[en] The structures of clean and hydrogen-adsorbed Rh(111) surfaces were investigated by dynamical low-energy electron-diffraction (LEED) analysis. Exposure of D₂ induced no additional LEED patterns except for (1x1). Surface-layer relaxation occurs vertically on both clean and D-saturated surfaces. On the clean surface, the interlayer distance between the first and second layers (d₁₂) is smaller by 1.2(±0.6)% than the corresponding bulk distance of 2.194 A. On the other hand, the contraction of d₁₂ is removed on the D-saturated surface. Detailed LEED analysis demonstrates that the D atoms are adsorbed on the fcc threefold hollow sites. The absolute saturation coverage of H on Rh(111) was determined to be 0.84 ML by nuclear reaction analysis (NRA). Moreover, the zero-point vibrational energy of H was derived from the analysis of the NRA resonance profile, which is discussed in comparison with the results of high-resolution electron-energy-loss spectroscopy

[en] This thesis is focused on a characterization of structural and electronic properties of two transition metal oxides, vanadium dioxide (VO2) and zirconium dioxide (ZrO2, zirconia), with the help of Density Functional Theory (DFT) calculations. Recent experimental findings have revealed that the rutile VO2(110) surface of supported thin films and single crystals undergo a (2x2) surface reconstruction. Since the vanadium dioxide is classified as a strongly correlated material, the performance and reliability of standard DFT functionals has been tested including the PBE, PBE+U (U = 2 eV), as well as the recently developed meta-GGA SCAN and SCAN+rVV functionals, all with nonmagnetic (NM), antiferromagnetic (AFM) and ferromagnetic (FM) spin ordering. The present work shows that the SCAN functional generally improves the bulk properties over PBE and PBE+U and also shows that the calculated stability of low-index VO2 surface orientations, is in most cases independent of the chosen spin configuration, with several identified exceptions. First, the nonmagnetic (NM) calculations on the (001) surface show a lowering of surface-decomposed V-t2g states, accompanied by a lowered surface energy. Second, a similar effect has been observed for all surface orientations when using the PBE+U functional, which opens an electronic band gap in the rutile phase when performing the spin-polarized calculations. Finally, the stoichiometric and off-stoichiometric (2x1) and (2x2) surface reconstructions have been characterized with the help of simulated annealing, an optimization of random structures and the adaption of a V2O5(001)-like monolayer. Polyhedral ring-like terminations, electronically and structurally related to a V2O5(001)-like monolayer are predicted. These proposed novel surface structures are more stable than pure oxygen adsorption phases on the rutile VO2(110) surface. Unfortunately, the stability of the predicted phases depends to some extent on the chosen functional, in which the spin-polarized SCAN functional offers the best compromise, as it gives both a reasonable description of the structural and electronic properties of the rutile VO2 bulk phase and the enthalpy of reaction for higher vanadium oxidation states. Furthermore, the predicted surface oxide models agree much better with experimental STM data than oxygen only adsorption models for the rutile VO2(110) surface. As concerns the insulator zirconia, the performance and accuracy of different DFT functionals was assessed in a first step. Both experimentally observed and theoretically predicted polymorphs have been treated comparing both structural and electronic properties to beyond-DFT benchmark calculations. For zirconia, the SCAN+rVV functional agrees best with the benchmark calculations, better than other (GGA, GGA+U and hybrid) functionals, which overestimate both energetic differences between the common phases and the stability of the predicted meta-stable phases. A subsequent investigation concerning oxygen vacancies in zirconia polymorphs revealed that the cost of forming a neutral oxygen vacancy is above 5.5 eV, depending on the bulk phase and functional used, which may raise this value up to 6.9 eV. However, the oxygen vacancy formation energy is lowered by 2 eV for positively charged defects considering a Fermi level situated in the middle of the band gap. The oxygen vacancies also leave behind a localized electronic charge density in vicinity of a defect, which induces electronic states within the electronic band gap found above mid-gap closer to the conduction band minimum (CBM). This is in agreement with recent experimental studies of defects in thin tetragonal films grown on Rh(111) substrate. (author)

[en] The adsorption of water and coadsorption with oxygen on Rh(111) under ultrahigh vacuum conditions was studied using synchrotron-based photoemission and photoabsorption spectroscopy. Water adsorbs intact on the clean surface at temperatures below 154 K. Irradiation with x-rays, however, induces fast dissociation and the formation of a mixed OH+H₂O layer indicating that the partially dissociated layer is thermodynamically more stable. Coadsorption of water and oxygen at a coverage below 0.3 monolayers has a similar effect, leading to the formation of a hydrogen-bonded network of water and hydroxyl molecules at a ratio of 3:2. The partially dissociated layers are more stable than chemisorbed intact water with the maximum desorption temperatures up to 30 K higher. For higher oxygen coverage, up to 0.5 monolayers, water does not dissociate and an intact water species is observed above 160 K, which is characterized by an O 1s binding energy 0.6 eV higher than that of chemisorbed water and a high desorption temperature similar to the partially dissociated layer. The extra stabilization is most likely due to hydrogen bonds with atomic oxygen.

