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H2-H2O system
Surface Technology, 16 (1982) 175 - 181
175
HYDROGEN ISOTOPE DISTRIBUTION EQUILIBRIA IN THE Pd/H2-H20 SYSTEM
S. G. MCKEE, J. P. MAGENNIS and F. A. LEWIS
Chemistry Department, Queen's University, Belfast BT9 5A G (Northern Ireland) (Received March 19, 1982)
Summary A method involving measurements of electrode potential and electrical resistivity is described for studying the overall isotopic equilibria established between the following: mixtures of H20 and D 2 0 ; molecular hydrogen species dissolved in the mixed solutions; the ratios of hydrogen isotopes absorbed by palladium electrodes immersed in the mixed solutions up to various combined isotope contents.
1. Introduction At temperatures in the ambient range palladium can absorb substantial contents of hydrogen at relatively low values of the hydrogen chemical potential [ 1 - 3]. This feature, taken in conjunction with the high substrate mobilities of the absorbed hydrogen and high catalytic activities of clean palladium surfaces for molecular hydrogen dissociation, has evoked interest with respect to possible uses of palladium and palladium alloys for the separation of hydrogen isotopes. In the cases of protium and deuterium, relationships between their chemical potentials and protium and deuterium contents respectively have been established in some detail [1 - 4]. Isotope separation factors ([H]/[D] )absorbed by Pd/([ H]/[D] )gas phase = SM with reference to isotope concentrations dissolved in the palladium and in the gas phase have also been reported [ 5 - 9]. (Concentrations are expressed in terms of atomic ratios.) For hydrided palladium electrodes immersed in hydrogen-saturated mixtures of H20 and D20, overall separation factors SL M, with higher values than S M, can be defined [6, 10, 11], where =
x
Here, S M = ([H]/[D] )adso,bedby Pd/([ HI/[D] )H,O-D~O and S~ = ([H]/[D] )gasin liquid/([ HI/[D] )H20--D20 0376-4583/82/0000-0000/$02.75
© Elsevier Sequoia/Printed in The Netherlands
176
Problems in measuring the values of S M are those of ensuring that overall equilibria have been fully established and of determining the concentrations of each of the isotopes in the palladium at equilibrium. In measurements by Sicking [11], the palladium was in the form of palladium black particles: these formed a slurry in the stirred aqueous phase and extracted samples were compacted before being outgassed. In a method suggested by Bucur and Lewis [10], pairs of palladium electrodes were employed in the form of discs, one of which was loaded initially with protium and the other with deuterium: the approach towards equilibrium was followed by measurements of the electrode potential difference. This general technique has now been modified to be employed with specimens in the forms of wires, which allow an additional monitoring of the approach towards equilibrium from measurements of electrical resistivity. It has also been possible to make a more extensive use of the higher catalytic activities of palladium black surfaces. Experimental details are outlined here, together with some preliminary experimental results which indicate the improved kinetic as well as thermodynamic information which it has now been possible to obtain.
2. Experimental details As illustrated in Fig. 1, specimens A are lengths (approximately 15 cm) of coiled palladium wires ( J o h n s o n - M a t t h e y Specpure). They are connected through short thin-walled platinum tubes B to platinum wires sealed into an electrode assembly constructed from soda glass. After spot welding the connections, a tube of soda glass is moved from further along the leads and sealed over the junction B. Specimens are surface preactivated by electrical heating to form a thin oxide layer and/or electroplated with palladium black. Subsequently electrodes are inserted, in pairs, into the borosilicate glass reaction vessel in Fig. 1 through polytetrafluoroethylene (PTFE)-sleeved
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Fig. 1. Reaction vessel and electrodes: S, magnetic stirrer; A, coiled p a l l a d i u m wire specimen; B, j u n c t i o n to platinum leads; C, hard-soldered junction to copper leads.
