Srirat Chuayboon

Stéphane Abanades
email: stephane.abanades@promes.cnrs.fr

Sylvain Rodat

Solar chemical looping reforming of methane combined with isothermal H 2 O/CO 2 splitting using ceria oxygen carrier for syngas production

Keywords: Chemical looping, Methane reforming, Ceria structure, Concentrated solar power, Syngas production, H 2 O/CO 2 splitting

The chemical looping reforming of methane through the nonstoichiometric ceria redox cycle (CeO 2 /CeO 2-δ ) has been experimentally investigated in a directly irradiated solar reactor to convert both solar energy and methane to syngas in the temperature range 900-1050 °C.

Experiments were carried out with different ceria shapes via two-step redox cycling composed of endothermic partial reduction of ceria with methane and complete exothermic re-oxidation of reduced ceria with H 2 O/CO 2 at the same operating temperature, thereby demonstrating the capability to operate the cycle isothermally. A parametric study considering different ceria macrostructure variants (ceria packed powder, ceria packed powder mixed with inert Al 2 O 3 particles, and ceria reticulated porous foam) and operating parameters (methane flow-rate, reduction temperature, or sintering temperature) was conducted in order to unravel their impact on the bed-averaged oxygen non-stoichiometry (δ), syngas yield, methane conversion, and solar reactor performance. The ceria cycling stability was also experimentally investigated to demonstrate repeatable syngas production by alternating the flow between CH 4 and H 2 O (or CO 2 ). A decrease in sintering temperature of the ceria foam was beneficial for increasing syngas selectivity, methane conversion, and reactor performance. Increasing both CH 4 concentration and reduction temperature enhanced δ with the maximum value up to 0.41 but concomitantly favored CH 4 cracking reaction. The ceria reticulated porous foam showed better performance in terms of effective heat transfer, due to volumetric absorption of concentrated solar radiation and uniform heating with lower solar power consumption, thereby promoting the solar-to-fuel energy conversion efficiency that reached up to 5.60%. The energy upgrade factor achieved during cycle was up to 1.19.

Stable patterns in the δ and syngas yield for consecutive cycles with the ceria foam validated material performance stability.

Introduction

Concentrated solar power is a sustainable and desirable renewable-energy source for process heat to drive high-temperature thermochemical reactions, e.g. redox cycles. Of particular interest is solar energy conversion into transportable and dispatchable chemical fuels by water (H 2 O) or carbon dioxide (CO 2 ) splitting using thermochemical redox cycles to produce syngas (mixture of hydrogen (H 2 ) and carbon monoxide (CO)). The resulting syngas can be further converted to liquid hydrocarbon fuel via Fischer-Tropsch [1] or utilized for production of methanol, ammonia, or dimethyl ether [2].

The conventional production of syngas through the conversion of methane (CH 4 ) involves steam reforming [3], dry reforming with CO 2 [4][5][6][7], and methane reforming over metal oxide redox materials [8][9][10] (partial oxidation of methane). The methane reforming over redox systems results in the partial oxidation of methane without the use of gaseous oxygen or catalysts and further allows operating the process as a cycle (because the solid oxide can be recycled back via oxidation with H 2 O or CO 2 ) while producing syngas. The heat required for such an endothermic reaction can be provided by solar energy using concentrating solar power technologies, thereby converting solar energy into transportable and storable chemical fuels [11][12][13][14]. This two-step combined process (methane reforming and H 2 O/CO 2 splitting) requires significantly lower reduction temperature as compared to the two-step oxide-based redox cycle [15,16] due to the aid of a reducing agent (CH 4 ). Since the metal oxide reduction with CH 4 and the H 2 O/CO 2 splitting steps usually proceed at similar temperatures, isothermal cycle operation is made possible, which reduces the constrains imposed by reactor materials as well as thermal radiation losses [17].

The feasibility of utilizing metal oxides (either non-volatile or volatile metals) as oxygen carriers for CH 4 partial oxidation has been experimentally reported for ceria (CeO 2 ) [9,18], cerium-based oxides [8,19,20], iron oxide [21], tungsten oxide [13], and zinc oxide [11,22]. Among them, the partial oxidation of CH 4 with either doped or undoped ceria is particularly attractive owing to its capabilities for both rapid oxygen storage and release through lattice transfer, while retaining a stable crystallographic structure over a wide range of reduction extents [15], with reversible shift between Ce 4+ and Ce 3+ oxidation states [23,24].

Prior experimental study considering the partial oxidation of CH 4 using the redox properties of ceria was first reported by Otsuka et al. [9], without the utilization of solar energy. They demonstrated that the conversion of methane into syngas with a H 2 /CO ratio of 2 was possible, and the reduced ceria could be re-oxidized with CO 2 to produce CO. Then, both thermodynamic and experimental studies [25,26] with the combination of concentrated solar energy were considered. Krenzke and Davidson [25] studied the thermodynamics of the ceria cycle with methane. They indicated that coupling the reduction of ceria with the partial oxidation of methane enables isothermal cycling at 950 °C with high-quality syngas produced during the reduction step and maximum predicted solar-to-fuel efficiency of 40%. Welte et al. [14] experimentally investigated the methane reforming over ceria in a particle-transport reactor. This reactor achieved a bed-averaged oxygen non-stoichiometry (δ) as high as 0.25 at the expense of unreacted ceria particle being entrained by the produced syngas.

The chemical-looping methane reforming over ceria can be represented by two steps.

First, the endothermic partial ceria reduction in the presence of CH 4 that reacts with lattice oxygen for the production of syngas with a H 2 /CO ratio of 2 suitable for methanol synthesis: CeO 2 + δCH 4 ⟶ CeO 2-δ + δCO + 2δH 2 (1) Second, the oxidation of partially-reduced ceria with H 2 O or CO 2 :

CeO 2-δ + δH 2 O ⟶ CeO 2 + δH 2 (2) CeO 2-δ + δCO 2 ⟶ CeO 2 + δCO (3)
The advantages of the process combining partial oxidation of CH 4 and ceria redox cycle are: (i) the utilization of CH 4 in the reduction step allows for isothermal operation, thereby avoiding sensible heat losses taking place during temperature-swing cycle and eliminating the need for heat recovery, (ii) solid oxide is used in place of gaseous oxygen which eliminates the need for oxygen production from air, (iii) reduced ceria can be subsequently oxidized with either H 2 O or CO 2 in an oxidation step to produce additional syngas and complete the cycle, (iv) deposited carbon on ceria structures can be concomitantly gasified and removed during the oxidation step, thus avoiding material deactivation and eliminating the requirement for expensive catalysts.

The ceria macrostructure plays a significant role on the performance of the combined two-step process in terms of conductive and radiative heat transfer across the material.

Various metal oxide structures such as porous foams [15,16,[26][27][START_REF] Siegel | ASME 2009 3rd International Conference on Energy Sustainability[END_REF][START_REF] Costa Oliveira | [END_REF], textured plates [START_REF] Siegel | ASME 2009 3rd International Conference on Energy Sustainability[END_REF],

vertical pins [START_REF] Siegel | ASME 2009 3rd International Conference on Energy Sustainability[END_REF], powder [30], powder mixed with inert material [23], multi-channeled honeycombs [31,32], felts [33] and three-dimensionally ordered macroporous (3DOM) ceramics [START_REF] Costa Oliveira | [END_REF]34] have been studied for two-step thermochemical H 2 O/CO 2 splitting cycles in order to provide an effective interface for uniform concentrated solar energy absorption and sufficient surface area for supporting rapid chemical reactions. The powder bed structure or powder mixed with inert promoter [23] exhibited rapid oxidation rates; however at the expense of high radiative opacity, which may lead to undesired temperature gradients across the bed. Such a barrier can be tackled by using porous foam structures with high specific surface area [15][16], although heat transfer limitation may arise from their high optical thickness. Such reactive structures could therefore be applied advantageously to the solardriven isothermal chemical looping reforming process with CH 4 and oxidant gas (H 2 O or CO 2 ) alternately flowing through the oxide structure. Besides, different solar reactor concepts have been developed for thermochemical solar fuel production and applied to e.g. solar gasification or metal oxide redox cycles [35][36][37][38][39][40][41]. They can mainly be categorized as particlebased (encompassing packed or fluidized beds, entrained flows or particle clouds) or structured reactor technologies (volumetric porous receiver). However, versatile solar reactor operation applied to a large variety of ceria materials for methane reforming has not been considered to-date. The feasibility of isothermal solar-driven chemical looping methane reforming using ceria structures thus needs to be demonstrated in a flexible and scalable solar-operated reactor able to achieve both process steps continuously with repeatable fuel production, reversible oxygen exchange and thermochemical performance stability.

Therefore, because of the noticeable advantages of the methane reforming over ceria and the beneficial effects of ceria structures, the present study aims to investigate the chemical looping process with different ceria oxygen carrier structures in a novel flexible solar reactor driven by real high-flux concentrated solar power, provided by a parabolic dish solar concentrator. The whole process was reliably operated and demonstrated for the first time in this work under real solar irradiation conditions. The effect of different ceria structures (ceria reticulated porous foam, packed-bed ceria powder, and blend of ceria mixed with inert Al 2 O 3 particles), CH 4 flow-rate, reduction temperature, and sintering temperature on the bed-averaged oxygen non-stoichiometry (δ), methane conversion, syngas production yield, and reactor performance was experimentally investigated and evaluated. A comprehensive experimental analysis of the solar process performance outputs was performed, encompassing quantification of gas yields produced during each step and by each side reaction, amounts of oxygen transferred during the redox process as well as energy conversion efficiencies. Besides, cycling stability of the ceria structure for the combined twostep process performed in this solar reactor was also assessed.

Experimental set up and methods

The solar reactor design and auxiliary components are schematically shown in Fig. 1.

The solar reactor concept is based on a directly-irradiated packed-bed solar absorber Three temperatures are measured by B-type thermocouples inside the alumina wool (T 1 ), in the middle of ceria structure (T 3 ), and at the external cavity wall surface (T 2 ). An optical pyrometer (operating at 4.8-5.2 µm in a H 2 O absorption band) is also utilized to directly measure the uppermost sample surface temperature through a fluorine window. The cavity pressure (P) is measured by a pressure transducer. Samples, either loose particles or foam (Fig. S1), are placed on the alumina wool support inside the cavity receiver directly exposed to concentrated solar irradiation. The ceria materials preparation is described in Supporting Information (materials synthesis). CH 4 , CO 2 , and Ar (gases purity of 99.999%) flow-rates are regulated by electronic Mass Flow Controllers (MFC, Brooks Instruments model SLA5850S, range 0-5 Nl/min ±0.2% of full scale), and liquid water is also supplied by a MFC (range 0-30 g/h ±1% of full scale). Reacting gases and Ar carrier gas are injected through the single inlet port at the cavity bottom. In addition, Ar protective gas is fed directly into the window area by two stainless tubes inserted in the insulation layer and subsequently enters the cavity via the aperture in the downward direction for preventing the hot gas contact with the transparent window. All the product gases with Ar exit the reactor via a single outlet port positioned at the upper cylindrical part of the cavity. They subsequently flow into a bubbler to condense steam and then through a gas filtering unit (two micro filters with 0.1 µm pore diameter) to remove moisture and solid carbon particles prior to gas analysis. Product syngas composition is monitored by an on-line syngas analyzer (GEIT 3100, uncertainty <±0.1% of full scale). Finally, all the measured data are recorded by an automated data acquisition system (BECKHOFF).

