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Surface phenomena and isotope effects at low temperature palladium hydride formation and during its decomposition
Surface Science 216 (1989) l-13 North-Holland, Amsterdam
SURFACE PHENOMENA AND ISOTOPE EFFECTS AT LOW IMPELS PAL~D~ HYDRIDE FORMATION AND DURING ITS DECOM~SI~ON Ryszard DUS, Ewa NOWICKA
and Zbigniew WOLFRAM
Institute of Physical Chemistry Polish Academy of Sciences, ul. Kmprzaka 44/52, Warszawa. Poland
Received 4 August 1988; accepted for publication 31 January 1989
Adsorption states of the isotopes: hydrogen and deuterium, arising in the process of low temperature (78 K) palladium hydride (deuteride) formation in thin palladium films are distinguished by simultaneous surface potential and pressure measurements. The precursor state for hydrogen (deuterium) incorporation into the bulk is determined, and the diffusion coefficients are calculated to be (2-4)X1O-13 cm2/s for hydrogen and (0.7-1.5)x10-t3 cm2/s for deuterium. Surface potential isotherms are obtained for palladium hydride (deuteride) showing a significant isotope effect.
1. Introduction Surface phenomena caused by hydrogen interaction with palladium have been studied less intensively than bulk phenomena [l]. The reason is probably the high solubility of hydrogen in palladium and the necessity of simultaneous studies of both adsorption and absorption processes. Experiments carried out under UHV conditions, when hydrogen interacts with a clean surface of palladium are of considerable importance and several experimental works have been performed using well defined single crystal surfaces. Conrad, Ertl and Latta observed by means of the TDS method dissolution of hydrogen in the bulk of palladium as a result of exposure of the Pd(l10) surface to hydrogen at room temperature [2]. A similar observation was made by Engel and Kuipers for the Pd(ll1) surface using the molecular beam technique 131, and by Behm, C~stmann and Ertl for the Pd~l~) surface with the use of TD spectroscopy [4]. These latter authors found that the open Pd(ll0) surface behaves quite differently when interacting with hydrogen at low temperatures (250 and 130 K) [5]. At hydrogen coverages close to monolayer coverage 8 - 1 a reconstruction of the palladium surface occurs. As a result of further exposure, at hydrogen consumption corresponding to 8 = 1.5 two new states appear in the TD spectrum. Such states have never been ~39-6028/89/$03.50 0 Elsevier Science Pub~shers B.V. (North-Holland Physics Publishing Division)
2
R. LG et al. / Low temperature pa~iadium h~~~de~oTrnatio~
observed with adsorption carried out at room temperature. The authors interpreted the new TD states as corresponding to subsurface hydrogen on Pd(ll0). Low temperature transformation of an entire palladium single crystal of a size applicable for conventional UHV studies into palladium hydride would be a long process because of kinetic reasons. It is well established on the other hand, that palladium hydride forms easily by interaction of hydrogen with palladium powder (palladium black) [6]. Therefore it could be expected that an investigation of surface phenomena associated with the transformation of palladium into palladium hydride can be performed using thin palladium films deposited at low temperature under UHV conditions. In this paper we present studies of surface phenomena caused by low temperature (78 K) hydrogen interaction with thin palladium films. The investigations were carried out by means of simultaneous surface potential and pressure measurements.
2. Experimental The experiments were performed using an UHV glass system capable of reaching routinely (l-2) X lo-*’ Torr during deposition of thin palladium
films, and pe~tting work with hydrogen or deute~um by meas~ng their pressures from 1 X 10e6 Torr up to 1 Torr during the adsorption. Measurements of the surface potential were carried out by means of a static capacitor system [7,8]. The static capacitor consists of two coaxial cylindrical electrodes; the inner one being the reference electrode was coated with a conducting layer of (SnO + Sb,O,). This electrode is movable and can be moved up during palladium film deposition on the inner wall of the outer cylinder which was maint~ned at 78 K. This palladium film is the active electrode of the static capacitor. Thin p~la~um films were deposited by evaporation from a fine palladium wire (Johnson-Matthey grade I) wound around a tungsten heater. To avoid alloy formation the palladium wire was never melted during the deposition. The pressure was read by means of modulated ionization gauge. The geometrical area of the films was 135 X lo-* m2, their average thickness lo-’ m, and their average roughness factor estimated by means of hydrogen-oxygen titration [9] was - 16. The films were sintered at 320 K for 20 min. Two versions of the electronic circuit for the static capacitor were applied. The first one for observation of long processes [7] had an overall response time of 0.1 s, with a stability of 1 mV/h, a sensitivity of f 1 mV, and a noise level of - 1 mV. The second for investigation of rapid processes [lo] had an overall response time of 1 ms, with a sensitivity of 0.1 mV and a noise level of - 0.2 mV. By means of these circuits it was possible to carry out long measurements
R. Dti et al. / L.ow temperature palladium hydride formation
3
as well as to observe rapid desorption which occurs e.g. within 1 s for a change of the surface potential ASP of several mV. Spectroscopically pure hydrogen (deuterium), purified additionally by diffusion through a palladium thimble was used. Hydrogen (deuterium) was introduced in calibrated, successive doses while the static capacitor was held at a constant temperature, and was separated from the pumps by means of a Dekker greasless valve. The surface potential and the pressure were continuously recorded. To avoid atomization of hydrogen on the hot filament and pumping effect of the ionization gauge during adsorption, the hydrogen (deuterium) pressure (P) was measured using an ultrasensitive, short response time (1 decade/s) Pirani type gauge capable of working within the range 1 X 10e6-1 Torr. Recording simultaneously ASP and P, while knowning the volume of the static capacitor, enables one to construct the calibration curve ASP versus hydrogen uptake n,. This allows us to transform the measured time dependence of ASP into a function: n a = f( t), and consequently the known kinetic equations can be applied to examine the surface processes. Knowing the weight of the thin palladium film deposited and the hydrogen uptake (adsorbed and absorbed) the ratio of consumed hydrogen H,,,, to the total amount of palladium atoms in the film (- 1000 ,& thick) can be determined and correlated to ASP.
