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H atom interaction with a 14 atom nickel cluster
Surface Science 137 (1984) 491-505 North-Holland, Amsterdam
H ATOM INTERACTION F. RUETTE, Deportment Received
491
WITH A 14 ATOM NICKEL CLUSTER
G. BLYHOLDER
of Chemistry,
University
and John D. HEAD
ofArkansas, Fayetteville, Arkansas
72701, USA
14 June 1983
The MINDO/SR calculational procedure gives reasonable charge, d band, and orbital properties for a Ni,, cluster. The density of states more closely resembles that from ab-initio calculations than from Xa-SW calculations. The order of stability of H atoms on the symmetric sites on the 100 face of the Ni,, cluster is four center > two center > one center. The four center and two center sites differ by only a few kcal/mol in adsorption energy and the most stable central cluster site has the H atom displaced 0.25 A from a four center site. The energies of the orbitals containing a contribution from the hydrogen orbital correlate well with UPS spectra. The H atom is always negative by about 0.1 e and the magnitude of the charge is inversely proportional to the coordination number of bonding Ni atoms. This negative charge facilitates catalytic hydrogenation and CO methanation reactions on Ni and its variation with surface sites explains the larger work function change observed on the Ni(ll0) surface than on other Ni faces for hydrogen adsorption.
1. Introduction Because of the ubiquitour presence of hydrogen in industrially important catalytic reactions [l], there is great interest in the interaction of hydrogen with transition metals in general and with Ni because it is a widely used hydrogenation catalyst. Experimental techniques have produced results for LEED, Auger, UPS, XPS, TDS and work function changes for H atoms adsorbed on single crystal planes of Ni [2-61. Several theoretical papers have appeared for the interaction of H atoms with nickel clusters [7-131. These calculations have either used semi-empirical methods like extended Htickel or CNDO which are not well suited to comparing different adsorption sites, or used such small clusters (less than 10 metal atoms) that comparisons of different sites is questionable, or used incomplete valence shell basis sets so that site comparisons are again questionable. The aim of this paper is to examine H atom adsorption sites on a cluster that is large enough to make site comparisons meaningful with a calculational procedure that is parameterized to give both good bond energy and geometry and is rapid enough that many situations on medium sized clusters can be examined. Hydrogen adsorption is examined here not only because of its intrinsic enterest, but because a new method should establish its credibility on simple systems before being used for more complex 0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
492
F. Ruetre et al. / H atom interaction wrth I4 amm nickel cluster
systems. The ability of the calculational procedure to handle atom species NiH and NiH, has been established [14].
2. Calculational
the single nickel
procedure
The calculations were done with a semi-empirical SCF method, which is a modification of MIND0 referred to as MINDO/SR. The details of the method as well as its ability to handle a wide variety of compounds have been reported previously [15]. The MINDO/SR procedure was selected for this work because it can readily handle clusters of 12 to 20 transition metal atoms using all s, p and d valence shell orbitals and is parametrized to give both good bond energies and bond geometries. The computer program used is based on QCPE Program 290 By Rinaldi as modified by Schmidling [16] to incorporate MIND0/3 and vibrational calculations. The Rinaldi program .has automatic geometry optimization using analytically calculated gradients. The Schmidling version was modified to incorporate transition metals [15], symmetry [17], and selective molecular orbital filling.
Table
1
Atomic
parameters
Orbital exponents s
for nickel
[18]
1.4277 Core parameters
P
d
1.4277
4.2
(eV) [19]
K
WPP
W,,
- 145.56
- 100.64
- 196.36
Slater-
Condon parameters (3d.W
FO
21.92910
F2
9.87705
F4
6.03675
;:
a)
(eo) [18]
@Us)
PUP)
15.33855
10.89516
(4%4PJ
17.44470 a)
10.42169
1.26907
0.21538 0.14276
from ref. [19].
(4P.4P) 8.99313 2.63654
0.64840
G3 ‘) Modified
=)
(4s,4s)
2.38044
f? Ruette et al. / H arom interaction with 14 atom nickel cluster
493
3. Parameter selection Atomic parameters for Ni are given in table 1 and are the same as those used for NiH and NiH,. These atomic parameters are taken directly from literature values [l&19] with only minor modification in 3 values of F”, which were made to bring the ionization potential of the Ni,, cluster into agreement with the experimental work function of bulk Ni. These changes did not affect the determination of the ground state for NiH or for Ni,. It has previously been noted that rather drastic changes in one center integrals, which are directly related to the Slater-Condon parameters, lead to only minor changes in MIND0 results [20]. The Ni-H bond parameters, which are those used for NiH and NiH, [14], are given in table 2. These were selected to give the experimental bond length of 1.47 A [21] and dissociation energy of 70.8 keal/mol ]22] for NiH. The Ni-Ni bonding parameters were selected to give properties for Ni, in agreement with experimental values where available and with ab-initio calculations otherwise and to give reasonable properties for Ni,,. The bond energy has been determined experimentally as 54.5 kcal/mol 1231, but re-evaluation has suggested a revised value of 46 _t 5 kcal/mol [24]. The internuclear distance is unknown. Theoretical results from ab-initio calculations give internuclear distances 2.20 [25], 2.26 [24], 2.36 [26], 2.04 1271 and 2.18 A [2X]. These calculations also all give a triplet ground state with holes in 8, and 8s orbitals. The Ni parameters in table 2 give a Ni, molecule -with a dissociation energy of 46 kcal/mol, an equilibrium bond length of 2.2 A, and a triplet ground state with holes in 8, and 8s orbitals for most satisfactory agreement with available experimental and theoretical data. 3.1. Ni,,
cluster
The cluster selected is shown in fig. 1 and C,, has symmetry with the top face representing the Ni(100) surface. In fig. 1 the solid circles represent atoms in the top layer and the dashed line circles represent atoms in the bottom layer. All calculations were done with the Ni-Ni distance fixed at the 2.5 A value for bulk nickel. One of the advantages of this cluster is that the molecular orbitals
Table 2 Bonding parameters Bond
for Ni-H
and Ni-Ni
Parameters (Y
Ni-H Ni-Ni
1.212 1.191
0.278 0.991
PP
pd
0.598 0.991
0.278 0.991
F. Ruette
Fig. 1. Ni,, I. THCE.
