A study on La 2 Mo 2 O 9 decomposition process
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A study on La2Mo2O9 decomposition process Bei Ruan • Baijun Yan • Jiayun Zhang
Received: 23 June 2012 / Accepted: 1 September 2012 / Published online: 19 September 2012 Ó Springer Science+Business Media, LLC 2012
Abstract The present paper reports the first stage of the study by the present group on the thermal decomposition of La2Mo2O9 under lower oxygen partial pressure. The stability of La2Mo2O9 at 1473 K in an oxygen partial pressure ðpO2 Þ ranges from 2.44 9 10-5 to 4.33 9 10-9 Pa was studied by XRD. The results showed that La2Mo2O9 could remain stable in a pO2 range greater than 2.44 9 10-5 Pa. While below this pO2 , the formation of several reduced and crystallized molybdates were found. By means of XRD, TEM, SAED, and EDS, the decomposition process of La2Mo2O9 at 1473 K under pO2 of 9.75 9 10-7 Pa was investigated. The intermediate and final products formed during the thermal decomposition were characterized. It was found that the La2Mo2O9 decomposition consisted of two steps. La2Mo2O9 first decomposed to intermediate products like LaMo2O5, La5Mo4O14, and La12Mo6O35 and then to final products like La5Mo3O16 and a new Mo-rich compound. Tilt series of SAED patterns of this new structure were recorded. These measurements were used for the reconstruction of the reciprocal space. The cell attained was monoclinic with parameters: a & 1.195 nm, b & 4.133 nm, c & 1.314 nm, and b & 95°.
B. Ruan B. Yan (&) J. Zhang Department of Physical Chemistry of Metallurgy, University of Science and Technology Beijing, 30 Xue Yuan Road, Beijing 10083, People’s Republic of China e-mail: [email protected] B. Ruan Department of Chemistry, Hebei Normal University, Shijiazhuang 050016, People’s Republic of China
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Introduction The lanthanum molybdates have attracted interests due to their potential applications in many fields such as solid oxide fuel cells (SOFCs), up-conversion materials, catalysts, magnetic materials, and so forth [1, 2]. The excellent physical chemistry properties of the lanthanum molybdates strongly depend on their crystal structures and the oxidation state of molybdenum. For instance, La2Mo3O12containing hexavalent molybdenum presents catalytic activity for selective oxidation of hydrocarbon compounds [3]. Many lanthanum molybdates in which the oxidation states of molybdenum are lower than ?6, such as LaMo8O14 [4], LaMo5O8 [5], and La5Mo32O54 [6], possess semiconducting and magnetic properties. As an oxide-ion conductor originally reported by Lacorre et al. [7], La2Mo2O9, exhibits good ionic conduction property at intermediate temperature range between 873 and 1073 K. In recent years, many lanthanum molybdates with different oxidation states of molybdenum were synthesized by means of solid-state reaction, fused salt electrolysis, hydrogen reduction, and thermal decomposition methods, e.g., the MoVI compounds, such as La2Mo4O15 [8], La2Mo3O12 [8], La2Mo2O9 [8], and La2MoO6 [8]; the MoIV compounds, such as La2MoO5 [9], and La2Mo2O7 [10]; the mixed molybdenum valence compounds, such as La5Mo3O16 [11],
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