J Abel

M Lamirand-Majimel

J Majimel

V Bellière-Baca

V Harlé

G André

C Prestipino

S Figueroa

E Durand

A Demourgues

Oxygen non-stoichiometry phenomena in Pr 1-x Zr x O 2-y compounds (0.02 < x < 0.5)

New Pr 1-x Zr x O 2-y oxides with x < 0.5 have been prepared by co-precipitation in basic medium and annealed under air at high temperatures (T ≤ 1200 °C).

Defined compositions with x = 0.02, 0.1, 0.2, 0.35, 0.40 and 0.5 have been characterized by XRD, Zr-K-edge EXAFS for the local structure, magnetic suscep-tibility measurements, and Pr L III -edge XANES in order to identify the variation of the cell parameter and Zr local environment versus Zr content and Pr n+ (4 < n < 3) oxidation states. The higher the Zr content, the lower the Pr valence state. The Zr amount stabilized in the distorted octahedral site is at the origin of the formation of defined compositions as discovered by Leroy Eyring et al. in the Pr n O 2n-2m series and the generation of oxygen vacancies stabilized in the fluorite-type network. TGA and TPR analyses help to follow the reduction properties under Ar/5% H 2 and show high Pr reducible rates at low temperatures (T < 250 °C). The identification of the fluorite-type superstructure (SG: Ia3 ¯) of reduced compositions annealed at T = 900 °C under Ar/5% H 2 shows the cationic and oxygen vacancy ordering. This feature plays a key role with Zr 4+ cations stabilized in flattened octahedral sites for the generation of oxygen vacancies and the stabilization of Pr 3+ in the reduced states.

Introduction

Rare earth-based oxides have been widely investigated considering various compositions and structural features related to ionic and redox properties for numerous applications such as catalytic oxidation 1,2 or fuel cells. 3 Those applications require materials which exhibit high oxygen mobility and redox properties at low temperatures associated with oxygen nonstoichiometry or vacancies and mixed valence states. Ceria (CeO 2 ) being well established as an oxygen buffer is widely used in catalysis in which oxidation-reduction processes are involved. [START_REF] Trovarelli | Catalysis by Ceria and related materials[END_REF] Ceria-zirconia mixed oxide materials have been developed as Three-Way-Catalysts (TWCs) and exhibit high oxygen storage capacity (OSC) with high oxygen release under reducing conditions and fast kinetics, high content of reducible Ce 4+ at T < 600 °C, and high thermal stability with good surface areas between 20 and 40 m 2 g -1 stabilized at T > 1000 °C. Various networks such as fluorite-type for high Ce rate, tetragonal dis-tortion of the fluorite-type structure for high Zr content and the pyrochlore-type structure [8][9][10][11][12][13][START_REF] Eyring | Handbook on the Physics and Chemistry of Rare Earths[END_REF][START_REF] Narula | [END_REF][16][17][18][19] for equimolar compositions have been characterized in the last 20 years. [5][6][7][8][9][10] The pyrochlore corresponding to a fluorite-type superstructure with Ce 4+ /Ce 3+ , eight-fold coordinated to oxygens and Zr 4+ cations in a distorted octahedral site as well as oxygen vacancy ordering exhibit the best redox properties. 18 Then the occurrence of cation site distortions as well as cationic ordering plays a key role for OSC, Ce reducibility and minimum temperature of reduction. 18 However, Pr-based oxides should be able to compete with Ce-based oxides. 11 Pr [Xe] 6s 2 4f 3 5d 0 has got one electron more than Ce [Xe]6s 2 4f 2 5d 0 . Its most stable defined compounds are Pr 7 O 12 (ref. 12) and Pr 24 O 44 (Pr 6 O 11 ) 13 containing mixed Pr 4+ / Pr 3+ valence states. The Pr n O 2n-2m defect structures were extensively studied by Eyring et al. [START_REF] Eyring | Handbook on the Physics and Chemistry of Rare Earths[END_REF] Considering ionization energies of Pr and Ce, Pr 4+ is less stable and more reducible than Ce 4+ . Moreover, in solution, the redox potential Pr 4+ /Pr 3+ is larger and equal to 2.7 V per ENH than that of Ce 4+ /Ce 3+ equal to 1.7 V per ENH. Hence, Pr 1-x Zr x O 2-y solid solutions are expected to exhibit mixed valence states and oxygen vacancies associated with oxygen mobility and redox processes at low temperatures.

In previous studies, [START_REF] Narula | [END_REF]16 no information has been given on Pr valence states in the oxidized as well as reduced phases. Moreover, no structural features of reduced compounds have been investigated and finally no defined compound (thermo-been indexed on the basis of fluorite-type structures (SG: Fm3 ˉm), pyrochlore-type structures (SG: Fd3 ˉm) and bixbyite-type structures (SG: Ia3 ˉ) in selected papers (ref. 5-10, 18-19).

Magnetic measurements

Curie constants and θp of various Pr-based oxides have been deduced from magnetic susceptibilities (1/χ = (T + θp)/C, χ = M/H) with magnetization recorded between 150 K and 300 K (increment 10 K) and the magnetic field going from 0 to 50 000 Oe. Magnetic measurements have been performed on a SQUID detector magnetometer Quantum Design MPMS 5. As the Curie constant is proportional to the concentration of paramagnetic species, Pr 3+ (4f 2 ) and Pr 4+ (4f 1 ) contents in the various compounds can be deduced from magnetic data.

Zr K-edge EXAFS and Pr L III -XANES experiments XAFS spectra at the Zr K-edge and Pr L III edge were collected on station BM23 at ESRF-Grenoble-France with beam currents between 150 mA and 250 mA. XAFS spectra were recorded at room temperature in transmission mode using a Si(111) monochromator crystal. EXAFS spectra at the Zr K-edge were reduced to normalized XAFS χ(k) using the VIPER software and weighted k 3 χ(k) data using the VIPER suite of program. Least squares analysis against model environments were calculated and compared with PrO 2 , ZrO 2 and Pr 2 Zr 2 O 7 as reference compounds. Four shells have been refined leading to the determination of nine parameters (radial distance R and Debye-Waller factors 2σ 2 for each shell + zero energy shift). The results obtained were compared with XRD analysis results and are reported in Table 3. In the case of Pr L III XANES spectra, the Pr 4+ and Pr 3+ rates were estimated on the basis of reference compounds containing 100% of Pr 4+ and 100% of Pr 3+ .

Powder neutron diffraction study of reduced samples Pr 0.98 Zr 0.02 O 1.51 oxides were characterized by powder neutron diffraction at room temperature on the 3T2 beamline with 1.2253 Å radiation and a graphite monochromator of the Léon Brillouin Laboratory (LLB) in Saclay (CEA)-France. The 2θ range was 10-120°with a step scan of 0.2°. Intensity treatments ( profile, cell parameters, atomic positions and isotropic thermal displacement) and refinements based on the Rietveld method were performed using the Fullprof package of program. The quality of the acquisition and the refinement was based on the conventional reliability factors R p , R wp and R Bragg , respectively.

TGA and TPR analyses

The thermogravimetric analysis (TGA) was carried out on a SETARAM Setsys Evolution under an Ar/3% H 2 flux (50 ml min -1 ) with a speed rate of 5 °C min -1 up to T = 900 °C. Samples were outgassed at T = 700 °C for 3 hours under air without moisture in order to eliminate carbonates and residual water. Temperature programmed reduction (TPR) was performed on a Micromeritics AutoChemII under Ar/10% H 2 (25 ml min -1 ) up to 900 °C (speed rate: 20 °C min -1 ).

dynamically stable) has been characterized or discussed in this series.

Narula et al. [START_REF] Narula | [END_REF] reported the synthesis, structure and redox properties of Pr 1-x Zr x O 2-y (x = 0.25, 0.5 and 0.75). They had shown great interest in such compositions for three way catalyst application and especially for high rate of praseodymium, such as Pr 0,75 Zr 0,25 O 2-y with a reduction temperature around T = 500 °C and around 50% Pr reduced (variation of oxygen content: ΔO = 0.16).

Bellakki et al. 16 investigated the Pr 1-x Zr x O 2-y (0 ≤ x ≤ 1) solid solutions synthesized by the single step combustion method. They had shown by XRD investigations that this solid solution crystallizes with the fluorite-type structure from x =0 to x =1. H 2 -TPR measurements were performed and the higher H 2 consumptions were obtained for the Pr-rich system (x ≤ 0.5). Finally, the achievement of pure phases is strongly related to synthesis routes.

In this study we focused on structural features and redox properties of Pr 1-x Zr x O 2-y defined compounds for high rates of praseodymium (0.02 ≤ x ≤ 0.5) and we detailed the impact of structural features and composition (Pr 4+ /Pr 3+ ) on the redox properties. XRD and Zr-K-edge EXAFS investigations were first carried out. Then magnetic susceptibility measurements and Pr L III -XANES studies were performed to follow the Pr oxidation states. Finally TPR and TGA measurements under Ar/5% H 2 were undertaken to evaluate the reduction properties and the reduced phases were characterized by neutron diffraction investigations.

Experimental section

Synthesis

Oxides have been prepared starting from nitrate precursors (SOLVAY), co-precipitated in basic medium (pH = 10) and annealed under air at temperatures of 700, 900 and 1200 °C for 12 hours. At pH = 10, all Pr-Zr hydroxides and oxyhydroxides precipitate. All these oxides annealed under air at 700 °C < T < 1200 °C crystallize with a fluorite-type structure and no phase transition is observed. Moreover these oxides were reduced in an oven as well as in TGA apparatus under Ar/5% H 2 flux at T = 900 °C in order to obtain and characterize reduced phases.

XRD analysis

Powder X-ray diffraction patterns were collected on a Phillips X'Pert MPD X-ray diffractometer with Bragg-Brentano geometry and using Kα1/Kα2 radiation (5° < 2θ < 120° step of 0.01° and a counting time of 150 s). Diffractograms were refined using the Rietveld method and the Fullprof package of programs with conventional reliability factors and on other hand with Kα1 radiation (5° < 2θ < 120° step of 0.017° and a counting time of 1500 s) for the reduced sample exhibiting the superstructure of the fluorite-type network. Unit cell parameters, atomic positions, occupancies and Debye-Waller factors have been refined in such a case. All the diffraction patterns have sitions equal to 2.54µ B and 3.58µ B , respectively, a proportion of Pr 4+ and Pr 3+ ions can then be evaluated in Pr 1-x Zr x O 2-y compounds. Furthermore taking into account that the theore-

Results and discussion

XRD analysis on each sample annealed at 1200 °C, using Cu Kα1/Kα2 radiation, led to identify the single phase corresponding to the fluorite type structure (SG: Fm 3m) for x = 0.02, 0.1 and 0.2 and to the pyrochlore one (SG: Fd 3m) for x = 0.35, 0.4 and 0.5 (Fig. 123and Table 1). Phase mixtures were obtained for 0.02 < x < 0.1 (2 fluorite phases) and 0.2 < x < 0.35 (1 fluorite and 1 pyrochlore phases). The cell parameter of Pr 1-x Zr x O 2-y compounds decreases gradually with the Zr content (Fig. 4). Magnetic measurements were performed on the single phase with Pr 1-x Zr x O 2-y compositions and the linear variation of the inverse magnetic susceptibility versus temperature (Fig. 5 and Table 2) allowed determining the effective magnetic moment per Pr ion. Taking into account the theoretical effective magnetic moments of pure Pr 4+ and Pr 3+ compotical effective magnetic momentum of isolated ions (Pr 4+ and Pr 3+ ) does not reflect the real values in the Pr 1-x Zr x O 2-y solid solutions (L-S spin-orbit coupling effect), two extreme compositions Pr 0.9 Zr 0.1 O 2 (one redox cycling) with 100% Pr 4+ and Pr 2 Zr 2 O 7 (x = 0.5) with 100% Pr 3+ were taken as references to evaluate the Pr 4+ /Pr 3+ corrected ratio. Furthermore, Fig. 6 indicates that the higher the Zr 4+ content, the lower the Pr 4+ rate. Actually, the zirconium content does involve a Pr 4+ /Pr 3+ ratio due to competitive bounds around Zr 4+ /Pr 3+ /Pr 4+ , i.e. the ionic radii of each cation in specific six-fold, seven-fold and eightfold oxygen environments. The larger Pr 3+ -O 2-bond is compensated by the smaller Zr 4+ -O 2-bond distance leading to the creation of oxygen vacancies.

