Evaluating Support Effects

Another potential advantage of the dendrimer route is the possibility of evaluating support effects. Because DENs are prepared ex situ from a support, it is possible to 

FIGURE 7.10. Selective catalytic reduction of NO with propylene by Pt32/Si02, Pti6Auig/Si02, and AU32/Si02. 

CO oxidation catalysis showed that, for all the supports, the bimetallic catalyst was more active at low temperatures than the corresponing monometallic and cometallic catalysts. Apparent activation energies for monometallic Pt and Au catalysts were very consistent, near 32 and 80 kJ/mole, respectively. The s)uiergism for PtieAuie catalysts also shows up in the apparent activation energies for these catalysts, which were consistently around 23 kJ/mole. 

Using the dendrimer route, it is possible to prepare supported catalysts not available via traditional routes. Dendrimer derived Pt-Au catalysts having compositions within the bulk miscibility gap can be prepared on several oxide supports. For all the supports studied, the bimetallic catalysts exhibited synergism with respect to mono- and cometallic catalysts for the CO oxidation and hydrocarbon NOx SCR reactions. The bimetallic Pt-Au catalysts also showed evidence of exchanging surface and subsurface atoms in response to strongly binding ligands such as CO. 

BDC and HL gratefully acknowledge the Robert A. Welch Foundation (Grant number W-1552) for financial support of this work. Acknowledgement is made to the donors for the American Chemical Society Petroleum Research Fund and to Research Corporation for partial support of this research. We are very grateful to Stephen Maldonado and Prof. Keith Stevenson at the University of Texas at Austin for their assistance in obtaining TEM data. We also thank Samuel Deutsch and Prof. Michael Amiridis at the University of South Carolina for performing the NOx experiments. 

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The software for the PC-based algorithm evaluating the effect of varying TI on unavailability is transferable by PC disk. The software required to support the analysis is Lotus 1-2-3. One hypothetical set of parameters for the model is included on the disk. 

The NACE Landrum Wheel velocity test, originally TM0270-72, is typical of several mechanical-action immersion test methods to evaluate the effects of corrosion. Unfortunately, these laboratory simulation techniques did not consider the fluid mechanics of the environment or metal interface, and service experience very seldom supports the test... 

Because the size regime of n=l-6 atoms is of great practical significance to the spectroscopic, chemical and catalytic properties of supported metal clusters in both weakly and strongly interacting environments (28), it is important to study very small metal clusters in various types of substrate as well as in the gas phase. In this way, one can hope to develop a scale of metal cluster-support effects (guest-host interactions) and evaluate the role that they play in diverse technological phenomena. 

Reaction forces and moments to be used in design of restraints and supports for a piping system, and in evaluating the effects of piping displacements on connected equipment, shall be based on the reaction range,... 

Many trials have evaluated the potential for (3-blocker therapy in patients with heart failure. The rationale is based on the hypothesis that excessive tachycardia and adverse effects of high catecholamine levels on the heart contribute to the downward course of heart failure patients. However, such therapy must be initiated very cautiously at low doses, since acutely blocking the supportive effects of catecholamines can worsen heart failure. Several months of therapy may be required before improvement is noted this usually consists of a slight rise in ejection fraction, slower heart rate, and reduction in symptoms. As noted above, bisoprolol, carvedilol, and metoprolol have been shown to reduce mortality. 

Julg et al.24 employed an approximation based on the self consistent field molecular orbital method to evaluate the average energy per atom for various structures. They calculate that whereas the normal b.c.c. structure is more stable for clusters containing more than 106 atoms, smaller clusters prefer to take up pentagonal symmetry. However, these authors make an important point, namely, that the calculated energies for different structures are very similar. Interconversion of different structures will be facile, and external factors such as the method of deposition, level of impurities, support effects, etc., may cause the less stable structure to grow. For example, impurities on... 

The efficiency and selectivity of a supported metal catalyst is closely related to the dispersion and particle size of the metal component and to the nature of the interaction between the metal and the support. For a particular metal, catalytic activity may be varied by changing the metal dispersion and the support thus, the method of synthesis and any pre-treatment of the catalyst is important in the overall process of catalyst evaluation. Supported metal catalysts have traditionally been prepared by impregnation techniques that involve treatment of a support with an aqueous solution of a metal salt followed by calcination (4). In the Fe/ZSM-5 system, the decomposition of the iron nitrate during calcination produces a-Fe2(>3 of relatively large crystallite size (>100 X). This study was initiated in an attempt to produce highly-dispersed, thermally stable supported metal catalysts that are effective for synthesis gas conversion. The carbonyl Fe3(CO) was used as the source of iron the supports used were the acidic zeolites ZSM-5 and mordenite and the non-acidic, larger pore zeolite, 13X. 

Catalytic Evaluation In order to investigate support effects in these iron/zeolite catalysts prepared from Fe3(C0)12 by the extraction technique, three catalysts of similar weight percent iron loading were evaluated for their ability to catalyze synthesis gas conversion these catalysts were 15.0% Fe/ZSM-5, 16.4% Fe/Mordenite andl5.0% Fe/13X. All catalysts were evaluated under similar conditions as described in the experimental section. Catalytic data is presented in the accompanying figures in each figure the first three points for each catalyst are data obtained at 280°C, the second three points are data at 300°C. 

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