J . Phys. Chem. 1991,95,4795-4800
4795
Pseudopotentlal Multlreference Single and Double Excited Configuration Interaction Calculations of NlckeCContalnlng Molecules. 4. HNiSIH, and HNiAlH, as Minimum Models of H Chemisorption on a Supported NI Atom Helmut Haberlandt, Friedrich Ritschl, Central Institute of Physical Chemistry, Rudower Chaussee 5, I I99 Berlin, Germany
and Gianfranco Pacchioni* Dipartimento di Chimica Inorganica e Metallorganica, Centro CNR, Universitci di Milano, via Venezian 21, I-20133 Milano. Italy (Received: September 21, 1990)
The chemisorption of atomic H on a Ni atom bound to SiH3or AIH, supporting fragments has been investigated by means of pseudopotential CI calculations. The two systems investigated, H-Ni-SiH, and H-Ni-AIH3, can be considered as “minimum” models of strong (SMSI) and weak (WMSI) metalsupport interactions, respectively. It is found that the presence on the SiH, support of a singly occupied dangling bond, not present on AIH,, is the key factor determining the strong metalsupport interaction. By this interaction, the Ni atom binds the adsorbed H atom less strongly than in the free NiH molecule. The chemisorption of H decreases also the Nisubstrate bond strength. This result, however, is found only after inclusion of correlation effects. Uncorrelated calculations show the opposite trend, that is, a reinforcement of both Ni-H and Nisupport bonds by H chemisorption. This contradictory result is probably the consequence of the different SCF and CI energy separation between the Ni 3d84s2and 3d94s’ configurations. The formation of two covalent bonds, as in H-Ni-SiH3, implies that the metal atom assuma a 3dn4s2-likeconfiguration more suitable to form directional 4sp hybrid orbitals. The case of Ni is compared with the limiting situations of Fe (3d64s2)and Cu (3d1°4s’) to generalize the conclusions about the electronic mechanisms of SMSI. For the model of WMSI, we found that in our system the interaction of a supported Ni with the approaching H atom induces the rupture of the Ni-substrate bond and the formation of a Ni-H molecule which easily desorbs from the surface.
Introduction It is a well-known fact and a subject of continuing fundamental as well as applied research1V2 that activity and selectivity of heterogeneous catalytic reactions depend on the nature of the support which can induce geometric as well as electronic effects. The electronic and physicochemical properties of the catalyst, including the chemisorption behavior, may be affected by the support. Bond3 has proposed a classification of the support effects as weak metalsupport interaction (WMSI), medium metalsupport interaction (MMSI), and strong metal-support interaction (SMSI).The so-called SMSI is usually defined in terms of strong decrease of H2 and CO chemisorption bond on transition metals supported on “reducible” oxide systems after high-temperature reduction in hydrogen a t m ~ s p h e r e . ~The SMSI has been found meanwhile also with transition metals supported on ‘nonreducible” oxides (e.g., Si02)‘ after reduction in hydrogen at temperatures near 1100 K. The electronic effects which are assumed to be the origin of this change in chemisorptive behavior are the subject of intensive debate (cf. refs 5-8 and references therein). Recently, we have derived several working hypotheses on the electronic effect that the supporting oxides SO2,Ti02, and A1203have on the properties of Ni atoms interacting with selected surface sites on these carriers and on the chemisorption of H atoms and CO molecules on top of NieS-* These considerations were based upon the results of (1) Burch, R. In Hydrogen Eflects in Catalysis. Fundamental and Practical Applications; Paal, 2..Ed.; Marcel Dekker: New York, 1988; p 347. (2) Imelik, B., Naccache, G., Coudurier, G., Praliaud, H., Meriaudeau, P., Gallezot, P., Martin, G. A., Vedrine, J. C., Eds. Metal-Supporr and Metal-Additive Effccrs in Catalysis; Elsevier: Amsterdam, 1982. (3) Bond, G. C. In ref 2, p 1. (4) Praliaud. H.; Martin, 0. A. J . Catal. 1981, 72, 304. (5) Haberlandt, H.; Ritschl, F. J . Phys. Chem. 1983, 87, 3244. (6) Haberlandt, H.; Ritschl, F. J . Phys. Chem. 1986, 90,4322. (7) Haberlandt, H.; Schle3inger, R.; Ritschl. F. In Proceedings ofrhe 6th International Symposium on Heterogeneous Catalysis;Shopov, D.,Andrew, A., Palazov, A., Petrov, L.,Eds.;Bulgarian Academy of Sciences: Sofia, 1987; Vol. 1, p 324.
