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FEM study of the interaction of silver whiskers and epitaxial layers with N2O and O2
SURFACE
SCIENCE 33 (1972) 624-629 0 North-Holland
LETTERS FEM STUDY
TO THE
EDITOR
OF THE INTERACTION
AND EPITAXIAL
LAYERS
Publishing Co.
OF SILVER WHISKERS
WITH N,O AND O2
Received 28 April 1972; revised manuscript
received 10 July 1972
The explanation of the reaction mechanism of ethylene oxidation over a silver catalyst largely depends on the answer to the question whether a diatomic oxygen species (0,) and/or a monoatomic oxygen species (O=) exist(s) at the surfacei). The aim of the present work is to obtain information on this point by a comparative study of the adsorption complexes formed on silver from nitrous oxide and molecular oxygen. Upon decomposition at the surface N,O will deposit atomic oxygen; adsorbed diatomic oxygen might be formed in the presence of 0,. Some preliminary results are reported here. The field-emission-microscopy (= FEM) technique was chosen because work function data enable differentiation between the various adsorbed species. Further, if the magnitudes of the polarizabilities of the negatively charged oxygen ions are assumed to be comparable, the more loosely bound species may desorb preferentially under the influence of the high electric field needed for electron emission (max. 5 V/rim). Further advantages are that the adsorptions are carried out at surfaces of controlled cleanliness and that information on crystal-face specific adsorption is obtained. Finally, this investigation offered an incentive to prepare clean silver surfaces suitable for FEM experiments up to 4OO”C, a temperature which is certainly required for studies of the silver/oxygen system to achieve complete oxygen desorption. The present work thus tries to extend the applicability of FEM to adsorption on soft metals. Single silver whiskers, [l lo] oriented and grown on a blunt (radius N lo3 nm) tungsten tip that was cleaned by heating to lOOO”C, as well as lo-50 atomic layers thick epitaxial silver layers [Ag( 111)//W (1 lo)] grown on a tungsten tip (radius 30-50 nm) that was first heated to max. 700°C and then field evaporated, were prepared. Silver whiskers2) and epitaxial silver layerss) have been mentioned before as potential FEM specimens to study gas adsorption, but to our knowledge they have never actually been used for this purpose. The following advantages of this method were found: (1) Because in the preparation of both whiskers and epitaxial layers welldegassed high-purity silver is deposited from its vapor in UHV (during evaporation the pressure is in the lo-* Pascal range) onto thoroughly cleaned tungsten tips, ultraclean silver surfaces are obtained in situ, the problem of 624
INTERACTION
OF
Ag
WITH
NaO
AND
02
625
diffusion of impurities from the bulk or from the shank of the tip to the emitting surface upon heating thus being eliminated. (2) Silver whiskers and epitaxial layers on tungsten substrates do not blunt after extensive heating at 400°C as was concluded from the fact that the emission voltage for a clean specimen remains virtually constant in the course of many experiments. Even heating up to 600°C for short periods is possible; at 700°C evaporation takes place. The W-Ag interaction was found negligible. Whiskers were preferred when high pressures of active gases were used; in the case of epitaxial layers tip failure occurred probably because a brittle tungsten oxide layer was built up at the W-Ag interface. The existence of this layer was shown by stepwise field evaporation/work function determination sequences. An additional advantage of using whiskers is that their diameter of 5-20 nm makes the tip surface similar to that of silver particles in actual supported catalysts. In figs. 1 (a) and 3 (a) emission patterns of a whisker and an epitaxial layer are shown. The (111) and (100) planes are well-developed, the bright triangular regions are centred around the (210) planes, the zone lines through (110) and (211) connecting the (111) planes and the (111) and (100) planes, respectively, are intermediately emitting. Before Fowler-Nordheim plots or photographs were taken the gas was always pumped off at 0°C (except when the adsorptions were carried out at - 196°C) until a pressure below lo-’ Pa was reached and the tip was cooled to - 196 “C to minimize field-induced migration and desorption. Maximum currents used were approx. 1 uA in the case of epitaxial layers and approx. 0.1 uA in the case of whiskers. Physical adsorption of N,O molecules on silver whiskers was found to cause a decrease in work function and was further characterized by complete desorption at - 196°C at 5 V/nm (tip negative). No accurate data on this work function change can be given yet. This non-dissociative adsorption occurred when the tip was exposed to N,O at - 196°C or when after exposure at higher temperature the N,O had been pumped off to an insufficient degree. Dissociative adsorption of N,O on Ag leaves oxygen at the surface, which causes an increase in work function. At temperatures from 0-200°C it proceeded at very low rates: at 0°C 5 min of N,O exposure at 130 Pa (= 1 torr) was required to produce any detectable amounts of chemisorbed species (fig. l), at lOO”C, 5 min at 13 Pa, and at 200°C 5 min at 1.3 Pa. This shows that the N,O decomposition on silver is an activated process; our preliminary data point to an activation energy smaller than the value of 120 kJ mole-’ (28 kcal mole-‘) indicated by Herzog4). The N,O used in this work was carefully purified by repeated freezing, pumping off and distilling.