The zirconium coordination number in the ideal pyrochlore structure (Pr 2 Zr 2 O 7 , pure Pr 3+ ) is 6. Due to the partial oxidation of pyrochlore leading to the stabilization of Pr 2 Zr 2 O 7+x , the coordination number of Zr 4+ changes from 6 to 7 then to 8. 17 For x = 0.2 in the case of the oxidized and reduced phases, the Zr 4+ coordination number seems to slightly decrease from a value between six and seven to six-fold coordination with an average Zr-O distance around 2.10 Å according to the Zr K-edge EXAFS study (Fig. 7 and Table 3). For x = 0.5, the pyrochlore phase, Zr 4+ occupies an octahedral site at a slightly shorter Zr-O radial distance than x = 0.2 composition in the oxidized state. Actually, Zr-K edge EXAFS refinements (Fig. 8 and 9, Table 3) of both of these compounds with x = 0.2 (oxidized and reduced states) show the same number of neighbors for the first shell but a smaller Zr-O radial distance for the reduced compound. The intensity of the modulus of the Zr-Pr 4+ and Zr-Pr 3+ at larger distance has been fixed at the correct value on the basis of magnetic measurements and the relative proportions of Pr 4+ /Pr 3+ in the oxidized and reduced samples. Moreover, one should have to note that the radial distances corresponding to the second shells increase in the reduced samples in good agreement with the XRD data refinements and the increase of the cell parameter.

Eyring et al. [START_REF] Eyring | Handbook on the Physics and Chemistry of Rare Earths[END_REF] highlighted the occurrence of defined compounds for PrO y compositions; the same phenomenon seems to appear in Pr 1-x Zr x O 2-y solid solution, but in this case the stabilization of compounds with specific Pr 4+ /Pr 3+ ratio can be tuned by the zirconium molar concentration.

XRD patterns of Pr 1-x Zr x O 2-y annealed at 700 °C (x = 0.2, 0.1 and 0.02) are represented in Fig. 10. In this case, the lowest annealing temperature has been considered in order to evaluate the redox properties of compounds exhibiting the lowest crystallinity related to the highest surface areas and reactivity. Fig. 11 shows the H 2 -TPR profiles of Pr 1-x Zr x O 2-y annealed at 700 °C (x = 0.2, 0.1 and 0.02). For these annealing temperatures, pure phases were obtained as can be seen from Fig. 10. Total hydrogen uptakes were deduced from the integrated values (from T = 30 °C to 900 °C) and are given in Table 4. The Pr 0.98 Zr 0.02 O 2-y compound was reduced below 600 °C and the H 2 volume uptake corresponds to reduction of 73% of Pr 4+ taking into account that 77% of Pr 4+ was titrated in the pristine material. TGA measurement (Ar/3% H 2 ) confirms (Fig. 12 and Table 4) the reduction steps at 250 °C, 350 °C, 450 °C and 500 °C of Pr 1-x Zr x O 2-y oxides. The lower temperature reduction was obtained for Pr 0.8 Zr 0.2 O 2-y and Pr 0.9 Zr 0.1 O 2-y compounds (below 500 °C) while H 2 uptake corresponds to reduction of only 53% of Pr 4+ (in the case of Pr 0.9 Zr 0.1 O 2-y oxide) taking into account that 75% of Pr 4+ was stabilized in the oxidized phase (Table 4). As the Zr content increases, the Pr 4+ reducible rate decreases because of the stabilization of high Pr 3+ content in the pristine phase. Since the surface areas were quite low (<10 m 2 g -1 ), it is assumed that the different reduction processes were not due to the surface but due to bulk phenomena and most probably due to various reduced phases with different cationic and oxygen ordering. Then, various coordination numbers from 6 to 8 for Pr 4+ /Pr 3+ /Zr 4+ cations can be stabilized in these reduced phases related to oxygen departure along <111> and <110> directions of the cubic fluorite-type structure.

Pr-L III XANES spectra in Fig. 13 and 14 of Pr 1-x Zr x O 2-y annealed at 1200 °C (x = 0.2 oxidized and reduced states, 0.1 and 0.02) illustrate the variation of Pr valence states with the two lines (5970 eV and 5981 eV with 4f 2 and 4f 1 electronic con- figurations of Pr 3+ and Pr 4+ respectively) and the increase of shake-down satellite (5981 eV) with the Pr 4+ content. Pr 0.98 Zr 0.02 O 2-y composition annealed at 900 °C has been reduced under Ar/H 2 (5%) at 900 °C for 15 hours. XRD and neutron diffraction experiments were performed (Fig. 15 and Table 5). The XRD and neutron patterns could be refined using the Ia3 ˉspace group; cations occupy two 24d and 8b Wyckoff positions and anions occupy 48e Wyckoff positions. The 8b sites correspond to a flattened octahedral site with six equal Pr2-O bond distances, whereas the 24d sites correspond to a distorted octahedral environment with three kinds of Pr1-O bond distances (Table 3). Depending on the composition, the 16c Wyckoff position could be occupied by oxygen atoms; in our case, the latter is empty leading to oxygen vacancy ordering.

The RE 2 O 3 sesquioxides adopt three allotropic forms:

• A-form RE 2 O 3 , with RE = La-Nd seven-fold coordinated to oxygen leading to the monocapped octahedral site (the most stable thermodynamically).

• B-form RE 2 O 3 , with RE = Sm-Dy six-fold and seven-fold coordinated to oxygen leading to an octahedron and monocapped trigonal prisms.

• C-form, with RE = Nd-Lu six-fold coordinated to oxygen leading to an octahedron and distorted trigonal prism.

Under our synthesis conditions, despite the large Pr content, the C-form corresponding to the reduced phase is always stabilized.

To conclude, investigations of reduced phases, where cationic as well as oxygen/vacancy ordering appears, show as observed for the pyrochlore network [17][18][19] the stabilization of the superstructure of the fluorite-type network. Such a phenomenon should explain the high oxygen mobility as well as the high oxygen vacancy rate and reduced cation content observed in such oxides.

Moreover, smaller Zr 4+ cations can be stabilized by larger Pr (3+n)+ cations in its nearest neighborhood. Both these

Conclusions

In this paper, we have explained the cell parameter variation versus the Zr content in the Pr 1-x Zr x O 2-y oxides for high content of Pr 4+ /Pr 3+ cations. The higher the Zr content, the higher the Pr 3+ rates in this series, thanks to competitive bonds around Zr 4+ cations and O 2-anions. The occurrence of defined compositions as shown by Leroy Eyring et al. in the Pr n O 2n-2m series is associated with the accurate amount of Zr 4+ cations stabilized in eight, seven as well as six-fold coordination sites and the existence of oxygen vacancies. The stabilization of Zr 4+ in flattened octahedral sites as shown by EXAFS investigation at the Zr K-edge is probably a key feature for the highest Zr content (atomic% Zr > 0.1) which explains the generation of oxygen vacancies around Zr 4+ centers and the migration of these vacancies allowing fast kinetics during the reduction steps of Pr 4+ cations. Magnetic measurements and Pr L III -edge XANES analysis give a perfect view of the variation of the Pr 4+ /Pr 3+ rate for the oxidized and reduced phases. The occurrence of a superstructure for the reduced oxides is also an important point leading to high OSC and high content of reducible Pr 4+ cations. TGA and TPR measurements show low first temperature reduction around T = 250 °C for the Zr content equal to 0.1 and 0.2. Finally, other synthesis routes such as sol-gel or self-combustion should be used in order to increase the surface area and obtain high rates of reducible species at lower temperatures for automotive exhaust catalysis.

Fig. 1

 1 Fig. 1 Powder XRD patterns of Pr 1-x Zr x O 2-y (0.02 < x < 0.5) annealed at 1200 °C under air.

Fig. 2

 2 Fig. 2 Enlargement at high 2θ values of powder XRD patterns of Pr 1-x Zr x O 2-y annealed at 1200 °C under air (0.02 < x < 0.5).

Fig. 3

 3 Fig. 3 Powder XRD patterns of Pr 1-x Zr x O 2-y annealed at 1200 °C under air ( pyrochlore hkl planes, a pyrochlore = 2a fluorite ).

Fig. 4

 4 Fig. 4 Plot of lattice (fluorite) parameter "a" versus Zr content (annealed at 1200 °C under air).

Fig. 5

 5 Fig. 5 Temperature dependence of the inverse magnetic susceptibility for Pr 1-x Zr x O 2-y compound annealed at 1200 °C under air.

Fig. 6

 6 Fig. 6 Variation of Pr 4+ concentration versus Zr 4+ content (annealed at 1200 °C under air).

Fig. 7

 7 Fig. 7 Zr-K edge EXAFS spectral comparison of Pr 0,8 Zr 0,2 O 2-y compound annealed at 1200 °C and annealed at 700 °C then reduced under Ar/H 2 (3%) (TGA).

Fourier

  transform of the EXAFS oscillations corresponding to the first Zr-O shell increases indeed in the case of the oxidized sample. Furthermore, the Debye-Waller corresponding to the first Zr-O shell decreases for the oxidized compound leading to consider that the real average coordination number is between 6 and 7. As far as the second shells are concerned, one should have to point out that the number of neighbors of

Fig. 8

 8 Fig. 8 EXAFS Zr K edge refinement of Pr 0,8 Zr 0,2 O 2-y annealed at 1200 °C. Fourier transform (above) and back Fourier transform (below).

Fig. 9

 9 Fig. 9 Zr-K edge EXAFS refinement of Pr 0,8 Zr 0,2 O 2-y annealed at 700 °C under air then reduced under Ar/H 2 (3%) (TGA). Fourier transform (above) and back Fourier transform (below).

Fig. 10

 10 Fig. 10 Powder XRD patterns of Pr 1-x Zr x O 2-y (x = 0.02, 0.1 and 0.2) annealed at 700 °C under air. Fig. 11 H 2 -TPR profiles of Pr 1-x Zr x O 2-y for x = 0.02, 0.1 and 0.2 annealed at 700 °C under air.

Fig. 12

 12 Fig. 12 Thermogravimetric curve (TG) and its first derivative (dTG) obtained for the Pr 1-x Zr x O 2-y compounds (x = 0.02, 0.1 and 0.2) annealed at 700 °C under air and reduced under Ar/3% H 2 atmosphere.

Fig. 13

 13 Fig. 13 Pr-L III edge XANES spectra of Pr 1-x Zr x O 2-y annealed at 1200 °C under air.

Fig. 14 L

 14 Fig. 14 L III edge Pr XANES spectra of Pr 0.8 Zr 0.2 O 2-y annealed at 1200 °C under air (oxidized) and annealed at 700 °C under air and then reduced under Ar/H 2 (3%) (TGA).

