湿氧化对活性炭表面化学性质的影响

Abstract

A series of activated carbons with different degrees of activation were oxidized with H,0,, (NH ),S,O, and HNO, inorder to introduce different oxygen surface complexes. Changes in the surface chemistry of the activated carbons after theroxidizing treatments were studied by different technigues including temperature-programmed desorption (TPD). X-ravphotoelectron spectroscopy (XPS), Fourier transformed infrared spectroscopy (FTIR, titrations with HCl and NaOHmeasurements of the pH of the point of zero charge and catalvtic dehvdration of methanol. Results showed that treatmentwith (NH,),S,O, fixed the lowest amount of both total oxygen and surface acid groups. However, this treatment yielded theacid groups with the highest acid strength. This could be because it favors fixation of carboxy! groups close to other groupssuch as carbonvl and hvdroxvl. which enhances their acidity. 

1. Introduction

The surface chemistry of carbon materials is basicallydetermined by the acidic and basic character of theirsurface, and can be changed by treating them withoxidizing agents either in the gas phase or in solution.These treatments fix a certain amount of oxygen surfacecomplexes such as: carboxyls, lactones, phenols, ketones.quinones, alcohols and ethers that make the carbon materi-als more hydrophilic and acidic, decreasing the pH of theirpoint of zero charge and increasing their surface chargedensity [l-11]. At the same time, the surface basicity hasbeen found to decrease suggesting that the surface basicsites are essentially of the Lewis type, associated with melectron-rich regions found on the basal planes. Thus, anincrease in the oxygen content of the carbon diminishes theelectronic density of the basal planes and, consequently.reduces the basicity of the carbon surface [11-19].

Different oxidizing agents can be used in aqueoussolutions to introduce oxygen surface complexes [12], andwe have shown recently [7,8] that not all of these behave in the same way. Thus, when activated carbons preparedalmond shells were oxidizedwith HNO. orfrom(NH , ),SO, the latter treatment yielded oxidized activatedcarbons with stronger acid groups than the former, in spiteof the fewer oxygen surface complexes [7,8]. The aim ofthe present work was to gain more insight into the changesoccurring in the surface chemistry of activated carbonswith different degrees of activation, after their wet oxida-tion with nitric acid, hydrogen peroxide and ammoniumperoxydisulphate. These changes were followed by tem-perature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), Fourier transformed infraredspectroscopy (FTIR), titrations with NaOH and HCl.measurements of the pH of the point zero charge, pHpzc;and by the behavior of the oxidized ccarbons in thedehydration reaction of methanol.

2. Experimental

Activated carbons were prepared from olive stones. Theraw material supplied by an olive mill was sieved to obtaina particle size between I and 1.5 mm, which was treatedwith a diluted H,SO. solution (10%) and washed withdistilled water till absence of sulphate ions in the washing water. This material was carbonized in a N, flow at 1273K for l h, and after that, steam activated at 1123 K fordifferent periods of time to obtain various degrees ofactivation, following the method explained elsewhere [8].The activated carbons so prepared will be referred to in thetext as BV followed by a number that indicates the degreeof activation, or percentage of burn-off, during the steamactivation step.

The samples obtained were oxidized with HNO,, H,Oand (NH,),S,O,. The oxidized samples, or the treatmentfollowed to obtain them, will be referred to in the text asN,H or S, respectively. The procedures followed tooxidize the activated carbons have been explained in detailelsewhere [7,8]. After the N and S treatments the sampleswere washed with distilled water till absence of nitrate andsulphate ions, respectively, in the washing water.

All samples were characterized by N, adsorption at 77K by applying the BET equation to the N, adsorptionisotherm. Mercury porosimetry data were obtained up to afinal pressure of 4200 kg cm-2 using an Austoscan 60equipment. From this technique, the pore volume of' poreswith a diameter between 3.6 and 50 nm, 以, and the porevolume of pores with a diameter greater than 50 nm wereobtained.

Surface chemistry of activated carbons was studied byTPD, XPS, FTIR, titration with NaOH and HCl and pHpzcmeasurements. TPD experiments were carried out byheating the samples up to 1273 K in He flow at a heatingrate of 20 K min ' and recording the amounts of CO andCO, evolved with a quadrupole mass spectrometer, firomBalzers, model Thermocube, as a function of temperatureas described elsewhere.

XPS measurements were made with a Physical Elec-tronic 5700 equipment with a Mg K. X-ray excitationsource (hy= 1253.6 eV) and hemispherical electron analyzer. Prior to the analysis, the samples were heated in situat 393 K for l h under high vacuum and introduced in theanalysis chamber without any contact with the atmosphereThe residual pressure in the analysis chamber was main-tained below 10- Torr during data acquisition. Surveyand multiregion spectra were recorded at Oi and C1sphotoelectron peaks. The atomic concentrations werecalculated from the photoelectron peak areas, using Shirleybackground subtraction 21] and sensitivity factors pro-vided by the spectrometer manufacturer PHl.