et ui. / H atom
cluster. Sites: A, FC;
rnreruction
wrth 14 atom
B, TC; C, OC; D, TCE;
E, TCEl;
mckel
cluster
F, TCEZ;
G. OCE;
H, OCEl;
are all singly degenerate, so problems with symmetry breaking from partially filled degenerate orbitals are eliminated. After considering a variety of electronic configurations, the lowest energy state found filled different symmetry orbitals from the lowest Huckel orbitals and had a multiplicity of 17. The high multiplicity obtained is not unexpected, since the atomic magnetic moment of nickel is 0.6 Bohr magnetons per atom. The characteristics of the cluster may be analyzed in terms of the following properties: bond energy, atomic charge, ionization potential, bond order, and band width. The experimental cohesive energy, defined as the binding energy per Ni atom divided by the average number of bonds per Ni atoms, is 0.75 eV for bulk Ni, while the calculated value for this Ni,, cluster is 0.86 eV. The cohesive energy for the cluster is expected to be larger, because with each Ni atom forming fewer bonds, the electron density that can be channeled into each bond is greater. Because of the symmetry of the cluster, there are five different sets of equivalent atoms. In table 3 are shown the equivalent sets of atoms, the charge on each atom and the orbital populations. The atomic charges are small compared with extended Huckel calculations for a Ni,, cluster [7] with 0, symmetry, which gives + 2.5 e and - 0.21 e for the central and edge atoms. The average of the charges calculated by MINDO/SR for central and edge
Table Charge
3 and orbital populations
of equivalent
sets of atoms of the cluster Ni,,
1
I,5
- 0.103
2
2. 3, 6. 7
+ 0.059
3
4. 8
+ 0.056
4 5 ‘) See fig. 1
9. 10 11.12.13,14
- 0.101 +0.015
F. Ruette et al. / H atom interaction with 14 atom nickel cluster Table 4 Bond orders
in Ni,,
Ni atom number in cluster
495
and Ni,,H Ni 14 Ni-Ni bond orders for Ni atoms
Ni,,H
(four center site)
Ni-Ni bond orders for Ni atom
H-Ni
No. 1
No. 2
No. 1
No. 2
1
0.0
0.76
0.0
0.74
0.45
2
0.76
0.0
0.74
0.0
0.39
3
0.76
0.01
0.74
0.01
0.00
4
0.79
0.22
0.77
0.22
0.00
5
0.69
0.22
0.56
0.22
0.45
6
0.22
0.00
0.19
0.00
0.00
7
0.22
0.75
0.22
0.70
0.39
8
0.00
0.00
0.01
0.00
0.00
9
0.75
0.85
0.69
0.87
0.08
10
0.75
0.01
0.73
0.01
0.01
11 12 13 14
0.02 0.85 0.85 0.02
0.01 0.03 0.87 0.04
0.03 0.88 0.84 0.01
0.01 0.02 0.96 0.05
0.00 0.00 0.01 0.01
bond orders
atoms are - 0.102 and + 0.043 respectively. As expected, the charges on atoms in homonuclear systems are small. Ab-initio calculations also show the edge atoms of Ni clusters to be electron deficient [9]. These calculations also give the average of the atomic orbital populations for the central and the edge atoms as qs0.h74p0.313d8.83
[2fjj.
The calculated bond orders for atoms 1 and 2 are presented in table 4. Bond orders are very similar, between 0.69 and 0.87, for nearest neighbors. The next nearest neighbors make an appreciable contribution to bonding with bond orders of 0.22. The bond order between atoms in different layers (upper and lower layer) are slightly greater than between atoms in the same layer. In fig. 2 the energy levels of the cluster are presented. They are separated into sp, d and spd types for the (Y and /3 spin sets and are labeled with their symmetry. The total band width for d electrons is 4.6 eV. Experimental values from 3.3 to 3.0 eV are reported for bulk nickel [29,30]. The Xa-SW method [31] gives a band width of 2 eV using a cluster of 6 atoms, while ab-initio calculations [26] yield a value of about 6 eV for a Ni, cluster and band theory calculations give about 5 eV [32]. The d band for the cy and /3 sets have three separated groups of d levels. The lowest group of the cy set has 9 energy levels which are built up essentially of atomic orbitals from edge atoms with a low coordination number (equivalent atom sets 2, 3 and 5 in table 3). The highest orbitals are composed of atomic orbitals from high coordination number central atoms (equivalent atom sets 1
F. Ruette et crl. / H atom uawzction
496
wrth 14 atom mckel cluster
u
spd
sp
-
0
B
d
sp -1 -
b, b;
=
spd
d
b, b, a,
_____________________ _--_____ _______ ______________-----_----___.
-0
F
z-o F c3 Ei Z w -0s
-0.
-
bz
-
b,
-
b,
-
b,
-
aI
-0.
-
a,
-I* Fig. 2. Energy
levels in the a and p spin sets of Ni,,.
and 4). The intermediate group is constructed from both low and high coordination number atoms. The separation in energy of the molecular orbitals that contain d atomic orbitals is due to the electron repulsion of the d electrons
497
F. Ruerre et al. / H atom interaction with 14 atom nickel cluster
-0:s
4.7
-05
-0.3
-0.1
0.1
Energy (a. u.) Fig. 3. Comparison of energy levels from MINDO/SR ab-initio [26] and Xu-SW calculations [31].
