Studies of the thermal decomposition of copper hydride
Introduction
Copper hydride, CuH, occupies a specific place among the hydrides. It does not belong to any of the three main hydride types (covalent hydrides, ionic hydrides and metallic hydrides) and can be synthesized at room temperature in aqueous solution. Copper hydride was first prepared by Wurtz [1] in 1844 by reaction of aqueous copper sulfate with hypophosphorous acid. It forms a red-brownish crystalline powder of the wurtzite structure with and as determined by Goedkoop and Andresen [2].
Several other methods have been used to obtain copper hydride including precipitation of the hydride from pyridine solution of CuJ and LiAlH4 [3], by reaction of copper sulfate with sodium borohydride in aqueous solution [4], and by electrolysis of alkaline solutions containing Cu (II) complexes [5]. The samples always contain impurities (water, alcohol, organic compounds) dependent on the method of preparation. The characteristic feature of this hydride is its relatively low stability. It slowly decomposes even at room temperature. Warf and Faitknecht [6] have shown that the kinetics of the decomposition of copper hydride can be expressed by a first-order kinetic equation. Under water, its half-life time is 30 min at 70°C and 70 h at 30°C. The stability of the hydride depends on the surrounding medium: organic solvent and aqueous solution of sodium chloride make it higher, but it is lower in base solution.
CuH decomposes exothermically, although different authors have reported contradicting data concerning the parameters of this process. For the copper hydride prepared according to Wurtz method, Mikheeva and Mal’tseva [7] found a single exotherm between 97 and 137°C for the completely dry material. Fitzsimons et al. [8] obtained three distinct transitions at 90°C (endothermic), 105°C (endothermic) and at 142°C (exothermic). CuH precipitated from organic solution is stable up to 60°C as reported by Wiberg and Hoenle [3]. Up to now, only two studies on the enthalpy of decomposition of CuH have been published. The data obtained by Sieverts and Gotta [9] (−21.4 kJ mol−1) and Warf [10] (−32.9 kJ mol−1) are in poor agreement and outside experimental error. These discrepancies in the light of the present knowledge could be attributed to the samples purity and the method of preparation used for those studies.
The purpose of this paper is to determine the decomposition enthalpy of copper hydride carried out on well-defined sample of CuH (without contamination) and the temperature dependence of this process.
Section snippets
Experimental
Copper hydride was prepared by reaction of aqueous copper sulfate with hypophosphorous acid at 45°C [6] based on the following equation:Small amount of sodium chloride was added to avoid decomposition of copper hydride during synthesis. The precipitate was cooled, filtered, and rinsed with ice water and cold absolute alcohol. The CuH was purified in vacuum at 0°C using liquid nitrogen trap. The powder was stored for further investigations in solid carbon
Results
TGA and DSC experiments were carried out on conventionally prepared sample as shown in Fig. 2, Fig. 3, respectively. The TGA curve shows several processes leading to a gradual weight loss which can hardly be connected with evolution of hydrogen from the sample. This coincidences with the DSC data where the main hydride decomposition and small unexpected thermal effects overlap. We obtained a large number of almost identical results for both DSC and TGA measurements from different runs and
Discussion
The determination of the thermodynamic properties of copper hydride is difficult due to the following reasons. Firstly, the problem of the thermal stability of CuH. Most authors agree that even at the temperature of 0°C it decomposes slightly. As a result, to avoid sample decomposition, CuH should be investigated at hydrogen pressure equal to or higher than the equilibrium pressure, which so far has not been experimentally determined. Furthermore, samples prepared in the proposed ways so far
Conclusions
The enthalpy of decomposition of copper hydride has been determined as equal to . Different methods of preparation resulting in different levels of contamination of CuH sample could be blamed for the discrepancies observed in previously published data. Our results obtained on the sample free of contamination reflect true value of enthalpy of decomposition of CuH. Excellent reproducibility of measured value of CuH observed for both different samples and different experiments
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