Fluorescence Yield Near-edge Spectroscopy Research Articles

Enhanced Low-Temperature CO Oxidation on a Stepped Platinum Surface for Oxygen Pressures above 10-5 Torr

The rate of CO oxidation has been characterized on the stepped Pt(411) surface for oxygen pressures up to 0.002 Torr, over the 100-1000 K temperature range. CO oxidation was characterized using both temperature-programmed reaction spectroscopy (TPRS) and in situ soft X-ray fluorescence yield near-edge spectroscopy (FYNES). New understanding of the important role surface defects play in accelerating CO oxidation for oxygen pressure above 10(-5) Torr is presented in this paper for the first time. For saturated monolayers of CO, the oxidation rate increases and the activation energy decreases significantly for oxygen pressures above 10(-5) Torr. This enhanced CO oxidation rate is caused by a change in the rate-limiting step to a surface reaction limited process above 10(-5) Torr oxygen from a CO desorption limited process at lower oxygen pressure. For example, in oxygen pressures above 0.002 Torr, CO(2) formation begins at 275 K even for the CO saturated monolayer, which is well below the 350 K onset temperature for CO desorption. Isothermal kinetic measurements in flowing oxygen for this stepped surface indicate that activation energies and preexponential factors depend strongly on oxygen pressure, a factor that has not previously been considered critical for CO oxidation on platinum. As oxygen pressure is increased from 10(-6) to 0.002 Torr, the oxidation activation energies for the saturated CO monolayer decrease from 24.1 to 13.5 kcal/mol for reaction over the 0.95-0.90 ML CO coverage range. This dramatic decrease in activation energy is associated with a simple increase in oxygen pressure from 10(-5) to 10(-3) Torr. Activation energies as low as 7.8 kcal/mol were observed for oxidation of an initially saturated CO layer reacting over the 0.4-0.25 ML coverage range in oxygen pressure of 0.002 Torr. These dramatic changes in reaction mechanism with oxygen pressure for stepped surfaces are consistent with mechanistic models involving transient low activation energy dissociation sites for oxygen associated with step sites. Taken together these experimental results clearly indicate that surface defects play a key role in increasing the sensitivity of CO oxidation to oxygen pressure.

In Situ Soft X-ray Studies of Ethylene Oxidation Mechanisms and Intermediates on the Pt(111) Surface

In situ studies of ethylene oxidation on Pt(111) have been performed using a powerful combination of fluorescence yield soft X-ray methods for temperatures up to 600 K and oxygen pressures up to 0.01 Torr. Absolute carbon coverages have been determined both in steady-state and dynamic catalytic conditions on the Pt(111) surface. Fluorescence yield near-edge spectroscopy (FYNES) and temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES) experiments above the carbon K edge were used to identify the structure and bonding of the dominant surface species during oxidation. TP-FYNES experiments of preadsorbed ethylene coverages in oxygen pressures up to 0.01 Torr indicate a stable intermediate is formed over the 215-300 K temperature range. By comparing the intensity of the C-H sigma resonance at the magic angle with the intensity in the carbon continuum, the stoichiometry of this intermediate has been determined explicitly. Based on calibration with known C-H stoichiometries, the intermediate has a C(2)H(3) stoichiometry for oxygen pressures up to 0.01 Torr, indicating oxydehydrogenation occurs before skeletal oxidation. FYNES spectra at normal and glancing incidences were performed to characterize the structure and bonding of this intermediate. Using FYNES spectra of ethylene, ethylidyne, and acetylene as reference standard, this procedure indicates the oxidation intermediate is tri-sigma vinyl. Thus, oxidation of ethylene proceeds through a vinyl intermediate, with oxydehydrogenation preceding skeletal oxidation.

