Temperature
Programmed Reduction (TPR) and Oxidation (TPO)
TPR/TPO techniques permit to find the most efficient reduction/oxidation conditions. Furthermore, the supported precursor phases and their interactions with the support can be identified. These experiments are particularly useful in case of multi-metallic systems, for the evaluation of the role of the added compounds or doping agents (alloy formations or promotion effects).
In the TPR technique an oxidized catalyst precursor is submitted to a programmed temperature rise, while a reducing gas mixture is flowed over it (usually, hydrogen diluted in some inert gas like argon). In the TPO technique, the catalyst is in the reduced form and is submitted to a programmed temperature increase, but in this case, an oxidizing mixture of gas (oxygen in helium) is flowed over the sample.
The reduction or oxidation rates are continuously measured by monitoring the change in composition of the reactive mixture after the reactor. The decrease in H2 or O2 concentration in the effluent gas with respect to the initial percentage monitors the reaction progress. An interesting application of this technique is that the TPR/O analysis may be used to obtain evidence for the interaction between the atoms of two metallic components, in the case of bimetallic system or alloy as already cited.
In general, TPR/TPO studies are carried out under low partial pressure of the reactive gas. In this way it is possible to observe the intermediate reactions, depending from analytical conditions such as temperature rate, flow rate and concentration of reactive gas.
The TPR/TPO methods are used for qualitative and quantitative analysis. In effect, the spectra produced are characteristic of a given solid. TPO is less commonly used than TPR, but the quantitative considerations for this type of analysis are more correct, in particular if the two analyses are performed in succession (hydrogen/oxygen titration).
When used in combination, the two techniques can provide useful information in the study of the reactivity and redox behavior of catalysts. The combination of the two reactions is a real titration of the hydrogen/oxygen consumption, permitting the calculation of the metal phase percentage in the catalyst (of course if the stoichiometric factor of the reaction is known). Another advantage of combining the two analyses is that the TPO permits to remove undesired contaminants to concentrate the attention on the characterisation of the catalyst active phase.