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Hydrogen chemisorption on Pd(100) studied with he scattering
139
Surface Science 148 (1984) 1399147 North-Holland, Amsterdam
HYDROGEN CHEMISORPI’ION SCATTERING K.H. RIEDER
ON Pd(100) STUDIED WITH He
and W. STOCKER
IBM Zurich Research Lahoratoty,
CH -8803 Riixchlrkon, Swrtrerland
Received
for publication
14 April 1984; accepted
4 June 1984
He scattering from the clean Pd(100) surface yields extremely weak diffraction beams relative to the specular, corresponding to a very small maximum corrugation amplitude of - 0.04 A. Hydrogen adsorption at a temperature of 110 K leads to the formation of a c(2 X 2) ordered phase at a coverage of 0.5 monolayers and a (1 Xl) phase at saturation coverage. The maximum corrugation amplitude of the c(2 X 2)H is - 0.13 A; surface charge density calculations using overlapping atomic charge densities indicate a normal distance of the hydrogens to the topmost Pd layer d, = 0.65-0.70 A corresponding to a H-Pd bonding distance of - 2.05 A in the fourfold hollow sites. The result that the maximum corrugation amplitude of the (1 x 1) hydrogen phase, with - 0.025 A, even smaller than that of the clean surface may indicate a movement of the hydrogens closer to the topmost metal layer, when the coverage is increased from 0.5 monolayers to saturation.
1. Introduction The H/Pd system has been the object of many investigations because of: (a) the ability of Pd to accommodate large amounts of hydrogen in its bulk [l-3], and (b) the catalytic activity of Pd [4]. The important role of the surface in both areas was emphasized in several recent studies of chemisorption on well-defined clean Pd surfaces [5-111. On Pd(llO), a hydrogen induced substrate reconstruction has recently been shown to provide effective channels for hydrogen to move from chemisorption sites into the crystal interior [g-lo]. For Pd(l1 l), a kinetic molecular beam study by Engel and Kuipers [ll] showed how surface and bulk processes can be disentangled. Interesting studies have also already been reported on the H/Pd(lOO) system [7]. Especially the order-disorder phase transformation observed for the c(2 X 2) hydrogen overlayer [7] attracted a number of theoretical efforts [12,13]. In this paper, we present He diffraction studies of the H/Pd(lOO) system, which were performed with the aim of gaining information on H/Pd bonding distances, as well as of inquiring whether the unique sensitivity of He diffraction for hydrogen adsorbates would allow a detailed study of the order-disorder transition of the c(2 x 2)H phase in the spirit of ref. [14]. 0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
2. Experimental
and diffraction data analyses
The Pd(lO0) specimen disk of 6 mm diameter and 1 mm thickness was prepared from a large Pd single crystal. The crystal was aligned to +OS”. cut with a wire saw and mechanically polished with successively finer grades of diamond paste. After several He bombardment and annealing cycles. LEED showed a pattern characteristic of the two domains of the c(2 x 4) superstructure Formed by carbon [15]. In order to remove the carbon, apart From Further sputtering and annealing treatments, the recipe given by Behm et al. [5] was applied: many oxidation-reduction cycles at 800 K with 2.10 ’ Tort- O2 and 5.10-’ Tot-r H, were performed until the sample reproduced the adsorption behavior of hydrogen as reported in ref. [7]. The He diffraction experiments were performed with beam energies between 20 and 63 meV. For the evaluation of diffraction data, spectra corresponding to angles of incidence not larger than 40” were chosen in order to avoid strong influences on the calculated hard-wall corrugtaion amplitudes owing to softness effects [16]. The intensity analyses were performed with the eikonal approximation [17], which is very reliable for the extremely small corrugations of the clean Pd(lOO) and the two ordered hydrogen phases.
