Benefit of LDH-Derived Mixed Oxides for the Co-Oxidation of Toluene and CO Exhausted from Biomass Combustion

Abstract

The proposed study is devoted to highlighting the importance of mixed oxides preparation through the layered double hydroxide route for undesirable gas pollutants abatement. Different series of Cu/Al/Ce mixed oxides with similar or different stoichiometrics were prepared and compared for toluene and/or CO oxidation. Catalyst synthesis methods influence material properties and activity for oxidation reactions. The high activity for the oxidation reactions of mixed oxides derived from LDH is explained by the Cu/Ce synergy. The presence of CO in the CO/toluene mixture does not affect the total toluene oxidation, and the toluene does not affect the total oxidation of CO conversion at low temperatures. The most effective catalytic material (Cu₆Al_1.2Ce_0.8) presents a long lifetime stability for total toluene oxidation and resistance to CO poisoning in mixtures.

1. Introduction

During the last decade, increasing interest has been directed towards the research of environmental pollution control and fossil fuel replacements for domestic heating using biomass as an alternative resource [1,2,3]. Organic raw feedstocks with low costs, WORLDWILD availability and suitability of use are ideal candidates and considered as a tool and technology for fulfilling the goals of carbon neutrality, preventing climate change and bioresource use [4]. However, the use of biomass as fuel during combustion can generate a mixture of pollutants containing several molecules of different types: CO, Volatile Organic Compounds (VOCs) and NOx, with various concentrations [5,6,7,8]. Thus, the emissions from non-efficient biomass combustion can be higher than those from fossil fuel burning [9]. The exhausted effluents address, in the presence of hazardous substances with different proportions in the environment, problems associated with healthiness while additionally having a negative impact on the greenhouse effect [10]. Particularly, CO and toluene are well known as highly toxic and seriously harmful to human health.

The catalytic oxidation of CO and toluene is a highly desirable solution [11,12,13,14]. The pollutants will be converted directly to harmless gases with a simple process and at relatively low temperatures [15]. Nevertheless, highly efficient, low-cost and environmentally friendly catalysts remain ambiguous. Toluene oxidation involves abundant intermediates with strong chemical activity and hence the rapid total degradation of CO₂ and H₂O with no release of side products, which are pollutants to the environment [16]. Oxidation with solid catalytic materials is a well-studied and industrially useful process [17,18,19]. The majority of heterogeneous catalysts employed for the pollutant remediation are based on Pt and Pd noble metals [20]. However, from an economic perspective, the high-priced catalysts make their industrial application challenging. Low-cost mixed metal oxides containing Co, Cu, Al and Mn or lanthanide metals with good catalytic performance and stability can offer an opportunity to carry out VOCs and CO oxidation at optimum reaction conditions [21,22,23,24,25,26,27,28,29,30].

Recently a significant achievement has been made in the catalytic oxidation of toluene and CO by copper- and cerium-based mixed oxides [31,32,33,34,35,36]. Nevertheless, it is still a challenge to adjust different frameworks to fully understand the reaction process and catalytically oxidize toluene and CO with high efficiency. Few studies have been published on the heterogeneous catalysis oxidation of toluene in the presence of others [37]. The reaction behavior and mechanism are probably quite different from those of the catalytic oxidation of individual pollutants. For instance, S. Mo et al. [38] found that toluene oxidation was improved in the presence of CO with Co₃O₄-based catalysts in contrast to Pt/Al₂O₃. Unfortunately, CO transformation over used catalysts was inhibited significantly by toluene, even though the reaction oxidation mechanism of both molecules was not affected. The same group studied a series of Pt-based supported metal oxide catalysts. One can prove that using CeO₂ support effectively alleviates the mutual inhibition between CO and toluene oxidation. The research paper describes the importance of oxygen storage in catalysts for the competitive adsorption of both molecules and, consequently, for enhancing the catalytic activity. Additionally, F. Bi et al. [39] found that when using non-oxide C₃N₄-supported Pd or Pt, the catalytic efficiency of CO was inhibited in the presence of mixed components. Furthermore, in S. B. Kang’s publication works [40,41], the CO poisoning effect on multi-VOCs (including toluene) over bimetallic Pd/Pt-supported alumina powders and monoliths was explored. The authors claimed that CO inhibition can be controlled by engineered monolith catalysts, where the metal ratio and the design of multiple zone systems were considered.

