# Influence of the CeO2 addition on the CuO/SBA-15 adsorbent performance for a DeSOx process under wet conditions

Grégory Guicheney, Sophie Dorge, Habiba Nouali, B. Lebeau, Philippe Fioux, M. Soulard, J. Patarin, M. Molière, M. Vierling, A.C. Houdon, et al.

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## To cite this version:

Grégory Guicheney, Sophie Dorge, Habiba Nouali, B. Lebeau, Philippe Fioux, et al. Influence of the CeO2 addition on the CuO/SBA-15 adsorbent performance for a DeSOx process under wet conditions. Microporous and Mesoporous Materials, 2026, 402, pp.113990. 10.1016/j.micromeso.2025.113990. hal-05469515

HAL Id: hal-05469515

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Submitted on 21 Jan 2026

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# Influence of the CeO₂ addition on the CuO/SBA-15 adsorbent performance for a DeSOx process under wet conditions

G. Guicheney<sup>a,b,c,f</sup>, S. Dorge<sup>a,c,*</sup>, H. Nouali<sup>b,c,**</sup>, B. Lebeau<sup>b,c ID</sup>,

Philippe Fioux<sup>b,c</sup>, M. Soulard<sup>d</sup>, J. Patarin<sup>d</sup>, M. Molière<sup>d,e</sup>, M. Vierling<sup>g</sup>, A.C. Houdon<sup>f</sup>,

J.F. Brilhac<sup>a,c ID</sup>

## ARTICLE INFO

### Keywords:

Desulfurization

SO₂ adsorption

CeO₂-CuO/SBA-15 adsorbents

Adsorption/regeneration cycles

Water vapor impact

CeO₂-CuO synergy effect

## ABSTRACT

In order to reduce the negative impact of water vapor on the cyclic SO₂ adsorption performance of a CuO/SBA-15 material, the addition of CeO₂ to the copper active phase was performed at different Ce/Cu molar ratios and temperatures of the adsorption/regeneration cycles. The addition of CeO₂ to the CuO/SBA15 adsorbent with a Ce/Cu molar ratio of 0.68 increases the SO₂ adsorption capacity at the breakthrough by a factor of 3.4 (387 mgSO₂/gCuO vs 114 mgSO₂/gCuO) with good performance over 20 cycles of SO₂ adsorption/regeneration under wet conditions at 450 °C. These enhancements are explained by a synergy effect between CuO and CeO₂ on the SO₂ adsorption reactions and a substantial decrease in the migration of the CuO active phase during adsorption/regeneration cycles under water vapor.

## 1. Introduction

In the fight against air pollution caused by human activities, Flue Gas Desulfurization (FGD) processes have been an effective means of reducing the harmful effects, on health [1–5] and ecosystems [6], of sulfur oxides (SOₓ with X = 2 or 3) for many years [7–9]. To remove these gaseous pollutants found mainly in the flue gases of fossil fuel-based energy production processes, there are numerous FGD processes, which vary depending on operating conditions, technologies used, and local regulations [10–14]. Generally, their operation involves passing the fumes through a bed of material. A large number of materials can be used for FGD [10–14]. Among the most commonly employed, are lime (quick or slaked), limestone, magnesia, activated carbons, and metal oxides. FGD processes can be classified according to the regenerable character of the material: (1) a FGD process is said to be “regenerative” when the captured SOₓ are released in gaseous form (mainly as SO₂) during the regeneration of the used material and then

used as a source of sulfur (e.g. for the production of sulfuric acid [7,15] or elemental sulfur [7,16]); (2) a FGD process is said to be “non-regenerative” when the used material is disposed of as waste or used directly as a by-product.

In the context of the work presented here, we are particularly interested in an SOₓ adsorbent material that can be used in a regenerative FGD process and that was developed by our team [17–24]. This adsorbent is composed of a CuO-based active dispersed on an ordered mesoporous SBA-15 silica support. This CuO active phase has the ability (1) to catalyze the oxidation of SO₂ to SO₃ and then react with this latter to form copper sulfate CuSO₄; (2) to be regenerable under a reducing atmosphere (with 0.5 vol% H₂ in N₂) and to release SO₂ for its valorization into sulfur products, all this under the temperature conditions of the combustion flue gases (300–600 °C). The effectiveness of this CuO/SBA-15 adsorbent for SOₓ abatement from a gas stream representative of flue gases from combustion installations using gas turbines, has been demonstrated in multicycle (desulfurization/regeneration cycles)

<footnote><sup>a</sup> Université de Haute-Alsace (UHA), LGRE UR 2334, 68100, Mulhouse, France</footnote>

<footnote><sup>b</sup> Université de Haute-Alsace (UHA), IS2M-CNRS UMR 7361, 68100, Mulhouse, France</footnote>

<footnote><sup>c</sup> Université de Strasbourg, 67000, Strasbourg, France</footnote>

<footnote><sup>d</sup> Zéphir Alsace SAS, 68450, Brunstatt-Didenheim, France</footnote>

<footnote><sup>e</sup> Université de Technologie de Belfort-Montbéliard (UTBM), ICB-LERMPS, 90000, Belfort, France</footnote>

<footnote><sup>f</sup> Agence de La Transition écologique (ADEME), 49000, Angers, France</footnote>

<footnote><sup>g</sup> GE Energy Products France, 90000, Belfort, France</footnote>

<footnote>* Corresponding author. Université de Haute-Alsace (UHA), LGRE UR 2334, 68100, Mulhouse, France.</footnote>

<footnote>** Corresponding author. Université de Haute-Alsace (UHA), IS2M-CNRS UMR 7361, 68100, Mulhouse, France.</footnote>

<footnote>E-mail addresses: sophie.dorge@uha.fr (S. Dorge), habiba.nouali@uha.fr (H. Nouali).</footnote>
and multigas (presence of SO₂, CO/CO₂, NO/NO₂, in N₂), at a relative high temperature (400 °C) [22]. However, it has been found that the FGD performance of the CuO/SBA-15 decreases significantly in the presence of water vapor [23,24]. This is a major disadvantage since water is an inherent compound of the combustion fumes. In the presence of 5 vol% of H₂O in a sulfurized gas stream (250 ppm of SO₂, in N₂), a significant loss in adsorption capacity of the CuO/SBA-15 adsorbent, reaching up to 70 %, has been shown both at 400 and 450 °C, which can reach up to 70 % of the adsorption capacities obtained without the presence of water vapor, depending on the content of the active CuO phase and the temperature [23]. The reasons exposed to explain this deactivation in the presence of water have been: (1) the blocking of the most active adsorption sites of the CuO phase, due to a competitive physisorption and/or chemisorption between H₂O and SO₂ molecules [25,26]; (2) the potential formation of hydrated copper sulfate species (hydrated copper oxyhydroxysulfates [27,28]) in the presence of H₂O that are difficult to decompose whatever the regeneration temperature used (400 or 450 °C); (3) the increase of the migration and coalescence phenomenon of the active phase into more or less large particles [23, 24].

Regarding the migration/coalescence phenomenon, it is known that the mobility of the atoms in matter (solid, liquid or gas) increases with increasing temperature. For solids, the temperature at which atoms begin to move and trigger a migration/coalescence phenomenon can be estimated approximately using the semi-empirical temperatures of Hüttig and Tamman [29–31], calculated from the material’s melting temperature (T<sub>m</sub>) [32]. At the Hüttig temperature (equal to 0.3 × T<sub>m</sub>), the surface atoms acquire mobility and at the Tamman temperature (equal to 0.5 × T<sub>m</sub>), bulk atoms in turn become mobile. The Hüttig and Tamman temperatures of copper species (Cu, CuO, and Cu₂O, species highlighted in the CuO/SBA-15 adsorbent during cyclic FGD tests) are reported in Table 1. We can notice that the Huttig temperatures of these species are close to the ones of the flue gases to be treated (from 350 to 450 °C), thus the migration/coalescence of the active phase of the CuO/SBA-15 adsorbent will be greatly facilitated. Therefore, to minimize deactivation of the CuO/SBA-15 adsorbent in the presence of water vapor related to migration/coalescence of the CuO active phase, its association with the cerium oxide (CeO₂), which has a higher melting point than copper species (Table 1), allows to improve and stabilize the dispersion of CuO phase on the siliceous support [17,33,34]. Moreover, CeO₂ exhibits the capacity to provide highly mobile oxygen atoms within its structure, making them readily accessible for catalytic reactions through the self-reduction process from Ce⁴⁺ to Ce³⁺ [35], thus, promoting the activity of Cu-Ce based catalyst. Indeed, for a CO oxidation application, Skarman et al. [36] shown that CuOₓ/CeO₂ materials present an increase of CO conversion rate in CO₂ at 150 °C from 10 % for a pure CeO₂ material to 75 % for a CuOₓ/CeO₂ material with 13 at.% of bulk Cu. The same trend was observed by Aguila et al. [34] with Cu-Ce supported on ZrO₂. They have shown that the variation of Cu/Ce loading weight ratio increased the CO conversion from 25 % (Cu/Ce = 0.125) to 60 % (Cu/Ce = 0.5) at 100 °C. In the same way, Nie et al. [37] noticed that the addition of 2 mol.% of Ce to a Cu-Fe catalyst induced a 12 % gain of the CO conversion rate. For a methanol steam reforming application, Bepari et al. [38] studied the effect of Ce and Zn on the activity of Cu-based mesoporous carbon and exposed that Ce-loaded

