theoretical study of underpotential deposition of foreign metal atoms on pt(111) single-crystal...

Journal of Molecular Structure (Theochem), 210 (1990) 353-364 Elsevier Science Publishers B.V., Amsterdam

353

THEORETICAL STUDY OF UNDERPOTENTIAL DEPOSITION OF FOREIGN METAL ATOMS ON Pt( 111) SINGLE-CRYSTAL CLUSTER SURFACES

Part I. The stability of the different metal underpotential-deposition layer structures as a function of surface coverage and applied electric potential*

M.B. LGPEZ, G.L. ESTIU, E.A. CASTRO and A.J. ARVIA

Instituto de Znvestigaciones Fisicoquimicas Tedricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, Universidad National de La Pk.&a, Sucursal4, Casilla de Correo 16, 1900 La Plata (Argentina)

ABSTRACT

Structures of underpotential deposited (UPD ) metal layers on a Pt ( 111) cluster surface are described through the application of the semiempirical atom superposition and electron delocal- ixation molecular orbital method (ASED-MO). Data predict that before the completion of the first metal monolayer a’second layer starts to grow. The growth of the second layer occurs simultaneously with the rearrangement of UPD metal lattice parameters. This process can be interpreted through the adsorption-nucleation and growth model. The early stages of three dimensional nucleation take place in the UPD potential region.

INTRODUCTION

Underpotential deposition (UPD) of foreign atoms onto metallic electrode surfaces is a well-known phenomenon which can he observed with a great va- riety of substrates in almost any type of environment [ 1,2]. UPD has been often considered as a two-dimensional adsorption process, leading to ordered surface layers exhibiting well defined equilibrium properties that in most cases fit readily into the framework of Gibbs surface thermodynamics. Both the Langmuir isotherm [ 31 and the Frumkim isotherm with a large attractive parameter have been employed to interpret the foreign atom-metallic-substrate interactions, and the foreign atom layer growth [ 41. However, it is now gen- erally accepted that the formation of a stable UPD layer structure implies an adsorption process followed by a phase transition [ 3-51. Data related to both

*Presented at the 18th International Congress of Theoretical Chemists of Latin Expression, held at La Plata, Argentina, 23-28 September, 1989.

0166-1280/90/$03.50 0 1990 - Elsevier Science Publishers B.V.

354

the optical properties [ 61 and the electrochemical characteristics of UPD metal deposits [ 71 reinforce the validity of the adsorption-desorption, nucleation and growth model to explain the kinetics of their formation. Accordingly, within certain specific operating conditions the simultaneous appearance of foreign atom clusters, foreign atoms adsorbed on the substrate surface, and bare substrate sites becomes possible in the UPD potential range. Nevertheless, at present it is insufficiently clear whether the deposition of the second atom layer begins before or after completion of the first one, although recent voltam- metric data favours the former possibility [ 51.

For certain bimetallic systems in contact with an electrolyte solution, the structural surface characteristics can be modified by changing the applied potential conditions. The presence of the UPD metal overlayer, either at the submonolayer or the monolayer level, changes the electrical properties of the substrate such as the capacitance and the potential of zero charge. Further- more, the catalytic activity of the bimetallic surface has often been found to be a surface-coverage-dependent property.

Semiempirical quantum mechanic calculations appear to be useful for de- scribing the stability of metal surface structures [ 81, as well as their reactivity towards a certain reaction [g-12]. In the present series of papers those meth- ods are used to determine the possible structure and reactivity of foreign metal atom UPD overlayers on Pt (111) cluster substrates, and their dependence on the applied electric potential, and on the degree of Pt surface coverage.

Nickel atoms were selected as foreign metal atoms because of the knowledge of the behaviour of Ni and Pt clusters towards Hz0 adsorption from previous work [9,10], despite the fact that no experimental data are available for the Ni/Pt system as the UPD of Ni on Pt in aqueous solutions is interfered with by the hydrogen evolution reaction. Nevertheless, the conclusions from the theoretical study become independent of the nature of the foreign atom.

