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# Adsorption of CO, NO and NO2 with H2O and O2 on nickel-exchanged sodium X- and Y-faujasites: DFT calculation, zeolite synthesis, characterization, isotherms and dynamic experiments – Application to the exhaust diesel gases

Hubert Monnier, Ioannis Karamanis, Mathieu Marchal, Jean-Louis Paillaud, Habiba Nouali, Michael Badawi, Gérald Chaplais

## ► To cite this version:

Hubert Monnier, Ioannis Karamanis, Mathieu Marchal, Jean-Louis Paillaud, Habiba Nouali, et al. Adsorption of CO, NO and NO2 with H2O and O2 on nickel-exchanged sodium X- and Y-faujasites: DFT calculation, zeolite synthesis, characterization, isotherms and dynamic experiments – Application to the exhaust diesel gases. Separation and Purification Technology, 2025, 378 (3), pp.134776. 10.1016/j.seppur.2025.134776. hal-05226646

HAL Id: hal-05226646

https://hal.science/hal-05226646v1

Submitted on 27 Aug 2025

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# Adsorption of CO, NO and NO₂ with H₂O and O₂ on nickel-exchanged sodium X- and Y-faujasites: DFT calculation, zeolite synthesis, characterization, isotherms and dynamic experiments – Application to the exhaust diesel gases

Hubert Monnier a,*, Ioannis Karamanis a,b, Mathieu Marchal a, Jean-Louis Paillaud c,d, Habiba Nouali c,d, Michael Badawi b, Gerald Chaplais c,d

a Institut National de Recherche et de Securité, INRS, 1, rue du Morvan, 54519 Vandœuvre-lès-Nancy, France

b Laboratoire de Physique et Chimie Théoriques, LCPT, CNRS, Université de Lorraine, Boulevard des Aiguillettes, 54500 Vandœuvre-lès-Nancy, France

c Institut de Science des Matériaux de Mulhouse, IS2M, UMR 7361 CNRS, Université de Haute-Alsace, 68100 Mulhouse, France

d Université de Strasbourg F-67000 Strasbourg, France, France

* Corresponding author: hubert.monnier@inrs.fr (H. Monnier)

**To cite this article:** *Adsorption of CO, NO and NO₂ with H₂O and O₂ on nickel-exchanged sodium X- and Y-faujasites: DFT calculation, zeolite synthesis, characterization, isotherms and dynamic experiments – Application to the exhaust diesel gases, Separation and Purification Technology* **2025**, *378* (Part 3), 134776, 1-17. DOI: <10.1016/j.seppur.2025.134776>, HAL: <hal-05226646>. Submitted: 8 July 2025; Received in revised form: 5 August 2025, Accepted: 14 August 2025; Available online: 15 August 2025

**Abstract:** This article investigates the adsorption of NO, NO₂ (NOₓ), CO and H₂O on Ni and Na exchanged Y and X zeolites (with Si/Al = 2.55 and 1.23 respectively), to limit the cold start pollutant emissions in diesel exhausts. Ni²⁺ exchanged Y and X zeolites were synthesized via cation exchange from NaY and NaX. Optical Emission Spec- troscopy was used to determine Ni/Al, Na/Al, and Si/Al ratios. Powder X-ray Diffraction confirmed framework crystallinity, while Brunauer–Emmett–Teller analysis and t-plot methods provided specific surface area and microporous volume, respectively. Adsorption isotherms were developed for CO, NO and H₂O at 278 K, 298 K and 318 K and were fitted using the Toth model, to estimate the equilibrium constant, the saturation capacity and Henry's constant. Adsorption enthalpies were calculated from the Clausius–Clapeyron relationship using data from the isosteres. High hysteresis was identified for NO suggesting chemisorption on Ni exchanged zeolites. Breakthrough experiments were conducted at 303 K and atmospheric pressure for gas mixtures composed of NOₓ, CO, O₂, H₂O in N₂, to measure breakthrough times and dynamic adsorption capacities. Density functional theory calculations were employed to predict adsorption energies between the gases and the frameworks. The study demonstrates that NiX is the best candidate and suitable for the selective trapping of CO and NO molecules in the presence of water and oxygen. This work is the first to integrate the density functional theory based molecular screening, targeted synthesis, and experimental validation, for the development of passive NOₓ adsorbers.

## 1. Introduction

Diesel exhaust gases are composed of a large amount of toxic and noxious compounds that have negative impacts on a person's health. NO, NO₂ and CO constitute three of the most toxic and noxious gases, causing a number of severe illnesses such as chronic obstructive pulmonary disorder, lung infections and lung cancer, insufficient blood oxygen and DNA mutation [1]. Each year, these pollutants lead to the deaths of more than half a million people in Europe [2]. Since humans spend more than 80 % of their lives indoors, including in dwellings and workplaces where pollutant levels are estimated to be typically five to 10 times higher than those in outdoor air, they are particularly exposed to indoor pollutants that tend to include NOₓ and CO [3]. Today, worldwide legislation tends to impose means to reduce the levels of these gases. The European Union, for instance, has set maximum NOₓ and CO concentration thresholds in order to ensure health protection at work. The Occupational Exposure Level Values (OELV) for NO, NO₂ and CO during a typical working day (eight hours) have been fixed at 0.5 ppm, 2 ppm and 20 ppm, respectively [4]. Nowadays, there is still a need to develop efficient technologies that will enable employers and employees to strictly adhere to these health standards and even go beyond them, drastically improving health and safety in the workplace. NOₓ and CO are mainly produced by the combustion of fossil fuels. They are produced anthropogenically by a variety of diesel vehicles and industrial machinery, as well as power plants, industrial thermal suppliers and non-road vehicles. To date, much research has been dedicated to finding NOₓ treatments for diesel exhaust gases. One possible strategy, Exhaust Gas Recirculation (EGR), re-injects a proportion of exhaust gases into the fresh charge air of the combustion chambers, leading to a NOₓ reduction of close to 85 %. There are a number of alternatives for treating the remaining 15 % of NOₓ emissions. One of the most commonly used treatments is catalysis. As a result, various materials have been developed and tested, including metal oxides, activated carbons [5] and zeolites doped or not by noble metals (Pt, Pd, etc.). These materials have been used in different catalytic reactors, such
<table><thead><tr><th colspan="2">Nomenclature</th><th></th></tr></thead><tbody><tr><td>A</td><td>prefactor K<sup>-1</sup></td><td>0 initial</td></tr><tr><td>A<sub>de</sub></td><td>area under the desorption isotherm (mol/g)</td><td>ads adsorption</td></tr><tr><td>A<sub>ad</sub></td><td>area under the adsorption isotherm (mol/g)</td><td>des desorption</td></tr><tr><td>A<sub>sf</sub></td><td>sorption area in the fully irreversible case (mol/g)</td><td>i, j specie</td></tr><tr><td>a, b, c</td><td>lattice parameters of the standard cubic cell –</td><td>sat saturation</td></tr><tr><td>b</td><td>equilibrium constant Pa<sup>-1</sup></td><td>Acronyms</td></tr><tr><td>C</td><td>concentration ppm or mol/m<sup>3</sup></td><td>BET Brunauer–Emmett–Teller</td></tr><tr><td>E<sub>int</sub></td><td>interaction energy kJ/mol</td><td>D2 correction of Grimme</td></tr><tr><td>F</td><td>gas flow velocity m<sup>3</sup>/s</td><td>D6R Double Six-Member Rings</td></tr><tr><td>K<sub>H</sub></td><td>Henry constant mmol/g Pa</td><td>DFT Density Functional Theory</td></tr><tr><td>m</td><td>masse kg</td><td>DNA Déoxyribo Nucléique Acid</td></tr><tr><td>P</td><td>pressure Pa</td><td>EGR Exhaust Gas Recirculation</td></tr><tr><td>P<sub>0</sub></td><td>atmospheric pressure Pa</td><td>FAU Faujasite</td></tr><tr><td>P<sub>elec</sub></td><td>electric power W</td><td>Fd3m faujasite crystallizes within the symmetry space group</td></tr><tr><td>pH</td><td>potential of hydrogen –</td><td>FRC Flow Rate Control</td></tr><tr><td>q</td><td>adsorbed amount mol/kg or kg/kg</td><td>FTIR Fourier Transform InfraRed</td></tr><tr><td>q<sub>0</sub></td><td>saturation capacity mol/kg or kg/kg</td><td>GGA Generalized Gradient Approximation</td></tr><tr><td>R</td><td>ideal gas constant 8.31447 J/mol K</td><td>HI Hysteresis Index (Eq. (5))</td></tr><tr><td>R<sup>2</sup></td><td>coefficient of linear regression (coefficient of Pearson) –</td><td>ICP OES Inductively Coupled Plasma-Optical Emission Spectroscopy</td></tr><tr><td>T</td><td>temperature K</td><td>IUPAC International Union of Pure and Applied Chemistry</td></tr><tr><td>t</td><td>time s</td><td>MAPE Mean absolute percentage error</td></tr><tr><td>t</td><td>Thoth parameter</td><td>NSR Nox Storage-Reduction</td></tr><tr><td>t<sub>10%</sub></td><td>breakthrough time s</td><td>OELV Occupational Exposure Level Values</td></tr><tr><td>V</td><td>Volume m<sup>3</sup> (or Å<sup>3</sup>)</td><td>PBE Perdew Burke Ernzerhof (functional density)</td></tr><tr><td colspan="2"><strong>Greek letters</strong></td><td>ppm parts per million</td></tr><tr><td>α, β, γ</td><td>Lattice parameters –</td><td>PXRD Powder X-Ray Diffraction</td></tr><tr><td>ΔH<sub>ads</sub></td><td>adsorption enthalpy J/mol</td><td>RH Relative Humidity</td></tr><tr><td>ΔE<sub>int</sub></td><td>interaction energy J/mol</td><td>SCR Selective Catalytic Reduction</td></tr><tr><td>Γ-point</td><td>Brillouin zone –</td><td>STP Standard Temperature and Pressure</td></tr><tr><td>σ</td><td>Gaussian smearing eV</td><td>TGA ThermoGravimetry Analyses</td></tr><tr><td>ν</td><td>frequency 1/s</td><td>TRC Temperature Regulator Controller</td></tr><tr><td colspan="2"><strong>Subscripts</strong></td><td>US EPA USA Environmental Protection Agency</td></tr><tr><td>*</td><td>dimensionless variable</td><td>VASP Vienna Ab initio Simulation Package</td></tr></tbody></table>