Table 1

 1 Compositions, structures, and refined cell parameters (compounds annealed at T = 1200 °C under air)

			a (Å)
	x	Structure	1st phase	2nd phase
	0.02	Fluorite	5.462(3)	-
	0.05	2 Fluorites	5.456(2)	5.465(2)
	0.07	2 Fluorites	5.446(2)	5.458(2)
	0.1	Fluorite	5.443(2)	-
	0.2	Fluorite	5.421(2)	-
	0.25	Fluorite + pyrochlore	5.417(2)	10.805(2)
	0.3	Fluorite + pyrochlore	5.406(2)	10.772(2)
	0.35	Pyrochlore	10.770(2)	-
	0.4	Pyrochlore	10.738(1)	-
	0.5	Pyrochlore	10.708(2)	-

Table 2

 2 Determination of magnetic parameters corresponding to the Curie-Weiss law (1/χ = (T + θp)/C and the Pr oxidation state in Pr 1-x Zr x O 2-y (0.02 < x < 0.5) compounds annealed at 1200 °C under air

	x	Thermal treatment	Slope	θp	Curie constant/Pr moles	µ eff (Pr)	%Pr 4+	%Pr 4+ corrected
	Pr 4+ theoretical	-	-	-	0.806	2.54β	100	-
	Pr 0.98 Zr 0.02 O 2-y	1200 °C	1.04 ± 0.01	-91 ± 3	0.96	2.77β	81 ± 5	64 ± 5
	Pr 0.98 Zr 0.02 O 2-y	700 °C	1.18 ± 0.01	-100 ± 3	0.84	2.59β	95 ± 5	77 ± 5
	Pr 0.9 Zr 0.1 O 2-y	1200 °C	1.00 ± 0.01	-92 ± 3	1.00	2.83β	75 ± 5	59 ± 5
	Pr 0.9 Zr 0.1 O 2-y	1 Redox cycle	1.58 ± 0.01	-150 ± 3	0,63	2,24β	-	100 ± 5
	Pr 0.9 Zr 0.1 O 2-y	900 °C	1.16 ± 0.01	-80 ± 3	0.86	2.62β	93 ± 5	75 ± 5
	Pr 0.8 Zr 0.2 O 2-y	1200 °C	0.894 ± 0.005	-67 ± 2	1.12	2.99β	60 ± 5	46 ± 5
	Pr 0.8 Zr 0.2 O 2-y	700 °C	1.15 ± 0.01	-86 ± 3	0.86	2.62β	92 ± 5	75 ± 5
	Pr 0.65 Zr 0.35 O 2-y	1200 °C	0.764 ± 0.004	-48 ± 1	1.31	3.24β	37 ± 5	25 ± 5
	Pr 0.6 Zr 0.4 O 2-y	1200 °C	0.734 ± 0.004	-44 ± 1	1.36	3.29β	30 ± 5	20 ± 5
	Pr 0.5 Zr 0.5 O 2-y Pr 3+ theoretical	1200 °C -	0.647 ± 0.003 -	-38 ± 1 -	1.54 1.602	3.51β 3.58β	8 ± 5 0	-	0 ± 5

Table 3

 3 EXAFS Zr K-edge refinement results for the two first shells

	Distances	Refined parameters	x = 0.5	x = 0.2	x = 0.1	x = 0.2 reduced	x = 0.02
		E 0	-7.5	-7.9	-8.2	-10.3	-4.2
	Zr-O	n 1	6	6	6	6	7
	Zr-Pr 4+	r 1 (Å) 2σ 1 (Å 2 ) n 2	2.101(4) 0.0056(5) -	2.113(5) 0.0065(6) 4.8	2.114(5) 0.0062(6) 6.6	2.090(7) 0.0076(8) 1.2	2.128(6) 0.0056(7) 8
	Zr-Pr 3+	r 2 (Å) 2σ 2 (Å 2 ) n 3	--6	3.65(1) 0.011(1) 4.8	3.708(8) 0.0116(9) 5.4	3.79(1) 0.003(1) 8.4	3.72(2) 0.014(2) 4
		r 3 (Å) 2σ 3 (Å 2 )	3.814(3) 0.0043(3)	3.88(2) 0.014	3.97(2) 0.023(2)	4.07(6) 0.025(8)	4.01(1) 0.0062(9)
	Zr-Zr	n 4	6	2.4	-	2.4	-
		r 4 (Å) 2σ 4 (Å 2 )	3.807(5) 0.0072(3)	3.881(8) 0.0049(7)	--	3.796(7) 0.0039(7)	--

Table 4

 4 Compositions, refined cell parameters, Pr 4+ rates, H 2 -uptakes (TPR measurements), Δm/m (%, TGA measurements) and final compositions after reduction (compounds annealed at T = 700 °C under air)

	Compositions	Pr 0.98 Zr 0.02 O 2-y		Pr 0.9 Zr 0.1 O 2-y		Pr 0.8 Zr 0.2 O 2-y
	Deduced oxidized formulae	(Pr 0.77	4+ Pr 0.23	3+ ) 0.98 Zr 0.02	4+ O 1.89	(Pr 0.75	4+ Pr 0.25	3+ ) 0.9 Zr 0.1	4+ O 1.89	(Pr 0.75	4+ Pr 0.25	3+ ) 0.8 Zr 0.2	4+ O 1.9
	TPR
	v(H 2 )	2070				1430				1125
	ΔO	0.36				0.24				0.18
	Deduced reduced formulae	(Pr 0.04	4+ Pr 0.96	3+ ) 0.98 Zr 0.02	4+ O 1.53	(Pr 0.22	4+ Pr 0.78	3+ ) 0.9 Zr 0.1	4+ O 1.65	(Pr 0.3	4+ Pr 0.7	3+ ) 0.8 Zr 0.2	4+ O 1.72
	ATG
	Δm/m (%)	-3.06				-2.8				-2.71
	ΔO	0.32				0.29				0.27
	Deduced reduced formulae	(Pr 0.12	4+ Pr 0.88	3+ ) 0.98 Zr 0.02	4+ O 1.57	(Pr 0.12	4+ Pr 0.88	3+ ) 0.9 Zr 0.1	4+ O 1.6	(Pr 0.18	4+ Pr 0.82	3+ ) 0.9 Zr 0.1	4+ O 1.63

environments are distorted. 8-fold coordinated Pr 4+ in the cubic environment which is highly reducible appears then probably far from Zr 4+ cations. So the oxygen release around Pr 4+ and migration of oxygen vacancies near Zr 4+ are assisted by the gradual variation of the Pr valence state in this series.


Hella Boumaiza

Romain Coustel

Ghouti Medjahdi

Christian Ruby
email: christian.ruby@univ-lorraine.fr

Latifa Bergaoui

Conditions for the formation of pure birnessite during the oxidation of Mn(II) cations in aqueous alkaline medium

Keywords: Birnessite, Hausmannite, Redox synthesis, Dissolved oxygen, XPS

Birnessite was synthetized through redox reaction by mixing MnO4 -, Mn 2+ and OH -solutions.

The Mn(VII):Mn(II) ratio of 0.33 was chosen and three methods were used consisting in a quick mixing under vigorous stirring of two of the three reagents and then on the dropwise addition of the third one. The obtained solids were characterized by XRD, FTIR and XPS spectroscopies.

Their average oxidation states were determined from ICP and CEC measurements while their surface properties were investigated by XPS. This study provides an increased understanding of the importance of dissolved oxygen in the formation of birnessite and hausmannite and shows the ways to obtain pure birnessite. The role of counter-ion ie. Na + or K + was also examined.

Introduction

Birnessite is a mixed Mn(III)-Mn(IV) oxide constituted by layers of edge-sharing MnO6 octahedra separated by planes of hydrated cations (e.g. Na + , K + , Ca 2+ ) and water molecules. It was found to be one of the most common and active occurring Mn oxide in soils and sediments [START_REF] Taylor | The association of manganese and cobalt in soils-further observations[END_REF][START_REF] Bargar | Structural characterization of terrestrial microbial Mn oxides from Pinal Creek , AZ[END_REF]. Jones and Milnes [START_REF] Jones | Birnessite, a new manganese oxide mineral from Aberdeenshire, Scotland[END_REF] proposed the formula Na0.7Ca0.3Mn7O14, 2.8H2O. This natural form presents a disordered structure [START_REF] Bargar | Structural characterization of terrestrial microbial Mn oxides from Pinal Creek , AZ[END_REF][START_REF] Silvester | Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: II. Results from chemical studies and EXAFS spectroscopy[END_REF][START_REF] Drits | Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: I. Results from X-ray diffraction and selected-area electron diffraction[END_REF]. Thereby, many studies were focused on ways to obtain birnessite. Besides the existing biogenic ways, the chemical methods could be divided in four groups based on the mechanism involved.

The first method is based on the Mn(II) oxidation in alkali media producing Mn(OH)2 and its oxidation by O2 bubbling to form buserite, the hydrated form of birnessite [START_REF] Giovanoli | Über Oxidhydroxide des vierwertigen Mangans mit Schichtengitter. 1. Mitteilung: Natriummangan (II,III) manganat (IV)[END_REF][START_REF] Post | Crystal structure determinations of synthetic sodium, magnesium, and potassium birnessite using TEM and the Rietveld method[END_REF]. To avoid hausmannite formation as by-product many authors studied the mechanism involved.

Intriguingly, Yang and Wang [START_REF] Yang | Syntheses and characterization of well-crystallized birnessite[END_REF] prepared pure birnessite by O2 oxidation of Mn(OH)2 precipitate that was prepared from deoxygenated NaOH and MnCl2 solutions. They suggested that the use of deoxygenated water prevented the presence of Mn(III) responsible of the hausmannite formation. Feng et al. [START_REF] Feng | Synthesis of birnessite from the oxidation of Mn 2+ by O2 in alkali medium: Effects of synthesis conditions[END_REF] showed that the formation of hausmannite was avoided when NaOH and MnCl2 were mixed below 10°C; this study also showed that an oxygen flow rate of 5 L min -1 during 5 hours is required to produce pure birnessite. Cai et al. [START_REF] Cai | Preparative parameters and framework dopant effects in the synthesis of layer-structure birnessite by air oxidation[END_REF] proposed a variant of Giovanoli's method by replacing O2 by air and they pointed that, in that case, a flow rate of 24 L min -1 is essential to avoid hausmannite formation. Pure birnessite was also obtained by using H2O2 instead of O2 as oxidant [START_REF] Feng | Synthesis of birnessite-type potassium manganese oxide[END_REF][START_REF] Luo | Mechanistic and kinetic studies of crystallization of birnessite[END_REF] The second way to obtain birnessite relies on the reduction of MnO4 -in concentrated HCl medium [START_REF] Mckenzie | The synthesis of birnessite, cryptomelane, and some other oxides and hydroxides of manganese[END_REF][START_REF] Golden | Ion exchange, thermal transformations, and oxidizing properties of birnessite[END_REF]. Numerous organic reducing agents were used such as fumaric acid [START_REF] Bach | Birnessite manganese dioxide synthesized via a sol-gel process: a new rechargeable cathodic material for lithium batteries[END_REF], sugars [START_REF] Ching | Sol-Gel synthesis of birnessite from KMnO4 and simple sugars[END_REF][START_REF] Ramalingam | Synthesis, spectral, thermal and CO2 absorption studies on birnessites type layered MnO6 oxide[END_REF], alcohols [START_REF] Luo | Mechanistic and kinetic studies of crystallization of birnessite[END_REF][START_REF] Ma | Syntheses of birnessites using alcohols as reducing reagents: Effects of synthesis parameters on the formation of birnessites[END_REF], ethylene glycol [START_REF] Yang | Synthesis of well-crystallized birnessite using ethylene glycol as a reducing reagent[END_REF] and more recently epoxypropane [START_REF] Zhang | Synthesis and capacitive property of δ-MnO2 with large surface area[END_REF] and lactate [START_REF] Händel | A simple method to synthesize birnessite at ambient pressure and temperature[END_REF]. This method could be constraining as it involved a relatively long time aging step [START_REF] Luo | Mechanistic and kinetic studies of crystallization of birnessite[END_REF][START_REF] Ma | Syntheses of birnessites using alcohols as reducing reagents: Effects of synthesis parameters on the formation of birnessites[END_REF], a calcination step at temperature above 400-450°C [START_REF] Ching | Sol-Gel synthesis of birnessite from KMnO4 and simple sugars[END_REF][START_REF] Ramalingam | Synthesis, spectral, thermal and CO2 absorption studies on birnessites type layered MnO6 oxide[END_REF] or a hydrothermal treatment [START_REF] Zhang | Synthesis and capacitive property of δ-MnO2 with large surface area[END_REF].

The third method is based on a direct conversion of hausmannite to birnessite involving a dissolution/recrystallization mechanism [START_REF] Cornell | Transformation of hausmanite into birnessite in alkaline media[END_REF] in alkaline medium. Birnessite was obtained for concentrations of OH -above 2 mol L -1 . The reaction required several weeks to be complete and the dissolution of hausmannite was found to be the rate-limiting step in this reaction.

