Langmuir 1995,11, 3828-3844
3828
Adsorption Modes of Acetonitrile on Ni(11l),Ni(loo), and Ni(110). A Semiempirical Theoretical Study Bernard Bigot,” Frangoise Delbecq, and Vincent-Henri Peuch Laboratoire de Chimie Thkorique, Ecole Normale Supkrieure de Lyon, 46, allke d’ltalie, 69364 Lyon Cedex 07, France, and Institut de Recherches sur la Catalyse, CNRS, L P 5401, 2 avenue Albert Einstein, 69 626 Villeurbanne Cedex, France Received December 12, 1994. I n Final Form: June 28, 1995@
The features of several adsorption modes of acetonitrile on three selected faces of nickel have been investigated by means of extended Hiickel calculations on finite-size clusters. The present results are consistent with the available experimental data on this system. The face for the stablest adsorption is the open (110) one, on which CH&N preferentially adsorbs parallel to the surface with a binding energy greater than 30 kcabmol-l. On the (100) face, acetonitrile is less strongly bound in the same q 2 geometry. Lastly, on the (111)face, there is a competition between two geometries: one parallel to the surface and the other perpendicular. The underlying electronic effects have been thoroughly analyzed and explained by the balance between stabilizing two-electron interactions and repulsive four-electron ones.
I. Introduction The hydrogenation of nitriles is of great industrial importance in the field ofthe preparation of amines. Some of t h e most important applications a r e the hydrogenation of nitriles of fatty acids a n d the one of adiponitrile (1-6 hexadinitrile), which leads to hexamethylenediamine and, hence, to Nylon 66. However, byproducts, such a s secondary a n d tertiary amines or cyclized products, are often obtained besides the desired primary amines. A high selectivity toward primary amines can nevertheless be obtained by a suitable catalyst choice: Raney nickel is one of the most frequently used a n d h a s been thoroughly tested1 under various operating conditions a n d with several additives. The knowledge of the mechanism of the hydrogenation reaction is a key point when attempting to rationalize the formation of undesired products, in order to eliminate them or, at least, reduce them. This requires the investigation of the precise nature a n d geometry of the adsorbed species on the catalyst. The first to be considered are the nitrile reactants. Several experimental works have dealt with the adsorption modes of nitriles on well-defined surfaces: acrylonitrile and benzonitrile on C U ,acetonitrile ~ 0nAg(110),~ acetonitrile a n d benzonitrile on Au( and acetonitrile on W(100),5Pt(111),6Ni(lll),’b8 Ni(llO), Ni(loo), a n d a stepped s u r f a ~ e substituent ;~ effects on the Abstract published in Advance A C S Abstracts, September 1, 1995. (1)For a review, see: Volf, J.;Pasek, J.In Studies in Surface Science and Catalysis; Cerveny, L., Ed.; Elsevier: Amsterdam, 1986; Vol. 27, p 105. ( 2 ) (a) Hoo, B. H.; Kato, T. Surf. Sci. 1993,284, 167. (b) Kordesch, M. E.; Feng, W.; Stenzel, W.; Weaver, M.; Conrad, H. J . Electron. Spec. Rel. Phenom. 1987, 44, 149. (3) Bange, K.; McIntyre, R.; Sass, J. K.; Richardson, N. V. J . Electroanal. Chem. 1984, 178, 351. (4) (a) Solomun, T.; Christmann, K.; Baumgartel, H. J . Phys. Chem. 1989, 93, 7199. (b) Solomun, T.; Baumgartel, H.; Christmann, K. J . Phys. Chem. 1991, 95, 10041. (5) Friend, C. M.; Serafin, J. G. J . Chem. Phys. 1988, 88, 4037. (6) (a) Sexton, B. A.;Avery, N. R. Surf. Sci. 1983,129,21. (b)Avery, N. R.; Matheson, T. W. Surf. Sci. 1984, 143, 110. (c) Avery, N. R.; Matheson, T. W.; Sexton, B. A. Appl. Surf. Sci. 1985,22/23, 384. (d) Ou, E. C.; Young, P. A.; Norton, P. R. Surf Sci. 1992,277, 123. (7) (a) Hemminger, J. C.; Muetterties, E. L.; Somorjai, G. A. J . A m . Chem. Sot. 1979,101,62. (b)Friend, C . M.; Muetterties, E. L.; Gland, J. L. J . Phys. Chem. 1981, 85, 3256. (c) Kishi, K.; Okino, Y.; Fujimoto, Y. Surf. Sci. 1986, 176, 23. (8)Gardin, D. E.; Barbieri, A,; Batteas, J. D.; Van Hove, M. A,; Somorjai, G. A. Surf Sei. 1994, 304, 316. @
adsorption process on Ni( 111)have also been considered,1° as well a s adsorption on evaporated nickel a n d palladium films’l a n d on Raney nickel.12 I t is usually estimated t h a t nitriles have two possible adsorption geometries, either perpendicular to the surface through the nitrogen lone pair (171 modes) o r parallel to the surface through the ic systems (172 modes). The adsorption mode on a given well-defined surface can be experimentally determined by spectroscopic methods. However, the most favorable adsorption mode of acetonitrile on N i ( l l 1 ) is much debated: it may well be t h a t the two species simultaneously exist i n a ratio depending on the temperature; some authors favor the 171 mode,7a,b,s while others favor the 172 On Ni(llO), the situation is clearer: the adsorption is stronger t h a n the one on N i ( l l l ) , a n d some decomposition of the adsorbed species occurs, suggesting a n 172 adsorption mode.s On Ni(100), the intensity of the adsorption strength is intermediate between those on N i ( l l 1 ) and Ni(llO), a n d t h e c(2x 2 ) ordered structure experimentally observedg favors the hypothesis of acetonitrile being adsorbed perpendicularly to the surface in a 4-fold site. Finally, on evaporated nickel films a n d on Raney nickel, acetonitrile is preferentially 172 adsorbed.”’l2 In this context, a theoretical approach is necessary to rationalize the trends observed in the adsorption of acetonitrile on these various surfaces. Moreover, to the best of our knowledge, there is only one theoretical study published on the adsorption of CH3CN on N i ( l l 1 ) as a part of a more general survey of amine d e h y d r ~ g e n a t i o n . ’ ~ We will present in this work the results of theoretical semiempirical calculations on the adsorption of acetonitrile on N i ( l l l ) , Ni(1001, and N i ( l l 0 ) .
11. Theoretical Model The calculation procedure employed for this study is the same as the one described elsewhere14l6 for the study ofthe adsorption of aldehydes and alkenes o n platinum and palladium surfaces, (9) Wexler, R. M.; Muetterties, E. L. J . Phys. Chem. 1984,88,4037. (10) ( a )Kishi, K.; Ikeda, S. Surf. Sei. 1981,107,405. (b) Nakayama, T.; Inamura, K.; Inoue, S.; Ikeda, S.; Kishi, K. Surf Sci. 1987,179,47. (11)Hochard, F.; Jobic, H.; Clugnet, G.; Renouprez, A. Catal. Lett. 1993, 21, 381. (12) Friend, C. M.; Stein, J . ;Muetterties, E. L. J . A m . Chem. SOC. 1981, 103, 767. (13)Ditlevsen, P. D.; Gardin, D. E.; Van Hove, M. A,; Somorjai, G. A. Langmuir 1993, 9, 1500. (14) ( a ) Sautet, P.; Paul, J. F. Catal. Lett. 1991,9,245. (b) Delbecq, F.; Sautet, P. Catal. Lett. 1994, 28, 89.