However, using a non-noble metal could be an interesting method. Indeed, in our previous studies, mixed oxides derived from a Layered Double Hydroxide (LDH) structure (denominated as Mg/Al/Ce and Co/Al/Ce) were tested for the toluene oxidation in the presence of CO [42,43]. These studies revealed an improvement in the toluene oxidation activity performance when toluene was in a mixture with other pollutants. It can be clearly observed that the reaction intermediates presented are quickly different from single to mixture reaction oxidation. Nonetheless, the catalytic CO oxidation in the presence of the mixture was not discussed.

It is therefore clear that proposing and developing more efficient catalysts and understanding the active sites on toluene and CO mixture oxidation are of fundamental importance. The present work approaches some degree of VOCs abatement in real conditions. The study reports different preparation methods of mixed oxide catalysts for toluene/CO simple or mixture oxidations. Mixed oxides with a metal molar ratio of Cu/Al/Ce of 6/1.2/0.8 with different textural and structural properties were synthetized and used in the same conditions. Their characterization and catalytic activity were investigated. Furthermore, the effect of Ce ratio loading on the mixed oxide structure-derived LDH was evaluated and optimized. In addition, the effect of the CO concentration on the mixture feed and, finally, the catalyst reuse was considered.

Thus, the novelty of this work is it exemplifying that preparing mixed oxides via the layered double hydroxide method is a promising strategy for developing robust, stable catalysts without noble metals to treat pollutants (VOCs and CO) issued from wood combustion.

2. Results and Discussions

The analytical section (Section 3) covers the preparation method and nomenclature of the studied materials.

2.1. Catalysts’ Characterization

The catalysts’ structural parameter values obtained by ICP are listed in Table S1 of the Supplementary Materials and showed the same nominal proportion of metal in the ternary oxides. X-ray patterns of the prepared mixed oxides and JPCDS cards of their plausible corresponding phases are shown, respectively, in Figure 1 and Figure S1. CuO and CeO₂ diffractograms showed that single metal-containing samples are found, respectively, as an oxide phase with XRD reflections corresponding to the pure monoclinic CuO (JCPDS card N°48-1584) and pure cubic CeO₂ (JCPDS card N°34-0394). A copper oxide phase appears for all the mixed oxides; the samples reveal the presence of CuO plane families with similar forms to that of the single metal oxide. The same goes for the Ce; when it is present, catalysts displayed several diffraction patterns corresponding to the CeO₂ structure. For the mechanically mixed pure metal oxides, the structure of the α-Al₂O₃ (JCPDS card N°46-1212) phase with a rhombohedral form was evidenced. Unfortunately, the Al diffractions were absconded for other catalysts, even though the element was loaded in the mixed oxide’s structure. This is mainly due to the high dispersion with a low average size or the Al element being in an amorphous form. Indeed, a close inspection of the mixed oxides derived from the LDH pics reveals q low intensity, which suggests that crystallites are divided nearby.

Table 1. Calculated structural and textural parameters of the prepared metal oxides.

Sample	Crystallite Size (nm)		BET Surface Area (m2/g)
	CuO	CeO2
CuO	44.4	-	14
CeO2	-	9.0	95
Cu6Al2	17.0	-	17
Cu6Al1.2Ce0.8	15.2	3.3	47
CuO-Al2O3-CeO2	44.0	9.4	14
(Cu6Al2-HT + Ce(NO3)3)500	25.1	9.4	27
(Cu6Al2 + CeO2)500	21.0	4.0	25

summarizes the calculated average crystallite size of both the CeO₂ and CuO phases. The XRD analysis of CuO powder indicated the presence of bulky nanocrystals or even suggested the presence of large CuO monocrystals. In contrast, the CeO₂ phase ratifies the presence of crystallite with less than 10 nm for all samples. The detected values for the LDH-derived metal oxides structure for CuO range between 15 and 21 nm, while the CeO₂ was around 3–4 nm. The size of both phases for the CuO-Al₂O₃-CeO₂ sample remain unchanged after the mechanical mixing of the three pure metal oxides. The cubic crystallite size was unaffected when ceria was mixed with the Cu₆Al₂ sample, but the CuO phase became slightly smaller. Copper’s presence in the LDH skeleton before calcination ensures the formation of a lower crystal size, with the presence of Ce becoming lower. Thus, a small crystallite size of these phases was observed for the Cu₆Al_1.2Ce_0.8 catalyst.

