Evidence for a Novel Chemisorption Bond: Formate (HCO2) on Cu(100)

Abstract

Surface extended-x-ray-absorption fine-structure measurements reveal that formate (HC0 2 ) groups on Cu(100) chemisorb via the two oxygen atoms in adjacent fourfold hollow sites with an average O-Cu nearest-neighbor bond length of 2.38 ±0.03 A. This distance is significantly ( ~-0.4 A) longer than typical O-Cu bonds in bulk compounds and all known surface complexes. The unusually large O-Cu distance is attributed to a steric effect involving the C atom in HC0 2 and the nearest-neighbor Cu surface atoms.PACS numbers: 78.70.Dm, 82.65.Jv The development of various surface structural probes over the last decade has allowed the determination of the precise location of chemisorbed atoms on surfaces. 1 Although it is still difficult to predict atomic chemisorption sites a priori the chemisorption bond lengths appear to be better understood. As shown by Madhukar 2 and Mitchell, 3 experimentally derived bond lengths for a variety of chemisorbed atoms on metal surfaces can be well accounted for (within 0.1 A) by Pauling's bonding concepts in molecular complexes and bulk compounds. 4 In contrast to the case of atomic chemisorption the structural data base for chemisorbed molecules is still relatively small as a result of the difficulty of many techniques to deal with such systems. From the available structural data 5 it appears, however, that the bond lengths between atoms in the molecule and surface substrate atoms are similar to those of the individual atoms in either surface or bulk complexes.Here we report the first observation that the molecular chemisorption bond can be significantly different from the corresponding bond for an atomic adsorbate or for bulk complexes. Surface extended-x-rayabsorption fine-structure (SEXAFS) 6 complemented by near-edge x-ray-absorption fine-structure (NEXAFS) 7 measurements reveal an unusually long O-Cu distance for the formate (HC0 2 ) species on Cu(l 00). The derived bond length is larger by about 0.4 A than the O-Cu nearest neighbor (nn) bond lengths for atomic oxygen and the methoxy (CH 3 0) species on the same surface and for various bulk compounds (e.g., oxides and formates). This unusual bonding is attributed to a steric effect involving the C atom in a bridge position between two Cu surface atoms. The nonbonding C-Cu interaction which coun-teracts the O-Cu bond pushes the molecule away from the surface and leads to the unusually long O-Cu distance.Experiments were performed on beam line 1-1 at the Stanford Synchrotron Radiation Laboratory with use of the grasshopper monochromator (1200 lines/mm holographic grating). SEXAFS and NEXAFS spectra were obtained by partial-electron-yield detection using a retarding voltage of -450 V. 6 The angle of incidence of the x rays on the sample could be varied from grazing (E vector close to surface normal) to normal (E in surface plane). The Cu(100) surface was cleaned by Ar" 1bombardment. Surface cleanliness and order after annealing were checked by Auger spectroscopy and LEED (base pressure 8xl0~n Torr). The formate was prepared 8,9 by p...

The local structure of molecular reaction intermediates at surfaces

“…The first structural study of these species was an O K-edge SEXAFS investigation of the formate species on Cu(100), in which the polarisation-direction dependence of the nearedge X-ray absorption structure (NEXAFS), the energy range in which intramolecular scattering effects dominate the spectra, provided information on the molecular orientation [14,15]. The NEXAFS data indicated that the molecular plane lies perpendicular to the surface.…”

Temporary Negative-Ion Formation in Chemisorbed Species: The Energy and Angular Dependence of Vibrational Losses for Electron Scattering Studies of HCOO/Ni(110)

“…21 For formate adsorption, the equivalent resonance drops to -15 eV. 8,9,21 It would appear that we are observing a similar resonance, involving the same type of final state, for HCOO/Ni(110) using electron scattering. In the NEXAFS results for HCOO on Cu(l00) and Cu (110) filled.…”

“…We relate this to the so-called a resonance in near-edge x-rayabsorption spectroscopy (NEXAFS) at -15 eV for HCOO on Cu(l 10) and Cu(lOO), involving transitions from the O-ls and C-\s core levels to a quasibound state, which is localized in the C-O bonds of the adsorbed species. 8,9 In addition, we find that there exists a strong angular dependence in the inelastically scattered electron distribution which is peaked at high emission angles away from the surface normal and along the (110) azimuth, corresponding to an enhanced emission in directions correlated with the C-O bonds of the formate species. This represents the first energy and angular study of resonant scattering from a chemisorbed system.…”

Investigation of the oxide shell forming on ɛ-Co nanocrystals

“…In this case, the carboxylic group would deprotonate on the NC surface to yield a resonant structure having both oxygen atoms of the -COO À molecule in the same chemical state. The O K edge of the formate molecule in such a chemical environment has been measured (Stöhr et al, 1985) using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy on a Cu(1 0 0) surface (Fig. 3b, green curve).…”