Langmuir 1995,11, 853-859
853
Adsorption of Formyl on Ni(100) Hong Yang* and Jerry L. Whitten Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 Received March 17,1994. I n Final Form: December 9,1994@ The adsorption of formyl radicals (HCO) on Ni(100)is treated using a many-electron embedding theory, modeling the lattice as a 30-atom, three-layer cluster with the Ni atoms fxed at bulk. Ab initio valence orbital configuration interaction (multiple parent) calculations carried out on a local surface region permit an accurate description of bonding at the surface. The 3d orbitals are explicitly used for six Ni atoms in the local surface region. The Ni(100)potential surface is very flat for ql-formyl4 adsorption. yl-Formyl-C binds to the surface mainly via the C atom, and the energy minimum occurs for an 0-C-surface normal angle of go", go", and 110" at four-fold, bridge, and atop sites, respectively. The HCO bond angle is 120". Calculated adsorption energies are 63.7, 63.5, and 63.6 kcallmol at four-fold,bridge, and atop sites, with C-surface distances of 1.93, 1.94, and 2.04 A, respectively. Calculated C - 0 stretching frequencies are around 1760 cm-l and C-H stretching frequencies are around 2940 cm-l for HCO at all adsorption sites and equilibrium geometries. The bonding of formyl to the nickel surface involves ionic and covalent contributions and substantial mixing with Ni 3d orbitals. Calculated yl-formyl-0, bonding to the surface via the 0 atom, is energetically less stable than ql-formy1-Cby 16.3 kcal/mol. Calculated C-0 and C-H stretching frequencies are 1370 and 2988 cm-l for ql-formyl-0 at four-fold sites. No energy barrier occurs for the conversion of ql-formyl-0 t o yl-formy1-C. 1. Introduction
The reaction of formyl, HCO, with transition-metal surfaces is of interest because of its role as a possible intermediate in catalytic oxidation and methanol decomposition.lS2 Early experimental studies of methanol decompositionon Ni(100)using temperature programmed desorption by Yates et aL3 and using IR by Baudais et aL4 indicated a quasistable HCO intermediate, while a COH intermediate has been proposed by Johnson and Madix5 using temperature programmed reaction spectroscopy. Surface-bound formyls resulting from formaldehyde decomposition have been identified by Weinberg and coworkers on Ru(001).6 Homogeneous catalysis studies based on organometallic complexes have indicated metal formyl compounds as important intermediates in the CO hydrogenation r e a c t i ~ n . ~ - Generalized l~ valence-bond Abstract published in Advance A C S Abstracts, February 1, 1995. (1)Herman, R. G.Stud. Surf. Sci. Catal. 1991,64,265. (2)Netzer, F. P.; Ramsey, M. G. Crit. Rev. Solid State Mater. Sci. 1992,17,397. (3)Yates, J. T.,Jr.; Goodman, D. W.; Madey, T. E. In Proceedings of the 7th International Vacuum Congress and 3rd International Conference on Solid Surfaces, Vienna, 1977;p 1133. (4)Baudais, F.L.;Borschke, A. J.; Fedyk, J. D.; Dignam, M. J. Surf. Sci. 1980,100,210. (5) Johnson, S.; Madix, R. J. Surf. Sci. 1981,103, 361. (6)Anton. A.B.: Parmeter, J. E.: Weinberg, - W. H. J . A m . Chem. SOC. 1986,108,i m . (7)Wolczanski, P. T.; Bercaw, J. E. ACC.Chem. Res. 1980,13, 121. (8)Wayland, B. B.; Woods, B. A,; Pierce, R. J . A m . Chem. SOC.1982, 104,302. (9)Floriani, C.Pure Appl. Chem. 1983,55, 1. (10)Curtis, M. D.; Shiu, K. B.; Butler, W. M.J . A m . Chem. SOC.1986, 108,1550. (11)Wong, W. K.; Wilson, T.; Strouse, C. E.; Gladysz, J. A.J . Chem. SOC.,Chem. Commun. 1979,530. (12)Wayland, B. B.; Woods, B. A. J . Chem. Soc., Chem. Commun. 1981,700. (13)Moloy, K. G.;Marks, T. J. J . A m . Chem. SOC.1984,106,7051. (14)Ziegler, T.; Versluis, L.; Tschinke, V. J . A m . Chem. SOC.1986, 108,612. (15)Tatsumi, K.;Nakamura, A.; Hoffmann, P.; Stauffert, P.; Hoffmann, R.J . A m . Chem. SOC.1985,107,4440. (16)Rappe, A.K. J . Am. Chem. SOC. 1987,109,5605, and references @
contained therein. (17)Koga, N.;Morokuma, K. J . A m . Chem. SOC.1986,108,6136. (18)Pacchioni, G.; Fantucci, P.; Koutechg, J.; Ponec, V. J . Cata2. 1988,112,34.
0743-7463/95/2411-0853$09.00/0
(GVB)calculations by Goddard et al. have shown that chemisorbed HCO radicals lead to a favorable chain reaction in the methanation of CO over Ni.19 There are three possible types of surface formyl, namely, +formyl-C,O, yl-formyl-C, and +formyl-0, bonding to the surface via both C and 0 atoms, via the C atom, and via the 0 atom, respectively, as shown in Figure 1. Surface formyl is usually characterized by its C-0 stretching frequency. The C-0 stretching frequencies fall between 1600 and 1750 cm-l for yl-formyl-C in metal formyl compoundss-10 and 1000and 1300 cm-l for $formyl-C,O in metal formyl compound^.^ The carbon-oxygen bond order is about 2 for ql-formyl-Cand about 1for $-formyl-
c,o.
Using high-resolution electron energy loss spectroscopy, Weinberg and co-workers have recently studied CO hydrogenation on Ru(001) a t low temperature.20!21Exposing gas-phase atomic H to a saturated CO overlayer on Ru(001) at 100 K results in reaction (via Eley-Rideal kinetics) under ultrahigh vacuum conditions. Both qlformyl-C and $-formyl-C,O are identified as initial reaction intermediates. The reported C-H stretching frequency is 2970 cm-l for both rl-formyl4 and $-formylC,O species. The observed C-0 stretching frequency is 1750 cm-l for yl-formy1-C and 1190 cm-l for q2-formylC,O, respectively.20,21 In this paper, we describe the adsorption of HCO on Ni(100) by a many-electron theory that permits a n accurate description of bonding a t the surface. The adsorbate and local surface region are embedded in the remainder of the lattice electronic distribution which is modeled as a 30-atom, three-layer cluster with boundary atom potentials determined from a 64-atom cluster by a n orbital localization transformation. Nickel 3d orbitals are explicitly included on the six nickel atoms of the surface region. Our theoretical approach allows us to calculate accurately the adsorption energies of HCO a t various types (19)Goddard, W. A,, 111; Walch, S. P.; Rappe, A. K.; Upton, T. H.; Melius, C. F. J . Vac. Sci. Technol. 1977,14,416. (20)Mitchell, W. J.;Wang,Y.;Xie,J.;Weinberg, W. H. J . A m . Chem. SOC.1993,115,4381.
(21)Mitchell, W. J.;Xie, J.; Wang, Y.; Weinberg, W. H. J . Electron Spectrosc. Relat. Phenom. 1993,64/65,427.
0 1995 American Chemical Society
Yang and Whitten
854 Langmuir, Vol. 11, No. 3, 1995
HCO and nickel atoms in the vicinity of the adsorption region, and 7 singly occupied orbitals, where 6 electrons are from the Ni 3d and 1electron is from the Ni 4s 4p band. The dominant SCF configuration with several other configurations, selected with a coefficient >0.05 in the initial expansion, defines the multiconfiguration expansion of I)O. Configuration interaction expansions, I), are generated from I)O, by single and double excitations, to give excited configurations, I),
+
q2-formyl-c,o
ly-formyl-c
7 1 ‘ f O w
Figure 1. Possible geometries of surface formyl, y2-formylC,O, yl-formyl-C, and rl-formyl-0,bondingtothe metal surface via both C and 0 atoms, via C atom, and via 0 atom, respectively.
of adsorption sites and to optimize the geometry of the HCO bound to the surface. The C-0 and C-H stretching frequencies are calculated from a one-dimensional harmonic oscillator model.
k
Configurations are retained if a n interaction threshold
2. Theory and Calculations
Total energy calculations are performed using a manyelectron embedding theory that permits the accurate computation of molecule-solid surface interaction^.^^-^^ Calculations are carried out at a n ab initio configuration interaction (CI)level, i.e., all electron-electron interactions are explicitly calculated and there are no exchange approximations or empirical parameters. The details of the method are discussed extensively in refs 25,26a, and 27. Calculations are performed by first obtaining selfconsistent-field (SCF)solutions for the metal cluster plus adsorbed species. The occupied and virtual orbitals of the SCF solution are then transformed separately to obtain orbitals spatially localized within the six-atom surface region. This unitary transformation of orbitals which is based upon exchange maximization with atomic valence orbitals enhances convergence of the configuration interaction (CI) e x p a n s i ~ n . ~ ~ - ~ ~ The selection of the localized occupied and virtual orbitals for the configuration interaction expansion calculations is based upon the degree of localization as measured by exchange interactions relative to the local region of the six Ni atom 4s orbitals plus HCO valence orbitals (excluding C and 0 1s). Letting {qh} denote the SCF orbitals and {sk} the valence atomic 4s orbitals of the six Ni atoms and the HCO valence orbitals, the positive definite exchange integral y = ($1’(1)(f)i’(2)lrl2-’lSk(l)ski!)) sk(2)) is maximized with respect to coefficients c1 of #1’ = &&,leading to a n eigenvalue problem. The orbitals that result are localized about the adsorption region (the six Ni atoms 4s plus the valence HCO orbitals) and, in the order of decreasing exchange eigenvalue, correspond to orbitals highly localized on the six Ni atoms region, orbitals describing bonds linking the six Ni atoms with the remainder of the lattice, and finally the interior bonds of the lattice. For a given localized orbital, +,’, a truncated orbital is defined by deleting all of the basis functions that belong to the adsorption region (i.e., the six Ni atoms plus the HCO). The resulting orbitals $Tis renormalized and the overlap (qY’l#’) with the original orbital is calculated. The value of the overlap is referred to a s the degree of localization. In the present case, orbitals with a degree of localization (+”I@’) greater than 0.5 are used in the CI. This criterion leads to 29 electrons and 28 virtual orbitals. There are 11doubly occupied orbitals involving (22) Whitten, J. L.; Pakkanen, T. A. Phys. Rev. B. 1980,21, 4357. (23)Whitten, J. L. Phys. Rev. B 1981,24, 1810. (24) Cremaschi, P.; Whitten, J. L. Sur6 Sci. 1985, 149, 273. (25) Whitten, J. L. Chem. Phys. 1993, 177, 387. (26) (a) Cremaschi, P.; Whitten, J. L. Theor. Chim. Acta 1987, 72, 485. Detailed localization procedure is discussed. (b) See an article by Siegbahn, P. E. M. Int. J . Quantum Chem. 1983,23, 1869. (27) Madhavan, P.; Whitten, J. L. J . Chem. Phys. 1982, 77, 2673.
is satisfied. In the present work CI expansions contain approximately 3500 configurations. Contributions of excluded configurations are estimated using second-order perturbation theory. The inclusion of several other configurations are reference states besides the SCF dominant configuration increases the correlation energy by increasing variational degrees of freedom.26b In calculation of the energies for HCO bonding at various sites on the nickel cluster, the positions of the Ni atoms in the cluster model were held fixed with a nearest neighbor Ni-Ni distance of 2.48 A taken from the bulk value. The geometry of adsorbed HCO is optimized point by point, first optimizing the C-surface or 0-surface distance by keeping the HCO bond angle and C-H and C-0 distances at their gaseous values and then optimizing the HCO bond angle by keeping all the distance parameters unchanged. Basis superposition contributions to the total energy were taken into account by calculating the energy of the Ni cluster with the adsorbed species’ virtual basis present (but not the adsorbate nuclei). Calculated basis superposition corrections to the total energy are between 4 and 6 kcallmol. The basis superposition contribution of the metal orbitals to HCO is negligible. All energies reported in the present work include the basis superposition contributions. The cluster geometry and surface region of the Ni(100) surface are shown in Figure 2. The three-layer, 64-atom cluster, consists of a surface layer of 30 atoms, a second layer of 18 atoms, and a third layer of 16 atoms. The embedding procedure is used to reduce the Nie4cluster to a 30-atom model depicted as shaded atoms: the surface layer of 16 atoms, a second layer of 8 atoms, and a third layer of 6 atoms. For the local surface region of six nickel atoms, a n effective l l s - 3 ~ 1core potential and valence 3d, 49, and 4p orbitals are used. Other Ni atoms are described by a n effective core potential for Lls-3dI electrons and a single 4s orbital. For all boundary atoms, and those in the third layer, the core potential is further modified to account for bonding to the bulk as defined by the embedding procedure (see refs 25 and 26a). The ground state of the final Ni30 cluster is 7A1with six singly occupied 3dzz orbitals and a closed-shell 4s band. The valence basis of 4s (five terms), 3d (four terms), and 4p (three terms) for nickel is a modified Gaussian basis of WachtersZ8and is published in our previous chemisorption studies of hydrogen on nickel.29 The procedure for construction and optimization of the Ni basis is reported in ref 30. The double zetas basis for hydrogen (28) Wachters, A. J. H. J . Chem. Phys. 1970, 52, 1033. (29)Yang, H.; Whitten, J. L. J . Chem. Phys. 1988, 89, 5329. (30) Madhavan, P.; Whitten, J. L. Chem. Phys. Lett. 1986,127,354.
Adsorption of Formyl on Ni(100)
Langmuir, Vol. 11, No. 3, 1995 855 a very recent He-atom diffraction value36of 1.86 f 0.07
A for H on Ni(111). In addition, the calculated H-surface
surface layer M
second layer
w third layer surface sites F: Cfold B: bridge A: atop
TTt
Figure 2. Cluster geometry and local region of the nickel cluster used to model the ( 100) crystal face of nickel. The three-layer 64-atom cluster consists of a surface layer of 30 atoms, a second layer of 18 atoms, and a third layer of 16 atoms. Embedding theory is used to reduce the Ni64 cluster to a 30-atom model depicted as shaded atoms. Atoms surrounding the six central atoms in the surface layer, those surrounding the two central atoms in the second layer, and the shaded atoms in the third layer are described by fixed electron distributions, corresponding to (ls-3p ~ o r e ) ( 3 d ) ~ ( 4configurations, sF x = 3/1~,'/6, and 2/3, respectively. Orbitals for the Ni atom ground state are used to define the configurations. All atoms have Phillips-Kleinman projectors z l Q m ) ( Q m l ( -e,,,) for the fixed electronic distribution, where Q m are Is, 2s, 2p, 3s, 3p, and 3d atomic orbital. The nearest neighbor Ni-Ni distance is 2.48 A. The notations, F, B, and A, refer to four-fold, bridge, and atop sites, respectively.
augmented with a set of 2p polarization functions with an exponent of 0.6 is taken from ref 29. The triple zeta s and p basis for carbon is taken from Whitten31and augmented with a set of d polarization functions with a n exponent of 0.626. The triple zeta s and p basis for oxygen is taken from our previous molecular studies of 0 2 and augmented with a set of d polarization fimctions with a n exponent of 0.8.32 The same basis and core potentials are used in all subsequent calculations on the Ni(100) surface of HCO adsorption calculations. The present embedding approach allows us to calculate accurately the energetics and geometries of adsorbed species on various types of adsorption sites on nickel surfaces. In the case of hydrogen adsorbed on a 28-atom7 three-layer cluster of Ni(11l),'i3 the calculated adsorption energy of 62 kcallmol for H a t three-fold sites is in agreement with a thermal desorption value of 63 kcall mol.34 The calculated H-Ni distance of 1.86 A at threefold sites is in excellent agreement with both a low-energy electron diffraction (LEED) value35of 1.84 f 0.06 A and (31)Whitten, J. L. J . Chem. Phys. 1966,44, 359. (32)Yang, H.; Hanson, D. M.; Trentini, F.; Whitten, J. L. Chem. Phys. 1990,147, 115. (33) Yang, H.; Whitten, J. L. Surf. Sci. 1991,255, 193. (34) Christmann, K.; Schober, 0.; Ertl, G.; Neumann, M. J . Chem. Phys. 1974,60,4528. (35) Christmann, K.; Behm, R. J.; Ertl, G.;Van Hove, M. A.; Weinberg, W. H. J . Chem. Phys. 1979, 70,4168.