<table><caption>**Table 1**
Melting, Hüttig and Tamman temperatures for some copper and cerium species [29–32].</caption><thead><tr><th>Metal species</th><th>Melting temperature (°C)</th><th>Hüttig temperature (°C)</th><th>Tamman temperature (°C)</th></tr></thead><tbody><tr><td>Cu</td><td>1084</td><td>325</td><td>542</td></tr><tr><td>CuO</td><td>1227</td><td>368</td><td>614</td></tr><tr><td>Cu₂O</td><td>1244</td><td>373</td><td>622</td></tr><tr><td>CeO₂</td><td>2480</td><td>744</td><td>1240</td></tr></tbody></table>

Cu-mesoporous carbon presented a methanol conversion rate 16 % higher than the one of a Cu-mesoporous carbon.

Therefore, the influence of adding CeO₂ on the FGD performance and the regeneration capacity of the CuO/SBA-15 material in the presence of water vapor has been evaluated by varying (1) the Ce/Cu molar ratio (from 0 to 0.68) in the adsorbent composition and (2) the process temperature (400–450 °C). Indeed, as shown by the different authors cited above, the use of Cu/Ce materials improves the catalytic activity in the oxidation of CO and maintain a high dispersion of the active phase CuO on the support. So, this strategy was followed for the SOₓ CuO/SBA-15 adsorbent, which loses its activity in the presence of water. The objective is to improve and maintain its performance in presence of water vapor along cycling experiments by adding CeO₂ and to determine particularly in what proportion the Ce heteroelement influences the performance and limit the agglomeration of copper-based active species.

## 2. Materials and methods

### 2.1. Adsorbent synthesis

#### 2.1.1. Reagents

Poly (ethylene oxide)-block-poly (propylene oxide)-block-poly (ethylene oxide) (Pluronic® P-123, 100 wt%, Sigma-Aldrich), hydrochloric acid (HCl, 37 wt%, Carlo Erba), tetraethylorthosilicate (TEOS, 98 wt%, Evonik), Cu(NO₃)₂·3H₂O (99.5 wt%, Sigma-Aldrich) and Ce(NO₃)₃·6H₂O (99 wt%, Sigma-Aldrich) were used without further purification. Demineralized water (H₂O) was obtained with a Aquadem system (18 MΩ cm).

#### 2.1.2. Synthesis of the organized mesoporous silica support

Adsorbents were made up of SBA-15 type silica support [39]. This SBA-15 support was synthesized according to the protocol employed in previous works [23,24] with the following molar composition 1 SiO₂: 0.017 Pluronic® P-123 : 5.71 HCl: 197H₂O. First, 196 g of pre-heated (60 °C for 15 h) porogen agent Pluronic® P-123 were dissolved in 1121 g of HCl and 6387 g of H₂O, at 45 °C for 12 h. Then, 425 g of silica precursor TEOS were added to the solution, which was kept under stirring for 4 h at 45 °C. After that, the mixture under static mode and autogenous pressure was heated to 90 °C for 18 h. After being recovered by centrifugation, the white precipitate was washed with H₂O until the pH of the washing water matched those of demineralized water (pH = 6.5), and it was dried in a ventilated oven for 24 h at 60 °C. The as-made material was then calcined at 300 °C under air flow (around 5 L/h) in a muffle furnace with a temperature ramp of 1 °C.min⁻¹ up to 300 °C and a plateau of 4 h at 300 °C to remove the porogen agent present in its mesopores. About 120 g of calcined SBA-15 were obtained.

#### 2.1.3. Deposition of the metal oxides on the support

The deposition of the metal oxide species CuO and CeO₂ on the SBA-15 support was realized by wet co-impregnation, using Cu(NO₃)₂·3H₂O and Ce(NO₃)₃·6H₂O as metal oxide precursors. The desired quantities of metal oxide precursors were first dissolved in 20 mL of demineralized water. Then, 3 g of SBA-15 support were introduced and the resulting suspension was mixed at room temperature for 4 h. Next, the most part of the water was completely evaporated by heating the mixture with a water bath at 60 °C under stirring. The recovered powder was then completely dried in a ventilated oven at 60 °C for 24 h before being calcined under the following conditions: in a fixed-bed reactor (internal diameter of 50 mm), with 10 mm adsorbent bed thickness, under an air flow, with a Gas Hourly Space Velocity of 15,000 h⁻¹, with a temperature ramp of 1 °C.min⁻¹ up to 550 °C followed by 6 h stage at 550 °C and a cooling of 1 °C.min⁻¹ up to room temperature. The obtained adsorbents impregnated with CuO, CeO₂ and CuO + CeO₂, are denominated as XCuO/SBA-15, YCeO₂/SBA-15 and XCuO-YCeO₂/SBA-15, respectively, where X and Y relates to the weight percentage of CuO and CeO₂ relative to the total mass of the product, respectively.
## 2.2. Characterization techniques

### 2.2.1. X-Ray Fluorescence

X-Ray Fluorescence (XRF) analyses were realized to determine the metal oxide contents of the adsorbents. The analyses were carried out with a X-Ray Fluorescence spectrometer Zetium from Panalytical on samples compacted in a 13 mm diameter pellet. Data were calibrated with homemade bulk CuO and CeO₂ standard compounds.

### 2.2.2. X-ray powder diffraction

The adsorbents' structural properties were evaluated using X-Ray powder Diffraction (XRD). A Panalytical X'Pert PRO MPD Powder diffractometer with a sample changer and X'Celerator detector was used for the analyses in reflection mode. For 20 angle values ranging from 0.7 to 70° (0.02°/step, 220 s/step), measurements were carried out using CuKα radiation (λ = 0.15418 nm). The samples were spread on a frosted glass plate.

### 2.2.3. Nitrogen adsorption/desorption measurements

To evaluate the adsorbents' textural properties, nitrogen adsorption-desorption isotherms were recorded. They were carried out at -196 °C using a Micromeritics Tristar sorptometer. Samples were outgassed for 15 h at 150 °C in a vacuum of 10⁻³ mbar prior to measurements. The total pore volume was measured at the relative pressure P/P₀ = 0.9 while the microporous volume was determined at P/P₀ = 0.005 according to Galarneau et al. [40] work. The difference between the microporous and total porous volumes was used to calculate the mesoporous one. The specific surface area SₐBET was determined using the BET (Brunauer-Emmett-Teller) method [41] and the BJH (Barrett-Joyner-Halenda) method applied to the desorption branch was used to determine the average mesopore size [42].

### 2.2.4. Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray mapping (EDX)

To assess the dispersion state of the metal oxides on the SBA-15 support, an ARM-200F microscope from Jeol with a LaB6 filament equipped with EDX Jeol Centurio detector was used to take transmission electron micrographs and record elemental mappings, respectively. Before analysis, samples suspended in chloroform were sonicated for 10 min. After that, two to three drops of the suspension were spread onto gold grids coated with a holey carbon film with a thickness of 5 nm.

