Mixed metal catalysts unsupported

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

The first Raman spectra of bulk metal oxide catalysts were reported in 1971 by Leroy et al. (1971), who characterized the mixed metal oxide Fe2(MoC>4)3. In subsequent years, the Raman spectra of numerous pure and mixed bulk metal oxides were reported a summary in chronological order can be found in the 2002 review by Wachs (Wachs, 2002). Bulk metal oxide phases are readily observed by Raman spectroscopy, in both the unsupported and supported forms. Investigations of the effects of moisture on the molecular structures of supported transition metal oxides have provided insights into the structural dynamics of these catalysts. It is important to know the molecular states of a catalyst as they depend on the conditions, such as the reactive environment. 

Unsupported carbon fibers are formed in a continuous process [47-48] where the metal catalyst particles are continuously mixed into the flow of the feed gases, and where fibers carrying a metal catalyst particle in their tips are continuously removed. A specific process [51] that is currently under commercial development uses iron pentacarbonyl as a catalyst and hydrocarbons such as methane, natural gas, or others which can be derived from coal, recyclate and discarded rubber tires. These fibers are only several pim long and have much lower diameters (0.1 to 0.2 jm). Apparently, less time is available for side growth (or thickening) in a continuous process. 

Before bismuth-promotion the Pt-on-alumina catalyst was pre-reduced in water with hydrogen. The pH was decreased to 3 with acetic acid and the appropriate amount of bismuth nitrate dissolved in water (10 - lO " M) was added into the mixed slurry in 15-20 min, in a hydrogen atmosphere. Promotion of unsupported Pt was carried out similarly. The metal composition of the bimetallic catalysts was determined by atomic absorption spectroscopy. 

---