Pergamon PII: SOOOS-6223(97)00171-l Carbon Vol. 36. Nos. l-2, pp. 145-151, 1998 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008-6223/97 $17.00+ 0.00 zyxwvuts EFFECTS OF NON-OXIDANT AND OXIDANT ACID TREATMENTS ON THE SURFACE PROPERTIES OF AN ACTIVATED CARBON WITH VERY LOW ASH CONTENT C. MORENO-CASTILLA,* F. CARRASCO-MAR~N, F. J. MALDONADO-H~DAR and J. RIVERA-UTRILLA Grupo de Investigacibn en Carbones, Departamento de Quimica InorgBnica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain zyxwvutsrqponmlkjihgfedcbaZYXWV (Received 7 March 1997; accepted in revised form 5 August 1997) Abstract-An activated carbon obtained from olive stones and with very low ash content (0.10%) was treated with either HCI, HF or HNO,. The changes in surface area and porosity resulting from the acid treatments were studied by N, and CO, adsorption at 77 and 273 K, respectively and by mercury porosimetry. The changes in surface chemistry were studied by temperature-programmed desorption and Fourier transformed infrared spectroscopy. The treatments with HCl yielded activated carbons on which some chlorine remained chemisorbed, whereas the HF treatment did not fix any fluorine. Due to this, the HCI treatment had a slight effect on the microporosity of the samples. Moreover, the HF treatment increased the amount of CO-evolving surface groups. The treatment with HNO, destroyed the pore walls to a large extent, fixing a large amount of oxygen surface groups. The nature and structure of the CO- and CO,-evolving groups will be discussed in detail. 0 1997 Elsevier Science Ltd All rights reserved zyxwvutsrqponmlkji Key Words-A. Activated carbon, B. chemical treatment, D. surface properties, D. functional groups. 1. INTRODUCTION Activated carbons and coal chars are frequently treated with non-oxidant acids such as HCl and HF in order to reduce their mineral matter content [l- 31. Depending on the amount and nature of the mineral matter as well as its distribution in the carbon matrix, the demineralization process can bring about changes in the surface area and pore texture of the sample, since the mineral matter can block a part of the carbon porosity. However, the acid treatment itself might modify the surface area and porosity of the sample independently of the changes introduced by removal of the mineral matter. On the other hand, the treatment of activated carbons and coal chars with oxidant acids, such as HNO,, apart from removal of the mineral matter introduces oxygen surface complexes that change the surface chemistry and can alter the surface area and porosity of the original sample [4-lo]. The objective of this work was to study the effects of acid treatments with HCl, HF and HNO, on the surface area, porosity and surface chemistry of an activated carbon with a very low ash content (0.10%) prepared from a lignocellulosic material. Thus, if changes occur in the surface properties of the samples after the acid treatments these will not be due to removal of the mineral matter. 2. EXPERIMENTAL An activated carbon was prepared from olive stones. The raw material, with a particle size between *Corresponding author. 145 1 and 1.4 mm was placed in a quartz boat inside a horizontal tubular furnace (from Heraeus) and was carbonized in N, flow (300 cm3 min-‘) at 1273 K for 1 hour and then activated in CO, flow (300 cm3 min-‘) at 1263 K for 10 hours up to 54% burn-off. The sample so obtained was ground and sieved to give a particle size between 0.5 and 0.8 mm. This activated carbon will be referred to in the text as A. Different portions of A were treated with HCl. HF, alternately with HCl and HF, and with HNO,. For treatment with HCl, the sample was mixed with concentrated HCl, 1 g/10 mL, and the suspension stirred for 1 hour at around 333 K. The HF treatment was carried out in a similar way. These two samples will be referred to in the text as AC1 and AF, respectively. The treatment with HCl and HF was as follows: the sample AC1 was treated with HF as described above. This sample will be referred to in the text as AClF. For the treatment with HNO,, activated carbon A was mixed with concentrated HN03, 1 g/10 mL, and the suspension was evaporated at around 333 K until dryness. This sample will be referred to in the text as AN. After acid treatments all samples were washed thoroughly with distilled water until halides or nitrate could no longer be detected by AgNO, or brucine, respectively, in the washing water. The halogen content of the samples was analyzed by X-ray photo- electron spectroscopy (XPS) with an Escalab 200R equipment using Mg Kcc (hv= 1253.6 eV) as a radia-