calculations
with the density
of states from
in atoms with different coordination numbers. Edge atoms are positively charged, so their electronic repulsion is smaller than central atoms which are negatively charged. These results suggest that the d bandwidth of nickel clusters which have nickel atoms with diverse coordination numbers and charges is wider than the bulk. This separation into groups according to the contributions of the atomic orbitals to the molecular orbitals is not followed by the sp orbitals. The energy levels of Ni calculated by MINDO/SR and the density of states as assessed by the Xcy-SW method and ab-initio calculations are presented in fig. 3. MINDO/SR calculations more closely resemble the ab-initio calculation than the Xa-SW. In the MINDO/SR calculation, as in the ab-initio calculation, the top of the d band is somewhat below the top of the sp band. The ionization potential of a d orbital using Koopmans’ theorem is 6.9 eV, which is high compared to the experimental values for bulk Ni of 4.75 [33] and 5.2 eV [34]. However ab-initio calculations for nickel clusters give a relaxation energy for 3d orbitals of 2.0 eV [26]. If the ionization potential is corrected by the relaxation energy, then the resultant value, 4.9 eV, is in good agreement with
498
F. Ruette et al. / H atom interaction with I4 atom nickel cluster
experimental results. MINDO/SR produces an sp band which is too broad compared to a band theory calculation [35]. It has been suggested [36] that one possible explanation for a very large bandwidth in CNDO methods is the approximation of the overlap matrix by a unit matrix. MINDO/SR also uses this approximation of neglecting differential overlap in the secular equation. Overall the MINDO/SR calculations produce a reasonable picture of metal cluster levels which are in good agreement with ab-initio HartreeeFock results for Ni and in general accord with calculations for Ag clusters [37,38]. 3.2. Ni,,H A summary of H atom binding energies, charges, and bond distances for interaction at various sites on the 14 atom Ni cluster are given in table 5. The location of the H atom for the various sites is given in fig. 1. In all calculations the positions of the 14 Ni atoms are kept fixed. To minimize the energy and determine the equilibrium bond length the H atom was allowed to move only in a vertical line to preserve the site symmetry for each site. In table 5 the column labeled CN gives the average coordination number of the Ni atoms to which the H atom is directly bonded. In labeling the sites, OC indicates one center, i.e. a H atom directly over one Ni atom; TC stands for two center, i.e. a H atom in a bridge between two Ni atoms; FC indicates a four center site; THC is for a three center site; and E indicates a site at the edge of the cluster. For non-edge sites, which presumably best represent adsorption on plane surfaces, the order of stability is FC > TC > OC. Experimental results have been interpreted as favoring multicenter sites for H atoms on Ni [5,6]. Previous theoretical calculations have also favored a multicenter site. For the 14 atom cluster MINDO/SR calculates that the four center position is the most stable. The coordination number of the atoms which form a four center site in Ni,, is less than that for bulk Ni. Thus the four center site on an extended surface could be less stable than the two center site, because the difference in energy Table 5 Properties of an adsorbed hydrogen atom over different sites on the nickel cluster Active site
BE (kcal/mol)
Charge (H)
BD(NiH) (I\)
CN (site)
oc OCE OCEl TC TCE TCEl TCE2 FC THCE
-40.8 - 69.2 - 56.3 -63.3 -81.1 - 62.5 - 73.8 - 66.0 - 68.6
- 0.29 -0.38 -0.39 -0.12 -0.23 - 0.20 - 0.22 - 0.09 -0.19
1.60 1.55 1.55 1.70 1.67 1.70 1.70 1.95 1.90
8 3 4 8 4 6 5.5 6 5
F. Ruette et al. / H atom interaction with 14 atom nickel cluster
499
between them is only 2.7 kcal in the Ni,, calculation. That edge adsorption is favored over center adsorption for the Ni,, cluster is indicated by the binding energies given in table 5. For a given type of site, i.e. one center or two center, as the average coordination number of the Ni atoms comprising the site decreases, the H atom binding energy increases. The transfer of electron charge from the Ni atoms to hydrogen also increases as the average coordination number of the bonding Ni atoms decreases. This result has been observed before and suggests edge sites may be important in catalysis. The hydrogen atoms adsorbed on the TC and FC sites have a charge of -0.12 and -0.09 units respectively. The transfer of charge from the metal to the hydrogen implies a positive work function change upon hydrogen chemisorption, as found experimentally by Christmann et al. [2]. The transfer of negative charge is in the order OC > TC > FC. The maximum work function changes upon hydrogen adsorption on Ni 100, 111 and 110 faces are 0.170, 0.195 and 0.530 eV respectively, which has the remarkable feature that the work function change for the 100 face is almost 3 times the value for the other two faces. The H atom charges in table 5 indicate that, as the coordination numbers of the Ni atoms to which the H atom is directly bonded decrease, the H atom charge increases. The Ni 110 face is a relatively rough face having surface atoms with lower coordination numbers than the 100 and 111 faces, so its high work function change is attributed to a larger charge transfer to H atoms than occurs on the other faces. Table 6 shows the charges, their changes and the atomic orbital populations for the different equivalent sets of atoms in the nickel cluster when hydrogen is adsorbed in the four center position. For the central atoms (1 and 5) the electronic charge decreases, while for the edge atoms (2 and 7) the electronic charge increases. There is also a transfer of electronic charge from the side of the cluster away from the adsorbed H atom to the side on which the H atom is adsorbed. The transfer of charge from nickel to hydrogen is a clearly calculated effect, but the
Table 6 Charge distribution above atom 9)
and atomic orbital
populations
Equivalent set of atoms
Atom number
Charge,
1 2 3 4 5 6 7 8
1,5 336 2,7 13,14 12,11 4, 8 9 10
+ + + + -
Q
for Ni atoms in Ni,,
Change
Q(W4W-QW14) 0.046 0.067 0.003 0.009 0.038 0.065 0.101 0.075
+ 0.057 + 0.008 - 0.062 - 0.006 + 0.023 + 0.009 0.00 + 0.026
H (four center adsorption
Orbital populations s0.4*p0.64d*.98 s0.70P0.*3d8.99 s0.63pcl.39d8.98 ,o.5sp0.42dS.98
so.ssp0.41d8.98 s0.75p0.19d8.99 so.56p0.56d8 98 so.56p0.53d8.98
500
F. Ruette et al. / H aiom interaction with 14 atom nickel cluster
question of which Ni atoms donate how much charge is masked by the use of a 14 atom cluster, since the magnitude of the Ni atom charge changes are less than the magnitude of the Ni atom charges due to their position in the cluster. When the Ni-Ni bond orders for Ni,,H (four center site) are compared to those for Ni,, in table 4, small changes in the bond orders are observed. The smallness of the changes agrees with the results obtained by Christmann et al. [2] who conclude that no appreciable reconstruction of the metallic surface is generated when hydrogen is adsorbed on the surface. The majority of bonds in the cluster are slightly weakened, including those of Ni atoms that are not directly linked to the hydrogen. This suggests the existence of some long range interaction effects. The Ni-H bond orders for the FC hydrogen average 0.42 for the four Ni atoms directly bonded to the H atom and are negligible for all other Ni atoms except for the Ni atom directly below the FC site, in which case the bond order has the small value of 0.08. This Ni atom is 2.6 A from the adsorbed H atom compared to 1.95 A for the four main bonding Ni atoms, so its small but nonnegligible bonding role is in keeping with its distance. The optimized bond distances for hydrogen chemisorbed on four and two center sites on Ni are 1.95 and 1.70 A respectively. The experimental Ni-H bond distance on a (100) surface is unknown. The bond distance for Ni(lll)-H is experimentally found to be 1.84 A. Because I-I is adsorbed over Ni(ll1) in a three-fold site and the Ni-H bond distance increases with the hydrogen coordination number, the values of 1.70 A for two center and 1.95 A for four center sites are reasonable. In fig. 4 energy levels of Ni,,H for the (Yand the /3 sets are depicted. They again are classified as sp, spd or d. Those orbitals that have a significant contribution from the hydrogen have a letter H on their left side. Coefficients of hydrogen in the molecular orbitals that have an important contribution are shown below the levels. These results demonstrate that the bond is mainly with sp molecular orbitals, with some interaction with the d orbitals through the hybridized spd orbitals. Mulliken bond orders indicate that the contribution of the d orbitals to the bonding is negligible even when d orbitals appear in molecular orbitals with the H atom orbital, because of the small d orbital overlap. Comparison of orbital levels in figs. 2 and 4 indicates that H atom interaction leaves the d band virtually unchanged and slightly lowers some sp levels. Difference UPS spectra for H on Ni(ll1) show very broad hydrogen peaks at about 0.7 eV and about 6 eV below the Fermi level [3,4]. Ignoring relaxation effects, fig. 4 suggests hydrogen adsorption would produce one feature at about 0.7 eV below the Fermi level and a very broad feature with its maximum contribution about 7 eV below the Fermi level, for reasonable agreement with the experimental data. While strong d orbital involvement in H atom bonding to (111) surfaces of Ni, Pd and Pt has been suggested, the photoelectron spectra only support this for Pd and Pt, with the Ni spectra only showing a small involvement of d orbitals compared to Pd and Pt [39].
