# Promoting NH3 synthesis by supporting Ru nanoparticles on hydroxyapatites of appropriate chemical composition: Towards structure-activity relationships

Sabrina Akrou, Guylène Costentin, Sandra Casale, Alexandre Vimont, Cyril Thomas

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## To cite this version:

Sabrina Akrou, Guylène Costentin, Sandra Casale, Alexandre Vimont, Cyril Thomas. Promoting NH3 synthesis by supporting Ru nanoparticles on hydroxyapatites of appropriate chemical composition: Towards structure-activity relationships. Applied Catalysis B: Environmental, 2025, 384, pp.126141. 10.1016/j.apcatb.2025.126141. hal-05351385

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https://hal.science/hal-05351385v1

Submitted on 6 Nov 2025

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# Promoting NH₃ synthesis by supporting Ru nanoparticles on hydroxyapatites of appropriate chemical composition: Towards structure-activity relationships

Sabrina Akrour<sup>a,1</sup>, Guylène Costentin<sup>a,2</sup>, Sandra Casale<sup>a,3</sup>, Alexandre Vimont<sup>b,4</sup>,
Cyril Thomas<sup>a,*,5</sup>

## ARTICLE INFO

### Keywords:

Heterogeneous catalysis

Ruthenium

Electron density

Structure-activity relationship

HAp

## ABSTRACT

Ammonia is the second most widely produced chemical in the world. It is essential for fertilizer manufacturing and has recently emerged as an energy carrier. Ammonia has been produced for over 100 years using the Haber-Bosch process, which is highly energy-intensive and accounts for nearly 3 % of annual CO₂ emissions. Efforts to improve the energy efficiency of this process, particularly the development of more efficient catalysts, are therefore of major interest. This study investigates how the chemical composition and basicity of hydroxyapatite (HAp) supports influence the catalytic performance of supported Ru metal nanoparticles in ammonia synthesis under mild conditions. Particular attention was paid to synthesizing catalysts with HAp supports of different chemical compositions, comparable specific surface areas, and with supported Ru metal nanoparticles of similar sizes (2.2 ± 0.2 nm) to allow for a reliable comparison. The HAp-supported Ru (2 wt%) catalysts exhibited optimal NH₃ performance with HAp supports precipitated at a pH ranging from 7.4 to 7.8. These catalysts exhibited an NH₃ TOF of 0.40 ± 0.01 min⁻<fnref n="1" /> at 400 °C and 1 bar, which is approximately 50 % higher than that of a Ru/MgO reference catalyst. The superior NH₃ synthesis performance of these catalysts is attributed to the minimization of defects incorporated in the HAp crystalline structure and the higher basicity of the corresponding HAp s. NH₃ TOF was remarkably correlated with the electron density of the Ru metal nanoparticles, as monitored by N₂ adsorption at low temperatures followed by FTIR, making this finding of great interest in the field of ammonia synthesis.

## 1. Introduction

Ammonia is a key chemical compound mainly used to produce fertilizers [1], which are essential for feeding the world's population. Ammonia is also used to produce explosives and polymers [2]. Recently, ammonia has been identified as an energy vector, due to its high energy density and hydrogen content, as well as the existing infrastructure for its storage and transportation [1,3]. NH₃ could therefore be used as a

chemical to store the hydrogen produced by water electrolysis powered by renewable energy sources, such as sunlight and wind [1].

The ever-increasing annual production of ammonia amounted to about 235 million tons in 2019, making it the second most produced chemical after sulfuric acid [2], is achieved via the well-known Haber-Bosch process industrialized as early as 1913 [4]. This process involves the reaction of molecular dihydrogen and dinitrogen (N₂ + 3 H₂ = 2 NH₃, ΔH = -45.7 kJ/mol) and was found to be originally catalyzed by

<footnote><sup>a</sup> Sorbonne Université, CNRS, Laboratoire Réactivité de Surface, LRS, Paris F-75005, France</footnote>

<footnote><sup>b</sup> Université de Caen Normandie, ENSICAEN, CNRS, LCS, Laboratoire Catalyse et Spectrochimie, Caen F-14000, France</footnote>

<footnote>* Correspondence to: Sorbonne Université, CNRS, Laboratoire Réactivité de Surface, LRS, UMR CNRS 7197, 4 Place Jussieu, Tour 43-53, 3ème étage, Case 178, Paris F-75252, France,</footnote>

<footnote>E-mail address: cyril.thomas@sorbonne-universite.fr (C. Thomas).</footnote>

<footnote n="1">¹ orcid.org/0009-0009-1845-3511,</footnote>

<footnote n="2">² orcid.org/0000-0003-1559-6890,</footnote>

<footnote n="3">³ orcid.org/0000-0003-2691-5391,</footnote>

<footnote n="4">⁴ orcid.org/0000-0001-9799-6024,</footnote>

<footnote n="5">⁵ orcid.org/0000-0003-4224-6095.</footnote>
Fe-based materials [1]. This process is thermodynamically-favored at high pressures and low temperatures, but kinetically-limited at low temperatures so that it is commonly operated at high temperatures (> 400 °C) and pressures (> 20 MPa), which makes it extremely energy intensive [2]. Additionally, the H₂ used in the Haber–Bosch process is produced by the catalytic steam reforming of natural gas (CH₄ + H₂O = CO + 3 H₂, ΔH = 206 kJ/mol), which is also an energy-intensive process due to its strong endothermic nature. Therefore, the production of ammonia through the steam reforming and the Haber–Bosch processes is responsible for large amounts of greenhouse gas emissions [2], which should be reduced in the context of global warming.

In the early 1970s, it was discovered that ammonia synthesis could be catalyzed more effectively than with the Fe-based catalyst, i.e., at lower temperatures and pressures using alkali metal-promoted Ru materials supported on activated carbons [1,5]. However, these catalysts not only suffered from hydrogen poisoning due to strong adsorption onto the Ru nanoparticles, but also from a burying/sintering of the Ru nanoparticles into/onto the activated carbons due to the methanation reaction (C<sub>support</sub> + 2 H<sub>2</sub> = CH<sub>4</sub>) at the interface between the carbon support and the Ru nanoparticles [1]. The latter drawback led to the study of oxide-supported Ru catalysts. Overall, it was concluded in this field that the catalytic performance of the Ru catalysts was strongly dependent on the size of the Ru nanoparticles (structure-sensitivity concept) [1,6,7] and on the catalyst basicity [1,7], which is defined as the electron-donation ability of the oxide promoters and/or the support to the Ru nanoparticles [1]. Since the rate-determining step of the ammonia synthesis catalytic reaction has been claimed to be the dissociation of N<sub>2</sub> [1,6,8], an increase in the catalyst basicity induced by the promoters and/or the support should result in an increase in the electron density of the Ru nanoparticles and the electron transfer from these nanoparticles to the π* anti-bonding orbital of N<sub>2</sub>, which in turn is supposed to favor the dissociation of N<sub>2</sub> [9]. The Ru-promoted-MgO catalytic system showed limitations, however, such as: (i) a strong sensitivity to the poisoning of the Ru surface by hydrogen [3], (ii) the loss of Cs interaction with the Ru nanoparticles after high-temperature reduction [10], (iii) Ru sintering due to changes in MgO support textural properties in the presence of the Ba promoter [10], and (iv) the deleterious effect of the presence of residual chlorides from the commonly used RuCl<sub>3</sub> precursor [9]. Recently, Miyahara et al. demonstrated that the hydrogen inhibiting effect could be significantly reduced in Ru/lanthanoid oxide catalysts and that the turnover frequency (TOF) of ammonia synthesis was correlated with the surface density of the basic sites in the catalysts [3]. Various studies have also reported that ammonia synthesis was sensitive to the size of the Ru particles [11–14] and that adding fullerene (C<sub>60</sub>) to supported Ru catalysts drastically promotes this reaction due to an “electron buffer” effect of C<sub>60</sub>, which would tune the electron density of the catalytic sites of Ru metal particles [15].

Hydroxyapatites (HApS), which are environmentally friendly calcium phosphate minerals (Ca<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub>), are the main mineral found in bones and teeth [16]. These materials have been shown to be thermally stable up to 900 °C [17] and to exhibit promising catalytic properties in reactions relevant to energy and environmental fields [18–22]. These materials are also well-known to exhibit basic properties that can be finely tuned by adjusting the Ca/P ratio; the higher the ratio is, the greater the basicity of HAp is [23].

As a "Never-Ending Story" [6], due to the increasing needs in ammonia [2] and the recent surge of interest in ammonia as an energy carrier [1], the development of improved materials for the catalytic synthesis of ammonia remains of great interest to increase the thermodynamic efficiency of this compound [6]. Overall, Ru-based catalysts promoted by basic additives and/or supports have been reported to be the most active materials in the catalytic synthesis of ammonia [1,2]. However, to our knowledge, one of the current promising basic supports, namely HAp, has not yet been reported as an oxide carrier of Ru nanoparticles for the catalytic synthesis of ammonia, despite the fact

that Ru/HAp materials have previously been reported to be beneficial in several reactions such as the oxidation of alcohols [24], the hydrogenation of renewable oils [25], and the water-gas shift reaction [26].

The aim of this study is to investigate how the chemical composition and basicity of HAp supports influence the catalytic performance of HAp-supported Ru metal nanoparticles for ammonia synthesis at atmospheric pressure. Particular attention is paid to synthesizing HAp supports with different chemical compositions but comparable specific surface areas and rod-like morphologies, and with supported Ru metal nanoparticles of similar sizes so as to provide a reliable comparison of the studied catalysts. The influence of the nature of the Ru precursor is also studied. Finally, by using a chloride-free Ru precursor and by considering the recent characterization of the electron density of the supported Ru metal nanoparticles via N<sub>2</sub> adsorption followed by FTIR [27], robust structure-activity relationships are obtained allowing for a comprehensive understanding of the catalytic performance of a series of HAp-supported Ru catalysts in ammonia synthesis.

