using a fresh catalyst charge. 

The mechanism and kinetics of methanol synthesis over Cu have been the subjects of extensive investigations [1-3]. Despite considerable research, there still remain controversies as to the exact mechanism by which methanol is synthesized over Cu-based catalysts and very little is agreed upon concerning the nature of active site and the role of ZnO phase. The present work was undertaken to obtain a more detailed mechanism of methanol synthesis from CO2/H2 over Cu/ZnO/Si02. To do this in situ FTIR was to observe the structure and surface concentration of adsorbed species during the reaction. Complementary TPD studies were also conduced to analyze the surface species. 

An interesting example of application of the Wulff rule is given by Ovesen et al. [61]. They have analyzed the kinetics of methanol synthesis on nm Cu particles supported by ZnO. The generalized surface tension for the particle-substrate interface was assumed to be dependent on the reduction potential of the gas phase. The latter resulted in the dependence of the areas of the (111), (100), and (110) facets on the gas-phase concentrations (such changes were observed by using EXAFS). The total reaction rate, represented as a sum of the reaction rates on different facets, was found to be affected by the changes in particle morphology. 

Fundamental Kinetics of Methanol Oxidation in Supercritical Water... 

Fourteen oxidation runs were conducted to determine the oxidation kinetics of methanol in supercritical water. The temperature range covered was 450—550 0 at a pressure of 243 atm (24.6 MPa). Reactor residence times ranged from 8.5 to 12.4 seconds. The oxidation run at 450 0 resulted in such low conversions of methanol (<1%) that experimental errors were too large to draw any quantitative conclusions. The data from this run were not included in... 

Samms, S.R. and Savinell, R.F. Kinetics of methanol-steam reformation in an internal reforming fuel cell. Journal of Power Sources, 2002, 112 (1), 13. 

At high anodic overpotentials, methanol oxidation reaction exhibits strongly non-Tafel behavior owing to finite and potential-independent rate of methanol adsorption on catalyst surface [244]. The equations of Section 8.2.3 can be modified to take into account the non-Tafel kinetics of methanol oxidation. The results reveal an interesting regime of the anode catalyst layer operation featuring a variable thickness of the current-generating domain [245]. The experimental verification of this effect, however, has not yet been performed. 

The early, primarily kinetic studies of methanol synthesis have been reviewed by Natta in 1955 (ref. 20), the mechanistic work by Kung in 1980 (ref. 21) and the characteristics of the Cu/ZnO/MxOy catalysts along with the available mechanistic information by the author in 1982 (ref. 22). Subsequently, a large number of papers emerged that indicate that the mechanism and kinetics of methanol synthesis are complex, may not be identical for different catalysts, and vary considerably with reaction conditions. 

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

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. 

Reaction mechanism and kinetics of methanol oxidation to fonnaldehyde... 

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 ... 

The three main barriers to the reduction of superior energy density of existing DMFC have been [35] (1) high methanol permeability of the commonly used ionomeric membranes (2) the balance of water challenge, due to the need of removing water produced in the cathode and needed in the anode (3) the moderate power density of the DMFC, due to the slow kinetics of methanol oxidation. 

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