# Molecular selectivity within mixed gas hydrates of climate-relevant guests: CO₂ and N₂O

Claire Petuya, Françoise Damay, Sophie Sobanska, Arnaud Desmedt

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## To cite this version:

Claire Petuya, Françoise Damay, Sophie Sobanska, Arnaud Desmedt. Molecular selectivity within mixed gas hydrates of climate-relevant guests: CO₂ and N₂O. Energy & Fuels, 2025, 39 (25), pp.12108-12115. 10.1021/acs.energyfuels.5c01889. hal-05363586

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Submitted on 13 Nov 2025

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Molecular Selectivity within Mixed Gas Hydrates of Climate Relevant

Guest: CO₂ and N₂O

Claire Petuya,¹*, Françoise Damay², Sophie Sobanska¹, Arnaud Desmedt¹,²

¹ Groupe Spectroscopie Moléculaire, Institut des Sciences Moléculaires UMR CNRS 5255, Talence Cedex, France.

² Laboratoire Léon Brillouin, UMR 12 CEA-CNRS, Gif-sur-Yvette Cedex, France.

* Corresponding Author:

Email: claire.petuya@u-bordeaux.fr

Phone: +(33) 5 4000 2937

KEYWORDS. Clathrate Hydrates, Cage Occupancy, Molecular selectivity, Electric Dipole Moment, Raman Spectroscopy, N₂O, CO₂.
ABSTRACT

This study investigates the selectivity properties of mixed CO₂-N₂O hydrates using Raman spectroscopy and neutron diffraction, revealing significant insights into selective capture and storage of greenhouse gases. We have shown that mixed CO₂-N₂O hydrates consistently form in the so-called SI structure irrespective of the gas mixture composition. Moreover, molecular selectivity is highly dependent on both the initial gas mixture composition and the hydrate formation pathway, with water solubility (CO₂ and N₂O gases are relatively soluble) and electric dipole moment (only for N₂O) playing key roles. Notably, the impact of these physicochemical properties varies depending on whether the hydrate forms from ice or liquid water. Such experimental results thus offer new opportunities for selective capture and storage of greenhouse gaseous species via hydrate-based processes.
INTRODUCTION

Clathrate hydrates are crystalline compounds consisting of water molecules forming cages inside of which guest molecules (like gas) are trapped¹. Two hydrate structures are most commonly formed and observed namely the SI structure and the SII structure. They are composed of two small cages (denoted SCs) and six large cages (denoted LCs) and sixteen SCs and eight LCs, respectively. If there are two different chemical species encapsulated within the cages, the commonly name used is “mixed” clathrate hydrate. On Earth, these ice-like systems are naturally formed in the permafrost or sediments of the continental margins when thermodynamic conditions (low temperature and high pressure) are available. Thereby, besides being a potential natural fuel energy reservoir (essentially with CH₄),¹ they also open up new perspectives for the storage and transportation of gases such as methane² or hydrogen³ under soft conditions when promoter co-guest⁴ is used (close to atmospheric conditions), for the capture and sequestration of carbon dioxide⁵⁻⁷ or for the conversion of methane hydrates to carbon dioxide hydrates.⁸⁻¹²

Nitrous oxide (N₂O) is considered the third greenhouse gas after carbon dioxide (CO₂) and methane (CH₄)¹³. It is regarded as an ozone-depleting substance, meaning that the continuous emission of N₂O may be responsible for the depletion of stratospheric ozone.¹⁴,¹⁵ In addition, N₂O is a greenhouse gas more efficient than CO₂.¹⁶,¹⁷ The increase level of N₂O emissions in the atmosphere comes from human activities (5 % of greenhouse gases emission in the U.S.),¹⁶,¹⁷ such as agriculture, fuel combustion, energy and chemical industries and waste management of nitrogen-based organic substances.¹⁸,¹⁹ So, it becomes more and more important to control and mitigate the N₂O gas emission, especially as a byproduct of burning fossil fuels in industrial applications.²⁰,²¹ Hydrate-base gas-separation (HBGS) technology is one of the N₂O recovery and/or sequestration process that has been recognized as one of the relevant environmentally approach²². Although challenging, selective engaging of N₂O from mixture of
gases through HBGS process is promising²³ but requires further detail investigations. Indeed,
previous works have already tried to understand the influence of fundamental physical and
chemical properties on the molecular selectivity of mixed gas hydrates like in CO-N₂,²⁴ CO₂-
N₂²⁵,²⁶ or CO₂-CO²⁵ mixed hydrates. It appears that water solubility (in case of CO₂) and dipole
moment (only CO has one) may influence the molecular selectivity. Thus, in case of CO₂-N₂O
binary mixture, both solubility and dipole moment are crucial parameters that need to be
unraveled to highlight the behavior of these two greenhouse gases. Indeed, CO₂ and N₂O own
both a good water solubility, even though the water solubility of CO₂ is better than the one of
N₂O (about one and a half times),²⁷,²⁸ but only N₂O has a dipole moment. Moreover, their
pressure-temperature equilibrium curves are similar, unlike the study of CO₂-CO system.²⁵
Hydrate structures may be formed either via ice or liquid water route. As an example, it was
demonstrated that CO₂ clathrate hydrate formation is favored in liquid water in agreement with
CO₂ water solubility. ¹,²⁷,²⁹ The formation process which may influence the structure and/or the
host gas selective property is not fully understood. In this work, we have investigated the
selectivity properties for mixed CO₂-N₂O hydrates by considering both initial gas mixture
composition and the hydrate formation process. High resolution Raman spectroscopy and
neutron diffraction were used to identify the behavior of CO₂-N₂O inclusion in the cage of
hydrate structures. These experimental results are compared with those obtained on pure N₂O
gas hydrate samples and previous work²⁵ performed on CO₂-based hydrates.