0743-746319512411-3828$09.00/0 0 1995 American Chemical Society
Adsorption Modes of 'Acetonitrile on Ni Chart I
Langmuir, Vol. 11, No. 10, 1995 3829 Chart 2
cp
i
1
Y
Ni,-Ni,
and it will only be briefly recalled. Extended surfaces of a selected metal are modeled by finite-size clusters; a correction for the resulting edge effects is performed by dividing those clusters into two parts: a core containing all the atoms directly involved in the adsorption with their entire first-neighbor environment and an outer shell which ensures the electroneutrality of the core. For the three nickel faces studied here, clusters of 70 nickel atoms have been set up, the core containing 22 atoms in the case of Ni(ll1) and 21 atoms for Ni(100) and Ni(ll0); these clusters are shown on Chart 1. Extended Hiickel calculations have been performed on these clusters. The electronic parameters for nickel have been chosen in order to get a description of this metal consistent with that of platinum and palladium previously used by us;14J5particular attention was paid to the value of the Fermi level and of the d bandwidth (see Table 4). We have found that the parameters for carbon, hydrogen, and oxygen had to be shifted from their standard values in order to get correct electron transfer between the surface and the adsorbed molecules; the same energetical shift has been applied for nitrogen. Because ofthe large number of orbitals to be considered, a Gaussian broadening ofthe discrete energy spectrum has been applied in order to present the results in terms of density of states (DOS) and crystal orbital overlap population (COOP)curves, as currently done when dealing with periodic infinite solids. Since the EH method does not allow any interatomic distance optimization, fixed values for all distances have been used. The geometries of some 71 and 72 complexes of nitriles are known from X-ray spectroscopy. After comparison of a large number of values measured for the Ni-N distance, it appears that 1.90 A is reasonable for both the 71 and 72 forms;l6J7following the values of the van der Waals radii, we have chosen a slightly longer distance for Ni-C (2.00 A). The metal atoms in the clusters are in their bulk geometry (Ni-Ni = 2.49 A). The other distances are standard: C i N = 1.15 A, C-CHs = 1.475 A, and C-H = 1.09 A. A few additional calculations have been performed on the most strongly bonded 72 forms with Ni-N = Ni-C = 1.9 A since the X-ray structures for most ofthe 72 complexes ofnitriles exhibit the same bond length between the metal and the carbon or nitrogen atoms;'* furthermore, this point is confirmed by the experimental structure obtained by LEED on the Ni(ll1) surface.8 Several calculations were performed on each 72 adsorption geometry, which is subject to pyramidalization, in order to determine the degree of hybridization. A hybridization parameter ( h )was used to describe all the geometries intermediate between the sp ( h = 0; C=N = 1.15 A; NCC = 180") and the sp2 ( h = 1;C-N = 1.30 A; NCC = 120")states, the CN bond length and the NCC bond angle varying linearly with h. The best geometry determined this way can directly be compared to experimental observations.sJ8 Last, for all calculations reported in this article, the metallic surface is taken as the ( x y ) plane, the x axis coinciding with Nil-Niz, and adsorption occurs in the z > 0 direction. (15)(a) Delbecq, F.;Sautet, P. Langmuir 1993,9,197. (b) Delbecq, F.;Sautet, P. Surf. Sci. 1993,295,353. (c) Delbecq, F.;Sautet, P. J . Catal., in press. (16)Bassi, I. W.; Calcaterra, M. J . Organomet. Chem. 1976,110, 129. (17)Bassi, I. W.; Benedicenti, C.; Calcaterra, M.; Intrito, R.; Rucci, G.; Santini, C. J . Organomet. Chem. 1978,144, 225. (18)(a) Wright, T. C.; Wilkinson, G.; Motevalli, M.; Hursthouse, M. B. J . Chem. SOC., Dalton Trans. 1986,2017.(b) Barrera, J.;Sabat, M.; Harman, W. D. J . A m . Chem. SOC.1991,113,8178.(c) Chetcuti, P.A,; Knobler, C. B.; Hawthorne, M. F. Organometallics 1988,7, 650. (d) Garcia Alonso, F. J.;Garcia Sanz, M.; Riera, V.; Abril, A. A,;Tiripicchio, A,; Ugozzoli, F. Organometallics 1992,11, 801.
7
3
N=C,
tl2P2
(di-a)
N/, -hiz
7
3
-4
5
N=C Nil-Ni,
72P3 (3 -fold)
6
,CH3
111. Adsorption on Ni(ll1) The set of different 72 or 71 geometries we have considered for comparison of the adsorption properties are depicted and defined on Chart 2. The calculated electronic data for these seven adsorption modes are given in Table 1. The best 71 geometry is the hollow one, and the best 72 one is the 4-fold one. One may immediately note that the values obtained for the binding energies are somewhat larger than the experimental ones: 21 kcabmol-l for acetonitrile and 18kcalmol-l for eth~1amine.l~ This trend is inherent to the EH method; however, this method is perfectly suitable for the qualitative comparison of different adsorption modes and for the understanding of the underlying electronic processes, which is precisely what we aim at in this study. In our previous works dealing with the adsorption of unsaturated molecules on metal surfaces, we have distinguished two kinds of interactions: the stabilizing twoelectron and the destabilizing four-electron ones.14J5 The former are quantified by the sum of the absolute values of the electron transfers between the organic molecule and the cluster, either by electron donation or by backbonding, and they involve the frontier orbital interactions. The latter are related to the interactions between occupied orbitals on the molecule and filled states on the metal surface; from perturbation theory, they are qualitatively described by the square of the overlap (noted S2 throughout this work) between the occupied (19)Ditlevsen, P.D.; Gardin, D. E.; Van Hove, M. A,; Somojai, G. A. Langmuir 1993,9,1500.
3830 Langmuir, Vol. 11, No. 10, 1995
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Table 1. Bonding Characteristics of the Various Adsorption Modes of Acetonitrile on Ni(ll1)
bare cluster and free CH3CN BE,a kcal/mol h (hybridization) overlap population C-N Ni-N Ni-C net charge CH3CN Nil Ni2 Ni3 Ni4 ETb p(N) z(CN)
71 top 71 bridge 1 2
r]l
-24
-32
-28
hollow fcc 3
72 di-a
~72-z
r]z-3-fold fcc 6
4
5
-14 0.7
0.5
-18 0.8
-1
r]2-4-fOld 7 -26 1
1.67
1.67 0.39
1.63 0.28 x 2
1.60 0.24 x 3
1.29 0.34 0.36
1.42 0.17 0.12
1.20 0.32 0.32
0 -0.10 -0.10 -0.10 -0.10
0.33 0.56 -0.10 0.00 -0.10
0.42 0.25 0.25
0.48 0.20 0.20 0.18 0.00
-0.34 0.68 0.21 0.00 0.00
-0.11 0.77 -0.10 0.00 -0.10
-0.41 0.70 0.22 0.69 -0.10
-0.30 0.89 0.89 0.07 0.57
pJN) $0.28
+0.12 +0.10 f0.02 -0.34 -0.03 0.65 4.6
f0.30 +0.07 +0.13 -0.73 -0.20 1.47 8.0
p,(N) +0.40
0.00 -0.10
pz(N) f0.33 +0.37 n, ?cy
n*(CN)
x*, x*,
fO.O1 fO.O1 -0.02 -0.02 0.39 2.8
f0.38 fO.11
+0.09 +0.05
+0.11
-0.05
-0.08 -0.08 0.77 8.3
-0.07 0.63 6.1
n, n,
x*~ x*?
f0.05 f0.02 -0.65 -0.06 1.10 4.9
1.09 0.18 x 2 f 0.34 0.16 x 2 f 0.24
+ 0.07 + 0.19
nz n, JC*~ I*,
+0.12 +0.24 -0.83 -0.27 1.89 11.8
total' of abs values 102S2total Taken as the differenceE(c1uster + adsorbed CH3CN)- E(c1uster) -k(free CH3CN). Anegative value means a stabilization. Expressed in electrons. ET > 0 means that the orbital loses electrons and ET < 0 that the orbital gains electrons. The total is larger than the sum of contributions from p(N), n(CN), and n*(CN) because it takes all orbitals into account.
