# N2 as a Quantitative FTIR Probe Molecule for Reliable Assessment of the Dispersion of Ru Supported on Hydroxyapatite

Sabrina Akrour, Alexandre Vimont, Guylène Costentin, Cyril Thomas

► To cite this version:

Sabrina Akrour, Alexandre Vimont, Guylène Costentin, Cyril Thomas. N2 as a Quantitative FTIR Probe Molecule for Reliable Assessment of the Dispersion of Ru Supported on Hydroxyapatite. Journal of Physical Chemistry C, 2025, 10.1021/acs.jpcc.5c03246. [http://dx.doi.org/10.1021/acs.jpcc.5c03246](http://dx.doi.org/10.1021/acs.jpcc.5c03246)

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Distributed under a Creative Commons Attribution 4.0 International License
N₂ as a Quantitative FTIR Probe Molecule for
Reliable Assessment of the Dispersion of Ru
Supported on Hydroxyapatite

Sabrina Akrour,ª Alexandre Vimont,*,b Guylène Costentin,ª and Cyril Thomasª,∗

* To whom correspondence should be addressed:

Dr. Alexandre Vimont: Université de Caen Normandie, ENSICAEN, CNRS, LCS, Laboratoire Catalyse et Spectrochimie, 14000 Caen, France.

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-mails: alexandre.vimont@ensicaen.fr, cyril.thomas@sorbonne-universite.fr

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

<footnote>ᵇUniversité de Caen Normandie, ENSICAEN, CNRS, LCS, Laboratoire Catalyse et Spectrochimie, 14000 Caen, France.</footnote>
ORCID:

Sabrina Akrour: orcid.org/0009-0009-1845-3511

Alexandre Vimont: orcid.org/0000-0001-9799-6024,

Guylène Costentin: orcid.org/0000-0003-1559-6890,

Cyril Thomas: orcid.org/0000-0003-4224-6095.
**Abstract:**

Accurate assessment of the metal dispersion is known to be important in the field of catalysis, as these data are used to calculate the turnover frequency of a given reaction on a given metal. This study shows that $N_2$ can be used as a Fourier transform infrared spectroscopy (FTIR) probe molecule not only qualitatively but also quantitatively. For the first time, it is found that the integrated molar absorption coefficient of $N_2$ adsorbed on the hydroxyapatite (HAp) supports ($\epsilon N_{2-HAp} = 0.092 \pm 0.008$ cm/µmol) does not depend on the chemical composition of HAp, while that of $N_2$ adsorbed on the HAp-supported $Ru^0$ nanoparticles of about 2.2 nm ($\epsilon N_{2-Ru^0}$: 0.5-1.03 cm/µmol) is sensitive to the chemical composition of HAp. This study also shows, for the first time, that $\epsilon N_{2-Ru^0}$ increases with increasing electron density of the $Ru^0$ nanoparticles, as monitored by the position of the maximum of the absorption band of the $N_2$ stretching vibration at about 2200 cm$^{-1}$ (v $N_{2-Ru^0}$), with a remarkable correlation between $\epsilon N_{2-Ru^0}$ and v $N_{2-Ru^0}$. The $\epsilon N_{2-Ru^0}$ values are two orders of magnitude smaller than the previously reported integrated molar absorption coefficients of CO ($\epsilon CO-Ru^0$). Finally, $\epsilon N_{2-HAp}$ and $\epsilon N_{2-Ru^0}$ allow us to estimate the Ru dispersion, which is in remarkable agreement with that determined by transmission electron microscopy (TEM).

**Keywords:** HAp, Ruthenium, Integrated molar absorption coefficient, Catalyst, TEM, XRF, XPS.
# 1. Introduction

Accurate assessment of the metal dispersion, i.e. the fraction of exposed surface metal atoms of nanoparticles, is known to be of paramount importance in the field of catalysis, as these data are used to calculate the turnover frequency of a given reaction on a given metal, which allows for a reliable comparison of the intrinsic efficiency of supported catalysts. In addition to the use of transmission electron microscopy (TEM), metal dispersion has long been assessed by volumetric chemisorption of probe molecules such as H₂ and CO, provided that the probe molecule-to-metal stoichiometry is known.¹ However, when the probe molecule exhibits multiple adsorption modes resulting in different probe molecule-to-metal stoichiometries, and/or when the probe molecule interacts with the catalyst support, volumetric chemisorption appears to be unreliable for assessing metal dispersion.

Alternatively, Binet et al.² and Primet et al.³ pioneered the estimation of Pd and Pt dispersion, respectively, by CO adsorption followed by Fourier transform infrared spectroscopy (CO-FTIR). This method has proven to be of particular interest not only when these metals are supported on reducible oxides, such as those related to ceria,³⁻¹⁰ in which case the H₂ chemisorption conditions must be adapted to avoid hydrogen spillover onto the support,⁷,¹¹ but also for the estimation of the dispersion/accessibility of Pd,¹²⁻¹⁵ Pt,¹⁴⁻¹⁸ Rh¹⁹⁻²¹ and Ir²², ²³ supported on non-reducible supports. Despite the numerous studies carried out to date on the characterization of supported Ru catalysts by CO adsorption followed by FTIR,²⁴⁻³³ probably partly because of the interest in Ru for Fischer-Tropsch synthesis,³⁴, ³⁵ the use of CO-FTIR to evaluate the Ru dispersion of supported catalysts has not been reported to our knowledge. This could be due to the complexity of the carbonyls formed with Ru³⁶, ³⁷ and/or to the fact that CO modifies the Ru nanoparticles by their oxidative disruption to metal carbonyls likely with the contribution of the neighboring OH groups of the supports,³⁰, ³⁶⁻³⁹ as originally demonstrated for Rh catalysts.⁴⁰⁻⁴²
As an isoelectronic molecule of CO, N₂ is a potential alternative FTIR probe molecule because it is known to be much less reactive than CO. However, N₂ obviously suffers from a much lower FTIR sensitivity than CO,⁴³ since N₂ only becomes IR active by creating a dipole moment when adsorbed on a surface. In addition, its use is generally more technically challenging than that of CO because low temperature or high pressure conditions are generally required to enable N₂ adsorption on surfaces. Furthermore, N₂ adsorption on metals is strongly conditioned by the absence of oxygen, which easily displaces N₂ adsorbed species⁴⁴ and may oxidize metal particles, potentially exposing cations that have been shown to adsorb N₂.⁴⁵,⁴⁶ Despite these drawbacks, the unreactive character of N₂ and the absence of IR gas-phase contribution have led few researchers to use this probe molecule qualitatively to essentially characterize the oxidation state/electron density of metals such as Ni,⁴⁷⁻⁴⁹ Pd⁴⁹, Pt,⁴⁹,⁵⁰ Rh,⁴³,⁴⁴,⁵¹⁻⁵⁸ Ir⁵⁹ and Ru.⁶⁰⁻⁶⁷ These works performed on the adsorption of N₂ followed by FTIR on Ru catalysts have been essentially motivated by the particular interest of using such a metal to improve the performance of ammonia synthesis under mild conditions.⁶⁸,⁶⁹

