Modeling of supported catalysts

Coppens and Froment (1995a, b) employed a fractal pore model of supported catalyst and derived expressions for the pore tortuosity and accessible pore surface area. In the domain of mass transport limitation, the fractal catalyst is more active than a catalyst of smooth uniform pores having similar average properties. Because the Knudsen diffusivity increases with molecular size and decreases with molecular mass, the gas diffusivities of individual species in... 

A. Cimino, D. Gazzoli, and M. Valigi, XPS quantitative analysis and models of supported oxide catalysts, Journal of Electron Spectroscopy and Related Phenomena 104, 1-29(1999). 

A few additional points have also been raised by specific surface-science work concerning the catalytic reduction of NO. For instance, it has been widely recognized that the reaction is sensitive to the structure of the catalytic surface. It was determined that rough surfaces such as (110), or even (100), planes enhance NO dissociation over flatter (111) surfaces, and also favor N2 desorption instead of N20 production. On the other hand, NO dissociation leads to poisoning by the resulting atomic species, hence the faster reaction rates seen with medium-size vs. larger particles on model rhodium supported catalyst (the opposite appears to be true on palladium). Also, at least in the case of palladium, the formation of an isocyanate (-NCO) intermediate was identified... 

For porous catalyst pellets with practical loadings, this quantity is typically much larger than the pellet void fraction e, indicating that the dynamic behavior of supported catalysts il dominated by the relaxation of surface phenomena (e.g., 35, 36). This implies that a quasi-static approximation for Equation (1) (i.e., e = 0) can often be safely invoked in the transient modeling of porous catalyst pellets. The calculations showed that the quasi-static approximation is indeed valid in our case the model predicted virtually the same step responses, even when the value of tp was reduced by a factor of 10. 

Figure 3.9 Models used in the quantitative analysis of XPS spectra of supported catalysts, a) particles on a semi-infinite support b) stratified layer model with cubic particles on sheets of support material as used by Kerkhof and Moulijn [30] c) particles with characteristic dimensions which have the same dispersion, giving nearly the same particle/support intensity ratio in XPS in the randomly oriented layer model according to Kuipers el al. [311.

A noteworthy line of research is the application of TEM on models for supported catalysts. Figure 7.6 shows a side view of Au particles (diameters <6 nm) on top of MgO crystals, taken from the work of Giorgio et al. [16]. The picture beautifully shows the shape of the particles together with the lattice fringes characteristic of certain orientations of the particles and the support. In addition, the authors obtained... 

It has previously been found (3., 11, 18, 31-3 ) that unsupported catalysts exhibit a HDS activity behavior quite similar to that of supported catalysts. This suggests that although the support is of importance, it does not have an essential role for creation of the active phase. Thus, it is very relevant to study unsupported catalysts, both in their own right and also as models for the more e-lusive supported catalysts. Many different explanations have been proposed to explain the similarity in behavior of unsupported and supported catalysts ( 3, 31-3b). Recently, we have observed that for both types of catalysts the HDS activity behavior can be related to the fraction of cobalt atoms present as Co-Mo-S (9-11 35). 

Supported organotransition metal complexes have important applications in the design and modeling of new catalysts. They are of interest in the modification of surface properties of solids, including electrodes, and as stoichiometric reagents. 

In general it should be noted that isothermal models are not strictly isothermal, but would typically display oscillations under nonisothermal conditions as well. In the case of supported catalysts at atmospheric pressure, oscillations are probably never purely isothermal. Usually, however, thermal effects tend to amplify instabilities, so that a model that predicts oscillations for the isothermal case will most probably predict oscillations under nonisothermal conditions. 

To date the surface science approach and techniques such as those described above have been used to study structure of ceria surfaces, the adsorption and desorption of several molecular species on ceria and model ceria supported catalysts, and the co-adsorption and reaction of certain of these molecular species. The results provide a basis for clarifying the elementary reaction steps underlying catalytic processes occurring on ceria based catalysts. In this Chapter it is attempted to review and summarize this research. 

The use of a synthetic model system has provided valuable mechanistic insights into the molecular catalytic mechanism of P-450. Groves et al. [34]. were the first to report cytochrome P-450-type activity in a model system comprising iron meso-tetraphenylporphyrin chloride [(TPP)FeCl] and iodosylbenzene (PhIO) as an oxidant which can oxidize the Fe porphyrin directly to [(TPP)Fe =0] + in a shunt pathway. Thus, (TPP)FeCl and other metalloporphyrins can catalyze the monooxygenation of a variety of substrates by PhIO [35-40], hypochlorite salts [41, 42], p-cyano-A, A -dimethylanihne A -oxide [43-46], percarboxylic acids [47-50] and hydroperoxides [51, 52]. Catalytic activity was, however, rapidly reduced because of the destruction of the metalloporphyrin during the catalytic cycle [34-52]. When (TPP)FeCl was immobilized on the surface of silica or silica-alumina, catalytic reactivity and catalytic lifetime both increased significantly [53]. There have been several reports of supported catalysts based on such metalloporphyrins adsorbed or covalently bound to polymers [54-56]. Catalyst lifetime was also significantly improved by use of iron porphyrins such as mew-tetramesitylporphyrin chloride [(TMP)FeCl] and iron mcA o-tetrakis(2,3,4,5,6-pentafluorophenyl)por-phyrin chloride [(TPFPP)FeCl], which resist oxidative destruction, because of steric and electronic effects and thereby act as efficient catalysts of P-450 type reactions [57-65]. 

AFM and XPS data for model Pd supported catalysts show that the morphology of these catalysts are greatly affected by reduction in hydrogen and by addition of sulfur compounds to the surface... 

R.T. Baker, C.H. Bartholomew, and D.B. Dadybuijor (J.A. Horsley, Editor), Stability of Supported Catalysts Sintering and Redispersion, Catalytic Studies Division, 1991. C.H. Bartholomew, Model Catdyst Studies of Supported Metal Sintering and Redispersion Kinetics, Catalysis, Specialist Periodical Report, Royal Society of Chemistry, Thomas Graham House, Cambridge, UK, Vol. 10, 1992. 

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