3. Results and discussion 3. I. Cleun Pd( 100)
He diffraction From the clean Pd(lO0) surface shows. beside the dominant specular, only extremely small in-plane and out-of-plane diffraction peaks of first order. The total diffraction intensity is about 30% of the incoming beam intensity. For E, = 63 meV and small 8,, their intensities are of the order of 0.9% relative to the specular. which corresponds to a corrugation parameter I&10) = 0.019-0.020 A, where the corrugation Function is assumed to be D(X,)j)
= ~D(lo)~cos(2~/~).~
+ cos(2n/a).‘]
(1)
Form of a (u = d/a = 2.74 A), which is the simplest possible analytical corrugation with full quadratic symmetry. The maximum corrugation amplitude (occurring along [ll] in the direction to next-nearest neighbors) equals 21>(10) and amounts to - 0.04 A for the clean Pd(lOO) surface. The value of the corrugation parameter D(10) is similar to the corresponding value for nearest neighbors in the Pd(ll0) surface where it was found to be - 0.02 A [18] in the same range of He energies. The maximum corrugation amplitude on Pd(lOO) is appreciably larger than for Ni(lOO), (0.015 A), which may simply be due to the smaller lattice constant of Ni (d = 3.52 A), so that the smearing-out effect of the Free electrons is more pronounced in the case of Ni. On the other
K. H. Rteder. W. Stocker / Hydrogen chemtsorption
on Pd(100)
141
hand, the corrugation amplitude is also larger than for Ag( 111) (0.022 A) [ 191, although silver has a larger lattice constant (d = 4.08 A); however, the (111) surface is more densely packed than the (100). The corrugation amplitude should increase with increasing He energy as the He atoms penetrate deeper and deeper into the sea of free electrons [16,20]. Because of the small diffraction intensities, we refrained from a detailed investigation of this effect in the case of Pd(lOO). Fitting the observed decrease of the specular intensity as a function of temperature for the clean Pd(lOO) surface, using a He energy of 63 meV and an angle of incidence of 25.5” with the traditional Debye-Waller formula [21] yields a Debye temperature of - 315 K for Pd(lOO) which is near to the bulk Debye temperature of 276 K [22] and has to be regarded as an effective surface Debye temperature characteristic for He scattering rather than the true surface Debye temperature which should be smaller than the bulk value. Effective Debye temperatures for He scattering close to the bulk value have also been observed for Ni(lOO) [23] and Pt(ll1) [24]. 3.2. The initial stage of hydrogen adsorption Fig. 1 shows the intensity behavior of the specular beam and a beam characteristic for the ordered c(2 x 2)H phase as a function of hydrogen dose for an adsorption temperature of 100 K. Immediately after opening the
HP on I
Pd (400) kHe = 0.63 r! 6i = 25.5’ T,= IfOK
‘*.J;
I 0.5
I
I
*.
(ig,
= (TO) or (07)
.I_
I
1.0
--__
I 4.5
H - EXPOSURE
-
_^_ 2.0
I 2.5
(L)
Fig. 1. Intensities of the specularly reflected and the (- l/2, tic of the c(2 X2)H phase as a function of H, exposure.
l/2)
He diffraction
beam characteris-
hydrogen valve, the specular intensity drops rapidly. This drastic effect is due to the giant cross-section of statistically distributed adatoms present in dilute concentrations on the surface caused by the different in- and out-refractions of the He particles at the attractive part of the potential between the He and the individual adsorbate atoms as recently demonstrated by Poelsema et al. [25]. Using the slope of the initial decrease and relating it to the number of particles present at the surface (according to ref. [7], the sticking coefficient of hydrogen on Pd(100) is not strongly dependent on the hydrogen coverage at small coverages), allows determination of the effective scattering cross-section 1251 of the individual H atoms, and yields 22 A’ in reasonable agreement with our previous result of 32 A* for H on Ni(ll0) [26]. At higher H, doses, the specular goes through a minimum, reaches a small maximum, again goes through a minimum and then rises until the saturation value is reached at very large H, doses. The (- l/2, f l/2) beam (denoted relative to the clean surface unit cell) increases steadily until it reaches a maximum, and decreases continuously as the coverage approaches saturation. The formation of the optimum c(2 X 2) phase was assumed to coincide with the maximum intensity of the fractional order beam; the fact that the maximum intensity of this beam does not coincide with either one of the minima of the specular intensity or the maximum in between, represents an unsolved puzzle at the moment. The small differences of relative intensities in the different regions near the (-l/2, l/2) maximum do not, however, influence the corrugation analyses appreciably. 3.3. The cf.? X 2) hydrogen phase and the order-disorder
