Microstructure, dangling bonds and impurities in activated carbons

Wide-angle X-ray diffraction (WAXD), magnetometry, electron spin resonance (ESR) and Raman spectroscopy are used to investigate three activated PICA c arbons (viz. GX203, P1400, Med 50) with surface areas of 1000, 1150 and 200 0 m(2)/g, respectively. Although WAXD showed the absence of any minerals in these carbons, temperature and magnetic field variations of the magnetizat ion shows significant magnetic impurities with concentrations in the order GX203 > Med 50 > P1400. Analysis of WAXD and Raman spectra shows these carb ons to consist of graphitic crystallites with L-a similar or equal to 30 An gstrom and L-c similar or equal to 10 Angstrom. All three carbons give an E SR line due to dangling bonds with g = 2.0028(3) but the linewidth Delta H and the spin concentration N-s are strongly affected by air/oxygen. The lin ewidths are reduced from Delta H similar to 4 Oe for the as-received sample s to Delta H similar to 1.2 Oe for the evacuated samples. The N-s values ar e enhanced upon evacuation, with the enhancement factors of about 40, 130 a nd 670 for GX203, P1400 and Med 50 respectively, correlating well with the increase in the relative surface areas of these carbons. Magnetization stud ies using a SQUID magnetometer show that below about 30 K, all three carbon s yield Curie-law variations with N-s similar to 10(19)/gram, in good agree ment with the N-s values obtained from the room temperature ESR studies on evacuated samples. (C) 1999 Elsevier Science Ltd. All rights reserved.