vibrational frequency and H orbital levels are also consistent with electron energy loss spectroscopy (EELS) and ultraviolet photoelectron spectroscopy(UPS) results. Thus, our previous calculations demonstrated that the characteristic features of H chemisorption on Ni(111) obtained from different experiments are well reproduced by the present embedding theory. Other adsorption studies of polyatomic molecules such as H20, NH3, C6H6, etc. on nickel surfaces have been r e p ~ r t e d ,where ~ ~ - ~the ~ difference of adsorption energy between our calculations and the experimental thermal desorption values is less than 2 kcallmol.
3. HCO Adsorption on Ni(100) By use of the same basis mentioned in the previous section, the calculated C-H and C-0 bond distances for gaseous HCO at the configuration interaction level are 1.16 and 1.19A;the HCO angle is 121",and the C-H and C-0 vibrational frequencies are 2885 and 1950 cm-l. Previous a b initio calculations, most using a double zeta s and p basis for C, 0 plus a set of d polarization functions and a double s basis for H plus a set of p polarization functions, showed that rc-0 ranges from 1.16 to 1.19 A, rC-H from 1.11 to 1.13 A, HCO angle from 123" to 130°, C-H stretching from 2815 to 2920 cm-l, and C-0 stretching from 1900 to 2050 cm-l, r e s p e ~ t i v e l y . ~Our ~-~~ calculated parameters of bond length and angle are used in the initial distance optimization of HCO to the surface. The sites considered for HCO adsorption are as follows: a four-fold hollow, a bridge site, and an atop Ni site, denoted by F, B, and A, respectively, in Figure 2. Oxygen or carbon distances from the surface are optimized for all the adsorption sites studied. Methanol decomposition on metal surfaces gives several possible intermediates with molecular formula HCO: the aforementioned three types of formyl, +formyl-C, qlformyl-0, and q2-formyl-C,0, plus the carbon bonded alcohol -COH. The -COH species has a broad O-H stretching vibration around 3400 cm-l and is fundamentally different than the formyls. We considered -COH adsorption on Ni(100) and found a significantly higher total energy than for the most stable + f ~ r m y l - C . ~ ~ Figure 3 shows the calculated minimum energy geometry for HCO at different adsorption sites. Adsorption energies, C- and O-surfaceequilibrium distances, and C-0 and C-H stretching frequencies are reported in Table 1. Our calculations show that there is essentially no adsorption energy difference between the four-fold, bridge, and atop sites-an unusual result for an adsorbate with unsaturated valence. The calculated adsorption energies are 63.7, 63.5, and 63.6 kcallmol for HCO at four-fold, bridge, and atop sites, respectively. The calculated (36) Gross, G.; Rieder, K. H. Surf. Sci. 1991,241, 33. (37)Yang, H.; Whitten, J. L. Surf. Sci. 1989,223, 131. (38) Chattopadhyay, A.;Yang, H.; Whitten, J. L. J . Phys. Chem.1990, 94, 6379. (39) Yang, H.; Whitten, J. L.; Markunas, R. J. Surf. Sci. 1993,294, L945. (40)Bruna, P. J.;Buenker, R. J.; Peyerimhoff, S. D. J . Mol. Spectrosc. 1976,32, 217. (41)Adams, G. F.; Bent, G. D.; Purvis, G. D.; Bartlett, R. J. J . Chem. Phys. 1979, 71,3697. (42) Dunning, T. H. Jr., J . Chem. Phys. 1980, 73, 2304. (43) Geiger, L. C.; Schatz, G. C. J . Phys. Chem. 1984,88,214. (44) Bowman, J. M.; Bittman, J. S.; Harding, L. B. J . Chem. Phys. 1986, 85, 911, and references contained therein. (45) The total energy of adsorbed -COH is higher by 76 kcaVmol than for ql-formy1-C on Ni(100). Relative to gaseous -COH, the adsorption is exothermic, 108 and 105 kcal/mol at the bridge and 4-fold sites, respectively.
856 Langmuir, Vol. 11, No. 3, 1995
Yang and Whitten 120"
\?
-
4-fold E a = 63.7(kdtnol)
top view at 4-fold:
-'
so
I
I
bridge
atop
63.5 (kcaVmd)
63.6 (kcaVmd)
I C
I I I rotational barrier less than 0.3 kcaVmol for all sites
H \ l a \
-
115
l.93*pTo
--e i
4-fold site equilibrium geometry
Figure 3. Calculated minimum energy geometries for HCO adsorbed at four-fold, bridge, and atop sites on Ni(100). Positive values are exothermic. Only the six Ni atoms of the surface layer are shown.
