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Langmuir 1988,4, 294-291
Isotope Dilution Neutron Spectroscopy: A Vibrational Probe of Hydrogen/Deuterium Adsorbate Interactions on Palladium Black? Jacqueline M. Nicol,*lv§J Terrence J. Udovic,§ J. J. Rush,§ and Richard D. Kelleye University of Maryland, College Park, Maryland 20742, and National Bureau of Standards, Gaithersburg, Maryland 20899 Received July 17, 1987. I n Final Form: October 23, 1987 Incoherent inelastic neutron scattering has been used to probe the vibrational spectra of both pure H and dilute H in D on Pd black. The occupation of both surface and subsurface sites at submonolayer coverage for pure H and the preferential H occupation of subsurface sites for dilute H in D are observed. For the pure H surface phase, considerable width and structure in the vibrational density of states for the degenerate parallel stretching modes reflect significant phonon dispersion, indicating the presence of strong H-H interactions. Isotope dilution of H with D replaces this dispersion with a narrower local mode feature shifted higher in energy. Comparison with simple mass-defect theory suggests the presence of some anharmonicity in the Pd-H bonding potentials for the surface H phase.
Introduction Understanding the interactions of hydrogen with catalyst surfaces is essential for elucidating the role of hydrogen in the surface processes that comprise many catalytic reactions. The interaction of hydrogen with Pd is of particular interest due to the useful catalytic properties of Pd, as well as to the variety of hydrogen species that may coexist. Indeed, unlike many other catalytic metals that only “adsorb” hydrogen, Pd can easily “absorb” hydrogen into the bulk to form hydride phases. In addition to the observation of surface and bulk hydrogen species, recent incoherent inelastic neutron scattering (IINS) results’s* have provided vibrational spectroscopic evidence for “subsurface” hydrogen in Pd (tentatively located in the octahedral sites just beneath the surface metal layer). The existence of subsurface hydrogen in Pd had previously been indicated by theoretical calculation^^-^ and other experimental studies on single-crystal surfaces employing a range of surface-sensitive UHV technique^.^-" In the present paper, IINS is used in conjunction with hydrogen and deuterium coadsorption on Pd black to further characterize the nature of the surface and subsurface hydrogen species. In general, IINS is a sensitive probe of the vibrational density of states of hydrogen in condensed-phase materials.12J3 Neutron spectroscopy is independent of the dipole selection rules present in photon and electron spectroscopies. Among other factors, the neutron scattering intensity for a particular vibrational transition is roughly proportional to the mean-squared displacement of H (or D) atoms in that mode. Since the total scattering cross section for H is ca. 12 times larger than that for D, H/D coadsorption spectra will be dominated by the vibrational density of states of H with much weaker scattering features due to D. Coupled with deuterium isotope dilution, IINS is capable of probing the dynamic H-H interactions that may be present in a particular adsorbate phase.14 The presence of H--H interactions can be seen clearly by the effect of isotope dilution on the vibrational density of states. Strong H-H dynamic coupling in an adsorbate layer can result Presented at the symposium entitled “Molecular Processes at Solid Surfaces: Spectroscopy of Intermediates and Adsorbate Interactions”, 193rd National Meeting of the American Chemical Society, Denver, CO,April 6-8, 1987. University of Maryland. National Bureau of Standards. Address correspondence to National Bureau of Standards. 0743-7463/88/2404-0294$01.50/0
in a significant dispersion of the corresponding hydrogen vibrational modes over momentum space. Although the isotope dilution of H with D leaves the adsorbate layer electronically and chemically unchanged, it can markedly change the dynamic coupling between adsorbate oscillators and, concomitantly,the degree of phonon dispersion. The usefulness of isotope dilution neutron spectroscopy has already been demonstrated in studies of /3-phase Pd hydride15 and chemisorbed H on both Raney Nil6 and Pt black.14 The purpose of this neutron spectroscopic study was to investigate the extent of dynamic H-H interactions within the surface and subsurface phases of Pd black.
