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Physi- &Chemisorption

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+

----------

----------

+ -

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-+e+e-

z

r

z

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Zaremba & Kohn, Phys. Rev. B 15, 1769 (1977)

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2 Molekulare Orbitale freien COs

-0.804-1.521 -0.639

-0.554 0.127

/*2px

2p

2p

2s

2s

CO

/*2py

m*2s

/2px /2py

m2s

m2pz

m*2pz

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2 Orbitale gebundenen COs

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2 Photoelektronenspektroskopie

4 H.-J. Freund and M. Neumann

ONE ELECTRON SCHEME OF AN ADSORBATE

8 @

f

C FI?, {"

EF

2~

/

//

N/|

2ft / \

\

/ /

/ /

~ 2 ix

~o

46 40

36 30

2@ 20

Fig. 1. Schematic one-electron diagrams for diatomic molecules (CO) interacting with a transition metal surface. The level scheme for a molecule-metal cluster (right) is correlated with the band scheme of a free unsupported molecular layer (extreme left) and the band scheme of the quasi-twodimensional adsorbate (mid- dle). The band structure of the metal projected onto the surface is schematically shown as the hatched area

below, allows us to study symmetry properties of the

electronic states of adsorbate systems, it may be

possible to disentangle via ARUPS in favourable cases

which of and how these two types of interactions are

active in the adsorbate. We note at this point that all of

our examples refer to adsorbate systems on transition

metal single crystal surfaces, because the majority of

data is available for these systems. There are very few

examples of ARUPS studies of molecular adsorbates

on semiconductor surfaces.

1. Molecular Aspects

Let us start with the "molecular aspect" of the CO-

molecule-substrate interaction, i.e. the right-hand side

of Fig. 1. What happens electronically can easily be

explained in terms of the so-called Blyholder model

[-4]: The carbon lone pair is donated into empty d or s

levels of the metal atom, establishing a o- metal-

molecule interaction; synergetically, metal d electrons

are donated into empty molecular orbitals (2~*) of CO

forming arc metal-molecule interaction. From the view

point of the molecule we can look at this process as a

o--donation-~-back-donation process. This means that

the distribution of electrons among the subsystems, i.e.

CO molecule and metal atom, in the metal-CO cluster

is considerably different to the non-interacting subsys-

tems. For example, the electron configuration of the

metal atom in the cluster may be different from the

isolated metal atom, or the electron distribution within

the CO molecule bonded towards the metal atom may

look more like the electron distribution of an "excited"

CO molecule rather than the ground state CO mole-

cule [5]. Nevertheless, as a consequence of the rela-

tively weak molecule-substrate interaction only cer-

tain electronic levels of the subsystems are strongly

influenced, so that it appears to be justified to classify

the electronic levels of the interacting adsorbate system

according to the nomenclature used for the isolated

J/"ij~ E~g - ~ COsoLID

-7 ~ ~COND - mot

C%AS

' 2() 15 ' 1CI . . . . . . 5 Evoc

Fig. 2. Set of normal emission CO adsorbate spectra [6-11]

(s,u.: shake-up satellite)

You can not do this adiabatically!

e-hν

“Surface System”

UPS

Freund and Neumann, Appl. Phys. A 47, 3(1988)

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2 CO Adsorption

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2 CO AdsorptionO O O O

C C C C

M M M MM

MM MM M

Terminal (”Linear”)(all surfaces)

Bridging (2f site)(all surfaces)

Bridging / 3f hollow(fcc(111))

Bridging / 4f hollow(rare - fcc(100)?)

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Bindungsenergie

Physisorption < 250 meV

Chemisorption > 500 meV

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2 Surface Science Microcalorimeter

Heat capacity: 4 µJ K-1

An ultrahigh vacuum single crystal adsorption microcalorimeter C. E. Borroni-Birda) and D. A. King Department of Chemistry, University of Cambridge, Cambridge CB2 IE W, United Kingdom

(Received 7 March 1991; accepted for publication 22 May 1991)