### 2.2.5. X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is used to identify the oxidation state of the Cu and Ce species present on the surface of the material. XPS spectra were recorded with a VG SCIENTA SES-2002 spectrometer equipped with a concentric hemispherical analyzer. The incident radiation used was generated by a monochromatic Al Kα x-ray source (1486.6 eV) operating at 420 W. Photo-emitted electrons were collected at a take-off angle of 90° from the surface substrate. Wide scan spectrum signal was recorded with a pass energy of 500 eV and for high resolution spectra pass energy was set to 100 eV in favor of energy resolution. The analyzed surface area was approximately 24 mm² and the base pressure in the analyze chamber during experimentation was about 10⁻⁹ mbar. Charging effects on these isolating sample were compensated by using of a Flood Gun The spectrometer energy scale was calibrated using the Ag 3d⁵/², Au 4f⁷/² and Cu 2p³/² core level peaks, set respectively at binding energies of 368.2, 84.0 and 932.6 eV. Spectra were subjected to a Shirley baseline and peak fitting was made using CASAXPS version 2.3.18 software. The surface composition expressed in atomic percentage (at%) was determined using integrated peak areas of each component and take into account transmission factor of the spectrometer, mean free path and Scofield sensitivity factors of each atom. All the binding energies (BE) are referenced to the C1s peak at 285.0 eV and given with a precision of 0.1 eV.

## 2.3. Evaluation of the cyclic SOx adsorption and regeneration performance of the adsorbents

Under DeSOₓ cyclic tests in wet conditions, the adsorbents' desulfurization and regeneration properties were evaluated with a fully automated system, well described in previous work [23]. Before the DeSOₓ tests, the adsorbent has been shaped into 32 mm diameter tablets (0.25 t/cm² for 2 min), manually crushed (with an agate mortar and pestle) and sieved within the 250–355 µm range. The DeSOₓ tests were carried out globally over 20 successive SO₂ adsorption (in the presence of water vapor) and adsorbent regeneration phases on a 20 mm thick adsorbent bed deposited on a frit in a quartz U-shaped reactor (internal diameter of 6 mm). Under pure N₂ flow, the adsorbent was first heated to the desired temperature (400 or 450 °C) for 1 h at a rate of 5 °C.min⁻¹. Next, the SO₂ adsorption step was launched during 10,800 s with an inlet gas stream composed of 250 ppm of SO₂, 10 vol% of O₂ and 5 vol% of H₂O diluted in N₂. Afterward, the regeneration step was carried out at the same temperature that the one of the adsorption step (isothermal process), under a reductive atmosphere composed of 0.5 vol% of H₂ diluted in N₂, during 1500 s. A pure N₂ flow was imposed for 600 s between the steps of adsorption and regeneration. At the outlet of the reactor, the concentration of SO₂ in the gas phase was determined online with a UV SO₂ adsorption analyzer (Rosemount X'Stream, detection range of 0–20.000 ppm) calibrated with a SO₂/N₂ gas mixture containing 500 ppm of SO₂. All along the tests, the gas was maintained at a Gas Hourly Space Velocity of 25,000 h⁻¹ (i.e. with a volumetric flow rate of 14 NL·h⁻¹, in laminar regime (Re ≈ 20)) and a temperature of 400 or 450 °C.

In this study, the evolution of two distinct data was used to evaluate the SO₂ adsorption and regeneration properties of the adsorbents through cycling experiments: the SO₂ adsorption capacity at the breakthrough Cₐbterhrough and the total SO₂ adsorption capacity Cₐtota. These capacities, expressed in mg of adsorbed SO₂ per gram of CuO active phase, were determined from the integration of the SO₂ breakthrough curves (established during the SO₂ adsorption stage), until the SO₂ concentration at the reactor outlet reaches 5 ppm and from the integration of the SO₂ desorption curves (established during the regeneration stage), according to equations (1) and (2), respectively:

$$C_{\text{SO}_2}^{\text{breakthrough}} = \frac{F_v \times \int_{t_1}^{t_b} (C_0 - C) dt}{10^3 \times V_m \times m_{\text{CuO}}} \times M_{\text{SO}_2} \quad (1)$$

$$C_{\text{SO}_2}^{\text{total}} = \frac{F_v \times \int_{t_2}^{t_f} C \, dt}{10^3 \times V_m \times m_{\text{CuO}}} \times M_{\text{SO}_2} \quad (2)$$

Where F_v (in NL·h⁻¹) is the gas volumetric flow rate. C₀ and C (in ppm) are the reactor inlet and outlet SO₂ mole fractions, respectively. t₁ and t₂ (in h) are the starting times of the adsorption step and the regeneration step, respectively. t_b and t_f (in h) are the time at the SO₂ breakthrough (when 5 ppm of SO₂ is detected from the analyzer) and the final time of the regeneration step, respectively. V_m is the molar volume of a gas (22.4 L·mol⁻¹ at 0 °C and 1 atm). mₐ₅O is the mass of CuO active phase in the adsorbent within the fixed-bed (in g). Mₐ₅O is the molar mass of SO₂ (64 g·mol⁻¹).

From the ratio of the molar quantity of SO₂ adsorbed (in molSO₂/gₐ₅O₄) to the molar quantity of CuO or CeO₂ content in the adsorbent (in molCuO/gₐ₅O₄), the sulfation ratio of the CuO or CeO₂ active phase was also calculated in percentage.
# 3. Results and discussion

## 3.1. Characterization of the fresh adsorbents

In order to evaluate the influence of the addition of the cerium (in its oxide form CeO₂) on the cyclic SO₂ adsorption performance of the CuO/SBA-15 adsorbent in the presence of water vapor, five materials based on CuO and/or CeO₂ supported on organized mesoporous silica of SBA-15 type, with only Ce, Cu or with Ce/Cu (molar ratios ranging from 0 to 0.68), were produced. The different Ce/Cu molar ratios were chosen to figure out how much cerium oxide should be added to the copper-based adsorbent in order to keep a high DeSOₓ performance and prevent copper-based species from agglomeration during cycling tests in presence of water vapor.

Named according to the metal oxide contents determined by XRF (Table 2), the following adsorbents were synthesized:

* A reference adsorbent containing only the active phase CuO: 7CuO/SBA-15;

* Three bimetallic adsorbents composed of the CuO and CeO₂ phases: 5CeO₂-7CuO/SBA-15, 9CeO₂-6CuO/SBA-15 and 10CeO₂-13CuO/SBA-15 (Ce/Cu molar ratio of 0.34, 0.68 and 0.36, respectively);

* A material based on only CeO₂ supported on SBA-15 silica: 10CeO₂/SBA-15, in order to identify the role that CeO₂ plays in SO₂ adsorption.

To evaluate the effect of adding CeO₂ to the copper-based adsorbent CuO/SBA-15 on its DeSOₓ performance under water vapor conditions, 7CuO/SBA-15, 5CeO₂-7CuO/SBA-15 and 9CeO₂-6CuO/SBA-15 adsorbents with a low initial mass content of about 7 wt% of CuO was used. Indeed, previous studies have demonstrated that an adsorbent with a low amount of CuO reduces the harmful effects of water vapor, particularly by limiting the migration/coalescence of copper-based species on the SBA-15 support [23,24]. The 10CeO₂-13CuO/SBA-15 adsorbent (Ce/Cu molar ratio of 0.36 similar to the 5CeO₂-7CuO/SBA-15 one) that presents a copper oxide content of 13 wt% (equivalent to the one of the 15CuO/SBA-15 adsorbent which has been extensively studied in previous works [23,24] and which presents the highest SO₂ adsorption capacities) was also studied to determine the impact of the metal oxides content on the SO₂ adsorption capacities.

The XRD diffractograms at low angles 2θ (from 0.7 to 5°) (Fig. 1a) show that for all adsorbents, the 2D hexagonal mesostructure of the SBA-15 silica support is preserved after incorporation of the metal oxides CuO and CeO₂ whatever the metal oxide contents. Indeed, the X-ray diffraction peaks corresponding to the (100), (110) and (200) reticular planes, characteristic of the 2D-hexagonal organization of the cylindrical mesopores of the support (symmetry group P6mm) [39] are observed. However, when compared to the ones of the SBA-15 silica support, these diffraction peaks are shifted toward higher 2θ angles, indicating a decrease in the hexagonal lattice parameter following the addition of the copper and cerium heteroelements (values reported in

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

Nomenclature and content in CuO and CeO₂ phases of the synthesized adsorbents.

