Coupling of Carbon Monoxide Molecules over Oxygen-Defected UO₂(111) Single Crystal and Thin Film Surfaces

Abstract

While coupling reactions of carbon-containing compounds are numerous in organometallic chemistry, they are very rare on well-defined solid surfaces. In this work we show that the reductive coupling of two molecules of carbon monoxide to C2 compounds (acetylene and ethylene) could be achieved on oxygen-defected UO2(111) single crystal and thin film surfaces. This result allows in situ electron spectroscopic investigation of a typical organometallic reaction such as carbon coupling and extends it to heterogeneous catalysis and solids. By using high-resolution photoelectron spectroscopy (HRXPS) it was possible to track the changes in surface states of the U and O atoms as well as identify the intermediate of the reaction. Upon CO adsorption U cations in low oxidation states are oxidized to U4+ ions; this was accompanied by an increase of the O-to-U surface ratios. The HRXPS C 1s lines show the presence of adsorbed species assigned to diolate species (-OCH=CHO-) that are most likely the reaction intermediate in the coupling of two CO molecules to acetylene and ethylene.

The reactions of water vapour on the surfaces of stoichiometric and reduced uranium dioxide: A high resolution XPS study

“…The stoichiometric surface, spectrum A, has the U4f 7/2 and U4f 5/ 2 peaks at 380.0 and 390.8 eV in addition to two satellite-peaks (S 1 and S 2 ) at 386.7 and 397.3 eV, respectively. The position of these peaks is in good agreement with a large number of XPS results of ''stoichiometric'' UO 2 surfaces where all U cations are in a +4 oxidation state [12][13][14][15]17,21]. Upon sputtering with argon ions at 300 K (spectrum B), a shoulder on the lower binding energy side of the 4f peaks is seen.…”

“…Complete surface oxidation occurs by farther annealing to higher temperature. In previous works, we have shown that this is due to oxygen diffusion from the bulk to the surface [13,15]. In other words, there are two unrelated phenomena presented in Fig.…”

“…1 shows the XPS spectra for the stoichiometric surface (A) before Ar + -sputtering, after Ar + -sputtering at 300 K, (B) and at 95 K (C). The details of the oxidation-reduction of the surface is reported and discussed elsewhere [15]. The stoichiometric surface, spectrum A, has the U4f 7/2 and U4f 5/ 2 peaks at 380.0 and 390.8 eV in addition to two satellite-peaks (S 1 and S 2 ) at 386.7 and 397.3 eV, respectively.…”

“…This region (the low binding energy side of the 4f U 4+ ) is relatively complex and contains contribution from U cations in different oxidation states including U metal [8,[13][14][15]. The maximum of the shoulder is found at 377.2 eV in the U4f 7/2 region.…”

“…The shift to higher binding energy of the spectrum is also consistent with increasing the doping concentration of the n-type semiconductor. The corresponding O1s spectra, not shown, are addressed elsewhere [15]. Assuming that the O/U of the nominally stoichiometric surface is 2.00, the heavily reduced surface would correspond to UO 1.44 .…”