FT-IR Spectra of Water in Microporous KBr Pellets and Water's Desorption Kinetics

Abstract

The structure and spectra of water adsorbed on microporous (pore size <3 μm) KBr pellets were investigated, with the use of an infrared absorption technique at 220 < T < 450 K; P = 0.13 Pa, and differential scanning calorimetry (DSC) measurements at 100 < T < 320 K. In addition, water's overall desorption kinetics from KBr pellets were determined by isothermal runs at 330 < T < 400 K, P = 0.13 Pa by means of in situ transmission Fourier transform infrared (FT-IR) spectroscopy. The results suggest that KBr/water interaction is complex and that water is retained by pellets in three modes. In the first, water forms a monolayer around KBr particles in which hydrogens of water are held at the surface via bonding between hydrogen and Br⁻ ions. This monolayer contains the dominant water in KBr pellets. In the second mode, small water clusters are trapped between the KBr particles and in micropores, and we believe that this type of water is trapped during pellet preparation. Additional water bands, observed at ∼3300 and ∼1670 cm⁻¹, have been assigned to a coordinated type of water, near or at the surface, which constitutes a third type. DSC results indicate that none of the three types of water are capable of forming ice in KBr pellets. On increasing the water content in the pellets, additional water is adsorbed on the top of the monolayer type water, with the other two types being uneffected. The water's desorption kinetics data were tested for ten different decomposition models, and the results can best be described by a 3-dimensional diffusion desorption model, suggesting that micropores are blocked by KBr particles and/or that pores do not run across the width of the pellet.