Metal carbides catalyst-support interaction

Most forms of carbon, except diamond, which are renowned as supports for precious metal catalysts in certain applications [3], interact strongly with MW [4]. Amorphous carbon and graphite, in their powdered form, irradiated at 2.45 GHz, rapidly (within 1 min) reach very high temperatures (>1300 K). This property has been used to explain MW-assisted syntheses of inorganic solids [5], In these syntheses carbon is either a secondary susceptor which assists the initial heating but does not react with other reactants, or is one of the reactants, e. g. in the synthesis of metal carbides. MW-carbon coupling has also been widely developed ... 

Leclercq, L., Almazouari, A., Dufour, M., and Leclercq, G. 1996. Carbide-oxide interactions in bulk and supported tungsten carbide catalysts for alcohol synthesis. In Chemistry of transition metal carbides and nitrides, ed. S. T. Oyama, 345-61. Glasgow Blackie. 

The intrinsic nature of tungsten carbide catalyst in CO-H2 reactions is to form hydrocarbons. This property can be modified by oxidic promoters as for the case of noble metals like Pt or Rh or by the presence of carbon vacancies at the surface. To increase the production of alcohols in the Fischer-Tropsch reaction, the catalyst should be bifunctional, with oxidic and carbidic components as in the case of WC on Ti02. Overcarburization of WC on supports like Si02 or Zr02 where the W-O-metal interaction is weak leads to C/W ratios close to unity and does not result in alcohol formation. 

One can use practically the same catalyst preparation methods with carbon blacks as with activated carbon supports to give comparable results. In fact, some precious metal compounds reduce upon contact with furnace black, resulting in a change of the furnace black structure around the metal particle [39]. It has been observed [40] that carbon blacks can be oxidized by the metal crystals supported on them at rather low temperatures (from 398 to 468 K). In fact, some metals may form carbides [41], and this type of metal-support interaction will clearly affect the performance of the catalyst. 

It is believed that some doped carbon nanotubes can provide the Pt catalyst with strong interaction. For example, highly dispersed Pt nanoparticles supported on surface of thiolation functional carbon nanotubes, such as S-doped CNTs (SH-CNTs), CNTs-CeHe-CHs-SH, CNTs-CONH-(CH2)2-SH, and CNTs—CONH—CeHe—SH, were explored. Furthermore, when some metal oxides such as NbTi02 and metal carbides were explored as the catalyst supports, strong interaction between catalyst and the support was also observed. ... 

The information on carbon-supported catalysts has been dominated by cata-lytically active metals that are part of the conventional hydroprocessing catalysts, i.e. Co(Ni)Mo(W). In a sulfided form, the structure of the Co(Ni)-Mo(W)-S active phase in these catalysts should approach that of Type-II phase observed on the y-Al203-supported catalysts after a high-temperature sulfidation. " Apparently, there is a sufficient driving force for a direct interaction of carbon with either Mo or sulfur leading to the formation of the Mo-C(S) bonds. Then, in carbon-supported catalysts, the presence of another active phase, i.e. Co-Mo C(S), appears to be plausible. The formation of metal carbides may take place if the supply of sulfur to maintain the catalyst surface in a sulfided form becomes limited, particularly if such a state persists for an extended period. ... 

A new preparation method is described to synthesize porous silicon carbide. It comprises the catalytic conversion of preformed activated carbon (extrudates or granulates) by reacting it with hydrogen and silicon tetrachloride. The influence of crucial convoaion parameters on support properties is discussed for the SiC synthesis in a ftxed bed and fluidized bed chemical vapour deposition reactor. The surface area of the obtained SiC ranges ftiom 30 to 80 m /g. The metal support interaction (MSI) and metal support stability (MSS) of Ni/SiC catalysts are compared with that of conventional catalyst supports by temperature programmed reduction. It is shown that a Ni/SiC catalyst shows a considnable Iowa- MSI than Ni/Si(>2- and Ni/Al203-catalysts. A substantially improved MSS is observed an easily reducible nickel species is retained on the SiC surface after calcination at 1273 K. 

With this respect, the work from Atanasoski and coworkers is promising (compare section Transition Metal Carbides, Nitrides and Chalcogenides ) [35], By performing a heat treatment of their sputtered C-N iFe Aims, the activity was drastically enhanced but still much lower compared to macrocycle-based catalysts. However, when titanium carbide was used as support instead of carbon, a high stability was obtained. The fact that by changing the support, an essentially better durability was obtained is an important result as it shows that even for catalysts based on molecular centers, alternative support materials can be utilized and that the interaction between the support and the catalytic centers might be cmcial for the optimization of those catalysts for a fuel cell application. 

In recent years, a lot of research effort has been directed towards dehydroaromatisation of methane in which methane is converted to aromatic products such as benzene and naphthalene in addition to hydrogen. Perhaps the most well studied system has been that employing Mo/ZSM-5 based catalysts, where the bifunctional interaction between the zeolite Bronsted acidity and molybdenum species is well recognised. Under reaction conditions, the active molybdenum species are known to be in the form of carbides or oxycarbides, and recently it has been proposed that the a-MoCi-x phase is the most active form. Deactivation, primarily due to coke formation, is well precedented in this reaction and represents a major obstacle to be overcome in the successful application of these catalysts. In this respect, it is interesting to note that Ichikawa and co-workers have published studies indicating that the inclusion of low levels of CO or CO2 in the feed can promote the reaction via the suppression of coke formation in the case of both Mo/HZSM-5 and Re/HZSM-5 catalysts. Other approaches adopted towards this aim have been the inclusion of second metal components and a reduction of the acid strength of the HZSM-5 support. ... 

Therefore, the loaded metals virtually interacted with an oxidized surface rather than the native carbide surface. Schweitzer et al. developed a synthesis method that could allow a direct contact of Pt with the genuine Mo C surface [65]. In the WGS reaction, the resulting Pt/MOjC catalysts exhibited a higher activity than the most active oxide-supported Pt catalysts and a commercial Cu-Zn-Al catalyst. Moreover, the experimental rates were more consistent with those predicted by the perimeter model than by the particle surface model, suggesting that the rate-determining step for WGS on Pt/Mo C catalysts occurred at the perimeters of the Pt particles. 

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