Doan Pham
email: doan.phamminh@mines-albi.fr

Nathalie Lyczko

Haroun Sebei

Ange Nzihou

Patrick Sharrock

Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: A comparative study

Keywords:

The synthesis of calcium hydroxyapatite (Ca-HA) starting from calcium carbonate and different orthophosphate sources, including orthophosphoric acid, potassium, sodium and ammonium dihydrogen orthophosphates, was investigated under ambient conditions. The reaction started with calcium carbonate dissolution in an acid medium, followed by rapid precipitation of calcium cations with orthophosphate species to form calcium phosphate based particles which were in the size range of 0.4-1 m. These particles then agglomerated into much larger ones, up to 350 m in diameter (aggregates). These aggregates possessed an unstable porous structure which was responsible for the porosity of the final products. The highest specific surface area and pore volume were obtained with potassium dihydrogen orthophosphate. On the other hand, orthophosphoric acid led to the highest dissolution of calcium carbonate and the complete precipitation of orthophosphate species. Under ambient conditions, calcium phosphate based solid products of low crystallinity were formed. Different intermediates were identified and a reaction pathway proposed.

Introduction

Calcium hydroxyapatite (Ca-HA) continues to attract much attention owning to its many potential applications in fields such as medicine, wastewater treatment, and catalysis. For example, synthetic Ca-HA particles are used for coating metallic implants or bone grafts [1,2]. In heterogeneous catalysis, Ca-HA was found to be active in different reactions: oxidative dehydrogenation of light alkanes, water gas shift reaction, total oxidation of CO, photodecomposition of organic pollutants, etc. [3][4][5][6]. Ca-HA also possesses very good retention capacities for heavy metals or organic pollutants in aqueous effluents [7][START_REF] Lemlikchi | Waste and Biomass Valorization[END_REF][START_REF] Nzihou | [END_REF][10][11] which confers a potential role for this calcium phosphate in environmental processes.

A great deal of effort has been devoted to the synthesis of Ca-HA and other apatitic products [12][13][14][15][16]. The genesis of carbonate apatites was first described by Ames [17], then by Krajewski [18] using calcium carbonate as insoluble calcium source. Carbonate apatites were also made using soluble salts as starting materials [19]. For Ca-HA, the standard synthesis calls for the use of soluble calcium salts [20,21]. The practically complete precipitation of calcium phosphate results from mixing the homogeneous solution of reactants. However, the use of soluble calcium salts requires the viscous Ca-HA precipitate to undergo a further purification step, so as to eliminate the associated anions. An alternate route for Ca-HA synthesis is the acid-base neutralization method using orthophosphoric acid and lime solutions as starting materials which contain not counter ions [22][23][24]. Calcium carbonate is also a good candidate for calcium source, thanks to its low cost, wide availability and the absence of interfering anions. Nevertheless, when it was used previously as a starting material, it was usually transformed into calcium oxide by a thermal treatment or into water soluble calcium salt by dissolution in an acid solution, prior to the Ca-HA synthesis [25,26].

In the present work, a comparative study of the synthesis of Ca-HA from calcium carbonate and different orthophosphate sources was investigated. The main purpose of this study was to compare the dissolution of calcium carbonate, the precipitation of orthophosphate and the formed solids when different orthophosphate sources were used. Different characterization techniques were used for the analysis of the resulting solid products. The dissolution of calcium carbonate was highest with orthophosphoric acid but could not yield a complete, quantitative and stoichiometric Ca-HA precipitate after a long reaction time (48 h) at ambient temperature. Nevertheless, this pathway presents interesting industrial outlets.

Materials and methods

A fine powder of calcium carbonate (CaCO 3 , 98%, Fisher Scientific) was used as calcium source. Ammonium dihydrogen orthophosphate (NH 4 H 2 PO 4 , 99%, Fisher Scientific), potassium dihydrogen orthophosphate (KH 2 PO 4 , 99%, Fisher Scientific), sodium dihydrogen orthophosphate (NaH 2 PO 4 , ≈100%, Fisher Scientific) and orthophosphoric acid (H 3 PO 4 , 85 wt.% in water, Merck) were used as orthophosphate sources without further modification.

Ca-HA synthesis was carried out in a 1.2 liter (L) U-form glass reactor (i.d.: 10 cm, length: 15 cm), equipped with a vertical stirrer. In each synthesis, 0.6 mol of orthophosphate salts or orthophosphoric acid and 500 mL of water were fed into the reactor and homogenized under stirring. Then 1.0 mol of calcium carbonate were added to the orthophosphate solution at a rate of 2.5 g min -1 , to avoid the overflowing of the liquid due to strong acid attack on calcium carbonate particles at low pH. These reactant quantities corresponded to the Ca/P molar ratio of 1.67, which is the theoretical Ca/P molar ratio of Ca-HA. The reaction was kept under stirring (400 rpm) for 48 h under ambient conditions (25 • C, atmospheric pressure). The pH of the reaction was monitored continuously throughout the reaction. During the reaction, samples were withdrawn from the reactor and filtered on a 0.45 m filter paper to separate liquid and solid phases. The solid was washed with permuted water and dried overnight in an oven at 105 • C under air atmosphere.

Thermogravimetric analysis (TG) was carried out with a TA Instruments SDTQ600 analyzer. Heat-flow and weight changes associated with transitions and reactions in the products over the temperature range ambient to 1000 • C (heating rate of 5 • C min -1 ) under air atmosphere (100 mL min -1 ) were measured simultaneously. X-ray diffraction (XRD) data were collected using a Phillips Panalytical X'pert Pro MPD diffractometer with a Cu K␣ (1.543 Å) radiation source and a nickel filter to suppress the Cu K␤ ray. Scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDX) measurement was performed on a Philips XL30 ESEM apparatus (FEI Company). The particle size distribution of solid powders was measured by laser scattering in a Mastersizer 2000 (Malvern Instruments Ltd., Malvern, UK) in the range from 0.020 to 2000 m. The specific surface area of the solids was measured by nitrogen adsorption following standard vacuum drying at 105 • C, using the BET method (MICROMETRICS Gemini Vacprep 061). The adsorption-desorption isotherm was determined with a MICROMETRICS ASAP 2010 using nitrogen as gas adsorbate with the data collection from relative pressure (P/P • ) of 0.03-0.99. Elemental analysis of the liquid phase was carried out using inductive coupled plasma atomic emission spectroscopy (ICP-AES) with a HORIBA Jobin Yvon Ultima 2 apparatus. The species distribution of orthophosphoric acid was computed using the solEq software (Academic Software, Yorks, UK) and the IUPAC stability constants database of 2004.

Results and discussion

pH evolution

All four initial orthophosphate solutions had acid pH values. When calcium carbonate was added, it reacted immediately with the acid solutions, liberating carbonic gas. Taking into account the amount and addition rate of calcium carbonate into the acid solutions, the time taken for the complete addition of CaCO 3 was 40 min. Fig. 1 shows the evolution of recorded pH values during this period where most notable changes in pH evolution were observed.

All three orthophosphate salts showed similar pH profiles. They had the initial pH of ca. 3.9 which increased rapidly in the first minutes, accompanied by strong release of carbonic gas. This release was due to the acid attack on the calcium carbonate powder (Eq. ( 1)) where calcium cations were formed and protons were consumed. In the presence of different orthophosphate species, calcium cations were then rapidly precipitated (Eq. ( 2)).

2H + + CaCO 3 → Ca 2+ + H 2 O + CO 2
(1)

Ca 2+ + orthophosphates → calcium orthophosphates (2)
Fig. 2 recalls the distribution of orthophosphate species as a function of pH in the range 0-14 in aqueous solution.

At the pH of 3.9 corresponding to the initial pH of aqueous solutions of orthophosphate salts, the species distribution was 2.46%, 97.46%, 0.05% and 0% for H 3 PO 4 , H 2 PO 4 -, HPO 4 2-, and PO 4

3-, respectively. With the presence of H 2 PO 4 -as the major orthophosphate anion in the solution, mono-calcium phosphate monohydrate (MCPM, Ca(H 2 PO 4 ) 2 •H 2 O) was expected to precipitate at this early stage [START_REF] Elliott | Studies in Inorganic Chemistry 18: Structure and Chemistry of the Apatites and Other Calcium Orthophosphates[END_REF]. Eqs. ( 1) and ( 2) can be combined as follows:

2H + + 2H 2 PO 4 -+ CaCO 3 → Ca(H 2 PO 4 ) 2 •H 2 O + CO 2 (3)
After the initial increase, the pH stabilized during the continued addition of calcium carbonate powder into the reactor (zone A, Fig. 1). This pH plateau could be explained by the formation of a second intermediate -dicalcium phosphate dihydrate (DCPD, CaHPO 4 •2H 2 O) or brushite (Eq. ( 4)). The formation of DCPD accompanied the release of protons H + . So the equilibrium between the formation of protons H + in Eq. ( 4) and the consumption of protons H + in Eq. ( 1) led to the pH plateau in zone A (Fig. 1). After this pH plateau, the pH increased gradually to about 6.7 after 40 min of reaction, which can be explained by the continuous acid consumption (Eq. ( 1)) and the evolution of MCPM and DCPD into other calcium phosphates.

