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Hydrogen-palladium interaction studies by reflection-absorption infrared spectroscopy
Surface Science 48 (1975) 549-560 © North-Holland Publishing Company
H Y D R O G E N - P A L L A D I U M INTERACTION STUDIES BY REFLECTION-
ABSORPTION INFRARED SPECTROSCOPY Irmina RATAJCZYKOWA Institute of Physical Chemistry of the Polish Academy of Sciences, Department of Catalysis on Metals, UI. Kasprzaka 44/52, 01-224 Warszawa,Poland Received 18 July 1974; manuscript received in final form 30 October 1974 The vibrational spectrum of hydrogen on a palladium film prepared under ultrahigh vacuum conditions has been studied using reflection-absorption infrared spectroscopy. No infrared band is observed for an electronegative form of adsorbed hydrogen. Two bands at 760 cm-1 and 880 cm-1 observed for hydrogen on a palladium hydride are attributed to an electropositive form of hydrogen bound in hydride-like positions at (100) and (111) crystal planes.
1. Introduction R e f l e c t i o n - a b s o r p t i o n infrared spectroscopy ( R A - I R ) is eminently suited for the study of well-defined metal surfaces, such as single crystal faces or thick thoroughly annealed evaporated films. Since the method was greatly expanded in its theoretical and practical bases by Greenler [ 1 , 2 ] , it has been successfully used in studies of carbon monoxide adsorption on metal single crystal faces [3] as well as on ribbons [ 4 - 6 ] or evaporated films [ 7 - 1 0 ] . In the case of CO (with its high extinction coefficient) chemisorbed on polycrystalline tungsten ribbon [6], it was shown that the adsorption band resulting from 4% o f a monolayer could be seen. It demonstrated a level of sensitivity which made the method appear to be a promising way of obtaining very specific information about structures o f a variery of adsorbed molecules on metals. However, as far as we know no paper on adsorption o f other than carbon monoxide molecules has appeared in literature and the question whether the method is capable or not to detect vibrations o f surface species with lower extinction coefficients has remained open. In the present work the R A - I R method was applied to the study of interaction of hydrogen with a polycrystalline palladium f'dm evaporated under UHV conditions.
550
L Ratajczykowa/Hydrogen-palladium interaction studies
2. Experimental 2.1. Optical system
It has been frequently pointed out [1, 2, 6, 9] that the sensitivity of the reflection-absorption infrared method is critically dependent on the angle of incidence of the radiation on the surface and that the optimum depth of the absorption band can be obtained at the angle of incidence in the vicinity of 88 ° . To satisfy this requirement the auxilary optical system shown in fig. 1 was built to modify the path of the sample beam in Perkin-Elmer Model 325 IR spectrometer. The system consists of three spherical (M 2, M4, M6) and four plane (M1, M3, Ms, MT) mirrors. With the mirror M4 infrared radiation is focused onto a palladium reflection rdm at an average angle of incidence of 82 ° with an incident beam angular spread of 5 °. All the mirrors are mounted on a stable base plate which can be removed and returned to its original position in the sample compartment of the spectrometer due to "cone, slot and flat" kinematic mounts. Mirrors M 1 and M 7 are mounted on a separate smaller casting that is kinematically mounted on the base plate. These two mirrors can be removed to allow the use of the instrument in the conventional transmission experiments without diffuculties. Transmission of the entire system with a copper mirror inserted in the beam path instead of a sample is 75% at 2000 cm -1 . Another optical system (not shown in the picture) was built to extend the optical path of the reference beam. It consists of two plane mirrors mounted on a similar casting as M 1 and M7, and of one big spherical mirror mounted above the instrument.
SA MPL E
~-
M6 Fig. 1. Schematic view of optical system used to obtain reflection-absorption spectra.
I. Ratajezykowa/Hydrogen-palladium interaction studies
551
It does not compensate all the attenuation of the sample beam resulting from the seven-mirror system, but it greatly decreases the disturbing influence of atmospheric water vapour and carbon dioxide as both the sample and the reference beam optical paths are equal. The optical systems are tightly covered and flushed with dry nitrogen during the experiments.
2.2. Infrared cell It has been shown [2] that there exists an optimum number of reflections from the surface with maximizes the depth of the reflection-absorption band. The number depends on the optical constants of the metal and is greater for the highly reflecting low-resistivity metals (such as copper and gold) than for the transition metals. Using the optical constants of palladium at 2100 cm -1 , it was calculated [ 11 ] that the optimum number of reflections for angles of incidence as used in this experiment (between 80 ° and 85 °) varies from 1 to 2 as shown in fig. 2. It appears that for palladium one reflection over the entire region is acceptably close to the theoretical optimum. In a lower wavenumber region the situation is less favourable, owing to the increase of reflectivity as the wavenumber decreases. PALLADiUH (24OOcm "4) n --63 ..
•
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k= 45.3 i
•
•
•
•
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ANGLE OF INCIDENCE (Degrees) Fig. 2. Reflection-absorption optimization for adsorbed hydrogen film on palladium at 2]00 cm-1 . The range of incident angles used in this experiment is enclosed by dotted lines.
552
L Ratajczykowa/Hydrogen-palladium interaction studies
I GLASSSCREEN
___L// TUNGSTENFILAMENT ~-
I ./-"~..,. / GLASSPLATE
- -'IF ------tI L-.,/ ~'V/-----A~" INFRARED ~.~ ~ ~'~- - ----~,~ RADIATION
X~
~ COPPERSUPPORT
I MANIPULATOR
Fig. 3. Ultrahigh vacuum one-reflection infrared cell.
