Currently, platinum is the most widely used catalyst for low temperature proton exchange membrane fuel cells (PEMFC). However, the kinetics at the cathode are slow, and the price of platinum is high. ...To improve oxygen reduction reaction (ORR) kinetics at the cathode, platinum can be alloyed with rare earth elements, such as yttrium. We report that Pt3Y has the potential to be over 2 times more active for the ORR compared with Pt inside a real fuel cell. We present detailed photoemission analysis into the nature of the sputtered catalyst surface, using synchrotron radiation photoelectron spectroscopy (SRPES) to examine if surface adsorbates or impurities are present and can be removed. Pretreatment removes most of the yttrium oxide in the surface leaving behind a Pt overlayer which is only a few monolayers thick. Evidence of a substochiometric oxide peak in the Y 3d core level is presented, this oxide extends into the surface even after Ar+ sputter cleaning in‐situ. This information will aid the development of new highly active nanocatalysts for employment in real fuel cell electrodes.
Interaction of water with fully oxidized and partially reduced CeO₂(111) thin film model catalyst grown on a Cu(111) surface was investigated by photoelectron spectroscopy (PES), scanning tunneling ...microscopy (STM) and temperature programmed desorption (TPD). On the stoichiometric surface water adsorbs molecularly at low temperatures (≤120K) while on the reduced surface the adsorption is partially dissociative with formation of OH groups. STM indicates no morphology variation and a very sensitive Ce 4d–4f resonant photoelectron spectroscopy (RPES) no noticeable change of the oxidation state of ceria upon water adsorption and subsequent complete desorption. Formation of co-adsorbed phase of residual water with OH during molecular water desorption, on the other hand, leads to a substantial resonance of the Ce³⁺ photoemission state around 170K. We propose that this behaviour indicating ceria reduction is in this case an electronic effect of the Ce 4f charge accumulation and Ce 5d charge depletion.
This photoelectron spectroscopy (PES) study compares the adsorption and subsequent decomposition of methanol on a Pd(111) single crystal and Pd particles on a CeO
2
(111)/Cu(111) thin film surface ...investigated by means of XPS and soft X-ray synchrotron-radiation-excited PES. Resonance PES (Ce 4d → 4f transitions) was used to determine the Pd deposit and cerium oxide surface interaction with high sensitivity. Methanol adsorbed at low temperature formed adsorbate multilayers, which were consecutively desorbed upon stepwise annealing to 600 K. The results obtained on Pd/CeO
2
(111) showed a more complex decomposition process involving different possible pathways compared to the single pathway decomposition to CO on the Pd(111) surface.
Graphical Abstract
Enhanced superconductivity of FeSe in the 2D limit on oxide surfaces as well as the prediction of topological superconductivity at the interface to topological insulators makes the fabrication of ...Fe-chalcogenide monolayers a topic of current interest. So far superconductive properties of the latter are mostly studied by scanning tunneling spectroscopy, which can detect gaps in the local density of states as an indicator for Cooper pairing. Direct macroscopic transport properties, which can prove or falsify a true superconducting phase, are yet widely unexplored due to the difficulty to grow monolayer films with homogeneous material properties on a larger scale. Here we report on a promising route to fabricate micron-scale continuous carpets of monolayer thick FeSe on Bi2Se3 topological insulators. In contrast to previous procedures based on ultraflat bulk Bi2Se3 surfaces, we use molecular beam epitaxy grown Bi2Se3 films with high step-edge densities (terrace widths 10-100 nm). We observe that step edges promote the almost strainless growth of coalescing FeSe domains without compromising the underlying Bi2Se3 crystal structure.