EXPERIMENTAL DETAILS

**Sample preparation.** The samples of pure N₂O hydrate are formed using two different pathways. From liquid water, a volume of *ca* 0.5 cm³ of liquid water is pressurized (during few minutes) at 298 K and 20 bar then cooling until ice was formed (around 240 K) and finally maintain at 270 K, 20 bar. From ice, the same volume of water is inserted and then cooled down (*ca* 240 K) until the ice is formed. The ice is pressurized at 270 K and 20 bar during several
hours (around 15h). Gas mixtures were prepared by using pure gas of CO2 and N2O (purity > 99.997%) and two syringe pumps.²⁴,²⁵ Each volume and pressure are measured with the help of a PM High Pressure pump (Top Industrie, Vaux-le-Penil, France) before mixing the two gases. Successive pressurization/depressurization were applied for homogenizing the gas mixtures. The initial gas mixture compositions were 10 %, 18 %, 25 %, 50 % and 75 % of CO2 (given [CO2]/[N2O] = 0.11; 0.22; 0.33; 1.00; 3.00, respectively). The CO2/N2O percentages refer to volume/volume ratios. To form the hydrate samples, Milli-Q water is first injected inside the lab-made high-pressure optical cell. The temperature is controlled using a modified temperature stage (Linkam Scientific Instruments Ltd., UK). Hexagonal ice is formed at *ca.* 240 K and the temperature is raised to 270 K (± 0.1 K). Finally, 20 bar of CO2-N2O gas mixture is applied with the high-pressure pump on the formed hexagonal ice. In the case of mixed hydrates formed from liquid water, the water injected is pressurized with the gas mixture and the cell is cooled until *ca.* 240 K to form hydrates. The temperature is raised to 270 K in the stability region of the hydrate (Figure 1). For neutron diffraction experiments, the sample was formed using a powder (grain size typically of the order of 100-200 µm) of deuterium oxide ice (99.9 % D) and applying a gas pressure (purity > 99.997%) of 10 bar of CO2-N2O during 3 days within a thermal bath maintained at 260 K. The whole amount of ice is not converted into hydrate during the three days. Kuhs et al.³⁰⁻³² work showed that this will be the case even after several weeks of pressurization.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Phase diagram of pure CO₂ (blue squares) and N₂O (red triangle) gas hydrates with data from references 33-35.</figcaption></figure>

**Neutron diffraction.** Experiments have been performed at Laboratoire Léon Brillouin (CEA-Saclay) using the cold neutron two-axis powder diffractometer G 4-1. Measurements were done with a wavelength of 2.428 Å. Due to a better instrumental resolution at low scattering angle 2θ, the angular range 2θ < 60° has been considered. The cold samples were transferred under nitrogen atmosphere into the cryostat pre-cooled at 100 K to ensure the preservation of the hydrate sample. The pattern-matching of the powder diffractogram were performed with the Le Bail algorithm³⁶ and the FullProf program from the FullProf suite.³⁷

**Raman spectroscopy.** Raman spectra were recorded with a Labram microspectrometer (Horiba Jobin Yvon, Villeneuve d'Ascq, France) using a 514 nm wavelength laser as excitation source. A 50X objective allowed focusing the incident laser beam and collecting the Raman scattering. The Raman scattering was dispersed by a high resolution holographic grating of 2400 lines/mm and analyzed using a Peltier-cooled CCD detector (Andor, Belfast, UK). Thus, the intensity of typical wavelengths of the spectrum can be measured with a spectral resolution
of 0.8 cm⁻¹. The calibration of the spectrometer was done using the 520.7 cm⁻¹ mode of a silicon sample and the 2348.4 cm⁻¹ mode of a neon light. The sample temperatures were maintained at the desired value (±0.1 K) during the acquisition by using a modified temperature controlled stage (Linkam Scientific Instruments Ltd., UK), including a lab-made high-pressure optical cell equipped with a 2 mm thick sapphire optical window. The sample pressures were controlled by a PM High Pressure pump (Top Industrie, Vaux-le-Penil, France) which can contain 100 cm³ of gas up to 500 bars. All spectra were collected at 270 K and 20 bar after around 15 h of pressurization.

## RESULTS AND DISCUSSION

**N₂O clathrate hydrate formation.** Neutron diffraction experiments were performed on a N₂O hydrate sample to verify the formed clathrate structure. It is important to note that Rietveld refinement will not be conducted due to pollution from normal ice peaks of the collected data. Indeed, the disorder of the guest molecules within the cage might make difficult any unambiguous response regarding the cage occupancy from Rietveld refinement. Figure 2 left represents the diffraction pattern of N₂O hydrate recorded at 100 K. Diamonds pointed peaks on the figure are typical of the SI structure. The un-marked Bragg peaks come from hexagonal ice (with a = 4.494 ± 0.001 Å, c = 7.318 ± 0.001 Å), unconverted into gas hydrate during the formation process. The lattice parameter of the sample formed at 100 K is 11.840 ± 0.001 Å. In addition, thermal variation experiments have been performed to observe the evolution of the lattice parameter with temperature from 20 K to 220 K (Figure 2 right). The melting temperature of the N₂O hydrate is observed at ca. 220 K at 1 bar. The thermal expansivity is typical of SI hydrates³⁸ with experimental points well-fitting with the analytical expression of the thermal expansivity given by Hester et al.³⁹ (see equation (1)):

$$a = a_0 \exp \left( a_1(x - T_0) + \frac{a_2}{2}(x - T_0)^2 + \frac{a_3}{3}(x - T_0)^3 \right) \quad (1)$$
$a_0$ is the lattice parameter at a reference temperature $T_0$, $a_1$, $a_2$ and $a_3$ are coefficients as defined by Hester *et al* ³⁹.

<table><caption>**Table 1.** Thermal expansion parameters of the experimental unit cell parameters (Equation 1) for the SI N₂O hydrate. The temperature $T_0$ has been fixed to 20 K in the fitting procedure.</caption><thead><tr><th>Structure</th><th>T<sub>0</sub> [K]</th><th>a<sub>0</sub> [Å]</th><th>a<sub>1</sub> [10<sup>-5</sup> K<sup>-1</sup>]</th><th>a<sub>2</sub> [10<sup>-7</sup> K<sup>-2</sup>]</th><th>a<sub>3</sub> [10<sup>-10</sup> K<sup>-3</sup>]</th></tr></thead><tbody><tr><td>SI</td><td>20</td><td>11.814 ± 0.001</td><td>1.5 ± 0.1</td><td>3.2 ± 0.1</td><td>-1.2 ± 1.0</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Left: Neutron diffraction pattern of the N₂O hydrate at 100 K. Diamonds indicate peaks representative of the SI structure and the remaining Bragg peaks are due to hexagonal ice. Right: Thermal expansivity of the SI structure N₂O hydrate. Experimental points are represented as squares when the continuous line represents the fitting procedure.</figcaption></figure>

Typical N₂O Raman spectra in its gas and liquid phases are shown on Figure 3. Three principal vibrational modes of gaseous N₂O are Raman active,⁴⁰⁻⁴² namely the N-O ($v_1$ around 1285 cm⁻¹) stretching mode, the N₂O ($v_2$ around 589 cm⁻¹) bending mode, and the N-N ($v_3$ around 2223 cm⁻¹) stretching mode. Our Raman measurements exhibit two additional vibration
modes with lower intensities at 1291.4 cm⁻¹ and 2208.8 cm⁻¹ in the gas phase and at 1290.3 cm⁻¹ and 2208.1 cm⁻¹ in liquid phase. These modes have also been reported by Langseth and Nielsen.⁴³ Anderson *et al.*⁴⁰ identified these peaks to the Raman signature of isotope ¹⁴N¹⁵N¹⁶O.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Spectrum of N₂O in its gas phase (298 K, 20 bar) and in its liquid phase (298 K, 60 bar).</figcaption></figure>

Figure 4 highlights the vibrational modes of N₂O molecules in the hydrate phase formed *in-situ*, i.e. under 20 bar N₂O gas pressure and at 270 K. Since the present high-resolution Raman measurements allow the observation of all gas phase Raman bands, the four bands of the N₂O gas phase are then observed at 1284.7, 1291.4, 2208.8 and 2223.3 cm⁻¹. The signatures of the hydrate-confined N₂O are shifted to lower frequencies in comparison with the gas peaks: they are observed at 1282.6 cm⁻¹, 1289.7 cm⁻¹ and 2220.3 cm⁻¹ for the samples formed from either liquid water and from ice. An additional band is observed at 2231 cm⁻¹. In a previous investigation,⁴⁴ this latter band was assigned to the encapsulation of N₂O in the SC of the SI structure. Since the N₂O hydrate is formed in the SI structure consisting of 6 LCs and 2 SCs, the most intense band is representative of the cage filling of the most populated type of cages. The signatures at 1282.6 cm⁻¹, 1289.7 cm⁻¹ and 2220.3 cm⁻¹ are then attributed to the encapsulation of N₂O molecules within LCs. The low intensity of the 2231 cm⁻¹ band would
indicate a small occupancy of the SCs. Moreover, the 1290 cm⁻¹ region exhibits a broad profile that might contain not only the down-shift of the 1291.4 cm⁻¹ gas band, but also negligible contribution due to up-shifted v₁ stretching mode of SC occupancy. As observed for various gas hydrates,²⁹,⁴⁵⁻⁴⁷ the down or up-shift of the Raman bands in the hydrate phase compared to the gas phase is characteristic of encapsulated guest molecules in attractive (down-shift) or repulsive (up-shift) interaction with the water cage. In the case of SC, the interaction between the cage and the N₂O guest would be repulsive (up-shift of the N₂O v₁ stretching mode), leading to a negligible SC occupancy. Thus, the N₂O SI hydrate exhibits N₂O molecules mainly occupying the LCs and a negligible occupancy of the SCs, as observed for the CO₂ SI hydrate.⁴⁷

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Raman signatures of N₂O encapsulated within clathrate hydrates formed from ice (red) and from liquid water (blue). Frequencies of the encapsulated N₂O are indicated on the figure. The Raman spectrum of the N₂O gas is shown at the bottom with the frequencies of the characteristic bands indicated with vertical dotted lines and arrows.</figcaption></figure>

