CO Molecular Beam Research Articles

Particle size effect on the Langmuir-Hinshelwood barrier for CO oxidation on regular arrays of Pd clusters supported on ultrathin alumina films.

The Langmuir-Hinshelwood barrier (ELH) and the pre-exponential factor (νLH) for CO oxidation have been measured at high temperatures on hexagonal arrays of Pd clusters supported on an ultrathin alumina film on Ni3Al (111). The Pd clusters have a sharp size distribution, and the mean sizes are 174 ± 13, 360 ± 19, and 768 ± 28 atoms. ELH and νLH are determined from the initial reaction rate of a CO molecular beam with a saturation layer of adsorbed oxygen on the Pd clusters measured at different temperatures [493 ≤ T(K) ≤ 613]. The largest particles (3.5 nm) give values of ELH and νLH similar to those measured on Pd (111) [T. Engel and G. Ertl, J. Chem. Phys. 69, 1267 (1978)]. However, smaller particles (2.7 and 2.1 nm) show very different behaviors. The origin of this size effect is discussed in terms of variation of the electronic structure and of the atomic structure of the Pd clusters.

Interactions of Hydrogen and Carbon Monoxide on Pd–Au Bimetallic Surfaces

Pd–Au bimetallic catalysts have shown potential applications in numerous heterogeneous reactions in which hydrogen and CO act as reactants, intermediates, or products. A fundamental understanding of the interplay between coadsorbed H and CO on the Pd–Au surface is necessary for improving the understanding of catalytic performance. In this study, the interactions of hydrogen and CO with Pd/Au(111) model surfaces were investigated by temperature-programmed desorption (TPD) and molecular beam scattering (MBS) experiments, carried out under ultra-high-vacuum conditions. Our results reveal that CO adsorbs competitively on the hydrogen-precovered Pd–Au surface, causing surface H adatoms to diffuse away from stronger-binding sites (e.g., Pd(111)-like islands) to weaker-binding sites (e.g., Pd–Au alloy sites and subsurface), as evidenced by a shift of the H2 desorption feature to lower temperatures in TPD measurements. Additionally, evolution of H2 was observed when a CO molecular beam was impinged onto the H-pre...

Gas–Surface Scattering Dynamics of CO2, NO2, and O3in Collisions with Model Organic Surfaces

High-energy (70 kJ/mol) molecular beams of CO(2), NO(2), and O(3) were scattered from long-chain methyl (CH(3)-), hydroxyl (OH-), and perfluoro (CF(3)(CF(2))(8)-, or F-) ω-functionalized alkanethiol self-assembled monolayers (SAMs) on gold to study the dynamics of energy exchange and thermal accommodation of atmospherically important triatomic molecules on model organic surfaces. Overall, the extent of energy transfer in gas collisions with all of the surfaces studied was substantial. Specifically, the triatomics scatter from each surface only after dissipating greater than 80% of their incident energy. Furthermore, although the OH-SAM is a more rigid surface, the extent of energy transfer and accommodation of these molecules to the CH(3)- and OH-SAMs were approximately the same. The similar scattering dynamics are likely due to significant gas-surface attractive forces between the triatomics and the OH terminal groups, which compensate for the rigidity of this monolayer. In contrast to the OH- and CH(3)-SAMs, the dominant pathway in collisions of the gases with the F-SAM was impulsive scattering. The portion of molecules that accommodated (<40%) to the F-SAM was about half of the amount that accommodated (∼70%) to the CH(3)- and OH-SAMs. Although differences in the surface properties had a significant effect on the dynamics, variances in the chemical and physical properties of the three gases, CO(2), NO(2), and O(3), were found to have little effect on the extent of energy transfer and accommodation for collisions with any one surface.

Photoemission and thermo‐programmed reaction study of the catalytic properties of Rh/CeO2 system

AbstractRhodium deposited onto thin films of cerium dioxide was exposed by molecular beams of CO and O2. Production of CO2 was studied as a function of the substrate temperature and structure and the oxidation state of the samples. Polycrystalline CeO2 films as well as oriented CeO2(111)/Cu(111) ones were used. The results show that CO2 production depends on the original conditions of the used samples. Morphology of their surface (oriented or polycrystalline) as well as their chemical state (oxidised or reduced) is important. The observed effects are discussed with a respect to the X‐ray photoelectron spectra obtained at particular steps of the experiment. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Gas–surface energy exchange and thermal accommodation of CO2 and Ar in collisions with methyl, hydroxyl, and perfluorinated self-assembled monolayers