The textural properties of the mixed oxides were investigated in terms of the BET-specific area (Table 1). The prepared single-metal oxide CeO₂ presented high values (95 m²/g), in contrast to the as-received CuO oxide calcined powder (14 m²/g). Similar to the structural parameters, CuO-Al₂O₃-CeO₂ have the same textural properties as the raw copper oxide. The specific surface area of the other mixed oxides ranges between the corresponding parameters of the pure metal oxide. Cerium’s presence in the sample increases the measured BET surface, and then Cu₆Al_1.2Ce_0.8 texture was two times higher compared to that of other mixed oxides. The porosity of the calcined LDH is ensured by the interlamellar space of the precursor. Moreover, in mixed oxides, the nearby separate aggregates of both crystallites with lower sizes permit the formation of a rich abundant-pores surface.

H₂-TPR analysis was used to determine the reducing capability of the catalysts (Figure 2). Ceria reduction was not observed above 500 °C. Pure CuO with bulky crystallites present one broad region with a maximum temperature T_max = 320 °C, corresponding to the reduction of the Cu²⁺ cation to Cu(0) [44] The same characteristic peak in this region appears in the case of CuO-Al₂O₃-CeO₂ mixed oxides but at lower T_max values with a higher consumption of hydrogen. Therefore, the direct physical mixing of metal oxide powders provides a negligible synergetic interaction between the presented metals.

The H₂-TPR profile for the other samples was deconvoluted into three regions, α, β and γ [45], using Gaussian fitting. The peak α is assigned to the interfacial strongly interacted copper with ceria or even the nearby aluminum issued from the oxidation of the LDH skeleton [46]. The second T_max peak appearing at a temperature range of 200 to 260 °C is most probably due to the reduction in separate CuO crystallites with a low average size. The α and β are assigned directly to the lattice vacant and mobile oxygen species. The third peak is associated with the bulky CuO clusters that are hardly reduced. The summit’s position and H₂ consumption (Table S2) change with the mixed oxides preparation method. The high-temperature positions of two peaks of the (Cu₆Al₂ + CeO₂)₅₀₀ are similar to that of their primary precursor Cu₆Al₂; however the lowest onset reduction temperature was extended, and the hydrogen consumption region was greater.

Mixing a cerium nitrate precursor with the LDH structure (Cu₆Al₂-HT) before oxidation generally downgraded the T_max of the mixed oxide. Nonetheless, the hydrogen consumption of the third region was still high due to the big crystallite size obtained with this sample. In contrast to others, the Cu₆Al_1.2Ce_0.8 T_max of the three-reduction peaks was significantly inferior, which indicates a huge reducibility character. A high consumption of H₂ was depicted for the α, β position, contrary to the γ part, where the hydrogen uptakes were at the bottom. With this sample, the redox properties are stronger and the reduction in the scarceness of the bulky crystallite is easier [47].

The amount of copper and the textural properties of the catalysts are other factors that may influence the reducibility of the catalyst. The H₂ consumption in the γ region was increased in the opposite direction to the T_max from single to ternary oxides. This is directly related to the reduced copper content associated with the increased specific surface area.

The catalyst’s characterization spotlights the influence of the preparation method and the Ce loading on the textural–structural properties of the mixed oxides. LDH structure precursors for mixed oxides preparation present highly well dispersed copper oxide. Moreover, Ce’s presence, in general, lowers the CuO average crystallite size and induces a new couple redox. The interfacial one is assured by the presence of the metal on the LDH skeletons before oxidation. Nearby Ce⁴⁺ and Cu²⁺ from the LDH calcination interface substitution had the same ionic radii and then improved the lattice oxygen vacancy [48].

2.2. Catalytic Tests

2.2.1. Individual Toluene Oxidation

The light-off curves of total toluene oxidation that present the conversion as a function of the temperature are shown in Figure 3. In general, the carbon balance for toluene conversion during all experiments was superior to 85%. The detected reaction products were CO₂ and H₂O, as well as traces of CO and benzene at a low conversion. No reaction intermediates are detected; small molecules are oxidized rapidly and are barely accumulated within the catalyst surface before the transformation to CO₂ and H₂O.

Toluene conversion under air was affected by the type of metal oxides used. The detailed data for 10, 50 and 90% toluene conversion are listed in Table S3. The T₅₀ (temperature corresponding to 50% of the conversion) of the toluene results indicated that CuO is the less active catalyst, among others. In the presence of Ce, the T₅₀ over prepared oxides shifted to a lower temperature by the presence of a new induced redox couple. The Cu₆Al₂ activity is comparable to that for ternary oxides prepared by a mechanical mixture. The T₅₀ is between 292 and 309 °C, while their corresponding T₉₀, in general, was higher than 320 °C.