Interestingly, a marked difference exists in the formation mechanism of the N₂O hydrate from liquid water and from ice. Although the Raman band positions are unchanged, Figure 4 clearly shows that the relative intensity of Raman bands representing N₂O molecules trapped
within the clathrate cages varies in comparison with the intensity of gas peaks (black spectra: at 1284.7 cm⁻¹, 1291.4 cm⁻¹ and 2223.3 cm⁻¹). This is particularly noticeable when the sample is formed from liquid water (blue spectra: at 1282.6 cm⁻¹, 1289.7 cm⁻¹ and 2220.3 cm⁻¹). This would highlight that the hydrate formation is favored from liquid water, in agreement with the high water solubility of N₂O. As a results, N₂O-hydrate formation is similar to this previously observed for CO₂ clathrate hydrate, which is not surprising since CO₂ and N₂O have similar physicochemical properties including solubility in water or steric hindrance (see Table S1 in Supporting Information).¹,²⁷,⁴⁸

**Formation of mixed N₂O-CO₂ hydrate and selectivity.** The neutron powder diffractogram of three samples (25 %, 50 % and 75 % of CO₂ in N₂O) of mixed gas hydrate formed at 10 bar and 260 K during 3 days is shown on Figure 5. The unmarked peaks are those of hexagonal ice (with $a = 4.496 \pm 0.003$ Å, $c = 7.325 \pm 0.008$ Å). The ice phase is only partially transformed into hydrate phase. The SI structure is the only hydrate phase formed as marked by diamonds on Figure 5 with a slight variation in parameter as function of gas mixture composition (i.e. $a_{25\%CO_2} = 11.846 \pm 0.001$ Å; $a_{50\%CO_2} = 11.836 \pm 0.001$ Å; $a_{75\%CO_2} = 11.849 \pm 0.001$ Å). This result is expected since pure CO₂³³ and pure N₂O³⁴,³⁵ hydrates have close equilibrium curves (Figure 1) and are both formed in the SI structure in these thermodynamic conditions of formation according to previous work.⁴⁵ It should be noticed that, within the kinetics of the experiment, no metastable phase is observed like in the case of N₂ hydrate.⁴⁶
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Neutron powder diffractograms of the CO₂-N₂O mixed gas hydrates at 100 K and 1 bar. Diamonds indicate peaks representative of the SI structure.</figcaption></figure>

To investigate the molecular selectivity, the Raman signatures of guest molecules within gas and hydrate phase has to be identified, as detailed in our previous works on mixed gas hydrates.²⁴,²⁵ The guest Raman signatures are represented on the Figure 6. The red and the blue spectra are characteristic of a sample consisting of 50 % CO₂ formed from liquid water and from ice, respectively. The difference between these two formation paths, as mentioned before, is that the formation of the hydrate should be facilitated from liquid water since CO₂ and N₂O gases have a good water solubility. The Raman peaks characteristic of CO₂ molecules trapped within cages, are related to the Fermi resonance bands ⁴⁹ observed at 1276.9 cm⁻¹ and 1381.6 cm⁻¹. Raman spectroscopy does not allow one to disentangle LCs and SCs occupancy due to the Fermi resonance between the symmetric stretching mode and the harmonic of the bending mode.⁴⁹

The β parameter has been introduced to define the molecular selectivity as follows (see details in our previous work)²⁴,²⁵:
$$ \beta = \frac{[CO_2]_{hyd} [N_2O]_{gas}}{[CO_2]_{gas} [N_2O]_{hyd}} = \frac{1}{\varepsilon} \frac{[CO_2]_{hyd}}{[N_2O]_{hyd}} = \frac{R_{hyd}}{R_{gas}} \quad (2) $$

where $[x]_y$ is the concentration of the species x (CO₂ or N₂O) in the phase y (hyd and gas denotes the hydrate phase and the gas phase, respectively) and $\varepsilon$ is the CO₂/N₂O molar concentration ratio of the gas mixture. The $\beta$ parameter is measured with the help of the measured Raman intensity ratios $R_{hyd}$ and $R_{gas}$, given by:

$$ R_y = \frac{l_{CO_2}^y}{l_{N_2O}^y} = \frac{[CO_2]_y \sigma_{CO_2,y}}{[N_2O]_y \sigma_{N_2O,y}} \quad (3) $$

$l_x^y$ is the intensity of a given Raman band characteristic of the species x in the phase y and $\sigma_{x,y}$ is the associated Raman scattering cross section (i.e., the square of the polarizability variation induced by the considered stretching vibration). In Eq. 2, the ratios of the Raman scattering cross sections $\sigma_{CO_2,y}/\sigma_{N_2O,y}$ are assumed to be similar in the gas phase and in the hydrate phase. Such an assumption has been validated in previous studies dedicated to selectivity in various mixed gas hydrate.²⁴,²⁵ In this frame, the measurements of the Raman intensities ratio through Eq. 2 and 3 give access to the molecular selectivity in the mixed gas hydrates. For these selectivity measurements, the $v_1$ and $v_3$ Raman bands (at 1282.8 cm⁻¹ and 2220.1 cm⁻¹, 2231 cm⁻¹, respectively) as well as the additional vibration modes at 1290 cm⁻¹ have been observed for the N₂O species and the Fermi resonance Raman bands (at 1381.6 cm⁻¹ and 1276.9 cm⁻¹ in the hydrate phase), for the CO₂ species.
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Raman signatures of guest molecules trapped within a gas hydrates formed from a gas mixture consisting of 50% CO₂ and 50% N₂O from liquid water (in red) and from ice (in blue) at 270 K and 20 bar. All spectra are in Supporting Information.</figcaption></figure>