## 2. Experimental

### 2.1. *Catalyst preparation*

To maintain consistency with a recent study [27], the HAp-supported Ru catalysts examined in this study are denoted as Ru/HAp-pH<sub>x</sub>, where x is the pH of the medium in which the HAp support was precipitated. The preparation of the HAp supports precipitated at pH 6.5, 7.4, 7.6, 8.0, 8.3, 8.5, and 9.0 was detailed in the aforementioned study. For this study, two additional HAp supports were precipitated at pH 7.2 and 7.8. In brief, an aqueous solution of the phosphate precursor (NH<sub>4</sub>H<sub>2</sub>PO<sub>4</sub>, ≥99.999 % purity, Acros Organics) was added to a calcium precursor solution (Ca(NO<sub>3</sub>)<sub>2</sub>·4 H<sub>2</sub>O, ≥ 99.0 % purity, Sigma-Aldrich) at 80 °C in a 0.5 L reactor of an Optimax 1001 Workstation (Mettler Toledo). This setup allowed for the control of the synthesis parameters (pH, temperature of the precipitation medium, and addition rate of the NH<sub>4</sub>H<sub>2</sub>PO<sub>4</sub> solution) [28]. The pH probe (InPRO 3250i from Mettler Toledo) used can measure pH between 0 and 100 °C and measures the temperature itself. The pH of the precipitation medium was measured every 3 s, and ammonia was automatically added if the pH deviated by more than 0.01 units. Thus, the reactor enables a pH regulation within 0.01 units. To ensure accurate pH measurements, the pH probe was calibrated at room temperature using buffer solutions prior to each synthesis. For pH synthesis lower than 7, pH 4 and 7 buffer solutions were used, while for pH synthesis higher than 7, pH 7 and 10 buffer solutions were used. The precipitation medium was heated to 80 °C, stirred continuously at 400 rpm and maintained at steady pH (measured at 80 °C). The pH of the medium in the reactor was controlled via automated addition of concentrated NH<sub>4</sub>OH (30%, Sigma-Aldrich) to offset the acidification of the medium associated with HAp precipitation (10 Ca<sup>2+</sup> + 6 HPO<sub>4</sub><sup>2-</sup> + 2H<sub>2</sub>O = Ca<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub> + 8 H<sup>+</sup>). The initial concentrations of the calcium and phosphate solutions, [Ca]<sub>0</sub> and [P]<sub>0</sub>, were 0.216 and 0.130 mol L<sup>-1</sup>, respectively, so as to theoretically achieve the preparation of materials with a Ca/P stoichiometric molar ratio of 1.67.

After precipitation, the HAp powders were recovered by centrifugation at 8500 rpm, washed 3 times with distilled water and centrifuged after each washing step. The HAp supports were then dried at 100 °C for 12 h in a laboratory oven on a glass watch before being crushed smoothly in an agate mortar.

2 wt% of Ru was deposited by wet impregnation of the dried HAp supports with a nitric acid solution of Ru(NO)(NO<sub>3</sub>)<sub>3</sub> (Merck, 1.4 wt% Ru). After drying at 80 °C for 1 h and at 100 °C for 12 h, the crushed and sieved (125–200 µm) samples were placed on the frit of a Pyrex fixed-bed reactor and reduced under 60 mL/min of H<sub>2</sub> (Air Liquide, Alpha-gaz 1, 99.999 %) for 2 h at 450 °C (5 °C/min).

The influence of the Ru loading deposited on an HAp-pH<sub>9.0</sub> support was studied by depositing 1, 4, and 6 wt% of Ru by wet impregnation with a Ru(NO)(NO<sub>3</sub>)<sub>3</sub> nitric acid solution (Merck, 1.4 wt% Ru). These
samples are denoted as Ru<sub>y</sub>/HAp-pH<sub>9.0</sub>, where y is the loading of Ru (wt %), and were dried and reduced as described previously for the samples containing 2 wt% of Ru.

The influence of the Ru precursor was investigated by using RuCl₃ (Tokyo Chemical Industry Co., 40 wt% minimum Ru content) and Ru(NH₃)₆Cl₃ (ThermoFisher, 32.1 wt% minimum Ru content) salts, whose appropriate weights were diluted in deionized water to deposit 2 wt% Ru by wet impregnation of an HAp-pH<sub>9.0</sub> support. The resulting samples are denoted as Ru<sub>Cl3</sub>/HAp-pH<sub>9.0</sub> and Ru<sub>(NH3)6Cl3</sub>/HAp-pH<sub>9.0</sub>, and were dried and reduced as previously described for the samples containing 2 wt% of Ru deposited with the Ru(NO)(NO₃)₃ precursor. Since most of the samples used in this study were prepared with the Ru(NO)(NO₃)₃ precursor, therefore, notification of this precursor was omitted from the labeling of the corresponding samples for clarity.

## 2.2. Catalyst characterization

The Ca and P content of the prepared HAp supports was determined according to a previously described quantitative X-ray fluorescence (XRF) protocol [28]. Briefly, the HAp supports were melted into “beads” and analyzed with a Panalytical Epsilon 3XL X-ray fluorimeter equipped with an Ag X-ray tube operating in a He atmosphere under two conditions: 12 kV and 25 µA for calcium analysis, and 10 kV and 30 µA for phosphorus analysis, for 120 s each. The beads were prepared by mixing approximately 0.1 g of the sample with 1.2 g of fluxing agent (LiBO₂/-Li₂B₄O₇) and 0.019 g of a non-wetting agent (LiBr) in a platinum crucible. The mixture was then heated to 1050 °C for 25 min in a melting device (LeNeo fluxer, Claisse). Calibration curves were obtained from beads of mechanical mixtures of the Ca(OH)₂ and NH₄H₂PO₄ precursors. The response was found to be perfectly linear over the entire concentration range. The XRF limit of detection was 0.03 and 0.004 wt% for Ca and P, whereas the XRF limit of quantification was 0.09 and 0.02 wt% for Ca and P, respectively. Three measurements were performed on each sample, resulting in the determination of the Ca and P content with a relative deviation of ± 1 %.

The X-ray diffraction (XRD) patterns of the dried HAp-pHₓ materials were recorded in the 2θ range of 10–80° with 0.01° steps and 0.5 s/step with a Bruker D8 ADVANCE diffractometer equipped with a copper source (λCu-Kα1 = 1.54056 Å and λCu-Kα2 = 1.54439 Å) and a LynxEye detector.

N₂ sorption measurements were carried out on a Belsorp max instrument (Bel Japan) at 77 K after evacuation of the samples at 300 °C for 3 h. The specific surface area of the samples was determined by using the Brunauer–Emmett–Teller (BET) method in the 0.05 < *p* / *p*₀ < 0.30 domain.

SEM–FEG imaging of the supporting oxides was done with a Hitachi SU-70 field emission gun scanning electron microscope, at magnifications ranging from × 10k to × 40k, corresponding to scalebars of 100–500 nm. The HAp-pHₓ powder samples were deposited directly onto carbon adhesive tape on an aluminum SEM support. Imaging was performed without any metallization, at 1 kV, and a working distance of approximately 2 mm, hence by an “in lens” secondary electron detector (SE–Upper).

Transmission electron microscopy (TEM) was performed using a JEOL JEM 2100Plus, operating at 200 kV and equipped with a Thermionic source LaB6 (single crystal). Micrographs were recorded with a Gatan Orius 4 K camera. The powder samples were ground in an agate mortar, suspended in ethanol, and dispersed in an ultrasonic bath. Then, they were rapidly deposited on a carbon-coated copper grid. The diameter of the Ru nanoparticles (d, in Å) was estimated by using the ImageJ program. *d* was determined by measuring at least 300 metal particles for most of the Ru/HAp-pHₓ samples, and 198 and 116 metal particles for the Ru/MgO and Ru(NH₃)₆Cl₃/HAp-pH₉.₀ samples, respectively, and was calculated using the following formula: $d = \frac{\sum n_i d_i^3}{\sum n_i d_i^2}$ [29] where nᵢ is the number of particles with a dᵢ diameter. The Ru dispersion, *D* (%), was calculated as follows: $D = (6 \nu_m / a_m) / d$, where νm

and aₘ are the volume occupied by a Ru atom in bulk metal (13.65 Å³) and the surface area occupied by a Ru atom on a hcp surface (6.35 Å²) [29], respectively. Statistical analysis of the Ru nanoparticles from the Ru/HAp-pH₈.₀ sample was performed twice on separate images, yielding a mean diameter of 2.2 ± 0.1 nm. The size of the HAp-supported Ru nanoparticles was essentially measured after the H₂ reduction step at 450 °C for 2 h (fresh). For comparison, the size of the HAp-supported Ru nanoparticles was also measured after catalytic NH₃ synthesis (spent).

The adsorption of N₂ followed by FTIR (N₂-FTIR) was performed on the Ru/MgO sample using the previously described procedure [27]. After the N₂ adsorption (Alphagaz™2, purity ≥ 99.9999 %, Air Liquide) on the Ru/MgO sample, the low-temperature (100 K) FTIR spectra were collected in transmission mode using a Nicolet iS50 infrared spectrometer equipped with a DTGS detector and an extended KBr beam splitter and purged under air previously treated by a CO₂ purification and drying unit. A total of 128 scans were averaged with a spectral resolution of 4 cm⁻¹. The samples were pressed into self-supporting wafers of approximately 10 mg/cm² (20 mg, 10⁷ Pa/cm², and 2.0 cm²). The wafer was loaded into an in situ quartz infrared PELICAEN cell [30], which was developed and commercialized at the Laboratoire Catalyse et Spectrochimie. The cell is equipped with a liquid nitrogen tank and CaF₂ windows on the bottom part for low-temperature experiments. The cell is connected to a vacuum line, equipped with two Pfeiffer vacuum pressure gauges (0–10 and 0–1000 mbar ranges with 0.2 % accuracy of the full scale), for sample pretreatment and the introduction of gaseous probe molecules. Prior to recording the spectrum, the samples underwent a reducing pretreatment (150 Torr of H₂, Alphagaz™2, ≥ 99.9999 % purity, Air Liquide) at 450 °C for 2 h (2 °C/min), followed by evacuation at 450 °C for 1 h (10⁻⁵ Torr). Finally, the samples were cooled to approximately 100 K by introducing liquid nitrogen into the tank under a dynamic vacuum of 10⁻⁵ Torr. To homogenize the temperature of the sample, an equilibrium pressure of approximately 0.2 Torr of helium (Alphagaz™2, ≥ 99.9999 % purity, Air Liquide) was introduced into the cell before adding accurate doses of N₂ through a small, calibrated volume (1.946 cm³) positioned at the front side of the cell. The band area measurements were typically accurate to within ± 5 % and normalized with respect to the weight of the self-supporting wafers.