Mechanism of acetylene oxidation on the Pt(111) surface using in situ fluorescence yield near-edge spectroscopy

In situ studies of acetylene oxidation on Pt(111) have been performed with both fluorescence yield near-edge spectroscopy (FYNES) and temperature-programmed FYNES (TP-FYNES) for temperatures up to 600 K and flowing oxygen pressures up to 0.009 Torr. Low-temperature spectroscopic (FYNES) results indicate that at 150 K acetylene adsorbs with the C C backbone tilted slightly up from the Pt(111) surface, consistent with the formation of an η 2 - μ 3 -CCH 2 surface intermediate. When acetylene is preadsorbed on the Pt(111) surface, forming the η 2 - μ 3 -CCH 2 intermediate, oxidation occurs in a single step over the 330–420 K temperature range. In the presence of excess oxygen, little effect on the rate of oxidation is seen when the oxygen pressure is increased, as observed previously for oxidation of adsorbed propyne on the Pt(111) surface. Comparison between the intensity of the C H σ ∗ resonance and the intensity in the carbon continuum clearly shows that the adsorbed hydrocarbon intermediate maintains a 1:1 C H stoichiometry throughout oxidation, suggesting that oxydehydrogenation and skeletal oxidation occur simultaneously above 330 K. Identical reaction temperature profiles observed for (1) reaction of coadsorbed acetylene and atomic oxygen, and (2) reaction of acetylene and oxygen from the gas phase signify that in both cases a direct, bimolecular surface reaction mechanism occurs during oxidation. For the catalytic studies, a large temperature hysteresis, which is associated with inhibition of oxygen adsorption by acetylene, is observed during thermal cycling. Detailed isothermal kinetic studies performed in flowing oxygen pressures indicate that the apparent activation energy for oxidation is 20.3 ± 2.0 kcal / mol with a pre-exponential factor of 10 10.3 ± 1.0 s −1 . Taken together, these results clearly indicate that surface properties and reactant properties both play major roles in controlling oxidation reactions over an extended range of reaction conditions.

Defect Enhanced Carbon Monoxide Oxidation at Elevated Oxygen Pressures on a Pt/Al2O3 Thin Film

Carbon monoxide oxidation has been studied, using in situ soft X-ray techniques and ultrahigh vacuum (UHV) temperature-programmed reaction spectroscopy (TPRS), on a well-characterized supported platinum thin film over the100-1000 K temperature range and in pressures ranging from UHV to 0.01 Torr. In high oxygen pressures the reaction is enhanced by oxidation at defect sites. Temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES) studies of preadsorbed CO coverages heated in pressures of flowing oxygen, ranging from I x 10 - 5 to I x 10 - 2 Torr, have been performed. Unlike with the Pt(111) surface, the onset temperature for oxidation of a saturated coverage of CO on the platinum thin film decreases dramatically with increasing oxygen pressure. The CO oxidation onset temperature decreased from 340 K in 1 x 10 - 5 Torr of flowing oxygen to 230 K in 1 x 10 - 2 Torr of flowing oxygen. Therefore, oxidation is not limited by CO desorption for high CO coverages, and reactions at defect sites control the rate of CO oxidation at high oxygen pressures. Isothermal oxidation experiments in 0.002 Torr of flowing oxygen have yielded an activation energy of 8.5 kcal/mol. TPRS of coadsorbed carbon monoxide and oxygen has indicated a primary oxidation channel at 350 K corresponding to the oxidation of CO on Pt( 111) terrace sites and minor oxidation channels above 400 K. The thin Pt/Al 2 O 3 film has been characterized using both chemical and structural methods. CO and O 2 temperature-programmed desorption (TPD) experiments indicate the Pt film surface is similar to Pt(111), but with a higher defect site concentration. Plane-view and cross-section transmission electron microscopy (TEM) experiments have confirmed a 100 A Pt film consisting of ∼450 A, well-ordered grains, as indicated by local area electron diffraction patterns, with the (111) orientation parallel to the substrate.

