Alumina, extinction coefficient

As expected for silica-alumina as a mixed oxide (see also Section IV.B.5), the PyH+ and PyL species are observed simultaneously (160, 205,206,221-223). Two distinct types of Lewis acid sites could be detected (19b mode at 1456 and 1462 cm-1, respectively) on a specially prepared aluminum-on-silica catalyst (160). On water addition, the Lewis sites can be converted into Br nsted sites (160, 205, 221), The effect of Na+ ions on the acidity of silica-aluminas has been studied by Parry (205) and by Bourne et al. (160). It can be concluded from Parry s results that Na+ ions affect both types of acid sites, so that alkali poisoning does not seem to eliminate the Br nsted sites selectively. For quantitative determination of the surface density of Lewis and Br nsted acid sites by pyridine chemisorption, one requires the knowledge of at least the ratio of the extinction coefficients for characteristic infrared absorption bands of the PyH+ and PyL species. Attempts have been made to evaluate this ratio for the 19b mode, which occurs near 1450 cm-1 for the PyL species and near 1545 cm-1 for the PyH+ species (160,198,206,221,224,225). The most reliable value as calculated from the data given by Hughes and White (198) seems to be... 

Hale and Bohn [252] measured the scattered radiation from a finite sample of reticulated alumina from an incident laser beam at 488 nm. They then matched Monte Carlo predictions of the scattered radiation calculated from various values of extinction coefficient and scattering albedo and chose the values that best matched the experimental data for reticulated alumina samples of 10, 20, 30, and 65 ppi. A scattering albedo of 0.999 and an assumed isotropic scattering phase function reproduced the measured data for all pore sizes. The large reported albedo value indicates that alumina is very highly scattering and that radiative absorption is extremely small for this material. 

FIGURE 7.30 Extinction coefficient versus pore diameter for alumina, partially stabilized zirconia, and silicon carbide. 

Absolute concentrations are, however, often difficult to establish due to the lack of information on extinction coefficients and due to different adsorption probabilities under the prevailing conditions. Instructive examples for research along these lines are the observation of Cu-Zn interactions in working methanol synthesis catalysts [28] and of coexistence of metal and ionic Ru sites in alumina-supported Ru catalysts for partial oxidation of methane [29]. 

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