3. Results and discussion Surface potential changes (ASP) measured as the result of the introduction of successive doses of hydrogen into the static capacitor with thin palladium films maintained at 298 and 78 K are shown in figs. 1 and 2, respectively. The different character of the adsorbate formed at these two temperatures can be clearly seen. At 298 K (fig. 1) negatively polarized adspecies (dipoles with the negative pole outward the surface) are formed, decreasing monotonically the surface potential. This is a commonly observed phenomenon caused by hydrogen interaction with many of the transition metals. The surface potential changes caused by deuterium adsorption on a thin palladium film at 298 K are very similar to those presented in fig. 1. The ASP values for deuterium are lo-20 mV lower than for hydrogen at the same coverage. Surface potential changes caused by hydrogen interaction at low temperatures with palladium [ll], and other transition metals forming hydrides [ll-141 have been studied in our laboratory for several years. In fig. 2 we present the result of one of many experiments carried out to study surface potential changes caused by hydrogen interaction with a thin palladium film at 78 K. All characteristic states of the adspecies described below were observed
4
R. Dd et al. / Low temperature palladium hydride formation
Fig. 1. Surface potential changes ASP caused by hydrogen adsorption 298 K. Arrows indicate introduction of the successive hydrogen constant
pressure
value is expressed
on a thin palladium films at doses. The corresponding
in Torr.
in all these experiments with very good reproducibility.Several hydrogen doses introduced into the static capacitor caused a decrease of the surface potential corresponding with the formation of negatively polarized adspecies (fig. 2A). This uptake corresponds to a H,,,/Pd ratio of - 1 X IO-‘. Thermal desorption shows that this form of deposited hydrogen corresponds to dissociative adsorption usually denoted as P-state. We denote this form the /3- state to emphasize its negative polarization. More than eighty further doses of hydrogen introduced into the static capacitor caused the appearance of positive transient of SP (fig. 2B). The value of the surface potential obtained as the result of the successive SP transients becomes increasingly more positive. In spite of the large amount of hydrogen consumed, the gas phase pressure corresponding to the constant value of SP after decay of the positive increment of SP is lower than 1 X 10e6 Torr. We denote this state of the adsorbate as /?’ state to emphasize its positive polarization. The p’ state is formed until the H,,,,/Pd ratio reaches 0.6-0.7; the phase diagram for the hydrogen-palladium system [16] shows that palladium hydride is formed.
R. Dti et al. / Low temperature palladium hydride formation
5
ASP[mV]
24
0
+380
48
time[S]
‘D A
B
+280+180-
+ 80.
‘32 j r,
,
0
,
2
/ 35
34’
4
6
8
10
time[min]
Fig. 2. ASP caused by hydrogen interaction with a thin palladium film at 78 K. Arrows indicate introduction of the successive hydrogen doses. The ASP caused by the isothermal desorption in the course of the evacuation is also shown.
We suppose [11,12] that the p’ adspecies create a precursor state for hydrogen incorporation into the bulk of transition metals with the formation of a hydride. Formation of positively charged hydrogen adspecies on the surface of a transition metal precovered with an amount of hydrogen can be expected on the basis of Grimley’s model [17]. The increase of the hydrogen population on the metal surface increases also the magnitude of the splitting between even and odd induced localized states. There is a critical value of the population above which the lower state merges into the metal conduction band. Then donation of electrons from hydrogen adatoms into unoccupied states of the conduction band occurs. The adspecies arising in this way incorporate easily into the bulk of the metal forming hydrides. This is observed as SP transients. If our supposition is valid, then the decay of the positive increments of SP caused by every hydrogen dose at this step of the process should be described by a diffusion equation. Assuming that within the
R. lb.8 et al. / Low te~eruture
~a~la~~
hydride fo~ation
020 032 -35 040 x 17 559
16--l
60 120 time [s] Fig. 3. Exa~nation of the validity of the diffusion equation for the fi’ hydrogen adspecies incorporation from the surface into the bulk of a thin palladium film at 78 K. Numbers indicate the successive hydrogen doses.
uptake of one hydrogen dose a linear relation between SP and the concentration of the fi’ adspecies on the surface exists we have for a chosen SP transient: ASP-
ASP,
8
where 7 = h2/v2 D, ASP, is the total change of the surface potential due to the formation of the p” adspecies at the moment when a successive hydrogen dose is introduced t = 0, ASP is the measured surface potential change after time r, ASP, corresponds to the stationary value of surface potential reached after sufficiency long time, h is the thin film thickness, and D is the diffusion coefficient. The validity of eq. (1) was observed for all transients. Examples of the validity of eq. (1) are presented in fig. 3. The calculated diffusion coefficients ratio are (2-4) X lo-l3 cm2/s for all successive doses within a H,,,/Pd interval of 0.1-0.6. This is shown in fig. 4. For a H,,/Pd ratio of 0.6-0.7 the diffusion coefficients seem to be higher. The calculated value is five orders of
R. Dti et al. / Low temperature palladium hydride formation
I
D[yl
x
0
hydrogen
.