## 2.3. Basic properties of the HAp-pHₓ supports

The basic properties of the HAp-pHₓ supports were estimated using the 2-methyl-3-butyn-2-ol (Methylbutynol, MBOH) model molecule [31]. The model gas-phase reaction was carried out using a differential-flow micro-reactor. The catalyst (20 mg) was placed on a fritted disk inside a U-type quartz reactor with an inner diameter of 10 mm. The temperature of the catalyst bed was controlled by a thermocouple located close to the wall of the quartz reactor. The samples were pretreated with a 50 mL/min helium (Alphagaz™1, ≥ 99.999 % purity, Air Liquide) flow up to 450 °C (5 °C/min) for 2 h before cooling to a reaction temperature of 140 °C. Then, the catalyst was exposed to the reacting feed, which was obtained by bubbling helium (100 mL/min) through liquid MBOH (Fluka, 99.9 % purity) at 20 °C (MBOH partial pressure of 1.73 kPa). The reaction products were analyzed every 2 min using a µ-GC (Varian, CP 2003 P) chromatograph equipped with a CPWAX 52 CB column. MBOH conversion (Conv.) was calculated as 0.5 (P<sub>acetone</sub> + P<sub>acetylene</sub>) / P<sub>MBOH</sub><sup>0</sup>, where P<sub>acetone</sub>, P<sub>acetylene</sub>, and P<sub>MBOH</sub><sup>0</sup> are the partial pressures of acetone and acetylene, and the initial partial of MBOH, respectively. For duplicate measurements, the deviation from the average value was found to be less than 5 % relative.

The number of OH groups in the HAp channels of some Ru/HAp-pHₓ samples was monitored using FTIR before introducing the N₂ doses, intended to characterize the electron density of the Ru nanoparticles supported on the HAp-pHₓ supports. The experimental conditions used are the same as those described in the N₂-FTIR paragraph at the end of Section 2.2.
## 2.4. NH₃ synthesis catalytic experiments

Unless otherwise stated, steady-state catalytic NH₃ synthesis experiments were carried out in a tubular stainless steel reactor (9.1 mm i.d.) operated from a FR-100 system (Micromeritics). In a typical run, 0.1 g of prereduced catalyst (125–200 µm) diluted in 4.9 g of silicon carbide (SiC, > 99 % purity, Prolabo, 484 µm) was placed in the middle of the reactor on a plug of quartz wool, which was supported by a stainless steel frit. A leak test up to 20 bar was performed under H₂. The catalyst was then exposed to a stoichiometric H₂–N₂ (3–1) flow from cylinders (Alphagaz™1, ≥ 99.999 % purity, Air Liquide) via Bronkhorst mass flow controllers (F-201CV-200-RAD-11-V) with a total flow rate of 60 mL·NTPR/min at atmospheric pressure. The temperature was controlled via a K-type thermocouple that was inserted into a thermocouple well within the catalyst bed. The catalyst was contacted with the reaction mixture at RT, and the temperature was increased (5 °C/min) to 450 °C, with steps of 25 °C from 300 to 450 °C, for approximately 2 h at each reaction step. After the 450 °C step, the temperature was decreased to 400 °C and was held at this temperature for at least 4 h. The temperature could also be increased up to 550 °C to verify the validity of the FR-100 setup on a thermodynamic basis. The outlet of the reactor was connected to a MKS2030 (MKS) FTIR analyzer, which allowed for the continuous monitoring of the NH₃ concentration with a sampling frequency of 0.2 Hz.

An NH₃ synthesis experiment was also performed on a Ru/HAp·pH₆.₅ catalyst using a homemade setup with a U-type quartz reactor (15 mm inner diameter) [32,33]. The operating conditions were the same as those detailed for the aforementioned FR-100 reactor. The H₂–N₂ (Alphagaz™1, ≥ 99.999 % purity, Air Liquide) gas feed was supplied with Brooks 5850TR mass flow controllers. The reactor temperature was controlled using a Eurotherm 2408 temperature controller and a K-type thermocouple located on the quartz reactor wall. NH₃ analysis was performed using the aforementioned MKS2030 FTIR analyzer.

Ammonia production is expressed as the measured NH₃ concentration ([NH₃], ppm), and the corresponding rate (Eq. 1), and turnover frequency (TOF, Eq. 2).

$$r \text{ NH}_3 = [\text{NH}_3] \times 10^{-6} \times F \times 10^3 / (22414 \times sw) \quad (1)$$

where r NH₃, F and sw are the NH₃ production rate (mmol NH₃/h g), the flow rate (3600 mL/h) and the catalyst sample weight (g), respectively,

$$\text{NH}_3 \text{ TOF} = r \text{ NH}_3 / \text{Ru}_s^0 = (r \text{ NH}_3 \times 10^{-3} / 60) / (\text{Ru}_L \times D_{\text{TEM}} / (10^4 \times M_{\text{Ru}})) \quad (2)$$

where NH₃ TOF, Ru⁰, RuL, DTEM, and MRu are the NH₃ turnover frequency (min⁻¹), the number of Ru⁰ surface atoms (mmol/g), the Ru loading (wt%), the Ru dispersion estimated by TEM (%), and the Ru molar weight (mol/g), respectively.

# 3. Results and discussion

## 3.1. Preparation of HAp·pHₓ materials with a similar specific surface area

As mentioned in the introduction, a reliable comparison of Ru/HAp·pHₓ catalysts in the ammonia synthesis catalytic reaction relies on the use of HAp supports with similar specific surface areas. However, synthesizing HAp·pHₓ materials has been shown to be challenging [20,28, 34]. A previous study has shown that the HAp's textural properties depend strongly on the synthesis operating parameters, such as the pH and the temperature of the precipitation medium, and the order in which precursor solutions are introduced [28]. In this study, the temperature was set to 80 °C, and the P-to-Ca synthesis route was used by adding the NH₄H₂PO₄ solution to the calcium solution [28]. The P-to-Ca synthesis route was chosen because it produces HAp materials with a

rod-like morphology over a wide pH range during precipitation, whereas the Ca-to-P synthesis route has been shown to produce HAp materials with ill-defined and platelet-like morphologies when the pH of the precipitation medium decreases from 9.0 to 4.2 [28]. Adding an NH₄H₂PO₄ solution at a rate (r NH₄H₂PO₄) of 4.4 mL/min to a 1 L reactor at pH between 8.3 and 9.0 yielded HAp materials (HAp·pH₈.₃, HAp·pH₈.₅, and HAp·pH₉.₀) with similar specific surface areas of approximately 38 m²/g (Table 1). Using the same addition rate in a 0.5 L reactor at pH 8.0 produced an HAp material (HAp·pH₈.₀) with a specific surface area of 34 m²/g (Table 1). As previously indicated [27], these 4 samples underwent a final 2-h maturation step. Under similar conditions but at a lower pH, the specific surface area of the synthesized HAp s was found to differ from the value targeted in this work. Reynaud et al. previously demonstrated in a 0.5 L reactor that the specific surface area of HAp s prepared at pH 6.5 increased significantly from 15 to 54 m²/g as the addition rate of the NH₄H₂PO₄ solution increased from 2.2 to 50.0 mL/min [34]. These results highlight the importance of the NH₄H₂PO₄ solution addition rate for controlling the specific surface area of HAp s at low pH. This work shows that synthesizing a material at pH 6.5 in a 0.5 L reactor with an NH₄H₂PO₄ solution addition rate of 50.0 mL/min, without a maturation step, produces HAp with a specific surface area of 58 m²/g (Fig. 1a). These results suggest that the maturation step did not lead to significant changes in the specific surface area of the synthesized HAp materials, given the accuracy with which such a property is determined (± 2 m²/g [27]). Fig. 1a also shows that using an intermediate NH₄H₂PO₄ solution addition rate of 25 mL/min produces an HAp·pH₆.₅ material with a specific surface area of 39 m²/g. An addition rate of 12 mL/min of the NH₄H₂PO₄ solution produced an HAp·pH₇.₂ support with a specific surface area of 38 m²/g (Table 1). Next, the regression line obtained from plotting the addition rate of the NH₄H₂PO₄ solution as a function of the synthesis pH of the HAp·pH₆.₅, HAp·pH₇.₂, and HAp·pH₈.₀ samples was used to estimate the addition rate of the NH₄H₂PO₄ solution for synthesizing the HAp·pH₇.₄, HAp·pH₇.₆, and HAp·pH₇.₈ samples (Fig. 1b), which have specific surface areas close to 38 m²/g (Table 1). As shown in Table 1, this approach enabled us to obtain a series of HAp samples with similar specific surface areas, ranging from 34 to 42 m²/g (38 ± 4 m²/g), under a wide range of synthesis pH conditions. Finally, Fig. 1b provides researchers with guidelines for synthesizing rod-like HAp s at 80 °C in a 0.5 L reactor with specific surface areas close to 38 m²/g at a pH ranging from 6.5 to 8.0, with the following dependency between the NH₄H₂PO₄ solution addition rate and the pH of the precipitation medium: r NH₄H₂PO₄ ~ -14 pH + 114.

