In this work we have studied the steady-state reaction of molecular and atomic hydrogen with oxygen on a Pd(111) surface at a low total pressure (<10(-7) mbar) and at sample temperatures ranging from 100 to 1100 K. Characteristic features of the water formation rate Phi(pH2; pO2; TPd) are presented and discussed, including effects that are due to the use of gas-phase atomic hydrogen for exposure. Optimum impingement ratios (OIR) for hydrogen and oxygen for water formation and their dependence on the sample temperature have been determined. The occurring shift in the OIR could be ascribed to the temperature dependence of the sticking coefficients for hydrogen (SH2) and oxygen (SO2) on Pd(111). Using gas-phase atomic hydrogen for water formation leads to an increase of the OIR, suggesting that hydrogen abstraction via hot-atom reactions competes with H2O formation. The velocity distributions of the desorbing water molecules formed on the Pd(111) surface have been measured by time-of-flight spectroscopy under various conditions, using either gas-phase H atoms or molecular H2 as reactants. In all cases, the desorbing water flux could be represented by a Maxwellian distribution corresponding to the surface temperature, thus giving direct evidence for a Langmuir-Hinshelwood mechanism for water formation on Pd(111).

At low tunneling resistance, scanning tunneling microscopy (STM) images of a Rh(100) surface with adsorbed hydrogen reproducibly show protrusions in all bridge sites of the surface, leading to a naive interpretation of all bridge sites being occupied with H atoms. Using quantitative low-energy electron diffraction and temperature programmed desorption we find a much lower H coverage, with most H atoms in fourfold hollow sites. Density functional theory calculations show that the STM result is due to the influence of the tip, attracting the mobile H atoms into bridge sites. This demonstrates that STM images of highly mobile adsorbates can be strongly misleading and underlines the importance of additional analysis techniques.

Exposure of Rh(100) to hydrogen (deuterium) in atomic form leads to the population of adsorption sites, not attainable with molecular species. Quantitative thermal desorption spectroscopy (TDS), high resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) calculations have been applied to investigate these new adsorption sites. In addition to the fourfold hollow sites (1 ML), which can be populated by dissociative adsorption, occupation of subsurface sites and the population of additional surface sites (for deuterium) have been observed (maximum coverage 3.4 ML). In TDS individual adsorption states show up in the form of three different peaks: Recombination of H (D) atoms from hollow sites around 300 K, desorption of subsurface species between 150–200 K, and recombinative desorption via a molecular precursor at about 120 K (for deuterium only). The exposure of the Rh(100) surface to atomic H (D) leads to a pronounced roughening of the surface, as evidenced in the HREELS spectra. Zero point corrected adsorption energies, activation barriers for adsorption, desorption, and diffusion into the subsurface sites, as well as vibrational energies have been calculated by DFT for a variety of adsorbate configurations of H and D and compared with the experimental data.

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