."
. I
Denrity of rtater
Denrity of rtatem
Density of staten
Figure 1. On-top geometry of Ni(ll1): DOS projected on s(Nil) (a) and dANid (b). Bare cluster: DOS projected on d,z(Nil) (c).
The dashed line corresponds to the Fermi level.
orbitals of the twointeracting parts. The binding energy (BE, see tables for definition) reflects the balance between these two types of interaction. However, the interaction of a filled orbital of the adsorbate and of the d band of nickel, which is not completely filled, divides into two contributions, as exposed on Chart 3, one stabilizing and the other repulsive. Hence, the stronger the stabilizing two-electron interactions, the larger the S2repulsive terms are. 1. 111 Modes. Not surprisingly, the total electron transfer increases with the coordination of the nitrogen with the surface: the lowest electron transfer is for the
on-top site, and the highest is for the hollow site. However, the overlap S2and the destabilizing four-electron interactions increase in the same trend, and a competition is thus expected. Considering the global effect, we find here that the attractive two-electron interactions prevail; this is related to the fact the d band of nickel is rather narrow: the opposite situation had been observed for the adsorption of formaldehyde on Pt(lll),15because platinum has a larger d bandwidth which gives a preeminent role to the repulsions. Amore detailed analysis of the two-electron interaction shows that the most important transfer is a donation from
Langmuir, Vol. 11, No. 10, 1995 3831
Adsorption Modes of Acetonitrile on Ni Chart 3
n
metal atom
molecule
the lone pair of nitrogen to the metal surface, leaving a positive charge on the adsorbate; surprisingly, the metal atoms directly involved in the adsorption are also positively charged (see further for an explanation). a. On-TopMode. The metal orbitals involved here are s, pz, and dz2, which have a substantial contribution along the z axis, as illustrated by the DOS curves of Figure l a and lb, where a peak correspondingto pz(N)in present in the DOS projected on s(Ni1) and dz2(Nil)(see Chart 2). The DOS projected on dz2(Nil),which was almost entirely below the Fermi level in the bare cluster (Figure IC),is now partly above the Fermi level (Figure lb): this means dz2(Nil)is destabilized to a large extent, resulting in a strong depopulation of this orbital (-0.76 e- compared with the situation in the bare cluster). The situation for p,(Nil) is opposite: it was empty and gains 0.14 e-. In the same time, the population of s(Ni1) doesn’t vary much. The global effect on the net molecular charge leads to a positive contribution in spite of the donation from the adsorbate. This can be explained in the following way: electrons are delocalized in the bulk metal, which acts as an electron reservoir for Nil. Nil, being in interaction with the adsorbate, is depleted because the interaction destabilizes some of its electronic level above the metal Fermi level (see Chart 4; for the sake of simplicity, the metal energy levels are taken as discrete). This point is further illustrated by the COOP curves of Figure 2: besides the bonding peak at low energy, the peak just above the Fermi level represents an in-phase interaction of pJN) with pz(Nil) and an out-of-phase interaction with d,z(Nil). The out-of-phase combination of pz(N) with dz2(Nil)is not totally destabilized above the Fermi level; the part which remains below is responsible for a repulsive four-electron interaction accompanying the stabilization. The electron loss of the d orbitals of the nickel atoms directly involved in the adsorption, discussed here, appears as a general feature and has been encountered in all the other adsorbed forms we have considered. b. Bridge and Hollow Modes. With the increase of the metallic coordination of the adsorbate and of the total Ni-N overlap population, ranging from the on-top position to the bridge and to the hollow positions, the electron donation from pz(N)only slightly increases; changes in the symmetry of the adsorption site allow different orbitals to interact, and for this last two modes, transfers involving the two n systems appear: 8-12 are the principal new in-phase or out-of-phase interactions between the nitrile system and the metal.
Antibondlng
Antibonding
Bondlng
Bonding
Figure2. On-top geometry of Ni( 111):COOP between pZ(Nil) and pANi) (a)and between d,z(Nil) and p,(N) (b). The dashed line corresponds to the Fermi level. n
8
9
11
10
12
These interactions can be precisely discussed from the COOP curves of Figure 3. Figure 3a (respectively 3d) presents the in-phase and out-of-phasemixing of the d, (d,2) orbitals of atoms Nil and Ni2. It can be pointed out that, in the bridge geometry, the in-phase combination interacts with pz(N) (see Figure 3b), while the out-of phase one (see 10,11,and Figure 3c and 3e) with ;rt,(CN). Last, Figure 3f presents the interaction of ny(CN)with dyz(Ni3)in the case of the hollow geometry. Owing to all these interactions, the n orbitals of CN are slightly depopulated, while the n* antibonding ones gain some electrons; this is consistent with the decrease in the CN overlap population (see in Table 1) in comparison to the on-top geometry. From the metallic cluster point ofview, Figure 3a clearly shows that, in the case of a bridge adsorption geometry, part of the bonding combination between d,(Nil) and d,(Ni2) is depopulated, being expelled above the Fermi level; in other words, the overlap population between Nil and Ni2 is lower when a molecule is adsorbed in a bridge position on the cluster than in the case of the bare cluster ( 0 . 0 4 ~0.09). s A similar situation is observed for Nil, Niz, and Ni3 in the case of an adsorption on a hollow site. Therefore, the increase in the number of metal atoms involved in the adsorption l‘esults in two competing effects: an increase of the surface-adsorbate bonding and a loss of bonding in the surface.20
Bigot et al.
3832 Langmuir, Vol. 11, No. 10, 1995
Antibonding
Bonding
Antibonding
Bonding
Antibonding
Bonding
Antibonding
Bonding
Antibonding
Bonding
Antibonding
Bonding
Figure 3. Bridge geometry on Ni(ll1): COOP between du(Nid and du(Nid (a),between d d N i d and PAN) (b), between du(Nid, n,(CN), and n*,(CN) (c),between d,Z(Nil) and dAN12) (d), and between d,z(Nil), n,(CN), and n*,(CN) (e). Hollow geometry: COOP between d,,(Nis), nY(CN),and n*,(CN) (0. The dashed line corresponds t o the Fermi level. 2. q2 Modes. As mentioned earlier, the acetonitrile molecule is hybridized in the 72 geometries; hence, the (20) (a) Hoffmann, R. In Solids and Surfaces: a Chemist's View of BondinginExtendedStructures, VCH: New York, 1988. (b)Hoffmann, R. Rev. Mod. Phys. 1988,60,601.