Experiments were performed at the focus of a vertical axis high-flux solar furnace of CNRS-PROMES, Odeillo. The reactor cavity was primarily flushed with Ar and simultaneously sucked by a Venturi pump to purge residual air from the system and maintain the pressure at ~0.9 bar (P atm = ~0.85 bar at site elevation 1,500 m above sea level).

Subsequently, the reactor was progressively solar-heated to the targeted reduction temperature. The solar power input was controlled by means of shutter opening to adjust the operating temperature. During heating, the Ar carrier gas (0.2 Nl/min) and Ar protective gas (2 Nl/min) were supplied to the reactor cavity and window area, respectively. S1).

After reaching the desired reduction temperature, the CH 4 flow-rate was delivered along with Ar carrier gas to drive the reduction reaction, and it was then stopped when H 2 and CO concentrations approached zero. Subsequently, oxidation was performed by injecting the reacting gases (either H 2 O or CO 2 ) at the same temperature. The produced syngas was continuously analyzed, and the flow rate of each gas specie ( i F ) was calculated from their measured mole fraction (y i ) and the known inlet flow rate of Ar (F Ar ):

(𝐹 𝑖 = 𝐹 𝐴𝑟 • 𝑦 𝑖 𝑦 𝐴𝑟 ⁄ ).
Then, the averaged oxygen non-stoichiometry in ceria (δ) and syngas yields were quantified by time-integration of the measured gas production rates over each cycle. The performance metrics of the solar reactor were determined from the measured gas production and solar power input.

In the first step, during partial oxidation of methane with ceria, the reaction of CH 4 with excess surface oxygen is also possible (especially at the beginning of the reaction when the amount of surface oxygen is maximum), then leading to the formation of H 2 O and CO 2 (by the following reaction: 4CeO 2 +δCH 4 ⟶4CeO 2-δ +δCO 2 +2δH 2 O). Thus, oxygen is recovered in the forms of CO, CO 2 , and H 2 O (twice the amount of CO 2 ). Therefore, the oxygen nonstoichiometry (δ red ) can be determined by:

𝛿 𝑟𝑒𝑑 = 𝑛 𝐶𝑂 +2𝑛 𝐶𝑂 2 +𝑛 𝐻 2 𝑂 𝑛 𝐶𝑒𝑂 2 (4)
Where n i are the mole amounts of species i.

The replenished oxygen (δ ox ) during ceria oxidation with H 2 O (Eq. ( 2)) can be calculated from the total amount of produced H 2 from which the amounts of H 2 produced by the reactions of carbon with H 2 O are subtracted (C+H 2 O⟶CO+H 2 and C+2H 2 O⟶CO 2 +2H 2 ):

𝛿 𝑜𝑥 = 𝑛 𝐻 2 -𝑛 𝐶𝑂 -2𝑛 𝐶𝑂 2 𝑛 𝐶𝑒𝑂 2 (5)
When using CO 2 as oxidant (Eq. ( 3)), the replenished oxygen (δ ox ) is calculated by the mass balance of oxygen:

𝛿 𝑜𝑥 = 2𝑛 𝐶𝑂 2,𝑖𝑛 -𝑛 𝐶𝑂 ,𝑜𝑢𝑡 -2𝑛 𝐶𝑂 2,𝑜𝑢𝑡 𝑛 𝐶𝑒𝑂 2 (6)
The performance metrics of the solar reactor for the chemical-looping process are encompassing methane conversion, solar-to-fuel energy conversion efficiency, and energy upgrade factor.

The solar-to-fuel energy conversion efficiency ( solar-to-fuel ) is defined as the ratio of the total chemical energy content of the produced syngas to the total energy input (including solar power input in both the reduction and oxidation steps and heating value of the converted methane):

𝜂 𝑠𝑜𝑙𝑎𝑟-𝑡𝑜-𝑓𝑢𝑒𝑙 = (𝑚̇𝐻 2 •𝐿𝐻𝑉 𝐻 2 +𝑚̇𝐶 𝑂 •𝐿𝐻𝑉 𝐶𝑂 ) 𝑐𝑦𝑐𝑙𝑒 𝑃 ̇𝑠𝑜𝑙𝑎𝑟 +(𝑋 𝐶𝐻 4 •𝑚 ̇𝐶𝐻 4 •𝐿𝐻𝑉 𝐶𝐻 4 ) (7)
where ).

LHV
The energy upgrade factor (U) is obtained by the ratio of the energy contained in the outlet gas species to the energy content of the inlet flow:

U= (𝑚̇𝐻 2 •𝐿𝐻𝑉 𝐻 2 +𝑚̇𝐶 𝑂 •𝐿𝐻𝑉 𝐶𝑂 ) 𝑐𝑦𝑐𝑙𝑒 +((1-𝑋 𝐶𝐻 4 )•𝑚̇ 𝐶𝐻 4 •𝐿𝐻𝑉 𝐶𝐻 4 ) (𝑚̇𝐶 𝐻 4 •𝐿𝐻𝑉 𝐶𝐻 4 ) (8)
All the operating conditions and experimental results for 44 runs with 6 different ceria samples performed in the solar reactor are summarized in Supporting Information (Table S1).

Results and discussion

Influence of sintering temperature for ceria foams

In order to experimentally study the effect of calcination temperature during ceria foam elaboration on the evolved syngas yield, two ceria reticulated porous foams were prepared and subsequently annealed under air at 1000 °C (for 6 h) and one of them was further sintered at 1400 °C (for 2 h) for densifying the structure. Thus, the obtained sintered foams were labeled as ST-1000 and ST-1400, respectively. The initial volumes of ST-1000 and ST-1400 were 46.76 and 83.13 cm 3 (after ceria coating process), and they were shrunk to 30.88 cm 3 (Fig. 3a, ST-1000) and 31.10 cm 3 (Fig. 3b, ST-1400) after heat treatment, representing a decrease of 34% and 63% of their initial volume, respectively. The final properties of the ceria foams are: porosity: 91.8 and 89.1 %, mean cell size: 3.5 and 2.5 mm, and apparent density: 0.595 and 0.780 g/cm 3 for ST-1000 and ST-1400, respectively. Both ceria foams were cycled in the temperature range 900-1050 °C to experimentally study the influence of temperature on syngas evolution as well as reactor performance.

Fig. 4 shows the syngas production rate along with nominal reactor temperature during ceria foam reduction (ST-1400) in the range 900-1050 °C (CH 4 flow-rate: 0. [9]. Moreover, the significant H 2 evolution still continuing after 12 min (Fig. 4e) was the result of the methane cracking reaction that is thermodynamically favorable at high temperature (>1000 °C) [23].

During oxidation step, the H 2 production rate increased with temperature while the CO and CO 2 production rates were negligible over the considered temperature range, thus demonstrating negligible impact of carbon formation associated with methane cracking for ST-1400. Fig. 5 compares the H 2 , CO, and CO 2 production rates (both reduction and oxidation steps) of ceria foam ST-1400 to those of ST-1000 at an operating cycle temperature of 1000 °C. During reduction step (Fig. 5a), increasing the sintering temperature (ST-1400) decreased syngas production rates while increasing the operating duration. This is because the high densification of the structure (at high sintering temperature of 1400 °C) leads to a decline of the porosity and hinders the access of the reacting gas to the solid surface and hollow struts of the foam. Likewise, higher sintering temperature (ST-1400) also led to lower reaction rates during oxidation step (Fig. 5b). Decreasing the sintering temperature (ST-1000) favored the oxidation rate and the formation of CO/CO 2 (because the formation of carbon during the reduction step was also favored). Fig. 6a shows that the syngas yields produced during reduction step decreased significantly with sintering temperature at the operating cycle temperatures of 1000 and 1050 °C, thus leading to a decline in δ red (e.g., from 0.36 for ST-1000 to 0.34 for ST-1400 at 1000 °C). Furthermore, the H 2 yield produced from CH 4 cracking reaction (quantified by the total H 2 yield measured by gas analysis minus the H 2 yield produced by the reaction of ceria with methane, which is equivalent to twice the quantity of produced CO, according to Eq.1)

decreased considerably when increasing the sintering temperature (e.g. from 0.67 mmol/g CeO ₂ for ST-1000 to 0.07 mmol/g CeO ₂ for ST-1400 at 1000 °C). This can be explained by the fact that increasing the sintering temperature lowers the available geometrical surface area for the heterogeneous reaction, which declines the surface concentration of adsorbed methane and alleviates the methane cracking reaction.

Likewise, Fig. 6b confirms that the methane cracking reaction is not favored for the sintered ceria foam (ST-1400), as evidenced by a sharp drop in the quantities of CO According to Fig 7a, the reduction yield (X red =δ red /δ max where δ max =0.5 for complete reduction of Ce 4+ into Ce 3+ ), oxidation yield (X ox =δ ox /δ red ), methane conversion (X CH₄ ) and solar-to fuel energy conversion efficiency (η solar-to-fuel ) were decreased when increasing the sintering temperature (e.g., X red , X ox , X CH ₄, and η solar-to-fuel ranging from 71.5%, 98.3%, 46.9%, and 3.8% for ST-1000 to 67.3%, 94.2%, 23.0%, and 2.8% for ST-1400 , respectively, during cycling at 1000°C). In addition, an increase in operating cycle temperature enhanced X red , X CH₄ , and η solar-to-fuel .

Fig. 7b compares the total syngas yields obtained for both reduction and oxidation steps of ceria foams (ST-1000 compared to ST-1400) during cycling at 1000 and 1050 °C.

Increasing sintering temperature considerably decreased total H 2 and CO yields (from 6.48 and 1.93 mmol/g CeO for ST-1000₂ to 5.29 and 1.68 mmol/g CeO₂ for ST-1400, respectively, at 1000 °C); however, the CO 2 and H 2 O yields tended to decrease slightly. A growth in the energy upgrade factor (U) with increasing sintering temperature was observed (e.g., from

1.03 for ST-1000 to 1.08 for ST-1400). This is because a high sintering of the ceria foam lowers the gaseous reactant access to the reactive surface and the ceria bulk reduction, thus increasing the unreacted CH 4 (much lower X CH₄ for ST-1400, Fig. 7a) and thereby leading to an increase in U. From these observations, it can be summarized that decreasing sintering temperature enhanced syngas selectivity; however, at the expense of weakened structure, with reduced thermo-mechanical resistance.

Influence of methane flow-rate and ceria macrostructure on syngas yield

The impact of methane flow-rate on syngas yields was experimentally studied with different ceria structures. Three ceria structures consisting of pure ceria powder (25.0052 g, bulk density: 1.12 g/cm 3 supply rate. In other words, when a lack of oxygen at the surface occurs, then carbon deposition is fastened, which is increasingly favored as oxygen is being depleted during the ceria reduction progress. It is interesting to highlight that when accounting for H 2 produced by CH 4 cracking reaction (Fig. 8b), the H 2 yield increased steeply (3.25-6.54 mmol/g CeO₂ for ceria foam) over the considered range, thus confirming that the CH 4 cracking reaction is favored when increasing CH 4 flow-rate, thus leading to a sharp increase in H 2 yield along with carbon deposition.

The CO and H 2 productions for each ceria structure were not significantly different, although a slightly higher H 2 and CO production was noticed for ceria powder (presumably due to non-uniform heating of the bed pyrometer >T 3 ) as evidenced by Fig. S2 for ceria powder and Fig. S3 for ceria powder mixed with inert Al 2 O 3 promoter). This advantageously confirms that the shaping of ceria as foam does not downgrade the reactivity. During subsequent ceria oxidation with H 2 O at 1000 °C (Fig. 9), H 2 yield (produced by Eq. 2) increased in accordance with an increase in CH 4 flow-rate during the reduction step, while the CO and CO 2 yields (produced by side reactions) also rose due to carbon deposition increase (Fig. 9a). For instance, the yields of H In order to emphasize the influence of CH 4 flow-rate on reactor performance, the evolution of the relevant metrics (δ red and δ ox , X red , X ox , X CH ₄, U, and η solar-to-fuel ) is presented in Fig. 10. Both δ red (Fig. 10a) and δ ox (Fig. 10b) are enhanced with a CH 4 flow-rate increase (e.g., maximum δ red and δ ox of 0.41 and 0.39 at 0.4 Nl/min, respectively, for ceria powder).

Besides, the δ ox values were consistent with δ red for any ceria structures, thereby confirming complete ceria re-oxidation with H 2 O. As expected, the δ red (Fig. 10a) of ceria powder (0.37-0.41) was slightly higher than those of other materials, in agreement with the higher syngas yields during reduction (Fig. 8). The δ ox values were not different (Fig. 10b) (0.34-0.39 for ceria powder and 0.35-0.37 for ceria reticulated foam), in agreement with the similar syngas yields during oxidation (Fig. 9). X red grew with increasing CH 4 flow-rate (Fig. 10c), e.g. from 74.7% at 0.1 Nl/min to 81.3% at 0.4 Nl/min for ceria powder, and no significant influence of ceria structure on X red can be observed. X ox values were close to ~100% for any ceria structures, thereby demonstrating complete ceria re-oxidation. X CH₄ was reduced noticeably with increasing CH 4 flow-rate for each ceria structure (Fig. 10d) (e.g., from 76.4 to 43.0% at 0.1-0.4 Nl/min, respectively, for ceria powder). The decrease in X CH₄ is attributable to the CH 4 supply rate that exceeds the rate of oxygen released by ceria, as noticed by a stable profile in X red at 0.3-0.4 Nl/min (Fig. 10c). U tended to decrease with CH 4 flow-rate (Fig. 10e).

This variation is attributed to the carbon formation increase with CH 4 flow-rate, and partial entrainment out of the reactor cavity, as confirmed by the presence of carbon particles in the filtering unit (Fig. S4), thus losing their heating value and lowering U. As expected, η solar-to-fuel rose with CH 4 flow-rate as a result of the substantial syngas yield improvement (Figs. 8 and9), and the highest η solar-to-fuel was obtained for ceria foam (3.1-5.6%), followed by CeO 2 -Al 2 O 3 blend (3.6-5.0%), and CeO 2 powder (3.1-3.6%), according to Fig. 10f. This can be explained by the different solar power inputs required for different ceria structures (1.06-1.14 kW for ceria powder, followed by 0.96-1.06 kW for CeO 2 -Al 2 O 3 blend, and 0.76-0.86 kW for ceria foam). Ceria foam thus requires lower solar power consumption than CeO 2 -Al 2 O 3 blend and CeO 2 powder. This is because the CeO 2 -Al 2 O 3 blend stands out from its high bed thickness (1.19 cm) and additional Al 2 O 3 heating (Fig. S5), while the CeO 2 powder layer (0.46 cm thickness) shows high opacity, which is not suitable for efficient radiative heat transfer in the whole volume. Both issues thus lead to temperature gradient, as evidenced in both Fig. S2 (for CeO 2 power) and Fig. S3 (for CeO 2 -Al 2 O 3 blend), with a higher temperature at the bed uppermost surface (T pyrometer >T 3 ). However, the CeO 2 -Al 2 O 3 blend offers a favored dispersion of ceria powder, which improves the gas flow through the reactive bed and heat transfer, and promotes the syngas yield (thereby leading to higher η solar-to-fuel than for ceria powder). The ceria foam consumed the lowest solar power input (thus enhancing η solar-to-fuel ), arising from the effective heat transfer through the semi-transparent medium and the uniform heating (as evidenced by the narrow temperature gap between ceria surface T pyrometer and T 3 in Fig. 2). In summary, the ceria foam structure is the most efficient in term of heat transfer (as reflected by uniform heating with lower solar power consumption), thereby leading to higher η solar-to-fuel , and it is thus the most suitable for the chemical-looping methane reforming.

Influence of temperature on syngas yield

The influence of temperature on syngas yield was investigated for the different ceria structures at 900, 950, 1000, and 1050 °C (T 3 is the nominal-mentioned temperature for experiments).

Fig. 11 shows the H 2 , CO, and CO 2 yields obtained from ceria reduction with CH 4 as a function of reduction temperature. As expected, both H 2 and CO yields increased sharply, while CO 2 tended to rise minimally with temperature (Fig. 11a), regardless of the ceria structures. For example, H Fig. 13 presents the evolution of δ red , δ ox , X red , X ox , X CH ₄, U, and η solar-to-fuel as a function of temperature for each ceria structure. The temperature increase improved steadily the reduction extent of ceria δ red (Fig 13a), e.g. from 0.24 at 950 °C to 0.39 at 1050 °C for CeO 2 -Al 2 O 3 blend owing to a significant beneficial enhancement of the reduction kinetics (Fig. S6).

δ ox also increased with temperature (ranging between 0.24-0.36 for CeO 2 -Al 2 O 3 blend, Fig. 13b) due to the oxygen vacancies consistently increasing with temperature. The impact of the considered ceria structures on both δ red and δ ox was not significant. δ ox values were similar to δ red values, thus validating complete ceria re-oxidation for any ceria structures. X red rose considerably with temperature (e.g., in the range 48.8-78.0% for CeO 2 -Al 2 O 3 blend), while X ox remained quite constant at ~100% for any ceria structures (Fig. 13c), thereby confirming complete ceria re-oxidation. X CH ₄ rose with temperature (Fig. 13d), and the highest X CH₄ was attained at 1050 °C (77.4% for CeO 2 -Al 2 O 3 blend). This is because the faster rate of oxygen release better matched the constant inlet flow of CH 4 , which leads to X CH₄ increase. The X CH₄ for both ceria foam and CeO 2 -Al 2 O 3 blend was higher than that of ceria powder, presumably due to both better gas flow through the structure and solid/gas contact between ceria and CH 4 .

Moreover, U (Fig. 13e) first increased slightly within 900-950 °C and then decreased above 950 °C, as a result of carbon formation issue. Indeed, the carbon deposition increased with increasing temperature, and some particles escaped from the reactor cavity via gas flow, thus lowering U. This issue can be tackled by decreasing temperature to favor U at the expense of lower syngas yield. η solar-to-fuel (Fig. 13f) was improved by increasing temperature (e.g., in the range of 3.0-4.3% for ceria foam). The lowest η solar-to-fuel values were observed for ceria powder (2.75-2.97% at 950-1000 °C) because of the higher solar power consumption (0.88-0.96 kW for ceria powder compared to 0.67-0.68 kW for ceria foam in the temperature range 950-1000 °C), confirming that ceria foam structure is the most suitable for the solar combined process. Fig. 15 shows the syngas yields quantified from the integration of the measured syngas production rates (Fig. 14) during reduction and oxidation steps. As expected, the CO (CeO 2 +CH 4 ), H 2 (CeO 2 +CH 4 ), H 2 (CH 4 cracking) yields, and δ red increased with temperature, while the CO 2 (CeO 2 +CH 4 ) yield remained the same (Fig. 15a). During oxidation step (Fig. 15b), an increase in the CO yield (2.89-3.12 mmol/g CeO₂ ) was observed when increasing the temperature in the range of 950-1050 °C, thereby enhancing the δ ox (0.37-0. A temperature increase significantly enhanced the reactor performance (Fig. 16a), especially methane conversion (37.2-75.8%). Fig. 16b compares the total syngas yield obtained from ceria cycles (sum of both steps) to the theoretical maximum yield (assuming that δ max =0.

Oxidation step with CO 2 during ceria cycling

Assessment of thermochemical stability during cycling

A pristine ceria reticulated foam (17.0152 g) sintered at 1000 for 6 hr was used (Fig. 17a) to experimentally investigate the cycling stability during 6 consecutive cycles at Fig. 18 shows the syngas yields and reaction extents over six consecutive cycles during ceria reduction with methane (Fig. 18a) and ceria oxidation with H 2 O (Fig. 18b). As expected, the H 2 (CeO 2 +CH 4 ), CO, and CO 2 yields were constant over the whole cycling (ranging between 3.39-3.68 mmol/g CeO ₂ for H 2 , 1.69-1.84 mmol/g CeO ₂ for CO, and 0.05-0.07 mmol/g CeO ₂ for CO 2 , Fig. 18a), thus validating ceria cycling stability. However, both small sintering and cracking lines within the sample were observed after the last cycle (Fig. 17b), presumably due to its weakened structure associated with low sintering temperature (1000 °C). Nevertheless, the redox cycling performance of ceria was not altered as reflected by a minimal fluctuation of reduction extent (δ red in the range of 0.32-0.36). The H 2 (CH 4 cracking) yield fluctuated slightly (0.64-1.16 mmol/g CeO ₂), except for cycle 1 (2.00 mmol/g CeO ₂) in which the H 2 yield and δ red were much higher due to a higher reduction temperature (1050 °C) than the other cycles (the CO and CO 2 formed in the oxidation step were thus also higher, Fig. 18b).

Likewise, the H 2 (CeO except for cycle 1 as mentioned above. Stable patterns in X red , X ox , X CH₄ ,  solar-to-fuel and total syngas yield were consistently noticed (Fig. S7). Thus, the cycling stability of ceria can fairly be validated.

Conclusion

A solar process for methane reforming using solid oxidants has been developed, with final aim of producing syngas according to the following reaction: CH 4 + M x O y  M x O y-1 + CO + 2H 2 . The key advantages of such a process with respect to the conventional process are:

(i) generation of a gaseous mixture suitable for methanol synthesis, (ii) utilization of a solid oxidant instead of gaseous oxygen or steam water, (iii) absence of costly catalysts, and (iv) possible production of H 2 (or CO)-rich gas in a second step enabling the regeneration of the starting oxide. This chemical looping reforming process has been fully demonstrated using -A high sintering temperature (1400 °C) adversely affects the syngas yield, methane conversion, and reactor performance, because of both lowered solid/gas interface area and lattice oxygen mobility, thus decreasing oxygen exchange capacity.

-Increasing the CH 4 flow-rate enhances δ (maximum value up to 0.41 for ceria powder), syngas production rate, and syngas yield. However, a remarkable decrease in CH 4 conversion is concomitantly observed (minimum value as low as 43% for CeO 2 powder).

High CH 4 flow rate also favors CH 4 cracking reaction and carbon deposition, since the rate of methane decomposition exceeds the rate of ceria reduction. Such carbon deposition is not detrimental for the whole process since carbon is gasified in the oxidation step.

-Increasing the temperature (between 900-1050 °C) accelerates the rate of ceria reduction, which in turn significantly enhances the methane conversion (up to 77.4% for CeO 2 -Al 2 O 3 blend) and syngas yield and decreases the reduction step duration. However, it comes at the expense of favoring methane cracking, especially at 1050 °C. The  solar-to-fuel is increased with both CH 4 flow-rate and temperature (values in the range 1.14-5.60%), while the energy upgrade factor up to 1.19 is accomplished with CO 2 as oxidant.

-The shape of ceria materials (packed-bed powder, foam) does not show any significant impact on both the syngas yield and δ but rather on CH 4 conversion and efficiency. The ceria foam shows better performance in terms of volumetric solar radiation absorption and uniform heating with lower solar power consumption compared to the other structures, thereby upgrading  solar-to-fuel (maximum value up to 5.6%). This implies that the foam structure is the most suitable to achieve high specific syngas production with reduced solar energy input.

-The ceria re-oxidation step is always complete (δ ox and δ red are similar), which means it is not kinetically limited, and it depends only on the extent of ceria reduction achieved during the previous reduction step. Ceria reduction (δ red ) is strongly dependent on temperature or methane flow rate, which thus denotes kinetically-controlled reaction rate.

-Stable patterns in the reduction/oxidation extents, syngas yields and  solar-to-fuel during consecutive cycles for the ceria reticulated foam validate excellent thermal cycling stability.

The solar reactor concept is expected to be flexible in processing different ceria structures with varying particle sizes or geometries. Further work should be performed

A 2 -

 2 m parabolic dish solar concentrator with a solar concentration ratio up to 10,551 suns (peak flux density of ~10.5 MW/m 2 for a DNI of 1 kW/m 2 ) is used to concentrate the solar radiation to the focal point.

Fig. 1 .

 1 Fig. 1. Schematic of the 1.5 kW th directly irradiated solar reactor and external components

Fig. 2

 2 represents the evolution of temperatures and cavity pressure in the directly irradiated solar reactor during heating phase, ceria foam reduction with methane, and subsequent oxidation with H 2 O at different operating cycle temperatures. While the cavity pressure remained stable (0.86 bar), the temperature increased gradually from the ambient temperature to the targeted temperature (1000 °C) for 35 min. It then changed in relation to endothermic and exothermic reactions and nominal operating cycle temperature defined, while the cavity pressure was constant (~0.9 bar) all over the cycles. The homogeneous temperature inside the ceria foam as well as the reactor cavity receiver was confirmed by narrow gaps between T 1 (below the foam), T 3 (inside the foam), and T pyrometer (upper surface of the foam) while the external cavity wall temperature was ~150 °C lower than those temperatures.

Fig. 2 .

 2 Fig. 2. Temperatures and pressure evolution in the reactor during heating phase, ceria foam

Fig. 3 .

 3 Fig. 3. Ceria reticulated foam fabricated for the solar cavity receiver: (a) after final heat

Fig. 4 .

 4 Fig. 4. Evolution of the production rates of H 2 , CO, CO 2 and CH 4 in the syngas along with

Fig. 5 .

 5 Fig. 5. Influence of sintering temperature on H 2 and CO production rates during both

Fig. 6 .

 6 Fig. 6. Effect of sintering temperature and cycle operating temperatures on syngas yields, δ red ,

Fig. 7 .

 7 Fig. 7. Effect of sintering temperature and cycle operating temperatures on (a) ceria reduction

Fig. 8 .

 8 Fig. 8. Effect of CH 4 flow-rate on H 2 and CO yields during reduction of different ceria

Fig. 9 .

 9 Fig. 9. Effect of CH 4 flow-rate on H 2 and CO yields during oxidation of different ceria

Fig. 10 .

 10 Fig. 10. Effect of CH 4 flow-rate on (a) δ red , (b) δ ox , (c) reduction yield (X red ) and oxidation

Fig. 11 .Fig. 12 .

 1112 Fig. 11. Effect of temperature on H 2 and CO yields during ceria materials reduction with CH 4

Fig. 13 .

 13 Fig. 13. Effect of temperature on (a) δ red , (b) δ ox , (c) reduction yield (X red ) and oxidation yield

Fig. 14 .

 14 Fig.14. Evolution of the production rates of H 2 , CO, CO 2 , and CH 4 in the syngas along

2 Fig. 15 .

 215 Fig. 15. Effect of temperature on syngas yields, δ red , and δ ox for (a) reduction with CH 4 and

Fig. 16 .

 16 Fig. 16. Effect of temperature on (a) ceria reduction yield, methane conversion, and solar-to-

Fig. 17 .

 17 Fig. 17. Ceria reticulated foam (ST-1000) (a) before the cycling stability test and (b) after six

Fig. 18 .

 18 Fig. 18. Syngas yield and δ for both (a) reduction and (b) re-oxidation of ceria during 6

CeO 2

 2 as the oxygen carrier material in the form of powders and reticulated porous foams within both fixed bed and volumetric solar reactor. Indeed, a directly irradiated 1.5 kW th solar reactor has been successfully operated for solar-driven chemical looping methane reforming and isothermal H 2 O/CO 2 splitting using different ceria structures as oxygen carriers, demonstrating the reliability and flexibility of the combined process towards syngas production. A parametric study was carried out focusing on the influence of sintering temperature of the foam structure, CH 4 flow-rate, operating temperature, type of oxidant (H 2 O or CO 2 ) and ceria structures on averaged oxygen non-stoichiometry (δ), CH 4 conversion, syngas production, reactor performance, and thermochemical cycling stability.

  2 Nl/min, Ar flow rate: 0.2 Nl/min, 50% CH 4 mole fraction). It was followed by subsequent ceria oxidation with H 2 O carried out at the same temperature (H 2 O: 200 mg/min, Ar: 0.2 Nl/min, 55% steam mole fraction at inlet).During reduction step, CO 2 production rate was maximal at the initial stage of the reaction, and it increased with temperature. In fact, H 2 O was also formed simultaneously[9,10,42]; however, it cannot be detected from gas analysis. An increase in the operating cycle temperature promoted both the syngas production rate (especially H 2 and CO) and ceria reduction rate (as evidenced by a shortened reaction duration). The peak rates of CO and H 2 produced were 0.02 and 0.04 Nl/min at 900 °C compared to 0.11 and 0.24 Nl/min at 1050 °C, and the operating duration was 25.4 min at 900 °C compared to 18.4 min at 1050 °C. It is interesting to note that the H 2 /CO mole ratio is constantly ~2 for any reduction temperatures

  (C+H 2 O), CO 2 (C+2H 2 O), H 2 (C+H 2 O), and H 2 (C+2H 2 O) formed by the side reactions (carbon deposit gasification with H 2 O). Note that the H 2 (C+H 2 O) yield is equal to the CO yield measured by gas analysis (C+H 2 O⟶CO+H 2 ), while the H 2 (C+2H 2 O) yield is equal to twice the CO 2 yield measured by gas analysis (C+2H 2 O⟶CO 2 +2H 2 ). In addition, an increase in the sintering temperature decreased the H 2 (CeO 2-δ +H 2 O) yield (e.g., from 2.04 mmol/g CeO₂ for ST-1000 to 1.84 mmol/g CeO₂ for ST-1400 at 1000 °C, then resulting in a decrease of δ ox from 0.35 to 0.32). Noticeably, δ red matched well δ ox values at both cycle temperatures of 1000 and 1050 °C, thereby confirming complete re-oxidation.

  2 , CO, and CO 2 for ceria foam were 2.01, 0.11, and 0.01 mmol/g CeO ₂ at 0.1 Nl/min compared to 2.16, 0.20, and 0.04 mmol/g CeO₂ at 0.4 Nl/min. Likewise, the H 2 yield increased sharper when including the amount of H 2

associated with carbon gasification (e.g. from 2.14 to 2.45 mmol/g CeO₂ at 0.1-0.4 Nl/min for ceria foam), according to Fig.

9b

. However, no significant effect of the ceria structure on H 2 , CO, and CO 2 yields can be evidenced whether or not accounting for H 2 yields from carbon gasification reactions.

