PERGAMON Carbon 38 (2000) 1257–1259 Letter to the Editor The effects of the electronic structure of micropores on the small angle scattering of X-rays and neutrons a, a a b * Peter J. Hall , Steve Brown , John Fernandez , J.M. Calo a Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G11XL, Scotland, UK b Chemical Engineering Program, Division of Engineering, Box D, Brown University, Providence, RI 02912, USA Received 6 May 1999; accepted 18 March 2000 Keywords: A. Porous carbon; C. Neutron scattering, X-Ray Scattering; D. Electronic properties, Microporosity Gas adsorption techniques represent what are perhaps number of potentially important applications of micropor- the most common approach to the characterisation of the ous carbons; e.g., for methane storage, for which it has it pore structure of porous materials. Within this general area, has been shown theoretically [5] that adsorption is a strong the most popular technique by far is to derive surface area function of pore size. and pore size distribution information from nitrogen Small angle scattering (SAS) techniques represent an adsorption isotherms obtained at 77 K. Surface areas are alternative to gas adsorption methods, with a number of most typically obtained from BET analyses (multipoint or advantages. SAS is sensitive to both closed and open single point), and pore size distributions from a number of porosity and in many cases offers a more complete picture different techniques. Mesopore size distributions are usual- of porosity. SAS can also be applied to ‘‘wet’’ samples. ly obtained from techniques based on various adaptations The two most common subatomic scattering particles used of the Kelvin equation, such as the BJH [1] and DM [2] in SAS are X-rays and neutrons, which can both be approaches. A number of different approaches have been produced at suitable wavelengths for small angle scattering proposed for micropore analysis, although it appears that analyses. The generally greater accessibility to, and availa- no single technique is applicable to all situations. These bility of X-ray instruments has made small angle X-ray range from micropore volume determinations using the scattering (SAXS) more common than small angle neutron so-called t -plot and a -plot methods, involving compari- scattering (SANS). sons of experimental isotherms to standard isotherms, to The physical mechanisms of scattering are completely pore size distribution determinations based on micropore- different for the two types of subatomic scattering par- filling arguments, such as those related to the Dubinin– ticles. For SAXS, interactions with electrons are most Radushkevitch equation, and others, like the Horvath– important, whereas interactions with nuclei are more Kawazoe [3] and density functional theory [4] methods. important for SANS. There have been no direct com- Often, pore size distribution determinations based on parisons between these two techniques for microporous micropore filling arguments use CO adsorption at 273 K, carbons and it has generally been implicitly assumed that 2 to circumvent activated diffusion problems which may be they yield essentially the same information. The purpose of encountered by nitrogen at 77 K in the smallest micropores the present communication is to explore this latter assump- (i.e., the ultramicropores). tion via a direct comparison between SAXS and SANS The theoretical limitations of these techniques are well data for a microporous carbon. documented, and often different techniques result in micro- SAXS measurements were carried out using a Kratky pore size distributions which may differ significantly from camera mounted on a fully stabilised Phillips generator, one another. This state of affairs can be inadequate for a Type PW 1010/1. The generator was operated at 40 kV and 20 mA. A Ni filter was used for the Cu radiation. b Both entrance and counter slits had slit widths of 250 mm. *Corresponding author. Tel.: 144-141-548-4084; fax: 144- Carbon samples were loaded into a 2 mm thick sample 141-548-4822. E-mail address: p.j.hall@strath.ac.uk (P.J. Hall). holder, 1 cm wide and 1.6 cm in height, covered with 0008-6223 / 00 / $ – see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(00)00096-8