Programmed temperature reduction
The temperature-programmed reduction (TPR) is used for examining the reducibility or the thermal behavior of solids in function of the temperature. Usually hydrogen is used as a reducing agent.
This method is particularly used in heterogeneous catalysis to characterize catalysts .
procedure
The solid or catalyst is first weighed into a sample vessel, which can be a simple U-tube, for example. This sample vessel is positioned in an oven that is equipped with a temperature control. In addition, a thermocouple records the temperature in the solid bed.
First, an inert gas ( nitrogen , argon ) flows through the sample vessel in order to remove the air present. With the help of flow regulators, hydrogen is added to the pure inert gas flow (for example 10 percent by volume hydrogen in nitrogen).
Now the sample is heated in the oven in a defined manner. The heating rates are mostly between 1 K / min and 20 K / min. The hydrogen content that is still present behind the sample vessel is measured and recorded by suitable detectors ( thermal conductivity detector , mass spectrometer ). If hydrogen was consumed, a reduction process took place at the temperature just reached.
Results
The hydrogen consumption is usually shown as a function of temperature. Information about the number of reduction processes can be derived from the diagram obtained with possibly several peaks. A carefully carried out measurement also allows a quantitative determination of the hydrogen consumption and thus (if the amount of substance weighed in) a determination of the degree of reduction.
If a mass spectrometer is used to detect the remaining hydrogen , the products of the reduction processes (water or organic products) can also be detected. This can lead to further insights into the reduction mechanism.
Even if the measurement cannot be quantified or the peaks obtained cannot be clearly assigned to the possible reduction processes, the TPR can be used to compare several samples with regard to their thermal behavior. This can be used in quality control.
evaluation
It should be noted that the quality of the measurement results depends crucially on the parameters heating rate β (in K / min), substance amount m (in g), gas flow (in mL / min) and hydrogen content in the reaction gas (in% by volume).