Understanding the Adsorption of Noble Gases in Metal-Organic Frameworks Using Diffuse Reflectance Infrared Fourier Transform Spectroscopy

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to study noble gas (i.e., He, Kr, and Xe) adsorption in metal-oxide frameworks (MOFs). To study the adsorption of infrared (IR)-inactive noble gases, we monitored the intensity of MOF stretching vibrations as a function of gas pressure using the DRIFTS spectra. Specific and nonspecific adsorption sites were identified in Ni/DOBDC (DOBDC = 1,4-dioxido-2,5-benzenedicarboxylate) and HKUST-1 (HKUST = Hong Kong University of Science and Technology) MOFs after noble gas adsorption, and the results were consistent with those obtained from single-crystal synchrotron X-ray diffraction (XRD) experiments and results published in the literature. Our results demonstrate that the DRIFTS technique can be an alternative approach for locating gas adsorption sites in MOFs. The adsorption selectivity of Xe/Kr for the two MOFs had qualitatively related to the peak saturation pressure in the DRIFTS spectrum, and the results are consistent with those obtained from isotherm data. In addition to the Xe adsorption process, we investigated the Xe desorption process by using the DRIFTS technique for HKUST-1 to validate the analysis of DRIFTS spectral data. We also studied competitive adsorption between Xe and trace amounts of water in the Ni/DOBDC.