as NOₓ storage-reduction (NSR) and selective catalytic reduction (SCR). However, catalysis processes have only proven to be especially efficient at high temperatures. The resulting technological solutions are, therefore, not well adapted for non-road vehicles that run discontinuously during the day and at low exhaust gas temperatures [6,7]. These solutions do not take into account the cold start cycle, therefore most non-road vehicles are not equipped with catalytic convertors. New technological advances are therefore required in order to minimize the concentration of pollutants in the workplace. Seeking these advances is the goal of several scientific laboratories that focus their efforts on investigating the efficiency of materials to adsorb NOₓ and CO combined with other gases like water vapour, oxygen, CO₂ and unburnt hydrocarbons. Indeed, the choice of the material has to take into account the possible competitive adsorption especially between NOₓ and water because water contributes between 1% and 2% of the volume in the diesel exhaust gas, while NOₓ concentration is close to 700 ppm, almost 10–40 times less. Therefore, concentration is close to 700 ppm, almost 10–40 times less. Therefore, the process proposed here is the capture of NOₓ and CO by adsorption. The aim is to use a material and to place it upstream of the catalytic reactors in order to trap NOₓ and CO during the cold start period. When the material is hot, the gas will desorb to be treated by catalysis. Solid-gas adsorption can be employed to trap gas which is highly efficient at low temperatures. An adsorbent has to be chosen to trap NOₓ and CO during a working day where a non-road vehicle is used in confined spaces, where ventilation is inadequate, such as on construction sites or in motorway tunnels. The adsorbent must be effective at low temperature and stable at high temperature to treat pollutants by catalysis. They have well-defined crystallinity, excellent thermal stability and high specific surface area. Zeolites comprise a total of 258 structural types, but only a small number of these are used to treat NOₓ given their adsorption effectiveness and selectivity [8–10]. Zeolites are porous alumino-silicate materials that are known for existing in a variety of Si/Al ratios and compensating cations. They have well-defined crystallinity [11,12], excellent thermal stability [13,14] and high specific surface area [15]. Indeed, BET surface areas close to 1,000 m²/g are often reported for zeolites, making them attractive candidates for solid-gas adsorption. There are especially effective passive NOₓ adsorbers (PNA) in the literature, including the zeolites Pd-exchanged ferrierite and Pd-exchanged chabazite, which also exhibit high hydrothermal stability [16–18]. Moreover, DFT results have previously highlighted the prominence of Cu⁺, Pt²⁺ and Pd²⁺ while incorporated in FAU [19]. However, Pd and Pt are rare and expensive metals and Cu⁺ gets easily oxidized to Cu²⁺ in zeolites with low Si/Al ratios in the presence of water, O₂ and room temperature [20,21]. Therefore, alternatives are being investigated, to treat the diesel pollutants with more viable solutions. Ni has been shown to be particularly interesting from a previous theoretical work regarding NOₓ selectivity, motivating the current study [19]. Faujasite (FAU), is a zeolite that seems particularly interesting due to its large pores, and its ability to carry high loads of compensating cations, which can act as strong adsorption sites [22–24]. Monovalent or divalent cations can be implemented in order to adjust their affinity towards the guest molecules (NOₓ, CO, H₂O). Very few studies to date have examined the ability of a cationic metal-coupled faujasite to remove NOₓ and CO [11,25–27]. To limit the possible routes, computational screening
is often implemented. DFT adsorption simulations, are used to select the most suitable existing faujasites by varying the nature of the compensating cations and the Si/Al ratio. After identifying the most interesting candidates, the materials are prepared and characterized and their adsorption effectiveness is evaluated [28–31].

In this study, DFT calculations were employed to evaluate the potential of NiY and NiX faujasites for trapping CO and NO in gas mixtures with high amount of water. Firstly, the syntheses of NiNaY and NiNaX are described, from Ni2+ aqueous salt and NaY and NaX [32,33]. Characterization of zeolite structures using PXRD, and ICP-OES revealed the atom composition. Nitrogen sorption was performed at 77 K to study the faujasite textural properties. NO, CO and water adsorption isotherms and dynamic experiments revealed the influence of the dual presence of nickel and sodium cations on the adsorption selectivity. The impact of the exchange degree between sodium and nickel and the loss of the crystallinity are also studied. NiNaX is the most promising zeolite in terms of developing a trap for diesel exhaust gases. This work integrates DFT-based molecular screening, targeted zeolite synthesis, and multi-scale experimental validation, all in one workflow, for the development of an effective passive NOx adsorber.

## 2. Methods and models

### 2.1 Simulation and models

Vienna Ab Initio Simulation Package (VASP) was used for the realization of the DFT calculations [34]. The Perdew-Burke-Ernzerhof (PBE) functional was used, within the Generalized Gradient Approximation (GGA). In order to provide a precise description of the adsorption process of the various molecules in the zeolite, dispersion interactions were taken into account by the implementation of Grimme D2 correction scheme [34,35]. The electron-ion interactions were described using the projector augmented plane wave method [36], and the Kohn-Sham equations [37] were solved self-consistently [38] until the energy difference between cycles dropped to below 10⁻⁶ eV. By plotting the evolution of the total energy of the zeolite as a function of the plane wave cut-off energy, convergence was reached for a cut-off value of 450 eV. A Gaussian smearing with σ = 0.1 eV was applied to the band occupancies in order to improve the total energy convergence. The atomic positions were fully optimized until all forces were smaller than 0.02 eV/Å per atom. Considering the large size of the unit cell, the Brillouin zone integration was made at the Γ-point only.

The adsorption energy of the gases on the zeolite has been calculated subtracting from the energy of the zeolite/gas adduct (E<sub>zeo+gas</sub>), the energy of the zeolite (E<sub>zeo</sub>) and the energy of the gas (E<sub>gas</sub>). In our case the adsorbent refers to the zeolite and the adsorbate, to the gases (Eq.1):

$$E_{ads} = E_{zeo+gas} - E_{zeo} - E_{gas} \quad (1)$$

E<sub>zeo</sub> and E<sub>gas</sub> refer to the relaxed zeolite system and the relaxed isolated gas, respectively. To calculate E<sub>zeo+gas</sub>, each gas was placed close to the adsorption site and the system was left to relax.

### 2.2 Structural model

Faujasite is a three-dimensional network belonging to the family of large-pore zeolites. Its framework is composed of sodalite cages with an internal pore diameter of 6.6 Å, which are interconnected via hexagonal prisms (D6R). This arrangement forms supercages that have a pore diameter of 12.4 Å. Fig. 1 shows the cubic arrangement of the sodalite cages (yellow polyhedra) and hexagonal prisms (red polyhedra). The 12 membered-ring openings between the supercages have a diameter of 7.4 Å. The faujasite framework belongs to the Fd3m space group [39]. Fig. 1 also illustrates the locations of the various cationic sites (Si, Si', SII, SII', SIII, and SIII'), following the nomenclature proposed by Smith and Flanigen [40]. The lattice parameters of the standard cubic cell (576 atoms, Si<sub>192</sub>O<sub>384</sub>) is a = 25.028 Å [41 42]. In the present study, the primitive rhombohedral cell is used, containing 144 atoms, to reduce the computational effort. Thus, the primitive cell contains only two supercages and four hexagonal prisms. The lattice parameters obtained after relaxation of the atoms and the cell shape are: a = 17.34 Å, α = β = γ = 60° and V = 3688.68 Å³.

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Structure of the faujasite zeolite framework showing the location of the different cationic sites Si, Si', SII, SII', SIII and SIII' present in zeolite faujasite. The silicon and aluminium atoms are localized at the vertices of the polyhedrons (yellow: hexagonal prisms and slight purple: sodalite cages) and the oxygen atoms have been omitted for clarity.</figcaption></figure>

## 3. Experimental methods

### 3.1. Materials

Binder-free NaY (Na<sub>59</sub>Al<sub>59</sub>Si<sub>133</sub>O<sub>384</sub>, structure type FAU, Si/Al: 2.55 CBV 100) and NaX (Na<sub>86</sub>Al<sub>86</sub>Si<sub>106</sub>O<sub>384</sub>, Si/Al: 1.23) faujasites
were purchased from Zeolyst USA and Sigma-Aldrich, respectively. Ni(NO₃)₂, 6H₂O (CAS n°13478-00-7 Sigma-Aldrich EC n° 236-068-5) were used for cation exchange. NaOH solution (0.01 mol/L) was used to adjust pH. High-purity gases, including N₂ (99.9999 % purity – Alphagaz2®), NO (99.990 %wt, N30), CO (99.9997 %, N47) and O₂ (99.9995 %wt) were purchased from Air Liquide and used to perform volumetric experiments (adsorption isotherms). For dynamic experiments, CO, NO and NO₂ were diluted with nitrogen, with the mixtures prepared and analysed in advance by Air Liquide. Deionized and degassed water was used in all experiments to ensure consistent and reliable results.

