Coadsorption of oxygen and hydrogen on a stepped nickel surface

55

Surface Science 215 (1989) 55-64 North-Holland, Amsterdam

COADSORPTION OF OXYGEN AND HYDROGEN ON A STEPPED NICKEL SURFACE A.-S. MARTENSSON Department Received

of Physics, Chalmers University of Technology S-412 96 Giiteborg, Sweden

28 June 1988; accepted

for publication

28 December

1988

Electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED) have been used to study the adsorption of oxygen on the stepped Ni(510) face at a surface temperature around 100 K. As the coverage increases three different adsorption states for the oxygen atoms are observed; initially a low coverage (0 < 0.25) layer of oxygen adsorbed in the hollow sites on the terraces is established, secondly also step sites get occupied, and finally c(2x 2) like oxygen overlayers are formed on the, probably microfaceted, terraces. Vibrational losses at 51, 62, and 44 meV, respectively, are related to these three adsorption states. Also coadsorption of oxygen and hydrogen has been studied. The Ni(510) surface, pre-exposed to different amounts of oxygen, was saturated with hydrogen. When only terrace sites are occupied with oxygen, both atomic and molecular hydrogen adsorption occur. The H atoms adsorb both in step sites and on the terraces while the H, molecules chemisorb at step edge sites. When the step sites are saturated with oxygen no hydrogen adsorption is observed any longer.

1. Introduction It is well known that even relatively small amounts of surface additives can have a large influence on the reaction rate in heterogeneous catalysis. For the dissociative adsorption of diatomic molecules like CO preadsorbed alkali atoms have a promoting effect [l]. On the other hand, adsorbed electronegative atoms (e.g. 0, C and Cl) act as poisons for CO and H, adsorption [2,3] and consequently e.g. the water forming reaction proceeds principally on the perimeters of oxygen islands since oxygen blocks hydrogen adsorption on neighbouring sites [4]. The poisoning effect is thought to be closely related to the binding of the molecule to the surface. The molecule is generally adsorbed at a large distance from the metal compared to its constituent atoms after dissociation and, partly as a consequence of the more efficient screening close to the surface, the electrostatic interaction of the poison with the adsorbate is larger at a position further out [5]. Thus, e.g. the adsorbed amount of hydrogen can be considerably increased on a poisoned surface by predissociating the H, molecules with a hot filament [3]. 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

56

A.-S. Mrirtensson / Oxygen adwrption on Ni(510)

On the clean Ni(510) surface both atomic and molecular chemisorption of hydrogen is observed [6,7] which makes it interesting to study the effect of preadsorbing this surface with a poisoning atom. This paper reports an electron energy loss spectroscopy (EELS) study of coadsorption of oxygen and hydrogen on the Ni(510) surface. Also adsorption of solely oxygen has been studied and these results are discussed in the beginning of the paper.

2. Experimental The experiments have been performed in an UHV chamber equipped with a Leybold ELS 22 spectrometer, a LEED system and a mass spectrometer. The base pressure was 1 X lo- ” Torr . The EEL spectra were recorded in the specular direction with an electron impact angle of 60” towards the specimen surface normal. The scattering plane was defined by the surface normal and the [OOl] crystal direction. The spectrometer was operated at an energy resolution of about 5 meV. The preparation of the crystal has been described elsewhere [7]. Between the experimental runs the crystal was cleaned by argon ion bombardment followed by heating to 1100 K. The specimen was cooled with liquid nitrogen and resistively heated. The temperature was measured with a Pt/Pt-13 at%Bh thermocouple.

3. Adsorption of oxygen 3. I. Background We will first shortly summarize the experimental vibrational data found in the literature for oxygen adsorption on the (loo), (111) and (110) surfaces of nickel. On all these faces oxygen is dissociatively adsorbed. Two ordered chemisorbed structures have been found on the (100) face, p(2 X 2) and c(2 x 2), corresponding to a coverage 0 = 0.25 ML (1 ML = 1 monolayer) and 8 = 0.5 ML, respectively. For both structures the oxygen atoms are adsorbed in the hollow positions, 0.8-0.9 A above the nickel surface [8]. The vibrational spectra for these two structures show however an unexpectedly large shift of the perpendicular oxygen mode. At 80 K specimen temperature the energy of this mode decreases by 12.5 meV, from 52.5 to 40 meV, in going from p(2 X 2) to c(2 x 2) [9]. The shift has been shown to be partly [9,10] due to differences in the coupling to the substrate phonon modes. Only in the p(2 x 2)0 case the oxygen motion can couple to a breathing motion of the surface nickel atoms and the corresponding surface phonon mode, S,(x), appears in the EEL spectrum, at 28.5 meV [9].

