Graphite-supported catalysts

In general, encapsulated metal particles were observed on all graphite-supported catalysts. According to Ref. [4] it can be the result of a rather weak metal-graphite interaction. We mention the existence of two types of encapsulated metal particles those enclosed in filaments (Fig. 1) and those encapsulated by graphite. It is interesting to note that graphite layers were parallel to the surface of the encapsulated particles. 

As was found in Ref. [13], the method of catalytic decomposition of acetylene on graphite-supported catalysts provides the formation of very long (50 fim) tubes. We also observed the formation of filaments up to 60 fim length on Fe- and Co-graphite. In all cases these long tubules were rather thick. The thickness varied from 40 to 100 nm. Note that the dispersion of metal particles varied in the same range. Some metal aggregates of around 500 nm in diameter were also found after the procedure of catalyst pretreatment (Fig. 2). Only a very small amount of thin (20-40 nm diameter) tubules was observed. 

As in the case of graphite-supported catalysts, some metal particles were also encapsulated by the deposited carbon (Fig. 4). However, the amount of encapsulated metal was much less. Differences in the nature of encapsulation were observed. Almost all encapsulated metal particles on silica-supported catalysts were found inside the tubules (Fig. 4(a)). The probable mechanism of this encapsulation was precisely described elsewhere[21 ]. We supposed that they were catalytic particles that became inactive after introduction into the tubules during the growth process. On the other hand, the formation of graphite layers around the metal in the case of graphite-supported catalysts can be explained on the basis of... 

The main effect of MW irradiation on the graphite- and charcoal-supported catalysts is to reduce the average temperature required for the reaction to occur. The authors believe this is the result of hot spots formed within the catalyst bed (Sect. 7.4.2). Graphite-supported catalysts, moreover, seem to be more selective than the equivalent charcoal-supported catalysts, especially under the action of MW irradiation - 83.6-97.7% compared with 68.4-86.3%. This might be because of the hydrophobic nature of the graphite which directs the reaction away from the production of water by dehydration of the alcohol. 

The results are exteded for the case of the Pt/Graphite supported catalysts. 

For graphite-supported catalysts, the presence of barium decreased the number of copper and of chromium atoms at the surface. From the relative intensity of the XPS signals it could be seen that the dispersion of copper varied with the barium content. In comparison with the alumina supported catalysts the Cu content was more significant whereas it was the contrary with Cr species which disappeared completely after addition of barium. These results are in agreement with XRD characteristics showing the presence of a CUC1O2 phase on the surface of the catalyst precursor. 

Equivalent results were obtained with alumina deposited catalysts (figure 5). However in this case ammonia has a very marked inhibiting effect on non-promoted catalysts. It can be seen then that the addition of the promotor strongly increases the stability of the catalysts in the presence of ammonia and of water. However contrary to graphite supported catalysts the addition of a promotor does not allow to stabilize completely the catalyst in the presence of ammonia and of water. 

Description of graphite-supported catalysts prepared using Pt and I Aujop.,... 

Guerrero-Ruiz, A. et al. 1998. Study of some factors affecting the Ru and Pt dispersions over high surface area graphite-supported catalysts. Applied Catalysis A General 173 313-321. 

As it was established by Geus et a/.[18, 19] the decrease of the rate of carbon deposition is a positive factor for the growth of fibres on metal catalysts. Si02 is an inhibitor of carbon condensation as was shown in Ref [20]. This support also provides possibilities for the stabilization of metal dispersion. Co and Fe, i.e. the metals that give the best results for the tubular condensation of carbon on graphite support, were introduced on the surface of siUca gel... 

Bi promoted Pt catalysts were prepared using JM proprietary methods. Aqueous solutions of Pt and Bi salts were co-precipitated onto the catalyst support and reduced using chemical reducing agents. Thereafter the materials were washed, filtered and retained as pastes. Graphite supported materials were dried prior to use. Similar preparation methods were used for all Pt and Pt-Bi catalysts. 

Its inert behavior towards numerous chemical compounds and its adsorbent properties (responsible for the retention of volatile or sublimable organic compounds), make graphite the choice support for thermal reactions. Among its impurities, magnetite was revealed to be an active catalyst, and some reactions can be performed without any added catalyst. Two processes are then possible, the graphite-supported reaction ( dry process), and the reaction in the presence of a small amount of graphite (solid-liquid medium). 

Over the last decade, novel carbonaceous and graphitic support materials for low-temperature fuel cell catalysts have been extensively explored. Recently, fibrous nanocarbon materials such as carbon nanotubes (CNTs) and CNFs have been examined as support materials for anodes and cathodes of fuel cells [18-31], Mesoporous carbons have also attracted considerable attention for enhancing the activity of metal catalysts in low-temperature DMFC and PEMFC anodes [32-44], Notwithstanding the many studies, carbon blacks are still the most common supports in industrial practice. 

The aqueous samples, H2L and 02L, were both exposed ex-situ to hydrogen saturated water at atmospheric pressure and 363 K for 20 hours prior to further treatment. The pH of the water after contacting with the catalyst sample amounted to 5 due to the presence of acidic groups on the graphite support. 

Consequently, since graphitized carbon-supports have lower carbon corrosion rates, the use of cathode catalysts with graphitized supports significantly reduces H2/air-front start-stop damage.12,22 Furthermore, if the ORR activity of the anode electrode is reduced by lowering anode Pt loading, H2/air-front start-stop degradation is decreased.22,23... 

J. H. Vleeming, B. F. M. Kuster, and G. B. Marin, Oxidation of methyl and n-octyl a-D-glucopyranoside over graphite-supported platinum catalysts effect of the alkyl substituent on activity and selectivity, Carbohydr. Res., 303 (1997) 175-183. 

In comparison to most other methods in surface science, STM offers two important advantages (1) it provides local information on the atomic scale and (2) it does so in situ [50]. As STM operates best on flat surfaces, applications of the technique in catalysis relate to models for catalysts, with the emphasis on metal single crystals. Several reviews have provided excellent overviews of the possibilities [51-54], and many studies of particles on model supports have been reported, such as graphite-supported Pt [55] and Pd [56] model catalysts. In the latter case, Humbert et al. [56] were able to recognize surface facets with (111) structure on palladium particles of 1.5 nm diameter, on an STM image taken in air. The use of ultra-thin oxide films, such as AI2O3 on a NiAl alloy, has enabled STM studies of oxide-supported metal particles to be performed, as reviewed by Freund [57]. 

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