Molecular beams of CO(2) and Ar were scattered from long-chain methyl (CH(3)-), hydroxyl (OH-), and perfluoro ((CF(2))(7)CF(3)-) functionalized alkanethiol self-assembled monolayers (SAMs) on gold to study the dynamics of energy exchange and thermal accommodation on model organic surfaces. Ar collisions, for incident energies ranging from 25 to 150 kJ mol(-1), exhibit final energy distributions that depend significantly on the terminal functional group of the SAM. The long-chain CH(3)-terminated monolayers serve as an excellent energy sink for dissipating the incident translational energy. For example, at 150 kJ mol(-1), greater than 90% of the collision energy is transferred to the CH(3)-SAM surface for specularly-scattered atoms (θ(i) = θ(f) = 30° from normal). However, the OH-SAM is a more rigid collision partner due to the formation of an intra-monolayer hydrogen bonding network and the (CF(2))(7)CF(3)-SAM (F-SAM) provides a high degree of rigidity due to the massive CF(3) groups. The final energies for the triatomic, CO(2), scattering from the three surfaces are remarkably similar to the results for Ar scattering. The only significant difference in the translational energy transfer dynamics for these two gases appears in collisions with the OH-SAM. Strong gas-surface attractive forces between CO(2) and the OH-SAM surface appear to counter the rigidity of the hydrogen-bonding network to help bring the majority of the molecules to thermal equilibrium at all incident energies up to 150 kJ mol(-1), resulting in increased energy transfer in comparison to Ar. The similarities in energy transfer for Ar and CO(2) final energy distributions in scattering from the CH(3)- and F-SAMs suggest that the internal degrees of freedom in the triatomic play only a small role in determining the outcome of the gas-surface collision under the scattering conditions employed in this work.

Quantum-State-Resolved CO2 Scattering Dynamics at the Gas−Liquid Interface: Incident Collision Energy and Liquid Dependence

Quantum-state-resolved dynamics at the gas-liquid interface are probed by colliding supersonically cooled molecular beams of CO(2) with freshly formed liquid surfaces in a vacuum. Translational, rotational, and vibrational state distributions of both incident and scattered fluxes are measured by high-resolution direct infrared absorption spectroscopy and laser dopplerimetry in the 00(0)0 and 01(1)0 rovibrational manifolds of CO(2) in the asymmetric stretch manifold. The present studies investigate the role of incident molecular beam energy (E(inc) = 1.6(1), 4.7(2), 7.7(2), and 10.6(8) kcal/mol) on these distributions for a series of perfluorinated, hydrocarbon, and hydrogen-bonded liquids. Boltzmann analysis of the internal quantum-state populations provide evidence for nonthermal scattering dynamics, as confirmed by Dopplerimetry on the absorption profiles. The data provide quantum-state-resolved support for a dual channel picture of the scattering process, consisting of either prompt impulsive scattering (IS) or longer duration trapping-desorption (TD) events, with the fraction observed in each channel dependent on incident kinetic energy and the physical properties of the liquid surface. The clear evidence that internal CO(2) rotational populations arising from the IS channel can be adequately described by a Boltzmann temperature (albeit with E(IS) > RT(S)) is consistent with previous gas-solid scattering studies and suggests that even nominally "prompt" IS events reflect both single (i.e. direct) and multiple impulsive interactions with the liquid interface.

Oxidation of CO on a Pt/Al 2O 3 catalyst: from the surface elementary steps to light-off tests : IV. Kinetic study of the reduction by CO of strongly adsorbed oxygen species

Transient experiments using mass and FTIR spectroscopy as detectors are performed at 300 K with a reduced 2.9% Pt/Al 2O 3 catalyst to study the reduction of strongly adsorbed oxygen species (denoted O sads) formed by O 2 chemisorption using several y% CO/ z% Ar/He mixtures ( y and z in the range 0.5–10). During the first seconds of the reaction C mass balances reveal that the CO consumption is mainly due to the formation of a strongly adsorbed CO species identified as a linear CO species (denoted L) interacting with the O sads species (IR band at 2084 cm −1). The evolution of the CO 2 production rate with time on stream presents different profiles according to the reaction temperature: decreasing exponential at T r<273 K and peak profiles for T r⩾300 K. The CO 2 production at T r<273 K is in agreement with a kinetic model considering two elementary steps: the adsorption of the L CO species without competition with O sads followed by a L–H elementary step (denoted S3b): O sads+L→CO 2ads, with a rate constant k 3b= ν 3bexp(− E 3b/ RT) and E 3b=65 kJ/mol at θ Osads≈1. For T r>300 K, mass transfer processes contribute to the apparent CO 2 production rate. At high θ Osads values, they compete with the surface reactions for 273 K< T r<360 K and finally dominate the CO 2 production at T r>360 K. However, kinetic studies can be performed at T r>300 K after a significant decrease in θ Osads due to the increase in E 3b: E 3b=110 kJ/mol at θ Osads=0.4. Several conclusions of the present study are in very good agreement with the reduction of O sads species on Pt single crystals using a CO molecular beam under UHV conditions.

Rh/Al bimetallic system with different thickness of Rh layer––AES characterisation and CO oxidation

CO oxidation over Rh/Al bimetallic system was studied by method of molecular beams (MBs). Rh films were evaporated in situ onto Al polycrystalline substrate. The samples with different amounts of deposited rhodium were prepared. Sample purity and composition were checked by Auger electron spectroscopy and electron energy loss spectroscopy. Rh–Al alloy forms on the sample surface during annealing of this system. CO oxidation was investigated in steady-state experiment using two separated MBs of CO and O 2. Experiments were performed in a dynamic mode (linear increase and decrease of the sample temperature to study the hysteresis effects). The influence of Rh layer thickness as well as of CO and O 2 MB intensity ratio on the CO 2 production was studied.