To the best of our knowledge, none of the previously reported studies on N₂ adsorption followed by FTIR have been carried out to evaluate the interest of using such a probe molecule to assess Ru dispersion, as has been previously demonstrated with the use of CO-FTIR on various metals.²⁻¹⁰, ¹²⁻²³ The present study addresses the challenging objective of using N₂ as an FTIR probe molecule (N₂-FTIR) to evaluate the dispersion of Ru supported on different hydroxyapatite materials (HAp: Ca₁₀(PO₄)₆(OH)₂). HAp, which crystalline structure is described in Figure S1, is the major component of bones and teeth and has recently emerged as an interesting and eco-friendly active phase and support in heterogeneous catalysis.⁷⁰,⁷¹ This support material was chosen for its intrinsic properties: (i) the ability to disperse metals that resist sintering,⁷² (ii) thermal stability up to 900 °C,⁷³ and (iii) tunable acid-base properties, via the Ca/P ratio,⁷⁴ that may influence the
electronic density of supported metal nanoparticles. Changes in the electronic density of supported metal nanoparticles via metal-support interactions have been shown to be of paramount importance in the field of catalysis,⁷⁵ as demonstrated by Aika et al. in ammonia synthesis catalyzed by supported Ru materials.⁶¹ In addition to the estimation of Ru dispersion on HAp, the Ca/P molar ratio of HAps, known to be sensitive to the pH of the synthesis medium,⁷⁴ is varied in this work so as to evaluate its influence on the integrated molar absorption coefficients of N₂ adsorbed on the HAp supports and on the supported Ru metal atoms, which are estimated for the first time. Finally, the integrated molar absorption coefficient of N₂ adsorbed on Ru metal atoms is compared with that of CO reported in a few previous studies.²⁹, ³⁰

## 2. Experimental

### 2.1. Material preparation

Rod-shaped HAp materials were prepared by coprecipitation. An aqueous solution of the phosphate precursor (NH₄H₂PO₄, ≥99.999% purity, Acros Organics) was added to a calcium precursor solution (Ca(NO₃)₂·4H₂O, ≥99.0% purity, Sigma-Aldrich) at 80 °C. The reaction mixture was stirred continuously at 400 rpm and maintained at a steady pH between 6.5 and 9.0. The pH of the medium in the reactor (pH<sub>R</sub>) was controlled via automated addition of concentrated NH₄OH (30%, Sigma-Aldrich) to offset the acidification of the medium associated with HAp precipitation (10 Ca²⁺ + 6 HPO₄²⁻ + 2 H₂O = Ca₁₀(PO₄)₆(OH)₂ + 8 H⁺). The initial concentrations of the calcium and phosphate solutions, [Ca]₀ and [P]₀, were 0.216 and 0.130 mol L⁻¹, respectively.

To facilitate the control of the pH of the medium during the synthesis, the pH of the phosphate precursor solution (pH<sub>P</sub>, Table S1) was adjusted at room temperature (RT) by adding concentrated NH₄OH (35%). For pH<sub>R</sub> ≥ 8, pH<sub>P</sub> was set to pH<sub>R</sub> + 1, and the Ca precursor solution
was basified with a few drops of concentrated NH₄OH. For pH< 8, pHₚ was set to pH< 2 and
no pH adjustment was applied to the Ca precursor solution (Table S1).

The volume of concentrated NH₄OH (Table S1) required to adjust the pH of the NH₄H₂PO₄ precursor solution (pHₚ) was taken into account to determine the volume of the NH₄OH-diluted phosphate precursor solution (noted as NH₄H₂PO₄ for the sake of clarity) to be added to the reactor containing the Ca precursor solution. This ensured a [Ca]/[P] final molar ratio of 1.67. All the operating parameters were controlled by using an Optimax 1001 workstation (Mettler Toledo),⁷⁶ as schematized in Figure S2A-1, and by following the flowchart diagram detailed in Figure S2B. As an example, the key experimental parameters (T, pH<sub>R</sub>, V NH₄H₂PO₄ and V NH₄OH) recorded during the synthesis of HAp-pH<sub>9.0</sub> are shown in Figure S2C.

The volume of the synthesis reactor and the addition rate were adjusted (Table S1) to
produce HAp materials with comparable specific surface areas. The detailed procedure for
selecting different addition rates of the phosphate solution to the calcium solution and for
characterizing the HAp samples by X-ray diffraction (XRD), which confirmed the presence of a
unique HAp crystalline phase in all of the HAp-pH<sub>x</sub> materials prepared in this study, and scanning
electron microscopy (SEM) is beyond the scope of this article and will be detailed in a forthcoming
article.

After precipitation, the HAp powders were recovered by centrifugation (8,500 rpm, Figure S2A-2), and washed 3 times with distilled water and centrifuged after each washing step (Figure S2A-3). Finally, the HAp supports were dried at 100 °C for 12 h in a laboratory oven on a glass watch (Figure S2A-4), crushed in an agate mortar and sieved (Figure S2A-5).

2 wt% of Ru was deposited by wet impregnation of the dried HAp supports with a
Ru(NO)(NO₃)₃ nitric acid solution (Merck, 1.4 wt% Ru) (Figure S2A-6). After drying at 80 °C for
12 h (Figure S2A-7), the crushed and sieved samples (Figure S2A-8) were placed on the frit of a
Pyrex fixed-bed reactor and reduced under 60 mL/min of H₂ (Air Liquide Alphagaz 1, 99.999%) for 2 h at 450 °C (5 °C/min) (Figure S2A-9). The materials are hereafter referred to as Ru/HAp-pHₓ, where x is the pH at which the precipitation was performed. The synthesis of the Ru/HAp-pHₓ materials is summarized in the flowchart shown in Figure S2A. For comparison, 2 wt% of Ru was also deposited by using the aforementioned procedure on an HAp-pH₈.₅ support that had been previously calcined for 2 h at 500 °C (5 °C/min) in a muffle furnace. This thermal treatment was performed to clean the HAp surface from potential contaminants. This sample is denoted as Ru/HAp-pH₈.₅c.

## 2.2. Catalyst characterization

**Chemical analysis of Ca and P by X-ray fluorescence (XRF).** The Ca and P contents of the prepared HAp supports were determined according to a previously described quantitative XRF protocol.⁷⁶ Briefly, all HAp supports were melted into “beads” and analyzed with an X-ray fluorimeter Epsilon 3x1 (Panalytical) equipped with an Ag X-ray tube operating in a He atmosphere under two conditions: 12 kV–25 µA for 120 s for calcium analysis and 10 kV–30 µA for 120 s for phosphorus analysis. The bead was prepared by mixing approximately 0.1 g of sample, 1.2 g of fluxing agent (LiBO₂/Li₂B₄O₇) and 0.019 g of non-wetting agent (LiBr) in a platinum crucible. The mixture was 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 precursors Ca(OH)₂ and NH₄H₂PO₄. 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 for each sample and resulted in the determination of the Ca and P contents with a relative deviation of ±1%.
**N₂ sorption.** N₂-sorption measurements were performed 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 BET method in the 0.05 < *p* /*p*⁰ < 0.30 range. The specific surface areas reported in this work were determined with a deviation of ± 2 m²/g.