Experimental Section Sample preparation and experimental conditions have been described in detail elsewhere.* Palladium black (27 g, 99.9%,40 m2 g;l, from Engelhard17)was washed in 1% nitric acid at 295 K, rinsed in distilled water, and dried at 323 K. Analyses by atomic emission spectroscopy and SIMS (0,’) indicated no significant concentrations of alkali, alkaline earth, or metal surface (1)Nicol, J. M.; Rush, J. J.; Kelley, R. D. Phys. Reu. B: Condens. Matter 1987,36,9315. (2)Nicol, J . M.; Rush, J. J.; Kelley, R. D. Surf. Sci., in press. (3)Chan, C. T.;Louie, S. G. Phys. Rev. B: Condens. Matter 1984,30, 4153. (4)Daw, M. S.; Foiles, S. M. Phys. Reu. B: Condens. Matter 1987,35, 2128. (5)Felter, T. E.; Foiles, S. M.; Daw, M. S.; Stulen, R. H. Surf. Sci. 1986,171, L379. (6)Gdowski, G. E., Felter, T. E.; Stulen, R. H. Surf. Sci. 1987,181, L147. (7)Behm, R. J.; Penka, V.; Cattania, M. G.; Christmann, K.; Ertl, G. J . Chem. Phys. 1983,78, 7486. (8) Baumberger, M.; Stocker, W.; Rieder, K. H. Appl. Phys. A 1986, 41, 151. (9)Eberhardt, W.;Louie, S. G.; Plummer, E. W. Phys. Rev. B: Condens. Matter 1983,28, 465. (10)Kubiak, G.D.; Stulen, R. H. J. Vac. Sci. Technol. A 1986,4,1427. (11)Gdowski, G. E.; Felter, T. E. J. Vac. Sci. Technol. A 1986,4,1409. (12)Cavanagh, R. R.; Rush, J. J.; Kelley, R. D. In Vibrational Spectroscopy of Molecules on Surfaces; Yates, J. T., Jr., Madey, T. E., Eds.; Plenum: New York, 1987;Vol. 1, p 183. (13)Howard, J.; Waddington, T. C. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1980;Vol. 7,p 87. (14)Rush, J. J.; Cavanagh, R. R.; Kelley, R. D.; Rowe, J. M. J.Chem. Phys. 1985,83,5339. (15)Rush, J. J.; Rowe, J. M.; Richter, D. Phys. Reu. B: Condens. Matter 1985,31,6102. (16)Cavanagh, R. R.; Kelley, R. D.; Rush, J. J. J. Chem. Phys. 1982, 77, 1540. (17)Manufacturers are identified in order to provide a complete description of experimental conditions. This is not intended as an endorsement by the National Bureau of Standards.
0 1988 American Chemical Society
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Langmuir, Vol. 4,No. 2, 1988 295 I
++ 70-76
i ++
2.
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++++
+
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.+eea
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60
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I
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Figure 1. Neutron energy loss spectra of (a) pure H and (b) H diluted in D adsorbed on Pd black. impurities. After the Pd black was loaded into a gold-sealed aluminum sample cell, surface oxygen was removed by repeated hydrogen titration cycles consisting of (1) reaction with hydrogen at 80 K, (2) evacuation at 295 K, and (3) further evacuation at 373 K. The resulting sample constitutedhydrogen-activatedPd black. Deuterium-activated Pd black was prepared from the hydrogen-activated sample by subjection to repeated deuterium exchange cycles to reduce the amount of residual H in the material. Hydrogen (Matheson Gas Products, Research Grade") and deuterium (Cambridge Isotope Laboratories, 99.98% isotopic purity") were used in these cycles and subsequent adsorptions without additional purification. The H (D) activation caused a stabilized reduction in surface area to ca. 18 m2g-' as measured by a N2 BET isotherm at 77 K. All experiments were performed at the National Bureau of Standards Research Reactor on the BT-4 Be-filter spectrometer with 40-min collimations before and after the Cu(220) monochromator. Energy resolution was ca. 5-6 meV (1meV = 8.065 cm-I) below 90 meV and ca. 6% above 90 meV. The sample cell was mounted in a variable-temperature cryostat equipped for in situ gas adsorptions. All IINS spectra were collected at 80 K to minimize contributions from multiphonon scattering. Spectra were corrected for fast neutron background contributions and differences in incident neutron monitor counts. Except for a scattering feature at 36 meV (fromthe AI sample cell), the spectra were dominated by the density of states of H.
