Hiroshi Uetsuka

Angular and velocity distributions of desorbing product carbon dioxide from two reaction sites on platinum(110)(1×2)

The angular and velocity distributions of desorbing product CO2 were studied on a platinum(110)(1×2) surface over a wide range of CO coverages by means of angle‐resolved thermal desorption combined with a cross‐correlation time‐of‐flight (TOF) technique. Heating the coadsorption layer of CO and oxygen yields four CO2 formation peaks P1–CO2 (∼400 K), P2–CO2 (300 K), P3–CO2 (250 K), and P4–CO2 (170 K)]. The angular distribution of each CO2 produced at high CO coverages consists of three desorption components. Two of them show desorption collimated along the inclined terrace normal; the other shows it along the bulk surface normal. The former is assigned to reaction on the inclined terrace, and the latter mostly to reaction on the bottom of the trough. The translational temperature of each desorption component is derived by deconvoluting the TOF spectrum. This temperature reaches 1000–1500 K. The maximum translational temperature is always observed in the normal direction of each reaction site. For P2–and P3...

Infrared chemiluminescence of CO and CO2 produced by molecular-beam surface reactions

Abstract Vibrationally excited molecules produced by molecular-beam surface reactions have been studied by using infrared emission spectroscopy. The product CO desorbed by partial oxidation of n-butane on Pt at 1500 K is substantially vibrationally excited, but rotationally very cool (rotational temperature; TR = 360 K), while the product CO2 desorbed by the CO + O2 reaction on Pt is both vibrationally and rotationally excited. The CO2 molecules produced by CO oxidation on Pt using an atomic oxygen beam are much more highly excited (both vibrationally and rotationally) than those by the CO + O2 reaction. These results showed that both reactant and product molecules do not necessarily thermally accommodate to the surface, and their energy distributions will give us information on the dynamics of surface-catalyzed reactions.

Mechanism of N2O decomposition over a Rh black catalyst studied by a tracer method the reaction of N2O with 18O(a)

Abstract N2O decomposition on an unsupported Rh catalyst has been studied using tracer technique in order to reveal the reaction mechanism. N216O was pulsed onto 18O/oxidized Rh catalyst at 220°C and desorbed O2 molecules (m/e=32,34,36) were monitored by means of mass spectrometer. The 18O fraction in the desorbed dioxygen was the same value as that on the surface oxygen. The result shows that the O2 molecules desorb via Langmuir–Hinshelwood mechanism, i.e., the desorption of dioxygen through the recombination of adsorbed oxygen. On the other hand, TPD measurements in He showed that desorption of oxygen from the Rh black catalyst occurred at the higher temperatures. Therefore, reaction-assisted desorption of oxygen during N2O decomposition reaction at the low temperature was proposed.

The dynamics of surface chemical reactions: infrared chemiluminescence of the product CO2 desorbed from metal surfaces

Abstract IR emission spectra of CO 2 were measured during decomposition of HCOOH, HCOOH + O 2 reaction and oxidation of CO on metal surfaces for the study of the dynamics of the catalytic reactions. Vibrationally excited CO 2 was produced by HCOOH + O 2 reaction on Pt foil. Internal energy distribution of the product CO 2 was the same as that from CO oxidation on Pt foil (a bimolecular reaction between CO ad + O ad ). On the contrary, CO 2 molecules desorbed during HCOOH + O 2 reaction on Ni foil and decomposition of HCOOH on Pt or Ni foil were not so excited (a unimolecular reaction). On the other hand, the IR chemiluminescence of the product CO 2 was measured for the first time during the oxidation of CO on single crystal Pd(111) and Pt(111) surfaces. The vibrational ( T V ) and rotational ( T R ) temperatures of the CO 2 from Pd(111) were much higher than those of the CO 2 from Pt(111). Furthermore, the comparison of the results with those on polycrystalline Pd and Pt surfaces revealed that the reaction dynamics of CO oxidation are structure-sensitive; i.e., the transition state of CO 2 formation depends on the structure of the catalyst surface.