**X-ray photoelectron spectroscopy (XPS).** XP spectra were collected on an Omicron Scienta (Argus) X-ray photoelectron spectrometer using a monochromated Al Kα (hv = 1486.6 eV) radiation source with a 280 W electron beam power. Charge compensation was performed using an electron beam with an energy of 1 eV and an emission current of 5 μA. The emission of photoelectrons from the sample was analyzed at a take-off angle of 45° under ultrahigh vacuum conditions (1 × 10⁻⁷ Pa). XP spectra were collected at a pass energy of 20 eV for the C 1s, O 1s, Ca 2p, and P 2p core XPS levels after calcining the samples at 500 °C (5 °C/min) for 2 h. The C 1s signal at 285 eV was used as a reference to calibrate the binding energies (BEs) of O 1s, Ca 2p₃/₂, and P 2p₃/₂. All of the BEs were measured to an accuracy of ±0.2 eV. Peak areas were determined after subtraction of a Tougaard background. Atomic ratio calculations were performed after normalization using Scofield factors.⁷⁷ Spectrum processing was performed by using the Casa XPS software package and Origin 7.1 (Origin Lab Corporation). The detection limit for XPS is typically 0.1–1 atomic percent, though it varies based on the sensitivity of the trace element and the sample matrix. XPS quantification is typically accurate to within ±10%, though the margin of error can range from 4 to 15%.

**Transmission electron microscopy (TEM).** Transmission electron microscopy was performed with a JEOL JEM 2100 operating at 200 kV, emitting with a LaB6 tip. Micrographs were taken with a 4 K camera (Gatan Orius). Samples were suspended in ethanol and dispersed in an ultrasonic bath before rapid deposition on a carbon-coated copper grid. The diameter of the Ru nanoparticles was estimated by using the ImageJ program. The mean diameter of Ru nanoparticles,
$d$ (Å), was determined by measuring at least 300 metal particles, and $d$ was calculated using the following formula: $d = \Sigma n_i d_i^3 / \Sigma n_i d_i^2$, where $n_i$ is the number of particles of diameter $d_i$. The Ru dispersion, $D$ (%), was calculated as follows: $D = (6 v_m/a_m)/d$, where $v_m$ and $a_m$ 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), respectively.¹ Statistical analysis of the Ru nanoparticles from the Ru/HAp-pH$_{8.0}$ sample was performed twice on separate images, yielding mean diameter of 2.2±0.1 nm.

**N$_2$-FTIR.** After adsorption of N$_2$ (Air Liquide, 99.9999%) on the Ru/HAp-pH$_x$ samples, 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. 128 scans were averaged with a spectral resolution of 4 cm$^{-1}$. Samples were pressed into self-supporting wafers of approximately 10 mg/cm$^2$ (20 mg, $10^7$ Pa/cm$^2$, 2.0 cm$^2$). The wafers were loaded into an *in situ* quartz IR PELICAEN cell,⁷⁸ developed and commercialized at the Laboratoire Catalyse et Spectrochimie, equipped with a liquid nitrogen tank and CaF$_2$ windows in the bottom part for low temperature experiments. The cell is connected to a vacuum line, equipped with two pressure gauges (Pfeiffer vacuum, 0-10 and 0-1000 mbar ranges, accuracy of 0.2 % of the full scale), for sample pretreatment and the introduction of gaseous probe molecules. Prior to spectrum recording, the samples were subjected to a reducing pretreatment (150 Torr H$_2$, Air Liquide, 99.9999%) at 450 °C for 2 h (2 °C/min) and then evacuated at 450 °C for 1 h ($10^{-5}$ Torr). Finally, the samples were cooled down to approximately 100 K under a dynamic vacuum ($10^{-5}$ Torr) by introducing liquid N$_2$ into the tank. Prior to the addition of accurate doses of N$_2$ through a small calibrated volume (1.9462 cm$^3$) positioned at the front side of the cell, an equilibrium pressure of about 0.2 Torr of helium (Air Liquide, 99.9999%) was introduced into the cell in order
to homogenize the temperature of the sample. The band area measurements were typically accurate to within ± 5%.

### 3. Results and discussion

#### 3.1. Specific surface area and bulk Ca/P ratio of the HAp-pHx supports

Table 1 shows that the synthesis protocols used to prepare the HAp supports at different pH resulted in specific surface areas of 38±4 m²/g. Overall, this table also shows that the bulk Ca/P molar ratio measured by XRF increases with increasing pH, in agreement with the previous study by Tsuchida et al.⁷⁴ Stoichiometric HAp, i.e. Ca/P molar ratio of 1.67, is obtained for the support prepared at pH 7.6. The slight increase in the Ca/P molar ratio at pH higher than 7.6 is attributed to the incorporation of carbonate defects at the phosphate crystallographic sites of HAp, as it has been reported that bulk carbonation of this material is favored under high pH precipitation conditions.⁷⁹

<table><caption>**Table 1.** Specific surface area (SSA) and bulk Ca/P molar ratio of the HAp-pHx supports, Ru particle size estimated by TEM and associated dispersion ($D_{TEM}$, %),¹ Ru dispersion estimated by N₂-FTIR ($D_{N₂-FTIR}$, %), and position of the maximum of the absorption band of N₂ adsorbed on Ru⁰ atoms (v N₂-Ru⁰).</caption><thead><tr><th rowspan="2">Samples</th><th rowspan="2">SSA<br>(m²/g)</th><th rowspan="2">Bulk Ca/P<br>molar ratio</th><th rowspan="2">Ru particle size<br>(nm)</th><th colspan="2">Ru dispersion</th><th rowspan="2">v N₂-Ru⁰<br>(cm⁻¹)</th></tr><tr><th>D<sub>TEM</sub><br>(%)</th><th>D<sub>N₂-FTIR</sub><br>(%)</th></tr></thead><tbody><tr><td>Ru/HAp-pH<sub>6.5</sub></td><td>39</td><td>1.61</td><td>2.1</td><td>61.4</td><td>54.8</td><td>2209</td></tr><tr><td>Ru/HAp-pH<sub>7.4</sub></td><td>39</td><td>1.65</td><td>2.2</td><td>58.6</td><td>64.8</td><td>2192</td></tr><tr><td>Ru/HAp-pH<sub>7.6</sub></td><td>42</td><td>1.67</td><td>2.3</td><td>56.1</td><td>61.1</td><td>2191</td></tr><tr><td>Ru/HAp-pH<sub>8.0</sub></td><td>34</td><td>1.64</td><td>2.2±0.1</td><td>60.0±1.4</td><td>55.0</td><td>2199</td></tr></tbody></table>
<table><tr><td>Ru/HAp-pH<sub>8.3</sub></td><td>38</td><td>1.70</td><td>2.1</td><td>61.4</td><td>60.4</td><td>2209</td></tr><tr><td>Ru/HAp-pH<sub>8.5</sub></td><td>39</td><td>1.69</td><td>2.5</td><td>51.6</td><td>58.5</td><td>2216</td></tr><tr><td>Ru/HAp-pH<sub>9.0</sub></td><td>38</td><td>1.68</td><td>2.3</td><td>56.1</td><td>52.0</td><td>2224</td></tr><tr><td>Ru/HAp-pH<sub>8.5c</sub></td><td>-</td><td>-</td><td>1.7</td><td>74.6</td><td>66.4</td><td>2205</td></tr></table>

## 3.2. Comparison of the surface (XPS) Ca/P and bulk (XRF) Ca/P ratios

In brief, the XP spectra of the HAp-pHₓ calcined samples were found to be similar to those recently reported for HAp materials.⁸⁰,⁸¹ The binding energies of O 1s, Ca 2p₃/₂ and P 2p₃/₂ were similar for the samples studied, close to 531.1, 347.2 and 133.3 eV, respectively, and in agreement with those previously reported.⁸⁰,⁸¹ Figure 1 shows that the XPS-determined surface composition is lower than that of the bulk of the HAp-pHₓ series. Previous studies have reported such discrepancies between the bulk and surface compositions of these materials.⁸⁰,⁸¹ These discrepancies have been attributed to the preferential HAp surface termination of HAp with a zig-zag configuration, as supported by DFT calculations.⁸⁰ In addition, Figure 1 shows that the surface Ca/P molar ratio remains essentially constant and close to 1.3 for the HAp-pHₓ series, so that their bulk composition exerts little influence on their surface composition in the limited range of bulk Ca/P molar ratios investigated in the present work from an XPS perspective.

