Activity, catalyst, definition

Attention should be paid to the fact that the ratio of Pd and phosphine ligand in active catalysts is crucial for determining the reaction paths. It is believed that dba is displaced completely with phosphines when Pd2(dba)3 is mixed with phosphines in solution. However the displacement is not eom-plcte[16]. Also, it should be considered that dba itself is a monodentate alkene ligand, and it may inhibit the coordination of a sterically hindered olefinic bond in substrates. In such a case, no reaction takes place, and it is recommended to prepare Pd(0) catalysts by the reaction of Pd(OAc)2 with a definite amount of phosphinesflO]. In this way a coordinatively unsaturated Pd(0) catalyst can be generated. Preparation of Pd3(tbaa)3 tbaa == tribenzylidene-acetylacetone) was reported[17], but the complex actually obtained was Pd(dba)2[l8],... 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

For the cases where gas phase cyclohexenes do not appear to be intermediates, the question arises as to the nature of the surface reaction. Thus, does cyclohexane simultaneously lose six hydrogen atoms via the sextet mechanism (T3) originally proposed by Balandin in 1929, or does the reaction take place in a stepwise fashion without desorption of intermediate products According to the sextet theory, the active catalyst unit is an aggregate of metal atoms which must be spaced within certain definite limits consistent with the geometry of the cyclohexane ring. While there... 

These mixed metal systems have also been tested with the transient method for catalytic activity in the Fischer-Tropsch reaction. We would like to remark here that the nature of the cation, anion, and zeolite are all important factors in the Fischer-Tropsch reactions that we have studied. Further details of these catalytic studies can be found elsewhere (23). We do observe here, however, that some catalysts that are completely reduced to the metallic state are not necessarily the most active catalysts. Also, even though the Mossbauer experiments suggest that 400°C is sufficient for complete reduction, higher activation temperatures can increase the activity and selectivity of these reactions. We have also observed that the cation definitely changes the product distribution and the activity. 

This expression also differs from the text book definition. Hence, the observed narrowing of polydispersity and reduction of observed catalytic activity in CCT under high concentrations of an active catalyst has, at least in part, kinetic origins. 

For such large intraparticle diffusion has a large effect on the rate. Practically these conditions mean that diffusion into the pellet is relatively slow, so that reaction occurs before the reactant has diffused far into the pellet. In fact, an alternate definition for is the fraction of the whole surface that is as active as the external surface. If 1, Eq. (11-43) shows that the rate for the whole pellet is the same as the rate if all the surface were available to reactant at concentration Q i.e., the rate at the center is the same as the rate at the outer surface—all the surface is fully effective. In this special case the concentration profile shown in Fig. 11-6 would be horizontal, with C = Cj. In contrast, if 77 1, only the surface near the outer periphery of the pellet is effective the concentration drops from Q to nearly zero in a narrow region near r, e, or high Ar. The latter factor shows that low effectiveness factors are more likely with a very active catalyst. Thus the more effective the active catalyst, the more likely it is that intrapellet diffusion resistance will reduce the rate per pellet. 

It has been established from these studies that the different catalytic properties of transition metal oxides (chromium, cobalt) on zirconium dioxide are attributed to their different acidic properties determined by TPDA and IR-spectroscopy. The most active catalyst is characterized by strong acidic Bronsted centers. The cobalt oxide deposited by precipitation on the zirconium-containing pentasils has a considerable oxidative activity in the reaction N0+02 N02, and for SCR-activity the definite surface acidity is necessary for methane activation. Among the binary systems, 10% CoO/(65% H-Zeolite - 35% Z1O2)... 

In the formation of valuable oxygen-containing compounds by the controlled or partial oxidation of hydrocarbons, such as benzene, two factors are of great importance, i.e., temperature and type of catahst. Other factors such as composition of hydrocarbon-air mixture and time of contact are also important. All of these factors are intimately related to each other and the successful operation of the process depends upon the control of each of them. It was early found that if mixtures of benzene apor and air in excess of tliat necessary for complete combustion were passed through heated tubes of such non-catalytic materials as iron, silica, aluminum, etc., and the temperature allowed to rise at will, only complete combustion products could be obtained, and no intermediate oxidation products could be isolated. On the other hand, if such mixtures of benzene vapor and air were passed over a catalyst such as platinum black, complete combustion also occurred but at a temperature far below that necessary in the empty tube made of non-catalytic material. However, only very small amounts of intermediate products could be obtained with such an active catalyst even when the temperatures were carefully controlled or the time of contact made very short. It is difficult to form any definite idea as to the temperatures which were actually attained by the reacting gases in most of the early experiments reported in the literature. Lack of uniformity in construction of reaction chambers, in displacement of cata-... 

Rh complexes of composition [Rh(nbd)(33)2]+X containing the phosphonium phosphines 33 (PHOPHOS II, III, VI, X) have been shown to be very active catalysts for the biphasic hydrogenation of n-hexene and maleic acid in water [103, 110]. A definite chain length effect was observed, the system where n = 6 being the most active. The biphasic systems containing longer chain ligands are not well behaved as catalysts since they are prone to formation of stable emulsions which are quite difficult to break. 

In our search for highly active catalysts effecting light-induced water decomposition, we have found that Pt particles supported by mineral carriers give optimal results. The fastest rate so far observed is k = 3 x llFs-1 at 10-4M Pt. In this case colloidal Ti02 served as a carrier. The MV+ reaction with Pt under these conditions is definitely much more rapid than the back reaction with oxidized sensitizer. Using Eq. (7.26) one can now calculate the size of the Pt cluster corresponding to such a rate... 

In all respects the relations are similar to those considered in the section concerning an active catalyst, but now there should be not one but several broken lines of the reactions and the same number of secant lines on Fig. 43. For the concrete definition let us analyze in Fig. 44a the dehydrogenation (I) and dehydration (II) of an alcohol on Ni where all the values are known (Section II,D). Therefore, Fig. 44a... 

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