In-Situ Soft X-ray Studies of Toluene Catalytic Oxidation on the Pt(111) Surface

The catalytic oxidation of toluene on the Pt(111) surface has been characterized, in flowing oxygen pressures up to 0.01 Torr, using temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES). During TP-FYNES oxidation experiments in flowing oxygen pressures, a dehydrogenated C7H7 η7-phenylmethylene intermediate is formed over the 190 to 300 K temperature range. Spectroscopic (FYNES) measurements clearly indicate that C−H bond activation occurs at the methyl group and the aromatic ring remains parallel to the surface. In contrast to the intermediates formed during benzene oxidation, no rearrangement of the aromatic ring occurs, signifying that the methyl group influences the structure of intermediates formed during toluene oxidation. This η7-phenylmethylene intermediate dominates over the 300−370 K temperature range, but is oxidized rapidly above 370 K. Strongly adsorbed η6-toluene and the stable η7-phenylmethylene intermediate inhibit oxidation below 370 K by inhibiting oxygen adsorption. A slight increase in the rate of rapid oxidation above 370 K is observed with increasing oxygen pressures over the range of 0.0005 to 0.01 Torr. Temperature-programmed reaction spectroscopy (TPRS) indicates that the deep oxidation products on the Pt(111) surface are CO2 and H2O. This combination of in-situ soft X-ray and UHV methods has provided a detailed mechanistic description of toluene catalytic oxidation on the Pt(111) surface.

Benzene Intermediates and Mechanisms during Catalytic Oxidation on the Pt(111) Surface Using In-Situ Soft X-ray Methods

The catalytic oxidation of benzene on the Pt(111) surface has been characterized, in flowing oxygen pressures up to 0.01 Torr, using temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES). During temperature-programmed oxidation experiments in flowing oxygen pressures, a series of four adsorbed intermediates are formed. The dominant intermediates with increasing temperature are η6-benzene, 1,4-di-σ-2,5-cyclohexadiene, 1,1,4-tri-σ-2,5-cyclohexadiene, and η5-cyclohexadienone. All of these intermediates are strongly adsorbed based on the molecular rehybridization indicated by spectroscopic (FYNES) measurements. Adsorbed benzene inhibits oxidation below 370 K by inhibiting oxygen adsorption. Over the temperature range 150−215 K, a 1,4-di-σ-2,5-cyclohexadiene surface intermediate with C6H6 stoichiometry is formed by rearrangement of the aromatic ring. Next, a 1,1,4-tri-σ-2,5-cyclohexadiene surface intermediate with C6H5 stoichiometry is formed by oxydehydrogenation over the temperature range 215−285 K. Above 350 K, a fraction of this intermediate is oxidized to form carbon dioxide and water, while the remainder is oxygenated to form a η5-cyclohexadienone intermediate with C6H5O stoichiometry, which is dominant at 390 K. The reactivity of this intermediate is clearly demonstrated by rapid oxidation with increasing temperature. No change in the onset temperature for rapid oxidation or the rate of carbon removal is observed with increasing oxygen pressures over the pressure range 0.0005−0.01 Torr. Temperature-programmed reaction spectroscopy (TPRS) has been used to identify the gas-phase products during deep oxidation between coadsorbed benzene and oxygen on Pt(111). Carbon dioxide and water are formed over the 300−500 K temperature range when benzene is in excess, and over the 380−620 K temperature range when oxygen is in excess. This combination of temperature-programmed UHV and in-situ soft X-ray methods has provided a detailed mechanistic description of catalytic benzene oxidation on the Pt(111) surface.

Polymer Conversion Measurement of Diacetylene-Containing Thin Films and Monolayers Using Soft X-ray Fluorescence Spectroscopy

Polymer formation within ordered monolayer and thin-film assemblies is measured using fluorescence yield near-edge spectroscopy (FYNES). This spectroscopic method measures soft X-ray absorption in the 270−330-eV energy range, probing core-electron transitions to unfilled molecular orbitals by detecting the resultant fluorescence emission. By observing transitions in the carbon 1s−π* region (280−286 eV), the diminution of monomer and formation of polymer are measured directly and simultaneously. In contrast with spectroscopic methods measuring partial electron yield, film damage caused by neutralization of sample charging is eliminated with FYNES. Using FYNES, monomer-to-polymer conversion efficiencies for ultrathin films and monolayer assemblies are successfully measured for the first time. Measured conversion efficiencies for diacetylene-containing assemblies is near 80%, demonstrating remarkable agreement among ultrathin films, surface-attached configurations, and previous studies of thicker film structures. Likewise, polymerization kinetics for these assemblies are also similar, exhibiting at least two distinct rate regimes. These results indicate that the polymerization properties of diacetylenes are not significantly dependent on molecular architecture and that induced-dipole interactions between neighboring diacetylenes predominate.