phase transition
A typical diffraction spectrum for in-plane and out-of-plane scattering for the ~(2 X 2) structure of hydrogen on Pd(100) is shown in fig. 2. The most intense beams are the (+1/2, 3: l/2) beams with about 11-12s of the specular intensity. All intensities observed can be fitted within a few percent by: (i) using the Fourier ansatz D(x,y)
= ;D(lO)
[cos(277/a)x
+ cos(2v/a)y]
+D(ll)
cos(27r/a)x
cos(2n/a)y
+ fD(20){cos(277/a)x
+ cos(2a/a)y)
(2)
(a = 3.88 A), where the best-fit Fourier coefficients are D(10) = 0.065 A, D(ll) = 0.020 A and D(20) = 0.015 A, and (ii) by modeling the adsorbates as Gaussian hills on a flat substrate, hwere the best-fit value for the amplitude is 0.14 A and that of the FWHM 2.1 A. In order to obtain a rough quantitative value for the position of the hydrogen atoms relative to the substrate, we have performed surface charge density calculations by using the simple method of overlapping atomic charge
K. H. Rieder, W. Stocker / Hydrogen chemisorption
on Pd(100)
143
densities [27,28]. We first checked the procedure for the case of the (2 X 1) phase of hydrogen on Ni(ll0) for which Hamann [29] has performed self-consistent calculations and has given a binding position for the adatoms. Comparison of the density contours with the corrugtaions, requires knowledge of the proportionality constant relating the incoming He energies with the surface charge density [30]. The most recent values correspond roughly to densities of 5 x 10e4 a.u. [31]. As compared to the experimental values of the clean surface, the corrugation amplitudes in the critical density range come out too large in Hamann’s and even larger in our own calculations, which shows that the smearing-out influence of the free electrons is not accounted for correctly. However, the density contours for the Ni(110)(2 X l)H phase obtained by Hamann and those of our own calculations are in good agreement, so that some confidence can be had in the simple method of overlapping atomic densities. For the clean Pd(lOO), the charge density overlap calculations yielded
II! I
T,= llOK
IllI
0 = 3.BBA a=45O
” 20
) (54)
0 =+-120
cti)
CO?,
I
I
I
” 20 I
I
I
4
(20) (02)
III1
Fig. 2. Typical He diffraction spectrum for the ~(2x2) structure of hydrogen on Pd(lOO). The beam indices refer to the unit cell of the superstructure rather than to that of the substrate as in figs. 1 and 3.
144
K.
H. Rleder,
W. Stocker
/ Hydrogen
chemrsorption
on Pd(100)
a corrugation amplitude (0.17 A) more than four times larger than the one observed (0.04 A). Therefore, we neglected the corrugation of the substrate and took the mean substrate densities at the different positions normal to the surface for the calculations of the adsorbate phases. With this procedure, we found that, at a density of 5 x 10ee4 a.u., the hydrogen adatoms produce a corrugation of - 0.1 A at a normal distance from the topmost Pd layer d, = 0.7 A. Assuming the H atoms to occupy the fourfold coordinated hollows as proposed by Behm et al. [7], this corresponds to a bond length of - 2.05 A. This value is in excellent agreement with the 2.04 A recently estimated by Nordlander et al. [32] on the basis of the measured vibrational adsorbate frequency [33]. Our result is in disagreement with the predictions of Muscat [34] which favor subsurface sites for hydrogen on Pd(lOO). Fig. 3 shows the intensity behavior of the (-l/2, l/2) beam of the c(2 X 2)H phase as a function of temperature. This measurement was performed, in order to investigate the possibilities of studying the well-known order-disorder transition of this phase [7] in more detail by using He diffraction. As can be seen from fig. 3, the phase transition causes only a slight change in the intensity decrease in the critical regime; nevertheless the value of the transition temperature determined by Behm et al. [7] is in reasonable
(a) Pd (WI) + c(2-2)H (b) NI (111)
I 130
I 160
I 190
I 220
I 250
260
TEMPERATURE Fig. 3. (a) Temperature
behavior
~(2 X 2)H phase on Pd(100).
+ (2.2)H
310
340
370
[OKI of the (-
l/Z,
(b) Temperature
characteristic
for the (2 X 2)H phase on Ni(l11).
temperatures
as determined
by LEED
[7,34].
l/2)
behavior
He diffraction
beam characteristic
of the (-l/2.
