Methanol concentration dependence

Fig. 15. CD-spectra of random (L-Leu0,48, L-Lys0,52) in water-methanol mixtures as a function of methanol concentration at 20 °C and pH 7,0. The insert shows [0]2O8 and —[0]2jo dependent on the methanol content112113 ...

The exchange current density for methanol oxidation depends on the methanol concentration, i.e. ... 

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... 

Using the colloidal Pt(i t ) + RU c/C catalysts described above, the optimal atomic ratio depends upon methanol concentration, cell temperature, and applied potential, as shown by the Tafel plots recorded with methanol concentrations of 1.0 and 0.1 M at T = 298K (Fig. 11.4) and 318K (Fig. 11.5). Some authors have stated that for potentials between 0.35 and 0.6 V vs. RHE, the slow reaction rate between adsorbed CO and adsorbed OH species must be responsible for the rate of the overall process [Iwasita et al., 2000]. From these results, it can be underlined that, at a given constant potential lower than 0.45-0.5 V vs. RHE, an increase in temperature requires an increase in Ru content to enhance the rate of methanol oxidation, and that, at a given constant potential greater than 0.5 V vs. RHE, an increase in temperature requites a decrease in Ru content to enhance the rate of methanol oxidation. 

The ratio QaJQ0X varies between 1 and 2 depending on methanol concentration and degree of coverage. In principle this can be interpreted in terms of a mixture of particles of composition (C, O) and (C, O, H). The nature of the Pt surface seems to influence also the charge ratio. In principle, the following configurations could be expected ... 

Which reaction channel is followed depends on the availability of neighboring free places. This could explain the effect of methanol concentration on adsorbate composition. It has been observed that the initial rate of adsorption is strongly enhanced by increasing methanol concentration [14]. From the adsorption steps given above, the first one, Eq. (2.7), is directly affected by the bulk concentration. At high methanol concentrations the Pt surface becomes very quickly covered with species like CH2OH or CHOH. Further reaction to a more stable state such as COH is inhibited because of the lack of free adjacent sites. Under these conditions CO should be formed with a greater probability. 

The degree of coverage, however, seems to influence the adsorbate composition at low methanol concentrations also. In particular, on smooth platinum, the dependence of surface composition on 6 is observed at concentrations as low as 5x 10-3 M (Fig. 2.7). In this case it could be possible that COH can be formed as long as adjacent sites are available. At high coverage (by all species involved in the adsorption process), the formation of COH should be geometerically prevented. 

The authors performed the above experiment at various methanol concentrations and charge ratios, QaaJQ0%, between I (COH or CHO) and 2 (CO) were obtained depending on the methanol concentration and coverage. By assuming that only CsC ds and COHttds (i.e. either CHO or COH) are present on the surface, the authors were able to calculate their mole fractions as a function of total coverage and methanol concentration. Thus, if the mole fraction of CO is. x and of COH is then ... 

The only dependencies noted in the kinetic studies were first-order dependencies on iodide promoter and rhodium concentrations. Thus there was no observed effect of varying methanol concentration, and the partial pressure of carbon monoxide had no effect on the reaction rate. Similarly, the concentration of the products, methyl acetate and acetic acid, has no effect on the reaction rate. Thus we have the unusual situation of a reaction, CH3OH + CO — CH3COzH, in which the concentrations of the reactants and product have no kinetic influence. 

Fig. 3-41 Temperature dependance of methanol oxidation current denai after 2 min. potential holding at 500 mV for Nafion SPE platinum electrode at various methanol concentration in the electrolyte or in the feed gas. 

This should be illustrated by the counterion concentration dependencies of three different model solutes, namely Af-3,5-dinitrobenzoylated serine (DNB-Ser), aspartic acid (DNB-Asp), and O-phosphoserine (DNB-PSer), which are structural analogs that differ in the number of nominal charges. The solutes have been analyzed in the RP mode with methanol-phosphate buffer (50 50 v/v) (pH 6.5) under variable phosphate concentrations. The results are shown in Figure 1.4. 

Further, Takahashi et al. [57] reported no spectral change in an air atmosphere between 200 nsec and 2 msec for 6-nitro-BIPS in methanol and acetonitrile, a result that is consistent with Lenoble and Becker s findings [56]. Kaliskey and co-workers [58] collected data for a nitro-substituted BIPS in acetonitrile with many similarities to that of Lenoble and Becker and further noted that the buildup of the 560 nm absorption is concentration dependent, which implies a bimolecular reaction such as aggregation leads to the absorption changes in this spectral region. 

The most notable feature about the data is the acetaldehyde rate, which increases with decreasing methanol conversion. For example, a rate of 17.5 K/hr is obtained at a conversion of 38%. There are numerous explanations for this trend, including product inhibition of the catalyst or rate dependence on methanol concentration. However, the important point is that when comparing any data dealing with this reaction, the conversions must be similar in order to draw meaningful conclusions. 

Figure 8.14. Dependence of initial polymerization rate on the methanol concentration in the reaction mixture. B0°C, [K2S2O8] = 3.7x10 mol/L, mol. weight PEG =15,000, 1-[MA]= 3.5x10, 2-[MA] = [PEG]= 3.5x10 3-[AA]=4.17xlO 4-[AA]=[PEG]=4.17xlO mol/L. Reprinted from I. M. Papisov, V. A. Kabanov, E. Osada, M. Leskano Brito, J. Reimont, and A. N. Gvozdeckii, Vysokomol. Soed., 14, 2462 (1972) with kind permission from Iz. Nauka.

Self-difFusion coefficients were measured with the NMR spin-echo method and mutual diffusion coefficients by digital image holography. As can be seen from Figure 4.4-3, the diffusion coefficients show the whole bandwidth of diffusion coefficient values, from 10 m s on the methanol-rich side, down to 10 on the [BMIM][PFg]-rich side. The concentration dependence of the diffusion coefficients on the methanol-rich side is extreme, and shows that special care and attention should be paid in the dimensioning of chemical processes with ionic Hquids. 

The intensity of the bands due to CH stretching modes in methanol and ethanol have been found to be temperature- and concentration-dependent, though not to as groat a degree as the OH vibrations. This intensity effect is due to perturbations by the H-bond which are probably transmitted only as far as the a carbon atom. 

M. A. Wendt, J. Meiler, F. Weinhold, and T. C. Farrar. Solvent and concentration dependence of the hydroxyl chemical shift of methanol. Mol. Phys. 93, 145-51... 

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