Propylene Oxidation Mechanisms and Intermediates Using in Situ Soft X-ray Fluorescence Methods on the Pt(111) Surface

The oxidation of propylene preadsorbed on the Pt(111) surface has been characterized in oxygen pressures up to 0.02 Torr using fluorescence yield near-edge spectroscopy (FYNES) and temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES) above the carbon K edge. During oxidation of adsorbed propylene, a stable intermediate was observed and characterized using these soft X-ray methods. A general in situ method for determining the stoichiometry of carbon-containing reaction intermediate species has been developed and demonstrated for the first time. Total carbon concentration measured during temperature-programmed reaction studies clearly indicates a reaction intermediate is formed in the 300 K temperature range with a surface concentration of 0.55 × 1015 carbon atoms/cm2. By comparing the intensity of the C−H σ* resonance at the magic angle with the intensity in the carbon continuum, the stoichiometry of this intermediate can be determined unambiguously. Based on calibration with molecular propylene (C3H6) and propylidyne (C3H5), the intermediate has a C3Η5 stoichiometry for oxygen pressures up to 0.02 Torr. A set of normal and glancing angle FYNES spectra above the carbon K edge was used to characterize the bonding and structure of this intermediate. Spectra of known coverages of adsorbed propylene and propylidyne served as standards. The spectra of di-σ propylene, propylidyne, and the intermediate were curve fit as a group with consistent energies and widths of all primary features. Based on this procedure, the intermediate is 1,1,2-tri-σ 1-methylvinyl. The stoichiometry and temperature stability range of the 1-methylvinyl intermediate formed in oxygen pressures up to 0.02 Torr is identical with the stoichiometry and stability of the same intermediate formed during oxidation of preadsorbed propylene by excess coadsorbed atomic oxygen.

Aniline Adsorption, Hydrogenation, and Hydrogenolysis on the Ni(100) Surface

The bonding and reactions of adsorbed aniline have been characterized on the Ni(100) surface both in hydrogen and in vacuum with a combination of surface spectroscopies. The structure of adsorbed aniline and derived intermediates has been characterized by near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoemission spectroscopy (XPS). The dominant surface reactions have been studied using temperature-programmed reaction spectroscopy (TPRS) and in-situ temperature-programmed fluorescence yield near-edge spectroscopy (TP FYNES). Competition between hydrogenation, hydrogenolysis, and dehydrogenation of aniline in the 300−400 K temperature range depends markedly on hydrogen pressures in the vacuum to 0.01 Torr range. In the absence of external hydrogen, dehydrogenation dominates with increasing temperature. Both hydrogenation and hydrogenolysis of aniline-derived surface intermediates are enhanced dramatically by hydrogen atmospheres. For aniline coverages up to 1 monolayer, hydrogenolysis to fo...

Aniline hydrogenolysis on nickel: effects of surface hydrogen and surface structure

Fluorescence yield near-edge spectroscopy (FYNES) above the carbon K edge and temperature programmed reaction spectroscopy (TPRS) have been used as the methods for characterizing the reactivity and structure of adsorbed aniline and aniline derived species on the Ni(100) and Ni(111) surfaces over an extended range of temperatures and hydrogen pressures. The Ni(100) surface shows appreciably higher hydrogenolysis activity towards adsorbed aniline than the Ni(111) surface. Hydrogenolysis of aniline on the Ni(100) surface results in benzene formation at 470 K, both in reactive hydrogen atmospheres and in vacuum. External hydrogen significantly enhances the hydrogenolysis activity for aniline on the Ni(100) surface. Based on spectroscopic evidence, we believe that the dominant aniline hydrogenolysis reaction is preceded by partial hydrogenation of the aromatic ring of aniline in the presence of 0.001 Torr of external hydrogen on the (100) surface. In contrast, very little adsorbed aniline undergoes hydrogen induced C-N bond activation on the Ni(111) surface for hydrogen pressures as high as 10−7 Torr below 500 K. Thermal dehydrogenation of aniline dominates with increasing temperature on the Ni(111) surface, resulting in the formation of a previously observed polymeric layer which is stable up to 820 K. Aniline is adsorbed at a smaller angle relative to the Ni(111) surface than the Ni(100) surface at temperatures below the hydrogenolysis temperature. We believe that the proximity and strong π-interaction between the aromatic ring of the aniline and the surface is one major factor which controls the competition between dehydrogenation and hydrogen addition. In this case the result is a substantial enhancement of aniline dehydrogenation relative to hydrogenation on the Ni(111) surface.

