Multivariate Analysis of Infrared Spectra for Monitoring and Understanding the Kinetics and Mechanisms of Adsorption Processes

Abstract

Water adsorption onto thin zeolite 3A wafers has been investigated as a function of time, water vapor concentration, and zeolite sample mass using mid-infrared spectroscopy coupled with multivariate data analysis. Principal component analysis (PCA) of the spectral region of the water combination band was used for quantitative characterization of water adsorption onto the zeolite. The kinetics of the adsorption of water are found to be very reproducible and nearly linear with time. The kinetics of water adsorption based on data from different masses of zeolite are consistent with a diffusion/immobilization model for which the interparticle diffusion rate is comparable to the rate of adsorption. The infrared zeolite bands (1340–1550 cm⁻¹) change during the adsorption process and yield more detail about the adsorption sites of the material. PCA applied to the zeolite bands was not directly interpretable. However, multivariate curve resolution applied to the spectral region containing the zeolite bands readily demonstrates that zeolite 3A has three water adsorption sites or environments that are sequentially occupied. Potential explanations for the observations of the multivariate curve resolution (MCR) analysis of these infrared (IR) kinetic adsorption experiments are presented. The explanation most consistent with our data suggests that water adsorbs sequentially on the zeolite to form single, double, and triple water adsorption on single zeolite adsorption sites. The combination of infrared spectroscopy and multivariate analysis is therefore demonstrated to be a powerful method to study detailed adsorption kinetics and mechanisms of the adsorption of molecules onto surfaces.