4090
J. Phys. Chem. B 1997, 101, 4090-4096
Energetics of Hydroxyl and Influence of Coadsorbed Oxygen on Metal Surfaces Hong Yang* and Jerry L. Whitten Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina, 27695-8204 ReceiVed: January 16, 1997X
The adsorption of the hydroxyl (OH) radical on Ni(100), Ni(111), and Fe(110) and coadsorption of OH + O on Ni(100) are treated using an ab initio configuration interaction embedding theory. The metal surfaces are modeled as a 30-atom cluster for Ni(100), a 28-atom cluster for Ni(111), and a 40-atom cluster for Fe(110), with the Ni and Fe atoms fixed at bulk lattice sites. The high-symmetry sites are found to be the most stable sites for OH adsorption with the H-O axis perpendicular to the surface. Calculated adsorption energies for OH are 94 kcal/mol at a hollow 4-fold site on Ni(100), 90 kcal/mol at a hollow 3-fold site on Ni(111), and 94 kcal/mol at a long-bridge site on Fe(110). On coadsorption of OH + O on Ni(100), the adsorbed atomic oxygen inhibits OH adsorption at the nearby surface sites. The adsorption energy of OH at a 4-fold site with O coadsorbed at the adjacent 4-fold site is 62 kcal/mol, and the OH axis can be tilted up to 50° toward O without an appreciable energy increase, indicating an attractive dipole interaction between coadsorbed OH and O. The present studies suggest that oxygen coverage and OH binding sites are important factors in the variation of the activation energy for OH desorption.
1. Introduction Hydroxyl radicals formed in the oxidation of hydrogen and hydrocarbons on transition metal surfaces have received a great deal of attention during the past decade.1-30 The desorption of hydroxyl from different metal catalysts has been reported for Ni,1,2 Pd,2,3 Pt,4-17 Rh,2,18 Ir,2 etc., using a laser-induced fluorescence technique. Studies by secondary ion mass spectrometry and thermal desorption spectrometry showed that OH was formed following the reaction of H2 with oxygen-covered polycrystalline nickel.19-21 Adsorbed OH was observed on an oxygen-dosed Ni(110) surface by electron-stimulated desorption ion angular distribution (ESDIAD) studies coupled with ultraviolet photoelectron spectroscopy methods for studying H2O adsorption.22 The formation of OH fragments was also observed from water decomposition on Ni(110) by high-resolution electron energy loss spectroscopy (HREELS) at 200 K.23 OH species were also identified by HREELS on the oxygen-covered Pt(111) surface24 and on oxygen-covered Pd(100).25,26 The desorption of OH from Pt has been extensively studied by the laser-induced fluorescence (LIF) method. In the LIF studies, the apparent activation energy for OH desorption were reported to range from 30 to 60 kcal/mol depending on the ratio of O2/H2 or O2/CH4 and the surface oxygen coverage. A similar variation of apparent activation energy is also found for OH desorption from Ni (27-41 kcal/mol), Pd (20-60 kcal/mol), Rh (27-42 kcal/mol), etc. The apparent activation energy in the experimental studies is calculated either from the slope of Arrhenius plots of laser-induced fluorescence intensities Versus reciprocal temperatures or from kinetic analysis which involves different steps in the mechanism of H2 oxidation by O2. The energetics of chemisorbed hydroxyl on Pt, Pd, Rh, and Ni surfaces have been recently studied by Patrito et al. using the bond order conservation-Morse potential (BOC-MP) method.27 Adsorbate-adsorbate interactions and oxygen coverage effects on the strength of metal-OH bond were considered. The calculated OH adsorption energy at zero coverage is 61, * Corresponding author: FAX, (919)515-5079; E-mail: hong yang@ ncsu.edu. X Abstract published in AdVance ACS Abstracts, April 15, 1997.
