Tuning of the Copper-Zirconia Phase Boundary for Selectivity Control of Methanol Conversion

Chemical-vapor deposition (CVD) of a Zr(O-tBu)4 precursor on different Cu substrates was used to prepare model systems for ZrOxHy-Cu catalysts and to test their reactivity and selectivity in methanol steam reforming (MSR). A partially hydroxylated and initially fully oxidized submonolayer ZrOxHy surface species results, exhibiting a pronounced catalytic synergism between the ZrOxHy overlayer and Cu only with respect to partial methanol dehydrogenation to formaldehyde. Thus, it differs strongly from in situ grown ZrOxHy layers on Cu formed from an initially bimetallic mixed Zr/ZrOx state under MSR conditions. CVD-grown Zr-OH groups are not stable under MSR conditions; thus reversible in situ hydroxylation and water-activating reaction channels are suppressed. Comparison of the two model systems indicates that only a dedicated Cu-ZrOxHy interface with in situ formed and reversibly hydroxylated sites (accessible only from initially (inter)metallic Cu/Zr species at the surface) leads to water activation, total oxidation of intermediate formaldehyde, and enhanced CO2 selectivity.(VLID)1371551Accepted versio

Complex oxide thin films: Pyrochlore, defect fluorite and perovskite model systems for structural, spectroscopic and catalytic studies

Well-ordered thin films of different defect fluorite and perovskite materials, namely lanthanum zirconate (La2Zr2O7), cerium zirconate (Ce2Zr2O7), lanthanum cerate (La2Ce2O7) and lanthanum strontium ferrite (La0.4Sr0.6FeO3), have been prepared by sputtering the respective powder targets onto vacuum-cleaved NaCl(001) single crystal facets. Characterization specifically also includes the sophisticateO7 and Ce2Zr2O7 result, whereas for the perovskite compound post-annealing procedures at 973K in air are necessary to obtain epitaxial ordering of orthorhombic structures. Epitaxial relationsd preparation of the initial target materials. For the defect fluorite materials, the target materials are the respective pyrochlore compounds, which reproducibly transform to the respective defect fluorite structures upon sputtering. At template temperatures of around 573K, well-crystallized epitaxial thin films of the defect fluorite compounds La2Zr2 for the well-ordered defect fluorite thin films are determined to be La2Zr2O7(001)//NaCl(001) and Ce2Zr2O7(001)//NaCl(001), respectively. Structural and spectroscopic characterization of the films by (high-resolution) electron microscopy (HR-TEM), selected-area electron diffraction (SAED) and depth-profiling X-ray photoelectron spectroscopy (XPS) reveal that the structures and compositions of the initial target materials are well-preserved during the sputtering process. Following this preparation routine, access to well-ordered thin films of complex oxide materials with defined stoichiometry is therefore granted, which can subsequently be used as model systems for studies of their materials or catalytic properties, as presented for simpler systems earlier. As a primary example, we show that for the defect fluorite thin films of Ce2Zr2O7 the defect fluorite-pyrochlore phase transformation can be observed at around 1123K as detected by in situ electron diffraction.(VLID)3593624Accepted versio

Structural and kinetic aspects of CO oxidation on ZnO_x-modified Cu surfaces

CO oxidation is studied on inverse clean metallic Cu0 and ZnOx-modified Cu0 model catalysts. ZnOx films with a coverage of 1.8 monolayers exhibit a pronounced low-temperature CO oxidation rate maximum at ˜500 K, followed by an intermediate rate minimum. Initially, a fully Cu°-shielding ZnOx layer, consisting of layer-by layer grown Zn° and Wurtzite-like ZnO domains at the Zn° island perimeter, shields the metallic Cu substrate from the reaction mixture and protects it from oxidative deactivation by oxygen up to ≈450 K. Above this temperature, thermal Zn desorption from Zn0 patches sets in, which leads to the in-situ formation of an active ZnOx-Cu0 phase boundary. Once formed, this boundary strongly speeds up the delivery of oxygen to the bare Cu0 surface and, thus, the reaction rate to CO2. In due course, also the oxidation of Cu0 to Cu2O is enhanced, leading to the observed deactivation. Structurally, the overall process resembles the breakdown of a passivating layer, leading to localized corrosion and fast oxidation to Cu2O, together with corresponding dewetting of Zn0 by desorption, ZnO formation and Zn0 alloying into the copper bulk at higher temperatures

