Number of total surface metal atoms

Measuring the number of total surface metal atoms by chemisorption... 

From the experimental value of chemical absorption capacity Vg), the number of total surface metal atoms Ns) can be calculated for the catalyst by the following equation ... 

For catal3dic purposes, once the correct surface average metal particle nanosize dav is known, it is convenient to calculate the number of total surface metal atoms per catalyst gram (Ns), given by the equation ... 

According to the number of total sirrface metal atoms on catalyst as shown in equation (7.71), the surface area Sm of active component can be calculated ... 

If the degree of coverage of the ruthenium by the copper is very high, the copper atoms should be coordinated extensively to ruthenium atoms. It is emphasized that the ruthenium-copper clusters are of such a size (average diameter of 32A by electron microscopy (33)) that the surface metal atoms constitute almost half of the total. Hence for a Cu/Ru atomic ratio of one, the number of copper atoms would correspond roughly to that required to form a monolayer on the ruthenium. 

On metal electrodes having coarse-crystalline structure, the number of special crystal sites (vertices, edges) is small relative to the total number of all surface atoms, so that their (positive or negative) contribution to the overall rate must be quite small. At highly disperse, fine-crystalhne deposits, however, the situation is different. Here special effects are really observed they are considered in more detail in Section 28.5.4. 

Osmium, iridium, and platinum catalysts with dispersions (ratio of surface metal atoms to total metal atoms) in the range 0.7 to 1 have been studied by Via, Sinfelt, and Lytle.As expected for small particles, the average co-ordination numbers were between 7 and 10, significantly lower than the value of 12 for the bulk metals. This result is in agreement with gas chemisorption data. Also, the disorder of the metal atoms, represented by the r.m.s. deviation of interatomic distance about its equilibrium value, was found to be greater by factors of 1.4—2.0 than for atoms in the bulk metals. Information on such disorder has not been available previously. 

The aim of any dispersion measurement (Dispersion, D) for a supported metal should be to determine the ratio of the number of surface metal atoms Ns) to the total number of metal atoms (Nt) within the system, that is,... 

The quantity H/M, i.e., H atoms irreversibly adsorbed divided by total metal atoms, is measured experimentally. H/Ms, where Ms refers to surface metal atoms, may be known on the basis of surface science studies [e.g., low-energy electron diffraction (LEED)] and is usually assumed equal to unity. However, we shall discuss some papers that show that H/Ms is also a function of d and other variables. In addition, reversibly adsorbed hydrogen may also be important (97, 98). For interpreting most data, nevertheless, we can only assume that FE = H/M. It seems reasonable to use surface metal atoms as a basis, and not surface sites. The number of surface atoms required to form a site is in many cases a subject of debate, whereas H/M is measured experimentally. 

Metal particles are defined as an ensemble of metal atoms of various sizes, and these atoms have different natures, depending on whether they are located in the core or at the surface of the particles. In general, the active centers are usually the surface metal atoms so that the properties of metal particles are related to the number of surface atoms (Ns), and characterized by the dispersion (D = NJN ) N = total number of atoms of the metal particle). Increasing the number of surface atoms (Vj) per total number of atoms (N ) requires particles of small sizes, typically in the nanometer range (Scheme 66 and Table 18). Van Hardeveld and Hartog have proposed that the shape of nanosized particles can be represented as cubooctahedrons. " ... 

Bifunctional Catalysts. One reason invoked (there are others) for why metal particles with high dispersion are desirable for catalysis is that the ratio of surface metal atoms to total number of atoms is quite high. Consider the potential cluster size for two zeolites faujasite and ZSM-12. Faujasite has supercages, which for the purposes of this question can be described as spherical with a diameter of 12 A, and ZSM-12 has a one-dimensional elliptical pore structure of dimensions 5.6 x 6.0 A. Assuming the metal atoms of inteiest have a diameter of 1.0 A and the cluster has a packing fraction corresponding to an FCC structure (0.74), estimate the number of atoms in a metal cluster in each of the two zeolites mentioned above. 

The dispersions (number of surface metal atoms/total metal atoms) of the samples are determined by chemisorptions of hydrogen, oxygen and CO at 25 °C [98]. Prior... 

There are two ways one might think of that could help to reach that goal further increasing the platinum dispersion (defined as the ratio of surface metal atoms to total number of atoms) by making finer platinum particles, if there is no decrease of Pt-specific activity (activity per tmit Pt surface area) or alternatively, increasing the Pt-specific activity. One could also seek a combination of these two approaches. 

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