Monitoring surfaces on the molecular level during catalytic reactions at high pressure by sum frequency generation vibrational spectroscopy and scanning tunneling microscopy

Sum frequency generation (SFG), using non-linear laser optics, detects vibrational spectra of submonolayer amounts of adsorbates with excellent energy and time resolution. Scanning tunneling spectroscopy (STM) is sensitive to the atomic surface structure; readily imaging defects, steps and kinks as well as stationary adsorbed species. Both of these techniques can be used during reactions at high pressures and temperatures to obtain molecular information in situ. We report studies of propylene hydrogenation over Pt(111) crystal surfaces at atmospheric pressures and 300 K using SFG and STM. Four surface species (2-propyl, -bonded propylene, di -bonded propylene, and propylidyne) were identified; the first two being implicated as reaction intermediates. The platinum surface structure remains unchanged during the reaction, consistent with the structure insensitive nature of olefin hydrogénation. Propylene decomposition induced substantial surface reconstruction.     

Characterization by scanning tunneling microscopy of silver oxydehydrogenation catalysts

A silicon carbide-supported silver catalyst used in the oxydehydrogenation of ethylene glycol to glyoxal has been studied by scanning tunneling microscopy. The surface morphology depends upon reaction conditions. Silver particles normally sinter into large plates covering the support. However, in the presence of diethylphosphite there is a chemical erosion which results in a tortuous and fractal-like surface.     

Imaging the structure and porosity of active carbons by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been employed for a comparative structural characterization, down to the atomic scale, of a representative set of porous carbons with different adsorption characteristics and prepared either by activation (physical or chemical) or templating techniques. The studied materials included a chemically activated, supermicroporous high surface area carbon, two activated carbons containing both micro- and mesopores synthesized by physical activation, an ultramicroporous carbon molecular sieve, and an ordered microporous carbon templated in the nanochannels of zeolite Y. In general, good agreement was found between the porous structures as imaged by STM and the porous texture derived from gas adsorption data for all the carbons investigated. The activated carbon samples were dominated by networks of slit type micropores and, in some cases, by mesopores of varied morphologies. By contrast, the zeolite-templated carbon exhibited rounded micropore morphology, and the carbon molecular sieve displayed a rather featureless conformation dominated by voids only below 1 nm wide. The structural differences observed by STM were interpreted in terms of the different preparation procedures of the studied carbons. In particular, the templated carbon consisted of minute clusters about 1 nm in diameter that were interpreted to be formed within the extremely confined, microporous spaces of the zeolite Y template.     

Zirconium oxide supported on Pd(100): Characterization by scanning tunneling microscopy and tunneling spectroscopy

Scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) were used to characterize the structure of a model metal-supported dispersed metal oxide, ZrO₂ on Pd(100). STM images illustrate changes in the surface morphology of the ZrO₂ resulting from various chemical treatments. When the sample was treated in O₂, the ZrO₂ appeared as a smooth, featureless overlayer of varying thickness wetting the Pd. After treatment in H₂, the ZrO₂ formed non-wetting particles on the Pd, with a sharp Pd-ZrO₂ interface. TS provided a fingerprint that verified the presence of a semiconducting overlayer on a metallic support. These results appear to be consistent with X-ray absorption spectra of ZrO₂ supported on Pd black, reported elsewhere.     

Study of antiphase boundaries and local 3 × 1 configuration on the (001) surface of homoepitaxial diamond films by scanning tunneling microscopy

Homoepitaxial diamond films have been studied using scanning tunneling microscopy. Boron-doped films were grown on diamond (001) substrates by microwave plasma-assisted chemical vapor deposition. Atomic resolution scanning tunneling microscopy (STM) images showed a dimer-type 2 × 1 reconstructed surface structure and features such as steps, kinks and terraces. Single steps where dimer rows on the upper terrace are normal to the step edge are ragged and the steps where dimer rows are parallel to the step edge are straight, indicating that the steps have different formation energies. The double-domain surface structure observed with STM was in agreement with low-energy electron diffraction pattern. Atomic images revealed two different types of antiphase boundary on the diamond (001) surface. These antiphase boundaries run parallel to or perpendicular to the dimer rows where dimer rows shift by only one lattice constant of the diamond (001) surface. We found that single dimer rows preferentially extended along an antiphase boundary. Local 3 × 1 configurations have been observed on homoepitaxially grown diamond (001) surface.     

