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The initial oxidation of the Rh(110) surface was studied by scanning tunneling microscopy, core level spectroscopy, and density functional theory. The experiments were carried out exposing the Rh(110) surface to molecular or atomic oxygen at temperatures in the 500-700 K range. In molecular oxygen ambient, the oxidation terminates at oxygen coverage close to a monolayer with the formation of alternating islands of the (10×2) one-dimensional surface oxide and (2×1)p2mg adsorption phases. The use of atomic oxygen facilitates further oxidation until a structure with a c(2×4) periodicity develops. The experimental and theoretical results reveal that the c(2×4) structure is a “surface oxide” very similar to the hexagonal O-Rh-O trilayer structures formed on the Rh(111) and Rh(100) substrates. Some of the experimentally found adsorption phases appear unstable in the phase diagram predicted by thermodynamics, which might reflect kinetic hindrance. The structural details, core level spectra, and stability of the surface oxides formed on the three basal planes are compared with those of the bulk RhO2 and Rh2O3. [ABSTRACT FROM AUTHOR]

The evolution of the structure of the adlayers and the substrate during adsorption of K and coadsorption of K and O on Rh(110) is studied by scanning tunneling microscopy and low-energy electron diffraction. The K adsorption at temperature above 450 K leads to consecutive (1×4), (1×3), and (1×2) missing-row reconstructions for coverage up to 0.12 ML, which revert back to (1×3) and (1×4) with increasing coverage up to 0.21 ML. The coadsorption of different oxygen amount at T>450 K and eventually following reduction-reoxidation cycles led to a wealth of coadsorbate structures, all involving substrate missing-row-type reconstructions, some including segmentation of Rh rows along the [110] direction. The presence of K stabilizes the (1×2) missing-row reconstruction, which facilitates the formation of a great variety of very open (10×2)-type reconstructions at high oxygen coverage, not observed in the single adsorbate systems. [ABSTRACT FROM AUTHOR]

Oxygen hydrogenation at 100 K by gas phase atomic hydrogen on Ni(110) has been studied under ultrahigh vacuum conditions by temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS). Formation of adsorbed water and hydroxyl species was observed and characterized. The coverage of the reaction products was monitored as a function of both temperature and initial oxygen precoverage. On the contrary, when high coverage oxygen overlayers were exposed to gas phase molecular hydrogen, no hydrogenation reaction took place. The results are compared to the inverse process, exposing the hydrogen covered surface to molecular oxygen. In this case, at 100 K, simple Langmuir–Hinshelwood modeling yields an initial sticking coefficient for oxygen adsorption equal to 0.26, considerably lower than for the clean surface. Moreover, formation of hydroxyl groups is found to be twice as fast as the final hydrogenation of OH groups to water. Assuming a preexponential factor of 1013 s-1, an activation barrier of 6.7 kcal/mol is obtained for OH formation, thus confirming the high hydrogenating activity of nickel with respect to other transition metals, for which higher activation energies are reported. However, oxygen is hardly removed by hydrogen on nickel: this is explained on the basis of the strong Ni–O chemical bond. The hydrogen residual coverage is well described including a contribution from the adsorption-induced H desorption process which takes place during the oxygen uptake and which is clearly visible from the TPD data. [ABSTRACT FROM AUTHOR]