  2 , CO, and CO 2 yields rose from 2.38, 1.21, and 0.05 mmol/g CeO₂ at 900 °C to 3.84, 1.92, and 0.09 mmol/g CeO₂ at 1050 °C, respectively, for CeO 2 -Al 2 O 3 blend. This is because increasing temperature accelerates the kinetic rate of ceria reduction with faster oxygen release, as evidenced by Arrhenius plot (Fig.S6). The activation energy obtained for each ceria structure (92.8-114.2 kJ/mol for H 2 and 92.8-95.1 kJ/mol for CO, Table. S2) is consistent with previously reported data[23]. When accounting for the H 2 produced by CH 4 cracking reaction (Fig.11b), the trends of H 2 became steeper, thus pointing out the significant influence of the reduction temperature on the H 2 formation from CH 4 cracking. Noticeably, the H 2 yield at 900 °C (CeO 2 -Al 2 O 3 blend) remained the same (2.38 mmol/g CeO₂ ) whether or not accounting for H 2 produced by cracking reaction, thus indicating that CH 4 decomposition was negligible at 900 °C. In comparison, the ceria powder showed the highest H 2 and CO yields at 950-1000 °C, as a result of the higher bed surface temperature and lower bed height, as previously mentioned.

  5 in Eqs. (1) and (3), thereby yielding both 1 mol CO and 1 mole H 2 per mole CeO 2 ). Increasing temperature promoted the overall H 2 and CO yields with maximum measured values of 5.11 and 5.04 mmol/g CeO₂ at 1050 °C, respectively, while the maximum theoretical H 2 and CO yields that can be expected with CO 2 are 5.81 mmol/g CeO₂ . Note that the amounts of both H 2 and CO produced by side reactions (CH 4 cracking: CH 4 →C+2H 2 and carbon gasification: C+CO 2 →2CO) in both steps were taken into account in Fig.16b. The CO yield was close to the H 2 yield throughout the range. In addition, U was in the range 1.11-1.19 (versus 1.31 for a stoichiometric reaction with δ max =0.5).

  2 (C+2H 2 O), H 2 (C+H 2 O), CO(C+H 2 O), and CO 2 (C+2H 2 O) remained similar

2-δ +H 2 O) yield produced by Eq. (2) (Fig.

18b

) was fairly stable (1.94-2.05 mmol/g CeO₂ ) thus leading to a similar stable δ ox pattern (0.33-0.35). In addition, the quantities of H

Acknowledgement

The King Mongkut's Institute of Technology Ladkrabang (KMITL), Thailand and the Franco-Thai scholarship program are gratefully acknowledged for fellowship granting.

Another ceria powder (27.0892 g) mixed with Al 2 O 3 (50 g) was employed to study the influence of temperature (950-1050 °C) on syngas yield and reactor performance during ceria oxidation with CO 2 . This ceria was first reduced with a constant CH 4 flow-rate of 0.2 Nl/min (50% CH 4 mole fraction at inlet) and subsequently re-oxidized with a constant CO 2 flow-rate of 0.2 Nl/min (50% CO 2 mole fraction at inlet) at the same temperature.

Syngas production rates along with reactor temperature for both steps are presented in Fig. 14. During reduction step, the syngas production rates were higher at 1050 °C than at 1000 and 950 °C. For instance, the peak H 2 and CO production rates were 0.32 and 0.14 Nl/min at 1050 °C compared to 0.21 and 0.10 Nl/min at 950 °C. The reaction duration declined considerably with increasing temperature due to improved ceria reduction kinetics (from 24.4 min at 950 °C to 15.1 min at 1050 °C). During oxidation step, the peak CO production rate increased minimally from 0.14 to 0.16 Nl/min while the oxidation duration decreased slightly (from 26 min at 950 °C to 22 min at 1050 °C). Therefore, the effect of temperature is not significant for the oxidation step with CO 2 , which thus suggests low impact of kinetic reaction control for this step.

regarding reactor upscaling and ceria porous foam structure tailoring to improve the global efficiency of the integrated isothermal solar process combining chemical looping reforming and H 2 O/CO 2 splitting.


German Montes-Hernandez
email: german.montes-hernandez@univ-grenoble-alpes.fr

François Renard

Dr German Montes

Nucleation of Brushite and Hydroxyapatite from Amorphous Calcium Phosphate Phases Revealed by Dynamic in situ Raman Spectroscopy

Keywords: Time-resolved Raman spectroscopy, Nucleation and growth processes, Amorphous calcium phosphate, Brushite, Hydroxyapatite

Since the 1970s, it is admitted that calcium phosphate crystals nucleate from one or several early amorphous calcium phosphate phases into several in vivo and in vitro systems. However, the precise chemical composition, structure and transformation mechanism of these amorphous phases remain controversial. Here, we characterize the reaction mechanism and kinetics of formation of two phosphate crystals, brushite and hydroxyapatite, by using in situ Raman spectroscopy in batch reactors at 25°C. We investigate three pH regimes to control the phosphate speciation in solution and used solutions with or without citric acid, a complexing agent that may stabilize the amorphous phases. As expected, brushite (CaHPO4.2H2O) forms at pH < 9.8. Amorphous calcium phosphate (ACP: CaHPO4.nH2O) with short lifetime (<2 minutes) and octocalcium phosphate (OCP: Ca8(HPO4)2(PO4)4.5H2O) are the main transient phases prior to brushite nucleation that occurs after ~8 minutes. At pH > 11, hydroxyapatite (HAp: Ca10(PO4)6(OH)2) nucleates after >35 minutes, depending on the experimental conditions. The reaction mechanism steps for hydroxyapatite are more complex compared to brushite. For hydroxyapatite formation, amorphous calcium phosphate phases with different chemical composition (Ca(HPO4)1-x(PO4)(2/3)x.nH2O, with x in the range 0.2-1, and with different lifetime may form. Amorphous tricalcium phosphate (ATCP: Ca(PO4)2/3.nH2O or Ca3(PO4)2.nH2O when x=1) is the most persistent phase which can transform either into OCP and then hydroxyapatite, or directly evolve into hydroxyapatite at pH > 12.2. The presence of citric acid retards the transformation kinetics by increasing the nucleation times of brushite and hydroxyapatite, but has little effect on the reaction mechanism steps. Finally, this study identifies new reactive pathways that characterize the formation of amorphous calcium phosphate phases and their transformation into brushite microcrystals or hydroxyapatite

nanocrystals under abiotic conditions, relevant to a wide range of technological applications and natural environments.

TOC Graphic

Synopsis: The nucleation of brushite and hydroxyapatite was monitored in real-time by Raman spectroscopy at 25 °C. For both minerals, amorphous calcium phosphate (Ca(HPO4)1x(PO4)(2/3)x.nH2O) with different Raman signature instantaneously formed in the initial solution.

Then, the amorphous phases transform into OCP, a crystalline transient phase. Finally, OCP transforms into brushite micro-crystals or into hydroxyapatite nanocrystals.