Single-oxide ceria with a high specific area set out a good activity for toluene oxidation with T₅₀ = 277 °C and T₉₀ = 312 °C, respectively. However, the results are still far from those obtained by Cu₆Al_1.2Ce_0.8. An excellent activity was achieved by ternary mixed oxide derived from LDH with a T₅₀ at ~254 °C and a T₉₀ at ~277 °C.

At higher temperatures the pure CuO ranks better than some mixed metal oxides. At high temperatures, the bulky CuO becomes active, but this can be expected for the sintering of highly separate, small CuO crystallites presented on the mechanically synthetized mixed oxides CuO-Al₂O₃-CeO₂.

In combination with the catalytic and characterization results, an increase in the specific surface area leads to an upward catalytic activity (Table 1). The higher surface leads to towering high oxygen storage, and consequently, more toluene transformation occurs. Such behavior is explained by the high activity of single ceria and Cu₆Al_1.2Ce_0.8 catalysts. The comparison results showed that mixed oxides samples cover the highest reducibility at low temperatures by interfacial oxygen vacancies, and the small crystallite size of copper oxide/cerium oxide with a good specific surface can monitor toluene oxidation easily.

2.2.2. Individual CO Oxidation

As for CO oxidation, which is directly transformed to CO₂, Cu₆Al_l.2Ce_0.8 still shows the best performance on individual molecule abatement, but this time at real low-grade temperatures (Figure 4).

The T₅₀ and T₉₀ values were reduced, respectively, from 254 and 277 °C for single toluene oxidation to 91 and 119 °C for single CO oxidation (Table S4). Outbidding oxidation performances for CeO₂ were totally decreased, and the catalyst reached no more than 20% CO conversion at T = 250 °C. The activity order for other catalysts also changed. Calcined commercial CuO powders reached 50% CO conversion before the (Cu₆Al₂ + CeO₂)₅₀₀ catalyst, and the T₉₀ was significantly less than that of both the mentioned and CuO-CeO₂-Al₂O₃ mixed oxides. (Cu₆Al_2-HT + Ce(NO₃)₃)₅₀₀ ranked third for CO total oxidation, and this likewise true for toluene. The CO oxidation rate is much higher for the Cu₆Al_1.2Ce_0.8 catalyst sample, followed by Cu₆Al₂, where the Ce is not present.

Herein, CO conversion to CO₂ occurred at distinct active sites regarding toluene, and the Cu₆Al_1.2Ce_0.8 sample exhibited particular crucial sites that are available in oxidizing both molecules with enhanced efficiency. It seems that CuO is the active phase for CO oxidation with regard to ceria and the raw binary metal oxides Cu₆Al₂ activities.

From the results, the most likely factor for CO transformation over mixed oxides is that the low crystallite size of CuO can be easily reduced, and therefore, the activity of CO oxidation is much higher.

Catalytic oxidation is related to the redox properties and structural–textural parameters. The redox couples in our catalysts are the Ce⁴⁺/Ce³⁺, Cu²⁺/Cu⁺ and Cu⁺ species results of the interfacial redox interactions between the CuO and CeO₂ phases [49]. It is suggested that, in the case of toluene oxidation, the three mentioned couples are active, and they have the same potential for oxidation reactions. However, it is accepted that the CO mechanism occurs mostly by the Mars–van Krevelen reaction mechanism over the Cu-based catalyst, in which Cu undergoes oxidation and reduction reactions via the oxygen vacancy lattice [50].

Thus, CO oxidation needs a catalyst reoxidation scope, given by both the surface oxygen exchange ability and oxygen mobility. Meanwhile, toluene oxidation indicates that just the oxygen swap capacity is sufficient. However, as the Cu/Ce synergistic effect accelerates the redox cycling rate for CO transformation, it could also improve the overall toluene oxidation by increasing the free energy adsorption of the reaction intermediates, facilitating rapid CO₂ production from the aromatic molecule.

The catalytic efficiency highlights once more the importance of the LDH route for CuCeAl mixed oxides preparation. With Cu₆Al_1.2Ce₈, toluene or CO molecules could be directly oxidized by adjacent active oxygens around the CuO/CeO₂ interfacial sites, and gaseous oxygen was then quickly captured and dissociated to continuously produce new active oxygen species. Also, satisficing the specific surface area leads to an increase in the defect site concentration, and therefore, the oxidation dispatch was improved.

