G. Paolucci

Real-time X-ray photoelectron spectroscopy of surface reactions

Abstract The experimental determination of the composition and structure of gas–solid surface interface at different stages of surface reactions is a crucial point in elucidating the reaction mechanism. This requires a quantitative surface sensitive technique, providing information on how the substrate surface, adsorbed species and their bonding configuration evolve at the time scale of the surface processes. The present review illustrates how the high performance levels achieved in X-ray photoelectron spectroscopy at the third generation synchrotron facilities, in particular the reduced data acquisition time down to a second range, have made possible studies of surface processes in real time. The article summarizes the wealth of knowledge that has been gained using representative examples of adsorption systems, where the relation between adsorption–desorption rate, adsorbate coverage, bonding configuration and interconversion between adsorption sites was established, and simple reaction systems, where the effects of the substrate structure and of the changes in the adsorbate layer under non-linear reaction conditions were probed.

Adsorption of oxygen on Rh(110): a LEED, Auger electron spectroscopy and thermal desorption study

Abstract The adsorption of oxygen on the Rh(110) surface has been studied by a variety of techniques. Low-energy electron diffraction shows the following patterns: (2 × 1)p2mg at 1 ML coverage and temperatures between 125 and 300 K; (2 × 2)p2mg at 0.5 ML coverage after heating to above 470 K; c(2 × 8) and complex streaked c(2 × 2 n ) patterns at coverages above 0.5 after heating to 470 K. These results are in partial agreement with previous work. Models for the first two structures are suggested. In the (2 × 2) structure, the oxygen is found to be much less reactive with CO at room temperature than in the (2 × 1) structure, suggesting that it is subsurface. A metastable (1 × 2) structure was produced from the (2 × 2) by reduction of the oxygen by CO at 450 K, and is interpreted as a surface reconstruction.

Temperature programmed X-ray photoelectron spectroscopy : a new technique for the study of surface kinetics

Abstract The desorption of CO from the (2 × 1)p2mg layer on Rh(110) was studied by means of a novel method — temperature-programmed X-ray photoelectron spectroscopy (TPXPS). The new information on the variation of the concentration of top and bridge CO during thermal desorption was used to evaluate the difference of the adsorption enthalpies. A model for the structural evolution of the adsorbed layer is suggested, taking into account the CO bonding configurations and the role of the CO CO repulsive interactions.

High-resolution XPS and NEXAFS study of SO2 adsorption on Pt(111): two surface SO2 species

Abstract The adsorption and condensation as well as the temperature-dependent decomposition of SO2 on Pt(111) have been studied by high-resolution core-level photoemission and near-edge X-ray absorption fine structure. The data analysis shows that SO2 adsorbs molecularly at 150 K forming a monolayer phase with two SO2 species of different binding energy and quite different molecular orientation. On increasing the temperature to 210 K only SO2 molecules of 0.43 monolayer coverage survive with molecular planes being tilted by α = 31° ± 10° with respect to the surface normal. For condensed SO2 multilayers an SO4 species is formed close to 300 K.

Adsorption of oxygen on Rh(110) and reactivity of different overlayer structures

Abstract The relation between surface structure and reactivity of oxygen on Rh(110) has been studied by means of AES, LEED and TPD. Various oxygen surface structures are produced by changing the oxygen coverage, θ o , and surface temperature. At temperatures below 300 K and oxygen coverages larger than 0.4 only a (2 × 1)p2mg LEED pattern was observed. Heating to 470 K leads to a c(2 × 8) pattern at nearly monolayer coverage and to c(2 × 2 n ) structures at 0.6 θ o 2 has been investigated and found to be higher for the (2 × 2)p2mg than for the c(2 × 2 n ) structure. After reduction of the (2 × 2)p2mg and c(2 × 2 n ) oxygen structures, metastable ( 1× n ) structures are produced which revenmt to (1 × 1) at temperatures above 470 K. Models for some of the oxygen structures are suggested on the basis of the LEED data and the observed differences in the reactivity of oxygen.

Oxygen on Pd(110): substrate reconstruction and adsorbate geometry by tensor LEED

Abstract The c(2 × 4) and (2 × 3) structures formed by oxygen adsorption on Pd(110) were studied in an LEED I–V experiment by employing the tensor LEED method. Adsorptiondashinduced reconstructions occur in both cases, giving rise to respectively (1 × 2) and (1 × 3) periodicities of the Pd atoms. Oxygen removal by H2 treatment leads to clean, metastable, reconstructed phases, which have been studied as starting points for the analysis of the oxygendashcovered surfaces. The reconstruction is of the missing-row type in both cases, with the removal of two rows of atoms out of every three for the (1 × 3) surface. In both cases a contraction of the first three interlayer spacings has been observed. The oxygen adatoms are adsorbed on the (111) facets formed as a consequence of the missing-row reconstruction, and form zig-zag chains along the [110] rows. These chains are in antiphase in the c(2 × 4) structure, and with random relative phase in the (2 × 3) structure. Compared to bulk Pd, the c(2 × 4) structure presents a slight increase in the first interlayer distance and a contraction of the second. On the other hand, the (2 × 3) structure shows a relatively large increase in the first interlayer distance and again a reduction in the second. Lateral changes of atomic positions and buckling in the second or third substrate layers have been found in all of the four structures analysed.