626
M. M. P. JANSSEN,
J. MOOLHUYSEN
AND
W. M. H. SACHTLER
Fig. 1. N20 decomposition on a silver whisker. Tip voltage approx. 4000 V. (a) Whisker cleaned by heating at 400°C in UHV: (b) whisker exposed to NzO at 0°C and 13 Pa for 5 min; (c) as (b) but N20 pressure 130 Pa.
Fig. 2. Adsorption of oxygen at 0°C and 200°C on the silver whisker of fig. 1. (a) Whisker exposed to oxygen at 0°C and 0.13 Pa for 5 min; (b) as (a) with additional 5 min exposure to oxygen at 200°C and 0.013 Pa.
INTERACTION
OF
Ag WITH N20
AND
Oa
621
Fig. 3. Adsorption of oxygen at 200°C on an epitaxial silver layer. Tip voltage approx. 700 V. (a) Clean Ag surface; (b) 9 min oxygen at 1.3 x 10e5 Pa; (c) as (b) +9 min at 6.5 x 1O-5 Pa; (d) as (c) + 1 min at 6.5 x 1O-4 Pa; (e) as (d) + 9 min at 6.5 x 1O-4 Pa; (f) as (e) + 3 min at 0.13 Pa.
628
M. M. P. JANSSEN,
J. MOOLHUYSEN
AND
W. M. H. SACHTLER
Adsorption of oxygen on silver whiskers and epitaxial layers proceeded much faster; moreover, the rate increase with higher adsorption temperatures reveals a low activation energy. Fig. 2 shows that the adsorption at high oxygen pressures leads to higher coverages at 200°C than at 0°C. Adsorption rates were measurable at oxygen pressures of lo-’ to 10T4 Pa. An example of the gradual covering of an epitaxial silver layer at 200 “C is shown in fig. 3. The bright areas darken first, then the (110) zone lines and finally the (211) zone lines. The (111) plane becomes smaller and ultimately the pattern consists of bright rings around the (111) and (100) planes. At high oxygen coverages (0.13 Pa for 5 min) the work function increased by approximately 0.4 and 0.8 eV at 0°C and 200 “C; the corresponding “log A changes (A = pre-exponential factor in the FN equation) were approximately -0.5 and - 1.0. A remarkable feature typical of adsorbed oxygen is that at high coverages some oxygen is desorbed at a field strength near that used for electron imaging. This observation would be in agreement with the presence of loosely bound diatomic oxygen such as 0; on the silver surface in addition to mono-atomic oxygen, as proved by infrared studies published by Gerei et al.5) and Kilty et al.6). Desorption of oxygen at 400°C in UHV was found to restore the clean surface. Partial cleaning, causing a decrease in work function, was achieved by exposure to hydrogen at low temperatures. The work function data are in good agreement with those measured by Rudnitskii et al.7) on a porous Ag catalyst by the vibrating condenser method. Exposure of an oxygen-covered surface to ethylene caused complexes which, again, are easily field-desorbed. This is consistent with the assumption of an organic peroxide complex as postulated in refs. 5 and 6. Summarizing, we can state that: (1) Soft metals like Ag can be studied as thermally stable clean adsorbents by FEM in the form of either whiskers or vapor-deposited layers on refractory metal tips. (2) The adsorption of N,O on Ag is non-dissociative at low temperatures, causing a decrease in work function; its dissociative adsorption at higher temperatures is activated and leads to an increase in work function. (3) Oxygen adsorption on Ag is face-specific and causes an increase in work function too. At high coverages a species is observed that is easily desorbed at electron imaging fields; it is tentatively described as 0 iads M. M. P. JANSSEN,J. MOOLHUYSEN and W. M. H. SACHTLER