The evolution of the β parameter with the equilibrium gas composition [CO₂]/[N₂O] is presented on Figure 7. Three domains can be distinguished: 1) β > 1, which means that the ratio of [CO₂]/[N₂O] is higher in the hydrate phase than in the gas phase and so the CO₂ molecules are preferentially encapsulated; 2) β < 1, which means that the ratio of [CO₂]/[N₂O] is higher in the gas phase than in the hydrate phase and so the N₂O molecules are preferentially trapped; 3) β = 1, which means there is no preferential encapsulation within hydrate cages. As observed on the Figure 7, the behavior of the β parameter is drastically different for the mixed gas hydrate samples formed from ice and from liquid water.
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Variation of the $\beta$ parameter with the equilibrium gas composition at 270 K and 20 bar for CO₂-N₂O mixed gas hydrate formed from liquid water (in red) and from ice (in blue).</figcaption></figure>

**Hydrate formed from ice.** The CO₂-N₂O mixed hydrate sample formed from ice is represented by the blue line on Figure 7. The $\beta$ parameter increases with the [CO₂]/[N₂O] ratio in the initial gas mixture until a plateau slightly above 1 from a mixture consisting of 50 % CO₂ ([CO₂]/[N₂O] = 1) pointing out that molecular selectivity highly depends on the initial gas mixture composition. At low CO₂ concentration, ([CO₂]/[N₂O] < 1, which means more N₂O in the gas mixture), the N₂O molecules are preferentially trapped inside cages since the $\beta$ parameter is less than 1, especially for gas mixture consisting of low CO₂ content (10%, 18% and 25% CO₂ ([CO₂]/[N₂O] = 0.11; 0.22; 0.33, respectively). As observed in the cases of the CO-N₂ mixed gas hydrate,²⁴ and CO-CO₂ mixed gas hydrate²⁵, the dipole moment - and hence the host-guest interaction - is known to be an intrinsic property influencing molecular selectivity. This work shows that it allows preferential trapping of N₂O inside hydrate cages.
Along the plateau the molecular selectivity slightly favors CO₂ molecules which means that CO₂ molecules begin to be preferentially encapsulated (β > 1) within hydrates cages when the amount of this gas is at least the same as N₂O in the initial gas mixture. However, increasing the fraction of CO₂ in the gas mixture from this point does not improve the molecular selectivity of CO₂ molecules. It has been already observed that molecular selectivity is closely related to the cage occupancy.²⁵ For instance, in the mixed CO₂-N₂ hydrate, CO₂ molecules start filling the small cages from a concentration in the initial gas composition of 33 % CO₂.²⁵,⁴⁹ At lower CO₂ concentration, only LCs would be occupied. However, it is difficult to make a direct analysis of the cage occupancy in the present work since Raman spectroscopy does not allow to observe the SCs occupancy signature of CO₂ gas, due to the Fermi resonance. In this case, the use of molecular selectivity data, and more precisely the study of the variation of these data with the initial gas mixture composition and the formation pathways, is crucial to better understand the behavior of the cage occupancy within the mixed CO₂-N₂O hydrate. Therefore, the plateau could give a first information about the cage occupancy namely that the increase of the fraction of CO₂ in the gas mixture does not influence the β parameter from concentration of 50 % CO₂ ([CO₂]/[N₂O] = 1). β being close to 1, the fraction of CO₂ and N₂O molecules trapped inside cages are equivalent and independent of the concentration. Moreover, we can make an analogy with the cage filling in CO₂-N₂ mixed hydrate since β parameter behaviors are similar (breaking point in the β variation around [CO₂]/[N₂O] = 0.5).²⁵,⁵⁰ In this way, N₂O molecules would be first trapped within the LCs and then be inserted in the SCs, at the same time as CO₂ molecules would begin to fill some LCs.

**Hydrate formed from liquid water.** The CO₂-N₂O mixed hydrate sample formed from liquid water is represented by the red line on Figure 7. At low concentration of CO₂ in the initial gas mixture (10 % CO₂, [CO₂]/[N₂O] = 0.11), the β parameter is lower than 0.5 meaning that [CO₂]/[N₂O] in the gas phase is higher than [CO₂]/[N₂O] in the hydrate phase and that N₂O
would be preferentially encapsulated within hydrate cages to CO2. CO2 gas having a water solubility about one and a half times²⁷,²⁸ greater than N2O we expected to have a CO2 preferred encapsulation even in N2O-enriched gas mixture. Once again, the dipole moment of N2O seems to have a stronger influence on the molecular selectivity when N2O gas is the major component of the gas mixture. Indeed, increasing the fraction of CO2 gas within the initial gas mixture (from 18 % CO2, $[CO_2]/[N_2O] = 0.22$) leads to the inversion of this trend with β becoming higher than 1. CO2 molecules are then preferentially trapped within hydrate cages although the proportion of CO2 is quite low. A maximum of β = 2 is observed for 25% of CO2 ($[CO_2]/[N_2O] = 0.33$). From this threshold, the water solubility of CO2 gas seems to have a stronger influence than the dipole moment of N2O gas.

