Two-Dimensional IR Pressure-Jump Spectroscopy of Adsorbed Species for Zeolites

摘要

Infrared spectroscopy of adsorbed probe molecules is a well established technique for the characterization of surface sites on zeolites and other solid catalysts. Time resolution in modern spectrometers increases the power of the technique for use with complex adsorption systems, with multiple adsorption sites and a microporous structure, by allowing the study of transient systems and of adsorption dynamics out of equilibrium. Step-scan Fourier transform interferometers can easily reach the microsecond timescale if the observation can be repeated with sufficient reproducibility. At this timescale, adsorbed molecules in a zeolite each have their eigen-response frequency to a pressure perturbation or modulation. Using pressure and temperature measurements, Rees and co-workers and Grenier and co-workers have explored the pressure modulation frequencies, indicated the frequencies for given adsorbates on given zeolites, and extracted diffusion kinetic parameters from their results. We thought we could use these eigen frequencies to extract spectral information for complex mixtures on surfaces. Thus, several adsorption sites, or several adsorbed molecules in a mixture, would each lead to a specific eigen-response frequency to a pressure modulation, and we could obtain the spectral signature for each individual species on each individual adsorption site. Instead of having to vary the modulation frequency progressively to explore the whole frequency domain, we decided to generate a square pressure wave on the surface (or more exactly a pressure jump), which would be equivalent to the generation of an infinite number of pressure oscillations with different frequencies (Figure 1). Such a pressure jump can be described as a shift of the adsorption in an excited state, with a relaxation by diffusion and adsorption equilibria. The relaxation process can be followed by fast time-resolved IR spectroscopy. The infrared spectra collected can be analysed by Fourier transform to extract the relaxation processes eigen frequencies, with correlations with the infrared frequencies. The technique is named IR pressure-jump spectroscopy of adsorbed species (PJAS-IR).