0
deuterium
1 d5
0
ci2
OL
0.6
0.8
H/Pd . D/I’d
Fig. 4. Diffusion coefficient for hydrogen and deuterium incorporation from the surface into the bulk of a thin palladium film at 78 K in the process of palladium hydride (deuteride) formation. (x) and (0) correspond to calculations based on examination of the SP transients. (0) and (0) correspond to calculations based on measurements of the rate of pressure decrease.
magnitude higher than expected by extrapolating to our conditions the data obtained for hydrogen diffusion in the bulk of palladium for temperatures in the region 230-470 K [18]. Assuming that as usually the pre-exponential factor for interstitial diffusion is of the order of lop3 cm*/s the activation energy for j3’ hydrogen adspecies incorporation into the bulk can be estimated to be 14.7 kJ/mol. This value does not differ very much from those determined experimentally (11 kJ/mol) [19] and calculated theoretically [20] for hydrogen diffusion in palladium hydride within the temperature interval 50-140 K. The low value of the activation energy for diffusion has been interpreted as the result of a tunnelling mechanism for hydrogen diffusion in palladium hydride [20]. Further doses of hydrogen introduced into the static capacitor caused monotonic increase of the surface potential without any transients (fig. 2C). The gas phase pressure now increased with every successive hydrogen dose. However the pressure value associated with every successive dose was not constant like the surface potential, but was slowly decreasing indicating further penetration of hydrogen into the film at constant concentration of hydrogen adspecies on the surface. Measurement of this pressure decrease
8
R DUS et al. / Low temperature palladium hydride formation
gives another, independent possibility of calculating the diffusion coefficients. Using a similar equation as (1) we calculated diffusion coefficients of (7-9) x lo-i3 cm2/s. At pressures of the order of lop2 Tort-, the H,,,,/Pd ratio approaches 0.9. Part of the deposit corresponding to H ,,,/Pd > 0.85 is weakly bound and desorbs in the course of isothermal desorption caused by evacuation at 78 K. This is observed by recording the SP changes as the result of evacuation (see fig. 2). Hydrogen redosing leads to readsorption giving the same value of ASP as before evacuation. Such a desorption-readsorption process can be repeated many times with good reproducibility. One can notice that the desorption is not a simple process. The rapid initial desorption rate is followed by a slower one. At H,ons/Pd > 0.7 the gas phase pressure is higher than 1 x lop6 Torr, so it is detectable by our ultrasensitive Pirani gauge. Hence we can determine volumetrically the hydrogen uptake n z reversibly adsorbed, and construct for this process the calibration curve ASP versus nz. This allows us to transform the relation ASP versus t recorded during isothermal desorption into a function nz = f( t), useful for examining the isothermal desorption kinetics. We found that the initial, rapid decrease of SP at the beginning of desorption is described by a first-order kinetic equation. This phenomenon could correspond to desorption of molecular, positively polarized adspecies denoted here cyE+[15]. The pumping speed of our system is however to low to calculate reliably the parameters of the kinetic equation. Examination of the slower part of the isothermal desorption [15] shown in fig. 5 revealed that, at the beginning, this process can be described by a second-order kinetic equation corresponding to desorption of weakly bound, positively polarized atomic adspecies /3+, and next by a diffusion equation similar to (1) except that (ASP - ASP,)/ASP, is replaced by nl(t)/nz, where n:, is the amount of hydrogen desorbed at very long time. It can be expected that for diffusion limited sorption and desorption processes the diffusion coefficients should be similar. Indeed, the diffusion coefficient calculated at isothermal desorption is 2 X lo-l3 cm2/s. It should be mentioned that deposition of thin palladium films at higher temperatures of the support, e.g. at 298 K influence the adsorption-absorption process carried out at 78 K lowering the hydrogen uptake via the j3’ precursor state. This suggests that surface defects are important for low temperature hydrogen interaction with palladium surface. Indeed, Gdowski, Felter and Stulen did not observed any rapid hydrogen consumption on the (111) single crystal palladium plane at 80 K [21]. The /3’ precursor state was not observed at all at 78 K on thin nickel films deposited at 298 K, while it was clearly seen on those deposited at 78 K [12]. Considering the very high value of the diffusion coefficient for hydrogen incorporation into the bulk it was interesting to examine the interaction of
R. Dti et al. / Low temperature palladium hydride formation
;
1
7
1o17
Inn;
II
70
60
time[s] Fig. 5. Examination
of the isothermal desorption of weakly bound hydrogen slower than the CI~ admolecules at 78 K.
adspecies
desorbing
deuterium with thin palladium films in order to find an isotope effect for this incorporation. This can help to understand the mechanism of this process. Surface potential changes caused by deuterium interaction with thin palladium films are shown in fig. 6. The picture is similar to that presented in fig. 2, and described below, however there are significant differences: (i) The rate of penetration of p’ deuterium adspecies from the surface into the bulk is lower than the rate of incorporation of /?’ hydrogen adspecies. The diffusion equation (1) is valid as indicated in fig. 7. The calculated diffusion coefficients for the successive doses are (0.7-1.5) X lo-l3 cm2/s as shown in fig. 4. This value is 2-3 times lower than for hydrogen. A similar isotope effect was observed by Mak et al. [22] for surface diffusion on a single palladium crystal. Thus the ratio of the diffusion coefficients for hydrogen and deuterium penetration from the surface into the bulk of thin palladium films is somewhat higher than expected for the classical
10
R. Du.4 et al. / Low temperature palladium hydride formation
+ 750-
, 2
0 +2300-
,
, 4
, 6
tim4-4 /evacuation
-o b i
+2200-
s
C$-
+2100-
/
+2000- 67, 0
69
d 7
68 2
x dd
,f--p
4
6
ir
8 10 time [min]
d 4 12
lk
Fig. 6. ASP caused by deuterium interaction with a thin palladium film at 78 K. Arrows indicate introduction of the successive hydrogen doses. The ASP caused by isothermal desorption is also
In(ASP-ASP,)
4 30
60
Fig. 7. Examination of the validity incorporation from a thin palladium
120
180
time [s]
of the diffusion equation for the j3’ deuterium adspecies film surface into the bulk at 78 K. Numbers indicate the successive deuterium doses.
