Direct Observation of the Xenon Physisorption Process in Mesopores by Combining In Situ Anomalous Small-Angle X-ray Scattering and X-ray Absorption Spectroscopy.

Eike Gericke,Michael Krumrey,Armin Hoell,Simone Raoux,Giorgia Greco,Dirk Wallacher,Simone Mascotto,Robert Wendt

doi:10.1021/acs.jpclett.1c00557

Abstract

The morphology and structural changes of confined matter are still far from being understood. This report deals with the development of a novel in situ method based on the combination of anomalous small-angle X-ray scattering (ASAXS) and X-ray absorption near edge structure (XANES) spectroscopy to directly probe the evolution of the xenon adsorbate phase in mesoporous silicon during gas adsorption at 165 K. The interface area and size evolution of the confined xenon phase were determined via ASAXS demonstrating that filling and emptying the pores follow two distinct mechanisms. The mass density of the confined xenon was found to decrease prior to pore emptying. XANES analyses showed that Xe exists in two different states when confined in mesopores. This combination of methods provides a smart new tool for the study of nanoconfined matter for catalysis, gas, and energy storage applications.

Highlights

The morphology and structural changes of confined matter are still far from being understood
As a result of the extensive presence of interfaces compared to the bulk, materials confined in mesopores experience a massive change in their physicochemical properties.[1−5] In general, the magnitude of these changes is directly correlated to the curvature of internal surfaces and the surface to volume ratio in these systems, and it is inversely proportional to the pore radius.[6−8] The understanding of these phenomena is required for a broad field of applications ranging from condensed matter physics[1,9] to catalysis and separation science,[10,11] passing through the characterization of mesoporous hosts using gas adsorption analysis.[2,12]
A major contribution was provided by computational methods[6,13−15] and gas adsorption experiments,[16−19] these approaches were necessarily limited by the assumption of simple pore structures and of macroscopic physicochemical parameters of the confined adsorbate.[2]

Summary

Introduction

The morphology and structural changes of confined matter are still far from being understood. The most significant steps of the physisorption process (e.g., capillary condensation) have to be interpreted using poorly resolved curves, leading to limited understanding.[22] a method for the direct investigation of adsorbate phase evolution is highly desirable.

Results

Conclusion