The design of an ultrahigh vacuum microcalorimeter enabling calorimetric heats of adsorption to be obtained on single crystal surfaces as a detailed function of coverage is discussed. The system comprises a pulsed supersonic molecular beam source, an ultrathin metal single crystal, and remote infrared temperature sensing. Sticking probabilities and coverages are determined pulsewise by the King and Wells method, and heat capacity calibrations are conducted in situ by laser beam pulsing. Results for CO and O2 on Ni{ 110) demonstrate excellent sensitivity to adsorption of - lo- l3 moles of gas ( -0.01 monolayer). The heat capacity of the calorimeter is 4.2 PJ K - I,

I. INTRODUCTION

The heat of adsorption is perhaps the most important experimental parameter related to studies of adsorption. It provides a direct link to the strength of bonding between and adsorbed species and the surface. However, despite the enormous body of data that has been obtained using mod- ern surface science techniques over the past thirty years,’ no calorimetric adsorption heats have been determined for adsorbates on single crystal surfaces. Isosteric heats of ad- sorption have been obtained for a number of systems, but these measurements can only be applied to perfectly revers- ible adsorbate systems. Thus, there are no studies, for ex- ample, giving adsorption heats for oxygen on metal single crystal planes; or for metastable states; or for the influence of promoters and poisons.

The heat evolved on adsorption of a submonolayer gas dose on a typical single crystal, several cm2 in surface area, is so small and the damping of the temperature rise is so effective that it has been impossible to accurately obtain such heats directly,2 and this has meant that several indi- rect methods have had to be used. For instance, measure- ments from an analysis of thermal desorption spectra yield approximate activation energies for desorption that will be equivalent to the heat of adsorption in cases where adsorp- tion is nonactivated.3-5 However, an accurate determina- tion requires a precise knowledge of the desorption rate law, including the variability of the various kinetic param- eters with coverage. Another disadvantage is that some gases, such as oxygen, dissolve in the metal bulk when heated and it has therefore been impossible to obtain de- sorption spectra for them. For several systems adsorption heats have been estimated from core level shift data, using a Born-Haber cycle.6 Calorimetric studies of the heat of adsorption, by contrast with all of these methods, are di- rect, more generally applicable, and do not rely on models.

Calorimetry requires that the heat capacity of the sam- ple should be minimized in order to yield a measurable temperature rise. Beeck, Cole, and Wheeler7 designed a calorimeter that consisted of a very thin polycrystallinejilm evaporated onto the inside of a thin-walled glass bulb. The

temperature rise is calibrated by dissipating known amounts of electrical power around the outside of the glass vessel, and the coverage for each gas dose is determined by volumetric analysis. Many studies have subsequently been made on the heats of adsorption on polycrystalline films,R89 and the discrepancies between results obtained on suppos- edly identical systems highlights the limitations of this method. For example, analysis of the measured tempera- ture rise assumes that the glass is uniformly thick, which is difficult to achieve and impossible to prove. The degree of sintering of the films will also alter the morphology and crystallinity and will have a significant effect on the heat of adsorption.

Calorimetric heats have also been obtained on poly- crystalline wire filaments,” with gas doses being produced by Rash heating a similar filament. More recently Kyser and Masel used thermistors attached to a platinum { 11 l} single crystal, and observed a small estimated temperature rise when as much as 0.4 monolayers of CO (a total of 6X lOI molecules on both faces) was adsorbed.2 The re- sults were not promising.

We have designed and developed a microcalorimeter that is capable of accurately measuring the heat released due to adsorption of less than 0.01 monolayers of gas on a metal single crystal surface. For example, experimental data for CO adsorption on Ni{l lo} yield a zero coverage heat of adsorption of 135 kJ mol- ‘, which is in good agreement with the isoteric heat previously reported for this system.1’V’2 The effective heat capacity of the micro- calorimeter is 4.17 PJ K - ’ which should be compared to a typical value for a Beeck calorimeter of 4 J K - ‘.13

II. DESIGN CRITERIA

In summary, the design criteria that form the basis of the instrument are as follows.

A. An unsupported single crystal with a low heat capacity

The single crystals, epitaxially grown on cut NaCl crystals and floated off in water, are approximately 2000 A

“Present Address: The Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato-Ku, Tokyo 106, Japan.

