Characterization of palladium species after γ-irradiation of a TBP–alkane–Pd(NO₃)₂ system

Abstract

The γ-irradiation of a biphasic system composed of tri-n-butylphosphate in tetrapropylene hydrogen (TPH) in contact with palladium(II) nitrate in nitric acid aqueous solution led to the formation of two precipitates. A thorough characterization of these solids was performed by means of various analytical techniques including X-Ray Diffraction (XRD), Thermal Gravimetric Analysis coupled with a Differential Scanning Calorimeter (TGA-DSC), X-ray Photoelectron Spectroscopy (XPS), InfraRed (IR), RAMAN and Nuclear Magnetic Resonance (NMR) Spectroscopy, and ElectroSpray Ionization Mass Spectrometry (ESI-MS). Investigations showed that the two precipitates exhibit quite similar structures. They are composed at least of two compounds: palladium cyanide and palladium species containing ammonium, phosphorous or carbonyl groups. Several mechanisms are proposed to explain the formation of Pd(CN)₂.

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Introduction

In the PUREX process, 30%_vol tri-n-butylphosphate (TBP) diluted in Tetra Propylene Hydrogen (TPH, a hydrocarbon diluent) is used to extract uranium and plutonium from fission products (ruthenium, molybdenum, cesium, palladium, etc.) and minor actinides contained in spent nuclear fuels.^1,2 Due to the biphasic nature of the chemical system and the presence of numerous solutes (nitric acid, uranium, plutonium, fission products…), the solvent is subject to hydrolytic and radiolytic degradation. The resulting damage can modify the solution composition, alter the physico-chemical properties, modify the extraction kinetics, and modify the redox properties of the metallic ions to be extracted.³ Since the 1950s, many studies have been focused on the investigation of the stability of TBP and diluent under radioactive stress.^3,4 In particular, the influence of the irradiation source (α-irradiation vs. γ-irradiation^5–12), diluent,^5,13–16 nitric acid concentration,^17–22 and temperature¹⁴ have been studied. It appears that irradiation of TBP in alkane diluents in contact with aqueous nitric acid solution produces dibutyl phosphoric acid (HDBP), monobutylphosphoric acid (H₂MBP), phosphoric acid (H₃PO₄), higher molecular weight dimers, acidic phosphates, hydroxylated and nitro-substituted phosphates, and low-molecular weight acid phosphates.^23–28 Among these products, HDBP, H₂MBP, H₃PO₄ and low-molecular weight acid phosphate species are generated by decomposition of the TBP radical cation produced by irradiation stresses or by hydrogen atom abstraction of TBP or the primary radiolysis products. Likewise, diluent decomposition leads to the formation of nitro-substituted alkanes in presence of nitrates and alkane oxidation products such as ketones and carboxylic acids while high-molecular weight products are formed by radical-addition reactions. Without treatment, accumulation of degradation products in the solvent could lead to (i) change of the physicochemical properties of the solutions (density and viscosity),²⁹ (ii) crud formation,^30–35 and (iii) changes in the extraction properties of the solvent (efficiency, selectivity, etc.).^{15,19,30,31,36–40}

In the literature, a particular interest has been focused on the complexing properties of the degradation products towards U(VI),^9,39,41,42 Pu(IV),^6,9,42–46 Zr(IV),^{30,32,35,45,47–55} Ln(III),^56,57 Fe(III)^58,59 as well as Ru.³⁷ Few studies have reported the effect of irradiation on the palladium behaviour.^{12,19,36,40,58–60} Guedon et al.^58,59 reported the formation of a precipitate containing palladium⁵⁸ under accelerated degradation conditions (higher than 0.2 MGy) of 30%_vol TBP diluted in n-dodecane in contact with 3 mol L⁻¹ nitric acid. They mentioned that formation of palladium precipitate could be related to diluent degradation. Moreover, Nowak et al.¹⁹ showed that solvent degradation is more intense in the presence of palladium(II) during irradiation of 30%_vol TBP diluted in n-dodecane in contact with nitric acid. In this case, palladium might be involved in the degradation mechanisms of the extraction solvent. Furthermore, degradation is accompanied by an increase of the distribution ratio of Pd(II) (D_Pd).⁴⁰ In order to explain this behaviour, the effect of some degradation products on the distribution ratio of Pd(II) were investigated.^40,60 The presence of alkenes and butyric aldehyde could be responsible for the increase of D_Pd. Conversely, no significant increase in the distribution ratio of palladium(II) was observed when HDBP, H₂MBP, methyl-ethyl ketone, 2-butanol, or iodoethane were added to the extraction solvent. Beside the increase of the distribution ratio of palladium(II), the formation of a black precipitate was also observed. According to the authors, such a precipitate could result from the reduction of Pd(II) into Pd(0) during the oxidation of alkenes into carbonyl compounds.⁶¹

In the ORANO La Hague reprocessing plants, after two or three decades of industrial operations, it is essential to learn how to cope with aging equipment in order to avoid process dysfunctions. In particular, precipitates containing a high proportion of palladium (>50%_w) and carbon (>25%_w) were found to be responsible for partial equipment clogging.⁶² Because of the high radioactivity of these solids, a thorough characterization is difficult to conduct. Therefore, in order to investigate the phenomena responsible for precipitate formation in the presence of Pd(II) in nuclear fuel reprocessing plants, inactive precipitates were synthetized in the laboratory by contacting 30%_vol TBP diluted in TPH with palladium(II) nitrate in 3 mol L⁻¹ nitric acid solution under γ-irradiation with a ⁶⁰Co source. X-Ray Diffraction (XRD) diagrams and InfraRed (IR) spectra of these inactive precipitates have demonstrated similarities between these precipitates and the solids observed in the ORANO plant.⁶² These solids have been thoroughly characterized in this work by X-Ray Diffraction (XRD), Thermal Gravimetric Analysis coupled with a Differential Scanning Calorimeter (TGA-DSC), X-ray Photoelectron Spectroscopy (XPS), InfraRed (IR), RAMAN and Nuclear Magnetic Resonance (NMR) spectroscopy as well as ElectroSpray Ionization Mass Spectrometry (ESI-MS). Finally, mechanisms responsible for precipitate formation have been discussed.

Results

A biphasic system composed of 30%_vol TBP diluted in TPH in contact with nitric acid solution containing palladium(II) nitrate was irradiated under accelerated degradation conditions of the PUREX process. Such conditions lead to the formation of two solids as illustrated in Fig. 1.

Fig. 1 Formation of two precipitates (Φ_S,1 and Φ_S,2) after γ-irradiation at 500 kGy with ⁶⁰Co source of the biphasic system. Initial composition of the organic phase: 30%_vol TBP diluted in TPH; initial composition of the aqueous phase: 0.02 mol L⁻¹ palladium(II) nitrate in 3 mol L⁻¹ nitric acid; organic : aqueous phase ratio = 1 : 1.

A thorough characterization of the two precipitates Φ_S,1 and Φ_S,2 was undertaken using complementary analytical techniques such as XRD, TGA, XPS and IR spectroscopy. Additional information has been deduced by dissolving the precipitates in appropriate solvents and by analysing them by NMR, IR and RAMAN spectroscopy as well as ESI-MS spectrometry.

Direct analysis of the precipitates

Samples were analysed by X-Ray Diffraction (XRD) to gain information about their structure. Fig. 2 shows XRD patterns of precipitate Φ_S,1, located at the liquid–liquid interface (green line) and the precipitate Φ_S,2 located at the bottom of the aqueous phase (blue line).

Fig. 2 XRD patterns of (a) Φ_S,1 (green color), Φ_S,2 (blue color).

