Thermo analytical techniques
For heterogeneous solid catalysis understanding the interaction between fluids and active sites on solid surface is very important as it determine the surface reactivity. These kinds of interactions are studied by many thermoanalytical techniques, carried out dynamically by application of linear temperature programme. These techniques involve recording the surface or bulk interaction of solid catalysts with gaseous environment as the temperature is increased, by continuously analyzing the gas phase composition. The temperature programmed reduction (TPR), temperature programmed oxidation (TPO), temperature programmed desorption (TPD), thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) are among the techniques used extensively for characterization of catalysts.
Temperature programmed reduction (TPR)
Temperature programmed reduction is used to determine the reducibility of the catalysts. In this method, reducible catalysts are exposed to flow of reducing gas mixture typically H2 in Ar. The sample is initially pretreated in an oxidative atmosphere (5% O2 in He) at predetermined temperature for 30 min and cooled to 2500C in the same mixture. The sample is then purged with helium and cooled to room temperature. Then the preoxidized sample is reduced in a flow of H2 in argon from room temperature to 9000C. The carrier gas flow is maintained at a specified value (usually 30 ml /min). Temperature is linearly increased at the desired heating rate (usually 10 K/ min). The extent of reduction is continuously followed by measuring the composition (H2 content) of the reducing gas mixture at the outlet of the reactor. The total amount of H2 consumed is determined from the area under the peak and is used to calculate the degree of reduction and average oxidation state of the solid material after reduction.
Fig.1. shows the arrangement used for TPR/TPO analysis. The same instrument as that used for chemisorption can be used for this study.
Fig. 1. Schematic diagram for TPR/TPO analysis
Parameters that affects TPR profiles
- Heating rate, K/s
- Initial amount of reducible species, μmol
- Flow rate of reducing gas mixture, cm3 /sec
- Concentration of H2 in carrier gas, ( μmol/cm3)
A higher heating rate increases the temperature of the sample faster compared to a slower heating rate, consequently reduction peaks for the sample appear at comparatively higher temperature than that at lower heating rate. On the other hand if the concentration of the hydrogen in the reducing gas mixture is increased the samples are reduced more easily resulting in sharper peaks at lower temperature compared to that at lower hydrogen concentration. Change in gas flow rate has small effect. As the initial amount of reducible species in samples increases, the area under the peak increases.
Hence the parameters have significant influence on shape of TPR profiles so that comparison between experimental results obtain under different conditions is difficult to make. Hence for comparison all TPR should be carried out at same process conditions.
Application:
Solid catalysts containing reducible metals are characterized using TPR. Following infomations can be gained from TPR curves:
1. Reduction peak temperature indicates the ease of reduction and degree of interaction between different species present in the catalyst sample. A higher reduction temperature indicates higher difficulty in reduction which can be attributed to the greater degree of interaction between the active metal and support.
2. Multiple peaks indicate the presence of metal in different forms on the support having different level of interaction between species and support.
3. The extent of reduction can be calculated as follows:
