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"100% Selective Cyclotrimerization of Acetylene to Benzene on Ag(111)"

Benzene, a high-volume chemical, is produced from larger molecules by inefficient and environmentally harmful processes. Recent changes in hydrocarbon feedstocks from oil to gas motivate research into small molecule upgrading. For example, the cyclotrimerization of acetylene reaction has been demonstrated on Pd, Pd alloy, and Cu surfaces and catalysts, but they are not 100% selective to benzene. We discovered that acetylene can be converted to benzene with 100% selectivity on the Ag(111) surface. Our temperature programmed desorption experiments reveal a threshold acetylene surface coverage of ∼one monolayer, above which benzene is formed. Furthermore, additional layers of acetylene increase the amount of benzene produced while retaining 100% selectivity. Our scanning tunneling microscopy images show that acetylene prefers square packing on the Ag(111) surface at low coverages, which converts to hexagonal packing when acetylene multilayers are present. Within this denser layer, features consistent with the proposed C4 intermediates of the cyclotrimerization process are observed. Density functional theory calculations demonstrate that the barrier for forming the crucial C4 intermediate generally decreases as acetylene multilayers are formed because the multilayer interacts more strongly with the surface in the transition state than in the initial state. Given that acetylene desorbs from Ag(111) at ∼90 K, the C4 intermediate on the pathway to benzene must be formed below this temperature, implying that if Ag-based heterogeneous catalysts can be run at sufficiently high pressure and low enough temperature, efficient and selective trimerization of acetylene to benzene may be possible. STM experiments were performed on a low-temperature scanning tunneling microscope (Infnity by Scienta Omicron) in UHV. 

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Open and Closed Cycle Sample Manipulators

When aiming at high experimental energy resolution for ARPES measurement, it is crucial to achieve ultra-low sample temperatures to quench thermal broadening. This is possible with state-of-the-art cryo manipulators reaching sample temperatures from < 3.5 K and featuring up to 6 fully motorized axes for a large range of movements. The manipulators are available as open and closed cycle. Open cycle manipulators reach lower temperature specifications and are rapidly cooled down from room temperature to 10 K in 15 min. The low He consumption below 1 l/h at ultimate temperature and the possibility to operate with liquid nitrogen at higher temperatures ensure a low operating cost. Closed cycle manipulators have no He consumption providing unlimited holding time.

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