Since it was not possible to predict exactly the position of the absorption band of hydrogen on palladium and to estimate the optimum number of reflections in that spectral region, the best way to meet the problem was to accept the requirements for 2100 cm -1 and to build one reflection infrared cell. The ultrahigh vacuum reflection cell is illustrated in fig. 3. It contained a glass plate 3 x 12 cm clipped to a copper support of the same dimensions. The copper support could be taken magnetically into a vertical position and then a palladium film could be deposited on the glass plate by evaporation of a palladium wire (0.01 cm diam.) wound around the tungsten filament. A glass screen of an appropriate shape enabled to evaporate the film on the plate itself but not on the walls of the cell. The windows were additionally protected during evaporation by thin steel discs held in position magnetically. The copper support had tungsten heaters inside the shown
L Ratajczykowa/Hydrogen-palladium interaction studies
553
holes and they permitted heating the sample up to temperatures of 650 K. The special glass cooling equipment was sealed into the cell. With liquid nitrogen the sample could be maintened at 270 K under vacuum or at 200 K with any gas present in the cell. The sample temperature was measured with a copper-constantan thermocouple inserted into a groove in the copper support just at the rear surface of the glass plate. The outer part of the cell was made of a spherical two-litre Pyrex bulb with two ports for the windows. Irtran-6 windows (3.7 cm dia. x 0.3 cm) transmitting infrared radiation down to 360 cm -1 were attached to the ports with Vacseal (Vacuum leak sealer - Vacuum Generators). The sealant, if properly cured, withstands prolonged heating at 473 K. The transmission of the entire system was 8% at 2000 cm -1 and 14% at 1000 cm-1 , due mainly to the high refractive index and resulting high reflection losses of the Irtran-6 material. The vacuum system was pumped using two high-speed Hg diffusion pumps and liquid nitrogen traps. A background pressure of order of 10-9 Torr ( 1 Torr = 133.3 N/m 2) could be obtained. The pressure was measured with a Redhead type ionization gauge. The infrared cell could be isolated from the gas and the vacuum lines by means of magnetically operated glass Dekker valves. The infrared cell, the vacuum and the gas handling systems were mounted on the same carriage and could be moved away from the optical system for baking. 2.3. Procedure
The system was baked at 343 K for 20 hr, followed by a rigorous outgassing of the copper support at 650 K and of the tungsten f'dament at a temperature just below the palladium evaporation temperature for several hours. After the second bakingoutgassing cycle the glass plate was taken into a vertical position and the whole system was moved into the optical path of the spectrometer; then the palladium film was slowly deposited on the glass plate maintained at 345 K. Spectroscopically pure grade I palladium wire (Johnson and Matthey) was used. At the beginning of the evaporation process, the pressure was 6 x 10-9 Torr and it did not change after the evaporation process was finished. The sample was then taken back into a horizontal position and the spectra were scanned. Electrolytical hydrogen of 99.99% purity was allowed to flow into the cell without any further purification except freezing in a series of two liquid-nitrogen traps. The pressure of the gas was measured either with a McLeod gauge or with a mercury manometer positioned on the gas handling line. The spectra were scanned at 0.32 cm-1 sec-1 with a spectral slit width of 5 cm -1. The reference beam attenuator and a recorder scale expansion equipment gave an overall absorption scale expansion of fourteen-fold.
554
L Ratajczykowa/Hydrogen-palladium interaction studies
3. Results and discussion
3.1. Hydrogen adsorption on palladium As it was mentioned, above, the background pressure of 6 x 10 -9 Tort was maintained during the evaporation process. Under these conditions one could expect a quick adsorption of residual gases on a freshly prepared clean palladium film. However, it must be taken into account that the main components of the gas phase are probably nitrogen and carbon monoxide. Nitrogen does not adsorb [12] on palladium in this pressure range at room temperature. Carbon monoxide which adsorbs on palladium associatively should give at low coverage an infrared band in the 1800 cm -1 region [13, 14]. The reflection spectra of the freshly evaporated film were scanned several times in this spectral region with 25-fold absorption scale expansion and no infrared band attributable to CO could be seen. Since it was possible with this scale expansion to detect the absorption band resulting from a coverage below 10-2 or a monolayer, it is believed that even if CO was present at the surface its coverage was extremely low. When hydrogen was introduced onto the clean film, after evaporation no absorption band was detected in the entire spectral region covered by the optical system used (4000 - 360 cm-1), even if the pressure of the hydrogen was as high as 690 Torr. The failure to observe any bands under these conditions confirms the satisfactory cleanliness of the hydrogen introduced into the cell. According to the results of the surface potential studies of the hydrogen-palladium interaction [15], hydrogen adsorption at room temperature on palladium films evaporated at a temperature above 78 K gives rise only to an electronegative/3- adspecies. Till now there have been no infrared data on adsorption of hydrogen on metals except those on supported platinum [ 1 6 - 1 8 ] . However, in the case of platinum an electropositive form of adsorbed hydrogen predominates [10] within the pressure range applied in the infrared studies. These observations would lead to the conclusion that the electronegative form of adsorbed hydrogen, which is probably much more strongly bound, is not active in infrared.