Study of CO adsorption on Rh/alumina model catalysts in dependence on substrate orientation

The interaction of a CO molecular beam with rhodium vapour-deposited onto (0001), (1–102) and (11–20) α-alumina was investigated. The dependence of the CO sticking coefficient on the sample temperature and the relative CO coverage for different Rh deposits were calculated from the intensity of the reflected CO molecular beam. The diffusion of CO over the uncovered substrate to the metal is discussed in relation to the Rh particle size and density.

Transient study of carbon monoxide oxidation on Pd(111) and Pd(110)

We report measurements of CO oxidation by CO molecular beam titration on Pd(110) and Pd(111) surfaces pre-covered with atomically bonded oxygen. The results are semi-quantitatively discussed by computer simulations with rate equations of the Langmuir–Hinshelwood type.

Characterization of oxygen phases created during oxidation of Ru(0001)

Thermal desorption spectroscopy, ultraviolet photoelectron spectroscopy, low energy electron diffraction (LEED), and the reactive scattering of a CO molecular beam have been applied to determine the relationship between the formation of the subsurface oxygen phase and the growth of oxides during oxidation of Ru(0001). Emission of RuOx (x&amp;lt;4) molecules observed in the thermal desorption spectra during the heating of the oxygen-rich sample has been used as a simple measure for the presence of bulk oxides. When performing the oxygen exposure at a temperature lower than the onset for oxygen desorption (Tp&amp;lt;850 K) a mobile atomic oxygen species is predominantly formed in the subsurface region. The conversion of these subsurface oxygen atoms into a regular RuxOy phase takes place within the temperature region of 900–1150 K. The growth of oxide films becomes the dominating reaction channel when performing the oxidation at temperatures higher than the onset for oxygen desorption. The oxide formation is strongly reduced when conducting the oxidation at temperatures higher than 1250 K. In this case only a relatively low amount of oxygen atoms adsorbed on the bare Ru surface can be achieved, neither oxides nor subsurface oxygen have been found. The presence of a RuO2 coating layer manifests itself by LEED patterns characteristic for a particular RuO2 single crystal face as well as by additional features in the valence ultraviolet photoelectron spectra. The oxidation of CO molecules reactively scattered at these oxygen-rich surfaces proceeds as long as mobile oxygen atoms are present in the subsurface region. The reaction is entirely quenched when the subsurface oxygen is replaced by an uniform film of RuO2.

Carbon monoxide oxidation on Ir (110)

We report measurements of CO oxidation by CO molecular beam titration of an Ir (110) surface precovered with atomically bonded oxygen. The CO2 formation rate increases with increasing CO fluence and depends strongly on both oxygen precoverage and surface temperature. The self-accelerating CO2 formation kinetics may be qualitatively discussed in the framework of coverage-dependent kinetics of the Langmuir–Hinshelwood type.

Molecular beam studies of CO oxidation and CO-NO reactions on a supported Pd catalyst

The surface reactivity of highly dispersed (85%) palladium cluster catalysts supported on γ-alumina (1.5 wt% loading) had been probed using the molecular beam scattering technique. This work illustrates the utility of molecular beam sources to study reaction kinetics on highly dispersed catalyst surfaces under UHV conditions. While CO adsorbs and desorbs molecularly throughout the coverage range, the Pd particles have been found to be extremely active with respect to NO dissociation forming a “mixed” layer of molecularly bound NO and atomic nitrogen and oxygen. Reaction kinetics of CO oxidation has been studied as a function of surface temperature by reaction of a pre-adsorbed oxygen layer with a CO molecular beam. A maximum reaction rate and percentage conversion to CO 2 was found to occur between 570 and 620 K. In contrast to previous studies of model Pd cluster catalysts, the highly dispersed Pd clusters have been found to be active for the CO/NO reaction probed by reaction of a CO beam with a NO pre-adsorbed layer.

Transitions induites par deux impulsions — Spectroscopie par transformée de Fourier

The transition probability induced between two energy levels, by two electrical pulses, is studied. The experimental method uses a H 2 CO molecular beam apparatus. It is shown that the spectrum is the Fourier Transform (TF) of the transition probability obtained by scanning the delay between both pulses. This method is an extension of the Ramsey method of separated fields; because the carrier frequency is zero, there is no Bloch Siegert shift. When the pulses are not rectangular, it is shown that the method is still applicable in the sudden approximation

Rotationally inelastic scattering from surfaces CO(g) + LiF(OO1)

The rotationally inelastic scattering of a supersonic, rotanonally cold ( T rot I = 4–30 k) CO molecular beam from a clean LiF(001) surface has been investigated by VUV laser-induced fluorescence. Rotational accommodation was incomplete for surface temperatures 350 < T s < 900 K. The degree of accommodation decreased with T s and was independent of collision energy.