2177 Rev. Sci. Instrum. 62 (9), September 1991 0034-6748/91/092177-09$02.00 @ 1991 American Institute of Physics 2177

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Rev. Sci. Instrum. 62, 2177 (1991)

d-1 /\a / c amplifier a/c Pre amplifier 1 1

Ge lens

Pulse stretcher

Paraboloid mirror

reverse - view

LEED -I-b,

“7G3

sample manipulator

A-- CRYSTAL/ - -. ~ ‘c w Mass Spectrometer

*- Gold flag

tB

\ -.

-. Beam stopper

cylindrical

mirror analyser

Stagnation

detector K

K skimmer -b.

3

nozzle ::; / O,~CQD,,N,

FIG. 1. Pictorial representation of the pulsed supersonic molecular beam single crystal adsorption microcalorimeter, with simultaneous sticking

probability and temperature rise measurements.

combination of a rotary-backed diffusion pump, titanium

sublimation pump, and liquid nitrogen cold trap.

Figure 3 shows an aerial view of the molecular beam

system. A piezoelectric crystal valve (Lasertechnics,

Albuquerque-model 203B) is used to provide a beam

source. It has a 0.2 mm nozzle diameter and can be oper-

ated in pulsed or continuous mode. A Lyons Instrument

pulse generator (PG71N) was used to trigger the valve

driver in order to create 1 Hz repetition rates. The nickel

skimmer (Beam Dynamics, Inc., Minnesota) has a 0.76- mm-diam entrance hole and is approximately 5 mm from the nozzle. The first collimating hole is 0.5 mm diameter and is 200 mm from the skimmer while the second colli-

mating hole has a diameter of 2 mm and is 780 mm from the skimmer. This second hole removes the penumbra gen-

erated by the nozzle source and first collimating hole, pro-

ducing a uniform beam flux. Calculations and photo-

graphic simulation were both used to measure the beam

diameter at the crystal. This was found to be 2.1 mm. The

nozzle-to-sample distance is approximately 800 mm. As-

suming a cos 8 distribution leaving the nozzle, the fraction

of the beam arriving at the crystal is calculated as

-2 x 10 - 6. The fraction intercepted by the skimmer is

-6x 10 - 3. Thus 1.4~ 10” oxygen molecules must leave

the valve in each pulse to produce a coverage change of 1% of a monolayer at the adsorbing surface. Calculations of the pressure changes in the three molecular beam stages and the chamber based on these estimates were found to be in good agreement with observation. The beam intensity

calibrations were performed using 50 ms pulses of deute-

FIG. 2. Side view of the system.

LEED optics; (B) infrared detector; linear drive stagnation detector.

(A) CC)

2179 Rev. Sci. Instrum., Vol. 62, No. 9, September 1991 Adsorption microcalorimeter 2179

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2 3 4 ‘rune /seconds

FIG. 11. Calorimetry results obtained simultaneously with the data in Fig. 9(b). (a) Infrared detector response with 0, pulsed onto an initially clean Ni( 110) surface at I Hz. (b) Infrared detector response averaged over the first three O2 pulses.

0.9, this gives a heat of adsorption of 135 kJ mol - ‘, which is in excellent agreement with the literature.“~12

Vi. FURTHER IMPROVEMENTS

Several improvements are being made to the system that will produce a higher signal, increase the accuracy, and reduce the noise level.

The first stage volume on the pulsed molecular beam source has been enlarged 15-fold to increase the maximum pulsed beam intensity by a factor of more than 15. This is clearly necessary where the sticking probability is low and/or the enthalpy change is small. Another improve- ment to the signal relates to the carbon backing. Tests using polycrystalline nickel films indicate that a 4000 A carbon film will produce an infrared signal approximately six times higher than the existing 2000 A carbon deposit. Different methods of carbon deposition are under exami- nation to reduce the stress on the sample. The combination of these factors will increase the infrared signal for calo- rimetry by between one and two orders of magnitude, de- pending on the sticking probability.

3.8

Time /seconds

FIG. 12. Infrared detector response averaged over the first seven CO pulses on Ni { 110). The data have been smoothed.

2185 Rev. Sci. Instrum., Vol. 62, No. 9, September 1991

The major source of inaccuracy in the heat of adsorp- tion determination lies in the accuracy of the coverage measurements, as opposed to the heat liberated. Uncertain- ties in the beam intensity are reduced by designing the stagnation detector as part of the third stage of the molec- ular beam source so that it can be used during the exper- iment by translating it into the beam. The use of thoriated iridium in the ionization gauge will remove any errors aris- ing from pumping which is very noticeable with deuterium and oxygen.