Then, from a fraction of 25 % CO2 ($[CO_2]/[N_2O] = 0.33$), the β parameter decreases with the increasing CO2 fraction until a plateau around 1.25 is reached. So, irrespective of the mixed gas hydrates (from ice or liquid water), a plateau is reached from a 50/50 initial gas mixture. The formation of the CO2-N2O mixed gas hydrates leads to a higher selectivity plateau in the case of liquid water-based formation than in the case of ice-based formation. This difference has probably to be correlated with the water solubility that is one and half times better for CO2 than for N2O. Indeed, while the formation of gas hydrate from ice involves consumption of guest species from the gas phase, the formation from liquid water involves consumption of guest species from both the gas phases and from the water-dissolved guest species. It is important to emphasize that the ice-based formation lowers this plateau, leading one to assume that the dipole moment has a strong effect on molecular selectivity. The behavior of the β parameter becomes similar for CO2-N2 and CO2-CO mixed gas hydrates²⁵,⁵⁰ (from the gas mixture consisting of 25 % CO2, $[CO_2]/[N_2O] = 0.33$) with a breaking point just before the point at $[CO_2]/[N_2O] = 1$ (50 % CO2). CO2 molecules could be trapped first within the LCs and then be inserted in the SCs, at the same time as N2O molecules would begin to fill some LCs. By
analogy, we can assume that, at the beginning of the process (10 % CO₂), N₂O molecules would
fill the LCs and the SCs and CO₂ would be only encapsulated within few LCs.

CONCLUSION

In summary, we highlighted the molecular selectivity in the CO₂-N₂O mixed hydrate thanks to Raman spectroscopy and neutron diffraction. Both pure N₂O and CO₂-N₂O mixed hydrate adopt the SI structure. The study of the molecular selectivity has revealed that its behavior is not the same whether the hydrate samples are formed from ice or from liquid water. Indeed, from ice, N₂O molecules are preferentially trapped within hydrates cages until reaching a plateau where CO₂ molecules could start to be slightly more encapsulated. N₂O molecules could have the same behavior as CO₂ one in CO₂-N₂ mixed gas hydrates,²⁵,⁴⁵⁰ filling first the LCs and then the SCs. From liquid water, the results are unusual since the sample formed with the lowest fraction of CO₂ (10 %, [CO₂]/[N₂O] = 0.11) is highly selective with the N₂O molecules, even though the water solubility of CO₂ is higher than N₂O. This seems to be due to the effect of the dipole moment of N₂O, which would compensate the influence of the water solubility on the molecular selectivity. Then, from a concentration of 18 % CO₂ ([CO₂]/[N₂O] = 0.22), the molecular selectivity parameter changes and becomes favorable to CO₂ molecules.

The cage occupancy of the CO₂ molecules starts to be the same as in the CO₂-N₂ mixed hydrate, from a CO₂ concentration of 25 % ([CO₂]/[N₂O] = 0.33), with the filling of the LCs and then of the SCs.²⁵,⁵⁰ Thus, the molecular selectivity and so the cage occupancy highly depend on the initial gas composition due to the influence of the intrinsic properties of guest molecules which are the water solubility and the dipole moment. Moreover, this work has demonstrated, for the first time, that whether the mixed gas hydrate is formed from ice or liquid water has a strong impact on its formation process. The specific behavior of the molecular selectivity observed in the case of formation of mixed hydrates formed from liquid water – that is the observation of optimum selectivity for a given gas mixture composition – opens, on the one hand, fundamental
interesting questions for which modeling and computational approaches might bring valuable information regarding the cage occupancy (e.g. by using the van der Waals and Platteeuw model) or any specific N₂O-cage interactions (e.g. by using molecular dynamics simulations)– and, in the other hand, new perspective for technologies of storage and capture of greenhouse gases through gas hydrates.

## ASSOCIATED CONTENT

**Supporting Information.** Table of physicochemical properties of CO₂ and N₂O molecules. Additional Raman spectra of CO₂-N₂O mixed hydrate (all composition) formed from ice and liquid water at 270 K and 20 bar. Nomenclature.

## AUTHOR INFORMATION

### Corresponding Author

*Claire Petuya. E-mail: claire.petuya@u-bordeaux.fr

### ORCID

Claire Petuya: 0000-0002-1530-7211

### Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

### Notes

The authors declare no competing financial interest.

### ACKNOWLEDGMENT
This paper falls in the frame of the MI2C project funded by the French ANR "Agence Nationale de la Recherche" (ANR-15CE29-0016).

REFERENCES

(1) Sloan, E. D.; Koh, C. A. *Clathrate Hydrates of Natural Gases* 3rd ed. Taylor & Francis-CRC Press: Boca Raton, FL, **2008**.

(2) Cao, X.; Su, Y.; Liu, Y.; Zhao, J.; Liu, C.J. Storage Capacity and Vibration Frequencies of Guest Molecules in CH₄ and CO₂ Hydrates by First-Principles Calculations *J. Phys Chem. A*, **2014**, *118*, 215-222.

(3) Koh, C. A.; Sloan, E. D. Natural Gas Hydrates: Recent Advances and Challenges in Energy and Environmental Applications. *AIChE J.* **2007**, *53*, 1636-1643.

(4) Florusse, L. J.; Peters, C. J.; Schoonman, J.; Hester, K. C.; Koh, C. A.; Dec, F.; Marsh, K. N.; Sloan, E. D. Stable Low-Pressure Hydrogen Clusters Stored in a Binary Clathrate Hydrate. *Science* **2004**, *306*, 469-471.

(5) Chatti, I.; Delahaye, A.; Fournaison, L.; Petitet, J.-P. Benefits and Drawbacks of Clathrate Hydrates: A Review of Their Area of Interest. *Energy Convers. Manage.* **2005**, *46*, 1333-1343.