The fourth process is based on a redox reaction between MnO4 -and Mn 2+ in alkaline conditions. A manganese salt (i.e. MnCl2, MnSO4) and sodium or potassium permanganate were generally used as Mn(II) and Mn(VII) suppliers respectively while NaOH or KOH were used to provide alkaline medium. This method was first reported by [START_REF] Murray | The surface chemistry of hydrous manganese dioxide[END_REF]; it consisted on adding slowly the MnCl2 solution to NaOH/NaMnO4 solution. The product presented a poor crystallinity similar to natural birnessite. Luo et al., [START_REF] Luo | Preparative parameters, magnesium effects, and anion effects in the crystallization of birnessites[END_REF] and Luo and Suib, [START_REF] Luo | Crystallization of sodiumbirnessite and accompanied phase transformation[END_REF] studied the influence of many parameters (temperature, basicity, MnO4 -/Mn 2+ ratio, presence of magnesium and anion effect) and showed that pure birnessite could be obtained in strong alkaline conditions with a MnO4 -/Mn 2+ ratio of 0.28-0.36 and at 40-65°C. The authors also pointed that feitknechtite (β-MnOOH) was an intermediate in birnessite formation. Villalobos et al. [START_REF] Villalobos | Characterization of the manganese oxide produced by Pseudomonas putida strain MnB1[END_REF] followed this work and showed the presence of small amount of manganite (γ-MnOOH) in addition to birnessite.

The redox method gained more interest on the past decades as it overcomes the inconvenient of the other methods such as high consumption of O2, use of boiling solution with concentrated acid and strong oxidant and long reaction times. In spite of an extensive literature [START_REF] Shen | Synthesis, characterization, and catalytic applications of manganese oxide Octahedral Molecular Sieve (OMS) nanowires with a 2 x 3 tunnel structure[END_REF][START_REF] Liu | Synthesis and delamination of layered manganese oxide nanobelts[END_REF][START_REF] Cui | Birnessites with different average manganese oxidation states synthesized, characterized, and transformed to todorokite at atmospheric pressure[END_REF][START_REF] Eren | Synthesis, structural characterization and Pb(II) adsorption behavior of K-and H-birnessite samples[END_REF] the role of dissolved oxygen on the redox method remains unclear. Actually a reactive medium with an average oxidation state (AOS) of manganese close to 3.25 (Mn(VII):Mn(II) molar ratio of 0.33) led to birnessite with an AOS of Mn equal to 3.53 [START_REF] Cui | Birnessites with different average manganese oxidation states synthesized, characterized, and transformed to todorokite at atmospheric pressure[END_REF]. This suggests that the oxidation of Mn(II) leading to birnessite is not related to a unique redox reaction between Mn(II) and Mn(VII) and that dissolved oxygen may intervene in it as already observed in other methods such as Giovanoli's studied by Yang and Wang [START_REF] Yang | Syntheses and characterization of well-crystallized birnessite[END_REF]. Besides, the mixing order of the three solutions generally involved (i.e. manganese salt, permanganate and base) varied from one study to another [START_REF] Murray | The surface chemistry of hydrous manganese dioxide[END_REF][START_REF] Cui | Birnessites with different average manganese oxidation states synthesized, characterized, and transformed to todorokite at atmospheric pressure[END_REF][START_REF] Eren | Synthesis, structural characterization and Pb(II) adsorption behavior of K-and H-birnessite samples[END_REF] and to our knowledge, no study has focused on the influence that could have this order of mixing on the nature of the obtained products. Moreover, the occurrence and possible role of hausmannite (Mn3O4), which can be observed as a by-product, is not clear.

The goals of this study are: i) to optimize the experimental conditions to obtain pure Nabirnessite or K-Birnessite through the redox method and ii) to get an insight into birnessite formation mechanism. A special attention is paid to the role of dissolved oxygen and to the mixing order of the reagents to prevent the side reaction leading to hausmannite. The X-ray photoelectron spectrometry is used in order to gain a more in-depth understanding of the target reaction mechanism.

Materials and methods

Chemicals

All chemicals were purchased from Sigma-Aldrich. Manganese(II) chloride tetrahydrate (MnCl2,4H2O, ACS reagent, ≥ 98%) was used as Mn 2+ supplier, sodium permanganate monohydrate (NaMnO4,H2O, ACS reagent ≥ 97%) or potassium permanganate (KMnO4 ACS reagent, ≥ 99.0%) as MnO4 -supplier and sodium hydroxide (NaOH BioXtra, ≥ 98% pellets anhydrous) or potassium hydroxide (KOH, ACS reagent, ≥ 85%, pellets) to provide alkaline medium. Double distilled water (DDW, 18.2 MΩ cm) was used for all the experiments.

Synthesis of birnessite

Three solutions were used for the synthesis of Na-birnessite: 125 mL of NaOH (8.8 mol L -1 ), 250 mL NaMnO4 (0.1 mol L -1 ) and 125 mL MnCl2 (0.6 mol L -1 ). Some syntheses were performed by using KMnO4 instead of NaMnO4 and KOH instead of NaOH. The Mn(VII):Mn(II) ratio of 0.33 was chosen accordingly to a previous study [START_REF] Cui | Birnessites with different average manganese oxidation states synthesized, characterized, and transformed to todorokite at atmospheric pressure[END_REF]. The typical procedure used in the present work consisted on a quick mixing under vigorous stirring of two of the three reagents and then on the addition of the third in one go (0 h) or during 2, 4 or 8 hours. The reaction mixture was then stirred for another 30 min and aged at 60ºC for 14 h. The product was finally centrifuged and washed until the pH of the solution was between 9 and 10 and dried at 60°C during 16 h. The designation of the three methods refers to the nature of the third reagent added. Thus, the first method, named the "reductant-last method", consists on a dropwise addition of the reductant (MnCl2) solution to the NaMnO4/NaOH mixture. The second one, the "alkali-last method" consists on a dropwise addition of the basic solution (NaOH) to the MnCl2/NaMnO4 mixture. The third one, called the "oxidant-last method", consists on a dropwise addition of the oxidant (NaMnO4) solution to the NaOH/MnCl2 mixture. Samples were denoted 𝐶 𝑡 𝑚 ∶ C = Na or K when NaMnO4 and NaOH or KMnO4 and KOH were used respectively, m = R, A or O when the reductant-last, the alkali-last or the oxidant-last method was used respectively and t corresponds to the addition time, in hour, of the third reactant. Three extra samples were synthesized using pre-deoxygenated solutions and N2 bubbling to study the influence of dissolved oxygen. These samples were synthesized following the oxidant-last method and the bubbling was maintained during only the mixture of NaOH and MnCl2 for the first sample (𝑁 2 𝑂 -a) and during all the reaction time of 2 hours for the second one (𝑁 2 𝑂 -b). A sample of hausmannite was prepared by the mixture of 250 mL NaOH (5.5 mol L -1 ) and 200 mL of MnCl2 (0.5 mol L -1 ) under air bubbling at a flow rate of 2 L min -1 during 5 hours.

Characterization techniques

The X-ray powder diffraction data were collected from an X'Pert MPD diffractometer (Panalytical AXS) with a goniometer radius 240 mm, fixed divergence slit module (1/2° divergence slit, 0.04 rd Sollers slits) and an X'Celerator as a detector. The powder samples were placed on zero background quartz sample holders and the XRD patterns were recorded at room temperature using Cu Kα radiation (λ = 0.15418 nm). FTIR analysis was performed using a Bruker Vertex 70v equipped with a DLaTGS detector. Spectra were recorded in transmission mode using KBr pellets containing 2-5 mg of sample. For each sample, 100 scans were collected in the 5000-220 cm -1 wavenumber with 4 cm -1 resolution. The background spectrum of KBr was also recorded at the same conditions. X-ray photoelectron spectra were recorded on a KRATOS Axis Ultra X-ray photoelectron spectrometer with Al Kα source monochromated at 1486.6 eV (spot size 0.7 mm × 0.3 mm). Photoelectrons were detected by a hemispherical analyzer at an electron emission angle of 90° and pass energy of 160 eV (survey spectra) and 20 eV (core level spectra). For the core-level spectra, the overall energy resolution, resulting from monochromator and electron analyzer bandwidths, was 800 meV. As an internal reference for the absolute binding energies, the C1s peak of hydrocarbon contamination set at 284.6 eV was used. Total contents of Mn was determined using Inductive Coupled Optical Emission Spectroscopy ICP-AES (Jobin Yvon-Horiba, Ultima) after complete digestion of about 5 mg of birnessite in NH3OHCl (0.7 mol L -1 , pH 1.9). The Cation Exchange Capacity (CEC) was determined following the cobalt-hexamine method [START_REF] Afnor | Normalisation française NF X31-130: Qualité des sols, méthodes chimiques, détermination de la capacité d'échange cationique[END_REF] in which 150 mg of birnessite were put in 20 mL of 16.67 M Co(NH3)6Cl3 solution. The mixture was shaken for an hour without temperature regulation and then micro-filtered. The cobalt-hexamine concentrations before and after contact with the birnessite were determined by measuring the solution absorption at 472 nm using an UV-visible spectrophotometer, i.e. a Cary 60 (Agilent Technologies).

Results and discussion

Mechanism of birnessite formation

Table 1 gives the nature of the products identified by XRD for all prepared samples.

Comparison of the three used methods

The three different orders of mixing were studied with an identical dropwise addition time (2 hours) of the third reactant. The XRD patterns of the samples 𝑁𝑎 2 𝑅 , 𝑁𝑎 2 𝐴 and 𝑁𝑎 2 𝑂 are shown in Fig. 1. The main product identified for the three methods is Na-birnessite (PDF 00-043-1456)

with two main reflections at 7.14 and 3.57 Å corresponding to d001 and d002 respectively. In addition to birnessite, hausmannite, Mn3O4, (PDF 04-007-9635) was also identified as byproduct appearing clearly in the oxidant-last method (Fig. 1c) and slightly in the two other methods (Fig. 1a and1b). Feitknechtite, β-MnOOH, (PDF 00-018-0804) with a small diffraction peak around 4.6 Å appeared as a second by-product for the oxidant method. Thus, when following the reductant-last and the alkali-last methods, the main product observed was Na-birnessite with only trace of hausmannite. In Fig. 2 The syntheses leading to the formation of Na-birnessite identified by XRD were further characterized by FTIR. The birnessites showed similar IR fingerprint (Fig. 3a and3b) for the different methods employed. Bands due to Mn-O lattice vibrations are located at 245, 360, 418, 478, 512 and 634 cm -1 with shoulders at ~550 and ~750 cm -1 , in good agreement with the reported IR adsorption of single phased birnessite [START_REF] Cui | Birnessites with different average manganese oxidation states synthesized, characterized, and transformed to todorokite at atmospheric pressure[END_REF][START_REF] Potter | The tetravalent manganese oxides : identification , hydration , and structural relationships by infrared spectroscopy[END_REF][START_REF] Feng | Pathways of birnessite formation in alkali medium[END_REF]. The hausmannite IR spectrum (Fig. 3d) presents a clearly distinct line shape with five bands located at 245, 345, 410, 515 and 621 cm -1 , as already reported [START_REF] Ishii | Infrared absorption spectra and cation distributions in (Mn, Fe)3O4[END_REF][START_REF] Potter | Mineralogy of manganese dendrites and coatings[END_REF][START_REF] Nohman | Characterization of the thermal genesis course of manganese oxides from inorganic precursors[END_REF][START_REF] Ocaña | Uniform particles of manganese compounds obtained by forced hydrolysis of manganese(II) acetate[END_REF][START_REF] Zhang | Controlled synthesis of Mn3O4 nanocrystallites and MnOOH nanorods by a solvothermal method[END_REF][START_REF] Ristić | Synthesis and long-term phase stability of Mn3O4 nanoparticles[END_REF]. In particular, the absence of absorption band close to 345 cm -1 rule out any contribution of hausmannite phase to 𝑁𝑎 2 𝑅 and 𝑁𝑎 2 𝐴 spectra. In contrast, the small band observed at 594 cm -1 for the samples prepared following oxidant-last or alkalilast method could be attributed to traces of MnOOH [START_REF] Potter | The tetravalent manganese oxides : identification , hydration , and structural relationships by infrared spectroscopy[END_REF][START_REF] Zhang | Controlled synthesis of Mn3O4 nanocrystallites and MnOOH nanorods by a solvothermal method[END_REF][START_REF] Kohler | Hydrogen Bonding and Jahn -Teller Distortion in Groutite , α-MnOOH , and Manganite , γ-MnOOH , and Their Relations to the Manganese Dioxides Ramsdellite and Pyrolusite[END_REF][START_REF] Sharma | The role of tetraethylammonium hydroxide on the phase determination and electrical properties of γ-MnOOH synthesized by hydrothermal[END_REF], the role of this manganese oxyhydroxide in birnessite formation will be discussed below.