lone pair of nitrogen and the n system perpendicular to t h e surface mix together to form t h e orbitals depicted in 13. 14. and 16 and called n,(CN), p,(N), and n*,(CN), respectively. The n system parallel to the surface and called nJCN)
Adsorption Modes ,of Acetonitrile on Ni
13 n,CN
14 p,N
Langmuir, Vol. 11, No. 10,1995 3833
,
15 n;*,CN
and n*JCN) is not modified. Table 1shows that the most important contribution to the interaction with the cluster involves n*,(CN): this interaction is enhanced by hybridization, which results in a lowering of the n*,(CN) energy level. As well as in the case of other adsorption geometries, the more numerous the metal atoms involved, the more new interactions that have to be considered: both the electron transfer (ET) and the repulsive S2term increase. However, due to the fact the acetonitrile molecule’s hybridization varies and the deformation energies are different,it is impossible to compare directly the BE of the four 572 modes to the one of the 571 modes only with regard to the relative values of ET and S2. a. Di-a Mode. The main interactions involve p,(N) and, above all, n*,(CN), resulting in a global negative charge on the adsorbate (see Table 1). The electornic population of the two nickel atoms participating in the bonding is strongly modified; in terms of orbitals, compared to the situation in the bare cluster, the most affected are pJNi1) (+0.11 e-), dz2(Nil)(-0.66 e-), d,(Nil) (-0.11 e-), s(Ni2) (-0.08 e-), p,(Ni2) (+0.05 e-), d22(Ni2)(-0.18 e-), and d,(Ni2) (-0.15 e-). The DOS’s projected on nz(CN), p,(N), and n*,(CN), on the one hand, and on d,z(Nil), d,2(Ni21, and dxZ(Ni2),on the other hand, are presented on Figure 4. Figure 4a-c shows that the three major interacting orbitals of acetonitrile are shifted from their original positions in the free but pyramidalized molecule;this effect is more pronounced for p,(N) and n*,(CN) since they are delocalized to a large extent over the d band of nickel. Both dz2and d,, metal orbitals interact with n*,(CN) (see Figure 4d-f), but dz2(Nil)also interacts with the occupied orbitals p,(N) and nz(CN) of acetonitrile. It should also be noted that the s and pz orbitals of Nil and Ni2 also take part in the adsorption process, as was the case for the modes, but their DOS’s are not presented here. Some of the interactions we have just mentioned are illustrated by the COOP curves of Figure 5. Figure 5a,b shows a three-body interaction between d,z(Nil), nz(CN), and p,(N); this can be schematically explained by Chart 5. The lowest peak is due to the bonding { dANi1) n,(CN) PAN)} combination, and the second one is due to { dz2(Nil) - n,(CN) p,(N)}; a large portion of dz2(Nil)is destablized beyond the Fermi level. Figure 5c accounts for the bonding interaction between d,2(Nil) and n*,(CN); finally,Figure 5d illustrates a totally repulsive interaction (here between d,,(Ni2) and n,(CN), but it is exactly the same between d,(Nil) and n,(CN)). The shape of orbitals 13,14,and 15 explains why the better oriented p,(N) and n*,(CN) are more involved than the nz(CN)(see 16,17, and 18)and why hybridization is rather high ( h = 0.7) for this adsorption mode.
+
+
+
16
17
18
The global effect of these interactions is that part of the bonding {d,(Nil) d,(Ni2)} combination gets over the Fermi level, leading to a weaker bonding between the two
+
PXN x,CN
-
nickel atoms (their overlap population is 0.056 vs 0.090 in the bare cluster). b. nMode. This geometry leads to a weaker adsorption than the di-a one: the shape of p,(N) or a*,(CN), due to the hybridization, is much less adapted to the interaction with one metallic center than with two, as was the case just before. For the n geometry, nz(CN) has the best orientation (compare 19-22),and this is the reason why
acetonitrile is less hybridizied than in the di-a mode; the situation here is a compromise between a large deformation of n*,(CN) and an optimal overlap with the metal orbital of Nil. Compared to the di-a geometry, the contribution of n,(CN) to the electron transfer increases, while contributions of p,(N) and n*,(CN) decrease: the net effect on the total ET is a decrease (see Table 1). The most affected metal orbitals of Nil are again s (-0.11 e-), dz2(-0.58 e-), and d, (-0.24 e-), which corresponds to the interactions depicted in 19-22. The corresponding DOS’s and COOP’S are not presented in this article since they are very similar to those given for the di-a mode. c. q 2 %Fold Geometry. This adsorption mode is intermediate between the di-a (as far as Nil and Ni2 are concerned) and the n ones (asfar as Ni3 is concerned); the distances of interest are NilN = 1.9 Ni3N = 2.1 and NilC = Ni2C = 2.0 There is no surprise that the ET is large; the fact that the n, system and pJN) orbital, principally responsible for the adsorption in the two previous modes we have exposed, are not as well oriented to interact with the dz2 or d, orbitals of the three nickel atoms is balanced by the appearance of new contributions to the interaction. The first contribution deals with the interaction of n,(CN) and p,(N) with the dyzorbitals of Nil, Ni2, and Ni3; the second contribution involves the nysystem which is parallel to the surface (see ny(CN)and n*JCN) in Table 1). The metal orbitals most perturbed from the situation on a bare surface are d,n(Nil) (-0.36 e-), dz4Ni3) (-0.27 e-), d,(Ni3) (-0.17 e-), dJNi1) (-0.30 e-), and dJNi3) (-0.16 e-). Some of the interactions can be further analyzed by the DOS of Figure 6. The comparison of Figure 6a and 6b shows that n*,(CN) is less delocalized over the d band than n*,(CN), and, hence, is less involved in the bonding (see also Table 1). The DOS projected on dz2(Nil)(Figure 6c) indicates interactions with nJCN), n*JCN), and n*,(CN), and the one projected on dJNi1) (Figure 6d) indicates interactions with n,(CN), p,(N), and n*,(CN). In a similar way, Figure 6e and 6f respectively shows that dz2(Ni3)interacts with both ny(CN)and n*,(CN), while dJNi3) only interacts with n(CN). It should also be noted that d,(Nis) interacts with n*,(CN) and n*,(CN), but the DOS’s are not drawn here. These interactions are depicted in 23-27.
A.
A,
A,
3834 Langmuir, Vol. 11, No. 10, 1995
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1
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24 -
23
27
26
Last, just like in the case of the previous adsorption modes involving several metallic atoms, the overlap population between the three nickel atoms considered in the 72 3-fold geometry is lower than in the bare cluster: 0.06 for Nil-Nia and Ni2-Ni3 and 0.03 for Nil-Ni3. This is explained, for instance, in 25, where the bonding combinations {dJNil) dJNi3)) or {dyz(Ni2) dJNi3)) are pushed above the Fermi level because of interactions with n,(CN). d. q 2 4-Fold Geometry. For this geometry, the CN bond is along the y axis, perpendicular to the Nil-Nia direction. Therefore, the n systems are alongx andz, and 14 is now called p (N). The nitrogen and carbon atoms are at 1.9 and 2.0 respectively, from both Nil and Niz. The molecule is tilted in the (yz) plane by 9.7" (see Chart 1)in order to have a N-Ni4 distance of 1.9 A also; the resulting C-Ni3 distance is 2.16 A. In this configuration, both nitrogen and carbon interact with three nickel atoms, explaining both very high ET and S2(the largest of the seven structures presented in Table 1). Like in the case of the 72 %fold structure, both n systems are involved, nz more than n,. As far as the metal orbitals' population are concerned, the major electron transfers are the following: s(Ni1) and s(Ni2) (-0.12 e-), d,z(Nil) and dz2(Ni2)(-0.28 e-), dANi4) (-0.17 e-), d,(Nil) and d,,(Nia) (-0.31 e-), dJNi1) and dJNi2) (-0.18 e-), dy(Ni3)(-0.12 e-), dJNi4) (-0.38 e-). The DOS's projected on n*,(CN) and n*,(CN) are very similar to those obtained for the 72 %fold structure and are not presented here. The DOS projected on d,n(Nil) (or d,z(Niz)) shows that the main interaction occurs with n,(CN) (see Figure 7a), while on the contrary, d,(Nil) as well as d,(Ni2) mainly interact with n,(CN) (see Figure 7b). Figure 7c and 7d presents the interactions of dJNi3) and dJNi4) with pJN) and n*,(CN). These interactions are schematized in 2831. Consequently, the overlap population between the
+
+
A,
3
3