A.-S. Mcb~~sson / Oxygen ndwrption on Ni(510)

5-l

Two ordered adsorbate layers are found also for oxygen adsorption on the Ni(ll1) face, p(2 X 2) and (6 X fi)R30” corresponding to coverages of 19= 0.25 ML and B = 0.33 ML, respectively. The oxygen atoms are located in the threefold hollow sites [ll]. Around T = 250 K substrate temperature the perpendicular vibration of the oxygen atoms is found at 72 meV [12]. Like on the Ni(lOO) surface, the ordered structures couple to surface phonons and, e.g. for the p(2 x 2) overlayer, losses attributed to the S,(M) and S,(M) modes are observed at 17 and 33 meV, respectively. Studies of oxygen adsorption on a Ni(ll0) substrate at temperatures at and above room temperature show that the surface reconstructs along the [OOl] direction giving rise to a (2 X 1) LEED pattern. A sawtooth model [13] for the reconstruction is consistent with the results from a scanning tunneling microscopy measurement [14]. The oxygen atoms are probably adsorbed in long bridge positions [14]. For small coverages, 0 < 0.1 ML, the oxygen atoms can adsorb on the unreconstructed Ni(ll0) surface [15]. Two different EELS studies of this low coverage region are published. Masuda et al. have found a loss peak at 60 meV which they assign to the perpendicular motion of oxygen atoms adsorbed in the short bridge sites [16]. Bar6 and 0116 got a conflicting result, their spectrum showing losses at 66 and 99 meV which are interpreted as being due to the symmetric and asymmetric stretching modes of oxygen chemisorbed in the long bridge sites [17]. 3.2. Results and discussion A sequence of EEL spectra for the oxygen exposed Ni(510) surface is shown in fig. 1. The adsorption was made by cumulative exposures with the target kept at - 100 K. Before each spectral recording the target was annealed to 470 K for 3 min, a treatment which sharpened the vibrational losses. After the lowest exposure shown in fig. 1,0.7 L O,_, only one energy loss, at 51 meV, is observed. The intensity of this loss has reached saturation in the 1.4 L 0, spectrum where also a second loss, at 62 meV, can be seen. In the spectrum recorded after a 3.5 L 0, exposure the intensity of the 62 meV loss is of the same magnitude as the 51 meV loss. Further increasing the exposure results in the growth of a new, intense, loss at 44 meV at the expence of the 51 meV loss which in the 6 L 0, spectrum has almost disappeared. Also in the phonon band peak structures appear. A loss at 16 meV is clearly seen in the 1.0 L 0, spectrum and in the 2.5 L 0, spectrum a shoulder is found at 25 meV. The energy of the initial loss peak, 51 meV, is close to what is found for the symmetric stretch motion of oxygen adsorbed in a p(2 x 2) structure on Ni(lOO), 52.5 meV [9]. We suggest that the oxygen adsorption on the Ni(510) surface starts on the (100) like terraces and assign the 51 meV loss to the

58

A. 4. Miirtensson

/ Oxygen adsorption on Ni(SI0)

;0.7,, 0

50 ENERGY

100 LOSS

imeV)

Fig. 1. EEL spectra from oxygen adsorbed on Ni(510). Cumulative exposures were used, 1 L = 1 x lo-6 Torr’s. The spectra were measured at - 100 K substrate temperature in the specular direction for an angle of incidence of 60” and an energy of 2 eV of the incident electron beam. The structure model shown in the inset is consistent with the LEED pattern seen at 6 L 0, exposure. White and grey circles are nickel atoms, black circles are oxygen atoms on the terraces. The adsorption site of the oxygen atoms at the steps is not known and is hence not indicated in the figure.

