Thick rhodium electrodeposition on copper backing as the target for production of palladium-103

The Rh target preparation for production of ¹⁰³Pd was investigated by using a thick electrodeposition of rhodium metal on a copper backing. The electrodeposition experiments were performed in acidic sulfate media using RhCl₃·3H₂O, Rh₂(SO₄)₃ (recovered from hydrochloric acid solution) and also in the commercially available Rhodex plating baths. For high current beam irradiation of a Rh target, the qualities of the deposit of the three baths were compared in terms of thermal shock, crack-free and morphology criteria. The quality of the plating obtained from a sulfate bath [Rh₂(SO₄)₃] was comparable with the one obtained from commercially available Rhodex bath. The optimum conditions of the electrodepositions were as follows: 4.8 g rhodium [as Rh₂(SO₄)₃], pH 2, DC current density of ca 8.5 mA·cm^–2, 1% sulfamic acid (w/v) and temperature 40–60 °C.The authors would like to thank their colleagues at the VUB-Cyclotron department for help and assistance in preparation of the electrodeposition equipment and taking the SEM photomicrographs and also K. Aardaneh (NRCAM) for his assistance.     

Crack-free,thick rhodium deposition on copper substrate using Rhodex solution

Summary Rhodium electrodeposition on a copper substrate was investigated for ¹⁰³Pd production. The electrodeposition was carried out by the commercially available Rhodex plating baths. The optimum conditions of the electrodeposition for complete depletion of Rh were: 6.2 g/l rhodium, DC current density of 12.83 mA ^. cm^-2 and 60 °C temperature.     

Nitration of rhodium(III) sulfates

Nitration of sulfate complexes of rhodium has been investigated by NMR ¹⁰³Rh, ¹⁴N, ¹⁵N, and ¹⁷O NMR. At high pH, [Rh(NO₂)₆]^3?, dimer [Rh₂(μ-OH)₂(NO₂)₈]^4?, and trimer [Rh₃(μ-OH)₄(OH)(NO₂)₉]^5? are the dominant species in solutions.     

State of rhodium(III) in sulfuric acid solutions

The formation of rhodium(III) sulfate complexes under moderately rigorous temperature conditions was studied by ¹⁰³Rh and ¹⁷O NMR spectroscopy. The complexes [Rh₂(μ-SO₄)₂(H₂O)₈]²⁺, [Rh₂(^*μ-SO₄)(H₂O)₈]⁴⁺, and [Rh₃(μ-SO₄)₃(μ-OH)(H₂O)₁₀]²⁺ were found to be the most stable species in aged solutions.     

Ethanol synthesis from syngas on Mn- and Fe-promoted Rh/γ-Al2O3

The catalytic hydrogenation of CO was studied over Mn- and/or Fe-promoted Rh/γ-Al₂O₃ catalysts. The catalysts were characterized by means of XRD, BET, H₂-TPR·H₂-TPD, XPS and DRIFTS. CO hydrogenation results showed that the doubly Mn- and Fe-promoted Rh/γ-Al₂O₃ catalysts exhibited superior catalytic activity and better ethanol selectivity. The DRIFTS results showed that Mn promoter stabilized the adsorbed CO on Rh⁺ and Fe stabilized adsorbed CO on Rh⁺ and Rh⁰, especially Rh⁰. The fact that doubly Mn- and Fe-promoted Rh/γ-Al₂O₃ owned more (Rhx⁰–Rhy⁺)–O–Fe³⁺·(Fe²⁺) active species was proposed to be a crucial factor accounting for its higher ethanol selectivity.     

Realization of Rhodium Metal‐Oxide Electrode in Indifferent Electrolytes

The pH‐dependence of the stationary open‐circuit potential E_i=0^st of rhodium electrode with a surface layer of anodically formed insoluble compounds has been studied in sulfate and phosphate solutions by means of cyclic voltammetry and chronopotentiometry. The range of potentials of the investigations performed has been confined to the region of rhodium electrochemical oxidation/reduction, i.e., 0.2<E<1.2 V (RHE) in order to prevent any possible interference of other reactions such as H₂ and O₂ evolution. It has been shown that rhodium electrode with a layer of surface compounds formed anodically at E<<1.23 V (RHE) behaves like a reversible metal‐oxide electrode within the range of pH values from ca. 1.0 to ca. 8.0. It has been presumed that the stationary potential of such electrode is determined by the equilibrium of the following electrochemical reaction: Rh+3H₂O??Rh(OH)₃+3H⁺+3e^?. The pH‐dependence of the reversible potential of Eequation/tex2gif-inf-6.gif electrode has been found to be: Eequation/tex2gif-inf-8.gif=E_i=0^st=0.69?0.059 pH, V. In acid solutions (pH<2.0) rhodium hydroxide dissolves into the electrolyte, therefore, to reach equilibrium, the solution must be saturated with Rh(OH)₃. This has been achieved by adding Rh³⁺ ions in the form of Rh₂(SO₄)₃. The solubility product of Rh(OH)₃, estimated from the experimental Eequation/tex2gif-inf-16.gif?pH dependence obtained, is ca. 1.0×10^?48, which is close to the value given in literature.     

