Reactivity and Stability of Ultrathin VO x Films on Pt(111) in Catalytic Methanol Oxidation
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ORIGINAL PAPER
Reactivity and Stability of Ultrathin VOx Films on Pt(111) in Catalytic Methanol Oxidation Bernhard von Boehn1 · Lena Scholtz1,2 · Ronald Imbihl1
© The Author(s) 2020
Abstract The growth of ultrathin layers of VOx ( 2 ML, a (3√3 × 6) was present. In particular the (√3 × √3)R30° that was the dominant structure in our experiments beyond 2 ML here, was not seen at all. A (2 × 2) structure was seen after deposition of submonolayer VOx films on Pd(111); it was described there as a VO
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Topics in Catalysis
wetting layer also denoted as s-V2O3 [17]. Based on their vibrational data Tang et al. followed the interpretation of the (2 × 2) as a surface V2O3 layer. Different from the results reported by Tang et al. we observed a (2 × 2) only in coexistence with a (√3 × √3)R30° structure after deposition of more than 1 ML (step-wise deposition) during V Ox growth. Furthermore we observed the (2 × 2) after methanol oxidation has been carried out on V Ox films of an initial thickness varying between 0.6 and several ML. From AES we estimate the VOx coverage of the (2 × 2) obtained after reaction conditions were applied to thick V Ox films to be roughly 1 ML. A (√3 × √3)R30° structure reported for thick V Ox layers (⪆1 MLE VOx) on Rh(111) and Pd(111) was assigned to the (0001) oriented surface of corundum V2O3 [17, 23]. Accordingly, we attribute the (√3 × √3)R30° structure observed here to three-dimensional crystallites of V 2O3 with (0001) orientation. Interestingly, no such corundum V 2O3 phase has been reported by Tang et al. for V Ox/Pt(111). Although some weak (√3 × √3)R30° diffraction spots were visible after deposition of 2.5 ML VOx on Pt(111) (Fig. 3f in [32]) they were not discussed further. For V coverages between 1 and 2 ML, a (3√3 × 6) pattern was observed by Tang et al. who interpreted this pattern as a bilayer structure. To conclude, compared with the results of Tang et al., we find differences in the growth mode—Stranski–Krastanow vs. layer-by-layer—as well in the structures at higher V coverage. Since different preparation procedures have been used the results are not in contradiction to each other. One could speculate whether the structures seen here are closer to thermodynamic equilibrium since reactive evaporation at high temperature was applied instead of post-oxidation. This, however, needs to be verified by future analyses.
4.2 Catalytic Activity of VOx/Pt(111) In catalytic methanol oxidation the system V Ox/Rh(111) exhibited a high catalytic activity and displayed remarkable dynamic behavior [25, 31], whereas VOx/Pt(111) showed negligible catalytic activity and practically no dynamics at all. In this comparison we refer to submonolayer coverages of VOx and reaction conditions in the 10−4 mbar range. If we take the picture of V Ox islands acting as catalytic microreactors on Rh(111), then we can explain the lower activity of VOx/Pt(111) in two ways: (i) the VOx islands are less active on Pt(111) and/or (ii) the surrounding metal surface, which in the
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