5090
J. Phys. Chem. 1996, 100, 5090-5097
Theoretical Adsorption Studies of HCN and HNC on Ni(111) Hong Yang* and Jerry L. Whitten Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204 ReceiVed: NoVember 2, 1995X
The interaction of hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) with Ni(111) is studied by using ab initio embedding theory. The Ni(111) surface is modeled as a three-layer, 28-atom cluster with the Ni atoms fixed at bulk lattice sites. The present calculations show that both HCN and HNC bind to the surface in an end-on geometry with the molecular axis perpendicular to the surface. Tilting either the H atom or the H-C bond or the H-N bond away from the surface normal destabilizes the system. The side-on bonded HCN and HNC with the C-N bond parallel to the surface are energetically less stable than the corresponding end-on bonded species. The calculated adsorption energy of the end-on HCN is 18 kcal/mol with the fcc 3-fold site favored over other sites by about 1-3 kcal/mol. The end-on HNC binds to the surface at the atop site with adsorption energy of 11 kcal/mol. The calculated adsorption energy for the side-on bonded HCN is only 7 kcal/mol. The side-on bonded HNC species is found to be unbound by 12 kcal/mol on Ni(111). Calculated C-N stretching frequencies are 2200 cm-1 and 2100 cm-1 for end-on bonded HCN and HNC and 2105 cm-1 and 1530 cm-1 for the side-on bonded HCN and HNC, respectively. Dipole moment calculations show that the bonding of HCN to the surface is relatively more ionic than HNC.
1. Introduction The determination of the structure of adsorbed CN-containing molecules on metal surfaces is essential to the understanding of their bonding and reactivity in catalysis and other surface phenomena. Hydrogen cyanide (HCN) is the simplest nitrile molecule containing the carbon-nitrogen triple bond. Thus, studies of the interaction of hydrogen cyanide and hydrogen isocyanide (HNC) with the Ni(111) surface can provide insight into the C-N, C-H, and N-H bond activation. The adsorption of HCN and other CN-containing molecules on single-crystal transition-metal surfaces has received a great deal of attention experimentally during the past decade.1-49 Hagans et al. studied the reaction of HCN on Ni(111) by using surface techniques including scanning kinetic spectroscopy (SKS), temperature-programmed desorption (TPD), and Auger electron spectroscopy (AES) in the temperature range 84-1000 K.2 It was found that HCN completely dissociates on Ni(111) to form Had, Nad, and Cad at low coverage, while at saturation coverage only one-third to one-half of the adsorbed HCN decomposes. Two energetically similar HCN desorption states were observed at 258 and 271 K in both SKS and TPD and the HCN desorption activation energies were estimated to be in the range 18-20 kcal/mol. Primarily from the TDS study, end-on bonding of HCN on Ni(111) was inferred.2 Hagans et al. also studied the adsorption, desorption, and decomposition of HCN on the Pt(111) and Pt(112) surfaces by SKS, TPD, LEED, and work function measurements (∆φ) in a similar temperature range, 87-1000 K.3 The high step density Pt(112) surface was found to be more effective in promoting dissociation of HCNad f CNad + Had. Both HCNad and CNad were found to cause an overall work function decrease on Pt(111) by -1.2 and -0.8 eV. LEED measurements in conjunction with TPD and ∆φ results predict that HCN binds to the surface via the nitrogen atom with the molecular axis normal to the surface.3 Hemminger and co-workers studied the adsorption of HCN on Pt(111) by X-ray photoelectron spectroscopy (XPS) as a function of anneal temperature.4 The N 1s spectra for the * Author to whom correspondence should be sent: Tel, (919)515-3351; FAX, (919)515-5079; E-mail, hong
[email protected]. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-5090$12.00/0
adsorption of HCN on Pt(111) at 200 K showed two distinct chemical species with N 1s binding energies of 398.6 and 396.9 eV. The weakly bonded species, with the higher N 1s binding energy, was interpreted as molecular HCN bonded to the Pt via the nitrogen atom with the CN bond perpendicular to the surface, while the more strongly bonded species, with the lower N 1s binding energy, was thought to be CNad formed from dissociative adsorption of the HCN at low temperature.4 The results were consistent with early TDS studies of HCN on Pt(111).5 Very recently, Jentz et al. studied the thermal chemistry of HCN on Pt(111) in temperature range 85-470 K by Fourier transform infrared absorption spectroscopy (FT-IRAS) and TDS.6 It was found that HCN is molecularly adsorbed at 85 K and is assumed to bond in an end-on geometry through the nitrogen lone pair. HCN began to dissociate into Had and CNad above 100 K, and some of the CNad rehydrogenated to form hydrogen isocyanide (HNC) between 200 and 250 K. Finally, HNC hydrogenated above 250 K, forming of a new surface intermediate, aminomethylidyne (CNH2).6 Kordesch et al. conducted a series of experiments on HCN on Pd(111) using high-resolution electron energy loss spectroscopy (HREELS) and TDS,7 near-edge X-ray absorption fine structure (NEXAFS) and HREELS,8 and angle-resolved photoelectron spectroscopy (ARUPS).9 They found that HCN is reversibly chemisorbed on Pd(111) as HCdN, which is only present in multilayers at low temperature. Desorption of the chemisorbed molecular HCN occurred at 425 K. Both HREELS and NEXAFS studies definitely showed that HCN bonds to Pd(111) with the CN axis parallel to the surface. The dominant species was identified as di-σ-bonded η2-HCN-C,N. ARUPS identified four adsorbate-induced features at 5.3, 8.5, 11.0, and 13.3 eV below the Fermi level EF for HCN on Pd(111). Only a tentative assignment of the spectral features was made in this synchrotron study.9 The C-N axis was also found to be parallel to the surface on Cu(111) by HREELS10 and on Cu(110) by ARUPS11 based upon the adsorption of HCN on the copper surfaces. Serafin and Friend studied the adsorption of HCN on the W(100)-(5×1)-C surface by using XPS, HREELS, and temperature-programmed reaction spectroscopy.12 Two surface species were present at 200 K. One was identified as the © 1996 American Chemical Society
