2.1 Catalysts synthesis

The zirconium oxide support was prepared by a precipitation method, adding dropwise an aqueous solution of zirconyl (IV) chloride to an ammonia solution under continuous stirring. The precipitate was filtered and washed to remove remaining chloride ions [16]. This solid was dried at 115 °C for 24 h and subsequently calcined under air flow (2 L h⁻¹) at 300 °C for 4 h [1,4,11].

The alkali/ZrO₂ systems were obtained by impregnation of aqueous alkali carbonates on a ZrO₂ support, with an initial specific area of 201 m² g⁻¹.

Alkali–Co_0.1/ZrO₂ catalysts were prepared by co-impregnation of cobalt (CoCO₃·nH₂O) and an alkali carbonate onto ZrO₂ solid. Alkali–Cu_0.1/ZrO₂ catalysts were prepared with the same method (using CuCO₃·nCu(OH)₂).

After drying, the samples were calcined under air flow (2 L h⁻¹) at 600 °C for 4 h. The as-obtained solids will be denoted by alkali_x–M_0.1/ZrO₂ where M is the symbol of the impregnated transition metal, x and 0.1 correspond respectively to alkali/Zr and M/ZrO₂ atomic ratios.

2.2 Catalysts characterization

The chemical composition of the samples was determined by ICP–MS with a Varian 820 MS apparatus.

TG–DTA experiments were studied with a Netzsch STA 409 apparatus. About 30 mg of dried catalyst were loaded in an alumina crucible and heated from room temperature to 1000 °C (heating rate: 5 °C min⁻¹) in air flow (75 mL min⁻¹).

BET surface areas were measured by nitrogen adsorption at 77 K on a Qsurf M1 apparatus (Thermo Electron Corporation).

FTIR experiments were performed with pellets prepared by pressing about 200 mg of catalyst powder (1 wt.%) diluted in KBr. Measurements were recorded on a Bruker Equinox 55 FTIR spectrometer at ambient temperature in the 4000–400 cm⁻¹ range. Spectra were recorded with a resolution of 2 cm⁻¹ and by co-addition of 64 scans. In situ DRIFT spectra were recorded on the same apparatus equipped with accessories for the diffuse reflectance and a room environment stainless steel manufactured by Harrick Scientific Corporation. A double thermocouple system can regulate the temperature inside the chamber and near the sample. The gaseous flow passing through the chamber is controlled by mass flow controllers. The pretreatment (350 °C for 4 h) and the catalytic test have been performed under the same conditions (gas flow rates, temperature ramp), but with 30 mg of pure sample. All spectra are presented in absorbance after subtracting the spectrum of the reference sample (KBr powder).

XRD measurements were performed by a Bruker D8 Advance diffractometer using CuK_α radiation (λ = 1.5406 Å) in the 2θ range 20–55°, with a step size of 0.02° and an integration time of 6 seconds.

Carbon black (CB) Printex U (from Evonik) was chosen as model for soot particles oxidation. Its chemical mass composition in percentage is: C = 92; H = 0.95; O = 4.4; N = 0.32 and S = 0.57. The specific area of the CB reaches 100 m² g⁻¹ [17]. The CB–catalyst mixture was performed and prepared in loose contact conditions [1,18]. Six percent of CB and 94% of catalyst were introduced in a small flask and were simply shaken for 20 minutes. The catalytic test towards CB oxidation was studied by TG–DTA measurements. About 30 mg of CB–catalyst mixture were loaded in an alumina crucible and heated from room temperature to 650 °C (heating rate: 5 °C min⁻¹) in air flow (75 mL min⁻¹). TG–DTA results will be used to determine catalytic performances and CB oxidation rate.

The oxidation of toluene was carried out in a continuous flow reactor with a fixed bed at atmospheric pressure. Before each test, the catalyst (100 mg) was dried in air (2 L h⁻¹) at 500 °C (1 °C min⁻¹) for 4 h in order to remove the hydroxyl groups, water or moisture adsorbed on the catalyst and thus to activate it [19]. The feed gas reactant mixture was made of 1000 ppm of toluene and 10% O₂ in N₂ (total flow rate: 100 mL min⁻¹). After a stable feed, it passed through the catalyst and the temperature was increased from room temperature to 500 °C at a rate of 1 °C min⁻¹. Toluene conversion was checked in parallel using a CP–4900 μGC (Varian).

