Catalyst deposition

The carbon deposited catalysts were treated both by oxidation and hydrogenation at temperatures in the range of 873-1173 K for various exposure times. Some results of oxidation treatment are presented in... 

Fig. 11. The loss of carbon rapidly increases with the increase of temperature. Heating of the catalysts in open air for 30 minutes at 973 K leads to the total elimination of carbon from the surface. The gasification of amorphous carbon proceeds more rapidly than that of filaments. The tubules obtained after oxidation of carbon-deposited catalysts during 30 minutes at 873 K are almost free from amorphous carbon. The process of gasification of nanotubules on the surface of the catalyst is easier in comparison with the oxidation of nanotubes containing soot obtained by the arc-discharge method[28, 29]. This can be easily explained, in agreement with Ref [30], by the surface activation of oxygen of the gaseous phase on Co-Si02 catalyst.

The preparation method of titania support was described in the previous paper [6]. Titanium tetraisopropoxide (TTIP 97%, Aldrich) was used as a precursor of titania. Supported V0x/Ti02 catalysts were prepared by two different methods. The precipitation-deposition catalysts (P-V0x/Ti02) were prepared following the method described by Van Dillen et al. [7], in which the thermal decomposition of urea was used to raise homogeneously the pH of a... 

GL 18] [R 6a] ]P 17] About 100% selectivity was achieved for the hydrogenation of p-nitrotoluene [17], with conversions of 58-98%. The conversion for the electro-deposited catalyst was 58%, whereas the impregnated catalyst gave a 58-98% conversion, depending on the process conditions (see Table 5.1). 

GL 18] [R 6a] [P 17] A sol-gel deposited catalyst used in a fixed-bed reactor gave higher conversion than a micro-channel catalyst impregnated on a porous alumina layer [17]. This was due to the higher geometric surface area of the sol-gel deposited catalyst. 

The apparent redox stoichiometry of O2 reduction catalysis [ av. Reaction (18.8)] is pH-independent, but for many catalysts depends strongly on the applied potential (Fig. 18.10). The apparent selectivity of Fe porphyrins deposited on the electrode surface typically increases with the amount of deposited catalyst. 

The Fe-only form of these metalloporphyrins is a highly selective ORR catalyst when adsorbed on a graphite or Au electrode. It operates at an overpotential of about 0.55 V at pH 7 and av >3.9 (Fig. 18.19) and retains these characteristics for >10 mrnovers the catalytic selectivity is independent of the amount of deposited catalyst. 

As discussed earlier, it is generally observed that reductant oxidation occurs under kinetic control at least over the potential range of interest to electroless deposition. This indicates that the kinetics, or more specifically, the equivalent partial current densities for this reaction, should be the same for any catalytically active feature. On the other hand, it is well established that the O2 electroreduction reaction may proceed under conditions of diffusion control at a few hundred millivolts potential cathodic of the EIX value for this reaction even for relatively smooth electrocatalysts. This is particularly true for the classic Pd initiation catalyst used for electroless deposition, and is probably also likely for freshly-electrolessly-deposited catalysts such as Ni-P, Co-P and Cu. Thus, when O2 reduction becomes diffusion controlled at a large feature, i.e., one whose dimensions exceed the O2 diffusion layer thickness, the transport of O2 occurs under planar diffusion conditions (except for feature edges). 

Co/A1203 catalysts that contain higher amounts of less reactive polymeric carbon not only exhibited enhanced deactivation when tested in FTS when compared to the fresh catalyst, but also showed an increase in selectivity to olefinic products.31 The authors postulated that this was probably due to the reduction in hydrogenation ability of the carbon deposited catalyst to convert primarily formed olefins into the corresponding paraffins. 

Now, by chemically, or electrochemically depositing catalyst atoms at these sites, a means is provided to reduce the activation energy. Figure 3 shows a schematic illustration of this new configuration. 

Figure 3. Possible configuration of strategically-deposited catalyst atom on nanoporous silicon...

The inner cavity of carbon nanotubes stimulated some research on utilization of the so-called confinement effect [33]. It was observed that catalyst particles selectively deposited inside or outside of the CNT host (Fig. 15.7) in some cases provide different catalytic properties. Explanations range from an electronic origin due to the partial sp3 character of basal plane carbon atoms, which results in a higher n-electron density on the outer than on the inner CNT surface (Fig. 15.4(b)) [34], to an increased pressure of the reactants in nanosized pores [35]. Exemplarily for inside CNT deposited catalyst particles, Bao et al. observed a superior performance of Rh/Mn/Li/Fe nanoparticles in the ethanol production from syngas [36], whereas the opposite trend was found for an Ru catalyst in ammonia decomposition [37]. Considering the substantial volume shrinkage and expansion, respectively, in these two reactions, such results may indeed indicate an increased pressure as the key factor for catalytic performance. However, the activity of a Ru catalyst deposited on the outside wall of CNTs is also more active in the synthesis of ammonia, which in this case is explained by electronic properties [34]. 

The first step of selectively depositing catalyst nanoparticies inside CNTs is their impregnation with a solution of the catalyst precursor. In the ideal case the solvent has a low surface tension, e.g., ethanol, to enable facile penetration into the CNT. Distilled water has also been reported for this purpose, however, with the aid of ultrasonic treatment [112]. The precursor solution can be added in excess or in defined volume. In any case, some weakly adsorbed metal species will remain on the outer CNT surface, which must be removed by washing with an appropriate solvent [111, 112]. A high selectivity to inside deposition is reported for this strategy, which is independent of the CNT diameter. 

