FT-IR Diffuse Reflectance Sampling with an Environmental Chamber for In Situ Reaction Monitoring

J.L. Briggs, PIKE Technologies

FT-IR diffuse reflectance sampling coupled with an environmental temperature chamber offers a convenient method to study sample degradation, to evaluate catalyst efficiency, and to determine reaction pathways as a function of temperature and pressure.

The aim of this application note is to present data from a model system to illustrate the FT-IR technique of diffuse reflectance used in combination with an environmental temperature chamber. Specifically, the dehydration of calcium oxalate monohydrate was monitored in situ.

Instrumentation

The PIKE DiffusIR™, a diffuse reflectance accessory, was used in conjunction with the PIKE environmental temperature chamber (Figure 1). The temperature range of the chamber used in this application was from ambient to 500 °C. Two other chambers are available, an ambient to 900 °C and a -150 to 500 °C low temperature version. Pressures up to 1500 psi may be achieved by using a high pressure adapter with the heated temperature chamber models. Furthermore, the chamber design allows for reacting or inert gases to be flowed through the sample.

Figure 1: The PIKE DiffusIR with environmental sampling chambers and temperature controller.

Measurement Parameters

Calcium oxalate monohydrate was mixed at a 1% level with KBr. The mixture was loaded into the porous-bottom ceramic sample cup and the cup was placed in the chamber. The loaded chamber was then introduced into the DiffusIR and secured with the push-lock pins for a tool-less insertion. The sample was held at 80 °C for 30 min to remove surface water from the sample followed by heating to 160 °C at a rate of 5 °C/min. Spectra were collected every 5 °C. A constant vacuum of 0.1 × 10^-6 Torr was applied to the chamber to remove water vapor throughout the experiment.

All spectra were collected at 8 cm^-1 spectral resolution over the range of 4000–400 cm^-1 . Twenty-four scans were co-added at each sampling temperature. A DTGS detector was used.

Application Results

The Kubelka-Munk transformation was used to linearize the spectral data. The spectra are shown in Figure 2. Most all of the absorbance bands between 3700–2800 cm^-1 are attributed to H-O-H stretching, while an absorbance band near 3258 cm^-1 is a result of the H-O-H bending overtone (1). H-O-H bending bands are located at 949 and 883 cm^-1 . Upon heating, bound water is released as may be observed by the decreasing absorbance bands of H-O-H vibrations as a function of temperature. The type of spectra collected in this application note could be used to determine kinetics parameters. Christy and colleagues offer kinetic models for a similar system (2). They show that the dehydration of calcium oxalate monohydrate is caused by two reactions.

Figure 2: FT-IR spectra of calcium oxalate monohydrate heated from 80 to 160 °C at 5 °C/min. All spectra displayed at equivalent y-scaling.

References

(1) I. Petrov and B. Soptrajanov. Spectrochim Acta Part A 31:309–316, (1975).

(2) A. Christy, et al. Appl. Spectroscopy 48(5), 561–568, (1994).

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