Scanning tunneling microscopy was applied to studies of adsorbates on model catalysts at elevated pressures where dense overlayers in equilibrium with the gas phase can be formed. A structure due an incommensurate hexagonal overlayer is observed for CO on the Pt(111) surface in the 200-750 Torr pressure range. This structure has not been observed before at similar coverage under low pressure and temperature conditions. The results illustrate the inequivalence in this case between the study of surfaces in catalytic conditions of high pressure and the surface science studies carried out in high vacuum.[S0031-9007 (98)05308-3] PACS numbers: 61.16.Ch, 68.35.Bs, 68.65. + gThe development of new atomic resolution imaging and spectroscopic techniques that can operate in environments other than vacuum is opening the important area of surface science at high pressures. Scanning tunneling microscopy (STM) is capable of atomic scale resolution in atmospheric pressure regimes, providing a much soughtafter link between decades of ultrahigh vacuum (UHV) studies and the results of catalysis research performed in the more industrial conditions of high reactant pressure [1]. The link over the pressure gap of many orders of magnitude that separates these two complementary fields is extremely important because many of the UHV results cannot be extrapolated to high pressure and vice versa. Examples of advances in high pressure imaging are the studies of the atomic structure of passivating sulfur monolayers on Mo(001) in air [2], the high pressure driven reconstructions of Pt(110) in atmospheric pressures of O 2 , H 2 , and CO [3] and the opening of the new field of tip-catalyzed reactions in O 2 and H 2 -hydrocarbon mixtures [4,5].None of these previous high pressure STM studies have determined the structure of molecular adsorbates in equilibrium with the gas phase. It has often been argued that the high surface coverage of adsorbates that is prevalent at high pressures can be simulated by operating at cryogenic temperatures in high vacuum conditions. However, the structures formed under these conditions are necessarily not in equilibrium with the gas phase. They might correspond to kinetically trapped structures with no similarity to structures that are thermodynamically stable only at high pressure and temperature. An example of this is the adsorption of CO on metal surfaces, which has been the object of numerous studies in the past [6,7]. Here we present the results obtained for this molecular adsorbate on Pt(111) in the pressure range of 200-750 Torr. Our data illustrate very clearly the fundamental differences that can be expected between the low temperature/low pressure and ambient temperature/high pressure regimes even if they correspond to a similar surface coverage.The UHV apparatus employed for these experiments is divided into two parts: a UHV analysis/preparation chamber (base pressure in the 10 210 Torr range) equipped with surface techniques, including Auger spectroscopy, low energy electron diffraction (LEED) and ion ...

Articles you may be interested inA new scanning tunneling microscope reactor used for high-pressure and high-temperature catalysis studies Rev. Sci. Instrum. 79, 084101 (2008); A high pressure, high temperature, scanning tunneling microscope for in situ studies of catalysts Rev.The design and performance of a high pressure, high temperature scanning tunneling microscope ͑HPHT-STM͒ system is described. The system combines an ultrahigh vacuum surface analysis/ preparation chamber with a variable pressure (5ϫ10 Ϫ10 Torr-1 atm͒ and temperature ͑300-675 K͒ STM. The STM chamber can be isolated by three gate valves and filled with 1 atm of any gas mixture. The composition of the gas can be monitored by gas chromatography. A load-lock mechanism allows the transfer of samples and tips into the chamber without exposing it to air. Heating at high pressure is achieved using a halogen lamp beneath the sample. By switching between tungsten and gold tips, the microscope can be used in both oxidizing and reducing environments at room temperature, and reducing environments at elevated temperatures. This instrument allows the exploration of surfaces in a pressure regime that few other surface sensitive techniques can work in.