## 3.2 Sorbent preparation and characterization

NiNaY and NiNaX samples were prepared by cation exchange from NaY and NaX zeolites, respectively [43–46]. Reported protocols showed that after one exchange with Ni²⁺ at 343 K, 60 % of the Na⁺ cations were exchanged, compared to only 20 % at room temperature [33,47]. The pH of the solution was kept slightly acidic, close to 7.0, to prevent the formation of Ni(OH)₂, and the nickel concentration of the solution was kept close to 0.2 mol/L in order to avoid the destruction of the zeolite framework (Supplementary data 1). 50 g batches of each faujasite were subjective to seven successive ion-exchanges until loss of crystallinity. Exchange experiments were carried out in aqueous solutions of 400 mL of 0.2 mol/L Ni(NO₃)₂ aqueous solutions, under stirring, at 400 rpm and 333 K for 2 h, or at room temperature for 12 h. The pH of the solution was kept higher than 6.0, using NaOH, to preserve the zeolite framework [33]. After the exchange, the zeolite suspensions were separated by decantation and centrifugation. The solid phases were washed with deionized water (five times). The powders were then dried at 373 K overnight. The solid phases were analysed using inductively coupled plasma-optical emission spectroscopy (ICP-OES iCap6500 series, Thermo Scientific) to determine their Ni concentrations and Si/Al ratios. To ensure that the faujasites' skeletons were maintained after the exchange, the samples were analysed by Powder X-Ray Diffraction (PXRD). Thus, a Malvern-Panalytical Empyrean Alpha1 diffractometer (Debye-Scherrer geometry) equipped with a single Ge symmetrical monochromator (Johansson assembly) emitting Cu Kα₁ radiation (λ = 1.5406 Å) was used for the data collection. The diffractometer featured a capillary spinner stage and a PiXcel¹D silicon strip direct detector (active length = 3.3482° 2θ). The datasets were obtained in the 5 to 65° 2θ range with a step size of 0.007° and an acquisition time of 2000 s per step. The unit cell parameters of the parent zeolites NaX and NaY, as well as those of the exchanged variants, were determined using the Le Bail method implemented in the program suite GSAS-II [48]. The exchanged zeolites were analysed by thermogravimetric analysis (Setsys Evo Cov apparatus – Setaram Instruments). About 50 mg of zeolite were placed in the sample holder and subjected to a gradual increase in temperature (1 K/min) from 303 K to 1 273 K under a nitrogen stream (Alphagaz2®, 200 mL/min STP). The textural properties were obtained by nitrogen adsorption and desorption experiments carried out by manometry, thanks to a Belsorp max II apparatus (BEL company), at 77 K. The total specific surface area, S<sub>BET</sub> (m²/g) was calculated using the BET method and the microporous volume, V<sub>micropores</sub> (cm³ STP/g), was measured by t-plot method [49]. Prior to each experiment, the samples were out-gassed in situ in a vacuum and activated at 773 and 723 K for X- and Y- faujasites, respectively (§5.3 Thermal stability) over 12 h (heating rate of 1 K/min), in order to ensure desorption of water and any other sub- stances from porosity.

## 3.3. Thermodynamic analysis

A thermodynamic analysis of gas and vapor adsorption on faujasites was conducted following the determination of equilibrium properties [50]. Adsorption isotherms of CO, NO and H₂O were obtained experi- mentally by manometry at three temperatures (278, 298 and 318 K). NO₂ was excluded from these measurements since manometry requires pure gas at atmospheric pressure and fixed equilibrium temperatures, and NO₂ is in diphasic form (gas and liquid) at atmospheric pressure and ambient temperature. For CO and NO gases, experiments were carried out in a pressure range between ~ 1 kPa to atmospheric pressure (P₀). In the case of water vapour, the maximum pressure used was the saturated vapour pressure (P<sub>sat</sub>) at the experimental temperature. For each gas sorption measurement, a fresh ~ 50 mg zeolitic sample was taken. Prior to the measurement, all samples were pre-treated following the same protocol (§3.2 Sorbent preparation and characterization), and then allowed to cool naturally to the adsorption temperature.

The Toth model was used to fit adsorption isotherms and to achieve equilibrium data at low pressures, mainly using Excel solver® [50] (Eq. (2)). The Toth equation was used because the classic Langmuir model was not relevant due to the presence of heterogeneous adsorption sites.

$$q = q_0 \frac{bP}{[1+(bP)^t]^{1/t}} \quad (2)$$

where q₀ (mmol/g) is the saturation capacity corresponding to complete monolayer coverage and b (Pa⁻¹) is the equilibrium constant of the Toth model. The Toth model is an empirical modification of the Langmuir equation that aims to reduce the gap between the experimental data and the predicted values. When t = 1, Eq. (1) simplifies to the Langmuir isotherm equation, indicating a homogeneous adsorption system. Consequently, the parameter t serves as an indicator of system hetero- geneity. When t deviates significantly from unity (t < 1), the system is considered heterogeneous. This model has been widely applied to empirically analyze heterogeneous adsorption systems and determine their adsorption enthalpies. Considering its temperature dependence, the parameter b can be expressed using an Arrhenius-type equation [51] (Eq. (3)):

$$b = \frac{A}{\sqrt{T}} \exp \left( -\frac{\Delta H_{ads}}{RT} \right) \quad (3)$$
Where ΔHads (J/mol) is the adsorption enthalpy and A (K⁻¹) is a pre-factor [50,52].

From Eq. (2) and Eq. (3), the Toth model can be written following Eq. (4) [50,53]:

$$q = q_0 \frac{\frac{A}{\sqrt{T}} \exp\left(-\frac{\Delta H_{ads}}{RT}\right) P}{\left(1 + \left(\frac{A}{\sqrt{T}} \exp\left(-\frac{\Delta H_{ads}}{RT}\right)\right)^{\frac{1}{T}}\right)^{\frac{1}{T}}} \quad (4)$$

ΔHads can be also calculated, with the classic isosteric method according to the Clausius-Clapeyron relationship (Eq. (5) [50].

$$\frac{\Delta H_{ads}}{RT} = - \left(\frac{\partial \ln p}{\partial T}\right)_q \quad (5)$$

Finally, a hysteresis loop was observed between the adsorption and desorption curves, which helps to characterize the presence of chemisorption. The mean deviation was evaluated using the adsorption and desorption isotherms. To quantify the hysteresis in gas adsorption on faujasites, a hysteresis index (HI) was applied as Eq. (6) [54,55]:

$$HI = \frac{A_{de}-A_{ad}}{A_{sf}-A_{ad}} \quad (6)$$

where Ade and Aad are the area under the desorption isotherm and adsorption isotherm, respectively. Asf represents the sorption area in the fully irreversible case. It corresponds to the surface of the rectangle which is equal to the maximum adsorption volume multiplied by the maximum gas pressure. HI allows us to give an order of magnitude to the hysteresis loop.

### 3.4. Dynamic experimental set-up

The adsorption of NO, NO₂ and CO and their mixtures with and without water and oxygen were studied at ambient temperature (303 K) under atmospheric pressure. The performances of Ni-exchanged faujasites were investigated, by evaluating the breakthrough times and the amounts of different types of gases adsorbed in dynamic operational conditions. The experimental setup that was used for these measurements in Fig. 2 and described below.

The composition of gases is based on what is emitted in the exhaust gases of non-road vehicles [56]. Therefore, 350 ppm of NO, 350 ppm of NO₂, 1,000 ppm of CO, 8.5 % of O₂ and 1 %-vol water were diluted in N₂. Experiments were carried out in at total gas flow rates equal to 30 L/h STP. The pipes that passed through the furnace were heated and insulated to make sure that the temperature of the gas mixture remained close to what was required during the experiment. The adsorbent was placed in a glass tube of 300 mm length and with an internal diameter of 10 mm (Supplementary data 1). To avoid any possible short-circuits into the granular fixed bed and to consider the tube as an ideal plug-flow separator in the studied conditions, the tubular diameter was at least ten times the particle diameter and the height of the granular bed was at least five times its diameter. Therefore, media had to be shaped to obtain pellets with a diameter between 0.355 and 1 mm. 5 g of pellets were required from the bed to reach a height of 50 mm. Before each experiment, adsorbents were calcined at 773 K for the X-type and 723 K for the Y-type zeolites for 12 h under a dry nitrogen flow to desorb water molecules and residual products. Breakthrough curves were obtained by measuring the outlet concentration of each gas as a function of time. They were recorded by an online FTIR analyser (Fourier Transform InfraRed) Antaris™ IGS analyser (Thermo Fisher Scientific™). Analyses were performed at 463 K and at atmospheric pressure, as recommended by US EPA for diesel exhaust gas investigations. The dynamic adsorption capacity of one species i can be calculated thanks to the following relationship (Eq. (8)):

$$q_m = \frac{FC_0}{m_{\text{faujasite}}} \left( t_{\text{sat}} - \int_0^{t_{\text{sat}}} \frac{C_i}{C_0} dt \right) \quad (8)$$

where, qm (kg/kg of faujasite) is the dynamic adsorption capacity of species i. F (m³/s) is the gas flow velocity, C₀ (kg/m³) is the initial concentration of the intake, Cᵢ (kg/m³) is the concentration of gas at t, m_faujasite (kg) is the amount of faujasite and tₐₜ (s) is the time of adsorption saturation.

