Peculiarities in the Water Adsorption and Morphology of Carbon Chars Assessed by NMR

Abstract

Proton spin–lattice relaxation and 1H NMR spectroscopic data are used for studies of the water adsorption and morphology of several newly synthesized carbon-based chars. Two NMR lines have been observed in accordance with a difference between macropore and micropore water in a system with heterogeneous pore structure. The macropores occupy about the half of the char particle volume. In aqueous suspension fast molecular exchange between water in macropores and bulk water leads to an intense NMR line with a single spin–lattice relaxation time T1, which exhibits a strong particle size effect. This effect has been used for the estimation of T1 of water molecules in the vicinity of paramagnetic centers at the surface of chars, as well as for macropore size estimation. The kinetics of water vapor adsorption in chars as seen by 1H NMR indicates that this process begins in micropores, which serve as primary adsorption centers. Water molecules in micropores exhibit specific properties which are characteristic of liquids in a small, restricted space, and they give rise to the observed accumulation as well as saturation effects.