The simultaneous oxidation of toluene and CB was studied, considering 1000 ppm of toluene and 10% O₂ in N₂ passed through CB–catalyst mixture (100 mg of catalyst + 6% of CB).

The effect of the presence of NO_x (NO₂ + NO) in the feed gas (900 ppm) was also studied for the both different oxidations. The analysis of combustion products (NO_x, NO, NO₂, CO and CO₂) was obtained with Xentra 4900C continuous emissions analyzer (Servomex). Toluene conversion was checked using a CP–4900 μGC.

The volume hourly space velocity (VHSV), calculated at ambient temperature and atmospheric pressure, is 105,000 h⁻¹. Since the work was always carried out in the same reaction conditions (same toluene and/or NO_x flow and catalyst weight), the conversion values, which are directly comparable, were given.

3.1 Effect of alkali metals on ZrO₂

The theoretical alkali metal content of 0.15 ratio molar in the catalysts was almost confirmed by ICP–MS. Indeed, the experimental molar ratio of the alkali metal in the samples was found to be slightly lower: 0.129 ± 0.015 (Table 1).

The specific area of the catalysts decreases with increasing temperature of calcination (Table 1). It decreases from 201 m² g⁻¹ for ZrO₂ calcined at 300 °C to 84 m² g⁻¹ for the same sample calcined at 600 °C. For the impregnated alkali samples before calcination, the specific area decreases when the molar mass of the alkali metal increases. It goes from 173 m² g⁻¹ in the case of Li_0.15/ZrO₂ to 86 m² g⁻¹ in the case of Cs_0.15/ZrO₂. The decrease of specific area can be attributed to the reorganization within the network of zirconia reducing the porosity of the solid [20]. After calcination at 600 °C, the specific surface of Li_0.15/ZrO₂ is four times (21 m² g⁻¹) lower than the value of the support ZrO₂ (84 m² g⁻¹). The addition of an alkali promoter leads to decrease markedly the specific surface of our catalysts. K and Na can migrate into the solid ZrO₂ and stabilize the tetragonal phase. Na promotes the good crystallization of the solid [20]. Other alkali metals (Li, Rb, Cs) facilitate the transformation of the tetragonal phase to the monoclinic phase. Indeed, the monoclinic phase (formed with Li, Cs) has a smaller surface area compared to the tetragonal phase, and then, the specific surface of the samples with Li and Cs is smaller than the value of the sample with Na.

A XRD study of ZrO₂ (calcined at 300 °C and 600 °C) and the different alkali_0.15/ZrO₂ catalysts calcined at 600 °C was carried out and the diffractograms obtained are displayed in Fig. 1. It shows that ZrO₂ calcined at 300 °C is an amorphous phase. The monoclinic phase (JCPDS 65.1023) appears significantly in the case of ZrO₂ calcined at 600 °C and Li_0.15/ZrO₂. For the tetragonal phase (JCPDS 50.1089), it is stabilized in the case of Na_0.15/ZrO₂, and less apparent in the case of K_0.15/ZrO₂. Indeed, according to Liu et al. [21], Na is able to diffuse inside the bulk of particles and stabilises the zirconia tetragonal phase. The tetragonal phase is predominant in the other catalysts. These results are in accordance with BET specific area results (Table 1).

DTA experiments (Fig. 2) of the different samples of alkali_0.15/ZrO₂ show the existence of exothermic peaks. These peaks are not accompanied by any significant mass loss (TG curves not shown). For the support calcined at 300 °C and the catalysts impregnated and dried at 115 °C, the first peak of crystallization of the tetragonal phase [16] is shifted to a higher temperature (428 °C in the case of the support and 433–600 °C in the case of promoted catalysts versus the molar mass of the alkali). Going from Li to Cs, the intensity of the first peak decreases; therefore the crystallization of amorphous ZrO₂ to tetragonal phase is inhibited. For the same type of preparation, the delay of crystallization increases when the molar mass of the alkali increases. A peak appears at high temperature (910 °C) in the case of Na_0.15/ZrO₂. It corresponds to the crystallization to monoclinic phase [22]. Indeed, the stability of the tetragonal phase (Fig. 1) can accelerate the appearance of monoclinic phase at this temperature. This observation is consistent with the observation of Liu et al. [22]. Thus, the presence of alkali metals on ZrO₂ calcined at 300 °C has an effect on the mode of crystallization of ZrO₂ during the calcination of catalyst up to 600 °C.

