Hydrogen Chemisorption on Ni(110) by High-Resolution Electron Energy Loss Spectroscopy

Journal of Electron Spectroscopy and Related Phenomena, 29 (1983) 273-278 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

273

HYDROGEN CHEMISORPTION ON Ni(llO) BY HIGH-RESOLUTION ELECTRON ENERGY LOSS SPECTROSCOPY M. NISHIJIMA, S. MASUDA, M. JO and M. ONCHI Dept. of Chemistry, Faculty of Science, Kyoto University, Kyoto (Japan)

ABSTRACT High-resolution electron energy loss spectra of hydrogen-covered Ni(llO) surfaces both at 100 and 300 Kare presented. The adsorbed sites of hydrogen atoms are discussed. INTRODUCTION The interaction of hydrogen with nickel surfaces has received considerable attention for many years. This is partly due to the technological importance, e.g., heterogeneous catalysis, hydrogen-storage technology, etc., but the main reason probably lies in the fact that the hydrogen-nickel interaction has been considered to be representative of gas-surface interactions. In the present investigation, the Ni(110)-H 2 system has been studied both at 100 and 300 K using high-resolution electron energy loss spectroscopy (HREELS). HREELS gives direct information on the adsorbed state (molecular or atomic), adsorbed structure, etc. The measurements were made in accompaniment with the in-situ combination of supplementary techniques: low-energy electron, diffraction (LEED) and Auger electron spectroscopy (AES). The adsorbed state and site location of the adsorbed hydrogen are examined. EXPERINENTAL The experiments were performed by the in-situ combined techniques of HREELS, LEED and AES, using an ultrahigh vacuum system with the base pressure of 6 x 10-11 Torr. The high-resolution electron spectrometer constructed for the present study consists of a monochromator and an energy analyzer, both of 127· cylindrical deflector type. For the HREELS measurements, the primary electron energy Ep of ...... 2-5 eV and the incidence angle 8i of 600 with respect to the surface normal were used. The Ni(llO) sample used was of 99.999 % purity and of 6 mm diam x 2 mm thick. The clean Ni{llO) surface, having the LEED p{l x 1) pattern, was carefully prepared by the standard technique (Ar+ ion bombardmentannealing-oxidation-flashing cycles). No impurities were observed on the clean surface thus prepared within the detection limit of AES. The Auger peak-height 0368-2048/83/0000-0000/$03.00 © 1983 Elsevier Scientific Publishing Company

274

ratio H(noise)/H(Ni L2,3VV) was less than 0.01, and the residual carbon, if any, was estimated to be less than 0.02 monolayer. Details of the experimental apparatus and techniques have been described in our previous paper [1]. RESULTS Fig. 1 shows the loss spectra in the specular mode of the Ni(llO) surface at 100 K with increasing hydrogen exposure. For small exposure of 0.25 L (1 L = 10-6 Torr sec), two losses are observed at 77 and ~135 meV. The loss values are reproducible to within ± 1.5 meV. The 77 meV loss is observed with strong intensity; the 135 meV loss is smaller and broad. As the hYdrogen exposure is increased to 0.6 L, a sharp but faint LEED (2 x 1) pattern is observed, and two losses at 77 and 135 meV are observed with relatively large intensities. The intensity ratio of the 77 and 135 meV losses is ~5. Angledependent measurements indicate that the two losses (including all losses discussed below) are peaked in the specular direction. For the exposure of 6 L, when a sharp LEED (1 x 2) pattern is formed, two losses are observed at 75 and 117 meV. Fig. 2 shows the loss spectra in the specular mode of hydrogen-covered Ni(llO) surfaces at 300 K. Measurements are made with the Ni(llO) surface exposed to hydrogen at constant pressures of 2.4 x 10-9-- 5 x 10-7 Torr. LEED (1 x 2) patterns are observed under these conditions. The half-order spots are streaky and elongated in the (100) Az; they are somewhat sharpened by the increase in the hydrogen pressure from 2.4 x 10-9 up to 5 x 10- 7 Torr. For the hydrogen pressure of 2.4 x 10-9 Torr, three losses are observed at 89, 115 and 135 meV. The intensities of the 89 and 135 meV losses are large, with the latter intensity slightly smaller; the 115 meV loss intensity is comparatively weak. As the hydrogen pressure is increased, the 89 meV loss is shifted to lower energies; 85, 83, 80 meV for 5 x 10-9, 5 x 10-8, 5 x 10-7 Torr, respectively. In addition, monotonic decrease in the 89 meV loss intensity is observed: the intensity for 5 x 10-7 Torr is roughly 1/3 of that for 2.4 x 10-9 Torr. The 115 meV loss increases its intensity without peak shift as the hydrogen pressure is increased from 2.4 x 10-9 to 5 x 10-7 Torr. The 135 meV loss shifts its peak position; 138 and 139 meV for 5 x 10-9 and 5 x 10-8--5 x 10-7 Torr, respectively. The intensity of the 135 meV loss remains almost unchanged in this pressure range. For the hydrogen exposure of 5 x 10-7 Torr, the intensity of the 115 meV loss is the largest, followed by that of the 135 meV (shifted to 139 meV) loss; the intensity of the 89 meV (80 meV) loss is the smallest. By heating to above 220 Kat 10-10 Torr, the loss spectrum for the lowtemperature (1 x 2)-H surface is changed irreversibly to the spectrum similar to that for the room-temperature (1 x 2)-H surface at 5 x 10-7 Torr hydrogen