177 joints (which have been carefully ground) into slightly offset central positions above the magnetic stirrer S which is driven by a m o t o r submerged in a thermostatted bath. The reaction vessel was previously filled with either argon-saturated or hydrogen-saturated mixtures of H 2 0 and D~O acidified with H2SO4-DzSO4. During insertion of the second electrode small excesses of solution are vented upwards through the PTFE-keyed tap. To prevent any subsequent loss of solution by evaporation through the ground joint, the lip of the joint was filled with a low melting point wax which could be easily stripped away at the end of the experiments. Before insertion into the reaction vessel, one or both specimens had been loaded electrolytically with either solely protium or solely deuterium up to desired contents n = [ H ] / [ P d ] or n = [ D ] / [ P d ] . After their insertion in the reaction vessel, the time dependence of changes in the electrical resistance of the specimens was measured with an a.c. bridge network and changes in the electrode potential differences between the specimens were measured with a Philips PM8251 high input impedance chart recorder. So far all experiments have been in approximately 1:1 mixtures of H~O and D 2 0 (volume, a b o u t 120 ml) in 0.01 N H2SO4-D2SO 4 at 25 + 0.2 °C. After removal from the reaction vessel at the conclusion of experiments, specimens were briefly (about 10 s) immersed in a solution of iodine in potassium iodide to inhibit spontaneous aerial oxidation [12 - 15] and then washed with water and acetone before being connected to a vacuum line which led to an AEI MS10 mass spectrometer by a further complementary PTFEsleeved ground joint. After evacuation, specimens were outgassed by being heated to approximately 350 °C by passing an electrical current through them.
3. Results and discussion
In the majority of measurements so far carried out, specimens were 0.0274 cm in diameter and overall values of the protium (deuterium) contents were restricted to n ~ 0.03, i.e. close to the upper limits of the ~ phase concentration ranges of the P d - H (Pd-D) system at 25 °C [1 - 4]. Restriction of protium (deuterium) contents n to near the ~ phase limits reduces problems arising from the distortion of specimens in cycles of absorption and desorption of hydrogen and from fracturing of the coating of glass over short lengths of the specimens adjacent to the junctions at B (Fig. 1) that would be caused by expansion in the specimen diameter due to longitudinally migrating hydrogen [ 1 ]. Experiments within these relatively low limits of n have been carried o u t in cases where one specimen had initially been loaded with protium and the other with deuterium and also in cases where only one specimen had initially been charged with protium or deuterium and the other had initially been uncharged. A final attainment of equilibrium in such experiments was taken to be indicated by a reduction in the electrode potential difference between them to less than 0.10 mV and,
178
in the cases where only one of the specimens had initially been loaded with either protium or deuterium, by a close final agreement between the relative electrical resistances R/Ro of both specimens. A detailed analysis of these results will be presented later. It has been found in these ~ phase experiments that close agreements were obtained between the ratios of protium to deuterium outgassed from the specimens at the assumed equilibria: calculations gave values of S M ~ 8. In view of the successful attainments of equilibria with contents n corresponding to the ~ phase composition range, some preliminary experiments have also been carried out in which one of the specimens had been loaded to ~ phase (n > 0.6) contents with either protium or deuterium and the other specimen had initially been hydrogen free. These experiments were carried o u t with wire specimens 0.0122 cm in diameter which permitted an improved sensitivity in the measurement of changes in R/Ro. Two examples of results of such experiments are shown in Fig. 2. For both experiments, it may be seen that the electrode potential difference between the specimens had declined from initial values of about 800 mV to a final difference of less than 0.10 mV after approximately 70 h. Over the same period, the values of R/Ro for the specimens initially containing ~ phase contents of protium (Fig. 2, point D) and deuterium (Fig. 2, point C) had declined to what within experimental reproducibility was again a c o m m o n value of about 1.74. The initially higher values of R/Ro for the deuterium-loaded specimen are consistent with steeper increases in R/Ro as a function of n for P d - D than for P d - H , as reported by Flanagan [4]. Complementarily, the initially unloaded specimens had shown increases in R/Ro with time to a final c o m m o n value of about 1.07. From a thermodynamic standpoint, these various changes seem to be qualitatively accounted for by the alterations in chemical potential and overall contents n indicated in the chemical potential versus composition diagram shown in Fig. 3. Thus it can be assumed from studies by Sieverts and Danz [16] of equilibrium pressure-composition relationships with protium and deuterium mixtures that the chemical potential-composition relationships for final mixed protium and deuterium contents in the pairs of electrodes will lie between those for unmixed protium and deuterium, the " a b s o r p t i o n " relationships [1 - 4] at 25 °C for which are included in the figure. The attainment of a final c o m m o n chemical potential by the initially unloaded specimens and by the initially protium-loaded (Fig. 3, point D) and deuterium-loaded (Fig. 3, point C) specimens can be expected from the diagram to correspond to substantially different combined isotope contents n, as represented by Fig. 3, points A and B. The ratios [ H ] / [ D ] in the gas released from the initially unloaded specimens corresponding to the contents at Fig. 3, point A, gave calculated values of S M = 8.2 + 0.2, in both cases in quite good agreement with values derived in the ~ phase experiments referred to above.
