290
S.S.Barton, B. H. Harrison, and J. Dollimore
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978
Surface Studies on Graphite. Desorption of Surface Oxides Formed on the Clean Surface at 300 K Stuart S. Barton," Brian H. Harrison, Depatiment of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, Ontario K17L 2 W3, Canada
and John Dollimore Department of Pure and Applled Physics, University of Salford, Salford, England (Received September 12, 1977) Publication costs assisted by the Defense Research Board of Canada
A study of the surface oxides formed on the clean surface of graphite at 300 K has revealed that the modes of formation of CO and COz, by thermal desorption of graphite up to 1223 K, are intimately related. Analysis of the desorption kinetics showed that at a given temperature both products were desorbed with the same activation energy. The variation of the activation energy of desorption of the major product CO with coverage was found to have a plateau region where the activation energy of desorption remained reasonably constant for large changes in the oxide coverage. The average desorption energy over this range was 275 kJ mol-'. The variation of the velocity constant of desorption with coverage has also been calculated and discussed in terms of a mobile surface oxide.
Introduction Surface oxides on carbon and graphite have a considerable influence on the surface properties of the substrate and are also important intermediates in the oxidation of these materials. These oxides desorb as CO and COz at temperatures up to 1273 K, and cover only a fraction of the total surface area. On graphite they are usually assumed to be located on the prismatic (or edge) planes of the crystallites where the graphitic layers terminate with unsaturated bonds.' Oxygen chemisorption has been used to measure this "active surface", and help to distinguish between different types of surface site^.^-^ Analysis of the kinetics of desorption of the oxides as CO and COz has also been used to investigate the nature of these oxides.6-10 During a recent studylo it became quite clear that the desorption energetics of the oxides originally present on the surface of graphite were different from those formed on its clean surface. The present communication reports the results of a more detailed study of the oxides formed on the clean surface of graphite at 300 K. Also to compliment current desorption studies on graphite using flash desorption techniques, it was felt desirable to measure the variation of the velocity constant of desorption with coverage, since the calculation of desorption energy distributions from flash desorption studies usually requires the assumption that the velocity constant for desorption does not vary with coverage.
Experimental Section The graphite sample and experimental procedures used in this work were the same as those used in an earlier study.1° The vacuum system, described previously, was rebuilt with UHV components, bakeable metal valves, and a VacIon pump to obtain pressures Torr over the sample during surface cleaning. In this procedure, the graphite was maintained at 1223 K for 15 h, and the whole vacuum system baked at 523 K. The clean graphite was then cooled to and thermostated at 300 K and surface oxides were formed by exposure to an oxygen pressure of 760 Torr for 1000 min. Since it has been established that after the removal of the oxides originally present on the graphite, the first two chemisorption-desorption cycles take up slightly more oxygen than subsequent ones," the 0022-3654/78/2082-0290$0 1.OO/O
TABLE I: Total Amount (pmol g-') of CO and C O , Desorbed after Oxygen Chemisorption at 300 K
co
co,
81.6
17.10 17.08
Experiment 1 Experiment 2 Experiment 3
80.9 81.8
AV
81.4
17.10 17.09
graphite was subjected to three chemisorption-desorption cycles before measurements were made. Desorption of the surface oxides was carried out by using a step heating program, in which the sample was held at a constant temperature for 30-min periods. At the end of each period the temperature was abruptly raised by 50 K and held at the new temperature for a further 30-min period. This procedure was repeated until the sample temperature was at 1223 K. The desorbed gases were transferred into calibrated reservior volumes using a diffusion pump. This procedure maintained a pressure
