19 research outputs found
Water adsorption on vanadium oxide thin films in ambient relative humidity.
In this work, ambient pressure x-ray photoelectron spectroscopy (APXPS) is used to study the initial stages of water adsorption on vanadium oxide surfaces. V 2p, O 1s, C 1s, and valence band XPS spectra were collected as a function of relative humidity in a series of isotherm and isobar experiments. Experiments were carried out on two VO2 thin films on TiO2 (100) substrates, prepared with different surface cleaning procedures. Hydroxyl and molecular water surface species were identified, with up to 0.5 ML hydroxide present at the minimum relative humidity, and a consistent molecular water adsorption onset occurring around 0.01% relative humidity. The work function was found to increase with increasing relative humidity, suggesting that surface water and hydroxyl species are oriented with the hydrogen atoms directed away from the surface. Changes in the valence band were also observed as a function of relative humidity. The results were similar to those observed in APXPS experiments on other transition metal oxide surfaces, suggesting that H2O-OH and H2O-H2O surface complex formation plays an important role in the oxide wetting process and water dissociation. Compared to polycrystalline vanadium metal, these vanadium oxide films generate less hydroxide and appear to be more favorable for molecular water adsorption
The effect of different InO(111) surface terminations on CO adsorption
InO-based catalysts have shown high activity and selectivity for
CO hydrogenation to methanol, however the origin of the high performance of
InO is still unclear. To elucidate the initial steps of CO
hydrogenation over InO, we have combined X-ray Photoelectron
Spectroscopy (XPS) and Density Functional Theory (DFT) calculations to study
the adsorption of CO on the InO(111) crystalline surface with
different terminations, namely the stoichiometric, the reduced, and the
hydroxylated surface, respectively. The combined approach confirms that the
reduction of the surface results in the formation of In ad-atoms and that water
dissociates on the surface at room temperature. A comparison of the
experimental spectra and the computed core-level-shifts (using methanol and
formic acid as benchmark molecules) suggests that CO adsorbs as a carbonate
on all surface terminations. We find that CO adsorption is hindered by
hydroxyl groups on the hydroxylated surface.Comment: 49 pages, 18 figure
Compression of a Stearic Acid Surfactant Layer on Water Investigated by Ambient Pressure X-ray Photoelectron Spectroscopy
We present a combined LangmuirâPockels trough and ambient pressure X-ray photoelectron spectroscopy (APXPS) study of the compression of stearic acid surfactant layers on neat water. Changes in the packing density of the molecules are directly determined from C 1s and O 1s APXPS data. The experimental data are fit with a 2D model for the stearic acid coverage. Based on the results of these proof-of-principle experiments, we discuss the remaining challenges that need to be overcome for future investigations of the role of surfactants in heterogeneous chemical reactions at liquidâvapor interfaces in combined LangmuirâPockels trough and APXPS measurements
Infrared surface spectroscopy and surface optical reflectance for operando catalyst surface characterization
Using a new sample environment, the two operando surface diagnostic techniques two-dimensional surface optical reflection (2D-SOR) and polarization-modulated infrared reflectionâabsorption spectroscopy (PM-IRRAS) have been combined with mass spectrometry (MS) to characterize a catalytic reaction. With both techniques integrated in a custom-built setup, we can correlate molecular information of the adsorbed surface species from PM-IRRAS with information about oxide formation on the sample from 2D-SOR. The new setup was evaluated by performing CO oxidation over a Palladium single crystal Pd(1 0 0) surface. The results reveal that the macroscopic and the microscopic molecular behavior correlate well. When the CO adsorption peak disappears in the PM-IRRAS spectrum, the formation of a well-defined ultra-thin surface oxide is observed in the 2D-SOR trend. We discuss the benefits and limitations of the two techniques as well as their potential for further studies of catalytic reactions at both gasâsolid and liquidâsolid interfaces
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Quantitative Characterization of a Desalination Membrane Model System by X-ray Photoelectron Spectroscopy.
