RHODIUM CARBOXYLATES

Rhodium (II) acetate hydrate [Rh(CH3COO)2]2·2 H2O

Fig. [Rh(OAc)2]2*2H2O dimeric structure

Rhodium (II) acetate dihydrate [Rh(CH3COO)2]2 ·2 H2O and its anhydrous analogue [Rh(CH3COO)2]2 is dark green powder slightly soluble in polar solvents including water.[[i]]

Rhodium(II) acetate is usually prepared by heating the hydrated rhodium(III) chloride in acetic acid (CH3COOH):[[ii]]

     Anhydrous rhodium(II) acetate has dimeric cage-like structure [Rh(CH3COO)2]2 with two Rh atoms bonded together by a strong Rh-Rh bond, and 4 acetate groups surrounding each Rh atom. The hydrated coplex has H2O molecules on Rh atoms at the edges of the dimer. (Fig.).

 [i] https://en.wikipedia.org/wiki/Rhodium(II)_acetate

[ii] G.A. Rempel, P. Legzdins, H. Smith, G. Wilkinson, Inorg. Synth. Inorganic Syntheses, 1972,  13. pp. 90–91, « Tetrakis(acetato)dirhodium(II) and Similar Carboxylato Compounds », doi:10.1002/9780470132449.ch16.

Characterisation of [Rh(OAc)2]2*2H2O by TGA/DSC

    Rhodium (II) acetate dihydrate [Rh(CH3COO)2]2 ·2 H2O was characterised by the thermogravimetric analysis/differential scanning calorimetry (TGA/DSC).

    Thermogravimetric (TG) curve of [Rh(CH3COO)2]2 · 2H2O in N2 atmosphere has two well defined weight loss steps. The first stage at 90–200 ◦C is mainly related to the dehydration leading to the formation of Rh2(CH3COO)4;  the weight loss at this stage is ca. 0.73 % (approximately matches to the theoretical value corresponding to the loss of two water molecules).

     The mechanism of formation of Rh(0) by the decomposition of rhodium acetate at 300–450◦C, at a relatively lower temperature in N2 atmosphere is due to the evolved CO, which reduces the Rh-O bond in situ.

Gradual weight loss of ~50 % up to 510 ◦C observed for pure rhodium acetate was attributed to the decomposition of organic framework from Rh and the evolution of pure metallic cubic shape by the disproportionation reaction.

Steady increase in the weight at ~590 ◦C up to ca. 9%, corresponding to RhO2 formation, which is in good agreement with results from XPS studies.

Above 825◦C, there is a gradual decrease in the weight loss due to the formation of Rh2O3 (i.e. decrease in the Rh: O ratio), which is consistent with the results of similar studies with other metal acetate systems.

Rh(OAC)2 DECOMPOSITION PATHWAY

 Proposed decomposition pathway of [Rh(CH3COO)2]2  · 2H2O  

Decomposition pathway of [Rh(CH3COO)2]2  · 2H2O  was determined, which can be summarized as below:

Reaction Pathway:

Stage I; (50−200◦C):  Rh2(CH3COO)4  · 2H2O → 2 Rh(CH3COO)2 + 2H2O 

Stage II; (300−450 ◦C):

2 Rh(CH3COO)2 → Rh2O3 + 4CH3COCH3 → 4CO + 4CH3−CH3 ↑

4 CH3COCH3 → 4CO + 4CH3−CH3 ↑

Rh2O3 + 3CO → 2Rh + 3CO2 ↑   

Stage III; (525−800 ◦C): Surface Rh + in situ ½ O2 → RhO2.

Rh(OAC)2 FOR MOCVD

Rh(OAc)2 for Rh nanostructures by CVD

  Rhodium acetate Rh(OAc)2 was applied as a rhodium metal-organic source for the deposition of Rh nanostructures by single step CVD. Shape selective growth of Rh nanostructures (as a function of temperature) was demonstrated, with different shapes like cubes at 500 °C, pyramids and hexagons at 500 °C, 700 °C and 900 °C obtained. The morphology, crystallinity and phase purity of the structures was studied by SEM, XRD, TGA, XPS and four probe conductivity measurements. The range of 2-7 kS/cm2 conductivity was measured in the synthesised nanostructures. The possible mechanistic pathway for the evolution of Rh nanostructures was discussed on the basis of the TGA and XPS studies.[i]

[i] B.R. Sathe, AIP Advances 2, 042122 (2012); « A facile approach for shape selective synthesis of rhodium nanostructures and conductivity studies », http://dx.doi.org/10.1063/1.4764867 http://www.researchgate.net/publication/232659766_A_facile_approach_for_shape_selective_synthesis_of_rhodium_nanostructures_and_conductivity_studies/file/d912f5089603cf1c1b.pdf

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