3.2. Hydrogen absorption by palladium 3.2.1. R eflectivity change upon hydrMe formation Palladium hydride is not easily formed with gaseous hydrogen and thick palladium films, even if the pressure of the hydrogen exceeds an equilibrium pressure, unless a film is evaporated at 78 K [ 15-21 ] and exposes mainly high index faces. The films used in the present work were rather thick ( 1 0 0 0 - 2 0 0 0 A) and evaporated at 345 K. The addition of hydrogen up to 690 Torr to the film at 303 K was not effective for a hydride formation. It was necessary to heat the film in hydrogen under that pressure at 393 K overnight. A great change in reflectivity of the palladium film which then appeared could be attributed solely to the transformation of the
L Ratajczykowa/Hydrogen-palladium interaction studies
555
f'dm into the/3-palladium hydride phase. The reflectivity of the hydride was approximately twice as low as that of palladium over the entire spectral region. Once palladium hydride was formed and afterwards decomposed, each new addition of hydrogen to an appropriate pressure caused its formation without any serious difficulties, a fact which is well known for the palladium-hydrogen system [24]. Though it is not possible to calculate optical constants from these measurements, it might be deduced from the simple expression [28] for the reflectivity of the parallel polarized component, _
(n -- sec 0) 2 + k 2
Rp-(n+sec0) 2+k 2 ' that an increase in the refraction index n, and a decrease in the extinction coefficient k, would take into account the reflectivity change Observed. This distinct reflectivity change could be a very useful additional tool in distinguishing between the palladium and palladium hydride adsorptive behaviour when samples of palladium could transform into ~PdHphase "in situ". The reflectivity change would then be clear evidence of the surface state of the palladium catalyst during the reaction process. 3.2.2. IR,absorption bands o f hydrogen on palladium hydride An electropositive form of hydrogen which is believed to be active in infrared can exist only on the surface of a palladium hydride [15, 21]. It has been suggested [22] that the nature of this form of adsorbed hydrogen is similar to that of hydrogen atoms (or screened protons) in a hydride [20]. Infrared spectra were scanned following the addition of hydrogen to a known pressure (hereafter referred to as hydride spectra) and upon pumping the gas phase away (background spectra). The spectra shown in this paper were obtained by subtracting the background from the hydride spectrum point-by-point at small frequency intervals. The development of the reflection-absorption bands for hydrogen is shown for increasing H 2 pressure in fig. 4 (spectra A to E). The shape of the band obtained suggested the existence of two overlapping bands. The procedure to resolve these two bands was essentially the same as that adopted by Yates et al. [6]. The centre of one of the bands'was located at 880 cm -1 and the band was constructed from its high wavenumber contour. The second band was then obtained from the experimental points by subtracting and it appeared at approximately 760 cm-1 . During the hydrogen absorption process no band and no reflectivity change were observed under a pressure of 15 Tore At 30 Torr, both bands appeared simultaneously at a quite high intensity. The spectrum F in fig. 4 was obtained upon pumping hydrogen away down to a pressure of 9 Torr. As it can be seen both bands are still present. At hydrogen pressures lower than 9 Tort, the hydride spectrum could not be obtained because of the time-dependent process of the hydride decomposition and the signal drift due to the reflectivity change.
1. Ratajczykowa/Hydrogen palladium interaction studies
556
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Fig. 4, Development of absorption-reflection bands of hydrogen on palladium. Spectra A to E were taken during absorption of hydrogen by palladium film; spectrum F was taken when the 1t2 pressure had been lowered to 9 Tort.
3.2.3. The intensity o f hydrogen bands It is well known [24] for the p a l l a d i u m - h y d r o g e n system (and for other similar h y d r o g e n - m e t a l systems) that even when "equilibrium" between the hydrogen in the gas phase and the hydrogen content in the sample can be established quite rapidly, the final value of pressure which is needed to transform the c~ solid solution into the /3 hydride phase, is much higher than the value at which the/3 ~ c~ transformation takes place. The existence of such an "hysteresis" between the absorption and desorption pressure - the band appearance - suggested that the two bands were due to hydrogen bonded in a hydride-like form. If this is true, one can expect that there should exist a direct correlation between the intensity of the bands and the concentration of hydrogen in palladium hydride. The integrated intensities were plotted on the equilibrium graph of palladium hydride as shown in fig. 5. The full line represents the pressure dependence of the H/Pd ratio as given by Wicke and Nernst [23]. It is evident that the integrated intensity follows the dependence very well. A question arises as to whether the two bands are characteristic of a bulk palladium
L Ratajczykowa/Hydrogen-palladium interaction studies
557
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oos
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Fig. 5. Part of the equilibrium diagram for the hydrogen-palladium system (after Wicke and Nernst [24] ). Points A to F represent integrated the intensities of the bands in fig. 4.
hydride or of hydrogen held at the surface in a hydride-like position. This question will be discussed in the next section.
3.2.4. Allocation ofiR-reflection bands to different crystal planes The large metal-to-hydrogen mass ratio makes it possible to describe the motion of the hydrogen atoms (or protons) in the hydride in terms of an Einstein model. The Einstein temperature of a bulk palladium hydride as estimated from the neutron scattering data [25, 26] is 650 K which corresponds to the fundamental mode of the local vibration of a proton positioned at 451.7 cm -1 . This value yields the force constant of the vibration, kb, calculated from the simple relation k b = (27rc)2~2M = 0.12 mdyn/)~,
(1)
where M is the proton mass. In the present work no band in the vicinity of 450 cm -1 could be seen and this implies that the bands a t Vl = 8 8 0 c m -1 and ~2 = 760 cm -1 might be considered to be due only to the vibrations of surface hydrogen atoms (or protons) without any interference from the bulk. For the description of the motion of protons at the surface, an Einstein model can be used as a good approximation, but it must be remembered that with the R - A infrared method one can see only those vibrations whose dipole moment change is per-
558
L Ratajczykowa/Hydrogen-palladium
interaction studies
pendicular to the surface. Eq. (1) for the force constants of the Vl and v2 vibrations: k 1 = 0.45 mdyn/A and k 2 = 0.34 mdyn/A. This would point to the fact that the proton is much more strongly held at the surface than it is in the bulk, which is in good agreement with the sequence of adsorption heats of different forms of hydrogen and the heat of hydride formation 2x/-/~-) > 2d-/(/3+) > zSJ-/(Pd--Hbulk) suggested for systems which easily form hydrides [21 ]. Another interesting feature can be observed when we look at the intensity of the bands. The ratio of the integrated intensities of 91 and ~2 bands remains constant as the pressure (and therefore the hydrogen population) changes. From this it might be deduced that the two bands are characteristic of two states of hydrogen bound at energetically different places on the surface. It has already been pointed out that well-annealed palladium films expose mainly (111) and (100) planes [12], and that the high index planes are responsible for infrared active adsorption of nitrogen [27]. In the present work no band of adsorbed nitrogen could be seen under pressures of nitrogen up to 300 Tort and temperatures down to 200 K. Therefore it was concluded that the high index planes were absent from the surface. In fig. 6 the hydride-like positions are shown for the (111) plane (fig. 6a) and the (100) plane (fig. 6b). On the (100) plane, one hydrogen atom interacts with five palladium atoms - four laying in the plane of the picture and one below it. On the (111) plane, there exist two possibilities of location of hydrogen in hydride-like positions. The positions shown as bigger open circles represent hydrogen held above the surface and the smaller full circles represent hydrogen held below the surface. The latter should be much more similar to hydrogen in the bulk hydride and it should give rise to an absorption band not shifted very much from 450 cm -1 . The hydrogen above the surface would be bound to four palladium atoms. As the frequency decreases with an increase of the number of metal atoms to which hydrogen atom is bonded [14], the higher frequency band (Pl) could be allocated to hydrogen bound on the (111) plane and 92 to that on the (100) plane. At present it is not possible to decide on the correctness of the suggested explana-
(a) (b) Fig. 6. Hydrogen positions on (a) the (11 l) planes, and (b) the (100) planes of palladium crystal. Large open circles: palladium atoms; small open and solid circles: hydrogen atoms.
