A Systematic Resolution of Sulfur in Reticulated Vitreous Carbon Using X-ray Absorption Spectroscopy
journal contributionposted on 27.11.2006, 00:00 by Patrick Frank, Serena DeBeer George, Elodie Anxolabéhère-Mallart, Britt Hedman, Keith O. Hodgson
Sulfur K-edge X-ray absorption spectroscopy (XAS) was used to characterize the ∼0.1% sulfur found both in native reticulated vitreous carbon (RVC) foam and in RVC oxidatively modified using 0.2 M KMnO4 in 2 M H2SO4. Sulfur valences and functional groups were assessed using K-edge XAS spectral curve-fitting and employing explicit sulfur compounds as models. For native RVC, these were episulfide (∼3%), thianthrene (∼9%), disulfide (∼10%), sulfenate ester (∼12%), benzothiophene (∼24%), N,N‘-thiobisphthalimide (∼30%), alkyl sulfonate (∼1.2%), alkyl sulfate monoester (∼6%), and sulfate dianion (∼6%). Permanganate oxidation of RVC diminished sulfenic sulfur to ∼9%, thianthrenic sulfur to ∼7%, and sulfate dianion to ∼1% but increased sulfate monoester to ∼12%, and newly produced sulfone (∼2%) and sulfate diester (∼5%). A simple thermodynamic model was derived that allows proportionate functional group comparisons despite differing (∼ ±15%) total sulfur contents between RVC batches. The limits of accuracy in the XAS curve-fitting analysis are discussed in terms of microenvironments and extended structures in RVC carbon that cannot be exactly modeled by small molecules. Sulfate esters cover ∼0.15% of the RVC surface, increasing to ∼0.51% following permanganate/sulfuric acid treatment. The detection of episulfide directly corroborates a proposed mechanism for the migration of elemental sulfur through carbon.