The first part of this paper refers to the stability of [ Pt (111) In (Ni), clusters for different applied electric potentials and degrees of Pt surface coverage by Ni atoms. The degree of surface coverage is defined by the value of m. The second part of the paper deals with the chemical reactivity of the various structures.

CALCULATION PROCEDURE

The stability of [ Pt ( 111) n (Ni), structures for different applied electric potentials and m values is theoretically determined by using the semiempirical atom superposition and electron delocalization molecular orbital (ASED-MO) technique [ 13,141. This procedure combines one-electron-orbital attractive energies, the latter calculated through a modified extended Hiickel method, with atom-atom repulsion energies. This calculation procedure, which has been

found useful for adsorption and electroadsorption studies, has been recently applied to describe UPD metal overlayer structures [ 151.

The attractive component energy matrix elements are:

Hh = - (VSIP); (1)

Hy=O (2)

Hf” =l.l25(H$+ +H$Jb)Sf’ exp( -0.13R) (3)

where i and j denote atomic orbitals whereas a and b correspond to different atoms. VSIP denotes the valence state ionization potential; S, is the overlap integral between orbital i on centre a, and orbital j on centre b; R is the inter- nuclear distance between centres a and b.

The VSIPs are experimentally based [ 16,171 and valence orbitals are of the Slater form with exponents based on SCF calculations [ M-211. To mimic work- function modifications during adsorption, the VSIPs are adjusted until the charge transfer at the equilibrium distance in the heteronuclear diatomic bond is close to that predicted by the electronegativity difference in Pauling’s ion- icity relationship [ 221.

As a first step, the deposition process has been ascribed to an adsorption mechanism [ 151 corresponding to the occupation of the most favourable sites of the metal cluster surface. Accordingly, Pt and Ni VSIPs and Slater exponents in the diatomic Pt-Ni bond are shifted until the predicted charge transfer has been reached. Parameters assembled in Table 1 define Ni adsorption ontouncharged [Pt(lll)],(Ni),substratesforO<n<17.

The zero potential value was selected as the potential at which the Ni-Pt charge transfer corresponds to equilibrium according to Pauling’s electronegativity relationship. This value of the zero potential falls within the potential range where the UPD of Ni on Pt takes place. This potential range is located on the positive side of the Ni/Ni (II) reversible electrode potential ( - 0.23 eV) [ 231. Applied electric potentials are simulated by either decreasing or increasing the VSIP from those of Table 1 for positive or negative charging, defining UPD or OPD (overpotential deposition) conditions, respectively. The resulting shift in the metal d band reproduces the actual electric charging effect on the metal. This approach has been used extensively for semiempirical calculations of several electrochemical systems [ 9,10,24-261 including surface do- pant effects [ 27,281. Changes of t 1.0 eV in VSIP are arbitrarily correlated to changes of + 1.0 V from the zero potential. This approach furnishes the correct trend for the variation in the electrode potential of metal electrodes in an electrochemical environment.

High spin, bulk superimposable [ Pt (111) ] n clusters, where n is equal to 18 or 24 (Fig. 1) were used throughout the present study to model the Pt (111) surfaces. The clusters were geometrically built up maintaining the Pt-Pt bond length constant at 2.77 A. This value, which has been used in previous Pt

356

TABLE 1

Atomic parameters used in ASED-MO calculations: ionization potential, VSIP, Slater-orbital exponents (Exp. 1 and Exp. 2)) and the corresponding linear coeffkient (C, and C,): modifications according to Pt-Ni charge transfer are included

CPt(lll) l,W), Orbital VSIP

(eV)

Exp. 1 Exp. 2 C1 CZ

5d 9.60 6.01 2.39 0.5715 0.6567 m=O, 6s 9.00 2.55 n=19 6~ 4.96 2.25

lGmG7 andn=12

5d 9.60 6.03 2.40 0.5755 0.6620 6s 9.00 2.56

6~ 4.96 2.27 3d 10.00 5.73 1.97 0.5696 0.6307 4s 7.64 1.78 4P 4.45 1.48

n=19, n=O

3d 10.00 5.75 2.00 0.5683 0.6292 4s 7.64 1.80

4P 4.45 1.50

Fig. 1. Clusters used to model Pt (111) surfaces. (a) Ptz4 single layer; (b) Ptls bilayer.