Results The isotope dilution results are illustrated by the IINS spectra in Figures 1and 2. The vibrational spectrum for hydrogen-activated Pd black is depicted in Figure l a with strong scattering features evident a t 58,94,101, and 120 meV and weaker scattering features evident at 75 and 111 meV. As was noted previously,2 0.5-0.7 monolayer (ML) of surface H remains chemisorbed on hydrogen-activated Pd black. (A monolayer is defined by the BET surface area, assuming a Pd atom surface density of 1.2 X 1019 atoms m-2 and a surface H/Pd ratio of unity.) Under the activation conditions employed, bulk hydride phases in palladium black are known to decompose spontaneously,l&l9although chemisorbed H will remain strongly bound. (18)Wicke, E.; Nernst, G. H. Ber. BunsenCes. Phys. Chem. 1964,68, 224.
I
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Figure 2. (a) Difference spectrum following the addition of 0.04 ML of H to deuterium-activated Pd black. (Solid line is intended only as a guide to the eye.) (b) The spectrum of pure H adsorbed on Pd black is shown for comparison. Spectral assignments have been made for these features.'P2 In particular, the 58-meV feature (and the possible weak overtone feature at ca. 111meV) has been assigned to the vibrations of subsurface H in octahedral sites. The remaining strong features have been assigned to surface H in threefold sites: the broad complex feature, which is peaked at 94 meV with a shoulder at 101 meV to the doubly degenerate parallel stretching modes, and the 120-meV feature to the perpendicular stretching mode. This assignment was in agreement with EELS measurements on Pd(111).20 Normal mode analysis has yielded a H-Pd bond distance of 2.1 f 0.05 A and a H-Pd force constant of 0.43 f 0.03 mdyn A-1. The origin of the weaker feature at 75 meV is still not clear but has been attributed to multiply bonded surface H. Braid et a1.21 have also observed this feature on P d black and have attributed it to H adsorbed on both terminal and bridge sites. Figure l b depicts the normalized and 5x-magnified vibrational spectra for deuterium-activated Pd black. Although the overall scattering intensity is significantly reduced with respect to Figure la, weak spectral features indicate that a small fraction of H is still present, diluted in surface and subsurface D. Hence, by incomplete H/D exchange, the deuterium exchange cycles created an isotope dilution of H with D on Pd black. The maximum of the subsurface feature for dilute H appears slightly shifted to 60 meV, ca. 2 meV higher than the corresponding feature for pure H. The apparent intensity of the dilute H surface feature at 123 meV is probably somewhat distorted by an overtone contribution from subsurface H. The location of the other dilute H surface feature (corresponding to the broad complex feature peaked around 94 meV for pure H) is obscured by the expected scattering contributions from surface D atoms. (19) Newbatt, P.; Sermon, P. A.; Luengo, M. A. M. Z . Phys. Chem. Neue Folge 1986, 147, 105. (20) Conrad, H.; Kordesch, M. E.; Scala, R.; Stenzel, W. J.Electron Spectrosc. Relat. Phenom. 1986, 38, 289. (21) Braid, I. J.; Howard, J.; Tomkinson, J. J. Chem. SOC.,Faraday Trans. 2 1983, 79, 253.
Nicol et al.