<table><thead><tr><th>Sample</th><th>CuO content<sup>a</sup> (wt. %)</th><th>CeO<sub>2</sub> content<sup>b</sup> (wt. %)</th><th>Ce/Cu molar ratio</th></tr></thead><tbody><tr><td>7CuO/SBA-15</td><td>7.1</td><td>–</td><td>0</td></tr><tr><td>10CeO<sub>2</sub>/SBA-15</td><td>–</td><td>10.2</td><td>–</td></tr><tr><td>9CeO<sub>2</sub>-6CuO/SBA-15</td><td>6.4</td><td>9.4</td><td>0.68</td></tr><tr><td>5CeO<sub>2</sub>-7CuO/SBA-15</td><td>6.9</td><td>5.1</td><td>0.34</td></tr><tr><td>10CeO<sub>2</sub>-13CuO/SBA-15</td><td>10.3</td><td>13.3</td><td>0.36</td></tr></tbody></table>

Table 3). This phenomenon of pore lattice contraction is probably related to dehydroxylation reactions [18,43,44] that occur during the thermal treatment at 550 °C. The X-ray diffractograms at wide angles 2θ (from 5 to 70°) of all the adsorbents and the silica support SBA-15 (Fig. 1b) show the characteristic halo of the amorphous SiO₂ phase constitutive of the SBA-15 support structure in the 15–40 2θ angle range. The adsorbents 7CuO/SBA-15, 10CeO₂/SBA-15, 5CeO₂-7CuO/SBA-15 and 9CeO₂-6CuO/SBA-15 do not show any X-ray diffraction peaks corresponding to crystalline copper or cerium phases, suggesting a good dispersion of these elements in the form of amorphous metal oxide particles or nanocrystallites, too small to be detectable by XRD. The X-ray diffractogram of the 10CeO₂-13CuO/SBA-15 adsorbent displays three diffraction peaks assigned to CeO₂ phase with very low intensity and barely stand out from the background. This indicates that CuO and CeO₂ are well dispersed in this material containing the highest metal oxides amount.

The N₂ physisorption isotherms of all adsorbents show that the porous texture of the SBA-15 support characterized by a hexagonal packing of cylindrical mesopores interconnected by micropores [45] is conserved after the incorporation of the metal oxides and the shaping of the adsorbents (Fig. 2). Indeed, all isotherms exhibit the combination of a type I(b) isotherm, indicating the presence of micropores, and a type IV (a) isotherm characteristic of materials with mesopores, in accordance with the IUPAC classification [46]. All the adsorbents exhibit a hysteresis loop. For the SBA-15 support, the hysteresis is a H1 type whereas for the adsorbents, it evolves progressively from H1 to H5 type (as shown by the elongation of the N₂ desorption branch) with the metal oxides loading. This indicates the presence of both open and partially blocked mesopores with metal oxides. The addition of CuO and/or CeO₂ metal oxides leads to a drastic decrease of both the specific surface area (SBET) and the porous volumes of the adsorbents. For the material with the highest loading of copper and cerium oxides (10CeO₂-13-CuO/SBA-15), the microporous volume of the SBA-15 silica support is reduced by more than 65 % (Table 3). This decrease in the mesoporous volume is probably due to the layer of metal oxides deposited on the wall of the SBA-15 mesopores. This is in agreement with the decrease of the average diameter of mesopores after CuO and/or CeO₂ metal oxides deposition on the SBA-15 silica support (Table 3).

Whatever the added metal oxides content, the characterization of the adsorbents by transmission electron microscopy shows a homogenous dispersion of the copper and cerium-based species. Indeed, only the mesostructure of the SBA-15 silica support is visible on the TEM images (Fig. S1) of the synthesized adsorbents, with the presence of parallel fringes corresponding to cylindrical pore channels packed in a 2-dimensional hexagonal arrangement ("honeycomb" zones). The absence of nanoparticles and the homogeneous atomic dispersion of copper and cerium on the EDX element mappings (Fig. 3) confirm the good deposition of the CuO and CeO₂ phases on the SBA-15 silica support.

XPS spectra of the Cu2p and Ce3d core levels of the fresh 7CuO/SBA-15, 10CeO₂/SBA-15 and 9CeO₂-6CuO/SBA-15 materials are shown in Figure S2 and Figure S3, respectively. As with previous studies [19,20], the deconvolution curves fitting of the Cu2p³/² (binding energy between 928 and 948 eV) region shows the presence of Cu⁺ and Cu²⁺ species in strong interaction with the silica support whatever the fresh Cu-containing materials, 7CuO/SBA-15 or 9CeO₂-6CuO/SBA-15. However, it is observed that the main copper species in the fresh materials are in the +1 oxidation state and that the cerium-containing material holds the higher Cu²⁺ amount. Indeed, the Cu⁺/Cu²⁺ ratio is 6.4 and 2.0 for 7CuO/SBA-15 or 9CeO₂-6CuO/SBA-15, respectively (Table S4). The Ce3d XPS spectra of 10CeO₂/SBA-15 and 9CeO₂-6CuO/SBA-15 fresh materials show the presence of Ce⁴⁺ and Ce³⁺ oxidation state with a higher proportion of Ce³⁺ species in the Ce-containing material: Ce³⁺/Ce⁴⁺ = 0.62 for 10CeO₂/SBA-15 and Ce³⁺/Ce⁴⁺ = 0.65 for 9CeO₂-6CuO/SBA-15 (Table S4). These observations indicate that redox reaction Cu⁺ + Ce⁴⁺ ⇌ Cu²⁺ + Ce³⁺ could appears in Ce/Cu-containing

<footnote>a Determined by XRF analyze.</footnote>
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** XRD patterns at low (a) and wide (b) 2θ angles (CuKα radiation) of the SBA-15 support, the fresh and spent 7CuO/SBA-15, 10CeO₂/SBA-15 and CeO₂-CuO/SBA-15 adsorbents after to 20 adsorption-regeneration cycles with 5 vol% H₂O during the SO₂ adsorption stage at 450 °C (and at 400 °C for the 9CeO₂-6CuO/SBA-15 adsorbent) and the simulated CuO, Cu₂O, CeO₂, Ce₂(SO₄)₃ patterns from ICDD database.</figcaption></figure>

<table><caption>**Table 3**
Structural and textural properties of the fresh and spent CuO/SBA-15, CeO₂/SBA-15 and CeO₂-CuO/SBA-15 adsorbents.</caption><thead><tr><th rowspan="2">Sample</th><th rowspan="2">Specific surface area<sup>a</sup> (m²/g<sub>adsorbent</sub>)</th><th colspan="3">Porous volumes<sup>b</sup> (cm³/g<sub>adsorbent</sub>)</th><th rowspan="2">Average mesopore diameter<sup>c</sup> (nm)</th><th rowspan="2">Hexagonal lattice parameter (nm)<sup>g</sup></th></tr><tr><th>Total<sup>e</sup></th><th>Micro.<sup>d</sup></th><th>Meso.<sup>e</sup></th></tr></thead><tbody><tr><td><strong>Support</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SBA-15</td><td>967</td><td>1.04</td><td>0.24</td><td>0.80</td><td>6.7</td><td>11.1</td></tr><tr><td><strong>Fresh adsorbents</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>7CuO/SBA-15</td><td>448</td><td>0.67</td><td>0.11</td><td>0.56</td><td>6.8</td><td>10.9</td></tr><tr><td>10CeO₂/SBA-15</td><td>501</td><td>0.58</td><td>0.12</td><td>0.46</td><td>6.3</td><td>10.8</td></tr><tr><td>5CeO₂-7CuO/SBA-15</td><td>474</td><td>0.58</td><td>0.11</td><td>0.47</td><td>6.2</td><td>10.8</td></tr><tr><td>9CeO₂-6CuO/SBA-15</td><td>467</td><td>0.52</td><td>0.11</td><td>0.42</td><td>5.9</td><td>10.8</td></tr><tr><td>10CeO₂-13CuO/SBA-15</td><td>343</td><td>0.44</td><td>0.08</td><td>0.36</td><td>6.3</td><td>10.6</td></tr><tr><td><strong>Spent adsorbents</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>450 °C</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>7CuO/SBA-15</td><td>446</td><td>0.61</td><td>0.10</td><td>0.51</td><td>6.0</td><td>10.3</td></tr><tr><td>10CeO₂/SBA-15</td><td>382</td><td>0.44</td><td>0.09</td><td>0.35</td><td>6.0</td><td>10.4</td></tr><tr><td>5CeO₂-7CuO/SBA-15</td><td>304</td><td>0.43</td><td>0.07</td><td>0.36</td><td>6.0</td><td>10.1</td></tr><tr><td>9CeO₂-6CuO/SBA-15</td><td>259</td><td>0.35</td><td>0.06</td><td>0.29</td><td>5.6</td><td>10.3</td></tr><tr><td>10CeO₂-13CuO/SBA-15</td><td>135</td><td>0.21</td><td>0.03</td><td>0.17</td><td>5.6</td><td>9.9</td></tr><tr><td>400 °C</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>9CeO₂-6CuO/SBA-15</td><td>235</td><td>0.31</td><td>0.05</td><td>0.25</td><td>5.6</td><td>10.3</td></tr></tbody></table>

<sup>a</sup> Determined by the BET method [41].

