Thermal , Structural and Morphological Characterisation of Freeze-dried Copper ( II ) Acetate Monohydrate and its Solid Decomposition Products

In the present study the thermal decomposition of a freeze-dried copper(II) acetate monohydrate powder, (CH 3 COO) 2 CuH 2 O, (FDCuAcH 2 O), was analysed by a combination of high-temperature X-ray diffractometry; differential scanning calorimetry and thermogravimetry, up to 700 °C. The structure and morphology of the calcined freeze-dried powders were analysed by scanning electron microscopy and X-ray diffractometry. The results showed that FDAcCuH 2 O decomposes during heating in two stages: I) (25-225 °C) FDCuAcH 2 O dehydrates giving rise to copper(II) acetate, (CH 3 COO) 2 Cu, (AcCu), and II) (225-525 °C) AcCu decomposes to CuO through complex oxidation reactions of Cu and Cu 2 O, simultaneously. SEM showed that FDCuAcH 2 O powder has a scale-like morphology, which is created in the freezing stage and retained after freeze-drying. After calcination at 125 and 225 °C, clusters of elongated tubes (or filaments) compose the resulting powder (AcCu). Subsequent calcination at temperatures above 325 °C resulted in hard clusters of spheroid-like CuO particles.


Introduction
In general, powders of hydrate metal acetates have the form (CH 3 COO) 2 M .xH 2 O, where M is a metal cation, CH 3 COO is an acetate group and x is the number of water molecules.These materials are useful reagents commercially available and are used mainly in industrial processes.Although metal acetates are technologically important materials only recently systematic data about their solubilities in water as a function of temperature have been published 1 .
The fact that metal acetates (including nitrates, oxalates, sulphates, etc.) are water-soluble salts indicates that they can be freeze-dried individually or in multi-components.In the conventional freeze-drying 2 (or cryochemical) process, the starting salts of high purity are initially dissolved in an adequate solvent, rapidly frozen to avoid precipitation or segregation of the components, followed by solvent sublimation under vacuum.The resulting powders obtained in this way are expected to be chemically more homogeneous, free of contamination by impurities and highly reactive.After thermal decomposition and solid state reactions in high temperature the precursors give rise to a more uniform mixture of metal oxides in atomic scale.Freeze-drying have been used to make improvements in the powder synthesis of catalysts 3 , PLZT 4,5 , ferrites 6 and superconductors [6][7][8][9] .Recently, scientific and technological efforts have been carried out through the utilisation of the freeze-drying technique to produce homogenous mixtures of powders such as oxides with copper(II) acetate monohydrate (CH 3 COO) 2 Cu .H 2 O (denoted CuAcH 2 O in the text for simplification) which have been used as precursors to preparation of ZnO-based varistors 10,11 .
In the last decades much work has been published in the literature 9,10,[12][13][14][15][16][17][18] on thermal decomposition of CuAcH 2 O, under various conditions, to understand the thermal events involved in the solid-gas-solid reactions and the products of such reactions.This material has been investigated by differential thermal analysis (DTA) 9,12,14,15,18 , thermogravimetry (TG) [13][14][15][16]18 , differential scanning calorimetry (DSC) 18 , X-ray diffractrometry (XRD) 9,12,16,18,[19][20][21] , scanning electron microscopy (SEM) 9,17,18 , both infrared (IR) and Raman spectroscopy 9,13,14,18,22  In this work, the thermal decomposition course of a freeze-dried (CH 3 COO) 2 Cu .H 2 O powder (denoted FDCuAcH 2 O) have been studied by DSC and TG, in static air, up to 700 °C.The structural and morphological changes have been characterised by RXD and SEM, at ambient temperature, after calcination in air, in various temperatures up to 625 °C. Fo the first time on literature the results of in situ characterisation by high-temperature X-ray diffractometry (HTXRD) up to 600 °C of a FDCuAcH 2 O powder are showed.The main aim of this study was to obtain more information about thermally-induced structural modifications to decide for a better condition of thermal treatment to get a desired phase.