S1089-5647(97)00231-9 CCC: $14.00
40, 39, and 51 kcal/mol on Ni, Pd, Pt, and Rh, respectively. This paper includes a critique of previously proposed explanations of the variation in the apparent activation energy for OH desorption. Hermann et al. recently studied the adsorption of OH on a cluster model of Cu(111) using ab initio theory.28 Adsorption energetics, geometries, and surface electronic structure are reported for CunOH (n ranging from 1 to 25). The adsorption energy for OH at the most stable fcc 3-fold site, calculated at the configuration interaction (CI), was found to be 74 kcal/ mol. We have recently reported studies of the adsorption of OH on Ni(111).29 OH was found to adsorb at the 3-fold site with the O-H axis perpendicular to the surface. The calculated adsorption energy was 90 kcal/mol and the oxygen-surface distance was 1.51 Å. For OH adsorbed at the bridge and atop sites, the adsorption energies are 85 and 74 kcal/mol with O-surface distances of 1.59 and 1.75 Å, respectively. The H-O axis is found to be nearly perpendicular to the surface at the bridge site, and inclined away from the surface normal by about 30° at the atop site. Much earlier, Bauschlicher also performed ab initio calculations of OH on a five-atom cluster of Ni(100) using a 4s effective potential description of the nickel atoms. Adsorption energies were not reported; however, geometries for OH on top Ni and above a 4-fold site were determined and the ionic and covalent characters of the bonding were analyzed.30 There is a disagreement between the calculated ab initio energetics for OH adsorbed on the clean metal surfaces and the observed desorption energies for OH from either reaction of O2 with H2 or CH4 or decomposition of H2O on the aforementioned transition metals, as well as a disagreement between ab initio results and the BOC-MP calculations. In order to investigate these discrepancies, ab initio calculations are carried out on the adsorption of OH on Ni(100), Ni(111), and Fe(110). The Ni(100), Ni(111), and Fe(110) surfaces are modeled as 30atom, 28-atom, and 40-atom, three-layer clusters, respectively, with embedding, boundary atom, potentials determined from larger clusters by an orbital localization transformation. Ni and Fe 3d orbitals are explicitly included on surface atoms near the adsorbate region. The oxygen coverage effect on Ni(100) is © 1997 American Chemical Society
Coadsorbed Oxygen on Metal Surfaces
J. Phys. Chem. B, Vol. 101, No. 20, 1997 4091
also investigated by calculations of the energetics of OH coadsorbed with O. These studies show that adsorbed O decreases the OH adsorption energy and causes the OH axis to tilt away from the surface normal. 2. Theory and Calculations Total energy calculations are performed using a many-electron embedding theory that permits the accurate computation of molecule-solid surface interactions.31,32 Calculations are carried out at an ab initio configuration interaction 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 extensively discussed in refs 33-35. Summarizing briefly the approach, calculations are performed by first obtaining self-consistent-field (SCF) solutions for the embedded metal cluster plus adsorbed species. The occupied and virtual (unoccupied) orbitals of the SCF solution are then transformed separately to obtain orbitals spatially localized with respect to six-atom, four-atom, and seven-atom surface regions on Ni(100), Ni(111), and Fe(110), respectively. This unitary transformation of orbitals which is based upon exchange maximization with atomic valence orbitals enhances convergence of the configuration interaction (CI) expansion.33-35 The CI calculations involve single and double excitations from multiple parent configurations. All configurations arising from excitations with an interaction energy greater than 1 × 10-5 hartree with the parent SCF configuration and other reference configurations with expansion coefficients > 0.05 are explicitly retained in the expansion. Approximately 30005000 configurations are generated and contributions of excluded configurations are estimated using second-order perturbation theory. In these systems, 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 metal cluster with the adsorbed species’ virtual basis present (but not the adsorbate nuclei). Calculated basis superposition corrections to the total energy are around 3-6 kcal/mol. All energies reported in the present work include the basis superposition contributions. In calculating the adsorption energies for OH and the coadsorption energies for OH + O, the positions of the metal atoms in the cluster model were held fixed at the bulk lattice value corresponding to a nearest-neighbor Ni-Ni distance of 2.48 Å and Fe-Fe distance of 2.48 Å. The clusters used to describe Ni(100), Ni(111), and Fe(110) are shown in Figure 1. After embedding, clusters are reduced in size to 30-, 28-, and 40-atom models for Ni(100), Ni(111), and Fe(110), respectively, depicted as shaded atoms in Figure 1. For the local surface region of six atoms on Ni(100), four on Ni(111), and five on Fe(110), an effective [1s-3p] core potential and valence 3d, 4s, and 4p orbitals are explicitly included in the calculations. Other Ni or Fe atoms are described by an effective core potential for [1s-3d] 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 33-35). The valence basis orbitals of Ni and H are reported in ref 36 and the Fe valence basis orbitals in ref 37. The exponent of H 2p functions is 0.6. The triple-ζ s and p basis for oxygen is taken from Whitten38 and augmented with a set of d polarization functions with exponent of 0.8.
Figure 1. Cluster geometries and local regions of the clusters used to model the Ni(100), Ni(111), and Fe(110) crystal faces. Embedding theory is used to reduce the larger clusters to 30-atom, 28atom, and 40-atom models for Ni(100), Ni(111), and Fe(110), respectively, depicted as shaded atoms. All shaded atoms are described by 4s orbitals, and 3d and 4p orbitals are also included for the central six nickel atoms on Ni(100), four nickel atoms on Ni(111), and five iron atoms on Fe(110) in the surface layer. Unshaded atoms have neutral atom potentials. All atoms have Phillips-Kleinman projectors Σ|Qm>