3.2 Impregnation of transition metals

The theoretical content of transition metals was confirmed by elemental analysis (Table 1). The molar ratio M/ZrO₂ was between 0.073 and 0.097. When the transition metal was impregnated alone or with a low amount of Cs, the molar ratio was between 0.086 and 0.097 molar ratio. The Cs/ZrO₂ molar ratio was 0.012. When the transition metal was impregnated with a high amount of Cs, the M/ZrO₂ molar ratio decreased to 0.073–0.074 and the corresponding Cs/ZrO₂ molar ratio was 0.094–0.1.

The impregnation of transition metal leads to materials with similar specific areas, comparable to that obtained from the support (in similar experimental conditions). The adding of a low amount of Cs leads also to the same observations, but in the case of the impregnation of Cu or Co with a high amount of Cs, the specific areas of corresponding catalysts are similar to those obtained from Cs_0.15/ZrO₂ (in similar experimental conditions).

A XRD study of Cs_x–M_0.1/ZrO₂ catalysts calcined at 600 °C was carried out and the obtained diffractograms are displayed in Fig. 3. For copper based catalysts (Fig. 3A), there is a stabilization of the tetragonal phase with all catalysts, but for the CuO phase (2θ = 38.8), CuO lines are observed (JCPDS 48.1484) in the presence of the alkali metal (Cs_0.15–Cu_0.1/ZrO₂). The same phenomenon was observed by Lick et al. [23]. Thus, the alkali metal allows the formation of the CuO phase and should influence the Cu segregation, leading to observable crystals with the XRD technique [23]. For catalysts with cobalt (Fig. 3B), in the absence of Cs or in its presence with a low amount, there is a line (at 2θ = 37.09) corresponding to Co₃O₄ (JCPDS 42.1467). The absence of a Co oxide phase peak in the XRD pattern for the Cs_0.15–Co_0.1/ZrO₂ sample shows that the oxide particles must be finely dispersed as amorphous or small sized (size < 3 nm) particles invisible by XRD [14]. Park et al. [24] demonstrated that the characteristic lines of Co₃O₄ are shown only with Na/Co ≤ 0.015 (atomic ratio) and for samples with Na/Co ≥ 0.03, characteristic peaks of β-Na_0.6CoO₂ increase with increasing Na content.

3.3 Oxidation tests

3.3.1 Catalytic oxidation of carbon black

Catalytic performance in CB oxidation was determined by the use of DTA–TG curves. The maximum of the curve corresponds to the temperature of the highest rate of CB oxidation T_max (Fig. 4). The oxidation of CB with ZrO₂ alone is between 414 and 617 °C with a maximum CB oxidation rate (975 μg s⁻¹ g⁻¹_CB) at T_max = 575 °C. A shift to lower T_max occurs in the presence of catalysts promoted with alkali, especially for Cs_0.15/ZrO₂ (T_max = 355 °C). The oxidation rate goes from 1966 μg s⁻¹ g⁻¹_CB with Li_0.15/ZrO₂ to 2516 μg s⁻¹ g⁻¹_CB with Cs_0.15/ZrO₂. The higher performance of Cs-loaded catalyst for soot ignition cannot be explained by the specific area [25]. The results show that the use of alkali-impregnated catalyst is beneficial for the oxidation of CB, reaching the best results with the “heavier” alkali. The addition of transition metals (especially Cu), provided for the enhancement of toluene oxidation, is also interesting for the oxidation of CB. However, for Co based catalyst (Cs_0.15–Co_0.1/ZrO₂), T_max value is lower than for Cs_0.15/ZrO₂. For the catalyst impregnated by Cu and Cs, the CB oxidation rate reaches a maximum value of 2346 μg s⁻¹ g⁻¹_CB and at T_max (347 °C), slightly lower than that achieved with Cs_0.15/ZrO₂. These experiments suggest a synergistic effect between the alkali metal and Cu already observed in a previous work [11]. The alkali metal promoter interacts with the supported copper(II) species and favours their participation following a redox mechanism. It is assumed that the formation of CuO phase, under the form of large or small particles, is responsible for this activity enhancement, CuO species being more active than isolated copper and copper clusters for CB oxidation reaction [26]. The performance of CuO based catalysts in the CB oxidation has been also attributed to the high mobility of the copper ions during burn-off [27]. Moreover, it was shown in the study of Laversin et al. [11] that the alkali promoters are species that not only favour CB–catalyst contact, but also enhance the participation of active oxygen from copper oxide and oxide carrier to oxidize CB. Furthermore, in the case of Cs_0.015–Cu_0.1/ZrO₂ and Cu_0.1/ZrO₂, the maximum CB oxidation rate is lower than those for the catalysts: Cs_0.15/ZrO₂ and Cs_0.15–Cu_0.1/ZrO₂. Moreover, T_max of these catalysts is also the highest among all the catalysts. This shows the contribution of Cs in the oxidation of carbonaceous particles. In fact, the presence of alkali metals favours contact between the catalyst and CB and enhances the catalyst's ability to release active oxygen species. Moreover, the presence of higher alkali content induces further activity improvement [11]. The XRD results (Fig. 3A) show that in the presence of high amount of Cs, the CuO phase appears on the XRD patterns. T_max found for Cs_0.15–Cu_0.1/ZrO₂ catalyst is lower than those of Cs_0.015–Cu_0.1/ZrO₂ and Cu_0.1/ZrO₂ catalysts. This suggests that the activity can be associated mainly with Cs and with the wetting effect of the catalytic surface that this metal confers. Cs- and Cu-based catalysts will then be used in the others tests of CB oxidation.

