A Simple Angle-Resolved Thermal Molecular Beam Reactor: Applied to CO Oxidation on Pt{110}

We developed a simple form of thermal molecular beam catalytic reactor system which is capable of measuring sticking probabilities and reaction probabilities, together with angle-resolved scattering of molecules and products during catalytic reactions at the surface. This includes very fast determination of the angle dependence of the reaction product flux at steady-state. It was employed to determine the oxidation of CO on Pt{110}-(1 × 2), including individual molecular sticking and scattering. The initial sticking probability of oxygen on Pt{110} shows a small variation between 140 and 750 K surface temperature, from 0.45 to 0.28. The saturation uptake drops from 1.5 ± 0.2 ML at 140 K to 0.6 ML at 300 K and to 0.23 ± 0.02 ML at 750 K. The initial sticking probability of CO at 300 K is 0.80 and decreases to 0.62 at 470 K. Beyond that temperature, it descends steeply down to near zero at 570 K, due to the high desorption rate of CO at that temperature. Kisliuk precursor mobility parameters K were calculated from shape of the sticking curves. For 300 K, a value of 0.11 ± 0.01 was found, which increases to 0.76 ± 0.01 at 470 K, indicating a change from considerable mobility in the precursor state, to more limited mobility before desorption at high temperature. In temperature-programmed CO-O2 reaction experiments, CO2 production was observed to initiate in the temperature region 460–510 K. Using isothermal angle-resolved experiments, the CO2 flux was determined in the [11¯0] plane at temperatures of 470–620 K. Two sharp scattering lobes at positions of ±16° off the surface normal were found, with a high cosine power angle dependence, which were attributed to desorption from the {111}-like microfacets of the 1 × 2 reconstructed surface, with products evolving over a high barrier.

OXYGEN AND POTASSIUM ADSORPTION ON A CARBIDE-MODIFIED STEPPED-W(100) IN CONTACT WITH THE CARBON SOLID SOLUTION: AN AES AND WF STUDY AT 300 K AND AT ELEVATED TEMPERATURES

The adsorption of oxygen and potassium on the two-phase system: carbide-modified stepped- W (100) surface (CMT) in contact with the solid solution of carbon in bulk tungsten, was investigated by AES and WF measurements. The CMT surface shows metallic behavior judging from its interaction with K . The expected dissociative adsorption of oxygen appears to occur with 1 - θ kinetics, possibly via a molecularly chemisorbed state. The "dispersed phase — two-phase" model is clearly applicable when oxygen adsorbs on the K -pre-covered carbide. The initial sticking coefficient of oxygen increases drastically from the dispersed to the condensed phase, at least four-fold with respect to s0 on the clean carbide. It is proposed that this two-phase carbon system can be advantageous compared with the bulk carbide since it can easily regenerate the surface if the latter is depleted from carbon.

ETHYLENE REACTIVITY WITH SILICON SURFACE

The clean Si (111)(7×7) surface has been exposed to ethylene ( C 2 H 4) doses, up to 7000 L (1 L =10-6 Torr ×1 s ) at most, under ultrahigh vacuum. The structural and electronic property changes of the surface have been studied by low energy electron diffraction (LEED), Auger electron spectrometry (AES) and photoemission yield spectroscopy (PYS). The interaction presents two types of kinetic adsorption, where the first is produced below 3000 L of C 2 H 4. In the first step, the ethylene molecule is adsorbed molecularly and the initial sticking coefficient S 0 is very low (S0≈2×10-3). At the saturation (≈ 5000–6000 L), the valence band is fitted by a power law (E — 4.42)2.5 eV .1,2 The surface is then a stronger scattering for photoemitted electrons. We attribute this result, produced at the second step of adsorption, to the C 2 H 4-π orbital and hydrogen liberated by this molecule, which break the Si – Si surface bonds.