On model materials designed by atomic layer deposition for catalysis purposes

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The aim of this work was to investigate the potential of model materials designed by atomic layer deposition toward applications in catalysis research. Molybdenum based catalysts promoted with cobalt were selected as target materials, considering their important roles in various industrial processes. Particular attention was paid to understand the growth dynamics of the ALD processes involved and further to characterize the obtained materials carefully. It was of main concern to verify the feasibility to coat porous materials by ALD with our equipment. Another ambition was to confirm the advantages of the atomic layer technique to create model materials for industrial research projects in catalysis. Thin film growth of molybdenum trioxide has been demonstrated by the atomic layer deposition technique using molybdenum-hexacarbonyl, water and ozone as precursors. A narrow ALD-window is observed at relatively low temperatures, leading to amorphous films as deposited. The effect of different oxygen precursors on the growth mechanism of the molybdenum oxide has been assessed by QCM investigations. The chemical composition and Mo-oxidation state in MoO3 thin films grown by ALD have been investigated by two XPS approaches. The sputtering based studies affects strongly the analysis results by inducing reduction of the Mo-O film prior to XPS data collection. The ARXPS proves that molybdenum is in oxidation state VI throughout the bulk of the film. Molybdenum in a lower oxidation state is observed at the substrate interface, representing the initial stage of film formation. A convenient process to achieve thin film model materials of MoO3 polymorphs has been proposed, describing the crystallization behavior of the thin films from the as-deposited amorphous state via metastable β-MoO3 to the orthorhombic α-MoO3 phase. A significant mass transport, in particular during recrystallization into α-MoO3 is demonstrated. By means of combined AFM/Raman studies we have been able to relate morphology and vibration mode of α-MoO3 features. The ability to coat porous materials with our ALD equipment has been confirmed by means of coating anodiscs with the current molybdenum process and a cobalt process. ALD thin film growth of cobalt oxide has been obtained using cobaltocene and ozone as precursors. SEM and EDS investigations of coated porous anodiscs show the specific coverage profile to be dependent on the fluid dynamics in the reactor. Cobalt molybdate has been grown by atomic layer deposition, varying the cobalt oxide precursor between Co(thd)2 and CoCp2. The growth dynamics of the films, their composition and crystallization have been examined as function of the subcycles ratio, proving CoMoO4 to be the preferential composition and the excess of molybdenum to crystallize into α-MoO3. The films catalytic activity in the ammonia decomposition process is assessed at the laboratory scale. The growth dynamics have been investigated using quartz crystal microbalance (QCM) where it is evident that the different precursor chemistries affect each other’s growth. When water is combined in the reactions, a surface controlled mechanism takes place which guides the deposited stoichiometry towards the CoMoO4 phase over a range of different cobalt rich pulsed compositions. This is a rare example of how surface chemistry can control stoichiometry of depositions in ALD. The catalytic properties of cobalt-molybdenum oxide thin films deposited by ALD on industrial alumina carriers have been studied as function of the thickness of the films. Cobalt molybdenum multilayered thin films activity has proven to increase with the thickness of the films up to a certain extend. Multilayered films show a better activity in the HDS conversion as compared with cobalt molybdate single phase films. Finally, TEM imaging characterization of copper particles on top of a zinc oxide film has been achieved by first depositing an underlayer of Al2O3, thereafter coated with zinc oxide and copper oxide thin films by ALD at low temperatures. In situ TEM imaging of the multilayered film at 250 °C under H2 shows crystallization of the ZnO grains and reduction of the copper oxide film leading to Cu particles formation.

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Henrik Sonsteby
University of Oslo
(Oslo, Norway)
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