## 3.2. Characterization of the HAp·pHₓ supports

### 3.2.1. XRD, XRF, SEM and specific surface area

Fig. 2a shows the XRD patterns of some of the dried HAp·pHₓ samples. The observed diffraction lines are representative of the crystalline hexagonal structure of HAp only, as indicated by comparison with the ICDD card No. 00–009–0432. Consistent with the work of Tsuchida et al. [23], Table 1 shows that the Ca/P molar ratio estimated by XRF increases as the pH of the precipitation increases up to HAp·pH₇.₈, before stabilizing to values close to 1.68–1.70 at synthesis pH higher than 8.0. The Ca/P ratio of HAp·pH₈.₀ displays a slightly lower Ca/P molar ratio in this trend. The latter peculiarity was confirmed using a newly synthesized HAp·pH₈.₀ sample. The changes in the composition of the HAp materials is due to the non-stoichiometric properties of the HAp structure associated with the real general formula Ca₁₀-xB(PO₄)₆-xB(HPO₄)ₓ(CO₃)ₐ+B(OH)₂(₂x-2A-B) with Ca/P = (10-x-B)/(6-B) [35]. Therefore, the Ca/P molar ratio can be found to be lower than that expected for a perfect, i.e., defect-free, crystalline structure (Ca/P = 1.67), due to the incorporation of hydrogenocarbonate (HPO₄²⁻) groups, resulting in Ca²⁺ and HO⁻ vacancies (mainly favored at intermediate pH synthesis). A Ca/P molar ratio higher than 1.67 reveals the incorporation of carbonates (B-type carbonates substituting phosphate groups), leading to HO⁻
<caption>**Table 1**</caption>

Specific surface area (SSA), bulk Ca/P molar ratio of the dried HAp-pHₓ supports, Ru particle size estimated by TEM and associated Ru dispersion (D<sub>TEM</sub>, %) [29], and NH₃ TOF at 400 °C and 1 bar.

<table><thead><tr><th rowspan="2">Samples*</th><th rowspan="2">SSA (m²/g)</th><th rowspan="2">Bulk Ca/P molar ratio</th><th colspan="2">Ru particle size (nm)</th><th colspan="2">Ru dispersion (D<sub>TEM</sub>, %)</th><th rowspan="2">NH₃ TOF<br>at 400 °C and 1 bar (min<sup>-1</sup>)</th></tr><tr><th>Fresh</th><th>Spent</th><th>Fresh</th><th>Spent</th></tr></thead><tbody><tr><td>Ru/MgO</td><td>21</td><td>-</td><td>2.4</td><td>2.5</td><td>53.7</td><td>51.4</td><td>0.26 ± 0.01<sup>§</sup></td></tr><tr><td>Ru/HAp-pH<sub>6.5</sub></td><td>39</td><td>1.61</td><td>2.1</td><td></td><td>61.4</td><td></td><td>0.22 ± 0.02<sup>§</sup></td></tr><tr><td>Ru/HAp-pH<sub>7.2</sub></td><td>38</td><td>1.64</td><td>2.3</td><td></td><td>56.1</td><td></td><td>0.32 ± 0.01<sup>§</sup></td></tr><tr><td>Ru/HAp-pH<sub>7.4</sub></td><td>39</td><td>1.65</td><td>2.2</td><td></td><td>58.6</td><td></td><td>0.40</td></tr><tr><td>Ru/HAp-pH<sub>7.6</sub></td><td>42</td><td>1.67</td><td>2.3</td><td></td><td>56.1</td><td></td><td>0.41</td></tr><tr><td>Ru/HAp-pH<sub>7.8</sub></td><td>35</td><td>1.68</td><td>2.3</td><td></td><td>56.1</td><td></td><td>0.39 ± 0.01<sup>£</sup></td></tr><tr><td>Ru/HAp-pH<sub>8.0</sub></td><td>34</td><td>1.64</td><td>2.2 ± 0.1<sup>§</sup></td><td>2.2</td><td>60.0 ± 1.4<sup>§</sup></td><td>58.4</td><td>0.30 ± 0.02<sup>§</sup></td></tr><tr><td>Ru/HAp-pH<sub>8.3</sub></td><td>38</td><td>1.70</td><td>2.1</td><td></td><td>61.4</td><td></td><td>0.24 ± 0.01<sup>§</sup></td></tr><tr><td>Ru/HAp-pH<sub>8.5</sub></td><td>39</td><td>1.69</td><td>2.5</td><td></td><td>51.6</td><td></td><td>0.24 ± 0.07<sup>§</sup></td></tr><tr><td>Ru/HAp-pH<sub>9.0</sub></td><td>38</td><td>1.68</td><td>2.3</td><td>2.2</td><td>56.1</td><td>57.6</td><td>0.18</td></tr><tr><td>Ru<sub>Cl3</sub>/HAp-pH<sub>9.0</sub></td><td>38</td><td>1.68</td><td>2.2</td><td></td><td>58.9</td><td></td><td>0.04</td></tr><tr><td>Ru<sub>(NH3)6Cl3</sub>/HAp-pH<sub>9.0</sub></td><td>38</td><td>1.68</td><td>2.5</td><td></td><td>52.4</td><td></td><td>0.01</td></tr><tr><td>Ru<sub>1</sub>/HAp-pH<sub>9.0</sub></td><td>38</td><td>1.68</td><td>2.1</td><td></td><td>60.6</td><td></td><td>0.18</td></tr><tr><td>Ru<sub>4</sub>/HAp-pH<sub>9.0</sub></td><td>38</td><td>1.68</td><td>2.6</td><td></td><td>49.0</td><td></td><td>0.22</td></tr><tr><td>Ru<sub>6</sub>/HAp-pH<sub>9.0</sub></td><td>38</td><td>1.68</td><td>2.4</td><td></td><td>54.2</td><td></td><td>0.24</td></tr></tbody></table>

* The samples for which (i) no Ru precursor is specified were prepared with Ru(NO)(NO₃)₃, and (ii) no Ru loading is specified contained 2 wt% Ru. The errors listed in this table for some samples result from duplicate measurements carried out on either the same batch<sup>§</sup> or on two different batches<sup>£</sup> of catalyst.

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** (a) Influence of the NH₄H₂PO₄ solution addition rate on the specific surface area of HAp-pH₆.₅ (green dots; the values shown by the black squares were extracted from the previous work of Reynaud et al. [34]), and (b) rates at which NH₄H₂PO₄ solutions were added to synthesize HAp s with specific surface areas of 38 ± 4 m²/g (Table 1) under various pH conditions using the 0.5 (circles) and 1 L (triangles) synthesis reactors.</figcaption></figure>

vacancies [35]. Note that A-type carbonates substituting HO⁻ groups are silent defects when considering the Ca/P molar ratio values as a macroscopic descriptor of the composition of HAp materials [35]. Even if intermediate and high pH syntheses favor under- and over-stoichiometric compounds [34], respectively, both types of defects (hydrogenocarbonates and carbonates) can be detected in the same sample, albeit in different amounts. Fig. 2b,c show SEM micrographs of

HAp-pH₆.₅ and HAp-pH₇.₆, respectively, as examples of the HAp-pHₓ series. Consistent with their similar specific surface areas (Table 1), Fig. 2b,c indicate that these HAp samples exhibit a rod-like morphology and that the rods are similar in size. As is well known in the HAp field [20,36], such HAp materials do not exhibit any microporosity that can be probed by N₂ sorption. The size of the channels (2.85 Å, [20]) is similar to, or slightly smaller than, the kinetic diameter of N₂ (3.64 Å, [37]), and these channels are filled with stacked OH groups. For this reason, N₂ sorption investigations were limited to determining the specific surface area of the HAp materials. Figure S1 shows the N₂ adsorption–desorption isotherm of HAp-pH₇.₆, which is an example of a nonporous material of the HAp-pHₓ series that exhibits a Type II isotherm [38].

### 3.2.2. Surface basicity of the HAp-pHₓ samples

The surface acid–base properties of the HAp-pHₓ samples were investigated using the model MBOH reaction [31]. Conversion of MBOH produced equimolar quantities of only acetylene and acetone, attesting to the basic reactivity [31] of such samples. As shown in Figure S2, the conversion of MBOH decreases moderately over time on stream for all samples, as also reported previously [39]. Fig. 3a plots the initial MBOH conversion and the Ca/P molar ratio of the HAp-pHₓ samples as a function of the synthesis pH. Overall, the initial MBOH conversion increases as the pH of the synthesis medium and the Ca/P molar ratio increase. These findings are consistent with earlier empirical relationships, initially reported by Tsuchida et al., established for series of HAp s with a maximum Ca/P molar ratio of 1.67 [23]. However, a closer examination of the two profiles indicates that the MBOH conversion exhibits two maxima for HAp-pH₇.₄ and HAp-pH₈.₅. Fig. 3b shows a low regression coefficient when reporting MBOH conversion as a function of Ca/P molar ratio. Previous reports have shown that the Ca/P molar ratio is not an effective descriptor of the MBOH-probed basicity of HAp s, especially for over-stoichiometric materials with carbonate defects, since the Ca/P ratio does not account for the proportion of A-type carbonates [35]. In contrast, HO⁻ groups have been identified as the basic sites for the MBOH reaction, and their concentration is significantly impacted by the presence of defects, particularly A-type carbonates [35]. Fig. 4 shows an intense and sharp FTIR contribution in the OH region found at 3573 cm⁻¹. This contribution is attributed to the stretching vibration of the HO⁻ groups located in the HAp channels [40]. The narrowness of this absorption band indicates an absence of hydrogen bonding, which is due to the long distance (0.344 nm, [41]) between neighboring HO⁻ groups inside the channels. This band combines the contributions from HO⁻ species located in the bulk and on the
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** (a) XRD patterns of part of the dried HAp-pHₓ samples, HAp-pH<sub>6.5</sub> (green), HAp-pH<sub>7.2</sub> (yellow), HAp-pH<sub>7.4</sub> (blue), HAp-pH<sub>8.0</sub> (red), HAp-pH<sub>8.3</sub> (magenta), HAp-pH<sub>8.5</sub> (gray), and HAp-pH<sub>9.0</sub> (black), and SEM micrographs of (b) HAp-pH<sub>6.5</sub>, and (c) HAp-pH<sub>7.6</sub>.</figcaption></figure>

surface of the HAp channels [42], preventing observation of a shift in its maximum position as the HAp composition and basicity change. However, as previously reported, the area of this band is directly affected by the HAp composition and can be used as a reliable indicator of the basicity [35]. Integrating this band for some of the samples studied here (Fig. 3c) confirms that the basicity of HAp s, as determined by the MBOH conversion, is governed by the number of HO⁻ groups, as shown by the significant increase in the regression coefficient in Fig. 3d compared to that obtained when considering the Ca/P molar ratio (Fig. 3b).