Ca(H 2 PO 4 ) 2 •H 2 O + Ca 2+ + 3H 2 O → 2[CaHPO 4 •2H 2 O] + 2H + (4)
In the case of orthophosphoric acid, the initial pH was 0.8. At this pH, the species distribution was the following: 96.81%, 3.19%, 0% and 0% for H 3 PO 4 , H 2 PO 4 -, HPO 4 2-, and PO 4 3-, respectively. The first increase in pH (before zone B, Fig. 1) was due to the consumption of protons H + (Eq. ( 1)) and the time-length of this increase was longer than that of orthophosphate salts. So the quantity of MCPM formed was logically more than that from orthophosphate salts. After this first increase, the pH decreased (zone B, Fig. 1) despite the addition of calcium carbonate powder into the reactor. This drop in pH could also be attributed to the release of protons H + coming from the transformation of MCPM into DCPD (Eq. ( 4)) at a rate higher than that of the consumption of protons H + in Eq. ( 1). This phenomenon was also observed by Kim et al. in the synthesis of Ca-HA from Ca(OH) 2 and orthophosphoric acid [START_REF] Kim | [END_REF]. After the decrease in zone B, the pH increased in zone C (Fig. 1) explained by the fact that in zone C, the transformation of MCPM into DCPD (Eq. ( 4)) reached equilibrium while protons H + were still continuously consumed by Eq. ( 1). After zone C, the pH increased slightly indicating the slow evolution of MCPM and DCPD into other calcium orthophosphates. From 40 min to 48 h of reaction, the pH increased continuously and stabilized to about 8 for orthophosphate salts and 6.8 for orthophosphoric acid.

Elemental analysis of the liquid phase

Much work has been done on the synthesis of Ca-HA but the liquid phase has usually not been analyzed although it is indispensable to evaluate the performance of the synthesis process.

Table 1 reports the contents of soluble phosphorus and calcium in the liquid phase, obtained by ICP-AES analysis. In all cases, the content of soluble calcium was very low after 48 h of reaction. So, most calcium existed in the solid state under either calcium phosphate products or non-dissolved calcium carbonate. The soluble phosphorus initially introduced in the reactor was 600 mmol. After 48 h of reaction, orthophosphoric acid led to a complete precipitation of soluble phosphate species into solid calcium phosphates. On the other hand, all three orthophosphate salts resulted in lower yields of precipitation of orthophosphate species which never exceed 64%. Probably the presence of K + , Na + and NH 4 + cations in the reaction mixtures caused this incomplete precipitation of orthophosphate species. In fact, after 48 h of reaction using orthophosphate salts, the concentration of OH -was of ca. 10 -6 mol L -1 because the final pH was of ca. 8. This concentration of OH -anions was very small in comparison with that of K + , Na + and NH 4 + cations. During the reaction, the initial solid carbonate was essentially transformed into carbonic gas by acid attack (Eq. ( 1)). Thus, the contents of HCO 3 -and CO 3 2-in the final liquid phase must also be low. Logically in the synthesis using orthophosphate salts, some of initial orthophosphate species had to be present in the liquid phase as counter ions of K + , Na + or NH 4 + cations. Under the present synthetic conditions, 36-40% of initial orthophosphate salts could not be converted into solid calcium phosphate products. This demonstrated the interest of the use of orthophosphoric acid instead of orthophosphate salts.

TG analysis

To better understand the evolution of intermediates during the reaction, solid samples withdrawn from the reaction mixture were washed, dried and then analyzed by different techniques. Fig. 3 shows the derivative thermogravimetry (DTG) curves in the temperature range from 25 • C to 1000 • C.

In this section, DTG max indicates the temperature corresponding to the maximum of the DTG signal. In Fig. 3(A), all samples had a small weight loss at the temperature lower than 100 • C, which was attributed to the elimination of physically absorbed surface moisture. The second weight loss occurred at the DTG max of 110 • C, which was due to the dehydration of MCPM (Eq. ( 5)). The next weight loss, much more abrupt, took place at the DTG max of 186 • C and could be attributed to the dehydration of DCPD to form dicalcium phosphate anhydrous -DCPA (Eq. ( 6)). This last product was later condensed at the DTG max of 430 • C into calcium pyrophosphate (Eq. ( 7)) [START_REF] Elliott | Studies in Inorganic Chemistry 18: Structure and Chemistry of the Apatites and Other Calcium Orthophosphates[END_REF]. The last and largest weight loss at the DTG max of 705 • C was due to the decomposition of the remaining calcium carbonate, according to reaction (Eq. ( 8)). The intensity of peaks related to calcium carbonate decomposition decreased progressively with the reaction time, indicating the continuous advancement of the reaction consuming the solid carbonate (Table 2).

Ca(H 2 PO 4 ) 2 •H 2 O → Ca(H 2 PO 4 ) 2 + H 2 O (5) CaHPO 4 •2H 2 O → CaHPO 4 + 2H 2 O (6) 2CaHPO 4 → Ca 2 P 2 O 7 + H 2 O (7) CaCO 3 → CaO + CO 2 (8)
Fig. 3(B) illustrates the DTG curves when sodium dihydrogen orthophosphate was used. MCPM was present in all samples in very small amounts while DCPD and DCPA decreased with the reaction time and had almost disappeared after 48 h of reaction. On the other hand, calcium carbonate was found at higher levels in comparison to those in Fig. 3(A), as was also observed for Fig. 3(C) and(D) corresponding to the use of potassium and ammonium dihydrogen orthophosphates. As illustrated in Fig. 1, the initial pH of the starting aqueous solution of orthophosphoric acid was lower than those of orthophosphate salts. Thus, orthophosphoric acid had a stronger acid attack on the calcium carbonate powder. In Fig. 3(C) and (D), all intermediates were present at low contents and had almost disappeared after 48 h of reaction.

From the results of TG analysis, the amount of remaining calcium carbonate in the solid samples was calculated (Table 2). As expected, orthophosphoric acid led to a higher dissolution of calcium carbonate than any of the three orthophosphate salts. In all cases, after a rapid acid attack during the first hour of reaction, in which 46-73% of the calcium carbonate was dissolved, the reaction rate decreased notably. Calcium phosphate precipitates, which were formed during the first reaction period, might coat the surface of the calcium carbonate particles to form a biphasic structure, as observed in previous work [29]. Calcium phosphate layers of low solubility limit the contact between the calcium carbonate and soluble orthophosphate reactants, slowing the reaction rates.

All weight losses in Fig. 3 were endothermic so the corresponding DSC curves were not shown in this report.

XRD characterization

XRD patterns of the solid samples withdrawn at different time intervals are presented in Fig. 4. Remaining calcium carbonate showed the clearest crystalline phases in all cases. As expected, different calcium phosphate based products were identified. DCPD and DCPA were mostly present in all samples using orthophosphoric acid or sodium dihydrogen orthophosphate (Fig. 4(A) and (B)). They reached the highest contents at 24 h for orthophosphoric acid and at 7 h for sodium dihydrogen orthophosphate. Then their amount decreased, confirming their transformation into other products. On the other hand, DCPD and DCPA appeared only at the beginning of the reactions with potassium and ammonium dihydrogen orthophosphates, which is coherent with the results of TG analysis. In no cases could MCPM be detected which was in accord with its very low quantity in these solid products.

In addition to the intermediates identified by TG analysis, XRD results allowed the identification of low crystallinity Ca-HA and other apatitic compounds such as octacalcium bis(hydrogenphosphate) tetrakis(phosphate) pentahydrate (OCP, Ca 8 (HPO 4 ) 2 (PO 4 ) 4 •5H 2 O) and B-type calcium carbonate-apatite (CAP). OCP could be formed because calcium carbonate was not totally consumed after 48 h of reaction, leading to a deficiency of available soluble calcium to construct the complete stoichiometric hydroxyapatite structure. The hydrolysis of the DCPD, which was the main intermediate in these syntheses, could also lead to the formation of OCP [30]. For the formation of CAP, the evolved carbonic gas saturated the solution with the increase in pH and participated in forming carbonate-apatite by the substitution of PO 4

3-groups by CO 3 2-groups (B-type substitution) [31][32][33]. This was confirmed by IR analysis of the solid products obtained after 48 h of reaction (Section 3.5).

The low crystallinity of the solid products, as shown by weak broad peaks of apatitic structure, was due to the low reaction temperature of synthesis, ca. 25 • C. Higher temperatures are required for the formation of well-crystallized Ca-HA [14,34,35]. As regards the largest dissolution of calcium carbonate by orthophosphoric acid, the resulting solid product after 48 h of reaction was calcined at 900 • C for 1 h under air atmosphere. Its XRD pattern is presented in Fig. 5.

In Fig. 5, all peaks could be attributed to the diffractions of beta-tricalcium phosphate (␤-TCP) and Ca-HA, which were both well-crystallized. Some active lime (CaO) also appeared in small amounts. In fact, the solid product from the reaction of calcium carbonate and orthophosphoric acid contained exactly the Ca/P molar ratio of 1.67 (Table 1). However, some of initial calcium carbonate was not dissolved as verified above by TG results. This remaining calcium carbonate decomposed under thermal treatment and resulted in the formation of active lime. Most of this active lime reacted with the intermediates such as MCPM, DCPD, DCPA to form TCP which is considered as the precursor of Ca-HA. The effect of temperature on the crystallinity of calcium phosphate based products and the formation of TCP was already observed [36][37][38].

IR study

Fig. 6 presents IR spectra of the solid products obtained after 48 h of reaction. In the wavelength range of 4000-1700 cm -1 , only a weak broad peak at about 3600-3000 cm -1 , which was attributed to water or OH -groups in apatitic channel, was observed (results not shown).

In the wavelength region of 1700-500 cm -1 , residual calcium carbonate was characterized by its main peaks at 711 cm -1 and 872 cm -1 . Two peaks at 1450 cm -1 and 1415 cm -1 were attributed to the presence of B-type carbonate-apatite [39] which confirmed again XRD results. The intensity of these peaks decreased in the following order: KH 2 PO 4 > NH 4 H 2 PO 4 > NaH 2 PO 4 > H 3 PO 4 . This was in accord with the observation of pH evolution. The highest pH after 48 h of reaction was obtained with KH 2 PO 4 which was the most favorable for the formation of carbonate anion in solution and therefore for the formation of B-type carbonate-apatite. Peaks in the wavelength ranges of 650-500 cm -1 and 1200-900 cm -1 could be attributed to different calcium phosphates present in these solid products [39,40].

Specific surface area measurement

From the point of view of the potential applications of Ca-HA based materials for the environmental or catalysis fields, specific surface area is an important parameter to consider. Table 3 summarizes the results obtained.