Both precipitates have the same crystallographic structure as they exhibit four peaks located at 2θ = 18.1°, 26.5°, 36.2°, 40.8° as well as 18.1°, 26.2°, 36.1° and 41.2°, respectively. A comparison of XRD patterns with the literature data indicates that the crystallographic structure of these precipitates is close to that of Pd(CN)₂·0.29H₂O.⁶³ Usually in an X-ray powder diffraction pattern, peak intensity and resolution (full-width half maximum) are related to the crystallinity state of the materials (crystallite size and micro-deformation of the crystal lattice). In Fig. 2, the width full-width half maximum of Φ_S,1 and Φ_S,2 are different: a finer and a higher intensity is observed for Φ_S,2 because the precipitate Φ_S,2 is probably better crystallized than the precipitate Φ_S,1.

The lattice structure of Φ_S,1 and Φ_S,2 is I4₁/amd and the lattice parameters are a = b = 4.949(29) Å and c = 8.486(68) Å (R_Bragg = 3.18, R_wp = 0.383) as well as a = b = 4.949(29) Å and c = 8.788(85) Å (R_Bragg = 3.35, R_wp = 0.336), respectively. The lattice parameters a, b and c are the same for both precipitates, but they are smaller than for the commercial compound Pd(CN)₂·0.29H₂O (a = b = 5.119 Å and c = 13.600 Å). Pd(CN)₂·0.29H₂O and Pd(CN)₂·0.29NH₃ contain water and NH₃ molecules to terminate the periodic structure of the solid, respectively (see Fig. 3 for the sake of illustration in the case of Pd(CN)₂·0.29H₂O).⁶³ The differences observed between the lattice parameter c for Φ_S,1, Φ_S,2 and Pd(CN)₂·0.29H₂O may be explained by difference in crystal network. For instance, it can be expected that water molecules located at the outer edges in the Pd(CN)₂ network may be replaced by carboxylates groups in Φ_S,1 and Φ_S,2. Furthermore, the difference in lattice parameters may be explained by the presence of at least two compounds in the precipitate since inclusions of a second compound (in minor quantity) could modify the lattice parameter c.

Fig. 3 Chemical structure proposed for Pd(CN)₂·xH₂O based on Pt(CN)₂·0.67H₂O proposed by Hibble.⁶³

In order to determine the surface elemental composition of the precipitate, X-ray photoelectron spectroscopy (XPS) was used. This technique provides information about surface composition of solids, their chemical environments and their oxidation states. The Pd-3d, C-1s and N-1s XPS spectra of Φ_S,1 and Φ_S,2 were recorded and compared with spectra of commercial compounds such as palladium acetate, palladium acetylacetonate, palladium pivalate and palladium cyanide. Atoms of Pd, C, O, N and P are present in both precipitates. Fig. 4 shows the Pd-3d core level spectra of the precipitates. It appears that the elemental composition and the nature of the chemical functions in Φ_S,1 and Φ_S,2 are similar. Table 1 depicts the binding energies of Pd-3d_5/2, N-1s and C-1s contributions for both precipitates and the reference samples.

Fig. 4 XPS core level spectra Pd-3d of (a) Φ_S,1, (b) Φ_S,2 and (c) commercial compounds (in red palladium acetate and in blue palladium cyanide).

Table 1 Binding energies (in eV) of Pd 3d, C-1s and N-1s for Φ_S,1, Φ_S,2, palladium acetate, palladium pivalate, palladium acétylacétonate and palladium cyanide. The environment of each element is given in brackets^a

Compound	Binding energy (eV)
	Pd 3d5/2	N-1s	C-1s
a *Pd–NR4^+: attributionbased on the ref. 64.
ΦS,1	338.8 ± 0.3 (Pd(OCO)	399.8 ± 0.3 (Pd(CN)2)	286.4 ± 0.3 (C≡N)
	339.9 ± 0.3 (Pd(CN))	401.9 ± 0.3 (*)	289.0 ± 0.3 (C–COO)
ΦS,2	338.9 ± 0.3 (PdOCO)	399.8 ± 0.3 (Pd(CN)2)	286.3 ± 0.3 (C≡N)
	340.0 ± 0.3 (Pd(CN))	401.9 ± 0.3 (*)	289.0 ± 0.3 (C–COO)
	338.8 ± 0.3 (Pd(OCOCH3))	—	285.6 ± 0.3 (C–COO)
			286.5 ± 0.3 (C–O)
	338.7 ± 0.3 (Pd(OCOC(CH3)3)	—	289.0 ± 0.3 (C–COO)
			285.8 ± 0.3 (C–COO)
	339.0 ± 0.3 (Pd(OC5H7O))	—	288.7 ± 0.3 (C–COO)
			285.8 ± 0.3 (C–CO)
	339.8 ± 0.3 (Pd(CN)2)	399.6 ± 0.3 (Pd(CN)2)	286.1 ± 0.3 (C≡N)

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Two different palladium species may be present in the precipitates since XPS spectra exhibit two contributions for Pd-3d_5/2. The comparison between reference samples and precipitates indicates the presence of Pd(II) bonded to –C≡N and O=C–O– groups. The presence of these two functions is confirmed by the C-1s core level spectra (Fig. S1, in ESI†): two contributions are observed at 286.3 eV for C≡N group and 289.0 eV for O=C–O group, respectively. In addition, NR₄⁺ compound is likely present in the precipitates (R = hydrocarbon chain). Indeed, the presence of a peak located at a binding energy of 401.9 eV in the N-1s XPS data is consistent with the presence of ammonium type compound in the precipitates.⁶⁴ These results confirm the presence of cyanide groups in close interaction with palladium but also show the presence of other interactions between palladium and carboxylate and/or ammonium groups. The calculation of the Pd/N atomic ratios by considering Pd and N bounded to cyano functions demonstrates that CN groups coordinate only palladium in Φ_S,1 and Φ_S,2, since Pd/N ratio are the same for the precipitates and commercial Pd(CN)₂ (Table 2). Finally, by assuming that Pd is only surrounding by two different environments (Pd–CN and Pd–O–C=O), the proportion of Pd in Pd-cyano and Pd–O=C–O can be calculated for Φ_S,1 and Φ_S,2 (Table 3). The composition of the two precipitates is slightly different, i.e. Φ_S,1 contains less Pd linked to CN⁻ than Φ_S,2.

Table 2 Pd/N atomic ratio determined by XPS

Ratio	Commercial compound Pd(CN)2	ΦS,1	ΦS,2
Pd/N	0.64	0.38	0.67

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Table 3 Palladium/CN and Pd/O–C=O ratios in Φ_S,1 and Φ_S,2 deduced by XPS

Sample	Pd–CN	Pd–O=C–O
ΦS,1	41	59
ΦS,2	80	20

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IR spectroscopy was then used to identify the functional groups involved in the precipitates. The IR spectra of (a) Φ_S,1 and (b) Φ_S,2 precipitates were analyzed and compared with the IR spectra of (c) palladium acetylacetonate, (d) palladium cyanide, (e) palladium acetate and (f) palladium pivalate (Fig. 5). The different bands are assigned in Table 4.

Fig. 5 IR spectra of (a) Φ_S,1 (b) Φ_S,2, (c) palladium acetylacetonate, (d) palladium cyanide, (e) palladium acetate and (f) palladium pivalate.

