Determination of catalyst surface area from desorption characteristics of physisorbed gases

Abstract

A new experimental method for the measurement of catalyst surface area of supported catalysts has been developed using selective physisorption. The desorption characteristics of a gas are studied separately on the catalyst, the support, and the supported catalyst by carrying out thermal desorption experiments in a continuous flow sorptometer. Differences in the coverage vs. temperature curves, obtained from the thermal desorption experiments, are a measure of the selectivity of the physisorbing gas, and allow calculation of the fraction of total surface area occupied by the catalyst.Two systems have been studied utilizing the thermal desorption with carbon dioxide as adsorbate: potassium carbonate/carbon black and silver/alumina. Supported catalyst surface area was determined for each system; the results were confirmed using physical mixtures of the two components (where the actual area of each component is known) and oxygen chemisorption for the silver/alumina system. The experimental technique allows for straightforward calculation of the catalyst area. D. J. MILLER and H. H. LEE Department of Chemical EngineeringUniversity of Florida Gainesville, FL 3261 1The exposed surface area of a catalyst on a support is an important parameter in the characterization of catalytic behavior, particularly in sintering and dispersion studies. Several experimental methods of determining catalyst area have been developed, including chemisorption, electron microscopy, and X-ray techniques. The difficulties involved in applying electron microscopy to actual catalysts and the sophistication of small angle X-ray scattering prevent their practical application in many cases. Chemisorption techniques have met with success in several catalyst systems, but the chemisorption techniques are limited to well-defined metal catalysts and are not in general applicable to nonmetallic catalysts. This paper presents a new method for determining catalyst area using physical adsorption. The method examines the adsorption characteristics of a gas separately on the catalyst (in powder or gauze form), the support, and the supported catalyst using a thermal desorption technique for the determination of the total exposed catalyst area as a fraction of the total catalyst and support area. This method has several advantages over other methods used in catalyst area measurements:1) The physisorption experiments are easy to carry out and the equipment needed is low cost. The physisorption experiments show good reproducibility, and sample preparation is the same as in conventional physisorption.2) The method can be applied to any catalyst system with the appropriate choice of adsorbing gas, since gases physisorb on all surfaces at low temperatures. This is in contrast to chemisorption, where the specific gas-solid interactions limit application to a few systems.3) The total catalyst area is determined directly from the experiments. Unlike chemisorption, there is no need to know the adsorption stoichiometry to determine the total area. CONCLUSIONS AND SIGNIFIC...

Further development of selective physisorption for measuring catalyst surface area

“…The use of packing factor in place of the fraction of total volume adsorbed (coverage 8 in Miller and Lee, 1984) results in a better sensitivity to small catalyst surface area and thus allows the fractional area to be determined even when the catalyst covers a small portion of the support surface.…”

“…An adsorbate exhibiting a large asymmetric directional polarizability is a good candidate for use in the selective physisorption technique. INTRODUCTION area of supported catalysts using selective physisorption of gases has been put forward recently by Miller and Lee (1984). The basic idea in the original development was that a mono-or submonolayer volume of a gas physisorbed on a two-component solid, as in supported catalysts, should distinguish between the two different surfaces upon thermal desorption if the interactions between the gas and the surfaces are strong, say 3 to 6 kcal/mol in terms of the heat of adsorption.…”

“…In the original development (Miller and Lee, 1984), thermal desorption of physically adsorbed carbon dioxide on the surfaces of potassium carbonate-carbon black mixtures was used to determine the fractional surface area of each component in the physical mixtures. A qualitative comparison between selective physisorption and oxygen chemisorption results was also given for supported silver catalysts.…”

“…Platinum supported on silica was chosen for the comparison since the hydrogen chemisorption has been well established as a reliable method of measuring the catalyst surface area. Although carbon dioxide was used as adsorbate in the original development, nitrous oxide was selected in this study in accordance with the recommendation made earlier (Miller and Lee, 1984) based on the directional polarizability data given by Ross and Oliver (1964). The criteria for the selection of adsorbate have been discussed in detail by Miller and Lee and will not be presented here.…”

“…A detailed description of the apparatus has been given by Miller and Lee (1984) and will not be presented here.…”