In all cases, S BET increased with the reaction time, indicating the evolution of the reaction at ambient conditions, which correlated with the change of calcium carbonate content in Table 2. The initial calcium carbonate used in this investigation had a very low S BET (<2 m 2 /g -1 ). Despite the lower elimination level of calcium carbonate, all three orthophosphate salts led to higher S BET in comparison to orthophosphoric acid. The solid product from the synthesis using Fig. 6. IR spectra of the solid products obtained after 48 h of reaction. potassium dihydrogen orthophosphate showed the highest S BET of 105 m 2 /g after 48 h of reaction.

SEM-EDX analysis

The solid products obtained after 48 h of reaction were examined by SEM-EDX and some examples are presented in Fig. 7. On the left-hand side (200 m scale), different groups of particles were present with a high occurrence of small particles. On the right-hand side (2 m scale), the surfaces of big particles are presented with the same enlargement. Particles of the solid products formed from orthophosphoric acid (A2) and sodium dihydrogen orthophosphate (B2) had a compact appearance with some sheet structure that could be due to the presence of DCPD in these solids [41]. On the other hand, particles formed from potassium dihydrogen orthophosphate (C2) and ammonium dihydrogen orthophosphate (D2) appeared much more porous and the primary platelets forming the aggregates could be clearly observed.

EDX analysis was also performed and in all cases, we observed the presence of Ca, P, O and C corresponding to calcium phosphate based products. The presence of carbon was partly due to the carbonic gas in air and CAP, but could also be due to the remaining calcium carbonate. On the other hand, no traces of potassium, sodium or nitrogen were detected. Thus, there was no incorporation of these elements into the first surface layers of calcium phosphates and simple washing prevented the possible inclusion of these elements on the surface of the solid products.

Particle size distribution and adsorption-desorption isotherm measurements

To confirm the results of S BET and SEM analysis, particle size distribution and isothermal adsorption-desorption measurements were investigated. Fig. 8 shows the particle size distribution of the initial calcium carbonate powder which could be considered as a Gaussian-like type for both volume and number distributions.

Fig. 9 shows the comparison of number (left) or volume (right) distributions for the solid products obtained at different reaction times. The number distributions revealed that most particles were present in the range from 0.4 to 1 m with more than 97% of the cumulative total. At each reaction time (3 h or 24 h or 48 h), there was very little difference between the four orthophosphate sources used. On the other hand, the volume distributions showed that more than 98% of accumulative volume belonged to the larger particles category (>1 m).

For the family of particles smaller than 1 m, the median diameter (d 50 ) increased slightly with the reaction time indicating their slow growth in the present reaction conditions. For the larger particles family (>1 m), in most cases, we observed the formation of three groups as follows: 1-10 m, 10-80 m and 80-350 m. The common tendency of these groups was the increase in volume of the 80-350 m group and the decrease in volume of the 1-10 m and 10-80 m groups during the course of the reaction.

Generally, the specific surface area of a material is attributed to both the external surface of particles and the internal surface of pores if the material is porous. From the results of the particle size distribution analysis in Fig. 9, the differences in the specific surface areas (S BET ) in Table 3 must be predominantly due to the porosity of the solids. Thus adsorption-desorption isotherms were carried out for four solids obtained after 48 h of reaction (Fig. 10).

All isotherms could be attributed to Type II solids with a hysteresis loop of Type H3 in the IUPAC classification of physisorption isotherms [42].

Using the BJH method [43] for both adsorption and desorption branches of isotherms, the porous volume (V p ), calculated specific surface area (S c ) and pore mean diameter (D p ) could be derived. They are presented in Table 4.

For each solid product, the calculated specific surface area from the desorption branch (S des c ) was higher than the specific surface area (S BET ). This result was characteristic of an unstable porous structure for aggregates as well as for the sheet structure of solids, which modify their pore volumes under the effect of high relative pressure. The pore mean diameters were all in the mesoporous range.

Reaction pathway

The formation of a large majority (97%) of small calcium phosphate particles (<1 m) from an initial calcium carbonate of much larger particle size confirmed the arguments in favor of dissolution of calcium carbonate to release calcium cations (Eq. ( 1)) and the formation of different calcium phosphate precipitates from these dissolved calcium cations and orthophosphate species (Eq. ( 2)). A reaction pathway could be proposed for the evolution of the reaction as given in Scheme 1.

First, calcium carbonate powder was dissolved in acid solution to form soluble calcium cations. These cations were then precipitated with orthophosphate anions to form MCPM and DCPD as the first intermediates. MCPM evolved to DCPD by the reaction with more calcium cations. DCPD was then transformed into DCPA as another intermediate. These intermediates had the Ca/P molar ratio of 0.5/1 or 1/1. With the on-going reaction, calcium carbonate continued to be dissolved, accompanying the increase in pH by the consumption of protons. Under these conditions, DCPD and DCPA developed an apatitic composition. This transformation was slow and depended on the orthophosphate source. The presence of carbonic gas or (bi)carbonate anions in the reaction mixture led to the formation of CAP in parallel with the formation of Ca-HA as the final product. Throughout the reaction, the agglomeration phenomenon of primary particles led to the formation of agglomerates with sizes up to 350 m. The final solid product after 48 h of reaction contained principally a mixture of Ca-HA, CAP and some left over CaCO 3 . Other intermediates including MCPM, DCPD and DCPA could also be present, depending on the initial orthophosphate source. Finally, the calcination at 900 • C for 1 h of the solid Scheme 1. Reaction pathway in the Ca-HA synthesis. product issue from the dissolution of calcium carbonate in the solution of orthophosphoric acid resulted in the formation of a biphasic mixture of well-crystalline TCP and Ca-HA.

Conclusion

For the first time, a comparative study on the direct synthesis of Ca-HA from calcium carbonate powder and different orthophosphates was undertaken. Orthophosphoric acid was the most interesting orthophosphate source since it was the most efficient for the dissolution of calcium carbonate and was the only reactant leading to complete consumption by precipitation of starting orthophosphate species within 48 h.

Porous solid products were formed with all orthophosphate reactants. The porosity of the products was due to the formation of large aggregates which had an unstable porous structure. The difference in the porosity explained the difference in the specific surface area (S BET ) of the solid products. On the other hand, particle size distributions were relatively similar to each other.

Using TG and XRD characterizations, different intermediates and their evolution during the reaction could be identified. A reaction pathway could be proposed.

Future work will focus on the improvement of the dissolution of calcium carbonate using orthophosphoric acid, for example at higher reaction temperature, together with TEM observations to confirm a simple one-step production route to economical Ca-HA.

Fig. 1 .

 1 Fig. 1. pH evolution during the first fourteen min of the reaction using orthophosphoric acid (red); sodium dihydrogen orthophosphate (green); potassium dihydrogen orthophosphate (black) and ammonium dihydrogen orthophosphate (pink). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2 .

 2 Fig. 2. Distribution of orthophosphate species in aqueous solution as a function of pH.

Fig. 3 .

 3 Fig.3. DTG curves (arbitrary units) of the solid powders in the synthesis using orthophosphoric acid (A), sodium dihydrogen orthophosphate (B), potassium dihydrogen orthophosphate (C) and ammonium dihydrogen orthophosphate (D) as orthophosphate sources.

Fig. 4 .

 4 Fig. 4. XRD patterns of the solid powders in the syntheses using orthophosphoric acid (A), sodium dihydrogen orthophosphate (B), potassium dihydrogen orthophosphate (C) and ammonium dihydrogen orthophosphate (D) as orthophosphate sources. Principal diffractions of calcium carbonate (A); DCPD (o); DCPA ( ); apatite structure ( ).

Fig. 5 .

 5 Fig. 5. XRD patterns of the calcined solid powders after 48 h of synthesis using orthophosphoric acid. Diffraction of TCP (♦); Ca-HA ( ); CaO ( ).

Fig. 7 .

 7 Fig. 7. SEM analysis of the solid products after 48 h of reaction using orthophosphoric acid (A1, A2), sodium dyhydrogen phosphate (B1, B2), potassium dihydrogen phosphate (C1, C2) and ammonium dihydrogen phosphate (D1, D2).

Fig. 8 .

 8 Fig. 8. Particle size analysis of the initial calcium carbonate powder; (1) and (2): particle number and volume distribution, respectively.

Fig. 9 .

 9 Fig. 9. Number (left) and volume (right) distribution as a function of particle size of the solid products at different reaction times and pure CaCO3; particle size is in base 10 logarithmic scale.

Fig. 10 .

 10 Fig. 10. Isotherm plots of the solid products obtained at 48 h of reaction from (A) orthophosphoric acid; (B) sodium dihydrogen orthophosphate; (C) potassium dihydrogen orthophosphate and (D) ammonium dihydrogen orthophosphate.

Table 1

 1 Elementary analysis of soluble phosphorus and calcium in the liquid phase after 48 h of synthesis; XP: conversion of initial soluble phosphorus into solid calcium phosphate products.

	Initial phosphate	P (mmol)	XP (%)	Ca (mmol)
	H3PO4	0.5	99.9	0.8
	KH2PO4	240	60.0	0.2
	NaH2PO4	220	63.3	0.3
	NH4H2PO4	229	61.8	0.6

Table 2

 2 Evolution of calcium carbonate content remaining in the solid product calculated from the results of TG analysis.

	Reaction time (h)	CaCO3 content (%)
		H3PO4	NaH2PO4	KH2PO4	NH4H2PO4
	1	27	54	50	52
	3	2 5	50	4 5	4 5
	7	25	48	43	43
	24	23	43	43	43
	31	20	41	41	41
	48	18	34	34	35

Table 3

 3 Specific surface area measured by BET method of the solid products after washing and drying.

	Reaction time (h)	Specific surface (SBET) (m 2 /g)
		H3PO4	NaH2PO4	KH2PO4	NH4H2PO4
	3	12	5	17	13
	24	14	23	68	21
	48	27	44	105	75

Table 4

 4 Results calculated from the isotherms of the solid products at 48 h of reaction: (A) orthophosphoric acid; (B) sodium dihydrogen orthophosphate; (C) potassium dihydrogen orthophosphate and (D) ammonium dihydrogen orthophosphate.