<sup>b</sup> Expressed in cm³ per gram of adsorbent (metal oxide phases + SBA-15 silica support).

<sup>c</sup> Total porous volume determined from the N₂ adsorbed volume at P/P₀ = 0.90.

<sup>d</sup> Microporous volume determined from the N₂ adsorbed volume at P/P₀ = 0.005 [40].

<sup>e</sup> Mesoporous volume calculated from the difference between the total porous volume and the microporous volume.

<sup>f</sup> Determined by the BJH method on the desorption branch between 0.6 and 0.8 P/P₀ [42] (taking the value of the largest peak).

<sup>g</sup> Determined from low 2θ angles XRD pattern.

materials.

It is surprising to find a higher proportion of low valent Cu species in 7CuO/SBA-15 adsorbent while the oxidizing conditions of the

calcination step is 20 vol% of O₂. In a previous study the Cu⁺ species were found at a lower proportion for a 15CuO/SBA-15 adsorbent [20]. To explain the presence of a higher proportion of Cu⁺, one explanation
<caption>**Table 4**</caption>

Adsorption energy (E<sub>ads</sub>) of H<sub>2</sub>O and SO<sub>2</sub> on the metal oxides Cu<sub>2</sub>O, CuO and CeO<sub>2</sub> [53–57].

<table><thead><tr><th>Metal oxide (lattice plane) | Oxidation state of the metal</th><th>E<sub>ads</sub> H<sub>2</sub>O (eV)</th><th>E<sub>ads</sub> SO<sub>2</sub> (eV)</th></tr></thead><tbody><tr><td>Cu<sub>2</sub>O(111) | +1</td><td>-1.22 [53]</td><td>-1.33 [54]</td></tr><tr><td>CuO(111) | +2</td><td>-0.71 [55]</td><td>-1.88 [56]</td></tr><tr><td>CeO<sub>2</sub>(111) | +4</td><td>-0.52<sup>a</sup> [57]</td><td>-2.02 [56]</td></tr></tbody></table>

<sup>a</sup> Average of the first 6 values of the configurations MS1 and MS2 from Table 1 of Fernández-Torre et al. work [57].

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Nitrogen adsorption/desorption isotherms at –196 °C of the SBA-15 support, the fresh and spent 7CuO/SBA-15, 10CeO<sub>2</sub>/SBA-15 and CeO<sub>2</sub>-CuO/SBA-15 adsorbents after 20 adsorption-regeneration cycles with 5 vol% H<sub>2</sub>O during the SO<sub>2</sub> adsorption stage at 450 °C (and at 400 °C for the 9CeO<sub>2</sub>-6CuO/SBA-15 adsorbent).</figcaption></figure>

can be the chemical interactions between Cu and the SBA-15 support that stabilizes Cu<sup>+</sup> and thus for a lower CuO content in the CuO/SBA-15 adsorbent the proportion of Cu<sup>+</sup> interacting with support related to the total amount of Cu species surface will be higher. Regarding Ce species, Ce<sup>3+</sup> is also present in high proportions as Cu<sup>+</sup> species. Again, this could be explained by the nature of the interactions with the SBA-15 support that stabilizes a part of total Ce species in Ce<sup>3+</sup> species.

According to all characterizations realized on the synthesized copper and cerium-based adsorbents, the wet impregnation technique used in this work appears suitable by leading to a uniform deposition of the CuO and CeO<sub>2</sub> metal oxides on the surface of the SBA-15 silica support, whatever the amount of metal oxides added.

## 3.2. Characterization of the adsorbents after DeSO<sub>x</sub> tests

The 7CuO/SBA-15, 5CeO<sub>2</sub>-7CuO/SBA-15, 9CeO<sub>2</sub>-6CuO/SBA-15, 10CeO<sub>2</sub>-13CuO/SBA-15 and 10CeO<sub>2</sub>/SBA-15 adsorbents were characterized after DeSO<sub>x</sub> tests by XRD, TEM, STEM, EDX and N<sub>2</sub> physisorption manometry.

Low-angle XRD patterns after DeSO<sub>x</sub> testing (Fig. 1a) of all the spent adsorbents, compared with those of the fresh adsorbents, show that the mesostructure is preserved after testing, despite a slight reduction in the hexagonal lattice parameter (Table 3). With the exception of the 9CeO<sub>2</sub>-6CuO/SBA-15 adsorbent with the highest Ce/Cu ratio of 0.68, the wide-angle XRD patterns of the 7CuO/SBA-15, 5CeO<sub>2</sub>-7CuO/SBA-15, and 10CeO<sub>2</sub>-13CuO/SBA-15 adsorbents show peaks that are attributed to the Cu<sub>2</sub>O or Cu<sup>0</sup> crystal structures (Fig. 1b). The appearance of these peaks after DeSO<sub>x</sub> tests, traduces a modification of the dispersion state of the Cu active phase in presence of water vapor for the 7CuO/SBA-15, 5CeO<sub>2</sub>-7CuO/SBA-15, and 10CeO<sub>2</sub>-13CuO/SBA-15 adsorbents, with a partial agglomeration that is confirmed by TEM analyses (Fig. S5). The peaks are narrower and more intense when the total metal oxide content is higher (23 wt% for the 10CeO<sub>2</sub>-13CuO/SBA-15 adsorbent), indicating the presence of larger particles as observed by TEM (Fig. S5).

The crystalline cerium sulfate phase Ce<sub>2</sub> (SO<sub>4</sub>)<sub>3</sub>, which is not degraded during the regeneration treatment in a reducing environment can be seen on the XRD pattern of the adsorbent 10CeO<sub>2</sub>-13CuO/SBA-15 unlike to the spent 10CeO<sub>2</sub>/SBA-15 adsorbent where the cerium species remain well dispersed on the support and below XRD equipment detection limits (Fig. 1). The observation of the cerium sulfate phase by XRD analysis confirm the fact that the regeneration treatment is ineffective in regenerating the sulfated cerium-based active phase.

After 20 cycles of adsorption-regeneration at 450 °C, Cu-based nanoparticles are clearly visible on the materials with a low Ce/Cu molar ratio of around 0.35 (5CeO<sub>2</sub>-7CuO/SBA-15 and 10CeO<sub>2</sub>-13CuO/SBA-15), as well as on the adsorbent without cerium (7CuO/SBA-15) (Fig. S5). The formation of these particles, which reach sizes up to about 50 nm and are substantially larger on the 10CeO<sub>2</sub>-13CuO/SBA-15 material, explain the decrease of the SO<sub>2</sub> breakthrough capacity over cycles. It is a result of the migration and coalescence of copper species into and out of the mesoporosity of the SBA-15 support during DeSO<sub>x</sub> tests. The 9CeO<sub>2</sub>-6CuO/SBA-15 adsorbent with the highest Ce/Cu ratio of 0.68 contains no Cu-based particles visible by TEM micrographs. The EDX analysis of this material clearly shows that copper and cerium remain well dispersed after DeSO<sub>x</sub> tests at 400 and 450 °C, unlike the other adsorbents (Fig. 4) where only the dispersion state of cerium remains homogeneous. Accordingly, the migration/coalescence phenomenon of the copper active phase occurred when the Ce/Cu was too low. It should be noted that the EDX sulfur mappings (Fig. 4) of the cerium-containing adsorbents overlap with those of the cerium, confirming that the cerium sulfates formed during the first adsorption-regeneration cycle do not decompose under our regeneration conditions, as already observed in XRD (Fig. 1b).

The N<sub>2</sub> adsorption-desorption isotherms, and the textural characteristics of the adsorbents before and after DeSO<sub>x</sub> tests, are reported in Fig. 2 and Table 3, respectively. The isotherms show that the textural
<figure><img src="image_7.png" /><figcaption>Fig. 3. STEM micrographs (Dark field mode) and Cu and Ce EDX mappings of the fresh 7CuO/SBA-15, 10CeO₂/SBA-15 and CeO₂-CuO/SBA-15 adsorbents.</figcaption></figure>

properties of the adsorbents are impacted after DeSOₓ test with a sig-
nificant loss of specific surface and pore volumes with the total CuO and
CeO₂ content (with -9, -24, -26, -33 to -52 % of total pore volume
loss for 7CuO/SBA-15, 10CeO₂/SBA-15, 5CeO₂-7CuO/SBA-15, 9CeO₂-
6CuO/SBA-15 and 10CeO₂-13CuO/SBA-15 adsorbents, respectively).