357

cluster studies, agrees with the Pt-Pt interatomic distance in the bulk metal. Nickel deposition originates the different [ Pt (111) ] n ( yi), structures depicted in Fig. 2, with a Ni-Ni interatomic distance of 2.5 A. The Pt-Ni interatomic distance was taken as 2.63 A.

RESULTS AND INTERPRETATION

The first UPD Ni monolayer on the Pt(ll1) surface

The growth of the first Ni monolayer implies the initial electroadsorption of Ni occupying the most favourable sites on Pt to proceed to a superlattice structure formation [ 4,151.

As a first step towards modelling a Ni covered Pt (111) surface, the most favourable binding sites for the Ni atoms on the Pt surface were determined. It was found that Ni atoms are bonded more strongly to the three-fold hollow sites at 2.15 A from the Pt( 111) surface. Likewise, the corresponding adsorption energy decreases with the decrease in coordination number of the surface site (Table 2 ) .

To attempt to describe the growth mechanism of Ni layers on the Pt sub-

TABLE 2

Binding-energy values (BE, eV) for the adsorption of Ni atoms on different Pt ( 111) cluster surface sites: the comparison of BE values for the first, second and third linearly bonded Ni atoms show a similar sequence on both Pt single layer and Pt bilaye+

Structure Atom number

1 W) 2 U-U 3 (H) 1 (B) 1 (T)

%J 6.2465 6.2588 6.2602 Pk?, 6.1387 6.1450 6.1612 Pt5 7.3486 6.0201 6.27106

“T, top site; B, bridge site; H, hollow site.

Fig. 2. Submonolayer of Ni deposited on [ Pt ( 111) ] 21 at zero potential. Numbers indicate the sequence of atom deposition.

358

strate, the number of Ni atoms involved in the Ni monolayer is required. It should be noted that a compromise between this number and computational possibilities has to be found. With the purpose of decreasing the number of metal atoms without affecting the results, the influence of the second under- lying Pt atom layer on Ni binding energy (BE) was evaluated. These values of BE for Ni were calculated for increasing Ni coverages on both Pt single layer and bilayers (Table 2). The corresponding BE values do not change appreciably, particularly with regard to the position sequence chosen to grow the Ni layer. Accordingly, substrate monolayers consisting of 24 Pt atoms (Fig. 1 (b) ) were used to investigate the first step (adsorption) of Ni atom deposition.

The first layer of Ni atoms consists of 10 Ni atoms adsorbed through coordination to Pt at well-defined sites, yielding the structure depicted in Fig. 2, where the Ni atom adsorption sequence is also indicated. The most favourable adsorption site for the eleventh Ni atom can be defined ambiguously as that comprising either three Pt ( Pt3) or three Ni (N&,) atoms. The latter brings up the structural and electronic conditions for the growth of the second Ni layer.

The possibility of Ni adsorption on a Ni, site was also investigated for lower Ni surface coverages (Table 3). The present data show that under zero potential condition and for the cluster size used in this work, the formation of the second Ni layer starts when the adsorption of the eleventh Ni atom on a Nig hollow site takes place. The Ni-Ni distance in the resulting [ Pt (111) ] 24 (Ni) i0 structure corresponds to the lattice constant of Pt. Hence, from ASED calculations, one can conclude that the structure of the first Ni layer corresponds to that of a close-packed submonolayer, the latter being only slightly distorted by the proper Pt substrate structure, i.e. the first Ni layer keeps essentially the lattice constant of the substrate. In this case the entire surface structure ex- hibits bare Pt sites and a degree of surface coverage by Ni atoms nearly equal to 0.5.