0022-3654/91/2095-4795%02.50/0
semiempirical CNDO/2 calculations on the adsorption of atomic H on a single Ni atom supported on small oxide cluster models.” The “interfacial” distance between the Ni atom and the surface site was estimated from atomic radii, and the Ni-H distance was, with some exceptions, kept a t a constant value. These working hypotheses can be summarized as follows. (a) An electron transfer to the metal is found for the interaction of nickel with surface sites on reduced support surfaces. The direction of the electron transfer is reversed for nickel on completely oxidized surfaces. Its amount can be roughly correlated with the ‘electronegativity of the clean surface sites”. (b) The metalsupport bond strength depends on the surface site; it is strong for SMSI and intermediate for WMSI models. (c) The binding energies of H and CO are lowered for SMSI models with respect to their isomorphous WMSI counterparts. These changes are connected with a weakening of both the metal-adsorbate and metalsupport bond strengths. This weakening is more pronounced for SMSI models, resulting in a larger loss of stability. Hence, in these models the metal is viewed as poisoned by the support. (d) A second explanation for the decrease of H binding energies in all types of models is a removal of electron density from the metal due to a mixing of “support” orbitals with the metal-H bonding orbitals. These hypotheses have to be verified in two directions: first, more realistic structures, e.g., larger supported and supporting clusters, have to be considered; second, carefully chosen ‘minimum models” should be studied with highly accurate calculations in order to avoid the deficiencies typical of semiempirical quantum-chemical methods and to provide reliable insight into the geometry and bonding nature at the interface as well as between the metal and the adsorbate.* The present paper is devoted to this second aspect. Recently (see part I11 of this ~ e r i e s we ) ~ have studied two selected minimum models of metalsupport systems, namely the (8) Haberlandt, H. Theoretical Investigation of Metal-Support Interactions and Their Influence on Chemisorption. In Theoretical Aspects of Hererogeneous Caralysis; Moffat, J. B., Ed.; Van Nostrand-Reinhold: New York, 1990. (9) Haberlandt, H.; Pacchioni, G. Chem. Phys. 1990, 142, 369.
0 1991 American Chemical Society
47%
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991
E:
Haberlandt et al. of TABLE I: Codigumtjoni Wcea (CS) and Tebl Wdgbta the Refeereace Configurationsin the F i ~ CI l Wave Function of Ground and First Excited States of HNiSiHl ud HNiAIHa Ni-H distances used to generate generated model state acommon CS, au nM' config E,c? HNiSiH' X 3E 2.8, 2.9, 3.0 1 5384 0.92 0.86c each distanceb 1 3295c IAl HNiAlH, X 'E 2.7, 2.8, 2.9 2 1192 0.92 2.9, 3.0, 3.1 1 5656 0.89 *E
'Number of reference configurations. tance was used. cAt r = 2.95 au.
Figure 1. Geometry of the H-Ni-MH3 (M = AI, Si) cluster.
Individual CS at each dis-
TABLE 11: Relative Emergy sepvitioll (eV) between Selected Atomic Ni States As Determined by Difftrent Computatid Mctbods HartrttFock state RHF" RHFb limit SD CI e x ~ t * ) 'F (3d8 4s2) 0.0 0.0 0.0 0.0 0.0 'D (3d9 4s') 1.42 0.11 1.28 -0.02 -0.03 ID (3d9 4s') 1.83 0.13 0.28 IS (3dLo) 6.12 2.23c 1.68
molecules NiSiH3 and NiAIH3, by means of a nonempirical 'Uncontracted GTO basis set. bContracted GTO basis set. 'Sixpseudopotential (PP)Io restricted Hartree-Fock (RHF) technique reference configurations have been used in the CI calculations. followed by configuration interaction (C1)ll calculations which take account of correlation effects. In this paper the same Gaussian-type orbital (GTO) basis sets are the same as those quantum-chemical method is applied to two adsorbate-metaldescribed in ref 9; they are of double zeta plus polarization quality support systems. The molecules HNiSiH3 and HNiAlH3 (Figure (DZ+P). A [4s,lp/2slp] basis set was employed for the chem1) are used as minimum models of SMSI and WMSI systems, isorbed H atom and for the H atoms of the SiH3 fragment.22 The respectively. H p polarization function has been omitted from AIH3 in HNiTo the best of our knowledge, neither experimental nor theoAIH3 because it has basically no effect on the description of the retical results on these two systems have been reported so far. H-Ni-M (M = AI, Si) interaction (see also ref 9). Recently, ab initio calculations have been performed on the The calculations were carried out within the C, point group transition-metal complexes HFeCH3I2and HCuCH3,I3." and on a subgroup of the full C, symmetry of the two systems the non-transition-metal HMgCH, and HAICH, m ~ l e c u l e s . ~ ~ ~ ' symmetry, ~ considered. Rigid pseudotetrahedral geometries were adopted for The HCuCH3 molecule has also been experimentally studied by the supporting SiH3 and AIH3 units with Si-H and AI-H distances means of matrix isolation techniques." fixed at 2.19 and 3.06 au, re~pectively.~ In this work the Ni-H equilibrium bond distances, re, and Correlation effects were introduced by means of the multiredissociation energies, De, in HNiSiH3 and HNiAIH3 have been ference single and double excited CI method" (MRD CI) which determined and compared with the corresponding values for the makes use of a configuration selection procedure and a subsequent free Ni-H molecule.'