F. Ruette et al. / H atom interaction with 14 atom nickel cluster a
sp
0.
-0,
-
spd
B
d
H%r2
SP
=
Hz55
d
-
1 -
HiGT
Hrn
‘13 22
sod
H..,,
“0’29
-0
501
i2 g-0 W
Hsz -0
-0
-I Fig. 4. Orbital
HO”
HTiZ
energy levels for Ni,,H.
Comparison of tables 6 and 3 indicates that our calculations do not show any H atom induced changes in d orbital occupancy. The differences in energies between the adsorption sites suggest that the mobility of the hydrogen is through the four and two center sites. In order to study the mobility of hydrogen, calculations for several locations between the TC and FC sites were carried out with the height of the H atom above the surface optimized at each location as usual. A potential energy curve is shown
502
F. Ruette et 01. / H atom rnteraction with 14 atom nickel cluster
0 E -62.0. \ 0 &30-
TC
G L
-1-25
-I-O
-075
-0.50
-0.25
0.00
Distance Fig.
5. Potential
and
the four center
energy
of a hydrogen
atom
over
0.?5
050
(A)
the Ni,,
cluster
between
o-75
I.0
the two center
I.25
position
position.
in fig. 5. A potential barrier of only 3.8 kcal was found. This low activation energy allows easy mobility on the surface. The calculations indicate an energy minimum between the four and two center sites. This result is supported by Anderson [6] who, using LEED and EELS, concluded that the hydrogen atom is adsorbed on four center sites, but with some attraction toward two Ni atoms. Putting aside edge sites, the most stable position calculated for the H atom in the central region of the 100 face of the cluster is not a symmetric site, but is a site between the FC and TC sites. From an experimental point of view the most stable site is closest to the FC site and would most appropriately be described as a displaced FC site. However, in the calculated most stable position, the H atom is only 1.836 A from the two central Ni atoms while being 2.15 A from the other 2 Ni atoms of the original FC site. At these distances the bonding of the H atom is primarily due to the two central Ni atoms and so from a bonding point of view, is mostly at a two center site augmented substantially by interaction with two next nearest neighbor Ni atoms. It should be remembered that only one H atom is considered in these calculations and when interactions between adsorbed hydrogen atoms are considered, the relative stability of different sites could change. Cluster edge effects could also affect results. Comparisons may be made to similar calculations that have been done from
F. Ruette et al. / H mom interaction
with 14 atom nickel cluster
503
an Fe,, cluster with a 100 face [40]. One striking similarity is that on both Fe and Ni, the most stable central site is a TC site displaced far enough to look like a FC site. For both Fe and Ni the TC and FC sites are separated by only a few kcal/mol, so hydrogen is readily mobile on both surfaces. There is a distinct difference between Fe and Ni in the amount of charge transfer to hydrogen. On Fe the H atom is positive on a FC site, close to neutral on a TC site and negative on a OC site, while on Ni the H atom is always negative. Although the matter needs more attention, the high catalytic activity of Ni as a hydrogenation catalyst may be in part due to the negative charge on the adsorbed hydrogen facilitating nucleophilic attack on other adsorbed molecules. Preliminary calculations for adsorbed CO and CH, groups show a positive charge on the carbon atom. Facilitation of the addition of a H atom to an adsorbed hydrocarbon group would result in the desorption of a saturated hydrocarbon and a high catalytic activity for hydrogenation. Facilitating the hydrogenation of CO and CH, groups would result in a good methanation catalyst which Ni is found to be. Thus the differences in catalytic selectivity between Fe and Ni may be in part attributed to the differences in H atom charge calculated for adsorbed H atoms.
4. Conclusions The ~INDO/SR calculational procedure produces reasonable properties for a Ni,, cluster, although the sp band is too broad. The d band width of 4.6 eV, being greater than the experimental values around 3.3 eV, is traced to the finite cluster having different environments for the different Ni atoms, which affects the d levels. The ionization potential for d orbitals corrected for relaxation energy (4.9 eV) is in excellent agreement with experimental results (4.75 eV 1331 and 5.2 eV [34]). The MINDO/SR density of states for clusters more closely resembles densities from ‘ab-initio calculations for clusters than from Xa-SW cluster calculations. The order of stability of H on the symmetric sites is four center > two center > one center. The FC and TC sites differ by only a few kcal/mole in adsorption energy and the most stable central cluster site has the H atom displaced 0.25 A from a FC site, with the binding best described as a bridge bond between two Ni atoms augmented by a substantial interaction with 2 more Ni atoms. For a cluster the hydrogen adsorption is stronger on the edge atoms than on the central atoms. The bonding is due to H atom interaction with Ni s and p orbitals and only a negligible contribution from the d orbitals. The energies of the orbitals containing a contribution from the H atomic orbital correlate with the UPS spectra of chemisorbed hydrogen. The chemisorbed hydrogen is always calculated to have a negative charge of about 0.1 e on Ni, whereas on Fe the adsorbed H atom is closer to neutral.
504
F. Ruette et al. / H atom interaction with 14 atom m&d
duster
This negative charge on the adsorbed H atom is postulated to facilitate catalytic hydrogenation of hydrocarbons and CO methanat~on on Ni surfaces. The larger work function change for hydrogen adsorbed on the NijllO) than on the other Ni faces is due to the larger negative charge on these H atoms caused by their bonding to low coordination number Ni atoms on the 110 face.
Acknowledgement is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research, to the Venezuelan Government for a fellowship for F.R., to the University of Arkansas for a computing time grant and to the National Science Foundation for partial support under Grant CPE-8105472.
