Yuemin Wang

Vibrational and structural properties of OH adsorbed on Pt(111)

OH species adsorbed on Pt(111) were studied in a combined investigation using scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS). OH was formed by two different reactions, by reaction of H2O with O, and as an intermediate in the reaction of O with hydrogen to H2O. In both cases, two ordered OH phases were observed, a (√3×√3)R30° and a (3×3) structure, for which models are proposed. Both structures have OH coverages of 2/3, and their formation is driven by hydrogen bond formation between the adparticles; the OH adsorption site is most likely on top. OH molecules at defects in the adlayer, in particular at island edges, are spectroscopically distinguishable and contribute significantly to the vibrational spectra in disordered OH layers. This is important for the water formation reaction, where the OH islands are small. The discrepancies between previous HREELS studies on OH can be explained by the different degree of order under the various formation conditions.

Structural Complexity in Metal–Organic Frameworks: Simultaneous Modification of Open Metal Sites and Hierarchical Porosity by Systematic Doping with Defective Linkers

A series of defect-engineered metal-organic frameworks (DEMOFs) derived from parent microporous MOFs was obtained by systematic doping with defective linkers during synthesis, leading to the simultaneous and controllable modification of coordinatively unsaturated metal sites (CUS) and introduction of functionalized mesopores. These materials were investigated via temperature-dependent adsorption/desorption of CO monitored by FTIR spectroscopy under ultra-high-vacuum conditions. Accurate structural models for the generated point defects at CUS were deduced by matching experimental data with theoretical simulation. The results reveal multivariate diversity of electronic and steric properties at CUS, demonstrating the MOF defect structure modulation at two length scales in a single step to overcome restricted active site specificity and confined coordination space at CUS. Moreover, the DEMOFs exhibit promising modified physical properties, including band gap, magnetism, and porosity, with hierarchical micro/mesopore structures correlated with the nature and the degree of defective linker incorporation into the framework.

Nanostructured WCx/CNTs as highly efficient support of electrocatalysts with low Pt loading for oxygen reduction reaction

Highly active Pt–WCx/carbon nanotube (CNT) electrocatalysts for the oxygen reduction reaction (ORR) have been developed by the combination of tungsten carbide with CNTs as electrocatalyst supports. The obtained WCx/CNT and Pt–WCx/CNT samples were characterized by XRD, TEM, XPS and electrochemical measurements. The results showed that nanostructured tungsten carbide particles on carbon nanotubes could be prepared by microwave-assisted thermolytic molecular precursor method, and the particle size of tungsten carbide increased with the increase of tungsten loading. The nanostructured WCx/CNTs showed electrocatalytic activity for oxygen reduction reaction. The deposition of Pt nanoparticles on the WCx/CNTs resulted in higher electrocatalytic activity for the oxygen reduction reaction and better immunity to methanol than Pt/CNT catalysts. The unique electrocatalytic properties of the novel Pt–WCx/CNT electrocatalyst were attributed to a synergistic effect between Pt, WCx and the CNTs. The findings also indicated that WCx/CNTs were efficient electrocatalyst supports that could reduce Pt usage while the same electrocatalytic properties were kept for the ORR in direct methanol fuel cells.

Energetics and Vibrational States for Hydrogen on Pt(111)

We present a combination of theoretical calculations and experiments for the low-lying vibrational excitations of H and D atoms adsorbed on the Pt(111) surface. The vibrational band states are calculated based on the full three-dimensional adiabatic potential energy surface obtained from first principles calculations. For coverages less than three quarters of a monolayer, the observed experimental high-resolution electron peaks at 31 and 68meV are in excellent agreement with the theoretical transitions between selected bands. Our results convincingly demonstrate the need to go beyond the local harmonic oscillator picture to understand the dynamics of this system.

Defects in MOFs: a thorough characterization.

As indicated by nearly perfect XRD data, but challenged by a two-signal IR spectrum of CO guest molecules, it is confirmed by computer simulations and XPS experiments that the most defect-free SURMOFs contain about 4% defective Cu sites.