L Ratajczykowa/Hydrogen-palladiurn
interaction studies
559
tion, but the experimental evidence presented above allows us to formulate at least the following conclusions: (i) The electronegative form of adsorbed hydrogen cannot be seen by infrared, either through a reflectivity change or as an r,bsorption band. (ii) The electropositive form of hydrogen which exists in the outermost surface layer of palladium hydride is active in infrared and it gives rise to absorption bands in the region below 1000 cm -1 ; the form remains in equilibrium with the bulk palladium hydride as it is shown by the straight correlation between the integrated intensity, the hydrogen pressure and the H/Pd ratio. (iii) The infrared active form can be observed at surface coverage lower than a monolayer.
Acknowledgements I am very grateful to Professor R.G. Greenler who was introducing me patiently to the arcana of the reflection-absorption method and who shared with me many discussions during designing of the optical system, I would like to express also my sincere thanks to Professor W. Palczewska for her continued interest, advice and encouragement during the performance of the experiments and for critical reading of the manuscript.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [ 11 ] [12] [13] [14] [15] [16] [17] [18] [19] [20]
R.G. Greenler, J. Chem. Phys. 44 (1966) 310. R.G. Greenler, J. Chem. Phys. 50 (1969) 1963. M.A. Chesters, J. Pritchard and M.L. Sims, Chem. Commun. (1970) 1454. M.J.D. Low and J.C. McManus,Chem. Commun. (1967) 1166. J.T. Yates and D.A. King, Surface Sci. 30 (1972) 601. J.T. Yates, R.G. Greenler, I. Ratajczykowa and D.A. King, Surface Sci. 36 (1973) 439. H.C. Eckstrom, G.G. Possley and S.E. Hannum, J. Chem. Phys. 52 (1970) 5435. E.F. McCoy and R.St.C. Smart, Surface Sci. 39 (1973) 109. J. Pritchard and M.L. Sims, Trans. Faraday Soc. 66 (1970) 427. M.L. Kottke, R.G. Greenler and H.G. Tompkins, Surface Sci. 32 (1972) 231. R.G. Greenler, private communciation. D.A. King, Surface Sci. 9 (1968) 375. J.C. Tracy and P.W. Palmberg, J. Chem. Phys. 51 (1969) 4852. R.P. Eischens and W.A. Pliskin, Advan. Catalysis 10 (1958) 1. R. Du~, private communication. D.D. Eley, D.M. Morgan and C.H. Rochester, Trans. Faraday Soc. 64 (1968) 2168. M. Primet, J.M. Basset, M.V. Mathieu and M. Prettre, J. Catalysis 68 (1973) 368. W.A. Pliskin and R.P. Eischens, Z. Physik. Chem. (NR) 24 (1960) 11. R. Dus'and F.C. Tompkins, submitted to J.C.S. Faraday Trans.I. A.C. Switendiek, Bet. Bunsenges. Physik. Chem. 76 (1972) 535; J. Friedel, Ber. Bunsenges. Physik. Chem. 76 (1972) 828.
560 [21] [22] [23] [24] [25] [26] [27] [28]
1. Ratajczykowa/Hydrogen-palladium interaction studies R. Dus] Surface Sci. 42 (1973) 324. R. Dus', J.C.S. Faraday Trans. I 70 (1974) 877. E. yon Wicke and G.H. Nernst, Ber. Bunsenges. Physik. Chem. 68 (1964) 224. F.A. Lewis, The Palladium--Hydrogen System (Academic Press, London, 1967). J. Bergsma and J.A. Goedkoop, Physica 26 (1960) 744. M.R. Chowdhury and D.K. Ross, Solid State Commun. 13 (1973) 224. R. van Hardeveld and A. van Montfoort, Surface Sci. 4 (1966) 396. R.S. Longhurst, Geometrical and Physical Optics (Longmans, Green and Co., London, 1967).
H Y D R O G E N - P A L L A D I U M INTERACTION STUDIES BY REFLECTION-
ABSORPTION INFRARED SPECTROSCOPY Irmina RATAJCZYKOWA Institute of Physical Chemistry of the Polish Academy of Sciences, Department of Catalysis on Metals, UI. Kasprzaka 44/52, 01-224 Warszawa,Poland Received 18 July 1974; manuscript received in final form 30 October 1974 The vibrational spectrum of hydrogen on a palladium film prepared under ultrahigh vacuum conditions has been studied using reflection-absorption infrared spectroscopy. No infrared band is observed for an electronegative form of adsorbed hydrogen. Two bands at 760 cm-1 and 880 cm-1 observed for hydrogen on a palladium hydride are attributed to an electropositive form of hydrogen bound in hydride-like positions at (100) and (111) crystal planes.