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F. Ruette et al. / H atom interaction with 14 atom nickel cluster
505
[28] J. Harris and R.O. Jones, J. Chem. Phys. 70 (1979) 830. [29] D.E. Eastman and J.K. Cashion, Phys. Rev. Letters 27 (1971) 1520. [30] S. Hufner, G.K. Wertheim, N.V. Smith and M.M. Traum, Solid State Commun. 11 (1972) 329. (311 N. Rosch and D. Menzel, Chem. Phys. 13 (1976) 243. [32] C. Wang and J. Callaway, Phys. Rev. B9 (1974) 4897. [33] R. Gerlach and T. Rhodin, in: The Structure and Chemistry of Solid Surfaces, Ed. G.A. Somojai (Wiley, New York, 1969). [34] A.A. Holscher, Surface Sci. 4 (1966) 891. [35] J.W. Connolly, Phys. Rev. 159 (1967) 415. [36] R.P. Messmer, B. McCarrol and C.M. Singal, J. Vacuum Sci. Technol. 9 (1972) 891. [37] J.D. Head, K.A.R. Mitchell and L. Noodleman, Surface Sci. 69 (1977) 714. [38] J.D. Head and K.A.R. Mitchell, Molecular Phys. 35 (1978) 1681. [39] W. Eberhardt, F. Greuter and E.W. Plummer, Phys. Rev. Letters 46 (1981) 1085. [40] G. Blyholder, J. head and F. Ruette, Surface Sci. 131 (1983) 403.
H ATOM INTERACTION F. RUETTE, Deportment Received
491
WITH A 14 ATOM NICKEL CLUSTER
G. BLYHOLDER
of Chemistry,
University
and John D. HEAD
ofArkansas, Fayetteville, Arkansas
72701, USA
14 June 1983
The MINDO/SR calculational procedure gives reasonable charge, d band, and orbital properties for a Ni,, cluster. The density of states more closely resembles that from ab-initio calculations than from Xa-SW calculations. The order of stability of H atoms on the symmetric sites on the 100 face of the Ni,, cluster is four center > two center > one center. The four center and two center sites differ by only a few kcal/mol in adsorption energy and the most stable central cluster site has the H atom displaced 0.25 A from a four center site. The energies of the orbitals containing a contribution from the hydrogen orbital correlate well with UPS spectra. The H atom is always negative by about 0.1 e and the magnitude of the charge is inversely proportional to the coordination number of bonding Ni atoms. This negative charge facilitates catalytic hydrogenation and CO methanation reactions on Ni and its variation with surface sites explains the larger work function change observed on the Ni(ll0) surface than on other Ni faces for hydrogen adsorption.
1. Introduction Because of the ubiquitour presence of hydrogen in industrially important catalytic reactions [l], there is great interest in the interaction of hydrogen with transition metals in general and with Ni because it is a widely used hydrogenation catalyst. Experimental techniques have produced results for LEED, Auger, UPS, XPS, TDS and work function changes for H atoms adsorbed on single crystal planes of Ni [2-61. Several theoretical papers have appeared for the interaction of H atoms with nickel clusters [7-131. These calculations have either used semi-empirical methods like extended Htickel or CNDO which are not well suited to comparing different adsorption sites, or used such small clusters (less than 10 metal atoms) that comparisons of different sites is questionable, or used incomplete valence shell basis sets so that site comparisons are again questionable. The aim of this paper is to examine H atom adsorption sites on a cluster that is large enough to make site comparisons meaningful with a calculational procedure that is parameterized to give both good bond energy and geometry and is rapid enough that many situations on medium sized clusters can be examined. Hydrogen adsorption is examined here not only because of its intrinsic enterest, but because a new method should establish its credibility on simple systems before being used for more complex 0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
492
F. Ruetre et al. / H atom interaction wrth I4 amm nickel cluster
systems. The ability of the calculational procedure to handle atom species NiH and NiH, has been established [14].
2. Calculational
the single nickel
procedure
The calculations were done with a semi-empirical SCF method, which is a modification of MIND0 referred to as MINDO/SR. The details of the method as well as its ability to handle a wide variety of compounds have been reported previously [15]. The MINDO/SR procedure was selected for this work because it can readily handle clusters of 12 to 20 transition metal atoms using all s, p and d valence shell orbitals and is parametrized to give both good bond energies and bond geometries. The computer program used is based on QCPE Program 290 By Rinaldi as modified by Schmidling [16] to incorporate MIND0/3 and vibrational calculations. The Rinaldi program .has automatic geometry optimization using analytically calculated gradients. The Schmidling version was modified to incorporate transition metals [15], symmetry [17], and selective molecular orbital filling.
Table
1
Atomic
parameters
Orbital exponents s
for nickel
[18]
1.4277 Core parameters
P
d
1.4277
4.2
(eV) [19]
K
WPP
W,,
- 145.56
- 100.64
- 196.36
Slater-
Condon parameters (3d.W
FO
21.92910
F2
9.87705
F4
6.03675
;:
a)
(eo) [18]
@Us)
PUP)
15.33855
10.89516
(4%4PJ
17.44470 a)
10.42169
1.26907
0.21538 0.14276
from ref. [19].
(4P.4P) 8.99313 2.63654
0.64840
G3 ‘) Modified
=)
(4s,4s)
2.38044
f? Ruette et al. / H arom interaction with 14 atom nickel cluster
493
3. Parameter selection Atomic parameters for Ni are given in table 1 and are the same as those used for NiH and NiH,. These atomic parameters are taken directly from literature values [l&19] with only minor modification in 3 values of F”, which were made to bring the ionization potential of the Ni,, cluster into agreement with the experimental work function of bulk Ni. These changes did not affect the determination of the ground state for NiH or for Ni,. It has previously been noted that rather drastic changes in one center integrals, which are directly related to the Slater-Condon parameters, lead to only minor changes in MIND0 results [20]. The Ni-H bond parameters, which are those used for NiH and NiH, [14], are given in table 2. These were selected to give the experimental bond length of 1.47 A [21] and dissociation energy of 70.8 keal/mol ]22] for NiH. The Ni-Ni bonding parameters were selected to give properties for Ni, in agreement with experimental values where available and with ab-initio calculations otherwise and to give reasonable properties for Ni,,. The bond energy has been determined experimentally as 54.5 kcal/mol 1231, but re-evaluation has suggested a revised value of 46 _t 5 kcal/mol [24]. The internuclear distance is unknown. Theoretical results from ab-initio calculations give internuclear distances 2.20 [25], 2.26 [24], 2.36 [26], 2.04 1271 and 2.18 A [2X]. These calculations also all give a triplet ground state with holes in 8, and 8s orbitals. The Ni parameters in table 2 give a Ni, molecule -with a dissociation energy of 46 kcal/mol, an equilibrium bond length of 2.2 A, and a triplet ground state with holes in 8, and 8s orbitals for most satisfactory agreement with available experimental and theoretical data. 3.1. Ni,,
cluster
The cluster selected is shown in fig. 1 and C,, has symmetry with the top face representing the Ni(100) surface. In fig. 1 the solid circles represent atoms in the top layer and the dashed line circles represent atoms in the bottom layer. All calculations were done with the Ni-Ni distance fixed at the 2.5 A value for bulk nickel. One of the advantages of this cluster is that the molecular orbitals
Table 2 Bonding parameters Bond
for Ni-H
and Ni-Ni
Parameters (Y
Ni-H Ni-Ni
1.212 1.191
0.278 0.991
PP
pd
0.598 0.991
0.278 0.991
F. Ruette
Fig. 1. Ni,, I. THCE.
