Catalysts, general support effect

As is shown in Figure 6 (experiments) and Table 4 (Monte-Carlo analysis), a general trend is that Pt catalysts with supports of higher acidity lead to a higher contribution of the a-T)1 (Dl) and di-o-T)2 (D2) intermediates. As the ASA and LTL supports have similar metal particle sizes, this cannot be explained by particle size effects. Apparently, acidic supports enhance... 

Among the more important catalysts are metals, which may be promoted by other metals, or by oxides and oxides, which are usually rendered more effective by mixing with other oxides. It is usual to distinguish between supported catalysts, generally metals in a finely divided condition on the surface of silicate minerals, and promoted catalysts, where an oxide, or occasionally some other compound, is mixed with the metal the mixture being sometimes also supported on an inert refractory support. The distinction is not, however, absolutely sharp. 

Since the formulation of his concepts in 1939 Kobozev has carried out many investigations in the attempt to verify their general validity. In the decomposition of H202 and the oxidation of Na2SCh, the specific effect (173) of small amounts of iron (0.0005 to 40%) added to copper on carbon and the converse in which small amounts of copper were added to iron on carbon were studied. The activity of these catalysts was very effectively promoted by these additives, the extent of promotion being proportional to the concentration of the additives. The catalytic synthesis of ammonia by iron supported on carbon or asbestos was also studied. The results of this study and similar studies of catalytic... 

Examples of the hydrogenation of various functional groups and reaction pathways are illustrated in numerous equations and schemes in order to help the reader easily understand the reactions. In general, the reactions labeled as equations are described with experimental details to enable the user to choose a pertinent catalyst in a proper ratio to the substrate, a suitable solvent, and suitable reaction conditions for hydrogenation to be completed within a reasonable time. The reactions labeled as schemes will be helpful for better understanding reaction pathways as well as the selectivity of catalysts, although the difference between equations and schemes is not strict. Simple reactions are sometimes described in equations without experimental details. Comparable data are included in more than 100 tables, and will help the user understand the effects of various factors on the rate and/or selectivity, including the structure of compounds, the nature of catalysts and supports, and the nature of solvents and additives. A considerable number of experimental results not yet published by the author and coworkers can be found in this Handbook. 

Various supported platinum group metal systems have been tested for the SCR process.101 Among them, supported platinum systems appear to be the most active when jointly considering the NOx reduction level achieved and the temperature range at which the catalyst is active, while palladium, rhodium and iridium also show catalytic activity for the process and Rh and Ir apparently present higher selectivity to N2.101>i03-i07 Support effects are observed which generally depend on the type of hydrocarbon employed, the presence or absence of SO2 in the reactant mixture or the type of impurities present in the support.101 In this respect, a variety of materials like SiC>2, AI2O3, ZrC>2, sulphated alumina, zeolitic materials and activated carbons have been employed as supports of the metals and tested for the process.101-112... 

In general, the effect of coke and poisons cannot be cleanly separated. On zeolite catalysts, trace amounts of heavy metal poisons such as V and Ni not only decrease the number of available active sites, they also increase the amount of coke formed. On supported metal catalysts, some poisons such as sulfur can decrease the amount of coke formed, as well as its distribution between metal and surface. 

Apart Ifom the molybdenum carbene complexes already listed in Tables 2.1 and 2.2 Mo-based catalysts are of three main types (i) other Mo complexes, activated by a suitable cocatalyst (ii) M0CI5, also activated by a cocatalyst and (iii) supported oxides, generated in various ways. For the metathesis of terminal olefins higher than propene. Mo-based catalysts are generally more effective than the corresponding W-based catalysts. 

The amount of hydrogen in relation of the estequiometric quantities of Ni to Ni and reduced to V is very interesting. These results are presented on Table 1. For 1-Ni catalyst this quantity is twice that required by the estequiometric. The amount of hydrogen consumption progressively decrease with the increase of vanadium concentration. This effect generally supports the idea of the nickel vanadium interaction. 

Chapter 4 contains the background of the development of effective modified Ni catalysts, discusses the methods of preparation of different types of stable and active metal catalysts, and discusses the selection of effective modifiers and the most suitable substrate molecules having practical interests. On the basis of these studies a reaction mechanism for the new effective catalytic systems was suggested and experimentally examined. The Chapter discusses the preparation variables for the development of this new type of effective chiral modified Ni catalyst, the supported metal catalysts, the chiral modified bimetal and multimetal catalysts including rare earth metals, and the new chiral modified nickel-ruthenium and palladium catalysts. Attempts are undertaken to elucidate the mechanism of enantioselectivity and to reveal the general regularities of asymmetric actions. 

In general, supported metal catalysts are less effective than Raney metal modified catalysts, and the enantioselectivities of supported catalysts are near to those of the Raney catalysts only in the cases when large amounts of metal are found on the surface of the support This can be explained, at least in the cases of bulk metal catalysts, as a consequence of an increase in crystallite sizes and diffusion of the tartaric acid modifier into the pores during modification of the catalyst (Sachtler " ). Detailed consideration of this problem is in discussed in Chapter 5. 

---