X-ray photoelectron spectroscopy as a tool for in-situ study of the mechanisms of heterogeneous catalytic reactions

Abstract

An ESCALAB High Pressure photoelectron spectrometer specially designed by Vacuum Generators Co. (UK) for in-situ measurements at high pressures has been modified for adsorption and catalytic experiments. New construction of a gas cell, which serves to create a high-pressure zone, allows us to measure photoemission spectra in-situ at pressures up to 0.2-0.5 mbar and to use a sample holder with independent heating of a sample. This makes it possible to measure temperature programmed desorption and reaction spectra and, as a consequence, to characterize the catalytic properties simultaneously with the acquisition of X-ray photoelectron spectroscopy (XPS) data. Capabilities of the modified spectrometer are demonstrated by the examples of studies of CO and methanol adsorption on Pd(111) single crystal and methanol oxidation over Cu poly .The XPS study of high-pressure adsorption of CO on perfect and sputtered Pd (111) showed that raising the pressure has the same effect as decreasing the temperature. The CO coverage is increased due to adsorption of CO to weakly bound (on-top) state, which appears in addition to more strongly bound three-fold and bridged species. No indications of CO dissociation or carbonyl formation were found under the given experimental conditions (high pressure and sputtering-induced defects), provided that the CO gas was sufficiently clean.Two pathways of methanol decomposition to CH x,ads (x 3) and to CO ads was unambiguously identified, with their contribution being dependent on P and T. The decomposition to carbon-originated species had a small contribution in UHV conditions at room temperature, but became more significant at higher pressures and temperatures.Analysis of the distribution of the reaction products and surface species during methanol oxidation over copper in the submillibar pressure range showed that an increase in catalytic activity observed on heating the sample from 420 to 670 K is accompanied by transformation of the adlayer composition. At low temperatures, the surface is covered by methoxy-and, to a lesser extent, formate-groups. The active surface (T > 520 K) is metallic copper with two adsorbed oxygen species. The possible nature of these species is discussed.

Oxidation of propylene over Pd(5 5 1): Temperature hysteresis induced by carbon deposition and oxygen adsorption

“…3). As to mention above, the first two peaks correspond to carbon species adsorbed on the surface and/or dissolved in the Pd bulk, while the last low-intensity peaks correspond to CO adsorbed species and carbon atoms in surface oxygenates, respectively [15,[19][20][21]28]. The relative intensity of the C1s peak at 283.8 eV increases with decreasing the incident photon energy from 905 to 425 eV, whereas the intensity of the C1s peak at 284.4 eV changes in the reverse manner.…”

“…The reasons for the decrease or increase in activity of palladium were unraveled by X-ray photoelectron spectroscopy (XPS) and temperatureprogrammed reaction spectroscopy (TPRS). XPS is one of the powerful tools in the catalytic surface science to investigate both the surface composition and the nature of adsorbed species; while TPRS can provide direct information about catalytic properties of samples under study [19][20][21][22]. A Pd(5 5 1) single crystal was used as a model catalyst.…”

Selective oxidation of methanol to form dimethoxymethane and methyl formate over a monolayer V2O5/TiO2 catalyst

“…Since in situ NAP XPS and XANES techniques can be used in the mbar pressure range [39,[54][55][56], the correct comparison of the spectroscopic data with the kinetics measurements performed in the flow reactor at atmospheric pressure (Table 2 and Fig. 3) could be possible only if there is no pressure gap.…”

“…The third component at ca. 287.8 eV can be assigned both to gas phase methanol (the pressure is high enough to observe the gas phase signals [39,55]) and to molecularly chemisorbed methanol [4]. The amount of the methoxy species decreases with increasing of the temperature.…”

Evolution of self-sustained kinetic oscillations in the catalytic oxidation of propane over a nickel foil

“…It allowed us to apply surface sensitive techniques such as X-ray photoelectron spectroscopy (XPS) coupled with on line mass spectrometry (MS) and gas chromatography (GC) for the in situ study of this phenomenon [25]. XPS is a powerful tool of the heterogeneous catalysis to investigate both the surface composition and the nature of adsorbed species, while mass spectrometry can provide the abundance of products and reactants in the gas phase [28,29].…”