Scanning tunneling microscopy and spectroscopy of silver and platinum catalysts

Scanning tunneling microscopic studies of silver catalysts dispersed on highly oriented pyrolitic graphite (HOPG) with 2, 5 and 10 wt% metal loading prepared by wet impregnation and hydrogen reduction show spherical crystallites, with the 2 wt% catalysts having an average crystallite size of 2 nm and the 5 and 10 wt% catalysts with size distributions of 2–5 and 4–12 nm respectively. The Ag catalysts prepared by NaBH₄ reduction show a narrower size distribution. Pt/HOPG catalysts with 2 and 5 wt% metal loading prepared by wet-impregnation and hydrogen reduction show large (2–11 nm) raft-like crystallites; small crystallites ( 1 nm) could be obtained by NaBH₄ reduction. Tunneling spectroscopic measurements reveal the nonmetallic nature of crystallites on the surface of Ag(2 wt%)/HOPG as well as Pt/HOPG prepared by NaBH₄ reduction.     

Imaging gold clusters on TiO2(110) at elevated pressures and temperatures

Variable temperature scanning tunneling microscopy (STM) has been used to image oxide-supported nanoclusters of Au at temperatures from 300 to 450 K and oxygen pressures from 10^–10 to 4 Torr. Oxygen-induced morphological changes of the TiO₂(1×2) reconstruction are apparent at room temperature and prolonged exposure (3×10³ L (langmuir)) at 10^–4 Torr oxygen. Gold clusters with diameters smaller than 4 nm are unstable toward sintering at ca. 450 K and oxygen pressures >10^–1 Torr. Oxygen at pressures >10^–4 Torr weakens the interaction between the gold cluster and the titania support. Increasing the sample temperature to >300 K facilitates disruption of the cluster–support interaction.     

Spatially (nanometer) controlled hydrogenation and oxidation of carbonaceous clusters by the platinum tip of a scanning tunneling microscope operating inside a reactor cell

Hydrocarbon clusters formed by the thermal decomposition of propylene on Pt(111) were rehydrogenated or oxidized with nanometer spatial resolution using the platinum tip of a scanning tunneling microscope (STM) at 300 K in atmospheric pressures of H₂ or O₂. The reaction rate shows a strong dependence on the oxygen or hydrogen pressures and on the tip-surface separation. The reaction stops when the Pt tip becomes contaminated with carbon, after the removal of 10⁷–10⁸ carbon atoms, but can be regenerated by removing material from the tip by application of a voltage pulse. Dissociative adsorption of H₂ and O₂ on the tip, followed by transfer of atoms to the surface is the proposed mechanism of these tip-catalyzed reactions.     

In situ STM study of surface catalytic reactions on TiO2(110) relevant to catalyst design

This paper briefly summarizes our recent work on the characterization of atom-resolved surface images of TiO₂(110) composed of Ti atoms, O atoms, defects and hydroxyls by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (NC-AFM). The paper also presents new kinetic aspects in the catalytic dehydration and dehydrogenation of formic acid on the characterized surface. Switchover of the reaction path from the dehydration to the dehydrogenation occurred by the presence of formic acid undetectable at the surface, where acidic formic acid molecules opened the basic catalysis. In situ STM observation revealed that the dehydrogenation reaction at 400–450 K was strongly suppressed in the vicinity of single-atom height steps. The suppressive effect of step ranged over 2.4 nm into terrace. It is likely that the catalyst with flat surfaces larger than 5 nm is active for the dehydrogenation of formic acid.     

Density of rocket propellant (RP-1 fuel) at high temperatures and high pressures

Density of rocket propellant (RP-1 fuel) has been measured with a constant-volume piezometer immersed in a precision liquid thermostat. Measurements were made in the temperature range from 301 to 745 K and at pressures up to 60 MPa. The uncertainty of density, pressure, and temperature measurements were estimated to be 0.1%, 0.05%, and 15 mK, respectively. The measured values of density were compared with the data reported in the literature and with the values calculated from a surrogate mixture model (equation of state). The average absolute deviation (AAD) between the present data and the values reported in the literature was 0.13%.     

