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Schlagwörter:
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Zusammenfassung:
Heterogeneous chemistry that develops on ultra-thin films such as bilayer SiO2/Ru(0001) is interesting as a model catalysis system.
We have studied the partial decomposition and hydrogenation of acetylene to ethylene and its trimerization to benzene on Pd-Cu
bimetallic alloy nano-particles (NPs)
supported on those thin silica films. In comparing the bilayer SiO2/Ru(0001) to thicker silica layers without metallic substrate, e.g., the native SiO2/Si(001), the size distribution of the clusters is narrower on the bilayer SiO2/Ru(0001) substrate, demonstrating the effect of the underlying metal in preventing cluster diffusion during their growth. In addition, the effect of medium pressure on NP shape has been investigated via transmission electron microscopy (TEM)
imaging of the NPs on relatively thick SiO2. The NPs become elongated when exposed to 0.2 mbar
acetylene inside a moderate-pressure cell embedded within an ultra-high vacuum (UHV) chamber. By changing the elemental composition of the NPs on both substrates, the important effect of the sub-oxide material on catalyst reaction selectivity has been emonstrated. However, the effect of the composition of the bare NPs is not enough to actually define the long-term activity of a catalyst. In order to address more realistic conditions, we performed consecutive reactivity cycles by adsorbing acetylene at 110K with subsequent annealing up to 400K in UHV on the same 1Pd:1Cu NPs/bilayer SiO2/Ru(0001) catalyst. This revealed a strong decrease in the selectivity towards ethylene, from an ethylene/benzene product yield ratio of 370±150 in the first cycle down to 50±15
during the third to fifth cycles. Carbon atoms accumulation on the metallic particles in the first and subsequent runs is the main reason for this modification in selectivity. Consecutive reactivity study uniquely demonstrates
how rapidly and significantly the catalyst’s performance is modified during the initial stages of its heterogeneous catalytic reactivity.