Experimental
From literature 1 , a solubility limit value of 0.38 mol.kg -1 , for AcCuH 2 O, in water, at 25 °C, corresponds to about 19 g of AcCuH 2 O for 250 ml of water.Thus, a certain mass of AcCuH 2 O was chosen in this wok to be below of its solubility limit.Initially, 15 g of copper(II) acetate monohydrate (CH 3 COO) 2 Cu .H 2 O powder (Mallinckrodt, 98.7%) was homogeneously diluted in 250 ml of distilled-deionized water, at ambient temperature.The pH value of the resulting aqueous mixture was around 6.7 indicating that there was no excess of acid products formed.In this work, the freeze-drying process consisted of two stages: i) freezing: the aqueous mixture was transferred to an appropriate glass flask and connected to a continuous freezing system (Edwards Shell Freezer).In this system, the flask (semidipped) rotated slowly and continuously (~45 min) in contact to a refrigerant fluid (-50 °C), at atmospheric pressure, until the aqueous mixture was completely frozen.After this step, overlapped thin frozen layers were created inside the flask; ii) drying: the flask was transferred and connected into the freeze-drier.A vacuum pump (Edwards E2M2) and a water trap (Edwards Micromodulyo) compose this equipment.During this stage the water sublimates and it was captured in the trap (-45 °C).After about 16 hours the powder was completely dry.During freeze-drying, the pressure inside the vacuum system (trap) was maintained at 2.4 × 10 -2 torr.The pH of the trapped water, at ambient temperature, after freeze-drying was around 6.2.
The thermal decomposition course of the resulting FDCuAcH 2 O powder was examined on heating up to 600 °C, at heating rate of 5 °C/min, in static air, by DSC and TG (Netzsch).The solid products from the thermal decomposition of FDCuAcH 2 O were identified in-situ using HTXRD (Siemens D5000).The powder was Joule-heated inside a Pt-crucible, at heating rate of 5 °C/min.The X-ray diffraction measurements were performed in a fixed range 34 < 2θ < 40°, at ambient temperature, and in intervals of 100 °C during heating up to 600 °C; at 300 °C and at ambient temperature, during cooling.
The FDCuAcH 2 O powders were calcined at various temperatures up to 625 °C, in intervals of 125 °C.The powders were calcined during 1h; in static air atmosphere, using a Al 2 O 3 -crucibles, in a electric furnace (EDG3000).During the calcinations, a constant heating and cooling rate of 5 °C/min was used.XRD measurements (Siemens D5000) were performed, at ambient temperature, for the FDCuAcH 2 O and calcined powders, in the range 5 < 2θ < 80°.The morphological aspects of the powders were analysed by SEM (Leica StereoScan 440) using secondary electrons, after Au-recovering.

Results and Discussion
The curves of thermal analysis of the FDCuAcH 2 O powder by DSC and TG are presented in the Fig. 1.DSC and TG measurements indicate that FDAcCuH 2 O decomposes via two processes of weight loss (thermal events I and II) over the temperature range 100-325 °C: Event I: (100-190 °C) corresponds to dehydration, with an endothermic maximum peak at 145 °C, corresponding to a weight loss about 11.9% at 190 °C.Normally, in both DTA and DSC, the obtained maximum or minimum temperature values are dependent on heating rate, shifting to right as heating rate is increasing 18 .Evidences from literature 9,12,[14][15][16]18 show that the first stage is mainly related to dehydration of (CH 3 COO) 2 Cu . H 2O (CuAcH 2 O) leading to formation of copper(II) acetate (CH 3 COO) 2 Cu (CuAc).The theoretical mass loss of water molecules of CuAcH 2 O corresponds to 9.0%.In this study, the greater weight loss than that expected for the formation of CuAc could be also attributed to the volatilisation of CuAc or water excess, in the FDCuAcH 2 O powder.The gas atmosphere significantly affects the mechanism of dehydration of CuAcH 2 O.Under vacuum, dynamic Ar or N 2 , CuAc sublimates and copper is lost 9,13,15,18 .
Event II: (220-325 °C) corresponds to decomposition of CuAc.In this range, DSC showed two double peaks: an exothermic at 245 °C, followed by an endothermic at 270 °C.These peaks are related to complex solid-gas-solid reactions involving probably the production of solid metallic copper (Cu 0 ) and volatile or gaseous products.TG showed that the mass loss at 375 °C was about 68.1%, which is equivalent to the theoretical mass loss to formation of Cu 0 (68.17%).The decomposition of CuAcH 2 O is greatly affected by the ambient atmosphere.In general, researchers [9][10][11][12][13][14][15][16][17][18] have been relating that in vacuum, N 2 or Ar, and in air, the main solid products have been Cu 0 and Cu 2 O, CuO or Cu 4 O 3 , respectively.The gaseous or volatile products have been mainly composed by acetone (CH 3 CH 3 CO), acetic acid (CH 3 COOH), acetaldehyde (CH 3 CHO), methane (CH 4 ), carbon dioxide (CO 2 ), and hydrogen gas (H 2 ).
The results of HTXRD for the range 34° < 2θ < 40° performed in air, at atmospheric pressure, are showed in Fig.The XRD spectra for the FDCuAcH 2 O calcined powders are presented in Fig. 3.At ambient temperature the     The morphological evolution of the FDCuAcH 2 O powder after calcination was examined by SEM. Figure 4a, 4b  and 4c) shows the morphological aspects of the FDCuAcH 2 O powders at ambient temperature (25 °C), and calcined from 125 to 225 °C/1h, respectively.The FDCuAcH 2 O powder, showed in Fig. 4a, has a clear scalelike aspect.The origin of this morphology is probably due to the freeze-drying processing technique used in this work.When the mixture is frozen overlapped solid thin layers are created inside the glass flask.During water sublimation that solid layers are retained giving rise to this interesting and unusual morphology.In a recent work, the characterisation by SEM of a freeze-dried CuAcH 2 O powder, obtained by spray-freeze-drying, showed morphology composed by very thin filaments or chained agglomerates 9 .Studies about the crystalline structure of CuAcH 2 O show that this material tends to form dimmers, (CH 3 COO) 4 Cu 2 , bonded by water molecules, which package in the monoclinic system [19][20][21][22] .The morphology of giant CuAcH 2 O single crystals have been reported as been composed by flat faces 17,18 .Analysis of both powder morphology, the FDCuAcH 2 O calcined at 125 and 225 °C, shows that after dehydration the powders form elongated solid tubes, which is characteristic of the open tetragonal structure of CuAc.After calcination at 325, 425 and 525 °C, CuAc decomposes to Cu 0 , Cu 2 O and CuO, in an oxidation reaction, resulting in hard clusters of spheroid-like CuO particles (Fig. 5).