<figure><img src="image_2.png" /></figure>
**Figure 1.** Comparison of the surface (XPS) and bulk (XRF) Ca/P molar ratios of Ru/HAp-pH<sub>6.5</sub> (green), Ru/HAp-pH<sub>7.6</sub> (orange), Ru/HAp-pH<sub>8.0</sub> (red), Ru/HAp-pH<sub>8.3</sub> (magenta), Ru/HAp-pH<sub>8.5</sub> (gray), Ru/HAp-pH<sub>9.0</sub> (black), and Ru/HAp-pH<sub>8.5c</sub> (gray).

**3.3. Ru dispersion estimated by TEM (D<sub>TEM</sub>)**

Representative TEM images of the HAp-supported Ru samples and corresponding Ru
particle size distributions are shown in Figure 2. After reduction at 450 °C, the micrographs show
that Ru is well dispersed with a rather narrow distribution on all rod-like HAp supports and that
the composition of the HAp support, in terms of Ca/P ratio, does not significantly affect the Ru
dispersion. The Ru/HAp-pH<sub>8.0</sub> sample was examined twice and showed good reproducibility
(Table 1). The average Ru particle size supported on HAp appears to be close to 2.2 nm with an
associated dispersion of about 60% (Table 1). In contrast, depositing Ru onto a precalcined HAp-
pH<sub>8.5</sub> support (Ru/HAp-pH<sub>8.5c</sub>) produces substantially smaller Ru nanoparticles with an average
size of 1.7 nm (Table 1 and Figure 2h).
<figure><img src="image_3.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>(e)</figcaption></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>(f)</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>(g)</figcaption></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>(h)</figcaption></figure>

<figure><img src="image_14.png" /></figure>
**Figure 2.** Representative TEM images and corresponding Ru particle size distributions recorded on (a) Ru/HAp-pH<sub>6.5</sub> (green), (b) Ru/HAp-pH<sub>7.4</sub> (blue), (c) Ru/HAp-pH<sub>7.6</sub> (orange), (d) Ru/HAp-pH<sub>8.0</sub> (red), (e) Ru/HAp-pH<sub>8.3</sub> (magenta), (f) Ru/HAp-pH<sub>8.5</sub> (gray), (g) Ru/HAp-pH<sub>9.0</sub> (black), and (h) Ru/HAp-pH<sub>8.5c</sub> (gray).

**3.4. Qualitative and quantitative N<sub>2</sub>-FTIR**

The FTIR spectra recorded at low temperature with incremental addition of N<sub>2</sub> on Ru/HAp-pH<sub>9.0</sub> are shown in Figure 3a. A narrow v(N≡N) band of increasing intensity with increasing amount of N<sub>2</sub> is observed at 2340±1 cm<sup>-1</sup>. Such a band is clearly shifted upward by about 10 cm<sup>-1</sup> with respect to the Raman frequency of N<sub>2</sub> in the gas phase.<sup>49</sup> According to a previous study, a v(N≡N) frequency higher than 2335 cm<sup>-1</sup> is generally assigned to N<sub>2</sub> interacting with Lewis acid sites, whereas v(N≡N) bands of H-bonded species are observed in the 2320-2335 cm<sup>-1</sup> frequency range.<sup>82</sup> Such a conclusion is supported by the lack of significant perturbation of the OH/POH signals in the OH-IR region upon introduction of N<sub>2</sub>. Moreover, it is known that most of the OH species of HAp are located inside the tunnels and are therefore not accessible to N<sub>2</sub>, except for those that emerge on the top surface of the crystallites.<sup>70,71</sup> In addition, these OH groups are known to be basic<sup>83, 84</sup> and, as a result, should not interact with N<sub>2</sub>. Therefore, the band at 2340 cm<sup>-1</sup> is assigned to N<sub>2</sub> adsorbed on Ca<sup>2+</sup>cationic sites of the HAp support (v N<sub>2</sub>-HAp), in agreement with the experiment carried out on HAp-pH<sub>9.0</sub> (Figure S3A) and previous studies performed on different supporting oxides.<sup>43, 44, 54-58</sup> As shown in Figure 3b, a broad absorption band of low intensity appears with a maximum at 2224 cm<sup>-1</sup> from the third dose of N<sub>2</sub> added (1.14 µmol of N<sub>2</sub>). The intensity of this band stabilizes with the 11<sup>th</sup> dose of N<sub>2</sub> added, corresponding to approximately 5.6 µmol of N<sub>2</sub> dosed on Ru/HAp-pH<sub>9.0</sub>. This broad absorption band is attributed to the stretching vibration of the N<sub>2</sub> molecule interacting with the exposed Ru<sup>0</sup> atoms<sup>60</sup> (v N<sub>2-Ru</sub><sup>0</sup>) via an end-on configuration.<sup>60, 64</sup> In agreement with previous literature studies on various metals<sup>43, 44, 50, 54, 59</sup> and Ru,<sup>60</sup> no shift of this band was observed with increasing introduction of N<sub>2</sub>, indicating that N<sub>2</sub> is
much less sensitive to dipole-dipole coupling than CO²⁹ with increasing coverage. This makes this particular probe molecule much more interesting than CO when the electron density of metallic particles is to be studied and compared between different samples.

<figure><img src="image_15.png" /><figcaption>**Figure 3.** (a) Spectra recorded at 100 K after introduction of 12 incremental doses of N₂ on Ru/HAp-pH₉.₀, (b) magnification of the 2100-2300 cm⁻¹ region attributed to the stretching vibration of N₂ adsorbed on Ru⁰ atoms. The amounts of N₂ introduced from the 1st to the 12th dose (dose number, and color of the spectrum given in parentheses) are 0.37(1, magenta), 0.39(2, light gray), 0.39(3, dark blue), 0.38(4, light green), 0.49(5, dark orange), 0.49(6, purple), 0.49(7, light blue), 0.49(8, light orange), 0.48(9, brown), 0.81(10, dark green), 0.81(11, black), and 0.63(12, red) µmol, respectively.</figcaption></figure>

The quantitative treatment of these contributions in the present work was done according to the previous work of Binet et al. using CO as a probe molecule.² The relationship between the integrated area of the band and the amount of the corresponding adsorbed species is given by the Beer-Lambert law:

$$A = \varepsilon l C \qquad \text{Eq. 1}$$
Where the integrated area of the band (A, cm⁻¹) is proportional to the product of the integrated molar absorption coefficient of the adsorbed species (ε, cm/µmol), the length of the diffusion path (l, cm), and the concentration of the adsorbed species (C, µmol/cm³).

