Electronic Surface States and the Hydrogen Dissociation Barrier

Abstract

Surface states contribute significantly to the charge density in front of metal surfaces. Owing to the Pauli principle, this yields a repulsive contribution in the physisorption potential for closed‐shell adsorbate particles, such as rare‐gases or H2. In the case of rare‐gas adsorption the surface state induced repulsion can be directly measured by means of high‐resolution photoemission. The additional repulsive contribution influences also the barrier height in dissociative adsorption. This is demonstrated for f.c.c. (111) and f.c.c. (110) surfaces by adjusting the surface state occupancy with a variety of suitable adsorbates and monitoring the associated changes in the dissociation probability of H2.

Bridging the structure gap: Chemistry of nanostructured surfaces at well-defined defects

“…E.g., for Ni(111) dissociative hydrogen chemisorption is activated and occurs directly [164], while for Ni (110), Ni(100), Ni(997) [165], Pd(100), Pd (111) and Pd(110)(1×2) [166] an additional, precursor mediated, channel opens up at low energy. The existence of a precursor to dissociation for Pd (110) and Pd (111) and its absence for Pt (111) and Ni (111) were ascribed to the different degree of occupation of the Shockley surface state at Γ for the different metals [167,168]. The lower reactivity of Pt(111) should be associated, accordingly, to the higher filling of the surface state.…”

Ballistic-electron-emission microscopy of conduction-electron surface states

“…STM current-voltage (I-V) measurements taken at various locations on the mesas and terraces of the samples invariably indicated the presence of the two-dimensional conduction-electron surface states, as seen previously in STM experiments on bulk single-crystal and epitaxial thinfilm Cu, Ag and Au ͑111͒ surfaces. [2][3][4][5][6][7][8][9][10] The noble-metal surface states have been measured by photoemission to have their band minima at 0.41, 0.39, and 0.12 eV below the Fermi level, for Au, Cu, and Ag, respectively. 1 In Fig.…”

“…These surface states arise from the lack of propagating bulk states at and about the Fermi surface of these metals in a cone centered on the ͑111͒ direction. The interest in these states is due in part to the roles they can play in surface phenomena and thin-film growth, 2,3 but also because of the ability of the scanning tunneling microscope ͑STM͒ to locally image and study these states with high precision. [4][5][6][7][8][9][10] With the STM spectacular images have been obtained of confinement of surface states by ''quantum corrals'' of individually placed atoms, 6,8 and by the formation of small mesas or terraces on the surface of a properly oriented metal crystal.…”

Two-dimensional electron gas at noble-metal surfaces

“…For example, surface states play an important role in shaping the physisorption potential, which in term determines chemical properties of surfaces, e.g. catalytic reactivity and dissociation [6][7][8]. Furthermore, surface states are responsible for long-range (r −2 ) substrate-mediated adsorbate interactions, which may dominate the bulk-state-mediated contribution (r −5 ) for large adsorbate-adsorbate separation [9].…”