Pyrolyzed catalysts

Pyrolyzed catalysts obtained by heat treatment in Argon of active carbon impregnated with solution of the compound Co-tetramethoxyphenylporphirine (CoTMPP) are studied [9], Air gas-diffusion electrodes with this catalyst show low polarisation in a wide interval of current densities (up to 100 mA/cm2) and stable long-term performance. These catalysts are more suitable for use in magnesium-air cells operating at high current drains, but unfortunately their price is comparatively high. 

Polymers have served roles in PEM fuel cell cathodes such as modifiers to macrocycle-based electrodes to improve conductivity and stability,165 composite materials with heteropolyacids,166 and as precursors to pyrolyzed catalysts.38,112,132,133 However, as discussed in the previous section, the activity of nitrogen-containing carbon raises the possibility of non-metal electrodes functioning in a cathode environment. Likewise, researchers have noted ORR activity for various conducting polymers containing nitrogen, and recently studies on their potential use in PEM fuel cell cathodes have been reported. 

Fig. 21. Arrhenius plots of methanol and methane formation over the pyrolyzed catalyst on MgO prepared from Rh4(CO)12 0.11 g (0.15 mmol) and MgO20g, 0.75% Rh wt. dispersion. The reactor volume was 320 ml CO H2 = 200 Torr 450 Torr (33). [Reprinted with permission from Bull. Chem. Soc. Jpn. 51, 2268 (1978). Copyright 1978 Chemical Society, Japan.]...

According to Eqns (12.2) and (12.3), the reciprocal of the current density, d, is plotted against the reciprocal of the square root of the rotation rate, w, to obtain the Koutecky-Levich plots, as shown in Fig. 12.6. It can be seen that the slope of the Koutecky-Levich plot with an unpyrolyzed catalyst is close to that of the theoretical 2-electron transfer reaction, and the slopes with pyrolyzed catalysts are close to that of the theoretical 4-electron transfer reaction, indicating that the ORR proceeds with a 2-electron transfer mechanism on an unpyrolyzed catalyst and with a 4-electron transfer mechanism on pyrolyzed catalysts. 

Homogeneous catalysts. With a homogeneous catalyst, the reaction proceeds entirely in the vapor or liquid phase. The catalyst may modify the reaction mechanism by participation in the reaction but is regenerated in a subsequent step. The catalyst is then free to promote further reaction. An example of such a homogeneous catalytic reaction is the production of acetic anhydride. In the first stage of the process, acetic acid is pyrolyzed to ketene in the gas phase at TOO C ... 

Direct elimination of a carboxylic acid to an alkene has been accomplished by heating in the presence of palladium catalysts.Carboxylic esters in which the alkyl group has a P hydrogen can be pyrolyzed, most often in the gas phase, to give the corresponding acid and an alkene. No solvent is required. Since rearrangement and other side reactions are few, the reaction is synthetically very useful and is often carried out as an indirect method of accomplishing 17-1. The yields are excellent and the work up is easy. Many alkenes have been prepared in this manner. 

Small solid seuaples can be analyzed directly by dynamic headspace sampling using a platinum coil and quartz crucible pyrolyzer and cold trap coupled to an open tubular column (341,369,379). This method has been used primarily for the analysis of mineral samples and of additives, catalysts and byproducts in finished polymers which yield unreliable results using conventional headspace techniques owing to the slow release of the volatiles to the headspace. At the higher temperatures (450-1000 C) available with the pyrolyzer the volatiles are more readily and completely removed from the sample providing for quantitative analysis. 

Several types of experimental magnesium-air cells were tested. These cells varied in their size (the working area of the air electrodes used) [10]. The current-voltage curves of an experimental Mg-air cell with two air electrodes (Sair = 80 cm2) with pyrolyzed CoTMPP catalyst and sandwich-type Mg anode (MA8M06) operating in NaCl-electrolytes with different concentrations are presented by Figure 2. 

The testing of the different magnesium anodes is performed in magnesium-air cells with two air electrodes with pyrolyzed CoTMPP catalyst (S = 65 cm2). One-sheet Mg anodes were used in 4N NaCl media. 

Figure 12. Polarization curves of air electrodes with catalysts from active carbon promoted with 5% Pt and with pyrolyzed CoTMPP catalyst.

The activity of air electrodes with pyrolyzed CoTMPP catalysts is investigated as a function of the heat treatment temperature. In Figure 13 we have presented the initial curve, and the curves obtained after long-term operation of the electrode at c. d. 100 mA/cm2. It is seen that a maximum in the activity is obtained at pyrolysis temperature ca. 800°C, the activity being practically preserved with the time of operation of the air electrode. 

Figure 13. Activity of air electrode with pyrolyzed CoTMPP catalysts as a function of the... 

Pyrolysis of pine wood biomass was conducted at 400°C followed by catalytic deoxygenation at 450°C. The yield of the different product phases was gravimetrically determined. The gas yield was calculated by the difference. The water content of the bio oil was measured by Karl Fischer titration. The yield of the different product phases is given in Table 3, calculated from the pyrolyzed biomass. The non-catalytic experiment was carried out in the same way as the catalytic ones with the exception that the upper catalyst bed was empty. 

Vniios A process for catalytically pyrolyzing hydrocarbons to low molecular weight alkenes, similar to catalytic cracking but more efficient. The catalyst is either potassium vanadate on corundum or indium oxide on pumice. Developed by the All-Union Research Institute for Organic Synthesis, Moscow. 

Another possible explanation for activity in these materials is that metal centers can reform once the catalyst is placed in the electrolyte. It has been established that a portion of the metal dissolves when pyrolyzed samples are placed in an acidic solution, and interestingly, such a wash improves activity.72 It is thus possible that... 

The objective of most research in the area of pyrolyzed metal/N/C materials has centered around understanding the nature of the active site for the ORR. Similar to heat-treated macrocycles, there has been a parallel controversy over the nature of the active sites and the role of Fe or Co in these metal-nitrogen-carbon catalysts. Based on the activity attainable from a wide-range of precursors, it seems safe to assume that above a certain temperature, the active site formed is the same regardless of the metal-nitrogen-carbon starting material (macrocycle or otherwise). Initially, some researchers believed that the metal clusters protected by a layer of carbon (which prevented leaching of the metal in the acidic electrolyte) were the source of catalytic... 

A final issue that faces this class of catalysts is stability in the fuel cell environment. Deactivation of materials in a fuel cell environment has been shown to be minimal in some studies,31,137 and severe in others.128,142 More active catalysts seem more susceptible to deactivation. Deactivation has been linked to the formation of peroxide and the loss of metal from the catalyst.128 On the other hand, demetallization has also been observed in pyrolyzed samples that did not lose activity with time.84 Another possible mode of deactivation could be due to the oxidation of the carbon surface. However, it seems reasonable that a complete understanding of the deactivation mechanism would first require a well-developed understanding of the active site. 

Conductive polymers do not perform particularly well as ORR catalysts compared to the most active pyrolyzed samples. While a measurable ORR current is often observed at potentials as high as 0.6 V v.y. NHE, the current is generally slow to take off.171 The PANI samples are only active when the polymer is in its reduced... 

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