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Fig. 2. Examples of measurements at 25 °C in initially argon-saturated 0.01 N H2SO 4 D2SO 4 with pairs of specimens 0.0122 cm in diameter electroplated with palladium black. One specimen of each pair had been initially hydrogen free. The other specimen of each pair had been loaded to a ~ phase content with deuterium (e) and protium (o). The plots show the differences in the electrode potential between the specimens (curves a), changes in R / R 0 for the initially charged specimens (curves b) and changes in R / R 0 for the initially uncharged specimens (curves c).
For the higher overall contents corresponding to Fig. 3, point B, the ratio [ H ] / [ D ] in the gas released from the specimen which had initially been loaded with deuterium gave S~ = 7.7 + 0.3 and from the initially protium-
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Fig. 3. Chemical potential vs. composition diagram representing changes occurring over the course of the experiments in Fig. 2. Points C and D represent approximate initial compositions of the deuterium-loaded and protium-loaded specimens respectively. Points A and B represent approximate common compositions of either of the pairs of electrodes at equilibrium; these are shown on isotherms with chemical potentials corresponding to the isotopic ratios in the electrodes. loaded specimen gave S M = 8.2 + 0.3. These values of SLM are larger than those (less than about 6.0) earlier reported by Bucur and Lewis [10] for spec imens with n ~ 0.3 and values of a b o u t 6 reported by von Stackelberg and Jahns [6] for specimens with overall values of n likely to have been in the/~ phase region. It thus seems likely that overall equilibria had generally not been completely established in these earlier experiments. Values of S M ~ 8 are closer to, but rather lower than, values of about 9 obtained by Sicking [ 11] in experiments with finely particulated palladium and under conditions where values of n at equilibrium were likely to be within the ~ phase composition range. An interesting and reproducible kinetic feature of the experiment with the initially deuterium-loaded specimen (Fig. 2) was that of the reversal of the sign of the electrode potential difference at quite an early stage. From consideration of the diagram in Fig. 3, this reversal of sign seems to indicate that ratios of deuterium to protium at the surface of the initially uncharged electrode thereafter exceeded those of the surface of the initially deuteriumloaded electrode over the course of the final approach to equilibrium. In turn, this suggests that the initially rapid desorption of deuterium molecules
181
from the loaded electrode, and their reabsorption from solution by the other electrode, was proceeding more rapidly than the simultaneous exchange reactions with the solution at their surfaces could reduce molecular deuterium to protium ratios to those corresponding to the final ratios at complete establishment of overall equilibria. This possibility seems to be supported by the fact that the R/Ro versus time plot for the initially unloaded specimen in this experiment exhibits a final region of decrease in R/Ro which could be taken to correspond [4] to incorporation of protium in place of deuterium in the course of the continuing subsequent exchange with the solution. Such an explanation also does not seem to be out of keeping with findings from experiments in which protium or deuterium gas was absorbed from either H20 or D20 respectively [ 17,18] where surface exchange reactions with the solution tended to proceed in conjunction with diffusive gain or loss of molecular hydrogen species, with somewhat imperfect initial efficiency, even at surfaces which were highly active for molecular dissociation and recombination.
Acknowledgments S.G.McK. acknowledges awards of Musgrave and Larmor Studentships by Queen's University. Financial support from the Science Research Council and technical assistance from J. H. Kirkpatrick is also gratefully acknowledged.