R. DUS et al. / L.ow temperature palladium
hydride formation
11
weakly
ASP[mV]
+ zooo-
+ 1500-
+ IOOO-
+ 500.
0.4
0.6
0.8
10
H/Pd D/Pd
Fig. 8. SP isotherms for hydrogen and deuterium interaction with thin palladium films at 78 K. Weakly bound part of the deposit is marked.
mass effect mechanism (1.41), but much lower than expected for tunnelling in the bulk of palladium [20,23]. (ii) The deuterium equilibrium pressure is higher than that of hydrogen at the same uptake. Probably for this reason the molecular form a; was not observed for deuterium-palladium interaction. (iii) The increase of the surface potential in the process of palladium deuteride formation is significantly higher than that recorded for palladium hydride. This is presented in the SP isotherm in fig. 8. The reason for this phenomenon is not clear yet. In the process of palladium hydride (deuteride) formation d-type metal is transfered into sp-type metal due to filling of the unoccupied states in the d-band by electrons donated from the hydrogen [24]. The work function of palladium strongly changes due to a shift of the Fermi level. This change does depend on the H/Pd (D/Pd) ratio. Our surface potential measurements are sensitive to the very surface composition. Significantly more positive SP for deuterium than for hydrogen at the same average H,,,,/Pd and D,,,,/Pd ratio can indicate higher surface segregation for deuterium than for hydrogen. Examination of the isothermal desorption of the reversibly adsorbed deuterium adspecies (fig. 9) showed that, similarly to hydrogen desorption, part of the deposit desorbs according to a second-order kinetic equation, while the other part follows a diffusion equation. The calculated diffusion coefficient for deuterium is 1 X lo- l3 cm2/s. Thus the isotope effect for diffusion is the same
12
R. Dus’ et al. / Low temperature palladium hydride formation
38
37
36
35
3L
33
OOi Fig. 9. Examination
of the isothermal
120 180 time [sl
X0
desorption
of weakly bound
300 deuterium
adspecies
0
Fig. 10. SP isotherms for hydrogen and deuterium adsorption on a thin palladium film at 298 K. The coverage is calculated knowing the hydrogen (deuterium) uptake, geometrical area and the roughness factor of the film.
R. Dus’ et al. / Low temperature palladium hydride formation
13
when calculated on the basis of adsorption and desorption data, as should be expected for diffusion limited processes. A weak isotope effect was observed in the surface potential isotherms for hydrogen adso~tion on palladium at higher temperatures when hydride (deuteride) is not formed (fig. 10).
The authors have pleasure in acknowledging the master glasswork done by Mr. J. Biechonski and Mr. R. Bojarski which made it possible to carry out our studies. This work was performed within Research Project Cl.
References [l] G. Alefeld and J. ViieIkl, Eds., Hydrogen in Metals, Vol. 29 of Topics in Applied Physics (Springer, Berlin, 1978). [2] H. Conrad, G. Ertl and E.E. Latta, Surface Sci. 41 (1974) 435. [3] T. Engel and H. Kuipers, Surface Sci. 90 (1979) 162. [4] R.I. Behm, K. Christmann and G. Ertl, Surface Sci. 99 (1980) 320. [5] R.I. Behm, V. Penka, M.G. Cattania, K. Christmann and G. Ertl, J. Chem. Phys. 78 (1983) 7486. [6] F.A. Lewis, The Palladium-Hydrogen System (Academic Press, London. 1967). [7] T. Delchar, A. Eberhagen and F.C. Tompkins, J. Sci. Instr. 40 (1963) 105. [8] E. Nowicka and R. DuS, Surface Sci. 144 (1984) 665. [9] R. DuS and W. Lisowski, Surface Sci. 59 (1976) 141. [lo] A. Bachtin, Vacuum 12 (1985) 519. [ll] R. DuS, Surface Sci. 42 (1973) 324. [12] R. DuS, J. Chem. Sot. Faraday Trans. I, 70 (1974) 877. [13] R. DuS, Surface Sci. 52 (1975) 440. 1141 R. Dug and W. Lisowski, Surface Sci. 61 (1976) 635. [15] E. Nowicka and R. Dug, Surface Sci. 185 (1987) 587. [Xi] H. Friske and E. Wicke, Ber. Bunsenges. Phys. Chem. 77 (1973) 48. [17] T.B. GrimIey, in: Chemisorption, Ed. W.E. Garner (Butterworths, London, 1957). [IS] E. Wicke and H. Brodowsky, in: Hydrogen in Metals, Vol. 29 of Topics in Applied Physics, Ed. G. Alefeld and J. VijeBd (Springer, Berlin, 1978). [19] J.K. Jacobs and F.D. Manchester, J. Phys. F (Metal Phys.) 7 (1977) 23. [20] LSvare, Physica 141 B (1986) 271. [21] G.E. Gdowski, T.E. Felter and R.H. Stulen, Surface Sci. 181 (1987) L147. [22] C.H. Mak, I.L. Brand, B.G. KoehIer and S.M. George, Surface Sci. 188 (1987) 312. [23] I.G. Lauderdale and D.G. Truhlar, Surface Sci. 85 (1986) 1676. [24] A.C. Switendick, Ber. Bunsenges. Phys. Chem. 76 (1972) 535.