Oxidation of the [Rh(CO)2Cl2] anion in aqueous solution

The reaction of rhodium(I) carbonyl chloride, [Rh(CO)₂Cl]₂, with dichromate, cerium(IV) sulfate, hexachloroplatinic acid or p-benzoquinone in aqueous hydrochloric acid proceeds by consumption of 4 equivalents of oxidizing agent per mole or rhodium(I) in accordance with the equation Rh^I(CO)₂  4e + H₂O → Rh^III(CO) + 2H⁺ + CO₂A “cyclic” oxidation mechanism is suggested.     

Kinetics of ammoniation of some halobis(ethylenediamine)-rhodium(III) perchlorates in liquid ammonia

Summary The ammoniation ofcis-[Rh(en)₂Cl₂] · (ClO₄) in liquid NH₃ was studied at constant ionic medium of 0.20 m perchlorate in the 0 to 35° range. The complex reacts in two distinct steps to givecis-[Rh(en)₂(NH₃)₂] · (ClO₄)₃, with the intermediate formation ofcis-[Rh(en)₂(NH₃)Cl] · (ClO₄)₂. Both steps follow a conjugate-base mechanism. Activation parameters were obtained for the acid-base preequilibrium and the rate-determining step. The entropies of activation for the rate-determining step are 0 and –42 JK^–1mol^–1 for the first and second ammoniations respectively. These values are considerably lower than those found for the cobalt(III) analogues. The entropy changes for the acid-base equilibria are –84 and –36 JK^–1mol^–1 respectively, which is less negative than those values found for the cobalt(III) analogues. Trans-[Rh(en)₂I₂] · (ClO₄) ammoniates totrans-[Rh(en)₂(NH₃)I] · (ClO₄)₂. The contribution of spontaneous ammoniation to the overall reaction oftrans-[Rh(en)₂I₂] · (ClO₄) is negligible, so the uniqueness oftrans-[Co(en)₂Cl₂] · (ClO₄) among cobalt(III) complexes in this respect is not reproduced for thetrans-dihalotetraamine structure in rhodium(III) complexes. A comparison of cobalt(III) and rhodium(III) amines with respect to activation parameters and the influence of formal charge of the metal complex on reactivity indicates a more associative type of activation for rhodium(III).     

High-yield syntheses of [Rh7(CO)16] and [Rh14(CO)25] working in ethylene glycol solution under 1 atm of CO

The anionic rhodium carbonyl clusters [Rh₇(CO)₁₆]³⁻ and [Rh₁₄(CO)₂₅]⁴⁻ can be easily prepared by a new simple and high yield one-pot synthesis starting from RhCl₃·nH₂O dissolved in ethylene glycol and involving two steps: (i) treatment of RhCl₃·nH₂O under 1 atm of CO at 50 °C to give [Rh(CO)₂Cl₂]⁻; (ii) addition of a base (CH₃CO₂Na or Na₂CO₃) followed by reductive carbonylation under 1 atm of CO at an adequate temperature (50 °C for [Rh₇(CO)₁₆]³⁻; 150 °C for [Rh₁₄(CO)₂₅]⁴⁻). These new syntheses are more convenient than those previously reported, especially since such clusters are not accessible via silica surface-mediated reactions. This different behavior is due to the particular stabilization on the silica surface and under 1 atm of CO of an anionic carbonyl cluster, called A, which does not allow the formation of a higher nuclearity carbonyl cluster, called B, which was shown to be the key-intermediate in the synthesis of [Rh₁₄(CO)₂₅]⁴⁻ working in ethylene glycol solution. Although it was not possible to isolate crystals of A and B suitable for X-ray structural determination, a combination of cyclovoltammetry, one of the few examples so far available of the use of this technique for anionic rhodium carbonyl clusters, infrared spectroscopy and elemental analyses suggest that A and B are probably the never reported [Rh₇(CO)₁₄]⁻ and [Rh₁₅(CO)₂₈]³⁻ clusters, respectively. In particular the tentative formulation of the two clusters was carried out by a non-conventional method based on the existence of a linear correlation between carbonyl frequencies of the main band and the [(charge/Rh atoms)/CO number] ratio.     

