Analysis Types for TCD Analyzers
The basic concept for all analyses is the same: the filament detects changes in the gas mixture flowing past it. The sample, gas selection, and analysis conditions determine what changes occur.
Pulse Chemisorption Analysis Tutorial
Temperature Programmed Desorption Analysis
The BET surface area analysis evaluates the total surface area of the catalyst before and after chemisorption. Pore-plugging phenomena, which might occur due to the irreversible adsorbed species during chemical reactions, and the occurrence of sintering1, can be studied.
After outgassing the sample, a mixture of nitrogen and helium (typically 5% to 30% N2) flows over the sample that is immersed in a liquid nitrogen (LN2) bath. Both the adsorption and desorption of the N2 are recorded. The amount of nitrogen desorbed at LN2 temperatures and the sample weight is used to calculate the total specific surface area.
The entire BET analysis — or even repeat analyses — is performed in situ.
- Langmuir Surface Area Analysis. The Langmuir surface area analysis allows for evaluating the total surface area of the catalyst and is especially useful for adsorbate/adsorbent systems that adsorb only a monolayer. These materials typically exhibit a Type 1 isotherm and are often microporous. The Langmuir surface area analysis may be best applied to zeolites and microporous carbons. Typically the Langmuir surface area will exceed the BET surface area for these materials and provide a more accurate estimation of the total surface area. The Langmuir surface area report may be applied to a wide range of gas concentrations and is not limited to the typical BET range (5% to 30% N2).
- Total Pore Volume Analysis. The total pore volume is a single-point estimate of the pore capacity of a material. The total pore volume analysis is usually conducted near the saturation pressure of the adsorbate (0.995 P/P0). The total pore volume of a material can be determined on both fresh and used materials. The difference in pore volume may indicate pore plugging and directly relate to changes in the performance of catalysts and adsorbents.
For a high surface area sample (> 100 m2/g), a quantity less than 50 mg is recommended. The high sensitivity combined with a large amount of adsorbed gas allows smaller sample quantities to be used while maintaining high precision. The reduced sample quantity will also reduce the likelihood of saturating the high sensitivity detector.
Additional Uses of the TCD Analyzer
The analyzer may also be used for temperature programmed reactions, catalyst pretreatment, and isothermal reactions. The tremendous flexibility of the analyzer allows the use of custom applications.
Temperature Programmed Reaction
A temperature programmed reaction monitors the products from the reaction between gases and a catalyst at a specified temperature. The analyzer can be programmed to raise the temperature of a catalyst bed at a constant ramping rate as the gases flow through the catalyst. At the optimal temperature, the gases react in the presence of the catalyst, creating products. The products of the reaction and excess reactants can be diverted to a gas chromatograph or to a mass spectrometer to be analyzed.
Catalyst Pretreatment
Catalyst pretreatment usually consists of activating a catalyst before its use in a chemical reaction. For example, a temperature programmed oxidation reaction may require reduction of the catalyst under a flow of H2 at a specific temperature.
Isothermal Reaction
An isothermal reaction is similar to a temperature programmed reaction except that the catalyst is kept at a constant temperature (isothermal) to perform the catalytic reaction. Both the product of the reaction and the excess reactants can be diverted to a gas chromatograph or to a mass spectrometer to be analyzed.
Applications
Catalytic processes that benefit from TPD/TPR analyses include:
|
|