Specifics of the desorption of oxygen from a silver surface promoted by phosphates

The interaction of oxygen with the surface of a polycrystalline silver catalyst promoted with phosphorus-containing compounds was studied by the temperature-programmed desorption method. It was demonstrated that the introduction of a phosphorus-containing promoter gives rise to the formation of predominantly high-temperature forms of oxygen, which are more strongly bonded to the catalyst surface. The chemical composition of the active silver surface at various stages of its evolution was determined by IR spectroscopy. Under the action of temperature and a reducer, the polyphosphate promoter layer was observed to produce metal silver particles, which participate in the formation of active sites at the catalyst surface.     

Transition state dynamics of the OH + H2O hydrogen exchange reaction studied by dissociative photodetachment of H3O2-

Dynamics in the transition state region of the bimolecular OH + H2O-->H2O + OH hydrogen exchange reaction have been studied by photoelectron-photofragment coincidence spectroscopy of the H3O2- negative ion and its deuterated analog D3O2-. The data reveal vibrationally resolved product translational energy distributions. The total translational energy distribution shows a vibrational progression indicating excitation of the antisymmetric stretch of the water product. Electronic structure calculations at the QCISD level of theory support this analysis. Examination of the translational energy release between the neutral products reveals a dependence on the product vibrational state. These data should provide a critical test of ab initio potential energy surfaces and dynamics calculations.     

Determination of silver carbonate in silver oxide(Ag2O) by infrared spectroscopy

Silver carbonate in silver oxide (Ag(2)O) is determined quantitatively by infrared spectroscopy. The deviation of the results from a mean line is +/-0.2% of Ag(2)CO(3) for samples containing only normal silver carbonate, and +/-0.4% of Ag(2)CO(3) for samples containing both normal and basic silver carbonate.     

Ultrafast vectorial and scalar dynamics of ionic clusters: azobenzene solvated by oxygen

The ultrafast dynamics of clusters of trans-azobenzene anion (A-) solvated by oxygen molecules was investigated using femtosecond time-resolved photoelectron spectroscopy. The time scale for stripping off all oxygen molecules from A- was determined by monitoring in real time the transient of the A- rise, following an 800 nm excitation of A- (O2)n, where n = 1-4. A careful analysis of the time-dependent photoelectron spectra strongly suggests that for n > 1 a quasi-O4 core is formed and that the dissociation occurs by a bond cleavage between A- and conglomerated (O2)n rather than a stepwise evaporation of O2. With time and energy resolutions, we were able to capture the photoelectron signatures of transient species which instantaneously rise (<100 fs) then decay. The transient species are assigned as charge-transfer complexes: A.O2- for A- O2 and A.O4-(O2)n-2 for A-(O2)n, where n = 2-4. Subsequent to an ultrafast electron recombination, A- rises with two distinct time scales: a subpicosecond component reflecting a direct bond rupture of the A- -(O2)n nuclear coordinate and a slower component (1.6-36 ps, increasing with n) attributed to an indirect channel exhibiting a quasistatistical behavior. The photodetachment transients exhibit a change in the transition dipole direction as a function of time delay. Rotational dephasing occurs on a time scale of 2-3 ps, with a change in the sign of the transient anisotropy between A- O2 and the larger clusters. This behavior is a key indicator of an evolving cluster structure and is successfully modeled by calculations based on the structures and inertial motion of the parent clusters.     

CuO and Ag2O/CuO catalyzed oxidation of aldehydes to the corresponding carboxylic acids by molecular oxygen

Furfural was oxidized to furoic acid by molecular oxygen under catalysis by 150 nm-sized Ag(2)O/CuO (92%) or simply CuO (86.6%). When 30 nm-size catalyst was used,the main product was a furfural Diels-Alder adduct. Detailed reaction conditions and regeneration of catalysts were investigated. Under optimal conditions, a series of aromatic and aliphatic aldehydes were oxidized to the corresponding acids in good yields.     

Electrochemically induced reversible solid state transformations: electrosynthesis of Ag2Cu2O4 by room temperature oxidation of Ag2Cu2O3

Electrochemical oxidation at room temperature of a slurry of Ag₂Cu₂O₃ yields a new silver copper oxide, formulated as Ag₂Cu₂O₄, with one more atom of oxygen per unit formula, that can in turn revert to the original precursor. The resulting oxide presents a different electronic and crystal structure from its precursor, as shown by XPS, X-ray and electron diffraction. This phase transformation involves a radical structural change from a 3D to a 2D network, as well as electronic changes involving silver and oxygen. The potential of electrochemical techniques to induce crystal-chemical solid state transformations is analyzed.     