The breakthrough time, t₁₀%, is defined as the time at which the normalized concentration, C/C₀, reaches 0.1, e.g. when C is equal to 10 % of C₀. Finally, the evolution of the gas concentration according to time is also converted to a set of non-dimensional equations with the help of the following dimensionless variables, Eq. (9) [57]:

$$C^* = \frac{C_i}{C_0} \quad (9)$$

### 4. DFT screening results

For each chemical system (pure-gas faujasite), calculations were performed to determine the interaction energy, Eint (kJ/mol), which is depicted in Fig. 3 [11,58,59]. Since adsorption is an exothermic phe-nomenon, the energies are negative. The contributions of the van der Waals forces to the total interaction energies were close to 10–15 % [60].
Whatever the Si/Al ratio, the adsorption energy calculations show that sodium cations exhibit a higher affinity for water than NOₓ and CO, indicating its unsuitability for the treatment of diesel exhaust gas. For each faujasite, Ni²⁺ behaves more efficiently than Na⁺ and Ni²⁺ shows better affinity for NO than water, for both Y (Fig. 3a) and X zeolite (Fig. 3b); this is not the case for NO₂. But, while NOₓ refers collectively to NO and NO₂, NO is the predominant species in diesel exhaust. As a result, it is common practice to focus solely on the selectivity toward NO in the presence of water, with NO₂ often omitted from the analysis [61,62]. Moreover, NiX shows a greater affinity for water than CO (Fig. 3b).

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Experimental setup used to study the adsorption and desorption of CO, NO, NO₂, O₂ and H₂O on nickel-exchanged faujasites. The symbols FRC (Flow Regulator Controller) denote the sensors that control the composition of the stream. P stands for pressure, RH for the relative humidity and T for the temperature of the pipes. Inside the furnace, TRC (Temperature Regulator Controller) refers to the temperature in the furnace and Pelec for its electric power.</figcaption></figure>

# 5. Experimental results

Following the screening performed by DFT, nickel was selected to be cation-exchanged on NaY and NaX zeolites.

## 5.1. Exchange efficiency

Fig. 4 show the Ni/Al ratios for nickel cation exchanges performed with NaY and NaX zeolites. The amount of nickel increases after each cationic exchange into the faujasites at 343 K, showing that the ex- change was successfully achieved. ICP OES analysis reveals the presence of the desired Ni²⁺ cation, but also that one part of native Na⁺ cations remain in all samples. Furthermore, ICP OES analysis also indicates that the Si/Al ratio decreases only after seven and ten exchanges for NaY and NaX, respectively.

The exchange procedure with Ni²⁺ reveal that the exchange capacity of zeolite NaY for Ni²⁺ is lower than with the zeolite NaX. As observed by Weidenthaler and Schmidt (2000), in both cases, the Ni content cannot be increased further by additional exchanges [33]. Indeed, the number of exchanged cations for Y stabilizes after eight exchanges. The final Ni/Al ratio is approximately 0.4, indicating an 80 % exchange rate (each Ni²⁺ cation replaces two Na⁺ ions). By contrast, the cation ex- change performed with NaX results in a constant Ni/Al ratio of 0.45 after ten exchanges, corresponding to a 90 % exchange rate of Na⁺ ions. The pH value of aqueous Ni(NO₃)₂ solution was adjusted close to 6 to too acidic conditions (see pH-oxidation-reduction potential graph in Supplementary data 2). This choice was made to avoid destroying the zeolite structure during the first exchanges [33].

Finally, the Si/Al ratios of NiY, NaY, NiX and NaX were not impacted by the exchange procedures. Indeed, the Si/Al ratios obtained by ICP OES analysis were equal to 2.58, 2.62, 1.24 and 1.23, respectively (Table 1, deviation < 2 %). The stability of faujasite with regard to the exchange number was studied by checking 3 factors: (i) Phase trans- formation processes, (ii) crystallinity loss and/or (iii) formation of by- products, such as quartz [41]. PXRD patterns were collected for each sample after Ni2+ exchange, washing and heating. According to the diffractograms (Supplementary data 3, Figs.a and b), the skeletons of Y and X became amorphous and collapsed beyond five and nine ex- changes, respectively. The amount of amorphous phase did not increase significantly by raising the exchange number. Despite the high amount of nickel in the samples, no extra phases such as nickel salts or nickel hydroxide were detected by PXRD and NaY and NaX were pure, with no detectable extra phases (Supplementary data 3, Fig.c). The unit cell parameters of the parent materials, determined using the Le Bail method, align with literature values with a = 24.6606(1) Å for NaY and a = 24.9660(1) Å for NaX [63]. The PXRD patterns of the nickel exchanged zeolites X and Y zeolites indicate that NaY retains high crystallinity even after ten exchange cycles. Degradation begins in NaX after the seventh cycle, leading to complete amorphization by the tenth exchange. The variation in the unit cell parameter a clearly shows an abrupt change for both zeolites after the third exchange (Supplementary data 3, Fig.d). This change corresponds to an increase in the case of Y zeolite and a decrease for X zeolite. After the third exchange, the unit cell parameter stabilizes at an average value of 24.674 Å for Y zeolite and 24.914 Å for X zeolite. This difference between the two
zeolites is attributed to a partial dealumination of the X zeolite during the ex- change process [64].

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Interaction energies Eint (kJ/mol) calculated at the PBE + D2 theory level for CO, NO, NO₂ and H₂O molecules at 0 K for (a) Y-faujasite and (b) X-faujasite [15,61,62].</figcaption></figure>

<caption>**Fig. 4.** Influence of the number of exchanges on the degree of exchange for Ni, on (a) NaY (Na₅₉Al₅₉Si₁₃₃O₃₈₄) and (b) NaX (Na₈₆Al₈₆Si₁₀₆O₃₈₄) at 333 K over 2 h and at room temperature over 12 h under stirring (this study). Comparison with literature [36]: room temperature (RT, white point) and 343 K (grey point).</caption>

The chemical compositions are summarized in Supplementary data 4. Thus, compensating sodium cations still remain in the zeolite with an exchange degree of 80 % and 90 % for Y- and X-faujasites, respectively. Few instances can be found in the literature with Ni2+ exchanges [65], and most studies reported to date concern exchanges of Y-faujasite [66]. In all previously reported studies, the authors observed that the cationic sites, i.e., the adsorption sites, are not fully occupied after exchange. Nickel cations are preferentially located in sites I and I', and the remaining Na⁺ ions are more stable in site II [66]. The chemical com- positions of exchanged zeolite reported in Supplementary data 4 are close to findings that have already been published [67–69].

## 5.2. Textural parameters

Fig. 5 displays the nitrogen adsorption–desorption isotherms of the four faujasites at 77 K: NaY, NaX, nickel-exchanged Y (NiYExch7NaY) and nickel-exchanged X (NiXExch9NaX). All zeolites exhibit a type I adsorption isotherm characteristic of microporous solids, except NiX-Exch9NaX, which exhibits a combination of type I and type IV isotherms, characteristic of micro-mesoporous material
according to the IUPAC classification [70]. Ni²⁺ exchanges lead to a drastic decrease of BET surface area (from 836 for NaY to 466 m²/g for NiY, and from 570 for NaX to 148 m²/g for NiX) and the microporous volume (from 0.329 for NaY to 0.228 cm³/g for NiY and from 0.252 for NaX to 0.01 cm³/g for NiX) of exchanged FAU-type zeolites (Table 1), as already observed by Oliveira et al. (2009) [71]. The decrease in microporous volume reaches 30 % and 95 % for exchanged NaY and NaX, respectively. At the same time, the cation-exchange carried out with NiXExch9NaX shows a hys-teresis loop of type H3, which is characteristic of aggregated particles or macropores that have not been completely filled. Previous studies on cation exchanged Y and X zeolites also reported similar results for ex-changes carried out with divalent cations [43,71,72]. The authors of these studies suggested that some cations, such as Fe²⁺ and Cu²⁺, may lead to pore blocking, which reduces material porosity, but not Ni²⁺. The ionic radii of the charge-compensating cations used in the ion exchange play a major role in the change of textural properties. Ni²⁺ (0.83 Å) has approximately the same radius as Na⁺ (0.90 Å), so the effect is small and is likely negligible. The wt.% of nickel reached 7.7 % and 13.4 % for NiYExch7NaY and NiXExch9NaX, respectively (Table 1).

<caption>**Table 1**</caption>

Textural parameters and metal content of NiYExch7NaY, NaY, NiXExch9NaX and NaX zeolites obtained by N2 at 77 K adsorption-desorption isotherms (BelSorp Max II (Bel)).