(6) Broseta, D.; Ruffine, L.; Desmedt, A. *Gas Hydrates: Fundamentals, Characterization and Modeling*, Wiley-ISTE, London **2017**, vol. 2.

(7) Xu, C.-G.; Li, X.-S. Research Progress of Hydrate-based CO₂ Separation and Capture from Gas Mixtures. *RSC Adv.* **2014**, *4*, 18301–18316.

(8) Goel, N. In Situ Methane Hydrate Dissociation with Carbon Dioxide Sequestration: Current Knowledge and Issues. *J. Pet. Sci Eng.* **2006**, *51*, 169-184.
(9) Golombok, M.; Ineke, E.; Rojas Luzardo, J.-C.; He, Y. Y.; Zitha, P. Resolving CO₂ and Methane Hydrate Formation Kinetics. *Environ. Chem. Lett.* **2009**, *7*, 325-330.

(10) He, Y. Y.; Rudolph, E. S. J.; Zitha, P.; Golombok, M. Kinetic of CO2 and Methane Hydrate Formation: An Experimental Analysis in the Bulk Phase. *Fuel* **2011**, *90*, 272-279.

(11) Glavastskiy, K. S.; Vlugt, T. J. H.; Kjelstrup, S. J. Towards a Possibility to Exchange CO2 and CH4 in sI Clathrate Hydrate. *J. Phys. Chem. B* **2012**, *116*, 3745-3753.

(12) Zhao, J.; Xu, K.; Song, Y.; Liu, W.; Lam, W.; Liu, Y.; Xue, K.; Zhu, Y.; Yu, X.; Li, Q. A Review on Research on Replacement of CH₄ in Natural Gas Hydrate by Use of CO₂. *Energies* **2012**, *5*, 399-419.

(13) Tian, H.; Pan, N.; Thompson, R.L.; Canadell, J.G.; Suntharalingam, P.; Regnier, P., Davidson, E.A.; Prather, M.; Ciais, P.; Muntean, M.; Pan, S.; Winiwarter, W.; et al. (2024). Global Nitrous Oxide Budget (1980–2020). *Earth System Science Data.* **2024**, *16*, 2543-2604.

(14) Ravinshankara, A. R.; Daniel, J. S.; Portmann, R. W. Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century. *Science* **2009**, *326*, 123-125.

(15) Portmann, R. W.; Daniel, J. S.; Ravishankara, A. R. Stratospheric Ozone Depletion due to Nitrous Oxide: Influences of Other Gases. *Philos. Trans. R. Soc. Lond., B, Biol. Sci.* **2012**, *367*, 1256-1264.

(16) United States Environmental Protection Agency (U.S. EPA). *Methane and Nitrous Oxide Emissions from Natural Sources*, U.S. EPA: Washington, D.C., 2010; http://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100717T.txt.
(17) United States Environmental Protection Agency (U.S. EPA). *Climate Change*; U.S. EPA:
Washington, D.C., 2013;
https://www3.epa.gov/climatechange/GHGemissions/gases/n2o.html.

(18) Lee, H.; Romero, J. et al. Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, IPCC 2023, Geneva, Switzerland.

(19) Davidson, E. A.; Kanter, D. Inventories and Scenarios of Nitrous Oxide Emissions.
*Environ. Res. Lett.* **2014**, *9*, 105012.

(20) U.S. Energy Information Administration (EIA). *International Energy Statistics*; EIA:
Washington, D.C., 2010; http://www.eia.gov/cfapps/ipdbproject/IEDIndex3.cfm.

(21) Houghton, J. T.; Ding, Y.; Griggs, D. J.; Noguer, N.; van der Linden, P. J.; Xiaosu, D.; Maskell, K.; Johnson, C. A. *Climate Change 2001: The Scientific Basis*; Cambridge University Press: Cambridge, U. K., 2001.

(22) Jang, J.; Lime, S. G.; Jeong, J. H.; Raghu, A. V.; Lee, J.; Cha, M.; Muromachi, S.; Yamamoto, Y. Yoon, J. Re recovery of N₂O: Energy-Efficient and structure driven clathrate-based greenhouse gas separation. *Environ. Sci. Technol.* **2021**, *55*, 3909-3917.

(23) Yang, Y.; Shin, D.; Choi, S.; Woo, Y.; Lee, J.; Kim, D.; Shin, H.; Cha, M.; Yoon, J. Selective encaging of N₂O in N₂O-N₂ binary gas hydrates via hydrate-based gas-separation, *Environ. Sci. Technol.*, **2017**, *51*, 3550-3557.

(24) Petuya, C.; Desmedt, A. Revealing Co-Preferential Encapsulation in the Mixed CO-N₂
Clathrate Hydrate. *J. Phys. Chem. C* **2019**, *123*, 4871-4878.
(25) Petuya, C.; Damay, F.; Desplanche, S.; Talaga, D.; Desmedt, A. Selective Trapping of CO₂ Gas and Cage Occupancy in CO₂-N₂ and CO₂-CO Mixed Gas Hydrates, *Chem. Commun.*, **2018**, *54*, 4290-4293.

(26) Chazallon, C.; Pirim, C. Selectivity and CO₂ capture efficiency in CO₂-N₂ clathrate hydrates investigated by in-situ Raman spectroscopy, *Chem. Eng. J.*, **2018**, *342*, 171-183.

(27) Kaye, G. W. C.; Laby, T. H. Tables of Physical and Chemical Constants. 15th ed., Longman, NY **1986**, p.219.