Coated Wall Reactors. Techniques for depositing catalyst onto the reactor walls include... 

In increasing the scale of operation, other problems will intrude. These are, a) resuspension of deposited catalyst, after some time of resting in wet condition ... 

Keywords Carbon nanotubes, Carbon nanofibers, Chemical vapor deposition, Catalyst, Activation. 

Auction of coke deposited. Catalysts reduced at 300°C Fig.3. Acetylene conversion as m... 

Figure 5. Relationship between amount of coke deposition and specific surface area of coke deposited catalyst. Catalyst H-Ga-Silicate.

Secondly, the coverage by species coming from methanol adsorption on each catalyst was evaluated (Fig. 20). It appears that this coverage is always higher at a co-deposited (non alloyed Pt+Ru electrode) than at a co-reduced catalyst, whatever the methanol adsorption potential, which can be an explanation for the higher activity of the co-deposited catalyst compared with the coreduced one. 

A number equal to 2 indicates that only CO is adsorbed at the surface, whereas a number higher that 2 indicates that both CHO and CO species are adsorbed at the surface. From the results given in Table 2, the calculated number of electrons is always close to 2, which indicates that the main adsorbed species is CO. However, this number is higher for the co-reduced than for the co-deposited catalyst, and becomes greater than 2, with an increase in the rathe-nium amount, which likely indicates that more CHOads is formed at co-reduced Pto gRuo 2/C and PC sRuq 5/C catalysts. 

In the case of the co-deposited catalyst (non-alloyed), the number of electron from the methanol stripping experiment is close to 2, indicating that almost only CO is formed at the electrode surface. The number of electrons for the oxidation of bulk methanol is close to 6 (6.6), which indicates that only a small amount of formic acid is formed, and that the main reaction path leads to the formation of CO2, according to the following steps ... 

Catalyst attached to membrane surface. When depositing catalyst particles on the surface of a catalytically inert, dense membrane, the membrane surface layer should be porous in nature to provide a high surface area catalyst support for strong adhesion of the catalyst particles. A layer or multi-layers of catalyst particles can be coated on inorganic membrane surfaces by several methods Pd by vapor deposition [Gryaznov et al, 1979], Pd and Pt by solution deposition [Gryaznov et al., 1983 Guther and Vielstich, 1982]. 

The deposition of a suitable catalyst in the intimate body of the above-described filters is controlled primarily by the structure of the filter itself, but it is also influenced by the nature of its constituent material. In fact, shear stresses may arise at the interface between this material and the deposited catalyst, owing to thermal expansion mismatch between the two phases. Since most catalyst supports are based on inorganic oxides, this problem would be particularly serious for metal-based filters, owing to their much higher thermal expansion coefficients. However, in some metal alloys, such as the FeCrAlloy, a thin surface layer of a metal oxide (e.g., ALOO is formed at high temperatures, which improves their thermal resistance and allows a proper basis for catalyst anchoring. 

In catalytic filters, since the catalyst layer is much thinner than the catalyst wall of honeycomb, SO3 formation is lowered proportionately. However, drawbacks for catalytic filters can be found in their more complex preparation procedure, in the probably lower long-term stability of the layer of the deposited catalyst, and in their higher pressure drop. However, concerning this last point, it has to be admitted that the catalytic filter on its own does not represent an alternative to the honeycomb converter alone, but to the combination of a traditional dedusting device and the honeycomb itself. In this context, the problems related to comparatively high pressure drops might be minor, if any, and in any case more or less critical, depending on the particular application of interest. 

Excess solution (Fp < Vg) is used for synthesis of powdered and granulated supported catalysts, if the adsorption is very intense, as well as for the preparation of powdered deposited catalysts. 

A 10-mL aliquot of the catalyst solution was diluted with 100 ml of freshly distilled toluene in a beaker. The solution was stirred for five minutes, and then the membrane sample was placed in the solution. The catalyst solution immediately penetrated the microporous structure. The sample was allowed to soak for about five minutes and then was allowed to dry in the box under blowing nitrogen for 10-15 minutes, giving a membrane with the solid state catalyst components deposited within the pore structure of the polypropylene. Weight of the sample plus the deposited catalyst was 0.8872 grams, or a wt % gain of 4.7%. The sample was placed in an empty Schlenk tube and removed from the dry box. 

Instead of rotating the catalyst. Ford and Perlmutter inserted a cylinder containing a deposited catalyst in a stirred vessel. Studies on the vapor-phase dehydrogenation of sec-butyl alcohol indicated that stirred-tank performance was achieved., ... 

Typical TEM images of the Au nanoparticles supported on Ti-MCM-41 and TiO-SiO(l) samples are shown in Figs. 3a and 3b, respectively. The TEM pictures for the Au deposited catalysts did not show the presence of any bulk titania phase in the samples. Au nanoparticles were found to be uniformly dispersed on the surface of titanosilicate samples. But the surface of mesoporous TiO-SiO(l) and Ti-Meso supports showed more... 

V. Meille, Review on methods to deposit catalysts on structured surfaces, Appl. Catal. A Gen. 315 (2006) 1. 

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