metal atoms involved in the adsorption is significantly reduced, especially between Nil and Ni2 (Nil-Ni2 = 0.012; Nil-NiB = 0.070; Nil-Nil = 0.040). In spite of a large S2value, the 72 4-fold structure is the stablest of all 72 forms; this behavior is radically different from that of formaldehyde on Pt(lll):15the modes with the highest metallic coordination are preferred by acetonitrile, while formaldehyde on platinum prefers the modes with the lowest coordination. There are two reasons. Firstly, acetonitrile has two n systems, and the highcoordination modes allow both systems to intervene, leading to large ET, while formaldehyde has only one n system and the interactions in the perpendicular direction are many repulsive four-electron ones. Secondly, the repulsiveness of the four-electron interactions is less on nickel than on platinum, because nickel has a smaller d bandwidth, and thus, these interactions do not prevail over the stabilizing two-electron ones. A satisfactory consideration is that the best 72 geometry we have found is in excellent agreement with the experimental result!.8 The hybridization observed by LEED (CN = 1.28 A; NCC = 123") corresponds to the optimized value of h = 1we have obtained (CN = 1.30 NCC = 120"). We may note that acetonitrile on Ni(ll1) behaves like acetylene, for which the same adsorption geometry has been found.21 3. Comparison between q1 and q 2 Geometries. As mentioned earlier in section 11, the EH method is not adequate for distance optimization; therefore, it is presently difficult to compare forms of different geometries. The point is the following: there is experimental evidence that shows that an interatomic distance may depend upon the adsorption mode; let us consider, for instance, CO adsorption for which the C-metal distance is longer for the bridge than for the on-top geometry.22Two reasonable attitudes can be adopted: either keep the metal-C and metal-N constant for all adsorption geometries or fit the geometry of the calculated structures to experimental values. We found the first solution is less likely to bias our aim (determine the trends concerning the hierarchy of different adsorption modes for acetonitrile), and we kept all distances fixed for all the calculations. However, we have made a few tentative calculations in order to see the quantitative importance of distance variations on the BEofound;for instance, with a longer Ni-N distance (2.0 A), the BE for the ~1 hollow mode decreases from 32 to 27 kcal-mol-l. Much in the same way, it appears experimentally that the Ni-C distance is often equal or even smaller than the Ni-N one in some 72 complexes of nitriles,8J8despite the inverse order of the atomic radii. All the 72 modes have been recalculated with Ni-C = Ni-N = 1.9 A. We find BE larger by 4-9 kcakmol-l, depending upon the mode; for the 72 4-fold, the new BE is as high as 32 kcal*mol-l. If we compare the best 71 mode (hollow)to the best 72 one (4-fold),we notice the BE of the former is in the 27-32 kcal*mol-l range, while that of the latter is in the 26-32 kcal*mol-l range. From these results, we conclude on a competition between the two modes; this is fully consistent with the fact that both modes have been experimentally detected in a ratio depending upon the temperature. In conclusion, there is a possible coexistence of two modes: 71 hollow and 72 4-fold, in agreement with experimental evidence. Moreover, the 72 geometry theoretically found is extremely close to the one measured by
A;
(21) Casalone, G.; Cattania, M. G.; Merati, F.; Simonetta, M. Surf. Sci. 1982, 120, 171. (22) (a)Sung, S.-S.; Hoffmann, R. J.Am. Chem. SOC.1985,107,578. (b) Wong, Y.-T.; Hoffmann, R. J . Phys. Chem. 1991,95, 859. (23) Christmann, K.; Schober, 0.;Ertl, G. J . Chem. Phys. 1974,60, 4719.
Langmuir, Vol. 11, No. 10, 1995 3835
Adsorption Modes of Acetonitrile on Ni
Denaity of atatea
Denrity of rtater
Denrity of rtater
Denrity of atatea
Denrity of atatea
Denrity of atater
Figure 4. Di-a geometry on Ni(ll1): DOS projected on n,(CN) 13 (a),p,(N) 14 (b), and n*,(CN) 15 (c) (the horizontal lines display the position of the corresponding orbitals in a free molecule of acetonitrile but with the same hybridization); DOS on dANid (d), d,z(Niz) (e), and d,(Niz) (0. The dashed line corresponds to the Fermi level.
LEED means. In the next sections, we will describe the adsorption modes of acetonitrile on Ni(ll0) and Ni(100) with less detail, since the orbitalar interactions are always of the type we have just exposed.
IV. Adsorption on Ni(ll0) In opposition to the (111)face, the (110) face of a fcc metal is an open face, presenting rows of metal atoms; these rows are not at a contact distance and allow the
Bigot et al.
3836 Langmuir, Vol. 11, No. 10, 1995
- 16 Antibonding
Bonding
'... I.......
L-
- 16
Antibonding
Bondlng
Antibonding
Bondlng
'......
- - - -..*
I Antibonding
L
Bonding
."
Figure 5. Di-o geometry on Ni(ll1): COOP between d,z(Nil) and n2CN 13 (a),between dANil) and px(N) (b), between d p (Nil)and n*,(CN) (c), and between dx2(Niz) and nz(CN)(d). The dashed line corresponds to the Fermi level.
rows of the second layer to be accessible for adsorption. Even if it is well-known that the (110) faces are easily reconstructed, we will only consider the unreconstructed face (see Chart 1). Because of this open geometry of the surface, a great variety of adsorption sites exist, and they are depicted in Chart 6 . Some of them are similar to some presented on the (111) face: top, bridge, di-a, and n. Due to the anisotropy ofthe surface, two n geometries can be distinguished, one along the atom rows (called nil) and the other perpendicular to them (called nl);like for ethylene14 and f~rmaldehyde,'~ the nl mode is significantly more stable than the nil one and will be the only one of the n modes considered in this study. Abridge structure between atoms ofadjacent rows can also exist (see 35,called the long bridge). In addition to the previous forms, some other modes similar to those studied on the face (111)are possible since one surface row and one row of the second layer just below it build a (111)step: we have considered a hollow site (36)and a form (39)similar to 7 on Ni(ll1)but with one nickel atom
less (Nid. Finally, some totally new structures involving the second layer have also been tested: 33 where acetonitrile is vl-bound to one atom and 40 where it is 72-bound to the same atom. For both of these forms, N and C are at close distances with the four Ni1,2,3,4 atoms, resulting in adsorbate-surface interactions. The electronic characteristics of these various modes are given in Table 2. The nl was found to be slightly stable (-6.0 kcal-mol-l) and is not reported in the table. The first remark is that the 72 5-fold geometry (40) is unambiguously the most stable, even more stable than the best form on Ni(ll1). This point is also in agreement with the experimental literature which attest that acetonitrile is more strongly bound on Ni( 110) than on Ni(1111,since it is not deplaced by CO or P(Me)3.9Moreover, it is reported that acetonitrile on Ni(ll0) yields 90% CH bond cleavage, suggesting a preferential adsorption parallel to the surface. The CO adsorption energy has been measured at roughtly 30 kcal*mol-l at low coverage on any ofthe three Ni surfaces:23adsorption of acetonitrile is below 30 kcal-mol-l on Ni(ll1) and above on Ni(ll0). 1. On-Top, Short Bridge, and Di-cr Geometries. For all these forms, with look-alike counterparts on Ni(1111,the BE form on Ni(ll0) is slightly larger than on Ni(ll1). The same trend had already been observed for ethylene and aldehydes, comparing Pt(ll1) and Pt(l10).14.15The comparison between the (111)and (110) faces of a d10 metal has already been carried out for p l a t i n ~ m ,and ' ~ similar explanations are valid for nickel. On a (110) face, each surface row atom has seven neighbors, while atoms on a (111)surface have nine. As a result, the d band of the surface row atoms of a (110) face is slightly narrower (see Figure 8a and 8b) and the spatial expansion of the d orbitals is smaller, leading to less overlapping with the possible adsorbate and reducing both two-electron and four-electron interactions. However, all d orbitals are not affected in the same way; especially there is much change for the orbitals which are highly involved in the bonding within the surface (dxz-y2 and dq), while changes are very small for the orbitals more dedicated to adsorption (d,z and dY2) (compare parts a-d of Figure 9.) Hence, the changes in the ET are only small when comparing similar forms on (110)and on (111). The key lies in the repulsion: up to now, we have only considered that repulsion arose from the interaction between occupied orbitals of the adsorbate and of the surface; repulsion can also occur between the adsorbate and some of the neighboring atoms, not directly involved in the adsorption, either by a direct through-space (TS) overlap or by an indirect through-bond (TB) overlap via the orbitals of the metal atoms taking part in the adsorption (see Charts 7 ) . These types of repulsive behavior have thoroughly been described for formaldehyde and acetone on ~ 1 a t i n u m . l ~ These four-electron repulsions due to metal atoms, not directly linked t o the adsorbate, give a large contribution to the total repulsion, and hence, the less coordinated the metal of the surface, the smaller is the repulsion. Comparable ET and reduced S2make (110)a better surface than (111)for adsorption. 2. f11Long-Bridge Geometry. In this structure (35), the molecule builds a bridge between Niz and Ni3 (Ni2N = Ni3N = 1.9A); the nitrogen atom is also at close distance from two nickel atoms of the underlying row (Ni5N = Ni6N = 2.3 A). The principal interaction of nitrogen with Niz and Ni3 is attributed to a side interaction with Nib and Ni6, as reflected by the four positive overlap population: 0.25 for N-Ni2 and N-Ni3; 0.06 for N-Nib and N-Nie. Since Niz and Ni3 are more distant than Nil and Nil and, hence, the overlap is weakened similarly as described in section V.2 (compare 55 and 561,the interaction of the
Adsorption Modes of Acetonitrile on Ni
Langmuir, Vol. 11, No. 10, 1995 3837
-Denrity of rtatea
Denrity of rtater
Denrity of rtster
a
-
-5 n
n
Y
Y
z
z R r
P
$
t
w
w
-10
-1
* PxN * ~g CN
- 16
-1
Denrity Of 8 h t a
Denrity of rtatm
Denrity of rtatea
Figure 6. 172 %foldgeometry on Ni(ll1): DOS projected on n*,(CN) (a),n*,(CN) (b),dp2(Nil)(c), dyz(Nil) (d),d,z(Nia) (e),and dyz(Nis) (0. The dashed line corresponds to the Fermi level.
