In-Situ Time-Dependent Attenuated Total Reflection Fourier Transform Infrared (ATR FT-IR) Spectroscopy of a Powdered Specimen in a Controlled Atmosphere: Monitoring Sorption and Desorption of Water Vapor

Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy is a powerful instrumental method of chemical analysis of solids and liquids. The majority of published studies by in-situ ATR-FTIR spectroscopy describe analysis of homogeneous samples, such as liquid solutions under circulation, or films on the ATR crystal that react with the gas of interest. The in-situ ATR-FTIR spectroscopic studies of specimens in physical shape of crystals or powder that react with a gas or vapor are rare. This work describes a modification of in-situ time-dependent ATR-FTIR spectroscopy to allow monitoring heterogeneous reaction “solid-gas” of powder in controlled atmosphere and in the time domain. Also, we describe a new facile gas flow chamber attachment to ATR-FTIR spectrometer which allows creating controlled atmosphere surrounding the specimen on the ATR crystal. Additionally, the capabilities of the described in-situ time-dependent ATR-FTIR spectroscopy experiment in controlled atmosphere are enhanced by the sensor for in-situ time-dependent monitoring the relative humidity (RH) of air surrounding the specimen. The operation of the setup for in-situ time-dependent ATR-FTIR spectroscopy in controlled atmosphere is demonstrated by monitoring reaction of gradual desorption of water vapor from color-indicating molecular sieves under controlled low air humidity. Further, the described spectroscopic method and apparatus is applied to monitor the reverse process, namely sorption of water vapor by color-indicating molecular sieves under controlled elevated air humidity. Water molecules are found to reversibly interact with two distinct sorption sites in the sorbent: the Si-O backbone and the color-indicating Co(II) centers. The reported variant of in-situ time-dependent ATR-FTIR spectroscopy in controlled atmosphere is powerful, yet facile and straightforward. It is promising for mechanistic, in-situ studies of sorption, desorption, chemo-sensing, heterogeneous catalysis and photocatalysis, and analysis of chemical kinetics of various “solid-gas” reactions.