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120. An electron microscopic investigation of the graphite-water reaction
REACTIVITY
AND SURFACES
225
revealed. The method enables the dimensions of each crystallite to be determined directly; the degree of parallelism of adjacent crystallites to be estimated; and the nature of the interface separating any given pair of crystallites to be characterized. Boundaries of pure tilt, of pure twist and of mixed character may be identified from the trace of the interface plane (developed by catalytic oxidation) on the basal surface. Large variations in the reactivity from one crystallite to another within a single crystal have been noted, there being evidence to suggest that some residual lattice impurities (analyzed mass-spectrographically) and localized accumulations of lattice defects (particularly vacancies) may be important in this connection. The implications of these findings in relation to studies of reactivity and diffusion are briefly discussed (15 min). l
Work supported by the U.K.A.E.A.,
Hanvell.
120. An electron microscopic investigation of the graphitewater
reaction.*
G. L. Montet and G. E. Myers (Argonne National Laboratory, Argonne, Illinois). The initial stages of oxidation of highly purified single crystals of natural graphite by water vapor at high temperatures has been studied by electron-microscopic examination of etch pits developed in the graphite surface. At a concentration of 2.2 x 104 VPM an activation energy of 66 kcal mol-1 was found. At concentrations of 104,4 x lo*,2 x 103, and 10s VPM an activation energy of 84 kcal mol-1 was found. Comparison of the rates at 950°C gives an apparent order of 0.7 over the concentration range studied. l
Based on work performed under the auspices of the U.S. Atomic Energy Commission.
121. Kinetics of oxygen chemisorption by Graphon. R. 0. Lussow, F. J. Vastola and P. L. Walker, Jr, (Pennsylvania State University, University Park, Pennsylvania). Samples of Graphon were activated to nine different levels by oxidation to burn-offs up to 35 per cent. These samples were reacted with oxygen in a static reactor at temperatures between 450 and 675°C with initial oxygen pressures varying from 6 to 73 millitorr. The surface of activated Graphon was found to be composed of groups of active sites with differing reactivities. The active surface areas of the samples were determined by chemisorption of oxygen and also calculated from the reactivity data. The chemisorptions were performed in the static system for twenty-four hours from an initial oxygen pressure of 0.5 torr at temperatures between 300 and 550°C. The amount chemisorbed did not increase significantly in the temperature range 300 to 450°C. However, between 450 and 550°C the amount chemisorbed increased sharply. The active surface area estimated from the low surface coverage data using an integrated form of the rate law was found to be in substantial agreement with the coverage attained by chemisorption in 24 hr at 300°C from an initial oxygen pressure of 0.5 torr. 122. Oxygen chemisorption on weIl cleaned carbon surfaces.* P. J. Hart,t F. J. Vastola and P. L. Walker, Jr. (Pennsylvania State University, UniversityPark, Pennsylvania). Graphon, a highly graphitized carbon black, was first oxidized to 14.4 per cent weight loss in 0, at 625°C to introduce significant active surface area. Following the cleaning of the activated Graphon surface by heating at 975°C in a vacuum of 10-a torr, chemisorption of oxygen between 25 and 400°C was studied. This was done by exposing the “cleaned” Graphon to a pressure of 500 millitorr of 0, until negligible further adsorption took place. For most temperatures, 24 hr was sufficient. Between 25 and 250°C the saturation amount of oxygen adsorbed increased only slightly with increasing temperature, between 250 and 300°C it increased sharply, and between 300 and 400°C it again increased only slightly. This behavior suggeststhe presence of at least two types of active sites on the activated Graphon surface. The maximum amount of oxygen adsorbed is estimated to occupy 2.8 ma/gor 2.6 per cent of the total surface. The rate of oxygen chemisorption on the more active sites is given by dn/dt = k&z, (I-O)‘, where k, ia a rate constant, C is the Oa concentration in the gas phase, na is the saturation coverage with oxygen on the more active sites, and 0 is the fraction of the more active sites that are covered at time t. The rate constant equalled 7.14 x lo9 exp (-7,40O/RT) cma/sec mole OS. l Work supported by tbc U.S. Atomic Energy Commi~on on Contract AT(30-l)-1710 and Petroleum Research Fund Grant PRF-1361-A2 various stages. t Present address: Dow Chemical Company, Midland, Michigan. NASA Fellow, 1964-1966.