In situ scanning tunneling microscopy study of selective dissolution of Au3Cu and Cu3Au (0 0 1)

We present an electrochemical study of Au₃Cu (0 0 1) single crystal surfaces in 0.1 mol dm⁻³ H₂SO₄ and 0.1 mol dm⁻³ H₂SO₄ + 0.1 mmol dm⁻³ HCl, and of Cu₃Au (0 0 1) in 0.1 mol dm⁻³ H₂SO₄. The focus is on in situ scanning tunneling microscopy experiments. The changes of the surface morphology, which are time- and potential-dependent, have been observed, clearly resolving single atomic steps and mono-atomic islands and pits. Chloride additives enhance the surface diffusion and respective morphologies are observed earlier. All surfaces have shown considerable roughening already in the passive region far below the critical potential.     

Temperature-Dependent Site Control of InAs/GaAs (001) Quantum Dots Using a Scanning Tunneling Microscopy Tip During Growth

Site-controlled InAs nano dots were successfully fabricated by a STMBE system (in situ scanning tunneling microscopy during molecular beam epitaxy growth) at substrate temperatures from 50 to 430°C. After 1.5 ML of the InAs wetting layer (WL) growth by ordinal Stranski–Krastanov dot fabrication procedures, we applied voltage at particular sites on the InAs WL, creating the site where In atoms, which were migrating on the WL, favored to congregate. At 240°C, InAs nano dots (width: 20–40 nm, height: 1.5–2.0 nm) were fabricated. At 430°C, InAs nano dots (width: 16–20 nm, height: 0.75–1.5 nm) were also fabricated. However, these dots were remained at least 40 s and collapsed less than 1000 s. Then, we fabricated InAs nano dots (width: 24–150 nm, height: 2.8–28 nm) at 300°C under In and As₄ irradiations. These were not collapsed and considered to high crystalline dots.     

Initial Stages of Growth of Alumina on NiAl(001) at 1025 K

The initial stages of growth of ordered layers of Al₂O₃ on NiAl(001) single-crystal surfaces at 1025 K and 10⁻⁷ mbar (10⁻⁵ Pa) in O₂ have been studied using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The STM results evidence the formation of elongated strips (26 Å wide and 11 Å high) of Al₂O₃ oriented along the [100] and [010] directions of the substrate. With longer oxidation, the substrate is increasingly covered by rectangular and striped islands resulting from the vicinal and parallel growth of the strips. On the ultrathin oxide film formed after 500 L (1 L = 10⁻⁶ torr·s (∼1.33 × 10⁻⁴ Pa·s)) of exposure, STM atomic resolution images have been obtained for the first time. They evidence the [001] orientation of the oxygen sublattice in Bain epitaxy on the substrate. The observation of one-dimensional atomic trenches together with the strips observed on the nanometer scale is consistent with the growth of θ-Al₂O₃. The STS local measurements evidence the insulating behavior of the oxide layer formed with a gap value ranging from 7 and 8 eV for amorphous and ordered domains, respectively.     

Heat capacity of rocket propellant (RP-1 fuel) at high temperatures and high pressures

Isobaric heat capacity of rocket propellant (RP-1 fuel) has been measured with a vacuum adiabatic calorimeter immersed in a precision liquid thermostat. Measurements were made in the temperature range from 293 to 671 K and at pressures up to 60 MPa. The uncertainty of heat capacity, pressure, and temperature measurements were estimated to be 2-2.5%, 0.05%, and 15 mK, respectively. The measured values of heat capacity were compared with the values calculated from a surrogate mixture model (equation of state, EOS). The average absolute deviation (AAD) between the present data and the values calculated with EOS was 0.81%.     

Generation of platinum microstructures on non-conducting surfaces by means of the scanning electrochemical microscope (SECM)

The scanning electrochemical microscope (SECM) was used to form platinum microstructures. For this purpose, a thin layer of platinum dichloride was deposited as precursor on different substrates by evaporating it in high vacuum. For the reduction of this precursor the SECM provided methyl viologen radical cations locally as reducing agent. The mechanism of the reduction is discussed.     

Scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) of heteropolyacid (HPA) self-assembled monolayers (SAMS): connecting nano properties to bulk properties

Nanoscale investigation of Keggin-type heteropolyacid (HPA) self-assembled monolayers (SAMs) was performed by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) in order to relate surface properties of nanostructured HPA monolayers to bulk redox and acid properties of HPAs. Cation-exchanged, polyatomsubstituted, and heteroatom-substituted HPAs were examined to see the effect of different substitutions. HPA samples were deposited on HOPG surfaces in order to obtain images and tunneling spectra by STM before and after pyridine adsorption. All HPA samples formed well-ordered monolayer arrays, and exhibited negative difference resistance (NDR) behavior in their tunneling spectra. NDR peaks measured for fresh HPA samples appeared at less negative potentials for higher reduction potentials of the HPAs. These changes could also be correlated with the electronegativities of the substituted atoms. Introduction of pyridine into the HPA arrays increased the lattice constants of the two-dimensional HPA arrays by ca. 6 A. Exposure to pyridine also shifted NDR peak voltages of HPA samples to less negative values in the tunneling spectroscopy measurements. The NDR shifts of HPAs obtained before and after pyridine adsorption were correlated with the acid strengths of the HPAs. This work demonstrates that tunneling spectra measured by STM can fingerprint acid and redox properties of HPA monolayers on the nanometer scale. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Electrodeposition of Ti from TiCl4 in the ionic liquid l-methyl-3-butyl-imidazolium bis (trifluoro methyl sulfone) imide at room temperature: study on phase formation by in situ electrochemical scanning tunneling microscopy

Titanium was electrodeposited from a nominal 0.24 M TiCl₄ in l-methyl-3-butyl-imidazolium bis (trifluoro methyl sulfone) imide ([BMIm]BTA) at room temperature on a Au(1 1 1) substrate. The process of electrodeposition was studied by cyclic voltammetry, chrono amperometry and in situ scanning tunneling microscopy (STM). In a first step TiCl₄ is reacted to TiCl₂, which is subsequently reduced to metallic Ti. Two dimensional (2D) clusters form preferentially on the terraces in the under potential deposition range. 2D clusters presumably of TiCl₃ precipitates grow and coalesce to cover the whole substrate with a 2D film at a substrate potential below −1.1 V versus ferricenium/ferrocene ([Fc]⁺/[Fc]) redox couple. At a potential of −1.8 V a dense layer of three dimensional (3D) clusters of titanium of 1-2 nm thickness is formed. The features of the I-U tunneling spectra and the relative reduction of the effective tunneling barrier by 0.8 eV with respect to gold clearly indicate the metallic character of Ti deposits. Observation of circular holes on the Au(1 1 1) substrate after dissolution of the deposited Ti indicates the formation of Au-Ti surface alloying.     

Atom-resolved noncontact atomic force microscopic and scanning tunneling microscopic observations of the structure and dynamic behavior of CeO2(1 1 1) surfaces

Atomic-scale structures and dynamic behaviors of CeO₂(1 1 1) surfaces were imaged by noncontact atomic force microscopy (NC-AFM) and scanning tunneling microscopy (STM). Hexagonally arranged oxygen atoms, oxygen point vacancies, multiple oxygen vacancies, and hydrogen adatoms at the surfaces were visualized by atom-resolved NC-AFM observations. Multiple defects were stabilized by displacement of the surrounding oxygen atoms around the multiple defects, which gave enhanced brightness in the NC-AFM image due to a geometric reason. Multiple defects without reconstruction of the surrounding oxygen atoms were reactive and were healed by exposure to O₂ gas and methanol at RT. Successive NC-AFM and STM measurements of slightly reduced CeO₂(1 1 1) surfaces revealed that hopping of surface oxygen atoms faced to the metastable multiple defects was thermally activated even at room temperature (RT) and more promoted at higher temperatures. Heterogeneous feature of the reactivity of surface oxygen atoms with methanol was imaged by successive NC-AFM observations. These observations gave a new insight for understanding the surface structures and behavior of CeO_2−x with the facile oxygen reservoir and oxidation–reduction properties related to the unique catalysis.     

Scanning Tunneling Microscopy (STM) at High Pressures. Adsorption and Catalytic Reaction Studies on Platinum and Rhodium Single Crystal Surfaces

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.