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The dynamical equilibrium in physical adsorption at low pressures
Abstracts of papers A sample with surface area a covered with an adsorbed population no is placed in a volume Vevacuated with a pumping speed C. The temperature of the sample rises rapidly according to a relation T = f (t). The gas density in volume V is recorded continuously. This density is related to the population on the sample by:
an
a ~ = -Cvdt
V~
dt oo a n = C f t vdt = - [t' ( v - vo)
O)
Assuming an unique binding energy and the validity of the Arrhenius equation for the desorption kinetics, we have; log~ --2 _dn = logeA _ --E n ~ dt RT
(lI)
This set of equations (I and II) allows the determination of the pre-exponentiel factor .4 and the desorption activation energy E. But these equations cannot be solved except for a few very simple cases (for instance C = 0, a = 1, simple relation T : f ( t ) ) and a numerical method has to be used. In order to investigate some possibilities of the method a certain number of experiments were simulated by an arithmetic calculator by injecting into the equations particular values of a, .4, E and no, obtaining in this way the evaluation of /~ and log e
~-~ ~ -
This allowed us to unravel the following problems : - (1) The determination of the influence of the parameters: speed of temperature rise and the ratio V/C (pumping time constant of the volume) on the precision of the results. This study enables us to optimize the values of these parameters as functions of the experimental cases in view. (2) The study of a system with several adsorbed phases. If the binding energies are close to each other, the desorption peaks are not separated; nevertheless in some cases it is still possible to gain information about the energies of these phases. (3) The study of a system having a continuously varying energy with the population. Two cases may be considered here: (a) The surface is homogeneous and the energy is a function of the population E = f (n). Thus the energy varies continuously during the desorption. It may be shown that equation I! remains linear in a large part of the experimental region even if the dependency has the form E = Eo--logn and that in this case one measures the energy corresponding to the initial population no. But this should not be so with a linear form
Finally these simulated experiments are compared with real experiments performed on the systems hydrogen-nickel and carbon monoxide nickel.
The dynamical equilibrium in physical adsorption at low pressures A Schram, Centre d'Etudes Nucldaires de Saclay, France
In our experiments on the physical adsorption of Ar, Kr and Xe on small nickel surfaces in the 80°K temperature range ~ we were faced with the difficulty of very long delays, generally several hours, for the attainment of the final equilibrium pressure for a given quantity of gas introduced into the system. This appears to be a very general phenomenon, and to our knowledge thus far no explanation has been offered which satisfies all cases. This paper presents the results of a great number of physical adsorption experiments of At, Kr and Xe at 77.4°K and 90.1 °K on the pyrex wall of the device described previously~-. The investigation was limited to coverages 0 < < 1. The experimental results could not be interpreted on the basis of diffusion phenomena or by a delay in the attainment of thermal equilibrium. The explanation of Deitz and Carpenter 3 who suggest a dependency of the adsorption properties on the thermal history of the adsorbent, is also inacceptable in our case. By thermodynamics it may be shown that an increase of the isosteric heat, at constant coverage leads indeed to a decrease of the equilibrium pressure. Starting from two different hypotheses the corresponding kinetics are derived. The first hypothesis assumes a surface migration of the adatoms through a potential barrier separating low-energy sites (mobile adsorption) from high-energy sites (more "localized" adsorption). With some simplification one finds an exponential pressure decrease with a time constant which is an exponential function of the potential barrier. If this hypothesis is correct we should expect a smaller time constant for adsorption at higher temperatures. Thus far our experiments at 77.4 ° and 90.1°K show no clear evidence for this for the three gases considered. In the second hypothesis the exchange between sites is assumed to take place only through the gas phase, by successive adsorptions and desorptions. This leads to a more difficult theoretical treatment, but it may be shown to result in a series of time constants, dependent on the chosen site distribution function. This prediction seems in better agreement with the experiments.
References 1 A Schrarn, Physical Adsorption of Argon on Nickel between 78° attd 120°K. lnt Syrup on Residual Gases in Electron Tubes and SorptionDesorption Phenomena in High Vacuum. Rome 14-17 March 1967. z j p Hobson, The Solid-Gas Interface. Vol I, ch 14 Ed by E A Flood, Marcel Dekker lnc, New York 1967. 3 V R Deitz, F G Carpenter, Solid Surfaces and the Gas-Solid Interface, p 146-159. Adv in Chem Series 33. Am Chem Soc, Washington DC, 1961.
E=Eo--B o.
(b) The surface is heterogeneous, the energy varies from site to site and the distribution of the adsorbed population among these sites change during the desorption. In this case the equation II should never be linear, even approximately. This is an important conclusion, because often the surface heterogeneity is evoked to explain many experimental results, without a direct proof of its real existence being given. We think that thermal flash desorption experiments may be able to settle this question.