SURFACE PHENOMENA AND ISOTOPE EFFECTS AT LOW IMPELS PAL~D~ HYDRIDE FORMATION AND DURING ITS DECOM~SI~ON Ryszard DUS, Ewa NOWICKA
and Zbigniew WOLFRAM
Institute of Physical Chemistry Polish Academy of Sciences, ul. Kmprzaka 44/52, Warszawa. Poland
Received 4 August 1988; accepted for publication 31 January 1989
Adsorption states of the isotopes: hydrogen and deuterium, arising in the process of low temperature (78 K) palladium hydride (deuteride) formation in thin palladium films are distinguished by simultaneous surface potential and pressure measurements. The precursor state for hydrogen (deuterium) incorporation into the bulk is determined, and the diffusion coefficients are calculated to be (2-4)X1O-13 cm2/s for hydrogen and (0.7-1.5)x10-t3 cm2/s for deuterium. Surface potential isotherms are obtained for palladium hydride (deuteride) showing a significant isotope effect.
1. Introduction Surface phenomena caused by hydrogen interaction with palladium have been studied less intensively than bulk phenomena [l]. The reason is probably the high solubility of hydrogen in palladium and the necessity of simultaneous studies of both adsorption and absorption processes. Experiments carried out under UHV conditions, when hydrogen interacts with a clean surface of palladium are of considerable importance and several experimental works have been performed using well defined single crystal surfaces. Conrad, Ertl and Latta observed by means of the TDS method dissolution of hydrogen in the bulk of palladium as a result of exposure of the Pd(l10) surface to hydrogen at room temperature [2]. A similar observation was made by Engel and Kuipers for the Pd(ll1) surface using the molecular beam technique 131, and by Behm, C~stmann and Ertl for the Pd~l~) surface with the use of TD spectroscopy [4]. These latter authors found that the open Pd(ll0) surface behaves quite differently when interacting with hydrogen at low temperatures (250 and 130 K) [5]. At hydrogen coverages close to monolayer coverage 8 - 1 a reconstruction of the palladium surface occurs. As a result of further exposure, at hydrogen consumption corresponding to 8 = 1.5 two new states appear in the TD spectrum. Such states have never been ~39-6028/89/$03.50 0 Elsevier Science Pub~shers B.V. (North-Holland Physics Publishing Division)
2
R. LG et al. / Low temperature pa~iadium h~~~de~oTrnatio~
observed with adsorption carried out at room temperature. The authors interpreted the new TD states as corresponding to subsurface hydrogen on Pd(ll0). Low temperature transformation of an entire palladium single crystal of a size applicable for conventional UHV studies into palladium hydride would be a long process because of kinetic reasons. It is well established on the other hand, that palladium hydride forms easily by interaction of hydrogen with palladium powder (palladium black) [6]. Therefore it could be expected that an investigation of surface phenomena associated with the transformation of palladium into palladium hydride can be performed using thin palladium films deposited at low temperature under UHV conditions. In this paper we present studies of surface phenomena caused by low temperature (78 K) hydrogen interaction with thin palladium films. The investigations were carried out by means of simultaneous surface potential and pressure measurements.
2. Experimental The experiments were performed using an UHV glass system capable of reaching routinely (l-2) X lo-*’ Torr during deposition of thin palladium
films, and pe~tting work with hydrogen or deute~um by meas~ng their pressures from 1 X 10e6 Torr up to 1 Torr during the adsorption. Measurements of the surface potential were carried out by means of a static capacitor system [7,8]. The static capacitor consists of two coaxial cylindrical electrodes; the inner one being the reference electrode was coated with a conducting layer of (SnO + Sb,O,). This electrode is movable and can be moved up during palladium film deposition on the inner wall of the outer cylinder which was maint~ned at 78 K. This palladium film is the active electrode of the static capacitor. Thin p~la~um films were deposited by evaporation from a fine palladium wire (Johnson-Matthey grade I) wound around a tungsten heater. To avoid alloy formation the palladium wire was never melted during the deposition. The pressure was read by means of modulated ionization gauge. The geometrical area of the films was 135 X lo-* m2, their average thickness lo-’ m, and their average roughness factor estimated by means of hydrogen-oxygen titration [9] was - 16. The films were sintered at 320 K for 20 min. Two versions of the electronic circuit for the static capacitor were applied. The first one for observation of long processes [7] had an overall response time of 0.1 s, with a stability of 1 mV/h, a sensitivity of f 1 mV, and a noise level of - 1 mV. The second for investigation of rapid processes [lo] had an overall response time of 1 ms, with a sensitivity of 0.1 mV and a noise level of - 0.2 mV. By means of these circuits it was possible to carry out long measurements
R. Dti et al. / L.ow temperature palladium hydride formation
3
as well as to observe rapid desorption which occurs e.g. within 1 s for a change of the surface potential ASP of several mV. Spectroscopically pure hydrogen (deuterium), purified additionally by diffusion through a palladium thimble was used. Hydrogen (deuterium) was introduced in calibrated, successive doses while the static capacitor was held at a constant temperature, and was separated from the pumps by means of a Dekker greasless valve. The surface potential and the pressure were continuously recorded. To avoid atomization of hydrogen on the hot filament and pumping effect of the ionization gauge during adsorption, the hydrogen (deuterium) pressure (P) was measured using an ultrasensitive, short response time (1 decade/s) Pirani type gauge capable of working within the range 1 X 10e6-1 Torr. Recording simultaneously ASP and P, while knowning the volume of the static capacitor, enables one to construct the calibration curve ASP versus hydrogen uptake n,. This allows us to transform the measured time dependence of ASP into a function: n a = f( t), and consequently the known kinetic equations can be applied to examine the surface processes. Knowing the weight of the thin palladium film deposited and the hydrogen uptake (adsorbed and absorbed) the ratio of consumed hydrogen H,,,, to the total amount of palladium atoms in the film (- 1000 ,& thick) can be determined and correlated to ASP.
