Low-temperature dissocation of O2 on Ag(110): Surface disorder and reconstruction

Are multiple oxygen species selective in ethylene epoxidation on silver?

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 in situ 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 (Oads) 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 Oads is present at very low O-coverages during in situ 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 Oads 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.

Molecular dynamics simulation of O2 adsorption on Ag(110) from first principles electronic structure calculations

State of the art simulations show that the physisorption state could be important for O₂/Ag(110) adsorption.

O(2) dissociation before the onset of added row nucleation on Ag(110): an atomistic scanning tunnelling microscopy view

We present here a scanning tunnelling microscopy (STM) study on O(2) adsorption at Ag(110) at T = 175 K, i.e. in the temperature range between the onset of O(2) dissociation and the formation of the added row reconstruction. In agreement with previous studies at lower surface coverage, we observe several structures forming upon O(2) dissociation, including Ag-O complexes randomly distributed on the surface. We suggest that the presence of the latter structures, characterized by a large cross section for low energy electrons, can account for the marked decrease of the surface electron reflectivity and for the corresponding increase of the diffuse elastic intensity previously reported in a HREELS investigation of the same system.

Dissociation of oxygen on Ag(100) induced by inelastic electron tunneling

Scanning tunneling microscopy (STM) is used to study the dissociation of molecular oxygen on Ag(100) induced by inelastic electron tunneling (IET) at 5 K. This dissociation is possible above 3.3 V with a yield of (3.63 ± 0.47) × 10(-9) per electron. Dissociation leads to three different types of hot atom motion: lateral motion, a cannon ball mechanism, and abstractive dissociation. Analysis of the I-t characteristics during dissociation suggests that the dissociation is proceeded by an adsorption site change.

Size-dependent surface reactions of Ag nanoparticles supported on highly oriented pyrolytic graphite

Various sizes of Ag particles were grown on highly oriented pyrolytic graphite (HOPG) surfaces, which had previously been modified with nanopits to act as anchoring sites. Surface reactions of O2, CHCl3, and CCl4 on the Ag particles and bulk Ag(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), and it has been shown that size dependence of O2 and CHCl3 reactions on Ag differs from that of CCl4. Weak reactions of O2 and CHCl3 were observed on the bulk Ag(111) surfaces, while strong reactions occur on Ag particles with medium Ag coverage, suggesting that the reactions are controlled by the number of surface defect sites. On the contrary, the dissociation of CCl4 is mainly determined by the exposed Ag facet area, mainly Ag(111) facet, and strong dissociation reaction happens on the bulk Ag(111) surface. The results suggest that the size effects, which are often discussed in heterogeneous catalysis, are strongly dependent on the reaction mechanism.

Chemisorption and dissociation of single oxygen molecules on Ag(110)

The chemisorption of single oxygen molecules on Ag110 and the dissociation of the adsorbed molecules induced by tunneling electrons were studied at 13 K using a variable-low-temperature scanning tunneling microscope. Two predominant types of chemisorbed O2 molecules were identified, one with the O2 molecular axis aligned along the [001] direction of the substrate [O2(001)], and the other with the molecular axis aligned along the [110] direction [O2(110)]. Tunneling of electrons between the scanning tunneling microscope tip and O2(001) caused the molecule either to rotate or dissociate, depending on the direction of electron tunneling. In contrast, electron tunneling caused O2(110) to dissociate regardless of tunneling direction. In addition to O2(001) and O2(110), several other oxygen species and their dynamical behaviors were observed.

State-resolved dynamics of oxygen atom recombination on polycrystalline Ag

Rotationally resolved, velocity distributions for desorbed O2 molecules formed by O-atom recombination on the surface of a polycrystalline Ag surface are reported. Surface O atoms are generated by oxygen permeation through a 0.25-mm-thick Ag foil heated to 1020 K. Desorbing O2 molecules are probed by (2 + 1) resonant multiphoton ionization via the C 3Pig (3ssigma), v' = 2 <-- <-- X 3Sigmag-, v" = 0 transition and time-of-flight mass spectrometry. Measured velocity distributions are near Maxwell-Boltzmann and yield average translational energies which are significantly lower than the surface temperature ([Et]/2kB approximately 515 K) and essentially independent of rotational excitation. Comparison of the observed C-X (2,0) resonantly enhanced multiphoton ionization spectrum with spectral simulations suggests that the v" = 0 rotational state distribution is more consistent with the surface temperature, but spectral congestion and apparent intensity perturbations prevent a more quantitative analysis. The calculated, sticking curves show a small barrier energy barrier (approximately 10 meV) beyond which sticking decreases. These observations are consistent with low energy desorption and adsorption pathways involving a weakly bound molecular O2 precursor.