<table><thead><tr><th>Adsorbents</th><th>Ni (%-wt) <sup>a</sup></th><th>S<sub>BET</sub> (m<sup>2</sup>/g) <sup>b</sup></th><th>V<sub>micropore</sub> (cm<sup>3</sup> STP/g) <sup>c</sup></th><th>Si/Al(mol/mol) <sup>a</sup></th></tr></thead><tbody><tr><td>NiYExch7NaY / NaY</td><td>7.7 / 0</td><td>466 / 836</td><td>0.228 / 0.329</td><td>2.58 / 2.62</td></tr><tr><td>NiXExch9NaX / NaX</td><td>13.4 / 0</td><td>148 / 570</td><td>0.01 / 0.252</td><td>1.24 / 1.23</td></tr></tbody></table>

## 5.3 Thermal stability

Thermogravimetry analyses (TGA, Setsys Evo Cov apparatus – Setaram Instruments) were performed to determine at which precise temperature the strongly adsorbed water can be released from the samples for adjusting pre-treatment temperature for adsorption experiments. About 100 mg of each zeolite were weighed very precisely and placed in the sample holder and subjected to a gradual increase in temperature (1 K/min) from 303 K to 1 273 K under nitrogen stream (Alphagaz2®, 200 Nml/min). The loss of weight was recorded in function of time. Thanks to the knowledge of the temperature according the time, the decomposition of the zeolite can be reached and studied. The TGA curves are exhibited in Fig. 6. Two weight-loss stages were observed. The first weight loss starts below 473 K and could therefore be assigned to physisorbed water removal: water depleted leading to a 13 % and 17 % weight loss for NiYExch7NaY and NiXExch9NaX, respectively. Then, a second weight loss was observed from 473 to 1 273 K which is attributed to the release of residual water and to dihydroxylation phenomena [73].

<figure><img src="image_6.png" /><figcaption>Fig. 6. TGA curves of NiYExch7NaY and NiXExch9NaX. Temperature of 303 K to 1 273 K at 1 K/min. Comparison carried out with NaY (Si/Al: 2.55) and NaX (Si/Al: 1.23).</figcaption></figure>

## 5.4. Gas adsorption isotherm and enthalpy

Based on these TGA results, the pre-treatment of NiYExch7NaY and NiXExch9NaX was been later adjusted at 723 K and 773 K, respectively to make sure that the samples used were free of water. Thus, the amount of water released is 19.4 % wt. and 21.2 % wt. for NiYExch7NaY and NiXExch9NaX, respectively. The NO, CO and H₂O adsorption isotherms over NiYExch7NaY, Si/Al: 2.58 and NiXExch9NaX, Si/Al: 1.23 at three temperatures are shown in Fig. 7. The study of NO₂ using manometry is challenging because NO₂ is corrosive and it is difficult to obtain pure NO₂ gas at a pressure necessary for the operation of the analytical apparatus. The Toth equation was used to describe the experimental data. The adsorbed amounts of NO and CO are plotted as a function of the relative pressure, P/P₀, where P₀ is the atmospheric pressure. In the case of H₂O, the adsorbed amount is plotted according to P/Psat. For each species, experimental isotherms – adsorption and desorption – have been carried out to observe potential hysteresis and chemisorption phenomena. A study of the repeatability and reproducibility of this procedure can be found in Supplementary data 5, where the data allow us to observe a maximum accuracy close to 12 %.

In almost all cases presented on Fig. 7, in the range of experimental accuracy, the curves show that adsorbed concentrations decrease when the temperature increases, which is expected due to the exothermic nature of the adsorption [50]. The adsorption capacity is higher when the Si/Al ratio is lower (for NiXExch9NaX). Indeed, Henry's constant of each probe molecule is weaker for NiYExch7NaY than in NiXExch9NaX (Table 2) at low pressure. The presence of Ni²⁺ increases the adsorption capacity compared to Na⁺ due to a higher interaction energy with the gases. Water's adsorption capacity, q and q₀, seems to be higher than those of both CO and NO for all nickel-exchanged zeolites.

Regardless of the zeolite and the gas studied, the adsorbed quantity, q₀ and equilibrium constant b, decrease with increasing temperature. ΔEint obtained from DFT/Toth calculations for NiY equal – 112.2/-90 kJ/mol for CO, – 157.0/-95 kJ/mol for NO, – 111.1/-

<footnote><sup>a</sup> Determined by the ICP-OES analysis, <sup>b</sup> Determined by the BET method, <sup>c</sup> Determined by the t-plot method.</footnote>
80 kJ/mol for H₂O, and ~ 72.5 kJ/mol for NO₂ – manometry results are not available for NO₂ (Fig. 3 and Table 3). The corresponding numbers for NiX are ~185.1/-90 kJ/mol for NO, ~135.9/-95 kJ/mol for CO, ~167.4/-80 kJ/mol for H₂O and ~2 133.4 kJ/mol for NO₂. There are two kind of discrepancies in these numbers. One arises from a big difference between the DFT interaction energy of Y and the ΔHads experimental values. ΔHads measurements, take into account both the interaction of Ni²⁺ and Na⁺ and the final result corresponds to a mean enthalpy resulting in weaker estimated interactions than the ones calculated with DFT [59]. Also, the DFT results are obtained at low coverage, which is the limit of the interaction energy that we would measure if one gas molecule adsorbs on one site Ni²⁺ cation. The other discrepancy originates from the comparison of the Y zeolite results and the experimental measurements with the X zeolite DFT results, because the simulated adsorptions take place on site III, where the cations are less stable and thus the interactions are much stronger. This is also the reason of the increased ΔHads measurements observed in the case of NiX, but once again, the value is taken as an average with site II and Na adsorptions. DFT results are not useful because the results are comparable with the experiment, but because they explain the mechanism of adsorption, the reason of the differences we measure and because they predict which combinations of adsorption sites/cations result in adsorptions selective towards NO and CO. Nevertheless, we observe the expected trend: NiYExch7NaY < NiXExch9NaX (Table 3). Finally, the Toth parameter, t, is well below unity for all the zeolite-gas systems studied, meaning that the adsorptions are heterogeneous. This may be due to the presence of two types of cations in both zeolites – sodium and nickel – whose behaviour towards the three gaseous species is quite different. To conclude, the experimental data show that the order of adsorption at low pressure is NO > CO > H₂O, and that NiYExch7NaY is less efficient adsorber than NiXExch9NaX.

CO.

Regardless of the temperature and the zeolite, the CO experimental adsorption curves fit a type-I isotherm. CO has stronger affinity for the two nickel faujasites than H₂O and slightly weaker than NO. KH and b are lower (Table 3). Moreover, its adsorption capacity is better when using NiXExch9NaX than when using NiYExch7NaY, as is shown in Table 3 where the equilibrium constant, b, increases when the Si/Al ratio decreases. DFT calculations corroborate the experimental data: adsorption is higher with a fully Ni²⁺-exchanged X-faujasite than a Y-faujasite. Finally, the Toth equation reveals well-fitting results with a MAPE less than 3 %. The value of the t-exponent, is far from unity (t ~ 0.15 – 0.18), enabling us to conclude that solid-gas interactions are heterogeneous whatever the Si/Al ratio.

NO.

The adsorption behaviour of NO according to the pressure is more difficult to explain. Like CO, regardless of the temperature, the curves seem to fit a type-I isotherm without reaching a plateau. At low pressure, i.e., below P/P₀ = 10⁻², the quantities adsorbed increased significantly until 0.2 mmol/g, i.e., almost twice the amount of CO. The Henry's constant corroborates this result. It is equal to KH = 1.38 10⁵ and 3.41 10⁵ mmol/g Pa at 298 K, for NiYExch7NaY and NiXExch9NaX, respectively (Table 2). However, regardless of the zeolite, the affinity with H₂O was much greater, as mentioned before. The slope of the curve became intermediate until P/P₀ was equal to 1 with a maximum uptake, q₀, close to 10 mmol/g (Table 3). In the case of CO, this value is 20 times lower than with NO, showing the great ability of nickel faujasite to trap NO. The findings of the adjustable Toth model deviate from some of the experimental data. Indeed, the parameters of the Toth model were determined by fitting adsorption isotherms with an accuracy higher than 30 % due to the nature of chemical specie and the shape of the isotherm; this gap is higher than those obtained with CO, for instance (<1%) (Table 3). The slight deviations of t-exponent from the unity (t ~ 0.1) suggest a heterogeneity of the adsorption sites. This conclusion agrees with data reported in Fig. 7, where a gap between adsorption and desorption isotherms is observed. It is worth noting that: (i) the predicted data obtained using the Toth model matches the experimental data well; (ii) the t-parameter of the Toth model, which characterizes the heterogeneity of the adsorption system, strongly deviates from unity, in comparison to CO experiments, and (iii) the observations of the HI index, which characterizes the gap between adsorption and desorption isotherms, are confirmed (Supplementary data 6). Considering NO adsorption in greater detail, regardless of the Si/Al ratio, the adsorption and desorption curves are separated. The HI index is observed between the adsorption and desorption isotherms when NO is used. This finding strongly suggests that adsorption of NO is fully impacted by a stronger physisorption or chemisorption process. To our knowledge, this phenomenon has never been observed in the literature. However, this kind of hysteresis may occur when insufficient time is given for the analytical system to attain equilibrium [74,75]. This phenomenon is often associated with a very slow diffusion of molecules through narrow pore entrances. Reversible adsorption isotherms may be achieved if some conditions of the experimental set-up are changed, such as increasing the time remaining for the manometer to reach the equilibrium or lowering the ΔP condition between two points of the isotherm. The experiments carried out allowed us to assume that chemisorption occurs between both nickel faujasites and NO (Supplementary data 8).

