Ming‐Cheng Wu

Synthesis and characterization of ultra-thin MgO films on Mo(100)

Abstract Ultra-thin MgO films have been synthesized under UHV conditions by evaporating Mg onto Mo(100) in various background pressure of oxygen. Low-energy electron diffraction (LEED) studies show that MgO films grow epitaxially in the 200–600 K substrate temperature range with the (100) face of MgO oriented parallel to Mo(100). The one-to-one stoichiometry of the MgO films has been confirmed using Auger electron spectroscopy (AES) and temperature programmed desorption (TPD). A typical loss pattern, characteristic of single-crystal MgO, has been obtained by high-resolution electron energy-loss spectroscopy (HREELS). At low oxygen pressures, the MgO film grows via a mechanism of island nucleation with domains that coexist with metallic Mg particles. The heat of sublimation of three-dimensional MgO islands is dependent on the oxygen pressure during growth and relates to the coordination number of the Mg cations.

Model surface studies of metal oxides : adsorption of water and methanol on ultrathin MgO films on Mo(100)

Model surface studies of magnesium oxide have been carried out using surface sensitive techniques. Ultrathin MgO films have been synthesized under ultrahigh vacuum (UHV) conditions by thermally evaporating Mg onto Mo(100) in the presence of oxygen. Low‐energy electron diffraction (LEED) studies indicate that the MgO films grow epitaxially with the (100) face of MgO oriented parallel to Mo(100). The MgO films, prepared under optimum synthesis conditions, have essentially one‐to‐one stoichiometry, are nearly free from pointlike surface defects, and have properties essentially identical to those of bulk, single‐crystal MgO. Adsorption of water and methanol onto the MgO films has been studied using high‐resolution electron energy‐loss spectroscopy (HREELS) and temperature programmed desorption (TPD). In order to circumvent the difficulty associated with intense multiple surface optical phonon (Fuchs–Kliewer modes) losses, a new approach to acquisition of HREELS data has been demonstrated. This new approach en...

Adsorption of CO on Pd/Al2O3/Ta(110) model catalysts

The adsorption of CO on Pd/Al2O3/Ta(110) model catalysts was studied using infrared reflection adsorption spectroscopy (IRAS) for two different Pd coverages, corresponding to 25–30 and 60–70 A average particle sizes. Striking similarities between the IRAS data for the larger particles and Pd single crystals indicate that the particles are comprised predominantly of 〈111〉 and 〈100〉 facets. The isosteric heat of adsorption of CO extrapolated to zero coverage was determined to be 38.9 kcal/mol on the 60–70 A particles. IRAS and Auger results provide evidence of CO dissociation on the smaller particles.

Preparation, characterization, and chemical properties of ultrathin MgO films on Mo(100)

Ultrathin MgO films have been synthesized under ultrahigh vacuum conditions by evaporating Mg onto Mo(100) in various background pressures of oxygen. Low‐energy electron diffraction and surface spectroscopic studies show that the MgO films, prepared under optimum oxidation conditions, grow epitaxially in the 200–600 K substrate temperature range and have an essentially one‐to‐one stoichiometry. The nature of the near‐surface defects of the MgO films grown at low oxygen pressures has been explored using electron energy‐loss spectroscopy. Finally, the chemical properties of the stoichiometric MgO films have been investigated using high‐resolution electron energy‐loss spectroscopy.

Adsorption and reaction of formic acid on NiO(100) films on Mo(100): Temperature programmed desorption and high resolution electron energy loss spectroscopy studies

Adsorption and reaction of formic acid on well‐defined NiO(100) films prepared on Mo(100) have been investigated using combined temperature programmed desorption (TPD)/high resolution electron energy loss spectroscopy (HREELS). A novel approach to HREELS of oxide materials is utilized to acquire spectral data. This approach enables the direct observation of weak loss features due to the excitation of adsorbates without serious interference from intense multiple surface optical phonon losses associated with oxide materials. The results indicate that formic acid initially adsorbs associatively on NiO(100) surfaces at 90 K and undergoes heterolytic dissociation upon heating to ≥200 K to form a formate intermediate. The adsorbed formate species is proposed to bond to a cation site via one of the oxygen atoms of the formate in a monodentate configuration. The presence of a dimeric formic acid species is also indicated and a bonding configuration for this species has been proposed.

Initial sticking probabilities of Cu vacuum deposited on ordered Al2O3 and MgO

The initial sticking probabilities, S0, of Cu on ordered Al2O3 and MgO have been measured using thermal desorption techniques. S0 is found to vary depending on the temperature and nature of the substrate. The sticking probabilities at room temperature are 0.84 ± 0.05 for CuAl2O3 and 0.82 ± 0.05 for CuMgO, values much greater than the S0 (0.3) previously reported for CuSiO2. S0 is independent of the Cu coverage in the range 0.2–1.5 monolayers. The present study shows that the weak adsorption of copper on silica may be anomalous and not general behavior for metals deposited onto oxides.

Methane coupling at low temperatures on Ru(0001) and Ru(11¯20) catalysts

AbstractThe conversion of methane to higher hydrocarbons on single crystal Ru catalysts has been investigated using combined elevated-pressure kinetic measurements/surface science studies. The reaction consists of activation of methane on Ru(0001) and Ru(11¯20) surfaces to produce carbonaceous intermediates at temperatures

CO interaction with ultrathin MgO films on a Mo(100) surface studied by infrared reflection–absorption spectroscopy, temperature programmed desorption, and x‐ray photoelectron spectroscopy

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