Table 4 Identification of the main vibration bands observed in the IR spectra of Φ_S,1, Φ_S,2, Φ_S,3, palladium cyanide, palladium acetate, palladium acetylacetonate and palladium pivalate and their attribution. w = weak; m = medium; s = strong

Assignment	Wavenumber (cm^−1)
	ΦS,1	ΦS,2	ΦS,3	Palladium cyanide^63	Palladium acetate^65	Palladium acetylacetonate^66	Palladium pivalate
H2O or amine + CH3, as stretch	3680 to 2999 (m)	3679 to 2991 (m)	3378 to 3015 (w)	—	2937 (w)	2921 (w)
CH–CHalkane	2960 to 2870 (w)	2956 to 2870 (w)	2975 to 2827 (w)	—
C≡N	2221 (m)	2221 (m)	—	2218 (m)
H2O or O–C=O	1610 (m)	1610 (m)	1610 (s)	1616 (w)	1593 (s)	1565 (m)	1595 (s)
P–O–C or H2O	1028 (s)	1028 (s)	1022 (s)	—
Pd–N or Pd–C	551 (m)	551 (m)	—	547 (s)
Pd–N or Pd–C, Pd–O	464 (m)	464 (m)	—	462 (s)		462 (s)	466 (s)
Pd–N, Pd–O, OPdO	266 (w)	264 (w)	—	266 (s)	268 (m)	262 (m)

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Φ_S,1 and Φ_S,2 exhibit the same vibration bands. The latter are very similar to those observed in the Pd(CN)₂ commercial compound. In particular, the vibration bands at 2220 cm⁻¹ (C≡N), 551 cm⁻¹ (Pd–N or Pd–C), 464 cm⁻¹ (Pd–N or Pd–C) and 264 cm⁻¹ (Pd–N) observed for the three compounds seem to confirm that Φ_S,1 and Φ_S,2 contain Pd(CN)₂. The vibration band around 1610 cm⁻¹ may indicate the presence of O–C=O functions in Φ_S,1 and Φ_S,2 as this band is also observed in the spectrum of commercial palladium acetate, palladium pivalate and palladium acetylacetonate, which contain O–C=O group (Fig. 5). The presence of the O–C=O group is in agreement with the previous XPS results, which showed the presence of carboxylate groups in both precipitates. A comparison of the spectra of the precipitate, palladium acetate, palladium pivalate and palladium acetylacetonate shows a slight shift of the band located at 1610 cm⁻¹. However, IR can only confirm the presence of carboxylate group in the compound without giving its exact composition.

Φ_S,1, Φ_S,2 and the commercial compound Pd(CN)₂ were analysed by thermogravimetric analysis coupled with a differential scanning calorimeter (TGA-DSC) (Fig. 6). The thermal behaviour of Φ_S,1 and Φ_S,2 are quite close. Two endothermic peaks observed between 25 °C and 110 °C, and between 350 °C and 650 °C as well as an exothermic peak located between 110 °C and 350 °C accompanied by weight loss were observed. The reaction between 25 °C and 110 °C, and between 110 °C and 350 °C can be attributed to water vaporization⁶³ and release of organic functions,^67,68 respectively. The weight loss between 350 °C and 650 °C may correspond to the release of (CN)_2(g).⁶³ TGA analyses are consistent with the previous results: the precipitates may be composed of palladium cyanide and organic compounds.

Fig. 6 TGA thermograms of (a) Φ_S,1 (in green), (b) Φ_S,2 (in blue) and (c) Pd(CN)₂,0.29H₂O (in red). Black line: heat flow (μV); measurements performed in nitrogen atmosphere.

TGA analyses allow calculating moisture content in the precipitates (% Moisture). Likewise, it is also possible to calculate the weight percentage of palladium in the precipitates (%_w Pd) by considering a complete degradation of the product when samples are heated at 650 °C in nitrogen atmosphere. At this temperature, there is no additional organic matter in the precipitate and the weight of the sample corresponds to the weight of palladium. Moreover, the weight of organic (%_w organic) and cyanogen (%_w (CN)₂) can also be calculated. Finally, the weight percentage of Pd-cyano groups (%_w Pd_(CN)2) and Pd-carboxylate groups (%_w Pd_(O–C=O)) in Φ_S,1, Φ_S,2 and Pd(CN)₂ can be deduced from TGA experiments by using the calculation method reported in the experimental part (eqn (13) and (14)). Table 5 reports these values.

Table 5 Quantitative analyses of the precipitates and Pd(CN)₂·xH₂O by TGA-DSC, i.e. weight percentages of moisture (% Moisture), palladium (%_w Pd), cyanogen (%_w (CN)₂), organic compounds (%_w organic)in Φ_S,1, Φ_S,2 and the commercial palladium cyanide. Calculation of the distribution Pd species in the precipitates: %_w Pd_(CN)2 corresponds to palladium coordinated to CN group and %_w Pd_(O–C=O) corresponds to palladium coordinated to carboxylate group

Sample	%w Moisture (between 25 °C and 110 °C)	%w Pd (at 650 °C)	%W (CN)2 (between 350 and 650 °C)	%W Organic (between 110 and 350 °C)	%w Pd(CN)2	%w Pd(O–C=O)
ΦS,1	3	54	10	33	39	61
ΦS,2	3	57	16	24	55	45
Pd(CN)2·xH2O	2	68	30	—	94	—

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The moisture contents are similar in the precipitates (2–3% for water). Moreover, total palladium contents are in the same order of magnitude in the solids Φ_S,1 and Φ_S,2 (around 54–57% for Pd). The major difference in composition between Φ_S,1 and Φ_S,2 is the distribution between the CN⁻ group and COO⁻ group. Φ_S,2 contains more palladium coordinated to CN⁻ than Φ_S,1 (55% vs. 39%). Conversely, more palladium coordinated to carboxylate group is observed in Φ_S,1 than Φ_S,2. Although the distribution of Pd species determined by TGA-DSC is slightly different from that calculated from XPS data (see Table 3), both techniques show that more Pd is linked to CN⁻ in Φ_S,2 than in Φ_S,1. This difference could be due to the difficulty by TGA-DSC to quantify precisely the (CN)_2(g) release between 350 °C and 650 °C in Φ_S,1 and Φ_S,2. In the commercial compound Pd(CN₂), the loss in mass of (CN)_2(g) is well defined in TGA-DSC data, whereas a broad signal is observed for the precipitates. Moreover, the XPS analysis allows analysing the extreme surface of the powder only. Some errors can come from the choice of the background, the recombination of the spectra and a different depth of analysis from one sample to another.

Analysis of the precipitates after solubilisation

Direct analyses of the precipitates by IR, Raman and XPS showed that Pd is coordinated to CN⁻ and COO⁻ while NR₄⁺ groups may be present in the precipitates. However, it is impossible to confirm the exact number of palladium and organic complexes present in the precipitates. In order to deeply characterize the precipitates, they were dissolved in several solvents (DMSO (dimethyl sulfoxide), NH₃ (1 mol L⁻¹), pyridine) and the obtained solutions were analysed. It was expected that NH₃ would be able to dissolve the precipitates as Pd(CN)₂ can be dissolved in NH₃ by forming the soluble complex Pd(CN)₂(NH₃)₂.⁶⁹ After 15 minutes of stirring of 20 mg of precipitate in 10 mL of 1 mol L⁻¹ NH₃, a filtrate (Φ_F1) and an insoluble fraction (Φ_S,3) were obtained. The insoluble fraction Φ_S,3 and the filtrate Φ_F1 were then analysed by IR and Raman spectroscopy (Fig. 7).

Fig. 7 IR spectra of (a) Φ_S,3 (solid residue after dissolution of the Φ_S,1 in 1 mol L⁻¹ NH₃) and (b) Φ_S,1.