References 1 F. A. Lewis, The Palladium~Hydrogen System, Academic Press, London, 1967; Platinum Met Rev., 26 (1982) 20. 2 E. Wicke, H. Brodowsky and H. Zuchner, Top. Appl. Phys., 29 (1978) 73. 3 R. V. Bucur, Rev. Phys. Acad. Rep. Populaire Roum., 6 (1961) 269. 4 T. B. Flanagan, J Phys. Chem., 65 (1961) 280. 5 E. Gleukauf and G. Kitt, in D. H. Desty (ed.), Vapour Phase Chromatography, Butterworths, London, 1957, p. 422. 6 M. yon Stackelberg and W. Jahns, Z. Elektrochem., 62 (1958) 349. 7 E. Wicke and G. H. Nernst, Ber. Bunsenges. Phys. Chem., 68 (1964) 224. 8 F. Botter, J. Less-Common Met., 49 (1976) 111. 9 H. Brodowsky and G. Repenning, Z. Phys. Chem. (Frankfurt am Main), 114 (1979) 141. 10 R. V. Bucur and F. A. Lewis, Z Phys. Chem. (Frankfurt am Mare), 75 (1971) 207. 11 G. Sicking, Ber. Bunsenges. Phys. Chem., 72 (1972) 790. 12 J. C. Barton, W. F. N. Leitch and F. A. Lewis, Electrochim. Acta, 11 (1966) 1171. 13 F. M. Mazzolai, M. Nuovo and F. A. Lewis, Nuovo Cimento B, 33 (1976) 242. 14 G. J. Zimmermann, J. Less-Common Met., 49 (1976) 49. 15 B. Abbenseth and H. Wipf, J. Phys. F, 10 (1980) 353. 16 A. Sieverts and W. Danz, Z. Phys. Chem. (Leipzig), Abt. B, 33 (1937) 46, 61. 17 G. L. Holleck and T. B. Flanagan, J. Phys. Chem., 73 (1969) 285; Trans. Faraday Soc., 65 (1969) 615. 18 F. A. Lewis, W. F. N. Leitch and A. Murray, Surf. Technol., 7 (1978) 385.
175
HYDROGEN ISOTOPE DISTRIBUTION EQUILIBRIA IN THE Pd/H2-H20 SYSTEM
S. G. MCKEE, J. P. MAGENNIS and F. A. LEWIS
Chemistry Department, Queen's University, Belfast BT9 5A G (Northern Ireland) (Received March 19, 1982)
Summary A method involving measurements of electrode potential and electrical resistivity is described for studying the overall isotopic equilibria established between the following: mixtures of H20 and D 2 0 ; molecular hydrogen species dissolved in the mixed solutions; the ratios of hydrogen isotopes absorbed by palladium electrodes immersed in the mixed solutions up to various combined isotope contents.
1. Introduction At temperatures in the ambient range palladium can absorb substantial contents of hydrogen at relatively low values of the hydrogen chemical potential [ 1 - 3]. This feature, taken in conjunction with the high substrate mobilities of the absorbed hydrogen and high catalytic activities of clean palladium surfaces for molecular hydrogen dissociation, has evoked interest with respect to possible uses of palladium and palladium alloys for the separation of hydrogen isotopes. In the cases of protium and deuterium, relationships between their chemical potentials and protium and deuterium contents respectively have been established in some detail [1 - 4]. Isotope separation factors ([H]/[D] )absorbed by Pd/([ H]/[D] )gas phase = SM with reference to isotope concentrations dissolved in the palladium and in the gas phase have also been reported [ 5 - 9]. (Concentrations are expressed in terms of atomic ratios.) For hydrided palladium electrodes immersed in hydrogen-saturated mixtures of H20 and D20, overall separation factors SL M, with higher values than S M, can be defined [6, 10, 11], where =
x
Here, S M = ([H]/[D] )adso,bedby Pd/([ HI/[D] )H,O-D~O and S~ = ([H]/[D] )gasin liquid/([ HI/[D] )H20--D20 0376-4583/82/0000-0000/$02.75
© Elsevier Sequoia/Printed in The Netherlands
176
Problems in measuring the values of S M are those of ensuring that overall equilibria have been fully established and of determining the concentrations of each of the isotopes in the palladium at equilibrium. In measurements by Sicking [11], the palladium was in the form of palladium black particles: these formed a slurry in the stirred aqueous phase and extracted samples were compacted before being outgassed. In a method suggested by Bucur and Lewis [10], pairs of palladium electrodes were employed in the form of discs, one of which was loaded initially with protium and the other with deuterium: the approach towards equilibrium was followed by measurements of the electrode potential difference. This general technique has now been modified to be employed with specimens in the forms of wires, which allow an additional monitoring of the approach towards equilibrium from measurements of electrical resistivity. It has also been possible to make a more extensive use of the higher catalytic activities of palladium black surfaces. Experimental details are outlined here, together with some preliminary experimental results which indicate the improved kinetic as well as thermodynamic information which it has now been possible to obtain.
2. Experimental details As illustrated in Fig. 1, specimens A are lengths (approximately 15 cm) of coiled palladium wires ( J o h n s o n - M a t t h e y Specpure). They are connected through short thin-walled platinum tubes B to platinum wires sealed into an electrode assembly constructed from soda glass. After spot welding the connections, a tube of soda glass is moved from further along the leads and sealed over the junction B. Specimens are surface preactivated by electrical heating to form a thin oxide layer and/or electroplated with palladium black. Subsequently electrodes are inserted, in pairs, into the borosilicate glass reaction vessel in Fig. 1 through polytetrafluoroethylene (PTFE)-sleeved
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Fig. 1. Reaction vessel and electrodes: S, magnetic stirrer; A, coiled p a l l a d i u m wire specimen; B, j u n c t i o n to platinum leads; C, hard-soldered junction to copper leads.