The inaccuracies involved in the determination of the sticking probability cannot be solved so easily. Increasing the emission current (above 2 mA) on the mass spectrom- eter is not recommended and does not appear to make any significant difference. Averaging over many beam pulses striking the gold flag reduces the error in the measurement of the zero coverage sticking probability. Repeating the experiment many times is feasible, as the experiment is quick and this will increase the signal-to-noise.

The signal-to-noise can also be improved by using more fine-tuned filtering. At present our system has a fixed bandwidth of 5 Hz to 1 kHz. A better signal-to-noise would be obtained if the center frequency and the band- width were varied. In this way the electronics can be di- rectly matched to the characteristic experiment rise and decay times. A special case of fine tuning is the removal of intermittent repetitive noise, associated with the turbomo- lecular pump vibration by using the computer fast fourier transform feature and filtering.

ACKNOWLEDGMENTS

We would like to thank Jacques Chevallier (Arhus University) for preparing and mounting the thin films, Professor Stig Andersson, Dr. Des Brennan, and Dr. Mike Bowker for many useful discussions and suggestions, and Nadia Al-Sarraf for experimental help. C.E.B-B gratefully acknowledges the award of a studentship by Liverpool University and the SERC; the SERC is also acknowledged for an equipment grant.

‘D. A. King and D. P. Woodruff, Eds., The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis (Elsevier, Amsterdam, 1980- 1990), Vols. l-5.

*D. A. Kyser and R. P. Masel, J. Vat. Sci. Technol. A 4, 1431 (1986). ‘D. A. King, Surf. Sci. 47, 384 (1975). 4E. Habenschaden and J. Kuppers, Surf. Sci. 138, L147 ( 1984). 5A. M. de Jong and J. W. Niemantsverdriet, Surf. Sci. 233, 355 (1990). 6W. F. Egelhoff, Jr., Phys. Rev. B 29, 3681 ( 1984). ‘0. Beeck, W. A. Cole, and A. Wheeler, Disc. Faraday Sot. 8, 314

(1950). ‘D. F. Klemperer and F. S. Stone, Proc. R. Sot. London A 243, 375

(1957). ‘D. Brennan, D. 0. Hayward, and B. M. W. Trapnell, Proc. R. Sot.

London A 256, 81 (1960). lo D. D. Eley and P. R. Norton, Proc. R. Sot. London A 319, 3 14 ( 1970). “J. L. Falconer and R. J. Madix, Surf. Sci. 48, 393 (1975). “5. Bauhofer, M. Hock, and J. Kuppers, Surf. Sci. 191, 395 ( 1987). 13S. Cerny and V. Ponec, Catal. Rev. 2, 249 ( 1968). 14D. A. King and M. G. Wells, Surf. Sci. 29, 454 (1972). ‘sD. Dayal, H.-U. Finzel and P. Weissman, Thin A-feral Films and Gas

Chemisorption (Elsevier Science, 1987). lbB N J. Persson, D. Schumacher, and A. Otto (private communica- .

tion).

Adsorption microcalorimeter 2185

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2 Langmuir Isotherme1.0

0.8

0.6

0.4

0.2

0.0

Cov

erag

e, Θ

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1

Pressure [Pa]

T = 300 K

T = 325 KT = 350 K

Ebind = 100 kJ mol-1

1.0

0.8

0.6

0.4

0.2

0.0

Cov

erag

e, Θ

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1

Pressure [Pa]

Ebind = 100 kJ mol-1Ebind = 90 kJ mol-1

Ebind = 80 kJ mol-1

T = 300 K

Irvine Langmuir

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Ertl & Koch, 1970

qst = 1.5 eV

CO/Pd(111)

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2D

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ed

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[TIB

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At:

21

:44

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20

07

HEATS O F CHEMISORPTION 139

FIG. 3a. Heats of adsorption of CO on polycrystalline transition

metal surfaces.

