Catalysts

Application of porous carbon to catalyst is not a new and nontraditional example, but the surfaces of porous carbons still have unsolved problems particularly in the participation of surfaces to catalysis. So the topic concerning catalyst is also included in this section. A concise review is available for surveying the important research in the last two decades by Rodriguez-Reinoso [168].

The keywords to understanding the catalytic natures of carbon support are oxidation activity, electron transfer, high surface area, and spillover of hydrogen.

The oxidation ability of the carbon surface has been known and already used in flue gas desulfurization and denitrification reactions. Figure 35 shows a scheme for desulfurization on a carbon surface. The emitted SO2 is adsorbed on the oxidation sites and then oxidized to SO3 by the action of oxygen gas. The formed SO3 is hydrated to sulfuric acid. Sulfuric acid first fills the micropores of porous carbon catalyst. After all of the micropores are filled with sulfuric acid, the reaction takes place on the macropore surface with lower reaction rate. Kisamori et al. claimed that the oxidation sites on the porous carbon are the surface defects induced by desorption of surface functional groups [169]. The desulfurization ability was not affected by surface areas of porous carbons, but porous carbons with higher nitrogen content showed excellent activity. The influence of surface treatments on the catalytic activity of carbon is an important topic in catalyst preparation [170-172]. Introduction of nitrogen onto the surfaces of porous carbons derived from a Victorian brown coal resulted in remarkable increases in the activity for both desulfurization and denitri-fication [173]. Kuhl et al. studied denitrification activity of sulfuric acid activated coke that was treated with ammonia [174]. They found that the denitrification activity increased when nitrogen was introduced in the range of 1-3%, and it saturated around 5%. However, the chemical structure and the mechanism of the introduced nitrogen are not clear.

Large surface areas of porous carbons have been used for catalyst supports. Porous carbon materials enable catalytic materials such as metals, metal oxides, and metal sulfides finely dispersed to exert high catalytic activity [168].

/regeneration H,O

/regeneration H,O

| Carbon substrate

aq.

: SO2 oxidation site

o

: SO3 adsorption site

A

: H2SO4 adsorption site

Figure 35. A schematic representation of desulfurization on a carbon surfaces.

graphite

Note: R.T. is room temperature.

Figure 35. A schematic representation of desulfurization on a carbon surfaces.

Porous silica, alumina, and titania are more common catalyst supports, but the most striking difference of carbon supports from them is that we can anticipate electronic interaction between the supported metal and the carbon supports, because carbons are electronic conductors. An example can be seen in the Hoechst-Wakker reaction of ethylene to produce acetaldehyde. The catalyst, palladium chloride, is reduced when it is used for oxidizing ethylene and is regenerated by the presence of copper chloride. If palladium chloride is loaded onto a carbon support, the regenerating reagent is not required. This means that the surface of the carbon support promotes the reoxidation of palladium metal to form a continuous catalytic cycle. Similar electronic effects were also found for ammonia synthesis reactions.

Spillover is very interesting phenomenon involving platinum or nickel catalysts supported on carbon. Porous carbons do not adsorb hydrogen in the temperature range of room temperature to ca. 673 K, and uptake begins above 723 K gradually. On the other hand, the metal supported porous carbons adsorb hydrogen at temperatures as low as 473 K, and the adsorbed amount of hydrogen was 1021 atom/g-C, corresponding to the several tens or several hundreds times the amount of hydrogen which is expected from the metal loading [175]. This has been explained by the hydrogen transfer from the metal surfaces to the carbon surfaces as shown in Figure 36. The adsorbed hydrogen can easily be desorbed from the catalyst. In the figure dehydro-genation of hydrocarbon takes place on the carbon surface, and the removed hydrogen becomes spillover hydrogen. The produced olefin and hydrogen are effectively separated to inhibit recombination reaction.

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