Influence of the NaMnO4 or NaOH addition rate

Further experiments were conducted following the oxidant-last method with varying the total addition time of the oxidant from 0 to 8 h. The XRD analyses of the different preparations are shown in Fig. 4. It indicates a coexistence of the two phases, i.e birnessite and hausmannite for any addition times. Furthermore, a global increase of the relative amount of hausmannite is observed for increasing NaMnO4 addition time. Indeed, the inversion of the balance between the intensity of the (001) birnessite peak and the (211) hausmannite is clearly observed between 2 and 8 hours of addition time (Fig. 4b and4d). When following the alkali-last method, the increase of the OH -addition time has no influence on the nature of obtained solid. The X-ray diffractograms observed for an addition time of 4 and 8 hours (results not shown) are similar to the one measured for an addition time of 2 hours (Fig. 1b).

Influence of dissolved oxygen on the product formation

This part is aimed to determine the influence of dissolved oxygen on the formation of Nabirnessite and hausmannite in the oxidant-last and alkali-last methods. As previously shown, the formation of pure birnessite was not possible for the oxidant-last method as the product consisted of a mixture of hausmannite and birnessite regardless of the addition rate used (Fig. 4). The reaction was carried out with N2 bubbling to remove oxygen from the solutions.

Thereby, the diffractogram of the 𝑁𝑎 2 𝑂 -a sample, prepared by N2 bubbling in the mixture of NaOH and MnCl2 solutions before adding the NaMnO4 solution, gives only birnessite (Fig. 5a).

The diffractogram of the 𝑁𝑎 2 𝑂 -b sample, prepared by N2 bubbling during all the reaction time gives both birnessite and manganite (γ-MnOOH) (Fig. 5b). FTIR measurements fully confirm this analysis (results not shown). Besides, a color difference was apparent between these experiments. Indeed, the initial mixture of NaOH and MnCl2 gave a white precipitate when the solutions are bubbled with N2 while it turned to beige when the water contained dissolved oxygen.

Discussion: conditions favoring the formation of birnessite vs hausmannite

It is first necessary to point out that Mn(II) and Mn(III) hydroxides probably precipitate initially since their solubilities are very low (Ksp[Mn(OH)2] = 2 x 10 -13 and Ksp[Mn(OH)3] = 2 x 10 -36 at 25°C) [START_REF] Kozawa | Solubilities of Mn(II) and Mn(III) ions in concentrated alkaline solutions[END_REF]. Indeed, for the oxidant-last method, the mixture of Mn 2+ and OH -gives instantly a precipitate of Mn(OH)2 containing both Mn 2+ and Mn 3+ if nitrogen is not purged [START_REF] Yang | Syntheses and characterization of well-crystallized birnessite[END_REF]. For the reductant-last method, the first Mn 2+ drop (Vdrop ≈ 0.09 mL) that falls in the mixture containing MnO4 -and OH -(the MnO4 -decomposition is very slow) gives a solubility product [Mn 2+ ].[OH - ] 2 = 1.2 x 10 -3 much higher than the Mn(OH)2 solubility constant. For the alkali-last method, the mixture of Mn 2+ and MnO4 -is metastable before the addition of OH -solution. Indeed, on one hand, Mn 2+ is stable in neutral or acidic medium and on the other hand, MnO4 -can react with H2O and Mn 2+ , but kinetics of these reactions are slow. As in the case of the reductant-last method, when the first drop of the OH -solution is added in the Mn 2+ /MnO4 -mixture in the alkali-last method, a product [Mn 2+ ].[OH -] 2 = 9 x 10 -7 much higher than Mn(OH)2 solubility constant is obtained. In summary, for all three methods, it can be supposed that Mn(OH)2 is the starting solid product for all other solids formation. Based on the work of Luo et al., [START_REF] Luo | Mechanistic and kinetic studies of crystallization of birnessite[END_REF], it is assumed that in oxygenated water, Mn(OH)2 oxidizes to hydrohausmannite which is a mixture of feitknechtite (β-MnOOH), and hausmannite (Mn3O4) and then the formation of birnessite occurs by the oxidation of hydrohausmannite. The pathways leading to birnessite and hausmannite are summarized in Fig. 6.

i. Influence of dissolved oxygen

Regardless of the method employed, oxidation conditions appear to be a key factor in directing the reaction towards the obtaining of a product or another. Thus, when producing birnessite from the oxidation of Mn(OH)2 with oxygen bubbling in alkali medium, Feng et al., [START_REF] Feng | Synthesis of birnessite from the oxidation of Mn 2+ by O2 in alkali medium: Effects of synthesis conditions[END_REF] showed that, for a flow rate lower than 5 L min -1 , hausmannite was obtained as by-product. This is in good line with our experiment in which hausmannite was obtained as a single phase when bubbling Mn(OH)2 with synthetic air under a flow rate of 2 L min -1 .

In the oxidant-last method, MnO4 -ions are used as oxidizer instead of oxygen bubbling.

In addition to birnessite, hausmannite is observed as by-product and its amount raised with the MnO4 -addition time (samples 𝑁𝑎 0 𝑂 , 𝑁𝑎 2 𝑂 , 𝑁𝑎 4 𝑂 and 𝑁𝑎 8 𝑂 ). This could be explained considering two competitive reaction pathways: the slow transformation of Mn(OH)2 to hausmannite by dissolved oxygen before the addition of MnO4 -via pathway A (cf. Fig. 6) as previously observed [START_REF] Yang | Syntheses and characterization of birnessite by oxidizing pyrochroite in alkaline conditions[END_REF] or the reaction leading to birnessite via pathway B (cf. Fig. 6). Lowering the MnO4 -addition rate indeed left time for the slow formation of hausmannite. Based on these observations, it can be assumed that Mn3O4 formation rate (pathway A, cf. Fig . 6) is lower than the birnessite one (pathway B, cf. Fig. 6).

These assumptions are supported by the sample 𝑁𝑎 2 𝑂 -a in which the formation of hausmannite was inhibited by the use of deoxygenated Mn(OH)2 suspension before adding the MnO4 -solution. Pathway A is thus suppressed and the only obtained solid is birnessite.

Thereby, the absence of dissolved oxygen avoided the presence of Mn(III) impurity in Mn(OH)2 responsible of hausmannite's formation as previously observed [START_REF] Yang | Syntheses and characterization of well-crystallized birnessite[END_REF].

But, when using deoxygenated Mn(OH)2 and MnO4 -solutions (N2 bubbling), both manganite (γ-MnOOH) and birnessite are observed (sample 𝑁𝑎 2 𝑂 -b). The oxidation of a fraction of Mn(OH)2 thus stops at the stage of manganite formed under less oxidizing conditions. This suggests that dissolved oxygen may participate in the oxidation reaction leading to birnessite and that the initial Mn(VII)/Mn(II) ratio here used is not sufficient to oxidize all Mn(OH)2 to birnessite when assuming a complete redox reaction between Mn(VII) and Mn(II) species.

Actually, for syntheses giving pure birnessite, the average oxidation state of Mn (AOS = 4 -Mn(III)/Mntot) determined via CEC and ICP measurements (Table 2) is situated close to 3.65, a value that is significantly higher than the expected one (the expected value, calculated on the basis of the used Mn(VII)/Mn(II) ratio, is equal to 3.25). Such a difference confirms that dissolved oxygen therefore acts as a co-oxidant in the formation of birnessite from a redox reaction with an initial Mn(VII)/Mn(II) ratio of 0.33.

In the reductant-last method, when the drop of Mn 2+ is put in an alkaline medium containing O2,aq and MnO4 -, it instantly precipitates into Mn(OH)2 and the birnessite is formed under the common effect of O2,aq and MnO4 -(sample 𝑁𝑎 2 𝑅 ) since pathway B is kinetically favored at the expense of pathway A.

In the alkali-last method, the addition of OH -into MnO4 -/Mn(II) solution leads instantly to the precipitation of Mn(OH)2. The conditions leading to birnessite formation are then favored (sample 𝑁𝑎 2 𝐴 ) in good agreement with the assumption that the rate of pathway B is higher than that of pathway A. Even if the flow rate of the alkaline solution decreases (samples 𝑁𝑎 2 𝐴 , 𝑁𝑎 4 𝐴 and 𝑁𝑎 8 𝐴 ), hausmannite does not form because of the permanent presence of MnO4 -in the reaction medium which kinetically favors birnessite formation.

It was thus possible to obtain pure birnessites following two methods: alkali-last and reductantlast. The obtaining of birnessite following the oxidant-last method was not successful as previously demonstrated [START_REF] Cui | Birnessites with different average manganese oxidation states synthesized, characterized, and transformed to todorokite at atmospheric pressure[END_REF]. Indeed, to obtain pure birnessite via the oxidant-last method, special precautions regarding the presence of dissolved oxygen when mixing NaOH and MnCl2 must be taken, otherwise, hausmannite appears as by-product.

ii. Thermodynamics of birnessite and mechanism of formation

Thermodynamic properties of birnessite were assumed to be equal to -MnO2 [44]. In fact, -MnO2, like its natural analog vernadite, is a turbostratic variety of birnessite. For sake of simplicity, the solid phases are assumed immiscible and activity of dissolved or ionic species were considered as equal to their respective concentrations. Table 3 presents the standard Gibbs free energies of formation (at 298 K) of various compounds involved in the reactions that may occur in the medium.

The reactions possibly occurring in the medium are (Fig. 6):

-oxidation of Mn(OH)2 into Mn3O4: The spontaneity of R1 and R2 depends only on the dissolved O2 concentration due to the constant activities of the other intervening species (solids or solvent). These reactions are thermodynamically favored even for very low dissolved O2 concentration ([O2,aq] higher than 3

x 10 -56 mol L -1 for R1 and higher than 3 x 10 -53 mol L -1 for R2). The spontaneity of R'2 and R3 depends on the [OH -]/[MnO4 -] ratio and the reactions are thermodynamically favored as long as the [OH -]/[MnO4 -] ratio is lower than 1.2 x 10 56 and 3.8 x 10 18 for R'2 and R3 respectively.

Then, the above mentioned reactions are thermodynamically favorable under the experimental conditions used. Besides, the transformation of hausmannite into birnessite possibly happening in the medium has to be considered as follow: According to R4, the transformation of hausmannite into birnessite only depends on the concentration of O2,aq: R4 is thermodynamically allowed for O2,aq concentration higher than 10 - 23 mol L -1 , while [O2,aq] is in the order of 10 -4 mol L -1 under ambient conditions. Furthermore, it was shown by Luo et al., [START_REF] Luo | Mechanistic and kinetic studies of crystallization of birnessite[END_REF] that the hausmannite does not form in the presence of a strongly oxidizing medium. Thermodynamic calculations showed that the oxidation of Mn3O4 by MnO4 - to give birnessite (R4') is favorable since the [OH -]/[MnO4 -] ratio is lower than 3 x 10 16 . But our experimental results suggested that, once produced, hausmannite did not turn into birnessite. In the hypothesis of a solid state transformation of Mn3O4, the kinetic constraint which arises from the oxygen diffusion into the compact spinel structure of hausmannite could stop these reactions (Fig. 6). This assumption is confirmed by the work of Cornell and Giovanoli [START_REF] Cornell | Transformation of hausmanite into birnessite in alkaline media[END_REF], who pointed that the transformation of hausmannite into birnessite, in alkaline medium, proceeds very slowly, involves a dissolution/re-precipitation mechanism and the dissolution of hausmannite is the rate limiting step.

iii. NaOH and NaMnO4 vs KOH and KMnO4

Solids obtained with KMnO4/KOH solutions were different from the ones obtained with NaOH/NaMnO4. Indeed, for the reductant-last method, with KOH and KMnO4, in addition to birnessite, hausmannite was observed (sample 𝐾 2 𝑅 ). Jin et al., [START_REF] Jin | Comparison of the oxygen reduction reaction between NaOH and KOH solutions on a Pt electrode: The electrolyte-dependent effect[END_REF] have shown that the nature of the alkaline solution influences the O2 solubility and its diffusion which increase when NaOH is changed by KOH. In this case, the easy diffusion of dissolved oxygen can enhance the rate of pathway A in aqueous solution in the expense of pathway B (Fig. 6).