Do

wn

loa

de

d B

y:

[TIB

-Liz

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136 TOYOSHIMA AND SOMORJAI

260 1 , 1 1 , , , 1 1 1 1 ,

0 240 -

220 -

200-

2 180-

-

-

- - 01

-

2 160- - r

-

0

2 140- 0

5 120- - 0

d 100- - U

80- - - 0

60-

40 - 0 -

20 -

-

-

o Slc 4 b C'r d n F:! Cb di CL i n Gb de 0

FIG. la. Heats of adsorption of oxygen on polycrystalline transi-

tion metal surfaces.

Toyoshima and Somorjai, Catal. Rev. Sci. Eng. 19, 105 (1979)

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Karlberg, Phys. Rev. B 74, 153414 (2006)

The low-coverage adsorption on the pure-metal surfacesis shown in panel !a" of Fig. 2. As expected, of all adsorbatesoxygen forms the strongest bond and water the weakest onall surfaces in the study. Hydroxyl and hydrogen, on theother hand, prove to have quite similar low-coverage adsorp-tion energies overall. In particular, the hydrogen adsorptionenergy is fairly constant on the noble and transition metals,respectively. Concerning which site is the most stable waterand O display clear preferences regardless of the metal; thetop site for water, and the fcc site for O. An exception isprovided by Ru where O adsorbs in the hcp site. On the otherhand, Ru is a hcp metal and hence the Ru!0001" surface isused. For hydroxyl and hydrogen the stable site alters be-tween fcc, bridge and fcc, top sites, respectively !Table I".

The zero-order effect of putting a layer of platinum on thesurfaces of this study is that the adsorption characteristicsbecome more platinumlike. This can be observed both fromthe low-coverage adsorption energies on the platinum skinsshown in panel !b" of Fig. 2, and from the site preferencesshown in Table I. This conclusion is strengthened by theobservation that to the next level of accuracy the adsorptionenergy varies linearly with lattice constant, increasing to theright of platinum and decreasing to the left. Such a trend wasobtained by Mavrikakis et al.23 for O and CO adsorption onRu when the lattice constant of Ru was varied. According toMavrikakis and co-workers the change in binding energy canbe understood by considering the effect of the strain or com-pression on the d-band states of the transition metal in ques-tion. From this they find a linear dependence on the latticeconstant. Hence, the linear trend underpins the conclusionthat the topmost layer determines the adsorption characteris-tics. The deviation from a linear dependence between theadsorption energy and the lattice constant can thus be seen asa measure of the influence of the next-uppermost layers inthe alloys.

On pure surfaces the H2O-H2O interaction increases al-most linearly with an increasing lattice constant #panel !c" inFig. 2$. This effect is unexpected since the smaller latticeconstants are the ones most similar to ice, and hence ought togive the strongest hydrogen bonds. Still, according to thepresent calculations, the water-surface interaction suffices toreverse the trend expected from pure ice. For the H2O-OHinteraction, on the other hand, the trend on pure metals in-deed is an increase in the hydrogen-bond strength with adecreasing lattice constant. A dependence on the underlyingsurface can be observed also in this case since there is a clear

FIG. 1. The structures used to calculate hydroxyl-water #!a" and!c"$ and water-water #!b" and !d"$ interaction as seen from aboveand from the side. The black rhomb indicates the supercell used forthe calculations. The hydroxyl-water mixed structures contain 1/3of a monolayer of water and 1/3 of a monolayer hydroxyl. The purewater structures contain 2/3 of a monolayer water.

FIG. 2. !Color online" Adsorption energies for H2O, OH, H, andO on !a" Ru, Rh, Ir, Pd, Pt, Ag, and Au, and !b" the same adsorptionenergies when the topmost layer of metal atoms is replaced by alayer of platinum. The coverage is 1 /4 of a monolayer for H and Oand 1/3 of a monolayer for OH and H2O. Furthermore, theadsorbate-adsorbate interaction energies for the two adsorbate over-layers shown in Fig. 1 are given both for !c" the pure metals and !d"for the platinum-skin alloys.

TABLE I. Adsorption site preferences for OH and H on Ru, Rh,Ir, Pd, Pt, Ag, and Au, and the same surfaces where the topmostlayer of atoms has been replaced by a layer of platinum. In general,the OH molecule adsorbs with the molecular axis upright in fccsites, whereas the axis is tilted upon bridge-site adsorption.

