Catalysts activity site distribution

Due to their multi-sited nature, Ziegler-Natta and chromium catalysts produce structurally heterogeneous ethylene homo- and copolymers. This means that the polymers have broad MWD and broad composition (short-chain branching) distribution (Fig. 9). Catalyst active sites that produce lower molecular weights also have a tendency to incorporate more comonomer... 

Catalyst Structural Characteristics. Structural features of AFS and USY materials have been characterized in this work in terms of unit cell size, presence of extraframework material, active-site distributions, and pore-size distributions. These features are similar for both sets of USY and AFS samples which indicates that structural characteristics are not related to the source of Y zeolite. 

Q. To proceed further at this point one has to specify a pore model for the catalyst, and a model for the active site distribution. Froment and co-workers have examined a variety of cases such as single pore models (single-ended pores and pores open on both sides) with both deterministic and stochastic active site distributions, the bundle of parallel pores model and various tree-like models of the porous structure, which were earlier used by Pismen (40) to describe transport and reaction in porous systems. Such treelike models contain interconnected pores but lack any closed loops and are usually called Bethe networks or lattices. They are completely characterized by their coordination number Z, which is the number of pores connected to the same site of the network. 

The olefin readsorption and CO transport models suggest the types of catalyst structures and active site distributions that lead to optimum values of C5+ selectivity in FT synthesis reactions (Fig. 20). The value of the structural parameter x required to give this optimum selectivity depends on the... 

Another method used to dampen the catalyst activity in the fluidized-bed process is to deliberately add poisons to the reactor, such as 02 in small amounts. It was discovered that these poisons sometimes cause a broadening or narrowing of the MW distribution of the polymer, because they also affect the active site distribution on the catalyst. 02, for example, tends to increase polymer MI and broaden the MW distribution, which makes the polymer more shear-thinning. Consequently, poisons are sometimes intentionally added to manipulate polymer properties in this process. 

The phosphorus deactivation curve is typical type C, and, according to the Wheeler model, this is associated with selective poisoning of pore mouths. Phosphorus distribution on the poisoned catalyst is near the gas-solid interface, i.e. at pore mouths, which confirms the Wheeler model of pore mouth poisoning for type C deactivation curves. Thus we may propose that in the fast oxidative reactions with which we are dealing, transport processes within pores will control the effectiveness of the catalyst. Active sites at the gas-solid interface will be controlled by relatively fast bulk diffusional processes, whereas active sites within pores of 20-100 A present in the washcoat aluminas on which the platinum is deposited will be controlled by the slower Knudsen diffusion process. Thus phosphorus poisoning of active sites at pore mouths will result in a serious loss in catalyst activity since reactant molecules must diffuse deeper into the pore structure by the slower Knudsen mass transport process to find progressively fewer active sites. 

Note that a near to similar ratio of rapidly and slowly exchangeable sulfur was determined when the catalyst was sulfided originally with thiophene instead of elemental irrespective of the 2.5 times lower S content of the sample, sulfided by thiophene. This indicates the similarity of active site distribution for catalysts of equal chemical composition. 

Site heterogeneity was investigated using a deconvolution technique based on inverse Laplace transform (ILT) method. As could be seen from the active site distribution shown in Fig. 11, the unpromoted catalyst had essentially only one kind of site (3). K-promotion increased the average... 

Fig. 11. Active site distribution for unpromoted and K promoted Ru/Si02 catalyst for ammonia synthesis. I ]...

Nature of Catalyst Active Site and Intermolecular Distribution of Molecular... 

The result above is obtained under strictly controlled conditions in laboratory, but the single surface of a-iron and fully exposed a-iron (111) surface are hard to get during the preparation and reduction of catalysts. Even more important, because of the existence of promoter, 50% or more surface of a-iron is covered by K2O or KOH and the structure of active phase changes because of the addition of the promoter, and even the active site distribution or active order on the crystal plane of a-Fe are greatly changed. As a result, the impact of microstructure changes on the mechanism of activity is worth exploring. 

The propane reaction is very exothermic and its enthalpy is even higher than both enthalpy steps added from the propylene process, since alkane dehydrogenation must be included in the former process. The propane reaction takes place via an eight-electron transfer, requiring a specific catalyst structure on which an adequate isolated active site distribution exists, in order to carry out the coordinated steps. Moreover, an appropriate element redox balance must be present to complete the catal3dic oxidation-reduction cycle, including in situ catalyst regeneration. 

The computational studies on surface chemistry of Co catalysts have offered significant supports to the investigation of FTS on Co catalysts. However, the work is far from decent. As the experimental studies indicated, surface reconstruction and phase transition were certain to take place under practical FTS conditions. The theoretical studies about surface reconstruction and phase transition of Co catalysts, however, are fairly rare. In addition, cluster models were less studied in the previous theoretical work compared to slab models. However, practical catalytic reactions do not always happen as proposed in ideal plane models, nor the active sites distribute homogeneously on the surface. The investigation on cluster models is acting a crucial role in the study of heterogeneous catalysis. Accordingly, more considerations on surface reconstruction, phase transition, and cluster models should be taken into account in future work. 

Aluminum distribution in zeolites is also important to the catalytic activity. An inbalance in charge between the silicon atoms in the zeolite framework creates active sites, which determine the predominant reactivity and selectivity of FCC catalyst. Selectivity and octane performance are correlated with unit cell size, which in turn can be correlated with the number of aluminum atoms in the zeolite framework. ... 

Influence of Metal Particle Size in Nickel-on-Aerosil Catalysts on Surface Site Distribution, Catalytic Activity, and Selectivity... 

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