Chemical vapor deposition (CVD)) is an important tool for production of a plethora of metal/metal oxide materials, which play important role in heterogeneous catalysis. With respect to purity and surface composition, films generated via gas-phase synthesis are preferred compared to materials from wet synthesis with surface active additives. This synthesis method open new avenues for expanding the unique experimental approaches applied in the field of gas-surface scattering dynamics, presently dominated by simple crystalline surfaces, to the investigation of fundamental molecular interactions at complex surfaces.
Vanadia-based catalysts are being used for selective catalytic reduction of NOx emissions, the partial oxidation of o-xylene to phthalic anhydride, dehydrogenation of propane and the oxidative destruction of volatile organic compounds. It provides a unique testing ground for molecular interactions at surfaces as it can morph itself from insulator to metal in only a few degrees temperature range (known as Mott transition). The dissipation of the energy stored in the vibrational coordinate to the surface differs strikingly between metals and insulators. The dependence of the metallic properties of VO2 films on their temperature should clearly bear on their propensity to interact with molecular vibrational coordinate and hence, possibly influence the chemistry of heterogeneous catalysis.
Here we report our first experiments focusing on state-to-state scattering of laser prepared molecular beams of NO(v=0) and NO(v=11) molecules from model catalyst films of VO2 fabricated by chemical vapor deposition (CVD). The CVD synthesis of VO2 films (1-3 µ thick) was based on sublimation of Vanadyl acetylacetonate (VO(acac)2) at ~150-190oC and its subsequent reaction with O2 at 360-500oC. The X-ray Diffraction (XRD) indicates that the predominant phase in the CVD fabricated samples is monoclinic Vanadium Dioxide. Scanning Electron Microscopy (SEM) analysis reveals grain sizes in the range of 0.5-2 µ. Four probe measurements indicate 3-4 orders of magnitude resistance drop across the insulator-to-metal transition. Remarkably, despite the topologically rough surface of the VO2 films rough the molecular beam scattering experiments indicate that both rotational temperature and kinetic energy of the scattered NO(v=11) molecules are strongly dependent on the incident kinetic energy, indicating direct scattering regime, i.e. retaining memory of the incident beam molecular parameters rather than thermalization with the surface. These findings suggest that catalytically relevant dynamical fingerprints can be inferred from these experiments.