Scanning Tunneling Microscopy (STM) at High Pressures. Adsorption and Catalytic Reaction Studies on Platinum and Rhodium
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Catalysis Letters Vol. 107, Nos. 3–4, March 2006 (Ó 2006) DOI: 10.1007/s10562-005-0015-5
Scanning Tunneling Microscopy (STM) at High Pressures. Adsorption and Catalytic Reaction Studies on Platinum and Rhodium Single Crystal Surfaces Max Montanoa,b, David C. Tanga,b, and Gabor A. Somorjaia,b,* a Department of Chemistry, University of California, Berkeley, CA 94720, USA Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
b
Received 12 September 2005; accepted 14 December 2005
In this paper, we review the experimental results that were obtained using our high pressure–high temperature STM system (HP/HT STM) for studies of gas adsorption in a broad pressure range. Measurements are carried out in equilibrium with the gas phase. We discovered ordered surface structures of adsorbates that do not exist at low pressures. It appears that small increases in coverage due to increased ambient pressures cause ordering due to repulsive adsorbate–adsorbate interactions. Adsorption isotherms, one of the oldest fields of surface thermodynamics, can be revisited using HP/HT STM to obtain molecular surface structures and surface phase diagrams as the gas pressure is altered. KEY WORDS: adsorption studies by STM; catalytic reaction by STM; platinum and rhodium crystal surfaces.
1. Introduction To obtain the surface structures of atoms and adsorbed molecules in equilibrium with the gas or liquid at the interface has been the dream of many surface scientists. Low energy electron diffraction (LEED) [1,2] or ion scattering [3] experiments carried out to obtain surface structures can only be performed at low ambient pressures because of the high scattering cross sections of both electrons and ions. Grazing angle X-ray diffraction [4] using high intensity tunable X-rays from a synchrotron shows promise for surface structure determination of atoms and molecules in the adsorbed monolayer, monitoring substrate restructuring at high pressures. Thus far, however, it has rarely been employed for these types of studies. Experiments in our laboratory with the scanning tunneling microscope (STM) that was built to operate at high pressures and temperatures as well as in ultra high vacuum (UHV) [5] have shown for the first time that atomic and molecular resolution can be obtained over a ten order of magnitude pressure range. The stability of our instrument is such that one can scan the same area as the pressure is being altered. Unfortunately, changes in temperature cause thermal drift and thus scanning the same area while temperature is varied is out of our reach at the present time. In this paper, we review the experimental results that were obtained using our high pressure-high temperature STM system (HP/HT STM) for studies of gas adsorption in a broad pressure range. Measurements are * To whom correspondence should be addressed. E-mail: [email protected]
carried out in equilibrium with the gas phase. We discovered ordered surface structures of adsorbates that do not exist at low pressures. It a
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