This may be related to the migration/coalescence of metal-based species
inside and/or outside the pores during cycling at 400 or 450 °C. A drastic
reduction of the mesopore volume for the 10CeO₂-13CuO/SBA-15
adsorbent (0.36 cm³/gadsorbent for fresh adsorbent vs 0.17 cm³/gadsorbent
for spent adsorbent) is observed, probably due to a very strong
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** STEM micrographs (dark field mode) and Cu, Ce and S EDX mappings of the spent 7CuO/SBA-15, 10CeO₂/SBA-15 and CeO₂-CuO/SBA-15 adsorbents after to 20 adsorption-regeneration cycles with 5 vol% H₂O during the SO₂ adsorption stage at 450 °C (and at 400 °C for the 9CeO₂-6CuO/SBA-15 adsorbent).</figcaption></figure>

migration/coalescence of the metal-based species and the presence of
cerium sulfate species.

3.3. Impact of CeO₂ addition on the SO₂ adsorption/regeneration performance of the CuO/SBA-15 adsorbent

3.3.1. Influence of the Ce/Cu molar ratio and the total metal oxide content
The adsorbents 7CuO/SBA-15, 5CeO₂-7CuO/SBA-15, 9CeO₂-6CuO/SBA-15, and 10CeO₂-13CuO/SBA-15 were tested in DeSOx in presence of water vapor (5 vol%) at a temperature of 450 °C in order to evaluate
the influence of CeO₂ addition and total metal oxide content (CeO₂ and CuO), on the SO₂ adsorption property of CuO/SBA-15 adsorbent and its regeneration. This temperature was chosen because, according to previous studies [23,24], it boosts the sulfation of the CuO active phase. First, the SO₂ adsorption capacities at the breakthrough ($C_{\text{SO}_2}^{\text{breakthrough}}$) are presented, followed by the SO₂ total adsorption ($C_{\text{SO}_2}^{\text{total}}$) and regeneration capacities of the different adsorbents.

### 3.3.1.1. SO₂ adsorption capacities at the breakthrough.
The SO₂ breakthrough curves for the four CuO-based adsorbents under investigation are shown in Fig. S6 and were obtained along 20 cycles of adsorption-regeneration. When comparing the SO₂ breakthrough curves evolution of the three adsorbents with the same amount of CuO phase, namely 7CuO/SBA-15, 5CeO₂-7CuO/SBA-15 and 9CeO₂-6CuO/SBA-15, along the 20 adsorption-regeneration cycles (Figs. S6a, b, and c), it is first noted that the breakthrough time increases with the presence of CeO₂ (5CeO₂-7CuO/SBA-15 and 9CeO₂-6CuO/SBA-15), reaching 1500 s for the both Ce-based adsorbents at first cycle, and that the adsorbent without ceria (7CuO/SBA-15) exhibits the shortest one, at 1000 s. Next, the SO₂ emissions after the breakthrough reach a plateau, indicating a maximum sulfation rate of the active species. This one is attained more rapidly in presence of CeO₂, at 3000 s for the 5CeO₂-7CuO/SBA-15 and 9CeO₂-6CuO/SBA-15 adsorbents, against 4250 s for the adsorbent without CeO₂, 7CuO/SBA-15. Finally, the breakthrough curves evolve greatly over the cycles for the 7CuO/SBA-15 and 5CeO₂-7CuO/SBA-15 materials, with a decrease of the breakthrough time (from 1000 and 1500 s down to 500 and 1000 s, respectively) and an increase of the SO₂ emissions at the end of the adsorption step (from 135 and 170 ppmv up to 170 and 210 ppmv, respectively). Whereas the 9CeO₂-6CuO/SBA-15 material displays a stable behavior of the breakthrough curves all over the cycles, with a breakthrough time of 1500 s and a SO₂ emissions at the end of the adsorption step of 160 ppmv. So, from these observations, we can conclude that the CuO/CeO₂ mixed oxides significantly promotes the sulfation reactions and particularly the oxidation of SO₂ to SO₃, which is the first step in the SO₂ trapping process. Indeed, within the fluorite-like structure of CeO₂ (CaF₂, space group Fm-3m), the twofold oxidation state of the element cerium, +III and +IV, leads to the reversible reduction of Ce⁴⁺ ions to Ce³⁺ ions. This reversible reduction of the cerium ions allows an easy transition from the stoichiometric form CeO₂ to the non-stoichiometric form CeO₂-x and vice-versa. This change in the stoichiometry of cerium oxide is directly related to the formation of oxygen vacancies and makes the lattice oxygen highly mobile and available for catalytic oxidation processes [47]. Thus, this oxide offers a facility to exchange oxygen with the surrounding environment and an exceptional oxygen storage capacity that improve the oxidation reaction of SO₂ to SO₃, which is then trapped under the form of sulfate. Also, the stable evolution of the SO₂ emissions over cycles of the 9CeO₂-6-CuO/SBA-15 adsorbent could be explained by the fact that CeO₂, which has a higher melting point than CuO (Table 1), stabilizes the dispersion of the latter on the SBA-15 support under these high temperature and humidity conditions.

A reference material based on cerium oxide on SBA-15 silica support (10CeO₂/SBA-15) was also synthesized and submitted to 20 adsorption-regeneration cycles, in order to determine the intrinsic role of CeO₂ in the removal of SO₂ present in the gas mixture to be treated, under the operating conditions defined previously. The SO₂ breakthrough curves of the 10CeO₂/SBA-15 presented in Fig. S7 indicate that this material presents a low SO₂ oxidation and trapping capacity during the first SO₂ adsorption step (cycle 1). During the following cycles, this capacity is strongly reduced, even close to 0. Indeed, from cycle 2, the SO₂ breakthrough appears immediately from the beginning of the SO₂ adsorption step and the SO₂ emissions quickly reach a high stable value corresponding to the injected SO₂ concentration of 250 ppmv (i.e. values increasing from about 235 ppmv for cycle 2, to 245 ppmv from cycle 7). This reflects the inefficiency of the 10CeO₂/SBA-15 material for both

SO₂ oxidation and trapping from cycle 2. This is probably due to the cerium sulfates formed during the first SO₂ adsorption (cycle 1) which do not decompose during the first regeneration step. These sulfates, inactive in SO₂ trapping, block the access on the active sites along next cycles and are responsible of the total deactivation of the CeO₂/SBA-15 adsorbent from cycle 2. This is confirmed by the absence of SO₂ emissions during the regeneration steps of the 10CeO₂/SBA-15 material under H₂ at 450 °C (Fig. S8). Indeed, cerium sulfates are more stable than copper sulfates and thus require higher regeneration temperatures, around 600 °C under H₂ flow [48]. To decompose these cerium sulfates, one solution would be to increase the temperature of the regeneration stage but in this case a possible decrease of the CuO based adsorbent performance could be observed, due to higher migration and coalescence of the CuO active phase with temperature [20]. This is the reason why in this work the regeneration stage is performed at temperatures not exceeding 450 °C [21]. The sulfation rate of the 10CeO₂/SBA-15 material was estimated at 18 %. Therefore, it is assumed that under these operational conditions, CeO₂ does not participate to the SO₂ trapping from cycle 2. Therefore, the SO₂ breakthrough adsorption capacities measured and reported below will correspond only to the capacities of the copper-based active phase to trap SO₂. Thus, they are expressed in milligrams of SO₂ per gram of CuO.

CeO₂ is inactive in SO₂ trapping in our conditions (after cycle one), but the addition of this metal oxide increases the SO₂ trapping capacity of the CuO/SBA-15 adsorbent in the presence of water vapor, as shown the evolution of the SO₂ breakthrough capacity as a function of the number of cycles of the three adsorbents 7CuO/SBA-15, 5CeO₂-7CuO/SBA-15 and 9CeO₂-6CuO/SBA-15, represented in Figs. 5 and 6. This SO₂ breakthrough capacity corresponds to the sulfation of the most easily accessible active sites (i.e. the surface sites).