Table 1. ttl-Formyl-CAdsorption on Ni(100) sitea [j (degIb a (deg)c E,dsd (kcaVmol) RC-sudacee Ro-sudacee
(A) (A)
C-0 stretching (em-') C-H stretching (em-') rc-o (A> rC-H
(A)
4-fold 90" 120" 63.7 1.93 1.93 1761 2950 1.27 1.17
bridge 90" 120" 63.5 1.94 1.94 1761 2945 1.26 1.17
I
atop
110" 120" 63.6 2.04 2.45 1758 2933 1.27 1.14
Four-fold site, F; bridge site, B; atop site, A; see Figure 2. /3 is defined as the 0-C-surface normal angle. When /3 is go", the C-0 bond is parallel to the surface; see Figure 3. These angles correspond to the minimum energy for HCO on Ni. a is the HCO bond angle; see Figure 4. E a & is relative to the ground state of HCO at infinite separation from Ni(100). Positive values are exothermic. Results are from configuration interaction calculations and are corrected for basis superposition effects. Adsorption energies without basis superposition correction are 67.6,66.9, and 68.6 kcaVmol for HCO a t 4-fold,bridge, and atop sites, respectively. e R c - s u d a c e and R o - s u d a c e are the perpendicular distances from the carbon nucleus and the oxygen nucleus, respectively, to the surface plane of nickel nuclei. a
minimum energy geometries for HCO at both four-fold and bridge sites have an 0-C-surface normal angle of go", Le., the C-0 bond is parallel to the surface with a HCO bond angle of 120". Calculated C- and 0-surface distances are 1.93 A a t four-fold sites and 1.94 A at bridge sites. For HCO adsorbed at the atop site, the O-Csurface normal angle is 110" with a HCO bond angle of 120"and a C-surface distance of 2.04 A. The bond-order conservation-Morsepotential (BOC-MP) calculations by Shustorovich predict an adsorption energy of HCO on Ni(111)surface of 50 kcaVm01.~~ The calculated C-0 stretching frequencies are around 1760 cm-l for HCO adsorbed at each site and C-H stretching frequencies are 2950 and 2945 cm-l at fourfold and bridge sites and 2933 cm-l at atop sites. Thus, the C-H stretching frequency for HCO adsorbed on Ni(100) is similar to the calculated gaseous value. The calculated C-0 stretching frequencies for HCO at the atop site with /3 = 110" and at the bridge and four-fold sites with /3 = 90" are in the range for a double carbonoxygen bond in metal formyl compounds.s-10 Thus, our (46) Shustorovich, E. In Advances in Catalysis;Eley, D. D., Pines, H., Weisz, P. B., Eds.; Academic Press: San Diego, CA, 1990; Vol. 37, p 133.
4.4
120
0.0
125
+2.7
130
6.6
135
+12.1
(4b)
Figure 4. (a)Rotation of 90" about t h e surface normal at fourfold sites on Ni( 100). (b) Energetics as a function of a,t h e HCO bond angle, at the four-fold site with the 0-C-surface normal angle fixed at 90".The distances ofC-H, C-0, and C-surface a r e kept at their equilibrium values. Only the six Ni atoms of the surface layer are shown.
calculations suggest that HCO binds to the nickel surface at each site mainly via the carbon atom and, based on the C-0 stretching, would be characterized as +formyl-C. Figure 4a shows that adsorbed ql-formyl-Con Ni(100) can freely rotate about the surface normal at all sites considered. The energy barrier for a 90" rotation is 0.1, 0.3, and 0.3 kcal/mol for HCO at four-fold, bridge, and atop sites, respectively (with the HCO angle and the distances of C-H, C-0, and C-surface kept at their equilibrium values). Figure 4b shows the energy change as a function of a,the HCO bond angle, for HCO adsorbed at the four-fold site with the 0-C-surface normal angle of 90". The energy minimum occurs at a = 120". Since a = 115"is only 0.4kcdmol higher in energy,this suggests that low-energy fluctuations in the HCO angle about 117 f 2" are allowed. Figure 5 shows the calculated energy change as a function of /3, the 0-C-surface normal angle for HCO adsorbed at a four-fold site. These results provide convincing evidence that tilting H toward the Ni surface is energetically unfavorable. Ifp = 150",the H-C-surface normal angle becomes go", and the calculated adsorption energy is 43 kcal/mol, about 21 kcal/mol less stable than the equilibrium angle of /3 = 90". A similar trend is also found for HCO adsorbed at bridge and atop sites; however, the energy vs /3 curve is flatter at the atop site. Figure 6 compares the calculated adsorption energies, C-0 and C-H stretching frequencies, angles, and bond distances for two typical HCO geometries adsorbed at fourfold, bridge, and atop sites. The HCO bond angle of 120" is kept unchanged in all calculations. In Figure 6, the results for an 0-C-surface normal angle and a H-Csurface normal angle each equal to 120" are compared with those for the calculated energy minimum geometries at each site. For HCO at the atop site with an O-Csurface normal angle of go", the calculated adsorption energy is 60.7 kcavmol. The adsorption of r,Aformyl-O, Le., HCO bonding to the surface via 0 atom, was also studied. In the initial surface distance optimization, there are two singly occupied orbitals on the C atom in the SCF calculations. Final CI calculations are performed for both high and low spin states relative to these singly occupied orbitals. The low
Adsorption of Formyl on Ni(100)
Langmuir, Vol. 11, No.3, 1995 857 Table 2. ql-Formyl-0 and ql-Formyl-CAdsorption on Ni(100)'" +formyl-O +formyl-C site 4-fold 4-fold &sa (kcal/mol) 47.4 63.7 RO-su*aceb
>,
4-fold site
F a3
rc-H
a
E
.-0
a
U
E 0 I
a
(A)
/3 (deg)c
E
W
U
(A)
C-0 stretching(cm-l) C-H stretching(cm-l) rc-o (8)
'pv
I
-70! 60
-
1
80
.