Reduced ceria can be reoxidized by CO2 at room temperature in complete absence of any noble metal co-catalysts, surface hydroxyl groups, or water on its surface. The activation of CO2 on ceria-based ...catalysts opens chemical routes to the use of CO2 as an oxidizing agent. Toward a better understanding of the underlying surface reactions, we have performed a model study on well-defined CeO2- x thin films on Cu(111) under ultra-high-vacuum conditions. Resonant photoelectron spectroscopy is used as a highly sensitive tool to monitor changes in the oxidation state of cerium as a function of CO2 exposure and thermal treatment. It is shown that partial reoxidation of CeO2- x by CO2 occurs with high reaction probabilities (i) even at temperatures as low as room temperature, (ii) on pure CeO2- x in the complete absence of any noble metal co-catalysts, and (iii) in the absence of surface hydroxyl groups or water. In contrast to previous studies on ceria powders, the findings clearly demonstrate the high activity of pure hydroxyl- and metal-free ceria surfaces for CO 2 activation, even at low reaction temperatures. PUBLICATION ABSTRACT
Adjustable morphology and degree of reduction represent desirable properties of model oxide substrates for heterogeneous catalysis. We investigate these properties in CeO2 (ceria) thin films on ...Cu(111) using scanning tunneling microscopy and photoelectron spectroscopy. We identify growth mechanisms of ceria on Cu(111): formation of incomplete oxide interfacial layer and formation of three-dimensional ceria pyramids by stacking of monolayer-high islands. Using these mechanisms, we control the coverage, the number of open monolayers, and the step density of ceria thin films on Cu(111). Annealing in vacuum allows us to control besides the morphology also the degree of ceria surface reduction. We find a correlation between surface reduction and morphological stability in annealed ceria layers. Oriented and stoichiometric thin films of ceria on Cu(111) can be prepared at temperatures as low as 150 and 250 °C. Both the morphology and the surface reduction of these films readily change with increasing temperature, which must be accounted for in considering temperature-programmed experiments with ceria on Cu(111).
The interaction of ultrathin bimetallic Ni–Co layers (0.25 and 1.5 nm) supported on polar (0001)Zn–ZnO and (0001̅)O–ZnO substrates was investigated by synchrotron-based photoelectron spectroscopy ...(PES) under ultrahigh vacuum (UHV) and O2 environments. Monometallic Ni and Co layers were also characterized to highlight the influence of Ni–Co synergetic effects on the metal–support interaction. At room temperature, cobalt is partially oxidized, while nickel is metallic. The effect of ZnO surface termination is minor, while the influence of surface hydroxyl groups is discussed. Annealing at 773 K in UHV promotes oxidation of monometallic Ni and Co layers but has little influence on bimetallic Ni–Co. In addition, significant agglomeration of the Ni–Co overlayer is observed, with a parallel increase in the surface Co concentration. Agglomeration of Ni–Co is more pronounced on O-terminated ZnO. Upon annealing in 1 × 10–6 mbar of O2, both Ni and Co readily oxidize and redisperse over the ZnO substrate. Moreover, cobalt tends to segregate over nickel, creating a concentration gradient between the two alloy constituents (probably a core–shell-like structure). Overall, our results indicate that the interaction at the Ni–Co/ZnO interface is influenced by the synergetic effects between the two metals and to a lesser extent by the substrate termination. Taking into account the substantial progress made in the synthesis of ZnO nanostructures and surfaces, this study can assist in the effort toward improved ZnO-based catalysts with tailored properties.
The electronic structure of cluster assembled nanostructured TiO(2) thin films has been investigated by resonant photoemission experiments with photon energies across the Ti L(2,3) edge. The samples ...were produced by supersonic cluster beam deposition with a pulsed microplasma cluster source. The valence band shows resonance enhancements in the binding energy region between 4 and 8 eV, populated by O 2p and hybridized Ti 3d states, and in the region about 1 eV below the Fermi level associated with defects related Ti 3d states. The data show that in as-deposited films Ti atoms are mainly fully (sixfolds) coordinated to oxygen atoms in octahedral symmetry and only a small fraction is in a broken symmetry environment. Since resonant photoemission is closely linked to the local electronic and structural configurations around the Ti atom, it is possible to correlate the resonant photoemission intensity and lineshape with the presence of defects of the films and with the degree of hybridization between the titanium and oxygen atoms.
Adsorption and desorption of methanol on a CeO2(1 1 1)/Cu(1 1 1) thin film surface was investigated by XPS and soft X-ray synchrotron radiation PES. Resonance PES was used to determine the occupancy ...of the Ce 4f states with high sensitivity. Methanol adsorbed at 110 K formed adsorbate multilayers, which were partially desorbed at 140 K. Low temperature desorption was accompanied by formation of chemisorbed methoxy groups. Methanol strongly reduced cerium oxide by forming hydroxyl groups at first, which with increasing temperature was followed by creation of oxygen vacancies in the topmost cerium oxide layer due to water desorption. Dissociative methanol adsorption and creation of oxygen vacancies was observed as a Ce4+ - > Ce3+ transition and an increase of the Ce 4f electronic state occupancy.
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