* The effect of H chemisorption on the energy extrapolation for the evaluation of the final CI energy. Ni-M (M = Si, AI) bond strength and the consequences of the The selection thresholds, T, used for HNiSiH3 and HNiAlH' are small size of the minimum models are critically discussed. Finally, T= and T = 5 X lod hartree, respectively. As previously some general conclusions are drawn about the reliability of the found for selected states of other Ni-containing mo1ecules,9J8+20 working hypotheses previously formulated.68 the extrapolated CI potential energy curves of HNiSiH' and HNiAIH3 are not smooth enough if these thresholds are used. Computational Method Hence, common configurational spaces have been generated from For the present calculations the nonempirical PP technique the configurational spaces obtained at selected Ni-H distances proposed by Durand and Barthelat'O has been used. The 1s to (Table I).'8 This procedure is equivalent to lowering the effective 3p core electrons of the Ni atom have been replaced by a nonthresholds reported above. relativistic PP operator19 as in our previous calculations on the For a thorough understanding of the mechanism of H chemNiH,'* NiSiH,, and NiAIH, molecules? The Si and AI isorption on a supported Ni atom it is necessary to analyze the PP operators were taken from ref 21. The Si, AI, and Ni spectrum of the free Ni atom as derived from R H F and CI calculations (Table 11). With the adopted uncontracted Ni GTO basis set, at the RHF level the Ni 3F 3d84s2state is preferred over (IO) Durand. P.; Barthelat, J. C. Theor. Chim. Acta. 1975, 38, 283. the 3D 3d94s' by 1.42 eV, in contrast with the experimental result Barthelat, J. C.; Durand, P.; Serafini, A. Mol. Phys. 1977, 33, 159. which gives the two states almost degenerate with 'D as the ground (1 1) Buenker, R. J.; Peyerimhoff, S. D. Theor. Chim. Acta 1974.35, 33; 1975.39.217. Buenker, R. J.; Peyerimhoff, S. D.; Butscher, W. Mol. Phys. state separated by only 0.03 eV from the 'F term?' This ordering 1978,35,771. Buenker. R. J. In Proceedings ojthe Workshopon Quantum is quantitatively reproduced after inclusion of correlation effects Chemistry and Molecular Physics, Burton, P., Ed.; University Press: Wolthrough the single-double CI procedure; the CI correctly reprolongong, 1980. Buenker, R. J. In Studies in Physical and Theoretical duces also the sequence and the energy separation of higher Ni Chemistry, Current Aspects of Quantum Chemistv,Carbo, R., Ed.;Elsevier: Amsterdam, 1981; Vol. 21, p 17. Buenker. R. J.; Phillips, R. A. J . Mol. excited states (Table 11). Struct. THEOCHEM 1985. 123, 291. The size-consistency error (SCE) of the CI procedure, which (12) McKee, M. L. J . Am. Chem. Soc. 1990, 112, 2601. largely affects the binding energy of an A-B system, was taken (13) Quelch, G. E.; Hillier, 1. Chem. Phys. 1988, 121, 183. into account by computing the De value as the difference of the (14) Poirier, R. A.; Ozin. 0. A.; McIntosh, D. F.; Csiunadia, 1. G. Chem. Phys. Lett. 1983, 101, 221. ( I S ) Quelch, G. E.; Hillier, 1. J . Chem. Soc., Faraday Trans. 2 1w17,83, 1637. (16) Quelch, G. E.; Hillier, 1. J . Chem. Soc., Faraday Trans. 2 1987,83, 2287. (17) hrnis, J. M.; Ozin, G. A. J . Phys. Chem. 1989, 93, 4023. (18) Haberlandt, H. J . Mol. Strucr. THEOCHEM 1990, 205, 25. (19) Barthelat, J. C. Personal communication. (20) Haberlandt, H. Chem. Phys. 1989, 138, 315.
(21) Technical Report of the Workshop on Quantum Molecular Calculations with Pseudopotentials. Laboratoire Physique Quantique: Toulouse, France, 1981. (22) Barthelat, J. C.; Trinquier, G.; Bertrand, G. J . Am. Chem. Soc. 1979. 101, 3785. (23) Moore, C. E. Atomic Energy Leuels; US.Government Printing Office: Washington, DC, 1952; Vol. 3, NBS Circular 467.
Chemisorption of H on Ni Bound to SiH, and AIH,
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4197 (E,ot* 44.01En
TABLE 111: Equilibrium Bond Distances, r , (nu),
and Dissociation Energies, D, (ev), for Ni-H Bod Stretchine in Free and Yhpr~rtcd"Ni-H
model HNiSiH, HNiAIHld
state
method
re
Dc"
'E
RHF
3.07 2.98 2.96 2.85 3.03 2.93 2.93 2.73
1.85 1.56 (2.07)c 1.32 1.26 ( 1 .60)c 0.27 1.17 1.41 2.45
'E
cI b
RHF CIb
*E NIH
RHF
cI b
RHF
2A
Clb
OC1 De values of A-B computed as E(A) + E(B) - ,?(AB). Extrapolated value. 'Size-consistency-corrected CI De values computed for a supermolecule at large internuclear separation (see text). 'Values computed for a fixed Ni-AI distance; by varying the Ni-AI distance the 2Ebecomes the ground state and the molecule dissociates into NiH and AIH, (see text).
energy of the AB system at the equilibrium distance with respect (a) to separate A and B fragments and (b) to the A*-B supermolecule computed at a large internuclear separationa The two computed energy values must be considered as upper and lower limits of the real CI binding energy, respectively. Unlike in our previous work,9.18J"the contribution of the basis set superposition error (BSSE)to the interaction energy has not been considered explicitly. However, the Ni-H equilibrium bond distance is slightly larger in HNiSiH, than in the diatomic NiH molecule; hence, the influence of the BSSE on the properties of the chemical bond is not expected to be larger than in the NiH molecule.18 Indeed, for a selected Ni-H distance of 2.9 au, we found that the BSSE correction for NiH and HNiSiH, at the R H F level is practically identical, 0.07 eV, while in CI the correction in HNiSiH, is even smaller than in NiH (0.13 vs 0.17 eV, respectively). Thus, it is justified to make use of the knowledge of the BSSE behavior of the NiH potential energy curve for the analysis of the results on HNiMH, (M = Si, A1);18 in particular, the BSSE correction is small and can be omitted without affecting the quality of the results.