	Product	SBET (m 2 /g)	Results from the adsorption			Results from the desorption
			V ads p	(cm 3 /g)	S ads c	(m 2 /g)	D ads p (nm)	V des p	(cm 3 /g)	S des c	(m 2 /g)	D des p (nm)
	(A)	27	0.25		25		39	0.26		32	32
	(B)	44	0.24		39		25	0.25		83	12
	(C)	105	0.68		98		28	0.65		136	19
	(D)	75	0.52		69		30	0.49		94	20

Acknowledgments

The authors gratefully acknowledge financial support from Solvay Group. Our gratitude goes to colleagues at RAPSODEE Center, Christine Rolland, Sylvie Del Confetto, Philippe Accart and Denis Marty for their technical help.


Abdoul Thanh Son Phan

Razac Sane

Bruna Rêgo De Vasconcelos

Ange Nzihou

Patrick Sharrock

Didier Grouset

Doan Pham
email: doan.phamminh@mines-albi.fr

Hydroxyapatite supported bimetallic cobalt and nickel catalysts for syngas production from dry reforming of methane

Keywords:

Hydroxyapatite (HAP, Ca 10 (PO 4 ) 6 (OH) 2 ) has all the criteria of a catalyst support, in particular its high thermal stability. However it is still less studied in the heterogeneous catalysis. For the first time, hydroxyapatite supported bimetallic Co-Ni catalysts were prepared and evaluated in the dry reforming of methane (DRM) process. Nanoparticles containing both nickel and cobalt were well formed on the surface of HAP by conventional impregnation methods. No modification of HAP structure was observed after metals deposition. DRM reaction was carried out at 700-750 °C and around 1.6 bar, using a fixed-bed reactor which was fed with a mixture of 20%vol CH 4 , 20%vol CO 2 and 60%vol N 2 . CH 4 and CO 2 conversion reached up to 60 and 68% at 700 °C, respectively, and 73 and 79% at 750 °C, respectively during long reaction times of 50-160 h. Water as a by-product could be quantified along the catalytic reaction indicating the implication of reverse water-gas-shift reaction. TEM-EDX analysis of the used catalysts recovered after catalytic tests showed that coke deposition was limited and there was slight modification of metals particle size. The results obtained were very promising for the design of an efficient catalytic system for DRM process.

Introduction

Waste management is up-to-date dominated by incineration and landfill technique. However, most of incinerators and landfill sites are not equipped with an energy or matter recovery system. In order to better valorize waste and biomass to energy, fuels and other added value and specialty products, new concepts are developed during these last decades including thermo-conversion route and biological route (Supporting information 1). Pyrolysis/gasification of biomass and waste is intensively studied during these last decades [START_REF] Brown | Thermochemical Processing of Biomass Conversion into Fuels, Chemicals and Power. Chapter I-Introduction to Thermochemical Processing of Biomass into Fuels[END_REF][START_REF] Collard | A review on pyrolysis of biomass constituents: mechanisms and composition of the products obtained from the conversion of cellulose, hemicelluloses and lignin[END_REF][START_REF] Sikarwar | Progress in biofuel production from gasification[END_REF][START_REF] Kersten | Biomass pyrolysis in a fluidized bed reactor. Part 1: literature review and model simulations[END_REF]. The highest challenge of this valorization route is the syngas cleaning. This last one is required to remove fine particles, condensable molecules and various gaseous pollutants (H 2 S, COS, HCl, NH 3 …), which depend on the nature of feedstock, gasifier, and downstream utilization [START_REF] Lai Fui Chin | Biomass Derived Syngas Cleaning Technologies[END_REF]. Much effort is still needed to make this route competitive and viable.

Methanisation is a biological process for the degradation of waste and biomass. Biogas is the main gaseous product which mainly contains CH 4 and CO 2 . This appears as a good solution for the management of non-hazardous wastes with minimization of the initial investment cost compared to thermal routes (incineration or pyro-gasification). Biogas can be upgraded to produce biomethane or transformed into syngas by reforming processes [START_REF] De Clercq | A review of global strategies promoting the conversion of food waste to bioenergy via anaerobic digestion[END_REF][START_REF] Hagos | Anaerobic co-digestion process for biogas production: progress, challenges and perspectives[END_REF][START_REF] Löfberg | Chemical looping dry reforming of methane: toward shale-gas and biogas valorization[END_REF].

Biogas reforming includes various processes such as steam reforming, dry reforming, partial oxidation, and auto-thermal oxidation. They generally need a solid catalyst. In fact, because of the chemical stability of CO 2 and CH 4 , biogas reforming needs high temperature (usually > 700 °C). Thus, side reactions such as Boudouard reaction and methane cracking take place leading to coke deposition. Steam reforming is found to be efficient for the conversion of biogas into syngas, with low catalytic deactivation rate. The presence of vapor allows gasifying coke deposition and so limiting catalytic deactivation. However, steam reforming is an energy-intensive process because of its high operational temperature and high water to hydrocarbon ratios used (3-4 for industrial installations). Partial oxidation and autothermal oxidation do not allow including carbon dioxide to the process. By these reasons, dry reforming of methane (DRM) has been intensively investigated these last years as new promising solution for biogas reforming. Generally, DRM is studied with an equimolar ratio mixture of CH 4 and CO 2 (Eq. ( 1)). Thermodynamic analysis of this reaction showed that it is favored at high temperature (i.e. above 700 °C) and low pressure (i.e. around atmospheric pressure) [START_REF] Jafarbegloo | Thermodynamic analysis of carbon dioxide reforming of methane and its practical relevance[END_REF][START_REF] Chein | Thermodynamic analysis of dry reforming of CH 4 with CO 2 at high pressures[END_REF]. The main products of this reaction are CO and H 2 , but coke deposition is inevitable below 900 °C at atmospheric pressure [START_REF] Chein | Thermodynamic analysis of dry reforming of CH 4 with CO 2 at high pressures[END_REF]. Water was usually reported as another by-product in the literature, and was recently quantified by Rêgo de Vasconcelos et al. [START_REF] De Vasconcelos | Catalytic transformation of carbon dioxide and methane into syngas over ruthenium and platinum supported hydroxyapatites[END_REF].

CH 4 + CO 2 → 2CO + 2H 2 (1)
The main challenge of DRM is related to the catalytic deactivation, together with side reactions such as reserve water-gas-shift reaction (RWGS). The deactivation is mostly due to coke deposition and catalyst modification (active phase and/or support) at high temperature [START_REF] De Llobet | Relationship between carbon morphology and catalyst deactivation in the catalytic decomposition of biogas using Ni[END_REF][START_REF] De Vasconcelos | Phosphates-based Catalysts for Synthetic Gas (syngas) Production Using CO 2 and CH 4[END_REF][START_REF] Luneau | Deactivation mechanism of Ni supported on Mg-Al spinel during autothermal reforming of model biogas[END_REF][START_REF] Ginsburg | Coke formation over a nickel catalyst under methane dry reforming conditions: thermodynamic and kinetic models[END_REF]. Thus, much work was devoted to the development of new performing catalysts to prevent catalytic deactivation. The minimization of the size of nickel particles as active phase was frequently attempted [START_REF] De Vasconcelos | Phosphates-based Catalysts for Synthetic Gas (syngas) Production Using CO 2 and CH 4[END_REF]. In fact, coke formation can be inhibited by using highlydispersed nickel particles [START_REF] Kim | Effect of metal particle size on coking during CO 2 reforming of CH 4 over Ni-alumina aerogel catalysts[END_REF][START_REF] Liu | Progresses in the preparation of coke resistant Nibased catalyst for steam and CO 2 reforming of methane[END_REF][START_REF] Mo | Highly dispersed supported metal catalysts prepared via in-situ selfassembled core-shell precursor route[END_REF]. Catalyst support plays also important role. Supports having high basicity and strong metal-support interaction are researched [START_REF] Alipour | Effect of alkaline earth promoters (MgO CaO, and BaO) on the activity and coke formation of Ni catalysts supported on nanocrystalline Al 2 O 3 in dry reforming of methane[END_REF][START_REF] José-Alonso | K and Sr promoted Co alumina supported catalysts for the CO 2 reforming of methane[END_REF]. High basicity favors CO 2 adsorption and limits coke formation while strong metal-support interaction limits metal particles sintering [START_REF] Goula | Syngas production via the biogas dry reforming reaction over Ni supported on zirconia modified with CeO 2 or La 2 O 3 catalysts[END_REF][START_REF] Dahdah | CO 2 reforming of methane over Ni x Mg 6-x Al 2 catalysts: effect of lanthanum doping on catalytic activity and stability[END_REF][START_REF] Liu | La-promoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF].

Conventional catalytic supports including Al 2 O 3 , MgO, TiO 2 , ZrO 2 , SiC, zeolites, and nanostructured carbons were frequently reported in the literature for DRM [START_REF] Löfberg | Ni/ CeO 2 based catalysts as oxygen vectors for the chemical looping dry reforming of methane for syngas production[END_REF][START_REF] Abdullah | Recent advances in dry reforming of methane over Ni-based catalysts[END_REF][START_REF] Taherian | Promotional effect of samarium on the activity and stability of Ni-SBA-15 catalysts in dry reforming of methane[END_REF][START_REF] Nahara | Hydrogen production from reforming of biogas: review of technological advances and an Indian perspective[END_REF][START_REF] Meric | Coke minimization via SiC formation in dry reforming of methane conducted in the presence of Ni-based core-shell microsphere catalysts[END_REF]. The combination of several metal oxides allowed improving catalytic performance [START_REF] Nahara | Hydrogen production from reforming of biogas: review of technological advances and an Indian perspective[END_REF][START_REF] Taherian | A comparative study of ZrO 2 , Y 2 O 3 and Sm 2 O 3 promoted Ni/SBA-15 catalysts for evaluation of CO 2 /methane reforming performance[END_REF][START_REF] Movasati | CO 2 reforming of methane over Ni/ZnAl 2 O 4 catalysts: influence of Ce addition on activity and stability[END_REF]. However, up-to-date and to the best of our knowledge, there is not application of DRM at large industrial scale yet. A highly-performing catalytic system is still to be developed and it is important to enlarge research area on other types of catalyst. For this reason, our current research is focused on synthetic calcium hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 , HAP)-based catalysts.