3. Results and discussion Surface potential changes (ASP) measured as the result of the introduction of successive doses of hydrogen into the static capacitor with thin palladium films maintained at 298 and 78 K are shown in figs. 1 and 2, respectively. The different character of the adsorbate formed at these two temperatures can be clearly seen. At 298 K (fig. 1) negatively polarized adspecies (dipoles with the negative pole outward the surface) are formed, decreasing monotonically the surface potential. This is a commonly observed phenomenon caused by hydrogen interaction with many of the transition metals. The surface potential changes caused by deuterium adsorption on a thin palladium film at 298 K are very similar to those presented in fig. 1. The ASP values for deuterium are lo-20 mV lower than for hydrogen at the same coverage. Surface potential changes caused by hydrogen interaction at low temperatures with palladium [ll], and other transition metals forming hydrides [ll-141 have been studied in our laboratory for several years. In fig. 2 we present the result of one of many experiments carried out to study surface potential changes caused by hydrogen interaction with a thin palladium film at 78 K. All characteristic states of the adspecies described below were observed
4
R. Dd et al. / Low temperature palladium hydride formation
Fig. 1. Surface potential changes ASP caused by hydrogen adsorption 298 K. Arrows indicate introduction of the successive hydrogen constant
pressure
value is expressed
on a thin palladium films at doses. The corresponding
in Torr.
in all these experiments with very good reproducibility.Several hydrogen doses introduced into the static capacitor caused a decrease of the surface potential corresponding with the formation of negatively polarized adspecies (fig. 2A). This uptake corresponds to a H,,,/Pd ratio of - 1 X IO-‘. Thermal desorption shows that this form of deposited hydrogen corresponds to dissociative adsorption usually denoted as P-state. We denote this form the /3- state to emphasize its negative polarization. More than eighty further doses of hydrogen introduced into the static capacitor caused the appearance of positive transient of SP (fig. 2B). The value of the surface potential obtained as the result of the successive SP transients becomes increasingly more positive. In spite of the large amount of hydrogen consumed, the gas phase pressure corresponding to the constant value of SP after decay of the positive increment of SP is lower than 1 X 10e6 Torr. We denote this state of the adsorbate as /?’ state to emphasize its positive polarization. The p’ state is formed until the H,,,,/Pd ratio reaches 0.6-0.7; the phase diagram for the hydrogen-palladium system [16] shows that palladium hydride is formed.
R. Dti et al. / Low temperature palladium hydride formation
5
ASP[mV]
24
0
+380
48
time[S]
‘D A
B
+280+180-
+ 80.
‘32 j r,
,
0
,
2
/ 35
34’
4
6
8
10
time[min]
Fig. 2. ASP caused by hydrogen interaction with a thin palladium film at 78 K. Arrows indicate introduction of the successive hydrogen doses. The ASP caused by the isothermal desorption in the course of the evacuation is also shown.
We suppose [11,12] that the p’ adspecies create a precursor state for hydrogen incorporation into the bulk of transition metals with the formation of a hydride. Formation of positively charged hydrogen adspecies on the surface of a transition metal precovered with an amount of hydrogen can be expected on the basis of Grimley’s model [17]. The increase of the hydrogen population on the metal surface increases also the magnitude of the splitting between even and odd induced localized states. There is a critical value of the population above which the lower state merges into the metal conduction band. Then donation of electrons from hydrogen adatoms into unoccupied states of the conduction band occurs. The adspecies arising in this way incorporate easily into the bulk of the metal forming hydrides. This is observed as SP transients. If our supposition is valid, then the decay of the positive increments of SP caused by every hydrogen dose at this step of the process should be described by a diffusion equation. Assuming that within the
R. lb.8 et al. / Low te~eruture
~a~la~~
hydride fo~ation
020 032 -35 040 x 17 559
16--l
60 120 time [s] Fig. 3. Exa~nation of the validity of the diffusion equation for the fi’ hydrogen adspecies incorporation from the surface into the bulk of a thin palladium film at 78 K. Numbers indicate the successive hydrogen doses.
uptake of one hydrogen dose a linear relation between SP and the concentration of the fi’ adspecies on the surface exists we have for a chosen SP transient: ASP-
ASP,
8
where 7 = h2/v2 D, ASP, is the total change of the surface potential due to the formation of the p” adspecies at the moment when a successive hydrogen dose is introduced t = 0, ASP is the measured surface potential change after time r, ASP, corresponds to the stationary value of surface potential reached after sufficiency long time, h is the thin film thickness, and D is the diffusion coefficient. The validity of eq. (1) was observed for all transients. Examples of the validity of eq. (1) are presented in fig. 3. The calculated diffusion coefficients ratio are (2-4) X lo-l3 cm2/s for all successive doses within a H,,,/Pd interval of 0.1-0.6. This is shown in fig. 4. For a H,,/Pd ratio of 0.6-0.7 the diffusion coefficients seem to be higher. The calculated value is five orders of
R. Dti et al. / Low temperature palladium hydride formation
I
D[yl
x
0
hydrogen
.