Theoretical Adsorption Studies R-HCN state with a maximum rate of desorption observed at 450 K corresponding to a desorption energy of ≈27 kcal/mol. The R-HCN was bonded at an on-top metal site through the nitrogen atom. Another species was determined to be HCNH above 400 K on the modified W(100) surface.10 Formation of HCNH was also found on the Si(111)-7×7 and Si(100)-2×1 surfaces by HREELS, XPS, and UPS.13,14 In the studies of chemisorption and thermal decomposition of methylamine on the Ru(001) surface by HREELS and TDS, Weinberg and co-workers identified the adsorbed hydrogen isocyanide species on Ru(001) as surface intermediates at 380390 K.16 Two adsorbed hydrogen isocyanide species were determined as terminally bound η1-(C)-CNH and bridge-bound µ-CNH. The assigned C-N stretching frequencies for the two configurations were 2275-2295 and 1660-1670 cm-1, respectively. The C-Ru surface stretching frequency was 350 cm-1 for the η1-(C)-CNH species and 600 cm-1 for the bridge-bound µ-CNH. Annealing the surface to 400 K causes dehydrogenation of the remaining NH bond and the conversion to side-on bonded cyanide, as η2-cyanide-C,N.16 The above experimental studies show that HCN behaves differently on Ni(111), Pd(111), and Pt(111) surfaces. One of the questions is how hydrogen cyanide bonds to these surfaces. Does the adsorbed HCN bond to the surface through the nitrogen atom with the C-N bond perpendicular to the surface or via both N and C atoms with the C-N bond parallel? We have recently carried out ab initio configuration interaction calculations for cyanide (CN) on Ni(111).50 Our calculations showed that CN is able to bind to the surface via either the carbon or nitrogen or in a side-on geometry with very small differences in total energy (≈2 kcal/mol). Adsorption energies at 3-fold, bridge, and atop sites are comparable, with the fcc 3-fold site more favorable over other adsorption sites by only ≈2 kcal/mol. At the fcc 3-fold site, adsorption energies, relative to the neutral CN radical, are 115, 113, and 113 kcal/mol for η1-cyanide-N, η1-cyanide-C, and η2-cyanide-C,N, respectively. Calculated C-N stretching frequencies are 2150, 1970, and 1840 cm-1 for η1-cyanide-N, η1-cyanide-C, and η2-cyanideC,N, respectively. Dipole moment calculations also showed that the bonding of CN to the Ni surface is largely ionic, while η2cyanide-C,N has more covalent character. Calculated energy barriers in going from η1-cyanide-C to η2-cyanide-C,N, and from η2-cyanide-C,N to η1-cyanide-N are very small, ≈2 kcal/mol. It was concluded that although CN is strongly bound to the surface (at ≈115 kcal/mol), within an energy range of ≈5 kcal/ mol, the molecule is free to rotate to other geometries. During this rotation there are large changes in the dipole moment.50 There are few theoretical results for HCN adsorption on transition-metal surfaces, although some calculations of CN adsorption on Cu and Ni surfaces have been reported.51-57 In this paper, the adsorption of hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) on Ni(111) is studied by using an ab initio configuration interaction theory. The adsorbate and local surface region are embedded in a larger cluster representing the metal lattice. Nickel 3d orbitals are explicitly included on the four nickel atoms of the surface region. Our theoretical approach allows us to optimize the geometry of adsorbed HCN and HNC and to calculate accurately the adsorption energy of HCN and HNC at various sites on Ni(111). 2. Theory and Calculations Total energy calculations are performed by using a manyelectron embedding theory that permits the accurate computation of molecule-solid surface interactions.58-60 Calculations are carried out at an ab initio configuration interaction (CI) level;
J. Phys. Chem., Vol. 100, No. 12, 1996 5091
Figure 1. Cluster geometry and local region of the nickel cluster used to model the (111) crystal face of nickel. The three-layer, 62-atom cluster consists of a surface layer of 28 atoms, a second layer of 17 atoms, and a third layer of 17 atoms. Embedding theory is used to reduce the Ni62 cluster to a 28-atom model depicted as shaded atoms. Atoms surrounding the four central atoms in the surface layer and those surrounding the one central atom in the second layer are described by effective potentials for (1s-3p core)(3d)9(4s)0.5 and (1s-3p core)(3d)9(4s)0.25 configurations, respectively. Effective potentials for the shaded atoms in the third layer describe the (1s-3p core)(3d)9(4s)0.6 configuration. Unshaded atoms have neutral atom (1s-3p core)(3d)9(4s)1 potentials. All atoms have Phillips-Kleinman projectors Σ|Qm >0.05 are explicitly retained in the expansion. Approximately 30004000 configurations are generated, and contributions of excluded configurations are estimated by using second-order perturbation theory. For all sites and geometries calculated, the SCF solution is the dominant configuration. 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 2 and 5 kcal/mol. All energies reported in the present work include the basis superposition contributions. All the reported vibrational frequencies are calculated by using a one-dimensional harmonic oscillator model. The geometry and surface region of the Ni(111) cluster are shown in Figure 1. The initial three-layer, 62-atom cluster consists of a surface layer of 28 atoms, a second layer of 17 atoms, and a third layer of 17 atoms. The embedding procedure is used to reduce the Ni62 cluster to a 28-atom model depicted as shaded atoms: the surface layer of 14 atoms, a second layer of nine atoms, and a third layer of five atoms. For the local surface region of four nickel atoms, an effective [1s-3p] core potential and valence 3d, 4s, and 4p orbitals are used. Other Ni atoms are described by an effective core potential for [1s-3d] electrons and a single 4s orbital. For all boundary atoms,
5092 J. Phys. Chem., Vol. 100, No. 12, 1996
Yang and Whitten TABLE 2: HCN Adsorption on Ni(111)a
Figure 2. Adsorption sites: the notations FT, HT, B, and A refer to fcc three-fold, hcp three-fold, bridge, and atop Ni sites, respectively. There is a second-layer Ni atom underneath the hcp three-fold site. Only the first layer of the three-layer cluster is shown.