HAP has all the criteria required for a catalytic support: high specific surface area, with or without mesopores depending on the synthesis conditions, very low water solubility, and high thermal stability [START_REF] De Vasconcelos | Catalytic transformation of carbon dioxide and methane into syngas over ruthenium and platinum supported hydroxyapatites[END_REF][START_REF] De Vasconcelos | Phosphates-based Catalysts for Synthetic Gas (syngas) Production Using CO 2 and CH 4[END_REF][START_REF] Galera Martínez | Synthesis, characterization and thermo-mechanical properties of copper loaded apatitic calcium phosphates[END_REF][START_REF] De Vasconcelos | Synthesis of carbon nanotubes/hydroxyapatite composites using catalytic methane cracking[END_REF][START_REF] Pham Minh | One-step synthesis of calcium hydroxyapatite from calcium carbonate and orthophosphoric acid under moderate conditions[END_REF]. In addition, it contains both acid and basic sites and the density of these sites can be controlled by varying the molar ratio of Ca to P. At high temperature, partial dehydration can take place but this phenomenon is reversible under the presence of water vapor [START_REF] Ababou | Influence des conditions de calcination sur l'évolution morphologique de l'hydroxyapatite[END_REF][START_REF] Bernache-Assollant | Sintering of calcium phosphate hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 I. Calcination and particle growth[END_REF]. Thus HAP becomes less sensitive to the presence of water, which is usually formed during DRM process. All these properties make HAP as new potential support for heterogeneous catalytic processes. Boukha et al. [START_REF] Boukha | Methane dry reforming on Ni loaded hydroxyapatite and fluoroapatite[END_REF] studied DRM over nickel loaded HAP in the temperature range of 600-800 °C. Encouraging results were obtained with high initial conversion of methane and carbon dioxide during 4 h of time-onstream. However, the catalytic stability was not investigated by these authors. Recently, we reported our last results on the catalytic performance of ruthenium and platinum supported HAP in DRM process. At low Pt loading of 0.73 wt.%, the catalyst Pt/HAP prepared by the simple incipient wetness impregnation (IWI) showed interesting catalytic activity and stability during 50 h of time-on-stream (at 700 °C and atmospheric pressure) [START_REF] De Vasconcelos | Catalytic transformation of carbon dioxide and methane into syngas over ruthenium and platinum supported hydroxyapatites[END_REF].

Although noble metals catalysts are known to have good catalytic performance, their high cost hinders their use at large scale. So, many researchers have focused on the use of transition metals such as nickel, cobalt, iron etc. as active phases. Despite showing high activity, these catalysts suffer from rapid deactivation mostly due to carbon deposition and metal sintering. In order to improve their stability, several approaches have been proposed such as the modification of methods for catalyst preparation or the addition of promoters. One of the most investigated approaches is the use of a second metal to enhance catalyst stability in DRM process. Takanabe et al. [START_REF] Takanabe | Titania-supported bimetallic catalysts for carbon dioxide reforming of methane[END_REF] showed that combining Ni and Co could enhance the catalyst activity and also suppress the carbon deposition during DRM. Zhang et al. [START_REF] Zhang | Development of stable bimetallic catalysts for carbon dioxide reforming of methane[END_REF] arrived at similar conclusions. They demonstrated that combining Ni and Co increased metal dispersion and generated stronger metal-support interaction when compared to the Ni and Co monometallic catalysts. Recent work of Wang et al. [START_REF] Wang | Template-free preparation of bimetallic mesoporous Ni-Co-CaO-ZrO 2 catalysts and their synergetic effect in dry reforming of methane[END_REF] highlighted again the beneficial effect of bimetallic Ni-Co catalysts compared to monometallic Ni or Co catalysts in DRM reaction.

This work investigated the catalytic performance of HAP supported nickel-cobalt bimetallic catalysts in DRM process. Nickel and cobalt were chosen for their high activity and their relative low cost compared to other metals while HAP appears as a new promising catalytic support as mentioned above.

Materials and methods

Catalyst preparation

Commercial HAP support (60 m 2 /g, fine powder) provided by PRAYON S.A. (Belgium) was used as catalytic support without further modification. Nickel nitrate (Ni(NO 3 ) 2 •6H 2 O, Fisher Scientific, > 98 wt %) and cobalt nitrate (Co(NO 3 ) 2 •6H 2 O, Acros Organics, > 98 wt%). Distilled water was also used for catalyst preparation.

Hydroxyapatite supported bimetallic nickel and cobalt catalysts were prepared by successive incipient wetness impregnation (SIWI) or incipient wetness co-impregnation (COI). For the catalysts prepared by SIWI method, HAP support was firstly impregnated with an aqueous solution of the first active metal. The volume of the solution and the concentration of this metal were calculated to reach the desired metal content in the final catalyst. After drying at 105 °C, the solid was calcined at 500 °C under air atmosphere. This procedure was repeated for the deposition of the second active metal. For the catalysts prepared by COI method, HAP support was impregnated with an aqueous solution of both active metals precursors. Table 1 summaries the catalysts prepared. The theoretical metals content was 2.85 wt.% for each metal in each catalyst.

Catalyst characterization

Elemental analysis was carried out using inductive coupled plasma atomic emission spectroscopy (ICP-AES) with a HORIBA Jobin Yvon Ultima 2 apparatus. Prior to the analysis, solid catalysts were dissolved in a strong acid mixture of HNO 3 and HCl. X-ray diffraction (XRD) data were collected using a Phillips Panalytical X'pert Pro MPD diffractometer with a Cu K α (1.543 Å) radiation source and a nickel filter to suppress the Cu Kβ ray. The specific surface area was measured by nitrogen adsorption following standard vacuum drying at 105 °C, using the BET method (MICROMETRICS Gemini Vacprep 061). Scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDX) measurement was performed on a Philips XL30 ESEM apparatus (FEI Company). Transmission electron microscopy coupled with energy dispersive spectroscopy (TEM-EDX) was done with a FEG JEOL JEM 2100F equipped with a HAADF detector. XRD, SEM and TEM analyses were done with fresh catalyst after calcination at 500 °C or with spent catalysts recovered after catalytic tests. From TEM image, metals particle size was evaluated with up to 300 particles.

Catalytic evaluation

Catalytic test was carried out with a tubular fixed-bed ceramic reactor (internal diameter of 8 mm, length of 25 cm), which was surrounded by an Inconel625 tube to protect against mechanical impacts. The details are described elsewhere [START_REF] De Vasconcelos | Catalytic transformation of carbon dioxide and methane into syngas over ruthenium and platinum supported hydroxyapatites[END_REF]. Briefly, the catalytic bed was set at the center of the reactor and was fixed by fine powder of inert support (α-alumina). The reaction temperature was controlled by a thermocouple set at the center of the catalyst bed. The reactor was heated by an electrical furnace. The reaction pressure was observed around 1.6 bar which was slightly higher than the atmospheric pressure. This was due to the pressure drop inside the reactor tube provoked by fine powders of inert support and catalyst. For a given catalytic test, 340 mg of catalyst, diluted two times with SiC, both under fine powder form, were used. Prior to the test, the catalyst was heated under N 2 atmosphere (70 NmL/min) to 700 °C and reduced in-situ at this temperature by 4vol.%H 2 /N 2 stream (70 NmL/min) for 2 h. Then, the reactor system was set at the working temperature under N 2 atmosphere. At the working temperature, the reactor was fed with a gas mixture of CH 4 (20 vol.%) and CO 2 (20 vol.%) diluted in N 2 (total gas flow rate of 90 Nml/min). This high dilution was applied due to security measures required in our laboratory, despite being far from the real biogas mixtures. The inlet gas composition and flow rate were controlled by mass flow controls (MFC). At the outlet of the reactor, a water trap was used to quantify periodically water formed during the reaction. Outlet gas composition was measured by a μ-GC (MyGC, Agilent). A gas counter was also used to measure the total outlet gas flow rate, which was used for the calculation of reaction conversion and selectivity.

The main products of the reaction were CO and H 2 . Water and coke were identified as by product. The quantification of CO, H 2 and H 2 O could be done during the reaction while it was not possible for coke. Thus, the reaction conversion and the selectivity in CO, H 2 , H 2 O, and solid carbon, as well as the molar ratio of H 2 /CO were calculated by the following formula.

= - CH conversion(%) Q ˙Q Q ˙x100 4 (CH ) (CH ) (CH ) 4 in 4 out 4 in (2)
= - CO conversion(%) Q ˙Q Q ˙x100 2 (CO ) (CO ) (CO ) 2 in 2 out 2 in (3) = CO selectivity(%) Q Q ˙x100 O(CO produced) O(CO2consummed) (4) = H selectivity(%) Q Q ˙x100 2 H(H2produced) H(CH4consumed) (5) = H O selectivity(%) Q Q ˙x100 2 O(H2O produced) O(CO2consummed) (6)
= + C selectivity(%) Q (Q ˙Q ˙) x100 (s) Cs(out) C(CH4consumed) C(CO2consumed) (7)
= H CO H CO / % % 2 2 (8)
where Q ˙(CH ) 4 in and Q ˙(CH ) 4 out : methane molar flow rate (mmol h -1 ) at the reactor inlet and outlet; Q ˙(CO ) 2 in and Q ˙(CO ) 2 out : carbon dioxide molar flow rate (mmol h -1 ) at the reactor inlet and outlet; Q ˙O(H2O produced) and Q ˙O(CO2consummed) : atomic oxygen flow rate (mmol h -1 ) at the reactor inlet (under H 2 O form) and outlet (under CO 2 form); Q ˙O(CO produced) : atomic oxygen flow rate (mmol h -1 ) at the reactor outlet (under CO form); Q ˙H(H2produced) and Q ˙H(CH4consumed) : atomic hydrogen flow rate (mmol h -1 ) at the reactor inlet (under H 2 form) and outlet (under CH 4 form); Q O(H2O produced) : atomic oxygen flow rate (mmol h -1 ) at the re- actor outlet (under H 2 O form).