l/2)
He diffraction
The arrows denote the order-disorder
of the beam
transition
K.H. Rieder, W. Stocker / Hydrogen chemrsorprion on Pd(100)
145
accordance with the steepest part of the decrease as measured with He diffraction. For comparison, we have also plotted the temperature behavior of the (-l/2, l/2) beam for the analogous situation of the (2 X 2) structure of hydrogen on Ni(ll1) [35]. In this case, the intensity drop in the critical temperature region is much more pronounced, so that this system appears to be a more favorable candidate for beam-shape measurements as a function of temperature [14]. The reason why Ni(ll1) presents a more favorable system probably lies simply in the fact that the effective Debye temperature is appreciably higher for Ni than for Pd, so that the decrease owing to phononic attenuation does not shade that of the order-disorder transition as strongly as in the case of Pd. This can be seen from fig. 3: The intensity decrease for the Pd/H system at the lowest temperatures sufficiently far away from the transition temperature is much steeper than for the Ni/H system. 3.4. The (I X I) hydrogen phase The final (1 X 1) diffraction pattern corresponding to saturation of the surface with hydrogen shows first-order diffraction peaks which are even smaller relative to the specular than in the case of the clean Pd(lOO). Using eq. (1) we arrive at a value for the corrugation parameter D(10) of - 0.012-0.013 A corresponding to a maximum corrugation amplitude of - 0.025 A, which is about two thirds that of the clean surface. Assuming an ideal (1 X 1) structure corresponding to a coverage of 1 ML and taking the Gaussians obtained by the best-fit of the c(2 X 2)H diffraction data to model the (1 X l)H corrugation, we would expect a maximum corrugation of 0.08 A, which is appreciably higher than the value observed experimentally. Performing again density calculations in the same manner as described above and looking at the normal distance of the hydrogens just necessary for the observed corrugation of 0.025, we arrive at a value of d, = 0.35 A, which would correspond to a shortening of the bond distance in the fourfold hollow from 2.05 A in the c(2 X 2) phase to 2.00 A in the (1 X 1). This is certainly only one of several possible explanations, but it does not appear unrealistic, especially as Behm et al. [7] have shown that the saturation coverage does not correspond to 1 ML exactly, but exceeds this value by - 30%. As the He diffraction pattern exhibited sharp diffraction beams with no disorder-induced background, we believe that the extra hydrogen goes subsurface. The subsurface hydrogen may affect the bond between the first Pd layer and the chemisorbed hydrogen sufficiently strongly to produce the slight change observed in bond length. 4. Conclusion He diffraction results were used to determine the corrugation shapes and amplitudes for the clean Pd(lOO) surface as well as for the c(2 X 2)- and
146
KM. Rieder, I+/.Stocker / H_vdrogenchemworptron on Pd(100)
(I X I)-ordered phases of hydrogen. Simple surface charge density contour calculations allowed determination of adsorbate-metal bond lengths, and the results are in good agreement with recent theoretical predictions [32]. Transition from the c(2 X 2) hydrogen phase to the saturation phase appears to be accompanied by a slight shortening of the hydrogen-metal bond length. The order-disorder phase transition occurring at elevated temperature for the c(2 x 2)H phase could be observed by using He diffraction, although it is heavily screened by strong Debye--Walter attenuation.
Acknowledgment The authors wish to express their thanks to M. Baumberger the surface density calculations.
for his help with
References [l] E. Wicke and H. Brodowski, in: Hydrogen in Metals, Vol. 2. Eds. G. Alefeld and J. Voelkl (Springer. Berlin, 1978) p. 73. [2] F.A. Lewis, The Palladjum-Hydrogen System {Academic, London, 1967). ]3] G. Comsa, R. David and B.J. Schuhmacher, Surface Sci. 9.5 (1980) L210. [4] G. Ertl, in: Catalysis, Science and Technology, Vol. 4. Eds. J.R. Andersen and M. Boudart (Springer, Berlin, 1983). [S] R.J. Behm, K. Christmann, G. Ertl and M.A. Van Hove, J. Chem. Phys. 73 (1980) 2984. [6] R.J. Behm, K. Christmann. G. Ertl. M.A. Van Hove, P.A. Thiel and W.H. Weinberg. Surface Sci. 88 (1979) L59. [7] R.J. Behm, K. (~hristmann and G. Ertl, Surface Sci. 99 (1980) 320. [8] K.H. Rieder, M. Baumberger and W, Stocker, Phys. Rev. Letters 51 (1983) 1799. [9] M.G. Cattania. V. Penka, R.J. Behm, K. Christmann and G. Ertl. Surface Sci. 126 (1983) 382. ]lO] R.J. Behm. V. Penka, M.G. Cattania, K. Christmann and G. Ertl, J. Chem. Phys. 78 (19X3) 7486. [ll] T. Engel and H. Kuipers. Surface Sci. 90 (1979) 162. [12] K. Binder and D.P. Landau, Surface Sci. 61 (1976) 577. 1131 K. Binder and D.P. Landau, Surface Sci. 108 (1981) 503. [14] L.D. Roelofs, A.R. Kortan, T.L. Einstein and R.L. Park, Phys. Rev. Letters 46 (1981) 1465. [15] A.M. Bradshaw and F. Hoffmann, Surface Sci. 72 (1978) 513. [16] K.H. Rieder. Surface Sci. 117 (1982) 13: K.H. Rieder and N. Garcia, Phys. Rev. Letters 49 (1982) 43. [17] U. Garibaldi, A.C. Levi, R. Spadacini and G.E. Tommei, Surface Sci. 4X (1975) 649. 1181 K.H. Rieder and W. Stocker. J. Phys. Cl6 (1983) L783. 1191 G. Boato, P. Cantini and R. Tatarek, J. Phys. F6 (1976) L237; J.M. Horne and D.R. Miller, Surface Sci. 66 (1977) 365. [20] J. Harris and A. Liebsch, Phys. Rev. Letters 49 (1982) 341. 1211 H. Hoinkes, H. Nahr and H. Wilsch. Surface Sci. 40 (1973) 457. [22] A. Konti and Y.P. Varshni, Can. J. Phys. 47 (1969) 2021. [23] K.H. Rieder and H. Wilsch, Surface Sci. 131 (1983) 245; 136 (1984) 666. 1241 B. Poelsema, L.K. Verheij and G. Comsa. Phys. Rev. Letters 49 (1982) 578.