Results a d Discussion 1 . The HNiSiH' Molecule. The NiSiH, molecule (Figure 1) has a 2E electronic ground state with the unpaired electron in the Ni 3 d ~ r b i t a lthe ; ~ chemisorption of an H atom on top of Ni occurs tirough the interaction of the H 1s orbital with both the doubly occupied Nisp-Sispbonding and the Ni dzz nonbonding orbitals. From this orbital interaction a doubly occupied H-Ni bonding orbital (3al) is formed, with the unpaired electron localized in the Ni d 9 orbital (4al); the 2al MO retains most of its Ni-Si bonding character, although some mixing with the H Is orbital occurs. The final valence electronic configuration of the 'E ground state can thus be schematically represented in terms of localized bonds as (2al, Nip-S,J2 (3al, N&p-H)2(4al, Ni dz)' (le, Ni M A ).I A spinpiring between the two unpaired electrons to give the A, state (le, Ni 3 d , ~ , 2 )is~ largely unfavorable as shown by the computed 'E IAl transition energy of 1.4 eV (CI extrapolated value). The loss of exchange energy caused by the coupling of the two unpaired electrons is not compensated by the formation of a stronger covalent bond with the two ligands. This is due to the very localized nature of the Ni 3d orbitals which are less easily involved in the covalent bonding with partners than the 4s, a fact with important consequences on the mechanism of SMSI, as will be discussed later. The bonding in HNiSiH' must be compared with that of the free separated NiH (2A)18 and NiSiH3 (2E)9molecules. In each of these molecules there is a two-center two-electron bond, either Ni-H or Ni-Si, mainly originating from the interaction of the Ni 4sp hybrid orbital with the H Is electron18or with the dangling bond of the SiH, fragment? respectively. In both NiH and
7
-
(24) Carsky, P.;Urban, M.Ab Initio Calculations. Methods and Applications in Chemistry. Lrcrures N o m in Chemistry; Springer Verlag: Berlin, 1980; Vol. 16.
- 0928 - 0.929
-1.108
r 2.80
2.90
3.00
3.lO
r,,,,,lo.u
Figure 2. RHF and CI potential energy curves for the stretching of the Ni-H bond in HNiSiH3.
NiSiH, the Ni atom assumes a 3d94s'-like configuration with the unpaired electron in the Ni 3 d ~ orbital. 9 The effect of the SiH, "support" on the strength of the Ni-H bond has been analyzed by computing the R H F and CI potential energy curves for the Ni-H stretching in HNiSiH, (Figure 2). The introduction of correlation effects reduces the Ni-H bond distance (see also Table 111). In terms of classical theory of chemical bond this should also correspond to an increase in the dissociation energy. However, this interpretation can be very misleading for molecules containing transition-metal atoms. The De value has been computed at both RHF and CI levels. The SCF calculations indicate that the Ni-H De increases from 1.41 to 1.85 eV on going from free to supported NiH (Tables 111 and IV). Hence, a bond elongation found in RHF for HNiSiH, with respect to NiH (Table 111) is not necessarily a sign of bond weakening. In order to account for the SCE correction, the H-NiSiH, CI De has been computed in two ways from extrapolated CI values, that is, as E(H) E(NiSiH3) - E(HNiSiH,) as well as with respect to the energy of the supermolecule H.-NiSiH,. The corresponding De values, 1.56 and 2.07 eV, respectively, must be considered as lower and upper bounds to the correct CI De values which can be estimated to be about 1.7-1.8 eV. This De is considerably smaller than that computed at the CI level for the NiH molecule, 2.41 eV,18 in contrast to what was found at the R H F level. The same discrepancy between CI and RHF is found when the strength of the Ni-Si bond is considered. The analysis of the RHF wave function indicates that the HNi-SiH3 bonding (1.64 eV) is stronger than the Ni-SiH, one (1.23 eV)9 (Table IV); when correlation effects are introduced, the situation is reversed, showing an easier dissociation of the Ni-Si bonding in HNiSiH, (0.99 eV) with respect to NiSiH, (1.79 eV) (Table IV). Apparently, two completely different bonding pictures arise from uncorrelated and correlated wave functions. In order to reconcile the two opposite results, it is useful to compare the present results with those obtained for other molecules of the same kind, namely HMgCH3,1SHFeCH3,12and HCuCHo (Table IV)." In terms of the one-electron picture, the stronger Ni-H and Ni-Si bonds found when the Ni atom assumes the linear coordination as in H-Ni-SiH3 (RHF results) can be explained by the formation of two singly occupied 4sp linear hybrid orbitals on the metal center and consequent localization of the bonding electron pairs. This implies, however, that the Ni atom assumes a 3d84sz-likeatomic configuration, which is appropriate for the formation of two 4sp hybrid orbitals. This configuration is the preferred one in RHF calculations (Table 11) and can explain why
-
+
Haberlandt et al.
