Reactions of acetone on the surfaces of the oxides of uranium

Abstract

The reactions of acetone were investigated on α U3O8 (stoichiometric and H2 reduced) by temperature programmed desorption. The surface and bulk characteristics of U oxides were investigated by x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD). The comparison between the XPS U 4f7/2 binding energy of β UO3, α U3O8, and UO2 indicated that the U cations in U3O8 are composed of +5/+6 (2/1) or +4/+6 (1/2) oxidation states. XPS of H2 reduced α U3O8 (at 800 K) indicated a shift of the U 4f7/2 binding energy from 381.6 (unreduced) to 380.5 eV (reduced) as well as the appearance of two satellites at 387.0 and 397.4 eV. These new XPS lines’ positions are characteristic of U+4 cations of UO2. Similar results were observed upon Ar-ion sputtering of α U3O8. Ar-ion sputtering of β UO3 also results in the reduction of U+6 cations to U+4 cations. XRD of H2 reduced α U3O8 indicated that all of the α U3O8 phase was transformed to the UO2 phase. A considerable difference between the reactivity of the surfaces of U3O8 and UO2 towards acetone was observed. A carbon–carbon bond formation reaction giving isobutene was observed only on the α U3O8 surface indicating its structure-sensitive nature and/or its sensitivity towards changing the oxidation state of U cations. This reaction has not been observed on the surfaces of actinide oxides before. On the other hand, acetone reacted on UO2 to give mainly propene. This latter reaction (which is a C–O bond dissociation) is most likely due to the capacity of UO2 to accommodate large numbers of atomic oxygen in interstitial positions while maintaining its structure intact.

Adsorption/Desorption Behavior of Ethanol Steam Reforming Reactants and Intermediates over Supported Cobalt Catalysts

“…Although butene and propanol formation from acetone has been reported over various metal oxides [47][48][49], these species were not observed in this study during acetone TPD over reduced Co/ZrO 2 . This may be due to C-C cleavage activity of the Co-based catalysts.…”

The interaction of uranium metal with nitrogen oxides: The formation of an oxynitride surface layer

“…The localized 5f peak is similar in intensity and located in a similar position to the localized 5f peak observed upon oxidation of uranium above UO 2 [21], suggesting that following reaction with NO (>10 L exposure) uranium is in a higher oxidation state than +4 and there is increased 5f electron involvement in bonding. Broadening of the core-level spectra would support this observation and may suggest the possible presence of a combination of +3 (UN) [18], +4 (380.0 eV) and +6 (381.2 eV) oxidation states [22,23] within the sampling depth of XPS.…”

“…This indicates that in the surface region the oxidation state of the uranium approaches U VI , as observed for UO 3 [26]. Since we would expect no localised U 5f feature in the UPS spectra for a U VI compound, the 5f peak observed at the highest exposures could be a result of an underlying lower oxide in agreement with the U 4f spectra, where the observed binding energy and peak broadening suggest a combination of +4 and +6 oxidation states [22,23]. …”

Acetaldehyde reactions over the uranium oxide system

“…2. We have previously shown that U oxides are active for several C-C bond formation reactions: making isobutene from acetone on α-U 3 O 8 [18]; making furan from acetylene [19], ethylene [20], acetaldehyde [21], or ethanol [22] on β-UO 3 ; and making ethylene from formaldehyde on UO 2 (111) single crystals [23].…”

“…Although there is considerable knowledge now of the bulk properties of this oxide system, our understanding of their surface reactions lags that of early transition metal oxides. Among the studies on uranium oxides conducted so far are organometallic investigations [1][2][3][4], theoretical calculations [5][6][7], surface spectroscopy [8,9], and catalytic studies [10][11][12][13][14][15][16][17][18]. Uranium oxides can be used as catalysts, promoters, or supports for other oxides and metals.…”