K. H. Rieder, W. Stocker / Hydrogen chemisorption on Pd(100) (25) [26] [27] [28] 1291 [30] [31] (321 1331
147
B. Poelsema. S.T. de Zwart and G. Comsa, Phys. Rev. Letters 49 (1982) 1731; 51 (1983) 522. H. Wilsch and K.H. Rieder, J. Chem. Phys. 78 (1983) 7491. M. Manninen. J.K. Nmskov and C. Umrigar, Surface Sci. 119 (1982) L393. D. Haneman and R. Haydock, J. Vacuum Sci. Technol. 21 (1982) 330. D. Hamann, Phys. Rev. Letters 46 (1981) 1227. N. Esbjerg and J.K. Nsrskov, Phys. Rev. Letters 45 (1980) 807. M. Manninen, J.K. Nurskov and C. Umrigar. J. Phys. F12 (1982) L7. P. Nordlander, S. Holloway and J.K. Nsrskov, Surface Sci. 136 (1983) 59. C. Nyberg and C.G. Tengstal. Solid State Commun. 44 (1981) 251; Surface Sci. 126 (1983) 163. [34] J.P. Muscat. Surface Sci. 110 (1981) 389. 1351 K. Christmann, R.J. Behm, G. Ertl, M.A. Van Hove and W.H. Weinberg, J. Chem. Phys. 70 (1979) 4168.
Surface Science 148 (1984) 1399147 North-Holland, Amsterdam
HYDROGEN CHEMISORPI’ION SCATTERING K.H. RIEDER
ON Pd(100) STUDIED WITH He
and W. STOCKER
IBM Zurich Research Lahoratoty,
CH -8803 Riixchlrkon, Swrtrerland
Received
for publication
14 April 1984; accepted
4 June 1984
He scattering from the clean Pd(100) surface yields extremely weak diffraction beams relative to the specular, corresponding to a very small maximum corrugation amplitude of - 0.04 A. Hydrogen adsorption at a temperature of 110 K leads to the formation of a c(2 X 2) ordered phase at a coverage of 0.5 monolayers and a (1 Xl) phase at saturation coverage. The maximum corrugation amplitude of the c(2 X 2)H is - 0.13 A; surface charge density calculations using overlapping atomic charge densities indicate a normal distance of the hydrogens to the topmost Pd layer d, = 0.65-0.70 A corresponding to a H-Pd bonding distance of - 2.05 A in the fourfold hollow sites. The result that the maximum corrugation amplitude of the (1 x 1) hydrogen phase, with - 0.025 A, even smaller than that of the clean surface may indicate a movement of the hydrogens closer to the topmost metal layer, when the coverage is increased from 0.5 monolayers to saturation.