Koninkl~ke~Shel~-L~bor~tori~m, Am~ter~m, ~etherl~~s
Amsterdam
(Shell Research N. Y.) ,
INTERACTION
OF
Ag
WITH
NaO
AND
02
629
References 1) 2) 3) 4) 5) 6)
W. M. H. Sachtler, Catalysis Rev. 4 (1970) 27. A. J. Melmed, J. Chem. Phys. 38 (1963) 607. A. J. Melmed and R. Gomer, J. Chem. Phys. 34 (1961) 1802. W. Herzog, Ber. Bunsenges. Physik. Chemie 74 (1970) 216. S. V. Gerei, K. M. Kholyavenko and M. Ya. Rubanik, Ukr. Khem. Zh. 31 (1965) 449. P. A. Kilty, N. C. Rol and W. M. H. Sachtler, paper submitted to 5th Intern. Congr. Catalysis, Miami (Fla.) USA, 1972. 7) L. A. Rudnitskii, L. I. Shakhovskaya, N. V. Kul’kova and M. I. Temkin, Dokl. Akad. Nauk SSSR 182 (1968) 786.
SCIENCE 33 (1972) 624-629 0 North-Holland
LETTERS FEM STUDY
TO THE
EDITOR
OF THE INTERACTION
AND EPITAXIAL
LAYERS
Publishing Co.
OF SILVER WHISKERS
WITH N,O AND O2
Received 28 April 1972; revised manuscript
received 10 July 1972
The explanation of the reaction mechanism of ethylene oxidation over a silver catalyst largely depends on the answer to the question whether a diatomic oxygen species (0,) and/or a monoatomic oxygen species (O=) exist(s) at the surfacei). The aim of the present work is to obtain information on this point by a comparative study of the adsorption complexes formed on silver from nitrous oxide and molecular oxygen. Upon decomposition at the surface N,O will deposit atomic oxygen; adsorbed diatomic oxygen might be formed in the presence of 0,. Some preliminary results are reported here. The field-emission-microscopy (= FEM) technique was chosen because work function data enable differentiation between the various adsorbed species. Further, if the magnitudes of the polarizabilities of the negatively charged oxygen ions are assumed to be comparable, the more loosely bound species may desorb preferentially under the influence of the high electric field needed for electron emission (max. 5 V/rim). Further advantages are that the adsorptions are carried out at surfaces of controlled cleanliness and that information on crystal-face specific adsorption is obtained. Finally, this investigation offered an incentive to prepare clean silver surfaces suitable for FEM experiments up to 4OO”C, a temperature which is certainly required for studies of the silver/oxygen system to achieve complete oxygen desorption. The present work thus tries to extend the applicability of FEM to adsorption on soft metals. Single silver whiskers, [l lo] oriented and grown on a blunt (radius N lo3 nm) tungsten tip that was cleaned by heating to lOOO”C, as well as lo-50 atomic layers thick epitaxial silver layers [Ag( 111)//W (1 lo)] grown on a tungsten tip (radius 30-50 nm) that was first heated to max. 700°C and then field evaporated, were prepared. Silver whiskers2) and epitaxial silver layerss) have been mentioned before as potential FEM specimens to study gas adsorption, but to our knowledge they have never actually been used for this purpose. The following advantages of this method were found: (1) Because in the preparation of both whiskers and epitaxial layers welldegassed high-purity silver is deposited from its vapor in UHV (during evaporation the pressure is in the lo-* Pascal range) onto thoroughly cleaned tungsten tips, ultraclean silver surfaces are obtained in situ, the problem of 624
