Controlling the oxidation is essential for tailoring the properties of carbon materials. Experiments on the one hand have shown that carbon materials become efficient catalysts for oxidative dehydrogenation of ethylbenzene by cofeeding with oxygen [1]. The complete oxidation of carbon materials on the other hand leads to combustion. Scanning tunnelling microscopy (STM) has accordingly shown that vacancies in highly oriented pyrolitic graphite (HOPG) are etched in an oxygen atmosphere [2]. Measurements of the growth rate of the vacancies led to an estimate of the activation energy of 1.7 eV for the etching, but the microscopic information about the etching process is still limited. Temperature-programmed desorption (TPD) experiments from graphite after oxidation, show predominantly CO desorption [3]. Multiple peaks due to CO desorption are observed above 900K, but it is uncertain which oxygen groups that are precursors for the desorption of these CO molecules. We have carried out an extensive study of oxidation of perfect and defective graphene in order to provide theoretical insight to oxidation process. The aim is to assign the oxygen groups that are formed during oxidation of graphene-based carbon materials to the desorption peaks observed in the TPD experiments and to determine the oxidation mechanism for the etching of vacancies. Our density-functional theory (DFT) calculations reveal that the O2 dissociation is endothermal on the defect free basal plane. The vacancies are more reactive towards oxygen in agreement with the STM experiments. The oxidation of the vacancies proceeds via a two-step mechanism: undercoordinated C-atoms that sit at the edge of the vacancy are quickly saturated by oxygen groups in an oxygen atmosphere. Our thermodynamic treatment shows that C-O-C (ether) and C=O (carbonyl) groups occur most frequently at oxidized vacancies. Based on a comparison of the calculated binding energies to the experimental desorption temperature, we identify the carbonyl group as the source of CO in the TPD experiments. Our calculations furthermore indicate that the etching of the vacancies is activated by adsorption of additional O2-molecules at vacancies already saturated by oxygen groups. The dissociative adsorption of O2 forms larger oxygen groups, such as C-O-C=O (lactones) and O=C-O-C=O (anhydrides) that can desorb as CO2. The desorption energy for CO2 from these oxygen groups is lower than for CO desorption from carbonyl groups. This suggests that the vacancies are always saturated by a variety of oxygen groups in an oxygen atmosphere. The etching then proceed via adsorption of additional O2-molecules at the vacancies that form the less stable oxygen groups that desorb as CO2.
[1] G. Mestl, N. I. Maksimova, N. Keller, V. V. Roddatis, and R. Schlügl, Angew. Chem. Int. Ed. 40, 2066 (2001).
[2] B. Marchon, J. Carazza, H. Heinemann, and G. A. Samorjai, Carbon 26, 507 (1988).
[3] F. Stevens, L. A. Kolodny, and T. P. Beebe Jr., J. Phys. Chem. B 102, 10799 (1998).
Controlling the oxidation is essential for tailoring the properties of carbon materials. Experiments on the one hand have shown that carbon materials become efficient catalysts for oxidative dehydrogenation of ethylbenzene by cofeeding with oxygen [1]. The complete oxidation of carbon materials on the other hand leads to combustion. Scanning tunnelling microscopy (STM) has accordingly shown that vacancies in highly oriented pyrolitic graphite (HOPG) are etched in an oxygen atmosphere [2]. Measurements of the growth rate of the vacancies led to an estimate of the activation energy of 1.7 eV for the etching, but the microscopic information about the etching process is still limited. Temperature-programmed desorption (TPD) experiments from graphite after oxidation, show predominantly CO desorption [3]. Multiple peaks due to CO desorption are observed above 900K, but it is uncertain which oxygen groups that are precursors for the desorption of these CO molecules. We have carried out an extensive study of oxidation of perfect and defective graphene in order to provide theoretical insight to oxidation process. The aim is to assign the oxygen groups that are formed during oxidation of graphene-based carbon materials to the desorption peaks observed in the TPD experiments and to determine the oxidation mechanism for the etching of vacancies. Our density-functional theory (DFT) calculations reveal that the O2 dissociation is endothermal on the defect free basal plane. The vacancies are more reactive towards oxygen in agreement with the STM experiments. The oxidation of the vacancies proceeds via a two-step mechanism: undercoordinated C-atoms that sit at the edge of the vacancy are quickly saturated by oxygen groups in an oxygen atmosphere. Our thermodynamic treatment shows that C-O-C (ether) and C=O (carbonyl) groups occur most frequently at oxidized vacancies. Based on a comparison of the calculated binding energies to the experimental desorption temperature, we identify the carbonyl group as the source of CO in the TPD experiments. Our calculations furthermore indicate that the etching of the vacancies is activated by adsorption of additional O2-molecules at vacancies already saturated by oxygen groups. The dissociative adsorption of O2 forms larger oxygen groups, such as C-O-C=O (lactones) and O=C-O-C=O (anhydrides) that can desorb as CO2. The desorption energy for CO2 from these oxygen groups is lower than for CO desorption from carbonyl groups. This suggests that the vacancies are always saturated by a variety of oxygen groups in an oxygen atmosphere. The etching then proceed via adsorption of additional O2-molecules at the vacancies that form the less stable oxygen groups that desorb as CO2.
[1] G. Mestl, N. I. Maksimova, N. Keller, V. V. Roddatis, and R. Schlügl, Angew. Chem. Int. Ed. 40, 2066 (2001).
[2] B. Marchon, J. Carazza, H. Heinemann, and G. A. Samorjai, Carbon 26, 507 (1988).
[3] F. Stevens, L. A. Kolodny, and T. P. Beebe Jr., J. Phys. Chem. B 102, 10799 (1998).
