Solid State Nuclear Magnetic Resonance 32 (2007) 109–117 A multiple-field 23 Na NMR study of sodium species in porous carbons Jair C.C. Freitas a,b,Ã , Miguel A. Schettino Jr. a , Francisco G. Emmerich a , Alan Wong b , Mark E. Smith b a Departamento de Fı´sica, Universidade Federal do Espı´rito Santo, 29075-910 Vito´ria, ES, Brazil b Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK Received 8 May 2007; received in revised form 3 September 2007 Available online 11 October 2007 Abstract The sodium environments in porous carbon materials prepared from NaOH activation of a char were investigated by means of multiple-field solid-state 23 Na NMR measurements, carried out at magnetic fields of 4.7, 8.45 and 14.1 T, with single-pulse excitation and magic angle spinning (MAS). The recorded spectra showed a relatively featureless resonance with linewidth and peak shift strongly dependent on the magnetic field strength and on the hydration level of the samples. The existence of second-order quadrupolar effects was inferred, although the structural disorder and the mobile character associated with the Na environment precluded the direct observation of typical quadrupolar features in the MAS NMR spectra. The analysis of the spectra collected at multiple magnetic fields yielded the values of 2.8 ppm for the isotropic chemical shift and 1.8 MHz for the quadrupole coupling constant, which were interpreted as due to Na + ions bonded to oxygenated groups at the edges of the graphene planes within the carbon pore network. r 2007 Elsevier Inc. All rights reserved. Keywords: Activated carbon; Nuclear magnetic resonance; Chemical structure; 23 Na NMR; Multiple magnetic fields 1. Introduction The occurrence and nature of surface groups in porous carbons constitutes an issue of great relevance for catalytic and electrochemical applications of carbon materials. This subject has been intensively studied in recent years using modern textural and spectroscopic techniques, as well as theoretical methods [1–5]. The presence of oxygenated surface Na groups in the porous network of carbon materials has been recently demonstrated by 23 Na NMR measurements in activated carbons prepared by reaction with NaOH [6]. Similar Na groups were detected also in raw coal samples naturally containing sodium in their mineral fraction [7,8]. In the case of the activated carbon materials, the reaction of the carbon-rich precursor (char, anthracite, coal, etc.) with NaOH leads to the formation of sodium carbonate as the main reaction by-product. After water and/or acid washing the carbonate is removed, leaving a porous structure and thus producing materials with increased surface area and microporous volume [9–11]. However, Na + ions remain bound to the carbon surface even after washing, in a disordered arrangement with chemical environments usually associated with oxy- genated surface groups [6,8]. The detailed nature of the bonding/interaction between metal elements (such as Fe, Mn, Li, Na, K, etc.) and carbon structures is still not completely understood. This issue, besides its genuine scientific interest, is of great importance in view of the role played by these metals in electrochemical processes [12,13], in catalytic [14], indus- trial [15], and biomedical [16] applications, as well as in the magnetic properties [17–19] of metal-containing carbon materials. The aim of the present work is to deepen the understanding about the chemical nature of the Na species present in activated carbons, by using 23 Na NMR measurements performed at multiple applied magnetic field strengths. With such an approach, it is possible to reliably extract the true isotropic chemical shift as well as the quadrupolar parameters associated with the Na moieties, and thus make inferences about the chemical ARTICLE IN PRESS www.elsevier.com/locate/ssnmr 0926-2040/$ - see front matter r 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.ssnmr.2007.10.002 Ã Corresponding author. Departamento de Fı´sica, Universidade Federal do Espı´rito Santo, 29075-910 Vito´ ria, ES, Brazil. Fax: +552733352823. E-mail address: jair@npd.ufes.br (J.C.C. Freitas).