2.2.3. Toluene and CO Co-Oxidation

The catalytic exhaustion of a toluene and CO gas mixture by oxidation over the prepared mixed oxides was investigated, and the tests results are shown in Figure 5. The T₅₀ of toluene (Table S5) for both single-metal oxides (CuO, CeO₂) were displaced to higher temperatures by 10–13 °C.

The worst CuO sample activity drops down more in the mixture, and CeO₂ catalytic behaviors becomes quasi-similar to the mechanically mixed solids. Mixed oxides show similar scenarios as the individual toluene oxidation, with a maximum shift to a lower temperature of 4 °C. This can indicate that CO did not inhibit the toluene oxidation, in contrast to some previously examined supported noble metal catalysts [37,39]. The Cu₆Al_1.2Ce_0.8 sample still led and achieved 50% conversion at a slightly lower temperature compared to when it was singly oxidized. The low-temperature moderate toluene conversion by the mixed oxide in a mixture is mainly due to some change in the oxidation mechanism and the presence of suitable and available active sites for converting toluene in the presence of CO. Also, it is suggested that toluene transformation to intermediates can be processed by the produced CO₂ or even CO [51,52].

On the other hand, all Cu-based catalysts reach total CO oxidation in the mixture before toluene (Figure 6). The unfunctionalized aromatic molecules adsorb relatively weakly on the mixed oxides regarding the CO. Exclusively for the Cu₆Al_1.2Ce_0.8 sample, CO oxidation performances in the mixture at conversion values inferior than 50% were significantly decreased, with small alterations in the catalytic order. Ternary mixed oxides rank better than Cu₆Al₂. However, despite being active and still reaching a high total CO oxidation before others, the Cu₆Al_1.2Ce_0.8 tendency for total CO conversion during the oxidation of a mixture was decreased compared to that observed for single oxidation. At low temperatures, the oxygen reaction with toluene exerted a high exothermicity character that would increase the local temperature on the catalyst surface, improving the CO transformation at temperatures below 115 °C. Nevertheless, the curve allure for Cu₆Al_1.2Ce_0.8 displayed a steady state at a temperature range of 115–166 °C for CO oxidation in the mixture. This is the well-known poisoning effects of second components on VOCs abatement relative to competitive adsorption [53]. This is likely due to the fact that non-reacted adsorbed bulky toluene molecules barely accumulated on the surface of the catalyst, reducing the available oxygen and the redox cycle rate. Therefore, the reaction entered the slowest step until the intermediate products of toluene were completely converted to small molecules and then oxides together with CO and CO₂ and, finally, gaseous oxygen replenished the active oxygen consumed in the reaction process.

It was demonstrated by several in situ insights that toluene and CO oxidation intermediates are bicarbonate bidentate with the active sites before CO₂ formation [53,54,55]. At the moment, when the temperature conditions favor high reactive adsorption, the active sites will be saturated and prevent a redox cycle. By increasing the temperature, CO₂ forms easily and the CO transformation process can be further promoted.

However, the T₅₀ of CO oxidation in the presence of toluene for the Cu₆Al_1.2Ce_0.8 catalyst is much lower than the ones obtained over recently studied catalysts [37,38,39,56].

2.2.4. Influence of the Ce Fraction on the CuAl_2−xCe_x Structure for Toluene and CO Co-Oxidation

The cerium proportion of the mixed oxides has a crucial impact on the catalytic performances of CO and VOCs abatement. In this sense, the influence of the Ce ratio loading on the Cu₆Al_2−xCe_x (x = 0, 0.2, 0.4, 0.6, 0.8) structure prepared through the LDH method for the toluene/CO mixture co-oxidation was studied. From Figure 7 with respect to the Cu₆Al₂ sample, all the series comply with the same order for single toluene and CO oxidation. In general, whatever the catalyst, T₅₀ values for toluene conversion in the mixture are consistent with those of simple feed (Table S6). Cu₆Al_1.2Ce_0.8 and Cu₆Al₁.₄Ce₀.₆ catalysts exhibit the highest catalytic activity (T₅₀ = 220 °C), while such decreases for the Ce fraction could slow down the oxidation process. The increase in the Ce content causes more amorphosity by promoting the presence of more defects. This is reflected by the decrease in the average pore size while the specific area and the total pore volume are increased (Table S7).

Here, this amorphosity of the materials could be used to induce more reactive coverage and more efficient reducibility, as can be seen by TPR (Figure 8).

