The NH3 + O2 reaction on a Pt(533) surface has been studied in the 10−4 mbar range and close to 1 mbar pressure with in situ X-ray photoelectron spectroscopy using synchrotron radiation. The ...coverages of the various O- and N-containing surface species have been followed in T-cycling experiments with varying mixing ratios O2/NH3 and varying total pressure. In heating/cooling cycles hysteresis of ∼50−100 K width occur. Adsorbed NOad already decomposes at T > 350 K. Under stationary conditions, no adsorbed NO could be detected. At no time during the experiments were Pt bulk oxides formed. A shift in the surface core level component of the Pt 4f spectrum by more than 0.5 eV toward higher binding energy is attributed to Pt atoms of the (100) step edges which are coordinated to more than one oxygen atom similar to the model proposed by Wang et al. Phys. Rev. Lett. 2005, 95, 256102.
SBA15–TiO
2 samples prepared by introducing titanium with a grafting method and having TiO
2 loadings below 15
wt.% have been characterized by XRF, XRD, IR, porosimetry, SEM, HRTEM, and UV–Visible ...diffuse reflectance. Differently from the samples reported in the literature characterized by a high TiO
2 loading, no evidences have been found for the presence of titania particles inside or outside the mesopores of SBA-15. Three different titanium species were instead evidenced to be present. The first two derive from the reaction of titanium with silanol groups in the corona area of inner SBA-15 walls leading to the formation of either TiO
4 tetrahedral sites (by reaction by hydroxyl nests of surface defect sites) and/or pseudo-octahedral surface sites anchored by two (or more) Si or Ti ions through bridging oxygens. The third species derives from the reaction of titanium in the regions with high sylanol density, e.g. in the micropores located in the corona of SBA-15 channels, leading to the formation of TiO
2-like nanoareas (probably Si-doped) with dimensions of around 1–2
nm maximum. The potential interest of these materials as photocatalysts, for the presence of a TiO
2-like nanoareas highly accessible by reactants, is discussed.
A novel type of temporal and spatial self-organization in a heterogeneous catalytic reaction is described for the first time. Using in situ x-ray photoelectron spectroscopy, gas chromatography, and ...mass spectrometry, we show that, under certain conditions, self-sustained reaction-rate oscillations arise in the oxidation of propane over Ni foil because of reversible bulk oxidation of Ni to NiO, which can be observed even with the naked eye as chemical waves propagating over the catalyst surface.
Simplified IrO x electrodes lacking typical mud crack structure have been produced on polycrystalline Ti cylinders with spin-coating using an iridium acetate solution and are compared to thicker ...samples in terms of stability, composition, and suitability as a model system. The spin-coating process forms smooth, thin islands of IrO x with limited cracking and decreases the surface-to-bulk ratio to allow a more intimate study of the growth, composition, and stability of the layer without the complications of the mud crack morphology. XPS and XRD measurements show a resulting (Ir,Ti)O x surface (x near 2) with OH– groups and H2O. Cyclic voltammetry measurements indicate the expected high catalytic activity for the oxygen evolution reaction as well as a dry IrO x phase resulting from the thermal manufacturing process, although evidence of hydrous phases are found in XPS. Both films required only small overpotentials for the oxygen evolution reaction, with the spin-coated sample showing a slightly lower activity. CO temperature desorption spectroscopy analysis showed CO → CO2 oxidation and, in combination with XPS, an unstable surface. The oxidation of CO was not due to the TiO x , and the absence of any evidence of an Ir-suboxide phase indicates the presence of near-surface active species present after synthesis or an active surface termination.
The locally-resolved reaction kinetics of CO oxidation on individual (100)-type grains of a polycrystalline Pt foil was monitored in situ using photoemission electron microscopy (PEEM). ...Reaction-induced surface morphology changes were studied by optical differential interference contrast microscopy and atomic force microscopy (AFM). Regions of high catalytic activity, low activity and bistability in a (p,T)-parameter space were determined, allowing to establish a local kinetic phase diagram for CO oxidation on (100) facets of Pt foil. PEEM observations of the reaction front propagation on Pt(100) domains reveal a high degree of propagation anisotropy both for oxygen and CO fronts on the apparently isotropic Pt(100) surface. The anisotropy vanishes for oxygen fronts at temperatures above 465K, but is maintained for CO fronts at all temperatures studied, i.e. in the range of 417 to 513K. A change in the front propagation mechanism is proposed to explain the observed effects.
► Kinetics of the CO oxidation on (100)-type grains of a polycrystalline Pt foil was studied in situ. ► Photoemission electron microscopy was used to study the local reaction kinetics on a mm-scale. ► Local kinetic phase diagrams for individual Pt(100) grains were obtained. ► Anisotropy of propagating reaction fronts on the apparently isotropic Pt(100) surface is explained.
Oxygen core excitations in different molecular titania–silica model clusters are evaluated using density-functional theory (DFT). These results are compared with in situ X-ray absorption fine ...structure (NEXAFS) measurements near the O K-edge of titania model catalysts supported on mesoporous silica, SBA-15, with low Ti loading. The comparison allows for an analysis of structural details of the supported titania species. The silica support is found to contribute to the NEXAFS spectrum in an energy range well above that of the titania units, allowing for a clear separation among the corresponding spectral contributions. The different bridging and terminal oxygen species can also be easily distinguished in the theoretical NEXAFS spectra. The experimental NEXAFS spectrum for 3 wt % Ti loading exhibits a single broad peak in the O 1s to the Ti 3d–O 2p excitation range of 530–534 eV. This can be explained by the theoretical data for tetrahedrally coordinated but not for triply coordinated TiO x species, suggesting the presence of OH groups at the expense of titanyl oxygen in the titania surface structure. The experimental NEXAFS spectrum for low Ti loading differs substantially from that of TiO2 anatase bulk with octahedral TiO6 units where the observed double-peak structure is also reproduced by the calculations.
Using Pt/CNT or Fe/CNT electrocatalysts it is possible to convert carbon dioxide to liquid fuels, particularly isopropanol. Fe/CNT shows a better behavior than Pt/CNT, although a faster deactivation. ...The main reason of deactivation is cross-over of the electrolyte, particularly of K ions. They react with iron particles and cause their dissolution and migration. In the case of Pt/CNT there is no, or minor, dissolution of metal, but potassium covers the Pt particles and/or induce deactivation. It is shown, however, that the electrolyte was necessary to simulate the half-cell of the full photoelectrocatalytic device, while in the latter no electrolyte is needed, being the protons and electrons produced by water splitting. The elimination of the electrolyte probably could eliminate or significantly reduce a main cause determining the observed fast deactivation.
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