The nature of the oxygen species active in ethylene epoxidation is a long-standing question. While the structure of the oxygen species that participates in total oxidation (nucleophilic oxygen) is ...known the atomic structure of the selective species (electrophilic oxygen) is still debated. Here, we use both
and UHV X-ray Photoelectron Spectroscopy (XPS) to study the interaction of oxygen with a silver surface. We show experimental evidence that the unreconstructed adsorbed atomic oxygen (O
) often argued to be active in epoxidation has a binding energy (BE) ≤ 528 eV, showing a core-level shift to lower BE with respect to the O-reconstructions, as previously predicted by DFT. Thus, contrary to the frequent assignment, adsorbed atomic oxygen cannot account for the electrophilic oxygen species with an O 1s BE of 530-531 eV, thought to be the active species in ethylene epoxidation. Moreover, we show that O
is present at very low O-coverages during
XPS measurements and that it can be obtained at slightly higher coverages in UHV at low temperature. DFT calculations support that only low coverages of O
are stable. The highly reactive species is titrated by background gases even at low temperature in UHV conditions. Our findings suggest that at least two different species could participate in the partial oxidation of ethylene on silver.
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•The oxidation of ethanol over a monolayer V2O5/TiO2 catalyst was examined.•The reaction involves reversible reduction of V5+ cations, whereas titanium cations remain in the Ti4+ ...state.•At 100–150°C: acetaldehyde is the main product; at higher temperatures: the reaction shifts toward acetic acid.•The oxidation of ethanol to acetaldehyde proceeds via the classical Mars–van Krevelen mechanism.•The production of acetic acid from acetate species proceeds only after the partial oxidation of the catalyst.
The selective oxidation of ethanol to acetaldehyde and acetic acid over a monolayer V2O5/TiO2 catalyst has been studied in situ using Fourier transform infrared spectroscopy and near-ambient-pressure X-ray photoelectron spectroscopy (XPS) at temperatures ranging from 100 to 300°C. The data were complemented with temperature-programmed reaction spectroscopy and kinetic measurements. It was found that under atmospheric pressure at low temperatures acetaldehyde is the major product formed with the selectivity of almost 100%. At higher temperatures, the reaction shifts toward acetic acid, and at 200°C, its selectivity reaches 60%. Above 250°C, unselective oxidation to CO and CO2 becomes the dominant reaction. Infrared spectroscopy indicated that during the reaction at 100°C, nondissociatively adsorbed molecules of ethanol, ethoxide species, and adsorbed acetaldehyde are on the catalyst surface, while at higher temperatures the surface is mainly covered with acetate species. According to the XPS data, titanium cations remain in the Ti4+ state, whereas V5+ cations undergo reversible reduction under reaction conditions. The presented data agree with the assumption that the selective oxidation of ethanol over vanadium oxide catalysts occurs at the redox Vn+ sites via a redox mechanism involving the surface lattice oxygen species. A reaction scheme for the oxidation of ethanol over monolayer V2O5/TiO2 catalysts is suggested.
Rhenium substantially promotes the rate of Pt-catalyzed glycerol hydrogenolysis to propanediols and shifts the product selectivity from 1,2-propanediol to a mixture of 1,2 and 1,3-propanediols. This ...work presents experimental evidence for a tandem dehydration–hydrogenation mechanism that occurs over a bifunctional Pt–Re catalyst. Infrared spectroscopy of adsorbed pyridine and the rate of aqueous-phase hydrolysis of propyl acetate were used to identify and quantify Brønsted acid sites associated with the Re component. Near-ambient-pressure XPS revealed a range of Re oxidation states on the Pt–Re catalysts after reduction in H2 at 393 and 493 K, which accounts for the presence of Brønsted acidity. A mechanism involving acid-catalyzed dehydration followed by Pt-catalyzed hydrogenation was consistent with the negative influence of added base, a primary kinetic isotope effect with deuterated glycerol, an inverse isotope effect with dideuterium gas, and the observed orders of reaction.
Gold nanoparticles on transition-metal oxides were synthesized by two different methods: precipitation and photoinduced decomposition of an intermediate gold–azido complex. Only samples prepared by ...the precipitation method showed significant CO conversion at low temperature. XPS shows the formation of two Au species (Au0 and Auδ+) on the surface of active Au/TiO2 and Au/Fe2O3 samples. The energy shift of the Auδ+ peak depends on the support and is 0.6 and 0.9 eV for Au/TiO2 and Au/Fe2O3, respectively. TEM images indicate the formation of overlayer on Au particles. These results prove Au activation via a strong metal–support interaction, on the basis of the strong influence of the support on the electronic structure of the gold through charge transfer and stabilization of low-coordinated Au atoms.
Silver’s unique ability to selectively oxidize ethylene to ethylene oxide under an oxygen atmosphere has long been known. Today it is the foundation of ethylene oxide manufacturing. Yet, the ...mechanism of selective epoxide production is unknown. Here we use a combination of ultrahigh vacuum and in situ experimental methods along with theory to show that the only species that has been shown to produce ethylene oxide, the so-called electrophilic oxygen appearing at 530.2 eV in the O 1s spectrum, is the oxygen in adsorbed SO4. This adsorbate is part of a 2D Ag/SO4 phase, where the nonstoichiometric surface variant, with a formally S(V+) species, facilitates selective transfer of an oxygen atom to ethylene. Our results demonstrate the significant and surprising impact of a trace impurity on a well-studied heterogeneously catalyzed reaction.