4798 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 TABLE I V Calculated Bond Masociation Energies for Uncorrelated (RHF)and Correlated (CI) Wave Functions bond broken De,eV ref bond broken De, eV Uncorrelated Calculations 1.41 1.85 this work H-Ni, 'A H-NiSiH,, >E 1.23 HNi-SiH,, 'E 1.64 this work Ni-SiH,, 'E 1.41 1.32 this work H-Ni, 'A H-NiAIH,, 'E Ni-AIH,, ,E 0.37 HNi-AIH,, 'E 0.17 this work 1.19 1.99 12 H-Fe, 6A H-FeCH,, JE 1.48 12 0.69 HFe-CH,, 5E Fe-CH,, 6E 13a H C u , 'L: 0.8 1 1.13 H-CUCH,, 'AI 0.68 0.36 13a CU-CH,, 'AI HCU-CH,, 'AI
Correlated Calculations" 1.56 this work H-Ni, 'A H-NiSiH3, 'E HNi-SiH,, ,E 0.99 this work Ni-SiH,, 'E H-NiAIH,, 'E 1.26 this work H-Ni, 'A Ni-AIH,, )E HNi-AIH,, 'E -0.78 this work H-Fe, 6A H-FeCH,, 5E 2.41 12 Fe-CH,, 6E HFe-CH,, 'E 2.25 12 'Present work and refs 9 and 13, CI calculations; ref 12 M0ller-Plesset perturbation theory. the R H F approach predicts bond strengthening. In fact, the situation is similar to that in the HMgCH, molecule where it was found that both the Mg-H and Mg-C bonds are reinforced with respect to the separated MgH and MgCH, fragmentsIs because the two ligands, H and CH,, force the 3s2 Mg atom to rehybridize to form strong localized bonds. The change in bond strength can be understood by assuming that the hybridization that the metal atom must undergo to bind the first fragment is not necessary to bind the second one. Due to the presence of 3d electrons on Ni, however, a comparison of the present results with those obtained for the HFeCH, and HCuCH, molecules is much more instructive. The energy required to break the bonds in this series of molecules is reported in Table IV. Moving toward the right of the transition series, from Fe to Ni to Cu, we observe a change in the effect of the "support" on the stability of the complex. In HFeCH,, in fact, the strength of both the H-Fe and Fe-CH, bonds is more than twice that in the free molecules (Table IV); inclusion of correlation effects through M~ller-Plesset (MP) perturbation theory does not change this result substantially (Table IV).I2 In HNiSiH, we found that, at the R H F level, both Ni-H and Ni-SiH, bonds are reinforced as compared with the bonds in the separated molecules, but now the increase in bond strength, about 30% is much smaller than for the Fe system. The correlated calculation, on the other hand, shows an opposite trend, indicating a bond weakening (Table IV). When we consider the HCuCH, molecule (only R H F results are available), we find that both Cu-H and Cu-CH, bonds are weaker than in the free counterparts (Table IV). Clearly, there is a continuous change upon passing from Fe to Cu and Ni is on the borderline between two electronically very different situations. The results can be explained as follows. The atomic ground state of Fe is 3d64s2(5D) with the 3d74s' (SF) term lying about 1 eV above it;23this energy separation is reproduced correctly by a M P calculation12and only qualitatively by the R H F calculations which overstabilizes the 5D term with respect to the sF.'2 This means that, to bind the H and the CH3 fragments, the Fe atom can use the two 4s electrons forming two 4sp hybrid orbitals with weak d character; these hybrid orbitals can overlap with the ligand orbitals forming two strong u bonds. In this respect the situation is completely similar to that of HMgCH,; the 3d electrons of Fe are not directly involved in the bonding and the molecular ground state exhibits high spin multiplicity (see Table IV and ref 12). Cu has a 3dI04s' 2S atomic ground state: when it forms a bond either with H or with CH3 the 4s electron can be used without requiring any electronic promotion, but in order to bind a second ligand the 3d electrons must be directly involved in the formation of the covalent bond. This requires the promotion of an electron from the 3d,r into a 4sp hybrid orbital; in this way a hole is formed in the 3d,z orbital as is found also by ESR measurement^.'^ The cost of the preparation of the Cu atom, which does not have to be paid to bind the first ligand, is not compensated by the for-
2.45 1.79 2.45 0.48 1.11 0.95
ref 18 9 18 9 12 12 13a 13a 18 9 18 9 12 12
mation of the second bond with consequent weakening of the structure (Table IV). Ni lies somewhat in between the two limiting cases of Fe and Cu; the two atomic states 3d84s2('F) and 3d94s' (,D)are almost degenerate, but their relative computed ordering largely depends on the details of the calculation. The R H F treatment with the uncontracted basis set favors the ,F term while the CI correctly reproduces the ,D-'F near degeneracy. This could explain the reinforcement of both Ni-Si and Ni-H bonds in RHF. The weight of the 3d84s2configuration in the R H F wave function is large, while the CI prefers the 3d94s' configuration. This increases the energy cost to prepare the Ni atom to bond two ligands, with consequent weakening of the Ni-Si and Ni-H interactions in CI. On the other hand, it must be noted that, when the 3D-3F separation is computed a t the R H F level with the contracted GTO basis set used for molecular calculations, the energy difference (Table 11) is too small to justify in a simple way the different trend in bond strength. The subtle interplay between energy gain occurring with bond formation, energy cost of the rehybridization, and atomic energy level separation, makes it hard to predict the behavior of an atom like Ni characterized by two almost degenerate atomic configurations. In this respect, correlation effects should play a less dramatic role in the HFeCH, and HCuCH, molecules. To summarize, the very localized nature of the 3d orbitals is the main reason for the different behaviors observed. When the 4s orbital is doubly occupied, as in Fe, the 3d do not necessarily take part in the bond formation and the behavior is similar to that of a non-transition-metal atom like Mg; when the 4s orbital is only partially occupied, a strong electronic reorganization involving the 3d electrons has to occur in order to bind two ligands; this is the case of Cu. Ni lies in between. 