perpendicular stretch motion of an oxygen atom chemisorbed in the hollow position. The high energy of the mode shows that the local coverage on the terraces has not gone beyond 8 = 0.25 ML. The new loss appearing for larger exposures is found at 62 meV, i.e. higher in energy than the 51 meV loss. Accordingly, we exclude that this new loss should be due to oxygen atoms in the hollow sites on the terraces since increasing the coverage of oxygen on Ni(lOO) above B = 0.25 ML is known to reduce the vibrational stretch energy [9]. On the other hand, the EEL measurements for small exposures of oxygen on Ni(ll0) gave loss frequencies at 60 [16] and 66 meV [17], respectively, and we suggest that the 62 meV loss is due to oxygen atoms adsorbed at the (110) like steps of the Ni(510) surface. For exposures larger than 3.5 L 0, the third loss at 44 meV gradually takes the place of the 51 meV loss. This is in accordance with what one expects for the symmetric stretch of oxygen when the coverage exceeds 8 = 0.25 ML on the terraces. The downshift of the frequency is less than found on the Ni(lOO)

A. 3.

M&tensson

/ Oxygen adsorption on Ni(510)

59

surface. This is however reasonable since the narrow terraces on the (510) surface only allow two or possibly three rows of adsorbed oxygen atoms. One can rule out that the peak structures in the phonon band, at 16 and 25 meV, should be caused by the parallel oxygen vibrational motion since this mode is found to lie around 80 meV for the p(2 X 2) structure [18]. Instead we assume that these two losses originate from the S, and S, phonon branches of the (100) face, respectively. The S,(M) mode is found at 19 meV for the clean Ni(lOO) surface [19] and the S,(X) mode appears at 28.5 meV for a p(2 X 2) oxygen overlayer on Ni(lOO) [9]. The observed LEED patterns are consistent with the assignments above. For low oxygen exposures the LEED pattern is quite similar to the LEED pattern for the clean Ni(510) surface though the background intensity has increased. However, around 2 L 0, exposure very weak and diffuse spots can be seen in between the [150] rows indicating a p(2 X 2) like ordering on the terraces. Since the unit mesh of a p(2 X 2) structure on a fcc(100) surface is of the same order as the width of the (510) terraces we cannot expect to get the long range order needed to observe a sharp pattern on the LEED screen. The growth of the 44 meV loss in the EEL spectrum is accompanied by new, “~(2 X 2)“, LEED spots in the [150] rows. However, these new spots are displaced half a unit mesh with respect to the LEED pattern for the clean Ni(510) surface. According to laser simulations [20] we have made, such a LEED pattern is consistent with an ABAB . . . ordering of the c(2 X 2) oxygen overlayers on neighbouring terraces. E.g. microfaceting into (410) and (610) terraces could be the reason for the doubled periodicity (see fig. 1, inset). The (410) face has been shown to be particularly stable when oxygen is adsorbed on vicinal Ni(lOO) surfaces [21]. The results above can be summarized as follows. For small exposures the oxygen atoms adsorb in the hollow sites on the terraces. The local coverage does not go beyond 0 = 0.25 ML, instead further oxygen exposure results in oxygen adsorption in step sites. Finally, the step sites get saturated and the oxygen coverage on the terraces increases again. A new LEED pattern emerges which is consistent with an ABAB.. _ ordering of c(2 X 2) like oxygen overlayers on neighbouring terraces, possibly due to a reconstruction of the nickel surface into (410) and (610) microfacets.

4. Adsorption of hydrogen on an oxygen pre-exposed Ni(510) surface 4. I. Background We have in an earlier study examined the adsorption of hydrogen on the clean Ni(510) surface [6,7]. At a substrate temperature of - 100 K both dissociative and molecular chemisorption exist. For small hydrogen doses the

A.-S. Mrirtensson

60

Table 1 EELS energies

/ Oxygen adsorption on Ni(510)

for a dense layer of hydrogen

State

Site

Atomic Atomic Atomic Molecular

Hollow, terrace Short bridge, step Long bridge, step Step edge

adsorbed

on Ni(510) (from ref. [7])

