Reactions of acetic acid on UO2(111) single crystal surfaces

Abstract

The reactions of acetic acid have been investigated on the (111) surface of uranium dioxide single crystals, which was characterized by low energy electron diffraction and Auger electron spectroscopy. Temperature programmed desorption (TPD) of this molecule displays a rich chemistry on both the stoichiometric and electron beam sputtered surfaces. Acetic acid-TPD on a stoichiometric surface yields ketene (dehydration) as the main product, plus acetaldehyde as the minor product. On an electron beam sputtered surface, two additional association products were observed—butene and crotonaldehyde. The carbon yield of ketene on the stoichiometric surface was comparable to that of CeO2(111) and TiO2(001), higher than that of ZnO(0001) and Cu2O(100), but lower than that of MgO(100). This investigation has shown the ability of UO2(111) surfaces to dehydrate, reduce, and reductively couple organic molecules.

Effect of Sr Substitution in LaMnO3(100) on Catalytic Conversion of Acetic Acid to Ketene and Combustion-Like Products

“…The PE-TPR experiments produced water, CO 2 , CO, and ketene, as the major products; accompanied by the formation of some acetaldehyde and H 2 . These same products have been observed over CeO 2 (100), CeO 2 (111), UO 2 (111), TiO 2 (100), and Fe 3 O 4 , with the main peaks of the organic products generally observed under similar temperature ranges as in this study (550–750 K). ,,,, For the experiments reported here, evidence of acetone and ethene formation were not observed.…”

“…The results observed over CeO 2 (111) and UO 2 (111) that reduction suppresses ketene formation may be counterintuitive to some readers (because it requires the removal of oxygen from the acetic acid), but is explained by a dehydration mechanism if “too many” oxygen vacancies suppress the dehydration mechanism to ketene. The currently accepted mechanism for catalytic conversion of acetic acid to ketene over metal oxides does involve a lattice oxygen as the adsorption site of hydrogen in eq , ,,,− with this lattice oxygen site becoming empty again when H 2 O is formed and desorbs (the final step in the catalytic cycle for conversion of acetic acid to ketene and water). If we assume that a dehydration mechanism is in fact responsible for ketene, this can explain the high-temperature H 2 O trend observed in Figure b.…”

“…On the basis of the literature reports on cerium oxide and titanium dioxide, there appear to be distinct kinetic pathways to ketene and acetone formation (though these may share a common ketene-like intermediate) . There is evidence that the number of coordination vacancies of the cations at the surface is involved in determining selectivity: that multiple coordination vacancies per cation leads to ketonization, and that single coordination vacancies per cation leads to ketene. ,, Another factor which has been used to successfully correlate the selectivity toward ketene is the Madelung potential, as experienced by the metal cations and the oxygen ions in the oxide lattice . The Madelung potentials experienced by the O and the Mn in LaMnO 3 are reported to be ∼+22.5 and ∼−37.5 eV, respectively .…”

“…The molecules in the ultrahigh vacuum chamber environment do not have a direct line of site to the sample face, and result in a flux on the sample that is ≤2 orders of magnitude lower than the directed reactant flux on the sample face, and are thus negligible relative to the intentional dosing. The masses monitored during experiments were subsets of: 2, 14, 16,18,26,27,28,29,31,32,42,43,44,45,58, and 60. The signals were baseline corrected.…”

“…47,48,72 Another factor which has been used to successfully correlate the selectivity toward ketene is the Madelung potential, as experienced by the metal cations and the oxygen ions in the oxide lattice. 42 The Madelung potentials experienced by the O and the Mn in LaMnO 3 are reported to be ∼+22.5 and ∼−37.5 eV, respectively. 73 On the basis of the abovementioned considerations, the lack of acetone observed in this study is thus attributed to a genuine selectivity difference relative to other oxides, with La 1−x Sr x MnO 3 (100) surfaces favoring ketene rather than acetone.…”