Temperature-dependent X-ray Photoelectron Spectroscopy Research Articles

Selective surface chemistry of allyl alcohol and allyl aldehyde on Si(1 0 0)2×1: Competition of [2 + 2] C [dbnd]C cycloaddition with O–H dissociation and with [2 + 2] C [dbnd]O cycloaddition in bifunctional molecules

Competition between the C C functional group with the OH group in allyl alcohol and with the C O group in allyl aldehyde in the adsorption and thermal chemistry on Si(1 0 0)2×1 has been studied by X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD), as well as density-functional theory (DFT) calculations. The similarities found in the C 1s and O 1s spectra for both molecules indicate that the O–H dissociation product for allyl alcohol and [2 + 2] C O cycloaddition product for allyl aldehyde are preferred over the corresponding [2 + 2] C C cycloaddition products. Temperature-dependent XPS and TPD studies further show that thermal evolution of these molecules gives rise to the formation of ethylene, acetylene, and propene on Si(1 0 0)2×1, with additional CO evolution only from allyl alcohol. The formation of these desorption products also supports that the [2 + 2] C C cycloaddition reaction does not occur. In addition, the formation of SiC at 1090 K is observed for both allyl alcohol and allyl aldehyde. We propose plausible surface-mediated reaction pathways for the formation of these thermal evolution products. The present work illustrates the crucial role of the Si(1 0 0)2×1 surface in selective reactions of the Si dimers with the O−H group in allyl alcohol and with the C O group in allyl aldehyde over the C C functional group common to both molecules.

Dissociative Adsorption and Thermal Evolution of Acetic Acid on Si(100)2×1: Surface-Mediated Formation of Ketene and Acetaldehyde from Unidentate and Bidentate Acetate Adsorbates

The dissociative adsorption of acetic acid on Si(100)2×1 at room temperature has been investigated by X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD), as well as density-functional theory (DFT) calculations. The three C 1s features obtained by XPS measurement can be attributed to the carboxyl C of bidentate acetate (at 286.9 eV) and unidentate acetate (at 289.8 eV), resulting from O−H dissociation upon adsorption, and the methyl C (at 285.7 eV). Furthermore, the formation of bidentate acetate at a low exposure is found to be followed by that of unidentate acetate at a higher exposure, with approximately equal population for both adstructures at the saturation exposure. This is also supported by our DFT calculations, which show that the bidentate acetate adstructure is the most stable configuration among the calculated adstructures. The combined temperature-dependent XPS and TPD studies provide strong evidence for the formation of ketene, acetaldehyde, and CO, with almost half of the adstructures dissociated into methyl groups that become SiC with increasing annealing temperature to 933 K. We propose plausible surface-mediated reaction pathways that could account for these products. The present work indicates the pivotal importance of the 2×1 surface in facilitating the thermally driven formation of ketene and acetaldehyde from the acetate adstructures.

Combined UHV and ambient pressure studies of 1,3-butadiene adsorption and reaction on Pd(1 1 1) by GC, IRAS and XPS

The hydrogenation of 1,3-butadiene on Pd(111) at 300K was studied at atmospheric pressure by infrared reflection absorption spectroscopy (IRAS) and gas chromatography (GC). Kinetic measurements showed 1-butene, trans-2-butene and cis-2-butene as primary products. Once 1,3-butadiene had been completely consumed, 1-butene was re-adsorbed on the surface producing trans-/cis-2-butene through isomerization and n-butane through hydrogenation. These results were corroborated by in situ IRAS spectroscopy. Post-reaction analysis by X-ray photoelectron spectroscopy (XPS) in the C1s region revealed a band at 284.2eV, corresponding to adsorbed butadiene and/or carbonaceous deposits. Quantification of this peak revealed a total carbon coverage of 0.3ML. Nevertheless, deactivation due to carbon deposition was a minor effect under our reaction conditions, as indicated by the kinetics of the subsequent butene hydrogenation reaction. Temperature-dependent XPS experiments after butadiene adsorption at 100K indicated a high stability of the diene molecule with hardly any desorption and/or decomposition up to 500K. Above this temperature, butadiene decomposed to carbon species that eventually dissolved in the Pd bulk above 700K.

Temperature dependent XPS study of CO dissociation on small Rh particles

We have used temperature dependent X-ray Photoelectron Spectroscopy (XPS) to study the heating-induced CO dissociation on oxide-supported Rh particles through observation of changes undergone by the adsorbate CO and the formation of atomic carbon from the dissociation process. Different behaviour upon heating was observed for the two CO species on the islands. Our study indicates that one of the species is desorptive and that the other is dissociative.

Researches on Polymer Surfaces: Structure and Dynamics. Surface Structure and Surface Molecular Motion of Solid Polymer Films.

Novel methods for the characterization of polymer surface structure and surface molecular motion have been presented. Scanning viscoelasticity microscopy (SVM) was used in the studies of the surface mechanical properties of polymeric solids. Employing dynamic secondary ion mass spectroscopy (DSIMS) for the depth profiling of the chemical composition of end-labeled polystyrene (PS) film, the end group segregation at the air/solid PS interface was verified. It was revealed that the molecular motion at the surface of glassy polystyrene (PS) was fairly activated, compared with that of bulk one due to the surface segregation of chain end groups. These results indicate that surface glass transition temperature is depressed due to the increase in free volume near surface region. Temperature dependent X-ray photoelectron spectroscopy (TDXPS) was utilized for the characterization of surface molecular motion of symmetric diblock copolymers. The activation of molecular motion at the surface was also confirmed by TDXPS. The two-dimensional mapping of the surface mechanical properties for the polymer blend ultrathin films was carried out on a nanometer scale by using SVM. Composition profile along the thickness direction of polymer blend thin film was also evaluated on the basis of neutron reflectivity (NR).

The Effect of ZDP's on the Surface Chemistry of Hydrocarbon Films on Oxidized Iron Substrates

The thermal chemistry of lubricant molecules adsorbed on poly-crystalline iron substrates has been studied using X-ray photoelectron spectroscopy (XPS). The lubricants examined were a synthetic, poly-α-olefin base oil and a base oil containing three different zinc dialkyldithiophosphates (ZDP's); isooctyl ZDP, isopropyl ZDP, and an aryl ZDP. The major focus of the work was to determine the influence of the base oil and additives on the thermal reduction of the native iron oxide passivating film. The kinetics of iron oxide film reduction were analyzed using a sequential reaction scheme, i.e., Fe111⇌Fe11⇌Fe0, with temperature-dependent rate constants determined by fitting to time- and temperature-dependent XPS data. The authors conclude that the presence of base oil accelerates oxide film reduction and that ZDP moderates this effect to some degree.