Loss energies (meV) Hydrogen

Deuterium

58 103 142 95 154 28 83147398

43 60 78 102 71 115 21 64 116 286

hydrogen atoms adsorb in both hollow sites on the terraces and in low symmetry bridge sites at the steps with a preference of the step sites. For larger exposures, - 0.9 L H,, also a second kind of step site gets occupied, presumably a low symmetry long bridge site. Finally, for exposures larger than 0.9 L H,, hydrogen molecules adsorb at step edge sites. The high coverage vibrational losses related to these four different adsorption states are summarized in table 1. These loss energies are used as fingerprints of the different adsorption states observed in this study of saturated hydrogen adsorption on the oxygen pre-exposed Ni(510) surface. Since the molecular hydrogen losses interfere with the atomic ones we have, apart from recording an EEL spectrum directly after the hydrogen exposure, also measured a spectrum after having heated the target to - 170 K whereby the H, molecules desorb from the surface. As seen in fig. 1, the adsorption of oxygen atoms in the step sites starts before the terrace coverage has reached B = 0.25 ML. Similarly, the oxygen coverage on the terraces locally exceeds B = 0.25 ML before all step sites are occupied. Hence, we cannot investigate hydrogen adsorption for the ideal surface conditions; a p(2 x 2) like oxygen coverage on the terraces and (a) no oxygen atoms in the steps and (b) saturation of oxygen in the steps. Instead we have chosen to pre-expose the Ni(510) surface to three different oxygen doses which enables us to follow how the hydrogen adsorption decreases as the oxygen coverage increases. 4.2. Results and discussion The vibrational spectra for adsorption of hydrogen on the Ni(510) surface pre-exposed to three different oxygen doses, 1.4, 2.5, and 5 L, are shown in fig. 2. For all oxygen exposures we show both the spectrum for the hydrogen saturated surface (15 L H,) and for the atomic hydrogen saturated surface, i.e. the spectrum recorded after having heated the sample to - 170 K. The corresponding spectrum for the oxygen exposed surface without any hydrogen is also shown in order to facilitate the interpretation of the data. In fig. 3 the analogous spectra for oxygen and deuterium on Ni(510) are shown.

A.-S. h4hensson

/ Oxygen ndrorption on Ni(510)

61

x

0

50 ENERGY

100 LOSS

0

150 (meV)

100 LOSS

50 ENERGY

(a)

150 (meV)

(b)

Fig. 2. EEL spectra from hydrogen adsorption on Ni(510) pre-exposed to different amounts of oxygen. The corresponding spectrum for the oxygen exposed surface without any hydrogen is shown below each coadsorption spectrum. Conditions as in fig. 1. (a) The surface is saturated with atomic hydrogen. (b) The surface is saturated with hydrogen.

When the Ni(510) surface is pre-exposed to 1.4 L 0, the vibrational spectrum for the atomic hydrogen saturated surface shows three hydrogen related features: the S, phonon loss at 26 meV emerges, the terrace and step

1

0

i0 ENERGY

lb0

LOSS

(m&d)

0

50 ENERGY

(a) Fig. 3. Analogous

spectra

100 LOSS (meV)

(b) to the ones in fig. 2 with deuterium

instead

of hydrogen.