A fluorescence-yield near-edge spectroscopy (FYNES) investigation of the reaction kinetics of NiO/Ni(100) with hydrogen

Abstract The method of fluorescence-yield near-edge X-ray absorption spectroscopy (FYNES) has been applied to investigate the thermal stability of NiO films on a Ni(100) substrate and the reaction kinetics of this surface with hydrogen gas. The FYNES results indicate that the total oxygen content near the surface region remains constant after heating a NiO/Ni(100) layer above a phase separation temperature of 525 K, indicating that there is no significant thermally induced diffusion of oxygen into the bulk Ni(100) lattice. These results support conclusions from previous EELS and XPS experiments that the NiO/Ni(100) layer prepared at 300 K undergoes a thermally induced phase separation to produce a surface with mixtures of roughly 50 A NiO(100) crystalline clusters and c(2 × 2)−O chemisorbed phases. The FYNES investigation of the reaction kinetics of NiO/Ni(100) with H2 in the temperature range of 600–800 K indicates that the oxygen removal rate from NiO clusters is roughly zero order with respect to the initial oxygen content, and that the subsurface oxygen is removed completely. In addition, the removal rate of oxygen is found to be approximately first order with respect to H2 pressure. The reaction of NiO/Ni(100) with H2 has a rather weak temperature dependence, as characterized by an activation energy of 1.8 kcal/mol with a pre-exponential factor of 9.4 × 1013 molecules cm−2 s−1. These observations indicate that the rate-determining step for the removal of oxygen is most likely the initial HH bond breaking, and that the removal of oxygen does not occur preferentially at the edges of NiO islands.

A fluorescence-yield near-edge spectroscopy (FYNES) investigation of the reaction kinetics of NiO/Ni(100) with hydrogen

The method of fluorescence-yield near-edge X-ray absorption spectroscopy (FYNES) has been applied to investigate the thermal stability of NiO films on a Ni(100) substrate and the reaction kinetics of this surface with hydrogen gas. The FYNES results indicate that the total oxygen content near the surface region remains constant after heating a NiO/Ni(100) layer above a phase separation temperature of 525 K, indicating that there is no significant thermally induced diffusion of oxygen into the bulk Ni(100) lattice. These results support conclusions from previous EELS and XPS experiments that the NiO/Ni(100) layer prepared at 300 K undergoes a thermally induced phase separation to produce a surface with mixtures of roughly 50 Å NiO(100) crystalline clusters and c(2 × 2)− O chemisorbed phases. The FYNES investigation of the reaction kinetics of NiO/Ni(100) with H 2 in the temperature range of 600–800 K indicates that the oxygen removal rate from NiO clusters is roughly zero order with respect to the initial oxygen content, and that the subsurface oxygen is removed completely. In addition, the removal rate of oxygen is found to be approximately first order with respect to H 2 pressure. The reaction of NiO/Ni(100) with H 2 has a rather weak temperature dependence, as characterized by an activation energy of 1.8 kcal/mol with a pre-exponential factor of 9.4 × 10 13 molecules cm −2 s −1. These observations indicate that the rate-determining step for the removal of oxygen is most likely the initial HH bond breaking, and that the removal of oxygen does not occur preferentially at the edges of NiO islands.

