Catalyst specific area

The above rate is expressed per unit of external surface. To express the rate per gramme of catalyst the flux has to be multiplied by the catalyst specific area (m surf g J). 

Support name Catalyst Specific area (SBEr)(mVg) Pore volume (mL/g) COD removal (%)... 

In order to compare the various catalysts it is necessary to take into account the catalyst specific area and the amount of carbon black introduced as the pellets are not always compacted exactly alike. Tlierefore, the activity is given as the number of moles of NO converted per unit of surface of catalyst and unit of mass of the solid mixture (catalyst + carbon black). 

Surface Area. Overall catalyst surface area can be determined by the BET method mentioned eadier, but mote specific techniques are requited to determine a catalyst s active surface area. X-ray diffraction techniques can give data from which the average particle si2e and hence the active surface area may be calculated. Or, it may be necessary to find an appropriate gas or Hquid that will adsorb only on the active surface and to measure the extent of adsorption under controUed conditions. In some cases, it maybe possible to measure the products of reaction between a reactive adsorbent and the active site. Radioactively tagged materials are frequentiy usehil in this appHcation. Once a correlation has been estabHshed between either total or active surface area and catalyst performance (particulady activity), it may be possible to use the less costiy method for quaHty assurance purposes. 

Disk holder material Polypropylene Catalyst specific surface area 3.6 0.4 m g ... 

Ru(bipy)3 formed in this reaction is reduced by the sacrificial electron donor sodium ethylenediaminetetra-acetic acid, EDTA. Cat is the colloidal catalyst. With platinum, the quantum yield of hydrogenation was 9.9 x 10 . The yield for C H hydrogenation was much lower. However, it could substantially be improv l by using a Pt colloid which was covered by palladium This example demonstrates that complex colloidal metal catalysts may have specific actions. Bimetalic alloys of high specific area often can prepared by radiolytic reduction of metal ions 3.44) Reactions of oxidizing radicals with colloidal metals have been investigated less thoroughly. OH radicals react with colloidal platinum to form a thin oxide layer which increases the optical absorbance in the UV and protects the colloid from further radical attack. Complexed halide atoms, such as Cl , Br, and I, also react... 

Specific surface areas are then obtained by dividing by the weight of catalyst employed in the experiments in question. It should be pointed out, however, that it is the BET adsorption isotherm that is the basis for conventional determinations of catalyst surface areas. (See Section 6.2.2.)... 

The physical properties of the catalyst (specific surface area, porosity, effective thermal conductivity, effective diffusivity, pellet density, etc.). 

The selected team initially established a specific plan of investigation procedures for this occurrence. This strategy session listed priorities and necessary actions to ensure that all required information was obtained in a prompt manner. Needless delays in evidence collection were avoided by the use of this plan which helped to accelerate the rebuilding/restarting of the catalyst preparation area. 

This observation was not so obvious on coke yields because the coke production is a contribution of mnltiple mechanisms and reactions. Thus, the coke yields are quite similar, probably because the catalytic coke is decreased while the contaminant coke is increased. The coke remarks are also observed on the CPS samples taking into account that the dehydrogenation degree is not strongly affected by the extended ReDox cycles, becanse the lower catalysts decay is limiting the effect of the required mass of catalyst (C/0 ratio). Thus, the small decrement of the coke yield on the CPS samples is possibly related to the descent of the catalyst (less specific area) leaving less available space for coke adsorption and less activity for catalytic coke production. It is clear that prolonging the deactivation procednres is not beneficial as far as the metal effects are concerned. 

In 2005, Rossi and coworkers investigated the effect of the carbon support on the activity of Au/C catalysts in the oxidation of glucose under atmospheric pressure of oxygen at 323 K and a pH of 9.5 [121]. Au nanoparticles (in a range of 1-6 nm) dispersed on carbon XC72R (specific area 254 m g pore volume 0.19 mL g ) were more active than Au nanoparticles dispersed on carbon X40S (specific area 1,100 m g pore volume 0.37 mL g ). However, if the nature of the carbon support played a role on the catalyst activity, authors pointed out that the major role was played by the size of the Au nanoparticles, the smaller particles being the most active. 