<caption>**Table 2**</caption>

Henry constant, KH (mmol/g Pa) of CO, NO and H₂O into NiYExch7NaY, Si/Al: 2.58 and NiXExch9NaX, Si/Al: 1.24 – T = 298 K (Figs. 8 and 9). R² is the coefficient of linear regression (coefficient of Pearson).

<table><thead><tr><th>Probe molecules</th><th>CO</th><th>NO</th><th>H<sub>2</sub>O</th></tr></thead><tbody><tr><td>K<sub>H</sub> (mmol/g Pa) at 298 K, NiYExch7NaY (R<sup>2</sup>)</td><td>0.68 10<sup>-5</sup> (0.90)</td><td>1.38 10<sup>-5</sup> (0.916)</td><td>2.58 (0.918)</td></tr><tr><td>K<sub>H</sub> (mmol/g Pa) at 298 K, NiXExch9NaX (R<sup>2</sup>)</td><td>1.23 10<sup>-5</sup> (0.875)</td><td>3.41 10<sup>-5</sup> (0.911)</td><td>0.53 (0.910)</td></tr></tbody></table>

To conclude, the interactions between NO and the investigated zeolites are stronger than classical physisorption, and may even be related to chemisorption. However, the adsorption hysteresis in faujasite can be the consequence of strong sorbate-sorbate interactions at high loadings and packing differences of the sorbed molecules on adsorption and desorption branches of the isotherm caused by heterogeneous strength of the adsorption sites [76]. In 1990, Pan and Mersmann corroborate this explanation [77]. They concluded that the phenomenon may result from the adsorbed molecules interacting among themselves and can be explained by the existence of many different energy levels of the adsorption sites. Another explanation is based on the adsorption gas which can induce
structural transformations to the adsorbent, leading to the formation of low-pressure hysteresis. This phenomenon has been observed with MOF material, where well-defined hysteresis loops are found in a pressure range far below the range associated with capillary condensation [74]. The mechanism is based on the pore network effect due to the shape of pores, where adsorbed molecules remain in the wide pores during desorption until they can pass through the narrow windows.

<figure><img src="image_7.png" /><figcaption>Fig. 7. CO (a and d), NO (b and e) and H₂O (c and f) adsorption/desorption isotherms at 278, 298 and 318 K (on NiYExch7NaY, Si/Al: 2.58 and NiXExch9NaX, Si/Al: 1.24). N₂ (99.9999 % Alphagaz2®), NO (99.990 %wt, N30), CO (99.9997 %, N47), O (99.995 %wt) – CO and NO experiments were carried out in a pressure range between ~ 1 kPa to atmospheric pressure (P₀). In the case of water, the maximum pressure used was the saturated vapour pressure (Psat). ~ 50 mg zeolitic sample was taken. All samples were pre-treated following the same protocol (§3.2 Sorbent preparation and characterization) – Predicted Toth curve is drawn in each case, according to (Eq. (4)).</figcaption></figure>

Adsorption enthalpies according to the coverage have been obtained from the slopes of the isotherms at three temperatures (278, 298 and 318 K) with the Clausius-Clapeyron relation. The adsorption enthalpy for each gas adsorption on each faujasite is given in Fig.
8, according to the coverage, and the corresponding DFT result is marked at "zero" loading for comparison. For CO@NiYExch7NaY, the shape of the adsorption enthalpy curve decreases with the coverage (Fig. 8a). Indeed, at low coverage ($q < 0.1$ mmol/g), in absolute value $|\Delta H_{ads}|$ is close to 100 kJ/mol and it decreases according to the coverage to settle around 19 kJ/mol beyond $q > 0.2$ mmol/g. By contrast, as Fig. 8d shows for NiX-Exch9NaX, the shape of the curve is flat in the range of the adsorbed amount studied ($5 \times 10^{-3} < q$ (mmol/g) < 0.25). The adsorption enthalpy value is around 35 kJ/mol. While this last value is low, it is below the DFT adsorption energy of both fully $Ni^{2+}$-exchanged X and NaX, with interaction energies equal to 200 and 70 kJ/mol, respectively. These observations can be explained with the three level of interaction energy met. The adsorption of CO on nickel cation leads to the higher interaction energy, while the second level is due to the adsorption on sodium cation because (i) the cationic exchange was not entire and (ii) the interaction energy with sodium is lower than with nickel cation. Indeed, this heterogeneity is due to the zeolite composition which contains both nickel and sodium atoms (Supplementary data 4). The reactivity of the nickel atom is much greater than that of the sodium atom. According to the adsorption enthalpies determined by DFT (red cross.), a factor of more than 50 can be observed between fully $Ni^{2+}$-exchanged Y and NaY: 118 kJ/mol against 23 kJ/mol for fully $Ni^{2+}$-exchanged Y and NaY, respectively [11,59]. Finally, the third comes from the filling of pores, which is less reactive than the interaction between CO and the sodium atom. Indeed, the first molecules of CO adsorb on the most reactive sites (nickel atoms at low coverage), followed by sodium atoms at higher coverage, after which the pores fill. Therefore, at low coverage, the curve tends toward the DFT NiY enthalpy, while at high coverage, a plateau can be observed close to the DFT NaY enthalpy or below. The steep slope of isotherms at low coverage ($q < 0.01$ mmol/g - Fig. 8ad) corroborate this comment. The same conclusions can be established with the behaviour of NO adsorption on nickel-exchanged faujasites (Fig. 8be). The experiments we carried out allowed us to investigate a high coverage close to 2.0 and 0.12 mmol/g for both nickel faujasites, respectively. For NiYExch7NaY, at this adsorbed amount, the curve is close to the enthalpy value under the DFT NaY enthalpy or below (because the process by which the pores fill is less reactive than the interaction with the sodium atom). However, the evolution of the curve agrees with calculations obtained by DFT: it tends toward a similar order of magnitude at low coverage for NO/NiY and medium coverage for NO/NaY. Any comparison with other studies is hard to make because there is no experimental value for NO that has been obtained either by this approach or by any other reported in the literature. When the Si/Al ratio decreases, the adsorption enthalpy is located between the DFT values of NiX and NaX. This last result shows that the experimental data are in agreement with the DFT calculation in regard to the molecular composition of the X-faujasite synthesized: $Ni_{39}Na_{6}Al_{86}Si_{106}O_{384}$ (Table 1). Nevertheless, the shape of the enthalpy curve is flat, regardless of the coverage. At the order of the magnitude close to the first data point ($5 \times 10^{-3}$ mmol/g), one would have expected that the curve tends towards higher enthalpy values to reach 180 kJ/mol for the lower coverage, the DFT value. The shape of the curve can be explained observing the gaps between three isotherms according temperature, which are very low, whatever amount is adsorbed. Consequently, using the Claudius-Clapeyron relationship (Eq. (3)) with the isostere method does not reveal any variation in enthalpy over the temperature and the coverage ranges studied. In the case of water, the influence of coverage on $|\Delta H_{ads}|$ is very low (Fig. 8cf). The shape of the curves changes very little depending on the covering rate in the studied range. Experimental values reach 60 to 40 kJ/mol and below 40 kJ/mol for Y- and X-exchanged faujasites, respectively. We observed the same trends as witnessed with NO: the gaps between the three isotherms on Fig. 5be according temperature are too low to reach enthalpy thanks to the Claudius-Clapeyron relationship. This is due to the accuracy of manometry at a very low pressure, close to $10^{-4}$ kPa. Therefore, the experiments that had been performed did not permit us to obtain the DFT value because we lacked experimental data at a very low pressure.

To conclude, the curves tend to lie between the enthalpies of nickel and pure sodium faujasites calculated using the DFT numerical tool. At points, they lean towards the nickel enthalpy at low coverage because the interactions between $Ni^{2+}$ and gases are highest. After that, the trend line flattens for medium coverage; this is due to the lower interactions between Na+ and gas than with $Ni^{2+}$. At high coverage, their shape can be considered flat, indicating that the interactions are sufficiently low to allow us to suppose a homogeneous adsorption and a filling pore. However, the adsorption enthalpy evolution according to coverage is undervalued and far lower than the results obtained by DFT. However, despite the adsorption enthalpy performed at 298 K using the Toth model (Table 3) gave a good agreement with DFT calculation, it is difficult to conclude for the possible reason to observe so small values. It is like the crystallinity is destroyed and the gases adsorb on an amor- phous aluminosilicate mass.

<caption>**Table 3**</caption>

Adsorption parameters of CO, NO and $H_2O$ on NiYExch7NaY and NiYExch9NaX adjusted by Toth equation. For the fit, the range of values is $10^{-10} - 10^{-13}$ K$^{1/2}$ Pa$^{-1}$ ($10^{-7}$ K$^{1/2}$) and 0.1 - 1, for A and t, respectively [54,56]. MAPE is the error function based on the mean absolute percentage error.