Results

Catalyst characterization

Table 2 shows ICP-AES and specific surface area results. The initial HAP support used in this work had the molar ratio of Ca to P of 1.7, which is close to the stoichiometry of calcium hydroxyapatite (Ca/ P = 10/6 or 1.67). The contents of nickel and cobalt in the final catalysts were also close to the theoretical value of 2.85 wt.%. The difference must be due to metals deposition on the flask walls during the preparation. Metals deposition led to a small decrease of the specific surface area of the support.

XRD patterns of HAP support and HAP supported metals catalysts are presented in Fig. 2. The initial support had only hydroxyapatite as crystalline phase with the main diffraction peaks at 2θ of 25.9, 31.8, 32.2, 32.9, 46.8 and 49.7°. NiO crystalline phase, which has the main diffraction peaks at 37.2 and 43.2°, was detected for Co-Ni/HAP_COI, which was prepared by co-impregnation method. The average crystallite size of NiO of this catalyst was estimated at 11 nm. CaO crystalline phase (with the main peak at 37.0°) was also observed for this co-impregnated catalyst. Co 3 O 4 crystalline phase (with the main diffraction peaks at 36.7°) appeared in all three catalysts. The estimation of the average crystallite size of CaO and Co 3 O 4 of the prepared catalysts is difficult because their main diffraction peaks were very close to each other, and were also close to the second diffraction peak of NiO. Finally, all three catalysts contained also new crystalline phase of cobalt exchanged hydroxyapatite (Ca 10-x Co x (PO 4 ) 6 (OH) 2 ) and nickel exchanged hydroxyapatite (Ca 10-x Ni x (PO 4 ) 6 (OH) 2 ).

The formation of CaO, Ca x Ni x (PO 4 ) 6 (OH) 2 could be explained by the cationic exchange capacity of HAP. In fact, HAP is well known for its capacity to replace Ca 2+ cations in its apatitic structure by other divalent cations such as Pb 2+ , Ni 2+ , Co 2+ , Cd 2+ …in an aqueous suspension [START_REF] Miyake | Synthetic carbonate apatites as inorganic cation exchangers[END_REF][START_REF] Pham Minh | Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF]. The cationic exchanges with Ni 2+ and Co 2+ can be illustrated by the following equations: Ca 10 (PO 4 ) 6 (OH) 2 + xNi(NO 3 ) 2 → Ca 10-x Ni x (PO 4 ) 6 (OH) 2 + xCa(NO 3 ) 2 [START_REF] Jafarbegloo | Thermodynamic analysis of carbon dioxide reforming of methane and its practical relevance[END_REF] Ca 10 (PO 4 ) 6 (OH) 2 + xCo(NO 3 ) 2 → Ca 10-x Co x (PO 4 ) 6 (OH) 2 + xCa(NO 3 ) 2 [START_REF] Chein | Thermodynamic analysis of dry reforming of CH 4 with CO 2 at high pressures[END_REF] Probably during the incipient wetness impregnation, the cationic exchange took place leading to the formation of cobalt and nickel exchange hydroxyapatites, as well as the liberation of Ca(NO 3 ) 2 (Eqs. ( 9) & ( 10)). Ca(NO 3 ) 2 decomposed during calcination to form CaO (Eq. ( 11)). 2Ca(NO 3 ) 2 → 2CaO + 4NO 2 + O 2 [START_REF] De Vasconcelos | Catalytic transformation of carbon dioxide and methane into syngas over ruthenium and platinum supported hydroxyapatites[END_REF] The formation of CaO was only detected by XRD for the catalysts prepared by co-impregnation method. The aqueous solution containing both nickel and cobalt nitrates used for co-impregnation could be more acidic than those containing only nickel or cobalt nitrate used for successive impregnation. High acidity could lead to a local surface dissolution of HAP layers, thus favor the cationic exchange between Ca 2+ and Ni 2+ and Co 2+ . However, CaO could be possibly formed for the two catalysts prepared by successive impregnation but its content was not enough for XRD detection.

SEM images of the fresh catalysts (after calcination) are presented in Fig. 3. For the catalyst prepared by cobalt deposition followed by nickel deposition (Fig. 3(a)), SEM allowed identifying metal-based particles (oxides) with particle size of dozens nm. These particles seemed to be well distributed on the surface of the HAP support. Similar results were observed with the catalyst prepared by nickel deposition followed by cobalt deposition (Fig. 3(b)). For the catalyst prepared by co-impregnation method, it seemed that metal-based particles of larger size were formed (Fig. 3(c)). In all case, metal deposition did not affect the morphology of HAP support. To evaluate the distribution of Ni-and Co-based particles on the support surface, TEM-EDX coupling was performed on the fresh catalysts. Fig. 4 shows TEM images of all three catalysts which confirmed SEM results presented above. From TEM images, particle size distribution was also evaluated using Image J software and the results are later shown in Fig. 11. Metal oxides particles of dozens nm were observed for all the prepared fresh catalysts which were similar to each other. They were also well distributed on the surface of the support.

To identify the distribution of each active metal on the support surface of the fresh catalysts, imaging of the main elements was performed with an EDX analyzer. Fig. 5 shows an example of EDX spot analysis on the catalyst prepared by nickel deposition followed by cobalt deposition and calcination. All spot analyses on bright metal-based particles demonstrated the presence of both Ni and Co (i.e. spots 001 and 002). This demonstrates the formation of "bi-metallic particles" with the simultaneous presence of the two active metals under oxides form. On dark particles (surface support), no trace of Ni or Co was observed (i.e. spot 003).

Fig. 6 shows an example of cartography of the main elements present in the catalyst Ni-Co/HAP_SIWI. As expected, Ca, P and O were observed as the three main elements of the HAP support. Most of Niand Co-based particles detected by EDX analysis were found to be superimposed to each other. This suggests the formation of bimetallic Ni-Co particles (yellow arrows in Fig. 6(e) and (f)). This catalyst was prepared by the successive impregnation method. Since nickel and cobalt have similar physico-chemical properties, nickel particles formed from the first deposition could promote and orientate the deposition and formation of cobalt particles during the second deposition. The superimposition of Ni and Co was also observed with the two other catalysts (Supporting information 2 and 3). Thus, the deposition order did not influence the formation of bimetallic Ni-Co particles.

Catalytic performance

Fig. 7 compares the catalytic activity of three catalysts under the same reaction conditions. The two catalysts prepared by SIWI method had the similar catalytic behavior with the comparable CH 4 and CO 2 conversions along the reaction. Both catalysts showed high initial catalytic activity (around 73-78%). Then, a deactivation was observed within the first ten hours of reaction. Finally, the reactivity seemed to be relatively stable at around 60% for CH 4 conversion and 68% for CO 2 conversion. For the catalyst prepared by co-impregnation method, the initial conversion of CH 4 and CO 2 were lower (around 60%) compared to those of the two other catalysts (around 73-78%). Then, the catalytic activity of this catalyst also became stable at around 47% for CH 4 conversion and 50% for CO 2 conversion. XRD results presented above showed diffraction peaks of CaO which resulted from the partial cationic exchange between Ca 2+ and Ni 2+ and Co 2+ for the co-impregnated catalyst (Fig. 2). It is well known that metallic Co and Ni were active for reforming reactions. On the other hand, exchanged Ni 2+ and Co 2+ in the rigid apatitic strucure (Ca 10-x Ni x (PO 4 ) 6 (OH) 2 and Ca 10- x Co x (PO 4 ) 6 (OH) 2 ) were not active in DRM reaction as confirmed by additional catalytic tests (results not shown). Thus, Ni-Co/HAP_COI catalyst prepared by co-impregnation was less active than two other catalysts prepared by successive impregnation.

The catalytic performance of the prepared bimetallic Co-Ni catalysts in this work was roughly comparable with that of the hydroxyapatite supported monometallic nickel catalyst reported by Rêgo de Vasconcelos et al. under the similar experimental conditions [START_REF] Rego De Vasconcelos | Regeneration study of Ni/hydroxyapatite spent catalyst from dry reforming[END_REF].

Table 3 shows the deactivation rates of each catalyst for 6 h and 50 h of time-on-stream (TOS). As expected, the deactivation rates of all three catalysts was relatively low during 50 h of TOS (0.23-0.32% h -1 ), which emphasizes the high performance of the bimetallic hydroxyapatite catalysts. The deactivation of these catalysts was higher within the first 6 h of TOS (1.15-2.13% h -1 ), especially for the Ni-Co/ HAP_SIWI catalyst (2.13% h -1 ). The deactivation of the catalysts might be related to different aspects such as coke deposition and sintering. These aspects will be addressed later in the manuscript.