275

lf)

Ni(110)-H z

I-

Z

lOOK

::J

co

a::

« >

l-

(h2)

ll)

6 L

Z W

I-

Z

( 2xl) 0.6 L

x200 0.25 L

o

50

100

ENERGY

150

LOSS

200

(meV)

Fig. 1. Loss spectra in the specular mode of the Ni(110) surface at 100 Kwith increasing hydrogen exposure.

Ni(110)-H z

lf)

I-

300 K I

Z

Ep:O

3.75eV

::J

cD

a::

« >

l-

ll) Z W

I-

10mE.'V

Z

-9

PH = 2.4xl0 TORR

x 200

o

50

100

ENERGY

LOSS

150

200

(meV)

Fig. 2. Loss spectra in the specular mode of hydrogen-covered tH(110) surfaces at 300 K (see text).

276

pressure (Fig. 2); the low-temperature (1 observed by the subsequent cooling to 100 becomes streaky upon heating to above 220 pattern is similar to the pattern for the

x 2)-H surface spectrum is not K. The sharp LEED (1 x 2) pattern 10 Torr, and the resulting K at 10room-temperature (1 x 2)-H surface.

DISCUSSION The observed losses associated with the adsorbed hydrogen are at 75--89, 115--117, and 135--139 meV. These loss energies are much lower than the energy of the vibrational fundamental of gaseous H2 (545 meV) [2], and much higher than the bulk-phonon energies of the Ni substrate (~36.6 meV) [3]. Therefore, it can be concluded that hydrogen is dissociatively adsorbed on the Ni(llO) surface. In addition, the observed vibrational energies are much lower than the vibrational fundamental of free diatomic NiH (239 meV) [2], which indicates that hydrogen atoms are not adsorbed in the on-top sites. Angle-dependent measurements indicate that the observed losses are peaked in the specular direction; the electron-surface interaction is predominantly of dipole character [4]. The (2 x l)-H surface produces two losses at 77 and 135 meV. The number of the observed peaks (two) implies that number of the adsorbed states of hydrogen is one or two, applying the "surface-normal-dipole selection rule" [4]. The (2 x l)-H surface is considered to be unreconstructed from our LEED results, and the adsorbed sites of high symmetry have C2v or Cs symmetry. The two losses at 77 and 135 meV can be assigned to the symmetric stretching mode and the symmetric bending mode, respectively, of hydrogen adsorbed in the three-coordinated sites of the inclined rudimentary (111) face of the unreconstructed Ni(llO) substrate. According to the simple valence-force field approximation, for the triangular-pyramidal HNi 3 "molecule", the vibrational-energy ratio of the asymmetric stretching mode and the symmetric stretching mode of hydrogen gives (tan a)/I2, where a is the angle between the H-Ni bond and the threefold rotational axis. The above identification shows a = 68° , which gives the H-Ni bond length of 1.55 Aand the hydrogen radius of 0.31 A. The hydrogen radius is somewhat smaller than the covalent radius of hydrogen (0.37 A). Our model is in reasonable agreement with the molecular-beam diffraction results of Rieder and Engel [5], and is compatible with the theoretical study of ~1uscat [6]. The low-temperature (1 x 2)-H surface produces two losses at 75 and 117 meV. For the (1 x 2)-H structure, the surface distortion model has been proposed from the LEED dynamical calculation, in which adjacent <110> rows of nickel atoms are alternately attracted or repelled 0.1 Atogether or apart and compressed into the bulk by 0.1 A[7]. Although the Ni(llO) substrate is slightly distorted, it seems quite reasonable to consider, as in the case of

