Precursor model

The precursor model of FAB applies well to ionic analytes and samples that are easily converted to ionic species within the liquid matrix, e.g., by protonation or deprotonation or due to cationization. Those preformed ions would simply have to be desorbed into the gas phase (Fig. 9.6). The promoting effect of decreasing pH (added acid) on [M+H] ion yield of porphyrins and other analytes supports the precursor ion model. [55,56] The relative intensities of [Mh-H] ions in FAB spectra of aliphatic amine mixtures also do not depend on the partial pressure of the amines in the gas phase, but are sensitive on the acidity of the matrix. [57] Furthermore, incomplete desolvation of preformed ions nicely explains the observation of matrix (Ma) adducts such as [M+Ma+H] ions. The precursor model bears some similarities to ion evaporation in field desorption (Chap. 8.5.1). 

Relevant to the synthesis of ammonia over iron catalysts is the observation of Ertl et al. (54) that potassium preadsorbed by an iron catalyst ((7 = 0.1) increased the rate of synthesis at 430 K by a factor of about 300. This effect the authors attributed to an enhancement of the heat of adsorption of molecular nitrogen due to transfer of electronic charge from potassium to the surface of the iron catalyst. This would be entirely in keeping with the precursor model proposed for nitrogen chemisorption (55). 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

Structure 2 precisely adheres to the analytical constraints of Table I, but a wide variety of aliphatic alicyclic terpenoids, steroids, and porphyrins may serve as its precursors. The Harvey et al. (20) precursor model was favored... 

Chemical systems of relevance to the molybdenum and tungsten enzymes include synthetic pterins, a-phosphorylated ketones (as precursor models), and a variety of molybdenum and tungsten oxido, sulfido, and 1,2-enedithiolate complexes. These compounds have been used to (1) confirm the identity of MPT derivatives (2) define steps in MPT biosynthesis (3) calibrate spectroscopic observations (4) give precise geometries and reactivities that can be used as input for theoretical studies and (5) provide options for mechanistic consideration. 

It can be seen that, for the N2—W system, the trapping-precursor model for adsorption is all-important. It should not be expected that this is so all the time. This is clearly shown by the work of Hayward and Walters [401] who measured the angular distribution of hydrogen scattered from W 100 at 300 K as a function of the amount of hydrogen... 

An extension of this approximation is the so-called "precursor" model [32,33], which is illustrated in Fig. 1.7. This model assumes a finite lifetime of particles in a second layer on the top of the chemisorbed phase ("extrinsic preciu-sor") during... 

The "precursor" model for adsorption on an already partly covered surface was introduced in Section 1.3. It assumes that an incident particle may also be weakly trapped on an already occupied site where it becomes thermally accommodated. There are, however, a series of observations whereafter even particles in the first layer are not immediately in thermal equilibrium but require some finite relaxation time for energy exchange with the surface during which novel dynamic effects may arise. 

K and 6 MPa in a flow reactor. The catalysts were prepared by the impregnation of the same AC. Initially, the Pd/AC was the most active, however, its activity abruptly declined after a few minutes on stream. For some catalysts, the activity recovered after an initial decline. In the series of the same catalysts, the Rh/AC was the most active during the HDS of thiophene. Therefore, the trends in Figsure 60 and 61 confirmed that the catalyst-deactivation patterns are influenced by several factors, e.g., the type of support and active metals, active-metal precursor, model reactions, method of catalyst preparation, etc. 

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