2. The HNiAlH, Molecule. To represent the interaction of an hydrogen atom with a model of a WMSI system we studied the H-NiAIH, cluster (Figure 1). In the NiAIH, molecule, the Ni atom is weakly bound to the substrate by means of a covalent dative bonding from the Ni 4sp hybrid orbital to the empty sp3 lobe of the AIH, fragment; the originating bonding orbital is singly occupied9 The second unpaired electron is localized in the 3 d ~ 9 Ni orbital and is not involved in the bonding. We have fixed the distance of the Ni atom at the optimal value found for the NiAIHl complex and we have considered the effect of adding an H atom on top of Ni (Figure 1). Two different electronic configurations, characterized by high and low spin multiplicity, can occur. In the high-spin structure the Ni 4sp electron is already involved in the bonding with the AlH, fragment, so that the bonding with the incoming H atom involves the 3d,~orbital with formation of a bonding combination with large H character and an essentially nonbonding singly occupied Ni 3d,z orbital; this state in C,? symmetry is a 4E and can be described as (2a,, Ni-H)2 (3al, NI 3 d ~ )(4al, ' Ni -AI)' (le, Ni 3d+2)'. In the low-spin structure, a spin coupling between
Chemisorption of H on Ni Bound to SiH3 and AlH3 the unpaired electrons occupying the Ni-A1 bonding orbital of NiAlH, and the largely H Is orbital occurs, with consequent formation of a strong Ni-H bond and considerable weakening of the Ni-AI interaction. The hole on the 3 d + 2orbital is still present and the ground state is 2E. At the R H F level, the 4E state is considerably lower than the 2E(the vertical ‘E 2Etransition energy is 1.OS eV); as must be expected, the introduction of correlation effects stabilizes the state of low spin multiplicity with respect to the high-spin one and the energy difference between the two states becomes very small (the 4Eis still the ground state but is separated by 0.08 eV only from the *Estate (Table 111). However, the most important aspect is not the determination of the molecular ground state but the analysis of the effect of the incoming H atom on the stability of the entire system. In fact, an inspection of the Ni-A1 binding energy at the R H F level shows that, in the 4Estate, the Ni-A1 De is reduced from 0.37 to 0.17 eV (Table IV), while the 2Estate, where the weak Ni-A1 bond has been broken in favor of the more strong Ni-H bond, is unstable toward dissociation into Ni-H and AlH,. The CI results confirm this indication. Both 4E and 2E states are unbound with respect to Ni-H and AlH,. The optimization of the Ni-A1 distance results, in both R H F and CI, in the stabilization of the 2Ewith respect to the 4E state and consequent formation of two separated Ni-H (ZA)and AlH, (‘A,) molecules. The results obtained with the HNiAlH, system give a clear indication of the different behavior of a Ni atom supported on a ‘partially reduced” (like SiH3) or on a ‘fully oxidized” (like AlH,) substrate. In the case of the fully oxidized substrate the effect of the support is not large enough to form a stable surface complex, differently from the partially reduced case. This consideration brings us back to the more general problem of metal-support interaction. 3. HNiSiH3 and HNiAIH3 as Minimum Models of Adsorbate-Metal-Support Systems. For catalytic systems exhibiting the SMSI behavior, the support is generally believed to be r e d u d , the interaction with a metal phase leads to intermetallic bonds like in silicides0 A corresponding surface site on the SiOz surface can be represented by the Si(OH)3 molecule showing a singly occupied dangling bond for a Si atom in the formal oxidation state +3.6*8This Si(OH), model of a partly reduced support has been further simplified in the present study by the omission of the three oxygen atoms resulting in the SiH3 minimum model previously discussed. Examples of WMSI behavior are found in catalysts consisting of metals supported by fully oxidized oxide carriers. A characteristic site on the surface of the support is the oxygen vacancy on the tetrahedrally coordinated A1 ion in A1203. This site is identified with a strong Lewis acid center and can be simulated by means of the Al(OH), molecule.’*8 The formal +3 oxidation state of an AI atom coordinated to three oxygen atoms justifies the classification as fully oxidized support. Again, in this work we have further simplified the support model by considering the AlH, substrate where the O H groups have been replaced by H atoms. A pronounced lowering of the De value of H bound to a Ni atom supported on a partly reduced carriers was found, but only after inclusion of correlation effects. This weakening is essentially due to the energy cost of reorganizing the Ni electronic configuration to increase the 3d84s2character. However, this conclusion cannot be generalized to other transition-metal atoms. In fact, the weakening or strengthening of the metal-hydrogen and metalsupport bond in the case of SMSI largely depends on the degree of involvement of the metal sp and d orbitals in the bonding with the two counterparts. The balance between the energy gained by the bond formation and the energy spent in promoting the d electrons into more diffuse and directional sp hybrid orbitals is the key factor governing the strength of the metal-hydrogen and metal-support bonds. For the case of WMSI (fully oxidized support) the formation on the surface of very stable molecules like transition-metal hydrides can easily result in a considerable weakening of the met-