[en] CH_x (x=1-3) adsorptions on clean and CO precovered Rh(111) surfaces were studied by density functional theory calculations. It is found that CH_x (x=1-3) radicals prefer threefold hollow sites on Rh(111) surfaces, and the bond strength between CH_x and Rh(111) follows the order of CH₃< CH₂< CH. A slight attraction between adsorbed CH_x and CH_x, CH_x and CO, CO and CO radicals/molecules at coverage of 1/9-1/4 ML is found, and considerable repulsion is built up at coverage higher than 1/4 ML. It is found that CH_x adsorption results in the reduction of work function due to charge transfer from the adsorbates to the substrate. For CH₃ adsorption, charge accumulation between the substrate and carbon atom is identified, which results in C-H stretch mode softening. For CO precovered surfaces, the mode softening is prevented due to the weakened interaction between CH_x and substrates from repulsive interaction between adsorbates. Conversely, the overall charge transfer from the CH_x (x=1-3) to the substrate enhances the charge back donation from the substrate to the empty antibonding states of adsorbed carbon monoxide, which results in the softness of the C-O stretch, respectively. The C 1s surface core-level shifts for CH_x with and without the presence of CO were calculated, and a negative shift with respect to the carbon in atop CO on Rh(111) is found, and the negative shift follows the order of CO< CH₃< CH₂< CH. The results are analyzed in details by difference of charge density and projected density of states

[en] The epitaxial growth of graphene on transition metal surfaces by ex situ deposition of liquid precursors (LPD, liquid phase deposition) is compared to the standard method of chemical vapor deposition (CVD). The performance of LPD strongly depends on the particular transition metal surface. For Pt(111), Ir(111) and Rh(111), the formation of a graphene monolayer is hardly affected by the way the precursor is provided. In the case of Ni(111), the growth of graphene strongly depends on the applied synthesis method. For CVD of propene on Ni(111), a 1 × 1 structure as expected from the vanishing lattice mismatch is observed. However, in spite of the nearly perfect lattice match, a multi-domain structure with 1 × 1 and two additional rotated domains is obtained when an oxygen-containing precursor (acetone) is provided ex situ. (paper)

[en] Infrared spectroscopy has been employed to study the reduction of carbon dioxide on supported catalyst films. The investigation included isotopic labeling using D₂ as the reduction gas. Isotopic exchange was observed for both CO₂/D₂ and CH₄/D₂ mixtures. The mechanism of this isotopic exchange involves migration of hydrogen from the support to the Rh sites, an ''inverse spillover effect''. A key intermediate in the dissociation of CO₂ on the supported Rh films was a carbonyl hydride species. 21 refs., 2 figs

[en] High-spin states in the neutron-rich 106,108,110,111,112Rh isotopes have been investigated in the fission of the compound system formed in three heavy-ion induced reactions. Four bands were assigned to 106,108,110,112Rh, respectively. Comparison with the lighter odd-odd Rh isotopes supports the assignment of the bands to the πg9/2 (multiply-in-circle sign) vh11/2 configuration. In 111Rh the level scheme consists of two rotational bands. In the ground-state band the odd-proton occupies the πg9/2 orbital

[en] Anionic adsorption on platinum and rhodium surfaces of different crystallographic orientations was studied by radioactive labelling technique and cyclic voltammetry. The free Gibbs energy of adsorption and the energy of lateral interaction of anions adsorbed on well-defined and disordered electrodes were calculated using the Bennes isotherm. It was shown that surface order generally leads to the increased amount of anions absorbed and affects substantially the potential dependence of adsorption. An electrocatalytic specificity of rhodium electrodes of different surface crystallography with respect of perchlorate reduction and chloride adsorption was found in a study performed on Rh(100), Rh(111) and polycrystalline Rh

[en] Levels in ¹¹¹Rh have been investigated via the γ-rays following the β^--decay of 2.1 s ¹¹¹Ru. The Ru activity was produced in the fission of ²⁴⁹Cf and separated chemically from the fission product mixture. The emitted γ-rays were studied by γ singles and γγ(t) coincidence measurements. Evidence for intruder states in ¹¹¹Rh has been obtained. Their properties are discussed and compared with those in the lighter Rh isotopes. (orig.)

[en] Using the Fast On-line Reaction Apparatus (FORA), the influence of various gas-purification columns onto the formation of metal carbonyl complexes (MCCs) under single-atom chemistry conditions was investigated. MCCs were synthesized from single atoms of Mo, Tc, Ru and Rh being produced by the spontaneous fission of ${}^{252}$Cf and recoiling into a CO-gas containing carrier gas atmosphere. The in-situ synthesized MCCs were volatile enough to be transported by the carrier gas to a charcoal trap where they were adsorbed and their subsequent decay was registered by γ-spectrometry. It was found that the type and combination of purification columns used to clean the applied CO-gas strongly influences the obtained formation and transport yields for all MCCs. With the exception of Rh-carbonyl, intense gas-purification strategies resulted in reduced formation and transport yields for MCCs in comparison with less efficient or even completely missing purification setups. It was postulated that the observed reduction in yield might depend on the content of Fe(CO)${}_5$ and Ni(CO)${}_4$, as well as potentially other MCCs, in the CO-gas, being formed by the interaction between CO and the steel-surfaces of FORA as well as from impurities in the used charcoal traps. Subsequently, it was shown that macro amounts of Fe(CO)${}_5$, Ni(CO)${}_4$, Mo(CO)${}_6$ and Re${}_2$(CO)${}_{10}$ added to the used process gas indeed increase significantly the overall yields for MCCs produced by ${}^{252}$Cf fission products. Ni(CO)${}_4$ appeared the most potent to increase the yield. Therefore, it was used in more detailed investigations. Using isothermal chromatography, it was shown that Ni(CO)${}_4$ does not affect the speciation of carbonyl species produced by the ${}^{252}$Cf fission product ${}^{104}$Mo. For ${}^{107}$Tc, ${}^{110}$Ru and ${}^{111}$Rh a speciation change cannot be excluded. For ${}^{111}$Rh a speciation change cannot be excluded. An inter-carbonyl transfer mechanism is suggested boosting the formation of MCCs. The current discovery might allow for new opportunities in various research fields, which are currently restricted by the low overall yields for MCCs produced under single-atom chemistry conditions. Examples are the chemical investigation of transactinides or the generation of radioactive ion beams from refractory metals at accelerators.