XPS and NMR studies of phosphoric acid activated carbons A.M. Puziy a, * , O.I. Poddubnaya a , R.P. Socha b , J. Gurgul b , M. Wisniewski c a Institute for Sorption and Problems of Endoecology, National Academy of Sciences of Ukraine, Naumov Street 13, 03164 Kyiv, Ukraine b Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30–239 Krako ´w, Poland c Physicochemistry of Carbon Materials Research Group, Department of Chemistry, Nicholas Copernicus University Gagarin Street 7, 87–100 Torun, Poland ARTICLE INFO Article history: Received 6 May 2008 Accepted 3 September 2008 Available online 11 September 2008 ABSTRACT Chemical structure of phosphorus species in two series of polymer-based and fruit-stone- based carbons obtained by phosphoric acid activation at 400–1000 °C were investigated by XPS and solid state 31 P-NMR and 13 C-NMR. It has been shown that the most abundant and thus thermally stable phosphorus species in all investigated carbons is phosphate-like structure bound to carbon lattice via C-O-P bonding. Small contribution of phosphonates (C-P-O linkage) was observed by 31 P NMR in carbons obtained at temperature range of 500–700 °C, phosphorus oxide was evidenced by XPS in carbon prepared at 900 °C and ele- mental phosphorus in carbon activated at 1000 °C. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction It is common knowledge that the properties of carbon adsor- bents are determined by porous structure and surface chem- istry [1]. Pore size and pore volume are important factors for physical adsorption [2–4], while surface chemistry plays a key role in specific adsorption and surface reactions [4–7]. Surface chemistry of carbon materials depends on the pres- ence of heteroatoms like hydrogen, oxygen, nitrogen, phos- phorus, chlorine etc. [4,5,8] that may come from carbon precursor or activating agent [9]. Heteroatoms affect acid– base characteristics of carbons and modify their electrochem- ical and catalytic properties [4,10–12]. Oxygen-containing sur- face groups confer hydrophilic and cation exchange properties [13–15]. Nitrogen-containing carbons show en- hanced anion exchange properties [15,16] and catalytic activ- ity in red–ox reactions [11,17,18]. Phosphorus-containing carbons show a number of specific characteristics that range from acid surface groups and cation exchange properties [19– 23] to enhanced oxidation stability [24]. Recent studies have shown that phosphoric acid activation results not only in developing porosity of carbons but also leads to inclusion of significant amount of phosphorus into carbon structure [19–23]. It has been shown that phosphorus compounds are responsible for enhanced cation exchange properties of phosphoric acid activated carbons. The studies of phosphoric acid activated carbons by chemical analysis, FTIR spectroscopy, XPS and pH-titration haveshown that sur- face groups of phosphoric acid activated carbons may be clas- sified as phosphorus-containing, carboxylic, lactone and phenol like groups [19–23,25]. Phosphorus-containing surface groups were ascribed to condensed phosphates formed dur- ing pyrolysis of carbonaceous precursor in presence of phos- phoric acid [19–22]. However, above studies were unable to show explicitly how phosphorus species are bound to phos- phoric acid activated carbons. To shed light on the chemical structure of phosphorus spe- cies and the way they are bound to carbon matrix two series of phosphoric acid activated carbons obtained at different temperatures from polymer and lignocellulosic precursors were examined by XPS and solid-state NMR methods. 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.09.010 * Corresponding author: Fax: + 380 44 4529328. E-mail address: alexander.puziy@ispe.kiev.ua (A.M. Puziy). CARBON 46 (2008) 2113 – 2123 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon