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230. Adsorption thermodynamics and the structure of crystal surfaces
128
Classified Abstracts 228--234
16. G a s e s and S o l i d s 16 228. The nature of adsorbed monolayers of oxygen and hydrogen on platinum. (U.S.A.) The partial molal enthalpies of adsorption of hydrogen and oxygen on platinum and on carbon-supported platinum have been measured calorimetrically as a function of temperature. At r o o m temperature the absorbed layer is mobile in both cases. In the case of oxygen adsorption, the system is mobile above 200 ° K. There is a m a x i m u m in the partial molal enthalpy of adsorption near half a monolayer at 200°K. The curve of heat capacity of the oxygen monolayer as a function of temperature is that to be expected for a system in which oxygen molecules move as hindered rotors and translators on the surface. Below 200°K the absorption of hydrogen and oxygen gives rise to an immobile layer with a resultant decrease in the energy of adsorption. Oxygen is not chemisorbed on a layer of platinum deposited over carbon black when the layer is only one molecule thick. Correspondingly, the partial molal enthalpies of adsorption of hydrogen are m u c h higher. (U.S.A.) (Author) J. G. Aston, J. Phys. Chem., 67, Oct. 1963, 2042-2045.
229. Steady-state sorption of gases
16 : 21 during vapor deposition.
(U.S.A.) An adjunctive method for sorption study is presented. The sorbate and sorbent are added simultaneously and continuously to obtain a steady-state condition. The sorbent is added as a vapor which condenses to a solid substrate on a cold surface in the system. A descriptive theory is given for the relationship between the steady-state pressure and the flow rate of sorbent vapor to sorbate gas. Experimental evidence for the validity of the theoretical equations is given for the water-air and water-nitrogen systems on a liquid nitrogen cooled surface. It is hoped that this method may serve as another tool for attacking the sorption problem. (U.S.A.) (Author) J. D. Haygood, J. Phys. Chem., 67, Oct. 1963, 2061-2064. 16 : 17 230. Adsorption thermodynamics and the structure of crystal surfaces. (U.S.A.) The relative magnitudes of the energies of evaporation of molecules absorbed at various types of sites on the surface of a simple cubic crystal have been calculated. The equilibrium structure of the surface is discussed in relation to the interaction between absorbed molecules and surface features such as the surface terminations of dislocations and the surface traces of tilt and grain boundaries. The theoretical isotherm for a gas absorbed at a step is given and the contributions of step and surface adsorption are compared ; the effect of adsorption at a step on the roughness of the step is considered. A n approximate theory is given for the dependance of surface roughness on adsorbate coverage and the integral entropy, isosteric heat of adsorption, and adsorption isotherm of a gas adsorbed on a surface with equilibrium roughness are derived. The change in the surface tension of a crystal with roughness and coverage is evaluated. A discussion is given of the kinetics of the change in surface and step roughness under the influence of changes in temperature and adsorbate coverage. (U.S.A.) (Author) W. J. Dunning, J. Phys. Chem., 67, Oct. 1963, 2023-2029. 16 231. An infrared spectroscopic study of the adsorption of water and carbon dioxide by Linde molecular sieve X. (U.S.A.) Infrared spectra at 20 ° are reported for small a m o u n t s of water and carbon dioxide (up to 1 molecule per cavity) adsorbed on Li, N a and KX zeolites. The water spectra show no evidence of structural surface hydroxyl but may be interpreted in terms of
isolated water molecules adsorbed simultaneously by an iondipole interaction with the exchangeable cation and by hydrogen bonding of one of the hydrogens to any oxygen of the zeolite surface. The remaining hydroxyl is free and gives a sharp O H band around 3700 cm 1. Carbon dioxide is both physically adsorbed in a linear configuration at a cation and also chemisorbed in one or more bent configurations which are assumed to be carbonate ions formed by interaction with surface oxide ions adjacent to exchangeable cations. One such species c o m m o n to all three zeolites gives bands around 1700 and 1340 cm 1. On N a X a second species with bands at 1485 and 1425 cm 1 is more strongly held at low pressures but is readily converted to the first form in the presence of any additional physically adsorbed molecules. Additional bands are also present at different frequencies for carbon dioxide adsorbed on LiX and K X but these do not show such behaviour. Small a m o u n t s of preadsorbed water greatly accelerate the rate of carbon dioxide a d s o r p t i o n - - p r e s u m a b l y by catalyzing the chemisorption step. (U.S.A.) (Author) L. Bertsch and H. W. Habgood, J. Phys. Chem., 67, Aug. 1963, 1621-1628. 16 232. Monolayer adsorption with lateral interaction on heterogeneoussurfaces. (U.S.A.) A general approach to the theory of physically adsorbed m o n o layers on heterogeneous surfaces is given. Explicit expressions are derived which give adsorption properties in terms of the distribution functions for single sites, pairs of sites at a given distance, and similar higher order functions. The lateral interaction terms are primarily dependent u p o n the pair and higher order distribution functions. A generalized form of the BraggWilliams equation is obtained, valid for any distribution of pairs of sites. A few specific site distribution functions are substituted into the general equations, and equations are obtained for the low coverage adsorption isotherms and isosteric heats. These expressions and equations are obtained for the low coverage adsorption isotherms and isosteric heats. These expressions are then compared with experimental data on two carbon black adsorbents (one h o m o g e n e o u s and one heterogeneous), and some information concerning the site distribution functions on the heterogeneous sample is deduced. (U.S.A.) (Author) W. A. Steele, J. Phys. Chem., 67, Oct. 1963, 2016--2023. 16 : 41 233. Electron optical observations of surface microstructure on iron single crystals. (U.S.A.) A study of the changes in surface topography of electro-polished iron single crystals during v a c u u m annealing and hydrogen treatment at 800 ° has been made. The surfaces were examined by reflection electron diffraction and electron microscopy. Large flat facets parallel to the low index planes (001), (011), (111), and (112) were found to develop as a result of both treatments. The height and distribution of the steps between the planes were both dependent on the deviation of the original electro-polished surface from the developing plane. The replica method used for electron microscopy was capable of revealing steps as small as 20A. high. The steps appear to be formed by surface migration rather than by evaporation and condensation of the iron. (U.S.A.)(Author) P. B. Sewell, E. G. Brewer, M. Cohen, J. Phys. Chem., 67, Oct. 1963, 2008-2015. 16 234. A study of physical adsorption at very low pressures using ultra-high vacuum techniques. (U.S.A.) Ultra-high v a c u u m techniques have been used in the measurement of the adsorption isotherms of nitrogen and argon on Pyrex glass in the range of relative pressures between 10 13 and 10 -6 and in
Classified Abstracts 228--234
16. G a s e s and S o l i d s 16 228. The nature of adsorbed monolayers of oxygen and hydrogen on platinum. (U.S.A.) The partial molal enthalpies of adsorption of hydrogen and oxygen on platinum and on carbon-supported platinum have been measured calorimetrically as a function of temperature. At r o o m temperature the absorbed layer is mobile in both cases. In the case of oxygen adsorption, the system is mobile above 200 ° K. There is a m a x i m u m in the partial molal enthalpy of adsorption near half a monolayer at 200°K. The curve of heat capacity of the oxygen monolayer as a function of temperature is that to be expected for a system in which oxygen molecules move as hindered rotors and translators on the surface. Below 200°K the absorption of hydrogen and oxygen gives rise to an immobile layer with a resultant decrease in the energy of adsorption. Oxygen is not chemisorbed on a layer of platinum deposited over carbon black when the layer is only one molecule thick. Correspondingly, the partial molal enthalpies of adsorption of hydrogen are m u c h higher. (U.S.A.) (Author) J. G. Aston, J. Phys. Chem., 67, Oct. 1963, 2042-2045.
229. Steady-state sorption of gases
16 : 21 during vapor deposition.
