The high catalytic activity of gold clusters on oxides has been attributed to structural effects (including particle thickness and shape and metal oxidation state), as well as to support effects. We ...have created well-ordered gold monolayers and bilayers that completely wet (cover) the oxide support, thus eliminating particle shape and direct support effects. High-resolution electron energy loss spectroscopy and carbon monoxide adsorption confirm that the gold atoms are bonded to titanium atoms. Kinetic measurements for the catalytic oxidation of carbon monoxide show that the gold bilayer structure is significantly more active (by more than an order of magnitude) than the monolayer.
In light of the many similarities between previous studies of the so-called strong metal support interaction (SMSI) involving Gr. VIII metals, and catalytically active Au, it is apparent that these ...two phenomena must be closely related. That active Au on titania spreads to form a bilayer structure, is electron-rich as determined by theory and experiment, nucleates on reduced Ti defects created by annealing to temperatures > 750 K, and is deactivated via sintering in oxygen, is convincing evidence that the same basic principles responsible for activation of Au on titania are operative for SMSI involving Gr. VIII metals.
Pd, Au, and Pd−Au mixtures were deposited via physical vapor deposition onto a Mo(110) substrate, and the surface concentration and morphology of the Pd−Au mixtures were determined by low-energy ion ...scattering spectroscopy (LEISS), infrared absorption spectroscopy (IRAS), temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). Pd−Au mixtures form a stable alloy between 700 and 1000 K with substantial enrichment in Au compared to the bulk composition. Annealing a 1:1 Pd−Au mixture at 800 K leads to the formation of a surface alloy with a composition Au0.8Pd0.2 where Pd is predominantly surrounded by Au. The surface concentration of this isolated Pd site can be systematically controlled by altering the bulk Pd−Au alloy concentration.
Au-based catalysts have great potential because of their unique activity and selectivity for a variety of important reactions. The special catalytic properties of supported Au nano-particles depend ...critically upon the particle morphology, i.e. size, shape and thickness, as well as support effects. This paper reviews the current understanding of CO oxidation on supported Au catalysts. The electronic structure of Au particles at various nucleation sites and on different supports is summarized, and the effect these changes have on catalytic performance is discussed. Recent results from our laboratories have demonstrated the synthesis of well-ordered Au mono- and bi-layer films on a titanium oxide support and show that the active Au structure for CO oxidation is an electron-rich, Au bi-layer. In contrast, the monolayer structure, which may involve the TiO
x
support, is significantly less active (by less than an order of magnitude) than the Au bi-layer. The oxidation state of the Au and how this relates to the catalytic activity are also discussed.
Gold clusters ranging in diameter from 1 to 6 nanometers have been prepared on single crystalline surfaces of titania in ultrahigh vacuum to investigate the unusual size dependence of the ...low-temperature catalytic oxidation of carbon monoxide. Scanning tunneling microscopy/spectroscopy (STM/STS) and elevated pressure reaction kinetics measurements show that the structure sensitivity of this reaction on gold clusters supported on titania is related to a quantum size effect with respect to the thickness of the gold islands; islands with two layers of gold are most effective for catalyzing the oxidation of carbon monoxide. These results suggest that supported clusters, in general, may have unusual catalytic properties as one dimension of the cluster becomes smaller than three atomic spacings.
CO oxidation on Pd(100), -(111), -(110), and Pt(110) single crystals was studied at steady-state conditions at low (≤2 × 10−3 Torr) and high (2−88 Torr) pressures at various reactant compositions. At ...low pressures the reaction fell into two regimes, one with a CO-dominant surface where the CO2 formation rate is low, and a second with an O-dominant surface where the reaction rate is high. Within this second regime, the reaction is collision-limited with no oxygen inhibition. Under high-pressure reaction conditions, three reaction regimes are evident: (i) a CO-inhibited metallic regime displaying a low CO2 formation rate; (ii) an oxygen-rich metallic regime with a high CO2 formation rate; and (iii) a high-temperature regime where the CO2 formation rate is either mass transfer limited on a metallic surface or limited by the reduced reactivity of the oxidized surface. The superior activity of Pt group metal oxides compared to the reduced metal, as proposed recently, was not observed in this study.
The EU-FP7-funded BioSHaRE project is using individual-level data pooled from several national cohort studies in Europe to investigate the relationship of road traffic noise and health. The detailed ...input data (land cover and traffic characteristics) required for noise exposure modelling are not always available over whole countries while data that are comparable in spatial resolution between different countries is needed for harmonised exposure assessment. Here, we assess the feasibility using the CNOSSOS-EU road traffic noise prediction model with coarser input data in terms of model performance. Starting with a model using the highest resolution datasets, we progressively introduced lower resolution data over five further model runs and compared noise level estimates to measurements. We conclude that a low resolution noise model should provide adequate performance for exposure ranking (Spearman's rank = 0.75; p < 0.001), but with relatively large errors in predicted noise levels (RMSE = 4.46 dB(A)).
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•The first implementation of CNOSSOS-EU for national scale noise exposure assessment.•Road traffic noise model performance with varying resolution of inputs is assessed.•Model performance is good with low resolution inputs (rs = 0.75).•This model will be applied in epidemiological studies of European cohorts.
The CNOSSOS-EU road traffic noise model estimates can be used for international scale exposure assessment when parameterised with freely available low resolution covering a large geographic area.
Sintering of supported metal catalysts is of primary interest in heterogeneous catalysis. The presence of reactant gases, especially oxidants, often accelerates the sintering of supported metal ...catalysts. Our in situ studies using scanning tunneling microscopy (STM) compare the sintering of Au particles supported on TiO2(110) in the presence of CO, O2, or mixtures of the two gases. The results reveal a synergetic effect of CO/O2 mixtures in accelerating the sintering of supported Au particles. By monitoring the morphological changes of individual Au particles, in situ studies allow investigation of the sintering kinetics of Au particles supported on TiO2(110) during CO oxidation. The results support a reactant-induced mechanism and provide a more complete kinetic description of the sintering of supported metal catalysts beyond a mean-field description.
In view of the stringent CO intolerance of the state-of-the-art proton exchange membrane (PEM) fuel cells, it is desirable to explore CO-free fuel processing alternatives. In recent years, step-wise ...reforming of hydrocarbons has been proposed for production of CO-free hydrogen for fuel cell applications. The decomposition of hydrocarbons (first step of the step-wise reforming process) has been extensively investigated. Both steam and air have been employed for catalyst regeneration in the second step of the process. Since, PEM is poisoned by very low (ppm) levels of CO, it is essential to eliminate even trace amounts of CO from the reformate stream. Preferential oxidation of CO (PROX) is considered to be a promising method for trace CO clean up. Related studies along with a discussion of catalytic ammonia decomposition (for applications in alkaline fuel cells) will be included in this review.