Introduction

Calcium phosphates (e.g. hydroxyapatite, brushite, octo-calcium phosphate and amorphous calcium phosphates) are critically important biogenic minerals constituting hard tissues (bones, teeth and cartilages) in vertebrates. Phosphorus (P) and calcium (Ca) are also essential elements for the development of living organisms. Moreover, calcium phosphates find wide medical and industrial applications in dentistry, orthopedics, reconstructive surgery, food additives, fertilizers, and water depollution [START_REF] Wang | Biomineralization of calcium phosphate revealed by in situ liquid-phase electron microscopy[END_REF][START_REF] Niu | Sustained delivery of calcium and orthophosphate ions from amorphous calcium phosphate and poly(L-lactic acid)based electrospinning nanofibrous scaffold[END_REF][START_REF] De Aza | Vibrational Properties of Calcium Phosphate Compounds. 1. Raman Spectrum of β-Tricalcium Phosphate[END_REF][START_REF] Combes | Amorphous calcium phosphates: Synthesis, properties and uses in biomaterials[END_REF][START_REF] Eliaz | Calcium Phosphate Bioceramics: A Review of Their History, Structure, Properties, Coating Technologies and Biomedical Applications[END_REF][START_REF] Bohner | Periodic Nucleation of Calcium Phosphate in a Stirred Biocatalytic Reaction[END_REF]. The formation of calcium phosphates and the interactions and associations of these minerals with organic macromolecules have been widely investigated under in vivo and in vitro conditions in the last decades [START_REF] Cross | Casein Phosphopeptide-Amorphous Calcium Phosphate Nanocomplexes: A Structural Model[END_REF][START_REF] Lotsari | Transformation of amorphous calcium phosphate to bone-like apatite[END_REF](9)[START_REF] Zhang | In vitro and in vivo evaluation of electrophoresis-aided casein phosphopeptide-amorphous calcium phosphate remineralization system on pH-cycling and acid-etching demineralised enamel[END_REF] in order to identify the nature of first primary precipitating particles, their lifetime and their chemical transformation into other minerals in aqueous media (11-20). For example, the formation of hydroxyapatite nanocrystals (Ca10(PO4)6(OH)2) under biotic or abiotic conditions involves the early formation of a transient amorphous calcium phosphate phase (ACP) (11)[START_REF] Habraken | Ion-association complexes unite classical and non-classical theories for the biomimetic nucleation of calcium phosphate[END_REF][START_REF] Lu | Short-Range Structure of Amorphous Calcium Hydrogen Phosphate[END_REF][START_REF] Tao | Evolution of Amorphous Calcium Phosphate to Hydroxyapatite Probed by Gold Nanoparticles[END_REF][START_REF] Ucar | Formation of Hydroxyapatite via Transformation of Amorphous Calcium Phosphate in the Presence of Alginate Additives[END_REF][START_REF] Boskey | Conversion of Amorphous Calcium Phosphate to Microcrystalline Hydroxyapatite. A pH-Dependent, Solution-Mediated, Solid-Solid Conversion[END_REF](17)[START_REF] Jiang | Effect of the aggregation state of amorphous calcium phosphate on hydroxyapatite nucleation kinetics[END_REF][START_REF] Kim | The role of confined collagen geometry in decreasing nucleation energy barriers to intrafibrillar mineralization[END_REF](20)[START_REF] Terraschke | In situ luminescence analysis: a new light on monitoring calcium phosphate phase transitions[END_REF][START_REF] Cadez | Amorphous Calcium Phosphate Formation and Aggregation Process Revealed by Light Scattering Techniques[END_REF][START_REF] Vecstaudza | Amorphous calcium phosphate materials: Formation, structure and thermal behavior[END_REF][START_REF] Wang | Crystallization at Multiple Sites inside Particles of Amorphous Calcium Phosphate[END_REF][START_REF] Zhang | In Situ Detection of Calcium Phosphate Clusters in Solution and Wet Amorphous Phase by Synchrotron X-ray Absorption Near-Edge Spectroscopy at Calcium K-Edge[END_REF][START_REF] Uskoković | Disordering the Disorder as the Route to a Higher Order: Incoherent Crystallization of Calcium Phosphate through Amorphous Precursors[END_REF]. This phase may either (1) transform directly by a solid-state transformation, [START_REF] Niu | Sustained delivery of calcium and orthophosphate ions from amorphous calcium phosphate and poly(L-lactic acid)based electrospinning nanofibrous scaffold[END_REF] dissolve and reprecipitate to produce nanocrystalline hydroxyapatite, or (3) nucleate inside other amorphous phases (e.g. 9, 11, [START_REF] Boskey | Conversion of Amorphous Calcium Phosphate to Microcrystalline Hydroxyapatite. A pH-Dependent, Solution-Mediated, Solid-Solid Conversion[END_REF][START_REF] Jiang | Effect of the aggregation state of amorphous calcium phosphate on hydroxyapatite nucleation kinetics[END_REF][START_REF] Wang | Crystallization at Multiple Sites inside Particles of Amorphous Calcium Phosphate[END_REF]. The first theory of phosphate formation from precursor amorphous phases was proposed by Posner and Betts (11) who observed pre-nucleation clusters of Ca9(PO4)6 with a size close to ~1 nm (so-called 'Posner's cluster'). These authors proposed that the clusters not only could be the building blocks for ACP but also could play an important role during the transformation of ACP into hydroxyapatite, suggesting that ACP only dissociates into these clusters rather than undergoing a complete ionic dissolution (11,[START_REF] Boskey | Conversion of Amorphous Calcium Phosphate to Microcrystalline Hydroxyapatite. A pH-Dependent, Solution-Mediated, Solid-Solid Conversion[END_REF]. However, the structure of pre-nucleation clusters during hydroxyapatite formation is still debated (e.g. [START_REF] Habraken | Ion-association complexes unite classical and non-classical theories for the biomimetic nucleation of calcium phosphate[END_REF][START_REF] Lu | Short-Range Structure of Amorphous Calcium Hydrogen Phosphate[END_REF][START_REF] Zhang | In Situ Detection of Calcium Phosphate Clusters in Solution and Wet Amorphous Phase by Synchrotron X-ray Absorption Near-Edge Spectroscopy at Calcium K-Edge[END_REF][START_REF] Uskoković | Disordering the Disorder as the Route to a Higher Order: Incoherent Crystallization of Calcium Phosphate through Amorphous Precursors[END_REF]. For example, crystalline metastable phases such as octo-calcium phosphate (OCP) (Ca8(HPO4)2(PO4)4.5H2O) and brushite (CaHPO4.2H2O) have been proposed to form as transient phases prior to hydroxyapatite (HAp) nucleation. Therefore, a convincing temporal and/or spatial characterization of phosphate formation in natural or synthetic samples is still lacking (e.g. 5, 8, 15, 27-28) despite the use of sophisticated time-resolved measurements and simulations performed in the last two decades, such as in situ X-ray absorption (XAS), in situ small/wide-angle X-ray scattering (SAXS/WAXS), electron transmission microscopy (Cryo-TEM or liquid-cell TEM), atomic force microscopy (liquid-cell AFM) and ab initio numerical simulations based on the density functional theory (e.g. 1, 12-13, 19, 25-27, 29-30). Despite all these available techniques, more knowledge is required to understand how phosphate minerals nucleate and grow from solutions and slurries. In this context, time-resolved Raman spectroscopy measurements provide invaluable information on the nucleation, transformation and growth of particles and crystals, as recently demonstrated for several carbonates [START_REF] Montes-Hernandez | Time-Resolved in Situ Raman Spectroscopy of the Nucleation and Growth of Siderite, Magnesite, and Calcite and Their Precursors[END_REF][START_REF] Montes-Hernandez | Mechanism of formation of engineered magnesite: A useful mineral to mitigate CO2 industrial emissions[END_REF][START_REF] Montes-Hernandez | Amorphous calciummagnesium carbonate (ACMC) accelerates dolomitization at room temperature under abiotic conditions[END_REF] and phosphates minerals [START_REF] Kazanci | Complementary Information on In Vitro Conversion of Amorphous (Precursor) Calcium Phosphate to Hydroxyapatite from Raman Microspectroscopy and Wide-Angle X-Ray Scattering[END_REF][START_REF] Stammeiera | In situ Raman spectroscopy of amorphous calcium phosphate to crystalline hydroxyapatite transformation[END_REF][START_REF] Han | Real-Time Raman Monitoring of Calcium Phosphate Precipitation in a Semi-Batch Stirred Crystallizer[END_REF][START_REF] Yu | An Evolutionarily Conserved Subdomain in Amelotin Promotes Amorphous Calcium Phosphate-to-Hydroxyapatite Phase Transition[END_REF]. This spectroscopy technique allows discriminating between amorphous and crystalline phases that form early during the nucleation and growth of more stable minerals.

In the present study, the nucleation of brushite and hydroxyapatite was monitored in real-time and in situ by using Raman spectroscopy at 25 °C. For both minerals, amorphous calcium phosphate (Ca(HPO4)1-x(PO4)(2/3)x.nH2O) with different Raman signature instantaneously formed as ionic solutions were rapidly mixed under mechanical stirring. Then, the amorphous phases transformed rapidly or slowly into OCP (crystalline transient phase). Finally, OCP transformed into brushite micro-crystals or into hydroxyapatite nanocrystals depending on the experimental investigated conditions. We varied the pH, the HPO4 2-/PO4 3-ratio and the Ca/P ratio. The lifetime of amorphous and crystalline transient phases, reaction mechanisms and kinetics of formation of brushite and hydroxyapatite were then measured. We acquired several ex-situ measurements and images of the products recovered at the end of the experiments in order to characterize the average particles size, crystal shape and mineral composition.

Materials and Methods

Time-lapse Raman spectroscopy

The formation of brushite and hydroxyapatite was monitored using a Raman probe (785 nm infrared laser) immersed into a C22 Hastelloy batch reactor with internal volume of 600 ml. This experimental setup is described in a previous study [START_REF] Montes-Hernandez | Time-Resolved in Situ Raman Spectroscopy of the Nucleation and Growth of Siderite, Magnesite, and Calcite and Their Precursors[END_REF]. Raman spectra were collected with an exposure time of three or two seconds and averaged over three or two scans. The delay between successive acquisitions varied between fifteen seconds and one hour, depending on the investigated reaction. The duration of experiments was arbitrarily imposed from 2h to 24h and all collected spectra were saved on the control computer.

Selected Raman peaks (strongest peak) corresponding to identified mineral phases (see Table S1 and Table 1) were fitted by using simple or combined Gaussian models in order to calculate the peak Full Width at Half-Maximum (FWHM) and the integrated peak surface area as a function of time (Movie S1). The Raman signals were fitted using the following procedure. First, we selected a small region of the entire Raman spectrum that contained only the peaks relevant for the reaction studied. Then, the baseline was removed to correct for temporal fluctuations of the intensity of the signal. Finally, we decomposed the Raman signal into two, three, or four Gaussian curves, depending on the number of species present in the system. For each species, the position of the peak was fixed and the amplitude and standard deviation of each Gaussian was fitted using a nonlinear least squares method (Movies S1, S2, S3). This calculation provides the peak decomposition necessary to propose relevant reaction mechanisms and to quantify the kinetics of formation of calcium phosphates phases in aqueous solutions or slurries, including nucleation time and lifetime of minerals and aqueous species.

Brushite (CaHPO4.2H2O) from a phosphate solution

200 ml of phosphate solution (0.2M of Na2HPO4, pH~9.8) were firstly placed in the reactor under continuous stirring (400 rpm) at a temperature of ~25°C. Raman spectroscopy monitoring reveals a strong peak at 993 cm -1 , characteristics of HPO4 2-, before the addition of a calcium solution (Figure S1a). Then, 200 ml of calcium solution (0.2M of CaCl2, pH~6.8) were rapidly added into the system with a syringe for about one to two minutes leading to the instantaneous formation of a gel-like amorphous calcium phosphate phase with a Raman peak at 957 cm -1 (Figure S1b). The chemical transformation of this amorphous phase to crystalline phases was then monitored for 23h with an acquisition frequency of one Raman spectrum every fifteen seconds during the first twenty minutes and every minute or five minutes in the remaining time. The same experiment was performed in presence of 1g of citric acid (C6H8O7), initially dissolved in the calcium solution in order to assess the effect of this acid on the kinetics and reaction mechanism. The citric acid is used here as a complexing agent that is expected to stabilize transient amorphous phosphate phases for longer durations than without acid. All chemical reactants with high purity (>99%) were provided by Sigma-Aldrich.

Hydroxyapatite from Na2HPO4-Ca(OH)2 slurry

200 ml of deionized water (18.2 megaohm of resistivity) and 3g of Ca(OH)2 (>96% of purity provided by Sigma-Aldrich) were placed in the reactor under continuous stirring (400 rpm) at a temperature of ~25°C. Direct Raman spectroscopy monitoring reveals a peak at 362 cm -1 , characteristics of calcium hydroxide, before the addition of a phosphate solution (Figure S2). After the addition of 200ml of a phosphate solution (0.2M of Na2HPO4, pH~9.8), the calcium hydroxide progressively dissolves (Figure S2) while hydroxyapatite forms under an alkaline pH higher than 12, constrained by the release of OH -ions, as shown in the following overall reaction:

10Ca(OH)2 + 6HPO4 2- Ca10(PO4)6(OH)2 + 12OH -+ 6H2O (1)
This overall reaction involves the formation of amorphous and crystalline transient phases that were monitored in real-time by Raman spectroscopy for 18h with an acquisition frequency of one Raman spectrum every fifteen seconds during the first twenty minutes, and every minute or five minutes in the remaining time. The same experiment was performed in presence of 1g of citric acid (C6H8O7), initially dissolved in the calcium hydroxide slurry

Hydroxyapatite from ionic solutions

200 ml of phosphate solution (0.2M of PO4 3-/HPO4 2-, pH~12.4 adjusted with NaOH) were firstly placed in the reactor under continuous stirring (400 rpm) at a temperature of ~25°C. Direct Raman spectroscopy monitoring confirms the presence of peaks for HPO4 2-at 992 cm -1 and PO4 3-at 939 cm -1 , before the addition of a calcium solution (Figure S3a). Then, 200 ml of calcium solution (0.33M of CaCl2) were rapidly added into the system with a syringe for about 1-2 minutes. Similar to the brushite experiment, the fast mixing of these two ionic solutions led to the instantaneous formation of a gel-like phase that contained an amorphous calcium phase with a Raman peak at 951 cm -1 (Figure S3b). The lifetime and chemical transformation of this gel into crystalline phases were monitored in real-time for 23h with an acquisition frequency of one Raman spectrum every fifteen seconds during the first twenty minutes and every minute or five minutes in the remaining time. The overall reaction to form hydroxyapatite can be written as follows:

2HPO4 2-+ 4PO4 3-+ 2OH -+ 10Ca 2+  Ca10(PO4)6(OH)2 + 2H + (2)
In contrast to the formation of hydroxyapatite from a calcium hydroxide slurry (reaction 1), the synthesis of hydroxyapatite using ionic solutions releases protons, which produces a decrease of pH during the experiment from ~12.4 to ~11.