The corresponding vibration bands are reported in Table 4. These spectra show the presence of vibration bands at 2218 cm⁻¹ (C≡N), 551 cm⁻¹, 464 cm⁻¹, and 266 cm⁻¹ (Pd–N or Pd–C) in Φ_S,1. These bands have not been observed in Φ_S,3. Therefore, this study confirms that 1 mol.L⁻¹ NH₃ allows solubilizing Pd(CN)₂. Moreover, further XPS analyses performed on the Φ_S,3 fraction confirm that NH₃ totally dissolved Pd(CN)₂ since the Pd contribution located at 339.3 eV has disappeared (Fig. S2, ESI†). This conclusion was also confirmed by Raman spectroscopy on the filtrate Φ_F1, since two vibration bands located at 2149 and 2140 cm⁻¹ attributed to cyano groups were observed (Fig. 8).

Fig. 8 Raman spectrum of Φ_F1 obtained after dissolution of Φ_S,1 in NH₃ (1 mol L⁻¹) at a solid : liquid ratio of 9 (mass of precipitate in g/volume of solvent in L).

Furthermore, the presence of an insoluble fraction Φ_S,3 (partial dissolution of Φ_S,1 in NH₃ 1 mol L⁻¹) seems to show that the precipitate is composed of at least of two different compounds. The second compound contains H₂O molecules (vibration bands 3680–3379 cm⁻¹ and 1610 cm⁻¹), CH–CH groups (vibration bands at 2975 and 2827 cm⁻¹ assigned to CH–CH_alkane vibration), O–C=O functions (vibration band at 1610 cm⁻¹), phosphate groups (vibration bands at 1274 cm⁻¹, 1022 cm⁻¹) or R–NH₂ functions (vibration bands at 1274 cm⁻¹).

ESI-MS experiments were undertaken to characterize the precipitates after dissolution in pyridine. This technique allows the transfer of pre-existing ions from the solution into the gas phase in order to obtain speciation information. Fig. 9 shows ESI-MS spectra obtained after partial dissolution in pyridine of Φ_S,1, Φ_S,2 and the commercial compound Pd(CN)₂. The main ions containing Pd were identified by comparison with a calculated isotopic pattern (Table 6).

Fig. 9 ESI-MS spectra of (a) Φ_S,1, (b) Φ_S,2 and (c) Pd(CN)₂ after partial dissolution in pyridine (solid : liquid ratio = 2 g L⁻¹) and dilution in acetonitrile ((a) 1/10^th, (b) 1/100^th and (c) 1/10^th). Experiment conditions: trap drive setting = 50; skimmer voltage = 30 V.

Table 6 m/z ratios and Pd species assignments in ESI-MS (positive ionization mode) for Φ_S,1, Φ_S,2 and Pd(CN)₂ in pyridine. pyr = pyridine = C₅H₅N; TBP = tri-n-butylphosphate = C₁₂H₂₇O₄P. Peak numbers (N°) are reported in Fig. 9^a

N° (Fig. 9)	m/z pattern	Identification	Pd(CN)2	ΦS,1	ΦS,2
a *, observed species; —, non detected species; n.i.: non identified species.
1	180.4–188.9	Pd^I(pyr)^+	—	*	*
2	211.1	(HDBP)H^+	—	*	*
3	259.9–268.9	Pd^I(pyr)2^+	*	*	*
	267.8	(TBP)H^+	—	*	*
4	285.9–294.8	Pd^I(pyr)2(CN)^+	*	*	*
5	294.8–303.9	Pd^I(pyr)2OH(H2O)^+	*	*	*
6	313.1–322.0	Pd^I(pyr)2OH(H2O)2^+	*	—	—
7	496.0–509.0	Pd^IPd^II(pyr)3(CN)2^+	—	*	*
8	523.0–533.9	Pd2^II(pyr)3(CN)3^+	*	*	*
9	552.2–561.2	Pd^I(pyr)2(CN)(TBP)^+	—	*	*
	555.2	(TBP)2Na^+	—	*	*
10	556.4	n.i.	*	—	—
11	601.1–614.0	Pd2^II(pyr)4(CN)3^+	*	*	*
12	610.9–622.9	Pd2^II(pyr)4(CN)2OH(H2O)^+	*	—	—

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Most ions contain pyridine indicating a strong solvation of Pd by pyridine during the solubilisation step. Moreover, the presence of ions of m/z = 180.4–188.9, 259.9–268.9, 285.9–294.8, 294.8–303.9, 313.1–322.0, 496.0–509.0 and 552.2–561.2 assigned to Pd(pyr)⁺ (pyr = C₅H₅N), Pd(pyr)₂⁺, Pd(pyr)₂(CN)⁺, Pd(pyr)₂OH(H₂O)⁺ Pd(pyr)₂OH(H₂O)₂⁺, Pd₂(pyr)₃(CN)₂⁺, and Pd^I(pyr)₂(CN)(TBP)⁺ indicates the partial reduction of Pd(II) into Pd(I). Nevertheless, it is not possible to say if Pd reduction occured during the dissolution step of the solid in pyridine or during the de-solvation/ionization step. The ions of m/z = 259.9–268.9, 285.9–294.8, 294.8–303.9, 523.0–533.9 and 601.1–614.0 corresponding to Pd(pyr)₂⁺, Pd(pyr)₂(CN)⁺, Pd(pyr)₂OH(H₂O)⁺, Pd₂(pyr)₃(CN)₃⁺ and Pd₂(pyr)₄(CN)₃⁺, respectively, were present in both the commercial compound and the precipitates Φ_S,1 and Φ_S,2. The ions of m/z = 313.1–322.0 and 610.9–622.9 assigned to Pd(pyr)₂OH(H₂O)₂⁺ and Pd₂(pyr)₄(CN)₂OH(H₂O)⁺, respectively, were only observed after dissolution of the Pd(CN)₂ commercial compound whereas the ions of m/z equal to 259.9–268.9, 267.8, 523.0–533.9, 552.2–561.2, 555.2 and 601.1–614.0 assigned to Pd(pyr)₂⁺, (TBP)H⁺, Pd₂(pyr)₃(CN)₂⁺ Pd^I(pyr)₂(CN)(TBP)⁺, (TBP)₂Na⁺, and Pd₂(pyr)₄(CN)₃⁺, respectively, were observed after dissolution of the precipitates in the pyridine. Thus, some differences are observed between the solution obtained after solubilisation of the commercial Pd(CN)₂ compound and the precipitates. Moreover, the ion of m/z equal to 556.4 observed in the commercial compound does not contain palladium and may be due to the presence of an organic impurity because of the very high sensitivity of the technique.

In the Pd(CN)₂ commercial compound, each palladium atom is surrounded by four CN groups. However, some water molecules can terminate the periodic structure (see Fig. 3). During the solubilisation step, pyridine molecules solvate the palladium and replace some CN group to produce Pd₂(pyr)₄(CN)₃⁺ and Pd₂(pyr)₄(CN)₂OH(H₂O)⁺. Then, during the transfer from the solution to the gas phase in the mass spectrometer, these ions can be modified by release of a pyridine, CN group or water molecules and Pd(II) could be reduced so that Pd₂^II(pyr)₄(CN)₃⁺; Pd₂^II(pyr)₃(CN)₃⁺. Pd^I(pyr)₂OH(H₂O)₂⁺; Pd^I(pyr)₂OH(H₂O)⁺; Pd^I(pyr)₂CN⁺ and Pd^I(pyr)₂⁺ can be formed.

