DRIFTS Spectroscopy
Diffuse Reflectance Infrared Fourier Transform spectroscopy means a special technique of infrared spectroscopy. The method infrared spectroscopy in general exploits the fact that molecules absorb specific frequencies that are characteristic of their structure. These absorptions are resonant frequencies, i.e. the frequency of the absorbed radiation matches the frequency of the bond or group that vibrates. The energies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms, and the associated vibronic coupling.
Infrared light (4000 cm-1 – 400 cm-1) is guided through an interferometer and then through a sample. A moving mirror inside the device alters the distribution of the infrared light. The recorded signal (the interferogram) represents the light output as a function of mirror position. The Fourier transform (a data processing technique) turn this raw data into the desired result (Light output as a function of the wavenumber).
The method of DRIFT spectroscopy is used for heterogeneous samples or powders and solids. In comparison to other IR techniques, only the diffusely scattered radiation from the sample (in our case: sensors or powders) is collected by a special mirror. By this, the surface phenomena of a material can be investigated. The full setup of the measuring system is outlined in Figure 1.
To take the infrared spectrum of a sample, it is necessary to measure both the sample and a reference. This is because each measurement is affected by not only the light-absorption properties of the sample, but also the properties of the instrument. The reference measurement makes it possible to eliminate the instrument influence. The reference spectrum is sampled in background conditions, before exposure to the test gas; the sample spectrum is taken after 15-60 min exposure to the test gas. Mathematically, the sample transmission spectrum is divided by the reference transmission spectrum. The resulting spectrum (transmission) is normally (in case of DRIFT spectroscopy) converted into an absorbance spectrum (absorbance = -log transmission). Figure 2 displays such a resulting absorbance spectrum. In order to verify the results obtained with sensors, it is recommended to perform measurements on samples in powder form; this allows for a better identification of the surface species and helps reduce the probability of taking artifacts into consideration. The method is exactly the same, only the test chamber is different. The powder is put in a small reservoir and heated at the desired temperature, controlled by a thermocouple.
Measuring set up and schedule:
-
Mounting the sensor in a special, homemade test chamber
-
The sensor is heated to the desired temperature by a power supply and connected to a multimeter, recording the resistance
-
Gas flow is controlled by a fully automated gas mixing system using mass flow controllers (MFCs)
-
The measurement starts with exposure to the background gas, followed by a test gas pulse. An IR spectrum is sampled every 15 min. The spectrum recorded during exposure to the background gas is used as the reference spectrum.
The combination of DRIFT spectroscopy and DC resistance measurements (Figure 3) is a powerful tool and offers valuable information about the reactions on the sensor surface (or powder) during exposure to a test gas, allowing the surface reactions that occur to be elucidated. Proposed reaction mechanisms during exposure to ethanol on WO3 at 150 °C are depicted in Figure 4.