However, at some point, further Ce loading allows for maintaining stability or slightly declining the toluene catalytic oxidation by extensively reducing the copper content, negatively affecting the same parameters. The Cu content decreases with increasing Ce on the catalyst. Nevertheless, the further loading of Ce from 0.6 to 0.8 allows for decreasing the specific surface area and the high hydrogen consumption in the β region. This is suggested by the presence of separate CeO₂ that decreases the number of oxygen vacancies related to the Cu/Ce interfacial surface.

Concerning the effect of the cerium content for the CO oxidation, Figure 9 illustrated that for the increase in CO₂ formation from single CO transformation, the higher reducibility at low temperatures is important. Meanwhile, CO oxidation in the mixture is more prone to inhibition by Cu/Ce interfacial oxygen poisoning.

CO conversion was displaced to a higher temperature with a great detected shift for the Cu₆Al₂ sample. The ternary oxides (Cu₆Al_2−xCe_x) reach total CO co-oxidation at approximately similar temperatures, although the catalytic performance at low temperatures is different and directly related to the reductive capacity at the peak position β, like toluene. Between 115 and 166 °C, the CO conversion rate begins to decrease. As the amount of cerium increases, it becomes more pronounced and weaker until the curve shows a stable pattern with cerium contents of 0.8 and 0.6. Therefore, the same interfacial oxygen vacancy lattice that participated for more activity was highly affected by strong CO poisoning. (Full Cu₆Al_2−xCe_x series characterization and detailed discussions have been reported in our previous study [57]).

2.2.5. CO Concentration Effect on the Toluene Oxidation in Mixture

Carbon monoxide is rottenly presented on the gas effluents and has a major impact on the oxidation efficiency. The co-oxidation of a mixture of different CO concentrations over our optimum catalyst does not affect the toluene transformation profile (Figure 10).

In general, at low temperatures, the mixture oxidation activity decreased with increasing CO concentrations. The CO conversion values at a temperature of 115 °C were diminished from 81% for 500 ppm CO to 55% for 2000 ppm. CO transformation is limited by the availability of active oxygen species. Interestingly, for a 1000 ppm concentration of CO, the conversion was slightly better in the mixture. The exothermic oxidation reaction of the mixture could induce the rapid conversion CO rate by the local surface temperature increasing. From another part, the steady region (115–166 °C) was affected by the same parameter where the CO rate decrease started to be more apparent. Without toluene, the empty surface-active sites have a scope for CO coverage and reaction rate promotion by the interfacial oxygen vacancy. However, when toluene is present and the total carbon concentration in the stream increases, the available oxygen is limited and the interfacial redox cycle is slowed down.

2.2.6. Catalyst Reuse

The catalyst’s stability was assessed by performing four consecutive light-off test cycles (Figure 11).

No significant change in the toluene oxidation was noticed with a 1000 ppm CO concentration in the mixture at lower temperatures. Nonetheless, a small drop-in at higher-temperature total toluene conversion followed by stable activity can be observed after the first cycle. It is therefore possible to reuse the catalyst after several heat treatments under toluene/CO flow. TGA-DSC is an useful tool for coke amount formation detection (Figure S2).

The characterization of the raw and used Cu₆Al_1.2Ce_0.8 sample with thermogravimetry analysis indicated that the presented physiosorbed water molecules after the reaction were hardly released, no carbon was deposited and no active sites leaching happened. This means that our catalyst should maintain its operation at a long duration after the first cycle without change and suggests that the slight loss of activity for the first time can tentatively be attributed to the further reduction of the Cu²⁺ small crystallite size to Cu metallic at higher temperatures.

3. Analytic Section

3.1. Reagents

Copper nitrate trihydrate (Cu(NO₃)₂ 3H₂O, 98%), aluminum nitrate nonahydrate (Al(NO₃)₃ 9H₂O, 98.5%), cerium nitrate hexahydrate (Ce(NO₃)₃ 6H₂O, 99.5%), sodium hydroxide (NaOH, 98%), sodium carbonate (Na₂CO₃, 99.5%), copper oxide (CuO, 100%), alumina (Al₂O₃, 100%) and toluene (C₇H₈, 100%) all purchased from Thermo Scientific Research (ThermoFisher Scientific, Illkirch, France), as well as carbon monoxide 1% CO/N₂ (Messer, Suresnes, France), were used for the chemical synthesis of materials or catalytic testing in the case of the last two named compounds.

3.2. Catalysts Preparation

A cerium hydroxide obtained by cerium nitrate hexahydrate precipitation in the presence of sodium hydroxide was calcined at 400 °C for 4 h using a 1 °C min⁻¹ ramp under an air flow of 4 L·h⁻¹ to finally obtain the pure cerium oxide (ceria) CeO₂ sample.