After solubilisation of the precipitates, the ions Pd₂(pyr)₄(CN)₂OH(H₂O)⁺ and Pd^I(pyr)₂OH(H₂O)₂⁺ were not observed. However, the ions Pd^I(pyr)₂(CN)(TBP)⁺, (TBP)₂Na⁺ and (TBP)H⁺ were observed and indicate the presence of the TBP in the solution. The presence of HDBP was also confirmed since an ion of m/z = 211.1 which corresponds to (HDBP)-H⁺ is observed. Comparing to the commercial compound in which water molecules terminate the periodic structure, it is possible that in the precipitate some TBP molecules terminate the structure. Thus, although not conclusive in providing structural information for the solid, the ESI-MS results provide evidence that the presence of TBP in the precipitates.

In order to get information about the chemical structure of the precipitates 1D and 2D ¹H NMR spectra were recorded after Φ_S,2 dissolution in DMSO-D6 (deuterated DMSO). Pyridine was not used for NMR analyses in spite of the high solubilization power of pyridine because of the two following drawbacks: (i) pyridine peak overlap ¹H signal in the aromatic area (about 7 ppm) and (ii) pyridine can easily dissociate Pd species. The 1D ¹H NMR spectrum of Φ_S,2 exhibits the reference peak at 0 ppm (TMS) and a peak located at 2.5 ppm attributed to residual protons of DMSO-D6 (Fig. 10). The triplet and the three multiplets located at 0.85, 1.31, 1.53 and 3.82 ppm, respectively, correspond to the butyl protons in TBP (for more information ESI – Fig. S3 and S4†). The three symmetric peaks of same intensity centred at 7.05 ppm indicate protons coupling with a nucleus of spin 1 (Fig. 10). Among the nuclei in the sample, only the nitrogen has a spin 1. The corresponding J_14N1H coupling constant of 104 Hz is in the magnitude order of ¹J_NH (i.e. ¹H one bond apart from ¹⁴N) and consequently could be due to the presence of an amine function.

Fig. 10 1D ¹H NMR spectrum of Φ_S,2 partially dissolved in DMSO-D6 m(Φ_S,2) = 9.8 mg in 650 μL of DMSO-D6, i.e. S/L = 15.07 g of precipitate/L of solvent.

In order to confirm the presence of amine functions in the precipitate Φ_S,2, a biphasic system composed of 30%_vol TBP diluted in TPH contacted with 3 mol L⁻¹ nitric acid solution containing palladium nitrate (5 g L⁻¹ Pd) and NaNO₃ (66 g L⁻¹) enriched with 98% atomic ¹⁵N was irradiated and the aqueous phase was analyzed. The ¹⁵N NMR spectrum shows a triplet centred on −360 ppm (¹⁵N scale is referenced to CH₃–NO₂) that is characteristic of NH₂ functions (Fig. S5, ESI†). The 2D ¹⁵N–¹H spectrum displayed in Fig. S7 (ESI†) shows a correlation spot (¹J_15N1H = 72 Hz) between the triplet at −360 ppm observed in the ¹⁵N NMR spectrum (Fig. S7†) and the peak located at 6.1 ppm observed in the ¹H NMR spectrum (Fig. S6 in ESI†). Therefore, protons observed at 6.1 ppm corresponds to NH₂ functions. Likewise, the peaks centred at 7.05 ppm in the ¹H NMR spectrum of Φ_S,2 can be assigned to an amine function.

To complete ¹H NMR peak assignments for Φ_S,2, gHSQCAD and gHMBCAD pulse sequences were performed. These 2D pulse sequences allow ¹³C assignments through ¹H detections and the corresponding coupling constants: (¹J_CH for gHSQCAD) and (ⁿJ_CH with n = 2 or 3 for gHMBCAD) (Fig. 11 and 12). In Fig. 11, positive red crosspeaks belong to CH– and –CH₃ groups whereas negative blue peaks belong to –CH₂– groups. The two red cross-peaks at (1.6 ppm; 13.8 ppm) (¹H scale; ¹³C scale) and (1.9 ppm; 13.8 ppm) are two unshielded –CH₃ groups. At 0.85, 1.31, 1.53 and 3.82 ppm (¹H scale), the set of large and small cross peaks are assigned to the CH₃–, two –CH₂– and –CH₂–O–P groups at 13.8, 18.6, 32.2 and 67.0 ppm (¹³C scale) of the butyl chains bearing on TBP (main compound) and HDBP peaks arising from TBP degradation. The presence of TBP or HDBP in the precipitate is consistent with IR and ESI-MS data. On the ¹H–¹³C gHMBCAD (Fig. 12), the –CH₃ chemical shift located at 1.9 ppm on the 1H scale is correlated to carbon peaks two or three bonds apart at 172 and 150 ppm which are assigned to carbonyl and cyano groups respectively. Such information allows to suggest the presence of methyl-oxo-cyanide in Φ_S,2 (CH₃–CO–CN). NMR analyses for Φ_S,1 have not been undertaken. However, the same results could be expected.

Fig. 11 2D NMR spectrum of ¹H and ¹³C (pulse sequence gHSQCAD) of Φ_S,2 partially dissolved in DMSO-D6 with m(Φ_S,2) = 9.8 mg in 650 μL of DMSO-D6 (S/L = 15.07 g of precipitate/L of solvent).

Fig. 12 2D NMR spectrum of ¹H and ¹³C (pulse sequence gHMBCAD) of Φ_S,2 partially dissolved in DMSO-D6 with m(Φ_S,2) = 9.8 mg in 650 μL of DMSO-D6 (S/L = 15.07 g of precipitate/L of solvent).

Discussion

In this work, γ-irradiation of a biphasic system containing 30%_vol TBP diluted in TPH (organic phase) in contact with palladium(II) nitrate dissolved in 3 mol L⁻¹ nitric acid (aqueous phase) led to the formation of a precipitate located at the liquid–liquid interface (Φ_S,1) and a precipitate that settled spontaneously (Φ_S,2) (Fig. 1). Elemental analyses showed that both precipitates contain palladium. Solubilisation tests demonstrated that the precipitates do not contain only one compound but a mixture of at least two compounds. XRD, XPS, TGA-DSC, IR, ESI-MS and NMR analyses allowed for the identification of palladium cyanide in the precipitates. Other compounds such as palladium carboxylates, phosphates (i.e. TBP or HDBP), compounds with ammonium and/or amine functions may also be present to a lesser extent. In the following, the presence of the different compounds in the precipitate is discussed.

The irradiation of a biphasic system leads to both radiolysis of nitric acid and water molecules in the aqueous phase, and radiolysis of the molecules present in the organic phase (TBP and TPH). Radiolysis of nitric acid primarily leads to the formation of HNO₂ (ref. 23 and 70) and H₂O₂ (ref. 70) whereas radiolysis of TBP and TPH can generate many different degradation products such as alkenes, carbonyl compounds (R–CO–R′, R–CO–H), carboxylic acids (R–COOH), alcohols (R–OH), nitro compounds (R–NO₂)²³ and phosphorus compounds such as HBDP, H₂MBP, H₃PO₄, etc.²⁶ In addition, several authors mentioned the formation of secondary products, such as oximes (R–C=N–OH), even though this molecule has never been formally identified.^23,71 These oxime compounds have a great affinity for Pd.⁷²

NMR and ESI-MS analyses showed the presence of TBP and HDBP in Φ_S,1 and Φ_S,2 but these molecules may not be directly bonded to the Pd since XPS analyses showed only two different environments around Pd that are not characteristic of phosphorous compounds but rather C≡N and O–C=O groups. However, the presence of TBP and HDBP in the precipitate may also explained by the inclusion of TBP/HDBP molecule in the solid since the precipitate are obtained in the biphasic system containing both TBP and HDBP (due to the radiolytic degradation of the TBP).

