Transient rates in the reaction of CO2 with highly oriented Pyrolytic graphite

✍ Scribed by G.S. Rellick; F. Rodriguez-Reinoso; P.A. Thrower; P.L. Walker Jr.

• Publisher: Elsevier Science
• Year: 1975
• Tongue: English
• Weight: 234 KB
• Volume: 13
• Category: Article
• ISSN: 0008-6223
• DOI: 10.1016/0008-6223(75)90261-4

✦ Synopsis

Transient gasification rates in the reaction of CO* with spectroscopically pure natural graphite have been shown by Shelef and Walker[l] to be closely related to the purity of the reacting system. In particular, hydrogen and hydrogen~ontain~ng gases prevent the occurrence of high transient rates, apparently through chemisorption on active sites involved in transient gasification. Even in highly purified CO,, vigorous outgassing conditions were needed to produce high rate transients. Shelef and Walker proposed that the kinetics of transient gasification could be explained by a model involving two kinds of surface oxides differing in reactivity. The more reactive oxide is responsible for the high initial rate. The decay of the transient to a steady-state value was postuiated to occur through the transition of the more reactive oxide into a stable type.

This same transient phenomenon has been examined by us using highly oriented, compression-annealed pyrolytic graphites, supplied by the Carbon Products Division of Union Carbide Corporation. The characterization of these graphites, using X-ray diffraction and 0, etching, has been reported on in detail elsewhere [2,3]. Samples were cut into parallelepiped sections, the basal surfaces were cleaved with cellophane tape, and the sample edges were polished with very fine emery paper. The polishing served to eliminate much of the roughness introduced during the cutting process [4]. Before reaction with CO,, samples were oxidized in O2 (to about 2 per cent burn-off) to further eliminate any loose or disordered material141 and then were outgassed under a vacuum of about 5 x 1O-6 Torr. Reactions were carried out at about 50Torr CO, pressure and at temperatures of 1050 and 1100°C. Samples were suspended by a quartz fiber from a Cahn RG Electrobalance. The signal from the balance was fed to a