Since C can be defined as the n/V ratio, where n and V represent the amount of N₂ adsorbed species and the volume of the self-supporting wafer, respectively, Eq. 1 simplifies to Eq. 2, where the l/V ratio is derived as the surface area (S, cm²) of the self-supporting IR wafer:

$$A = \varepsilon l n / V = \varepsilon n / S \qquad \text{Eq. 2}$$

### 3.4.1. Determination of the integrated molar absorption coefficient of N₂ adsorbed on the HAp_pHₓ supports

Figure S3B shows that the integrated area of the band at 2340 cm⁻¹ of HAp-pH₉.₀ is linearly correlated with the cumulative amount of N₂ introduced in each dose up to the 6th dose, corresponding to approximately 2.7 µmol of N₂ dosed. Figure 4a also shows such a correlation on Ru/HAp-pH₉.₀ with the cumulative amount of N₂ introduced in each of the first four doses (●), while the integrated area of the 5th dose is not correlated with the cumulative amount of N₂ introduced (○). From the slopes of the corresponding straight lines (Figures S3B and 4a), the integrated molar absorption coefficients of N₂ adsorbed on the HAp support (εN₂-HAp) can be determined by using Eq. 2. For HAp-pH₉.₀ and Ru/HAp-pH₉.₀, the εN₂-HAp-pH₉.₀ integrated molar absorption coefficient are estimated to be 0.094 and 0.091 cm/µmol, respectively. The fact that such a straight line was obtained with an excellent correlation coefficient within the first four doses of N₂ on Ru/HAp-pH₉.₀ (Figure 4a), while a band at 2224 cm⁻¹, attributed to the stretching vibration of N₂ adsorbed on Ru⁰ atoms, appears after the third dose (Figure 3b), indicates (i) that the amount
of N₂ adsorbed on the Ru�0 atoms must be negligible compared to the amount of N₂ adsorbed on the HAp-pH9.0 support for the first four N₂ doses, so that N₂ should essentially interact with the HAp-pH9.0 support, and (ii) that the absorption coefficient of N₂ adsorbed on the Ru⁰ atoms should be much larger than that of N₂ adsorbed on the HAp-pH9.0 support. After the 5ᵗʰ dose, the amount of N₂ adsorbed on the Ru⁰ atoms should no longer be negligible.

<figure><img src="image_16.png" /></figure>

$y = 0.0454x$

$R^2 = 0.9997$

Amount of N₂ dosed (µmol)

<figure><img src="image_17.png" /></figure>

$y = 0.2513x$

$R^2 = 0.9544$

Amount of N₂ adsorbed on Ru⁰
atoms (µmol)

**Figure 4.** (a) Integrated area of the absorption band at 2340 cm⁻¹ assigned to the interaction of N₂ with the HAp support of Ru/HAp-pH₉.₀ as a function of the amount of N₂ introduced in the first five doses (first four doses ● and 5ᵗʰ dose ○), and (b) integrated area of the absorption band at 2224 cm⁻¹ assigned to the interaction of N₂ with Ru⁰ atoms of Ru/HAp-pH₉.₀ as a function of the amount of N₂ adsorbed on Ru⁰ atoms from the 5ᵗʰ to the 12ᵗʰ dose. The amounts of N₂ adsorbed on Ru⁰, as plotted in Figure 4b, result from the calculations detailed in Section 3.4.2. These calculations introduce more uncertainty into these data than into the amounts of N₂ adsorbed on HAp, as plotted in Figure 4a, which were extracted directly from the measurements.

The integrated area of the band at 2340 cm⁻¹ upon the introduction of a single dose of N₂ for the other Ru/HAp-pHₓ samples prepared with the dried HAp-pHₓ supports is in good agreement with
that of Ru/HAp-pH9.0 (Figure 5a). This indicates that εN2-HAp-pHx is essentially constant over the different HAp-pHx supports (Figure 5b) with an average value of 0.092±0.008 cm/μmol. This also shows that the changes in the surface composition of the HAp support, which appear to be beyond the sensitivity of XPS (Figure 1), have little influence on the integrated molar absorption coefficient of N2 interacting with the support in the Ca/P range investigated in this work. This is consistent with the absence of support electron back donation⁸⁵ in the Π* anti-bonding orbital of N2 and the absence of shift of the band at 2340 cm⁻¹ (Figure 3a). Finally, it can be noted that the value of the integrated molar absorption coefficient of N2 adsorbed on Ca²⁺ cations (0.092±0.008 cm/μmol) determined in this study is in good agreement with that estimated from N2 adsorbed on Zr⁴⁺ cations by Malyshev et al. (0.123 cm/μmol), whose absorption band was also observed at 2340 cm⁻¹.⁸²

### 3.4.2. Determination of the integrated molar absorption coefficient of N₂ adsorbed on the Ru⁰ atoms of the Ru/HAp_pHₓ samples

The determination of the integrated molar absorption coefficient of N₂ species adsorbed on Ru⁰ surface atoms (εN₂-Ru⁰) is less straightforward than in the case study of Binet et al. using CO as a probe molecule of Pd⁰ atoms.² While CO interacts exclusively with metal atoms at RT, the present work shows that a significant amount of N₂ interacts with the HAp supports (Figure 3a). However, by determining the integrated molar absorption coefficient of N₂ adsorbed on the HAp supports (Figure 5b, εN₂-HAp-pHₓ = 0.092±0.008 cm/μmol), the amount of N₂ adsorbed on the Ru⁰ surface atoms at each incremental dose of N₂ can be estimated:

$$n_{\text{N2-Ru}^0} = n_{\text{N2 dosed}} - n_{\text{N2-HAp}} \qquad \text{Eq. 3}$$
Where $n_{N_2-Ru^0}$, $n_{N_2 \text{ dosed}}$, and $n_{N_2-HAp}$ are the amount of N₂ adsorbed on Ru⁰ surface sites, the cumulative amount of N₂ dosed in the FTIR cell, and the amount of N₂ adsorbed on the HAp support (µmol), respectively.

