Size effect in the CO chemisorption on palladium clusters supported on magnesium oxide

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.

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“…And indeed, there are papers, authors of which could not observe CO dissociation for Pd/Al 2 O 3 and Pd/MgO samples [43][44][45][46].…”

Section: Co Adsorption On Pdmentioning

confidence: 93%

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

2005

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“…The d-band model thus explains the many observations that steps form stronger chemisorption bonds than flat surfaces [25,26,[30][31][32][33][34]. …”

Section: Variations Due To Changes In Surface Structurementioning

confidence: 95%

Ligand effects in heterogeneous catalysis and electrochemistry

Bligaard

Nørskov

2007

Electrochimica Acta

549

491

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“…This process is important for technical catalysis since carbon deposition is a major reason for catalyst deactivation. However, there are also conflicting reports on supported Pd, where CO dissociation was not observed [6][7][8][9].…”

Section: Introductionmentioning

confidence: 96%