Design of a variable-temperature scanning tunneling microscope to study reaction intermediates in heterogeneous catalysis

Abstract

Catalytic processes and in particular heterogeneous catalysis are vital for todays industry. However, many industrial catalytic processes require high temperatures and pressures to work efficiently. This stands in contrast to biological catalysts, which function under ambient temperatures and atmospheric pressures and excel in catalytic activity and selectivity. We may learn something from nature by studying the size-dependent reactivity of small metal particles resembling the active centers of biological catalysts. The catalytic properties of such metal clusters grown on suitable substrates can be investigated by scanning tunneling microscopy. Single-molecule vibrational spectroscopy at low temperatures allows one to identify adsorption sites and measure adsorption energies of reaction intermediates. We have designed and mounted a variable-temperature scanning tunneling microscope dedicated to the study of reaction intermediates. Our beetle-type microscope is protected against external vibrations by a double-stage spring suspension system. It hangs inside two copper radiation shields, which are directly screwed onto a liquid He-flux cryostat. The microscope ramps for coarse approach are made of sapphire and reside on copper-beryllium balls. Direct electrical contacts to the sample for exact temperature reading exist. The microscope eigenresonances all lie beyond the critical frequency range of 1-10 kHz and rattling resonances are efficiently suppressed. The microscope has proved to work in air. Images of highly-oriented pyrolytic graphite in air were acquired, which showed that spectroscopic measurements are feasible. At the same time, during the microscope mounting period, several mechanical problems were identified and therefore suggestions for future improvements are made. With the goal of finding a suitable substrate for our future model catalysts, we investigated ultra-thin MgO layers on Mo(100) by low-energy diffraction and Auger electron spectroscopy. Defect-free thin oxide film with a superstructure can serve as templates for the growth of a regular array of small metal clusters. We report on the formation of a c(2 × 14) superstructure of 2.5 ± 0.4 ML of MgO on Mo(100)-(1 × 1)-O upon annealing at 1300 K. The observed structure is most probably induced by an interfacial reconstruction of the oxygen-covered Mo(100) surface since the annealed Mo(100)-(1 × 1)-O substrate exhibits a similar LEED pattern. This means that the reordering of the interface upon annealing is decisive for the oxide overlayer structure. To investigate the growth of small metal islands we studied the two systems Co/Pt(111) and Pt/Pt(111). 0.1 ML Co or Pt were evaporated onto Pt(111) at 50 K and the island size as a function of annealing temperature was imaged using a variable-temperature scanning tunneling microscope. For both systems we observe a stepwise increase of the mean island size, which can be well reproduced by kinetic Monte-Carlo simulations and mean-field nucleation theory calculations. This behaviour stands in contrast to the Ostwald ripening previously observed for Ag/Pt(111). It indicates that for Co/Pt(111) and Pt/Pt(111) the energy migration barriers of monomers, dimers and trimers must be significantly lower than the respective dissociation barriers. Finally, high-pressure scanning tunneling microscopy was used to study the adsorption of CO on Pt(111) at room temperature and in equilibrium with the gas phase. The coverage is found to vary continuously and over the whole range from 10-6—1 bar pressure-dependent moire patterns are observed. This stands in contrast to the CO/Pt(111) lattice gas structures found at low temperatures in the same coverage range. Nevertheless, a true pressure gap cannot be established since sufficient cooling of the sample leads to the formation of hexagonal CO overlayers similar to those at high pressure. We found that below 1.3. 10-2 mbar the moire superlattice is oriented along a 30° high symmetry direction of the substrate while near to 1 bar it becomes compatible with a (19-1/2 × 19-1/2) R23.4°-13CO commensurate structure.