Production of the industrial chemical propylene oxide is energy-intensive and environmentally unfriendly. Catalysts based on bulk silver surfaces with direct propylene epoxidation by molecular oxygen ...have not resolved these problems because of substantial formation of carbon dioxide. We found that unpromoted, size-selected Ag₃ clusters and approximately 3.5-nanometer Ag nanoparticles on alumina supports can catalyze this reaction with only a negligible amount of carbon dioxide formation and with high activity at low temperatures. Density functional calculations show that, relative to extended silver surfaces, oxidized silver trimers are more active and selective for epoxidation because of the open-shell nature of their electronic structure. The results suggest that new architectures based on ultrasmall silver particles may provide highly efficient catalysts for propylene epoxidation.
Iridium oxide based electrodes are among the most promising candidates for electrocatalyzing the oxygen evolution reaction, making it imperative to understand their chemical/electronic structure. ...However, the complexity of iridium oxide's electronic structure makes it particularly difficult to experimentally determine the chemical state of the active surface species. To achieve an accurate understanding of the electronic structure of iridium oxide surfaces, we have combined synchrotron-based X-ray photoemission and absorption spectroscopies with ab initio calculations. Our investigation reveals a pre-edge feature in the O K-edge of highly catalytically active X-ray amorphous iridium oxides that we have identified as O 2p hole states forming in conjunction with Ir(III). These electronic defects in the near-surface region of the anionic and cationic framework are likely critical for the enhanced activity of amorphous iridium oxides relative to their crystalline counterparts.
Alloying provides a means by which to tune a metal catalyst's electronic structure and thus tailor its performance; however, mean-field behaviour in metals imposes limits. To access unprecedented ...catalytic behaviour, materials must exhibit emergent properties that are not simply interpolations of the constituent components' properties. Here we show an emergent electronic structure in single-atom alloys, whereby weak wavefunction mixing between minority and majority elements results in a free-atom-like electronic structure on the minority element. This unusual electronic structure alters the minority element's adsorption properties such that the bonding with adsorbates resembles the bonding in molecular metal complexes. We demonstrate this phenomenon with AgCu alloys, dilute in Cu, where the Cu d states are nearly unperturbed from their free-atom state. In situ electron spectroscopy demonstrates that this unusual electronic structure persists in reaction conditions and exhibits a 0.1 eV smaller activation barrier than bulk Cu in methanol reforming. Theory predicts that several other dilute alloys exhibit this phenomenon, which offers a design approach that may lead to alloys with unprecedented catalytic properties.
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•TiO2 bulk defects are identified by in-situ electrical conductivity measurements.•Role of bulk defects on electronic metal-support interactions in Au/TiO2 catalysts.•CO adsorption ...strength on Au NPs is strongly reduced due to EMSIs.•The effect of EMSIs on CO oxidation is detrimental at 80°C, but promoting at −20°C.•The dominant reaction mechanism changes with temperature.
We report results of a comprehensive study on the effect of bulk defects on the catalytic behavior of Au/TiO2 catalysts in the CO oxidation reaction, combining quantitative information on the amount of surface and bulk defects from in situ non-contact electrical conductivity measurements after pretreatment and during reaction with information on the electronic/chemical state of the Au nanoparticles (NPs) provided by in situ IR spectroscopy. Treating the catalyst in strongly reducing atmosphere (10% CO/90% N2) at 400°C results in a distinct increase in electrical conductivity, indicative of the formation of defects (oxygen vacancies), which are stable at 80°C in N2. Long-term kinetic measurements performed at 80°C show a distinctly lower activity of the bulk reduced catalyst, which increases slowly with time on stream, directly correlated with the decreasing abundance of bulk defects. The detrimental effect of bulk defects on the CO oxidation activity is shown to originate from the lowered CO adsorption strength and hence very low COad coverage on the Au NPs due to electronic metal-support interactions (EMSIs) induced by the presence of TiO2 bulk defects, in good agreement with our recent proposal (Wang et al., ACS Catal. 7 (2017) 2339). For reaction at −20°C, EMSIs lead to a promoting effect on the CO oxidation, pointing to a change in the dominant reaction mechanism, away from the Au-assisted Mars-van Krevelen mechanism dominant at 80°C. The role of EMSIs in the CO oxidation reaction and its temperature dependence is discussed in detail.
Replacing noble metals in heterogeneous catalysts by low-cost substitutes has driven scientific and industrial research for more than 100 years. Cheap and ubiquitous iron is especially desirable, ...because it does not bear potential health risks like, for example, nickel. To purify the ethylene feed for the production of polyethylene, the semi-hydrogenation of acetylene is applied (80 × 10(6) tons per annum; refs 1-3). The presence of small and separated transition-metal atom ensembles (so-called site-isolation), and the suppression of hydride formation are beneficial for the catalytic performance. Iron catalysts necessitate at least 50 bar and 100 °C for the hydrogenation of unsaturated C-C bonds, showing only limited selectivity towards semi-hydrogenation. Recent innovation in catalytic semi-hydrogenation is based on computational screening of substitutional alloys to identify promising metal combinations using scaling functions and the experimental realization of the site-isolation concept employing structurally well-ordered and in situ stable intermetallic compounds of Ga with Pd (refs 15-19). The stability enables a knowledge-based development by assigning the observed catalytic properties to the crystal and electronic structures of the intermetallic compounds. Following this approach, we identified the low-cost and environmentally benign intermetallic compound Al(13)Fe(4) as an active and selective semi-hydrogenation catalyst. This knowledge-based development might prove applicable to a wide range of heterogeneously catalysed reactions.
The morphology, crystallinity, and chemical state of well-defined Ir oxide nanoscale thin-film catalysts prepared on Ti substrates at various calcination temperatures were investigated. Special ...emphasis was placed on the calcination temperature-dependent interaction between Ir oxide film and Ti substrate and its impact on the electrocatalytic oxygen evolution reaction (OER) activity. The Ir oxide films were characterized by scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and cyclic voltammetry. Furthermore, temperature programmed reduction was applied to study the Ir oxide species formed as a function of calcination temperature and its interaction with the Ti substrate. A previously unachieved correlation between the electrocatalytic OER activity and the nature and structural properties of the Ir oxide film was established. We find that the crystalline high temperature Ir oxide species is detrimental, whereas low temperature amorphous Ir oxy-hydroxides are highly active and efficient catalysts for the OER. Moreover, at the highest applied calcination temperature (550°C), Ti oxides, originating from the substrate, strongly affect chemical state and electrocatalytic OER activity of the Ir oxide film.
The electrocatalytic oxidation of small organic molecules is of general importance for energy-related issues such as the fuel cells and electrochemical re-formation. The common emergence of ...current/potential oscillations in these reactions has implications on mechanistic aspects as well as on the overall conversion, and thus on the performance of practical devices. We investigate in this paper some general features of the electro-oxidation of formaldehyde, formic acid, methanol, and ethanol on platinum and in acidic media, with emphasis on the comparison of the activity under conventional and oscillatory regimes. The comparison is carried out by different means and generalized by the use of identical experimental conditions in all cases. In all four systems studied, the occurrence of potential oscillations is associated with excursions of the electrode potentials to lower values, which noticeably decreases the overpotential of the anodic reaction, when compared to that in the absence of oscillations. Quantitatively speaking, a 2-fold enhancement in the power density was observed in an idealized fuel cell operated with formaldehyde. This aspect, together with spontaneous self-cleaning processes, presents important advantages to the use of autonomous oscillations to reach both higher and long-term activities. Finally, some mechanistic aspects of the studied reactions are also discussed.
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