On the nature of the active site for the ethylbenzene dehydrogenation over iron oxide catalysts

The dehydrogenation of ethylbenzene to styrene was studied over single-crystalline iron oxide model catalyst films grown epitaxially onto Pt(111) substrates. The role of the iron oxide stoichiometry and of atomic surface defects for the catalytic activity was investigated by preparing single-phased Fe3O4(111) and α-Fe2O3(0001) films with defined surface structures and varying concentrations of atomic surface defects. The structure and composition of the iron oxide films were controlled by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), the surface defect concentrations were determined from the diffuse background intensities in the LEED patterns. These ultrahigh vacuum experiments were combined with batch reactor experiments performed in water–ethylbenzene mixtures with a total gas pressure of 0.6 mbar. No styrene formation is observed on the Fe3O4 films. The α-Fe2O3 films are catalytically active, and the styrene formation rate increases with increasing surface defect concentration on these films. This reveals atomic surface defects as active sites for the ethylbenzene dehydrogenation over unpromoted α-Fe2O3. After 30 min reaction time, the films were deactivated by hydrocarbon surface deposits. The deactivation process was monitored by imaging the surface deposits with a photoelectron emission microscope (PEEM). It starts at extended defects and exhibits a pattern formation after further growth. This indicates that the deactivation is a site-selective process. Post-reaction LEED and AES analysis reveals partly reduced Fe2O3 films, which shows that a reduction process takes place during the reaction which also deactivates the Fe2O3 films

Adsorption and dehydrogenation of ethylbenzene on ultrathin iron oxide model catalyst films

Thin iron oxide model catalyst films with defined stoichiometries were grown onto a Pt(111) single crystal substrate. On clean and potassium covered monolayer films with FeO stoichiometry as well as on clean Fe3O4 and Fe2O3 multilayer films the adsorption of ethylbenzene (EB) at T=120 K and the catalytic dehydrogenation of EB to styrene was studied by temperature programmed desorption (TPD) and stationary mass spectrometry measurements. On all films weakly chemisorbed EB desorbs molecularly with first order kinetics at temperatures between T=200 and 250 K. On potassium covered FeO monolayer films the EB desorption temperature increases to 260 K. Desorption energies and frequency factors of these adsorption states were determined by a numerical analysis of the TPD curves. Between 2 and 2.5 langmuir (L) exposures these weakly bound states get saturated. With further increasing exposures condensed EB multilayers desorbing at T=155 K form and stronger chemisorbed adsorption states are occupied. For 7 L exposure we observe about 0.5 monolayers of EB desorbing between T=300 and 500 K from the FeO monolayer and Fe3O4 multilayer films and around T=900 K from the Fe2O3 films. The latter temperature coincides with the reaction temperature of the technical iron oxide catalyst. Stationary measurements in a water-EB mixture at T=873 K reveal a catalytic styrene formation only on the Fe2O3 film, demonstrating that only this oxide phase is active for the dehydrogenation of EB

Energetics and kinetics of ethylbenzene adsorption on epitaxial FeO(111) and Fe₃O₄(111) films studied by thermal desorption and photoelectron spectroscopy

The adsorption of ethylbenzene (EB) has been studied on thin films of FeO(111) and Fe3O4(111) grown epitaxially on Pt(111) using thermal desorption spectroscopy (TDS), ultraviolet photoelectron spectroscopy (UPS) and low energy electron diffraction (LEED). Applying a threshold analysis of the TDS data, desorption energies Edes and the corresponding frequency factors are deduced. The UPS measurements are performed under adsorption–desorption equilibrium conditions: The spectra are taken at varying sample temperature at constant EB gas phase pressures. From the spectra, the EB-coverages ΘEB are deduced. From the adsorption isobars obtained in this way, isosteric heats of adsorption qst(ΘEB) are obtained which are compared to the desorption energies Edes deduced from TDS. On the oxygen-terminated FeO(111) surface, two adsorption states are observed, a physisorbed first layer (β-EB) followed by condensation (α-EB). Their UP spectra are almost identical and very similar to the spectrum of gas phase EB. On Fe3O4(111), a more strongly chemisorbed species (γ1-EB) is adsorbed first, followed by physisorbed β- and condensed α-EB. The chemisorbed phase exhibits a strong shift and split of the highest occupied π orbitals of the phenyl group. This indicates a strong interaction between the substrate and the adsorbed molecules that are adsorbed with the phenyl ring lying flat on the surface. The desorption energies Edes and the isosteric heats of adsorption qst, respectively, are 91 (85) kJ/mol for γ1-, 55 (58) kJ/mol for β- and 50 (52) kJ/mol for α-EB and agree generally well. The differences are discussed in terms of different coverage ranges accessible for both methods, the nonequilibrium character of the TDS method and to the threshold analysis which yields only data for the most loosely bound molecules desorbing first in each desorption track

Correction approach of detector backlighting in radiography

In various kinds of radiography, deficient transmission imaging may occur due to backlighting inside the detector itself arising from light or radiation scattering. The related intensity mismatches barely disturb the high resolution contrast, but its long range nature results in reduced attenuation levels which are often disregarded. Based on X ray observations and an empirical formalism, a procedure is developed for a first order correction of detector backlighting. A backlighting factor is modeled as a function of the relative detector coverage by the sample projection. Different cases of sample transmission are regarded at different backlight factors and detector coverage. The additional intensity of backlighting may strongly affect the values of materials attenuation up to a few 10 . The presented scenario provides a comfortable procedure for corrections of X ray or neutron transmission imaging dat