TABLE 1: Molecular Properties for Gaseous HCN and HNC
∆E, kcal/mol rC-N, Å rH-C, Å rH-N, Å ωC-N, cm-1 ωH-C, cm-1 ωH-N, cm-1
ground state HCN, 1Σ+
ground state HNC, 1Σ+
this work
ref 70a
this work
ref 70a
exptl
0.00 1.158 1.068
0.00 1.165 1.068
1.153b 1.066b
14.5 1.163
14.3 1.182
1.169b
2241 3421
2210 3543
2130c 3433c
0.995 2117
0.997 2135
0.994b 2061c
3695
3950
3810c
exptl
siteb
fcc 3-fold
hcp 3-fold
bridge
atop
Eads,c kcal/mol RN-surface,d Å RN-Ni,d Å ωC-N, cm-1 ωC-H, cm-1 ωHCN-surface, cm-1 rC-N, Å rC-H, Å
18 2.24 2.65 2220 3418 256 1.16 1.07
16 2.25 2.66 2225 3429 266 1.16 1.07
15 2.28 2.60 2215 3419 270 1.16 1.07
17 2.06 2.06 2200 3428 280 1.16 1.07
a Results are for the perpendicular HCN geometry. b Figure 2 shows the adsorption sites. There is a second-layer Ni atom underneath the hcp 3-fold site where there is no second-layer Ni atom underneath the fcc 3-fold site. c Eads is relative to HCN at infinite separation. Positive values are exothermic. Results are from configuration interaction calculations and are corrected for basis superposition effects. d RN-surface are the perpendicular distances from nitrogen to the Ni surface, and RN-Ni are the corresponding distances from the nitrogen nucleus to the nearest Ni nucleus.
a Ab initio configuration interaction calculations with all single and double excitations using a standard double-ζ plus polarization basis set. Stretching frequencies are for harmonic vibrational frequencies. b Listed in ref 70. c Reference 71.
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 60 and 61). The sites considered for HCN and HNC adsorption on Ni(111) are as follows: a hollow 3-fold site with no second layer Ni atom underneath (fcc extension of the lattice); a filled 3-fold site with a second layer Ni atom underneath (hcp extension of the lattice); a bridge site and an atop Ni site, denoted by HT, FT, B, and A, respectively, as shown in Figure 2. The basis orbitals of Ni and H are listed in ref 68. The H basis set includes a double-ζ s and p plus a set of 2p functions with an exponent of 0.6. The triple-ζ s and p basis for carbon and nitrogen is taken from Whitten69 and augmented with a set of d polarization functions with an exponent of 0.626 for carbon and 0.913 for nitrogen. In Table 1 are listed the present calculated molecular properties for HCN and HNC along with the early ab initio calculations by Schaefer and co-workers70 and experimental values.71 The ground state of HNC, 1Σ+, is calculated to be 14.5 kcal/mol higher in energy than the ground state of HCN, 1Σ+, which is in excellent agreement with the value of 14.3 kcal/mol obtained from the early ab initio configuration interaction calculations.70 Both ground states are linear. Calculated bond distances and vibrational frequencies are also comparable, and the values are in good agreement with experiment. This indicates that the present basis set is able to describe both HCN and HNC. 3. Adsorption of HCN on Ni(111) Calculated adsorption energies, N-surface equilibrium distances, vibrational frequencies, and bond distances for perpendicular HCN are reported in Table 2. The potential curves obtained by varying the N-surface distance are presented in Figure 3. The calculations show that the potential surface is fairly flat for HCN adsorption. Calculated adsorption energies
Figure 3. Adsorption energy of end-on bonded HCN at various adsorption sites versus the HCN-surface distance (in au). The fcc three-fold and atop sites are of comparable stability.
are 18, 16, 15, and 17 kcal/mol at the fcc three-fold, hcp threefold, bridge, and atop sites, with the N-surface distances of 2.24, 2.25, 2.28, and 2.06 Å, respectively. Thus, for this endon bonded HCN species, adsorption energies for different surface sites are within 3 kcal/mol. Hagans et al. studied the reaction of HCN on Ni(111) by using scanning kinetic spectroscopy (SKS), temperatureprogrammed desorption (TPD), and Auger electron spectroscopy (AES) in the temperature range 84-1000 K.2 It was found that HCN completely dissociates on Ni(111) to form Had, Nad, and Cad at low coverage, while at saturation coverage only onethird to one-half of the adsorbed HCN decomposes. Two energetically similar HCN desorption states were observed at 258 and 271 K in both SKS and TPD and the HCN desorption activation energies were estimated to be in the range 18-20 kcal/mol. It was concluded that the two states could be due to HCN adsorbed at different sites or due to repulsive lateral interaction effects.2 The calculated HCN adsorption energies are also close to the experimental desorption energy and the 3 kcal/mol variation is within the experimental range. The calculated C-N stretching frequencies are 2220, 2225, 2215, and 2200 cm-1 at the fcc three-fold, hcp three-fold, bridge, and atop sites, respectively. It is red-shifted about 20 cm-1 at the three-fold site and 40 cm-1 at the atop site compared to the gaseous value calculated at 2241 cm-1. This indicates only
Theoretical Adsorption Studies
J. Phys. Chem., Vol. 100, No. 12, 1996 5093
Figure 5. Calculated net electronic charge in units |e| from a Mulliken population analysis of CI wavefunctions for the end-on bonded HCN at various adsorption sites for the equilibrium N-surface distances.