Partial stripping of Ag atoms from silver bilayer on a Au(111) surface accompanied with the reductive desorption of hexanethiol SAM

The interfacial structures of Ag bilayer prepared by underpotential deposition on Au(111) (Ag(2ML)/Au(111)) were determined by ex situ scanning tunneling microscopy and in situ surface X-ray scattering measurements before and after oxidative adsorption and after reductive desorption of a self-assembled monolayer (SAM) of hexanethiol (C₆SH) in alkaline ethanol solution. While no structural change was observed after oxidative formation of C₆SH SAM on the Ag(2ML)/Au(111) in an ethanol solution containing 20 mM KOH and 0.1 mM C₆SH, some of the Ag atoms in the bilayer were stripped when the SAM was reductively desorbed. Dedicated to Professor J. O’M. Bockris on the occasion of his 85th birthday.     

Structure and dynamics of water at a clay surface from molecular dynamics simulation

We report a molecular dynamics study of the structure and dynamics of water at a clay surface. The negative charge of the surface and the presence of surface oxygen atoms perturbs water over two to three molecular layers, while the nature of the counterions (Na(+)or Cs(+)) has only a small effect. In the first molecular layer, approximately half of the water molecules are H-bonded to the surface. We also analyze the H-bond network between surface water molecules. The diffusion of water molecules along the surface is slowed down compared to the bulk case. As far as the orientational order and dynamics of the water dipole are concerned, only the component normal to the clay surface is perturbed. We investigate the surface H-bond formation and dissociation dynamics and their coupling to the release of molecules from the first molecular layer. We introduce a simple kinetic model in the spirit of Luzar and Chandler [Nature, 1996, 379, 55] to allow for a comparison with bulk water dynamics. This model semi-quantitatively reproduces the molecular simulation results and suggests that H-bond formation is faster with the surface than in the bulk, while H-bond dissociation is slower.     

Ultrafast O2 activation by copper oxide for 2,4-dichlorophenol degradation: The size-dependent surface reactivity

This study demonstrated interesting ultrafast activation of molecular O₂ by copper oxide (CuO) particles and very rapid elimination of aqueous 2,4-dichlorophenol (2,4-DCP) within reaction time of 30 s. Electron paramagnetic resonance (EPR) characterization indicated that OH, Cu³⁺, ¹O₂ and O₂⁻ were generated in the CuO/O₂ systems, wherein O₂⁻ would be the main reactive species responsible for 2,4-DCP degradation. It was further found that the catalytic ability of CuO for O₂ activation was highly size dependent and nano-CuO was far reactive than micro-CuO. H₂ temperature-programmed reduction (H₂-TPR), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer (VSM) analyses revealed that both the quantity and the reactivity of the surface reaction sites (surface Cu⁺ and O₂) could determine the catalytic ability of CuO affecting efficient Cu⁺-based molecular oxygen activation. Moreover, the O₂ activation ability of CuO would depend on not only the dimension, but also crystalline factors, for example, the exposed facets.     

Ultrafast dynamics of the azobenzene-coumarin complex: investigation of cooling dynamics measured by an integrated molecular thermometer

The energy dissipation mechanism from photoexcited azobenzene (Az) was studied by femtosecond time-resolved UV absorption spectroscopy using 7-amino-4-trifluoromethylcoumarin (ATC) as a probe. The distance between the probe molecule and Az was fixed by covalently linking them together through a rigid proline spacer. Picosecond dynamics in THF solutions were studied upon excitation into the S1 state by a 100 fs laser pulse at 480 nm. Transient absorption spectra obtained for Az-Pro-ATC combined the S1 state absorption and vibrationally excited ground-state absorption of ATC. Correction of the transient spectrum of Az-Pro-ATC for the S1 absorption provided the time-resolved absorption spectrum of the ATC hot band. Three major components were observed in the transient kinetics of Az-Pro-ATC vibrational cooling. It is proposed that in ca. 0.25 ps after the excitation, the S1 state of azobenzene decays to form an initial vibrationally excited nonthermalized ground state of Az-Pro-ATC that involves vibrational modes of both azobenzene and coumarin. This hot ground state decays in ca. 0.32 ps to the next, vibrationally equilibrated, transient state by redistributing the energy within the molecule. Subsequently, the latter state cools by transferring its energy to the closest solvent molecules in ca. 5 ps; then, the energy diffuses to the bulk solvent in 13 ps.     