In summary, three different pH regimes were investigated in this study using equimolar Ca/P ratio or hydroxyapatite stochiometric ratio (Ca/P=1.66) and two different Ca sources (CaCl2 and Ca(OH)2). The pH in the solutions and slurries was systematically measured at the beginning and the end of each experiment. We used a conventional pH meter (Mettler Toledo MA235). However, the in-situ pH variations during precipitation experiments were not recorded. Table 1 summarizes all experiments performed in the present study. Each experiment was repeated at least three times to verify the reproducibility of the results.

Ex situ characterization of precipitates

At the end of each experiment, the solid product was recovered by centrifugation and washed twice with ultrapure water and once with ethanol. Then, it was dried at 60°C for 48h. The dry solid products were stored in plastic flasks for subsequent characterization of selected samples by field emission gun scanning electron microscopy (FESEM) and powder X-ray diffraction (XRD).

For high resolution imaging, the solid products were dispersed by ultrasonic treatment in absolute ethanol for five to ten minutes. One or two droplets of the suspension were then deposited directly on an aluminum support and coated with gold and platinum. The morphology of the crystals was imaged using a Zeiss Ultra 55 field emission gun scanning electron microscope (FESEM) with a maximum spatial resolution of approximately 1nm at 15kV.

XRD analyses were performed using a Siemens D5000 diffractometer in Bragg-Brentano geometry, equipped with a theta-theta goniometer with a rotating sample holder. The XRD patterns were collected using Cu kα1 (kα1=1.5406 Ǻ) and kα2 (kα2=1.5444 Ǻ) radiations in the range 2 = 10 -70° with a step size of 0.04° and a counting time of six seconds per step.

Results and discussion

Brushite formation: Reaction mechanism and kinetics

The quick mixing of two equimolar ionic solutions, one containing HPO4 2-(0.2M, pH~9.8) and the other containing Ca 2+ (0.2M, pH~6.8), leads the instantaneous formation of an amorphous calcium phosphate (ACP: CaHPO4.nH2O) with a peak at 957 cm -1 (Figure S1b). This Raman signature is transient with a lifetime of about two minutes. Then, ACP transforms into octo-calcium phosphate (OCP: Ca8(HPO4)2(PO4)4.5H2O) with a peak at 962 cm -1 . This crystalline phase nucleates from ACP after two minutes and its maximum Raman intensity is reached after five minutes. The OCP is also a transient phase that progressively transforms into brushite (CaHPO4.2H2O), with a peak at 990 cm -1 , after a nucleation time of 7.5 minutes. Brushite reaches a maximum Raman intensity after 24 minutes. The progressive increase of the brushite Raman peak is proportional the decrease of OCP Raman intensity until the total consumption of OCP. This sequence of reactions and kinetics are summarized in Figure 1, Figure S4 and Movie S1. Based on these data, the following reaction mechanism is proposed:

1. Instantaneous formation of ACP:

HPO4 2-+ Ca 2+ + nH2O  CaHPO4.nH2O (3)
Amorphous calcium phosphate containing only HPO4 2-(Ca/P =1) is assumed at this investigated pH regime where only HPO4 2-ions may exist in solution. However, the incorporation of phosphate into the amorphous phase (Ca(HPO4)1-x(PO4)(2/3)x.nH2O) and the removal of protons (H + ) at the solid-fluid interface are not excluded.

2. Transformation of ACP into OCP via a solid-state crystallization pathway:

8CaHPO4.nH2O + 5H2O  Ca8(HPO4)2(PO4)4.5H2O + 2H2PO4 -+ 2H + + 8nH2O (4)
This reaction implies the protonation of residual phosphate ions as confirmed by the peaks at 880 and 1080 cm -1 assigned to H2PO4 -(Figure S5 and Table S1). This means that a significant decrease of pH (probably, pH<6.7) takes place in first minutes that favor then the OCP transformation into brushite as directly monitored by Raman spectroscopy (Figure 1) and described below by reaction 5.

In addition, a solid-state crystallization is assumed because the overall transformation of ACP into OCP takes place abruptly in less than fifteen seconds, which corresponds to the time resolution in our experiments, as illustrated in Figure 1.

3. Transformation of OCP into brushite via a dissolution-recrystallization pathway:

Ca8(HPO4)2(PO4)4.5H2O + 2H2PO4 -+ 2H + + 11H2O  8CaHPO4.2H2O (5)
Dissolution-recrystallization pathway is here assumed because the OCP transient phase is progressively consumed by the formation and growth of brushite (Figure 1). This interpretation is supported by a temporal increase the FWHM of the main OCP Raman peak until its total consumption (Figure S6). This result can be interpreted by an average decrease of the crystals size, z, of OCP with z  1/FWHMOCP, due to a progressive dissolution process. Conversely, the temporal decrease of FWHM for brushite formation indicates a crystal growth process.

Ex-situ characterization of solids recovered at the end of experiments by FESEM and XRD confirmed the presence of elongated microcrystals (>20m) of brushite (see Figure S7).

In the presence of citric acid, the reaction mechanism remains very similar because the reactions 3 to 5 occur and the only measurable difference is a very slight transformation of brushite into hydroxyapatite confirmed by XRD in the recovered solid product (experiment 2 in Table 1 and Figure S8). In presence of citric acid, the transformation kinetics, and mainly the transformation of OCP into brushite (reaction 5), are significantly retarded from 15 (Figure 1) to 400 minutes (Figure 2). Conversely, the formation of ACP (reactions 3) and the transformation of ACP into OCP (reaction 4) are only slightly retarded, as displayed in Figure 2.

Hydroxyapatite formation from Ca(OH)2 slurry: Reaction mechanism and kinetics

In this case, a phosphate ionic solution (0.2M of HPO4 2-, pH~9.8) was rapidly added to a Ca(OH)2 slurry (0.015g/gH2O, pH~12.2). As expected, an amorphous calcium phosphate (ACP1) with a peak at 954 cm -1 forms instantaneously in the first fifteen seconds, and has a lifetime of less than one minute. The ACP1 rapidly transforms into another amorphous calcium phase with a peak at 943 cm -1 (ACP2), a transformation accompanied by a transient release of HPO4 2-ions with a peak at 993 cm -1 (Figure 3). Based on this result, we assume that the shift of the peak position of the calcium amorphous phases is exclusively related to the HPO4 2-or PO4 3-structural content into the amorphous phase (Ca(HPO4)1-x(PO4)(2/3)x.nH2O; with x that varies between 0 and 1). Therefore, the calcium amorphous phase can show a Raman signature with broad peaks between 943 cm -1 for Ca3(PO4)2.nH2O (ATCP), and 957 cm -1 for CaHPO4.nH2O (ACP). This result is illustrated in Figure S9, where the parameter x can be calculated by a linear interpolation of the peak position in the interval 943-957cm -1 (x=68.357 -(1/14)*PACP: where PACP is the position in cm -1 comprised between 943 and 957). Then, the ATCP slowly transforms into hydroxyapatite with a peak at 963 cm -1 . The OCP clearly detected in the brushite systems as a transient phase (experiments 1 and 2)

is also expected to form in more alkaline systems. However, the strongest Raman peak of OCP (962 cm -1 ) is very close to the main peak of hydroxyapatite (963 cm -1 ). For this reason, the OCPlike layers transient phase is also considered in the deconvolution peak fitting, as illustrated in the Movie S2, and in the kinetics behavior summarized in Figure S10. Note that the OCP-like layers formation in this particular case remains speculative because we did not detect this phase in the solid product after the experiment. However, we needed this phase to fit the Raman data. Based on these time-resolved Raman spectroscopy measurements, the following reaction mechanism is suggested:

1. Instantaneous formation of ACP1: HPO4 2-+ Ca 2+ + nH2O  Ca(HPO4)0.8(PO4)0.13.nH2O + 0.07PO4

Solid-state transformation is assumed because ACP1 has a short lifetime, less than one minute, and its transformation into ATCP takes place abruptly, as shown in Figure 3. The phase Ca(PO4)2/3.nH2O is equivalent to Ca3(PO4)2.nH2O reported in the literature and is constituted mainly by the so-called Posner's clusters Ca9(PO4)6 with ~1 nm size (11). The reaction 7 implies a release of HPO4 2-ions and protons (H + ) that are also detected by Raman spectroscopy (Figure 3).

These ions rapidly react to form OCP as illustrated in the following reaction, and residual phosphate ions are transformed to PO4 3-, the main phosphate aqueous species in high alkaline aqueous media.

Residual protons (H + ) are neutralized to water (H + + OH - H2O).

Transformation of ATCP to OCP-like layers:

8Ca(PO4)2/3.nH2O + 0.67HPO4 Both reactions (9 and 10) participate to the formation of hydroxyapatite, as shown by the temporal behavior of the Raman intensity of selected peaks (Figure 4) and the integrated peak surface areas of concerned phases (Figure S10 and Movie S2). However, the low temporal variation of the FWHM parameter (Figure S11) does not allow to conclude if the reactions takes place via a dissolution-recrystallization pathway, as clearly deduced for brushite system. In this case, partial dissolution, i.e. conserving Ca9(PO4)6 clusters of ~1 nm size as building primary units, combined with self-assembly aggregation could operate to form hydroxyapatite, as suggested in previous studies (11 -12, 14, 16, 18). Unfortunately, Raman dynamic in situ monitoring cannot provide clear or complementary information on this specific important interfacial process.

Ex-situ characterization of solids recovered after the experiments by FESEM and XRD confirm the presence of acicular nanocrystals of hydroxyapatite, with size <100nm, and, probably, some residual amorphous calcium phosphate, as attested by the presence of very fine particles with size < 20nm (Figure S12).

Concerning the effect of citric acid, Raman data reveal that the reaction mechanism remains very similar because the reactions 6 to 10 can be confirmed. The main difference concerns reaction 6

where the transient amorphous calcium carbonate (ACP1) has a Raman signature at 957cm -1 , and therefore contains only HPO4 2-in its structure (CaHPO4.nH2O), as illustrated in Figure S9. The other difference is that the transformation kinetics of transient phases into hydroxyapatite, including the nucleation time (reactions 8-10) is significantly retarded, as illustrated in Figure 5.

Conversely, the formation of ACP1 (reaction 6) and the transformation of ACP1 to ATCP (reaction 7) are slightly retarded only.

