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O: Fachverband Oberflächenphysik

O 31: Surface Chemical Reactions and Heterogeneous Catalysis I

O 31.1: Talk

Tuesday, March 27, 2007, 15:45–16:00, H38

Theoretical insight into the oxidation of nanoporous carbon materials — •Johan M. Carlsson and Matthias Scheffler — Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin

Nanoporous carbon(NPC) may be visualized as crumpled graphene sheets with a significant amount of voids. Oxygen can be used to modify its properties and STM indicates that O₂ etches edges and vacancies in graphitic materials[1]. TPD shows desorption of CO and CO₂ indicating the presence of C-O-groups on the surface[2]. However, the characterization of the actual surface structure of oxidized NPC is unsatisfactory and the individual reaction steps for the oxidation are still not well known. We have therefore carried out an extensive study of NPC using ab-initio thermodynamics. Our density-functional theory (DFT) calculations show that O₂ dissociation is endothermal on defect free graphene surfaces, but exothermal at vacancies. The O-atoms attach to the undercoordinated C-atoms at vacancies forming C-O-groups. C-O-C (Ether) groups dominate and C=O (Carbonyl) groups may also appear. The C=O groups are precursors for CO-desorption, but two C-C bonds needs to be broken to desorb the CO-molecule. Our thermodynamic treatment shows that CO therefore only desorbs above 700 K from these sites in agreement with the TPD experiment[2]. Based on these results we propose that the oxidative etching of NPC materials proceeds via a sequence of dissociative O2 adsorption and CO desorption. [1] J. J. Paredes et al. , Carbon 38, 1183, (2000). [2] M. J. Nowakowski et al., J. Am. Ceram. Soc. 76, 279 (1993).