Production and Characterization of Heterogeneous Catalysts

In principle, solids found in nature can display activity as heterogeneous catalysts (e.g., iron ores are applied in high temperature Fischer-Tropsch catalysis). However, after 100 years of intense research in catalysis it has been found that in most cases synthetic materials are more suitable as the reproducibility of their synthesis is higher and their specific properties can be adjusted by the synthetic procedure. 

The most important method to produce catalyst supports or solid catalysts is precipitation. The precipitation process is realized by controlled addition of a precipitation reagent (very often a change of pH by addition of acid or base). Depending on the desired composition of the catalyst material, co-precipiation of more than one component is frequently applied. An alternative process is the precipitation of metal salts onto preformed support particles. 

Precipitation is a complex sequence of supersaturation, seeding, and coagulation. All three steps determine particle size and size distribution of the obtained particles. The process is strongly dependent on the exact precipitation conditions and is still difficult to model. Table 2.3.6 displays examples of technically important catalysts or catalyst supports produced by precipitation. 

A few industrial important heterogeneous catalysts are prepared by melt processes. Examples are the Fe-catalyst for the Haber-Bosch process and the Pt/Rh-net for the ammonia oxidation in the Ostwald process (see also Section 6.4). Melting is also the initial process step for the preparation of Raney-nickel and Raney-copper catalysts. For these catalysts an alloy of Ni/Cu and A1 is prepared by melting. This alloy is later treated with NaOH to dissolve the A1 from the solid to create pores and reactive surface sites. Raney-Ni and Raney-copper are very important hydrogenation catalysts. 

For the synthesis of zeolites and related compounds, hydrothermal synthesis and sol-gel processes are of importance. In the hydrothermal synthesis a hydrogel is prepared from the Si- and Al-sources in an aqueous medium. The latter often 

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