The preparation method of the mixed oxides Cu₆Al_2−xCe_x with x = 0–0.8 through the LDH route is described elsewhere [41]. In brief, an appropriate amount of metal salts with the required molar ratio of Cu²⁺/M³⁺ (M = Al_2−xCe_x) of 3/1 was dissolved in 200 mL of an aqueous solution and dropwise added to a beaker containing 50 mL of a Na₂CO₃ solution (1 mol·L⁻¹). The pH of the solution was maintained constant (10.5) by sodium hydroxide. When proceeding, the final mixture was continuously aged for 18 h in an atmospheric environment at room temperature. Then, the precipitate formed was filtered and washed with hot distilled water (60 °C) several times. The hydrotalcite solid collected was then dried in an oven for 72 h at 60 °C before grinding. Finally, the mixed oxides were obtained by the calcination of the LDH at 500 °C for 4 h.

The properties of the mixed oxides obtained through the LDH method were compared using other methodologies. Different oxides were prepared with the same molar ratio of the Cu₆Al_1.2Ce_0.8 sample. The CuO-Al₂O₃-CeO₂ catalyst was prepared by the calcination of the physically mixed prepared CeO₂ with the as-received, commercial CuO and Al₂O₃. The individual metal oxide precursors were well mixed and treated for 4 h at 500 °C.

On the other hand, a sample of (Cu₆Al₂ + CeO₂)₅₀₀ was obtained by mechanically mixing the Cu₆Al₂ sample with an appropriate amount of CeO₂. The mixed oxides then also calcined at the same conditions. Finally, the as-prepared Cu₆Al_2-HT material precursor of Cu₆Al₂, with a desired amount of Ce(NO₃)₃, was mechanically mixed and grinded and then treated by heating under air flow at 500 °C to obtain the finally labelled (Cu₆Al₂-HT + Ce(NO₃)₃)₅₀₀ catalyst.

3.3. Catalyst Characterization

The real metal proportions on the sample were analyzed using inductively coupled plasma-optical emission spectroscopy (ICP-OES, iCAP-6300-DUO, Thermo Electron (Thermo Fisher Scientific, Illkirch, France). The metal oxide was dissolved in aqua regal under a microwave for 30 min and then diluted extensively before the analysis.

The XRD analysis of the crystalline structure was carried out at room temperature with a Bruker D8 Advance diffractometer (Bruker AXS, Billerica, MA, USA) equipped with a LynxEye Detector and Cu anode (λ = 1.5406 Å). The scattering intensities were obtained by a continuous scan mode at an angular range from 10 to 80º using a 0.02° step-size and a count time of 4 s per step. For each pattern, structural determination was performed by comparison with the “Joint Committee on Powder Diffraction Standards” (JCPDS) files.

The Scherrer equation was applied using the signals corresponding to the (111) of CeO₂ and (11-3) of CuO planes (Equation (1)) to estimate the size of the presented crystalline oxide phases:

$$Lc=\frac{K \lambda }{\beta \cos \theta }$$

(1)

$\lambda$= 1.1506Å, $K$= 0.9, $\theta$ is the peak position of the detected plan and $\beta$ is the full width at half maximum (FWHM) of the peak plane $\theta$ position.

The catalysts’ textural properties were obtained by nitrogen physisorption conducted on QSurf M1 apparatus thermoelectron equipment (Thermo Fisher Scientific, Illkirch, France). Prior to the N₂ molecule being adsorbed, the sample was degassed at 300 °C under vacuum before cleaning the surface, and the pressure remained constant. The specific surface area was calculated from the N₂ adsorption–desorption at −196 °C using the Brunauer–Emmett–Teller (BET) equation [58,59].

The temperature reduction of the catalytic materials was identified by means of a temperature-programmed reduction (H₂-TPR) performed using an AMI-200 instrument (ZETON ALTAMIRA, Cumming, GA, USA). In total, 30 mg of the catalyst emerged in a U-shaped quartz tube reactor. The catalyst was first pretreated at 250 °C for 2 h using pure Argon at a flow rate of 30 mL·min⁻¹ to remove water adsorbed on the catalyst surface. The steam outlet from the reactor was connected to the thermal conductivity detector (TCD) for H₂ analysis. After surface cleaning, the temperature of the reactor was cooled down, and then the gas was switched to 5% v/v H₂/Ar with a 20 mL·min⁻¹ constant flow rate and kept constant until the detector indicated a stable baseline. The sample was then heated from room temperature to 400 °C under the same flow (5 °C·min⁻¹), and simultaneously, the hydrogen uptake behavior profile of the sample was registered and analyzed.