The interaction of atomic hydrogen with clean and deuterium precovered Ru(1010) was studied by means of temperature-programmed desorption (TPD) spectroscopy. Compared to molecular hydrogen experiments, after exposure of the clean surface to gas-phase atomic hydrogen at 90 K, two additional peaks grow in the desorption spectra at 115 and 150 K. The surface saturation coverage, determined by equilibrium between abstraction and adsorption reactions, is 2.5 monolayers. Preadsorbed deuterium abstraction experiments with gas-phase atomic hydrogen show that a pure Eley–Rideal mechanism is not involved in the process, while a hot atom (HA) kinetics describes well the reaction. By least-squares fitting of the experimental data, a simplified HA kinetic model yields an abstraction cross section value of 0.5±0.2 Å2. The atomic hydrogen interaction with an oxygen precovered surface was also studied by means of both TPD and x-ray photoelectron spectroscopy: oxygen hydrogenation and water production take place already at very low temperature (90 K). © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The composition, stability, and thermal evolution of nitric oxide on Rh(100) have been characterized by means of synchrotron radiation fast x-ray photoelectron spectroscopy and low-energy-electron diffraction. At 140 K, there is a single NO chemisorbed species (α[sub 2]–NO) at saturation, forming a disordered phase which develops in an ordered p(4[square_root_of_2]×[square_root_of_2])R45° structure after heating the layer to 373 K. A further annealing induces desorption and a progressive dissociation of the NO molecules, which is completed above 425 K, where a c(2×2) phase consisting of coadsorbed N and O atoms is observed. At low NO coverage, a different NO molecular species (α[sub 1]–NO), characterized by ≈1.7 eV lower N1s binding energy, is detected on Rh(100). α[sub 1]–NO is observed to undergo dissociation under the photon beam. The N1s line shape modifications due to irradiation by 500 eV photons were monitored in real time by using fast photoemission. The different stability of the α[sub 1]–NO and α[sub 2]–NO species is discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A detailed experimental and theoretical investigation of the structure of nitric oxide adsorption layers formed at 140 K on Rh(100) has been carried out by means of x-ray photoelectron spectroscopy, x-ray photoelectron diffraction, near-edge x-ray absorption fine structure, and ab initio calculations. At saturation, a single NO species is present. Both theory and experimental results indicate a bridge adsorption site for NO in this phase, with the molecules standing upright on the surface. At low NO coverage, the presence of a different molecular species is experimentally revealed and it is fully characterized by ab initio calculations. This species has been identified with a NO molecule with the molecular axis almost parallel to the surface, lying above a Rh(100) hollow site with the two atoms in asymmetric bridge sites. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The bonding geometry of the C–N molecule in the saturated c(2×2) layer on the Pd(110) surface has been determined by combining polarization-dependent near edge x-ray absorption fine structure and full-solid-angle x-ray photoelectron diffraction (PED). The N K-edge spectra display a strong dependence on the polar and azimuthal orientation of the light polarization with respect to the sample surface. A strong forward scattering peak along the [001] direction is present in the full-solid-angle photoelectron diffraction data of the C1s core level. Both the position of the C1s PED forward scattering peak and the angular dependence of the N K-shell absorption spectra provide direct evidence that the CN molecules is oriented with the molecular axis along the [001] surface direction, at variance with earlier conclusions based on angle-resolved valence level photoemission data. The forward scattering peak in the C1s PED data further indicates that the N atoms lie above the C atoms, with the C–N molecular axis tilted by 25°±4° with respect to the surface plane. The close analogy of this geometry with the results of previous structure determinations of CN adsorbed on Ni and Rh (110) surfaces is discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The nature of bonding in amorphous carbon nitride was studied with core level spectroscopies such as x-ray absorption, photoemission, and Auger electron spectroscopy, using synchrotron radiation as the excitation source. The changes in local structure were systematically studied as a function of sample preparation conditions. The correspondence between nitrogen local environment and spectral features is discussed. It is found that thermal annealing increases the average coordination number and decreases bond multiplicity at nitrogen sites. [ABSTRACT FROM AUTHOR]

