Dispersed catalysts

Radial density gradients in FCC and other large-diameter pneumatic transfer risers reflect gas—soHd maldistributions and reduce product yields. Cold-flow units are used to measure the transverse catalyst profiles as functions of gas velocity, catalyst flux, and inlet design. Impacts of measured flow distributions have been evaluated using a simple four lump kinetic model and assuming dispersed catalyst clusters where all the reactions are assumed to occur coupled with a continuous gas phase. A 3 wt % conversion advantage is determined for injection feed around the riser circumference as compared with an axial injection design (28). 

Research on catalytic coal Hquefaction was also carried out using an emulsified molybdenum catalyst added to the slurry medium to enhance rates of coal conversion to distiUate (26). Reaction at 460°C, 13.7 MPa (1980 psi) in the presence of the dispersed catalyst was sufficient to greatiy enhance conversion of a Pittsburgh No. 8 biturninous coal to hexane-soluble oils ... 

It will also be shown that the absolute electrode potential is not a property of the electrode but is a property of the electrolyte, aqueous or solid, and of the gaseous composition. It expresses the energy of solvation of an electron at the Fermi level of the electrolyte. As such it is a very important property of the electrolyte or mixed conductor. Since several solid electrolytes or mixed conductors based on ZrC>2, CeC>2 or TiC>2 are used as conventional catalyst supports in commercial dispersed catalysts, it follows that the concept of absolute potential is a very important one not only for further enhancing and quantifying our understanding of electrochemical promotion (NEMCA) but also for understanding the effect of metal-support interaction on commercial supported catalysts. 

Successful and reproducible preparation of highly dispersed catalysts crucially depends on the state of the carrier surface and on the concentration and pH of the impregnating solution. It is an art and a science for which several goodbooks and reviews exist.1 5... 

Consequently the proven functional identity of classical promotion, electrochemical promotion and metal-support interactions should not lead the reader to pessimistic conclusions regarding the practical usefulness of electrochemical promotion. Operational differences exist between the three phenomena and it is very difficult to imagine how one can use metal-support interactions with conventional supports to promote an electrophilic reaction or how one can use classical promotion to generate the strongest electronegative promoter, O2, on a catalyst surface. Furthermore there is no reason to expect that a metal-support-interaction-promoted catalyst is at its best electrochemically promoted state. Thus the experimental problem of inducing electrochemical promotion on fully-dispersed catalysts remains an important one, as discussed in the next Chapter. 

I. Material cost minimization The main consideration here is the problem of efficient catalyst material utilization which requires the use of thin (e.g. 10 nm thick) catalyst electrodes or dispersed catalysts.7... 

Figure 12.1, Principle of electrochemical promotion of a finely dispersed catalyst deposited on an electronically conductive material.7,li 15...

Figure 12.3. Principle of electrochemical promotion of a fully dispersed catalyst.

A third approach, not yet fully demonstrated at the limit of dispersed catalysts, is the induction of electrochemical promotion without an intermediate conductive phase (Fig. 12.3). This approach will be discussed in Section 12.3 in relation to the concept of the bipolar design. 

Electrochemical Promotion with Highly Dispersed Catalysts... 

This study, in conjunction with that discussed in 12.2.1.2, show that when using aqueous electrolytes or Nafion saturated with H20, the induction of NEMCA on finely dispersed noble metal catalysts is rather straightforward. The role of the electronically conducting porous C support is only to conduct electrons and to support the finely dispersed catalyst. The promoting species can reach the active catalyst via the electrolyte or via the aqueous film without having to migrate on the surface of the support, as is the case when using ceramic solid electrolytes. 

Catalytic hydrogenation is typically carried out in slurry reactors, where finely dispersed catalyst particles (<100 (tm) are immersed in a dispersion of gas and liquid. It has, however, been demonstrated that continuous operation is possible, either by using trickle bed [24] or monoHth technologies [37]. Elevated pressures and temperatures are needed to have a high enough reaction rate. On the other hand, too high a temperature impairs the selectivity of the desired product, as has been demonstrated by Kuusisto et al. [23]. An overview of some feasible processes and catalysts is shown in Table 8.1. 

This study relates to a continuous process for the preparation of perfluoroalkyl iodides over nanosized metal catalysts in gas phase. The water-alcohol method provided more dispersed catalysts than the impregnation method. The Cu particles of about 20 nm showed enhanced stability and higher activity than the particles larger than 40 nm. This was correlated with the distribution of copper particle sizes shown by XRD and TEM. Compared with silver and zinc, copper is better active and stable metal. 

As can be seen in table 1, with different preparation methods and active metals, the average size of the copper particle for the catalysts A and D were 20.3 nm and 50.0 nm. While those of the catalysts B and C were 51.3 nm and 45.4 run, respectively. CuO, non-supported metal oxide, made by impregnation is sintered and cluster whose particle size was 30 pm. The water-alcohol method provided more dispersed catalysts than the impregnation method. 

