Temperature desorption spectroscopy (TDS) and infrared reflection absorption spectroscopy (IRAS) were used to study CO adsorption on the terrace and step sites of Pt nanoparticles grown in ultrahigh vacuum on SiO 2 . The percentage of terrace sites obtained by TDS measurements from four average particle sizes (>4, 4.2, 3.3, 2.6, and 2.5 nm) are compared to a simple hard sphere counting model of a truncated cuboctahedron. It is demonstrated experimentally that when the average Pt particle size is reduced from 4.2 to 2.5 nm, the percentage of terrace sites decreases by ∼50%, consistent with the hard sphere models. Pt particle morphology is further explored by blocking the terrace sites with ethylidyne (derived from ethylene). CO adsorption on the unoccupied sites of Pt nanoparticles precovered with ethylidyne demonstrates a continuous red-shift in IRAS with increasing particle size. Similarities and differences between Pt nanoparticles and high-index single crystals are discussed.

In this study, we present evidence for the existence of a molecularly chemisorbed oxygen species on a Au/TiO2 model catalyst and a Au(111) single crystal following exposure of these samples to an oxygen plasma-jet molecular beam. We present evidence for the molecularly chemisorbed oxygen species from thermal desorption, collision-induced desorption, and heat of adsorption/reaction-induced desorption measurements. Thermal desorption measurements reveal a peak desorption temperature at approximately 145 K which corresponds to an activation energy for desorption of approximately 0.35 eV.

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Iron-monosulfide oxidation and associated S transformations in a natural sediment were examined by combining selective extractions, electron microscopy and S K-edge X-ray absorption near-edge structure (XANES) spectroscopy, The sediment examined in this study was collected from a waterway receiving acid-sulfate soil drainage. It contained a high acid-volatile sulfide content (1031 micromol g(-1)), reflecting an abundance of iron-monosulfide. The iron-monosulfide speciation in the initial sediment sample was dominated by nanocrystalline mackinawite (tetragonal FeS). At near-neutral pH and an 02 partial pressure of approximately 0.2 atm, the mackinawite was found to oxidize rapidly, with a half-time of 29 +/- 2 min. This oxidation rate did not differ significantly (P < 0.05) between abiotic versus biotic conditions, demonstrating that oxidation of nanocrystalline mackinawite was not microbially mediated. The extraction results suggested that elemental S (S8(0)) was a key intermediate S oxidation product Transmission electron microscopy showed the S8(0) to be amorphous nanoglobules, 100-200 nm in diameter. The quantitative importance of S8(0) was confirmed by linear combination XANES spectroscopy, after accounting for the inherent effect of the nanoscale S8(0) particle-size on the corresponding XANES spectrum. Both the selective extraction and XANES data showed that oxidation of S8(0) to SO4(2-) was mediated by microbial activity. In addition to directly revealing important S transformations, the XANES results support the accuracy of the selective extraction scheme employed here.

In this study we present results of an investigation into the reactivity of molecularly chemisorbed oxygen species on a Au/TiO2 model catalyst. We have previously shown that a Au/TiO2 model catalyst sample can be populated with both atomically and molecularly chemisorbed oxygen species following exposure to a radio frequency-generated oxygen plasma-jet. To test the reactivity of the molecularly chemisorbed oxygen species, we compare the CO2 produced from a sample that is populated with both oxygen species to the CO2 produced from a sample that has been given an identical exposure but has been cleared of molecularly chemisorbed oxygen employing collision-induced desorption. We observe that samples that are populated with both oxygen species consistently result in greater CO2 production. For the data presented in this paper, we observe a difference of 41% in the CO2 production. We interpret this result to indicate that molecularly chemisorbed oxygen can react directly with CO to form CO2.

C 2 H 4 ∕CO∕H 2 reaction is investigated on Rh∕SiO 2 model catalyst surfaces. Kinetic reactivity and infrared spectroscopic measurements are investigated as a function of Rh particle size under near atmospheric reaction conditions. Results show that propionaldehyde turnover frequency (TOF) (CO insertion pathway) exhibits a maximum activity near hd p i ¼ 2.5 nm. Polarization modulation infrared reflection absorption spectroscopy under CO and reaction (C 2 H 4 ∕CO∕H 2 ) conditions indicate the presence of Rh carbonyl species (RhðCOÞ 2 , Rh(CO)H) on small Rh particles, whereas larger particles appear resistant to dispersion and carbonyl formation. Combined these observations suggest the observed particle size dependence for propionaldehyde production via CO insertion is driven by two factors: (i) an increase in propionaldehyde formation on undercoordinated Rh sites and (ii) creation of carbonyl hydride species (Rh(CO)H)) on smaller Rh particles, whose presence correlates with the lower activity for propionaldehyde formation for hd p i < 2.5 nm.Ethylene hydroformylation | CO insertion | polarization modulation infrared reflection absorption spectroscopy | Rh/SiO2 A complicating effect of understanding the structure-activity relationships of heterogeneous catalytic reactions at ambient pressures (near 1 atm or above) is the known ability of reactant gas environments to alter the morphology, particle dispersion, and even the types of adsorbates present on certain supported nanoparticle (NP) systems (1-4). For example, elevated pressure CO ambients have been shown to oxidatively disrupt and disperse Rh nanoparticles, creating highly dispersed gem-dicarbonyl species RhðCOÞ 2 on oxide supports (1-3, 5-8). Under elevated pressure (CO∕H 2 ) and (CO 2 ∕H 2 ) hydrogenation reaction conditions, Rh(CO)H carbonyl hydride species can also be formed (3, 9-12). The creation and stability of such carbonyl species (RhðCOÞ 2 , Rh(CO)H) can depend on particle size, gas pressure, and surface temperature. The cumulative effect of such factors could potentially have an important effect on the overall catalytic properties of the supported Rh NP surface, especially for catalytic reactions involving surface bound CO. Developing an understanding of how such factors can affect the catalytic properties of supported Rh NPs is important from a fundamental surface chemistry perspective. Unraveling these details will require the ability to conduct spectroscopic and kinetic investigations of an informative probe reaction (i) under near atmospheric reaction conditions and (ii) on supported Rh nanoparticles surfaces with well defined initial particle size distributions.CO insertion into adsorbed alkyl groups (R-C x H y ) to form oxygenates (e.g., alcohols, aldehydes) is an important reaction step in many heterogeneous catalytic reactions. For example, C 2 H 4 hydroformylation (C 2 H 4 þ CO þ H 2 ) is a well known reaction for the synthesis of aldehydes via the CO insertion reaction (13). Insightful studies by Chuang and coworkers (14-17) and others (...

This paper presents results of an investigation of low-temperature CO oxidation and the role of moisture on an atomic oxygen covered Au(111) surface by employing molecular beam scattering techniques under ultrahigh vacuum (UHV) conditions. The effect of atomic oxygen precoverage on CO oxidation was examined at sample temperatures as low as 77 K. Prompt CO 2 production was observed when the CO beam impinges on the sample followed by a rapid decay of CO 2 production in all cases. At oxygen precoverages above 0.5 ML, CO 2 production decreases with increasing oxygen precoverage primarily due to the decrease in CO uptake. CO oxidation at 77 K goes through a precursor mediated reaction mechanism, where CO is in a precursor or trapped state and oxygen atoms are in a chemisorbed state. The role of adsorbed water was studied by using isotopically labeled water [H 218 O] to distinguish the oxygen species from that used in oxygen atom exposures [ 16 O]. Evidence is presented that shows activated water or OH groups formed from water can directly participate in oxidizing CO on an atomic oxygen covered Au(111) surface.