INTERACTION
OF
Ag
WITH
NaO
AND
02
625
diffusion of impurities from the bulk or from the shank of the tip to the emitting surface upon heating thus being eliminated. (2) Silver whiskers and epitaxial layers on tungsten substrates do not blunt after extensive heating at 400°C as was concluded from the fact that the emission voltage for a clean specimen remains virtually constant in the course of many experiments. Even heating up to 600°C for short periods is possible; at 700°C evaporation takes place. The W-Ag interaction was found negligible. Whiskers were preferred when high pressures of active gases were used; in the case of epitaxial layers tip failure occurred probably because a brittle tungsten oxide layer was built up at the W-Ag interface. The existence of this layer was shown by stepwise field evaporation/work function determination sequences. An additional advantage of using whiskers is that their diameter of 5-20 nm makes the tip surface similar to that of silver particles in actual supported catalysts. In figs. 1 (a) and 3 (a) emission patterns of a whisker and an epitaxial layer are shown. The (111) and (100) planes are well-developed, the bright triangular regions are centred around the (210) planes, the zone lines through (110) and (211) connecting the (111) planes and the (111) and (100) planes, respectively, are intermediately emitting. Before Fowler-Nordheim plots or photographs were taken the gas was always pumped off at 0°C (except when the adsorptions were carried out at - 196°C) until a pressure below lo-’ Pa was reached and the tip was cooled to - 196 “C to minimize field-induced migration and desorption. Maximum currents used were approx. 1 uA in the case of epitaxial layers and approx. 0.1 uA in the case of whiskers. Physical adsorption of N,O molecules on silver whiskers was found to cause a decrease in work function and was further characterized by complete desorption at - 196°C at 5 V/nm (tip negative). No accurate data on this work function change can be given yet. This non-dissociative adsorption occurred when the tip was exposed to N,O at - 196°C or when after exposure at higher temperature the N,O had been pumped off to an insufficient degree. Dissociative adsorption of N,O on Ag leaves oxygen at the surface, which causes an increase in work function. At temperatures from 0-200°C it proceeded at very low rates: at 0°C 5 min of N,O exposure at 130 Pa (= 1 torr) was required to produce any detectable amounts of chemisorbed species (fig. l), at lOO”C, 5 min at 13 Pa, and at 200°C 5 min at 1.3 Pa. This shows that the N,O decomposition on silver is an activated process; our preliminary data point to an activation energy smaller than the value of 120 kJ mole-’ (28 kcal mole-‘) indicated by Herzog4). The N,O used in this work was carefully purified by repeated freezing, pumping off and distilling.
626
M. M. P. JANSSEN,
J. MOOLHUYSEN
AND
W. M. H. SACHTLER
Fig. 1. N20 decomposition on a silver whisker. Tip voltage approx. 4000 V. (a) Whisker cleaned by heating at 400°C in UHV: (b) whisker exposed to NzO at 0°C and 13 Pa for 5 min; (c) as (b) but N20 pressure 130 Pa.
Fig. 2. Adsorption of oxygen at 0°C and 200°C on the silver whisker of fig. 1. (a) Whisker exposed to oxygen at 0°C and 0.13 Pa for 5 min; (b) as (a) with additional 5 min exposure to oxygen at 200°C and 0.013 Pa.