Electrochemical Properties of a Rhodium(III) Mono-Terpyridyl Complex and Use as a Catalyst for Light-Driven Hydrogen Evolution in Water

Molecular hydrogen (H₂) is considered one of the most promising fuels to decarbonize the industrial and transportation sectors, and its photocatalytic production from molecular catalysts is a research field that is still abounding. The search for new molecular catalysts for H₂ production with simple and easily synthesized ligands is still ongoing, and the terpyridine ligand with its particular electronic and coordination properties, is a good candidate to design new catalysts meeting these requirements. Herein, we have isolated the new mono-terpyridyl rhodium complex, [Rh^III(tpy)(CH₃CN)Cl₂](CF₃SO₃) (Rh-tpy), and shown that it can act as a catalyst for the light-induced proton reduction into H₂ in water in the presence of the [Ru(bpy)₃]Cl₂ (Ru) photosensitizer and ascorbate as sacrificial electron donor. Under photocatalytic conditions, in acetate buffer at pH 4.5 with 0.1 M of ascorbate and 530 μM of Ru, the Rh-tpy catalyst produces H₂ with turnover number versus catalyst (TON_Cat*) of 300 at a Rh concentration of 10 μM, and up to 1000 at a concentration of 1 μM. The photocatalytic performance of Ru/Rh-tpy/HA^–/H₂A has been also compared with that obtained with the bis-dimethyl-bipyridyl complex [Rh^III(dmbpy)₂Cl₂]⁺ (Rh2) as a catalyst in the same experimental conditions. The investigation of the electrochemical properties of Rh-tpy in DMF solvent reveals that the two-electrons reduced state of the complex, the square-planar [Rh^I(tpy)Cl] (Rh^I-tpy), is quantitatively electrogenerated by bulk electrolysis. This complex is stable for hours under an inert atmosphere owing to the π-acceptor property of the terpyridine ligand that stabilizes the low oxidation states of the rhodium, making this catalyst less prone to degrade during photocatalysis. The π-acceptor property of terpyridine also confers to the Rh-tpy catalyst a moderately negative reduction potential (Ep_c(Rh^III/Rh^I) = −0.83 V vs. SCE in DMF), making possible its reduction by the reduced state of Ru, [Ru^II(bpy)(bpy^•−)]⁺ (Ru⁻) (E_1/2(Ru^II/Ru⁻) = −1.50 V vs. SCE) generated by a reductive quenching of the Ru excited state (*Ru) by ascorbate during photocatalysis. A Stern–Volmer plot and transient absorption spectroscopy confirmed that the first step of the photocatalytic process is the reductive quenching of *Ru by ascorbate. The resulting reduced Ru species (Ru⁻) were then able to activate the Rh^III-tpy H₂-evolving catalyst by reduction generating Rh^I-tpy, which can react with a proton on a sub-nanosecond time scale to form a Rh^III(H)-tpy hydride, the key intermediate for H₂ evolution.     

Multinuclear NMR studies on substituted derivatives of Rh6(CO)16 in solution

Multinuclear NMR data (¹³C, ³¹P, ¹³C–{³¹P}, ¹³C–{¹⁰³Rh} and ³¹P–{¹⁰³Rh}) for a series of mono- and di-substituted derivatives of Rh₆(CO)₁₆ containing neutral two electron donor ligands [Rh₆(CO)₁₅L, (L=NCMe, py, cyclooctene, PPh₃, P(OPh)₃,1/2(μ₂,η¹:η¹-dppe)); Rh₆(CO)₁₄(LL), (LL=cis-CH₂=CMe-CMe=CH₂, dppm, dppe, (P(OPh)₃)₂)] are reported; these data show that the solid state structure is maintained in solution. Detailed assignments of the ¹³CO NMR spectra of Rh₆(CO)₁₅(PPh₃) and Rh₆(CO)₁₄(dppm) clusters have been made on the basis ¹³C–{¹⁰³Rh} double resonance measurements and the specific stereochemical features of the observed long range couplings in these clusters have been studied. The stereochemical dependence of ³J(P–C) for terminal carbonyl ligands is discussed and the values of ³J(P–C) are found to be mainly dependent on the bond angles in the P–Rh–Rh–C fragment; these data enable the fine structure of the complex multiplets in the ¹³C–{¹H} and ³¹P–{¹H} NMR spectra of Rh₆(CO)₁₄ (dppm) to be simulated. Variable temperature ¹³C–{¹H} NMR measurements on Rh₆(CO)₁₅(PPh₃) reveal the carbonyl ligands in this complex to be fluxional. The fluxional process involves exchange of all the CO ligands except the two terminal CO's associated with the rhodium trans to the substituted rhodium and can be explained by a simple oscillation of the PPh₃ on the substituted rhodium atom aided by concomitant exchange of the unique terminal CO on this rhodium with adjacent μ₃-CO's.     