Figure 4. Calculated dipole moment (1 au ) 2.54 D) of end-on bonded HCN along the surface normal at various adsorption sites versus the HCN-surface distance. Calculated values of the first derivative of the dipole moment curve, dµ/dR, are -0.44, -0.39, -0.40, and -0.29 for HCN at the fcc three-fold, hcp three-fold, bridge, and atop sites, respectively.
weak metal-π back-bonding. For HCN on W(100)-(5×1)-C12 at 200 K, the observed C-N stretching frequency is 2080 cm-1, which is only a 50-cm-1 red-shift compared to the experimental value of 2130 cm-1 for gaseous HCN.71 Very little change of the C-N stretching frequency for HCN adsorbed on the Si(111) and Si(100) surfaces was also observed (≈20 cm-1) in the temperature range 100-220 K.13,14 Based upon the close resemblance between the IR spectrum of HCN adsorbed Pt(111) at 85 K and the IR spectrum of HCN in the gas phase and trapped in an argon matrix, Jentz et al. concluded that the HCN molecule is weakly adsorbed at 85 K and remains largely unperturbed.6 Our calculated C-H stretching frequency of 3420-3430 cm-1 for HCN on Ni(111) is also very close to the calculated value of 3421 cm-1 and to the experimental value of 3433 cm-1 for gas-phase HCN.71 The C-H stretching frequency for HCN on Pt(111) was found to be nearly identical to that of HCN in the gas phase and in an Ar matrix.6 A Weakly perturbed C-H stretching frequency was also observed for HCN adsorbed on the carbon-modified W(100) at 200 K12 and on the Si(111) and Si(100) surfaces at 100 to 220 K.13,14 The above results suggest that the C-H and C-N stretching vibrations are only weakly perturbed by the surface for the endon bonded HCN species. Calculated HCN-Ni surface perpendicular stretching frequencies, ωHCN-surface, are 256 and 266 cm-1 at the fcc and hcp three-fold sites and 270 and 280 cm-1 at the bridge and atop sites, respectively. Figure 4 shows the variation in dipole moment along the surface normal with respect to HCN-surface distance for the end-on HCN adsorbed at different sites on the Ni(111) surface. The dipole moment curves are quite similar for HCN at all sites. The values for the first derivative of the dipole moment, dµ/ dR, are -0.44, -0.39, -0.40, and -0.29 for HCN at the fcc three-fold, hcp three-fold, bridge, and atop sites, respectively. By this measure, it would appear that HCN at the atop sites has the least ionic character. Figure 5 depicts the calculated charge for HCN on the surface as calculated from a Mulliken population analysis of the configuration interaction wavefunctions. For HCN at the atop
site in the equilibrium N-surface distance, the total charge on HCN is +0.13 |e|, indicating that electrons are transferred to nickel. This is consistent with a decrease in work function upon HCN adsorption. The work function of the clean surface is 5.3 eV, calculated as ENi+-ENi, while 4.9 eV is obtained for HCN at the atop site. There are no experimental work function measurements for HCN on Ni(111), however, adsorbed HCN species caused a work function decrease on Pt(111) yielding ∆φ ) -1.2 eV at saturation coverage.3 Our calculated value of ∆φ ≈ -0.4 eV is for very low coverage. For HCN adsorbed at high symmetry sites, the net charges on HCN and the work function changes of the surface are -0.06 |e| and -0.2 eV at the fcc three-fold site, -0.07 |e| and -0.3 eV at the hcp three-fold site, and -0.09 |e| and -0.3 eV at the bridge site, respectively. Thus, for these sites, there is a work function decrease but little apparent electron transfer to CN. Figure 5 indicates that the transferred electrons are mainly localized on the carbon atom. Comparing HCN at the atop and fcc three-fold sites, the net charges on carbon are +0.07 |e| and -0.11 |e|, respectively. It appears that the metal-π backbonding is greater at high symmetry sites than for the atop site. Mulliken populations show very little change in the Ni 3d population; thus, the π back-bonding is accomplished mainly through the 4s orbitals in nearest-neighbor atoms defining the high-symmetry sites. 4. Adsorption of HNC on Ni(111) Calculated adsorption energies, C-surface equilibrium distances, vibrational frequencies, and bond lengths for perpendicular HNC are reported in Table 3. The potential curves obtained by varying the C-surface distance are presented in Figure 6. The adsorption energy, Eads, is relative to HCN (hydrogen cyanide) at infinite separation. The ground state of HNC in the gas phase is calculated to be 14.5 kcal/mol higher in energy than the ground state of HCN. Calculated HNC adsorption energies are 8.2, 6.5, 8.6, and 11 kcal/mol at the fcc three-fold, hcp three-fold, bridge, and atop sites, respectively. The atop site is the more favorable for the hydrogen isocyanide adsorption, while the fcc three-fold site is favorable for hydrogen cyanide adsorption. The calculated C-surface perpendicular distances are consistently shorter than the N-surface distances for HCN by ≈0.15 Å. The calculated C-N stretching frequencies are 2110, 2120, 2120, and 2100 cm-1 at the fcc three-fold, hcp three-fold, bridge, and atop sites, respectively. These values are essentially the same as calculated for gaseous HNC, 2117 cm-1, indicating
