Controlled physisorption (IGC): new ways of catalyst characterizations

Do you know, that highly quantitative values for diffusion coefficients, adsorption isotherms of methanol on zeolites and energy distribution functions, as well as detailed characterization of recycled and artificially aged catalysts can all be performed by perfectly controlled adsorption and desorption processes?
​We have entirely redefined an instrument also known under the name “Inverse Gas Chromatography”, short IGC. Today, our setup is the most versatile instrument to study all kind of physisorption processes – not only for catalysts, but for many other powders, fibers and plates too. 

We were so much looking forward to showing our poster (download below) about catalyst applications at the “53. Jahrestreffen Deutscher Katalytiker” scheduled for 11-13 March in Weimar – usually attracting around 600 experts! But: Due to the Corona crisis, this conference was cancelled. Now we are taking the online opportunity to explain the key features of our unique setup and explain the use by three very different applications. 

Key feature of our IGC instrument is very flexible injection of any kind of volatile “probe” substance into the gas stream – either at very low concentrations, ideally “infinite dilution”, or at high concentrations at defined p/p0, e.g. 0.3, to allow a saturation of the surface. In IGC terms, these two regions are called IGC-ID and IGC-FC (finite concentration). An autosampler and a most accurate dosing system allows up to 48 different probe substances. We usually use 15 different apolar and polar molecular probes to characterize surfaces in its different dimensions. 

A standard IGC characterization (IGC-ID) is done by using different n-alkanes at infinite dilutions to determine the disperse (apolar) surface energy 𝛾sd. Usually, we are adding seven different polar molecules ranging from chloroform to diethylether thereby covering electron acceptor (acidic) and donor (basic) probes. In the example, the customer wanted to characterize the changes of catalysts from fresh to used and the processes of recycling and artificially aging.  The analysis revealed very clearly, that the recycling process is not only far from restoring the original surface properties but also increasing apolar properties far beyond those of fresh or used catalysts. 

The ability to saturate the entire surface at a given p/p0 (IGC-FC) with any kind of probe molecule and to interpret the desorption isotherm in detail was used in a study of zeolites and methanol adsorption. The IGC-FC setup determines the adsorption isotherm, the specific surface area (BET), any irreversible adsorption and the Adsorption Energy Distribution Function (AEDF). The zeolites are very close in performance, but the adsorption of methanol and subsequent desorption revealed significant differences in the amount of irreversible bound methanol and in the distribution of the high energy sites. This way, it enabled a better selection and further optimization of the zeolites for application. 

Also, saturation at a given p/p0 and desorption allows the determination of diffusion coefficients (D) in porous systems. In a setup called Zero Length Column (ZLC), a very small amount of sample is exposed to a given p/p0 and the sudden desorption front interpreted using a ln(c/c0) plot to obtain D by fitting the desorption curve. In the given example, the diffusion coefficients of toluene, cyclooctane and n-octane were determined for two different alumina beads. 
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These examples show the power of the experimental setup by injecting any kind of volatile probes at different concentrations and accurately determine the physisorption behavior. The IGC framework provides a solid background to determine disperse surface energies but many more values can be obtained, like diffusion coefficients or adsorption energy distribution functions. 

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