Hydroxyapatite formation from mixture of ionic solutions: Reaction mechanism and kinetics

The quick mixing of two ionic solutions with hydroxyapatite stochiometric ratio, Ca/P=1.66, one containing 0.2M of PO4 3-/HPO4 2-(pH~12.4 adjusted with NaOH) and the other containing Ca 2+ (0.33M, pH~6.8), leads to the instantaneous formation of amorphous calcium phosphate (ACP: Ca(HPO4)1-x(PO4)(2/3)x.nH2O; x=0.43) with a peak at 951cm -1 (Figure 6). This Raman signature is transient with a short lifetime of three to four minutes. The total lifetime of ACP is about 30 minutes, and the position of its Raman peak varies from 951 cm -1 (x=0.43) to 954 cm -1 (x=0.21) prior to its transformation into OCP. Then, OCP is transformed into hydroxyapatite with a nucleation time of 36 minutes (Figure 6). This result is supported by the deconvolution fitting of selected Raman peaks (Figure S13 and Movie S3). The formation of OCP-like layers in this case remains speculative, but in agreement with literature [START_REF] Eliaz | Calcium Phosphate Bioceramics: A Review of Their History, Structure, Properties, Coating Technologies and Biomedical Applications[END_REF][START_REF] Simon | First evidence of octacalcium phosphate@osteocalcin nanocomplex as skeletal bone component directing collagen triple-helix nanofibril mineralization[END_REF]28). Based on these time-resolved Raman spectroscopy measurements, the following reaction mechanism steps are proposed:

1. Instantaneous formation of ACP:

(1-x)HPO4 2-+ (2/3)xPO4 3-+ Ca 2+ + nH2O  Ca(HPO4)1-x(PO4)(2/3)x.nH2O (11)
Here, x varies between 0.43 and 0.21, i.e. the ACP is structurally enriched with HPO4 2-as a function of time prior to its transformation into OCP-like layers. Clusters aggregation process combined with partial dissolution could be the responsible to this variation of phosphate content into ACP.

Transformation of ACP into OCP-like layers:

8Ca(HPO4)0.8(PO4)0.13.nH2O + 2.96PO4 3-+ 5H2O  Ca8(HPO4)2(PO4)4.5H2O + 4.4HPO4 2-+ 8nH2O (

A solid-state transformation is proposed because the transformation of ACP into OCP-like layers takes place over a short duration, in less than two minutes, and the position of the Raman peak shifts rapidly from 954 cm -1 to 959 cm -1 (Figure 6).

3. Transformation of OCP-like layers into hydroxyapatite:

Ca8(HPO4)2(PO4)4.5H2O + 2Ca 2+ + 2OH - Ca10(PO4)6(OH)2 + 2H + + 5H2O (13)
As explained above, the partial dissolution of OCP-like layers and ACP conserves Ca9(PO4)6

clusters of ~1 nm size as primary building unit (11, 16). These clusters can aggregate to form hydroxyapatite as described in the literature (11-12, 14, 16, 18). This interpretation is supported by the slight progressive shift of the position of the Raman peak from 960 to 962cm -1 and the slight decrease of FWHM of hydroxyapatite (Figure S14).

Concluding remarks

Time-resolved Raman measurements reveal that amorphous calcium phosphate (Ca(HPO4)1-

x(PO4)(2/3)x.nH2O) has specific Raman signature comprised between 943 cm -1 for ATCP (Ca3(PO4)2.nH2O, x=1) and 957 cm -1 for ACP (CaHPO4.nH2O, x=0), as shown in the Figures 1,3, 6 and S9. This specific Raman signature does not overlap with the strongest Raman peaks of crystalline calcium phosphates phases formed at ambient temperature (OCP at 960-962 cm -1 , brushite at 990 cm -1 and hydroxyapatite at 962-963 cm -1 ) and aqueous species (H2PO4 -at 880 cm - signatures monitored in real-time and under in situ conditions, we demonstrate that the ACP that contains only HPO4 1, Figure 5). These results are supported by experiment 5 (Table 1), where the ACP contains initially both PO4 3-and HPO4 1). (right) Main peaks of transient phases (ACP and OCP) and brushite, the most stable phosphate phase under the investigated conditions.

Figure 2 .

 2 Figure 2. Time-lapse Raman spectroscopy monitoring of the formation of brushite in experiment

Figure 3 .

 3 Figure 3. Time-lapse Raman spectroscopy monitoring of the formation of hydroxyapatite (HAp)

Figure 4 .

 4 Figure 4. Time-lapse Raman spectroscopy monitoring of the formation of hydroxyapatite (HAp)

Figure 5 .

 5 Figure 5. Time-lapse Raman spectroscopy monitoring of the formation of hydroxyapatite (HAp)

Figure 6 .

 6 Figure 6. Time-lapse Raman spectroscopy monitoring of the formation of hydroxyapatite (HAp)

  As indicated above, only OCP-like layers were suspected in the raw Raman data, but, it is necessary to improve the deconvolution fitting, particularly in the first 90 minutes (see FigureS10and Movie S2). Moreover, the ATCP phase has a long lifetime and coexists with hydroxyapatite until the end of the experiment. For this reason, two different mechanisms may occur simultaneously to form

	hydroxyapatite in high alkaline media.
	4a. Transformation of OCP-like layers into hydroxyapatite:
	Ca8(HPO4)2(PO4)4.5H2O + 2Ca 2+ + 2OH - Ca10(PO4)6(OH)2 + 2H + + 5H2O	(9)
	4b. Transformation of ATCP into hydroxyapatite:
	10Ca(PO4)2/3.nH2O + 2OH - Ca10(PO4)6(OH)2 + 0.66PO4 3-+ 10nH2O	(10)
	2-+ 1.33H + + 5H2O  Ca8(HPO4)2(PO4)4.5H2O + 8nH2O	(8)

  2-in its structure (CaHPO4.nH2O, x=0) has a short lifetime (<5min) in solution (experiments 1-2 in Table1, Figures1-2). Conversely, the ATCP that contains only PO4 3-in its

	structure (Ca(PO4)(2/3).nH2O or Ca3(PO4)2.nH2O, x=1) has a much longer lifetime (>24h) in
	solution (experiments 3-4 in Table

Table 1 .

 1 2-in its structure with x=0.[START_REF] Combes | Amorphous calcium phosphates: Synthesis, properties and uses in biomaterials[END_REF]. This phosphate composition in ACP varies with time until x=0.2 prior to its crystallization into a more stable phase. In this case, an intermediate lifetime of ~35 minutes is measured (Figure6). InThe early formation of amorphous phases and their transformation into transient or stables crystalline phases is now considered to be a generalized reaction pathway during the formation of calcium carbonate and phosphate minerals and/or biominerals in aqueous systems. This crystallization pathway agrees with Ostwald's rule, but transient phases are not necessary polymorphs; in fact, they can have different chemical compositions and different particle sizes.Moreover, thermodynamic and kinetic arguments should be considered in the crystallization tendency (e.g. 38, this study). In this way, multi-steps nucleation events may exist and combine various reaction mechanisms at the solid-fluid interfaces. Thus, solid-state transition, dissolutionrecrystallization, self-assembly aggregation and partial dissolution-aggregation may operate together during nucleation and growth processes (e.g. 31-33 and the present study). In this case, List of dynamic in situ Raman spectroscopy experiments performed to study the nucleation of brushite and hydroxyapatite. Experiments were performed under three different pH regimes, equimolar Ca/P ratio or hydroxyapatite stochiometric ratio (Ca/P=1.66), and two different calcium sources (CaCl2 and Ca(OH)2).

	addition to these fundamental insights, the reaction mechanism steps and kinetics during
	crystallization of brushite and hydroxyapatite can be determined under specific experimental
	conditions. For example, brushite (CaHPO4.2H2O) forms when the pH is < 9.8; amorphous calcium
	phosphate (ACP: CaHPO4.nH2O) with short lifetime (<2 minutes) and octocalcium phosphate
	(OCP: Ca8(HPO4)2(PO4)4.5H2O) are the main transient phases prior to the nucleation of brushite
	that occurs after ~8 minutes (see reactions 3-5). When pH is > 11, the hydroxyapatite
	(Ca10(PO4)6(OH)2) nucleates rapidly, after about 35 minutes, depending on the experimental
	conditions. Herein, amorphous calcium phosphate with different chemical compositions (ATCP:
	Ca(HPO4)1-x(PO4)(2/3)x.nH2O; x from 0.2 to 1) and with different lifetimes can form and then
	transform into OCP-like layers. Then, the OCP transient phase transforms into hydroxyapatite. The
	ATCP phase can also transform directly into hydroxyapatite when pH is > 12.2, without forming
	OCP as an intermediate phase (see reactions 6-10 and/or 11-13). In conclusion, the present study
	reports fundamental insights on the formation of amorphous calcium phosphate phases and their
	transformation into brushite micro-crystals or hydroxyapatite nanocrystals under abiotic
	conditions, relevant to technological applications and/or natural settings.

the classical nucleation theory (CNT), the most common theoretical formalism to quantify nucleation and growth rates (including variation of particles size distribution) cannot successfully be applied in its original formalism as proposed in the last three decades by various research groups (e.g.

[START_REF] Gebauer | Stable prenucleation calcium carbonate clusters[END_REF][START_REF] Gebauer | Pre-nucleation clusters as solute precursors in crystallization[END_REF][START_REF] Wang | Nonclassical crystallization: Mesocrystals and morphology change of CaCO3 crystals in the presence of a polyelectrolyte additive[END_REF][START_REF] Demichelis | Stable prenucleation mineral clusters are liquid-like ionic polymers[END_REF]

. These groups have proposed an alternative descriptive approach called non-classical nucleation and crystallization pathways (including pre-nucleation clusters). However, in the absence of a theoretical quantitative formalism to describe the so-called nonclassical nucleation, the improved and/or adapted classical nucleation theory remains the best tool to simulate nonclassical nucleation of crystals and/or particles (e.g. 12, 43-45).

and 1080 cm -1 , HPO4

2-at 992-993 cm -1 , and PO4 3-at 939 cm -1 ). Based on these spectral Raman

Acknowledgements

The authors acknowledge funding from the French National Centre for Scientific Research (CNRS), the Université Grenoble Alpes (UGA) and the Labex OSUG@2020 (Investissement d'avenir-ANR10-LABX56). The FUNMIN project funded an additional Raman probe for ex-situ measurements in suspensions and powders. We thank Nathaniel Findling for technical assistance.

Supporting Information Available

Table S1 and Figures S1-S14 as described in the text (PDF). Time evolution of the peaks of the main mineral phases during experiments 1, 3 and 5 (AVI)