Overall, the characterization of the basicity of the HAp-pHₓ supports shows that the series of materials exhibits different surface basicities. Under the activation conditions used for the HAp-pHₓ materials, MgO was found to poorly catalyze the transformation of MBOH, achieving

only about 3 % conversion. This may be due to the poisoning of the basic sites of MgO by strongly held carbonates/hydrogenocarbonates that could not be released by pretreating the sample at 450 °C. Since previous studies have shown that the MBOH conversion strongly depends on the pretreatment of MgO [43], MgO was not tested under different pretreatment conditions than those applied to the HAp-pHₓ series because the results would be difficult to compare. While measuring the basicity of supports is important, their ability to transfer electrons to supported metal nanoparticles is crucial for promoting catalytic reactions [44], such as NH₃ synthesis [1,7,9,45].

### 3.3. Characterization of the supported Ru nanoparticles by TEM

The TEM images of most of the supported Ru nanoparticles of the Ru/HAp-pHₓ samples have been provided in a recent study [27], and their corresponding particle sizes and dispersions are listed in Table 1. Fig. 5a-c shows that the sizes of the Ru nanoparticles supported on MgO, HAp-pH<sub>7.2</sub>, and HAp-pH<sub>7.8</sub> are similar to those previously measured on the other Ru/HAp-pHₓ samples [27], as listed in Table 1. Fig. 5d-f and Table 1 demonstrate that the Ru nanoparticles in the spent Ru/MgO, Ru/HAp-pH<sub>8.0</sub>, and Ru/HAp-pH<sub>9.0</sub> materials did not sinter during the NH₃ catalytic reaction, as their size varied within the margin of error of the TEM estimation, i.e., ± 0.1 nm (Table 1).

### 3.4. Characterization of the MgO-supported Ru nanoparticles by N₂-FTIR

Fig. 6a shows the N₂-FTIR spectra recorded with the incremental addition of N₂ to the Ru/MgO reference sample. Unlike the Ru/HAp-pHₓ samples [27], no contribution from N₂ species interacting with the MgO support is observed (the very weak contributions around 2340 cm⁻¹ are attributed to a slight instability of residual atmospheric CO₂ concentration in the IR bench optical path). The broad band of increasing intensity with incremental doses of N₂ is attributed to the stretching vibration of N₂ (ν(N≡N)) adsorbed on reduced Ru sites (νN₂–Ru�0) with limited interaction with the MgO support [46]. The maximum of this band is observed at 2208 cm⁻¹, which is consistent with previous studies in which N₂ adsorbed onto Ru(001) model surfaces exhibited absorption frequencies in the 2209–2198 cm⁻¹ range at monolayer coverage [47, 48]. This is also in rather good agreement with the absorption frequencies of N₂ adsorbed on Ru⁰ sites supported on MgO (2168 [46] and 2210 cm⁻¹ [49]). Fig. 6a also shows that saturation surface coverage of the Ru metal nanoparticles is reached after the 8th dose (red spectrum) corresponding to a cumulated amount of N₂ dosed of 2.8 µmol. Since N₂ does not interact with the MgO support, the estimation of the integrated molar absorption coefficient of adsorbed on the Ru⁰ sites (ε N₂–Ru⁰) can be estimated more directly than for the Ru/HAp-pHₓ samples [27]. The slope of the straight line obtained from the integrated area of the absorption band at 2208 cm⁻¹ versus the amount of N₂ dosed [27] in the first 8 doses (full circles, Fig. 6b) yields an estimation of ε N₂–Ru⁰ of 0.75 cm/µmol. This value falls within the range of ε N₂–Ru⁰ values found for the Ru/HAp-pHₓ samples [27]. Additionally, the Ru dispersion (62.1 %) estimated from the cumulated amount of N₂ adsorbed on the Ru nanoparticles after addition of 8 N₂ doses is in good agreement with the dispersion estimated from TEM (53.7 %, Table 1). These results indicate that the methodology developed for estimating Ru dispersion using N₂-FTIR for HAp-supported catalysts [27] can be extended to the MgO-supported system. Since νN₂–Ru⁰ reflects the electron density of the Ru⁰ particles, and the N₂ bond cleavage is claimed to be the rate-determining step in NH₃ synthesis and influenced by the electron transfer from the metal to the π* antibonding orbital of N₂ [46], it can be anticipated that Ru/HAp-pHₓ samples with νN₂–Ru⁰ values [27] close to that found for Ru/MgO in this study should exhibit NH₃ synthesis rates similar to that measured for Ru/MgO in this study (Section 3.5.).
<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Influence of (a) the pH of the HAp-pHₓ precipitation medium on the Ca/P molar ratio determined by XRF and on the initial MBOH conversion, (b) the Ca/P molar ratio of the HAp-pHₓ samples on the area of the band at 3573 cm⁻¹ reported in Fig. 4 and on the initial MBOH conversion, (c), the pH of the HAp-pHₓ synthesis medium on the area of the band at 3573 cm⁻¹ reported in Fig. 4 and on the initial MBOH conversion, and (d) the area of the OH band at 3573 cm⁻¹ reported in Fig. 4 on the initial MBOH conversion (the error bars in panel (d) represent 5% relative error).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Weight-normalized FTIR contribution of the OH groups located in the channels of some of the HAp-pHₓ samples, HAp-pH<sub>6.5</sub> (green), HAp-pH<sub>7.4</sub> (blue), HAp-pH<sub>7.6</sub> (orange), HAp-pH<sub>8.0</sub> (red), HAp-pH<sub>8.3</sub> (magenta), HAp-pH<sub>8.5</sub> (gray), and HAp-pH<sub>9.0</sub> (black), recorded after reduction and evacuation at 450 °C for 2 and 1 h, respectively.</figcaption></figure>

### 3.5. NH₃ synthesis

#### 3.5.1. Thermodynamic validation of the experimental setup, reproducibility of the measurements and stability of Ru/HAp-pHₓ catalysts for NH₃ synthesis

First, it was confirmed that the SiC-filled reactor was inactive for NH₃ production from RT to 450 °C. Then, the validity of the FR-100 experimental setup, which was used for catalytic ammonia synthesis, was validated using a Ru/HAp-pH₈.₀ material. In the first experiment, the ex situ-reduced sample was exposed to a H₂-N₂ (3–1) feed, and the temperature was increased from RT to 450 °C at a rate of 5 °C/min, with 2 h dwell periods from 300 to 450 °C every 25 °C. Fig. 7 shows that the NH₃ concentration in the outlet stream (1st experiment, black triangles) approaches thermodynamic equilibrium at 450 °C, which is the maximum temperature studied in this experiment. The second experiment was run on a new aliquot of Ru/HAp-pH₈.₀ and was performed

using a procedure similar to that of the first experiment but the temperature was increased from 400 to 550 °C in 100 °C increments. As shown in Fig. 7 (2nd experiment, red dots), this experiment demonstrates that the concentration of NH₃ reaches thermodynamic equilibrium at temperatures greater than or equal to 500 °C. This result provides support for the correct operation of the reactor. Additionally, comparing NH₃ production at 400 and 450 °C in both experiments shows a fairly good reproducibility of the measurements. Figure S3 shows that Ru/MgO behaved similarly to Ru/HAp-pH₈.₀, approaching thermodynamic equilibrium at 450 °C.

Fig. 8 shows that NH₃ production with the Ru/HAp-pH₇.₈ catalyst stabilizes rather quickly within 2 h at each temperature step. After measuring NH₃ production at 450 °C, the reaction temperature was decreased and stabilized at 400 °C. NH₃ production at 400 °C after the reaction at 450 °C agrees well with NH₃ production measured before the reaction at 450 °C. This indicates that the catalyst did not deactivate after the reaction at 450 °C. Furthermore, the catalyst was found to be stable on stream at 400 °C for approximately 60 h.

#### 3.5.2. Influence of the Ru precursor on the ammonia synthesis production of Ru/HAp-pH<sub>9.0</sub> catalysts

Consistent with previous studies [1,9,50,51], using chlorinated Ru precursors significantly decreased ammonia synthesis performance compared to the RuNO(NO₃)₃ precursor (Fig. 9a), while the size of the Ru nanoparticles obtained from the chlorinated Ru precursors was similar (Figure S4) to that of Ru/HAp-pH<sub>9.0</sub> (Table 1, and ref. [27]). At 400 °C, the NH₃ production decreases by about 73 and 95 % on RuCl₃/HAp-pH<sub>9.0</sub> and Ru(NH₃)₆Cl₃/HAp-pH<sub>9.0</sub>, respectively, compared to Ru/HAp-pH<sub>9.0</sub> (Fig. 9a). The decrease in catalytic performance of these samples is due to the presence of retained chlorides in the HAp support, which has a well-known affinity for forming chloroapatites [20]. The electron affinity of the chlorides decreases the electron density of neighboring Ru nanoparticles, resulting in the observed loss of NH₃ production on such catalysts.
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Representative TEM images and corresponding Ru particle size distributions recorded on fresh (a) Ru/MgO (brown), (b) Ru/HAp-pH<sub>7.2</sub> (yellow), (c) Ru/HAp-pH<sub>7.8</sub> (purple), and spent (d) Ru/MgO (brown and white checked pattern), (e) Ru/HAp-pH<sub>8.0</sub> (red and white checked pattern), and (f) Ru/HAp-pH<sub>9.0</sub> (black and white checked pattern) samples.</figcaption></figure>

### 3.5.3. Influence of the Ru loading on the ammonia synthesis catalytic performance of Ru<sub>y</sub>/HAp-pH<sub>9.0</sub> catalysts

Fig. 9b shows that NH<sub>3</sub> production increases with Ru loading and that Ru<sub>6</sub>/HAp-pH<sub>9.0</sub>, which is the most highly loaded catalyst, approaches NH<sub>3</sub> thermodynamic equilibrium much more rapidly than the other samples. Figure S5 shows that the density of Ru nanoparticles

increases as the Ru loading increases. However, as the size of the Ru nanoparticles remains essentially constant (2.4 ± 0.2 nm) for all catalysts (Figure S5 and Table 1), resulting in a similar NH<sub>3</sub> TOF at 400 °C for all samples (0.21 ± 0.03 min<sup>-1</sup>, Table 1). These data emphasize the unique ability of the HAp support to disperse metals, here Ru, as previously reported by Akri et al. for Ni-based HAp-supported catalysts
<figure><img src="image_10.png" /><figcaption>Fig. 6. (a) Spectra recorded at 100 K after introduction of 9 incremental doses of N₂ on Ru/MgO where the amounts of N₂ introduced from the 1st to the 9th dose (dose number, and color of the spectrum given in parentheses) are 0.15(1, magenta), 0.24(2, gray), 0.48(3, dark blue), 0.40(4, green), 0.41(5, orange), 0.31(6, light blue), 0.43(7, brown), 0.39(8, red), and 0.61(9, black) µmol, respectively, and (b) integrated area of the absorption band at 2208 cm⁻¹ assigned to the interaction of N₂ with Ru⁰ surface atoms of Ru nanoparticles supported on MgO as a function of the amount of N₂ adsorbed on the Ru⁰ surface atoms from the 1st to the 9th dose (1st to 8th dose: full circles, 9th dose: open circle).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 7. NH₃ production after 2 h on stream at different temperatures for two experiments performed on Ru/HAp-pH₈.₀.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 8. NH₃ production on Ru/HAp-pH₇.₈ with time on stream.</figcaption></figure>

[52].

