Carbonaceous support materials

There exist three different ways to dope the carbonaceous support material with platinum or other catalytically active noble metals. 

Figure 7.4 Images of the most prominent carbonaceous support materials (a) graphite and (b) Vulcan carbon. (Taken from Ref. [14]. Copyright (2014), with permission from Elsevier Limited).

In most industrial processes, the electrodes are prepared from a so-called ink consisting of the supported catalyst powder, ionomer, and additives. Already here, the chosen carbonaceous support material will affect the ink properties, such as homogeneity and viscosity, and therefore necessitate adapted ink recipes for each support material. The ink is then either airbrushed, sieve-printed, inkjet-printed, or DECAL transferred onto the membrane or the GDL and - like the ink ingredients - also the chosen manufacturing process strongly affects the electrode properties. 

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 reaction often is accompanied by the growth of carbon nanofibers of different textures (see Figure 5.3). Notice that the supported metal cat alysts are normally used for the purpose and that processes Hke (5.20) can lead to the increase of the catalyst granules weight by a factor of 300 and more compared to their initial weight to allow extremely pure carbonaceous filamentous materials to be produced. 

Owing to the advantageous properties of CNTs and CNFs as supports, several studies have been carried out on different catalytic reactions. In particular, much attention has been dedicated to liquid-phase reactions with MCWNT- and CNF-supported catalysts indeed, their high external surface and their mesoporos-ity would result in a significant decrease in mass-transfer limitations compared with activated carbon. It is worth noting that as for fullerenes [28], few studies on SWCNT-supported catalytic systems have been reported, due either to their microporosity or to the fact that it is still very difficult to obtain the large amounts of pure material required to conduct catalytic studies. In this section we present the results obtained for each type of reaction and, when possible, we will try to rationalize these results by comparison with other carbonaceous supports. 

Many support materials have been studied, with a variation in the extent to which this interacts with the metal component. Thus, silica ° and alumina ° have been largely studied along with inert supports such as carbonaceous materials. ... 

XPS was used to study the modifications of the surface chemistry of carbonaceous materials after chemical treatments [111]. For example, recent studies were devoted to the preparation of active carbons modified by incorporation of nitrogen aiming at the development of new solid catalysts [33,34], new catalyst support materials [155], and new competitive adsorbents [111]. 

Surface properties of the support material influence the adsorption states of Pt(acac)2, Cr(acac)3 and V(acac)3 as well as the decomposition pathways of the adsorbates and finally the dispersion of the catalytic compound. The deposited particles are more mobile on silica and hence, more capable to agglomerate. Alumina-supported Pt may be stabilized by coordinativdy unsaturated Al " " surface ions similar arguments may apply for the stabilization of amorphous Cr203 on alumina. Because the metal acetyl acetonate decomposition is accompanied by deposition of carbonaceous compounds an additional air treatment of the samples is required. Finally, the fluidized-bed technique has been proven to be applicable for preparation of catalyst particles of uniform dispersion of the catalytic compound throughout the whole bed of particles. 

Data from the NIST consortium work also supports the assertion that a homogeneous residue is critical for providing the most effective heat-transfer barrier. Comparison of the mass loss rate data for three different PP/PP-g-MA/clay nanocomposites and pure PP-g-MA is shown in Figure 3.15. As the images of the residues show, little or no carbonaceous (black) material is present after gasification. The fluorinated synthetic mica (FSM) has the lowest mass loss rate this... 

For multicomponent porous materials with mixed wettability, which are widely used in applied electrochemistry (e.g., fuel cell electrodes containing platinum particles on carbonaceous supports along with different additives), it is possible to investigate separately the structure (pore volume distribution versus pore size) for hydrophilic and hydrophobic (liophilic and liophobic) pores and to evaluate some important parameters, such as the dependence of the fraction p of the pore surface occupied by the hydrophobic (liophobic) components on the pore size. 

Electrodes used in PEM fuel cells typically employ an electrocatalyst layer with a porous, carbonaceous, electronically-conductive substrate that has been rendered hydrophobic. The catalyst layer usually comprises platinum, or a platinum-containing alloy, on a carbonaceous support (typically carbon black), dispersed ionomeric material similar in constituency to the electrolyte-membrane, and dispersed hydrophobic polymer such as polytetrafluoroethylene. The substrate, which serves as a reactant-gas diffusion layer, may be a carbon paper or a woven or non-woven cloth. The catalyst layer may be deposited directly onto the electrolyte-membrane or onto the substrate and later placed in contact with the membrane. 

Finally, the abatement of NO pollution by using sorbing catalytic materials [59,60] must also be cited. Several solid sorbents for NO removal (metal oxides, spinels, perovskites, double-layered cuprates, zeolites, carbonaceous materials, heteropolyacids and supported heteropolyacids) have been tested. The results are interesting, but not competitive to actual technologies. To mention that the use of sorbing materials allows... 

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