Second-shell coordination numbers

The EXAFS data recorded after exposure to air of the unsupported Co-Mo catalysts with different cobalt content allow one to examine the effect of cobalt. In spite of a great uncertainty in the coordination numbers, the promoted catalysts seem to have a somewhat smaller domain size than the unpromoted catalyst as indicated both by the smaller second shell coordination numbers and by the larger effect of air exposure (i.e., reduced sulfur coordination number in first shell). This influence of cobalt on the domain size may be related to the possibility that cobalt atoms located at edges of M0S2 stabilize the domains towards growth in the basal plane direction. Recent results on C0-M0/AI2O3 catalysts indicate that Co may also have a similar stabilizing effect in supported catalysts (36). 

In the case of hydrated Al(III) the first shell consisting of six water molecules was shifted from 1.8 to 1.9 A when changing from conventional QM/MM to QMCF treatment. The second shell distances coincide showing a value of 4.1 A, the average second shell coordination number increases slightly from 12.2 to 12.5 when the QMCF scheme is applied. However, the third shell as well as the transition to the bulk reveals significant changes. In the case of the conventional QM/MM MD simulation the third shell forms a plateau whereas a distinct peak results when the QMCF framework is applied. 

In the case of Zn(II) the first and second shells show similar shapes and the maxima are found at 2.1 and 4.4 A, respectively. The average second shell coordination number decreased from 13.8 to 13.2 when changing from the QM/MM to the QMCF scheme, whereas the first shell coordination number remains at 6. The main structural... 

Munoz-Paez et al. [114] performed extended X-ray absorption fine structure studies on aqueous solutions of Cr and Zn +. They detected second coordination shells in both cases with coordination numbers of 13.3 + 1 (Cr +) and 11.6 + 1.5 (Zn ). The same group performed Monte Carlo [115] and molecular dynamic [116, 117] simulations of [Cr(H20)6] " in dilute aqueous solutions using an ab initio Cr + hydrate-water interaction potential. They found second shell coordination numbers of 14 from both simulations. Furthermore, from simulations and EXAFS measurements they concluded that chloride ions are situated beyond the second hydration shell. 

It is important to distinguish first-sheU coordination of the adsorbate and second-shell coordination number of surface atoms with surrounding metal atoms, as is shown in Table 10.1. 

QEXAFS scans (30-90 seconds per scan) is suitable to study reactions like pretreatment processes. The whiteline area gives information concerning the oxidation state. The EXAFS region represents the geometrical structure of the Pt atom. The final metal particle size was obtained from the Pt-Pt first shell coordination number, H2 chemisorption26 as well as HRTEM. 

From MD simulations it can be concluded that for trivalent ions (Cr, lanthanides) in general, first shell water molecules form two hydrogen bonds to second shell water oxygens. In that way the number of second shell water molecules (CNu) is roughly twice that of the first shell coordination number (CNi). For [Cr(H20)6] a mean second shell number of 12.9 H2O molecule was found and for lanthanides, CNii-values of 17.61 (for [Nd(H20)9] ) and 16.74 (for [Yb(H20)g] +) were obtained (Fig. 4.7). 

Madelung constant in the surface layer has decreased relative to that of the bulk, but less than the decrease in first-shell coordination number. Interestingly, the ion potential in the layer of ions next to the surface the Madelung constant increases. The reason is that in this layer there is no change in coordination number in the first coordination shell, but there is a change in the second coordination shell. Due to the presence of the surface the number of the next-nearest-neighbor cations or anions... 

First, the hydrogen bond is a bond by hydrogen between two atoms the coordination number of hydrogen does not exceed two.7 The positive hydrogen ion is a bare proton, with no electron shell about it. This vanishingly small cation would attract one anion (which we idealize here as a rigid sphere of finite radius—see Chap. 13) to the equilibrium intemuclear distance equal to the anion radius, and could then similarly attract a second anion, as shown in Figure 12-1, to form... 

EXAFS has been used to determine the second hydration shell of zinc in aqueous solution. Aqueous solutions of zinc nitrate over a range of concentrations were examined and a Zn—O distance of 2.05 A for the first shell of the six-coordinate zinc center found, which is unaffected by concentration. The second hydration shell shows a Zn—O distance which has no systematic trend but an average distance of 4.1 A. The coordination number for the second shell is 11.6 1.6 with unusual behavior for the most concentrated 2.7 M solution, which has a decrease in coordination number to 6.8 1.5 340... 

Because of their having larger sizes and more filled shells of electrons between the outer shell and the nucleus, the ionization energies of second- and third-row metals are lower than those of first-row metals. Consequently, it is easier for the heavier metals to achieve higher oxidation states, which also favors higher coordination numbers. In general, there is also a greater tendency of the heavier metals... 

In order to obtain data with reduced temperature smearing, experiments were also carried out at 77 K. However, such experiments could not be carried out in. situ and the catalysts were thus exposed to air before the measurements. EXAFS data of three catalysts with Co/Mo atomic ratios of 0.0., 0.25, and 0.50 were obtained. The results show many similarities with the data recorded in situ and were fitted in a similar fashion using phase and amplitude functions of the well-crystallized model compound M0S2 recorded at 77 K. The results, which are given in Table III, show that the bond lengths for the first and second coordination shell are the same for all the catalysts and identical to the values obtained for the catalyst recorded in situ (Table II). The coordination numbers for both shells appear, however, to be somewhat smaller. Although coordination numbers determined by EXAFS cannot be expected to be determined with an accuracy better than + 20, the observed reduction... 

The molecular structure of liquids is best analyzed using the concept of RDF. This is of particular importance in solute-solvent structures as it defines the solvation shells around the solute molecule. Therefore, we analyzed the solvation of the anion F using the RDF between the anion and the oxygen of the water molecules, as shown in Fig. 2. At least three solvation shells are well defined. The integration of these peaks defines the coordination number, or the number of water molecules in each solvation shell. The first shell that ends at 3.15 A with a maximum at 2.65 A has, on average, 6.6 molecules of water. The second shell,... 

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