1. Introduction The H/Pd system has been the object of many investigations because of: (a) the ability of Pd to accommodate large amounts of hydrogen in its bulk [l-3], and (b) the catalytic activity of Pd [4]. The important role of the surface in both areas was emphasized in several recent studies of chemisorption on well-defined clean Pd surfaces [5-111. On Pd(llO), a hydrogen induced substrate reconstruction has recently been shown to provide effective channels for hydrogen to move from chemisorption sites into the crystal interior [g-lo]. For Pd(l1 l), a kinetic molecular beam study by Engel and Kuipers [ll] showed how surface and bulk processes can be disentangled. Interesting studies have also already been reported on the H/Pd(lOO) system [7]. Especially the order-disorder phase transformation observed for the c(2 X 2) hydrogen overlayer [7] attracted a number of theoretical efforts [12,13]. In this paper, we present He diffraction studies of the H/Pd(lOO) system, which were performed with the aim of gaining information on H/Pd bonding distances, as well as of inquiring whether the unique sensitivity of He diffraction for hydrogen adsorbates would allow a detailed study of the order-disorder transition of the c(2 x 2)H phase in the spirit of ref. [14]. 0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
2. Experimental
and diffraction data analyses
The Pd(lO0) specimen disk of 6 mm diameter and 1 mm thickness was prepared from a large Pd single crystal. The crystal was aligned to +OS”. cut with a wire saw and mechanically polished with successively finer grades of diamond paste. After several He bombardment and annealing cycles. LEED showed a pattern characteristic of the two domains of the c(2 x 4) superstructure Formed by carbon [15]. In order to remove the carbon, apart From Further sputtering and annealing treatments, the recipe given by Behm et al. [5] was applied: many oxidation-reduction cycles at 800 K with 2.10 ’ Tort- O2 and 5.10-’ Tot-r H, were performed until the sample reproduced the adsorption behavior of hydrogen as reported in ref. [7]. The He diffraction experiments were performed with beam energies between 20 and 63 meV. For the evaluation of diffraction data, spectra corresponding to angles of incidence not larger than 40” were chosen in order to avoid strong influences on the calculated hard-wall corrugtaion amplitudes owing to softness effects [16]. The intensity analyses were performed with the eikonal approximation [17], which is very reliable for the extremely small corrugations of the clean Pd(lOO) and the two ordered hydrogen phases.
3. Results and discussion 3. I. Cleun Pd( 100)
He diffraction From the clean Pd(lO0) surface shows. beside the dominant specular, only extremely small in-plane and out-of-plane diffraction peaks of first order. The total diffraction intensity is about 30% of the incoming beam intensity. For E, = 63 meV and small 8,, their intensities are of the order of 0.9% relative to the specular. which corresponds to a corrugation parameter I&10) = 0.019-0.020 A, where the corrugation Function is assumed to be D(X,)j)
= ~D(lo)~cos(2~/~).~
+ cos(2n/a).‘]
(1)
Form of a (u = d/a = 2.74 A), which is the simplest possible analytical corrugation with full quadratic symmetry. The maximum corrugation amplitude (occurring along [ll] in the direction to next-nearest neighbors) equals 21>(10) and amounts to - 0.04 A for the clean Pd(lOO) surface. The value of the corrugation parameter D(10) is similar to the corresponding value for nearest neighbors in the Pd(ll0) surface where it was found to be - 0.02 A [18] in the same range of He energies. The maximum corrugation amplitude on Pd(lOO) is appreciably larger than for Ni(lOO), (0.015 A), which may simply be due to the smaller lattice constant of Ni (d = 3.52 A), so that the smearing-out effect of the Free electrons is more pronounced in the case of Ni. On the other
K. H. Rteder. W. Stocker / Hydrogen chemtsorption
on Pd(100)
141
hand, the corrugation amplitude is also larger than for Ag( 111) (0.022 A) [ 191, although silver has a larger lattice constant (d = 4.08 A); however, the (111) surface is more densely packed than the (100). The corrugation amplitude should increase with increasing He energy as the He atoms penetrate deeper and deeper into the sea of free electrons [16,20]. Because of the small diffraction intensities, we refrained from a detailed investigation of this effect in the case of Pd(lOO). Fitting the observed decrease of the specular intensity as a function of temperature for the clean Pd(lOO) surface, using a He energy of 63 meV and an angle of incidence of 25.5” with the traditional Debye-Waller formula [21] yields a Debye temperature of - 315 K for Pd(lOO) which is near to the bulk Debye temperature of 276 K [22] and has to be regarded as an effective surface Debye temperature characteristic for He scattering rather than the true surface Debye temperature which should be smaller than the bulk value. Effective Debye temperatures for He scattering close to the bulk value have also been observed for Ni(lOO) [23] and Pt(ll1) [24]. 3.2. The initial stage of hydrogen adsorption Fig. 1 shows the intensity behavior of the specular beam and a beam characteristic for the ordered c(2 x 2)H phase as a function of hydrogen dose for an adsorption temperature of 100 K. Immediately after opening the
HP on I
Pd (400) kHe = 0.63 r! 6i = 25.5’ T,= IfOK
‘*.J;
I 0.5
I
I
*.
