Heterogeneous chemistry that develops on ultrathin films such as bilayer SiO 2 /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 nanoparticles (NPs) supported on those thin silica films. In comparing the bilayer SiO 2 /Ru(0001) to thicker silica layers without a metallic substrate, for example, the native SiO 2 /Si(100), the size distribution of the clusters is narrower on the bilayer SiO 2 /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 the NP shape has been investigated via transmission electron microscopy imaging of the NPs on relatively thick SiO 2 . The NPs become elongated when exposed to 0.2 mbar acetylene inside a moderate-pressure cell embedded within an ultrahigh vacuum (UHV) chamber. By changing the elemental composition of the NPs on both substrates, the important effect of the suboxide material on catalyst reaction selectivity has been demonstrated. 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 110 K with subsequent annealing up to 400 K in UHV on the same 1Pd:1Cu NPs/bilayer SiO 2 /Ru(0001) catalyst. This revealed a strong decrease in the selectivity toward 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 atom accumulation on the metallic particles in the first and subsequent runs is the main reason for this modification in selectivity. A consecutive reactivity study uniquely demonstrates how rapidly and significantly the catalyst's performance is modified during the initial stages of its heterogeneous catalytic reactivity.

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The morphology and surface elemental composition of Au-Pd bimetallic nanoclusters are reported to be sensitive to and affected by reduced silicon defect sites and structural corrugation on SiO2/Si(100), generated by argon ion sputtering under ultrahigh vacuum (UHV) conditions. Metastable structures of the bimetallic clusters, where Au atoms are depleted from the top surface upon annealing, are stabilized by the interaction with the reduced silica sites, as indicated from CO temperature programmed desorption (TPD) titration measurements. Acetylene conversion to ethylene and benzene has been studied as a probe reaction, revealing the modification of selectivity and reactivity enhancement in addition to improved thermal stability on substrates rich in reduced-silica sites. These observations suggest that these unique sites play an important role in anchoring thermodynamically metastable conformations of supported Au-Pd bimetallic catalysts and dictate their high-temperature activity.

Low-energy electrons are known to play a fundamental role in activating small molecules in interstellar chemistry. Here we illustrate the electron-induced activation of the inert molecule methane while sandwiched between two 50 monolayers thick layers of amorphous solid water (ASW) on ruthenium substrate at 25 K by employing externally supplied low-energy electrons (5 eV) under ultrahigh vacuum conditions. We demonstrate how electron transmission through ASW layers under cryogenic conditions is strongly affected in the presence of cosandwiched oxygen molecules. We conclude that the resonant nature and direct electron attachment process leads to a higher degree of conversion in the presence of embedded oxygen molecules along with methane. Cross sections ranging from 1 × 10–18 to 1 × 10–19 cm2/electrons were obtained from the post-irradiation temperature-programmed desorption spectra.

The ability to correlate industrial high-pressure catalysis with high-vacuum research has been of great interest for decades. We employed a double-chamber vacuum system to study the self-hydrogenation of acetylene to ethylene and its trimerization to benzene at medium pressures to compare the reactivity in this pressure range to the known model catalytic acetylene reactivity in ultrahigh vacuum (UHV). We measured the reactivity of Pd–Cu bimetallic alloy nanoparticles (ANPs) with different elemental compositions deposited on top of native SiO2/Si­(100) and on bilayer SiO2/Ru­(0001) surfaces, where the latter was shown to contribute to ANP stability. Following exposure to 0.5 mbar of acetylene, ANPs on both surfaces catalyze the formation of ethylene and benzene, with ethylene as the more probable product. The ANPs on bilayer SiO2/Ru­(0001) were highly selective toward ethylene formation, with an ethylene/benzene ratio of more than 2 orders of magnitude, whereas on the native SiO2/Si­(100) there was a significantly lower selectivity (about 5) at the same temperature range and catalyst elemental composition. Interestingly, these selectivity values are similar to those found under UHV conditions. In addition, ANPs grown on native SiO2/Si­(100), unlike SiO2/Ru­(0001), revealed an optimal temperature for ethylene and benzene formation due to the limited stability of the particles.