0
deuterium
1 d5
0
ci2
OL
0.6
0.8
H/Pd . D/I’d
Fig. 4. Diffusion coefficient for hydrogen and deuterium incorporation from the surface into the bulk of a thin palladium film at 78 K in the process of palladium hydride (deuteride) formation. (x) and (0) correspond to calculations based on examination of the SP transients. (0) and (0) correspond to calculations based on measurements of the rate of pressure decrease.
magnitude higher than expected by extrapolating to our conditions the data obtained for hydrogen diffusion in the bulk of palladium for temperatures in the region 230-470 K [18]. Assuming that as usually the pre-exponential factor for interstitial diffusion is of the order of lop3 cm*/s the activation energy for j3’ hydrogen adspecies incorporation into the bulk can be estimated to be 14.7 kJ/mol. This value does not differ very much from those determined experimentally (11 kJ/mol) [19] and calculated theoretically [20] for hydrogen diffusion in palladium hydride within the temperature interval 50-140 K. The low value of the activation energy for diffusion has been interpreted as the result of a tunnelling mechanism for hydrogen diffusion in palladium hydride [20]. Further doses of hydrogen introduced into the static capacitor caused monotonic increase of the surface potential without any transients (fig. 2C). The gas phase pressure now increased with every successive hydrogen dose. However the pressure value associated with every successive dose was not constant like the surface potential, but was slowly decreasing indicating further penetration of hydrogen into the film at constant concentration of hydrogen adspecies on the surface. Measurement of this pressure decrease
8
R DUS et al. / Low temperature palladium hydride formation
gives another, independent possibility of calculating the diffusion coefficients. Using a similar equation as (1) we calculated diffusion coefficients of (7-9) x lo-i3 cm2/s. At pressures of the order of lop2 Tort-, the H,,,,/Pd ratio approaches 0.9. Part of the deposit corresponding to H ,,,/Pd > 0.85 is weakly bound and desorbs in the course of isothermal desorption caused by evacuation at 78 K. This is observed by recording the SP changes as the result of evacuation (see fig. 2). Hydrogen redosing leads to readsorption giving the same value of ASP as before evacuation. Such a desorption-readsorption process can be repeated many times with good reproducibility. One can notice that the desorption is not a simple process. The rapid initial desorption rate is followed by a slower one. At H,ons/Pd > 0.7 the gas phase pressure is higher than 1 x lop6 Torr, so it is detectable by our ultrasensitive Pirani gauge. Hence we can determine volumetrically the hydrogen uptake n z reversibly adsorbed, and construct for this process the calibration curve ASP versus nz. This allows us to transform the relation ASP versus t recorded during isothermal desorption into a function nz = f( t), useful for examining the isothermal desorption kinetics. We found that the initial, rapid decrease of SP at the beginning of desorption is described by a first-order kinetic equation. This phenomenon could correspond to desorption of molecular, positively polarized adspecies denoted here cyE+[15]. The pumping speed of our system is however to low to calculate reliably the parameters of the kinetic equation. Examination of the slower part of the isothermal desorption [15] shown in fig. 5 revealed that, at the beginning, this process can be described by a second-order kinetic equation corresponding to desorption of weakly bound, positively polarized atomic adspecies /3+, and next by a diffusion equation similar to (1) except that (ASP - ASP,)/ASP, is replaced by nl(t)/nz, where n:, is the amount of hydrogen desorbed at very long time. It can be expected that for diffusion limited sorption and desorption processes the diffusion coefficients should be similar. Indeed, the diffusion coefficient calculated at isothermal desorption is 2 X lo-l3 cm2/s. It should be mentioned that deposition of thin palladium films at higher temperatures of the support, e.g. at 298 K influence the adsorption-absorption process carried out at 78 K lowering the hydrogen uptake via the j3’ precursor state. This suggests that surface defects are important for low temperature hydrogen interaction with palladium surface. Indeed, Gdowski, Felter and Stulen did not observed any rapid hydrogen consumption on the (111) single crystal palladium plane at 80 K [21]. The /3’ precursor state was not observed at all at 78 K on thin nickel films deposited at 298 K, while it was clearly seen on those deposited at 78 K [12]. Considering the very high value of the diffusion coefficient for hydrogen incorporation into the bulk it was interesting to examine the interaction of
R. Dti et al. / Low temperature palladium hydride formation
;
1
7
1o17
Inn;
II
70
60
time[s] Fig. 5. Examination
of the isothermal desorption of weakly bound hydrogen slower than the CI~ admolecules at 78 K.
adspecies
desorbing
deuterium with thin palladium films in order to find an isotope effect for this incorporation. This can help to understand the mechanism of this process. Surface potential changes caused by deuterium interaction with thin palladium films are shown in fig. 6. The picture is similar to that presented in fig. 2, and described below, however there are significant differences: (i) The rate of penetration of p’ deuterium adspecies from the surface into the bulk is lower than the rate of incorporation of /?’ hydrogen adspecies. The diffusion equation (1) is valid as indicated in fig. 7. The calculated diffusion coefficients for the successive doses are (0.7-1.5) X lo-l3 cm2/s as shown in fig. 4. This value is 2-3 times lower than for hydrogen. A similar isotope effect was observed by Mak et al. [22] for surface diffusion on a single palladium crystal. Thus the ratio of the diffusion coefficients for hydrogen and deuterium penetration from the surface into the bulk of thin palladium films is somewhat higher than expected for the classical
10
R. Du.4 et al. / Low temperature palladium hydride formation
+ 750-
, 2
0 +2300-
,
, 4
, 6
tim4-4 /evacuation
-o b i
+2200-
s
C$-
+2100-
/
+2000- 67, 0
69
d 7
68 2
x dd
,f--p
4
6
ir
8 10 time [min]
d 4 12
lk
Fig. 6. ASP caused by deuterium interaction with a thin palladium film at 78 K. Arrows indicate introduction of the successive hydrogen doses. The ASP caused by isothermal desorption is also
In(ASP-ASP,)
4 30
60
Fig. 7. Examination of the validity incorporation from a thin palladium
120
180
time [s]
of the diffusion equation for the j3’ deuterium adspecies film surface into the bulk at 78 K. Numbers indicate the successive deuterium doses.