Reactions of hydrated electrons (H2O)n- with carbon dioxide and molecular oxygen: hydration of the CO2- and O2- ions

The gas-phase reactions of hydrated electrons with carbon dioxide and molecular oxygen were studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Both CO2 and O2 react efficiently with (H2O)n- because they possess low-lying empty pi* orbitals. The molecular CO2- and O2- anions are concurrently solvated and stabilized by the water ligands to form CO2(-)(H2O)n and O2(-)(H2O)n. Core exchange reactions are also observed, in which CO2(-)(H2O)n is transformed into O2(-)(H2O)n upon collision with O2. This is in agreement with the prediction based on density functional theory calculations that O2(-)(H2O)n clusters are thermodynamically favored with respect to CO2(-)(H2O)n. Electron detachment from the product species is only observed for CO2(-)(H2O)2, in agreement with the calculated electron affinities and solvation energies.     

Ag4V2O6F2: an electrochemically active and high silver density phase

Low-temperature hydrothermal techniques were used to synthesize single crystals of Ag(4)V(2)O(6)F(2). This previously unreported oxide fluoride phase was characterized by single-crystal X-ray diffraction and IR spectroscopy and was also evaluated as a primary lithium battery cathode. Crystal data: monoclinic, space group P2(1)/n (No. 14), with a = 8.4034(4) A, b = 10.548(1) A, c = 12.459(1) A, beta = 90.314(2) degrees , and Z = 4. Ag(4)V(2)O(6)F(2) (SVOF) exhibits two characteristic regions within the discharge curve, an upper plateau at 3.5 V, and a lower sloped region around 2.3 V from reduction of the vanadium oxide fluoride framework. The material has a nominal capacity of 251 mAh/g, with 148 mAh/g above 3 V. The upper discharge plateau at 3.5 V is nearly 300 mV over the silver reduction potential of the commercial primary battery material, Ag(2)V(4)O(11) (SVO).     

Phase transition of a DPPC bilayer induced by an external surface pressure: from bilayer to monolayer behavior. a molecular dynamics simulation study

Understanding the lipid phase transition of lipid bilayers is of great interest from biophysical, physicochemical, and technological points of view. With the aim of elucidating the structural changes that take place in a DPPC phospholipid bilayer induced by an external isotropic surface pressure, five computer simulations were carried out in a range from 0.1 to 40 mN/m. Molecular dynamics simulations provided insight into the structural changes that took place in the lipid structure. It was seen that low pressures ranging from 0.1 to 1 mN/m had hardly any effect on the structure, electrical properties, or hydration of the lipid bilayer. However, for pressures above 40 mN/m, there was a sharp change in the lipid-lipid interactions, hydrocarbon lipid fluidity, and electrostatic potential, corresponding to the mesomorphic transition from a liquid crystalline state (L(alpha)) to its gel state (P'(beta)). The head lipid orientation remained almost unaltered, parallel to the lipid layer, as the surface pressure was increased, although a noticeable change in its angular distribution function was evident with the phase transition.     

Kinetics of the thermal desorption of O2, HCl,and H2O from the surface of cobalt chromite

A study has been carried out on the kinetics of the temperature-programmed desorption of O₂, HCl, and H₂O from the surface of cobalt chromite. A mathematic model was obtained for the kinetics of the thermal desorption and a method was developed for determining the kinetic constants. The kinetic parameters of the desorption of O₂, HCl, and H₂O were calculated using these experimental data.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 5, pp. 449–455, September–October, 1993.     

Reaction of metallotetraphenylporphyrins on hydroxyl-modified silver colloid and Ag2O colloid by surface-enhanced Raman scattering.

This paper reports a novel reaction of metallotetraphenylporphyrins on hydroxyl-modified silver colloid and Ag2O colloid. Surface-enhanced Raman spectra of Ag(II) and Cu(II) complexes of tetraphenylporphyrin (TPP) adsorbed on the hydroxyl-modified Ag colloid and Ag2O colloid have been studied. The time-dependent SERS spectra of MTPP (M = Ag, Cu) on hydroxyl-modified Ag colloid were recorded and dramatic change on SERS spectra was observed. The final spectra were found to be strikingly different from the corresponding normal Raman spectra (NRS), with the appearance of new Raman bands at 1614. 1417, 947, 674 and 292 cm(-1). The UV-visible absorption spectrum of MTPP on hydroxyl-modified Ag colloid exhibits a broad shoulder near 460 nm. Similar spectral phenomena were also observed for AgTPP and CuTPP adsorbed on Ag2O colloid. The observed spectral alterations were ascribed to new species formation due to the irreducible oxidation of MTPP on the colloids.     