The second UPD Ni overlayer growth

A higher coverage by Ni atoms under the same applied potential conditions results in their migration to Nig sites, defining the second Ni atom layer growth. The stability of the Ni layer increases as the Ni-Ni interatomic distance approaches that of bulk Ni. Then, from the first adsorbed Ni layer whose lattice parameter is distorted by the Pt substrate, the appearance of the second Ni layer induces a rearrangement in the direction of shortening the distance per- pendicular to the surface. In addition, there is a clustering effect which results from atom displacements parallel to the substrate surface until the Ni-Ni interatomic distance reaches 2.5 A. This fact leads to the stabilization of a Ni7 based layer structure adsorbed on Pt (111). The clustering effect occurs simultaneously with the migration of Ni atoms from the first to the second layer (atom 4, Figs. 2 and 3 ). For the P& cluster there are only two Ni atoms at the

TA

BL

E 3

Val

ues

of

the

bin

din

g en

ergy

(B

E,

eV )

for

the

adso

rpti

on o

f ea

ch N

i at

om o

n d

isti

ngu

ish

able

non

-equ

ival

ent

Pt

( 111

) h

ollo

w s

ites

: th

e re

sult

ing

stru

ctu

res a

re d

epic

ted

in F

igs.

2-4

Ato

m

Sit

e n

um

ber

No.

1

2 3

4 5

6 7

a 9

lo

lo’

11

12

13

14

15

16

1 2 3 4 5 6 7 8 9 10

11

12

13

14

15

16

6.15

6 6.

146

6.14

8 6.

167

6.16

9 6.

158

6.17

8 6.

16s

6.18

0 6.

179

6.19

6 6.

187

6.18

5 6.

173

6.11

4 6.

138

6.16

1 6.

171

6.14

6 6.

185

6.18

2 6.

180

6.16

1 6.

170

6.16

8 6.

166

6.15

2

6.17

1 6.

149

6.12

3 6.

121

6.14

5 6.

117

6.13

8 6.

132

6.12

1 6.

112

6.11

8 6.

102

6.10

0 6.

008

6.09

0 6.

030

6.00

1

360

Fig. 3. Ni overlayer structures on [ Pt ( 111) ] 24’ Numbers indicate the sequence of the second layer Ni atom deposition.

r X

Fig. 4. Sequence of Ni atoms deposition for building up the third Ni layer.

first layer (numbered 8 and 9, Fig. 2) outside the Ni, hexagonal structure, which also share a contraction to a Ni, structure, providing a structure more stable than that corresponding to both atoms separated from the rest without lateral attractive (stabilizing) interactions. Accordingly, the second Ni layer approaches the N& configuration on top of the Ni, layer instead of the Nig configuration which corresponds to the Ni, pyramidal structure. Both Ni, and Nig based structures are characterized by bulk Ni lattice constants. When the Ni, structure is formed on the Pt (111) surface, the periodicity of the hexagonal nucleous which characterizes the (111) crystal faces is accomplished.

The comparatively more stable Ni7 based pyramidal structure formed on the Pt (111) surface is produced after the initial reversible adsorption process by means of a rearrangement of the close-packed Ni layer distorted by the Pt substrate through a phase transition involving a nucleation and growth mechanism.

The growing sequence and the corresponding adsorption energies for the second Ni layer, and for the first adsorption stage of the third Ni layer are illustrated in Figs. 3 and 4, and in Table 3, respectively.

361

The Ni UPD on Pt(ll1) at high positive potentials

The increase in the electric potential applied to the Pt substrate (positive charging) produces a decrease in Ni atom coverage which defines the first layer (submonolayer level). Although, qualitatively, the characteristics of the Ni deposits are not appreciably modified, quantitative differences in the stabilization energy at the Ni submonolayer level are observed, and the growth of the second Ni layer starts earlier than in the cases described previously. On the basis of the parameters assembled in Table 1, the increase in substrate charge of 1.0 V gives rise to the structure depicted in Fig. 2. The stabilization of the latter again results from a phase transition which occurs in a similar way to that described for a nucleation and growth mechanism. Because of the cluster size, the limiting number of Ni atoms which determines the growth of the second Ni layer does not change for a positive charge of 1.0 V.