By combining equations 2 and 3, the amount of N₂ adsorbed on the Ru⁰ surface atoms at each dose of N₂ is estimated to be:

$$n_{\text{N}_2\text{-Ru}^0} = n_{\text{N}_2 \text{ dosed}} - A_{\text{N}_2\text{-HAp}} S / \varepsilon \text{N}_{2\text{-HAp-pHx}} \quad \text{Eq. 4}$$

Where $A_{\text{N}_2\text{-HAp}}$, S and $\varepsilon \text{N}_{2\text{-HAp-pHx}}$ are the integrated area of the band at 2340 cm⁻¹ (cm⁻¹), the surface area of the wafer (2 cm²) and the integrated molar absorption coefficient of N₂ adsorbed on the HAp support (0.092 cm/µmol), respectively. The amounts of N₂ interacting with the Ru⁰ surface atoms of Ru/HAp-pH₉.₀ derived from the above calculation method are shown in Figure 4b. This

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>
<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /><figcaption>**Figure 5.** (a) Comparison of the integrated area of the band at 2340 cm⁻¹ assigned to the interaction of N₂ with the HAp-pHₓ supports upon the introduction of a single dose containing between 0.60 and 0.93 µmol of N₂ for Ru/HAp-pH₆.₅ (green), Ru/HAp-pH₇.₄ (blue), Ru/HAp-pH₇.₆ (orange), Ru/HAp-pH₈.₀ (red), Ru/HAp-pH₈.₃ (magenta), Ru/HAp-pH₈.₅ (gray), and Ru/HAp-pH₈.₅c (○, gray) with those of Ru/HAp-pH₉.₀ (black), and evolution of (b) the integrated molar absorption coefficient of N₂ adsorbed on the HAp supports (εN₂-HAp) of the Ru/HAp-pHₓ samples as a function of the Ca/P molar ratio, (c) the integrated molar absorption coefficient of N₂ adsorbed on the Ru�0 atoms (εN₂-Ru⁰) of the Ru/HAp-pHₓ samples as a function of the Ca/P molar ratio, and (d) εN₂-Ru⁰ as a function of the stretching vibration of N₂ adsorbed on the Ru⁰ atoms (v N₂-Ru⁰) of the Ru/HAp-pHₓ samples.</figcaption></figure>

figure shows that the estimation of such quantities is more difficult than that of CO² due to the much lower intensity of the N₂ absorption band compared to that of the CO absorption band and the competitive adsorption of N₂ on the HAp support. By applying the same methodology to the N₂ species adsorbed on Ru⁰ atoms (Figure 4b) as that used for the N₂ species adsorbed on the HAp supports to estimate εN₂-HAp-pHₓ (Figure 5a), the εN₂-Ru⁰ integrated molar absorption coefficient is estimated to be 0.50 cm/µmol for Ru/HAp-pH₉.₀. Figure 5c shows that the εN₂-Ru⁰ integrated molar
absorption coefficients vary between 0.50 and 1.03 cm/µmol for the Ru/HAp-pHₓ series, with
maximum values found for Ru/HAp-pH₇.₄ and Ru/HAp-pH₇.₆. The fact that εN₂-Ru�0/HAp-pHₓ is from
about 5 to 10 times larger than that of εN₂-HAp-pHₓ (Figure 5b, εN₂-HAp-pHₓ = 0.092±0.008 cm/µmol)
is consistent with the expected electron back-donation from the *d*-orbitals of the Ru metal atoms to
the Π* anti-bonding orbital of the N₂ molecule, which is absent on the HAp support, and must
result in a red shift of the N-N stretching vibration that increases with increasing electron density
of the Ru metal nanoparticles.⁶⁰ While CO as a probe molecule has been used much more frequently
than N₂ as a probe molecule to characterize Ru catalysts²⁸, ³⁰, ³⁶, ⁸⁶, ⁸⁷ and the CO integrated molar
absorption coefficient has been discussed qualitatively,²⁴⁻²⁷ to the best of our knowledge, the only
two works in which such an integrated molar absorption coefficient has been quantified are those
of Bell and coworkers.²⁹, ³⁰ Winslow and Bell showed that the integrated molar absorption
coefficient of CO adsorbed on Ru⁰ atoms supported on SiO₂ increased from 45 to 90 cm/µmol as
the CO surface coverage decreased from 1 to 0.²⁹ Although it is generally accepted that the
integrated molar absorption coefficient of N₂ should be much lower than that of CO⁴³, ⁵¹ due to the
intrinsically much smaller dipole moment of N₂ compared to CO, the magnitude by which such an
integrated molar absorption coefficient varies between N₂ and CO has, to the best of our
knowledge, never been reported. The comparison of the integrated molar absorption coefficient of
N₂ adsorbed on Ru⁰ atoms supported on HAp determined in the present study (0.5-1.03 cm/µmol)
with that of CO adsorbed on Ru⁰ atoms supported on SiO₂ reported by Winslow and Bell (45-
90 cm/µmol)²⁹ shows for the time that the integrated molar absorption coefficient of N₂ is about
two orders of magnitude lower than that of CO when adsorbed on Ru⁰ atoms.
### 3.4.3. Evolution of the N₂ absorption bands with the chemical composition of the HAp support of the Ru/HAp\_pHₓ samples

Figure 3b (11th and 12th dose) shows that the maximum N₂ coverage is reached after dosing about 5.6 µmol of N₂ on Ru/HAp-pH₉,₀, as the spectra do not change significantly and the amount of N₂ adsorbed on Ru�0 atoms remains essentially constant between the last two doses of N₂ (Figure 4b). Figure 6 compares the N₂-FTIR spectra of the Ru/HAp-pHₓ samples at N₂ saturation coverage. Contrary to what is observed at the position of the absorption band at 2340 cm⁻¹, the position of the maximum of the absorption band at about 2200 cm⁻¹, related to N₂ adsorbed on Ru⁰ atoms, is more or less shifted with the nature of the HAp-pHₓ support (Figure 6) and is found to be the lowest for Ru/HAp-pH₇.₄ and Ru/HAp-pH₇.₆ (Table 1). It should be noted that the observed shifts cannot be attributed to changes in the morphology of the Ru nanoparticles,⁴⁹ as their size was similar in all samples prepared with dried HAp-pHₓ supports (Table 1 and Figure 2). However, the relatively high full width at half maximum (FWHM) of the absorption band at about 2200 cm⁻¹ may indicate heterogeneity among the Ru⁰ sites of the nanoparticles. Overall, the shift of the maximum of the absorption band related to N₂ adsorbed on Ru⁰ atoms supported on HAps is lower than those previously reported for Ru⁰ atoms interacting with different supports,⁶⁰⁻⁶³, ⁶⁵⁻⁶⁷ suggesting that the electron density of the present Ru metal nanoparticles would be lower than those previously reported. Since the electron density of the Ru metal nanoparticles affects both the position of the absorption band⁶⁰ and the integrated molar absorption coefficient (Figure 5c) of N₂ adsorbed on Ru⁰ atoms, a more relevant correlation could be obtained by plotting the integrated molar absorption coefficient of N₂ adsorbed on Ru⁰ atoms (εN₂-Ru⁰) as a function of the stretching vibration of N₂ (νN₂-Ru⁰). Figure 5d shows quantitatively for the first time that the integrated molar absorption coefficient of N₂ adsorbed on Ru⁰ atoms increases remarkably linearly with decreasing
$N_2$ stretching vibration, in agreement with the earlier qualitative statements of Schwank et al. for $CO^{25}$ and Wang and Yates for $N_2^{43}$ probe molecules for Ru/MgO and Rh/$Al_2O_3$ samples, respectively. Finally, Figure 5c shows that the Ca/P molar ratio cannot be taken directly as a descriptor of the electron transfer from the HAp supports to the Ru metal nanoparticles.