et ui. / H atom
cluster. Sites: A, FC;
rnreruction
wrth 14 atom
B, TC; C, OC; D, TCE;
E, TCEl;
mckel
cluster
F, TCEZ;
G. OCE;
H, OCEl;
are all singly degenerate, so problems with symmetry breaking from partially filled degenerate orbitals are eliminated. After considering a variety of electronic configurations, the lowest energy state found filled different symmetry orbitals from the lowest Huckel orbitals and had a multiplicity of 17. The high multiplicity obtained is not unexpected, since the atomic magnetic moment of nickel is 0.6 Bohr magnetons per atom. The characteristics of the cluster may be analyzed in terms of the following properties: bond energy, atomic charge, ionization potential, bond order, and band width. The experimental cohesive energy, defined as the binding energy per Ni atom divided by the average number of bonds per Ni atoms, is 0.75 eV for bulk Ni, while the calculated value for this Ni,, cluster is 0.86 eV. The cohesive energy for the cluster is expected to be larger, because with each Ni atom forming fewer bonds, the electron density that can be channeled into each bond is greater. Because of the symmetry of the cluster, there are five different sets of equivalent atoms. In table 3 are shown the equivalent sets of atoms, the charge on each atom and the orbital populations. The atomic charges are small compared with extended Huckel calculations for a Ni,, cluster [7] with 0, symmetry, which gives + 2.5 e and - 0.21 e for the central and edge atoms. The average of the charges calculated by MINDO/SR for central and edge
Table Charge
3 and orbital populations
of equivalent
sets of atoms of the cluster Ni,,
1
I,5
- 0.103
2
2. 3, 6. 7
+ 0.059
3
4. 8
+ 0.056
4 5 ‘) See fig. 1
9. 10 11.12.13,14
- 0.101 +0.015
F. Ruette et al. / H atom interaction with 14 atom nickel cluster Table 4 Bond orders
in Ni,,
Ni atom number in cluster
495
and Ni,,H Ni 14 Ni-Ni bond orders for Ni atoms
Ni,,H
(four center site)
Ni-Ni bond orders for Ni atom
H-Ni
No. 1
No. 2
No. 1
No. 2
1
0.0
0.76
0.0
0.74
0.45
2
0.76
0.0
0.74
0.0
0.39
3
0.76
0.01
0.74
0.01
0.00
4
0.79
0.22
0.77
0.22
0.00
5
0.69
0.22
0.56
0.22
0.45
6
0.22
0.00
0.19
0.00
0.00
7
0.22
0.75
0.22
0.70
0.39
8
0.00
0.00
0.01
0.00
0.00
9
0.75
0.85
0.69
0.87
0.08
10
0.75
0.01
0.73
0.01
0.01
11 12 13 14
0.02 0.85 0.85 0.02
0.01 0.03 0.87 0.04
0.03 0.88 0.84 0.01
0.01 0.02 0.96 0.05
0.00 0.00 0.01 0.01
bond orders
atoms are - 0.102 and + 0.043 respectively. As expected, the charges on atoms in homonuclear systems are small. Ab-initio calculations also show the edge atoms of Ni clusters to be electron deficient [9]. These calculations also give the average of the atomic orbital populations for the central and the edge atoms as qs0.h74p0.313d8.83
[2fjj.
The calculated bond orders for atoms 1 and 2 are presented in table 4. Bond orders are very similar, between 0.69 and 0.87, for nearest neighbors. The next nearest neighbors make an appreciable contribution to bonding with bond orders of 0.22. The bond order between atoms in different layers (upper and lower layer) are slightly greater than between atoms in the same layer. In fig. 2 the energy levels of the cluster are presented. They are separated into sp, d and spd types for the (Y and /3 spin sets and are labeled with their symmetry. The total band width for d electrons is 4.6 eV. Experimental values from 3.3 to 3.0 eV are reported for bulk nickel [29,30]. The Xa-SW method [31] gives a band width of 2 eV using a cluster of 6 atoms, while ab-initio calculations [26] yield a value of about 6 eV for a Ni, cluster and band theory calculations give about 5 eV [32]. The d band for the cy and /3 sets have three separated groups of d levels. The lowest group of the cy set has 9 energy levels which are built up essentially of atomic orbitals from edge atoms with a low coordination number (equivalent atom sets 2, 3 and 5 in table 3). The highest orbitals are composed of atomic orbitals from high coordination number central atoms (equivalent atom sets 1
F. Ruette et crl. / H atom uawzction
496
wrth 14 atom mckel cluster
u
spd
sp
-
0
B
d
sp -1 -
b, b;
=
spd
d
b, b, a,
_____________________ _--_____ _______ ______________-----_----___.
-0
F
z-o F c3 Ei Z w -0s
-0.
-
bz
-
b,
-
b,
-
b,
-
aI
-0.
-
a,
-I* Fig. 2. Energy
levels in the a and p spin sets of Ni,,.
and 4). The intermediate group is constructed from both low and high coordination number atoms. The separation in energy of the molecular orbitals that contain d atomic orbitals is due to the electron repulsion of the d electrons
497
F. Ruerre et al. / H atom interaction with 14 atom nickel cluster
-0:s
4.7
-05
-0.3
-0.1
0.1
Energy (a. u.) Fig. 3. Comparison of energy levels from MINDO/SR ab-initio [26] and Xu-SW calculations [31].
