An Example of Use
Consider a temperature-programmed reduction (TPR) in which a metal oxide is reacted with hydrogen to form a pure metal, in this case platinum. Argon, which has a very low thermal conductivity, is used as a carrier gas. It is blended in a fixed proportion with hydrogen, an analysis gas with a much higher thermal conductivity. Then the gas mixture flows through the analyzer, through the sample, and past the detector.
When the gas blend begins flowing over the sample, a baseline reading is established by the detector. This baseline is established at a low enough temperature so that no reduction of the sample is occurring. The proportion of gases flowing over the detector is the same as the proportion of gases entering the analyzer, because at the low temperature, there is no interaction.
The temperature is then gradually increased and when a critical temperature is reached, hydrogen atoms in the gas flow react with the sample, forming H2O molecules which are removed from the gas stream using a cold trap. As a result, the amount of hydrogen in the argon/hydrogen gas blend decreases and the proportion between the two gases shifts in the direction of argon, as does the mixture’s thermal conductivity.
Since argon has a lower thermal conductivity than hydrogen, the mixture’s thermal conductivity consequently decreases. The flowing gas removes heat from the filament more slowly, requiring less electricity to maintain a constant filament temperature. The instrument records the electrical demands as it changes (this is called the detector signal). The detector signal is recorded continuously over a range of temperatures. The resulting signals may present either positive or negative peaks.
This example illustrates the fundamental concept upon which the analyzer operates. Of course, the various types of analyses the AutoChem can perform result in different types of traces. For example, a pulse chemisorption analysis results in a series of peaks that gradually increases in size as the sample is dosed with separate but equal increments of gas. Initially, the gas uptake by the sample results in smaller peaks. But when all the active sites are saturated, no more gas can be taken up and the peaks become equal.
Chemical adsorption is an interaction much stronger than physical adsorption. In fact, the interaction is an actual chemical bond where electrons are shared between the gas and the solid surface. While physical adsorption takes place on all surfaces if temperature and pressure conditions are favorable, chemisorption only occurs on certain surfaces and only if these surfaces are clean. Chemisorption, unlike physisorption, ceases when the adsorbate can no longer make direct contact with the surface; it is therefore a single layer process.
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