The AutoChem II Technique:
During the TPR, a metal oxide reacts with hydrogen to form a pure metal. This reaction is referred to as a reduction; for example, TPR of a catalyst containing platinum. Argon, which has a very low relative thermal conductivity, is used as a component in the carrier gas.
It is blended in a fixed proportion with hydrogen, the reducing gas with a much higher thermal conductivity. Then the gas mixture flows through the analyzer, through the sample, and past the detector. When the hydrogen and argon 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 occurs. The baseline level indicated by the detector is that of the thermal conductivity of the two gases in their fixed proportion.
The temperature is then raised and, when a critical temperature is reached, hydrogen atoms in the gas flow react with the sample, forming H2O molecules. The H2O molecules are removed from the gas stream using a cold trap.
As a result, the amount of hydrogen in the argon/hydrogen gas blend inside the analyzer 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 demand as it changes (this is called the detector signal). The detector signal is recorded continuously over a range of temperatures. When these readings are graphed, the data form one or more peaks. Peaks can be positive or negative.
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.
Click on image above to view the Chemisorption Poster