Spectroscopic evidence for the partial dissociation of H2O on ZnO(100)

The interaction of water with the non-polar ZnO(1010) surface has been studied by high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). Adsorption of water at room temperature leads to the partial dissociation of water molecules giving rise to a well defined (2x1) superstructure. This observation was confirmed by the HREELS data which show the water-induced O-H stretching modes at 396 and 460 meV (3193 and 3709 cm-1) as well as the peak at 456 meV (3677 cm-1) arising from the OH species. The large red shift of the loss at 396 meV indicates unusually strong hydrogen bonding interactions of water to both neighbouring adsorbate molecules and the surface O atoms which are responsible for the partial dissociation of water molecules on the perfect ZnO(1010) surface.

ZnO@ZIF-8: stabilization of quantum confined ZnO nanoparticles by a zinc methylimidazolate framework and their surface structural characterization probed by CO2 adsorption

The microporous and activated zeolitic imidazolate framework (Zn(MeIM)2; MeIM = imidazolate-2-methyl; ZIF-8) was loaded with the MOCVD precursor diethyl zinc [Zn(C2H5)2]. Exposure of ZIF-8 to the vapour of the volatile organometallic molecule resulted in the formation of the inclusion compound [Zn(C2H5)2]0.38@ZIF-8 revealing two precursor molecules per cavity. In a second step the obtained material was treated with oxygen (5 vol% in argon) at various temperatures (oxidative annealing) to achieve the composite material ZnO0.35@ZIF-8. The new material was characterized with powder XRD, FT-IR, UV-vis, solid state NMR, elemental analysis, N2 sorption measurements, and transmission electron microscopy. The data give evidence for the presence of nano-sized ZnO particles stabilized by ZIF-8 showing a blue-shift of the UV-vis absorption caused by quantum size effect (QSE). The surface structure and reactivity of embedded ZnO nanoparticles were characterized viacarbon dioxide adsorption at different temperatures monitored by ultra-high vacuum FTIR techniques. It was found that the surface of ZnO nanoparticles is dominated by polar O–ZnO and Zn–ZnO facets as well as by defect sites, which all exhibit high reactivity towards CO2 activation forming various adsorbed carbonate and chemisorbed CO2δ− species.

Chemical Activity of Thin Oxide Layers: Strong Interactions with the Support Yield a New Thin‐Film Phase of ZnO

Small Cu particles supported on and most likely activated by a ZnO substrate are the active component in the industrial catalyst used to convert syngas (H2, CO, CO2) into methanol, the third most important chemical product worldwide. Although a topic of intense research, the nature of the active site is still under debate. Recently, it has been pointed out that Zn atoms present at the surfaces of the Cu particles exhibit pronounced chemical activity and could explain some of the experimental findings. Another interesting suggestion is the presence of a thin layer of ZnOx species which forms on the surface of the Cu particles under reaction conditions. The importance of such thin oxide layers on the surface of metals under reaction conditions has already been pointed out in other contexts, where it was found that their chemical properties may differ substantially from those of the corresponding bulk oxides. In the case of ZnO this question is particularly interesting, since strong interactions between ZnO and the supporting metal have been reported for ZnO/ Cu and in recent work thin layers of ZnO have been shown to adopt a depolarized, graphitic structure, ZnO(gr), different from the wurtzite-type bulk. The properties of oxide thin films supported on metal substrates have been successfully studied in a number of cases, for example, for thin aluminum oxide films grown by oxidation of Ni/Al alloys. In contrast, the chemical activity of ZnO thin films supported on Cu single crystals has been investigated in a few cases only. Maroie et al. have investigated the adsorption and oxidation processes for single-crystal brass(110), brass(100), and brass(111) surfaces by X-ray photoelectron spectroscopy (XPS). Brass(110) and brass(111) show the same behavior with regard to the interaction with oxygen: the dissociative adsorption of oxygen on the surface is followed by the growth of thin ZnO layers. Wiame et al. reported that after oxidation of (111)-oriented Cu0.7Zn0.3 samples at room temperature the surface is covered by ZnO islands; it was suggested that these islands have (0001) and (0001) surface terminations. A more detailed characterization of the chemical properties of the thin ZnO layers was not carried out in this early work. In the present paper we report a detailed multitechnique investigation of a brass(111) single-crystal substrate (Cu/Zn ratio 9:1) subjected to different oxidation procedures using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) under ultrahigh vacuum (UHV) conditions. The experimental findings are then interpreted by comparison with the results of a rather extensive set of density functional theory (DFT) calculations. Our results reveal the growth of thin ZnO adlayers with chemical properties that are markedly different from those of normal, wurtzite-type ZnO substrates. XPS data recorded for the brass(111) surface before and after different oxidation procedures for two different exit angles of the photoelectrons are shown in Figure 1. Since it is difficult to discriminate between Cu and Cu on the basis of XPS data, also the corresponding results from Auger electron spectroscopy (AES) are shown. For the clean brass(111) substrate the data indicate a Zn atom concentration of 5%, clearly lower than the 10% expected based on the bulk Cu/Zn ratio. Upon oxidation, the XPS data reveal an increase of the surface Zn concentration. Since it is a crucial question whether, in addition to Zn, also Cu or Cu is present, we have carefully analyzed the XPS and AES data. Neither in the Cu2p XPS data nor in the Cu L3M45M45 Auger data were the characteristic signatures of Cu or Cu species resulting from an oxidation of copper atoms detected. Cu exhibits a L3M45M45 peak at electron kinetic energies of 915 eV– 917 eV, which is clearly absent in the present data (see Figure 1). This observation, which agrees with the conclusions presented in a previous study by Rameshan et al., is expected, since in the presence of the less noble Zn one would expect the formation of ZnO to precede that of CuxO. Oxidation at elevated temperatures results in a substantial increase of the Zn signal, revealing the formation of thicker ZnO adlayers. The thickness of these thin ZnO layers was determined from the intensity of the Zn2p3/2 and Cu2p3/2 XPS signals. Exposure of the samples to 500 L of O2 at room temperature yields a ZnO adlayer with an average thickness of about 1.7 , consistent with the presence of a monolayer. More extended exposures to oxygen at room temperature did not result in a significant further increase of the thickness of the ZnO layer. Even the oxidation of the brass substrate at [*] Dr. V. Schott, Dr. A. Birkner, Dr. M. Xu, Dr. Y. Wang Chair of Physical Chemistry Ruhr-University Bochum (Germany)