1. Introduction R e f l e c t i o n - a b s o r p t i o n infrared spectroscopy ( R A - I R ) is eminently suited for the study of well-defined metal surfaces, such as single crystal faces or thick thoroughly annealed evaporated films. Since the method was greatly expanded in its theoretical and practical bases by Greenler [ 1 , 2 ] , it has been successfully used in studies of carbon monoxide adsorption on metal single crystal faces [3] as well as on ribbons [ 4 - 6 ] or evaporated films [ 7 - 1 0 ] . In the case of CO (with its high extinction coefficient) chemisorbed on polycrystalline tungsten ribbon [6], it was shown that the adsorption band resulting from 4% o f a monolayer could be seen. It demonstrated a level of sensitivity which made the method appear to be a promising way of obtaining very specific information about structures o f a variery of adsorbed molecules on metals. However, as far as we know no paper on adsorption o f other than carbon monoxide molecules has appeared in literature and the question whether the method is capable or not to detect vibrations o f surface species with lower extinction coefficients has remained open. In the present work the R A - I R method was applied to the study of interaction of hydrogen with a polycrystalline palladium f'dm evaporated under UHV conditions.
550
L Ratajczykowa/Hydrogen-palladium interaction studies
2. Experimental 2.1. Optical system
It has been frequently pointed out [1, 2, 6, 9] that the sensitivity of the reflection-absorption infrared method is critically dependent on the angle of incidence of the radiation on the surface and that the optimum depth of the absorption band can be obtained at the angle of incidence in the vicinity of 88 ° . To satisfy this requirement the auxilary optical system shown in fig. 1 was built to modify the path of the sample beam in Perkin-Elmer Model 325 IR spectrometer. The system consists of three spherical (M 2, M4, M6) and four plane (M1, M3, Ms, MT) mirrors. With the mirror M4 infrared radiation is focused onto a palladium reflection rdm at an average angle of incidence of 82 ° with an incident beam angular spread of 5 °. All the mirrors are mounted on a stable base plate which can be removed and returned to its original position in the sample compartment of the spectrometer due to "cone, slot and flat" kinematic mounts. Mirrors M 1 and M 7 are mounted on a separate smaller casting that is kinematically mounted on the base plate. These two mirrors can be removed to allow the use of the instrument in the conventional transmission experiments without diffuculties. Transmission of the entire system with a copper mirror inserted in the beam path instead of a sample is 75% at 2000 cm -1 . Another optical system (not shown in the picture) was built to extend the optical path of the reference beam. It consists of two plane mirrors mounted on a similar casting as M 1 and M7, and of one big spherical mirror mounted above the instrument.
SA MPL E
~-
M6 Fig. 1. Schematic view of optical system used to obtain reflection-absorption spectra.
I. Ratajezykowa/Hydrogen-palladium interaction studies
551
It does not compensate all the attenuation of the sample beam resulting from the seven-mirror system, but it greatly decreases the disturbing influence of atmospheric water vapour and carbon dioxide as both the sample and the reference beam optical paths are equal. The optical systems are tightly covered and flushed with dry nitrogen during the experiments.
2.2. Infrared cell It has been shown [2] that there exists an optimum number of reflections from the surface with maximizes the depth of the reflection-absorption band. The number depends on the optical constants of the metal and is greater for the highly reflecting low-resistivity metals (such as copper and gold) than for the transition metals. Using the optical constants of palladium at 2100 cm -1 , it was calculated [ 11 ] that the optimum number of reflections for angles of incidence as used in this experiment (between 80 ° and 85 °) varies from 1 to 2 as shown in fig. 2. It appears that for palladium one reflection over the entire region is acceptably close to the theoretical optimum. In a lower wavenumber region the situation is less favourable, owing to the increase of reflectivity as the wavenumber decreases. PALLADiUH (24OOcm "4) n --63 ..
•
,
k= 45.3 i
•
•
•
•
•
i
[
I
i J
p I
t
~'o
•
I
OPT N
•
•
|
•
•
I
I
i
i
,
°
I
] i i
[
6,
•
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[ j I
O
iI
i
AR (OPT)
:2
.13
42 ee
44 O
"
90
•
,
•
•
,
85
i
•
•
•
80
•
•
75
•
•
70
ANGLE OF INCIDENCE (Degrees) Fig. 2. Reflection-absorption optimization for adsorbed hydrogen film on palladium at 2]00 cm-1 . The range of incident angles used in this experiment is enclosed by dotted lines.
552
L Ratajczykowa/Hydrogen-palladium interaction studies
I GLASSSCREEN
___L// TUNGSTENFILAMENT ~-
I ./-"~..,. / GLASSPLATE
- -'IF ------tI L-.,/ ~'V/-----A~" INFRARED ~.~ ~ ~'~- - ----~,~ RADIATION
X~
~ COPPERSUPPORT
I MANIPULATOR
Fig. 3. Ultrahigh vacuum one-reflection infrared cell.