nitrogen lone pair with this two atoms in the long-bridge geometry is slightly smaller than its interaction in the short-bridge one: the electron transfer is 0.31 e- compared to 0.36 e-. Even though the additional interactions with Nib and Ni6 make the global ET larger than in the short-
bridge geometry, the S2terms are increased even more and make the long-bridge form less stable than the shortbridge one. 3. Structures Involving the Underlying Rows. a. ql Hollow Mode. Acetonitrile lies in the hollow site
3838 Langmuir, Vol. 11, No. 10, 1995
Bigot et al. Chart 6
cp
lk?
(short bridge)
3
I
(long bridge)
Density of states
Density of statw
7
N=C,
~ i, tii,
Density of rtntes
Density of states
Figure 7. ~2 4-fold geometry on Ni( 111): DOS projected on d,z(Nil) (a), d,(Nil) (b), dyz(Ni3)(c),and dy2(Ni4)(d). The dashed line corresponds to the Fermi level.
formed by atoms Nil, Ni2, and Ni5, perpendicular to the plane formed by these three atoms (36). Due to the fact that the molecule is tilted in the (yz) plane, the combinations of orbitals involved in the adsorption are different from the one involved for 3 (see 9 or 12 (for example), where the molecule was perpendicular to (xy). In this case, the orbitals mainly involved are, in terms of ET, s(Ni1)and s(Ni2)(-0.08 e-), s(Ni5)(-0.13 e-), dxy(Nil)and dxy(Ni2)(-0.11 e-), and dzz(Ni5)(-0.36 e-) and dyz(Ni5) (+0.12 e-), as illustrated in 41-44.
41
42
35
43
44
One may immediately note that Ni5 has more contributions than Nil or Ni2. Being in the second layer, this atom
3
T2P2
(di-o)
38
has its complete coordination shell and, therefore, a larger d band than the first-layer atoms. As a result, both the two-electron and the four-electron interactions will be stronger. The DOS projected on Ni5 is presented in Figure 8c. The comparison of the dzzand dyzorbitals with the dyz orbital of Nis for the adsorption on (111)is significant: Figure loa-c shows that the DOS's ofthe two first orbitals are broader than that of the third. On top of this, dzz(Ni5) and dyz(Ni5)have a peak just below the Fermi level, which leads to even better ET with the adsorbate. , The dxy(Nil)on this face can directly be compared to the d,(Nil) on the (111)face for the same hollow adsorption mode: see the DOS curves on Figure 10d and 10e, which are almost identical, except for a peak near the Fermi level on the (110) face, resulting again in a better ET. All these facts considered, one can understand why the ET is much larger in the case of the hollow site on (110)than on (111): 0.90 vs 0.77. They also explain why the S2 contributions are larger. The global consequences is that the BE on the (110) face for the hollow geometry is only slightly larger than the one for the similar form on the (111)plane, and it is in competition with the short-bridge mode. b. q 2 %FoldMode. In this geometry (see 39), the N-C bond is perpendicular to NilaNi2, with NilN = Ni2N = 1.9 and NilC = Ni2C = 2.0 A, and the molecule is
Adsorption Modes of Acetonitrile on Ni
Langmuir, Vol. 11, No. 10, 1995 3839
Table 2. Bonding Characteristics of the Various Adsorption Modes of Acetonitrile on Ni(l1O)a Vl
top
32
BE, kcallmol
-26
h (hybridization) overlap population C-N Ni-N Ni-C
71 top 111 short ql hollow subjacent33 bridge34 36 -21
1.67 0.39
1.61 0.30
0.32
0.50
-33
+ 0.08 x 4
pJN) f 0 . 3 2 f0.36 Rx
xY
n*(CN)
R*,
x*?
fO.O1 fO.O1 -0.02 -0.02 0.38 2.1
+0.10 f0.13
-0.05 -0.06 0.69 7.0
total of abs values 102S2total The same footnotes as in Table 1 are valid.
72 %fold
37
39
- 17
-33
0.7
-27 0.2
1.63 0.29 x 2
1.57 0.25 x 3
1.30 0.33 0.36
1.45 0.14 x 2 0.13 x 2
0.41
0.48
-.0.33
0.33
+0.36 fO.ll fO.O1 -0.05 -0.05 0.59 4.9
+0.39 +0.13 +0.14 -0.10 -0.12 0.90 9.6
net charge CH3CN ET p(N) dCN)
q 2 di-a
p,(N) +0.28
py(N) +0.34
+0.05 zY +0.01 R*, -0.63 z * ~ -0.06 1.05 3.8
R,
R,
n, R*, R*,
+0.20 f0.14 -0.16 -0.20 1.06 10.1
qz &fold
40 -40 1 1.05 0.33 x 2 f 0.18 0.30 x 2 0.13
+ 0.30
+
-0.43
p,(N) $0.42 R,
x, R*=
x*,
+0.14 +0.32 -0.84 -0.51 2.26 11.7
Denaity of etatea Denalty of rtater Denrity of stater Figure 8. DOS projected on Nil in the bare (111) cluster (a), on Nil (first-layer atom) in the bare (110) cluster (b), and on Nis (second-layeratom) in the same bare (110) cluster (c).
tilted by 12.8" around this axis in order to reach Ni5N = 1.9 This geometry looks like the 72 4-fold one, already studied on (lll),except that only three metal atoms are directly involved here (instead of four) and the optimized hybridation parameter is smaller ( h= 0.2). This geometry makes it possible that all d orbitals are involved in the ET: everydorbitalloses0.10-0.13 e-, exceptford,,which loses 0.19 e-; they interact with the n, and n, systems, in the way described in 28 and 29. As far as the Ni5 atom is concerned, the main interacting orbital is again d,z, which loses 0.53 e-. The DOS projected on this orbital (Figure 11)helps us to understand that this orbital mainly interacts with the nitrogen lone pair (now directed along the y axis) and only very little with the unoccupied n* orbitals. Hence, the nature of the interaction is very close to the one for the on-top adsorption: this is consistent with both facts that the hybridization is found to be small on the one
A.
hand and on the other hand that the net charge on acetonitrile is positive like for all 71 adsorption modes and in contrast to the other 7 2 forms. The total ET is the same as for the di-ageometry but S2is significantly larger since more nickel atoms are directly involved. Nevertheless, the 72 %fold geometry is much more stable than the di-a one, beause of a smaller deformation energy, related to the weaker hybridization we have already mentioned. c. Top Subjacent Geometry (33). The nitrogen is 1.9 above Ni5 and, therefore, not far from the mean surface; as a result, it is close to the four Nil to NL atoms (2.25A). Like in the on-top geometry (32),the main orbital involved is dzz(Ni5),losing 0.5 e-. As exposed earlier, both the two- and four-electron interactions with Ni5 and the second layer atoms are enhanced; the ET from the nitrogen lone pair is 0.36 e-, while it is only 0.32 e- for the on-top mode. Moreover, the interactions ofthe nxandn, systems, which were very small in the on-top geometry, are greatly
1
Bigot et al.