INTERACTION
OF
Ag WITH N20
AND
Oa
621
Fig. 3. Adsorption of oxygen at 200°C on an epitaxial silver layer. Tip voltage approx. 700 V. (a) Clean Ag surface; (b) 9 min oxygen at 1.3 x 10e5 Pa; (c) as (b) +9 min at 6.5 x 1O-5 Pa; (d) as (c) + 1 min at 6.5 x 1O-4 Pa; (e) as (d) + 9 min at 6.5 x 1O-4 Pa; (f) as (e) + 3 min at 0.13 Pa.
628
M. M. P. JANSSEN,
J. MOOLHUYSEN
AND
W. M. H. SACHTLER
Adsorption of oxygen on silver whiskers and epitaxial layers proceeded much faster; moreover, the rate increase with higher adsorption temperatures reveals a low activation energy. Fig. 2 shows that the adsorption at high oxygen pressures leads to higher coverages at 200°C than at 0°C. Adsorption rates were measurable at oxygen pressures of lo-’ to 10T4 Pa. An example of the gradual covering of an epitaxial silver layer at 200 “C is shown in fig. 3. The bright areas darken first, then the (110) zone lines and finally the (211) zone lines. The (111) plane becomes smaller and ultimately the pattern consists of bright rings around the (111) and (100) planes. At high oxygen coverages (0.13 Pa for 5 min) the work function increased by approximately 0.4 and 0.8 eV at 0°C and 200 “C; the corresponding “log A changes (A = pre-exponential factor in the FN equation) were approximately -0.5 and - 1.0. A remarkable feature typical of adsorbed oxygen is that at high coverages some oxygen is desorbed at a field strength near that used for electron imaging. This observation would be in agreement with the presence of loosely bound diatomic oxygen such as 0; on the silver surface in addition to mono-atomic oxygen, as proved by infrared studies published by Gerei et al.5) and Kilty et al.6). Desorption of oxygen at 400°C in UHV was found to restore the clean surface. Partial cleaning, causing a decrease in work function, was achieved by exposure to hydrogen at low temperatures. The work function data are in good agreement with those measured by Rudnitskii et al.7) on a porous Ag catalyst by the vibrating condenser method. Exposure of an oxygen-covered surface to ethylene caused complexes which, again, are easily field-desorbed. This is consistent with the assumption of an organic peroxide complex as postulated in refs. 5 and 6. Summarizing, we can state that: (1) Soft metals like Ag can be studied as thermally stable clean adsorbents by FEM in the form of either whiskers or vapor-deposited layers on refractory metal tips. (2) The adsorption of N,O on Ag is non-dissociative at low temperatures, causing a decrease in work function; its dissociative adsorption at higher temperatures is activated and leads to an increase in work function. (3) Oxygen adsorption on Ag is face-specific and causes an increase in work function too. At high coverages a species is observed that is easily desorbed at electron imaging fields; it is tentatively described as 0 iads M. M. P. JANSSEN,J. MOOLHUYSEN and W. M. H. SACHTLER
Koninkl~ke~Shel~-L~bor~tori~m, Am~ter~m, ~etherl~~s
Amsterdam
(Shell Research N. Y.) ,
INTERACTION
OF
Ag
WITH
NaO
AND
02
629
References 1) 2) 3) 4) 5) 6)
W. M. H. Sachtler, Catalysis Rev. 4 (1970) 27. A. J. Melmed, J. Chem. Phys. 38 (1963) 607. A. J. Melmed and R. Gomer, J. Chem. Phys. 34 (1961) 1802. W. Herzog, Ber. Bunsenges. Physik. Chemie 74 (1970) 216. S. V. Gerei, K. M. Kholyavenko and M. Ya. Rubanik, Ukr. Khem. Zh. 31 (1965) 449. P. A. Kilty, N. C. Rol and W. M. H. Sachtler, paper submitted to 5th Intern. Congr. Catalysis, Miami (Fla.) USA, 1972. 7) L. A. Rudnitskii, L. I. Shakhovskaya, N. V. Kul’kova and M. I. Temkin, Dokl. Akad. Nauk SSSR 182 (1968) 786.