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Free keywords:
Palladium; Pd(111); Ethene oxidation; High pressure in situ XPS; Subsurface carbon; Dissolved carbon; Temperature-programmed reaction; Kinetic hysteresis
Ethylene Oxidation over Pd(111)
Abstract:
The oxidation of ethene on the Pd(111) surface was studied in the temperature range of 330–923 K by in situ XPS and mass spectrometry both heating and cooling in a reaction mixture of 5 × 10−4 mbar C2H4 and 1.5 × 10−3 mbar O2. Carbon-containing surface species were found to be strongly predominant over oxygen species within the whole temperature range, despite the excess of oxygen in the gas phase. Diffusion of carbon into the palladium bulk started at 480 K, leading to the appearance of an electronically altered, dissolved carbon phase with a C1s energy of 284.45 eV at ~500 K, which extended over several layers in the near-surface region and was stable up to ~650 K. This spectroscopic trend was clearly related to a pronounced shift of catalytic selectivity toward CO. Above 660 K, the dissolved carbon species decomposed,
the reaction occurred on an adsorbate-depleted Pd metal surface, with CO as the main product. During the cooling ramp, the same near-carbon modification formed at a 70-degree-lower threshold temperature, inducing pronounced hysteresis of the catalytic selectivity.
