Kinetic and characterization studies of the formation of barium monomolybdate in equimolar powder mixture of BaCO 3 and
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Mohamed I. Zakia) Chemistry Department, Faculty of Science, Minia University, El-Minia 61519, Egypt (Received 13 January 2003; accepted 7 July 2003)
The formation of barium monomolybdate (BaMoO4) in equimolar powder mixtures of BaCO3 and MoO3 was examined under isothermal and nonisothermal conditions upon heating in air at 25–1200 °C, using thermogravimetry. Concurrence of the observed mass loss (due to the release of CO2) to the occurrence of the formation reaction was evident. Accordingly, the extent of reaction (x) was determined as a function of time (t) or temperature (T). The x-t and x-T data thus obtained were processed using a well-established mathematical apparatus and methods to characterize the nature of the reaction rate-determining step and derive isothermal and nonisothermal kinetic parameters (rate constant, frequency factor, reaction order, and activation energy). Moreover, the reaction mixture quenched at various temperatures (450–575 °C) in the reaction course was analyzed by various spectroscopic (x-ray diffractometry, infrared spectroscopy, and laser Raman spectroscopy) and microscopic (scanning electron microscopy and x-ray energy dispersive spectroscopy) techniques for material characterization. The results obtained indicated that the reaction rate may be controlled by unidirectional diffusion of MoO3 species through the product layer (BaMoO4), which was implied to form on the barium carbonate particles. The nonisothermally determined activation energy (156 kJ/mol) was found to be close to the isothermally determined one (164–166 kJ/mol).
I. INTRODUCTION
Over the past three decades, applied solid state chemistry has been established as a recognizable field in chemical and physical sciences,1 wherein more and more methods and procedures of preparation of new materials of distinct properties and applications are being devised.2 Challenges in understanding this field, discovering its merits, and probing its ambiguities are growing with some excitement.1 What is definite about it, however, is that this field owes its existence to advancing knowledge about kinetics and mechanisms of reactions in the solid state.3 A general consensus has been developed, that kinetic studies of reactions in the solid state are hampered by a number of theoretical and practical problems.3,4 The most prominent of these problems are that (i) the mixing of solids at atomic dimensions, as required by chemical reactions, is less obvious than of gases or liquids;3 (ii) a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 10, Oct 2003
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controlling parameters of reaction kinetics in the solid state, such as fraction of reaction completed, changes in reactant particle shape and size, influence of reaction reversibility, and temperature inhomogenieties, are difficult to determine or account for;4 and (iii) neither isothermal nor nonisothermal kinetic analysis is alladvantageous in the absolute sense.5,6 Whereas the isotherma
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