Since it was not possible to predict exactly the position of the absorption band of hydrogen on palladium and to estimate the optimum number of reflections in that spectral region, the best way to meet the problem was to accept the requirements for 2100 cm -1 and to build one reflection infrared cell. The ultrahigh vacuum reflection cell is illustrated in fig. 3. It contained a glass plate 3 x 12 cm clipped to a copper support of the same dimensions. The copper support could be taken magnetically into a vertical position and then a palladium film could be deposited on the glass plate by evaporation of a palladium wire (0.01 cm diam.) wound around the tungsten filament. A glass screen of an appropriate shape enabled to evaporate the film on the plate itself but not on the walls of the cell. The windows were additionally protected during evaporation by thin steel discs held in position magnetically. The copper support had tungsten heaters inside the shown
L Ratajczykowa/Hydrogen-palladium interaction studies
553
holes and they permitted heating the sample up to temperatures of 650 K. The special glass cooling equipment was sealed into the cell. With liquid nitrogen the sample could be maintened at 270 K under vacuum or at 200 K with any gas present in the cell. The sample temperature was measured with a copper-constantan thermocouple inserted into a groove in the copper support just at the rear surface of the glass plate. The outer part of the cell was made of a spherical two-litre Pyrex bulb with two ports for the windows. Irtran-6 windows (3.7 cm dia. x 0.3 cm) transmitting infrared radiation down to 360 cm -1 were attached to the ports with Vacseal (Vacuum leak sealer - Vacuum Generators). The sealant, if properly cured, withstands prolonged heating at 473 K. The transmission of the entire system was 8% at 2000 cm -1 and 14% at 1000 cm-1 , due mainly to the high refractive index and resulting high reflection losses of the Irtran-6 material. The vacuum system was pumped using two high-speed Hg diffusion pumps and liquid nitrogen traps. A background pressure of order of 10-9 Torr ( 1 Torr = 133.3 N/m 2) could be obtained. The pressure was measured with a Redhead type ionization gauge. The infrared cell could be isolated from the gas and the vacuum lines by means of magnetically operated glass Dekker valves. The infrared cell, the vacuum and the gas handling systems were mounted on the same carriage and could be moved away from the optical system for baking. 2.3. Procedure
The system was baked at 343 K for 20 hr, followed by a rigorous outgassing of the copper support at 650 K and of the tungsten f'dament at a temperature just below the palladium evaporation temperature for several hours. After the second bakingoutgassing cycle the glass plate was taken into a vertical position and the whole system was moved into the optical path of the spectrometer; then the palladium film was slowly deposited on the glass plate maintained at 345 K. Spectroscopically pure grade I palladium wire (Johnson and Matthey) was used. At the beginning of the evaporation process, the pressure was 6 x 10-9 Torr and it did not change after the evaporation process was finished. The sample was then taken back into a horizontal position and the spectra were scanned. Electrolytical hydrogen of 99.99% purity was allowed to flow into the cell without any further purification except freezing in a series of two liquid-nitrogen traps. The pressure of the gas was measured either with a McLeod gauge or with a mercury manometer positioned on the gas handling line. The spectra were scanned at 0.32 cm-1 sec-1 with a spectral slit width of 5 cm -1. The reference beam attenuator and a recorder scale expansion equipment gave an overall absorption scale expansion of fourteen-fold.
554
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3. Results and discussion
3.1. Hydrogen adsorption on palladium As it was mentioned, above, the background pressure of 6 x 10 -9 Tort was maintained during the evaporation process. Under these conditions one could expect a quick adsorption of residual gases on a freshly prepared clean palladium film. However, it must be taken into account that the main components of the gas phase are probably nitrogen and carbon monoxide. Nitrogen does not adsorb [12] on palladium in this pressure range at room temperature. Carbon monoxide which adsorbs on palladium associatively should give at low coverage an infrared band in the 1800 cm -1 region [13, 14]. The reflection spectra of the freshly evaporated film were scanned several times in this spectral region with 25-fold absorption scale expansion and no infrared band attributable to CO could be seen. Since it was possible with this scale expansion to detect the absorption band resulting from a coverage below 10-2 or a monolayer, it is believed that even if CO was present at the surface its coverage was extremely low. When hydrogen was introduced onto the clean film, after evaporation no absorption band was detected in the entire spectral region covered by the optical system used (4000 - 360 cm-1), even if the pressure of the hydrogen was as high as 690 Torr. The failure to observe any bands under these conditions confirms the satisfactory cleanliness of the hydrogen introduced into the cell. According to the results of the surface potential studies of the hydrogen-palladium interaction [15], hydrogen adsorption at room temperature on palladium films evaporated at a temperature above 78 K gives rise only to an electronegative/3- adspecies. Till now there have been no infrared data on adsorption of hydrogen on metals except those on supported platinum [ 1 6 - 1 8 ] . However, in the case of platinum an electropositive form of adsorbed hydrogen predominates [10] within the pressure range applied in the infrared studies. These observations would lead to the conclusion that the electronegative form of adsorbed hydrogen, which is probably much more strongly bound, is not active in infrared.