Fig. 8 shows the selectivity of the reaction into H 2 , CO, H 2 O, and the molar ratio of H 2 to CO. In all cases, the selectiviy was roughly stable in the range of 80-90% for H 2 and CO, and 10-20% for H 2 O. In the literature [START_REF] Xu | Biogas reforming for hydrogen production over nickel and cobalt bimetallic catalysts[END_REF][START_REF] Aw | Strategies to enhance dry reforming of methane: synthesis of ceria-zirconia/nickelecobalt catalysts by freeze-drying and NO calcination[END_REF], Ni-and Co-based catalysts usually had high selectivity into H 2 and CO as found in this work. For both H 2 and CO selectivity, the catalyst prepared by the co-impregnation method was more selective (around 90%) than the catalysts prepared by the successive impregnation method (around 83-86%). These high H 2 and CO selectivity of Ni-Co/HAP_COI catalyst must be due to its physico-chemical and structural properties. As mentioned above, Ni-Co/HAP_COI prepared by co-impregnation was different from two other catalysts prepared by successive impregnation, mostly with the presence of diffraction peaks of calcium oxide, resulting from the cationic exchange between Ca 2+ and Ni 2+ and Co 2+ . The presence of calcium oxide might have some beneficial effects on the selectivity of the reaction. Previous work of Bellido et al. [START_REF] Bellido | Effect of adding CaO to ZrO 2 support on nickel catalyst activity in dry reforming of methane[END_REF] showed that the addition of CaO to Ni/ZrO 2 catalysts allowed decreasing the implication of CO 2 in RWGS reaction, as well as limiting coke deposition from Boudouard reaction or methane cracking. This improves H 2 and CO selectivity. The beneficial effect of CaO addition on H 2 and CO selectivity in DRM reaction was also highlighted in other works using nickel as active metal [START_REF] Amin | Study of coking and catalyst stability over CaO promoted Ni-based MCF synthesized by different methods for CH 4 /CO 2 reforming reaction[END_REF][START_REF] Bachiller-Baeza | Transient studies of low-temperature dry reforming of methane over Ni-CaO/ZrO 2 -La 2 O 3[END_REF].

About the H 2 to CO morar ratio, its evolution was similar for all three catalysts. This ratio decreased within the first 20 h of reaction from roughly 100% to appromaxively 90% and was then stable at this value. The initial decrease in the H 2 /CO ratio might be related to the occurrence of Boudouard reaction (Eq. ( 12)) and of RWGS (Eq. ( 13)).

CO 2 + H 2 → CO + H 2 O ( 13
)
Despite numerous papers on DRM reaction in the literature, no study reported the experimental quantification of water as a by-product. Water formation was only deduced from the quantification of H 2 , CO, CO 2 and CH 4 at the reactor outlet [START_REF] Aw | Strategies to enhance dry reforming of methane: synthesis of ceria-zirconia/nickelecobalt catalysts by freeze-drying and NO calcination[END_REF][START_REF] Serrano-Lotina | Influence of the operating parameters over dry reforming of methane to syngas[END_REF]. The reactor setup used in this work (Fig. 1) allowed measuring water content formed along the catalytic reaction, as shown in Fig. 8. Under the experimental conditions used in this work, water formation from RWGS (Eq. ( 13)) is inevitable as predicted by thermodynamic calculation [START_REF] De Vasconcelos | Phosphates-based Catalysts for Synthetic Gas (syngas) Production Using CO 2 and CH 4[END_REF][START_REF] Soria | Thermodynamic and experimental study of combined dry and steam reforming of methane on Ru/ZrO 2 -La 2 O 3 catalyst at low temperature[END_REF]. The water selectivity was roughly similar for all three catalysts and was stable around 15% within the last 30 h of reaction. This high amount of water indicated the omnipresence of RWGS for these catalytic systems. This explains also the higher CO 2 conversion compared to CH 4 conversion for each catalyst (Fig. 7) and the molar ratio of H 2 to CO smaller than 1 (Fig. 8). In the literature, Ni-and Co-based catalysts were well reported as efficient catalysts for RWGS [START_REF] Goncalvesa | Selective hydrogenation of CO 2 into CO on a highly dispersed nickel catalyst obtained by magnetron sputtering deposition: a step towards liquid fuels[END_REF][START_REF] Wang | Reverse water-gas shift reaction over co-precipitated Co-CeO 2 catalysts: effect of Co content on selectivity and carbon formation[END_REF].

The solid carbon (C (s) ) formed during the reaction could be deduced from the mass balance. The results are shown in Fig. 9. Selectivity to solid carbon was relatively low (< 13%) for all three catalysts investigated. Both catalysts prepared by SIWI method showed carbon selectivity around 13% during 50 h of TOS. However, the catalyst prepared by the co-impregnation method (Ni-Co/HAP_COI) showed initial carbon selectivity around 8%, which decreased to 4% during the last 25 h of TOS. The decrease in the carbon selectivity is simultaneous to the increase in CO selectivity, which suggests the occurrence of the reverse Boudouard reaction, in which the solid carbon reacts with CO 2 to form CO. These results also emphasize the high performance of the bimetallic hydroxyapatite catalysts. Indeed, there are several reports in the literature concerning the benefits of bimetallic Ni-Co catalysts. The formation of a Ni-Co alloy would enhance not only the catalytic activity but also the supression of carbon deposits during the reaction [START_REF] Takanabe | Titania-supported bimetallic catalysts for carbon dioxide reforming of methane[END_REF][START_REF] Huo | Reduction time effect on structure and performance of Ni-Co/ MgO catalyst for carbon dioxide reforming of methane[END_REF][START_REF] Fan | Catalytic technology for carbon dioxide reforming of methane to synthesis gas[END_REF].

TEM analysis of the used catalysts was also carried out. TEM results are illustrated in Fig. 10. In all cases, the used catalysts contained only few carbonaceous species on their surface. The growth of carbon nanotubes (CNTs) was observed at low frequency compared to the results reported in the literature for DRM reaction [START_REF] Abdullah | Recent advances in dry reforming of methane over Ni-based catalysts[END_REF][START_REF] Taherian | Promotional effect of samarium on the activity and stability of Ni-SBA-15 catalysts in dry reforming of methane[END_REF][START_REF] Shamskar | The influence of Ni loading on the activity and coke formation of ultrasound-assisted co-precipitated Ni/Al 2 O 3 nanocatalyst in dry reforming of methane[END_REF]. In some cases, we can see the embedding of metals particles inside CNTs, causing the partial loss of the catalytic activity. Particularly, other kinds of carbon (i.e. core-shell carbon, amorphous carbon) leading to the embedding of metals particles and so the catalytic deactivation during DRM reaction [START_REF] Abdullah | Recent advances in dry reforming of methane over Ni-based catalysts[END_REF][START_REF] Wang | Coking and deactivation of a mesoporous Ni-CaO-ZrO 2 catalyst in dry reforming of methane: a study under different feeding compositions[END_REF][START_REF] Wolfbeisser | Methane dry reforming over ceria-zirconia supported Ni catalysts[END_REF], was rarely observed. This explained the catalytic stability of these catalysts. Fig. 11 compares metals particle size of the prepared catalysts before and after catalytic tests at 700 °C. We particularly note that for the fresh catalysts which were pre-treated by air calcination, TEM analysis showed the particle sizes of cobalt and nickel oxides. On the other hand, for the used catalysts, TEM analysis showed the particle sizes of metallic particles, which could be partially oxidized in free contact with air after DRM reaction. Co-Ni/HAP_SIWI (Fig. 11A) and Ni-Co/HAP_COI (Fig. 11B) fresh catalysts showed a d 50 of 10-20 nm. However, after 50 h of TOS, the percentage of particles of more than 30 nm was relatively higher, which indicates that sintering/agglomeration of the active phase took place during the reaction. This decreased particles dispersion and, along with the carbon deposition, led to the initial deactivation of the catalysts in the first hours of TOS. On the other hand, Ni-Co/HAP_SIWI fresh catalyst showed higher percentage of particles around 20-30 nm and its distribution did not significantly changed after 50 h of TOS (Fig. 11C). This might also explain the very low C (s) selectivity presented by this catalyst. Indeed, it has been widely reported in the literature that small metal particles are less prone to carbon deposition [START_REF] Kim | Effect of metal particle on coking during CO 2 reforming of CH 4 over Ni-alumina aerogel catalysts[END_REF][START_REF] Usman | Dry reforming of methane: influence of process parametersa review[END_REF] and thus favors the catalyst stability. In all cases, the size of Ni-Co particles only slightly increased, which indicated that sintering of metal particles took place in a low extent. This explains also the overall catalytic stability observed during DRM reaction.

The high performance of the catalysts is related to its physicalchemical properties. The hydroxyapatite support gives thermal stability to the catalyst and its well-known basic properties helps coke removal through Boudouard reaction (Eq. ( 12)) [START_REF] De Vasconcelos | Synthesis of carbon nanotubes/hydroxyapatite composites using catalytic methane cracking[END_REF][START_REF] Pham Minh | One-step synthesis of calcium hydroxyapatite from calcium carbonate and orthophosphoric acid under moderate conditions[END_REF][START_REF] Boukha | Methane dry reforming on Ni loaded hydroxyapatite and fluoroapatite[END_REF][START_REF] Zhang | Development of stable bimetallic catalysts for carbon dioxide reforming of methane[END_REF]. Moreover, using an active phase composed of Ni and Co probably enhanced the catalyst activity and the suppression of carbon deposits. In fact, it has been widely reported in the literature that combining Co and Ni results in better metal dispersion and strong metal-support interaction, which enhances the coke resistance of the catalysts [START_REF] Zhang | Development of stable bimetallic catalysts for carbon dioxide reforming of methane[END_REF][START_REF] Ay | Dry reforming of methane over CeO 2 supported Ni Co, and Ni-Co catalysts[END_REF]. Further characterizations such as acido-basic titration, TPO, TPD and TG-MS coupling seem to be necessary to understand why bimetallic Ni-Co/HAP allowed limiting the formation of amorphous carbon during DRM reaction. Surface properties of HAP support such as its interaction with metals particles, its acido-basic properties need to be highlighted.

