Characterization of the nanoporous carbon material using EPR spin probe method

摘要

Electron paramagnetic resonance spectroscopy was used for the determination of adsorption properties of activated carbon material (ACM) with high effective surface and nano-sized pores. We have studied carbonized material synthesized from natural and synthetic precursors - coconut shell and phenol-formaldehyde pitch; stable nitroxide radicals TEMPO(2,2,6,6-tetramethyl-1-piperidinyloxyl) and TEMPOL(4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxyl) were used as spin probes. It was shown that the measured EPR spectra of nitroxides in susupensions of ACM were a superposition of two components, triplet and singlet. The first component belongs to the fast tumbling of nitroxides in solution, and the second is a result of strong spin exchange interaction between the radicals adsorbed in carbon nanopores. Double integration of the EPR spectra of initial solutions and suspensions as well allowed to evaluate the adsorption capacity of the carbons examined. Adsorption kinetics was investigated using measurements of EPR triplet intensity decay in time. Temperature dependence of rotational diffusion correlation times of spin probes revealed the phase transition of water incapsulated into ultramicropores at the temperature approximately 200 K.