Catalyst layer with additives

GP 6] [R 5] With a stabilized CU2O catalyst layer, by addition of bromomethane (ppm level), 20% selectivity at 5% conversion was found (0.5 vol.-% propene 0.1 vol.-% oxygen 2.25 ppm promoter 350 °C) [37]. This is far better than with non-conditioned copper oxide catalysts which contain CuO besides CU2O. It is expected that the first species promotes more total oxidation, whereas the latter steers partial oxidation. In the above experiment, selectivity rises from 7 to 30% at slightly reduced conversion after 3 h of promoter conditioning. 

UTC) has been using SiC for 50 years as an electrolyte matrix in PAFC because of its extreme stability in hot phosphoric acid. The system could not be used in fuel cell electrodes due to its poor catalytic activity and electrical conductivity. However, SiC has been evaluated as a catalyst support with addition of carbon black to enhance conductivity in the catalyst layer. The approach included the deposition of Pt particles on SiC by chemical route followed by mixing with carbon to formulate catalyst. Authors claimed that a higher Pt loading has led to improved electrode performance even with large particles of Pt. This indicated that electrode performance depends not only on surface area of Pt but also on the interaction nature between support and metal catalyst [19]. 

Both reactions were carried out under two-phase conditions with the help of an additional organic solvent (such as iPrOH). The catalyst could be reused with the same activity and enantioselectivity after decantation of the hydrogenation products. A more recent example, again by de Souza and Dupont, has been reported. They made a detailed study of the asymmetric hydrogenation of a-acetamidocin-namic acid and the kinetic resolution of methyl ( )-3-hydroxy-2-methylenebu-tanoate with chiral Rh(I) and Ru(II) complexes in [BMIM][BF4] and [BMIM][PFg] [55]. The authors described the remarkable effects of the molecular hydrogen concentration in the ionic catalyst layer on the conversion and enantioselectivity of these reactions. The solubility of hydrogen in [BMIM][BF4] was found to be almost four times higher than in [BMIM][PFg]. 

The kinetic principles operating during the initiation and advance of interface-controlled reactions are identical with the behaviour discussed for the decomposition of a single solid (Chaps. 3 and 4). The condition that overall rate control is determined by an interface process is that a chemical step within this zone is slow compared with the rate of arrival of the second reactant. This condition is not usually satisfied during reaction between solids where the product is formed at the contact of a barrier layer with a reactant. Particular systems that satisfy the specialized requirements can, however, be envisaged for example, rate processes in which all products are volatilized or a solid additive catalyzes the decomposition of a solid yielding no solid residue. Even here, however, the kinetic characteristics are likely to be influenced by changing effectiveness of contact as reaction proceeds, or the deactivation of the catalyst surface. 

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

The results have been compared with the earlier proposal of a dual-pathway mechanism for Cl oxidation, and, together with previous experimental and theoretical results, summarized in a comprehensive reaction scheme that explicitly includes also the (reversible) exchange between adsorbed species, dissolved product species in the catalyst layer, and similar species in the bulk electrolyte. The traditional dualpathway mechanism, where both the direct and indirect pathways lead to CO2 formation, has beenextended by adding a third pathway that accounts for formation and desorption of incomplete oxidation products. In the mechanistic discussion, we have focused on the role in and contribution to the Ci oxidation process of the formation/desorption and re-adsorption plus further oxidation of incomplete oxidation products. This not only leads to faradaic currents exceeding that for CO2 formation, but may result in additional COad and CO2 formation, via adsorption and oxidation of the incomplete oxidation products. 

In practice, the catal5Tic layers are prepared by brushing or spraying catalyst ink (a suspension of the catalyst particles in water and/or an organic solvent with addition of ionomer) either onto diffusion media (carbon paper or carbon cloth, also referred to as substrates), resulting in so-called catalyst-coated substrates (CCS), or directly onto... 

When processing flue gas containing dust, the reactors are typically vertical, with down flow of flue gas. The catalyst is typically arranged in a series of two to four beds, or layers (Figure 1.2). For better catalyst utilization, it is common to use three or four layers, with provisions for an additional layer which is not initially installed. 

In addition, the more the number of piled catalyst sheet, the larger the dehydrogenation activities in the liquid-film state especially, as evident in Figures 13.23 and 13.24. It is easy for the liquid reactant to penetrate into the catalyst layer consisting of the ACC with lots of air space. By piling the ACC, therefore, decalin would penetrate into catalyst layer further and be kept in it for a long time, resulting in an enhanced catalytic performance. 

Typically, Nation ionomer is the predominant additive in the catalyst layer. However, other types of CLs with various hygroscopic or proton conductor additives have also been developed for fuel cells operafed xmder low relative humidity (RH) and/or at elevated temperatures. Many studies have reported the use of hygroscopic y-Al203 [52] and silica [53,54] in the CE to improve the water retention capacity and make such CEs viable for operation af lower relative humidity and/or elevated temperature. Alternatively, proton conducting materials such as ZrP [55] or heteropoly acid HEA [56] have also been added... 

In addition to Nafion-based catalyst layers, additional types have been developed, including CLs with different ion exchange capacities (lECs) [57,58] or with other hydrocarbon-type ionomers such as sulfonated poly(ether ether ketone) [58-60], sulfonated polysulfone [61,62], sulfonated polyether ionomers [63], and borosiloxane electrolytes [64], as well as sulfonated polyimide [65]. These nonfluorinated polymer materials have been targeted to reduce cost and/or increase operating temperature. Unfortunately, such CLs still encounter problems with low Pt utilization, flooding, and inferior performance compared wifh convenfional Nafion-based CLs. 

Although the sputter deposition technique can provide a cheap and directly controlled deposition method, the performance of PEM fuel cells with sputtered CLs is still inferior to that of conventional ink-based fuel cells. In addition, other issues arise related to the physical properties of sputtered catalyst layers, such as low lateral electrical conductivity of the thin metallic films [96,108]. Furthermore, the smaller particle size of sputter-deposited Ft can hinder water transport because of the high resistance to water transport in a thick, dense, sputtered Ft layer [108]. Currently, the sputter deposition method is not considered an economically viable alternative for large-scale electrode fabrication [82] and further research is underway to improve methods. 

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