Effect of the reaction conditions on the stability and structure of nitrogen layers on Rh(110) surfaces

Abstract Parallel TPD and LEED studies are used to compare the behaviour of atomic nitrogen layers produced by NH3 dissociation or by NO + H2 reaction on Rh(110)1 × 1 and Rh(110)1 × 2 surfaces at reaction temperatures ranging from 310 to 480 K. Special attention is paid to the effect of “subsurface” oxygen and coadsorbed oxygen on the stability and desorption kinetics of the nitrogen layers. Depending on the reaction temperature, Tr, nitrogen deposited by dissociative adsorption of NH3 on a well reduced Rh(110)1 × 1 surface results in two N2 TPD peaks, α and β1, located at ≈470 and ≈530 K, respectively. The α-peak, which is the dominating feature for Tr ≦ 330 K, is completely removed by the β1-peak when the exposure is performed at T r ≧ 400 K. The nitrogen layers built by NO + H2 reaction at T > 450 K are the most stable and desorb in a very sharp β2-peak at 580 K. The same “explosive” desorption is shown by nitrogen layers deposited by dissociative adsorption of NH3 at T r ≧ 400 K on Rh(110)1 × 2 oronRh(110)1 × 1 with traces of “subsurface” oxygen. The desorption of the β1- and β2-states follows first-order and zeroth-order desorption kinetics, respectively. Ordered 3 × 1 and 2 × 1 LEED patterns characterize both higher temperature β-states. For the β-states the intensity of the fractional spots is comparable to that of the integer ones and almost does not change with increasing primary energy, but the Debye-Waller factor of the β2-state is smaller. It is suggested that the accommodation of nitrogen atoms in these states involves a rearrangement of the substrate atoms. The observed differences in the behaviour of the atomic nitrogen layers indicates that a substantial restructuring of the substrate is required for optimization of nitrogen bonding. The degree of restructuring depends on the actual reaction conditions, the reaction temperature, the substrate surface symmetry and the presence of oxygen contaminants.

Structural determination of molecules adsorbed in different sites by means of chemical shift photoelectron diffraction: c(4×2)-CO on Pt(111)

Abstract In this letter we present a chemical shift photoelectron diffraction (CS-PED) study of the c(4×2) phase of CO on Pt(111), in which two molecular species are present. The angular dependence of both chemically shifted C1s core levels was independently determined in a high resolution photoemission experiment. By means of a full multiple scattering analysis, the local structure around each of the two CO species has been quantitatively evaluated. A configuration with half of the CO molecules on top and half in the bridge position gives good agreement with the experimental data, in accord with previous structural studies of this system. Our result highlights the capabilities of CS-PED for the study of complex adsorption systems where the same molecule occupies different adsorption sites.

CO adsorption on unreconstructed and reconstructed Rh(110) surfaces: LEED and XPS studies

Abstract The adsorption of CO on the (1 × 1) unreconstructed and (1 × 2) reconstructed Rh(110) surfaces has been studied by means of XPS and LEED. The O 1s spectra of CO were used for measuring the coverage and determining the binding sites of the adsorbed molecules. It has been found that on both surfaces CO adsorption occurs in two bonding configurations characterized by O 1s binding energies at 530.8 and 531.9 eV. They are assigned to occupation of bridge and on-top sites, respectively. On the (1 × 1) surface initially the on-top sites are occupied yielding an ordered c(2 × 2) layer at a coverage of 0.5 ML. At higher coverage CO changes its bonding configuration and a (2 × 1)p2mg structure is formed near saturation when the coverage approaches 1 ML. It is suggested that the (2 × 1)p2mg structure consists of zig-zag chains of bridging tilted CO. When CO adsorbs on the (1 × 2) surface simultaneous occupation of on-top and bridge sites is observed. Molecules in both bonding configurations contribute to the observed (2 × 2), c(2 × 4) and (2 × 2)p2mg structures formed on the (1 × 2) surface. The intensities of the CO O 1s spectra indicate that the amount of CO adsorbed on the (1 × 1) and (1 × 2) surfaces is the same. Comparable amounts of CO adsorbed on the [110] rows and (111) microfacets of the (1 × 2) surface are suggested in order to explain the observed (2 × 2), c(2 × 4) and (2 × 2)p2mg structures.

Metastable (1×2) and (1×3) reconstructions of Pd(110)

Abstract By adsorption and subsequent reduction of oxygen on Pd(110), metastable (1 × 2) and (1 × 3) reconstructed surfaces have been produced. Oxygen was not present after the reduction but a small amount of residual hydrogen (