### 3.5.4. Comparison of the NH₃ synthesis catalytic performance of Ru/MgO with previous studies

The NH₃ rates and TOF measured on a Ru/MgO reference catalyst are compared to previously reported values in Table 2. As shown in Table 2, the data from the present study are in good agreement with previous findings below 400 °C [46,53,54]. At 400 °C, the NH₃ TOF of this study (0.26 ± 0.01 min⁻¹) falls within the range of values reported previously [9,55–59], which varies by an order of magnitude (0.05–0.50 min⁻¹, Table 2). Since the NH₃ synthesis catalytic reaction has been suggested to be structure sensitive [11], NH₃ TOF comparisons should be made between catalysts with similar Ru nanoparticle sizes. In this case, the

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 9. Influence of (a) the Ru precursor with 2 wt% Ru on HAp-pH₉.₀, and (b) Ru loading for HAp-pH₉.₀ samples synthesized with a RuNO(NO₃)₃ precursor, on NH₃ synthesis performance.</figcaption></figure>

NH₃ TOF measured on 2.4-nm Ru nanoparticles of our Ru/MgO catalyst (0.26 ± 0.01 min⁻¹) compares well with that reported on the 1.9-nm Ru nanoparticles by Ertl and co-workers. [55] (0.22 min⁻¹, Table 2). However, it is puzzling that our NH₃ TOF appears to be 2.6 times greater than that reported for 1.8-nm Ru nanoparticles (0.10 min⁻¹, Table 2) by Croisé et al. [59]. The Ru/MgO catalyst in the latter study was synthesized with an identical MgO support and Ru precursor (RuNO(NO₃)₃), and was evaluated under similar experimental NH₃ synthesis experimental conditions as ours (Table 2). This discrepancy cannot be attributed to differences in catalyst grain size, as Croisé et al. demonstrated that the catalyst granulometry had essentially no influence on the NH₃ synthesis rate [59]. Additionally, it is difficult to attribute this difference to changes in the Ru particle size, 2.4 nm in this study (Table 2) vs 1.8 nm in the study of Croisé et al. [59], as the NH₃ synthesis rate measured by Ertl and co-workers [55] on 1.9-nm Ru nanoparticles was
<table><caption>**Table 2**

NH₃ synthesis data reported for the reaction of N₂–H₂ (3–1) stoichiometric mixtures at 1 bar over Ru/MgO catalysts.</caption><thead><tr><th rowspan="2">MgO (m²/g)</th><th rowspan="2">Precursor</th><th rowspan="2">Loading (wt %)</th><th rowspan="2">Dispersion (%)</th><th rowspan="2">Particle size (nm)</th><th colspan="6">NH₃ synthesis catalytic reaction</th></tr><tr><th>T (°C)</th><th>F (mL/min)</th><th>Sample weight (g)</th><th>[NH₃] (ppm)</th><th>Rate (mmol/g h)</th><th>TOF (min⁻¹)</th><th>Refs.</th></tr></thead><tbody><tr><td>21</td><td>RuNO (NO₃)₃</td><td>2.0</td><td>53.7</td><td>2.4</td><td>300</td><td>60</td><td>0.1</td><td>30</td><td>0.048</td><td>0.01</td><td>This work</td></tr><tr><td>90</td><td>Ru₃(CO)₁₂</td><td>2.0</td><td></td><td></td><td>315</td><td>60</td><td></td><td></td><td>0.060</td><td></td><td>[46]</td></tr><tr><td rowspan="2">21</td><td>RuNO (NO₃)₃</td><td>2.0</td><td>53.7</td><td>2.4</td><td>325</td><td>60</td><td>0.1</td><td>98</td><td>0.156</td><td>0.02</td><td>This work</td></tr><tr><td>Ru₃(CO)₁₂</td><td>4.3</td><td>23.5</td><td>5.5</td><td>350</td><td>60</td><td></td><td></td><td>0.684</td><td>0.10</td><td>[53]</td></tr><tr><td>21</td><td>RuNO (NO₃)₃</td><td>2.0</td><td>53.7</td><td>2.4</td><td>350</td><td>60</td><td>0.1</td><td>253</td><td>0.402</td><td>0.06</td><td>This work</td></tr><tr><td>56</td><td>Ru₃(CO)₁₂</td><td>8.0</td><td>32.0</td><td></td><td>380</td><td>60</td><td>0.2</td><td></td><td>2.017</td><td>0.13</td><td>[54]</td></tr><tr><td>21</td><td>RuNO (NO₃)₃</td><td>2.0</td><td>53.7</td><td>2.4</td><td>375</td><td>60</td><td>0.1</td><td>550</td><td>0.875</td><td>0.14</td><td>This work</td></tr><tr><td>16</td><td>Ru₃(CO)₁₂</td><td>2.0</td><td>68.0</td><td></td><td>400</td><td>60</td><td></td><td></td><td>0.379</td><td>0.05</td><td>[9]</td></tr><tr><td>52</td><td>Ru₃(CO)₁₂</td><td>5.0</td><td>52.5</td><td>1.9</td><td>400</td><td>120</td><td>0.138</td><td></td><td></td><td>0.22</td><td>[55]</td></tr><tr><td rowspan="2">200</td><td>Ru₃(CO)₁₂</td><td>5.0</td><td>8.7</td><td>15.4</td><td>400</td><td>60</td><td>0.2</td><td></td><td>1.296</td><td>0.50</td><td>[56]</td></tr><tr><td>Ru₃(CO)₁₂</td><td>2.0</td><td>20.8</td><td>6.2</td><td>400</td><td>60</td><td>0.2</td><td></td><td>0.115</td><td>0.05</td><td>[57]</td></tr><tr><td>37</td><td>Ru(NO₃)₃</td><td>1.0</td><td>28.7</td><td>4.5</td><td>400</td><td>22<sup>$</sup></td><td>0.2</td><td>344</td><td>0.101</td><td>0.06</td><td>[58]</td></tr><tr><td>21</td><td>RuNO (NO₃)₃</td><td>2.1</td><td>53.4</td><td>1.8</td><td>400</td><td>60</td><td>0.1</td><td>420</td><td>0.675</td><td>0.10</td><td>[59]</td></tr><tr><td>21</td><td>RuNO (NO₃)₃</td><td>2.0</td><td>53.7</td><td>2.4</td><td>400</td><td>60</td><td>0.1</td><td>1025 ± 20</td><td>1.630 ± 0.03</td><td>0.26 ± 0.01</td><td>This work</td></tr></tbody></table>

consistent with that measured in this study (Table 2).

An additional NH₃ synthesis experiment was carried out on Ru/HAp-pH₆.₅ using a homemade experimental setup with a quartz reactor. This setup was connected to the same analyzer as that used with the FR-100 setup. Overall, Fig. 10 shows a similar NH₃ production for both reactors, albeit one may argue that the NH₃ produced in the homemade quartz reactor appears to be slightly lower than that recorded with the stainless steel FR-100 reactor. Apart from the reactor geometry, which may affect catalytic performance, NH₃ synthesis catalysts, including Ru ones, have recently been shown to be extremely sensitive to traces of oxygen impurities in the reactant feed [60]. The slightly lower NH₃ production observed in the homemade quartz reactor compared to the stainless steel reactor (Fig. 10) may be due to a higher concentration of oxygen impurities in the quartz reactor setup. The homemade quartz reactor is connected using ultra-torr fittings (Swagelok), while the stainless steel FR-100 reactor is connected using either VCR or stainless steel fittings (Swagelok), which are known to be much more leak-resistant than the ultra-torr fittings. Using ultra-pure gases as reactants is crucial for catalytic reactions sensitive to oxygen traces, such as NH₃ synthesis [60]. However, it is also important to properly manage the supply chain of these gases to the catalyst bed to prevent oxygen contamination on their way from the cylinder to the catalyst bed and ensure that gases remain

ultra-pure.

In this study, we did not use the purest reactant gases, as Alphagaz™1 (purity ≥ 99.999 %) was used, while Alphagaz™2 (purity ≥ 99.9999 %) is available at a much higher cost. The long-term activity of the Ru/HAp-pH₇.₈ with no deactivation at 400 °C for approximately 60 h (Fig. 8) suggests that the oxygen impurities in our reactant gases must not be the determining factor under our experimental conditions. Therefore, one might wonder if the discrepancy between the previously reported NH₃ synthesis rates [9,57–59] and the rates measured in this study on Ru/MgO catalysts (Table 1) could be tentatively attributed to an oxygen contamination of the experimental setups used in former studies. This study also emphasizes the importance of comparing the catalytic performance of newly prepared catalysts with a reference catalyst, under the same experimental conditions and setup, to fully appreciate their effectiveness.

