A systematic resolution of sulfur in reticulated vitreous carbon using X-ray absorption spectroscopy.
Sulfur K-edge X-ray absorption spectroscopy (XAS) was used to characterize the approximately 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 (approximately 3%), thianthrene (approximately 9%), disulfide (approximately 10%), sulfenate ester (approximately 12%), benzothiophene (approximately 24%), N,N'-thiobisphthalimide (approximately 30%), alkyl sulfonate (approximately 1.2%), alkyl sulfate monoester (approximately 6%), and sulfate dianion (approximately 6%). Permanganate oxidation of RVC diminished sulfenic sulfur to approximately 9%, thianthrenic sulfur to approximately 7%, and sulfate dianion to approximately 1% but increased sulfate monoester to approximately 12%, and newly produced sulfone (approximately 2%) and sulfate diester (approximately 5%). A simple thermodynamic model was derived that allows proportionate functional group comparisons despite differing (approximately +/-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 approximately 0.15% of the RVC surface, increasing to approximately 0.51% following permanganate/sulfuric acid treatment. The detection of episulfide directly corroborates a proposed mechanism for the migration of elemental sulfur through carbon.
Frank
Patrick
P
Department of Chemistry, Stanford University, Stanford, California 94305, USA. Frank@ssrl.slac.stanford.edu
George
Serena DeBeer
SD
Anxolabéhère-Mallart
Elodie
E
Hedman
Britt
B
Hodgson
Keith O
KO
D000042:Absorption; D002244:Carbon / Q000737:chemistry* / Q000378:metabolism; D004220:Disulfides / Q000737:chemistry; D006571:Heterocyclic Compounds / Q000737:chemistry; D008956:Models, Chemical; D010084:Oxidation-Reduction; D013057:Spectrum Analysis / Q000379:methods*; D013431:Sulfates / Q000737:chemistry; D013455:Sulfur / Q000032:analysis* / Q000737:chemistry; D013457:Sulfur Compounds / Q000737:chemistry; D013816:Thermodynamics; D013876:Thiophenes / Q000737:chemistry; D014965:X-Rays
D004220:Disulfides; D006571:Heterocyclic Compounds; D013431:Sulfates; D013457:Sulfur Compounds; D013876:Thiophenes; C088015:benzothiophene; C009449:thianthrene; D013455:Sulfur; D002244:Carbon