Exposed atoms

Each precious metal or base metal oxide has unique characteristics, and the correct metal or combination of metals must be selected for each exhaust control appHcation. The metal loading of the supported metal oxide catalysts is typically much greater than for nobel metals, because of the lower inherent activity pet exposed atom of catalyst. This higher overall metal loading, however, can make the system more tolerant of catalyst poisons. Some compounds can quickly poison the limited sites available on the noble metal catalysts (19). 

The turnover frequency (TOP) based on surface-exposed atoms significantly increases with a decrease in the diameter of the gold particle from 5 nm [66]. This feature is unique to gold, because other noble metals usually show TOFs that decrease or remain the same with a decrease in the diameter [7]. The decrease in particle size gives rise to an increase in corner or edge and perimeter of NPs and change in electronic structure however, the origin of size effects on catalytic activity for CO oxidation is not clear. 

Given a certain metal, what can we do to alter its reactivity First there is the structure of the surface. More open surfaces expose atoms of lower coordination. This narrows the d band, and shifts its position (up if it is more than half filled, down if the d band is less than half filled). To illustrate the point Tab. 6.2 shows experimentally determined activation energies of NO dissociation on the (111) and (100) surfaces of rhodium. 

The bcc structure contains more empty space than the fee structure, as Fig. A.l clearly shows. The closest packing is observed in the (110) plane. The (111) plane forms a rather open surface, which exposes atoms from three different levels. Values of Zs are given in Table A. 1. 

Highly dispersed metals expose atoms to the gas phase in unusual geometric arrangements, which leads to particular low coordination (unsaturated) etc. (84-93). 

Very small particles expose atoms of a lower coordination and therefore with a population of d-electrons different from that of atoms on flat... 

Figure 15-19 illustrates the mechanism by which a CdS crystal responds selectively to certain ions. The crystal can be cleaved to expose planes of Cd atoms or S atoms. The Cd plane in Figure 15-19a selectively adsorbs HS- ions, whereas an S plane does not interact strongly with HS. Figure 15-19b shows strong response of the exposed Cd face to HS, but only weak response when the S face is exposed. The opposite behavior is observed in response to Cd2+ ions. The partial response of the S face to HS- in the upper curve is attributed to about 10% of the exposed atoms actually being Cd instead of S. 

It is well known that not all of the exposed atoms of a metal crystallite are catalytically active. Establishment of the number and nature of the actual active sites is an important step in the determination of the complete mechanisms of surface reactions and in prediction and preparation of more active and selective metal catalysts. 

Besides, the geometric factor is important in the catalytic activity of iron, that is, the catalyst surface has irregularities such as steps and kinks that expose atoms with low coordination numbers which appears to be particularly reactive [22],... 

The Pt particles were formed by physical vapor deposition onto 10 nm thick Fe304(lll) films grown on Pt(lll). The particles become more uniform in size and well-faceted upon heating to elevated temperatures. Figure 4.4.6 shows typical morphology of the Pt nanoparticles vacuum-annealed at temperatures above 800 K. The particles, about 1 nm in height and 5 nm in width, expose atomically... 

Coupling agents presently used in glass fiber-reinforced plastics were designed to have an outermost exposed atomic grouping which is... 

The third concern of those who are searching for better catalysts might be the metal crystallite size effect. As shown in Figure 5, with a decrease in the diameter of metal particles the fractions of edges, corners, and surface exposed atoms increase, markedly in the range below 2 nm. The electronic state of metal particles may also... 

Metal accessibility was determined by the H2-O2 titration method in a static volumetric equipment. Hydrogen preadsorbed at 500°C was titrated at 25°C by oxygen. Isotherms have been determined in the 0-50 torr range. Extrapolation to the original values was used to calculate the number of exposed atoms. 

The rate of reaction expressed as molecules reacted (or formed) per unit time per catalytic site (or per exposed atom of active metal for metal catalysts) is called the turnover frequency. For supported metal catalysts the calculation requires knowledge of the dispersion, i.e., the fraction of the active metal available for adsorption of reactants. Boudart coined the term demanding (structure-sensitive) for catalyzed reactions for which the turnover frequency varies with the dispersion. Related to this is the ensemble effect, where the active site requires a specific multiatom grouping.f ... 

For optimal use of Pt, a preferred size of Pt crystal can be predicted. For catalytic activity of exposed atoms (STY), this would take place around point C (Fig. 6.7 d). The opposite trends of dispersion and catalytic activity (related to metal character of atom in the dispersion) give the resulting curve C for overall catalysis observed as function of cluster size. Catalysts prepared by different methods will, generally, yield different crystallite size distribution and shape of metal crystals. This will result in dissimilarities in sorptive and catalytic behavior, even if allowance is made for the difference in active surface area285. ... 

Methods are now emerging that can provide an accurate and practical alternative to explicit solvent inclusion. A common way to avoid dealing with this problem is to concentrate on differences between similar systems, each modeled without water. Assuming that solvent affects each system in the same way, it is hoped that errors will cancel. Another way is to treat the solvent as a statistical continuum,examining the interaction of each fully or partially exposed atom with solvent,and adding a solvation energy function to existing molecular mechanics potentials. ... 

We note that the use of N2 chemisorption on iron, for example, really amounts to measuring the atoms per active site as a function of FE. This procedure is merely an alternative to the one we have adopted, in which we reason about the number of sites involving a given configuration of atoms per unit of exposed atoms. This quantity has been called the Taylor fraction (367). Similar concepts have been discussed recently by Carberry (368). 

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