### 3.5.5. Influence of the nature of the HAp support on the catalytic performance of Ru/HAp-pHₓ catalysts in ammonia synthesis

The NH₃ concentration profiles recorded on the Ru/HAp-pHₓ catalysts are similar to those shown on the Ru/HAp-pH₈.₀ (Fig. 7) and the Ru₃/HAp-pH₉.₀ (Fig. 9b) catalysts. Comparison of the production of NH₃ on the different catalysts is made at 400 °C. Fig. 11a shows that the NH₃ production rate of Ru/MgO at 400 °C (1.63 ± 0.03 mmol/g h) is slightly higher than that measured on Ru/HAp-pH₆.₅ and Ru/HAp-pH₉.₀ but lower than that measured on the HAp-supported Ru catalysts for which the HAp supports were prepared from pH 7.2–8.5. Among these catalysts, Ru/HAp-pH₇.₄ exhibits the highest NH₃ production rate, followed closely by Ru/HAp-pH₇.₆ and Ru/HAp-pH₇.₈. Normalizing the rates of the Ru/HAp-pHₓ catalysts per Ru surface atom reveals that the NH₃ TOF at 400 °C of Ru/MgO (0.26 ± 0.1 min⁻¹) is higher than that of Ru/HAp-pH₆.₅, and Ru/HAp-pH₉, and comparable to that of Ru/HAp-pH₈.₃ and Ru/HAp-pH₈.₅. The three most active catalysts are Ru/HAp-pH₇.₄, Ru/HAp-pH₇.₆ and Ru/HAp-pH₇.₈ with a NH₃ TOF of 0.40 ± 0.01 min⁻¹. These catalysts appear to be approximately 50 % more active than the Ru/MgO reference catalyst and the least active HAp-supported samples, namely Ru/HAp-pH₆.₅ and Ru/HAp-pH₉.₀.

### 3.5.6. N₂-FTIR-NH₃ TOF structure-activity relationships

Since NH₃ synthesis has been shown to be strongly influenced by the electron density of the Ru nanoparticles [1,45,46], and since the stretching vibration frequency of N₂ adsorbed on the surface atoms of Ru

<figure><img src="image_15.png" /><figcaption>**Fig. 10.** NH₃ production after 2 h on stream at different temperatures for experiments performed on Ru/HAp-pH₆.₅ with two different reactors: stainless steel FR-100 (full symbols) and quartz homemade (open symbols). The data for the stainless steel reactor are based on an average of two experiments, with a maximum deviation of 8 relative% at most, as indicated by the black error bars.</figcaption></figure>

<footnote><sup>$</sup> Estimated with a 24,000 h⁻¹ Hour Space Velocity [64] and a MgO density of 3.58 g/cm³.</footnote>
<figure><img src="image_16.png" /><figcaption>**Fig. 11.** (a) NH₃ production rate and (b) NH₃ TOF after 2 h on stream at 400 °C and 1 bar on approximately 0.1 g of Ru/MgO (brown), HAp-pH₆.₅ (green), HAp-pH₇.₂ (yellow), HAp-pH₇.₄ (blue), HAp-pH₇.₆ (orange), HAp-pH₇.₈ (purple), HAp-pH₈.₀ (red), HAp-pH₈.₃ (magenta), HAp-pH₈.₅ (gray), and HAp-pH₉.₀ (black). 60 mL/min flow rate of H₂–N₂ (3–1). At 400 °C and 1 bar, the equilibrium concentration of NH₃ is 4439 ppm to which corresponds a NH₃ rate of approximately 7.1 mmol/h g.</figcaption></figure>

nanoparticles, denoted in this work as $v$ N₂–Ru⁰, is a reliable descriptor of Ru metal nanoparticles’ electron density [45,46], the NH₃ production rates and TOF measured in this study (Section 3.5.4. and 3.5.5.) were plotted as a function of $v$ N₂–Ru⁰ and the integrated molar absorption coefficient of N₂ adsorbed on Ru⁰ surface atoms ($ε$ N₂–Ru⁰) (Fig. 12). The $v$ N₂–Ru⁰ and $ε$ N₂–Ru⁰ descriptors were determined in our previous work and were found to be correlated [27].

Fig. 12a shows that the NH₃ production rate correlates well with $v$ N₂–Ru⁰, as indicated by a reasonable correlation coefficient. To our knowledge, Aika et al. are the only researchers to date to report a comparable correlation in this field by plotting the NH₃ synthesis rate measured at 315 °C as a function of $v$ N₂–Ru⁰ of Cs-promoted Al₂O₃- and MgO-supported Ru catalysts [45]. The much wider range of N₂ absorption stretching frequencies observed in the study of Aika et al. [45], compared to this study, is due to the use of different supporting oxides and the promotion of the Ru metal nanoparticles by Cs [61]. Fig. 12b shows that the quality of the correlation between the NH₃ production rate and $ε$ N₂–Ru⁰ is similar to that observed between the NH₃ production rate and $v$ N₂–Ru⁰ (Fig. 12a). This is not surprising, as both FTIR descriptors of the electron density of the Ru metal nanoparticles were previously shown to be correlated [27]. Fig. 12c shows that the correlation coefficient for the NH₃ TOF at 400 °C data plotted as a function of $v$ N₂–Ru⁰ is the lowest, while that for the NH₃ TOF data plotted as a function of $ε$ N₂–Ru⁰ is by far the greatest (Fig. 12d). Overall, Fig. 12 shows that the NH₃ synthesis activity at 400 °C increases as $v$ N₂–Ru⁰/$ε$ N₂–Ru⁰ decreases/increases. In other words, this occurs as the strength of the N≡N bond decreases due to an enhanced electron donation from the d-orbitals of the Ru⁰ atoms to the π* antibonding orbital of N₂. This must result from the greater electron density of the Ru nanoparticles due

to an enhanced support-metal electron transfer, i.e., a higher basicity/electron density of the support. Therefore, this study shows that the most robust correlation between the NH₃ synthesis activity and Ru nanoparticle electron density is obtained when the NH₃ synthesis activity is expressed as TOF, and electron density of the Ru nanoparticles is expressed as the integrated molar absorption coefficient of N₂ adsorbed on Ru⁰ surface atoms ($ε$ N₂–Ru⁰), as depicted in Fig. 12d. It is well known in the field of catalysis that the TOF of a catalytic reaction is the best descriptor for reliably comparing catalyst performance [62]. The increase in the correlation coefficient from Fig. 12c to Fig. 12d suggests that the integrated molar absorption coefficient of N₂ adsorbed on Ru⁰ surface atoms ($ε$ N₂–Ru⁰) is a more robust descriptor of the Ru electron density than the maximum frequency of the N₂ stretching vibration absorption band ($v$ N₂–Ru⁰). This peculiarity is attributed to the fact that the maximum frequency of the broad absorption band due to the stretching vibration of N₂ adsorbed on Ru⁰ surface atoms ($v$ N₂–Ru⁰) represents only the electron density of the adsorption sites related to $v$ N₂–Ru⁰. In contrast, the integrated molar absorption coefficient ($ε$ N₂–Ru⁰) provides the average electron density of all of the Ru⁰ surface sites responsible for the broadness of the absorption band. Therefore, $ε$ N₂–Ru⁰ is shown to be a more suitable descriptor of the electron density of the Ru⁰ surface atoms than $v$ N₂–Ru⁰ for correlating with the NH₃ TOF values, which report on the average NH₃ reaction rate per Ru⁰ surface atom of the metal nanoparticles.

The existence of an optimal pH range for synthesizing the HAp supports yielding to the most active Ru/HAp-pHₓ catalysts, with x ranging from 7.4 to 7.8, in NH₃ synthesis on a TOF basis (Fig. 11b), may tentatively be attributed to the HAp composition that was previously shown to influence its basic properties [23]. HAp s have been shown to have a complex composition [35], incorporating defects such as carbonates (CO₃²⁻) and hydrogenophosphates (HPO₄²⁻) [35,42]. Interestingly, the above mentioned most active samples in NH₃ synthesis are also those with Ca/P ratios (1.67 ± 0.02, Table 1) very close to the theoretical Ca/P stoichiometric ratio of defect-free HAp (10/6 = 1.67). HAp samples synthesized at the higher pH levels are known to incorporate mainly carbonate defects that substitute for either OH⁻ (A-type) of PO₄³⁻ (B-type) groups. At this stage, the relationship between the electron transfer ability of the HAp support to the Ru nanoparticles and the basicity of the HAp support, as determined by the MBOH conversion, is unclear. This could first be attributed to the fact that the MBOH reaction has been shown to probe Brønsted basicity (i.e., proton abstraction ability) rather than Lewis basicity (i.e., electron donation ability) [63], while the ability of the HAp support to transfer electrons to the Ru nanoparticles is more likely to involve Lewis basicity. Second, since carbonate defects are incorporated at high synthesis pH, the increased basicity observed for HAp-pH₈.₅ and HAp-pH₉.₀ (Fig. 3a) may be due to the presence of carbonates acting as additional basic sites that do not contribute to the electron enrichment of the Ru nanoparticles. Carbonate incorporation in the HAp structure results in both stoichiometric and over-stoichiometric HAp s (Ca/P ≥ 1.67) [35]. This is typically the case for the Ru catalysts supported on the HAp-pHₓ materials prepared from pH 8.3–9.0 in this study. Indeed, these catalysts show the most intense FTIR contributions typical of carbonates among the studied samples (Figure S6a, 1550–1250 cm⁻¹ [35]). In contrast, the HAp samples synthesized at the lower pH values are under-stoichiometric (Ca/P < 1.67, Table 1). This is due to the substitution of PO₄³⁻ groups with HₓPO₄³⁻ groups, which creates Ca²⁺ vacancies for balance charge requirements [35]. Due to the very weak intensity of the P-OH contributions, the incorporation of HₓPO₄³⁻ defects can be difficult to be ascertained by FTIR (Figure S6b). The incorporation of these defects is favored at the lower pH levels, since it has recently been shown that the formation of HAp results from the rapid hydrolysis of octacalcium phosphate (OCP; Ca₈(HPO₄)₂(PO₄)₄·5 H₂O, Ca/P = 1.33) [34]. Therefore, this study shows that the electron transfer from the HAp-pHₓ supports to the Ru metal nanoparticles and associated NH₃ synthesis catalytic performance (Fig. 12d), is hindered by introducing structural defects into the HAp
<figure><img src="image_17.png" /><figcaption>Fig. 12. NH₃ production rate plotted as a function of (a) v N₂–Ru�0, and (b) ε N₂–Ru⁰ and NH₃ TOF plotted as a function of (c) v N₂–Ru⁰, and (d) ε N₂–Ru⁰, on Ru/MgO (brown triangle), HAp-pH₆.₅ (green), HAp-pH₇.₄ (blue), HAp-pH₇.₆ (orange), HAp-pH₈.₀ (red), HAp-pH₈.₃ (magenta), HAp-pH₈.₅ (gray), and HAp-pH₉.₀ (black). NH₃ production rate measured after 2 h on stream at 400 °C and 1 bar on approximately 0.1 g of catalyst with a 60 mL/min flow rate of H₂–N₂ (3–1).</figcaption></figure>

support, as the electron density of the Ru metal nanoparticles has been found to be maximum for stoichiometric HAp-pHₓ prepared at inter-mEDIATE pH of 7.4 and 7.6 [27].

