We report the observation and manipulation of hydrogen atoms beneath the surface of a Pd{111} crystal by using low-temperature scanning tunneling microscopy. These subsurface hydride sites have been postulated to have critical roles in hydrogen storage, metal embrittlement, fuel cells, and catalytic reactions, but they have been neither observed directly nor selectively populated previously. We demonstrate that the subsurface region of Pd can be populated with hydrogen atoms from the bulk by applying voltage pulses from a scanning tunneling microscope tip. This phenomenon is explained with an inelastic excitation mechanism, whereby hydrogen atoms in the bulk are excited by tunneling electrons and are promoted to more stable sites in the subsurface region. We show that this selectively placed subsurface hydride affects the electronic, geometric, and chemical properties of the surface. Specifically, we observed the effects of hydride formation on surface deformation and charge and on adsorbed hydrogen on the surface. Hydrogen segregation and overlayer vacancy ordering on the Pd{111} have been characterized and explained in terms of the surface changes attributable to selective hydrogen occupation of subsurface hydride sites in Pd{111}.atomic manipulation ͉ catalysis ͉ palladium ͉ hydrogen ͉ scanning tunneling microscopy

We present an atomic-scale study of substituent effects in the Ullmann coupling reaction on Cu{111} using low-temperature scanning tunneling microscopy and spectroscopy. We have observed fluorophenyl intermediates and phenyl intermediates as well as biphenyl products on Cu{111} after exposure to 4-fluoro-1-bromobenzene (p-FC(6)H(4)Br) and bromobenzene (C(6)H(5)Br), respectively. When p-FC(6)H(4)Br dissociatively chemisorbs at 298 K on Cu{111}, the relatively weakly bound Br dissociates, and fluorophenyl intermediates are formed. These intermediates couple to form 4,4'-difluorobiphenyl and desorb at temperatures below 370 K. However, by cooling the substrate to low temperature (4 K), we have observed unreacted fluorophenyl intermediates distributed randomly on terraces and at step edges of the Cu{111} surface. Alternatively, at similar coverages of C(6)H(5)Br, we have observed biphenyl distributed on terraces and step edges. In each case, Br adatoms were randomly distributed on the surface. Chemical identification of fluorophenyl and phenyl intermediates and biphenyl products was achieved by vibrational spectroscopy via inelastic tunneling spectroscopy. The strongest vibrational mode in the phenyl species disappears when the tilted intermediates couple to form biphenyl products. We infer that the surface normal component of the dipole moment is important in determining the transition strength in inelastic electron tunneling spectroscopy.

Long-range electronic interactions between Br adatom islands, which are formed at approximately 600 K, on Cu(111) are mediated by substrate surface-state electrons at that elevated temperature. Using scanning tunneling microscopy at 4 K, we have quantified nearest neighbor island separations and found favored spacings to be half-multiples of the Fermi wavelength of Cu(111). The strong interaction potential and decay length of the interisland interactions are discussed in terms of the interaction of Br with the substrate surface state.

The reaction of methanol on the (100) surfaces of single crystal vanadium carbide (VC) and titanium carbide (TiC) has been studied using high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). Methanol forms a mixed monolayer of molecular methanol and a methoxy intermediate upon adsorption at 153 K on both VC(100) and TiC(100). With increasing temperature, methanol is evolved from both surfaces through molecular and recombinative desorption. Approximately half of the methoxy intermediate reacts with the VC surface to produce formaldehyde and hydrogen, with a small amount of methane and persistent oxygen surface species. By contrast, very little of the methoxy intermediate reacts with the TiC surface, producing methane and hydrogen. A model of the surface reactions has been constructed based upon differences in the electronic structures of the carbide substrates.

Detailed spectroscopic studies of the interaction of carbon monoxide (CO) with the (100) surfaces of titanium carbide (TiC) and vanadium carbide (VC) have been performed for the first time and analyzed to provide insight into the nature of the surface chemical interactions. The carbide materials are technologically important in extreme applications due to their remarkably high hardness and melting points. This work was pursued to develop a fundamental understanding of the surface bonding and reaction properties to enhance the use of TiC and VC as tribological materials and to gain insight into their potential use as catalysts. VC and TiC are both rocksalt materials but differ fundamentally in their electronic structure as the additional electron present in a formula unit of VC presents a significantly different surface bonding environment. CO has been used as a probe molecule to determine the relative electron accepting and donating tendencies of the substrates. Temperature-programmed desorption (TPD) has demonstrated that CO has a significantly higher heat of desorption on VC compared to TiC. High-resolution energy loss spectroscopy (HREELS) was used to measure surface vibrational frequencies, and the CO stretch of reversibly adsorbed CO is 2060 cm-1 on VC, and 2120 cm-1 on TiC, indicative of greater π-back-bonding on the VC surface. This enhanced back-bonding interaction is also observed in core level X-ray photoelectron spectroscopy satellite structure, and in valence band perturbations observed with ultraviolet photoelectron spectroscopy. Detailed analyses of these data show that CO has a slightly stronger σ-donor interaction with VC, but the stronger VCCO bond is due primarily to the π-interaction that is essentially absent on the TiC surface. Density functional theory (DFT) has also been applied to small MC clusters that qualitatively reproduce the observed experimental trends. DFT also provides compelling evidence of the impact of the electronic structure difference on the CO interaction, as occupied d-orbitals in VC participate in the back-bonding interaction, but these levels are unoccupied in TiC. The results are entirely consistent with a simplified molecular orbital description of the materials that results in the surface metal atoms of TiC behaving like d0 species and those of VC as d species. These formal occupations are greatly tempered by covalent mixing with carbon atoms in the lattice, but the electronic structure clearly plays a dominant role in the surface bonding of the carbides, controlling their reactivity with lubricants and reactants with which they come into contact.

Chemisorption of hydrogen on Pd{111} is a relatively simple, yet important surface chemical process. By using low-temperature scanning tunneling microscopy, tip-induced motion of adsorbed atomic hydrogen at 4 K has been observed at low coverage. The motion has been ascribed to excitation of vibrational modes that decay into translational modes; vibrational spectroscopy via inelastic electron tunneling corroborates this assignment, and the barrier to hydrogen atom motion has been determined. At higher coverages, tip-induced motion of vacancies in the hydrogen overlayer is observed, and the associated barrier has also been determined.

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