Ru Rh Ir Pd Pt Ag Au

OH !pure" fcc bri bri bri bri fcc fccOH !Pt skin" bri bri bri bri bri fcc fccH !pure" fcc fcc top fcc top fcc fccH !Pt skin" top top top fcc top fcc fcc

BRIEF REPORTS PHYSICAL REVIEW B 74, 153414 !2006"

153414-2

The low-coverage adsorption on the pure-metal surfacesis shown in panel !a" of Fig. 2. As expected, of all adsorbatesoxygen forms the strongest bond and water the weakest onall surfaces in the study. Hydroxyl and hydrogen, on theother hand, prove to have quite similar low-coverage adsorp-tion energies overall. In particular, the hydrogen adsorptionenergy is fairly constant on the noble and transition metals,respectively. Concerning which site is the most stable waterand O display clear preferences regardless of the metal; thetop site for water, and the fcc site for O. An exception isprovided by Ru where O adsorbs in the hcp site. On the otherhand, Ru is a hcp metal and hence the Ru!0001" surface isused. For hydroxyl and hydrogen the stable site alters be-tween fcc, bridge and fcc, top sites, respectively !Table I".

The zero-order effect of putting a layer of platinum on thesurfaces of this study is that the adsorption characteristicsbecome more platinumlike. This can be observed both fromthe low-coverage adsorption energies on the platinum skinsshown in panel !b" of Fig. 2, and from the site preferencesshown in Table I. This conclusion is strengthened by theobservation that to the next level of accuracy the adsorptionenergy varies linearly with lattice constant, increasing to theright of platinum and decreasing to the left. Such a trend wasobtained by Mavrikakis et al.23 for O and CO adsorption onRu when the lattice constant of Ru was varied. According toMavrikakis and co-workers the change in binding energy canbe understood by considering the effect of the strain or com-pression on the d-band states of the transition metal in ques-tion. From this they find a linear dependence on the latticeconstant. Hence, the linear trend underpins the conclusionthat the topmost layer determines the adsorption characteris-tics. The deviation from a linear dependence between theadsorption energy and the lattice constant can thus be seen asa measure of the influence of the next-uppermost layers inthe alloys.

On pure surfaces the H2O-H2O interaction increases al-most linearly with an increasing lattice constant #panel !c" inFig. 2$. This effect is unexpected since the smaller latticeconstants are the ones most similar to ice, and hence ought togive the strongest hydrogen bonds. Still, according to thepresent calculations, the water-surface interaction suffices toreverse the trend expected from pure ice. For the H2O-OHinteraction, on the other hand, the trend on pure metals in-deed is an increase in the hydrogen-bond strength with adecreasing lattice constant. A dependence on the underlyingsurface can be observed also in this case since there is a clear

FIG. 1. The structures used to calculate hydroxyl-water #!a" and!c"$ and water-water #!b" and !d"$ interaction as seen from aboveand from the side. The black rhomb indicates the supercell used forthe calculations. The hydroxyl-water mixed structures contain 1/3of a monolayer of water and 1/3 of a monolayer hydroxyl. The purewater structures contain 2/3 of a monolayer water.

FIG. 2. !Color online" Adsorption energies for H2O, OH, H, andO on !a" Ru, Rh, Ir, Pd, Pt, Ag, and Au, and !b" the same adsorptionenergies when the topmost layer of metal atoms is replaced by alayer of platinum. The coverage is 1 /4 of a monolayer for H and Oand 1/3 of a monolayer for OH and H2O. Furthermore, theadsorbate-adsorbate interaction energies for the two adsorbate over-layers shown in Fig. 1 are given both for !c" the pure metals and !d"for the platinum-skin alloys.

TABLE I. Adsorption site preferences for OH and H on Ru, Rh,Ir, Pd, Pt, Ag, and Au, and the same surfaces where the topmostlayer of atoms has been replaced by a layer of platinum. In general,the OH molecule adsorbs with the molecular axis upright in fccsites, whereas the axis is tilted upon bridge-site adsorption.

Ru Rh Ir Pd Pt Ag Au

OH !pure" fcc bri bri bri bri fcc fccOH !Pt skin" bri bri bri bri bri fcc fccH !pure" fcc fcc top fcc top fcc fccH !Pt skin" top top top fcc top fcc fcc

BRIEF REPORTS PHYSICAL REVIEW B 74, 153414 !2006"

153414-2