Binuclear rhodium(II) complexes with leucine and proline

The dimeric rhodium(II) complexes [Rh₂(leu)₄(H₂O)₂]- (ClO₄)₄ and [Rh₂(pro)₄(H₂O)₂](ClO₄)₄ have been prepared and characterized by elemental analyses, i.r., u.v.–vis. and ¹H-n.m.r. spectroscopy. The amino acid molecules are coordinated as bridging ligands via their carboxylato groups. Cyclic voltammetry in DMF has shown that the complexes undergo a quasi-reversible reduction to yield dimers containing a Rh ₂ ³⁺ core. Oxidation processes within the 0–1.5V range were not observed.     

Soluble {[Rh2(μ-OOCCH3)2(dbbpy)2][BF4]}n molecular wire and [Rh2(μ-OOCCH3)2(dbbpy)2L2] complexes; dbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine: Synthesis and physicochemical characterization

Binuclear Rh(II) compounds [Rh₂(μ-OOCCH₃)₂(dbbpy)₂(H₂O)₂](CH₃COO)₂ (1) (dbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine), [Rh₂(μ-OOCCH₃)₂(dbbpy)₂(H₂O)₂](BF₄)₂·H₂O·CH₃CN (2), [Rh₂(CH₃COO)₂(C₁₈H₂₄N₂)₂(CH₃CN)₂](BF₄)₂·4CH₃CN (3) and {[Rh₂(μ-OOCCH₃)₂(dbbpy)₂][BF₄]}_n (4) have been synthesized and characterized with spectroscopic methods. Structure of complex 3 has been determined using X-ray crystallography. Rhodium atoms in compound 3 have distorted octahedral coordination with O and N atoms in equatorial positions and Rh atom and CH₃CN molecule in axial coordination sites. Reduction of rhodium(II) compounds with aqueous 2-propanol leads to the formation of polymetallic compound {[Rh₂(μ-OOCCH₃)₂(dbbpy)₂][BF₄]}_n (4) containing [Rh₂]³⁺ core. Compound 4 shows strong antiferromagnetic properties, μ = 0.18–1.73 M.B. in the range 1.8–300 K, J = −597 cm⁻¹. Electrochemistry of compounds 3 and 4 in CH₃CN has been investigated. Compound 4 exhibits a poorly reversible oxidation system at E_1/2 = −0.92 V (ΔE_p = 0.19 V) and in solution in DMF is slowly oxidized to 3 even in total absence of oxygen. Complex 3 is irreversibly oxidized to Rh(III) compound at E_pa = 1.48 V and irreversibly reduced at E_pc = −1.02 V to lead to the unstable polynuclear complex 4 in CH₃CN.     

Aspects of ZnSe electrosynthesis from selenite and selenosulfite aqueous solutions

Cathodic electrodeposition of ZnSe is studied in aqueous acidic selenite (Se^(IV)O₂) baths of free Zn²⁺ ions and alkaline selenosulfite (Se⁰SO₃^2–) baths of zinc complex ions. The synthesis of polycrystalline, cubic ZnSe by Se^(IV)-diffusion-controlled electrodeposition from acidic solutions is optimized in terms of selenite concentration, while the effect of Ti, Ni, anodized Ti and CdSe substrates on the obtained layers growth and structural properties is investigated. Furthermore, the production of stoichiometric though amorphous ZnSe is demonstrated using selenosulfite solutions. The results are valuated by means of X-ray diffraction, scanning electron microscopy and reflectance spectroscopy techniques.     

Structure and properties of di-μ-acetato-dichlorobis(2,2′-bipyridine)dirhodium(II)-trihydrate

Reduction of [Rh₂(-OAc)₂(bpy)₂(H₂O)₂](OAc)₂ and [Rh₂Cl₂(-OAc)₂(bpy)₂] · 3H₂O complexes with ethanol and [Cr₂(OAc)₄(H₂O)₂] has been investigated using e.p.r. and u.v.–vis. spectra. The results indicate that stable complexes containing the [Rh₂ ³⁺] entity are not formed. The X-ray structure of [Rh₂Cl₂(-OAc)₂(bpy)₂] · 3H₂O has been determined. Coordination around the Rh atom is in the form of a distorted octahedron. The complex shows an almost ideal eclipsed conformation. The equatorial coordination sites are occupied by bridging carboxylato ligands and 2,2-bipyridine and axial positions by the Cl ligand and the rhodium atom. The Rh–Rh distance is 2.574 Å.     