DRM at 750 °C

Previous study on the thermodynamic of DRM reaction showed that high reaction temperature favors the conversion of CH 4 and CO 2 and limits the deposition of carbonaceous species [START_REF] De Vasconcelos | Phosphates-based Catalysts for Synthetic Gas (syngas) Production Using CO 2 and CH 4[END_REF][START_REF] Soria | Thermodynamic and experimental study of combined dry and steam reforming of methane on Ru/ZrO 2 -La 2 O 3 catalyst at low temperature[END_REF][START_REF] Challiwala | A combined thermo-kinetic analysis of various methane reforming technologies: comparison with dry reforming[END_REF]. However, increasing the reaction temperature favors also the sintering of active metallic nanoparticles. It is needed to keep a compromise between the temperature effect on the thermodynamic and on the sintering of nanoparticles as well as the operational cost. Thus, another catalytic test was done at 750 °C with Ni-Co/HAP_SIWI. Despite not showing the better catalytic performance among the three catalysts investigated at 700 °C, Ni-Co/HAP_SIWI catalyst suffered of sintering of metals particles in a very low extent, which resulted in very low solid carbon selectivity. Thus, this catalyst was chosen for the investigation of the influence of temperature. The results are presented in Fig. 12. As expected, increasing the reaction temperature from 700 to 750 °C allowed increasing the conversion of both CH 4 and CO 2 from around 60 and 68% (Fig. 7) to around 73 and 79% (Fig. 12A), respectively. An initial catalytic deactivation was also observed at the beginning of the reaction. Then the catalytic activity was kept stable for a long reaction time of 160 h. Water was also formed as a by-product of the reaction and its selectivity was stable around 12%. This was slightly lower than the values observed at 700 °C (Fig. 8). Thermodynamic prediction shows that both water and carbon deposition decrease with the increase of the reaction temperature [START_REF] De Vasconcelos | Phosphates-based Catalysts for Synthetic Gas (syngas) Production Using CO 2 and CH 4[END_REF][START_REF] Soria | Thermodynamic and experimental study of combined dry and steam reforming of methane on Ru/ZrO 2 -La 2 O 3 catalyst at low temperature[END_REF]. This could be resulted from the reaction between coke and water vapor (Eq. ( 14)).

C + H 2 O → 2CO (14)
Fig. 12C shows the selectivity into H 2 and CO. The results were similar to those obtained at 700 °C. After an initial deactivation during the first 20 h of reaction, the selectivity into both H 2 and CO was stable around 75 and 85%, respectively. The H 2 selectivity was lower than the CO selectivity which was explained by RWGS reaction. Also, C (s) selectivity was very low (≈10%) during the 160 h of TOS (result not shown here). Finally, the H 2 to CO molar ratio was close to 90%, as already observed at 700 °C.

As conclusion, the increase of the reaction temperature from 700 to 750 °C allowed significantly increasing the conversion of both CH 4 and CO 2 with the similar catalytic selectivity and stability of Ni-Co/ HAP_SIWI catalyst.

Table 4 compares the performance of hydroxyapatite-based catalysts to those of catalysts prepared with conventional supports for DRM reaction under similar experimental conditions. For both the temperature of 700 and 750 °C, hydroxyapatite supported bimetallic Ni-Co catalysts prepared in this work were found to be competitive versus other nickel and cobalt catalysts supported on the conventional supports like alumina, silica, zirconium oxide, ceria, and SiC. So, hydroxyapatite-based catalysts merit to be further studied for DRM reaction as well as for other reforming and heterogeneous catalytic processes.

Conclusions

For the first time, hydroxyapatite supported bimetallic Ni-Co catalysts were prepared, characterized and studied in DRM reaction. In all cases, bimetallic nanoparticles containing both Ni and Co were formed, regardless of the preparation methods. Ni-Co particles of dozens nm could be clearly observed on the surface of HAP support by TEM-EDX analysis. No modification of HAP support structure was observed after Ni-Co deposition, except the superficial cationic exchange of Ca 2+ with Ni 2+ and Co 2+ .

DRM reaction was carried out at 700 and 750 °C and 1.6 bar with an equimolar mixture of CO 2 and CH 4 diluted in N 2 (60%vol.). All the prepared catalysts showed high catalytic activity and stability. For each catalytic test, an initial catalytic deactivation took place within the first 10 h of time-on-stream (TOS). Then the catalytic activity was stable up to 50 or 160 h of TOS. CO and H 2 were the main products of the reaction and their selectivity reached about 80-90%. Reverse WGS also occurred, which led to the formation of water as by-product, to the higher conversion of CO 2 compared to that of CH 4 , and to the H 2 /CO molar ratio close to 90%.

All the prepared catalysts showed good catalytic stability during DRM reaction. This was explained by the formation of carbonaceous species under CNTs form at low content. Other kinds of carbon such as amorphous carbon or core-shell carbon, which mainly causes the catalytic deactivation in DRM reaction, were not observed.

Fig. 1 .

 1 Fig. 1. Illustration of the fixed-bed reactor used in this work.

Fig. 2 .

 2 Fig. 2. XRD patterns of the prepared catalysts; ♦: HAP, *: cobalt and nickel exchanged hydroxyapatites, ◊: NiO, □: C o 3 O 4 , Δ: CaO.

Fig. 3 .

 3 Fig. 3. SEM images of: (a) Co-Ni/HAP_SIWI; (b) Ni-Co/HAP_SIWI; (c) Ni-Co/HAP_COI; and (d) support HAP.

Fig. 4 .

 4 Fig. 4. TEM images of: (a, b) Co-Ni/HAP_SIWI; (c, d) Ni-Co/HAP_SIWI; and (e, f) Ni-Co/HAP_COI.

Fig. 5 .

 5 Fig. 5. Example of EDX analysis of Ni-Co/HAP_SIWI.

Fig. 6 .

 6 Fig. 6. Imaging of the main elements present in the catalyst Ni-Co/HAP_SIWI; (a): TEM image of the cartography zone; (b), (c), (d), (e), (f): imaging of O, Ca, P, Co, Ni, respectively.

Fig. 7 .Fig. 8 .Fig. 9 .

 789 Fig. 7. Reaction conversions at 700 °C obtained with different catalysts; gas flow rate of CH 4 , CO 2 and Ar: 20, 20 and 50 NmL/min, respectively; 340 mg of catalyst.

Fig. 10 .

 10 Fig. 10. TEM images the used catalysts recovered after DRM reaction at 700 °C: (a, b) Co-Ni/ HAP_SIWI; (c, d) Ni-Co/HAP_SIWI; and (e, f) Ni-Co/ HAP_COI.

Fig. 11 .Fig. 12 .

 1112 Fig. 11. Comparison of metals particle size distribution before and after catalytic test at 700 °C; (A): Co-Ni/HAP_ SIWI; (B) Ni-Co/HAP_ SIWI; and (C) Ni-Co/HAP_COI.

Table 1

 1 Catalysts prepared in this work.

	Catalyst	Preparation method	Preparation procedure
	Co-Ni/HAP_SIWI	SIWI	Cobalt deposition, followed by nickel
			deposition
	Ni-Co/HAP_SIWI	SIWI	Nickel deposition, followed by cobalt
			deposition
	Ni-Co/HAP_COI	COI	Co-impregnation of cobalt and nickel

Table 2

 2 Elemental analysis and specific surface area of the prepared catalysts.

	Catalyst	Co (wt.%)	Ni (wt.%)	Ca/P	S BET (m 2 /g)
	Ni-Co/HAP_SIWI	2.4	2.4	1.7	46
	Co-Ni/HAP_SIWI	2.6	2.8		49
	Ni-Co/HAP_COI	2.8	2.6		48
	HAP support	0	0		60

Table 3

 3 Deactivation rates of the hydroxyapatite-based catalysts.

	Catalyst	Deactivation rate (% h -1 ) at	Deactivation rate (% h -1 ) at
		TOS = 6h	TOS = 50h
	Ni-Co/HAP_SIWI	2.13	0.23
	Co-Ni/HAP_SIWI	1.15	0.32
	Ni-Co/HAP_COI	1.45	0.23

Table 4

 4 Examples of the catalytic performance of Ni-and Co-based catalysts at 700-750 °C in DRM. Space velocity, cm 3 h -1 g -1 .

	Metal and its content	Support	Catalyst weight,	Inlet gas composition,	Gas feeding T (°C)/P	CH 4 conv.,	CO 2 conv.,	Time-on-	Ref.
	(wt.%)		mg	vol.%		(bar)	%	%	stream, h
	5.1%Ni 5.9%Ni	Ce 0.15 Zr 0.85 O 2 Y 2 O 3 -ZrO 2 Al 2 O 3 Al 2 O 3	50 50 50 100	60,000 a 4%CO 2 , 4%CH 4 , 92%He 120,000 a 50%CO 2 , 50%CH 4 4%CO 2 , 4%CH 4 , 92%He 120,000 a 40%CO 2 , 40%CH 4 , 20% 100 b	750/1 750/1 750/1 750/1	≈85 ≈93 ≈10 48	≈85 ≈90 ≈20 48	70 25 5 100	[61] [61] [61] [62]
	10%Ni	Phosphate-modified	100	N 2 40%CO 2 , 40%CH 4 , 20%	100 b	750/1	58	57	100	[62]
	10%Ni-3%Co 2%Ni-1%Co 2.8%Ni-2.8%Co	Al 2 O 3 Al 2 O 3 -ZrO 2 CeZrO 2 /β-SiC HAP	100 500 340	N 2 50%CO 2 , 50%CH 4 50%CO 2 , 50%CH 4 20%CO 2 , 20%CH 4 , 60%	40 b 100 b 90 b	750/1 750/1.2 750/1.6	≈80 ≈65 73	≈90 ≈75 79	n.r. 100-550 160	[63] [64] This
	10%Ni 10%Co 2.8%Ni-2.8%Co	SiO 2 SiO 2 HAP	200 200 340	N 2 50%CO 2 , 50%CH 4 50%CO 2 , 50%CH 4 20%CO 2 , 20%CH 4 , 60%	40 b 40 b 90 b	700/1 700/1 700/1.6	≈50 ≈21 60	≈62 ≈35 68	5 5 160	work [65] [65] This
				N 2						work

a b Ncm 3 /min; n.r.: not reported.

Acknowledgments

The authors gratefully thank ADEME (Agence de l'environnement et de la maîtrise de l'énergie, France, for the financial support of the VABHYOGAZ3 project) and PRAYON for their supports. The authors also thank our colleagues at the RAPSODEE center (CNRS UMR 5302) for technical help.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcatb.2017.10.063.