calculations
with the density
of states from
in atoms with different coordination numbers. Edge atoms are positively charged, so their electronic repulsion is smaller than central atoms which are negatively charged. These results suggest that the d bandwidth of nickel clusters which have nickel atoms with diverse coordination numbers and charges is wider than the bulk. This separation into groups according to the contributions of the atomic orbitals to the molecular orbitals is not followed by the sp orbitals. The energy levels of Ni calculated by MINDO/SR and the density of states as assessed by the Xcy-SW method and ab-initio calculations are presented in fig. 3. MINDO/SR calculations more closely resemble the ab-initio calculation than the Xa-SW. In the MINDO/SR calculation, as in the ab-initio calculation, the top of the d band is somewhat below the top of the sp band. The ionization potential of a d orbital using Koopmans’ theorem is 6.9 eV, which is high compared to the experimental values for bulk Ni of 4.75 [33] and 5.2 eV [34]. However ab-initio calculations for nickel clusters give a relaxation energy for 3d orbitals of 2.0 eV [26]. If the ionization potential is corrected by the relaxation energy, then the resultant value, 4.9 eV, is in good agreement with
498
F. Ruette et al. / H atom interaction with I4 atom nickel cluster
experimental results. MINDO/SR produces an sp band which is too broad compared to a band theory calculation [35]. It has been suggested [36] that one possible explanation for a very large bandwidth in CNDO methods is the approximation of the overlap matrix by a unit matrix. MINDO/SR also uses this approximation of neglecting differential overlap in the secular equation. Overall the MINDO/SR calculations produce a reasonable picture of metal cluster levels which are in good agreement with ab-initio HartreeeFock results for Ni and in general accord with calculations for Ag clusters [37,38]. 3.2. Ni,,H A summary of H atom binding energies, charges, and bond distances for interaction at various sites on the 14 atom Ni cluster are given in table 5. The location of the H atom for the various sites is given in fig. 1. In all calculations the positions of the 14 Ni atoms are kept fixed. To minimize the energy and determine the equilibrium bond length the H atom was allowed to move only in a vertical line to preserve the site symmetry for each site. In table 5 the column labeled CN gives the average coordination number of the Ni atoms to which the H atom is directly bonded. In labeling the sites, OC indicates one center, i.e. a H atom directly over one Ni atom; TC stands for two center, i.e. a H atom in a bridge between two Ni atoms; FC indicates a four center site; THC is for a three center site; and E indicates a site at the edge of the cluster. For non-edge sites, which presumably best represent adsorption on plane surfaces, the order of stability is FC > TC > OC. Experimental results have been interpreted as favoring multicenter sites for H atoms on Ni [5,6]. Previous theoretical calculations have also favored a multicenter site. For the 14 atom cluster MINDO/SR calculates that the four center position is the most stable. The coordination number of the atoms which form a four center site in Ni,, is less than that for bulk Ni. Thus the four center site on an extended surface could be less stable than the two center site, because the difference in energy Table 5 Properties of an adsorbed hydrogen atom over different sites on the nickel cluster Active site
BE (kcal/mol)
Charge (H)
BD(NiH) (I\)
CN (site)
oc OCE OCEl TC TCE TCEl TCE2 FC THCE
-40.8 - 69.2 - 56.3 -63.3 -81.1 - 62.5 - 73.8 - 66.0 - 68.6
- 0.29 -0.38 -0.39 -0.12 -0.23 - 0.20 - 0.22 - 0.09 -0.19
1.60 1.55 1.55 1.70 1.67 1.70 1.70 1.95 1.90
8 3 4 8 4 6 5.5 6 5
F. Ruette et al. / H atom interaction with 14 atom nickel cluster
499
between them is only 2.7 kcal in the Ni,, calculation. That edge adsorption is favored over center adsorption for the Ni,, cluster is indicated by the binding energies given in table 5. For a given type of site, i.e. one center or two center, as the average coordination number of the Ni atoms comprising the site decreases, the H atom binding energy increases. The transfer of electron charge from the Ni atoms to hydrogen also increases as the average coordination number of the bonding Ni atoms decreases. This result has been observed before and suggests edge sites may be important in catalysis. The hydrogen atoms adsorbed on the TC and FC sites have a charge of -0.12 and -0.09 units respectively. The transfer of charge from the metal to the hydrogen implies a positive work function change upon hydrogen chemisorption, as found experimentally by Christmann et al. [2]. The transfer of negative charge is in the order OC > TC > FC. The maximum work function changes upon hydrogen adsorption on Ni 100, 111 and 110 faces are 0.170, 0.195 and 0.530 eV respectively, which has the remarkable feature that the work function change for the 100 face is almost 3 times the value for the other two faces. The H atom charges in table 5 indicate that, as the coordination numbers of the Ni atoms to which the H atom is directly bonded decrease, the H atom charge increases. The Ni 110 face is a relatively rough face having surface atoms with lower coordination numbers than the 100 and 111 faces, so its high work function change is attributed to a larger charge transfer to H atoms than occurs on the other faces. Table 6 shows the charges, their changes and the atomic orbital populations for the different equivalent sets of atoms in the nickel cluster when hydrogen is adsorbed in the four center position. For the central atoms (1 and 5) the electronic charge decreases, while for the edge atoms (2 and 7) the electronic charge increases. There is also a transfer of electronic charge from the side of the cluster away from the adsorbed H atom to the side on which the H atom is adsorbed. The transfer of charge from nickel to hydrogen is a clearly calculated effect, but the
Table 6 Charge distribution above atom 9)
and atomic orbital
populations
Equivalent set of atoms
Atom number
Charge,
1 2 3 4 5 6 7 8
1,5 336 2,7 13,14 12,11 4, 8 9 10
+ + + + -
Q
for Ni atoms in Ni,,
Change
Q(W4W-QW14) 0.046 0.067 0.003 0.009 0.038 0.065 0.101 0.075
+ 0.057 + 0.008 - 0.062 - 0.006 + 0.023 + 0.009 0.00 + 0.026
H (four center adsorption
Orbital populations s0.4*p0.64d*.98 s0.70P0.*3d8.99 s0.63pcl.39d8.98 ,o.5sp0.42dS.98
so.ssp0.41d8.98 s0.75p0.19d8.99 so.56p0.56d8 98 so.56p0.53d8.98
500
F. Ruette et al. / H aiom interaction with 14 atom nickel cluster
question of which Ni atoms donate how much charge is masked by the use of a 14 atom cluster, since the magnitude of the Ni atom charge changes are less than the magnitude of the Ni atom charges due to their position in the cluster. When the Ni-Ni bond orders for Ni,,H (four center site) are compared to those for Ni,, in table 4, small changes in the bond orders are observed. The smallness of the changes agrees with the results obtained by Christmann et al. [2] who conclude that no appreciable reconstruction of the metallic surface is generated when hydrogen is adsorbed on the surface. The majority of bonds in the cluster are slightly weakened, including those of Ni atoms that are not directly linked to the hydrogen. This suggests the existence of some long range interaction effects. The Ni-H bond orders for the FC hydrogen average 0.42 for the four Ni atoms directly bonded to the H atom and are negligible for all other Ni atoms except for the Ni atom directly below the FC site, in which case the bond order has the small value of 0.08. This Ni atom is 2.6 A from the adsorbed H atom compared to 1.95 A for the four main bonding Ni atoms, so its small but nonnegligible bonding role is in keeping with its distance. The optimized bond distances for hydrogen chemisorbed on four and two center sites on Ni are 1.95 and 1.70 A respectively. The experimental Ni-H bond distance on a (100) surface is unknown. The bond distance for Ni(lll)-H is experimentally found to be 1.84 A. Because I-I is adsorbed over Ni(ll1) in a three-fold site and the Ni-H bond distance increases with the hydrogen coordination number, the values of 1.70 A for two center and 1.95 A for four center sites are reasonable. In fig. 4 energy levels of Ni,,H for the (Yand the /3 sets are depicted. They again are classified as sp, spd or d. Those orbitals that have a significant contribution from the hydrogen have a letter H on their left side. Coefficients of hydrogen in the molecular orbitals that have an important contribution are shown below the levels. These results demonstrate that the bond is mainly with sp molecular orbitals, with some interaction with the d orbitals through the hybridized spd orbitals. Mulliken bond orders indicate that the contribution of the d orbitals to the bonding is negligible even when d orbitals appear in molecular orbitals with the H atom orbital, because of the small d orbital overlap. Comparison of orbital levels in figs. 2 and 4 indicates that H atom interaction leaves the d band virtually unchanged and slightly lowers some sp levels. Difference UPS spectra for H on Ni(ll1) show very broad hydrogen peaks at about 0.7 eV and about 6 eV below the Fermi level [3,4]. Ignoring relaxation effects, fig. 4 suggests hydrogen adsorption would produce one feature at about 0.7 eV below the Fermi level and a very broad feature with its maximum contribution about 7 eV below the Fermi level, for reasonable agreement with the experimental data. While strong d orbital involvement in H atom bonding to (111) surfaces of Ni, Pd and Pt has been suggested, the photoelectron spectra only support this for Pd and Pt, with the Ni spectra only showing a small involvement of d orbitals compared to Pd and Pt [39].