5094 J. Phys. Chem., Vol. 100, No. 12, 1996
Yang and Whitten
TABLE 3: HNC Adsorption on Ni(111)a siteb
fcc 3-fold
hcp 3-fold
bridge
atop
Eads,c kcal/mol RC-surface,d Å RC-Ni,d Å ωC-N, cm-1 ωN-H, cm-1 ωHNC-surface, cm-1 rC-N, Å rN-H, Å
8.2 2.08 2.52 2110 3700 250 1.16 1.00
6.5 2.09 2.53 2120 3690 255 1.16 1.00
8.6 2.16 2.51 2120 3680 260 1.16 1.00
11 1.92 1.92 2100 3671 280 1.16 1.00
a Results are for the perpendicular HNC geometry. b Figure 2 shows the adsorption sites. There is a second-layer Ni atom underneath the hcp 3-fold site, where there is no second-layer Ni atom underneath the fcc 3-fold site. c Eads is relative to HCN at infinite separation. In the gaseous ground state, HCN is calculated to be 14.5 kcal/mol more stable than HNC. The adsorption energies, with respect to HNC at infinite separation, are 22.7, 21.0, 23.1, and 25.5 kcal/mol at the fcc 3-fold, hcp 3-fold, bridge, and atop sites, respectively. Positive values are exothermic. Results are from configuration interaction calculations and are corrected for basis superposition effects. d RC-surface are the perpendicular distances from carbon to the Ni surface and RC-Ni are the corresponding distances from carbon nucleus to the nearest Ni nucleus.
Figure 7. Calculated dipole moment (1 au ) 2.54 D) of end-on bonded HNC along the surface normal at various adsorption sites versus the HNC-surface distance. Calculated values of the first derivative of the dipole moment curve, dµ/dR, are -0.06, 0.06, and 0.03 for HNC at the fcc three-fold, hcp three-fold, and bridge sites, respectively. For HNC at the atop and bridge sites, the dipole moment curve is no longer linear.
Figure 6. Adsorption energy of the end-on bonded HNC at various adsorption sites versus the HNC-surface distance (in au). The atop site is energetically favorable over other adsorption sites.
Figure 8. Calculated net electron charge in units |e| from a Mulliken population analysis of CI wavefunctions for the end-on bonded HNC at various adsorption sites for the equilibrium C-surface distances.
that the C-N bond is unperturbed upon end-on adsorption. The calculated H-N stretching frequencies are around 3700 cm-1 and the HNC-Ni surface stretching frequencies, ωHNC-surface, are between 250 and 280 cm-1 for HNC at different sites. Figure 7 shows the variation in dipole moment along the surface normal with respect to HNC-surface distance for the end-on HNC adsorbed at different sites on the Ni(111) surface. The values for the first derivative of the dipole moment, dµ/ dR, are -0.06, 0.06, and 0.03 for HNC at the fcc thrre-fold, hcp three-fold, and bridge sites, respectively. Thus, the slope is ≈0. At the bridge and atop sites, the dipole moment curves are nonlinear. On comparison of Figure 7 (for HNC) and Figure 4 (for HCN), the difference in slopes is large enough to suggest that HNC on the Ni(111) surface has much more covalent character than HCN. Figure 8 depicts the calculated charge for HNC on the surface as calculated from a Mulliken population analysis of the CI wavefunctions. The net charges on HNC are +0.11 |e| at the atop site and +0.04 |e| at both the three-fold and bridge sites, indicating only a slight electron transfer to the surface. Work
function decreases (∆φ ≈ -0.2 eV) at all sites for this nearly zero HNC coverage. Our calculations suggest that adsorption of hydrogen isocyanide, as the case of hydrogen cyanide, will cause a work function decrease. Figure 8 shows that substantial π back-bonding occurs when HNC is adsorbed, causing the nitrogen atom to become more negatively charged. For HNC at the atop site, the π orbitals gain 0.16 electrons compared to the gaseous value, while the Ni 3d orbitals from the Ni atom underneath lose 0.13 electrons compared to the clean surface. This is typical d-π backbonding, and it causes the net charge on the nitrogen atom to be -0.41 |e|. In contrast, for the case of N-bonded HCN at the atop site, the π orbitals gained only 0.02 electrons, the net charge on the carbon atom is +0.07 |e|, and the Ni 3d orbitals from the Ni atom underneath lose only 0.03 electrons. For HNC at the fcc three-fold site, the π orbitals gain 0.12 electrons and the Ni surface loses 0.03 3d electrons from the three neighboring Ni atoms (0.01 |e| per Ni atom). In this case, since the net charge on the nitrogen atom is -0.04 |e|, it appears that the metal-π back-bonding at the three-fold site is accomplished mainly through the Ni 4s orbitals.
Theoretical Adsorption Studies
Figure 9. Adsorption energy of HCN at the fcc three-fold site with respect to β, the angle between the H-C bond and the surface normal. The N-surface distance is kept unchanged at R ) 2.24 Å for β ) 0°. Tilting the H atom away from the C-N axis destabilizes the HCN species.