Besides, NMR analyses showed the presence of carboxylate and CN group. XPS indicated that these two groups are directly bonded to palladium. The formation of Pd-carboxylate compounds could be easily explained by the reaction of carboxylic acid (radiolytic degradation products of the TBP–alkane solution^73–76) with the palladium leading to a precipitate.

Regarding the palladium cyanide formation, an assumption would be the reaction of HCN with the palladium present in the organic/aqueous solutions. Ketones and oximes seem to be a valuable precursor to the formation of HCN among the degradation products formed during γ-irradiation.^77,78 In particular, it is interesting to highlight that heating of aldehydes, ketones or alcohols in the presence of hydroxylamine (NH₂OH) or nitrous acid (HNO₂) in acidic media can be responsible for HCN formation.⁷⁷ An oxo-oxime can be formed by the addition of HNO₂ or NH₂–OH to the carbon in α position of the carbonyl group (eqn (1)). This species becomes dehydrated and leads to the formation of an α-keto-cyanide (eqn (2)), which can be hydrolyzed into carboxylic acid (R–CO–OH) with the departure of HCN (eqn (3)).⁷⁷

Other ways to produce oxime that are consistent with the experimental conditions of the PUREX process were also reported in the literature.⁷⁷ In presence of HNO₂, alcohols can and form nitrous acid ester by esterification (eqn (4)). A rearrangement can occur to produce a compound containing both an hydroxyl (–OH) and a nitroso (–N=O) groups, i.e. an oxime (eqn (5)). The oxime can be afterward hydrolysed into a ketone (eqn (6)). Finally, the ketone can form HCN by reacting with HNO₂ or NH₂OH according to eqn (1) to (3).

Another alternative pathway for oxime formation could be the reaction of CH₃˙ radical with NO (eqn (7)) since NO radicals can be formed by irradiation of TBP or alkanes.⁷⁹

CH₃˙ + NO → H₃C–N=O⇄H₂C=N–OH (7)

In this particular case, CH₂=N–OH can lead to the formation of HCN in the presence of Pd(0) according to Scheme 1. The presence of Pd(0) is likely and has already been reported by Vialard and Germain¹² after radiolysis of TBP in alkane solution in contact with aqueous phase.

Scheme 1 Proposition of catalytic mechanism for HCN formation in the presence of Pd⁰ and formaldehyde oxime.

Finally, HCN can react with Pd²⁺ to form Pd(CN)₂ as shown in eqn (8) since the stability constant is quite high (K = 10^+41.8).⁸⁰

Pd² + 2HCN = Pd(CN)₂ + 2H⁺ (8)

Several formation pathways for Pd(CN)₂ have been proposed. An organic degradation product, such as alcohol, carbonyl compound or methyl radical, in the presence of HNO₂ or NH₂OH allows the formation of HCN. Then, in presence of Pd²⁺ in solution and HCN, Pd(CN)₂ can be formed with a thermodynamic constant of 10^+41.8 (Scheme 2).

Scheme 2 Proposition of a formation pathway for Pd(CN)₂ after irradiation of a biphasic system composed of TBP in alkane in presence of nitric acid solution containing palladium(II) nitrate.

Conclusion

XRD, XPS, TGA, IR, ESI-MS and NMR have been used to characterize the two precipitates (Φ_S,1 and Φ_S,2) produced by γ-irradiation at 500 kGy of 30% _vol tri-n-butylphosphate diluted in TPH in contact with nitric acid containing palladium(II) nitrate. Φ_S,1 and Φ_S,2 exhibit similar structures and contain a mixture of Pd(CN)₂ and other compounds (Pd-carboxylate, ammonium or amine functions and phosphorous compounds). The degradation products formed during irradiation of the two-phase biphasic system may be responsible for precipitates formation of palladium. Among the degradation products formed by irradiation, ketones and oximes could be responsible for precipitate formation.

Future studies designed to validate the proposed Pd(CN)₂ formation mechanism and to identify the other Pd compounds are currently under progress. Nitrate group arising from nitric acid or palladium nitrate may be responsible for the presence of CN⁻ in the precipitates since only these two products contain nitrogen atoms in the biphasic system. Therefore, γ-irradiation in the presence of ¹⁵HNO₃ may be very helpful to identify the different intermediates involved in HCN formation. Specifically, analyses of the organic and the aqueous phases could provide information on the nitrogen species formed by γ-irradiation and allow for the confirmation or invalidation of the proposed mechanism for palladium cyanide formation. Biphasic systems containing an organic phase either of pure TBP or pure dodecane in contact with a nitric acid aqueous solution containing palladium(II) nitrate will be irradiated under the same conditions as the biphasic system {TBP–TPH–HNO₃–Pd} described in this study in order to determine which organic molecule is responsible for the precipitate formation.

Experimental

Reagent

Palladium nitrate (purity = 99.9%, Sigma Aldrich), 68.5%_wt nitric acid (purity = 99.8%, VWR), TBP (purity ≥ 99.9%, VWR) and tetrapropylene hydrogen (TPH, petroleum fraction, Novasep) were used without further purification. The extraction solvent was prepared by mixing appropriate amounts of TBP and TPH (30%_vol TBP diluted in TPH). Palladium(II) nitrate was weighted in order to obtain a concentration of 0.02 mol L⁻¹ (or 2 g L⁻¹) of Pd in 3 mol L⁻¹ nitric acid aqueous solution prepared by dilution of 68.5%_wt nitric acid in Milli-Q deionized water. The solution was filtered to remove traces of metallic palladium.

Palladium(II) cyanide (purity = 99.9%), palladium(II) acetate (purity ≥ 99.9%) palladium(II) pivalate (purity = 97%), palladium(II) acetylacetonate (purity = 99%) were purchased from Sigma Aldrich and used without further purification.

Dimethyl sulfoxide (DMSO, purity = 99.5%, VWR), pyridine (pyr, purity = 99.7%, VWR), ammonia (purity = 99.8%, VWR), dichloromethane (purity = 99.9%, Sigma Aldrich), tetrahydrofuran (THF, purity ≥ 99.9%, Acros Organic), methanol (purity = 99.8%, VWR), acetone (purity = 99.9%, VWR) were used without further purification.

An aqueous solution of 1 mol L⁻¹ sodium hydroxide was purchased from VWR and used for potentiometric titrations.

A palladium solution of 1000 μg mL⁻¹ in 10%_wt HCl provided by Fischer Scientific was used to prepare the standard solutions for ICP-AES measurement.

Synthesis of the palladium precipitates

A 3 mol L⁻¹ nitric acid aqueous phase containing 0.02 mol L⁻¹ palladium(II) nitrate (2 g L⁻¹ of Pd) was contacted with an organic phase containing 30%_vol TBP diluted in TPH with a volume ratio organic to aqueous phase of 1. The organic phase was not pre-equilibrated before extraction of Pd(II). The biphasic system was shaken for 5 minutes at ambient temperature before γ-irradiation to 500 kGy by using a ⁶⁰Co source at the Synergy Health company (Marseille, France) with a dose rate of about 5 kGy h⁻¹. The integrated dose was chosen to accelerate the degradation process and to simulate long term behaviour of the solvent under irradiation stress. After γ-irradiation of the biphasic system, four phases were observed: an organic phase (Φ_org), an aqueous phase (Φ_aq), a light precipitate at the liquid–liquid interface (Φ_S,1) and a heavy precipitate (Φ_S,2) at the bottom of the flask as illustrated in Fig. 1. Precipitates were recovered and filtered under vacuum. Afterwards, they were washed twice with 2 mol L⁻¹ nitric acid and dried at 40 °C for 24 hours. The palladium concentration and the acidity of the liquid phases, before and after irradiation are reported Table 7. The acidity of the organic and the aqueous phases scarcely decreased while the palladium concentration in the aqueous phase decreased drastically.