F. Ruette et al. / H atom interaction with 14 atom nickel cluster a
sp
0.
-0,
-
spd
B
d
H%r2
SP
=
Hz55
d
-
1 -
HiGT
Hrn
‘13 22
sod
H..,,
“0’29
-0
501
i2 g-0 W
Hsz -0
-0
-I Fig. 4. Orbital
HO”
HTiZ
energy levels for Ni,,H.
Comparison of tables 6 and 3 indicates that our calculations do not show any H atom induced changes in d orbital occupancy. The differences in energies between the adsorption sites suggest that the mobility of the hydrogen is through the four and two center sites. In order to study the mobility of hydrogen, calculations for several locations between the TC and FC sites were carried out with the height of the H atom above the surface optimized at each location as usual. A potential energy curve is shown
502
F. Ruette et 01. / H atom rnteraction with 14 atom nickel cluster
0 E -62.0. \ 0 &30-
TC
G L
-1-25
-I-O
-075
-0.50
-0.25
0.00
Distance Fig.
5. Potential
and
the four center
energy
of a hydrogen
atom
over
0.?5
050
(A)
the Ni,,
cluster
between
o-75
I.0
the two center
I.25
position
position.
in fig. 5. A potential barrier of only 3.8 kcal was found. This low activation energy allows easy mobility on the surface. The calculations indicate an energy minimum between the four and two center sites. This result is supported by Anderson [6] who, using LEED and EELS, concluded that the hydrogen atom is adsorbed on four center sites, but with some attraction toward two Ni atoms. Putting aside edge sites, the most stable position calculated for the H atom in the central region of the 100 face of the cluster is not a symmetric site, but is a site between the FC and TC sites. From an experimental point of view the most stable site is closest to the FC site and would most appropriately be described as a displaced FC site. However, in the calculated most stable position, the H atom is only 1.836 A from the two central Ni atoms while being 2.15 A from the other 2 Ni atoms of the original FC site. At these distances the bonding of the H atom is primarily due to the two central Ni atoms and so from a bonding point of view, is mostly at a two center site augmented substantially by interaction with two next nearest neighbor Ni atoms. It should be remembered that only one H atom is considered in these calculations and when interactions between adsorbed hydrogen atoms are considered, the relative stability of different sites could change. Cluster edge effects could also affect results. Comparisons may be made to similar calculations that have been done from
F. Ruette et al. / H mom interaction
with 14 atom nickel cluster
503
an Fe,, cluster with a 100 face [40]. One striking similarity is that on both Fe and Ni, the most stable central site is a TC site displaced far enough to look like a FC site. For both Fe and Ni the TC and FC sites are separated by only a few kcal/mol, so hydrogen is readily mobile on both surfaces. There is a distinct difference between Fe and Ni in the amount of charge transfer to hydrogen. On Fe the H atom is positive on a FC site, close to neutral on a TC site and negative on a OC site, while on Ni the H atom is always negative. Although the matter needs more attention, the high catalytic activity of Ni as a hydrogenation catalyst may be in part due to the negative charge on the adsorbed hydrogen facilitating nucleophilic attack on other adsorbed molecules. Preliminary calculations for adsorbed CO and CH, groups show a positive charge on the carbon atom. Facilitation of the addition of a H atom to an adsorbed hydrocarbon group would result in the desorption of a saturated hydrocarbon and a high catalytic activity for hydrogenation. Facilitating the hydrogenation of CO and CH, groups would result in a good methanation catalyst which Ni is found to be. Thus the differences in catalytic selectivity between Fe and Ni may be in part attributed to the differences in H atom charge calculated for adsorbed H atoms.
4. Conclusions The ~INDO/SR calculational procedure produces reasonable properties for a Ni,, cluster, although the sp band is too broad. The d band width of 4.6 eV, being greater than the experimental values around 3.3 eV, is traced to the finite cluster having different environments for the different Ni atoms, which affects the d levels. The ionization potential for d orbitals corrected for relaxation energy (4.9 eV) is in excellent agreement with experimental results (4.75 eV 1331 and 5.2 eV [34]). The MINDO/SR density of states for clusters more closely resembles densities from ‘ab-initio calculations for clusters than from Xa-SW cluster calculations. The order of stability of H on the symmetric sites is four center > two center > one center. The FC and TC sites differ by only a few kcal/mole in adsorption energy and the most stable central cluster site has the H atom displaced 0.25 A from a FC site, with the binding best described as a bridge bond between two Ni atoms augmented by a substantial interaction with 2 more Ni atoms. For a cluster the hydrogen adsorption is stronger on the edge atoms than on the central atoms. The bonding is due to H atom interaction with Ni s and p orbitals and only a negligible contribution from the d orbitals. The energies of the orbitals containing a contribution from the H atomic orbital correlate with the UPS spectra of chemisorbed hydrogen. The chemisorbed hydrogen is always calculated to have a negative charge of about 0.1 e on Ni, whereas on Fe the adsorbed H atom is closer to neutral.
504
F. Ruette et al. / H atom interaction with 14 atom m&d
duster
This negative charge on the adsorbed H atom is postulated to facilitate catalytic hydrogenation of hydrocarbons and CO methanat~on on Ni surfaces. The larger work function change for hydrogen adsorbed on the NijllO) than on the other Ni faces is due to the larger negative charge on these H atoms caused by their bonding to low coordination number Ni atoms on the 110 face.
Acknowledgement is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research, to the Venezuelan Government for a fellowship for F.R., to the University of Arkansas for a computing time grant and to the National Science Foundation for partial support under Grant CPE-8105472.
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