Aromatic polyamide films form the active layer in reverse osmosis desalination membranes. Despite widespread use of this technology, it suffers from low rejection rates for certain water contaminants and from membrane fouling. Through a better understanding of the fundamental surface chemical processes during reverse osmosis desalination, advances in membrane and material design are expected. The recent invention of a molecular layer-by-layer (mLbL) preparation technique [ Johnson , P. M. ; Molecular Layer-by-Layer Deposition of Highly Crosslinked Polyamide Films . J. Polym. Sci., Part B: Polym. Phys. 2012 , 50 ( 3 ), 168 - 173 ] yields films that are sufficiently smooth to warrant investigation with high-resolution microscopy and spectroscopy methods. We present high-resolution, quantitative X-ray photoelectron spectroscopy (XPS) data on the surface chemistry of ultrathin polyamide films that can serve as a model system for desalination membranes. We show that a quantitative analysis of the XPS spectra gives information about the functional groups of the film as well as other compounds present due to the synthesis under ambient conditions. Unpolymerized functional groups are identified and aid in understanding the degree of cross-linking. Investigation of polymers with synchrotron-based XPS requires taking beam-induced changes into account. We quantify X-ray beam damage and show that beam damage to the polyamide is limited, allowing long-term investigation of thin polyamide films. Characterizing mLbL-grown films via high-resolution XPS is the basis for a better understanding of the chemical interplay of polyamide surface functional groups with the major components of desalination systems
Critical Step in the HCl Oxidation Reaction over Single-Crystalline CeOâââ(111): Peroxo-Induced Site Change of Strongly Adsorbed Surface Chlorine
The catalytic oxidation of HCl by molecular oxygen (Deacon process) over ceria allows the recovery of molecular chlorine from omnipresent HCl waste produced in various industrial processes. Previous density functional theory (DFT) model-calculations1 proposed, that the most critical reaction step in this process is the displacement of tightly bound chlorine at a vacant oxygen position on the CeOâ(111) surface Cl(vac) towards a less strongly bound cerium on-top Cl(top) position. This step is highly endothermic by more than 2 eV. On the basis of a dedicated model study, namely the re-oxidation of a chlorinated single crystalline Cl(vac)-CeOâââ(111)-(â3 Ă â3)R30° surface structure, we provide unique spectroscopic data (high resolution core level spectroscopy (HRCLS) and X-ray adsorption near edge structure (XANES)) for this oxygen-induced de-chlorination process. Combined with theoretical evidence from DFT calculations, the Cl(vac) â Cl(top) displacement reaction is predicted to be induced by a surface-adsorbed peroxo species (OâÂČâ»), making the displacement step concerted and exothermic by 0.6 eV with an activation barrier of only 1.04 eV. The peroxo species is shown to be important for the re-oxidation of Clvac-CeOâââ(111) and is considered essential for understanding the function of ceria in oxidation catalysis
Operando Observation of Oxygenated Intermediates during CO Hydrogenation on Rh Single Crystals
The CO hydrogenation reaction over the Rh(111) and (211) surfaces has been investigated operando by X-ray photoelectron spectroscopy at a pressure of 150 mbar. Observations of the resting state of the catalyst give mechanistic insight into the selectivity of Rh for generating ethanol from CO hydrogenation. This study shows that the Rh(111) surface does not dissociate all CO molecules before hydrogenation of the O and C atoms, which allows methoxy and other both oxygenated and hydrogenated species to be visible in the photoelectron spectra
Operando Reflectance Microscopy on Polycrystalline Surfaces in Thermal Catalysis, Electrocatalysis, and Corrosion
We have developed a microscope with a spatial resolution of 5 ÎŒm, which can be used to image the two-dimensional surface optical reflectance (2D-SOR) of polycrystalline samples in operando conditions. Within the field of surface science, operando tools that give information about the surface structure or chemistry of a sample under realistic experimental conditions have proven to be very valuable to understand the intrinsic reaction mechanisms in thermal catalysis, electrocatalysis, and corrosion science. To study heterogeneous surfaces in situ, the experimental technique must both have spatial resolution and be able to probe through gas or electrolyte. Traditional electron-based surface science techniques are difficult to use under high gas pressure conditions or in an electrolyte due to the short mean free path of electrons. Since it uses visible light, SOR can easily be used under high gas pressure conditions and in the presence of an electrolyte. In this work, we use SOR in combination with a light microscope to gain information about the surface under realistic experimental conditions. We demonstrate this by studying the different grains of three polycrystalline samples: Pd during CO oxidation, Au in electrocatalysis, and duplex stainless steel in corrosion. Optical light-based techniques such as SOR could prove to be a good alternative or addition to more complicated techniques in improving our understanding of complex polycrystalline surfaces with operando measurements
Operando Observation of Oxygenated Intermediates during CO Hydrogenation on Rh Single Crystals
The CO hydrogenation reaction over the Rh(111) and (211) surfaces has been investigated operando by X-ray photoelectron spectroscopy at a pressure of 150 mbar. Observations of the resting state of the catalyst give mechanistic insight into the selectivity of Rh for generating ethanol from CO hydrogenation. This study shows that the Rh(111) surface does not dissociate all CO molecules before hydrogenation of the O and C atoms, which allows methoxy and other both oxygenated and hydrogenated species to be visible in the photoelectron spectra