Monolayer dispersion capacities

Samples of Mo03/Ti02 prepared by an impregnation method using a solution of (NH4)2Mo207 have been studied recently by Quincy et al. (39). They obtained a similar plot, which allows us to obtain the same monolayer dispersion capacity, namely, 0.12 g MoO3/100 m2 of Ti02 surface. 

Fig. 23. Radial distribution around Ni. (a) Crystalline NiO. Two monolayer-dispersed samples (b) 0.10 g NiO/g -y-A Oj (c) 0.20 g NiO/g y-Al20,. Monolayer dispersion capacity is 0.26 g NiO/g y-Al203. See text for discussion of peaks.

Fig. 27. TEM micrographs and HEED patterns (a) and (a ) y-Al203 (b) and (b ) 0.15 g NiO/g -y-Al203 (c) and (c ) 0.30 g NiO/g y-Al203. Samples are prepared by the impregnation method. Their monolayer dispersion capacity is 0.25 g NiO/g y-Al2Q3,...

Fig. 29. Relation between acidity and Mo03 content in Mo03/Si02. Monolayer dispersion capacity is 0.12 g MoQ3/g Si02.

Table 8.2. Empirical monolayer dispersion capacity on activated alumina...

Monocoordination, versus dicoordination, BOC-MP, 37 125-127 Monolayer-dispersed, 37 2-4 adsorption, 37 33-34 capacities, 37 13-14... 

According to a simple model based on the assumption that the anions of oxide or salt form a close-packed monolayer on the surface of the support and the cations occupy the interstices left over by anions, one can figure out the close-packed monolayer capacity for oxide or salt on a unit area of the support. We estimate it at 0.10 g/100 m2 or higher for various active components (see later, Table II). The specific surface of the support is about 200 m2/g for y-Al203, 300 m2/g for silica gel, and 1000 m2/g for active carbon. Although each of the catalysts in Fig. 1 contains a considerable amount of active component, its content is still lower than that estimated on the basis of a close-packed monolayer. Therefore, the monolayer dispersion in many of these catalysts does not correspond to the full coverage of the support surface, and more precisely is known as submonolayer dispersion. 

Plots for these systems, similar to one shown in Fig. 3, have been obtained on the basis of the data from XRD quantitative phase analyses, but they are omitted here. Each plot contains a threshold dispersion capacity. The dispersion capacities so derived in our work are listed in Table II (Section II,G). We can see that within the limits of experimental error the dispersion capacities are either equal to or lower than the respective close-packed monolayer capacities. So we come to the conclusion that these compounds disperse spontaneously onto the surface of the support to form submonolayers more often than monolayers. 

The threshold value derived from a plot of the residual amount of crystalline NaCl versus the total amount of NaCl in each sample, as shown in Fig. 15, is 0.39 g NaCl/g NaY. This value corresponds to a dispersion capacity of 10.5 NaCl molecules Vsodalite cage in the NaY zeolite. In view of the fact that a sodalite cage can accommodate only one NaCl molecule, most of the NaCl molecules are dispersed on the wall of the supercages. On the basis of the close-packed monolayer capacity of NaCl (0.085 g/100 m2) taken from Table II and the BET surface area of NaY zeolite (800 m2/g), we estimate the utmost monolayer capacity at 0.68 g/g NaY, which is reasonably higher than the experimental value of 0.39 g NaCl/g NaY, because in our calculation we have neglected heterogeneity of the internal surface of the zeolite. 

The ethylene adsorption capacity increases with the content of CuCl and reaches the highest value at the point near the monolayer dispersion threshold. Similar plots have been reported by Zhao et al. (57) in the investigation of ethylene adsorption on Cu0/y-Al203 and by Duan et al. 

Figure 3.51. CO chemisorption by pulse response of a reduced 5 wt% Pt/Al203 a. Thermal Conductivity Detector (TCD) signals after the CO pulses, b. Cumulative amount of CO chemisorbed. The monolayer capacity is 0.06 mmol/g Pt, corresponding with a dispersion of 24%.

V2O3 corresponding to twice the monolayer capacity was dispersed on silica by bulk melting at 973... 

It should be checked whether Sn affects the dispersion of Rh particles or not. We have already studied the effect of Sn addition on the dispersion of Rh by comparing the XPS intensity and the adsorption capacity of H2 and CO [11]. The relative XPS intensity of Rh3d5/2 to a monolayer catalyst was larger than the theoretical value calculated from the adsorption capacity of H2 and CO by the method of Kerkhof and Mouljin [13]. These results indicated that the decrease of the adsorption capacity of Rh-Sn/Si02 catalysts was ascribed not to the increase of isolated Rh particle size, but to surface composition of Rh-Sn bimetallic... 

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