62

A. 3. M&?ensson / Oxygen adsorption on Ni(Sl0)

site oxygen losses get concealed in a broad structure centered at 51 meV, and finally, a new loss appear at 95 meV. The corresponding deuterium spectrum confirms the phonon loss at 26 meV and bring out clearly that the 95 meV loss is due to adsorbed hydrogen since the analogous new deuterium loss is found at 71 meV. We can assign the 95 (71) meV loss to hydrogen (deuterium) atoms adsorbed in low symmetry long bridge sites at the steps in accordance with what was found for hydrogen adsorption on the clean Ni(510) surface, see table 1. We also notice that the second loss related to this site is discernible in the deuterium case, at 118 meV. It is clearly seen in the deuterium spectrum for the 1.4 L 0, pre-exposed surface that the vibrational structure for the oxygen atoms on the terraces is affected by the coadsorption of the deuterium atoms. The 51 meV loss is broadened and the weak 62 meV step site loss cannot be resolved any longer. The broadened structure is also shifted towards lower energy in the deuterium spectra. A dense layer of deuterium (hydrogen) atoms on Ni(510) showed a rather broad loss at 43 (58) meV. This loss is assigned to the lateral motion of deuterium (hydrogen) atoms in hollow sites on the terraces, see table 1, and we conclude that hydrogen atoms can be adsorbed in between the oxygen atoms on the terraces. For the Ni(510) surface pre-exposed to 2.5 L O,, losses due to hydrogen (deuterium) atoms at the steps are no longer discernible when the surface is saturated with atomic hydrogen or deuterium. However, the loss structure around 50 meV still gets shifted showing that the oxygen vibrational structure is influenced by adsorbed hydrogen (deuterium) atoms on the terraces. When the Ni(510) surface is pre-exposed to 5 L 0, prior to the hydrogen exposure no hydrogen induced changes of the EEL spectrum are observed. All oxygen step sites are occupied which apparently implies that no atomic hydrogen can be adsorbed on the terraces any longer although the oxygen coverage on the terraces only locally has passed 8 = 0.25 ML. When the 1.4 L 0, exposed Ni(510) surface is saturated with H, (or D,) molecules as well, the EEL spectrum changes drastically. Molecular hydrogen (deuterium) losses emerge at 29, 84, and 148 meV (21, 64, and 116 meV> which are essentially at the same energies as for the clean Ni(510) surface, see table 1. The loss at 95 (71) meV associated with hydrogen (deuterium) atoms in low symmetry long bridge sites in the steps disappears, just like on the oxygen-free surface. The terrace and step site oxygen losses are still concealed in a broad loss structure. In the EEL spectrum recorded after having saturated a 2.5 L 0, surface with hydrogen (deuterium) the molecular H, (D2) losses are less intense and for the even higher oxygen exposure, 5 L O,, no molecular losses can be observed. Hence, the adsorption of hydrogen (deuterium) molecules decreases when the concentration of oxygen atoms adsorbed in the steps increases. We can thus conclude that hydrogen adsorption, both atomic and molecu-

A.-S. M&rensson

/ Oxygen adsorption on Ni(5IO)

63

lar, occurs readily on the stepped Ni(510) surface even when the oxygen coverage on the terraces corresponds to a p(2 x 2) structure on the flat (100) surface. The H atoms are found partly on the terraces, in between the oxygen atoms, and partly in low symmetry long bridge sites at the steps while the H, molecules only adsorb at step edge sites in the same geometry as on the H covered Ni(510) surface [6] as judged from the vibrational energies observed. When the step region is saturated with oxygen atoms the hydrogen adsorption ceases. In the introduction it was argued that the poisoning effect of oxygen for hydrogen adsorption should be due to an electrostatic repulsion between the adsorbed oxygen atoms and the incoming hydrogen molecule [5]. Such a picture, with the dissociation being the crucial step, agrees rather well with the results in the present study. While a p(2 x 2)0 covered Ni(lOO) surface is essentially inert to hydrogen adsorption [22] the same oxygen coverage on the (100) like terraces on Ni(510) permits coadsorption of hydrogen atoms since the steps provide sites where the dissociation can take place.

5. Summary Oxygen adsorption on the stepped Ni(510) surface has been studied with the use of EELS and LEED. As a function of exposure, the adsorption behaviour can essentially be divided into three parts. Initially a low coverage (8 < 0.25) layer of oxygen atoms in hollow sites is formed on the terraces, secondly sites at the steps get occupied and saturated, and finally the coverage on the terraces increases again. c(2 x 2) like oxygen overlayers are formed on the terraces and this high oxygen coverage probably brings about a surface reconstruction into (410) and (610) microfacets. The Ni(510) surface, pre-exposed to different amounts of oxygen, has also been saturated with hydrogen. Both hydrogen atoms and hydrogen molecules can be adsorbed as long as the step sites are not occupied with oxygen. Part of the H atoms adsorbs in sites at the steps, but the H atoms also occupy sites on the terraces, in between the oxygen atoms. The H, molecules chemisorb at step edge sites as they do when the Ni(510) surface is saturated with a dense layer of atomic hydrogen.

Acknowledgements The author wishes to thank Stig Andersson, Curt Nyberg, and Mats Persson for fruitful discussions. Financial support from the Swedish Board for Technical Development and the Natural Science Research Council is gratefully recognized.

64

A.-S. M&iensson

/ Oxygen adsorption on Ni(Sl0)

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[S] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] (22)

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