In situ studies of catalysts under reaction conditions by total electron-yield XAS. Possibilities and limitations of a new experimental technique

Abstract

Total electron-yield (TEY) detection has long been regarded a method for XAS investigations of surface adsorbates under ultra-high vacuum conditions. Recent work has shown, however, that the technique can also be applied to in situ studies of catalysts under reaction conditions. We have thoroughly investigated the TEY signal formation process under gas-flow conditions with a view to (i) characterizing the surface sensitivity of the technique, (ii) assessing the accuracy of the information acquired, and (iii) establishing the practical possibilities and limitations of true in situ measurements at elevated pressures and temperatures. We conclude that the technique opens up new possibilities particularly for XAS investigations of unsupported and/or concentrated materials that were hitherto difficult to study by other XAS detection techniques. I n situ TEY detection is most promising for studies with medium and hard X-rays ( E > 3 keV), which penetrate the gaseous environment of catalytic reactors most readily. Data are presented for iodine monolayers on Au(ll1): these show that monolayer studies are, in principle, possible, but represent the present limit of the technique in the medium and hard X-ray ranges. Results for a Cu-based catalyst for the synthesis of methanol are discussed to demonstrate what information can be achieved by the technique.

F K-edge X-ray absorption near-edge structure (XANES) of AlF3 polymorphs: combining ab initio calculations with Walsh correlation diagrams

“…The presence of He permits measurements of total electron-yield spectra without the interference of charging effects from the electrically insulating samples because secondary charges generated in the gas phase neutralise any charges accumulating at the surface of the sample. 28,29 A welcome side effect is the amplification of the total electron-yield signal through ionisation of He by energetic Auger electrons. [28][29][30] The gas-flow TEY detection mode probes essentially the bulk of the samples, with an exponential attenuation length in the region of 10 nm at the F K-edge.…”

“…28,29 A welcome side effect is the amplification of the total electron-yield signal through ionisation of He by energetic Auger electrons. [28][29][30] The gas-flow TEY detection mode probes essentially the bulk of the samples, with an exponential attenuation length in the region of 10 nm at the F K-edge. 31,32 This is significantly longer than surface-and near-surface changes that may be expected as a result of the acetone treatment during sample preparation.…”

The rise of electrochemical NAPXPS operated in the soft X-ray regime exemplified by the oxygen evolution reaction on IrOx electrocatalysts

“…The TFY signal is considered bulk sensitive owing to the large penetration depth of the photons, in the µm range in the soft X-ray regime. Meanwhile TEY is accepted as a surface sensitive technique due to the short total escape depth of the photoelectrons (elastic and inelastic) [24,25]. While useful for probing catalytic reactions, this surface sensitivity makes the collection of spectra especially challenging in the soft X-ray regime in presence of liquids, as the photoelectrons and secondary electrons are strongly scattered in dense media [10].…”

Active sites on an oxide catalyst for F/Cl-exchange reactions: X-ray spectroscopy of fluorinated γ-Al2O3

“…The presence of He permits measurements of total electron-yield spectra without the interference of charging effects. 19,20 A welcome side effect is the amplification of the total electron-yield signal through ionisation of He by energetic Auger electrons. [19][20][21] Sample activation and catalytic Cl/F-exchange Gas chromatographic analysis of the gas composition was performed with a Hewlett Packard HP 5890 GC Series II, using a polar PONA column and a flame ionisation detector (FID).…”

“…19,20 A welcome side effect is the amplification of the total electron-yield signal through ionisation of He by energetic Auger electrons. [19][20][21] Sample activation and catalytic Cl/F-exchange Gas chromatographic analysis of the gas composition was performed with a Hewlett Packard HP 5890 GC Series II, using a polar PONA column and a flame ionisation detector (FID). A temperature programme with a heating rate of 20 C min À1 was applied.…”