Chemical surface properties of birnessite

Surface properties of the obtained solids were investigated by XPS. Overview XPS spectra (see supplementary materials) show core-level photoelectron peaks at ~285 eV (C1s), ~530 eV (O1s), ~642 eV (Mn2p3/2), ~654 eV (Mn2p1/2), ~770 eV (Mn2s). Additional peaks at ~1071 eV (Na1s) or ~293 eV (K2p) and ~376 eV (K2s) are observed for Na-or K-birnessite, respectively. The C1s peak should be attributed to a hydrocarbon contamination in ambient atmosphere before introduction into the XPS chamber.

Na-birnessite and K-birnessite prepared by reductant-last, oxidant-last and alkali-last method are quite similar in their Mn 2p3/2 spectra (Fig. 7) with a maximum at 642 ± 0.2 eV and two broad shoulders, at 643 eV and 641 eV. For samples prepared with oxidant-last method, additional weak contribution is observed at lower binding energy. According to literature [START_REF] Yin | Characterization of Ni-rich hexagonal birnessite and its geochemical effects on aqueous Pb 2+ /Zn 2+ and As(III)[END_REF][START_REF] Wang | Sorption behavior of heavy metals on birnessite: Relationship with its Mn average oxidation state and implications for types of sorption sites[END_REF][START_REF] Wang | Preparation, characterization and adsorption performance of cetyltrimethylammonium modified birnessite[END_REF][START_REF] Nesbitt | Interpretation of XPS Mn ( 2p ) spectra and constraints on the of Mn oxyhydroxides mechanism of MnO2 , precipitation[END_REF], Mn 2p3/2 spectrum of birnessite is composed of multiplet peaks associated to Mn(III) and Mn(IV). Experimental spectra, shown in Fig. 7, were fitted considering the multiplet peak characteristics (positions and relative intensities) determined previously [START_REF] Nesbitt | Interpretation of XPS Mn ( 2p ) spectra and constraints on the of Mn oxyhydroxides mechanism of MnO2 , precipitation[END_REF]. For the samples prepared with oxidant-last method (Na 2 O and K 2 O ) the weak contribution at low binding energy can be attributed to a third multiplet associated to Mn(II). Similar component is observed for K-birnessite prepared by reductant-last method (K 2 R ). This is consistent with the presence of hausmannite whose reference Mn 2p3/2 spectrum is given in Fig. 7g. Indeed, XPS probes Mn(II) in Na 2 O , K 2 O and K 2 R samples while XRD measurements evidence hausmannite. This suggests that Mn 2p3/2 profile provides a fair assessment of birnessite purity.

It should be noted that when N2 bubbling is used before adding the NaMnO4 solution (𝑁𝑎 2 𝑂 -a), Mn(II) contribution is absent in Mn 2p3/2 spectrum. Moreover, when N2 bubbling is used during all the reaction time (𝑁𝑎 2 𝑂 -b) stronger Mn(III) contribution attributed to γ-MnOOH is observed (results not shown). This is perfectly in line with XRD and FTIR results that showed, for the oxidant-last method, that the initial deoxygenation prevents hausmannite formation and leads to pure birnessite phase while N2 bubbling during all reaction time favored the formation of manganite (γ-MnOOH). Considering the multiplet fitting, the Mn(III)/Mntot surface ratio was determined to be above 0.5 (Table 2). The corresponding bulk ratio was established from the combined results of elemental analyses leading to the Mn concentration and CEC measurements giving the necessary amount of Na + to counter-balance the negative charge of the layers. The Mn(III)/Mntot bulk ratio was found closed to 0.3 (Table 2). This is in line with previous reports [START_REF] Yin | Characterization of Ni-rich hexagonal birnessite and its geochemical effects on aqueous Pb 2+ /Zn 2+ and As(III)[END_REF][START_REF] Nesbitt | Interpretation of XPS Mn ( 2p ) spectra and constraints on the of Mn oxyhydroxides mechanism of MnO2 , precipitation[END_REF][START_REF] Lanson | Crystal Structure Determination of Synthetic Na-Rich Birnessite: Evidence for a Triclinic One-Layer Cell[END_REF] pointing to an Mn(III) excess at birnessite surface with respect to bulk content. This suggests that birnessite formation occurs through a dissolution/precipitation process rather than a solid state reaction. Actually, in the latter case, oxidation would progress inward from the particle surfaces leading to higher oxidation state of the surface with respect to the bulk.

Conclusion

Na-birnessites with different chemical compositions were synthesized following three different methods based on the use of the same reagents, i.e. OH -, MnO4 -and Mn 2+ but differing from each other in the order of mixing the reagents. Among the three methods, only two give rise to the formation of birnessite as single phase. The third (oxidant-last method) leads to the formation of hausmannite, in addition to birnessite. Moreover, lowering the addition time of the oxidant solution decreases the amount of hausmannite; nevertheless pure phase birnessite is still not obtained indicating that the purity depends on kinetic constrains not thermodynamic ones. The formation of pure birnessite is successful by following oxidant-last method when the initial mixture of NaOH and MnCl2 is bubbled with N2. Therefore, this study points out the key role played by dissolved oxygen in the formation of birnessite via redox reaction and how its careful control allowed avoiding hausmannite as by-product. Moreover, the ratio MnO4 -/Mn(II) = 0.33 used in this study is not sufficient to oxidize the total amount of manganese into birnessite as manganite is observed when the N2 bubbling is maintained during all the reaction time. Finally, the use of KMnO4 and KOH instead of NaMnO4 and NaOH leads to K-birnessite only for the alkali-last method, showing the key role played by the counter-ion in the birnessite formation. A or O when the reductant-last, the alkali-last or the oxidant-last method is used and t corresponds to the time addition, in hour, of the third reactant. a and b under N 2 bubbling (during the initial mixture for "a" and 2 h for "b"). bir for birnessite, haus for hausmannite, mang for manganite and feit for feitknechtite. * : trace and (c) 𝑁𝑎 2 𝑂 (oxidant-last method). The addition time of the third reagent: 2h, *birnessite,

• hausmannite and ▼ feitknechtite.

  are shown the XRD patterns of samples 𝐾 2 𝑅 , 𝐾 2 𝐴 and 𝐾 2 𝑂 . In this series, KMnO4 and KOH were used instead of NaMnO4 and NaOH in order to investigate the role of the counter-ion. As in the previous series, two products were identified: K-birnessite (PDF 01-080-1098) with two main diffraction peaks corresponding to d001 = 7.02 and d002 = 3.51 Å and hausmannite. But, in contrast to the Na-birnessite, the formation of K-birnessite single phase, with only traces of a crystalline by-products, was only possible in the alkali-last method (sample 𝐾 2 𝐴 ).

Mn 3 O 4 , 1 3

 341 sd + O 2,aq = 3 MnO 2,sd (R 4 ),  r G°4 = -128 kJ mol -Mn 3 O 4,sd + 2 MnO 4 -+ 4 H 2 O liq = 8 MnO 2,sd + 3 Mn(OH) 2 + 2 OH - (R' 4 ),  r G°4 ' = -188 kJ mol -1

Fig. 1 .

 1 Fig. 1. XRD patterns of sample (a) 𝑁𝑎 2 𝑅 (reductant-last method), (b) 𝑁𝑎 2 𝐴 (alkali-last method)

Fig. 3 .

 3 Fig. 3. IR spectra of sample (a) 𝑁𝑎 2 𝑅 (reductant-last method), (b) 𝑁𝑎 2 𝐴 (alkali-last method), (c) 𝑁𝑎 2 𝑂 (oxidant-last method) and (d) hausmannite.

Fig. 4 . 2 𝑂 , 𝑁𝑎 4 𝑂

 424 Fig. 4. XRD patterns of the products obtained by the oxidant-last method with different addition time of the oxidant solution, i.e. a) 0 hours, b) 2 hours, c) 4 hours and d) 8 hours (samples 𝑁𝑎 0 𝑂 ,

Fig. 5 .

 5 Fig. 5. XRD pattern of a) 𝑁𝑎 2 𝑂 -a and b) 𝑁𝑎 2 𝑂 -b, *Birnessite and ¤ Manganite.

Fig. 6 .

 6 Fig. 6. Pathways of birnessite and hausmannite formations.

Fig. 7 .

 7 Fig. 7. XPS Mn (2p3/2) spectra of: a) 𝑁𝑎 2 𝑂 , b) 𝐾 2 𝑂 , c) 𝑁𝑎 2 𝑅 , d) 𝐾 2 𝑅 , e) 𝑁𝑎 2 𝐴 , f) 𝐾 2 𝐴 and g) hausmannite. For comparison, the spectra obtained from deconvolution (red line) and their components (Mn(II): grey, Mn(III): blue, Mn(IV): green) are shown.

Table 1 .

 1 Nature of the product obtained in the different syntheses performed Na or K when NaMnO 4 and NaOH or KMnO 4 and KOH were used, m = R,

	Sample	𝑁𝑎 2 𝑅	𝑁𝑎 2 𝐴	𝑁𝑎 2 𝑂	𝐾 2 𝑅	𝐾 2 𝐴	𝐾 2 𝑂	𝑁𝑎 0 𝑂	𝑁𝑎 4 𝑂	𝑁𝑎 8 𝑂 𝑁𝑎 4 𝐴 𝑁𝑎 8 𝐴 𝑁𝑎 2 𝑂 -a 𝑁𝑎 2 𝑂 -b
	Main product observed by XRD	Bir Haus*	bir	bir haus feit*	bir haus	bir	bir haus	bir haus	bir haus	bir haus	bir bir	bir	bir mang*
	Samples denotation 𝐶 𝑡 𝑚 ∶ C =

Table 2 .

 2 Chemical composition of Na-birnessite

	Sample	CEC (meq/g) §	x = R = Mn(III)/Mntot (bulk)	Mn(III)/Mntot (surface) ¤
	𝑵𝒂 𝟐 𝑺	3.33	0.35	0.53
	𝑵𝒂 𝟐 𝑨	3.32	0.35	0.55
	𝑵𝒂 𝟐 𝑶 -a	3.25	0.34	0.57
	§ expressed on a dry weight basis ¤ XPS measurements

Table 3 .

 3 Standard Gibbs free energies of formation of different species involved in studied reactions(Dean, 1999).

	Compound	Mn 2+ (aq)	H2O (lq)	OH -(aq)	O2 (aq)	MnO4 -(dis)	Mn(OH)2 (sd)	Mn3O4 (sd)	MnO2 (sd)	MnO(OH) (sd)
	fG° (kJ/mol)	-227.8 -237	-157	16.3	-447	-615	-1283	-465	-567

  Fig. 2. XRD patterns of sample (a) 𝐾 2 𝑅 (reductant-last method), (b) 𝐾 2 𝐴 (alkali-last method) and (c) 𝐾 2 𝑂 (oxidant-last method). The addition time of the third reagent: 2h, *birnessite and

	7.14 Å *
	3.57 Å *			2.52 Å 2.42 Å * * * * 2.14 Å 2.22 Å	1.87 Å *	1.81 Å *	1.47Å 1.44 Å * *
	4.92 Å	28.9 Å	Å 32.32	36.09 Å

• hausmannite.


domain: chim sde phys.cond.cm-ms

text:
Hella Boumaiza

Romain Coustel

Christelle Despas

Christian Ruby

Latifa Bergaoui

Interaction of ammonium with birnessite: Evidence of a chemical and structural transformation in alkaline aqueous medium

The ammonium cation interaction with Na-birnessite in aqueous alkaline medium was studied. Solution and solid analysis give evidence that birnessite is not only acting as a cationic exchanger toward NH4 + . The surface analysis performed by XPS showed that N1s spectra are characterized by the existence of two different environments: one assignable to an interlayer NH4 + and the second to a chemisorbed N-species. Structural and chemical transformations were observed on birnessite with nitrogen mass balance deficit. The monitoring of NH4 + , Na + , Mn 2+ , NO3 -and NO2 -and solid changes (average oxidation state of Mn, cation exchange capacity, solid nitrogen content and symmetry evolution identified by XRD and FTIR) indicate unambiguously that NH4+ reacts chemically with the birnessite.

