Clusters catalyst

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). 

Trends in the electronic structure of the chalcogenide catalysts have proved to be helpful in the design and understanding of the catalyst clusters. During ORR, the molecular oxygen has been found to react with the cluster as a whole, rather than individual metal atoms.177 The overall number of electrons per cluster unit (NEC) in the valence bond has been shown to have a factor in the activity and stability of the cluster catalysts.177,181 The unsubstituted Chevrel phases have a NEC of 20.177,181 Substituting or intercalating other transition metals into the crystal lattice to make ternary or pseudo-binary Chevrel phases allows for the increase of NEC. It has been found that as the NEC approaches 24, the catalytic activity improves.181 Alonso-Vante compiled the results from his previous studies to show the effect of NEC in... 

However, the technique suffers the drawback that in real catalyst systems, particles are randomly distributed over the support and thus will not all be in the correct orientation for diffraction into the angular collection range subtended by the objective aperture, and that the smaller clusters diffract too weakly to be detected against the support. A further difficulty is that random superposition of atoms in amorphous support materials, such as charcoal or silica can give rise to "speckle" which may be easily confused with small catalyst clusters. (16,13). 

Scattering from Thin Crystals. The simple arguments given above hold only if samples are amorphous. Although some catalyst supports are non-crystalline, such as charcoal and silica, others such as alumina are not. Furthermore, the metal catalyst clusters themselves are generally crystalline and thus the above arguments must be modified to account for Bragg reflections from crystalline areas. 

Molecular precursors for tailored metal catalysts, 38 283-392 see also Bimetallic catalysts, cluster-derived Zeolites carbon-supported, 38 389-390 chemical interaction between clusters and supports, 38 295-296... 

HDS catalysts have been characterized extensively with a wide variety of tools, and several extensive reviews of the subject have been presented (85,88-91). Substantial effort has been aimed at relating catalytic activity and selectivity to microscopic properties such as catalyst composition, electronic structure, and geometric structure. EXAFS investigations of working catalysts have provided information about the composition, average local coordination, and interatomic distances of atoms in the catalyst clusters. It has been concluded that the active phase under operating conditions is MoS2-like particles with a dimension of 10—20 A (92-94). 

Recently, a laser ablation-condensation technique was used to produce nanometer-sized catalyst clusters to grow nanowires by the VLS method. A schematic of the laser ablation apparatus used by Morales and Lieber (1998) to produce silicon nanowires is shown in Fig. 11. The target consists of silicon and the catalyst material (e.g., Sii AFeA), and a pulsed laser is used to produce nanometer-sized catalyst clusters within a reaction chamber at 1200°C. The ablated materials are carried by an argon gas flow, and the... 

Properties of supported catalysts by bimetallic substrates depend on the changes in geometry of the catalyst material by the strain of the substrate. Using a bimetallic substrate multiphes the possibilities to tune the catalyst to specific requirements. The chemistry of the nanosized overlayer is affected by the different orbital overlaps of atoms from the catalyst cluster and those from the substrate. Additionally, small supported metallic islands show low coordination and reduced near-neighbor distances thus their chemical properties are different with respect to those of flat surfaces. " Reactivity of several bimetallics were also studied by Balbuena et al., including bimetallics systems . Norskov et al. found several relations for the bimetallic systems considering local and nonlocal effects have also been reported. ... 

We consider now highly divided (FE > 0.5) supported metals, which may be model or actual working catalysts. Clusters supported by frozen inert gas matrices are not discussed here. Possible interactions with the support now complicate the interpretation of experiments, but we shall see that for silica and alumina the behavior of supported and unsupported clusters seem to be similar. 

On irregular surfaces with high defect concentrations (e.g. catalysts, clusters), H is preferentially adsorbed at sites of high coordination and near steps [34]. This leads to an adsorption energy which may be initially several kJ (mol H) higher and shows that H absorption can be used as a probe for surface defects. 

An early stimulus to cluster chemistry was the cluster-surface analogy which proposed that cluster chemistry would resemble the surface chemistry of metals, because both surfaces and clusters consist of arrays of metal atoms. Supported metals such as Pd/C are very active catalysts. Clusters have so far not shown the high catalytic activity of either metal surfaces or mononuclear homogeneous catalysts, probably because clusters are poisoned by the pres-... 

The other catalyst supports as a nonstoichiometric mixture of several titanium oxide phases that have hypo-d-electron character have an ability to interact with inert noble metals, like platinum, changing additionally the catalytic activity of the platinum as catalyst. However, very low specific surface area (the maximum value referred is 15 m g ) prevents high dispersion and good compositional homogeneity of the catalyst clusters which is an important request for the activity of the catalyst. 

Watanabe M, Makita K, Usami H, Motoo S (1986) New preparation method of a high performance gas diffusion electrode working at 100 % utilization of catalyst clusters and analysis of the reaction layer. J Electroanal Chem 197 195-208... 

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