TGA-DTG thermogravimetries analysis was performed under the presence of synthetic air from ambient temperature to 1000 °C (5 °C·min⁻¹ ramp) in SDT Q600 (TA Instruments, New Castle, DE, USA) equipment.

3.4. Catalytic Tests

The catalytic toluene and/or CO oxidation tests were carried out in a U-shaped quartz continuous flow reactor at atmospheric pressure with 100 mg of a fixed-bed catalyst. Before the experiment, the sample was activated in the presence of air flow (2 L·h⁻¹) at 500 °C for 2 h. After pretreatment, a 100 mL·min⁻¹ reactive gas flow (1000 ppm of C₇H₈ and 20% O₂ balanced by helium) was adjusted by a Michell apparatus consisting of a saturator and mass flow controllers. After stabilization, the total gas flow was passed through the catalyst bed reactor, the temperature was increased from room temperature to 400 °C (1 °C·min⁻¹), and simultaneously, the quantitative analysis started to be carried out. The resulting gas mixture was analyzed online by an agilent 490 Micro gas chromatography (Agilent Technologies Inc., Santa Clara, CA, USA) for toluene quantification and an infrared analyzer 4400 IR (ADEV, Cesano Maderno, Italia) for CO and CO₂.

Toluene conversion was calculated as a function of the carbon number for each compound of the reaction [39].

$$X_{Tol}^T(\mathrm{\%})=100\times \left(\frac{\left({{CO}_2}_{Ana}^T-{{CO}_2}_{CO}^T\right)+{6\times C}_6{H}_{6,T}}{\left({{CO}_2}_{Ana}^T-{{CO}_2}_{CO}^T\right)+{6\times C}_6{H}_{6,T}+{7\times C}_7{H}_{8,T}}\right)$$

(2)

where

• $X_{Tol}^T$: toluene conversion at temperature T;
• $C_6{H}_{6,T}$: analyzed benzene concentration at temperature T;
• $\left({{CO}_2}_{Ana}^T-{{CO}_2}_{CO}^T\right)$: CO₂ concentration produced from toluene by the following equation:

$${{CO}_2}_{CO}^T={CO}_{converted}=({CO}_0-{CO}_{Ana}^T)$$

(3)

where

• ${CO}_{converted}$: CO converted at temperature T;
• ${CO}_0$: initial inlet concentration of CO;
• ${CO}_{Ana}^T$: obtained CO concentration at temperature T by the infrared analyzer;
• ${{CO}_2}_{CO}^T$ refers to the detected concentrations of CO₂ produced from CO.

The CO conversion was calculated based on the ppm concentration indicated by the infrared analyzer using Equation (4).

$$X_{CO}^T(\mathrm{\%})=100\times \frac{\left({CO}_0-{CO}_{Ana}^T\right)}{{CO}_0}$$

(4)

4. Conclusions

The synthesis method affects the distribution of active sites in the mixed oxides. Mixed oxides derived from LDH result in highly dispersed small crystallites. The presence of Ce in the LDH seems to narrow the CuO phase size of the final obtained mixed oxides. The cerium active sites in the mixed oxides not only increase the reducibility of the materials but also create an interesting new redox couple.

Toluene oxidation over CuAlCe-based mixed oxides is related mainly to the number of different oxygen vacancies. However, the conversion of CO to CO₂ is linked directly to the presence of oxygen lattice mobility, where the CuO is the active center. CO inhibition in toluene/CO mixture co-oxidation is attributed to the competing adsorption of the molecule but also to the active site’s saturation and the redox cycle ignition. Consequently, the Cu/Ce fraction in the final oxide-derived LDH preferentially influences catalyst properties and can highly alter the CO oxidation in mixtures. CuAlCe mixed oxides derived from LDH make the CO poisoning of toluene oxidation insignificant.

Within all series, the superior catalytic performance of toluene and/or CO oxidation is achieved by the Cu₆Al_1.2Ce_0.8 sample due to a distinguished diversity of active centers, such as a good crystallite shape and specific area and a great Cu/Ce synergy. The best operated mixed oxide is stable and maintains toluene oxidation with a high concentration of CO in the mixture stream during four cycles of reutilization. Thus, the material Cu₆Al_1.2Ce_0.8 appears to be a promising catalyst for application in the treatment of pollutants issued from biomass combustion.