Formation of mononuclear rhodium(III) sulfates: <Superscript>103</Superscript>Rh and <Superscript>17</Superscript>O NMR study

It was shown that the monomeric rhodium sulfate complexes [Rh(H₂O)₄(SO₄)]⁺, trans-[Rh(H₂O)₂(SO₄)₂]^?, cis-[Rh(H₂O)₂(SO₄)₂]^?, and [Rh(SO₄)₃]^3? were not predominant forms in aqueous solutions. The ¹⁰³Rh NMR chemical shifts of the complexes were assigned, and the conditions for their formation in solutions, concentration parameters, and acidity at which the fraction of the monomers was maximal were determined. The constants of formation of the complexes and ion pair (IP) were estimated: K _IP = 8 ± 3.5, K ₁ ≈ 8, K _2trans ≈ 1, K _2cis ≈ 1, and K ₃ ≈ 2.     

Radiolytic synthesis of Rh2(CO)4Cl2 and Rh6(CO)16 from RhCl3 in ethanol under CO atmosphere

Ethanolic solutions of Rh^III chloride exposed to γ-radiation under CO atmosphere are shown to be totally reduced into Rh^I complexes (Rh₂(CO)₄Cl₂ and Rh(CO)₂Cl^-₂) within a few hours with a radiolytic reduction yield of about 6.0 elementary reductions/100 eV (6.2·10^-7 mol·J^-1). The chloride ions freed in the medium inhibit further reduction through Rh(CO)₂Cl^-₂ formation. On addition of copper metal under the same conditions, Rh^III is transformed into Rh₆(CO)₁₆ with a conversion yield 50%. This cluster is formed via Rh₂(CO)₄Cl₂ although Rh(CO)₂Cl^-₂ is also present under these conditions. Rh₆(CO)₁₆ cluster is also formed under radiolysis by direct reduction of Rh₂(CO)₄Cl₂, but metallic rhodium and other reduced products are obtained at the same time.     

Aluminium anodising in low acidity sulphate baths: growth mechanism and nanostructure of porous anodic films

Overall kinetic and chronopotentiometric studies were performed during Al anodising in H₂SO₄, 0–5% w/v, bath solutions pure and saturated by Al₂(SO₄)₃. Peculiarities in film growth mechanism and nanostructure in these cases appeared, like significant differences of porosity and its dependence on film thickness, different critical current density above which pitting appears, salt deposition on pitted surface regions in saturated bath, etc. The different conditions inside pores are responsible for this behaviour like almost depletion of H⁺ during a long initial transient stage in the first case, supersaturation and formation of Al₂(SO₄)₃ nanoparticle micelles on pore surface in the second case, etc. Differences in film growth mechanism also appeared between these and alike baths at higher acidity. Anodising in low acidity saturated baths shows superiority for growing low porosity films at specific conditions. New technologies may be suggested to produce optimal films of desired structure.     

Complexation of Rh(III) in diluted sulfuric acid solutions

Complex formation in a system Rh(III)-H₂SO₄-H₂O was studied by the ¹⁰³Rh and ¹⁷O NMR spectroscopy at room temperature. The formation of two interrelated systems of mononuclear and polynuclear complexes was established in the above solutions. The predominant species in the first system is a labile ionic pair {Rh(H₂O) ₆ ³⁺ SO ₄ ^2? }⁺, while in the second system, two inert binuclear complexes [Rh₂(μ-SO₄)₂(H₂O)₈]²⁺ and [Rh₂(μ-SO₄)(μ-OH)(H₂O)₈]³⁺ prevail.     

Characterization of Au-Rh/TiO2 catalysts by CO adsorption; XPS, FTIR and TPD experiments

On X-ray photoelectron spectra of the Au-Rh/TiO₂ catalysts the position of Au4f peak was practically unaffected by the presence of rhodium, the peak position of Rh3d, however, shifted to lower binding energy with the increase of gold content of the catalysts. Rh enrichment in the outer layers of the bimetallic crystallites was experienced. The bands due to Au⁰-CO, Rh⁰-CO and (Rh⁰)₂-CO were observed on the IR spectra of bimetallic samples, no signs for Rh⁺-(CO)₂ were detected on these catalysts. The results were interpreted by electron donation from titania through gold to rhodium and by the higher particle size of bimetallic crystallites.