The Ni OPD on Pt(ll1)

The negative charging of the Pt (111) substrate which results on shifting the potential above -0.23 eV becomes sufficient to achieve OPD conditions for Ni on Pt (111) . The calculations based on the parameters assembled in Table 1 indicate that the OPD of Ni becomes largely favoured. In agreement with experimental data and thermodynamic predictions, bulk Ni deposition contin- ues after completion of the Ni monolayer produced on the Pt (111) substrate.

Molecular-orbital interpretation

The molecular-orbital interactions related to the stabilization of Ni atoms on either Pt3 or Nig hollow sites (Fig. 5) can be interpreted in the following

E (4

-2 I

-4

-6

-6

Ni Pt ,Ni Ni,Ni

Fig. 5. Molecular-orbital interactions involved in the adsorption of a Ni atom on Pt:, and Ni3 hollow sites.

362

Fig. 6. Dependence of energy levels on the applied electric potential for a Ni atom adsorbed on a PtS hollow site.

Fig. 7. Dependence of energy levels on the applied electric potential for a Ni atom adsorbed on a NiS hollow site.

-13

way. The d orbitals of each Ni atom are similarly stabilized through bonding interactions on either Pt, or Ni3 metal structures (Fig. 5). Otherwise, 4s Ni orbitals participate in both bonding and antibonding interactions. The antibonding interaction energy of Ni 4s orbitals with Pt3 unoccupied orbitals becomes lower than that resulting through the interaction with Ni3 structures. This result offers an explanation for the preferential stabilization of a Ni atom through adsorption on Ni3 sites as compared with Pt3 sites. Accordingly, when a stable Ni structure at the submonolayer level has grown on the Pt substrate (Fig. 2) another Ni atom coordinates on a hollow site defined through Ni atoms instead of that defined through Pt atoms.

Positive metal charging (Ni UPD on Pt (111) ) shifts the metal d band down-

363

wards, reinforcing the effect of antibonding interactions. Hence, due to the lower stability of the Pt,Ni ensemble, the second Ni layer starts to grow for smaller degrees of Ni surface coverages. This, in principle, seems to be con- firmed through the trends seen in the BE values. This effect should imply a different limiting Ni surface coverage for the second Ni layer growth, but this cannot be definitely concluded for the cluster size used in the present work.

The smaller influence of antibonding interactions for negative electrode charging (Ni OPD conditions) (Figs. 6 and 7) allows a complete Ni monolayer to be formed on the Pt ( 111) surface before the second Ni layer starts to grow.

CONCLUSIONS

The semiempirical analysis of the structures which represent stable Ni deposits onto Pt (111) cluster substrates grown under UPD allows the following conclusions to be drawn.

Underpotential Ni deposition conditions define different stable Ni layer configurations on Pt (111) for surface coverages smaller than that of the Ni monolayer, with the simultaneous existence of bare Pt sites. Molecular orbital calculations are consistent with the Ni deposition pathway previously concluded from experimental work. Basically, the pathway consists of an initial adsorption step followed by a phase transformation involving a nucleation and growth mechanism, which finally yields the stable Ni layer structure.

The growth of the second Ni layer begins before completion of the first Ni layer. This process is favoured because of the greater stabilization of a Ni atom on the first Ni layer than on the Pt substrate structure. According to the molecular-orbital scheme this fact is related to the lower stability of the Pt-Ni bond caused by the stronger antibonding interactions.

High positive UPD conditions increase the difference between Ni-Ni and Pt-Ni antibonding interaction energies. This fact facilitates the earlier appearance of the second Ni layer.

Otherwise, for Ni deposition under OPD conditions the opposite effect is found. In this case the calculations predict that a complete Ni monolayer is firstly formed, and later the subsequent bulk Ni deposition takes place.