3.3.3 Catalytic removal of CB in the presence of toluene or NO_x

3.3.3.1 Oxidation of carbon black in the presence of toluene

For comparison reasons, the tests of CB oxidation were all recorded in the same conditions by the use of a microreactor (Table 3). The ignition temperature T_i for CB oxidation was determined as light-off temperature, at which a significant evolution of CO₂ started. In a previous study [28], Aouad et al. showed that the presence of propene in reactant gases flow was shown to be beneficial to CB elimination in the presence of Ru_xCe catalysts, knowing that the temperature of total oxidation of propene is lower than that of CB (NC 330, Evonik). In this work, the tests were performed with toluene and CB (Printex U). For all the catalysts, a decrease between 10–20 °C of T_i and T_max is recorded for CB oxidation in the presence of toluene in comparison with simple CB oxidation. Thus, the presence of the hydrocarbon in the reactant gases mixtures is not only beneficial to the oxidation of a part of CB at low temperatures but also to the faster oxidation of CB [28]. The exothermic effect of the toluene oxidation and/or toluene adsorption on CB can explain this enhancement. However, the toluene oxidation seems not to be improved by the CB oxidation. The CB oxidation is enhanced in the presence of toluene but especially for the catalysts with a high amount of Cs (Cs_0.15/ZrO₂ and Cs_0.15–Cu_0.1/ZrO₂). However, the catalyst Cs_0.15–Cu_0.1/ZrO₂ is the best one. This can be attributed to the both participation of a high content of alkali metal and CuO phase.

Table 3

CB oxidation and NO_x conversion values.

Catalyst	Tmax and (Ti) for CB oxidation (°C)	Maximum of NOx conversion (%)
	CB + O2	CB + O2 + toluene	CB + O2 + NO
Cs0.15/ZrO2	349 (275)	325 (235)	323 (270)	22.6 (340–450 °C)
Cs0.15–Cu0.1/ZrO2	321 (268)	308 (239)	355 (255)	25.9 (at 370 °C)
Cs0.015–Cu0.1/ZrO2	496 (362)	451 (410)	384 (291)	22.6 (at 390 °C)
Cu0.1/ZrO2	498 (335)	497 (329)	405 (200)	16.3 (at 410 °C)