177 joints (which have been carefully ground) into slightly offset central positions above the magnetic stirrer S which is driven by a m o t o r submerged in a thermostatted bath. The reaction vessel was previously filled with either argon-saturated or hydrogen-saturated mixtures of H 2 0 and D~O acidified with H2SO4-DzSO4. During insertion of the second electrode small excesses of solution are vented upwards through the PTFE-keyed tap. To prevent any subsequent loss of solution by evaporation through the ground joint, the lip of the joint was filled with a low melting point wax which could be easily stripped away at the end of the experiments. Before insertion into the reaction vessel, one or both specimens had been loaded electrolytically with either solely protium or solely deuterium up to desired contents n = [ H ] / [ P d ] or n = [ D ] / [ P d ] . After their insertion in the reaction vessel, the time dependence of changes in the electrical resistance of the specimens was measured with an a.c. bridge network and changes in the electrode potential differences between the specimens were measured with a Philips PM8251 high input impedance chart recorder. So far all experiments have been in approximately 1:1 mixtures of H~O and D 2 0 (volume, a b o u t 120 ml) in 0.01 N H2SO4-D2SO 4 at 25 + 0.2 °C. After removal from the reaction vessel at the conclusion of experiments, specimens were briefly (about 10 s) immersed in a solution of iodine in potassium iodide to inhibit spontaneous aerial oxidation [12 - 15] and then washed with water and acetone before being connected to a vacuum line which led to an AEI MS10 mass spectrometer by a further complementary PTFEsleeved ground joint. After evacuation, specimens were outgassed by being heated to approximately 350 °C by passing an electrical current through them.
3. Results and discussion
In the majority of measurements so far carried out, specimens were 0.0274 cm in diameter and overall values of the protium (deuterium) contents were restricted to n ~ 0.03, i.e. close to the upper limits of the ~ phase concentration ranges of the P d - H (Pd-D) system at 25 °C [1 - 4]. Restriction of protium (deuterium) contents n to near the ~ phase limits reduces problems arising from the distortion of specimens in cycles of absorption and desorption of hydrogen and from fracturing of the coating of glass over short lengths of the specimens adjacent to the junctions at B (Fig. 1) that would be caused by expansion in the specimen diameter due to longitudinally migrating hydrogen [ 1 ]. Experiments within these relatively low limits of n have been carried o u t in cases where one specimen had initially been loaded with protium and the other with deuterium and also in cases where only one specimen had initially been charged with protium or deuterium and the other had initially been uncharged. A final attainment of equilibrium in such experiments was taken to be indicated by a reduction in the electrode potential difference between them to less than 0.10 mV and,
178
in the cases where only one of the specimens had initially been loaded with either protium or deuterium, by a close final agreement between the relative electrical resistances R/Ro of both specimens. A detailed analysis of these results will be presented later. It has been found in these ~ phase experiments that close agreements were obtained between the ratios of protium to deuterium outgassed from the specimens at the assumed equilibria: calculations gave values of S M ~ 8. In view of the successful attainments of equilibria with contents n corresponding to the ~ phase composition range, some preliminary experiments have also been carried out in which one of the specimens had been loaded to ~ phase (n > 0.6) contents with either protium or deuterium and the other specimen had initially been hydrogen free. These experiments were carried o u t with wire specimens 0.0122 cm in diameter which permitted an improved sensitivity in the measurement of changes in R/Ro. Two examples of results of such experiments are shown in Fig. 2. For both experiments, it may be seen that the electrode potential difference between the specimens had declined from initial values of about 800 mV to a final difference of less than 0.10 mV after approximately 70 h. Over the same period, the values of R/Ro for the specimens initially containing ~ phase contents of protium (Fig. 2, point D) and deuterium (Fig. 2, point C) had declined to what within experimental reproducibility was again a c o m m o n value of about 1.74. The initially higher values of R/Ro for the deuterium-loaded specimen are consistent with steeper increases in R/Ro as a function of n for P d - D than for P d - H , as reported by Flanagan [4]. Complementarily, the initially unloaded specimens had shown increases in R/Ro with time to a final c o m m o n value of about 1.07. From a thermodynamic standpoint, these various changes seem to be qualitatively accounted for by the alterations in chemical potential and overall contents n indicated in the chemical potential versus composition diagram shown in Fig. 3. Thus it can be assumed from studies by Sieverts and Danz [16] of equilibrium pressure-composition relationships with protium and deuterium mixtures that the chemical potential-composition relationships for final mixed protium and deuterium contents in the pairs of electrodes will lie between those for unmixed protium and deuterium, the " a b s o r p t i o n " relationships [1 - 4] at 25 °C for which are included in the figure. The attainment of a final c o m m o n chemical potential by the initially unloaded specimens and by the initially protium-loaded (Fig. 3, point D) and deuterium-loaded (Fig. 3, point C) specimens can be expected from the diagram to correspond to substantially different combined isotope contents n, as represented by Fig. 3, points A and B. The ratios [ H ] / [ D ] in the gas released from the initially unloaded specimens corresponding to the contents at Fig. 3, point A, gave calculated values of S M = 8.2 + 0.2, in both cases in quite good agreement with values derived in the ~ phase experiments referred to above.