<figure><img src="image_22.png" /><figcaption>**Figure 6.** $N_2$-FTIR spectra recorded at 100 K after $N_2$ saturation of the Ru/HAp-$pH_x$ samples and normalized to the sample weight of the wafers, Ru/HAp-$pH_{6.5}$ (green), Ru/HAp-$pH_{7.4}$ (blue), Ru/HAp-$pH_{7.6}$ (orange), Ru/HAp-$pH_{8.0}$ (red), Ru/HAp-$pH_{8.3}$ (magenta), Ru/HAp-$pH_{8.5}$ (gray), Ru/HAp-$pH_{9.0}$ (black), and Ru/HAp-$pH_{8.5c}$ (dashed gray).</figcaption></figure>
### 3.5 Ru dispersion estimated by N₂-FTIR ($D_{N2-FTIR}$)

Consistent with the previous work of Aika and Tamara,⁸⁸ Kerpal et al. reported on a σ-bonded adsorption mode of N₂ perpendicular to the surface of Ru clusters of various sizes.⁸⁹ Therefore, a stoichiometry of one N₂ molecule per surface Ru atom was assumed. The amount of N₂ adsorbed on the Ru metal nanoparticles determined at N₂ saturation coverage on the wafers allows to estimate the dispersion of Ru on the different samples, by considering that the Ru dispersion ($D_{N2-FTIR}$, %) is the quotient of the amount of N₂ adsorbed at saturation coverage normalized per g of sample ($n_{N2-FTIR}$, µmol N₂/g) by the amount of Ru in the sample ($n_{Ru}$ = Ru weight loading (wt%) / $M_{Ru}$ (101.07 g/mol) = 2 / 100 / 101.07 = 197.9 µmol Ru/g):

$$D_{N2-FTIR} = n_{N2-FTIR} / n_{Ru} \times 100 \quad \text{Eq. 5}$$

Table 1 and Figure 7 show that there is a remarkably good agreement between the dispersion of Ru estimated by N₂-FTIR and that determined by TEM. Figure 7 also shows that the data are randomly scattered on either side of the unity dashed line, which prevents any bias in such measurements related to the pH of the HAp synthesis coprecipitation medium. Finally, the fact that the dispersion of the sample prepared with a calcined support (Ru/HAp-pH8.5c) aligns reasonably with the unity line, despite having smaller Ru particles than the other samples in this study (Table 1 and Figure 2), provides further robustness to the proposed method.

<figure><img src="image_23.png" /></figure>
**Figure 7.** Ru dispersion estimated by N₂-FTIR as a function of the Ru dispersion determined by TEM of the Ru/HAp-pHₓ samples; Ru/HAp-pH₆.₅ (green), Ru/HAp-pH₇.₄ (blue), Ru/HAp-pH₇.₆ (orange), Ru/HAp-pH₈.₀ (red), Ru/HAp-pH₈.₃ (magenta), Ru/HAp-pH₈.₅ (gray), Ru/HAp-pH₉.₀ (black), and Ru/HAp-pH₈.₅c (O, gray).

**5. Conclusion**

This work shows that N₂ can be used as an FTIR probe molecule not only qualitatively to compare the electron density of the Ru metal nanoparticles supported on HAp supports with different bulk compositions, which were varied by changing the synthesis pH of the HAp precipitation medium, but also quantitatively to provide a reliable determination of the integrated molar absorption coefficient of N₂ adsorbed on the HAp supports, the integrated molar absorption coefficients of N₂ adsorbed on the HAp-supported Ru�0 nanoparticles and their associated dispersion. For the first time, it is found that the integrated molar absorption coefficient of N₂ adsorbed on the HAp supports (εN₂-HAp) does not depend on the chemical composition of these materials with an average value of 0.092±0.008 cm/µmol. In contrast, the integrated molar absorption coefficient of N₂ adsorbed on the HAp-supported Ru⁰ nanoparticles (εN₂-Ru⁰) of about 2.2 nm is found to be extremely sensitive to the chemical composition of the HAp support. The latter integrated molar absorption coefficients were found to increase linearly with the downward shift of the band position and varied from 0.5 to 1.03 cm/µmol. These integrated molar absorption coefficients thus appear to be from 5 to 10 times higher than those found on the HAp supports, which is attributed to the electron back-donation of the Ru⁰ nanoparticles in the Π* anti-bonding N₂ orbital. The present study also allowed, for the first time, to provide quantitative arguments showing that εN₂-Ru⁰ increases with increasing electron density of the Ru⁰ nanoparticles, as monitored by the position of the maximum of the absorption band of the N₂ stretching vibration at
about 2200 cm⁻¹ (ν N₂-Ru⁰), with a remarkable correlation between εN₂-Ru⁰ and ν N₂-Ru⁰. These εN₂-Ru⁰ data are found to be two orders of magnitude smaller than those of the integrated molar absorption coefficients of CO (εCO-Ru⁰) on SiO₂-supported Ru⁰ nanoparticles as the CO coverage decreases from 1 to 0 (45-90 cm/µmol), as reported in the only previous study to date.²⁹ Using the determined εN₂-HAp and εN₂-Ru⁰ absorption coefficients, it was possible to estimate the Ru dispersion, which was found to be in remarkable agreement with that determined by TEM. N₂ chemisorption on Ru catalysts may pave the way for using such a technique in a more quantitative approach, as far as the methodology proposed in this work would also apply for Ru catalysts using supports different from HAp, especially in the field of ammonia synthesis.

**CRediT author statement**

**Sabrina Akrour:** Investigation, Data Curation, Visualization, Validation, Writing – Review & Editing, Resources. **Alexandre Vimont:** Conceptualization, Methodology, Investigation, Data Curation, Writing – Review & Editing. **Guylène Costentin:** Conceptualization, Supervision, Writing – Review & Editing. **Cyril Thomas:** Conceptualization, Methodology, Data Curation, Writing – Original Draft, Writing – Review & Editing, Visualization, Supervision, Project Administration, Funding Acquisition. All co-authors have read and agreed on the submitted version of the manuscript.

**Supporting Information:** Figure S1: HAp crystalline structure; Table S1: operating parameters used to synthesize HAp-pHₓ supports; Figure S2: flowchart of the elementary steps in Ru/HAp-pHₓ synthesis, flowchart of the successive steps for precipitation and maturation of HAp-pHₓ supports, and key parameter profiles recorded during the synthesis of Ru/HAp-pH₉.₀; and
Figure S3: N₂-FTIR spectra recorded on HAp-pH₉.₀ and integrated area of the absorption band at 2340 cm⁻¹ (DOC).

## Acknowledgments

This work was supported by the French National Research Agency (H2CASTORAMA project: ANR-22-CE05-0002). SA 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 authors thank A. Miche for the XPS measurements. The TEM 2100 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. The authors also thank S. Casale and Dr. F. Ben Romdhane for the TEM measurements. J-M. Krafft is also thanked for the preliminary N₂-FTIR measurements carried out at the Laboratoire de Réactivité de Surface.

## Conflicts of interest

There is no conflict of interest to declare.

## Data Availability Statement

Data supporting the results of this article are available upon request from the corresponding author.
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Table of Content Image

<figure><img src="image_24.png" /></figure>
Supporting Information

N₂ as a Quantitative FTIR Probe Molecule for
Reliable Assessment of the Dispersion of Ru
Supported on Hydroxyapatite

Sabrina Akrour,ª Alexandre Vimont,*,b Guylène Costentin,ª and Cyril Thomasª,∗

Dr. Alexandre Vimont: Université de Caen Normandie, ENSICAEN, CNRS, LCS, Laboratoire Catalyse et Spectrochimie, 14000 Caen, France.

