Surface diffusion of stearic acid on aluminum oxide

Abstract

By means of infrared internal reflection spectroscopy we measured the time dependence of the surface concentration of stearic acid molecules over a well defined region of the surface. The molecules were diffusing into this region from a source comprising a compressed monolayer or bulk crystalline material outside the field of view. By these measurements in conjunction with solutions to the diffusion equations we determined the diffusion coefficient D, of stearic acid on the (0001) plane of a-alumina.We also determined the dependence of D, upon the amount of water CO-adsorbed on the surface. When the surface was free of reversibly adsorbed water, D, had a value of about 2 x 10-7 cm2/s. As the surface coverage of coadsorbed water increased to about a monolayer D, increased to a maximum value of about 4 x 10-5 cm2/s. With further additions of surface water D, then decreased and became less than 10-8 cm2/s at coverages slightly less than about two monolayers. In the region of its maximum value the temperature dependence of D, indicated SCOPEAn accurate experimental method is needed to measure surface diffusion rates on nonporous surfaces. Plane surfaces provide simple models for the study of the general phenomenon of surface flux which contributes to transport through catalyst pellets, adsorbent beds, and the diffusion barriers used in important separation processes. Most previous studies of surface diffusion have been based on a determination of total diffusive flow through porous materials. Gas chromatography, nuclear magnetic resonance, and neutron scattering spectroscopy have been employed for this purpose. The contribution due to Knudsen flow is then calculated and subtracted from the total. The difference is attributed to surface diffusion. Clearly, this approach is not applicable to nonvolatile species, but it is the least volatile component in any system which is most affected by surface diffusion.The direct measurements of surface diffusion on solids previously reported were by Beischer (1951) and Rideal and Tadayon (1954). They determined the diffusion coefficient of stearic acid on mica by means of radioactive tracer techniques. Our method is also based on a direct observation of transport in adsorbed layers on planar surfaces. It uses the somewhat simpler and more generally applicable technique of Internal Reflection Spectroscopy (IRS) to measure time dependent surface concentration. The results are combined with approximate solutions of the Fick diffusion equations under appropriate boundary conditions to give surface diffusion coefficients.Water adsorbed on a solid surface often has a dramatic effect upon the rates of interphase exchange or chemical reactions at that surface. In at least one case, where surface transport is the slow step, it has been shown that the marked effect of adsorbed water on the rate of reaction is due to its influence on the rate of diffusion (Levy and Boudart, 1974). In this paper we report the effect of coadsorbed water on the surface diffusion of stearic acid on aluminum ...

Kinetic Modeling of Spillover and Temperature-Programmed Oxidation of Oxy-Carbon Surface Species on Pt/Al2O3

“…The rate expression for diffusion of oxy-carbon species on Al 2 O 3 is therefore given by eq : which represents the average diffusion rate of all the different oxy-carbon surface species. The value of E a from this work (24 kJ/mol) is within the range of values previously reported for diffusion of carbonaceous species on different phases of Al 2 O 3 : 5 kJ/mol for propene diffusion on η-Al 2 O 3 ; 8–12 kJ/mol for CO 2 diffusion on γ-Al 2 O 3 ; 12 kJ/mol for benzene diffusion on η-Al 2 O 3 ; and 109 kJ/mol for stearic acid diffusion on α-Al 2 O 3 …”

Adsorption in Biodiesel Refining - A Review

“…Yang et al (1974) found that stearic acid had a surface diffusivity on alumina of about 10 -9 -10 -11 m 2 s -1 depending on the hydration degree of the alumina. Allara and Nuzzo (1985) reported values of D s of 10 -10 -10 -11 for different alkanoic acids on alumina.…”

Applications of Internal Reflection Spectroscopy to Surface Studies

“…The final result is 66 l/(mole)(cm). Given the fact that the first two o r three CH, groups will be perturbed by the surface and therefore have a lower than average extinction coefficient, the agreement with the peak molar extinction coefficient of 77 l/(mole) (cm) per methylene group for the asymmetric stretch of n-alkanes measured in CC1, solution [17] is fairly good.…”