High-pressure 129Xe NMR study of supercritical xenon confined in the mesopores of FSM-16

Abstract

Pressure (0–10 MPa) and pore size dependence of 129Xe NMR spectra of xenon confined in FSM-16 (pore diameter d = 1.9, 2.7, 4.1 nm) were measured by a high-pressure NMR probe. In the sample with d = 1.9 nm pores, the 129Xe spectrum of the confined xenon had a peak at ca. 95 ppm (P = 0.3 MPa) and consisted of two components above 2 MPa, a high field one due to relatively mobile xenon and a low field one due to strongly adsorbed xenon. The line-width of the weakly confined xenon broadened with increase in pressure and reached a maximum in the vicinity of the critical point of free xenon gas, while that of the bulk xenon remained almost constant in the whole pressure range of the measurement. In the sample with d = 4.1 nm pores, the line shape of the confined xenon could be described by a single component, implying that a type of critical phenomenon took place in the mesopores with a wide opening in the vicinity of the critical point of the bulk xenon. In order to investigate the exchange between the free and the confined xenon, 2D-exchange NMR spectra was measured.