Heterogeneous catalysis definition

The surface basicity of a solid catalyst can be defined in a way analogous to that applied to conventional bases. Thus, a surface Lewis base site is one that is able to donate an electron pair to an adsorbed molecule. If we take the definition of surface basicity in a more general way, it could be said that the active surface corresponds to sites with relatively high local electron densities. This general definition will include not only Lewis basicity but also single electron donor sites. We emphasize that the literature of heterogeneous catalysis often reports that both single-electron and electron-pair donor sites exist on basic catalysts. 

By definition, the turnover frequency is expressed per number of active sites. So, catalytic samples that differ only in the amount active sites must exhibit the same values of turnover frequency. If not, heat and mass transfer phenomena are present. Specifically, the correct measurement of intrinsic kinetic data in heterogeneous catalysis is difficult due to the effect of heat and mass transfer, especially inside the pores of high specific-area materials. The turnover frequency reveals these phenomena. In other words, in the case of supported... 

As shown by Taylor and Roy (1) the behavior of small-pore zeolites does not necessarily conform to the classical definition of a zeolite. Rather, the properties evidenced by the P-zeolites, and perhaps other classes of small-pore zeolites as well, constitute a basis for possible future technical innovation in selective adsorption and heterogeneous catalysis. The zeolite structure, and hence the size and shape of its cell apertures and cavities and disposition of mobile cations may differ substantially at elevated temperatures from what it is under ambient conditions. 

Modeling of reaction and diffusion in heterogeneous catalysis begins with a definition of the structure or geometry of the internal porous media. 

The catalyst turnover number (TON) and the turnover frequency (TOF) are two important quantities used for comparing catalyst efficiency. Their definitions, however, vary slightly among the three catalysis fields. In homogeneous catalysis, the TON is the number of cycles that a catalyst can run through before it deactivates, i.e., the number of A molecules that one molecule of catalyst can convert (or turn over ) into B molecules. The TOF is simply TON/time, i.e., the number of A molecules that one molecule of catalyst can convert into B molecules in one second, minute, or hour. In heterogeneous catalysis, TON and TOF are often defined per active site, or per gram catalyst. This is because one does not know exactly how many... 

The reach of hydrophobicity in the model can severely complicate its synthesis. This criterion makes sense in the case of the task of achieving as close a correspondence as possible between the model and the simulated enzyme. However, a more particular objective is often assigned for the synthesis of mimics. It consists of synthesizing an effective catalyst simulating a definite (useful) enzyme property. Therefore, it seems more desirable to replace hydrophobic tub synthesis by more accessible methods, developed in chemical catalysis, particularly in coordination and heterogeneous catalysis. 

The preparation of catalysts is often easier in heterogeneous catalysis, and products are definitely easier to separate. Nevertheless, homogeneous catalysis has the substantial advantage of typically requiring milder conditions and yielding a higher selectivity. This increases the purity of the products and reduces the amount of waste, two concepts which are very useful considering the desires to develop a more environmentally friendly chemical industry. 

Traditionally, a comparison between homogeneous and heterogeneous catalysis for each methodology stresses a number of weak and strong points.[1] The definition of the active site at the molecular level and the variability in design are generally... 

The present manual is based on the same general principles as those used in the Manual of Symbols and Terminology for Physicochemical Quantities and Units of the Commission on Symbols, Terminology and Units of the Division of Physical Chemistry, Definitions, Terminology and Symbols in Colloid and Surface Chemistry of the Commission on Colloid and Surface Chemistry, Appendix II Part 1 Definitions, Terminology and Symbols in Colloid and Surface Chemistry, Part II Heterogeneous Catalysis, and Recommendations in Reporting Physisorption Data for Gas/Solid Systems [1-3]. 

Although recent literature gives evidence for a homogeneous reaction on the catalyst surface (cf. Section 3.3.5.3.2) the commercial processes show typical features of heterogeneous catalysis and do not comply with the definition of homogeneous catalysis given in the preface. Therefore a description of the manufacturing process is not included here but it is dealt with in [59]. 

In extension of the definition of reaction order in heterogeneous catalysis, the order of an electrocatalytic reaction, a,, with respect to a reactant, j, is defined as... 

In biocatalysis /C2 is the rate measured when all enzyme molecules are complexed with reactant divided by the total concentration of enz)one present. This is the Turn-Over Number according to biochemists definition. Note that this differs from the Turn-Over Frequency as defined in heterogeneous catalysis where it is simply the rate normalised to the total number of surface sites present. In the latter case it is a function of the gas phase composition. 

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