(U.S.A.) An adjunctive method for sorption study is presented. The sorbate and sorbent are added simultaneously and continuously to obtain a steady-state condition. The sorbent is added as a vapor which condenses to a solid substrate on a cold surface in the system. A descriptive theory is given for the relationship between the steady-state pressure and the flow rate of sorbent vapor to sorbate gas. Experimental evidence for the validity of the theoretical equations is given for the water-air and water-nitrogen systems on a liquid nitrogen cooled surface. It is hoped that this method may serve as another tool for attacking the sorption problem. (U.S.A.) (Author) J. D. Haygood, J. Phys. Chem., 67, Oct. 1963, 2061-2064. 16 : 17 230. Adsorption thermodynamics and the structure of crystal surfaces. (U.S.A.) The relative magnitudes of the energies of evaporation of molecules absorbed at various types of sites on the surface of a simple cubic crystal have been calculated. The equilibrium structure of the surface is discussed in relation to the interaction between absorbed molecules and surface features such as the surface terminations of dislocations and the surface traces of tilt and grain boundaries. The theoretical isotherm for a gas absorbed at a step is given and the contributions of step and surface adsorption are compared ; the effect of adsorption at a step on the roughness of the step is considered. A n approximate theory is given for the dependance of surface roughness on adsorbate coverage and the integral entropy, isosteric heat of adsorption, and adsorption isotherm of a gas adsorbed on a surface with equilibrium roughness are derived. The change in the surface tension of a crystal with roughness and coverage is evaluated. A discussion is given of the kinetics of the change in surface and step roughness under the influence of changes in temperature and adsorbate coverage. (U.S.A.) (Author) W. J. Dunning, J. Phys. Chem., 67, Oct. 1963, 2023-2029. 16 231. An infrared spectroscopic study of the adsorption of water and carbon dioxide by Linde molecular sieve X. (U.S.A.) Infrared spectra at 20 ° are reported for small a m o u n t s of water and carbon dioxide (up to 1 molecule per cavity) adsorbed on Li, N a and KX zeolites. The water spectra show no evidence of structural surface hydroxyl but may be interpreted in terms of
isolated water molecules adsorbed simultaneously by an iondipole interaction with the exchangeable cation and by hydrogen bonding of one of the hydrogens to any oxygen of the zeolite surface. The remaining hydroxyl is free and gives a sharp O H band around 3700 cm 1. Carbon dioxide is both physically adsorbed in a linear configuration at a cation and also chemisorbed in one or more bent configurations which are assumed to be carbonate ions formed by interaction with surface oxide ions adjacent to exchangeable cations. One such species c o m m o n to all three zeolites gives bands around 1700 and 1340 cm 1. On N a X a second species with bands at 1485 and 1425 cm 1 is more strongly held at low pressures but is readily converted to the first form in the presence of any additional physically adsorbed molecules. Additional bands are also present at different frequencies for carbon dioxide adsorbed on LiX and K X but these do not show such behaviour. Small a m o u n t s of preadsorbed water greatly accelerate the rate of carbon dioxide a d s o r p t i o n - - p r e s u m a b l y by catalyzing the chemisorption step. (U.S.A.) (Author) L. Bertsch and H. W. Habgood, J. Phys. Chem., 67, Aug. 1963, 1621-1628. 16 232. Monolayer adsorption with lateral interaction on heterogeneoussurfaces. (U.S.A.) A general approach to the theory of physically adsorbed m o n o layers on heterogeneous surfaces is given. Explicit expressions are derived which give adsorption properties in terms of the distribution functions for single sites, pairs of sites at a given distance, and similar higher order functions. The lateral interaction terms are primarily dependent u p o n the pair and higher order distribution functions. A generalized form of the BraggWilliams equation is obtained, valid for any distribution of pairs of sites. A few specific site distribution functions are substituted into the general equations, and equations are obtained for the low coverage adsorption isotherms and isosteric heats. These expressions and equations are obtained for the low coverage adsorption isotherms and isosteric heats. These expressions are then compared with experimental data on two carbon black adsorbents (one h o m o g e n e o u s and one heterogeneous), and some information concerning the site distribution functions on the heterogeneous sample is deduced. (U.S.A.) (Author) W. A. Steele, J. Phys. Chem., 67, Oct. 1963, 2016--2023. 16 : 41 233. Electron optical observations of surface microstructure on iron single crystals. (U.S.A.) A study of the changes in surface topography of electro-polished iron single crystals during v a c u u m annealing and hydrogen treatment at 800 ° has been made. The surfaces were examined by reflection electron diffraction and electron microscopy. Large flat facets parallel to the low index planes (001), (011), (111), and (112) were found to develop as a result of both treatments. The height and distribution of the steps between the planes were both dependent on the deviation of the original electro-polished surface from the developing plane. The replica method used for electron microscopy was capable of revealing steps as small as 20A. high. The steps appear to be formed by surface migration rather than by evaporation and condensation of the iron. (U.S.A.)(Author) P. B. Sewell, E. G. Brewer, M. Cohen, J. Phys. Chem., 67, Oct. 1963, 2008-2015. 16 234. A study of physical adsorption at very low pressures using ultra-high vacuum techniques. (U.S.A.) Ultra-high v a c u u m techniques have been used in the measurement of the adsorption isotherms of nitrogen and argon on Pyrex glass in the range of relative pressures between 10 13 and 10 -6 and in