1
-
1
-
I
I
-
I
100 120 1 4 0 160 180
Angle of 0-C-Surface Normal,
p
1.51 1370 2988 1.33 1.13 180 110
1.93 1761 2950 1.27 1.17
90 120
Ea&is relative to the ground state of HCO at infiniteseparation
from Ni(100). Positive values are exothermic. Results are from configuration interaction calculations and are corrected for basis superposition effects. Ro-surfaceis perpendicular distance from oxygen to the Ni surface. pis defined as the 0-C-surface normal angle and a is the HCO bond angle. 8 = C-0-surface normal angle
(")
Figure 5. Adsorption energy versus /?,the 0-C-surface normal angle, for +formyl-C at the four-fold site. The HCO angle and the distances of C-H, C-0, and C-surface were kept at their equilibrium values. Negative value is exothermic. The energy minimum occurs at /3 = go", where the C - 0 bond is parallel to the surface. I
e= E&=
54.7
I
I
180" 47.4(kcal)
160" 51.1 (kcal)
120" 54.9 ()ccal)
63.7
e- Ea&(kcaVmOl) -->
H
C
1 1761 cm" 0
I i.94A
56.2
1.94A
e--- Ea& (kcalrrnol) --->
I
63.5
e= b=
1100 56.8(kcal)
I
90" 63.5 (W
Figure 7. A conversion pathway from 7'-formyl-0 to +formylC. 8 is the C-0-surface normal angle. 8 = 180"refers to the
calculated energy minimum structure of +formyl-O adsorbed at four-fold sites with an adsorption energy of 47.4 kcaumol. 8 = 90"refers t o the calculated energy minimum geometry of 7'-formy1-C adsorbed at bridge sites with an adsorption energy of 63.5 kcaymol. The oxygen atom above the four-fold site is unchanged during the rotation. I
61.7
63.6
Figure 6. Calculated adsorption parameters for two typical HCO geometries at four-fold, bridge, and atop sites. Positive values of adsorption energy are exothermic.
spin state, which is a singlet carbene-like state with one doubly occupied carbon orbital, is the lowest energy state for most of the geometries considered. The 0-surface distance, C-0 and C-H bond distances, and HCO bond angle are optimized. The calculated adsorption energy for ql-formyl-0, relative to the ground state of HCO at infinite separation from the surface, is 47.4 kcaymol at the four-fold sites,
32.3 kcal/mol at the bridge sites, and 22.8 kcaYmol at the atop sites. Thus, t h e latter two sites are significantly less favorable than the four-fold site. Table 2 reports the calculated results for 7'-formyl-0 adsorbed at a four-fold site along with results for yl-formyl-C at the same site. The calculated C-0 and C-H stretching frequencies are 1370 and 2988 cm-l for yl-formyl-0, compared to 1761 and 2950 cm-l for yl-formyl4 at a four-fold site. The calculated 0-surface distance is 1.51 A for r,J-formyl-O, whereas it is 1.93 A for 7'-formyl-C. Our calculations therefore show that yl-formyl-0 is less stable than +formyLC by 16 kcal/mol on Ni(100). When the C-0 bond is tilted toward the surface, as shown in Figure 7, yl-formyl-0 converts to yl-formyl-C with essentially no energy barrier. Figure 7 shows the calculated energetics and geometries for some of the intermediates.
858 Langmuir, Vol. 11, No. 3, 1995
Yang and Whitten
Table 3. HCO and Nickel Mulliken Charge Distributiona yl-formy1-C site 4-fold bridge atop
Pb (deg) 120 90 120 90 120 110
H -0.06 -0.05 -0.07 -0.06 -0.03 -0.04
C -0.55 -0.41 -0.47
-0.40 -0.21 -0.19
Ni Layers
0 -0.07 -0.25 -0.11 -0.24 -0.15 -0.18
I (Ni 3dT f0.77 (+0.02) f0.72 (+0.03) +0.68 (+0.05) +0.65 (+0.03) +0.48 (+0.10) +0.49 (+0.10)
I1 +0.04
+0.07 f0.22 +0.24 +0.06 +0.06
I11 -0.13 -0.14 -0.27 -0.25 -0.15 -0.14
pzd (D) 1.46 3.10 3.03 4.35 1.33 1.73
a Net charges are in units le1 from a Mulliken population analysis of CI wavefunctions containing 3500 configurations compared to infinite separation between HCO and clean surface including basis superposition corrections. In gaseous HCO, the electronic charges for H, C, and 0 atoms are -0.961e1, -5.711e1, and -8.331e1, respectively. I, 11, and I11 refer to the Ni surface layer, the second layer, and the third layer, respectively. ,b is defined as the 0-C-surface normal angle. Total amount of Ni 3d electrons transferred from the neighboring four Ni atoms, two Ni atoms, and the underneath Ni atom for HCO at four-fold, bridge, and atop sites, respectively. pz is the calculated surface, ((-''adsorbate. dipole moment component normal to the surface. Positive values indicate a dipole pointing toward HCO, Le.,
"+"
Starting with yl-formyl-0 adsorbed a t a four-fold site on Ni(100), as shown in Figure 7a, where the C-0-surface normal angle, 8, is 180", the calculated adsorption ener is 47.4 kcallmol for a n oxygen-surface distance of 1.51 Tilting HCO counterclockwise by 20", 60", 70", and go", corresponding to 8 = 160", 120", 110", go", causes the system to become progressively more stable. The optimized 0-surface distance also increases to 1.55, 1.65, 1.79, and 1.94A, respectively. The position of the oxygen atom above the four-fold site is unchanged during the rotation. The final geometry of 6 = 90" corresponds to r'-formyl-C adsorbed near the bridge site.