277

the (2 x l)-H surface, that the 75 meV loss is associated with the symmetric stretching mode of hydrogen atoms in the three-coordinated sites. The newlyobserved 117 meV loss may be attributed to the symmetric bending mode of hydrogen atoms in the three-coordinated sites. This assignment indicates that the hydrogen radius is 0.33 Ausing the valence-force field approximation described above. The present model is compatible with the molecular-beam diffraction results [5J. The molecular-beam diffraction study, further, indicates that hydrogen atoms are also adsorbed in the two-fold hollow sites [5J. The loss associated with these hydrogen atoms seems not observed. The room-temperature (1 x 2)-H surface produces three losses at 80, 116 and 139 meV. The 80 meV loss is associated with the symmetric stretching mode of hydrogen atoms in the three-coordinated sites of the distorted Ni(llO) substrate. The 116 meV loss may be associated with the symmetric bending vibration of hydrogen atoms in the three-coordinated sites. This assignment indicates o that the hydrogen radius is 0.35 A using the valence-force field model. The reason is not understood why, as the hydrogen pressure is increased from 2.4 x 10-9 to 5 x 10-7 Torr (Fig. 2), the 116 meV loss intensity is increased, whereas the 80 meV loss intensity is decreased. The 139 meV loss may be associated with the stretching vibration of hydrogen in the bridge sites. For transition metal hydrides with hydrogen in a position bridging two (or more) metals, the hydrogen vibration is observed at 136 ± 37 meV using infrared and Raman spectroscopies [8J. The 139 meV loss would not be associated with the symmetric bending vibration of hydrogen atoms in the three-coordinated sites, because (1) the intensity ratio of the 80 and 139 meV losses for the roomtemperature (1 x 2)-H surface is ~1/2, which is much smaller than the corresponding ratios for the low-temperature (1 x 2)-H surface (-v6) and for the (2 x l)-H surface (~5), and (2) the hydrogen-pressure dependent measurements (Fig. 2) may indicate that the 80 and 139 meV losses are not associated with the different vibrational modes of a single species: the 80 meV loss intensity is decreased as the hydrogen pressure is increased from 2.4 x 10-9 to ,5 x 10-7 Torr, whereas the 139 meV loss intensity remains unchanged. The loss spectrum for the low-temperature (1 x 2)-H surface is changed irreversibly to that for the room-temperature (1 x 2)-H surface by heating to above 220 K. Similar irreversible change has also been observed in the molecular-beam diffraction study [5J. The irreversibility may be understood if hydrogen atoms are absorbed in the nickel subsurface by heating to above 220 K. The most likely sites for hydrogen atoms are the octahedral sites below the three-coordinated sites. These are the sites that hydrogen atoms occupy in NaCl-type nickel hydride [9J. The vibrational energy for hydrogen atoms in the octahedral sites is considered to be ~80 meV [lOJ; the corresponding loss, however, may not be detected due to the screening by conduction electrons. Molecular-beam diffraction study [5J

278

shows that the hydrogen overlayer is strongly disordered. Thus, for the roomtemperature (1 x 2)-H surface, hydrogen atoms may be randomly adsorbed in the three-coordinated, two-fold hollow and short-bridge sites and, possibly, in the octahedral sites of the distorted Ni(llO) substrate. sur·lf;1ARY High-resolution electron energy loss spectra of hydrogen covered Ni(llO) surfaces have been studied. Tentative models for the adsorbed sites of hydrogen atoms are as follows: (1) For the (2 x l)-H surface, hydrogen is adsorbed in the three-coordinated sites of the rudimentary (111) face of the unreconstructed Ni(llO) substrate. (2) For the low-temperature (1 x 2)-H surface, hydrogen is adsorbed in the three-coordinated sites and, probably, in the two-fold hollow sites of the distorted Ni(llO) substrate. (3) For the room-temperature (1 x 2)H surface, hydrogen is disorderedly adsorbed in the three-coordinated, two-fold hollow and short-bridge sites and, possibly, in the octahedral sites of the distorted Ni(llO) substrate. Some of the unresolved problems in the above assignments are summarized: (1) Strictly, the three-coordinated sites above are somewhat different from those discussed in the molecular-beam diffraction study [5]. (2) For the low-temperature (1 x 2)-H surface, the loss associated with hydrogen in the two-fold hollow sites is apparently not observed. (3) Intensity changes of the three losses for the room-temperature (1 x 2)-H surface with increasing hydrogen pressure (Fig. 2) are not well understood.

REFERENCES 1 M. Nishijima, S. Masuda, H. Kobayashi and M. Onchi, Rev. Sci. Instrum., 53 (1982) 790. 2 G. Herzberg, Molecular Spectra and Molecular Structure I. Spectra of Diatomic Molecules, Van Nostrand, Princeton, New Jersey, 1950. 3 R.J. Birgeneau, J. Cordes, G. Dolling and A.D.B. Woods, Phys. Rev., 136 (1964) A1359. 4 E. Evans and D.L. Mills, Phys. Rev., B5 (1972) 4126. 5 K.H. Rieder and T. Engel, Phys. Rev. Lett., 43 (1979) 373; Surface Sci., 109 (l98l) 140. 6 J.P. Muscat, Surface Sci., 110 (1981) 85. 7 J.E. Demuth, J. Colloid and Interface Sci., 58 (1977) 184. 8 H.D. Kaesz and R.B. Saillant, Chem. Rev., 72 (1972) 231. 9 E.O. Wollan, J.W. Cable and W.C. Koehler, J. Phys. Chem. Solids, 24 (1963) 1141. 10 R.D. Kelley, J.J. Rush and T.E. Madey, Chem. Phys. Lett., 66 (1979)159.