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The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4199 al-support bond. In the case examined here, HNiAlH3, the Ni-A1 bond strength is reduced to such an extent that the NiH molecule is desorbed from the surface. However, in this case the minimum model adopted, AlH,, does not sufficiently account for the important electrostatic interaction between substrate and adsorbate metal atom. Of course, the same holds for the SiH3 minimum model of SMSI,but here the presence on the substrate of a singly occupied dangling bond results in the formation of a strong covalent bond so that the improper description of the electrostatic term has probably a much smaller effect. Indeed, UHF calculations of the HNiSi(OH), molecule by Kobayashi and YamaguchiZ5exhibit an elongation of the Ni-H bond (0.17 au), comparable to that observed in this work by means of the HNiSiH, model. A generalization of the present conclusions drawn for the case of a single supported metal atom to a supported metal cluster is not straightforward. The working hypothesis previously formulated on the basis of semiempirical C N D 0 / 2 calculations&8is not contradicted by the present study based on accurate electronic structure calculations of a minimum model system. In particular, two general trends were suggested for the case of on-top chemisorption:8 (i) The binding energy of atomic hydrogen to a supported metal particle is lowered for models of SMSI (partly reduced) with respect to their isomorphous models of WMSI (fully oxidized). These reductions of H binding energies are accompanied by a weakening of the metal-support bond. (ii) The decrease of metal-hydrogen bond strength is closely related to a removal of electron density from the metal due to the mixing of support orbitals with metal-H bonding orbitals. Indeed, our ab initio minimum models show that the Ni-H bond is weakened on the SiH, support (SMSI) while the HNi-AlH, system (WMSI) is unbound, so that an indication of the effect of the fully oxidized support on the strength of the Ni-H bond is not possible. On the other hand, the weakening of the metal-support bond is confirmed for both models of strong and weak metal-support interaction. Clearly, more work is necessary in order to confirm these trends, in particular by considering larger models of supported metal particles.
Conclusions We have performed RHF and CI calculations on HNiSiH, and HNiAlH, molecules as minimum models of the on-top chemisorption of atomic hydrogen on a supported Ni atom. The two Ni-Si and Ni-A1 systems investigated can be considered as very simplified models of strong (SMSI) and weak (WMSI) metalsupport interactions, respectively. The electronic ground state of HNiSiHS (SMSI) is ’E with the two unpaired electrons localized in the 3 d t and 3d+9 Ni orbitals. In the ground state, two-center two-electron Ni-H and Ni-Si bonds are formed; an increase or reduction of the strength of the Ni-H and Ni-SiH3 bonds with respect to the separated free molecules depends on the balance between the cost of hybridization of the Ni atom and the energy gain accompanying the bond formation. At the R H F level the presence of two ligands favors the formation of two 4sp hybrid orbitals with small 3d,z contribution, thus reinforcing the two covalent bonds. In CI, on the contrary, the formation of two bonds results in a bond weakening. The different energy separation between )D and ’F atomic states in RHF and CI represents a possible explanation of this behavior. The energy balance between cost of rehybridization and bond formation accounts also for the different behavior of transitionmetal atoms like Fe, Ni, or Cu. The basic rule for metal atoms supported on a partly reduced substrate is that the outer s orbital must be doubly occupied to form bonds with both the adsorbed H atom and the substrate. In the case of Fe, which has a 3d64s2 atomic configuration, the effect of the support is to reinforce the bonding with the substrate as well as the bonding with H; the (25) Kobayashi, H.; Yamaguchi, M. In Churucterizurion of Metal Cumlysts; Philips, M. J., Ternan. M., Eds.; Proceedings of the 9th International Congress of Catalysis; The Chemical Industry of Canada: Ottawa, 1988; p
1098.
4800
J. Phys. Chem. 1991,95, 4800-4803
hybridization that Fe must undergo to be bound to the substrate is not necessary to bind hydrogen. In Cu, with a 3dI04s' configuration, the formation of the metalsubstrate and metal-H bonds can occur only at the cost of a profound electronic rearrangement toward a 3d94sZconfiguration with consequent reduction of the bond strength. Thus, the hypothesis formulated on the basis of semiempirical calculations that a weakening of metal-hydrogen and metalsupport interactions occurs in SMSI* is confirmed by the present results, at least for the case of a single Ni atom. In the HNiAIH3, model of WMSI, we found two electronic states, 4Eand 2E, nearly degenerate in energy when CI wave functions are considered. The 4Eleading electronic configuration corresponds to a H*Ni*AIH3structure with two weak Ni-H and Ni-AI oneelectron bonds and the third unpaired electron localized on the Ni 3d9-9 orbital; in the 2E,on the contrary, a Ni-H two-electron bond is formed a t the expense of the weak Ni-AIH3 bond, which is broken. As a consequence, the 2Eis unstable with respect to NiH (2A)and AIH3 ('A,) and spontaneously dissociates as the Ni-AI distance is varied. The absence on A1H3 of a singly occupied dangling bond prevents the formation of a covalent Nisupport bond as in Ni-SiH3. The Ni-AIH3 interaction is
essentially electrostatic in nature and its strength is underestimated in our model because of the omission of the electronegative oxygen atoms. Possibly, a more refined model of the oxide substrate would prevent the desorption of the NiH molecule because of a stronger electrostatic attraction. To summarize, a distinct indication of the different local electronic mechanisms in SMSI and WMSI can be obtained from the minimum cluster models employed here. The fundamental role of the dangling bond on the surface of partly reduced versus fully oxidized supports on the energetics of the chemisorption process has been shown. Further work is necessary to understand the nature of this electronic substrate effect for the more relevant case of supported metal clusters.
