Most active reaction intermediates

IN SSITKA, Np and the mean surface residence time of these most active reaction intermediates (xp) are determined. After a step-change between two reactant streams containing different isotopes of a reactant without disturbing other reaction conditions or reaction (as long as an H2/D2 switch is not used), the distributions of isotopically labeled products are monitored using a mass spectrometer. Tp is first determined by integration of the normalized isotopic transient of a product relative to an inert tracer (usually Ar) that delineates gas phase hold-up (see Figure 1 for the case of methanation). Np is then calculated from... 

A dramatic reduction of undesirable catalysis by-products requires higher catalytic selectivities, which at present are mostly attainable only with en me like catalysts or by solid catalysts with nearly uniform active sites. This latter catalyst characteristic is specially important for catalytic transformations involving strongly activated reaction intermediates, such as the carbocations formed on strong and super acids. Namely, in this activated state competing reaction paths are likely. Clearly, catalysts with uniform or at least nearly uniform active sites should be important objectives for future catalysis. 

MASI) or most abundant reaction intermediate (MARI) are terms to denote the (reactive) intermediate, which is present in the highest concentration on the surface. If the concentration of the MASI/MARI sufficiently exceeds that of the other surface species, simplified rate expressions can be obtained.)(13,14,23). As a result of this microscopic vuiderstanding of the macroscopic phenomena, that is, the observed catalytic activity and selectivity, a more rational design of new catalysts becomes possible, provided that relations can be established between the catalyst synthesis procedure and the surface phenomena on the catalyst (see Fig. 8). 

Using a Langmuir equation Eq. (74), i.e. Nads being the most abundant reaction intermediate. Brill obtained a good fit to the data and calculated the activation energy on the doubly promoted catalyst to be 17.4 kcal/mol. 

Pyrrole can be reduced catalyticaHy to pyrroHdine over a variety of metal catalysts, ie, Pt, Pd, Rh, and Ni. Of these, rhodium on alumina is one of the most active. Less active reducing agents have been used to produce the intermediate 3-pyrroline (36). The 2-pyrrolines are ordinarily obtained by ring-closure reactions. Nonaromatic pyrrolines can be reduced easily with to pyrroHdines. 

Previous reports on FMSZ catalysts have indicated that, in the absence of added H2, the isomerization activity exhibited a typical pattern when measured as a function of time on stream [8, 9], In all cases, the initial activity was very low, but as the reaction proceeded, the conversion slowly increased, reached a maximum, and then started to decrease. In a recent paper [7], we described the time evolution in terms of a simple mathematical model that includes induction and deactivation periods This model predicts the existence of two types of sites with different reactivity and stability. One type of site was responsible for most of the activity observed during the first few minutes on stream, but it rapidly deactivated. For the second type of site, both, the induction and deactivation processes, were significantly slower We proposed that the observed induction periods were due to the formation and accumulation of reaction intermediates that participate in the inter-molecular step described above. Here, we present new evidence to support this hypothesis for the particular case of Ni-promoted catalysts. 

Attempts to determine how the activity of the catalyst (or the selectivity which is, in a rough approximation, the ratio of reaction rates) depends upon the metal particle size have been undertaken for many decades. In 1962, one of the most important figures in catalysis research, M. Boudart, proposed a definition for structure sensitivity [4,5]. A heterogeneously catalyzed reaction is considered to be structure sensitive if its rate, referred to the number of active sites and, thus, expressed as turnover-frequency (TOF), depends on the particle size of the active component or a specific crystallographic orientation of the exposed catalyst surface. Boudart later expanded this model proposing that structure sensitivity is related to the number of (metal surface) atoms to which a crucial reaction intermediate is bound [6]. 

Professor Stewart Trippett is an initiator and has led the development of modern organophosphorus chemistry. He is also a founder and the original Senior Reporter of this Report and we were all concerned to hear of his recent illness. All his friends and colleagues wish him well. It is appropriate and significant that the areas where he has contributed so much, those of the Wittig reaction and hypervalent intermediates, are still among the most actively investigated topics. 

Ammonia oxidation catalysts (sometimes called slip catalyst) are conventional oxidation catalysts based on precious metals. The most active types are based on Pt. Then-activity is strongly dependent on the temperature and, thus, relatively large catalyst volumes are required for the ammonia oxidation below 250°C. At rising temperatures, their oxidation power increases and this leads to the formation of N20 and NO. Especially undesired is their strong tendency to form N20 at intermediate temperatures (250-300°C) [2] if the gas coming from the SCR catalyst also contains unreacted NO, which allows for the reaction ... 

The carbodiimide of choice used to couple cystamine to carboxylate- or phosphate-containing molecules is most often the water-soluble carbodiimide, EDC hydrochloride Chapter 3, Section 1.1). This reagent rapidly reacts with carboxylates or phosphates to form an active ester intermediate, which is highly reactive toward primary amines. The reaction is efficient from pH 4.7 to 7.5, and a variety of buffers may be used, providing they don t contain competing groups. 

The proposed mechanism of the hydrovinylation is supported by available evidences, but so far no study has established clearly all the reaction intermediates. RajanBabu has proposed a mechanism of nickel-catalyzed hydrovinylation, which seems to be one of the most efficient processes, involving a cationic nickel hydride species 144 complexed with a weakly coordinated counterion (Scheme 40). The active catalyst species can be generated through... 

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