The SO₂ breakthrough capacity of the reference adsorbent without cerium, 7CuO/SBA-15, decreases over the cycles: from a maximum value of 250 mgSO₂/gCuO in cycle 2 to a stabilized value of 115 mgSO₂/gCuO from cycle 12 (Fig. 5) correspond to 54 % retained capacity. This decrease is related to the migration and coalescence along cycles, of the CuO active phase into larger particles as already observed in previous work [24]. In the presence of cerium oxide, a much higher SO₂ breakthrough capacity is observed for the 5CeO₂-7CuO/SBA-15 and the 9CeO₂-6CuO/SBA-15: 400 mgSO₂/gCuO at cycle 2 for both adsorbents (Fig. 5). This very important gain in SO₂ adsorption capacity of 60 % at cycle 2 for the two CeO₂-containing materials clearly highlights an important synergistic effect between copper and cerium oxides with respect to SO₂ capture compared to the 7CuO/SBA-15 performance. Indeed, the SO₂ adsorption capacity at the breakthrough of these adsorbents is significantly greater than the sum of the adsorption

<figure><img src="image_9.png" /><figcaption>Fig. 5. Evolution of the SO₂ adsorption capacities at the breakthrough of the 7CuO/SBA-15 (Ce/Cu = 0), 5CeO₂-7CuO/SBA-15 (Ce/Cu = 0.34) and 9CeO₂-6CuO/SBA-15 (Ce/Cu = 0.68) adsorbents, submitted to 20 adsorption-regeneration cycles with 5 vol% of H₂O during the SO₂ adsorption stage at 450 °C.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Evolution of the SO₂ adsorption capacities at the breakthrough of the 5CeO₂-7CuO/SBA-15 (Ce/Cu = 0.34) and 10CeO₂-13CuO/SBA-15 (Ce/Cu = 0.36) adsorbents, submitted to 20 adsorption-regeneration cycles, with 5 vol% H₂O during the SO₂ adsorption stage at 450 °C.</figcaption></figure>

capacities of the 7CuO/SBA-15 (250 mgSO₂/gCuO at cycle 2) and 10CeO₂/SBA-15 adsorbents (around 4 mgSO₂/gCeO₂ at cycle 2). For the 9CeO₂-6CuO/SBA-15 adsorbent, this gain reaches even 248 % from cycle 12 to cycle 20. A similar behavior was reported by Gaudin et al. [17] for CeO₂-CuO/KIT-6 adsorbent. In the literature, some studies also underline that combining CuO and CeO₂ can result to the formation of mixed metal oxides of CuO-CeO₂ type, which may have better catalytic properties than the metal oxides alone [49–51]. Coupled with the fact that CeO₂ is an “oxygen tank”, this Cu-Ce synergy thus increases the oxidation of SO₂ to SO₃. This increase in the catalytic activity of CuO in the presence of CeO₂ has already been observed previously for other applications such as the catalytic oxidation of hydrocarbons, volatile organic compounds [49] and in particular for the catalytic oxidation of CO studied by Tang et al. [51]. In view of the promising results obtained, the Cu-Ce synergy effect seems possible even if the cerium oxide is partially sulfated.

The evolution of the SO₂ breakthrough capacities (Fig. 5) shows a decrease from 400 mgSO₂/gCuO at cycles 2–7 to 310 mgSO₂/gCuO at cycle 20 for the 5CeO₂-7CuO/SBA-15 adsorbent with the lowest Ce/Cu molar ratio (0.34). Nevertheless, this loss of performance (23 % retained capacity) is less important than the one of the CuO based adsorbent. The deactivation of the 5CeO₂-7CuO/SBA-15 adsorbent can be attributed to an insufficient amount of CeO₂ to preserve the good dispersion state of the CuO active phase along cycles in presence of water vapor and prevent its agglomeration. Tang et al. [51] have indeed shown that the amount of metal oxide strongly impacts the interactions between CeO₂ and CuO and thus the dispersion state and the redox properties of these mixed metal oxides, responsible for their catalytic performance.

In contrast, for the 9CeO₂-6CuO/SBA-15 adsorbent with the highest Ce/Cu molar ratio (0.68), the SO₂ breakthrough capacity is constant and close to 400 mgSO₂/gCuO over the entire 20 cycles test (2 % retained capacity), as the SO₂ adsorption curves, which are overlapped (Fig. 5). Therefore, this material does not deactivate during cycling in the presence of water vapor. The presence of CeO₂ can stabilize the dispersion state of copper-based particles, as observed by Alvarez et al. [33] with CuCe/SiO₂ (i.e. Aerosil 130) catalyst. They showed that contrary to Cu/SiO₂ material, no CuO particles are formed in CuCe/SiO₂. Skårman et al. [50] have also shown that the presence of CeO₂ prevents the agglomeration of copper-based particles on nanocomposite powders of CuOₓ/CeO₂ mixed oxides.

The influence of the total amount of CuO and CeO₂ metal oxides deposited on the SBA-15 silica support has also been studied. Comparing the SO₂ breakthrough capacities of the 5CeO₂-7CuO/SBA-15 (total metal oxides content of 12 wt%) and of the 10CeO₂-13CuO/SBA-15 (total metal oxides content of 23 wt%), adsorbents with similar Ce/Cu molar ratios (0.34 and 0.36 respectively), the SO₂ breakthrough capacity of the most loaded material in metal oxides, 10CeO₂-13CuO/SBA-

15, is unstable along the 20 cycles (Fig. 6). It decreases progressively from 330 mgSO₂/gCuO at cycle 2 to 110 mgSO₂/gCuO at cycle 20, corresponding to a loss of 67 % retained capacity. This decrease is less important for the lower CuO and CeO₂ loaded material (5CeO₂-7CuO/SBA-15), for whom the decrease reaches only 22 % after 20 cycles. The difference in behavior can be explained by the accentuation of the migration/coalescence phenomenon of the copper-based active phase with the increase of the total content metal oxides deposited on the SBA-15 silica support [23].

From these different observations, a Ce/Cu molar ratio of 0.68 for a total content of metal oxides of 15 wt% could guarantee in the presence of water vapor in the gas mixture to be treated, not only a good state of dispersion of the CuO and CeO₂ metal oxides on the SBA-15 silica support, but also a high and stable SO₂ breakthrough capacity along all cycles. These trends have also been observed by Astudillo et al. [52] who determined that the CO oxidation activity of a CuO-CeO₂ catalyst supported on SiO₂ (i.e. Aerosil 200) is optimal for a Ce/Cu molar ratio of 1 and a total oxide content of 10 wt%.

### 3.3.1.2. Desorption curves and total adsorption capacities.
The total SO₂ adsorption capacities (Fig. 7), obtained by integrating the SO₂ regeneration curves (Fig. 8), correspond only to the capacities of the copper-based active phase to trap SO₂. They are expressed in milligrams of SO₂ per gram of CuO because the cerium sulfates formed during the first adsorption-regeneration cycle are not decomposable under a reducing atmosphere at 450 °C.

Cerium-containing adsorbents present higher total SO₂ adsorption capacities (from 520 to 610 mgSO₂/gCuO) than the one of the 7CuO/SBA-15 adsorbent (420 mgSO₂/gCuO) at cycle 1 (Fig. 7). Thus, the Cu-Ce synergy effect is also visible on the total SO₂ adsorption capacities. Moreover, doubling the Ce/Cu molar ratio leads to a good stabilization of the total SO₂ adsorption capacity. Indeed, for the 9CeO₂-6CuO/SBA-15 adsorbent with Ce/Cu molar ratio of 0.68, this stabilization occurs from cycle 1 while it happens only from cycle 12 and 7 for the 5CeO₂-7CuO/SBA-15 and 10CeO₂-13CuO/SBA-15 (Ce/Cu molar ratio close to 0.35), respectively. This finding could be explained by a stronger interaction between copper and cerium oxides species for the 9CeO₂-6CuO/SBA-15 adsorbent, which therefore maintains an optimal copper dispersion state from the beginning of the SO₂ adsorption-regeneration experiments and along the cycles.