(ig,
= (TO) or (07)
.I_
I
1.0
--__
I 4.5
H - EXPOSURE
-
_^_ 2.0
I 2.5
(L)
Fig. 1. Intensities of the specularly reflected and the (- l/2, tic of the c(2 X2)H phase as a function of H, exposure.
l/2)
He diffraction
beam characteris-
hydrogen valve, the specular intensity drops rapidly. This drastic effect is due to the giant cross-section of statistically distributed adatoms present in dilute concentrations on the surface caused by the different in- and out-refractions of the He particles at the attractive part of the potential between the He and the individual adsorbate atoms as recently demonstrated by Poelsema et al. [25]. Using the slope of the initial decrease and relating it to the number of particles present at the surface (according to ref. [7], the sticking coefficient of hydrogen on Pd(100) is not strongly dependent on the hydrogen coverage at small coverages), allows determination of the effective scattering cross-section 1251 of the individual H atoms, and yields 22 A’ in reasonable agreement with our previous result of 32 A* for H on Ni(ll0) [26]. At higher H, doses, the specular goes through a minimum, reaches a small maximum, again goes through a minimum and then rises until the saturation value is reached at very large H, doses. The (- l/2, f l/2) beam (denoted relative to the clean surface unit cell) increases steadily until it reaches a maximum, and decreases continuously as the coverage approaches saturation. The formation of the optimum c(2 X 2) phase was assumed to coincide with the maximum intensity of the fractional order beam; the fact that the maximum intensity of this beam does not coincide with either one of the minima of the specular intensity or the maximum in between, represents an unsolved puzzle at the moment. The small differences of relative intensities in the different regions near the (-l/2, l/2) maximum do not, however, influence the corrugation analyses appreciably. 3.3. The cf.? X 2) hydrogen phase and the order-disorder
phase transition
A typical diffraction spectrum for in-plane and out-of-plane scattering for the ~(2 X 2) structure of hydrogen on Pd(100) is shown in fig. 2. The most intense beams are the (+1/2, 3: l/2) beams with about 11-12s of the specular intensity. All intensities observed can be fitted within a few percent by: (i) using the Fourier ansatz D(x,y)
= ;D(lO)
[cos(277/a)x
+ cos(2v/a)y]
+D(ll)
cos(27r/a)x
cos(2n/a)y
+ fD(20){cos(277/a)x
+ cos(2a/a)y)
(2)
(a = 3.88 A), where the best-fit Fourier coefficients are D(10) = 0.065 A, D(ll) = 0.020 A and D(20) = 0.015 A, and (ii) by modeling the adsorbates as Gaussian hills on a flat substrate, hwere the best-fit value for the amplitude is 0.14 A and that of the FWHM 2.1 A. In order to obtain a rough quantitative value for the position of the hydrogen atoms relative to the substrate, we have performed surface charge density calculations by using the simple method of overlapping atomic charge
K. H. Rieder, W. Stocker / Hydrogen chemisorption
on Pd(100)
143
densities [27,28]. We first checked the procedure for the case of the (2 X 1) phase of hydrogen on Ni(ll0) for which Hamann [29] has performed self-consistent calculations and has given a binding position for the adatoms. Comparison of the density contours with the corrugtaions, requires knowledge of the proportionality constant relating the incoming He energies with the surface charge density [30]. The most recent values correspond roughly to densities of 5 x 10e4 a.u. [31]. As compared to the experimental values of the clean surface, the corrugation amplitudes in the critical density range come out too large in Hamann’s and even larger in our own calculations, which shows that the smearing-out influence of the free electrons is not accounted for correctly. However, the density contours for the Ni(110)(2 X l)H phase obtained by Hamann and those of our own calculations are in good agreement, so that some confidence can be had in the simple method of overlapping atomic densities. For the clean Pd(lOO), the charge density overlap calculations yielded
II! I
T,= llOK
IllI
0 = 3.BBA a=45O
” 20
) (54)
0 =+-120
cti)
CO?,
I
I
I
” 20 I
I
I
4
(20) (02)
III1
Fig. 2. Typical He diffraction spectrum for the ~(2x2) structure of hydrogen on Pd(lOO). The beam indices refer to the unit cell of the superstructure rather than to that of the substrate as in figs. 1 and 3.
144
K.