J. Phys. Chem., Vol. 100, No. 12, 1996 5095
Figure 10. Adsorption energy of HCN at the fcc three-fold site with respect to β, the angle between the H-C-N bond and the surface normal. The N-surface distance is kept unchanged at R ) 2.24 Å for β ) 0°. Tilting the H-C bond away from the surface normal slowly destabilizes the HCN species.
5. Other HCN and HNC Adsorption Geometries In the previous sections, we have studied N-bonded HCN and C-bonded HNC on Ni(111) with the HCN axis and HNC axis perpendicular to the surface. Both configurations are found to bind weakly to the surface and the HCN species is about 7 kcal/mol more stable than the C-bonded HNC species. On Pd(111) experimental studies concluded that the C-N bond is parallel to the surface for HCN, while on the Ni(111) and Pt(111) surfaces it was suggested that HCN in an end-on N-bonded geometry. Figure 9 shows the variation in adsorption energy with respect to the angle, β, between the H-C bond and the surface normal for HCN at the fcc three-fold site. The C-N bond is still perpendicular to the surface with the N-surface distance kept unchanged at R ) 2.24 Å, the equilibrium value for β ) 0°. The corresponding adsorption energy at β ) 0° is 18 kcal/mol. Our calculations indicate that tilting H away from the surface normal destabilizes the system. The adsorption energy decreases to 17 and 15 kcal/mol at β ) 10° and 20° and then decreases sharply to 11, 5.6, and 0.3 kcal/mol at β ) 30°, 40°, and 50°, respectively. If H-C-N is tilted away from the surface normal, the corresponding adsorption energy change is depicted in Figure 10. The N-surface distance for HCN at the fcc three-fold site is still kept at the equilibrium distance of R ) 2.24 Å for untilted HCN. Although tilting the H-C-N away from the surface normal still destabilizes the HCN species, it is much less repulsive than tilting only the H away from the surface normal as shown in Figure 9. For instance, the latter case is unbound by about 5.5 kcal/mol at β ) 60°, while on tilting H-C-N away from the surface normal to β ) 80°, the HCN is still bound to the surface by about 5.2 kcal/mol. The calculated equilibrium geometry for side-on bonded HCN, with N at the fcc three-fold site, has a HCN bond angle of 145° and a C- and N-surface distance of 2.01 Å. The C-N bond is assumed to be parallel to the surface. The corresponding adsorption energy is only 7 kcal/mol, about 11 kcal/mol less stable than the end-on HCN. The calculated C-N stretching frequency for the side-on bonded HCN is 2105 cm-1 and the C-N bond distance slightly increases from 1.16 to 1.19 Å. For the side-on bonded HCN, the π orbitals gain 0.16 electrons compared to the perpendicular
Figure 11. Adsorption energy of the side-on bonded HCN with respect to R, the H-C-N bond angle. The N atom in HCN is at the fcc threefold site. The energy minimum occurs at R ≈ 145° with the C- and N-surface distances of 2.01 Å.
HCN at the fcc three-fold site and the Ni 3d orbitals lose 0.1 electrons. The net charge on HCN is -0.31 |e|, while only -0.06 |e| occurs for end-on bonded HCN. Our results lead to a conclusion that the metal-π back-bonding for the side-on bonded HCN is much stronger than the end-on bonded species and is accomplished through both the Ni 3d and 4s orbitals. Figure 11 shows the adsorption energy change with respect to the HCN bond angle, R, for side-on bonded HCN at the fcc three-fold site. The C- and N-surface distances are kept at 2.01 Å. The energy change between R ) 130° and 160° is only ≈2 kcal/mol. Thus, low-energy fluctuations in the HCN angle about an equilibrium value of 145 ( 15° can occur. Similar calculations are also carried out for tilted hydrogen isocyanide (HNC) on Ni(111). Energy changes for tilting the H atom and the H-H bond away from the surface normal are very similar to the ones depicted in Figures 9 and 10. However, one interesting geometry, the side-on bonded HNC, is worth mentioning. The calculated geometry for side-on bonded HNC with the C at the fcc three-fold site has a HNC bond angle of 135° and a C- and N-surface distance of 2.11 Å. This chemisorption
5096 J. Phys. Chem., Vol. 100, No. 12, 1996
Yang and Whitten (4) Dipole moment calculations show that the bonding of HCN to the surface is relatively more ionic than that of HNC. Acknowledgment. Support of the work by the U.S. Department of Energy is gratefully acknowledged. We thank Dr. Jentz and Dr. Trenary for their helpful discussions. References and Notes
Figure 12. Calculated equilibrium geometries for end-on and side-on bonded HCN and HNC on Ni(111), respectively. Eads is relative to HCN at infinite separation from the surface and positive values are exothermic. In the gas phase the ground state of HNC is calculated to be 14.5 kcal/mol higher in energy than HCN.