The regeneration curves (Fig. 8) reveal different SO₂ emission profiles, depending on whether the adsorbents contain cerium or not. Indeed, for all cerium-containing materials, a peak of SO₂ emission is observed during the first 200 s of the regeneration stage, which probably corresponds to a decomposition of the surface copper sulfates, as shown in previous works on CuO/SBA-15 without water vapor [20,23]. The presence of CeO₂ limits the migration and coalescence of copper species

<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Evolution of the total SO₂ adsorption capacities of the 7CuO/SBA-15 and CeO₂-CuO/SBA-15 adsorbents, submitted to 20 adsorption-regeneration cycles with 5 vol% H₂O during the SO₂ adsorption stage at 450 °C.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 8.** SO₂ desorption curves of the (a) 7CuO/SBA-15 and (b, c and d) CeO₂-CuO/SBA-15 adsorbents, submitted to 20 adsorption-regeneration cycles with 5 vol% H₂O during the SO₂ adsorption stage at 450 °C.</figcaption></figure>

and thus stabilize the initial reactive surface copper sites even in presence of water vapor, which is not the case for the adsorbent without Ce species (7CuO/SBA-15) [24].

### 3.3.2. Influence of the DeSOₓ test temperature

The effect of the operating temperature was studied using the 9CeO₂-6CuO/SBA-15 adsorbent, which was discovered to be the best performing with regard to the SO₂ adsorption along the cycles. A DeSOₓ test at 400 °C was carried out over 20 adsorption-regeneration cycles to determine if the synergistic effect between copper and cerium oxides, is also detectable at lower temperatures.

As already seen in a previous work for an adsorbent based on 16 wt% of CuO without CeO₂ supported on SBA-15 [23], the SO₂ adsorption capacity at the breakthrough for the 9CeO₂-6CuO/SBA-15 adsorbent is lower at 400 °C than at 450 °C (Fig. S9). It is divided by about two, going from 400 mgsO₂/gCuO at 450 °C to 210 mgsO₂/gCuO at 400 °C in cycle 20 due to the decreasing in the copper-based active phase's reactivity toward the oxidation of SO₂ with the temperature. Indeed, as shown by the breakthrough curves obtained at 400 and 450 °C with the 9CeO₂-6CuO/SBA-15 adsorbent (Fig. S10), SO₂ emissions are higher at 400 °C (230 ppmv) than at 450 °C (150 ppmv) at the end of the SO₂ adsorption stage (10,800 s). Moreover, like at 450 °C, the breakthrough curves

overlap at 400 °C from cycle 2, indicating stable performance of the material even at lower temperatures.

The total SO₂ adsorption capacities are similar at both temperatures, close to 625 mgsO₂/gCuO (Fig. 9). Indeed, even if the reactivity of the copper-based active phase towards SO₂ capture is lower at 400 °C than at 450 °C, for this low CuO content (6 wt%), the duration of the SO₂ adsorption stage (10,800 s) is long enough to sulfate most of the copper-based sites (70 % CuO sulfation rate). The SO₂ desorption curves show a different behavior between regeneration at 450 and 400 °C (Fig. 10). At 450 °C the profile of the curves presents the peak of SO₂ emissions at the beginning of regeneration (before 200 s) which is due to the fast decomposition of the surface copper sulfates. The lower emission intensity indicates reduced decomposition kinetics at 400 °C.

### 3.4. Physico-chemical phenomena involved

Based on the DeSOₓ performance and the previous characterization of the fresh and spent 7CuO/SBA-15 and 9CeO₂-6CuO/SBA-15 materials, the enhanced SO₂ trapping performance in presence of Ce and under humid conditions can be outlined in Fig. 11 and explained as the following three physico-chemical phenomena:

- A competitive adsorption of SO₂ versus H₂O in favor of SO₂ (Fig. 11a): from literature [53–57], DFT-calculations show that the adsorption energy (Table 4) is the lower for H₂O on Cu⁺ and also the lower for SO₂ on Ce⁴⁺. Moreover, it appears that at 400–450 °C under 5 vol% H₂O, 2H₂O molecules can occupy each CuO(111) site [55] and 2–8 H₂O each Cu₂O(111) site [58] while CeO₂ sites remain totally available for SO₂ capture (no H₂O molecules adsorbed on the surface [59]). Therefore, the SO₂/H₂O ratio at the surface can vary depending on the metal species, offering to 9CeO₂-6CuO/SBA-15 material the greatest number of SO₂ adsorption sites, due to the presence of CeO₂ which adsorb only SO₂ on this surface in the experimental conditions and a higher quantity of Cu²⁺, which are less sensitive to H₂O adsorption than Cu⁺.

- The addition of cerium species improves the sulfation rate of the Cu species (Fig. 11b): the adsorbed SO₂ molecules react with Cu and Ce species to form CuSO₄ and Ce(SO₄)₂. However, the electronic transfer reaction Cu⁺ + Ce⁴⁺ ⇌ Cu²⁺ + Ce³⁺ and the highly mobile

<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Evolution of the total SO₂ adsorption capacities of the 9CeO₂-6CuO/SBA-15 adsorbent, submitted to 20 adsorption-regeneration cycles with 5 vol% H₂O during the SO₂ adsorption stage at 450 °C and 400 °C.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Fig. 10.** SO₂ desorption curves of the 9CeO₂-6CuO/SBA-15 adsorbent, submitted to 20 adsorption-regeneration cycles at (a) 450 °C and (b) 400 °C, with 5 vol% H₂O during the SO₂ adsorption stage.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 11.** Representation of the physico-chemical mechanisms explaining the performance gain in SO₂ absorption in the presence of water for 9CeO₂-6CuO/SBA-15 material.</figcaption></figure>

oxygen atoms provided by Ce lead to an increase of the sulfation rate of the Cu species, particularly when Cu and Ce species are in contact.

- A more stable dispersion of the Cu active species over cycles (Fig. 11c): the active phase dispersion evolves over cycles. Without ceria, Cu nanoparticles sinter during cycling, reducing active surface area and then SO₂ trapping capacity. It is observed that CeO₂ addition stabilizes Cu dispersion through Cu-Ce interactions, strongly mitigating agglomeration and maintaining stable SO₂ uptake performance over multiple cycles.

## 4. Conclusion

The present study has demonstrated that the addition of the heteroelement Ce in the form of CeO₂ (molar ratio Ce/Cu of 0.68 and a CuO loading of 6 wt%) improves the SO₂ adsorption and regeneration capacities of CuO/SBA-15 adsorbents along cycling experiments. The high performance is maintained along cycles, even in the presence of water vapor. The interaction between CeO₂ and the CuO active phase allows

the stabilization of the dispersion of this latter along the adsorption and regeneration cycles of the adsorbent. In addition, the synergy between cerium and copper oxides drastically improves the SO₂ trapping capacities at the breakthrough of the CuO active phase with a gain of 248 % after 20 cycles of SO₂ adsorption/regeneration under wet conditions at 450 °C. Nevertheless, the results obtained for the adsorbents with the same molar ratio Ce/Cu (0.34) clearly show that the total amount of metal oxides (12 vs 25 wt%) is an important parameter which must be optimized to avoid the phenomena of migration and coalescence of the CuO active phase. Therefore, adding a heteroelement like cerium seems to be a very promising way to enhance the DeSOₓ activity in wet conditions of CuO/SBA-15 adsorbents under cycling experiments.

### CRediT authorship contribution statement

**G. Guicheney:** Writing – original draft, Methodology, Investigation, Formal analysis. **S. Dorge:** Writing – review & editing, Validation, Supervision, Methodology, Funding acquisition, Formal analysis,
**Conceptualization.** **H. Nouali:** Writing – review & editing, Validation, Supervision, Methodology, Investigation, Funding acquisition, Conceptualization. **B. Lebeau:** Writing – review & editing, Validation, Supervision, Methodology, Investigation, Formal analysis, Conceptualization. **Philippe Fioux:** Writing – review & editing, Formal analysis. **M. Soulard:** Writing – review & editing, Methodology, Conceptualization. **J. Patarin:** Writing – review & editing, Methodology, Conceptualization. **M. Molière:** Writing – review & editing, Methodology, Conceptualization. **M. Vierling:** Validation, Conceptualization. **A.C. Houdon:** Validation, Funding acquisition. **J.F. Brilhac:** Writing – review & editing, Validation, Supervision, Methodology, Investigation, Formal analysis, Conceptualization.

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

The French Environment and Energy Management Agency (ADEME), Zephyr Alsace S.A.S., and the "Fondation pour l'ENSCMu" are all acknowledged for their financial support but also the technical platforms of IS2M for the XRD analyses, nitrogen adsorption/desorption measurements and electron microscopy analyses and the LGRE for cyclic DeSOₓ tests. Authors want to acknowledge Safouan Zaïd and Léa Musard for their help in materials synthesis.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.micromeso.2025.113990.

Data availability

Data will be made available on request.

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