XRD analysis of the solid product showed three main peaks at 28.5 , 47.4 and 56.3 , which indicated that pure crystalline CuCl was formed [3]. Several well-known dispersants polyvinyl pyrrolidone (PVP), sodium hexameta phosphate (SHP), the sodium salt of EDTA (EDTA-Na), sodium dodecyl sulfonate (SDS), and sodium dodecyl benzene sulfonate (SDBS), were introduced to obtain a highly dispersed catalyst. The X-ray patterns obtained with these were basically the same as the patterns obtained with the solids prepared in the other experiments described here. 

In ecent years the utility of extended X-ray absorption fine structure UXAFS) as a probe for the study of catalysts has been clearly demonstrated (1-17). Measurements of EXAFS are particularly valuable for very highly dispersed catalysts. Supported metal systems, in which small metal clusters or crystallites are commonly dispersed on a refractory oxide such as alumina or silica, are good examples of such catalysts. The ratio of surface atoms to total atoms in the metal clusters is generally high and may even approach unity in some cases. 

It is also convenient to compare the behavior of different dispersed catalysts with the help of the two following quantities the mass activity, MA, in Ag", and the specific activity, SA, in pA The values ofMA... 

Methods for Preparing Electrodes with Disperse Catalysts... 

Direct Chemical or Electrochemical Deposition of the Disperse Catalyst This method of direct deposition from a solution of its salt on a suitable conducting substrate is simpler and more practical than the preparation of electrodes from the hnished powders. It has the merit of being able to provide better contact between the catalyst and substrate, and multicomponent metal catalysts can be deposited from a solution containing a mixture of salts of several metals. 

Incorporation into a Polymer Layer In recent years a new electrode type is investigated which represents a layer of conducting polymer (such as polyaniline) into which a metal catalyst is incorporated by chemical or electrochemical deposition. In some cases the specific catalytic activity of the platinum crystallites incorporated into the polymer layer was found to be higher than that of ordinary dispersed platinum, probably because of special structural features of the platinum crystallites produced within the polymer matrix. A variant of this approach is that of incorporating the disperse catalyst directly into the surface layer of a solid polymer electrolyte. 

As a rule, the dispersed catalysts are polydisperse (i.e., contain crystallites and/or crystalline aggregates of different sizes and shapes). For particles of irregular shape, the concept of (linear) size is indehnite. For such a particle, the diameter d of a sphere of the same volume or number of metal atoms may serve as a measure of particle size. 

Macrokinetic Limitations in Electrodes with Disperse Catalyst... 

One of the main reasons for a lower specific activity resides in the fact that electrodes with disperse catalysts have a porous structure. In the electrolyte filling the pores, ohmic potential gradients develop and because of slow difiusion, concentration gradients of the reachng species also develop. In the disperse catalysts, additional ohmic losses will occur at the points of contact between the individual crystallites and at their points of contact with the substrate. These effects produce a nonuniform current distribution over the inner surface area of the electrode and a lower overall reaction rate. 

Another factor producing an apparent decrease in the activity of disperse catalysts is steric hindrance. In reactions involving relatively large molecules, not aU of the inner surface area of the catalyst may by accessible for these molecules, so that the true working surface area is smaller than that measured by BET or hydrogen adsorption. 

Apart from these macrokinehc hmitahons, two more effects exist that may influence the overaff efectrochemicaf characteristics of an electrode with disperse catalyst in the posihve or negahve direchon (1) an influence of the crystallite size itself on the intrinsic catalyhc achvity (Section 28.5.4) and (2) an influence of the catalyst substrate (Sechon 28.5.5). These two effects have great importance for the prachcaf use of disperse catalysts and for the theoretical analysis of electrocatalytic effects. 

A quantitative investigation of the influence exerted by a substrate on the properties of disperse catalysts is hampered by the distorting effects of many other factors, particularly the macrokinetic limitations and the size effects mentioned in Section 28.5.4. 

In every case, large particles of metal are more active in oxidation than the smallest ones. CO oxidation is moderately structure-sensitive (less than one order of magnitude between metal foil and much dispersed catalysts). By contrast, propane oxidation (and in general oxidation of small alkanes) are strongly stmcture-sensitive (two orders of magnitude between large and small particles). Rate equations were also expressed as... 

The high sensitivity of the reaction to particle size of Rh is confirmed at 230°C, in a mixture of 0.5% NO+1% CO, the turnover frequency increases from 0.017 s-1 for a highly dispersed catalyst to 0.74 s 1 for a catalyst dispersed at 1.7%, the activity per metal site on unsupported Rh catalysts being still much higher. 

A mild structure sensitivity accompanies hydrogenation of nitrosobenzene over a series of dispersed 1% Pd/Si02.294 The lower dispersed catalysts (larger particle sizes) catalyze faster rates, suggesting that planes are more active than either edges or corners for catalyzing the hydrogenation of nitrosobenzene. 

On Pt, Pd, Ni, and Rh the regioselectivity depends on the conditions. Mainly ester is formed at low hydrogen pressures, whereas mainly diol monoether is formed at higher pressures.30 31 Selectivity is structure-sensitive more ester is formed on highly dispersed catalysts. 

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