Hydrogenolysis of adsorbed methylthiolate on the Pt(111) surface

A combination of temperature programmed reaction experiments and fluorescence yield near edge spectroscopy (FYNES) has been used to characterize hydrogen induced C-S bond activation in adsorbed methylthiolate on the Pt(111) surface. The low temperature methane peak observed at 310 K during temperature programmed decomposition of methanethiol clearly involves reaction with adsorbed hydrogen since deuterium incorporation is observed in the presence of coadsorbed deuterium. Some multiple deuteration is observed suggesting that a mixture of carbon containing species may be involved in the hydrogenolysis process. In situ FYNES above the carbon K edge has been used to characterize the kinetics of this hydrogenolysis process in more detail. Transient FYNES measurements were used to characterize changes in the total carbon concentration during the hydrogenolysis reaction. In situ temperature programmed reaction experiments performed in 0.02 Torr of hydrogen clearly indicate that hydrogenolysis begins near 290 K and all carbon containing species are removed by 360 K. In situ isothermal kinetic studies indicate that the hydrogenolysis reaction is first order in adsorbed thiolate coverage and approximately half order in hydrogen pressure. The apparent activation energy for the hydrogenolysis reaction is 18 kcal/mol in the presence of 0.02 Torr of gaseous hydrogen. This activation energy represents a lower bound for the hydrogenolysis activation energy since the apparent heat includes the heat of adsorption for hydrogen. These direct kinetic measurements of this hydrogenolysis reaction under reactive gaseous environments highlight the role that in situ methods can play in characterizing surface reaction processes.

Hydrogen-induced low temperature CO displacement from the Pt(111) surface

A new low temperature displacement mechanism for CO on the Pt(111) surface has been observed in the presence of high pressures of hydrogen (0.001 to 0.1 Torr H 2). Temperature-programmed fluorescence yield near-edge spectroscopy (TP FYNES) was used to continuously monitor the CO coverage as a function of temperature both with and without hydrogen. For hydrogen pressures above 0.01 Torr, removal of CO begins at 130 K ( E d = 10.6 kcal/ mol) instead of near the desorption temperature of 400 K ( E d = 26 kcal/ mol). The large decrease in CO desorption energy appears to be caused by substantial repulsive interactions in the compressed monolayer induced by coadsorbed hydrogen. The new low temperature CO desorption channel appears to be caused by displacement of the compressed CO adlayer by coadsorbed hydrogen. In addition, the desorption activation energy for the main desorption channel of CO near 400 K is lowered by ~ 1 kcal/mol for hydrogen pressures in the 0.001 to 0.1 Torr range. These new results clearly emphasize the importance of in-situ methods capable of performing kinetic experiments at high pressures on well characterized adsorbed monolayers on single crystal surfaces. High coverages of coadsorbed hydrogen resulting from substantial overpressures may substantially modify desorption activation energies and thus coverages and kinetic pathways available even for strongly chemisorbed species. These phenomena may play an important role in surface reactions which occur at high pressure.

Fluorescence Yield Near Edge Spectroscopy (FYNES) of benzene adsorbed in Na-Y zeolite

Benzene adsorbed in sodium-Y zeolite has been investigated using Fluorescence Yield Near Edge Spectroscopy (FYNES). These studies demonstrate that FYNES measurements are feasible as a direct probe of the local electronic structure of hydrocarbons adsorbed in and on insulating, microporous practical catalysts. The distribution of π energy levels for adsorbed benzene was not significantly altered as compared to the condensed phase.

Ultrasoft (C,N,O) x-ray fluorescence detection: Proportional counters, focusing multilayer mirrors, and scattered light systematics (invited)

Ultrasoft x-ray detection is discussed as applied to the fluorescence yield near edge spectroscopy (FYNES) technique. We describe a progression of improved ultra-high vacuum compatible soft x-ray proportional counters for FYNES measurements. The importance of energy dispersive detection is discussed and examples shown. Scattered light background problems are quantified and compared for several real samples. Finally, focusing multilayer mirror collection is introduced for reducing scattered light background and improved elemental differentiation in the fluorescence signal.