A quantitative colorimetric assay of H2O2 and glucose using silver nanoparticles induced by H2O2 and UV

A simple spectrophotometric assay of H2O2 and glucose using Ag nanoparticles has been carried out. Relying on the synergistic effect of H2O2 reduction and ultraviolet （UV） irradiation, Ag nanoparticles with enhanced absorption signals were synthesized. H2O2 served as a reducing agent in the Ag nanoparticles formation in which Ag＋ was reduced to Ago by O2- generated via the decomposition of H2O2 in alkaline media. On the other hand, photoreduction of Ag＋ to Ago under UV irradiations also contributed to the nanoparticles formation. The synthesized nanoparticles were characterized by TEM, XPS, and XRD. The proposed method could determine H2O2 with concentrations ranging from 5.0× 10^-7 to 6.0× 10^-5 tool/ L The detection limit was estimated to be 2.0 × 10^-7 mol/L. Since the conversion of glucose to gluconic acid catalyzed by glucose oxidase was companied with the formation of H2O2, the sensing protocol has been successfully utilized for the determination of glucose in human blood samples. The results were in good agreement with those determined by a local hospital. This colorimetric sensor thus holds great promises in clinical applications.     

Collisional dynamics of Ag19 on Pd(100): a molecular dynamics study

By means of molecular dynamics simulations based on realistic n-body potentials we investigate structural and dynamical features inherent to the energetic collision of a silver cluster (Ag₁₉) on the Pd(100) substrate. Both the system and the impact energy (E_i = 95 eV) adopted have been chosen to parallel an experimental study of size selected Ag cluster deposition on Pd(100). Our results indicate that the experimental cross section obtained via thermal energy atom scattering at the same collision energy is well reproduced by the simulations.The modeling allows to rationalize the collision outcome in terms of defect production and cluster atoms implantation. The adsorbed structures have an heterogenous nature and are mostly two-dimensional.     

Formation mechanism of nanostructured Ag films from Ag2O particles using a sonoprocess

Nanostructured Ag films composed of nanoparticles and nanorods can be formed by the ultrasonication of ethanol solutions containing Ag₂O particles. The present work examined the formation process of these films from ethanol solutions by two different agitation methods, including ultrasonication and mechanical stirring. The mass-transfer process from Ag₂O particles to ethanol solvent is accelerated by the mechanical effects of ultrasound. Ag⁺ ions and intermediately reduced Ag clusters were released into the ethanol. These Ag⁺ ions and Ag clusters provide absorption bands at 210, 275 and 300 nm in UV-vis spectra. These bands were assigned to the absorption of Ag⁺, Ag ₄ ²⁺ and Ag_n (n?≈?3). The Ag_n clusters that readily grow to become Ag nanoparticles were formed due to the surface reaction of Ag₂O particles with ethanol under ultrasonication. The reactions of Ag⁺ ions in ethanol to form Ag nanomaterials (through the formation of Ag ₄ ²⁺ clusters) were also accelerated by ultrasonication.     

Charge-transfer-to-solvent-driven dissolution dynamics of I- (H2O)2-5 upon excitation: excited-state ab initio molecular dynamics simulations

In contrast to the extensive theoretical investigation of the solvation phenomena, the dissolution phenomena have hardly been investigated theoretically. Upon the excitation of hydrated halides, which are important substances in atmospheric chemistry, an excess electron transfers from the anionic precursor (halide anion) to the solvent and is stabilized by the water cluster. This results in the dissociation of hydrated halides into halide radicals and electron-water clusters. Here we demonstrate the charge-transfer-to-solvent (CTTS)-driven femtosecond-scale dissolution dynamics for I-(H2O)n=2-5 clusters using excited state (ES) ab initio molecular dynamics (AIMD) simulations employing the complete-active-space self-consistent-field (CASSCF) method. This study shows that after the iodine radical is released from I-(H2O)n=2-5, a simple population decay is observed for small clusters (2 相似文献   

H2O2-mediated oxidation of zero-valent silver and resultant interactions among silver nanoparticles, silver ions, and reactive oxygen species

The H(2)O(2)-mediated oxidation of silver nanoparticles (AgNPs) over a range of pH (3.0-14.0) is investigated here, and an electron charging-discharging model capable of describing the experimental results obtained is developed. AgNPs initially react with H(2)O(2) to form Ag(+) and superoxide, with these products subsequently reacting to reform AgNPs (in-situ-formed AgNPs) via an electron charging-discharging mechanism. Our experimental results show that the AgNP reactivity toward H(2)O(2) varies significantly with pH, with the variation at high pH (>10) due particularly to the differences in the reactivity of H(2)O(2) and its conjugate base HO(2)(-) with AgNPs whereas at lower pH (3-10) the pH dependence of H(2)O(2) decay is accounted for, at least in part, by the pH dependence of the rate of superoxide disproportionation. Our results further demonstrate that the in-situ-formed AgNPs resulting from the superoxide-mediated reduction of Ag(+) have a different size and reactivity compared to those of the citrate-stabilized particles initially present. The turnover frequency for AgNPs varies significantly with pH and is as high as 1776.0 min(-1) at pH 11.0, reducing to 144.2 min(-1) at pH 10.0 and 3.2 min(-1) at pH 3.0.