Iron Metal–Organic Frameworks MIL‐88B and NH2‐MIL‐88B for the Loading and Delivery of the Gasotransmitter Carbon Monoxide

Crystals of MIL-88B-Fe and NH2-MIL-88B-Fe were prepared by a new rapid microwave-assisted solvothermal method. High-purity, spindle-shaped crystals of MIL-88B-Fe with a length of about 2 μm and a diameter of 1 μm and needle-shaped crystals of NH2-MIL-88B-Fe with a length of about 1.5 μm and a diameter of 300 nm were produced with uniform size and excellent crystallinity. The possibility to reduce the as-prepared frameworks and the chemical capture of carbon monoxide in these materials was studied by in situ ultrahigh vacuum Fourier-transform infrared (UHV-FTIR) spectroscopy and Mössbauer spectroscopy. CO binding occurs to unsaturated coordination sites (CUS). The release of CO from the as-prepared materials was studied by a myoglobin assay in physiological buffer. The release of CO from crystals of MIL-88B-Fe with t(1/2) = 38 min and from crystals of NH2-MIL-88B-Fe with t(1/2) = 76 min were found to be controlled by the degradation of the MIL materials under physiological conditions. These MIL-88B-Fe and NH2-MIL-88B-Fe materials show good biocompatibility and have the potential to be used in pharmacological and therapeutic applications as carriers and delivery vehicles for the gasotransmitter carbon monoxide.

A new dual-purpose ultrahigh vacuum infrared spectroscopy apparatus optimized for grazing-incidence reflection as well as for transmission geometries.

A newly designed ultrahigh vacuum (UHV) infrared spectroscopy apparatus dedicated to the spectroscopic characterization of oxides, singles crystals as well as powders, is described. It combines a state-of-the-art vacuum Fourier transform infrared (FTIR) spectrometer (Bruker, VERTEX 80v) with a novel UHV system (PREVAC) consisting of load-lock, distribution, measurement, and magazine chambers. The innovative design allows carrying out both reflection-absorption IR spectroscopy experiments at grazing incidence on well-defined oxide single crystal surfaces and FTIR transmission measurements for powder particles. A further unique feature of the apparatus is the entirely evacuated optical path to avoid background signals from gas phase H(2)O, CO(2), and other species, thus creating the possibility to record high-quality IR data with high sensitivity and stability, an essential prerequisite for monitoring molecular species adsorbed on oxide single-crystal surfaces. The unique performance of this new apparatus with regard to the spectroscopic characterization of adsorbates on oxide single crystals as well as on powder particles is demonstrated by case studies for two different materials, TiO(2) and ZnO.