3840 Langmuir, Vol. 11, No. 10, 1995
A.
Nil-C = Ni4-C = 2.0 The interaction with Nil to Ni4 is far more pronounced than in the previous case. Like in the n geometry described on Ni(111)(see section III.2.b), the main orbitals of Ni5 involved are s (-0.19 e-), dz2(-0.62 e-), and d, (-0.44 e-). The other major interacting orbitals are dZ2-,2(Ni2 or Ni3) (-0.28 e-), dX2-,2(Nil or Ni4) (-0.10 e-), dJNi2 or Ni3) (-0.17 e-), and d,,(Nil or Ni4) (-0.06 e-). This is no surprise, since the combinationof the d+,2 and dyzorbitals of the four Nil to Ni4 atoms provides an orbital suitable for interaction with acetonitrile as well as with the lone pair p,(N), as with the n*zand n*,(see Figure 12 and 45-47).
Denaity of stater
Density of states
--
-Denaity of states
For the sake of simplicity, only the orbitals of Nil and Ni2 are drawn in 45-47; the situation is symmetrical for Ni4 and Ni3. Because of these numerous strong interactions, the total ET is also the largest of all the structures studied; the S2are consequently very important, but the two-electron interactions prevail, and the q 2 &fold is the stablest of all forms. If we compare this geometry to the q 2 4-fold one on Ni(l l l ) , already studied, it is seen that the ET is greater since five nickel atoms are directly involved here, as opposed to four in the other case. Still, the S2terms are the same for both modes, making the 472 5-fold far more stable, in spite of the increase in the number of metal atoms involved; this is related to the fact that (110) is an open face, hence, less sensitive to repulsive interactions. In addition to this, when the CH3 conformation is such that one hydrogen atom is directed toward the surface, a positive overlap population of 0.06 is created between this atom and the nickel atom neighbors of Nil and Ni4. This accounts for the fact that a C-H bond can be broken from this adsorption mode, leading to a Ni-H bond; this is in full agreement with the experimentally observed decomposition of acetonitrile on this face. Like in the case of the 572 4-fold mode in Ni( 11l), we have recalculated this form but with NiC = Ni-N = 1.9 A; the binding energy is, of course, even larger: -42.0 kcalomol-l.
Density of stat-
Figure 9. DOS projected on d,z(Nil) (a) and on dyz(Nil)( c ) in the bare (111)cluster; DOS projected on dg(Ni1) (b) and on dyz(Nid(d) in the bare (110) cluster. Chart 7
increased due to the proximity with Nil to Ni4. However, the overlap population between the methyl group of the nitrile and each of the four first layer nickel atoms is negative, and the large ET is more than balanced by a larger S2term. As a result, the BE found for this mode is smaller than for the on-top geometry. d. q 2 5-Fold Mode. In this geometry (40), the acetonitrile molecule lies parallel to the atom rows and very close to the surface since it is n-bonded with the subjacent Ni5 (Ni5-N = 1.9 A; Ni5-C = 2.0 By a small 7.5" tilt of the NC bond in the yz plane, it is potsible simultaneously to achieve Ni2-N = Ni3-N = 1.9 A and
A).
V. Adsorption on Ni(100) A (100)face of a fcc metal is a square lattice where each metal atom has eight neighbors (four in the first layer and four in the second layer); therefore, the coordination of the surface atoms is intermediate between that on a (111)face and that on a (110)face. The various adsorption modes that can be considered are depicted on Chart 8. Some forms are identical to those found on the other faces: ql on top, ql bridge, q2 di-a, 572 n. The others are specific to this (100) face: ql 4-fold7di-a diagonal, and 72 4-fold. The bonding characteristics of all these geometries are given in Table 3. The most stable form is the q 2 4-fold7 where acetonitrile lies parallel to the surface in a quaternary site. Its BE is intermediate between those calculated for the most stable forms on Ni(ll1) and Ni(110), in agreement with the experimental result^.^ 1. On-Top, Bridge, and Di-a Geometries. The BE of these three modes is intermediate between those on Ni(ll1) and those on Ni(llO), on which they also exist. The total ET is roughly the same on Ni(100) and Ni(ll0) and a little smaller than on Ni( lll),while the repulsive S2term decreases from Ni(ll1) to Ni(100) and Ni(ll0). As already exposed in section IV. 1,a smaller coordination implies smaller overlaps with the molecule and, hence,
Adsorption Modes of Acetonitrile on Ni
Langmuir, Vol. 11, No. 10, 1995 3841
a
-6 n
%i
U
PB
Y
-to
-IC
. I
Denrity of rtater
Denrity of atate8
Denrity of a t a h a
-Denrity or r t e t e ~ Figure 10. DOS projected on d,z(Nis) (a)and on dyz(Ni5) (b) in the bare (110)cluster; DOS projected on dy,(Nis)(c) in the bare (111) cluster; DOS projected on d,(Nil) in the bare (110) cluster (d) and on d,(Nil) in the bare (111)cluster (e).
smaller two- and four-electron interactions: the relative order ofS2follows well the coordination; the two-electron interactions are less sensitive so they are almost identical on the two open faces (110) and (100). The orbitals involved, of course, are the same as on the other faces, and no additional comment will be made on these forms.
2. q14-FoldGeometry. The nitrogen atom is equally distant (1.9 A) from the four nickel atoms; as one could expect, both the total ET and the S2 contributions are high. The orbitals involved for each ofthe four interacting nickel atoms are s (-0.08 e-), d,z, d,,, and dyz(-0.04 e-), and d, (-0.10 e-). The participation of this last orbital
Bigot et al.
3842 Langmuir, Vol. 11, No. 10, 1995 Chart 8
cp
G
VlPl
Y Ni,-Nil
(top)
tllP2
49
(bridge)
1
TIP3 (4-fold)
Ni3-;Ni, \ Ni, ' \ Ni,
7
3
N=C
50
tl2k
51
(70
I
Ni,-Nil
7
3
N=C, Ni,
-Ni,
N Y c A H 3
Figure 11. d,z(Nis).
~2
Density of states 3-fold geometry on Ni(ll0): DOS projected on
V2P2
52
(di-o)
Ni3-:Ni, \ ' \ Ni, -Nil
tl2P2
Ni3-:Ni, \ ' \ Ni, -Nil
q2P4
(di-o diag)
53
54
(4-fold)
while the second does not interact with the nitrogen lone pair, as was the case for the hollow site on Ni(ll1). This difference is due to the shorter N surface distance, which yields smaller overlaps between p,(N) and the combinations of d,, and dyzorbitals of the nickel atoms; this point is illustrated in 55 and 56, drawn in the plane
n
n
N
Ni
Ni I
55 q hollow on Ni( 111)
N *"
Figure 12. d+2(Nil).
r,72
Density of states &fold geometry on Ni(ll0): DOS projected on
is surprising, but it is due to the short N surface distance which allows a nonnegligible overlap. In Figure 13, the DOS's projected on dXy(Ni1)and d,,(Nil) are compared: the first one is much more involved in the interaction as well with the nitrogen lone pair as with the .n,or nysystem,
ql
56 4-fold on Ni( 100)
Ni
57 q1 hollow on
58
Ni( 111)
ql 4-fold on Ni( 100)
containing both CN and Nil. The opposite occurs for the n systems, for which a smaller distance to the surface favors overlapping (see 57 and 58). However, the global effect is that the BE is smaller than what could be expected for a 4-fold site, and the bridge form is the stablest of the geometries on this face.
