Oxygen-17 and deuterium NMR investigation into the adsorption of water on activated carbon

Abstract

Magic-angle spinning (MAS) 2H and 17O NMR spectra were obtained for water, following addition to an activated carbon. Shortly after addition two peaks are seen: a 'free liquid' resonance and a second broad resonance shifted to low frequency. The latter is attributed to water in the pores of the activated carbon. Over time, some intensity from the free water peak shifts into the lower frequency peak and a further decrease in the average chemical shift is seen. This is ascribed to gradual diffusion of water into the smaller pores with time. Comparisons between the 2H and 17O spectra are drawn with regard to the likely orientation of water molecules on the carbon surface in relation to the effects on chemical shifts by ring currents due to the graphene sheets. Spectra are presented for sealed samples spun at different MAS rates, and it is demonstrated that rates above 3 kHz induce loss of water from the pores, increasing the intensity of the free water peak. The implications of this centrifugation effect are discussed. Copyright © 2000 John Wiley & Sons, Ltd.