(28) Weiss, R. F.; Price, B. A. Nitrous Oxide Solubility in Water and Seawater. *Marine Chem.* **1980**, *8*, 347-359.

(29) Petuya, C.; Damay, F.; Talaga, D.; Desmedt, A.; Guest Partitioning in Carbon Monoxide Hydrates by Raman Spectroscopy, *J. Phys. Chem. C*, **2017**, *121*, 13798-13802.

(30) Kuhs, W. F.; Staykova, D. K.; Salamatin, A. N. Formation of Methane Hydrate from Polydisperse Ice Powders. *J. Phys. Chem. B* **2006**, *110*, 13283-13295.

(31) Qin, J.; Kuhs, W. F. Quantitative Analysis of Gas Hydrates Using Raman Spectroscopy. *AIChE Journal* **2013**, *59*, 2155-2167.

(32) Qin, J.; Kuhs, W. F. Calibration of Raman Quantification Factors of Guest Molecules in as Hydrates and Their Application to Gas Exchange Processes Involving N₂. *J. Chem. Eng. Data* 2015, *60*, 369-375.

(33) Larson, S. D. Phase Studies of the Two-Component Carbon Dioxide-Water System Involving the Carbon Dioxide Hydrate, *Thesis 1955*, University of Illinois, Urbana, IL.
(34) Mohammadi, A. H; Richon, D. Equilibrium Data of Nitrous Oxide and Carbon Dioxide Clathrate Hydrates. *J. Chem. Eng. Data* **2009**, *54*, 279-281.

(35) Sugahara, T.; Kawazoe, A.; Sugahara, K.; Ohgaki, K. High-Pressure Equilibrium and Raman Spectroscopic Studies on the Nitrous Oxide Hydrate System. *J. Chem. Eng. Data* **2009**, *54*, 2301-2303.

(36) Le Bail, A. *Accuracy in Powder Diffraction II: Proceedings of the International Conference May 26-29 edited by E. Prince & JK Stalick*. NIST Special Publication, **1992**, 846, 142-153.
(37) Rodriguez-Carvajal, J. FULLPROF: a program for Rietveld refinement and pattern matching analysis. In *satellite meeting on powder diffraction of the XV congress of the IUCr Toulouse (France)*, **1990**, p. 127.

(38) Petuya, C; Martin-Gondre, L.; Aurel, P.; Damay, F.; Desmedt, A. Unravelling the Metastability of the SI and SII Carbon Monoxide Hydrate with a Combined DFT-Neutron Diffraction Investigation. *J. Chem. Phys.* **2019**, *150*, 184705.

(39) Hester, K. C.; Huo, Z.; Ballard, A. L.; Koh, C. A.; Miller, K. T.; Sloan, E. D. Thermal Expansivity for sI and sII Clathrate Hydrates. *J. Phys. Chem. B* **2007**, *111*, 8830-8835.

(40) Anderson, A; Sun, T. S. Raman Spectra of Molecular Crystals. Carbon Dioxide and Nitrous Oxide. *Chem. Phys. Lett.* **1971**, *8*, 537-542.

(41) Cahill, J. E.; Lerol, G. E. Raman Spectra of Solid CO₂, N₂O, N₂, and CO. *J. Chem. Phys.* **1969**, *51*, 1324-1332.

(42) Olijnyk, H.; Daufer, H.; Rubly, M.; Jodl, H.-J.; Hochleimer, H. D. Effect of Pressure and Temperature on the Raman Spectra of Solid N₂O. *J. Chem. Phys.* **1990**, *93*, 45-54.

(43) Langseth, A.; Nielsen, J. R. Raman Spectrum of Nitrous Oxide. *Nature* **1932**, *130*, 92-93.
(44) Yang, Y.; Shin, D.; Choi, S.; Lee, J.-W.; Cha, M.; Kim, D.; Yoon, J.-H. Cage Occupancy and Stability of N2O-Encaged Structure I and II Clathrate Hydrates. *Energy Fuels* **2016**, *30*, 9628-9634.

(45) Kyung, D.; Lee, W. Structure, Stability and Storage Capacity of CO2-N2O Mixed Hydrates for the Storage of CO2+N2O Mixture Gas. *Int. J. GreenH. Gas Con.* **2018**, *76*, 32-38.

(46) Petuya, C.; Damay, F.; Chazallon, B.; Bruneel, J.-L.; Desmedt, A. Guest Partitioning and Metastability of the Nitrogen Gas Hydrate. *J. Phys. Chem. C*, **2018**, *122*, 566-573.

(47) Schober, H.; Itoh, H.; Klapproth, A.; Chihaia, V.; Kuhs, W.F. Guest-Host Coupling and Anharmonicty in Clathrate Hydrates. *Eur. Phys. J. E* **2003**, *12*, 41-49.

(48) Takeya, S.; Hondoh, T.; Uchida, T. In-Situ Observation of CO₂ Hydrate by X-ray Diffraction *Presented at the Third International Conference on Gas Hydrates*, Salt Lake City, UT, **1999**.

(49) Sum, A. K.; Burruss, R. C.; Sloan, E. D. Measurement of Clathrate Hydrates via Raman Spectroscopy. *J. Phys. Chem. B* **1997**, *101*, 7371-7377.

(50) Seo, Y.-T.; Lee, H. J. Structure and Guest Distribution of the Mixed Carbon Dioxide and Nitrogen Hydrate as Revealed by X-rays Diffraction and ¹³C NMR Spectroscopy. *J. Phys. Chem. B* **2004**, *108*, 530-534.
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