The titration of a (2×2)p2mg–O/Rh(110) surface by hydrogen and the subsequent deconstruction of the nonequilibrium (1×2) reconstructed surface has been studied by scanning tunneling microscopy (STM) in a temperature range between 380 and 405 K. The titration reaction is seen to occur via reaction fronts, which start at steps, are elongated in the [11¯0] direction and leave behind a surface covered by diffusing oxygen atoms. Two deconstruction mechanisms are active: A long-range row pairing of adjacent (1×2) rows, which propagates over several hundreds of Å in the [11¯0] direction, and a local hole formation in which Rh atoms detach from the ends of broken [11¯0] rows, diffuse over up to 100 Å and condense into (1×1) islands. Both mechanisms lead to the formation of correlated up–down islands; at step edges, the second one leads to a step retraction. The deconstruction is strongly defect mediated and the STM results indicate that the (1×2) reconstructed surface is most probably metastable. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The oxygen adsorption on the unreconstructed Rh(110) surface has been studied by scanning tunnelling microscopy (STM), by imaging the evolution of the surface reconstruction in situ during oxygen uptake at 390 K. The dynamics terminate with the formation of a strained (10x2)-O structure with high local oxygen coverage. Initially, kink atoms are detached from the steps and diffuse over the surface, most probably as Rh-O units, as inferred by comparison to x-ray photoelectron spectroscopy (XPS) data, until they condense in zigzag structures characteristic for the (2x2)p2mg-O structure (0.5 ML). At oxygen doses above 0.12 L, missing rows begin to be dug along the [11¯0] direction into the steps, detaching further Rh atoms. The reconstruction proceeds as a missing/added row reconstruction. With ongoing surface reconstruction, a second reconstruction mechanism of local nature sets in: Rh atoms start to be ejected out of the terraces and condense at the ends of the so formed troughs. Dosing further oxygen on the (2x2)p2mg-O structure leads to the formation of the strained (10x2)-O structure. The ejection of two of every 10 Rh atoms proceeds only if the Rh rows are not neighbored by troughs. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Oxygen adsorption on the (1x2) missing row reconstructed Rh(110) surface has been studied by means of low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). Starting from the already known (2x2)p2mg oxygen overlayer in which the substrate is (1x2) reconstructed, further oxygen has been dosed at room and lower temperatures. Upon heating, additional substrate reconstruction takes place and the surface forms a new structure with (10x2) periodicity and high local oxygen coverage. Oxygen 1s XPS measurements show a binding energy shift from 530.25 eV in the (2x2)p2mg to 529.75 eV in the (10x2) layer. Analysis of STM images reveals that in the (10x2) layer, the rhodium close-packed rows are strained and segmented in the [11¯0] direction. On the basis of the experimental results, models for the (10x2) structure and its formation process are proposed and compared with "pseudo-oxide" structures. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The normal state temperature dependence of a metallic K[sub 3]C[sub 60] phase pure film has been investigated by means of core levels and valence band photoemission, carbon K edge x-ray absorption, and electron energy loss spectroscopies and compared with that of a Mott-Hubbard insulating K[sub 4]C[sub 60] film. The anomalous temperature behavior of K[sub 3]C[sub 60], in the range 30 K-600 K, can be consistently interpreted considering the presence of orientational disorder of the C[sub 60] molecules together with the inhomogeneity of the conduction electron distribution at the molecular level. In particular, the changes observed in the photoemission spectra near the Fermi level, are consistent with the behavior predicted for strongly correlated metallic systems in presence of disorder. Also in the case of K[sub 4]C[sub 60] we observe anomalies in the temperature dependence of core levels photoemission data. These anomalies can be explained with the freezing of the molecular motions at low temperature and point to large thermal fluctuations of the alkali atoms around their equilibrium position at high temperature. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The adsorption energy of carbon monoxide on Ni ad-islands and ultra-thin films grown on the Cu(110) surface can be finely tuned via a complex interplay among diffusion, site change mechanisms, and coverage effects. The observed features of CO desorption can be explained in terms of migration of CO molecules from Cu to Ni islands, competition between bridge and on-top adsorption sites, and repulsive lateral adsorbate-adsorbate interactions. While the CO adsorption energy on clean Cu(110) is of the order of 0.5 eV, Ni-alloying allows for its controlled, continuous tunability in the 0.98-1.15 eV range with Ni coverage. Since CO is a fundamental reactant and intermediate in many heterogeneous catalytic (electro)-conversion reactions, insight into these aspects with atomic level detail provides useful information to potentially drive applicative developments. The tunability range of the CO adsorption energy that we measure is compatible with the already observed tuning of conversion rates by Ni doping of Cu single crystal catalysts for methanol synthesis from a CO 2 , CO, and H 2 stream under ambient pressure conditions.

The surface and interface structure of the RhO(2) ultrathin film grown on Rh(100) is investigated by means of x-ray photoelectron diffraction. Experimental and simulated one- and two-dimensional angular distribution intensities of the O1s and Rh3d(5/2) chemically shifted core levels are quantitatively analyzed. The previously proposed O-Rh-O trilayer model is independently confirmed. A rippled buckling of the metal surface is observed at the oxide-metal interface, with a mean interfacial Rh-O distance which is 0.2 Å larger with respect to previous findings. The link between the local atomic rearrangement and the overall geometric and electronic properties of the oxide is discussed on the basis of a thorough comparison with the corresponding RhO(2) rutile structure.

We present the results of high-energy resolution core level photoelectron spectroscopy experiments paralleled by density functional theory calculations to investigate the electronic structure of highly undercoordinated Pt atoms adsorbed on Pt(111) and its correlation with chemical activity. Pt4f(7/2) core level binding energies corresponding to atoms in different configurations are shown to be very sensitive not only to the local atomic coordination number but also to the interatomic bond lengths. Our results are rationalized by introducing an indicator, the effective coordination, which includes both contributions. The calculated energy center of the valence 5d-band density of states, which is a well known depicter of the surface chemical reactivity, shows a noteworthy correlation with the Pt4f(7/2) core level shifts and with the effective coordination.

The (1 X 1) --> quasihexagonal (HEX) phase transition on a clean Pt(100) surface was investigated by monitoring the time evolution of the Pt4f(72) core level photoemission spectra. The spectral component originating from the atoms forming the (1x1) metastable unreconstructed surface was found at -570+/-20 meV with respect to the bulk peak. Ab initio calculations based on density functional theory confirmed the experimental assignment. At temperatures above 370 K, the (1 X 1) phase irreversibly reverts to the more stable HEX phase, characterized by a surface core level shifted component at -185 +/ -40 meV. By analyzing the intensity evolution of the core level components, measured at different temperatures in the range of 393 - 475 K, we determined the activation energy of the phase transformation, E = 0.76 +/- 0.04 eV. This value is considerably lower than the one previously determined by means of low energy electron diffraction. Possible reasons for this discrepancy are discussed.