Acknowledgment. We thank Professor S. D. Peyerimhoff (University of Bonn) for placing at our disposal the MRD CI program package, and Professor J. C.Barthelat (University of Toulouse) for sending us unpublished pseudopotentials and basis sets. H.H. thanks the Italian C N R for the financial support of his stay at the University of Milan. Registry No. H, 12385-13-6; Ni, 7440-02-0 SiH,, 13765-44-1; AIH3, 7784-21-6.
Llnear Chains of Pt(bpy)(CN), at Electrode Surfaces by Partial Reduction of the Blpyrldlne ?r System J. B. Cooper, S.M.Rhodes, and D. W . Wertz* Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 (Received: July 30, 1990)
The spectrochemical properties of Pt(bpy)(CN), are reported on both roughened Ag and polished Pt electrodes. On the basis of the presence of a weak emission and a very strong resonance Raman spectrum (RRS) with visible excitation, it is concluded that Pt(bpy)(CN), forms linear chains on both surfaces. Since both the emission and RRS are very potential dependent, being observed at -500 f 50 mV, the approximate El of the bipyridine reduction, and the RRS consists exclusively of bipyridine modes, we conclude that linear chain growth is the result of partial reduction of the bipyridine 7r system and visible absorption is due to a x T* transition that has red shifted considerably due to the band formation resulting from bipyridine-bipyridine interaction along the chain.
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Introduction The most extensively studied inorganic one-dimensional compounds are those based on tetracyanoplatinate anions.'-3 In these materials, the square-planar ions are stacked face-teface to form linear chains of the platinum atoms. The d t band that is formed from the overlap of the platinum 5dzz orbitals along the chain is filled, and interaction between the platinums is enhanced by removal of some of the electron density from this band, i.e. by partial oxidation. In principle, e n h a n d bonding between adjacent monomeric units can also be accomplished by partial r e d ~ c t i o n . ~ In fact, TTF-TCNQ exists in segregated stacks of TTF donors and TCNQ acceptors involved in a partial electron t r a n ~ f e rand ,~ conductivity is dominated by the partially reduced TCNQ stacks6 in which overlap of the 7r systems is optimized by a face-to-face orientation of the TCNQ molecules. In addition, solid-state effects S.;Eptein, A. J. P frog. Inorg. Chem 1976, 20, 1. (2) Krogmann, K. Angew. Chem., Inr. Ed. Engl. 1969, 8, 35. (3) Tanner, D. B. In Extended Unear Chain Compounds; Miller, J. S., Ed.;Plenum: New York, 1982; Vol. 2. (4) Hoffman, B. M.; Ibers, J. A. Acc. Chem. Res. 1983* 16, 15. (5) Ferrais, J.; Cowan, D. 0.;Walatka, V.: Perlstein, J. H. J . Am. Chem. soc. i m , 9 5 , 9 4 a . (6) (a) Tomkiewicz, Y.; Taranko. A. Phys. Reo. krr. 1976, 36,751. (b) Berlinsky, A. J.; Carolan, J. F.; Weilcr, L. Solid State Commun. 1974, 15, 795. (c) Soda, G.;Jerome, D.; Weger, M.; Alizon, J.; Gallice, J.: Robert,H.: Fabre, J . M.; Giral, L. J . Phys. 1977, 38, 931. (1) Miller, J .
0022-3654/9 1/2095-48OOS02.50/0
on the emission spectra of [Pt(bpy)z](C104)2and [Pt(phen)z]C12 are attributed to an excimeric interaction between the 7r systems of the diimine ligands of adjacent monomers in the unit cell and not to Pt-Pt electronic interactions.' Recently, Che and co-workers* reported that Pt(bpy)(CN), consists of linear chains with the bipyridines in a face-to-face configuration and a platinum-platinum distance of 3.3296 A. They noted that the compound is colorless in solution and gives no room-temperature emission, but when it is crystallized from hot DMF, deep red, needlelike crystals are obtained, which have a very intense, long-lived ( 7 = 2 ms) emission at 16 300 cm-'. Although they did not assign the emissive state, they did attribute it to metal-metal interactions. Previously, Biederman et a1.9 used polarized emission spectrocopy to assign both the emitting and absorbing states, but their analysis neglected to include any type of molecular interaction along the chain as is indicated by Che, and they also incorrectly assumed a trans configuration for the bipyridines. Since the separation between the bipyridines is less than the sum of the van der Waals radii for aromatic groups (3.7 AloJ') (7) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1989, 28, 1529. (8) Che C-M.; He, L-Y.; Poon, C-K.; Mak, T. C. W. Inorg. Chem. 1989, 28. 308 1. (9) Biderman, J.; Wallfahrcr, M.; Glieman, G.J . Lumin. 1987, 37, 323. (IO) Miller, J. S. Inorg. Chem. 1976, 15, 2357. ( I 1) Miller, J. S.Ann. N. Y.Acad. Sci. 1978, 313, 25.
0 1991 American Chemical Society