3.3.3.2 Oxidation of carbon black in the presence of NO_x

The results concerning the CB oxidation in the presence of NO_x are shown in Table 3 and Fig. 6. Dotted lines in the figure represent the NO_x concentration in the gas feed in the presence of 10% O₂, i.e., the points below and above this line mean the occurrence of NO_x sorption/reduction and NO_x desorption respectively. For the sample Cs_0.15/ZrO₂, T_i of CB oxidation in the presence of NO_x is slightly enhanced compared to that in the test of CB oxidation alone, but lower than that in presence of toluene. The same phenomenon is recorded for the values of T_max. An increase of NO_x is also observable. Cs_0.15/ZrO₂ exhibits sorptive NO_x, which is followed by a broad NO_x desorption at higher temperatures. It gives rise to a sharp NO_x desorption, which starts before the beginning of CO₂ evolution. Unlike the case of the catalyst alone (not shown), very sharp and temporary desorption of NO_x appears at 323 °C (also T_max of CB oxidation). After this, the NO_x concentration drops suddenly below the inlet level suggesting the occurrence of reactions between NO and CB [25] in the temperature range 323–450 °C. This is consistent with a larger area below the dotted line than that above the line (Fig. 6A). This phenomenon is in accordance with the observation of Ito et al. [25]. First, a sharp desorption of NO_x should result from a very fast and local exotherm due to soot ignition. As described above, the temperature of soot ignition is lowered when a large amount of NO is pre-adsorbed on the catalysts. If soot would directly be reacted with desorbed NO_x, the ignition must be observed after the beginning of NO_x desorption. However, this is not the case. This leads to another important suggestion that soot ignition is presumably promoted by adsorbed NO_x species [25]. For Cs_0.15–Cu_0.1/ZrO₂, T_max of CB oxidation is less good; it rises from 321 °C to 355 °C, but a conversion of NO_x is detected at 370 °C, which is close to the maximum CB oxidation rate. The presence of NO_x is unfavourable for toluene oxidation with this catalyst although this catalyst records the high NO_x conversion. For Cs_0.015–Cu_0.1/ZrO₂, T_i and T_max of CB oxidation are considerably enhanced. A decrease of about 110° is observed for T_max and 70° for T_i. A good NO_x conversion is recorded with this catalyst. In the case of the catalyst Cu_0.1/ZrO₂, the CB oxidation is enhanced with a shift to a lower temperature in the presence of NO_x (Table 3), but there is a low NO_x conversion for Cu_0.1/ZrO₂ compared to Cs_0.015–Cu_0.1/ZrO₂. As described on Fig. 6, the ignition temperature is lowered when a large amount of NO_x is pre-adsorbed on these catalysts. If the CB directly reacts with desorbed NO_x, the ignition must be observed after the beginning of NO_x desorption. The addition of Cs not only activates CB ignition, but also increases NO_x adsorption, which is also effective for CB ignition [25].

Fig. 8 shows the infrared spectra after CB oxidation tests without (spectra (a)) and in the presence of NO_x (spectra (b)). The NO_x sorption leads to the formation of bidentate and/or monodentate nitrates species on the surface. Two bands at 1380 and 1335 cm⁻¹ are detected after tests in the presence of NO_x for Cs_0.15/ZrO₂ and Cs_0.15–Cu_0.1/ZrO₂. These bands are attributed to free NO₃⁻ groups (1380 cm⁻¹) and to monodentate NO₃⁻ groups (1335 cm⁻¹) [1]. Presence of bridged NO₃⁻ groups could not be excluded; their characteristic band (∼1620 cm⁻¹) could be masked by adsorbed water band [1]. The amount of nitrate is large for the catalyst Cs_0.15–Cu_0.1/ZrO₂ and lower for Cs_0.15/ZrO₂. For the samples Cs_0.015–Cu_0.1/ZrO₂ and Cu_0.1/ZrO₂, there is not any significant adsorption. The large amount of nitrate adsorbed in the case of Cs_0.15/ZrO₂ and Cs_0.15–Cu_0.1/ZrO₂ is due to the strong basicity of Cs. The oxidative adsorption of NO was also increased to form nitrate on the solid surface [25]. The use of a low amount of Cs impregnated with Cu, leads to a great shift of T_i and T_max of CB oxidation to a low range of temperatures. The catalysts Cs_x–Cu_0.1/ZrO₂ possess the combination of NO_x–oxidation activity and moderate basicity, yielding in significant synergism in the oxidative adsorption as to yield nitrite (NO₂⁻) and/or nitrate (NO₃⁻) on the surface. This explains the high NO_x conversion in the case of these catalysts, compared to Cs_0.15/ZrO₂ or Cu_0.1/ZrO₂. The maximum NO_x conversion corresponds to T_max of CB oxidation (Table 3). Comparing the curves of NO_x in the absence of CB (Fig. 7B) and in the presence of CB (Fig. 7C), the NO_x conversion is just observed when a maximum of CB is oxidized at T_max. This conversion is accompanied with a decrease of NO₂ quantity (curve a). Thus, NO₂ participates in the CB oxidation. This is explained by the need for NO₂ as an intermediate for the catalyst to work efficiently [6], especially in the oxidation of CB [29]. The detailed mechanism of CB–NO_x interaction is not clear at this stage, but this may be explained by considering adsorbed nitrate species as an oxidizing agent [25]. The oxidation of NO to the nitrate is supposed to proceed in two steps: [15]

NO+12O2→NO2

(1)

2NO2+O2−→2NO3−

(2)

where O²⁻ means lattice oxygen in oxides. Reaction (1) is the essential step to form the nitrate ion; thus, the high activity of the oxidation catalyst is important for the first step. If the oxidation activity is insufficient, adsorption of NO_x as nitrate ion, NO and NO₂ species, proceeds. The second step (reaction (2)) also depends strongly on the kind of the catalyst [15]. The reaction (1) is a simple catalytic oxidation (Fig. 6B), whereas the reaction (2) is oxidative NO_x uptake in a sorbent [15]. The formed nitrate ions are stored on the surface and/or in the bulk of zirconia [15].