296 Langmuir, Vol. 4 , No. 2, 1988 Table I. Estimates of H and D Occupation (ML) of Surface and Subsurface Sites in Deuterium-Activated Pd Black H D total H + D _surface subsurface total
0.04 0.03 0.07
0.56 0.09 0.65
0.60 0.12 0.72
In the harmonic oscillator approximation, the D mode energies would be shifted by 0.707 (i.e., 1/2’12)times the H mode energies. This would result in D scattering features centered at 41 meV for subsurface D and a t 66-71 and 85 meV for surface D. Careful examination of Figure l b locates weak scattering maxima a t ca. 43, 70-76, and 90 meV, suggesting that the D mode energies are shifted somewhat less than predicted for a harmonic potential. If true, this would indicate that some anharmonicity exists in the Pd-H bonding potentials for the surface and subsurface phases. As the deuterium-activated Pd black was evacuated at 373 K for a number of hours, the coadsorbed H and D should be well equilibrated between the surface and subsurface sites. A comparison of Figure l a and lb-indicates that the subsurface/surface intensity ratio is markedly higher for dilute H in D than for pure H. This strongly suggests that, for dilute H in D, the subsurface H/D ratio is larger than the surface H/D ratio (i.e., there is preferential occupation of H in the subsurface sites). A rough estimate of these H/D ratios can be made from a more quantitative analysis of the neutron scattering intensities in Figure 1. From the relative intensities of the spectral features for hydrogen-activated Pd black, the occupation of subsurface H has previously been estimated to be ca. 20% that of the surface H coverage.’ Hence, assuming a H surface coverage of 0.6 ML in the current analysis implies that there are 0.12 ML of subsurface species (totalling 0.72 ML of species) present. The relative scattering intensities for subsurface and surface H features in Figure l a and l b indicate an estimated population of 0.03 ML of subsurface H and 0.04 ML of surface H (i.e., 0.07 ML of total H) for dilute H in D. Since the activation conditions for deuterium-activated Pd black were identical with those of the hydrogen-activated sample, it is assumed for the purpose of this analysis that the total H + D occupation after activation is also 0.72 ML for the deuterium-activated sample. This leads to a total H/D ML ratio of 0.0710.65 on the deuterium-activated sample. In addition, if the subsurface/surface ratio is assumed to be 0.12/0.6 H + D, similar to the pure H sample, the isotope ratios in the two adsorbate phases can be estimated. This yields an H/D subsurface ratio of 0.03/0.09 ( ~ 0 . 3and ) an H/D surface ratio of 0.04/0.56 (-0.07). Table I summarizes these results. Again, due to all the inherent assumptions, the calculated occupations are only rough estimates intended to demonstrate the extent of preferential H occupation in the subsurface phase of deuterium-activated Pd black. Nonetheless, these ratios are consistent with H/D scattering intensity ratios for the subsurface and surface phases estimated from Figure 1b. Figure 2a illustrates the difference spectrum following the adsorption of an additional 0.04 ML of H on the deuterium-activated Pd black. After hydrogen adsorption at 80 K, the sealed sample was annealed to 300 K for 3 h to ensure equilibration and then recooled to 80 K before collection of the scattering spectrum. Spectral features represent the incremental increase in H site population due to hydrogen addition. The presence of considerable experimental scatter in the differecce spectrum precludes a rigorous quantitative analysis of the spectral features. Nonetheless, several observations can be made. A com-
parison of the scattering intensities with Figure 2b (the spectrum for hydrogen-activated Pd black) suggests that an additional 0.01 ML of subsurface and 0.03 ML of surface H atoms have adsorbed, yielding total H/D ratios of 0.04/0.09 ( ~ 0 . 4and ) 0.07/0.56 (-0.1) for the subsurface and surface phases, respectively (assuming the distribution of D remains unchanged). Moreover, the absence of D scattering features in the difference spectrum permits a closer comparison with the pure H spectrum. In particular, it appears that the 60-meV subsurface feature for dilute H is a t least as narrow (ca, 6-meV fwhm corrected for instrumental resolution) as that for pure H.’** A similar examination of the dilute surface H feature at ca. 101 meV (Le., the degenerate parallel stretching modes) indicates that it has narrowed considerably compared to that for pure H. In particular, a single-Gaussian fit of the surface H feature for dilute H in D yielded a peak position of 101 f 1 meV and a fwhm of 11 & 3 meV. A similar analysis for pure H yielded a peak position of 95 f 1 meV and a fwhm of 21 f 1meV. No similar conclusion can be drawn for the 125-meV surface H feature (i.e., the perpendicular stretching mode) since it is distorted by the presence of the subsurface H overtone.
