Adsorption processes on a carbonaceous surface: Electron spin resonance study

Abstract

The effect of pyrolysis at T ∼700 °C of the low-rank coal samples was studied using scanning electron microscopy (SEM), electron paramagnetic resonance (EPR), and Raman scattering of light methods. The kinetics of adsorption-desorption processes on a carbon surface was first investigated based on the dynamics of the spin characteristics of paramagnetic defects (carbon dangling bonds). The EPR signals of these defects in the air are characterized by the anomalously large broadening up to ΔBpp = 32 mT, nearly two orders of magnitude in comparison with the initial samples. It is established that broadening is caused by the interaction of the spins of defects with molecular oxygen. The latter is adsorbed on the surface of the samples in a high concentration because the samples have a high porosity formed with the participation of ash nanoclusters, including FexOy. The observed EPR dynamics are very complex. It depends on the initial conditions of signal observation, the occurrence of other gases, e.g. nitrogen, and drastically on the humidity of the environment. For example, holding samples in the water vapor at T = 100 °C reduces the ΔBpp from ∼17.5 mT to 3.2 mT for a time of nearly t ∼24 s. Using the strong effect of oxygen on the broadening of the studied samples, the kinetics of competitive processes of adsorption/desorption of N2↔O2↔H2O on the carbon surface is studied in detail. The unusual adsorption kinetics for the mixture of O2 and N2 gases, caused by a difference in the diffusion rates, was found.