Table 7 Palladium and proton concentrations before and after γ-irradiation at 500 kGy of the biphasic system. Initial composition: 30%_vol TBP in TPH in contact with 0.02 mol L⁻¹ palladium(II) nitrate in 3 mol L⁻¹ nitric acid aqueous phase with an organic:aqueous phase ratio of 1 : 1 (γ-irradiation performed with ⁶⁰Co)

[Pd]aq (mmol L^−1)		[H^+]aq (mol L^−1)		[H^+]orga (mol L^−1)
Before irradiation	After irradiation	Before irradiation	After irradiation	Before irradiation	After irradiation
18 ± 10^−3	4 ± 10^−3	2.63 ± 0.07	2.45 ± 0.07	0.62 ± 0.07	0.60 ± 0.07

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The volumes of the aqueous and organic phases before γ-irradiation and the weight of the precipitates ϕ_S,1 and ϕ_S,2 recovered after irradiation are reported in Table 8.

Table 8 Volumes of irradiated aqueous phase (V_aq) and organic phase (V_org), and weights of precipitates ϕ_S,1 and ϕ_S,1 before and after γ-irradiation (m_ϕS,1 and m_ϕS,1, respectively)

Volume of the solutions before irradiation		Weights of precipitates after irradiation
Vaq (mL)	Vorga (mL)	mϕS,1 (mg)	mϕS,2 (mg)
200	170	264	312

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In order to characterize the composition of the precipitates, dissolution tests were performed by using DMSO, CH₂Cl₂, THF, methanol, acetone, NH₃ (1 mol L⁻¹) and pyridine. No complete dissolution of Φ_S,1 and Φ_S,2 with a S/L of 2 (solid/liquid: mass of precipitate (g)/volume of the solvent (L)) for 15 minutes at room temperature was achieved with these solvents. DMSO, pyridine and 1 mol L⁻¹ NH₃ lead to the best dissolution of the precipitates. Table 9 shows the experimental conditions requested to dissolve partially the precipitates in DMSO, pyridine or 1 mol L⁻¹ NH₃ and the weight of the insoluble part (after the partial dissolution, a liquid phase (Φ_F1) and a solid (Φ_S,3) were obtained).

Table 9 Volume (V) of solvent (DMSO, pyridine or 1 mol L⁻¹ NH₃) used to dissolve partially m (mg) of precipitates ϕ_S,1 or ϕ_S,2 and weight of insoluble fraction of solid (m′). Experimental conditions: room temperature; stirring time = 15 minutes

Sample	Solvent
	DMSO			Pyridine			1 mol L^−1 NH3
	m (mg)	V (mL)	m′ (mg)	m (mg)	V (mL)	m′ (mg)	m (mg)	V (mL)	m′ (mg)
a m′ could not be determined since no enough insoluble fraction was obtained.
ΦS,1	11.2	5.6	6.3	10.7	5.35	—^a	11.3	5.65	0.3
ΦS,2	11.8	5.9	9.5	11.4	5.7	0.5	12.8	6.4	0.2

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Table 10 gathers palladium concentration in these solvents after partial dissolution of the precipitates Φ_S,1 and Φ_S,2. The accuracy on the palladium concentration and the weight percent of palladium solubilized was 5%.

Table 10 Palladium concentrations and weight percent of Pd solubilized in the filtrates after partial dissolution for 15 minutes stirring of Φ_S,1 and Φ_S,2 in DMSO, pyridine and 1 mol L⁻¹ NH₃ with a solid/liquid (S/L) ratio of 2 (mass of precipitate used for the dissolution (g)/volume of the solvent (L))

Solvents used for the dissolution	%w Pd solubilized^a		[Pd] solubilized (mol L^−1)
	ΦS,1	ΦS,2	ΦS,1	ΦS,2
a The weight percent of palladium solubilized in each solvent DMSO, pyridine and NH3 is calculated as shown in eqn (9).
DMSO	10	9	9.2 × 10^−4	1.3 × 10^−3
Pyridine	80	81	6.8 × 10^−3	1.0 × 10^−2
NH3	99	83	9.1 × 10^−3	9.6 × 10^−3

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The calculation of m_Pd(0) is described in the TGA-DSC analysis, it corresponds to the mass remaining at 650 °C.

Equipment and analysis

ThermoGravimetric analysis coupled with a differential scanning calorimeter (TGA-DSC). Thermogravimetric analyses coupled with a differential scanning calorimeter (TGA-DSC) were performed with a LABSYS EVO model from SETARAM under nitrogen at a flow rate of 20 mL min⁻¹ between 25 °C and 650 °C with a ramp of temperature of 5 °C min⁻¹. An alumina crucible (90 μL) was used as reference crucible for TGA measurements. Experiments were conducted under inert nitrogen atmosphere in order to avoid side reactions between palladium salts and dioxygen responsible for PdO formation as reported by Gallager et al.⁶⁷

The weight loss observed around 80 °C attributed to the H₂O vaporization allows calculating the moisture content in the precipitates according to eqn (10)

where m_initial denotes the initial mass of the precipitate used for TGA analysis and m₁ corresponds to the remaining mass at 110 °C.

By considering that a complete degradation is achieved when the samples are heated at 650 °C, the weight percent of palladium in the precipitates can thus be calculated as follows eqn (11).

where m_Pd(0) corresponds to the weight of sample at 650 °C.

The weight loss observed between 25 °C and 110 °C, from 110 °C to 350 °C, and between 350 °C and 650 °C are attributed to H₂O vaporization, organic compounds, and (CN)₂ respectively. The weight percent calculations are detailed below. Based on the TGA analyses, a quantification of each loss in mass can be deduced.

By measuring the loss in mass of (CN)₂ produced between 300 °C and 650 °C, TGA analysis allows the determination of the mass associated to gaseous (CN)₂ thanks to eqn (12).

m_(CN)2 = m₂ − m_Pd(0) (12)

where m_(CN)2 denotes the mass of (CN)₂ in the sample, m₂ the remaining mass of the sample at 350 °C and m_Pd(0) the remaining mass at 650 °C.

By assuming all the (CN)₂ is in palladium cyanide form, it is possible to calculate the fraction of Pd linked to cyano functions present in the total amount of the Pd in the precipitate as shown in eqn (13).

where M_(CN)2 corresponds to the molecular weight of (CN)₂ and M_Pd denotes the molecular weight of palladium.

The weight percent of palladium linked to carboxylate present in the total amount of the Pd in the precipitate can be deduced as follows by considering that only carboxylate and CN functions are linked to palladium in the precipitates (eqn (14)).

%_w Pd_(O−C=O) = 100 − %_w Pd_(CN)2 (14)

At 350 °C, the loss in mass is attributed to organic compounds. The weight percent of organic compound can be deduced thanks to eqn (15).

where m_initial corresponds to the initial mass of the precipitate used for TGA analysis, m_Pd(0), m_(CN)2 and m_H2O represent weight of palladium in the sample, weight of (CN)₂ released, and weight of water present in the sample (m_H2O = m_initial − m₁), respectively.

In general, it may be considered that the TGA measurement uncertainties lead to an error of approximatively 10% for the difference weight percent calculations.