Conclusions
Results obtained in this study show that the freeze-drying synthesis process is reliable for producing copper(II) acetate monohydrate powder (FDAcCuH 2 O), with an unusual scale-like morphology created in the freezing stage and retained after freeze-drying.Results of the solid decomposition products show that the copper(II) acetate monohydrate powder (FDAcCuH 2 O) decomposed during heating in two stages: I) (25-225 °C) FDCuAcH 2 O dehydrates giving rise to copper(II) acetate, (AcCu), and II) (225-525 °C) AcCu decomposes to CuO through complex oxidation reactions of Cu and Cu 2 O, simultaneously.
The morphological aspect of the products from decomposition of copper(II) acetate monohydrate powder (FDAcCuH 2 O), between 125 and 225 °C, are elongated solid tubes or filaments of AcCu and after subsequent calcination above 325 °C the morphological aspect resulted in hard clusters of spheroid-like CuO particles.

2 .
This range of study was especially chosen because it includes the diffraction planes of relative maximum intensity for the crystalline phases CuO and Cu 2 O, i.e., (-111) at 2θ = 35,55° and (111) at 2θ = 36,41°, respectively.Three phases were monitored and identified by card number from powder diffraction files (PDF) of the joint committee on powder diffraction standards (JCPDS): CuO (tenorite) or copper(II) oxide (PDF 5-0661); Cu 2 O (cuprite) or copper(I) oxide (PDF 5-0667); (CH 3 COO) 2 Cu.H 2 O (copper(II) acetate monohydrate) or copper acetate hydrate (PDF 27-0145).At ambient temperature (25 °C), XRD showed that the FDCuAcH 2 O powder is CuAcH 2 O. Comparing the XRD data for FDCuAcH 2 O powder with previous XRD data for the commercial material powder (not presented here) did not indicate any secondary phase.At 100 °C occurs the dehydration of FDCuAcH 2 O leading to formation of CuAc.At 200 °C is the beginning of the thermal decomposition process of CuAc leading probably to formation of Cu 2 O and CuO phases, simultaneously.In the range 200-400 °C, these amorphous phases crystallise to CuO in an oxidation process.At 500 and 600 °C, on heating and on cooling from 600 °C to ambient temperature the present phase is CuO.

Figure 1 .
Figure 1.Curves of DSC and TG for a freeze-dried copper(II) acetate monohydrate powder.

Figure 3 .
Figure 3. XRD spectra for the copper(II) acetate monohydrate powder: freeze-dried; and calcined at 125 to 625 °C, in air for 1h.