A small angle neutron scattering study of porous graphitic materials before and after adsorption and condensation of C 6D 6 within the accessible pores

Abstract

Compact specimens of the binder-free Poco graphite AXM-5Q1 (POCO) and of the graphitic Matrix A3-3 (A3-3) were oxidized to a weight loss of 2.3 and 2.0%, and subsequently exposed to deuterobenzene vapor at 323 K. Coherent macroscopic differential neutron scattering cross sections dΣdΩ were measured as a function of the scattering vector h for specimens with either an adsorbed C 6D 6 monolayer or with the accessible pores completely filled by liquid C 6D 6. Filling of all open pores results in a downward parallel shift of the scattering curve (dΣ/dΩ)(h) with respect to the curve measured for the C 6D 6 free specimens, which indicates the similarity of size distribution and shape of open and closed pores. The fraction x of closed pores with a radius R 2 ⩽ 200A˚ is x = 0.57 for A3-3 and x = 0.6 for POCO. The Porod plots 1n (dΣ/dΩ) vs 1n h for POCO are linear in a wide range of h for the specimen with and without an adsorbed C 6D 6 monolayer. Adsorption leads to a slight increase in the slope from −3.0 to −3.1, whereas Porod's law predicts a slope of −4. Theoretical (dΣ/dΩ)(h) data were calculated using the volume fraction/radius distribution function F(R 2) = 0.018R 2 −0.9 and assuming spherical pore shape and a C 6D 6 layer with a thickness of 3.7A˚adsorbed on a smooth boundary between pores and solid. The model reproduces the slight increase in the slope of the Porod plots due to C 6D 6 adsorption. The observed deviation from Porod's law is compatible with a power law volume/radius distribution of smooth walled mesopores in the size range of 12 to 180A˚. The fractal dimension of the accessible surface area in POCO and A3-3 was estimated as D s < 2.2 from adsorption isotherm data.