x
4. Discussion The calculated C-Ni distance of 2.04 A for ql-formyl-C a t a n atop site atp = 110"is very similar to the determined C-metal distance of 2.055 A for the metalloformyl compound of v ~ - C ~ H ~ R ~ ( P H ~ P ) ( N O )and ( C Hclose O ) ~to~ the value of 2.01 f 0.01 A for terminally bonded sixcoordinated $-acyl complexes of V, Mo, and W,l0 but relatively longer than the 1.869 A of RhOEP(CH0) (OEP = octaethylporphyrin).8 The observed C-0 stretching frequencies are 1558 and 1700 cm-' for the Re and Rh metalloformyl complexes and 1500 to 1600 cm-' for the +acyl complexes. For HCO adsorbed at bridge and four-fold sites, our calculations show that the energy minimum occurs at /3 = go", i.e., both C-surface and 0-surface distances are the same. The calculated C-0 stretching frequency of 1760 cm-l, which is in the range of a carbon-oxygen bond order of about 2 in metal formyl compounds, leads to the conclusionthat +formyl-C,O with a carbon-oxygen bond order near 1 is not formed in our calculations. To date, yl-formyl-C and $-formyl-C,O on Ru(001), which were very recently identified by Weinberg and cow o r k e r ~ ,are ~ ~the , ~ only ~ surface formyls observed spectroscopically. The observed C-0 stretching frequency for $-formyl-C,O is 1190 cm-l and that for r,J-formy1-C is 1750 cm-'. On the basis of the assigned C-0 stretching frequency, +formyl-C,O corresponds to a carbon-oxygen bond order =l.15J6 Our calculated geometry for HCO a t bridge and fourfold sites, shown in Figure 3, would suggest a v2-formylC,O formation. Both C-surface and 0-surface distances are =1.94 A for the bridge and four-fold sites. The corresponding distances to Ni nuclei Rc-N~ and RO-Ni are 2.30 and 2.60 A a t the bridge site and 2.61 and 2.29 A a t the four-fold site, respectively. The van der Waals 0-Ni and C-Ni distances are 3.15 and 3.33A, respectively (1.70 A for C, 1.52 A for 0, and 1.63 A for Ni). Among the transition-metal +acyls (both C and 0 in -COR are bound to the metal atom) which have been structurally characterized, there are no correlations between the parameters, Rc-M,rc-0, A = (Rc-M- Ro-MI,
and the acyl C-0 stretching frequency (M = V, Mo, W, Ru, Tp, Ti, Zr, and Th).l0 The acyl C-0 distance varies irregularly between 1.18 and 1.27 A, while A varies from 0.44to -0.07 A. The acyl C-0 stretchingfrequency varies from 1469 to 1620 cm-l. The acyl C-0 stretching frequency is lowered by at least 150 cm-' to 400 cm-l compared to the terminally bound CO group in the same compound;1° thus these complexes are characterized as +acyl. Our calculated C-0 stretching frequencies for adsorbed formyl are shifted down about 200 cm-l compared to the gaseous value of 1950 cm-'. This probably reflects the contribution of metal-n back-bonding. Table 3 lists the net charges from Mulliken population analyses of CI wavefunctions containing 3500 configuration^.^^ For qlformyl-C adsorbed a t an atop site with the 0-C-surface normal angle of 110", our calculations show that HCO gains 0.4 electron from the Ni surface, about 0.10 e of which comes from the 3d shell of the Ni atom underneath. This is a typical d-n* interaction between Ni and formyl CO. In contrast, the 3d electron transfer is much smaller a t four-fold and bridge sites. However, electron transfer from Ni 4s to n* orbitals occurs for adsorption a t each site. For HCO adsorbed a t a four-fold site, about 0.7 electron is transferred from the surface (mainly from the 4s and 4p electrons) to HCO for p = 120" and 90". The Ni 3d electron transfer from the neighboring four Ni atoms is very small. As HCO is tilted so that oxygen is closer to the surface, oxygen also gains electrons and HCO becomes more stable. The increase in stability is also accompanied by a n increase in the dipole moment normal to the surface. For p = 120" and go", the normal component of the dipole moment is 1.46 and 3.10 D, respectively, for HCO a t fourfold sites, a net increase of 1.64 D, when 0 approaches the surface. The SCF eigenvalue spectra for gaseous HCO and for adsorbed $-formyl-C on Ni(100) are shown in Figure 8. These results and the Mulliken populations provide information about orbital mixing on HCO adsorption. Table 4 summarizes the Mulliken populations of Ni and HCO orbitals for 7'-formy1-C. On bonding to the surface, the 5a',6a',la", and 7a'(unpaired electron in the 7a' orbital in the gas phase) symmetry arbitals of formyl (C, classification) are the orbitals principally involvedin bonding. Figure 8 shows that HCO orbital levels are similar for HCO adsorbed a t four-fold and bridge sites. When ,8 is 120", the 5a', 6a', and la" orbitals mix substantially with the Ni 3d orbitals, and the 7a' orbital forms a strong a-type bond with the Ni 4s and 4p orbitals. When p is go", i.e., the C-0 bond is parallel to the surface, the 5a', 6a', and (47) Although not useful for the precise assignment of atomiccharges, these populations provide qualitative information on major shifts in the charge density.