4. Conclusion

This work provides researchers with valuable guidelines for syn-
thesizing hydroxyapatite (HAp) supports with different acid-base
properties while maintaining similar specific surface areas (38
± 4 m²/g) and rod-like morphologies. This can be achieved by adjusting
the pH of the precipitation medium and the rate at which the phos-
phorus precursor solution is added to the calcium solution. Changing the
pH of the precipitation medium from 6.5 to 9.0 and the addition rate of
the phosphorus precursor solution from 25.0 to 4.4 mL/min resulted in
HAp supports with a Ca/P ratio that increased from 1.61 to 1.70.
Overall, the basic properties of these materials increased with the pH of
the precipitation medium, as determined by the conversion of MBOH as
a model reaction. However, this increase was not monotonous, with two
maxima observed for the HAp supports prepared at pH 7.4 and 8.5.

Ru deposition via wet impregnation of an HAp-pH₉ support by using various Ru precursors showed that the chlorinated precursors strongly hindered the ammonia synthesis catalytic reaction and that the RuNO (NO₃)₃ chloride-free precursor was preferable. The deposition of Ru loadings increasing from 1 to 6 wt% on HAp-pH₉ with the latter Ru precursor confirmed the high dispersion capacity of such a material, producing 2.4 ± 0.2 nm Ru nanoparticles and a comparable NH₃ TOF of 0.23 ± 0.03 min⁻¹. The deposition of 2 wt% of Ru with RuNO(NO₃)₃ on the HAp-pHₓ supports yielded Ru nanoparticles measuring 2.2 ± 0.2 nm. A Ru/HAp-pH₇.₈ catalyst was shown to efficiently catalyze ammonia synthesis at 400 °C for approximately 60 h on stream, without deactivation. Measurement of the size of the metal nanoparticles of some of the supported Ru catalysts by TEM after NH₃ synthesis up to 450 °C revealed no sintering.

The Ru/HAp-pHₓ samples with 2 wt% Ru showed optimal NH₃

performance when the HAp supports were precipitated at a pH ranging
from 7.4 to 7.8. These catalysts exhibited an NH₃ TOF of 0.40
± 0.01 min⁻¹ at 400 °C and 1 bar, which is approximately 50 % higher
than that of a Ru/MgO reference catalyst with 2.4 nm Ru nanoparticles
(0.26 ± 0.01 min⁻¹). The superior NH₃ synthesis performance of these
catalysts is attributed to the absence of defects incorporated into the
HAp crystalline structure, which is consistent with their stoichiometric
Ca/P ratios (1.67 ± 0.02). NH₃ TOF measured at 400 °C and 1 bar could
be remarkably correlated with the electron density of the Ru metal
nanoparticles monitored by N₂ adsorption at low temperatures followed
by FTIR from a previous study [27]. The structure-activity relationship
between NH₃ TOF and the integrated molar absorption coefficient of N₂
adsorbed on the Ru metal particles (ε N₂–Ru⁰) was found to be much
better than that between NH₃ TOF and the maximum N₂ stretching
frequency of the broad absorption band of adsorbed N₂ on the Ru metal
particles (v N₂–Ru⁰). This is attributed to the fact that v N₂–Ru⁰ relates to
the Ru⁰ surface atoms with the highest electron density of the Ru metal
nanoparticles, while ε N₂–Ru⁰ provides an average of the electron den-
sity of all the Ru⁰ surface atoms of the metal nanoparticles, which agrees
better with the average NH₃ activity provided by the NH₃ TOF. The
superior NH₃ performance of the catalysts with Ru metal nanoparticles
exhibiting the highest electron density (ε N₂–Ru⁰) is consistent with
previous statements in the field suggesting that rate-determining N₂
dissociation is favored by the electron transfer from the d-orbital of the
Ru surface metal atoms to the π* antibonding orbital of N₂.

Finally, the characterization of the basic properties of the supports could not be directly correlated with the found NH₃ TOF, suggesting that the electron transfer from the support to the Ru metal nanoparticles depends not only on the intrinsic basicity of the support, but also on other factors that remain to be elucidated. Nevertheless, it is noticeable that the electron density of the Ru metal particles and the NH₃ TOF of the Ru/MgO reference catalyst align perfectly with the structure-activity relationship established for the Ru/HAp-pHₓ catalysts. This finding demonstrates that the previously proposed methodology to
characterize electron density of the Ru/HAP-pHₓ samples using N₂-FTIR [27] can be generalized to other supports, making it of great interest in the field of ammonia synthesis.

### CRediT authorship contribution statement

**Sabrina Akrour:** Investigation, Validation, Resources, Data curation, Visualization, Writing – review & editing. **Guylène Costentin:** Conceptualization, Data curation, Writing – review & editing, **Supervision. Sandra Casale:** Investigation, **Alexandre Vimont:** Investigation, Writing – review & editing, Data curation. **Cyril Thomas:** Conceptualization, Data curation, Writing – original draft, Writing – review & editing, Visualization, Supervision, Project administration, Funding acquisition.

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

This work was supported by the French National Research Agency (H2CASTORAMA project: ANR-22-CE05-0002). S.A. thanks the French National Research Agency for granting her PhD through the H2CASTORAMA project. The authors thank the Sorbonne Université, the Laboratoire Catalyse et Spectrochimie (Labelled CNRS Platform VISIO) and the CNRS for providing funding (Crédits récurrents) and access to the analytical equipment used in this work, which was not funded by the H2CASTORAMA project. The authors also thank S. Nowak (Plateforme Rayons X, UFR de Chimie, Université Paris Cité, Paris, France) for the quantification of Ca and P by X-ray fluorescence spectroscopy. The TEM 2100Plus microscope at the Fédération de Chimie et Matériaux de Paris-Centre (FCMat, Fédération de Recherche 2482) used in this work was funded by the UPMC, the CNRS, and the Région Ile de France. Dr. F.C. Meunier is acknowledged for providing the NH₃ thermodynamic concentrations shown in Fig. 7 as a function of the temperature (HSC Chemistry software). The authors would like to thank Dr. P. Bazin for his support with the N₂-FTIR experiments.

### Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcatb.2025.126141.

### Data Availability

Data will be made available on request.

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**Supporting Information**

Promoting NH₃ synthesis by supporting
Ru nanoparticles on hydroxyapatites
of appropriate chemical composition: Towards
structure-activity relationships

Sabrina Akrour,ª Guylène Costentin,ª Sandra Casale,ª Alexandre Vimontᵇ

and Cyril Thomasª,\*

Dr. Cyril Thomas

Sorbonne Université, CNRS, Laboratoire Réactivité de Surface, LRS, UMR CNRS 7197, 4 Place Jussieu, Tour 43-53, 3ème étage, Case 178, F-75252, Paris, France

e-mail: cyril.thomas@sorbonne-universite.fr

Tel: + 33 1 44 27 36 30

Fax: + 33 1 44 27 60 33

<footnote><sup>a</sup>Sorbonne Université, CNRS, Laboratoire Réactivité de Surface, LRS, F-75005 Paris, France.</footnote>

<footnote><sup>b</sup>Université de Caen Normandie, ENSICAEN, CNRS, LCS, Laboratoire Catalyse et Spectrochimie, F-14000 Caen, France.</footnote>

<footnote>\* To whom correspondence should be addressed:</footnote>
<figure><img src="image_18.png" /><figcaption>**Figure S1.** N₂ adsorption (full symbols) and desorption (open symbols) isotherms on HAp-pH7.6.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Figure S2.** MBOH conversion with time on stream on HAp-pH<sub>6.5</sub> (green), HAp-pH<sub>7.2</sub> (yellow), HAp-pH<sub>7.4</sub> (blue), HAp-pH<sub>7.6</sub> (orange), HAp-pH<sub>7.8</sub> (purple), HAp-pH<sub>8.0</sub> (red), HAp-pH<sub>8.3</sub> (magenta), HAp-pH<sub>8.5</sub> (gray), HAp-pH<sub>9.0</sub> (black), and MgO (brown triangle).</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Figure S3.** NH₃ production after 2 h on stream at different temperatures for two experiments performed on Ru/MgO.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Figure S4.** Representative TEM images and corresponding Ru particle size distributions recorded on fresh (a) RuCl3/HAp-pH9.0, and (b) Ru(NH3)6Cl3/HAp-pH9.0</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Figure S5.** Representative TEM images and corresponding Ru particle size distributions recorded on fresh (a) Ru₁/HAp-pH₉.₀, (b) Ru₄/HAp-pH₉.₀, and (c) Ru₆/HAp-pH₉.₀.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Figure S6.** (a) Carbonate and (b) P-OH FTIR contributions recorded on HAp-pH<sub>6.5</sub> (green), HAp-pH<sub>7.4</sub> (blue), HAp-pH<sub>7.6</sub> (orange), HAp-pH<sub>8.0</sub> (red), HAp-pH<sub>8.3</sub> (magenta), HAp-pH<sub>8.5</sub> (gray), HAp-pH<sub>9.0</sub> (black) after reduction and evacuation at 450 °C for 2 and 1 h, respectively.</figcaption></figure>