<table><thead><tr><th>Gas specie</th><th>zeolite</th><th>T(K)</th><th>q<sub>0</sub> (mmol/g)</th><th>b (1/Pa)</th><th>T (-)</th><th>|ΔH<sub>ads</sub>| (kJ/mol)</th><th>MAPE (%)</th></tr></thead><tbody><tr><td rowspan="6">CO</td><td rowspan="3">NiYExch7NaY</td><td>278</td><td>0.77</td><td>7.32</td><td>0.15</td><td>91</td><td>3</td></tr><tr><td>298</td><td>0.59</td><td>365</td><td>0.18</td><td>90</td><td>1</td></tr><tr><td>308</td><td>0.56</td><td>21</td><td>0.17</td><td>86</td><td>0.4</td></tr><tr><td rowspan="3">NiXExch9NaX</td><td>278</td><td>0.40</td><td>232.69</td><td>0.18</td><td>99</td><td>1.3</td></tr><tr><td>298</td><td>0.37</td><td>1.130</td><td>0.18</td><td>93</td><td>0.4</td></tr><tr><td>308</td><td>0.30</td><td>47</td><td>0.18</td><td>88</td><td>0.1</td></tr><tr><td rowspan="6">NO</td><td rowspan="3">NiYExch7NaY</td><td>278</td><td>10</td><td>232.69</td><td>0.07</td><td>99</td><td>3</td></tr><tr><td>298</td><td>11</td><td>6.30</td><td>0.07</td><td>97</td><td>1</td></tr><tr><td>308</td><td>11</td><td>21.42</td><td>0.09</td><td>86</td><td>2</td></tr><tr><td rowspan="3">NiXExch9NaX</td><td>278</td><td>13</td><td>851.29</td><td>0.06</td><td>102</td><td>0.2</td></tr><tr><td>298</td><td>11</td><td>28.50</td><td>0.07</td><td>101</td><td>0.05</td></tr><tr><td>308</td><td>11</td><td>2.99</td><td>0.07</td><td>99</td><td>0.1</td></tr><tr><td rowspan="2">H<sub>2</sub>O</td><td rowspan="2">NiYExch7NaY</td><td>278</td><td>22</td><td>106</td><td>0.26</td><td>70</td><td>9</td></tr><tr><td>298</td><td>22</td><td>3</td><td>0.32</td><td>69</td><td>6</td></tr></tbody></table>
<table><tr><td></td><td>308</td><td>22</td><td>1</td><td>0.45</td><td>69</td><td>11</td></tr><tr><td rowspan="3">NiXExch9NaX</td><td>278</td><td>149</td><td>9.14</td><td>0.08</td><td>81</td><td>9</td></tr><tr><td>298</td><td>81</td><td>327</td><td>0.10</td><td>78</td><td>8</td></tr><tr><td>308</td><td>35</td><td>19</td><td>0.14</td><td>78</td><td>3</td></tr></table>

<figure><img src="image_8.png" /><figcaption>Fig. 8. Adsorption enthalpy of CO (a / d), NO (b / e) and H₂O (c / f) in three temperatures 278 K, 298 K and 318 K for NiYExch7NaY (Si/Al: 2.58) / NiXExch9NaX (Si/Al: 1.24), versus the amount of gas adsorbed. Isostere method has been used. DFT calculation at "zero" coverage are denoted with (×) for NiY, NiX and (+) for NaY, NaX. Red colour is used for CO, green for NO and blue for H₂O.</figcaption></figure>
## 5.5. Dynamic experiments

The performance of Ni exchanged Y and X zeolites was studied, by exposing them in a realistic diesel gas mixture. The breakthrough time, t₁₀% (min), and the adsorption capacity, q (g/g), have been evaluated from breakthrough experiments with granular beds.

### 5.5.1. Breakthrough curve of CO, NO and NO₂

The adsorption properties of the exchanged zeolites were investigated in a cyclic breakthrough experiment (Supplementary data 9). Between each breakthrough experiment, a regeneration (desorption) step was performed, consisting of a temperature ramp at 1 K/min to 573 K, followed by a 4-hour isothermal hold and natural cooling under Alphagaz2® nitrogen flow. The inlet concentration of 350 ppm was chosen for all pollutants. The breakthrough times and adsorption capacities of isolated gases and gas mixtures for Y- and X-faujasite are depicted in Fig. 9ab, respectively and their measurements are given in Supplementary data 10. Fig. 9ab show that the trends in adsorption capacity follow closely the trends in breakthrough times. In addition, both the breakthrough times and the capacities are reduced in the case of the mixtures, regardless of the Si/Al ratio, with NO₂ being the least sensitive to these changes. These figures illustrate that X zeolite exhibits much better adsorption capacity than Y, which is also correlated with higher breakthrough times. The breakthrough times are not close, and their order of breakthrough is independent of the zeolite nature and a fortiori the Si/Al ratio. The corresponding order is CO < NO < NO₂. The adsorption capacities follow the same evolution. These results highlight that NiXExch9NaX is more efficient than NiYExch7NaY. Of course, the mass of the NiXExch9NaX granular bed used into the column is higher by another 20%; however, the gap in adsorption capacity between the two zeolites is close to 99% for each gas. In other words, the adsorption increases when the Si/Al ratio decreases, which is attributed to the higher number of accessible Ni²⁺ cations. Fig. 9b compares the efficiencies of NiXExch9NaX and parent NaX. Regardless of which gas is tested, the exchanged zeolite outperforms NaX: all NaX breakthrough times are lower than those achieved with NiXExch9NaX. These results clearly demonstrate that the presence of nickel increases the adsorption capacity of CO, NO and NO₂.

<figure><img src="image_9.png" /><figcaption>**Fig. 9.** Breakthrough curves of pure CO, NO and NO₂ gases for (a) NiYExch7NaY and (b) NiXExch9NaX with NaX for comparison. C~CO,0~ = C~NO,0~ = C~NO₂,0~ = 350 ppm, Qv = 30 L h⁻¹ STP, T = 303 K, P = 101.3 kPa, granular bed length ~ 70 mm, diameter = 10 mm, 0.355 < d~p~ < 1 mm, m~NiYExch7NaY~ = 4.366 g and m~NiXExch9NaX~ = 5.480 g. Exchanged faujasites have previously been calcined (1 K/min and 573 K during 4 h). The outlet concentration was determined by FTIR analysis.</figcaption></figure>

The difference between the shape of the breakthrough curve containing Ni²⁺ and Na⁺ can be attributed to the accessibility of the pores. In general, divalent cations perform better than alkaline metal ions, such as sodium, in terms of adsorption capacity, but they lead to broader breakthrough profiles. This can be explained by the shape of the breakthrough curve, which depends on the thermodynamic equilibrium, while the width derives from the mass transfer resistances in the porous granular bed [78,79].

Fig. 10 represents results obtained with a similar experiment carried out in the same operating conditions, but with a mixture consisting of CO, NO and NO₂. The previous order based on single-compound high throughput experiments (CO < NO < NO₂) is respected. However, the results differ from single gas adsorption in that the concentration of NO and CO exceed the input concentration as the adsorption progresses, namely C/C₀ became greater than 1.0 (Fig. 9). This phenomenon is due to the roll-ups and indicates that NO₂ is more favourably adsorbed than CO and NO [80–82]. Finally, the comparison between results from Figs. 9 and 10 allows us to observe that a mixture tends to reduce the adsorbed amount and the breakthrough time of all gases. This is due to an increased total gas concentration while the number of faujasite adsorption sites remains constant. It should be noted that the NO₂ breakthrough curve tends towards the inlet concentration, CO, without reaching it. This could be explained by the reaction of adsorbed NO₂ with residual H₂O (Alphagaz2®) to form NO [16,83].

### 5.5.2 Case of diesel exhaust gases

In order to study the adsorption capacities of NiYExch7NaY and NiXExch9NaX for NO, NO₂, CO, O₂ and H₂O, the gas concentrations were adjusted to those found in diesel exhaust gases of non-road vehicles: 350 ppm of NO, 350 ppm of NO₂, 1 000 ppm of CO, 1 %-vol of H₂O and 8.5 %-vol of O₂. Fig. 11 shows breakthrough curves obtained for the mixture and can explain physical interactions and chemical reactions between all species. For each given gas species, the shapes of the curves appear similar for NiYExch7NaY and NiXExch9NaX. Similar to the breakthrough curve experiments based on a mixture but without O₂ and water (Fig. 10), the breakthrough
order CO < NO < NO₂ is maintained and several roll-ups are still observed, especially for CO and NO. The affinity for H₂O seems to be higher than for the pollutants: its break-through time is more than twice that of NO, for instance (Table 4). However, an intriguing observation was made during these experiments: the shape of the NO₂ breakthrough curve shows a first peak before a second that increases towards a plateau close to the inlet concentration.

CO.

<caption>**Table 4**</caption>

Breakthrough time, t₁₀% (min), and adsorption capacity, q (g/g), calculated from breakthrough curves (Fig. 11). Q = 30 L/h STP, T = 303 K, P = 101.3 kPa.

<table><thead><tr><th rowspan="2">Zeolite species</th><th colspan="4">NiYExch7NaY (Calc 11)</th><th colspan="4">NiXExch9NaX (Calc 18)</th></tr><tr><th>CO</th><th>NO</th><th>NO₂</th><th>H₂O</th><th>CO</th><th>NO</th><th>NO₂</th><th>H₂O</th></tr></thead><tbody><tr><td>breakthrough time, 4 t₁₀%(min)</td><td>52</td><td>191</td><td>359</td><td></td><td>9</td><td>116</td><td>255/1850</td><td>240</td></tr><tr><td>adsorption capacity, q (g/g)</td><td>3.51 10<sup>-5</sup></td><td></td><td></td><td></td><td>5.64 10<sup>-6</sup></td><td>5.16 10<sup>-2</sup></td><td>2.61 10<sup>-1</sup></td><td></td></tr></tbody></table>

The adsorption of CO is marked by two roll-ups, which are likely due to NO and NO₂. This was already observed during experiments performed on a mixture without oxygen and water (Fig. 10). Indeed, the roll-ups end close to the breakthrough time of NO and NO₂. CO concentration is similar to its inlet one, meaning that there are no chemical reaction occurring for CO and other species like NO, NO₂ or O₂ to generate CO₂ for instance; the mass balance of CO is respected. The shape of the breakthrough curves seems to be related to the physical interaction between CO and NOₓ.