Adsorption of Formyl on Ni(100)
- HCO
For HCO at the atop site a t ,8 = 110", there are appreciable contributions of Ni 4s, 4p, and 3d orbitals mixing with the 6a' orbitals. The la" orbitals mix mainly with Ni 3d orbitals, and show the near degeneracy feature seen for adsorption at four-fold and bridge sites a t /3 = 90". The 4a' orbital retains its gas phase character, with Ni contributions of less than 2% for all sites studied. Although the oxygen atom attracts more electrons a t p = 90" than at /3 = 120" for HCO adsorbed a t both four-fold and bridge sites, this does not necessarily mean than an 0-surface bond is formed. In fact, the calculated C-0 stretching frequency around 1760 cm-I a t p = 90" suggests the C-0 bond order remains ~ 2 At@ . = go", the stabilized 5a', 6a', and la" orbitals are also responsible for the stability of yl-formyl-C adsorbed a t both four-fold and bridge sites.
HCO/Ni( 100) Sites
- +Ni
-0.1
Langmuir, Vol. 11, No. 3, 1995 859
P= -4
- --
-
-0.3 -
-
- --
-
-70'
- -
A
-
?
0 W
-0.9
'
Figure 8. SCF eigenvalues for ql-formyl-C adsorbed on Ni(100). The symmetry of orbitals (4a', 5a', 6a', la", and 7a') for adsorbed HCO (C, classification) is indicated. The left most eigenvalue spectrum is for the clean surface plus gaseous HCO. For gaseous HCO, the orbital levels are -0.823, -0.688, -0.592, -0.572, and -0.380 hartree for 4a', 5a', 6a', la", and 7a', respectively. The Fermi level for the clean surface,&, is -0.192 hartree. The notation s, p, and d refers to the Ni 4s, 4p, and 3d orbitals. /3 is the 0-C-surface normal angle. Table 4. Ni 3d, 4s, and 4p Mulliken Populations of q1-Formyl-COrbitalsa site atop
5a'
6a'
1a'@
7a'
(p = 110") 0.10(d) 0.14(sp) + 0.15(d)
bridge
(p = 120") 0.07(d) 0.36(d) (p = 90") 0.07(d) O.O4(sp)+ 0.16(d)
4-fold
(p = 120") 0.05(d) 0.4(d) (p = 90") 0.10(d) O.O5(sp)+ 0.2(d)
a Corresponding eigenvalue spectra are shown in Figure 8. The HCO 4a' orbital retains its gas phase character, with Ni contributions of less than 2% for all sites studied. There are two nearly degenerate la"-Ni 3d orbital levels for HCO at the atop site at p = 110" with ~ 0 . eV 1 spacing, and a t the four-fold and bridge sites 4 spacing. a t p = 90" with ~ 0 . eV
la" orbitals are stabilized and shified down approximately by 0.9,0.6, and 1.6 eV, respectively, for HCO a t four-fold and bridge sites. In addition, another la"-Ni 3d orbital level with smaller la" contributions occurs, split off by ~ 0 . eV. 4
5. Conclusions The adsorption of HCO radicals on the Ni(100) surface is studied by using a many-electron embedding theory a t a n ab initio configuration interaction level. The results are summarized as follows: (1) The Ni(100) potential surface is very flat for yl-formy1-C. The calculated adsorption energies are 63.7, 63.5, and 63.6 kcal/mol a t four-fold, bridge, and atop sites, respectively. (2) The energy minimum occurs for an 0-C-surface normal angle of go", go", and 110" a t four-fold, bridge, and atop sites, respectively. The HCO bond angle is 120", and the corresponding C-surface distances are 1.93,1.94, and 2.04 A. (3)Calculated C-0 stretching frequencies are around 1760 cm-I and C-H stretching frequencies are around 2940 cm-I for yl-formyl-C a t all adsorption sites and geometries considered. The calculated C-0 stretching frequency is in the range for a carbon-oxygen double bond suggesting that y2-formyl-C,Ois not formed. (4) The bonding of yl-formyl-C to the nickel surface involves both ionic and covalent contributions with substantial mixingwith Ni 3d orbitals. Ni 3d orbitals are involved in the formation of the HCO bond a t atop sites. The stabilized 5a', 6a', and la" orbitals make tilted ylformyl4 a t both four-fold and bridge sites as stable a s at the atop site. (5) yl-Formyl-0 is energetically less stable than ylformy1-C by 16 kcaymol. Calculated adsorption energies for yl-formyl-0 are 47.4, 32.3, and 22.8 kcaumol a t fourfold, bridge, and atop sites. The calculated C-0 and C-H stretching frequencies are 1370 and 2988 cm-I a t fourfold sites. No energy barrier occurs for the conversion of yl-formyl-0 to yl-formyl-C.
Acknowledgment. Support of the work by the U.S. Department of Energy is gratefully acknowledged. H.Y. thanks Professors C. M. Friend, R. J. Madix, and W. H. Weinberg for helpful discussions and encouragement during the course of the work. LA940234U