state is, however, unbound by about 12 kcal/mol with respect to hydrogen cyanide (HCN) at infinite separation. If we recall that the ground state of HNC in the gas phase is calculated to be 14.5 kcal/mol higher in energy than the ground state of HCN, this side-on bonded HNC is barely bound, by only 2 kcal/mol, relative to hydrogen isocyanide (HNC) at infinite separation. The side-on bonded HNC is less stable by about 23 kcal/mol than the end-on bonded HNC. Our calculated C-N stretching frequency for side-on bonded HNC is low, ≈1530 cm-1. The C-N stretching frequency is decreased about 570 cm-1 compared to the end-on bonded HNC, indicating a weakening of the C-N bond. The net charge on HNC is -0.34 |e|, with only 0.06 |e| from the Ni 3d shells. Based upon the electrons transferred between the HNC species and the surface, our calculations imply a work function increase for side-on bonded HNC and a decrease for end-on bonded HNC. The low C-N stretching frequency is consistent with a longer C-N bond length, which increases from 1.16 Å in the end-on geometry to 2.26 Å in the side-on configuration. 6. Conclusions The adsorption of HCN and HNC on the Ni(111) surface is studied by an ab initio electronic structure theory. The Ni(111) surface is modeled as a three-layer, 28-atom cluster with the Ni atoms fixed at the bulk distances. The results are summarized in Figure 12 and as follows: (1) Both HCN and HNC bind to the surface in an end-on geometry with the molecular axis perpendicular to the surface. Tilting either the H atom or H-C bond or the H-N bond away from the surface normal destabilizes the chemisorption system. The side-on bounded HCN and HNC with the C-N bond parallel to the substrate are energetically less stable than the corresponding end-on bonded species. (2) Calculated adsorption energy of the end-on HCN is 18 kcal/mol with the fcc three-fold site favorable over other sites by about 1-3 kcal/mol. The end-on HNC binds to the surface at the atop site with adsorption energy of 11 kcal/mol. Calculated adsorption energy for the side-on bonded HCN is only 7 kcal/mol. The side-on bonded HNC species is found unbound by 12 kcal/mol on Ni(111). (3) Calculated C-N stretching frequencies are 2200 and 2100 cm-1 for the end-on bonded HCN and HNC and 2105 and 1530 cm-1 for the side-on bonded HCN and HNC, respectively.
(1) Netzer, F. P.; Ramsey, M. G. Crit. ReV. Solid State Mater. Sci. 1992, 17, 397. (2) Hagans, P. L.; Chorkendorff, I.; Winkler, A.; Yates, J. T., Jr. J. Phys. Chem. 1988, 92, 471. (3) Hagans, P. L.; Guo, X.; Chorkendorff, I.; Winkler, A.; Siddiqui, H.; Yates, J. T., Jr. Surf. Sci. 1988, 203, 1. (4) Lindquist, J. M.; Ziegler, J. P.; Hemminger, J. C. Surf. Sci. 1988, 210, 27. (5) Somers, J. S.; Bridge, M. E. Surf. Sci. 1985, 159, L439. (6) Jentz, D.; Celio, H.; Mills, P.; Trenary, M. Surf. Sci. 1995, 341, 1. (7) Kordesch, M. E.; Stenzel, W.; Conrad, H. Surf. Sci. 1988, 205, 100. (8) Kordesch, M. E.; Linder, Th.; Somers, J.; Conrad, H.; Bradshaw, A. M.; Williams, G. P. Spectrochim. Acta 1987, 43A, 1561. (9) Somers, J. S.; Kordesch, M. E.; Hemmen, R.; Lindner, Th.; Conrad, H.; Bradshaw, A. M. Surf. Sci. 1988, 198, 400. (10) Kordesch, M. E.; Feng, W.; Stenzel, W.; Weaver, M. J.; Conrad, H. J. Electron. Spectrosc. Relat. Phenom. 1987, 44, 149. (11) Connolly, M.; McCabe, T.; Lloyd, D. R. Taylor, E. In Structure and ReactiVity of Surfaces; Morterra, C., Zecchina, A., Costa, G., Eds.; Elsevier: Amsterdam, 1989; p 30. (12) Serafin, J. G.; Friend, C. M. J. Phys. Chem. 1988, 92, 6694. (13) Bu, Y.; Ma, L.; Lin, M. C. J. Phys. Chem. 1993, 97, 11797. (14) Bu, Y.; Ma, L.; Lin, M. C. J. Phys. Chem. 1993, 97, 7081. (15) Weinberg, W. H.; Johnson, D. F.; Yang, Y. Q.; Parmeter, J. E.; Hills, M. M. Surf. Sci. 1990, 235, L299. (16) Johnson, D. F.; Wang, Y.; Parmeter, J. E.; Hills, M. M.; Weinberg, W. H. J. Am. Chem. Soc. 1992, 114, 4279. (17) Friend, C. M.; Serafin, J. G. J. Chem. Phys. 1988, 88, 4037. (18) Stevens, P. A.; Madix, R. J.; Sto1 hr, J. J. Chem. Phys. 1989, 91, 4338. (19) Stevens, P. A.; Madix, R. J.; Friend, C. M. Surf. Sci. 1988, 205, 187. (20) Erley, W.; Hemminger, J. C. Surf. Sci. 1994, 316, L1025. (21) Lloyd, K. G.; Hemminger, J. C. Surf. Sci. 1987, 179, L6. (22) Kingsley, J. R.; Dahlgren, D.; Hemminger, J. C. Surf. Sci. 1984, 139, 417. (23) Somers, J.; Kordesch, M. E.; Linder, Th.; Conrad, H.; Bradshaw, A. M.; Williams, G. P. Surf. Sci. 1987, 188, L693. (24) Kordesch, M. E.; Stenzel, W.; Conrad, H. J. Electron. Spectrosc. Relat. Phenom. 