A Short Review of the Separation of Iridium and Rhodium from Hydrochloric Acid Solutions by Solvent Extraction

Synthesis of Atomically Dispersed Rh Catalysts on Oxide Supports via Strong Electrostatic Adsorption and Characterization by Cryogenic Infrared Spectroscopy

“…This is consistent with the exchange of Cl ligands for H 2 O, OH, or NH 3 and likely results in decreasing negative charge or a switch to positive charge for the Rh ions in solution. 69,71,72 These results are consistent with previous examples where RhCl 3 •xH 2 O undergoes hydrolysis when diluted with water and then, when mixing with desired reagents, results in the formation of new ionic complexes in solution. 69−74 Given the broadness of the bands at low pH and the resulting featureless spectra at higher pH, the results suggest that RhCl 3 initially speciates into a range of anionic complexes, which then undergo ligand exchange to begin producing predominantly cationic species above a pH of about 7.…”

“…The speciation of RhCl 3 is known to be fast under these conditions and results in a distribution of the charged metal−aquo complexes. 30,64,70,71 The broadness of the bands observed in the UV−vis spectra here in comparison to previous measurements on well-defined Rh species suggests that the initial precursor solution contains a range of anionic Rh species with varying Cl, H 2 O, and OH coordination. 64 As pH of the solution was increased with NH 4 OH addition, the absorbance features associated with anionic Rh−Cl aquo complexes shifted to lower wavelengths (higher energy) and became less resolved, and the overall absorbance of the solution increased.…”

“…For example, diluting RhCl 3 · x H 2 O in liquid produces a range of Rh ions with varying Cl, OH, and H 2 O coordination, as well as overall charge, which exhibit distinct UV–vis absorbance spectra. − UV–vis analysis of a dilute (125 ppm Rh with a resulting pH of 3) solution of RhCl 3 · x H 2 O in water results in a spectrum with broad bands at ∼408 and 518 nm, Figure a. Experimental and theoretical analyses have assigned these features to RhCl 6 3– or RhCl 5 (H 2 O) 2– . − The RhCl 6 3– complex favorably forms in an acidic solution when excess Cl is present but undergoes fast aquation into RhCl 5 (H 2 O) 2– . ,, In this experiment, no additional Cl was added to the solution, which suggests that RhCl 6 3– species either form due to excess Cl concentration in the stock solution or from disproportionation of RhCl 3 into species with higher and lower Cl coordination as compared to the starting precursor. The speciation of RhCl 3 is known to be fast under these conditions and results in a distribution of the charged metal–aquo complexes. ,,, …”

“…A key component and complication to the SEA-based synthesis of supported Rh catalysts is the speciation of Rh ions 67,70,71 In this experiment, no additional Cl was added to the solution, which suggests that RhCl 6 3− species either form due to excess Cl concentration in the stock solution or from disproportionation of RhCl 3 into species with higher and lower Cl coordination as compared to the starting precursor. The speciation of RhCl 3 is known to be fast under these conditions and results in a distribution of the charged metal−aquo complexes.…”

Recovery of Iridium by Solvent Extraction and Direct Electrodeposition Using Phosphonium-Based Ionic Liquids

“…11 Additionally, the separation of Ir(IV) from Rh(III) in a hydrochloric acid solution by solvent extraction (SX) using amines has been reported. [12][13][14] The electrochemical behavior of Ir electrodeposited in 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF 4 ] 15 or in the composite IL 1-butyl-3-methylimidazolium chloride tetrafluoroborate [BMIC] [BF 4 ] 16 has also been reported.…”