Infrared chemiluminescence study of vibrationally excited CO2 formed by catalytic oxidation of CO over single crystal and polycrystalline Pt surfaces

Abstract The infrared chemiluminescence technique has been applied to the catalytic oxidation of CO on Pt(110), Pt(111) and polycrystalline Pt surfaces. The CO 2 molecules produced by the oxidation of CO on Pt(110)-(1 × 2) and Pt(111) were vibrationally excited but rotationally rather cool. However, the vibrational temperature ( T v ) is higher on Pt(110)-(1 × 2) than on Pt(111), while T v is much lower on Pd(110) than on Pd(111). On polycrystalline Pt surfaces, the vibrational and rotational energy states of product CO 2 were altered by the pretreatment (annealing in vacuo or in air), which strongly affects the surface structure. These results suggest that the vibrational and rotational states of the CO 2 product are sensitive to the surface structure, i.e. the transition state of CO oxidation is strongly affected by the structure of the reaction sites.

Isotopic study of nitrous oxide decomposition on an oxidized Rh catalyst: mechanism of oxygen desorption

N2O decomposition on an oxidized Rh catalyst (unsupported) has been studied using a tracer technique in order to reveal the reaction mechanism. N216O was pulsed onto an 18O/oxidized Rh catalyst at 493 K and desorbed O2 molecules were monitored. The 18O fraction in the desorbed oxygen had the same value as that on the surface oxygen. The result shows that the oxygen molecules do not desorb via the Eley–Rideal mechanism, but via the Langmuir–Hinshelwood mechanism. On the other hand, desorption of oxygen from Rh surfaces (in vacuum or in He) occurs at higher temperatures, which suggests reaction-assisted desorption of oxygen during the N2O decomposition reaction at low temperature.

Dynamics and kinetics of CO oxidation on Pd(335): infrared chemiluminescence of CO2

Abstract Infrared chemiluminescence technique has been applied to steady-state carbon monoxide oxidation on a stepped Pd(335) surface, which consists of four-atom wide terraces of (111) orientation and monatomic-height steps, and the results are compared with those of a flat Pd(111) surface. The activity for CO oxidation was higher on Pd(335) than on Pd(111). The vibrational temperatures ( T v ) of the product CO 2 molecules were substantially different on the stepped Pd(335) surface from on the Pd(111) surface, and the difference in T v between on Pd(335) and on Pd(111) is much larger at the surface temperature ( T s ) of 850xa0K than at T s =650xa0K. These results suggest that, for the steady-state CO oxidation on the Pd(335) surface, the reaction takes place mostly on the step sites at T s =850xa0K, but the contribution from the terrace sites is increased significantly at T s =650xa0K.

The dynamics of CO oxidation on Pt(110) studied by infrared chemiluminescence of the product CO2: effect of CO coverage

The infrared chemiluminescence technique has been applied to the catalytic oxidation of CO on a Pt(110)(1×2) surface. The vibrational and the rotational states of CO2 formed on the reconstructed Pt(110)(1×2) surface are more excited than those on the terrace Pt(111) surface. The vibrational state of the product CO2 strongly depends on the CO coverage: the vibrational temperature (TV) of the product CO2 becomes higher, as the coverage of CO increases.

Infrared chemiluminescence study of the dynamics of steady-state CO oxidation on single-crystal Pd surfaces : effect of oxygen pretreatment

Abstract Infrared chemiluminescence has been applied to obtain information on the dynamics of steady-state CO oxidation over well-defined Pd(110) and Pd(111). The dynamics of the CO oxidation is structure sensitive, i.e., the internal energy states (vibration and rotation) of the product CO 2 depend on the surface structure of the reaction sites. Furthermore, the effect of exposures of Pd(110) to high-pressure O 2 (10 −2 Torr) on the internal energy states of the CO 2 was investigated, but the dynamics of CO 2 formation appeared to be the same between clean and oxidized Pd(110) surfaces.

Infrared chemiluminescence study of CO produced by partial oxidation of butane on platinum

The selective formation of CO and H2 was observed by a molecular-beam catalytic reaction betweenn-C4H10 and O2 on a Pt surface from around 1000 to 1500 K. The infrared emission of the product CO desorbed from the surface showed that the CO molecules are vibrationally substantially excited but rotationally very cool (rotational temperature;TR = 360 K). The present molecular-beam study showed that CO and H2 were formed directly from the hydrocarbon and O2 without involving formation of CO2 and H2O as primary products. The implications of these results are discussed for the partial oxidation of methane (and other alkanes) to synthesis gas using practical supported metal catalysts.