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Fig. 2. Examples of measurements at 25 °C in initially argon-saturated 0.01 N H2SO 4 D2SO 4 with pairs of specimens 0.0122 cm in diameter electroplated with palladium black. One specimen of each pair had been initially hydrogen free. The other specimen of each pair had been loaded to a ~ phase content with deuterium (e) and protium (o). The plots show the differences in the electrode potential between the specimens (curves a), changes in R / R 0 for the initially charged specimens (curves b) and changes in R / R 0 for the initially uncharged specimens (curves c).
For the higher overall contents corresponding to Fig. 3, point B, the ratio [ H ] / [ D ] in the gas released from the specimen which had initially been loaded with deuterium gave S~ = 7.7 + 0.3 and from the initially protium-
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Fig. 3. Chemical potential vs. composition diagram representing changes occurring over the course of the experiments in Fig. 2. Points C and D represent approximate initial compositions of the deuterium-loaded and protium-loaded specimens respectively. Points A and B represent approximate common compositions of either of the pairs of electrodes at equilibrium; these are shown on isotherms with chemical potentials corresponding to the isotopic ratios in the electrodes. loaded specimen gave S M = 8.2 + 0.3. These values of SLM are larger than those (less than about 6.0) earlier reported by Bucur and Lewis [10] for spec imens with n ~ 0.3 and values of a b o u t 6 reported by von Stackelberg and Jahns [6] for specimens with overall values of n likely to have been in the/~ phase region. It thus seems likely that overall equilibria had generally not been completely established in these earlier experiments. Values of S M ~ 8 are closer to, but rather lower than, values of about 9 obtained by Sicking [ 11] in experiments with finely particulated palladium and under conditions where values of n at equilibrium were likely to be within the ~ phase composition range. An interesting and reproducible kinetic feature of the experiment with the initially deuterium-loaded specimen (Fig. 2) was that of the reversal of the sign of the electrode potential difference at quite an early stage. From consideration of the diagram in Fig. 3, this reversal of sign seems to indicate that ratios of deuterium to protium at the surface of the initially uncharged electrode thereafter exceeded those of the surface of the initially deuteriumloaded electrode over the course of the final approach to equilibrium. In turn, this suggests that the initially rapid desorption of deuterium molecules
181
from the loaded electrode, and their reabsorption from solution by the other electrode, was proceeding more rapidly than the simultaneous exchange reactions with the solution at their surfaces could reduce molecular deuterium to protium ratios to those corresponding to the final ratios at complete establishment of overall equilibria. This possibility seems to be supported by the fact that the R/Ro versus time plot for the initially unloaded specimen in this experiment exhibits a final region of decrease in R/Ro which could be taken to correspond [4] to incorporation of protium in place of deuterium in the course of the continuing subsequent exchange with the solution. Such an explanation also does not seem to be out of keeping with findings from experiments in which protium or deuterium gas was absorbed from either H20 or D20 respectively [ 17,18] where surface exchange reactions with the solution tended to proceed in conjunction with diffusive gain or loss of molecular hydrogen species, with somewhat imperfect initial efficiency, even at surfaces which were highly active for molecular dissociation and recombination.
Acknowledgments S.G.McK. acknowledges awards of Musgrave and Larmor Studentships by Queen's University. Financial support from the Science Research Council and technical assistance from J. H. Kirkpatrick is also gratefully acknowledged.
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