Requirements for Carbon Materials as Catalyst Supports in Industrial Applications

2 REQUIREMENTS FOR CARBON MATERIALS AS CATALYST SUPPORTS IN INDUSTRIAL APPLICATIONS 

The properties and requirements of activated carbon supports are closely related to the requirements of the catalyst. There are a series of parameters that are important for the selectivity and activity of a catalyst for example, the surface area of the support, the distribution of the pores, and the pore volume, the purity of the activated carbon, and the number and functionality of the surface groups. In regard to engineering aspects, particle size distribution plays an important role for the filtration of the catalyst, and the mechanical stability needs to be considered for the recyclability of the catalyst. 

These parameters are discussed in the following paragraphs. Besides those parameters the availability of the activated carbon and the reproducibihty of the manufacturing process are crucial. 

Surface Area Compared to bulk catalysts the supported catalysts have the advantage that they utilize the catalytically active metal much more efficiently. In bulk catalysts most of the metal is used to actually support the active metal and thus is lost for the catalytic process. Therefore, the support offers a high surface area for maximum dispersion of the metal crystallites. This is very important for precious metal catalysts, where the value of the precious metal (Pd, Pt, Rh, Ru) is very high. For example 100 kg of a commercial 5 wt% platinum catalyst contains 5 kg of platinum metal with a very high value. Current state-of-the-art precious metal powder catalysts have crystallite sizes in the range 2 to 20 nm. The crystallite size can be adjusted by the preparation method (e.g., it is possible to achieve 2.5-nm crystallites with a very tight size distribution on a commercial 

Purity The purity of activated carbon is essential for the performance of the final catalyst. Impurities of activated carbon originate from the raw material and the process conditions. Ash contents of up to 20% can be possible. Wood-based activated carbons have ash contents as low as 1 wt% [7]. The ash content can be lowered further by acid treatment of the activated carbon [8]. Typically, the ash consists of alkaline and alkahne earth metal oxides, silicates, and smaller amounts of other compounds (e.g., iron). The presence of the alkaline and alkaline earth metal oxides makes those carbons more basic in nature, so that some additional adjustments are necessary during catalyst manufacturing to meet the constant quality requirements. Since the supports are used in catalysts, the presence of catalytically active compounds that could have a potential influence on the performance of the final catalyst has to be considered as well. For the manufacture of catalysts, activated carbon based on wood, peat, nut shells, and coconut are commonly used. Due to a relatively high sulfur content in activated carbons derived from coal, those carbons are typically not used as catalyst support. 

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