Adsorption Modes of Acetonitrile on Ni
Langmuir, Vol. 11, No. 10, 1995 3843
Table 3. Bonding Characteristics of the Various Adsorption Modes of Acetonitrile on Ni(lOO)=
71 top 48 BE, kcal/mol h (hybridization) overlap population C-N Ni-N Ni-C net charge CH3CN ET p(N) dCN)
5
n*(CN)
Jt*X
total of abs values 102S2total a
PAN) JtX
Jt*Y
71 bridge 49
71 4-fold 50
~2
di-a 52
72 di-a diag 53
-24
-31
-28
- 15 0.7
-18 0.8
1.67 0.39
1.64 0.29 x 2
1.55 0.21 x 4
1.29 0.33 0.36
1.23 0.38 0.34
0.32
0.40
0.42
-0.33
-0.38
+0.33 +0.01 +0.01 -0.02 -0.02 0.39 2.4
+0.36 +0.09 +0.04 -0.07 -0.04 0.59 5.1
+0.32 +0.21 +0.21 -0.10 -0.10 0.96 10.9
pJN) Jc,
Jc, Jt*z Jt*Y
+0.28 +0.05 +0.01 -0.63 -0.06 1.06 4.1
~
t
*
JC*,
4-fold 57
-35 0.9 1.12 0.30 x 2 0.32 x 2
+ 2 x 0.01 + 2 x 0.07
-0.57
p(N)
p(N) +0.37 zz nXy
~2
+0.03 +0.06 ~ -0.76 -0.11 1.35 5.9
JtZ JtX
Jt*z Jt*x
+0.37 +0.04 +0.19 -0.90 -0.32 1.87 9.0
The same footnotes as in Table 1 are valid. Table 4. Parameters Used for the Calculations
atom H C N
Ni
a
=
level
energy, eV
1s 2s 2P 2s 2p 4s 4P 3d
- 12.1
-19.9 -9.9 -24.5 -11.9 -7.95 -3.85 -10.05
l;2(bIa 1.3 1.625 1.625 1.950 1.950 2.100 2.100 6.785(0.6462)
N
2.359(0.5504)
Slater-type orbitals: R(r)= Cr" expi-&) for ns and np; R(r) + b exp(-5;2r) for nd.
CrTa exp(-5;lr)
Density of states
by NilC and Ni3N with the surface favor strong overlapping with the {d,-dyz} of Nil or Ni3 (see 58);this leads to better ET transfer for p(N) and n*,(see Table 3) than in the di-a geometry. The second n system, parallel to the surface, is in the diagonal plane and is therefore called nxy.Both nq and n*q also lead to better ET, due to the secondary interactions with Ni2 and Ni4,than nyand n*yin the di-a geometry. The larger ET, in part balanced by a large S2, make the ~2 di-a diagonal geometry slightly more stable than the di-a one. 4. 9 4 4-Fold Geometry. In this geometry (54), both the carbon and the nitrogen atoms have strong interactions with two nickel atoms, which explains that the total ET and the S2are the largest of all the studied forms. The orbitals involved in the bonding are principally dyz(Ni2)or dJNi3) (-0.35 e-), d,,(Ni2) or d,(Ni3) (-0.06 e-), dYz(Nid or dJNi4) (-0.14 e-), dxz(Nil)or dxz(Ni4)(-0.09 e-), or d,z(Niz) or d24Ni3)(-0.08 e-). The higher weight of the dyz metal orbitals favors the interaction with the adsorbate (see 60). This form turns out to be the stablest of all, due to the strong ET; there are no specific interactions, and this case will not be studied here in more detail.
Density of states
Figure 13. q 2 4-fold geometry on Ni(100): DOS projected on d,(Nil) (a) and on d,(Nil) (b).
3. 7 2 Di-a Diagonal Geometry. The nitrogen and the carbon atoms are linked to Nil and Ni3, respectively (see 53);they !re also at close distances from Ni2 and Ni4 (2.48and 2.43 A, respectively). The strongest interactions, of course, involve Nil and Ni3, but Ni2 and Ni4 both exhibit positive overlap with C (0.07) and N (0.01). The most perturbed orbitals are dz2(Ni3)(-0.17 e-), d,(Ni3) and dyz(Ni3)(-0.24 e- each), and d,(Nil) and dJNi1) (-0.13 e- each). The interactions are the same as those occurring for the di-a geometry (see 16-18), except that d, is replaced here by its combination with dyz,in order to be in the so-called diagonal plane (59). As in the case of the ~1 4-fold geometry, the carbon and nitrogen are close to the surface and the angles described
I
I
N-C
60
VI. Conclusion By means of extended Huckel type calculations on metallic clusters, the face dependence of the adsorption of acetonitrile on nickel has been studied. Our results are in agreement with the experimental evidence: the adsorption energy decreases in the order Ni( 1lo), Ni(100), and Ni(ll1). The stablest geometry is parallel to the
3844 Langmuir, Vol. 11, No. 10, 1995 surface (qz)forNi(llO)andNi(100). OnNi(lll),two forms compete: the one perpendicular and the other parallel to the surface. The interesting feature on this face is that the best 7 2 geometry we predict is very similar to that experimentally found by LEED means. The binding energies are slightly overestimated, but the trends are consistent with the experiments. The interpretation of the results lies on the balance between the stabilizing two-electron and the destabilizing four-electron interactions occuring between the surface and the adsorbate. The differences between the faces are not due to the two-electron interactions (ET) since their amount varies in opposition to the BEs. The face differences are essentially due to the four-electron interactions, which are very sensitive to the coordination of the metal surface atoms. High coordinated surfaces, as the close-packed (111)one, yield high repulsion, both through-space and through-bond; for lower coordinated atoms, this four-electron term is strongly reduced, due to the increase of the free space around the site (TS contribution goes down) and by the reduction of the TB repulsion, since these atoms have less neighbours. Moreover, nickel exhibits a remarkably narrow d bandz4among the transition metals, and the repulsive interactions play a less important role than for other metals such as (24) Norlander, P.;Holloway, S.;Norskov, J. K. Surf. Sci. 1984,136, 59.
Bigot et al. platinum or even palladium. As a result, on each single face, the preferential adsorption sites are, rather, those involving a large number of metal atoms in order to yield high ET. This tendency is pronounced with acetonitrile since it has two n systems; this was not the case for aldehydes and ketones on platinum, since repulsions prevail and they only have one n system. From Tables 1-3, one notices that the overlap population between C and N is strongly reduced in the 72 geometries of acetonitrile compared to the value in the free molecule, especially for the polycoordinated forms. In these forms, the C-N bond is weakened and, therefore, in adequate condition to react with hydrogen. A study of the coadsorption of acetonitrile and hydrogen on nickel surfaces is under progress in order to start on some hydrogenation processes.
Acknowledgment. This work has been supported by Rhhe-Poulenc, and we owe much to discussions with Dr. J . Jenck and Dr. M. Joucla, from R-P research center in Decines (France). J.J. and M.J. should be further acknowledged for suggesting this work. Finally, the Institut du Developpement et des Ressources en Informatique Scientifique (formerly Centre Inter-RBgional de Calcul Electronique) is acknowledged for a generous allocation of computer time. LA940975V