R. DUS et al. / L.ow temperature palladium
hydride formation
11
weakly
ASP[mV]
+ zooo-
+ 1500-
+ IOOO-
+ 500.
0.4
0.6
0.8
10
H/Pd D/Pd
Fig. 8. SP isotherms for hydrogen and deuterium interaction with thin palladium films at 78 K. Weakly bound part of the deposit is marked.
mass effect mechanism (1.41), but much lower than expected for tunnelling in the bulk of palladium [20,23]. (ii) The deuterium equilibrium pressure is higher than that of hydrogen at the same uptake. Probably for this reason the molecular form a; was not observed for deuterium-palladium interaction. (iii) The increase of the surface potential in the process of palladium deuteride formation is significantly higher than that recorded for palladium hydride. This is presented in the SP isotherm in fig. 8. The reason for this phenomenon is not clear yet. In the process of palladium hydride (deuteride) formation d-type metal is transfered into sp-type metal due to filling of the unoccupied states in the d-band by electrons donated from the hydrogen [24]. The work function of palladium strongly changes due to a shift of the Fermi level. This change does depend on the H/Pd (D/Pd) ratio. Our surface potential measurements are sensitive to the very surface composition. Significantly more positive SP for deuterium than for hydrogen at the same average H,,,,/Pd and D,,,,/Pd ratio can indicate higher surface segregation for deuterium than for hydrogen. Examination of the isothermal desorption of the reversibly adsorbed deuterium adspecies (fig. 9) showed that, similarly to hydrogen desorption, part of the deposit desorbs according to a second-order kinetic equation, while the other part follows a diffusion equation. The calculated diffusion coefficient for deuterium is 1 X lo- l3 cm2/s. Thus the isotope effect for diffusion is the same
12
R. Dus’ et al. / Low temperature palladium hydride formation
38
37
36
35
3L
33
OOi Fig. 9. Examination
of the isothermal
120 180 time [sl
X0
desorption
of weakly bound
300 deuterium
adspecies
0
Fig. 10. SP isotherms for hydrogen and deuterium adsorption on a thin palladium film at 298 K. The coverage is calculated knowing the hydrogen (deuterium) uptake, geometrical area and the roughness factor of the film.
R. Dus’ et al. / Low temperature palladium hydride formation
13
when calculated on the basis of adsorption and desorption data, as should be expected for diffusion limited processes. A weak isotope effect was observed in the surface potential isotherms for hydrogen adso~tion on palladium at higher temperatures when hydride (deuteride) is not formed (fig. 10).
The authors have pleasure in acknowledging the master glasswork done by Mr. J. Biechonski and Mr. R. Bojarski which made it possible to carry out our studies. This work was performed within Research Project Cl.
References [l] G. Alefeld and J. ViieIkl, Eds., Hydrogen in Metals, Vol. 29 of Topics in Applied Physics (Springer, Berlin, 1978). [2] H. Conrad, G. Ertl and E.E. Latta, Surface Sci. 41 (1974) 435. [3] T. Engel and H. Kuipers, Surface Sci. 90 (1979) 162. [4] R.I. Behm, K. Christmann and G. Ertl, Surface Sci. 99 (1980) 320. [5] R.I. Behm, V. Penka, M.G. Cattania, K. Christmann and G. Ertl, J. Chem. Phys. 78 (1983) 7486. [6] F.A. Lewis, The Palladium-Hydrogen System (Academic Press, London. 1967). [7] T. Delchar, A. Eberhagen and F.C. Tompkins, J. Sci. Instr. 40 (1963) 105. [8] E. Nowicka and R. DuS, Surface Sci. 144 (1984) 665. [9] R. DuS and W. Lisowski, Surface Sci. 59 (1976) 141. [lo] A. Bachtin, Vacuum 12 (1985) 519. [ll] R. DuS, Surface Sci. 42 (1973) 324. [12] R. DuS, J. Chem. Sot. Faraday Trans. I, 70 (1974) 877. [13] R. DuS, Surface Sci. 52 (1975) 440. 1141 R. Dug and W. Lisowski, Surface Sci. 61 (1976) 635. [15] E. Nowicka and R. Dug, Surface Sci. 185 (1987) 587. [Xi] H. Friske and E. Wicke, Ber. Bunsenges. Phys. Chem. 77 (1973) 48. [17] T.B. GrimIey, in: Chemisorption, Ed. W.E. Garner (Butterworths, London, 1957). [IS] E. Wicke and H. Brodowsky, in: Hydrogen in Metals, Vol. 29 of Topics in Applied Physics, Ed. G. Alefeld and J. VijeBd (Springer, Berlin, 1978). [19] J.K. Jacobs and F.D. Manchester, J. Phys. F (Metal Phys.) 7 (1977) 23. [20] LSvare, Physica 141 B (1986) 271. [21] G.E. Gdowski, T.E. Felter and R.H. Stulen, Surface Sci. 181 (1987) L147. [22] C.H. Mak, I.L. Brand, B.G. KoehIer and S.M. George, Surface Sci. 188 (1987) 312. [23] I.G. Lauderdale and D.G. Truhlar, Surface Sci. 85 (1986) 1676. [24] A.C. Switendick, Ber. Bunsenges. Phys. Chem. 76 (1972) 535.