Discussion For the degenerate parallel stretching modes of surface H on Pd black, the change from a broad complex feature peaked at ca. 94 meV for pure H to a much sharper feature centered at ca. 101 meV for dilute H in D is strongly suggestive of dynamic H-H interactions within the pure H surface phase. In other words, these interactions cause a dispersion of the surface optical modes similar to that for bulk p-Pd hydride15and adsorbed H on Pt black.14 One can rule out mixed-site population and anharmonicity in explaining the origin of the complex feature for pure H since the presence of these factors would yield a vibrational density of states structure that would be largely independent of isotope dilution. The particular sensitivity of dynamic H-H interactions to the parallel stretching modes is intuitively consistent with the fluctuations in adsorbate-adsorbate distances. These fluctuations are more pronounced for concerted motions parallel rather than perpendiciilar to the surface plane. The present data preclude a reliable estimation of the relative dispersion effects for the perpendicular stretching mode. Yet, results for H/Raney Nil6 have previously suggested a significant repulsive H-H interact,ion for parallel vibrations compared with a smaller interaction for perpendicular vibrations. Except for the small shift to higher energy, there appears to be little effect of isotope dilution on the density of states assigned to subsurface H. This is consistent with the concentration and degree of isotope dilution of the subsurface phase. Firstly, the total subsurface phase concentration was on the order of 0.1 ML, a low-concentration regime where adsorbate-adsorbate interactions would be weak. Secondly, the preferential occupation of dilute H into the subsurface phase resulted in a high H/D ratio of ca. 0.4, which is far from being “dilute” compared to the surface H/D ratio of ca. 0.1. The selective occupation of subsurface hydrogen in the presence of deuterium has, indeed, been reported elsewhere in the l i t e r a t ~ r e . ’ ~ ~ ~ ~ ~ It is informative to compare the isotope dilution results with simple mass-defect theory. This theory was previously used to describe the results of the isotope dilution (22) Gdowski, G. E.; Stulen, R. H.; Felter, T. E. J. Vac. Sci. Technol.
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studies of @-Pdhydride15and H / P t black.14 For isotope dilution of H with D, the dynamics of the isolated mass defects (i.e., dilute H atoms) are described by23v24
JgD(E) =-
EH2
- dE E2
(1)
d H 2
where gb (E)is the energy-dependent density of states of the surface D (host) phase (with .fgD(E)dE = l), E H is the H (defect) local mode energy, and = (MD- MH)/MD = (where MH and M D are the masses of the H (defect) and D (host) atoms, respectively). In effect, this equation indicates that the lighter H defect atom cannot vibrate within the dispersion-broadened phonon density of states of the surrounding heavier D host atoms (i.e., E H 2 - E2in eq 1 would become zero at E = E H ) . Rather, the energy of the H-defect mode must be located outside and above the energy cutoff for gD(E). It is evident from Figure l b that &(E) for the parallel stretching modes of surface D is obscured by the strong scattering features from the H defect atoms, Yet, gD(E) must be known to calculate a value for E H by eq 1. In view of this, gD(E)was first approximated from gH(E)for pure H by rescaling the energy axis of gH(E)by 0.707, the shift factor corresponding to vibrational harmonicity. The resultant gD(E)was then used in eq 1to yield a value of 96 meV for the H local mode energy, significantly lower than the observed value of 101 meV. To obtain the H local mode energy of 101 meV, the appropriate rescaling factor was determined by a reiterative process to be 0.75. This rescaling factor is consistent with the positions of the scattering maxima due to the surface D modes in Figure l b and supports the earlier observation that some anharmonicity is present in the surface phase. The presence of adsorbate-adsorbate interactions at high