The effects of contact patterns and genetic specificity on host and parasite evolution

<p>Many bacteria, viruses and other parasites cause severe morbidity or mortality in their host populations, creating strong selection for physiological or behavioural mechanisms to avoid disease. Likewise, changes in host susceptibility and contact patterns can dramatically influence the spread of infectious diseases, and hence selection for traits such as virulence and infectivity range in parasites. Understanding how ecological and evolutionary changes in one population affect selection in another represents a key challenge for theoreticians and empiricists alike, and is essential for gaining further insights into host-parasite relationships.</p> <p>This thesis contains theoretical models that explore how genetic and environmental factors shape the evolutionary and coevolutionary dynamics of hosts and parasites. In particular, the roles of genetic specificity (i.e. genotype-by-genotype interactions) and population mixing patterns are investigated, using both mathematical models and computer simulations. A broad range of scenarios are covered, including the coevolution of broad resistance and infectivity ranges (generalism), the persistence of coevolutionary cycling and the maintenance of sex, the effects of mating behaviour on disease prevalence and evolution, and the evolution of sexual and social behaviour. The models presented herein develop our understanding of host-parasite relationships and highlight the importance of genetic interactions and ecological feedbacks.</p>This thesis is not currently available on ORA

Chemical vapor deposition prepared sub nanometer Zr clusters on Pd surfaces promotion of methane dry reforming

An inverse Pd–Zr model catalyst was prepared by chemical vapor deposition (CVD) using zirconium-t-butoxide (ZTB) as an organometallic precursor. Pd–Zr interaction was then investigated with focus on the correlation of reforming performance with the oxidation state of Zr. As test reactions, dry reforming of methane (DRM) and methanol steam reforming (MSR) were chosen. Depending on treatments, either ZrO_xH_y or ZrO₂ overlayers or Zr as sub-nanometer clusters could be obtained. Following the adsorption of ZTB on Pd(111), a partially hydroxylated Zr⁴⁺-containing layer was formed, which can be reduced to metallic Zr by thermal annealing in ultrahigh vacuum, leading to redox-active Zr⁰ sub-nanometer clusters. Complementary density functional theoretical (DFT) calculations showed that a single layer of ZrO₂ on Pd(111) can be more easily reduced toward the metallic state than a double- and triple layer. Also, the initial and resulting layer compositions greatly depend on gas environment. The lower the water background partial pressure, the faster and more complete the reduction of Zr⁴⁺ species to Zr⁰ on Pd takes place. Under methanol steam reforming conditions, water activation by hydroxylation of Zr occurs. In excess of methanol, strong coking is induced by the Pd/ZrO_xH_y interface. In contrast, dry reforming of methane is effectively promoted if these initially metallic Zr species are present in the pre-catalyst, leading to a Pd/ZrO_xH_y phase boundary by oxidative activation under reaction conditions. These reaction-induced active sites for DRM are stable with respect to carbon blocking or coking. In essence, Zr doping of Pd opens specific CO₂ activation channels, which are absent on pure metallic Pd

Boosting Hydrogen Production from Methanol/Water by in situ activating Bimetallic Cu-Zr

A bimetallic Cu/Cu51Zr14 precatalyst, activated insitu, for hydrogen generation from methanol and water provides very high CO2 selectivity (>99.9%) and high H2 yields. Referenced to the geometric surface area of our model surface, higher activity of at least one order of magnitude was observed in comparison to supported Cu/ZrO2 and Cu/ZnO/ZrO2 catalysts. Evolution of structural activation monitored by Xray diffraction (XRD), Xray photoelectron spectroscopy (XPS), and electron microscopy indicates transformation of the bimetallic Cu/Cu51Zr14 precatalyst into an active, selective, and selfstabilizing state with coexistence of dispersed Cu and partially hydroxylated tetragonal ZrO2. The outstanding performance is assigned to the presence of a high interfacesite concentration following insitu decomposition of the intermetallic compound. These active sites result from the cooperation of Cu, responsible for methanol activation, and tetragonal ZrO2, which activates the water by surface hydroxylation.(VLID)1371547Accepted versio

