Synergistic catalytic effect

Recently, effective eatalytic amidation has been examined at redueed re-aetion temperatures. Tertiary amine-eontaining boronic acid 16 catalyses several amidation reaetions in fluorobenzene under reflux (85 °C). In a recent study, 3,4,5-trifluorophenylboronic acid (13) and o-nitrophenyl-boronic acid (19) were found to be the best catalysts at low temperature (65-68 °C) for amidations as well as peptide and dipeptide formation (though higher catalyst loadings were required for slower dipeptide formations). This method conserved the enantiopurity of the ehiral starting material in most cases. An interesting synergistic catalytic effect... 

In 1965, synergistic (nonadditive) catalytic effects were discovered in elecho-chemical reactions. It was shown in particular that the electrochemical oxidation of methanol on a combined platinum-ruthenium catalyst will occur with rates two to three orders of magnimde higher than at pure platinum even though pure ruthenium is catalytically altogether inactive. 

Results, again, show net gain over the same cracking catalyst used previously for cerium/alumina case. Cerium, moreover, seems to act as a promotor for other rare earths as could be implied from the synergistic effect observed between cerium and lanthanum (27). Our conclusions about the catalytic effect of cerium have been confirmed recently by others (28). 

Synergistic Promotion Effects. As was already mentioned promoter elements are not considered themselves to be catalytically active, but it is fair to say that this is not always the case. This promoter activity may indirectly affect the behaviour of the catalytic active element since it will alter, e.g., the local feed composition or may, due to its catalytic properties, influence the overall reaction product distribution. The following effects, illustrated in Figure 5, are expected to occur in a promoted Co F-T catalyst. 

It is far from easy to distinguish structural, electronic and synergistic promotion effects. Structural promotion is, in this respect, the most easily to observe. Most synergistic elfects are also widely discussed in the literature in enhancing the catalytic performance of supported cobalt nanoparticles. Instead, promotion as a result of electronic effects are much more difficult to detect. The main reason is that one has to discriminate between the number of surface cobalt sites and the intrinsic activity of a surface cobalt site (turnover frequency). This is especially difficult in view of the complexity of the catalyst material. It also requires spectroscopic tools, which are able to detect changes in the electronic structure of the supported cobalt nanoparticles. 

Catalysed substitution reactions of an unusual kind are collected together in this section. In each case, the catalysis of the reaction by a homogeneous entity is assisted by the surface of a solid. The resulting reinforcement of catalytic effects is frequently described as synergistic. The homogeneous and heterogeneous catalysts quite often possess a species in common, for example Ag+ ions and solid Agl, and many of the homogeneously catalysed reactions exhibit autocatalysis as a result. 

Similar effects were demonstrated by them for palladium and ruthenium in the reduction of pyridine, and for ruthenium and platinum in the reduction of nitriles and of nitrobenzenes. Since they show similar although not as large synergistic effects when the two metals are introduced on separate carbon particles, one may be sure that the enhancement is not an intrinsic property of some chemical combination or contact between the different metals. As the authors point out, the various metals are known to have differing catalytic effectiveness for the reduction of different functional groups. Thus, when reduction takes place via more than one chemical step, such as in the case... 

Often multicomponent catalyst systems are utilized to carry out reactions consisting of two or more active metal components or both oxide and metal constituents. For example, a Pt-Rh catalyst facilitates the removal of pollutants from car exhausts. Platinum is very effective for oxidizing unburned hydrocarbons and CO to H2O and CO2, and rhodium is very efficient in reducing NO to N2, even in the same oxidizing environment. Dual functional or multifunctional catalysts are frequently used to carry out complex chemical reactions. In this circumstance the various catalyst components should not be thought of as additives, since they are independently responsible for different catalytic activity. Often there are synergistic effects, however, whereby the various components beneficially influence each other s catalytic activity to provide a combined additive and multifunctional catalytic effects. 

The previous sections indicated that at present PtRu remains the most effective binary catalyst for methanol oxidation. A significant amount of work has been carried out and various theoretical and experimental techniques have been brought to bear in order to reveal the details of the Pt-Ru catalytic/co-catalytic effect. For DMFC performance enhancement, which is the prevalent point of view adopted in this review, the situation is further complicated since in addition to intrinsic kinetic effects, the anode performance depends in a synergistic and often poorly understood manner on the PtRu catalyst preparation method, PtRu atomic ratio, surface morphology (e.g., roughness), presence and type of support, operating anode potential range, methanol concentration, and temperature. 

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