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| Home > Documents in process > Cu Nanoparticles on Vicinal and Basal ZnO as Model Catalysts for Methanol Synthesis |
| Dissertation / PhD Thesis | PUBDB-2025-04540 |
; ;
2023
Universität Hamburg
Hamburg
Abstract: This cumulative dissertation addresses Cu/ZnO-based model catalysts for the het-erogeneous synthesis of methanol. The catalysts were prepared following a surfacescience approach by growing Cu nanoparticles (NPs) under ultra-high vacuum(UHV) conditions on basal and vicinal ZnO single crystals. From the combineduse of high-energy surface-sensitive X-ray diffraction, X-ray reflectometry, X-rayphotoelectron spectroscopy under both UHV and ambient pressure, scanning tun-neling microscopy, scanning electron microscopy, and computational simulationsbased on density functional theory, an understanding of the catalysts structureand its surface intermediates under UHV and operando conditions is obtained onan atomic scale.The growth and morphology of Cu NPs on the basal ZnO(0001), ZnO(000¯1),and vicinal ZnO(10¯14) surfaces are studied. It is shown, that the choice of ZnOfacet affects the size distribution, crystallographic orientation, morphology, andfacet termination of the Cu NPs. The vicinal ZnO support guides the formation ofCu NPs with densely stepped facets that have a high abundance of low-coordinatedsurface atoms, which are regarded as an important prerequisite for high catalyticactivity.Secondly, the three model catalysts were measured under oxidation/reductionconditions at 1 bar in air at room temperature, Ar at 420 K and 500 K as wellas under H2 and CO2 conditions at 500 K to investigate their structural stabilityunder these experimental conditions. Mass transport in the form of sintering andagglomeration can already be observed for oxidized Cu2O NPs on all ZnO surfacesat 500 K in Ar, prior to the activation of the catalyst. The (111) orientation ofCu NPs on basal ZnO is largely maintained during oxidation to Cu2O and duringreduction by H2, whereas Cu NPs on vicinal ZnO rotate around an axis parallelto the substrates’ surface steps during oxidation to Cu2O. The initial orientationixx Abstractof Cu NPs on ZnO(10¯14) is partially reestablished upon reduction in H2, thus,an enhanced abundance of low-coordinated Cu surface sites is expected to remainpresent after the oxidation-reduction cycle.Subsequently, the formation of reaction intermediates of methanol synthesison the Cu/ZnO model catalysts under H2:CO:CO2 flow was studied by ambientpressure photoelectron spectroscopy. A new component normalization techniqueis employed to obtain reliable trends of C 1s components as a function of exper-imental conditions. Supported by DFT calculations, we observe a larger amountof chemisorbed versus physisorbed CO2 on the vicinal ZnO surface, which bindsin a carbonate-like geometry. Under the applied experimental conditions, hydro-carbons and carbonates are observed, whose formation is enabled by photo- andcatalyst-induced dissociation of CO and CO2. Under CO-rich conditions, the Cusurface is covered by formyl, which is particularly prevalent on the vicinal Cu/ZnOmodel catalyst. At ambient pressure, no formate formation is observed; instead,formaldehyde is formed at the Cu–ZnO interface. These results indicate that theformate pathway of methanol synthesis from CO2 is inhibited at ambient pres-sure and that methanol synthesis may only be achieved via the CO hydrogenationpathway under the employed experimental conditions.To overcome the pressure gap from ambient pressure up to the industriallyrelevant methanol synthesis conditions of about 500 K and 50 bar, I designeda novel reaction cell that enables the use of high energy surface sensitive X-raydiffraction, X-ray reflectometry, and online mass spectrometry for single crystallinesamples, adlayers and supported NPs. Signals from the least intense, i.e. mostsurface sensitive, locations of crystal truncation rods from a Cu single crystal aredisplayed as a proof-of-concept for this measurement technique.
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