The increasing number of environmental zones in large towns with stricter motor vehicle entry allowances, in combination with discussions concerning particulate emissions, is drawing increasing attention to pollutants released from diesel exhaust particulates. Particle filters in diesel vehicles have to be pyrolytically purged of the soot particles that have accumulated. This purge process can be optimized using the results of TGA/FTIR analyses.

Greifensee, Zurich -- (SBWire) -- 06/11/2012 --TGA/FTIR hyphenation for combustion analysis
In order to determine both the quantity and type of gases released during such a heating process, a combination of thermogravimetric analysis (TGA) and a gas analyzer is ideal. TGA is an experimental technique in which the weight of a sample is measured as a function of temperature or time in isothermal experiments. The Fourier transform infrared spectroscopy (FT-IR) excels in gas analysis, which can be used to continuously examine a gas flow both qualitatively and, with the appropriate calibration, quantitatively.

How it works
The sample gas is rinsed from the furnace of a TGA/DSC 1 using a carrier gas stream (usually consisting of nitrogen or argon) and channeled into a gas vessel in the IR beam path via a heated transfer tube. IR spectra are recorded and saved as series at short intervals. Single spectra are then analyzed at particular points in the measurement in relation to typical IR absorption bands of functional groups in the gas mixture. A comprehensive library of gas phase IR spectra can be of huge benefit here. If the gas types have been identified in this way, height profiles can be created across specific absorption bands as a function of the measuring time. These can then be compared with the TGA and DTG curves as shown in the following diagram.

Combustion analysis
The exhaust particulate sample was heated to 800 °C at a rate of 20 K/min under nitrogen atmospheres followed by a switch to an oxygen purging gas to determine the carbon content via combustion.

Since carbon dioxide continues to accumulate after changing to oxygen at 800 °C, the height profile for the wave number 2360cm-1 is hidden in this phase of the measurement. It is clear that, initially, both free and associated water from the diesel exhaust particulates accumulate to a combined total of 3.2%. With further heating, carbon dioxide is identified as the main gas emission together with low quantities of aliphatic hydroTGA/DSC 1 carbons. Above 300 °C, carbon monoxide and methane are also present. The total amount of gas released between 150 °C and 800 °C in this example is 27.3%. The carbon content following combustion in oxygen atmospheres is 67.6% leaving a residue of less than 2%.

The diagrams on the right side show IR single spectra with the reference spectra of the identified gases.

Conclusion
As this example has shown, TGA combined with a FTIR spectrometer allows not onlythe quantity of the released gases to be determined but also the nature (identity) of the evolved gases formed in the TGA to be investigated online. This allows diesel exhaust particles to be analyzed more accurately. Based on this information, the regeneration (purge) process in catalytic converters of diesel vehicles can be greatly optimized.