One of the important applications of mono- and multimetallic clusters is to be used as catalysts [186]. Their catalytic properties depend on the nature of metal atoms accessible to the reactants at the surface. The possible control through the radiolytic synthesis of the alloying of various metals, all present at the surface, is therefore particularly important for the catalysis of multistep reactions. The role of the size is twofold. It governs the kinetics by the number of active sites, which increase with the specific area. However, the most crucial role is played by the cluster potential, which depends on the nuclearity and controls the thermodynamics, possibly with a threshold. For example, in the catalysis of electron transfer (Fig. 14), the cluster is able to efficiently relay electrons from a donor to an acceptor, provided the potential value is intermediate between those of the reactants [49]. Below or above these two thresholds, the transfer to or from the cluster, respectively, is thermodynamically inhibited and the cluster is unable to act as a relay. The optimum range is adjustable by the size [63]. 

Large particles (diskettes) were formed by applying a spark plasma sintering process to leached catalyst particles. Almost all of the catalyst specific surface area was retained after sintering. This may open a way to manufacture pellet type catalysts or electrodes with high specific surface area. 

Measurements can be made either statically or dynamically and selective gases like CO (or H2) can be used. In the static experiment, a known amount of gas is added to a known amount of metal catalyst. Each gas molecule takes up a specific area on the metal surface (typically the gas is not adsorbed on the inert support surface). In the dynamic experiment, gas is pulsed over a catalyst. The amount of gas remaining in the pulse after contact with the metal is measured and, as before, the gas on the metal surface is calculated by difference to estimate the surface area available for catalysis. 

The value of the turnover frequency can be reproduced in different laboratories, if the method of measurement of the rate and the counting of sites are kept the same. Moreover, the use of turnover frequency allows the comparison between two catalysts that differ in metal or size for a specific reaction. The great advantage of such a comparison is that the activity of different catalysts is compared at active site level without the considerations of catalyst arrangement. To be more specific, using turnover frequency, we can compare the activity of the pure active site, ignoring the specific area of the catalyst. 

Another important aspect of electrocatalysis is the study of dispersed high specific area and supported, both metal and non-metal, electrocatalysts. A high degree of dispersion brings about enhancement of the catalytic activity because of the specific area and energetics of active sites [140] and decrease of susceptibility of poisoning because of the improved ratio of catalyst area to impurities in solution. 

A series of Chromia-Alumina aerogel catalysts containing different contents of chromium was prepared by autoclave method. The specific areas of the catalysis were measured with Ng at 77°K according to the BET method. Their structural properties were determined from the X ray diffraction patterns recorded on a philips diffractometer PW 1050/70. EPR measurements were performed with a 8ruker ZOO TT spectrometer at 77°K operating in X band. DPPH was used as the g value standard. Kinetic data were obtained in dynamic pyrex microreactor operating at atmospheric pressure as described elsewhere (ref. 3). 

Required properties of the catalyst carriers are high specific area, low pressure drop and high mechanical resistance at temperatures up to 1000°C (1830°F). The catalysts are usually in the form of rings (e.g., outer diameter 16 mm, height 16 mm, inner diameter 8 mm) but other forms, such as saddles, stars, and spoked wheels are also commercially available. 

The important properties of small particles are defined in Figure 1.3. For unsupported catalysts and for support materials, it is necessary to know how much material is exposed to the gas phase. This property is expressed by the specific area, in units of m2 g 1. Typical supports such as silica and alumina have specific areas on the order of 200 to 300 m2 g-1, while active carbons may have specific areas of up to 1000 m2 g 1, or more. Unsupported catalysts have much lower surface areas, typically in the range of 1 to 50 m2 g 1. 

Fig. 1.3 Specific area and dispersion are important characteristic properties of a supported catalyst.

---