3.2. Hydrogen absorption by palladium 3.2.1. R eflectivity change upon hydrMe formation Palladium hydride is not easily formed with gaseous hydrogen and thick palladium films, even if the pressure of the hydrogen exceeds an equilibrium pressure, unless a film is evaporated at 78 K [ 15-21 ] and exposes mainly high index faces. The films used in the present work were rather thick ( 1 0 0 0 - 2 0 0 0 A) and evaporated at 345 K. The addition of hydrogen up to 690 Torr to the film at 303 K was not effective for a hydride formation. It was necessary to heat the film in hydrogen under that pressure at 393 K overnight. A great change in reflectivity of the palladium film which then appeared could be attributed solely to the transformation of the
L Ratajczykowa/Hydrogen-palladium interaction studies
555
f'dm into the/3-palladium hydride phase. The reflectivity of the hydride was approximately twice as low as that of palladium over the entire spectral region. Once palladium hydride was formed and afterwards decomposed, each new addition of hydrogen to an appropriate pressure caused its formation without any serious difficulties, a fact which is well known for the palladium-hydrogen system [24]. Though it is not possible to calculate optical constants from these measurements, it might be deduced from the simple expression [28] for the reflectivity of the parallel polarized component, _
(n -- sec 0) 2 + k 2
Rp-(n+sec0) 2+k 2 ' that an increase in the refraction index n, and a decrease in the extinction coefficient k, would take into account the reflectivity change Observed. This distinct reflectivity change could be a very useful additional tool in distinguishing between the palladium and palladium hydride adsorptive behaviour when samples of palladium could transform into ~PdHphase "in situ". The reflectivity change would then be clear evidence of the surface state of the palladium catalyst during the reaction process. 3.2.2. IR,absorption bands o f hydrogen on palladium hydride An electropositive form of hydrogen which is believed to be active in infrared can exist only on the surface of a palladium hydride [15, 21]. It has been suggested [22] that the nature of this form of adsorbed hydrogen is similar to that of hydrogen atoms (or screened protons) in a hydride [20]. Infrared spectra were scanned following the addition of hydrogen to a known pressure (hereafter referred to as hydride spectra) and upon pumping the gas phase away (background spectra). The spectra shown in this paper were obtained by subtracting the background from the hydride spectrum point-by-point at small frequency intervals. The development of the reflection-absorption bands for hydrogen is shown for increasing H 2 pressure in fig. 4 (spectra A to E). The shape of the band obtained suggested the existence of two overlapping bands. The procedure to resolve these two bands was essentially the same as that adopted by Yates et al. [6]. The centre of one of the bands'was located at 880 cm -1 and the band was constructed from its high wavenumber contour. The second band was then obtained from the experimental points by subtracting and it appeared at approximately 760 cm-1 . During the hydrogen absorption process no band and no reflectivity change were observed under a pressure of 15 Tore At 30 Torr, both bands appeared simultaneously at a quite high intensity. The spectrum F in fig. 4 was obtained upon pumping hydrogen away down to a pressure of 9 Torr. As it can be seen both bands are still present. At hydrogen pressures lower than 9 Tort, the hydride spectrum could not be obtained because of the time-dependent process of the hydride decomposition and the signal drift due to the reflectivity change.
1. Ratajczykowa/Hydrogen palladium interaction studies
556
p &I=4% L
~
'
-
:3OTorr A
N2
q
-7"
, ""~
,
,
,'~;'-r ~ ~ ~ , - - r ~ r ~ T - - - - PH# ~OOTorr B
PH2=/i~+6T°rr ....
C
...._._.. PH2
= 248 Torr
D
PH2=730T°rr ~
- " ~ ?
E
. . . . . . . . . .....4...
PH#9T°rr -
-
,---- - r - - ' - 7
600
_.,...,--'- ~"c'.~.~'J . . . . . . ,
,
,
,
,
800 W A V E NUMBER
.--...~. *
,
~--°--Y-
4000
*
F -
-
P
~
4200
CM -4
Fig. 4, Development of absorption-reflection bands of hydrogen on palladium. Spectra A to E were taken during absorption of hydrogen by palladium film; spectrum F was taken when the 1t2 pressure had been lowered to 9 Tort.
3.2.3. The intensity o f hydrogen bands It is well known [24] for the p a l l a d i u m - h y d r o g e n system (and for other similar h y d r o g e n - m e t a l systems) that even when "equilibrium" between the hydrogen in the gas phase and the hydrogen content in the sample can be established quite rapidly, the final value of pressure which is needed to transform the c~ solid solution into the /3 hydride phase, is much higher than the value at which the/3 ~ c~ transformation takes place. The existence of such an "hysteresis" between the absorption and desorption pressure - the band appearance - suggested that the two bands were due to hydrogen bonded in a hydride-like form. If this is true, one can expect that there should exist a direct correlation between the intensity of the bands and the concentration of hydrogen in palladium hydride. The integrated intensities were plotted on the equilibrium graph of palladium hydride as shown in fig. 5. The full line represents the pressure dependence of the H/Pd ratio as given by Wicke and Nernst [23]. It is evident that the integrated intensity follows the dependence very well. A question arises as to whether the two bands are characteristic of a bulk palladium
L Ratajczykowa/Hydrogen-palladium interaction studies
557
1000 500 1"=303 K 200 lO0 I-
g
so
J ,D
Tocr/
20
17.3 ABSORPTION
~0
8/-~ Torr DESORP'flON~INTEr..~RA'IED INTENSITY(arb.units) 0i i 20 t,O 60 80 i i i i L L
o'.so o ss
oos
H/Pd
o. o
Fig. 5. Part of the equilibrium diagram for the hydrogen-palladium system (after Wicke and Nernst [24] ). Points A to F represent integrated the intensities of the bands in fig. 4.
hydride or of hydrogen held at the surface in a hydride-like position. This question will be discussed in the next section.
