Catalyst, general particles

The available surface area of the catalyst gready affects the rate of a hydrogenation reaction. The surface area is dependent on both the amount of catalyst used and the surface characteristics of the catalyst. Generally, a large surface area is desired to minimize the amount of catalyst needed. This can be accomphshed by using either a catalyst with a small particle size or one with a porous surface. Catalysts with a small particle size, however, can be difficult to recover from the material being reduced. Therefore, larger particle size catalyst with a porous surface is often preferred. A common example of such a catalyst is Raney nickel. 

The mechanical incorporation of active nanoparticles into the silica pore structure is very promising for the general synthesis of supported catalysts, although particles larger than the support s pore diameter cannot be incorporated into the mesopore structure. To overcome this limitation, pre-defined Pt particles were mixed with silica precursors, and the mesoporous silica structures were grown by a hydrothermal method. This process is referred to as nanoparticle encapsulation (NE) (Scheme 2) [16] because the resulting silica encapsulates metal nanoparticles inside the pore structure. 

To probe the details of this correlation, the hyam activity was plotted against the dso and % fines of each catalyst (Figures 10.2 and 10.3). In general it was found that increasing dso lead to lower activity catalysts while increasing fines (% particles < 3 pm) yielded more active catalysts. Therefore, particle size, particularly the amount of fines, seems to be important to the catalyst activity. 

In general, TPR measurements are interpreted on a qualitative basis as in the example discussed above. Attempts to calculate activation energies of reduction by means of Expression (2-7) can only be undertaken if the TPR pattern represents a single, well-defined process. This requires, for example, that all catalyst particles are equivalent. In a supported catalyst, all particles should have the same morphology and all atoms of the supported phase should be affected by the support in the same way, otherwise the TPR pattern would represent a combination of different reduction reactions. Such strict conditions are seldom obeyed in supported catalysts but are more easily met in unsupported particles. As an example we discuss the TPR work by Wimmers et al. [8] on the reduction of unsupported Fe203 particles (diameter approximately 300 nm). Such research is of interest with regard to the synthesis of ammonia and the Fischer-Tropsch process, both of which are carried out over unsupported iron catalysts. 

Other important physical measurements are bulk densities used to estimate hopper contents and circulation factors, and particle size analysis. The correct distribution of fine particles (30 - 180 microns) is essential to proper fluidization and transfer within the FCC unit. Generally, particles less than 30 microns are lost to the atmosphere or fines recovery system and are destined for a landfill. If the catalyst is too coarse, it may not circulate through the unit, necessitating a shutdown. Both problems are costly to the refiner and must be avoided. In addition, observation of particle size distribution changes at various points within the unit can pinpoint equipment malfunctions that might otherwise go undetected. 

The activity and selectivity of heterogeneous catalysts generally depend on the state of metal dispersion (particle size), structure (shape and morphology), metal composition, and metal-support interactions. If the catalytic centers include many metallic atoms, the electronic and geometric distributions of the constituents elements may be strongly related to the chemical reactivities and catalytic performances of the bimetallic and alloy catalysts (5). 

For Pt catalysts, generally to say, the smaller the Pt particles and the larger SSA the particles possess, the better the electro-catalytical performance would be. However, whether or not there exists a critical size that can give the maximum Pt-mass activity is still no clear answer yet, although extensive research efforts have been done on this subject. Calculation using density fimctional theory (DFT) is often used for providing insight into the effect of nanoparticle size on the electrocatalytical activity. ... 

In general, carbon nanotubes are produced in one of two ways either on a supported catalyst or on a free-floating catalyst. In the supported catalyst method, particles such as iron are attached to a substrate and a carbon source such as a hydrocarbon or CO is passed over the substrate in a high-temperature environment. The nanotubes then grow from the catalyst particles. With the free-floating (aerosolized) catalyst method, particles are produced in the gas phase by flowing a catalyst precursor into the high-temperature environment with a source of carbon. The precursor typically decomposes to form the catalyst particles as an aerosol. 

Catalytic materials for MRs have some particular requirements compared to a conventional tube flow reactor. The catalytic material should be in a form that can be inserted easily into the membrane reactor, and the catalyst should not have any mechanical failure or properties which are not suitable for a MR. Very hne powder form catalysts cannot be used, as the small particulates may block the pores of the membrane however, small particulates (>0.2 mm) have been considered (e.g., Li et al., 2010). Thus, in many cases, the catalysts generally used in MRs are pellets, extrudates or tablets. In addition to these forms, novel hbre type or foam catalysts have been studied as support materials for active metals. Li et al., (2010) have presented in their study one kind of a method of encapsulating the catalyst particles (diameter 0.2-1.7 mm) which combines a catalyst particulate with a membrane layer. This has been reported to increase the selectivity of the reaction, and thus the separation process is much easier. 

As on previous occasions, the reader is reminded that no very extensive coverage of the literature is possible in a textbook such as this one and that the emphasis is primarily on principles and their illustration. Several monographs are available for more detailed information (see General References). Useful reviews are on future directions and anunonia synthesis [2], surface analysis [3], surface mechanisms [4], dynamics of surface reactions [5], single-crystal versus actual catalysts [6], oscillatory kinetics [7], fractals [8], surface electrochemistry [9], particle size effects [10], and supported metals [11, 12]. 

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