ATR-FTIR spectroscopy. IR spectra were recorded with a Fourier Transform Infrared Spectrometer Vertex 70 from BRUKER equipped with a wide range beam splitter (scan number = 32; resolution = 4 cm⁻¹) and the DTGS wide range (diamond) allowing scanning between 80 and 4000 cm⁻¹. Samples were deposited on an ATR diamond crystal.

Raman. Raman spectra were recorded with a labRAM HR evolution from HORIBA equipped with a laser with a wavelength of 633 nm. The acquisition time and the number of accumulations were of 100 seconds and 3, respectively. A quartz cell placed in the macro lens UV-Vis NIR was used. Raman spectroscopy was used to analyse the filtrate (ϕ_F1) obtained after a partial dissolution of the precipitate ϕ_S,1 in NH₃ 1 mol.L⁻¹ for 15 minutes of stirring at ambient temperature.

X-ray photoelectron spectroscopy. X-ray Photoelectron Spectroscopy (XPS) analyses were carried out with a ThermoFisher Escalab 250 XI spectrometer using a monochromatic Al Kα X-ray source (1486.6 eV). A dual beam charge compensation flood gun was used when important charge effects occurred and the C-1s signal for adventitious carbon was used to correct the charge effect. The C–C/C–H component of C-1s spectra was fixed at 285.0 eV. The spectrometer was calibrated to the silver Fermi level (0 eV) and to the 3d_5/2 core level of metallic silver (368.3 eV). The following core levels were recorded with a 20 eV pass energy: Pd-3d, C-1s and N-1s. Data processing was performed using the commercial Avantage® software (ThermoFisher Scientific Inc.). Shirley background subtraction method was used for fitting procedure. With our results, the Pd/N ratio was calculated. The Pd/C ratio could not be calculated due to the possible presence of contaminants. An intensity ratio of the peak can be calculated supposing the atoms considered for the calculation are not pollution. This ratio is then corrected by sensitivity factors (Scofield factors have been used for quantification) and spectroscopic factors (ability of the spectrometer to detect an electron of a defined energy). Moreover, the palladium proportion of the two contributions Pd–CN and Pd–O=C–O for Φ_S,1 and Φ_S,2 can be calculated based on XPS analysis. By calculating the maximum intensity ratio of Pd linked to CN function and Pd linked to carboxylate function and by considering only two kinds of palladium are present in the precipitates, it is possible to obtain the palladium proportion of the two kinds of palladium. For XPS analysis, there are several sources of error in the weight percent calculated by this technique. First of all, the XPS analysis only gives information to relative quantifications. The errors can come from the choice of the background, the recombination of the spectra and a different depth of analysis from one sample to another. Finally, this technique allows analysing the extreme surface of the powder only and does not allow going deep into the sample. All this errors can explain the large uncertainty error in the value obtain by calculated of the proportions of Pd linked to cyano functions (C≡N) and Pd linked to carboxylate functions (O–C=O).

X-ray powder diffraction analysis. X-ray powder diffraction analyses (XRD) were performed by means of a D8 Advance BRUKER diffractometer equipped with a LynxEyes™ detector. A copper cathode tube was used at a voltage of 40 kV and a current of 40 mA. XRD patterns were recorded between 2θ = 5° and 2θ = 140° with a step of 0.01° and an acquisition time of 0.5 s per step. Powders were deposited onto a mirror sample holder (monocrystal of silicon) wetted with ethanol to paste the powders. The Diffrac software, Evaluation V2.1 and database ICDD PDF 2016 were used for indexing XRD patterns.

The precipitates were analyzed by Rietveld refinement, using TOPAS 4.2 software and Le Bail pattern matching.⁸¹ The profile parameters (cell dimension, peak shape, background, sample displacement correction and asymmetry) were defined. The peak shape was described by pseudo-Voigt function with the formulation of Caglioti.⁸²

The Rietveld method⁸³ was used to determine the crystallographic structure of solid samples. The refinement program minimizes the residual between experimental and calculated XRD patterns by the method of least squares. The quality of a refinement was determined by comparing the experimental XRD pattern and the calculated one by means of the following parameters, which must be as low as possible:

- The R_Bragg factor associated with a given structure is based on the integrated intensities and is calculated by eqn (16). The structural model is assumed to be correct when the ratio R_Bragg value is lower than about 5.

where I_i(obs) and I_i(calc) denote the intensities of the peak i in the experimental and calculated XRD-pattern, respectively.

- The factor R_wp weighted profile is associated with the whole pattern and is calculated as follows in eqn (17):

where y_i(obs) denotes the intensity observed to the angle 2θ_I, y_i(calc) corresponds to the calculated angle 2θ_i, and w_i denotes the statistical weight.

Electrospray-ionization mass spectroscopy. Electrospray-ionization mass spectrometry spectra (ESI-MS) were recorded in the positive ionization mode using a BRUKER Esquire-LC quadrupole ion trap equipped with an electrospray interface. A syringe infusion pump (Cole Palmer) delivered the sample at a rate of 180 μL h⁻¹ to the electrospray source. The capillary voltage was set to −4000 V. Nitrogen was employed as the drying and nebulizing gas. The drying gas flow rate was set to 4.0 L min⁻¹ and the nebulizing gas pressure was set to 5.0 psi. The source temperature was set to 250 °C. Spectra were acquired over a mass/charge (m/z) range of 45–2200 with a trap drive setting of 50. All samples were diluted by a factor of 10 or 100 in acetonitrile prior to any injection into the ESI-MS. Species containing Pd atom(s) were identified by comparison with an isotopic pattern calculated using the software DataAnalysis 4.0 (seen Fig. 13 as an example).

Fig. 13 Experimental (at the top) and simulated (at the bottom) isotopic pattern of [Pd(pyr)₂(CN)]⁺.

Nuclear magnetic resonance spectroscopy. Nuclear magnetic resonance spectra (¹H NMR) were recorded on a 400 MHz Agilent DD2 spectrometer equipped with a OneNMR probe. Solutions were diluted using deuterated dimethylsulfoxide (DMSO-D6, Aldrich) containing traces of tetramethylsilane (0.03%). The measurements were performed in a 5 mm-diameter tube. Solutions were analysed by using 1D hydrogen spectra and 2D gHSQBC and gHMBC methods.

Inductively coupled plasma – atomic emission spectroscopy. Analyses by Inductively Coupled Plasma – Atomic Emission Spectroscopy (ICP-AES) were carried out with an Ultima 2 from HORIBA. The pressure of argon and nitrogen were 6 bar and 0.5 bar, respectively. The flux of plasma-forming and sheathing gas were 12 L min⁻¹ and 0.2 L min⁻¹, respectively. ICP-AES was used to measure palladium concentrations in the aqueous phases. The calibration curve was determined by using 8 standard solutions containing palladium at concentrations ranging from 0 to 20 mg L⁻¹ prepared from 1 g L⁻¹ commercial standard solution. The samples were diluted in 2%_w nitric acid (prepared from 68.5%_w nitric acid). The uncertainty obtained by ICP-MS is approximately 5%.

Potentiometric titration. The acidity of the organic phases was measured by 0.04 or 0.1 mol L⁻¹ NaOH standard solution (purity > 99.9% certified grade, provided by VWR) using a 785 DMP Titrino autotitrator from Metrohm. Aqueous samples were diluted in 80 mL of water (deionized water, 18.2 Ω). For organic samples analysis, samples were diluted in 80 mL of aqueous phase and 1 mL of acetone was added before titration to obtain a homogenous solution. The accuracy obtained by this method is approximately 3%.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was financially supported by ORANO Cycle and Commissariat à l’Énergie Atomique et aux Énergies Alternatives (CEA). The authors would like to thanks Ashleigh Kimberlin for helpful comments and editing assistance.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra02695e

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