NO.

When oxygen and water are used in the gaseous mixture, the breakthrough curve of NO shows a different shape compared with experiments carried out without O₂ and H₂O (Fig. 10). After the breakthrough, close to 52 min and 116 min for NiYExch7NaY and NiXExch9NaX, respectively, the curve started declining (Table 4). This is attributed to a chemical reaction between NO and O₂, generating NO₂ and reducing the NO concentration in the granular bed. This situation is often encountered with other faujasites [16,82]. Fig. 11 depicts a first roll-up where the NO concentration rises, exceeding its inlet concentration to reach C/C₀ > 1.8 at a time close to the H₂O breakthrough. Then, instead of beginning to decline and to tend towards its inlet concentration, as expected, the NO concentration stabilizes close to C/C₀ = 1.45 and 1.25 for NiYExch7NaY and NiXExch9NiX, respectively. The shape of the curve differs considerably from what was usually observed. The literature explains that the gap between the concentration plateau and the inlet concentration is likely due to the conversion of NO₂ to NO with the involvement of faujasite [82,83]. At the end of the experiment, NO₂ and NO amounts approached to their initial concentrations: C/C₀ = 1. Thus, gas phase is in equilibrium with solid phase and the chemical reactions described above no longer occur between all the species. The inlet NO concentration (C/C₀ = 1) was reached at the same time as the second NO₂ breakthrough occurred. Finally, the adsorbed amount of NO is very low. However, the breakthrough time is equal to 52 and 116 min for NiYExch7NaY and NiXExch9NaX, respectively (Table 4).

<figure><img src="image_10.png" /><figcaption>Fig. 10. Breakthrough curve of mixture CO, NO and NO₂ for (a) NiYExch7NaY and (b) NiXExch9NaX. C₀,₀ = CNO,₀ = CNO₂,₀ = 350 ppm, Qv = 30 L h⁻¹ STP, T = 303 K, P = 101.3 kPa, granular bed length = 70 mm, diameter = 10 mm, 0.355 < dₚ < 1 mm, mNiYExch7NaY = 4.366 g and mNiXExch9NaX = 5.480 g. Exchanged faujasites had previously been calcined (1 K/min and 573 K during 4 h). The outlet concentration was determined by FTIR analysis.</figcaption></figure>

NO₂.

For both exchanged Y- and X-faujasites, NO₂ curves show an intriguing shape. A first “hump” is observed between the time that NO and water breakthrough occurred. Specifically, the NO₂ concentration declines towards zero, at the point of the water breakthrough. The comparison between exchanged faujasites suggests that the first part of the hump results from the beginning of a roll-up. In both cases, the phenomenon is attributed to the roll-up formation. Then, the sharp decrease in the concentration of NO₂ indicates that chemisorption is a removal route for NO₂. This is consistent with the NO concentration profile: these slope changes observed for NO and NO₂ breakthrough curves pertain with the conversion of NO₂ to NO due to water presence with the involvement of faujasites [82]. After the hump, as long as NO₂ and H₂O adsorb in the exchanged faujasite pores, the reaction takes place and NO₂ is consumed. Consequently,
the NO₂ concentration remains close to zero. The chemical surface reaction then stops due to the breakthrough of H₂O. The chemical reactions persist briefly, and NO₂ undergoes a second breakthrough, gradually reaching its inlet concentration. At the same time, NO concentration decreases towards the same value.

Usually, breakthrough curves exhibit an "S"-shaped profile before gradually reaching their peak at C/C₀ = 1, where they stabilize, regardless of the gas being analysed. However, the situation of C/C₀ > 1 (roll-up) and slope changes suggest a strong competitive physisorption and chimisorption between species, where water is the gas molecule which possesses the highest affinity with faujasites since it is the only one that shows a breakthrough curve without a roll-up [79,80]. However, at the beginning of adsorption (before 4 min and 10 min, respectively), exchanged faujasites provide sufficient adsorption sites for both water and the pollutants to be adsorbed simultaneously (Fig. 11). All gas concentrations in the outlet of the granular bed are also equal to zero, and an accumulation of CO, NO and NO₂ appears in the zeolite. However, the breakthrough time achieved with NiXExch9NaX is higher than that of NiYExch7NaY, making Ni²⁺-exchanged X-faujasite a better candidate to trap target gases from diesel exhaust fumes in the presence of water and oxygen.

Established Passive NOₓ Adsorbers (PNAs) from the literature include Ru exchanged NaY and Pd exchanged beta zeolites [45,67], where they reported adsorption capacities of q = 3.2 ± 0.1 mg NOₓ g⁻¹ with a gas flow of 1000 ppm NO, 5 % O₂, 3 % H₂O and balance N₂ (T = 250 °C) and q = 65 µmol/g (1.95 mg/g if we consider the mass of NO) of NOₓ, in a feed of 200 ppm NO, 200 ppm CO, 5 % H₂O, 10 % O₂, balance N₂ (room temperature) respectively. If we compare them to our values (Table 4) of 1.19 mg/g for NiYExch7NaY (Calc 11) and approximately 51.6 mg/g for NiXExch9NaX (Calc 18), we see that our materials have excellent performance. However, we have to keep in mind that we have added NO₂ in the gas feed, which was not present in the feeds of the current articles.

<figure><img src="image_11.png" /><figcaption>Fig. 11. Breakthrough curves obtained for a mixture of CO, NO, NO₂, O₂ and H₂O for (a) NiYExch7NaY after 11 use cycles and (b) NiXExch9NaX after 18 use cycles (C₀: 0 = 1000 ppm, CNO,0 = CNO₂,0 = 350 ppm, CO₂,0 = 8.5 %-vol, CH₂O,0 = 1 %-vol, Qv = 30 L·hr⁻¹·STP, T = 303 K, P = 101.3 kPa, granular bed length = 70 mm, diameter = 10 mm, 0.355 < Od < 1 mm, mNiYExch7NaY = 4.366 g and mNiXExch9NaX = 5.480 g). Exchanged faujasites had previously been calcined (1 K/min and 573 K during 4 h). The outlet concentration was determined by FTIR analysis.</figcaption></figure>

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

## 6. Conclusion

This study aimed to investigate the adsorption of CO, NO and NO₂ from a gas mixture containing water vapour and oxygen, similar to the exhaust fumes from diesel. DFT calculations highlighted that Ni²⁺ is a good candidate for trapping the target gases. X- and Y-faujasites were thus prepared from sodium X- (Si/Al: 1.23) and Y-faujasites (Si/Al: 2.55). The characterization of the Ni-exchanged materials performed by ICP-OES and XRD showed zeolites to have been over-exchanged. Nevertheless, a mass balance performed on the sodium cation demonstrated that the cation-exchange was successful. The resulting zeolites contained both nickel and sodium cations: NiNaX with seven exchanges (Ni₃₉Na₆Al₈₆Si₁₁₀₆O₃₈₄, Si/Al: 1.24); and NiNaY with nine exchanges (Ni₂₀Na₁₃Al₅₃Si₁₃₆O₃₈₄ Y, Si/Al: 2.58). Moreover, for CO, NO and NO₂ adsorption, Ni²⁺-exchanged zeolites present an improvement in adsorption capacity compared to their sodium-based analogue. Besides, the nickel-exchanged X-faujasite outperforms its Y-faujasite counterpart because of the presence of more accessible cations or more adsorption sites. Indeed, NiNaX contains around twice the amount of Ni²⁺ cations that are found in NiNaY. Therefore, the choice of modified X-faujasite is best suited to treat diesel exhaust gases. These experiments also corroborate the results of our preliminary DFT investigation: the nickel cation is more effective than the sodium cation. Thus, the adsorption energies are sufficiently high to trap NO and CO while making gas desorption possible at moderate temperatures without collapsing the material. Finally, even if the Ni²⁺-exchanged X-faujasite has been identified as the most promising adsorbent for NOₓ adsorption, both its capacity to be regenerated by thermal treatment and reused for repeated cycles have to be studied, and its implementation in a separation process at a higher scale readily and easily still remain to be explored. Researching both outcomes will require large-scale synthesis and shaping studies.
CRediT authorship contribution statement

**Hubert Monnier:** Writing – original draft, Validation, Investigation, Conceptualization. **Ioannis Karamanis:** Investigation. **Mathieu Marchal:** Writing – original draft. **Jean-Louis Paillaud:** Investigation. **Habiba Nouali:** Investigation. **Michael Badawi:** Validation, Supervision, Investigation. **Gérald Chaplais:** Validation, Supervision.

Declaration of competing interest

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

Acknowledgements

This work is partially funded by the French National Research Agency NOA Project (ANR-20-CE08-0024) and the COMETE project (COception in silico de Matériaux pour l'Environnement et l'Energie), supported by the French government and the European Union through the program FEDER-FSE Lorraine et Massif des Vosges 2014-2020.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.seppur.2025.134776.

Data availability

No data was used for the research described in the article.

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