The simultaneous adsorption of hydrogen and nitrogen on polycrystalline tungsten J H Singleton, Westinghouse Research Laboratories Pittsburgh, Pa. 15235, USA
The simultaneous adsorption of hydrogen and nitrogen on a polycrystalline tungsten ribbon has been studied at 300°K. The objective was to investigate the change in adsorption characteristics of each gas in the presence of the second gas. The sticking 123
an
a ~ = -Cvdt
V~
dt oo a n = C f t vdt = - [t' ( v - vo)
O)
Assuming an unique binding energy and the validity of the Arrhenius equation for the desorption kinetics, we have; log~ --2 _dn = logeA _ --E n ~ dt RT
(lI)
This set of equations (I and II) allows the determination of the pre-exponentiel factor .4 and the desorption activation energy E. But these equations cannot be solved except for a few very simple cases (for instance C = 0, a = 1, simple relation T : f ( t ) ) and a numerical method has to be used. In order to investigate some possibilities of the method a certain number of experiments were simulated by an arithmetic calculator by injecting into the equations particular values of a, .4, E and no, obtaining in this way the evaluation of /~ and log e
~-~ ~ -
This allowed us to unravel the following problems : - (1) The determination of the influence of the parameters: speed of temperature rise and the ratio V/C (pumping time constant of the volume) on the precision of the results. This study enables us to optimize the values of these parameters as functions of the experimental cases in view. (2) The study of a system with several adsorbed phases. If the binding energies are close to each other, the desorption peaks are not separated; nevertheless in some cases it is still possible to gain information about the energies of these phases. (3) The study of a system having a continuously varying energy with the population. Two cases may be considered here: (a) The surface is homogeneous and the energy is a function of the population E = f (n). Thus the energy varies continuously during the desorption. It may be shown that equation I! remains linear in a large part of the experimental region even if the dependency has the form E = Eo--logn and that in this case one measures the energy corresponding to the initial population no. But this should not be so with a linear form
Finally these simulated experiments are compared with real experiments performed on the systems hydrogen-nickel and carbon monoxide nickel.
The dynamical equilibrium in physical adsorption at low pressures A Schram, Centre d'Etudes Nucldaires de Saclay, France
In our experiments on the physical adsorption of Ar, Kr and Xe on small nickel surfaces in the 80°K temperature range ~ we were faced with the difficulty of very long delays, generally several hours, for the attainment of the final equilibrium pressure for a given quantity of gas introduced into the system. This appears to be a very general phenomenon, and to our knowledge thus far no explanation has been offered which satisfies all cases. This paper presents the results of a great number of physical adsorption experiments of At, Kr and Xe at 77.4°K and 90.1 °K on the pyrex wall of the device described previously~-. The investigation was limited to coverages 0 < < 1. The experimental results could not be interpreted on the basis of diffusion phenomena or by a delay in the attainment of thermal equilibrium. The explanation of Deitz and Carpenter 3 who suggest a dependency of the adsorption properties on the thermal history of the adsorbent, is also inacceptable in our case. By thermodynamics it may be shown that an increase of the isosteric heat, at constant coverage leads indeed to a decrease of the equilibrium pressure. Starting from two different hypotheses the corresponding kinetics are derived. The first hypothesis assumes a surface migration of the adatoms through a potential barrier separating low-energy sites (mobile adsorption) from high-energy sites (more "localized" adsorption). With some simplification one finds an exponential pressure decrease with a time constant which is an exponential function of the potential barrier. If this hypothesis is correct we should expect a smaller time constant for adsorption at higher temperatures. Thus far our experiments at 77.4 ° and 90.1°K show no clear evidence for this for the three gases considered. In the second hypothesis the exchange between sites is assumed to take place only through the gas phase, by successive adsorptions and desorptions. This leads to a more difficult theoretical treatment, but it may be shown to result in a series of time constants, dependent on the chosen site distribution function. This prediction seems in better agreement with the experiments.
References 1 A Schrarn, Physical Adsorption of Argon on Nickel between 78° attd 120°K. lnt Syrup on Residual Gases in Electron Tubes and SorptionDesorption Phenomena in High Vacuum. Rome 14-17 March 1967. z j p Hobson, The Solid-Gas Interface. Vol I, ch 14 Ed by E A Flood, Marcel Dekker lnc, New York 1967. 3 V R Deitz, F G Carpenter, Solid Surfaces and the Gas-Solid Interface, p 146-159. Adv in Chem Series 33. Am Chem Soc, Washington DC, 1961.
E=Eo--B o.
(b) The surface is heterogeneous, the energy varies from site to site and the distribution of the adsorbed population among these sites change during the desorption. In this case the equation II should never be linear, even approximately. This is an important conclusion, because often the surface heterogeneity is evoked to explain many experimental results, without a direct proof of its real existence being given. We think that thermal flash desorption experiments may be able to settle this question.
The simultaneous adsorption of hydrogen and nitrogen on polycrystalline tungsten J H Singleton, Westinghouse Research Laboratories Pittsburgh, Pa. 15235, USA
The simultaneous adsorption of hydrogen and nitrogen on a polycrystalline tungsten ribbon has been studied at 300°K. The objective was to investigate the change in adsorption characteristics of each gas in the presence of the second gas. The sticking 123