A new scanning tunneling microscope reactor used for high-pressure and high-temperature catalysis studies

This is the published version. Copyright 2008 American Institute of PhysicsWe present the design and performance of a homebuilt high-pressure and high-temperature reactor equipped with a high-resolution scanning tunneling microscope STM for catalytic studies. In this design, the STM body, sample, and tip are placed in a small high pressure reactor 19 cm3 located within an ultrahigh vacuum UHV chamber. A sealable port on the wall of the reactor separates the high pressure environment in the reactor from the vacuum environment of the STM chamber and permits sample transfer and tip change in UHV. A combination of a sample transfer arm, wobble stick, and sample load-lock system allows fast transfer of samples and tips between the preparation chamber, high pressure reactor, and ambient environment. This STM reactor can work as a batch or flowing reactor at a pressure range of 10−13 to several bars and a temperature range of 300–700 K. Experiments performed on two samples both in vacuum and in high pressure conditions demonstrate the capability of in situ investigations of heterogeneous catalysis and surface chemistry at atomic resolution at a wide pressure range from UHV to a pressure higher than 1 atm

Imaging and manipulation of nanometer-size liquid droplets by scanning polarization force microscopy

Using atomic force microscopy in noncontact mode, we have imaged nanometer-size liquid droplets of KOH water solutions on the surfaces of highly oriented pyrolitic graphite and mica. On graphite the droplets prefer to be adsorbed on atomic step edges. Droplets on the same step tend to be evenly spaced and of similar size. The droplets can be manipulated by the atomic force microscopy tip allowing the controllable formation of droplet patterns on the surface

When Langmuir is too simple: H-2 dissociation on Pd(111) at high coverage

Recent experiments of H2 adsorption on Pd(111) [T. Mitsui et al., Nature (London) 422, 705 (2003)] have questioned the classical Langmuir picture of second order adsorption kinetics at high surface coverage requiring pairs of empty sites for the dissociative chemisorption. Experiments find that at least three empty sites are needed. Through density functional theory, we find that H2 dissociation is favored on ensembles of sites that involve a Pd atom with no direct interaction with adsorbed hydrogen. Such active sites are formed by aggregation of at least 3 H-free sites revealing the complex structure of the "active sites.

A new scanning tunneling microscope reactor used for high-pressure and high-temperature catalysis studies

This is the published version. Copyright 2008 American Institute of PhysicsWe present the design and performance of a homebuilt high-pressure and high-temperature reactor equipped with a high-resolution scanning tunneling microscope STM for catalytic studies. In this design, the STM body, sample, and tip are placed in a small high pressure reactor 19 cm3 located within an ultrahigh vacuum UHV chamber. A sealable port on the wall of the reactor separates the high pressure environment in the reactor from the vacuum environment of the STM chamber and permits sample transfer and tip change in UHV. A combination of a sample transfer arm, wobble stick, and sample load-lock system allows fast transfer of samples and tips between the preparation chamber, high pressure reactor, and ambient environment. This STM reactor can work as a batch or flowing reactor at a pressure range of 10−13 to several bars and a temperature range of 300–700 K. Experiments performed on two samples both in vacuum and in high pressure conditions demonstrate the capability of in situ investigations of heterogeneous catalysis and surface chemistry at atomic resolution at a wide pressure range from UHV to a pressure higher than 1 atm

Ambient pressure X-ray photoelectron spectroscopy study of room-temperature oxygen adsorption on Cu(1 0 0) and Cu(1 1 1)

We investigated the room-temperature chemisorption of oxygen on Cu(100) and Cu(111) using ambient-pressure X-ray photoelectron spectroscopy (APXPS). A shoulder-to-shoulder comparison between the oxygen-gas titration on the two surfaces reveals that Cu(100) is the more active for oxygen dissociative chemisorption when the surfaces are clean. The (2 root 2 x root 2)R45 degrees missing-row reconstruction appears in Cu(100)'s LEED image after about 10(4) Langmuir of oxygen exposure, whereas on Cu(111), no long-range ordering was observed throughout the whole experiment. An oxide layer consisting of cuprous and cupric oxide shows up on Cu(111) at an oxygen exposure that is significantly lower than for Cu(100). This observation suggests that the presence of (2 root 2 x root 2)R45 degrees missing-row reconstruction layer slows down Cu(100) oxidation. Literature has widely reported that surface morphology influences the copper oxidation process. This study provides an XPS demonstration that copper surface oxide formation in O-2 at room temperature depends on the surface crystallographic orientation

Hydrogen migration at restructuring palladium-silver oxide boundaries dramatically enhances reduction rate of silver oxide.

Heterogeneous catalysts are complex materials with multiple interfaces. A critical proposition in exploiting bifunctionality in alloy catalysts is to achieve surface migration across interfaces separating functionally dissimilar regions. Herein, we demonstrate the enhancement of more than 104 in the rate of molecular hydrogen reduction of a silver surface oxide in the presence of palladium oxide compared to pure silver oxide resulting from the transfer of atomic hydrogen from palladium oxide islands onto the surrounding surface formed from oxidation of a palladium-silver alloy. The palladium-silver interface also dynamically restructures during reduction, resulting in silver-palladium intermixing. This study clearly demonstrates the migration of reaction intermediates and catalyst material across surface interfacial boundaries in alloys with a significant effect on surface reactivity, having broad implications for the catalytic function of bimetallic materials

In-situ X-ray photoelectron spectroscopy studies of water on metals and oxides at ambient conditions

X-ray photoelectron spectroscopy (XPS) is a powerful tool for surface and interface analysis, providing the elemental composition of surfaces and the local chemical environment of adsorbed species. Conventional XPS experiments have been limited to ultrahigh vacuum (UHV) conditions due to a short mean free path of electrons in a gas phase. The recent advances in instrumentation coupled with third-generation synchrotron radiation sources enables in-situ XPS measurements at pressures above 5 Torr. In this review, we describe the basic design of the ambient pressure XPS setup that combines differential pumping with an electrostatic focusing. We present examples of the application of in-situ XPS to studies of water adsorption on the surface of metals and oxides including Cu(110), Cu(111), TiO2(110) under environmental conditions of water vapor pressure. On all these surfaces we observe a general trend where hydroxyl groups form first, followed by molecular water adsorption. The importance of surface OH groups and their hydrogen bonding to water molecules in water adsorption on surfaces is discussed in detail