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-mails: alexandre.vimont@ensicaen.fr, cyril.thomas@sorbonne-universite.fr

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

<footnote>ᵇUniversité de Caen Normandie, ENSICAEN, CNRS, LCS, Laboratoire Catalyse et Spectrochimie, 14000 Caen, France.</footnote>

<footnote>* To whom correspondence should be addressed:</footnote>
<figure><img src="image_25.png" /><figcaption>**Figure S1.** HAp crystalline structure illustrated as a projection along the *c*-axis of 4 adjacent unit cells. While Ca²⁺(1) cations form linear columns along the *c*-axis, Ca²⁺(2) cations are arranged in triangles (dashed and dotted blue lines) alternately rotated by 60° at different z-values, which delimit channels of about 2.85 Å diameter¹ with a pseudo-hexagonal cross-section that can be described as a series of ovoid cavities² where the OH⁻ anions are stacked.</figcaption></figure>
<table><caption>**Table S1.** Operating parameters used in the synthesis of the HAp-pHₓ materials carried out at 80 °C and using 200 and 400 mL of the Ca(NO₃)₂ solution in the 0.5 and 1.0 L reactors, respectively. The pH of the Ca(NO₃)₂ solution was adjusted by adding a few drops of concentrated NH₄OH except for Ru/HAp-pH₆.₅, Ru/HAp-pH₇.₄, and Ru/HAp-pH₇.₆.</caption><thead><tr><th>Sample</th><th>RV<sup>a</sup><br>(L)</th><th>pH<sub>R</sub><sup>b</sup></th><th>V Ca(NO<sub>3</sub>)<sub>2</sub>·4H<sub>2</sub>O<br>into the reactor<sup>c</sup></th><th>pH<sub>P</sub><sup>d</sup></th><th>V NH<sub>4</sub>OH added to<br>NH<sub>4</sub>H<sub>2</sub>PO<sub>4</sub> (mL) at RT<sup>e</sup></th><th>V NH<sub>4</sub>H<sub>2</sub>PO<sub>4</sub> added into the<br>reactor (mL)<sup>f</sup></th><th>r<br>(mL/min)<sup>g</sup></th><th>NH<sub>4</sub>H<sub>2</sub>PO<sub>4</sub></th><th>Maturation<br>time (h)</th></tr></thead><tbody><tr><td>Ru/HAp-pH<sub>6.5</sub></td><td>0.5</td><td>6.5</td><td>200</td><td>8.5</td><td>1.5</td><td>200.6</td><td>25.0</td><td></td><td>0</td></tr><tr><td>Ru/HAp-pH<sub>7.4</sub></td><td>0.5</td><td>7.4</td><td>200</td><td>9.4</td><td>3.3</td><td>202.0</td><td>11.4</td><td></td><td>0</td></tr><tr><td>Ru/HAp-pH<sub>7.6</sub></td><td>0.5</td><td>7.6</td><td>200</td><td>9.6</td><td>3.6</td><td>202.3</td><td>8.6</td><td></td><td>0</td></tr><tr><td>Ru/HAp-pH<sub>8.0</sub></td><td>0.5</td><td>8.0</td><td>200</td><td>9.0</td><td>3.1</td><td>201.9</td><td>4.4</td><td></td><td>2</td></tr><tr><td>Ru/HAp-pH<sub>8.3</sub></td><td>1.0</td><td>8.3</td><td>400</td><td>9.3</td><td>7.4</td><td>404.5</td><td>4.4</td><td></td><td>2</td></tr><tr><td>Ru/HAp-pH<sub>8.5</sub></td><td>1.0</td><td>8.5</td><td>400</td><td>9.5</td><td>11.1</td><td>407.6</td><td>4.4</td><td></td><td>2</td></tr><tr><td>Ru/HAp-pH<sub>9.0</sub></td><td>1.0</td><td>9.0</td><td>400</td><td>10.0</td><td>13.5</td><td>409.5</td><td>4.4</td><td></td><td>2</td></tr></tbody></table>

<footnote>(a)Reactor volume, (b)pH of the medium in the reactor measured at 80 °C and maintained during the precipitation and maturation steps, (c)Volume of the Ca(NO₃)₂·4H₂O solution introduced into the reactor, [Ca]₀ = 0.216 mol L⁻¹, (d)pH adjustment of the NH₄H₂PO₄ solution at RT to help control pHₘ, (e)Volume of NH₄OH added to the NH₄H₂PO₄ solution at RT to reach pHₚ, (f)Volume of NH₄OH-diluted NH₄H₂PO₄ solution added into the reactor to reach a [Ca]/[P] final value of 1.67, (g)NH₄OH-diluted NH₄H₂PO₄ addition rate (mL/min) in the reactor.</footnote>
<figure><img src="image_26.png" /><figcaption>(B)</figcaption></figure>

<figure><img src="image_27.png" /></figure>
<figure><img src="image_28.png" /><figcaption>**Figure S2.** (A) Flowchart of the elementary steps of Ru/HAp-pHₓ synthesis, (B) Flowchart of the successive steps for the precipitation and maturation of the HAp supports, and (C) Key parameter profiles (T, red trace), volumes of NH₄H₂PO₄ (blue trace) and NH₄OH (green trace) introduced into the reactor, pH of the medium into the reactor (black trace) recorded during the synthesis of Ru/HAp-pH₉.₀.</figcaption></figure>
<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

**Figure S3.** (A) Spectra recorded at 100 K after introduction of 9 incremental doses of N₂ on HAp-pH₉.₀, the amounts of N₂ introduced from the 1st to the 9th dose (dose number, color of the spectrum given in parentheses) are 0.39(1, magenta), 0.39(2, light gray), 0.39(3, dark blue), 0.39(4, green), 0.39(5, orange), 0.77(6, red), 0.77(7, purple), 0.77(8, brown), and 0.76(9, black) µmol, respectively, and (B) integrated area of the absorption band at 2340 cm⁻¹ due to the interaction of N₂ with the Ca²⁺ cations of the HAp-pH₉.₀ support as a function of the amount of N₂ introduced in the 9 doses.

In addition to the contribution at 2340 cm⁻¹, which is attributed to the interaction of N₂ with the Ca²⁺ cations of HAp, three contributions - two negative contributions and one positive - can be seen at approximately 2183, 2176 and 2170 cm⁻¹, respectively. These contributions were not observed in the Ru/HAp-pH₉.₀ sample (Figure 3b) and are attributed to the interaction of N₂ with the P-OH surface species, as well as a slight shift in the corresponding overtone and/or combination bands of the P-O bands.³ This attribution is supported by the presence of negative bands also recorded at 3730 and 3680 cm⁻¹ (not shown).
References

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(2) Uskoković, V. The Role of Hydroxyl Channel in Defining Selected Physicochemical Peculiarities Exhibited by Hydroxyapatite. *RSC adv.* **2015**, *5* (46), 36614-36633.

(3) Diallo-Garcia, S.; Osman, M. B.; Krafft, J.-M.; Casale, S.; Thomas, C.; Kubo, J.; Costentin, G. Identification of Surface Basic sites and Acid-Base Pairs of Hydroxyapatite. *J. Phys. Chem. C* **2014**, *118* (24), 12744-12757.