Adsorbed hydroxyl is a key intermediate present in many catalytic reactions and electrochemical processes. In particular, hydroxyl adsorbed on noble metal surfaces has attracted attention due to its ...role in water-gas shift, selective oxidation of hydrocarbons and water splitting. In this work, from a well-defined oxygen covered Ag(110) surface with O-
p
(2 × 1) reconstruction, we prepared a fully hydroxylated surface phase in equilibrium with water and oxygen in the gas phase under near ambient conditions.
In situ
soft X-ray spectroscopy combined with density functional theory revealed distinctive modifications in the electronic structure of the adsorbate layer upon hydroxylation. We show that both the core and valence electronic states of OH adsorbates have higher binding energies relative to the Fermi level than the states for the O adsorbate. The OH orbitals interact with the d band of Ag giving rise to hybridized orbitals with bonding and anti-bonding symmetry, with larger energy splitting than the oxygen adsorbate.
Electronic structure and bonding configuration of surface hydroxyl probed by
in situ
X-ray spectroscopy combined with DFT.
Common methods to produce supported catalysts include impregnation, precipitation, and thermal spray techniques. Supported electrocatalysts produced by a novel method for thermal spray deposition ...were investigated with respect to their structural properties, elemental composition, and electrochemical performance. This was done using electron microscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry. Various shapes and sizes of catalyst particles were found. The materials exhibit different activity towards oxidation and reduction of Fe. The results show that this preparation method enables the selection of particle coverage as well as size and shape of the catalyst material. Due to the great variability of support and catalyst materials accessible with this technique, this approach is a useful extension to other preparation methods for electrocatalysts.
A single‐step rapid hydrothermal low‐temperature process for the formation of high purity polycrystalline Ag‐based delafossite oxides 3R‐AgBO2 (B: Al, Ga, In) is reported. For the synthesis process ...reusable and widely available PTFE‐lined pressure vessels are used. The presence of an elemental Ag by‐phase is traced back to the metastability of the Ag‐delafossite under the reaction conditions. High‐purity products were obtained by decreasing the synthesis time requirements by up to 90 %. The effect of the isoelectronic B‐site atoms on the structure and bonding situation were comparatively studied experimentally and by ab‐initio calculations. Distorted coordination environments are observed in all cases, and the nature of the chemical bond changes qualitatively along the series of group 13 B‐site atoms Al, Ga, and In. This is reflected in systematic changes of the (optical) band gap, the contactless conductivity, the thermal stability, and the energetic position of Raman modes, which are strongly affected and decrease with increasing atomic number. The establishment of a facile synthesis strategy allows the application of these Ag‐delafossite systems in many disciplines, i.e. in heterogeneous catalysis and as optoelectronic devices.
Ag‐Delafossites AgBO2 (B: Al, Ga, In) were obtained by a facile and scalable hydrothermal low‐temperature synthesis strategy decreasing the synthesis time by up to 90 %. The selected B‐site atom strongly alters the properties of the materials, such as band gap, conductivity, and thermal stability, as evidenced by experimental observations and ab‐initio calculations.
Invited for the cover of this issue are Dr. Elias Frei and co‐workers from the Inorganic Chemistry Department of the Fritz‐Haber‐Institute of the Max‐Planck‐Gesellschaft, Berlin, and Dr. Frank ...Rosowski from BasCat/BASF, Ludwigshafen, Germany. The cover image shows three different hourglasses underlining the importance of the reaction time for the hydrothermal preparation of the metastable delafossites AgBO2 (B: Al, Ga, In).
Now, with a time‐saving and facile synthesis procedure at hand, the field of Ag‐delafossites might attract a broader audience …
Read more about the story behind the cover in the Cover Profile and about the research itself in the Full Paper by E. Frei et al.
We address the question of the nature of Au NP activation and through a combination of experimental and theoretical techniques. In‐situ XPS measurements of Au/TiO2 during CO oxidation show high ...catalytic activity can be associated with the formation of an ionic Au species. DFT calculations performed on Au/TiO2 show that the formation of such ionic Au is due to a strong metal‐support interaction between Au and reduced and defective TiO2. TEM supports these findings, indicating the formation of an overlayer of transition metal oxide support on Au NPs after CO oxidation. These results suggest TiO2 lattice oxygen is involved directly in CO oxidation, which was confirmed with labeled 18O2 experiments.
Only a supporting role? To address the question over the nature of Au activation XPS measurements, DFT calculation, mass‐spectroscopy and TEM were performed. The results show the formation of an ionic Au species during CO oxidation. DFT calculations show that oxidized TiO2 and Au interact weakly, whereas reduced and defective TiO2 show a strong metal‐support interaction. Mass‐spectroscopy confirms CO oxidation on supported Au via a Mars–van Krevelen like mechanism.
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