In situneutron diffraction study of adsorbed carbon dioxide in a nanoporous material: Monitoring the adsorption mechanism and the structural characteristics of the confined phase

The behavior of molecules confined in a nanoporous material was studied by adsorption from the gas phase with in situ neutron diffraction, with the aid of a high-pressure adsorption apparatus and the General Materials diffractometer (GEM, ISIS). The aim of this work is to establish the combined adsorption/neutron diffraction technique as a unique tool to elucidate both the adsorption mechanism and the structure of molecules confined in nanoporous materials. For this reason, a well-defined mesoporous system (MCM-41) was selected as adsorbent and carbon dioxide as adsorbate, while diffraction measurements have been carried out along a CO₂ adsorption isotherm (T=253 K) in the pressure range of 0--18 bar. Small-angle results at low pressures favor the development of an adsorbed film rather than filling of wall microporosity, while diffraction data provide evidence of a distinct molecular arrangement of this adsorbed layer. At higher pressures diffraction data reveal that the structure of the condensed phase is directly comparable to that of bulk liquid CO₂.