1986, 39, 89. (25) Kordesch, M. E.; Stenzel, W.; Conrad, H. Surf. Sci. 1987, 186, 601. (26) Besenthal, K.; Chiarello, G.; Kordesch, M. E.; Conrad, H. Surf. Sci. 1986, 178, 667. (27) Kordesch, M. E.; Stenzel, W.; Conrad, H.; Weaver, M. J. J. Am. Chem. Soc. 1987, 109, 1878. (28) Ramsey, M. G.; Rosina, G.; Netzer, F. P.; Saalfeld, H. B.; Lloyd, D. R. Surf. Sci. 1989, 218, 317. (29) Kordesch, M. E.; Stenzel, W.; Conrad, H. Surf. Sci. 1986, 175, L687. (30) Hwang, S. Y.; Seebauer, E. G.; Schmidt, L. D. Surf. Sci. 1987, 188, 219. (31) Avery, N. R.; Matheson, T. W. Surf. Sci. 1984, 143, 110. (32) Sexton, B. A.; Avery, N. R. Surf. Sci. 1983, 129, 21. (33) Avery, N. R.; Matheson, T. W.; Sexton, B. A. Appl. Surf. Sci. 1985, 22/23, 384. (34) Gudde, N. J.; Lambert, R. M. Surf. Sci. 1983, 124, 372. (35) Jentz, D.; Trenary, M.; Peng, X. D.; Stair, P. Surf. Sci. 1995, 341, 282. (36) Gardin, D. E.; Barbieri, A.; Batteas, J. D.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1994, 304, 316. (37) Gardin, D. E.; Somorjai, G. A. J. Phys. Chem. 1992, 96, 9424. (38) Chorkendorff, I.; Russell, J. N.; Yates, J. T., Jr. J. Chem. Phys. 1987, 86, 4692. (39) Kishi, K.; Yoshikazu, O.; Fujimojto, Y. Surf. Sci. 1986, 176, 23. (40) Friend, C. M.; Stein, J.; Muetterties, E. L. J. Am. Chem. Soc. 1981, 103, 767. (41) Friend, C. M.; Muetterties, E. L.; Gland, J. L. J. Phys. Chem. 1981, 85, 3256. (42) Hemminger, J. C.; Muetterties, E. L.; Somorjai, G. A. J. Am. Chem. Soc. 1979, 101, 62. (43) Outka, D. A.; Jorgensen, S. W.; Friend, C. M.; Madix, R. J. J. Mol. Catal. 1983, 21, 375. (44) Solymosi, F.; Kiss, J. Surf. Sci. 1981, 108, 368.
Theoretical Adsorption Studies (45) Benziger, J. B. Appl. Surf. Sci. 1984, 17, 309. (46) Kishi, K.; Ikeda, S. Surf. Sci. 1981, 107, 405. (47) Inamura, K.; Inoue, Y.; Ikeda, S.; Kishi, K. Surf. Sci. 1985, 155, 173. (48) Nakayama, T.; Inamura, K.; Inoue, Y.; Ikeda, S.; Kishi, K. Surf. Sci. 1987, 179, 47 and references contained therein. (49) Ramsey, M. G.; Steinmuller, D.; Netzer, F. P.; Kostlmeier, S.; Lauber, J. Ro1 sch, N. Surf. Sci. 1994, 307-309, 82. (50) Yang, H.; Caves, T. C.; Whitten, J. L. J. Chem. Phys. 1995, 103, 8756. (51) Zhou, X.-Y.; Shi, D.-H.; Cao, P.-L. Surf. Sci. 1989, 223, 393. (52) Bagus, P. B.; Nelin, C. J.; Muller, W.; Philpott, M. P.; Seki, H. Phys. ReV. Lett. 1987, 58, 559. (53) Hermann, K.; Muller, W.; Bagus, P. B. J. Electron. Spectrosc. Relat. Phenom. 1986, 39, 107. (54) Philpott, M. P.; Bagus, P. B.; Nelin, C. J.; Seki, H. J. Electron. Spectrosc. Relat. Phenom. 1987, 45, 169. (55) Nelin, C. J.; Bagus, P. B.; Philpott, M. P. J. Chem. Phys. 1987, 87, 2170. (56) Bagus, P. B.; Nelin, C. J.; Hermann, K.; Philpott, M. P. Phys. ReV. B 1987, 36, 8169. (57) Rodriquez, J. A.; Campbell, C. T. Surf. Sci. 1987, 185, 299. (58) Whitten, J. L.; Pakkanen, T. A. Phys. ReV. B 1980, 21, 4357. (59) Whitten, J. L. Phys. ReV. B 1981, 24, 1810.
J. Phys. Chem., Vol. 100, No. 12, 1996 5097 (60) Cremaschi, P.; Whitten, J. L. Theor. Chim. Acta 1987, 72, 485. (61) Whitten, J. L. Chem. Phys. 1993, 177, 387. (62) Yang, H. Surf. Sci. 1995, 343, 61. (63) Yang, H.; Whitten, J. L. Langmuir 1995, 11, 853. (64) Yang, H.; Whitten, J. L.; Friend, C. M. Surf. Sci. 1994, 313, 295. (65) Yang, H.; Caves, T. C.; Whitten, J. L.; Huntley, D. R. J. Am. Chem. Soc. 1994, 116, 8200. (66) Yang, H.; Whitten, J. L.; Markunas, R. J. Appl. Surf. Sci. 1994, 75, 12. (67) Selection of the localized occupied and virtual orbitals for the configuration interaction expansion calculations relative to the local region of the four Ni atom 4s orbitals plus HCN valence orbitals (excluding C and N 1s) is based upon the degree of localization as measured by exchange interactions. In the present case, 34 electrons (15 double occupied and four singly occupied orbitals) and 30 virtual orbitals are used. See detailed discussion in refs 60 and 61. (68) Yang, H.; Whitten, J. L. J. Chem. Phys. 1988, 89, 5329. (69) Whitten, J. L. J. Chem. Phys. 1966, 44, 359. (70) Laidig, W. D.; Yamaguchi, Y.; Schaefer, H. F., III, J. Chem. Phys. 1984, 80, 3069. (71) Ross, S. C.; Bunker, P. R. J. Mol. Spectrosc. 1983, 101, 199.
JP953238W