These conclusions apply reasonably well to experimental data derived for the Pt-Ag and Pt-Cu bimetallic systems in acid electrolyte solutions. These bimetallic systems are structurally similar to the Pt-Ni system. Unfortunately for the latter no experimental results are available because Ni electrodeposits on Pt are produced in the potential range where the hydrogen evolution reaction takes place simultaneously.

ACKNOWLEDGEMENT

This research project was financially supported by the Consejo National de Investigaciones Cientificas y Tdcnicas of Argentina and the Comision de In- vestigaciones Cientificas de la Provincia de Buenos Aires.

364

REFERENCES

2 A.M. Kolb, in H. Gerischer and C.W. Tobias (Eds.), Advances in Electrochemistry and Elec- trochemical Engineering, Vol. 11, Wiley, New York, 1978.

2 K. Juttner and W.J. Lorentz, Z. Phys. Chem. N.F., 122 (1980) 163. 3 R.C. Salvarezza, D.V. Vdsquez Moll, M.C. Giordano and A.J. Arvia, J. Electroanal. Chem.,

213 (1986) 301. 4 A. Bewick and B. Thomas, J. Electroanal. Chem., 85 (1977) 329. 5 D. Margheritis, R.C. Saivarezza, M.C. Giordano and A.J. Arvia, J. Electroanal. Chem., 229

(1987) 327. 6 D.M. Kolb and R. Kotz, Surf. Sci., 64 (1977) 698. 7 W.J. Lorenz, E. Schmidt, G. Staikov and H. Bast, Faraday Symp. Chem. Sot., (1977) 15. 8 A.B. Anderson, J. Chem. Phys., 68 (1978) 4. 9 G.L. Estiu, S.A. Maluendes, E.A. Castro and A.J. Arvia, J. Phys. Chem., 92 (1968) 2512.

10 G.L. Estiri, S.A. Maluendes, E.A. Castro and A.J. Arvia, J. Electroanal. Chem., in press. 11 A.B. Anderson, Surf. Sci., 105 (1981) 159. 12 A.B. Kang and A.B. Anderson, J. Am. Chem. Sot., 107 (1985) 7858. 13 A.B. Anderson, J. Chem. Phys., 62 (1975) 1187. 14 A.B. Anderson, R.W. Grimes and S.Y. Hong, J. Phys. Chem., 91 (1987) 4242. 15 S.P. Mehandru and A.B. Anderson, Surf. Sci., 216 (1989) 105. 16 W. Lotz, J. Opt. Sot. Am., 60 (1970) 206. 17 C.E. Moore, Atomic Energy Levels, NBS Circular 467, National Bureau of Standards, US

Government Printing Office, Washington, DC, 1958. 18 E. Clementi and D.L. Raimondi, J. Chem. Phys., 38 (1963) 2686. 19 E. Clementi and C. Roetti, Atomic Data and Nuclear Data Tables, Vol. 14, Academic Press,

New York, 1974. 20 J.W. Richardson, W.C. Nieuwpoort, R.R. Powell and W.F. Edgell, J. Chem. Phys., 36 (1962)

1057. 21 H. Basch and H.B. Gray, Theor. Chim. Acta 4 (1966) 367. 22 L. Pauling, The Nature of the Chemical Bond, 3rd edn, Cornell University Press, Ithaca, NY,

1960. 23 Handbook of Chemistry and Physics, 55th edn., CRC Press, Cleveland, OH, 1973. 24 G.L. Estiu, S.A. Maluendes, E.A. Castro and A.J. Arvia, J. Electroanal. Chem., 283 (1990)

303. 25 A.B. Anderson and N.K. Ray, J. Phys. Chem., 86 (1982) 488. 26 A.B. Anderson and D.P. Onwood, Surf. Sci., 154 (1985) L261. 27 A.B. Anderson and M.K. Awad, J. Am. Chem. Sot., 107 (1985) 7854. 28 N.K. Ray and A.B. Anderson, Surf. Sci., 125 (1983) 803.

theoretical study of underpotential deposition of foreign metal atoms on pt(111) single-crystal...

Documents