Introduction

Among the various manganese oxides, Birnessite was found to be the most common occurring form in soils and sediments [START_REF] Bargar | Structural characterization of terrestrial microbial Mn oxides from Pinal Creek , AZ[END_REF]. The crystal system of birnessite was shown to be either triclinic or hexagonal [re] but monoclinic system was also sometimes reported [ref].

All the forms consist of a lamellar structure of edge-shared MnO6 octahedra in which the layers are negatively charged and with an interlayer space containing a compensating cation (e.g.. Na + , K + , Mn 2+ , Mn 3+ ).

In the triclinic form, the charge deficit of the layers is due to the mixed valence states of manganese: +III and +IV [START_REF] Post | Crystal structure determinations of synthetic sodium, magnesium, and potassium birnessite using TEM and the Rietveld method[END_REF][START_REF] Post | Rietveld refinement of a triclinic structure for synthetic Na-birnessite using synchrotron powder diffraction data[END_REF]. The layers have been described as an alternation of two Mn IV O6 rows and one Mn III O6 along the b axis [START_REF] Drits | Structure of Synthetic Monclinic Na-rich Birnessite and Hexagonal Birnessite: results from X-ray diffraction and selected-area electron diffraction[END_REF][START_REF] Silvester | Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: II. Results from chemical studies and EXAFS spectroscopy[END_REF]Lanson, V. A. Drits, et al., 2002). The triclinic symmetry is caused by the presence of the Mn III O6 octahedra distorded by the Jahn Teller effect [START_REF] Lanson | Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH[END_REF]. The manganese cations present in the layers of the hexagonal form are mainly in the oxidation state +IV, the negatively charged layers being mainly caused by the presence of vacancies [START_REF] Lanson | Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH[END_REF]. The charge deficit is compensated by the presence of interlayer manganese in the oxidation state +II and/or +III. These protons are located above and below the vacancies [START_REF] Silvester | Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: II. Results from chemical studies and EXAFS spectroscopy[END_REF]. Consequently, different concentrations of Mn II , Mn III and Mn IV are present in the two forms of birnessite.

Both forms have been known to participate in a large variety of reactions: ion exchange and sorption reaction [START_REF] Golden | Ion Exchange, Thermal Transformations, and Oxidizing Properties of Birnessite[END_REF][START_REF] Goff | Synthesis, ion exchange and electrochemical properties of lamellar phyllomanganates of the birnessite group[END_REF][START_REF] Peacock | Sorption of Ni by birnessite: Equilibrium controls on Ni in seawater[END_REF][START_REF] Jiang | Sorption and desorption of tetracycline on layered manganese dioxide birnessite[END_REF], as well as redox reaction with heavy metals, organic and/or (?) inorganic molecules [START_REF] Scott | Reactions at oxide surfaces. -1. Oxidation of As(III) by synthetic Birnessite[END_REF][START_REF] Scott | Reactions at oxide surfaces. -2. Oxidation of Se(IV) by synthetic Birnessite[END_REF][START_REF] Renuka | Investigation on layered birnessite type manganese oxides for battery applications[END_REF][START_REF] Rao | Oxidative transformation of aqueous phenolic mixtures by birnessite-mediated catalysis[END_REF][START_REF] Remucal | A critical review of the reactivity of manganese oxides with organic contaminants[END_REF]. Given that the birnessite reactivity is influenced by the manganese valence state, determining the Mn II , Mn III and Mn IV ratios is of great importance.

It is possible to determine the manganese average oxidation state (AOS) of birnessite by chemical titrations [START_REF] Lingane | New Method for Determination of Manganese[END_REF][START_REF] Vetter | potentialausbildung an der manga-dioxid-elektrode als oxidelektrode mit nichtstöchiometrischem oxid[END_REF][START_REF] Murray | The oxidation state of manganese in marine sediments and ferromanganese nodules[END_REF][START_REF] Silvester | Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: II. Results from chemical studies and EXAFS spectroscopy[END_REF] or by solid characterization via Mn K-and L-edge XANES analysis [START_REF] Manceau | Structural chemistry of Mn, Fe, Co, and Ni in manganese hydrous oxides. 2.Information from EXAFS spectroscopy and electron and X-Ray-Diffraction[END_REF][START_REF] Chalmin | A pre-edge analysis of Mn K-edge XANES spectra to help determine the speciation of manganese in minerals and glasses[END_REF]. These methods are bulk techniques that donot provide information about the near-surface chemistry of the oxide where the reactions actually take place.

To this purpose, the X-ray Photoelectron spectroscopy appears to be the technique of choice allowing the determination of the valence state of manganese located near the surface..Despite the existence of many studies that focused on the structural determination of triclinic and hexagonal birnessites, and the conversion from one phase to another [START_REF] Post | Crystal structure determinations of synthetic sodium, magnesium, and potassium birnessite using TEM and the Rietveld method[END_REF][START_REF] Drits | Structure of Synthetic Monclinic Na-rich Birnessite and Hexagonal Birnessite: results from X-ray diffraction and selected-area electron diffraction[END_REF][START_REF] Silvester | Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: II. Results from chemical studies and EXAFS spectroscopy[END_REF][START_REF] Lanson | Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH[END_REF][START_REF] Lanson | Structure of synthetic Na-birnessite: Evidence for a triclinic one-layer unit cell[END_REF], there are only a few that reported on the determination of Mn II , Mn III and Mn IV relative concentrations in birnessite at the surface. The oldest one [START_REF] Nesbitt | Interpretation of XPS Mn ( 2p ) spectra and constraints on the of Mn oxyhydroxides mechanism of MnO2 , precipitation[END_REF] was based on the fitting of the Mn2p3/2 of different K-birnessite. Accordingly, some studies focused on the AOS determination by XPS using the decomposition of the Mn2p3/2 line [START_REF] Yin | Characterization of Nirich hexagonal birnessite and its geochemical effects on aqueous Pb2+/Zn2+ and As(III)[END_REF][START_REF] Mckendry | Decoration of the layered manganese oxide birnessite with Mn(ii/iii) gives a new water oxidation catalyst with fifty-fold turnover number enhancement[END_REF][START_REF] Boumaiza | Conditions for the formation of pure birnessite during the oxidation of Mn(II) cations in aqueous alkaline medium[END_REF] but the obtained AOS values measured on the surface were systematically lower then the bulk determination indicating the presence of a Mn III excess on the surface. More recently, [START_REF] Ilton | XPS determination of Mn oxidation states in Mn (hydr)oxides[END_REF] determined the manganese distribution for several manganese oxide surfaces by comparing the mutiplet splitting of Mn 2p3/2, 3s and 3p lines. This study revealed that the use of the 3s splitting was not appropriate for birnessite, but the determination of the AOS via the 3p splitting was more accurate in comparison with the charge balancing cations in the interlayer region measured by bulk analysis. In contrast the distribution obtained by the 2p3/2 splitting which values were systematically more reduced. The authors raised the possibility that the quantification extracted from the analyses of the Mn2p3/2 XPS peaks are not precise because of the well-known sensitivity of the peak shape to differences in the Mn bonding environment. They concluded that for an accurate AOS determinations, the Mn(3p) could be combined with the Mn(2p) to provide a greater precision of the Mn speciation in the birnessite.

When submitted to an acidic treatment, the triclinic birnessite undergoes a symmetry conversion to hexagonal [START_REF] Drits | Structure of Synthetic Monclinic Na-rich Birnessite and Hexagonal Birnessite: results from X-ray diffraction and selected-area electron diffraction[END_REF][START_REF] Silvester | Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: II. Results from chemical studies and EXAFS spectroscopy[END_REF][START_REF] Lanson | Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH[END_REF]. This change is the result of the release of Mn II in the solution accompanied by a modification of the Mn II , Mn III and Mn IV proportions in the solid. The study of this change by XRD and FTIR analyses is well documented in the literature [START_REF] Lanson | Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH[END_REF][START_REF] Ling | Fourier-transform infrared spectroscopy (FTIR) analysis of triclinic and hexagonal birnessites[END_REF]. But, to the very best of our knowledge, no study focused on the variation of the Mn II , Mn III and Mn IV surface proportions by using XPS during this acidic treatment. The main goal of this study is thus to follow the changes of triclinic birnessite placed at pH 3 by monitoring both the solid and liquid (properties. The reliability to use XPS for the determination of the Mn Average Oxidation State (AOS) the near-surface will be evaluated and compared to bulk analysis.

Materials and methods

Chemicals

All chemicals were purchased from Sigma-Aldrich. Manganese(II) chloride tetrahydrate (MnCl2,4H2O, ACS reagent, ≥ 98%), sodium permanganate monohydrate (NaMnO4,H2O, ACS reagent ≥ 97%) and sodium hydroxide (NaOH, BioXtra, ≥ 98% pellets anhydrous) were used for birnessite's synthesis. A hydrochloridric acid solution (0.2 mol L -1 ) was used to stabilize the birnessite suspension at the desired pH.

Synthesis of birnessite

The triclinic birnessite was obtained following the alkaline method of [START_REF] Boumaiza | Conditions for the formation of pure birnessite during the oxidation of Mn(II) cations in aqueous alkaline medium[END_REF] in which 125 mL of NaOH (8.8 mol L -1 ) is added dropwise during 2 hours to a mixture of 250 mL of NaMnO4 (0.1 mol L -1 ) and 125 mL of MnCl2 (0.6 mol L -1 ). The mixture is then mixed during ? an additional 30 minutes, aged at 60 °C during 14h, washed until the supernatant pH is 9 -10 and dried at 60 °C during 16 hours. The obtained product consisted on triclinic Nabirnessite with a general formula [Mn III 0.35 Mn IV 0.65 O2][Na + 0.35 0.7 H2O], an Average Mn Oxidation State (AOS) equal to 3.65 and a Cation Exchange Capacity (CEC) equal to (2.3 ± 0.1) meq g -1 .

The hexagonal birnessite was obtained from the transformation of Na-birnessite in acid solution: Na-birnessite suspension (6 gL -1 ) was stabilized at pH 3 by adding hydrochloridric acid solution (0.2 mol L -1 ). An additional experiment without pH stabilization was also performed (pH = 10). For the liquid analysis, samples were collected at different contact times and filtered through 0.22 µm syringe filter. After 24 h of contact time, the remaining birnessite was collected for solid state characterizations.

Characterization techniques

Powder X-ray Diffraction (XRD) was performed at room temperature by placing the powders on zero background quartz sample holders and by using an X'Pert MPD diffractometer (Panalytical AXS) and an X′Celerator as a detector. The goniometer radius is 240 nm and the divergence slit module is fixed (1/2° divergence slit, 0.04 rd Sollers slits). The XRD patterns are recorded using CuKα radiation (λ=0.15418 nm).

FTIR analysis was performed using a Bruker Vertex 70 v equipped with a DLaTGS detector.

The transmission mode was used to record spectra in which KBr pellets contained 2 -5 mg of sample. In the 5000 -200 cm -1 wavenumber range, 100 scans were collected for each sample with 4 cm -1 resolution. The background spectrum of KBr was recorded in the exact same conditions.

X-ray photoelectron spectra were recorded on a KRATOS Axis Ultra X-ray photoelectron spectrometer (Kratos Analytical, Manchester, UK) equipped with a monochromated Al Kα source (hυ =1486.6 eV, spot size 0.7 mm×0.3 mm). The samples were pressed onto a Cu tape fixed on a holder and introduced into the spectrometer. The detector is a hemispherical analyzer at an electron emission angle of 90° and pass energy of 160 eV (survey spectra) or 20 eV (core level spectra). For the core-level spectra, the overall energy resolution, resulting from monochromator and electron analyzer bandwidths, was 800 meV. Charge correction was carried out using the C 1s core line, setting adventitious carbon signal (H/C signal) to 284.6 eV.

