The influence of pre-adsorbed water on adsorption of methane on fumed and nanoporous silicas

Co-adsorption of water and methane onto fumed (A-300, A-380) and micro/mesoporous (Gasil 200DF) silicas was studied. FTIR and 1H NMR spectroscopy with layer-by-layer freezing-out of bound water were used at different levels of hydration (h = 0.005–1.0 g of water per gram of silica). Methane adsorption was largest (1–2 wt% at T < 280 K) for nanosilica A-300 (SBET = 337 m2/g) at hydration h = 0.1 g of water per gram of silica for a non-equilibrated system. This sample was characterised by a large amount of weakly associated water (δH ≈ 1 ppm), and maximal clustering of all bound water. These conditions provide the increased microporosity necessary for enhanced methane adsorption. Heating and subsequent wetting, or long equilibration of nanosilica, decreased the adsorption of methane. The adsorption of methane on silica 200DF decreased with increasing amounts of pre-adsorbed water, characterised by significant associativity (δH ≈ 5 ppm) at h ≥ 0.005 g/g.