Microstructural and Chemical Evolution and Analysis of a Self-Activating CO2-Selective Cu-Zr Bimetallic Methanol Steam Reforming Catalyst

The microstructure of the CO2-selective self-activating and self-stabilizing Cu-Zr bimetallic compound Cu51Zr14 has been studied by a combination of high-resolution electron microscopy and energy-dispersive X-ray spectroscopy both before and after entering the CO2 selective state in methanol steam reforming. Prior to catalysis, the phase composition of the catalyst is characterized by a microstructural mixture of Cu51Zr14 and metallic Cu. The structure appears in a distinct needle-like morphology with a characteristic microstucture of small Cu particles embedded in the intermetallic matrix. In contrast, entering the CO2-selective state goes along with oxidative decomposition - investigated by differential thermal analysis (DTA), thermogravimetry (TG), and mass spectrometry (MS) - and therefore massive structural and compositional changes of the Cu51Zr14 compound both in the near-surface and bulk regions. The final state is then composed of a structurally very heterogeneous sample with Zr-rich and Cu-rich regions within the material bulk with a characteristic lamellar structure. Most importantly, the catalytically relevant surface regions are drastically corroded and depleted in Zr and are characterized by a majority of Cu in intimate contact with oxidized ZrO2 exhibiting a well-ordered, predominantly tetragonal structure. This newly created Cu-ZrO2 interface is believed to be the most significant descriptor steering the CO2 selectivity. In due course, this new method for self-adjustment of the microstructure starting from well-defined intermetallic compounds in the catalytic reaction mixture might pave the way for a more systematic approach of controlled oxidative decomposition of intermetallic compounds acting as promising catalyst precursors.(VLID)2628490Accepted versio

Zirconium Assisted Activation of Palladium To Boost Syngas Production by Methane Dry Reforming

C‐saturated Pd0 nanoparticles with an extended phase boundary to ZrO2 evolve from a Pd0Zr0 precatalyst under CH4 dry reforming conditions. This highly active catalyst state fosters bifunctional action: CO2 is efficiently activated at oxidic phase boundary sites and PdxC provides fast supply of C‐atoms toward the latter

Zirconium assistierte Aktivierung von Palladium zur Steigerung der Produktion von Synthesegas in der Trockenreformierung von Methan

Kohlenstoff‐gesättigte Pd0‐Nanopartikel mit ausgedehnter Phasengrenze zu ZrO2 bilden sich unter CH4‐Trockenreformierbedingungen aus einem intermetallischen Pd0Zr0‐Präkatalysator. Der resultierende, hochaktive Katalysatorzustand arbeitet bifunktionell: CO2 wird an oxidischen Phasengrenzplätzen effizient aktiviert, und PdxC gewährleistet die rasche Versorgung der Phasengrenze mit reaktiven C‐Atomen

Treading in the limited stability regime of lanthanum strontium ferrite Reduction, phase change and exsolution

We present an in situ investigation of the iron exsolution from lanthanum strontium ferrite perovskites. Using in situ X-ray diffraction experiments at the synchrotron, the exact onset of exsolution was determined by a change in the lattice parameter before any iron reflexes become visible. For an initially orthorhombic thin film, on the other hand, a phase transition to a fluorite/rock-salt structure is observed to occur during the exsolution. Also, a difference in the iron oxidation states between bulk and surface is found since photoelectron spectroscopy and X-ray absorption spectroscopy both indicate the existence of the Fe(III)/Fe(II) couple in oxidation/reduction cycles, whereas magnetic measurements would suggest Fe(IV)/Fe(III)