Mn content in the supernatant was determined using Inductive Coupled Optical Emission Spectroscopy ICP-AES (Jobin Yvon-Horiba, Ultima).

The Mn AOS of the bulk birnessite was measured by potentiometric titration using a Mohr salt, potassium permanganate and sodium pyrophosphate dibasic [START_REF] Lingane | New Method for Determination of Manganese[END_REF][START_REF] Vetter | potentialausbildung an der manga-dioxid-elektrode als oxidelektrode mit nichtstöchiometrischem oxid[END_REF].

3. Results and discussion.

Analysis of soluble manganese

It is first necessary to point out that, the manganese in solution is assumed to be Mn(II) since Mn(III) and Mn(IV) are esssentially stable in solid-phase [START_REF] Langston | Structure of synthetic Nabirnessite: Evidence for a triclinic one-layer unit cell[END_REF]. The Mn(II) content in the supernatant at pH 3 as a function of time is shown in Fig. 1. The trend/ evolution (?) is characterized by two distinct steps: during the first 30 minutes, the Mn(II) concentration rises rapidly to a maximum of 1.6 mmol L -1 . Then, the concentration decreases gradually to 1.32 mmol L -1 at 180 minutes and this value is retained until 24 h reaction time. At this pH, the amount of Mn(II) released in the solution only represents 3% of Na-birnessite initially introduced in the solution while at pH 10, no Mn(II) was detected in the supernatant. This is consistent with the previous observation showing thatthe reaction of r triclinic birnessite placed in an acidic medium [START_REF] Silvester | Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: II. Results from chemical studies and EXAFS spectroscopy[END_REF] could be explained by a two steps reaction.

The first step corresponds to the disproportionation of Mn(III) in the layers into Mn(II) and Mn(IV), the Mn(II) formed being then dissolved in acidic medium by creating vacancies in the layers by following reaction (1): 2 Mn 3+ layer → Mn 4+ layer + Mn 2+ layer → Mn 4+ layer + vacancy + Mn 2+ aq (1)

In a second step, a part of the dissolved Mn(II) re-adsorbs on the surface, above and below vacancies, resulting in the diminution of the amount of aqueous Mn(II) [START_REF] Silvester | Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: II. Results from chemical studies and EXAFS spectroscopy[END_REF].

Another study also suggested that the absorbed Mn(II) is able to react with Mn(IV) ions in the layers to regenerate Mn(III) [START_REF] Zhao | Redox Reactions between Mn(II) and Hexagonal Birnessite Change Its Layer Symmetry[END_REF] The XRD patterns of the triclinic Na-birnessite is presented in figure 2a in comparison to the one maintained at pH 3 during 24 h (Fig. 2b).

Solid analysis • XRD and FTIR characterizations

For 2θ range from 10 to 30 °, the two XRD patterns are quite similar and are both characterized by the presence of two main peaks at around 7 and 3.5 Å assigned respectively to the basal reflections 001 and 002. In the triclinic form, the two main peaks are located at angles corresponding to the distances 7.02 and 3.54 Å. When submitted to acidic treatment, a slight shift toward lower 2θ values is observed and the two main peaks correspond henceforth to the distances 7.10 and 3.59 Å. For 2θ range from 30 to 70 °, the initial reflections of the triclinic form collapse when submitted to acidic treatment and the product maintained at pH 3 during 24 h presents other reflection peaks corresponding to hexagonal birnessite [START_REF] Drits | Structure of Synthetic Monclinic Na-rich Birnessite and Hexagonal Birnessite: results from X-ray diffraction and selected-area electron diffraction[END_REF][START_REF] Lanson | Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH[END_REF].

The solids were further characterized by FTIR analysis and the spectra are presented in figure 3. The initial triclinic Na-birnessite is characterized by the presence of bands located at 245, 360, 418, 478, 512 and 634 cm -1 with a shoulder at 550 cm -1 . These bands are assigned to the Mn-O lattice vibration, in good agreement with the reported spectra of Na-birnessite [START_REF] Potter | The tetravalent manganese oxides : identification , hydration , and structural relationships by infrared spectroscopy[END_REF][START_REF] Feng | Pathways of birnessite formation in alkali medium[END_REF][START_REF] Cui | Birnessites with different average manganese oxidation states synthesized, characterized, and transformed to todorokite at atmospheric pressure[END_REF][START_REF] Ling | Fourier-transform infrared spectroscopy (FTIR) analysis of triclinic and hexagonal birnessites[END_REF].

The spectrum of the birnessite maintained at pH 3 during 24 h is sensibly/clairly different and is only characterized by the presence of two large bands located at 430 and 495 cm -1 characteristics of the hexagonal symmetry [START_REF] Johnson | Water in the interlayer region of birnessite: Importance in cation exchange and structural stability[END_REF][START_REF] Ling | Fourier-transform infrared spectroscopy (FTIR) analysis of triclinic and hexagonal birnessites[END_REF].

The liquid and solid analyses thus showed that, a triclinic Na-birnessite is converted into hexagonal H-birnessite when maintained at pH 3 during 24 h, as it has been previously observed [START_REF] Drits | Structure of Synthetic Monclinic Na-rich Birnessite and Hexagonal Birnessite: results from X-ray diffraction and selected-area electron diffraction[END_REF][START_REF] Silvester | Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: II. Results from chemical studies and EXAFS spectroscopy[END_REF][START_REF] Lanson | Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH[END_REF]. • XPS analysis Surface properties of Na-birnessite and H-birnessite were examined by XPS. Overview XPS spectra (see supplementary materials) show core-level photoelectron peaks at ~50 eV (Mn3p), ~285 eV (C1s), ~530 eV (O1s), ~642 eV (Mn2p3/2), ~654 eV (Mn2p1/2), ~770 eV (Mn2s) and ~1071 eV (Na1s). The C1s peak should be attributed to a hydrocarbon contamination of the birnessite surface before introduction into the XPS chamber.

The latter peak disappears for the hexagonal form, indicating the total exchange of the interlayer sodium. are given in Figure 4 and Table 1. This analysis confirms that Na-birnessite presents only Mn(III) and Mn(IV) while H-birnessite presents Mn(II). It appears also that Mn(III) contain decreases while Mn(IV) contain increases after acidic treatment, as expected from the mechanism proposed by Silvester et al.[ref].

Recently, [START_REF] Ilton | XPS determination of Mn oxidation states in Mn (hydr)oxides[END_REF] claimed that a more reliable determination of Mn(II), Mn(III) and Mn(IV) distribution on the surface can be obtained from the Mn3p signal. Corresponding experimental results are showed in Figure 4: Mn3p peaks for Na-birnessite and H-birnessite are located à 50 eV with a shoulder at 48 eV. These authors proposed an empirical approach where each valence state contribution is described as a multiplet whose shape is experimentally determined from XPS measurement on monovalent Mn oxide (MnO, MnOOH and MnO2).

Results obtained from such Mn3p fitting procedure are given in Figure 4 and table 1. As determined from the Mn2p 3/2 spectra, the Ilton et al's procedure indicate that Mn(II) appears after acid treatment while Mn(III) and Mn(IV) contain decreases and increases, respectively. The quantitative analysis shows that the Na/Mn ratio decreases from 0.5 for the triclinic form to 0 for the hexagonal one indicating that all the initial sodium was released in the solution after acidic treatment.

The determination of Mn II , Mn III and Mn IV distribution on the surface based on the Mn2p3/2 and Mn3p are summarized in Table 1 in comparison to the bulk determination of the Mn AOS. The Mn AOS determined by XPS analysis of both forms of birnessite are similar for the two fitting methods. Indeed, the triclinic form presented aMn AOS of 3.43 and 3.47 for the Mn2p3/2 and Mn3p fittings respectively. The same applies to the hexagonal form which Mn AOS at the surface were equal to 3.60 and 3.56 for the Mn2p3/2 and Mn3p fittings respectively. These obtained values from surface analysis are considerably more reduced than the Mn AOS determined by bulk analysis corresponding to 3.65 and 4.00 for the triclinic and hexagonal forms respectively. However, it is necessary to point out that the triclinic form presented systematically a Mn AOS more reduced than the hexagonal form, in both bulk and surface analyses, in good agreement with the proposed conversion mechanism between the two forms [START_REF] Lanson | Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH[END_REF]. Indeed the symmetry conversion is the result of the exchange of the interlayer Na + by H3O + from the solution and the disproportionation of Mn(III) to Mn(IV) in the layers and Mn(II) in the solution. The hexagonal birnessite thus presents higher amounts of Mn(IV) and consequently a higher Mn AOS value.

The use of the Mn2p3/2 fitting combined with the Mn3p gave systematically lower Mn AOS values than the bulk, unlike what has been observed in the study of [START_REF] Ilton | XPS determination of Mn oxidation states in Mn (hydr)oxides[END_REF]. This suggests that manganese at the near-surface of all types of birnessite is significantly more reduced than in the bulk.

Conclusion

The conversion of the triclinic birnessite into hexagonal one was monitored using liquid and solid characterization. The ICP, FTIR and XRD characterization were similar to what have been previously observed. The determination of Mn II , Mn III and Mn IV distribution on the surface by XPS based on the Mn2p3/2 and Mn3p fitting parameters gave similar results that were systematically more reduced than the bulk analysis. The XPS analyses appears to be not an accurate tool to monitor and determine the bulk Mn AOS in the birnessites.

Fig. 1 .

 1 Fig. 1. Evolution of Mn(II) concentration in function of time at pH 3 ([Na-bir] = 6 g L -1 , T = 25 °C, reaction time = 24 h)

Fig. 2 .

 2 Fig. 2. XRD patterns of a) triclinic Na-birnessite and b) hexagonal H-birnessite ([Na-bir] = 6 g L -1 , T = 25 °C, reaction time = 24 h, pH= ?)

Fig. 3 .

 3 Fig. 3. FTIR spectra of a) triclinic Na-birnessite and b) hexagonal H-birnessite ([Na-bir] = 6 g L -1 , T = 25 °C, reaction time = 24 h, pH = ?).

  Peculiar attention was paid to Mn2p 3/2 and Mn3p peaks from which it has been claimed that the valence state of Mn can be extracted [citer Nessbitt et Ilton]. The Mn2p3/2 spectrum of Na-birnessite is centered at 642 eV with shoulders at 641 eV and 643 eV, as expected for Mn(III)-Mn(IV) birnessite [citer Boumaiza et al 2017]. Similar, but widened, feature is observed for Hbirnessite (Figure 4.a) with additional shoulder at 639 eV. This low binding energy component is attributed to Mn(II) contribution [citer Nessbitt et Ilton]. Then XPS results confirm that Hbirnessite presents Mn(II) according to the mechanism of Na-birnessite conversion in acidic medium proposed by Silvester et al. [citer la ref]. Nesbitt et al. have proposed a method to isolate the Mn(II), Mn(III) and Mn(IV) contributions of Mn2p 3/2 peak observed for binessite : based on the earlier work of Gupta and Sen [Gupta, RP and Sen, S.K. (1974) According to calculations of multiplet structure core p-vacancy levels Physical Reviews B, 10,7119] each valence state contribution is modeled as a multiplet whose parameters are derived from ab-initio calculations. Mn2p 3/2 spectra of Na-birnessite and Hbirnessite were fitted following the recommendations of Nesbitt et al. and corresponding results

Fig. 4 .

 4 Fig.4. Mn2p3/2 and Mn3p XPS spectra of the triclinic Na-birnessite (a & c) and the hexagonal H-birnessite (b & d) ([Na-bir] = 6 g L -1 , T = 25 °C, reaction time = 24 h, pH= 3).

Table 1 :

 1 Mn II , Mn III and Mn IV distribution on the surface compared to the bulk determination

		Triclinic Na-birnessite	Hexagonal H-birnessite
		Mn2p3/2 fitting	Mn3p fitting	Mn2p3/2 fitting	Mn3p fitting
	Mn(II) (%)	0	0	7	2
	Mn(III) (%)	57	53	26	40
	Mn(IV)	43	47	67	58
	AOS surface	3.43	3.47	3.60	3.56
	AOS bulk	3.65		4.00