Catalyst, carbon diffusion

The book focuses on three main themes catalyst preparation and activation, reaction mechanism, and process-related topics. A panel of expert contributors discusses synthesis of catalysts, carbon nanomaterials, nitric oxide calcinations, the influence of carbon, catalytic performance issues, chelating agents, and Cu and alkali promoters. They also explore Co/silica catalysts, thermodynamic control, the Two Alpha model, co-feeding experiments, internal diffusion limitations. Fe-LTFT selectivity, and the effect of co-fed water. Lastly, the book examines cross-flow filtration, kinetic studies, reduction of CO emissions, syncrude, and low-temperature water-gas shift. 

First, the catalyst is meant to leach out of the capsules into a reaction solution. In this case, the capsules ate not meant to break open but are semipermeable to the catalyst, which diffuses into the reaction mixture over time. This method is t) pically used for metal catalysts or catalyst precursors where the metals leach out and perform the desired reaction. This method is useful because metal-catalyzed reactions typically require lower catalyst loading than organocatalysts (< 1 mol%), and highly loaded capsules can be isolated and reused until exhausted. Such metal catalysts are often touted for their decreased pyrophoricity relative to such catalysts as palladium on carbon (Coleman and Royer 1980 Bremeyer et al. 2002). One could simply use resins, microspheres, or other solid supports as catalyst reservoirs, but capsules are well suited because of their inherently higher surface areas (Royer et al. 1985 Wang et al. 2006). 

Hydrocarbon distributions in the Fischer-Tropsch (FT) synthesis on Ru, Co, and Fe catalysts often do not obey simple Flory kinetics. Flory plots are curved and the chain growth parameter a increases with increasing carbon number until it reaches an asymptotic value. a-Olefin/n-paraffin ratios on all three types of catalysts decrease asymptotically to zero as carbon number increases. These data are consistent with diffusion-enhanced readsorption of a-olefins within catalyst particles. Diffusion limitations within liquid-filled catalyst particles slow down the removal of a-olefins. This increases the residence time and the fugacity of a-olefins within catalyst pores, enhances their probability of readsorption and chain initiation, and leads to the formation of heavier and more paraffinic products. Structural catalyst properties, such as pellet size, porosity, and site density, and the kinetics of readsorption, chain termination and growth, determine the extent of a-olefin readsorption within catalyst particles and control FT selectivity. 

Nanoparticles have also been used successfully as amperometric gas sensors. Chiou and co-workers developed a dispersed catalyst gas-diffusion electrode for SO2 sensing [195]. Chloroauric acid is adsorbed on carbon black and subsequently reduced in a stream of hydrogen to obtain nanometer-sized particles. These are then shaped in the form of an electrode and used as a sensor. The electrochemical oxidation of SO2 gas is catalyzed by the nanoparticles with a fast response time. A... 

In a recent study, Baker and Chludzinski (10) used the basis of this proposed mechanism to develop methods of inhibiting the growth of filamentous carbon. The approach was aimed at the introduction of additives into the metal catalyst particles, which had the potential of reducing the rate of the critical steps involved in the growth process i.e., carbon solubility or carbon diffusion through the catalyst particle. Among several additives investigated, silica was the most effective as it reduced the rates of both of these processes. 

Carbon Gasification Rates. Because the reforming rates we observed during this work were often controlled by diffusion, it was not possible to. determine individual reaction rates and rate constants. However, from the TPSR measurements we were able to estimate rate constants for the gasification of catalyst and noncatalyst carbons. These rates are listed in Table VII along with selected results taken from the literature (29, 30, 31). We found that the catalyst carbon gasification rates were first order in carbon amounts up to equivalent (CO adsorption) monolayer... 

In an overall view, many different parameters markedly influence nanotube growth in chemical vapor deposition. They include, among others, the size and shape of the catalyst particles, the metal s ability to form carbides, or the question of whether the carbon diffuses on the surface or through the bulk phase of the catalyst particle. Only a deeper understanding of the interplay between these factors will enable the reproducible production of a specific type of nanotube. 

CNFs, resulting from carbon diffusion through the metal particles, and formed at temperatures above 723 K (interestingly, CNF formation does not induce catalyst deactivation, thus indicating that the metal nanoparticle is still accessible to reactants)... 

The fomation of carbon on iron and iron-copper catalysts by the reaction 2C0 = C02+C has been studied by several investigators (70-73). The most significant result of this work (in so far as the Fischer-Tropsch synthesis is concerned) is the fact that neither an iron-free nor a copper-free carbon deposit was obtained. The data show that cai-bon is deposited in the crystal lattice of the catalyst and the inability to obtain a copper-free carbon deposit from tests with an iron-copper catalyst shows that iron carbonyl formation will not explain the results. It is very probable that carbon is formed from carbon monoxide b3 way of iron carbide as an intermediate. Carbidic carbon diffuses rapidly throughout the crystal lattice and subsequently decomposes to yield elemental carbon, thus disrupting the lattice structure. 

The formation of carbon to carbon bonds in the lattice of the catalyst must occur to obtain elemental carbon from metal carbide. If the diffusion of carbidic carbon to nuclei where such carbon to carbon bonding has started is retarded by the penetration of the lattice by atomic hydrogen, the rate of elemental carbon formation would decrease with increasing partial pressure of hydrogen. This explanation, based on rate of diffusion of carbidic carbon in the lattice, may serve also to account for the effect of alkali and other promoters or impurities. Thus the presence of alkali tends to preserve a structure similar to that of the spinels, and it is possible that carbidic carbon diffuses more readily through such a lattice than through that obtained when little or no alkali is present. 

Carbon diffusion on catalytic particles Baird and Fryer in 1974 [97] and Oberlin et al. in 1976 [98] postulated that carbon filaments are formed by surface diffusion on the particle (Fig. 13). This process should be easier if the catalyst particles are liquid [74]. 

In the preparation of MEA, there are two options. One is to coat the CL onto the DM such as carbon paper, or carbon cloth, the other is to coat the CL onto the PEM, as shown in Figure 3.17. The CL coated on the diffusion medium is called Catalyst-coated Diffusion Medium (CDM), and the CL coated on the membrane is called Catalyst Coated Membrane (CCM). Using a hot-press process, by sandwiching PEM between two CDMs, or CCM between two DMs, an MEA can be fabricated for fuel cell testing. 

Table 8.2 Measured activation energies for filament growth with those for carbon diffusion in the corresponding metal catalysts.

For most PEM fuel cell catalysts, carbon black and other order carbon materials (such as carbon nanotubes) are usually used as support materials. These supports can give catalysts good electron conductivity, a very important feature in a fuel cell catalyst. Platinum and its alloys are popular active components, generally highly dispersed on the surface of support materials as micro- and nano-particles. Catalyst performance is related not only to the conductivity and supporting amounts of noble metals, but also, and more importantly, to the dispersion and composition of the active components. Because the hydrogen molecule is small and easily diffused in catalysts, in general the catalyst pore structure is not more important than the surface area. 

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