at
AND SURFACES
225
revealed. The method enables the dimensions of each crystallite to be determined directly; the degree of parallelism of adjacent crystallites to be estimated; and the nature of the interface separating any given pair of crystallites to be characterized. Boundaries of pure tilt, of pure twist and of mixed character may be identified from the trace of the interface plane (developed by catalytic oxidation) on the basal surface. Large variations in the reactivity from one crystallite to another within a single crystal have been noted, there being evidence to suggest that some residual lattice impurities (analyzed mass-spectrographically) and localized accumulations of lattice defects (particularly vacancies) may be important in this connection. The implications of these findings in relation to studies of reactivity and diffusion are briefly discussed (15 min). l
Work supported by the U.K.A.E.A.,
Hanvell.
120. An electron microscopic investigation of the graphitewater
reaction.*
G. L. Montet and G. E. Myers (Argonne National Laboratory, Argonne, Illinois). The initial stages of oxidation of highly purified single crystals of natural graphite by water vapor at high temperatures has been studied by electron-microscopic examination of etch pits developed in the graphite surface. At a concentration of 2.2 x 104 VPM an activation energy of 66 kcal mol-1 was found. At concentrations of 104,4 x lo*,2 x 103, and 10s VPM an activation energy of 84 kcal mol-1 was found. Comparison of the rates at 950°C gives an apparent order of 0.7 over the concentration range studied. l
Based on work performed under the auspices of the U.S. Atomic Energy Commission.
121. Kinetics of oxygen chemisorption by Graphon. R. 0. Lussow, F. J. Vastola and P. L. Walker, Jr, (Pennsylvania State University, University Park, Pennsylvania). Samples of Graphon were activated to nine different levels by oxidation to burn-offs up to 35 per cent. These samples were reacted with oxygen in a static reactor at temperatures between 450 and 675°C with initial oxygen pressures varying from 6 to 73 millitorr. The surface of activated Graphon was found to be composed of groups of active sites with differing reactivities. The active surface areas of the samples were determined by chemisorption of oxygen and also calculated from the reactivity data. The chemisorptions were performed in the static system for twenty-four hours from an initial oxygen pressure of 0.5 torr at temperatures between 300 and 550°C. The amount chemisorbed did not increase significantly in the temperature range 300 to 450°C. However, between 450 and 550°C the amount chemisorbed increased sharply. The active surface area estimated from the low surface coverage data using an integrated form of the rate law was found to be in substantial agreement with the coverage attained by chemisorption in 24 hr at 300°C from an initial oxygen pressure of 0.5 torr. 122. Oxygen chemisorption on weIl cleaned carbon surfaces.* P. J. Hart,t F. J. Vastola and P. L. Walker, Jr. (Pennsylvania State University, UniversityPark, Pennsylvania). Graphon, a highly graphitized carbon black, was first oxidized to 14.4 per cent weight loss in 0, at 625°C to introduce significant active surface area. Following the cleaning of the activated Graphon surface by heating at 975°C in a vacuum of 10-a torr, chemisorption of oxygen between 25 and 400°C was studied. This was done by exposing the “cleaned” Graphon to a pressure of 500 millitorr of 0, until negligible further adsorption took place. For most temperatures, 24 hr was sufficient. Between 25 and 250°C the saturation amount of oxygen adsorbed increased only slightly with increasing temperature, between 250 and 300°C it increased sharply, and between 300 and 400°C it again increased only slightly. This behavior suggeststhe presence of at least two types of active sites on the activated Graphon surface. The maximum amount of oxygen adsorbed is estimated to occupy 2.8 ma/gor 2.6 per cent of the total surface. The rate of oxygen chemisorption on the more active sites is given by dn/dt = k&z, (I-O)‘, where k, ia a rate constant, C is the Oa concentration in the gas phase, na is the saturation coverage with oxygen on the more active sites, and 0 is the fraction of the more active sites that are covered at time t. The rate constant equalled 7.14 x lo9 exp (-7,40O/RT) cma/sec mole OS. l Work supported by tbc U.S. Atomic Energy Commi~on on Contract AT(30-l)-1710 and Petroleum Research Fund Grant PRF-1361-A2 various stages. t Present address: Dow Chemical Company, Midland, Michigan. NASA Fellow, 1964-1966.
at