On the thermal decomposition mechanism of uranyl and uranium(IV) acetates

Abstract

Abstract The methods of isothermal kinetics. X-ray spectrography, mass spectrometry and IR spectroscopy were used to study the thermal decomposition of uranyl acetate, UO 2 (CH 3 COO) 2 , and uranium(IV) acetate, U(CH 3 COO) 4 . The data obtained were supported by the results of the chemical analysis of products formed in decomposition processes and in model reactions. The compounds studied are assumed to be decomposed by a single mechanism including the following elementary stages: (1) breaking the carbon—oxygen bond of the carboxyl group, (2) forming a new carbon—oxygen bond with the appearance of an intermediate compound (co-ordinated to uranium atoms) with the structure of acetic anhydride, (3) breaking the uranium—oxygen bond with a subsequent rotation of the acetyl group, (4) transferring a hydrogen atom from a methyl radical of one acetate group to the oxygen of another and evolving a molecule of acetic acid which is the primary product of metal acetate decomposition. Arguments are presented in favour of the suggested mechanism. Despite a great number of papers on the thermal decomposition of metal salts of organic acids, there is no commonly recognized picture of its mechanism even for such relatively simple systems as metal formates and acetates. This is especially true for uranium compounds which are insufficiently studied in general. In the present work, an attempt is made, by using a variety of methods such as elementary chemical and X-ray phase analysis. IR spectroscopy, modelling separate reactions, measurement of the vapour pressure of volatile decomposition products and time-of-flight mass spectrometry, as well as some methods of isothermal and non-isothermal kinetics, to inquire into the process of the thermal decomposition of hexa-and tetravalent uranium salts of acetic acid, viz. UO 2 (CH 3 COO) 2 , UO 2 (CH 3 COO) 2 - CH 3 COOH, UO 2 (CH 3 COO) 2 - 2H 2 O, U(CH 3 COO) 4 . Comparison of regularities in the decomposition of uranium tetraacetate and uranyl acetate is of interest in the sense that in each case the solid products contain one and the same oxide, viz. UO 2 1,2 . It could be expected, therefore, that this comparison would lead to an understanding of the process of formation of decomposition products, to get a more comprehensive idea of the thermal destruction of metal acetates and to draw more convincing conclusions about the separate elementary stages. According to the X-ray spectrographic and IR spectroscopic data obtained by the authors on decomposing the above four compounds, no intermediate stable solid substances are formed 3, 4 . As to the final product, it is found to contain 0.45 g at C/g at. U in the case of uranyl acetate and one order less (≈ 0.04 g at C/g at. U) in the case of uranium tetraacetate. Of the volatile products of decomposition in vacuum, low temperature traps cooled by liquid nitrogen were used to collect and then to separate and identify (by vapour pressure and IR spectra) the following substances: acetic acid, acetic anhydride, ketene, acetone, carbon dioxide, carbon monoxide and water 3, 4 . To the authors' knowledge, there is only one paper available 3 in which ketene is mentioned as a product of metal (potassium) acetate decomposition. The radical mechanism of the thermal decomposition of metal acetates suggested by Bell and Reed 6 cannot explain the formation of acetic acid as the main decomposition product of acetates of metals such as copper, silver and others, including uranium acetate. Poppl's explanation 3 , which attributes the formation of acetic acid to hydrolysis, can hardly be satisfactory. At the same time, the occurrence of acetone among the thermal decomposition products of acetates of uranium (as well as other metals) may be conditioned by both the catalytic transformation of acetic acid and the processes leading to the elimination of acetone as an initial product. These points were to be verified and this was done by means of model reactions 1 , Study was made of the transformation of vapours of acetic acid, acetic anhydride and acetone on uranium dioxide and trioxide at 300°C ± 2°C. The experiments 4 have shown that (1) acetone does not undergo any transformation under these conditions; (2) acetic acid yields acetone, carbon dioxide and water in accordance with the well-known equation of ketonization