129Xe Nuclear Magnetic Resonance Study of Pitch-Based Activated Carbon Modified by Air Oxidation/Pyrolysis Cycles: A New Approach to Probe the Micropore Size

Reevaluation of the Suitability of 129Xe Nuclear Magnetic Resonance Spectroscopy for Pore Size Determination in Porous Carbon Materials

Xenon isotope nuclear magnetic resonance (129Xe-NMR) spectroscopy has been widely used to evaluate the pore structure of materials. However, determining how to apply this technique to investigate porous carbon materials is sometimes challenging, partly due to the structural disorder and heterogeneity of the surface properties of these materials, and partly due to the lack of reliable methods for controlling and assessing the density of adsorbed Xe. In this study, we designed and constructed a temperature- and pressure-controllable 129Xe-NMR system to evaluate the interaction between activated carbon (AC) and adsorbed Xe molecules. Based on a confirmation of surface-covering adsorption form of Xe molecules in AC pores, the extrapolation to the ordinate in plots of the surface area-normalized Xe adsorption amount (ρXe) and measured 129Xe-NMR chemical shifts of adsorbed Xe molecules, δ(Xe), to remove the influence of the Xe-Xe interaction allowed us to estimate the interaction between the AC pore surface and a single Xe molecule. These data confirmed that interactions between the pore surface and adsorbed Xe molecules depend on a parameter related to the AC pore size, regardless of the content of oxygen-containing surface functional groups, and an empirical equation to estimate the average pore size of ACs from the 129Xe-NMR results was proposed. Downward deviations of the linear correlation between ρXe and δ(Xe) were attributed to the influence of paramagnetism presumably derived from oxygen-containing functional groups on the surfaces of ACs and to changes in the adsorption form in low- and high-ρXe regions, respectively. These findings confirm the suitability of 129Xe NMR for pore size determination in porous carbon materials.

129Xe: A Wide-Ranging NMR Probe for Multiscale Structures

Porous materials are ubiquitous systems with a large variety of applications from catalysis to polymer science, from soil to life science, from separation to building materials. Many relevant systems of biological or synthetic origin exhibit a hierarchy, defined as spatial organization over several length scales. Their characterization is often elusive, since many techniques can only be employed to probe a single length scale, like the nanometric or the micrometric levels. Moreover, some multiscale systems lack tridimensional order, further reducing the possibilities of investigation. 129Xe nuclear magnetic resonance (NMR) provides a unique and comprehensive description of multiscale porous materials by exploiting the adsorption and diffusion of xenon atoms. NMR parameters like chemical shift, relaxation times, and diffusion coefficient allow the probing of structures from a few angstroms to microns at the same time. Xenon can evaluate the size and shape of a variety of accessible volumes such as pores, layers, and tunnels, and the chemical nature of their surface. The dynamic nature of the probe provides a simultaneous exploration of different scales, informing on complex features such as the relative accessibility of different populations of pores. In this review, the basic principles of this technique will be presented along with some selected applications, focusing on its ability to characterize multiscale materials.

Optimization of activated carbon fiber preparation from Kenaf using K2HPO4 as chemical activator for adsorption of phenolic compounds

The present work reports the preparation of activated carbon fiber (ACF) from Kenaf natural fibers. Taguchi experimental design method was used to optimize the preparation of ACF using K(2)HPO(4). Optimized conditions were: carbonization at 300 degrees C, impregnation with 30%w/v K(2)HPO(4) solution and activation at 700 degrees C for 2h with the rate of achieving the activation temperature equal to 2 degrees C min(-1). The surface characteristics of the ACF prepared at optimized conditions were also studied using pore structure analysis, scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy. Pore structure analysis shows that micropores constitute the most of the porosity of the prepared ACF. The ability of the ACF prepared at optimized conditions to adsorb phenol and p-nitrophenol from aqueous solution was also investigated. The equilibrium data of phenol and p-nitrophenol adsorption on the prepared ACF were well fitted to the Langmuir isotherm. The maximum adsorption capacities of phenol and p-nitrophenol on the prepared ACF are 140.84 and 136.99 mg g(-1), respectively. The adsorption process follows the pseudo-first-order kinetic model.