H. Rleder,
W. Stocker
/ Hydrogen
chemrsorption
on Pd(100)
a corrugation amplitude (0.17 A) more than four times larger than the one observed (0.04 A). Therefore, we neglected the corrugation of the substrate and took the mean substrate densities at the different positions normal to the surface for the calculations of the adsorbate phases. With this procedure, we found that, at a density of 5 x 10ee4 a.u., the hydrogen adatoms produce a corrugation of - 0.1 A at a normal distance from the topmost Pd layer d, = 0.7 A. Assuming the H atoms to occupy the fourfold coordinated hollows as proposed by Behm et al. [7], this corresponds to a bond length of - 2.05 A. This value is in excellent agreement with the 2.04 A recently estimated by Nordlander et al. [32] on the basis of the measured vibrational adsorbate frequency [33]. Our result is in disagreement with the predictions of Muscat [34] which favor subsurface sites for hydrogen on Pd(lOO). Fig. 3 shows the intensity behavior of the (-l/2, l/2) beam of the c(2 X 2)H phase as a function of temperature. This measurement was performed, in order to investigate the possibilities of studying the well-known order-disorder transition of this phase [7] in more detail by using He diffraction. As can be seen from fig. 3, the phase transition causes only a slight change in the intensity decrease in the critical regime; nevertheless the value of the transition temperature determined by Behm et al. [7] is in reasonable
(a) Pd (WI) + c(2-2)H (b) NI (111)
I 130
I 160
I 190
I 220
I 250
260
TEMPERATURE Fig. 3. (a) Temperature
behavior
~(2 X 2)H phase on Pd(100).
+ (2.2)H
310
340
370
[OKI of the (-
l/Z,
(b) Temperature
characteristic
for the (2 X 2)H phase on Ni(l11).
temperatures
as determined
by LEED
[7,34].
l/2)
behavior
He diffraction
beam characteristic
of the (-l/2.
l/2)
He diffraction
The arrows denote the order-disorder
of the beam
transition
K.H. Rieder, W. Stocker / Hydrogen chemrsorprion on Pd(100)
145
accordance with the steepest part of the decrease as measured with He diffraction. For comparison, we have also plotted the temperature behavior of the (-l/2, l/2) beam for the analogous situation of the (2 X 2) structure of hydrogen on Ni(ll1) [35]. In this case, the intensity drop in the critical temperature region is much more pronounced, so that this system appears to be a more favorable candidate for beam-shape measurements as a function of temperature [14]. The reason why Ni(ll1) presents a more favorable system probably lies simply in the fact that the effective Debye temperature is appreciably higher for Ni than for Pd, so that the decrease owing to phononic attenuation does not shade that of the order-disorder transition as strongly as in the case of Pd. This can be seen from fig. 3: The intensity decrease for the Pd/H system at the lowest temperatures sufficiently far away from the transition temperature is much steeper than for the Ni/H system. 3.4. The (I X I) hydrogen phase The final (1 X 1) diffraction pattern corresponding to saturation of the surface with hydrogen shows first-order diffraction peaks which are even smaller relative to the specular than in the case of the clean Pd(lOO). Using eq. (1) we arrive at a value for the corrugation parameter D(10) of - 0.012-0.013 A corresponding to a maximum corrugation amplitude of - 0.025 A, which is about two thirds that of the clean surface. Assuming an ideal (1 X 1) structure corresponding to a coverage of 1 ML and taking the Gaussians obtained by the best-fit of the c(2 X 2)H diffraction data to model the (1 X l)H corrugation, we would expect a maximum corrugation of 0.08 A, which is appreciably higher than the value observed experimentally. Performing again density calculations in the same manner as described above and looking at the normal distance of the hydrogens just necessary for the observed corrugation of 0.025, we arrive at a value of d, = 0.35 A, which would correspond to a shortening of the bond distance in the fourfold hollow from 2.05 A in the c(2 X 2) phase to 2.00 A in the (1 X 1). This is certainly only one of several possible explanations, but it does not appear unrealistic, especially as Behm et al. [7] have shown that the saturation coverage does not correspond to 1 ML exactly, but exceeds this value by - 30%. As the He diffraction pattern exhibited sharp diffraction beams with no disorder-induced background, we believe that the extra hydrogen goes subsurface. The subsurface hydrogen may affect the bond between the first Pd layer and the chemisorbed hydrogen sufficiently strongly to produce the slight change observed in bond length. 4. Conclusion He diffraction results were used to determine the corrugation shapes and amplitudes for the clean Pd(lOO) surface as well as for the c(2 X 2)- and
146
KM. Rieder, I+/.Stocker / H_vdrogenchemworptron on Pd(100)
(I X I)-ordered phases of hydrogen. Simple surface charge density contour calculations allowed determination of adsorbate-metal bond lengths, and the results are in good agreement with recent theoretical predictions [32]. Transition from the c(2 X 2) hydrogen phase to the saturation phase appears to be accompanied by a slight shortening of the hydrogen-metal bond length. The order-disorder phase transition occurring at elevated temperature for the c(2 x 2)H phase could be observed by using He diffraction, although it is heavily screened by strong Debye--Walter attenuation.
Acknowledgment The authors wish to express their thanks to M. Baumberger the surface density calculations.
for his help with
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