coverage on Pd black is consistent with the observed population of both surface and subsurface sites at submonolayer coverage. Assuming that the differential heat of adsorption, AH, for the surface phase at zero coverage is slightly larger than that for the subsurface phase,5 repulsive adsorbate-adsorbate interactions can lead to a decrease in the AH of the surface phase as coverage is increased, so that it becomes less than the AH for the empty subsurface phase. This would allow some population of subsurface sites before saturation of surface sites. In recent theoretical calculations using the embeddedatom method: submonolayer occupation of both threefold surface and octahedral subsurface sites for H/Pd(lll) has, in fact, been predicted by the inclusion of substrate-mediated adsorbate interactions. Moreover, the existence of repulsive adsorbate interactions on the Pd surface has previously been suggested by experiments performed on single-crystal surfaces such as TPD studies of H (D)/Pd~
~
~~
~~
(23) Lovesey, S. W. In Frontiers in Physics; Pines, D., Ed.; Benjamin Cummings: Reading, MA, 1980; Vol. 49. (24) Elliot, R. J.; Maradudin, A. A. In Inelastic Scattering of Neutrons; IAEA: Vienna, 1965; Vol. I, p 231.
(111)11and TPD, work function, and LEED studies of H/Pd(100).25 Although caution must be exercised whenever extrapolating the behavior of single-crystal surfaces to the behavior of polycrystalline surfaces, previous neutron spectroscopic s t ~ d i e s ' ~have ~ ~ ~illustrated *~' that the adsorption of H on polycrystalline Ni and Pt surfaces occurs at well-defined sites with vibrational energies consistent with those found for adsorption on single-crystal surfaces. Likewise, the vibrational spectrum for H/Pd black was successfully modeled2by assuming that the majority of surface H was situated in Pd(ll1)-like threefold sites and the majority of subsurface H was situated in bulk-like octahedral sites just below the surface metal atom plane. Hence, it seem reasonable both to assume that the repulsive H-H interactions suggested from Pd(ll1) surface studies will be present on the Pd black surface and to expect that these interactions will give rise to phonon dispersion in the H surface vibrational modes as was previously observed for H on Pt black.14
Summary In conjunction with isotope dilution of H with D, incoherent inelastic neutron scattering has been successfully used to investigate the existence and degree of H-H interactions for H adsorbed on the Pd black surface. The occupation of both surface and subsurface sites at submonolayer coverage for pure H and the preferential H occupation of subsurface sites for dilute H in D are observed. Significant interactions are reflected by the presence of considerable width and structure (i.e., dispersion) in the phonon density of states for the degenerate parallel stretching modes of the pure H surface phase. The transformation from a broad complex feature peaked at ca. 94 meV for the pure H surface phase to a sharper local mode feature centered at ca. 101 meV for dilute H in D was modeled using simple mass-defecttheory. The results suggest that some anharmonicity exists in the Pd-H bonding potentials for the surface H phase. The present study again illustrates the usefulness of isotope dilution neutron spectroscopy for probing the interactions of hydrogenous adsorbates in polycrystalline materials such as catalysts. Future experiments are planned to more quantitatively investigate the effects of isotope dilution on the phonon density of states for surface H on Pd black.
Acknowledgment. We acknowledge, with thanks, the partial support of this work by the Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division. Registry No. H2, 1333-74-0; Pd, 7440-05-3. ~~
(25) Behm, R. J.; Christmann, K.; Ertl, G . Surf. Sci. 1980, 99,320. (26) Graham, D.; Howard, J.; Waddington, T.C. J . Chem. SOC.,Faraday Trans. 1 1983, 79, 1281. (27) Udovic, T. J.; Kelley, R. D. In Hydrogen Effects in Catalysis: Fundamentals and Practical Applications; Paal, Z., Menon, P. G., Eds.; Marcel Dekker: New York, 1988; p 167.
