Catalytic activity, principles

Chiral H-bond donors and acids have proven their potential many times over several decades. Some useful apphcations in natural product synthesis have been reported, using either hydrogen bonding activation as the sole catalytically active principle, or utilizing bifunctional catalysts. With respect to the catalytic moiety of choice, the considerable potential of thioureas can be emphasized, especially those based on Cinchona alkaloids (Table 6). 

The enhanced concentration at the surface accounts, in part, for the catalytic activity shown by many solid surfaces, and it is also the basis of the application of adsorbents for low pressure storage of permanent gases such as methane. However, most of the important applications of adsorption depend on the selectivity, ie, the difference in the affinity of the surface for different components. As a result of this selectivity, adsorption offers, at least in principle, a relatively straightforward means of purification (removal of an undesirable trace component from a fluid mixture) and a potentially useflil means of bulk separation. 

Remarkably, seventy years after Houdry s utilization of the catalytic properties of activated clay and the subsequent development of ci ystalline aluminosilicate catalysts that arc a magnitude more catalytically active, the same fundamental principles remain the basis for the modern manufacture of gasoline, heating oils, and petrochemicals. 

Since the catalytically active phase is frequently quite expensive (e.g. noble metals) it is clear that it is in principle advantageous to prepare catalysts with high, approaching 100%, catalyst dispersion Dc. This can be usually accomplished without much difficulty by impregnating the porous carrier with an aqueous solution of a soluble compound (acid or salt) of the active metal followed by drying, calcination and reduction.1... 

Assuming that substituted Sb at the surface may work as catalytic active site as well as W, First-principles density functional theory (DFT) calculations were performed with Becke-Perdew [7, 9] functional to evaluate the binding energy between p-xylene and catalyst. Scalar relativistic effects were treated with the energy-consistent pseudo-potentials for W and Sb. However, the binding strength with p-xylene is much weaker for Sb (0.6 eV) than for W (2.4 eV), as shown in Fig. 4. 

The oxidation rate should, in principle, be described by a law using a rate constant independent of pH, as long as a single reaction mechanism is involved. The rate law (28.4) is unusual in that the rate varies with the concentration of the Mn11 component, rather than an individual species. If we hypothesize that the catalytic activity is promoted by a surface complex >MnOMnOH, a slightly different form of the rate law may be appropriate. Since the surface complex would... 

High metal dispersion results in exposure of metal sites of enhanced catalytic activity. I. Stranski et al. used the Gibbs-Wulff principle " ... 

As an aside, we should mention that the same principles apply to the formation of bimetallic clusters on a support. In the case of Pt-Re on AI2O3 it has been shown that hydroxylation of the surface favors the ability of Re ions to migrate toward the Pt nuclei and thus the formation of alloy particles, whereas fixing the Re ions onto a dehydroxylated alumina surface creates mainly separated Re particles. As catalytic activity and selectivity of the bimetallic particles differ vastly from those of a physical mixture of monometallic particles, the catalytic performance of the reduced catalyst depends significantly on the protocol used during its formation. The bimetallic Pt-Re catalysts have been identified by comparison with preparations in which gaseous Re carbonyl was decomposed on conventionally prepared Pt/Al203 catalysts. ... 

The structure and dynamics of clean metal surfaces are also of importance for understanding surface reactivity. For example, it is widely held that reactions at steps and defects play major roles in catalytic activity. Unfortunately a lack of periodicity in these configurations makes calculations of energetics and structure difficult. When there are many possible structures, or if one is interested in dynamics, first-principle electronic structure calculations are often too time consuming to be practical. The embedded-atom method (EAM) discussed above has made realistic empirical calculations possible, and so estimates of surface structures can now be routinely made. 

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