3.2.4. Allocation ofiR-reflection bands to different crystal planes The large metal-to-hydrogen mass ratio makes it possible to describe the motion of the hydrogen atoms (or protons) in the hydride in terms of an Einstein model. The Einstein temperature of a bulk palladium hydride as estimated from the neutron scattering data [25, 26] is 650 K which corresponds to the fundamental mode of the local vibration of a proton positioned at 451.7 cm -1 . This value yields the force constant of the vibration, kb, calculated from the simple relation k b = (27rc)2~2M = 0.12 mdyn/)~,
(1)
where M is the proton mass. In the present work no band in the vicinity of 450 cm -1 could be seen and this implies that the bands a t Vl = 8 8 0 c m -1 and ~2 = 760 cm -1 might be considered to be due only to the vibrations of surface hydrogen atoms (or protons) without any interference from the bulk. For the description of the motion of protons at the surface, an Einstein model can be used as a good approximation, but it must be remembered that with the R - A infrared method one can see only those vibrations whose dipole moment change is per-
558
L Ratajczykowa/Hydrogen-palladium
interaction studies
pendicular to the surface. Eq. (1) for the force constants of the Vl and v2 vibrations: k 1 = 0.45 mdyn/A and k 2 = 0.34 mdyn/A. This would point to the fact that the proton is much more strongly held at the surface than it is in the bulk, which is in good agreement with the sequence of adsorption heats of different forms of hydrogen and the heat of hydride formation 2x/-/~-) > 2d-/(/3+) > zSJ-/(Pd--Hbulk) suggested for systems which easily form hydrides [21 ]. Another interesting feature can be observed when we look at the intensity of the bands. The ratio of the integrated intensities of 91 and ~2 bands remains constant as the pressure (and therefore the hydrogen population) changes. From this it might be deduced that the two bands are characteristic of two states of hydrogen bound at energetically different places on the surface. It has already been pointed out that well-annealed palladium films expose mainly (111) and (100) planes [12], and that the high index planes are responsible for infrared active adsorption of nitrogen [27]. In the present work no band of adsorbed nitrogen could be seen under pressures of nitrogen up to 300 Tort and temperatures down to 200 K. Therefore it was concluded that the high index planes were absent from the surface. In fig. 6 the hydride-like positions are shown for the (111) plane (fig. 6a) and the (100) plane (fig. 6b). On the (100) plane, one hydrogen atom interacts with five palladium atoms - four laying in the plane of the picture and one below it. On the (111) plane, there exist two possibilities of location of hydrogen in hydride-like positions. The positions shown as bigger open circles represent hydrogen held above the surface and the smaller full circles represent hydrogen held below the surface. The latter should be much more similar to hydrogen in the bulk hydride and it should give rise to an absorption band not shifted very much from 450 cm -1 . The hydrogen above the surface would be bound to four palladium atoms. As the frequency decreases with an increase of the number of metal atoms to which hydrogen atom is bonded [14], the higher frequency band (Pl) could be allocated to hydrogen bound on the (111) plane and 92 to that on the (100) plane. At present it is not possible to decide on the correctness of the suggested explana-
(a) (b) Fig. 6. Hydrogen positions on (a) the (11 l) planes, and (b) the (100) planes of palladium crystal. Large open circles: palladium atoms; small open and solid circles: hydrogen atoms.
L Ratajczykowa/Hydrogen-palladiurn
interaction studies
559
tion, but the experimental evidence presented above allows us to formulate at least the following conclusions: (i) The electronegative form of adsorbed hydrogen cannot be seen by infrared, either through a reflectivity change or as an r,bsorption band. (ii) The electropositive form of hydrogen which exists in the outermost surface layer of palladium hydride is active in infrared and it gives rise to absorption bands in the region below 1000 cm -1 ; the form remains in equilibrium with the bulk palladium hydride as it is shown by the straight correlation between the integrated intensity, the hydrogen pressure and the H/Pd ratio. (iii) The infrared active form can be observed at surface coverage lower than a monolayer.
Acknowledgements I am very grateful to Professor R.G. Greenler who was introducing me patiently to the arcana of the reflection-absorption method and who shared with me many discussions during designing of the optical system, I would like to express also my sincere thanks to Professor W. Palczewska for her continued interest, advice and encouragement during the performance of the experiments and for critical reading of the manuscript.
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R.G. Greenler, J. Chem. Phys. 44 (1966) 310. R.G. Greenler, J. Chem. Phys. 50 (1969) 1963. M.A. Chesters, J. Pritchard and M.L. Sims, Chem. Commun. (1970) 1454. M.J.D. Low and J.C. McManus,Chem. Commun. (1967) 1166. J.T. Yates and D.A. King, Surface Sci. 30 (1972) 601. J.T. Yates, R.G. Greenler, I. Ratajczykowa and D.A. King, Surface Sci. 36 (1973) 439. H.C. Eckstrom, G.G. Possley and S.E. Hannum, J. Chem. Phys. 52 (1970) 5435. E.F. McCoy and R.St.C. Smart, Surface Sci. 39 (1973) 109. J. Pritchard and M.L. Sims, Trans. Faraday Soc. 66 (1970) 427. M.L. Kottke, R.G. Greenler and H.G. Tompkins, Surface Sci. 32 (1972) 231. R.G. Greenler, private communciation. D.A. King, Surface Sci. 9 (1968) 375. J.C. Tracy and P.W. Palmberg, J. Chem. Phys. 51 (1969) 4852. R.P. Eischens and W.A. Pliskin, Advan. Catalysis 10 (1958) 1. R. Du~, private communication. D.D. Eley, D.M. Morgan and C.H. Rochester, Trans. Faraday Soc. 64 (1968) 2168. M. Primet, J.M. Basset, M.V. Mathieu and M. Prettre, J. Catalysis 68 (1973) 368. W.A. Pliskin and R.P. Eischens, Z. Physik. Chem. (NR) 24 (1960) 11. R. Dus'and F.C. Tompkins, submitted to J.C.S. Faraday Trans.I. A.C. Switendiek, Bet. Bunsenges. Physik. Chem. 76 (1972) 535; J. Friedel, Ber. Bunsenges. Physik. Chem. 76 (1972) 828.
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1. Ratajczykowa/Hydrogen-palladium interaction studies R. Dus] Surface Sci. 42 (1973) 324. R. Dus', J.C.S. Faraday Trans. I 70 (1974) 877. E. yon Wicke and G.H. Nernst, Ber. Bunsenges. Physik. Chem. 68 (1964) 224. F.A. Lewis, The Palladium--Hydrogen System (Academic Press, London, 1967). J. Bergsma and J.A. Goedkoop, Physica 26 (1960) 744. M.R. Chowdhury and D.K. Ross, Solid State Commun. 13 (1973) 224. R. van Hardeveld and A. van Montfoort, Surface Sci. 4 (1966) 396. R.S. Longhurst, Geometrical and Physical Optics (Longmans, Green and Co., London, 1967).