3.3.4 Catalytic oxidation of toluene in the presence of CB or NO_x

3.3.4.1 Oxidation of toluene in the presence of CB

The results of comparison between toluene oxidation alone and toluene oxidation in the presence of CB are summarized on Table 2. T_i of toluene oxidation is determined when a significant toluene conversion started. For Cs_0.15/ZrO₂, T_50% of toluene conversion is less good in the presence of CB, but T_i is slightly enhanced. This is due to the oxidation of CB proceeding before that of toluene oxidation. The same result is obtained with Cs_0.15–Co_0.1/ZrO₂. For Cs_0.015–Co_0.1/ZrO₂ and Co_0.1/ZrO₂, T_50% and T_i are both enhanced in the presence of CB. In this case, adsorption of toluene on CB should be important. The results show the interest of adding a low amount of alkali metal to cobalt in comparison with the simple catalyst Co_0.1/ZrO₂.

3.3.4.2 Oxidation of toluene in the presence of NO_x

The oxidation of toluene in the presence of NO_x is presented in Fig. 9 and Table 2. For Cs_0.15/ZrO₂, T_i and T_50% of toluene oxidation decrease in the presence of NO_x. The addition of Co to Cs_0.15/ZrO₂ enhances toluene oxidation, but not enough to reach the conversion obtained without NO_x. The low NO_x concentration observed at low temperatures (Table 2) should be due mainly to the adsorption of NO_x species by the catalyst with a alkali amount, by the formation of nitrates [15]. The diminution of the amount of Cs leads to enhance the performance, greatly for NO_x conversion and slightly for toluene oxidation compared to the same oxidation without presence of NO_x. Co_0.1/ZrO₂ catalyst allows us to enhance NO_x conversion in comparison with Cs_0.015–Co_0.1/ZrO₂, but the oxidation of toluene is more efficient in the presence of NO_x over the Cs_0.015–Co_0.1/ZrO₂ catalyst. Moreover, on Fig. 9B (b), toluene oxidation increases when the concentration of NO_x reaches its minimum. Therefore, the adding of a low amount of Cs to cobalt based catalysts leads to enhance toluene oxidation.

FTIR measurements for catalysts tested (Fig. 10) are carried after simple toluene oxidation at 500 °C (spectra (a)) and after oxidation of toluene in the presence of NO_x (spectra (b)). For Cs based catalysts, a band at 1335 cm⁻¹ corresponding to monodentate NO₃⁻ [1] is observed for the tests of toluene oxidation in the presence of NO_x. So, the catalysts with the higher alkali content show the presence of a strong absorbance band ascribed to monodentate NO₃⁻ species (1335 cm⁻¹). Therefore, NO_x could be stored as nitrate species on catalysts with higher alkali content. This phenomenon is in good agreement with previous works revealing the strong interaction of NO_x over alkali containing catalysts [1]. In this case, active oxidation sites of toluene could be occupied by NO_x. This could explain the lower performance of Cs high amount catalysts. On the contrary, nitrite species (1265 cm⁻¹) band showed in the case of Cs_0.015–Co_0.1/ZrO₂ and Co_0.1/ZrO₂ catalysts demonstrates that the transition metals have the ability to form NO₂ which is a stronger oxidizer than O₂ [23].

3.4 Operando DRIFT study

Fig. 11 shows in situ FT–IR spectra of Cs_0.015–Co_0.1/ZrO₂ at 350 °C. When introducing toluene and O₂, there are bands at 1660 and 1425 cm⁻¹ corresponding to substituted aromatic compounds, a band at 1305 cm⁻¹ corresponding to skeletal C–C stretching vibration and a band at 1220 cm⁻¹ corresponding to carbon single bonded oxygen stretching. In the presence of NO in the gas mixture (toluene + O₂ + NO), the bands at 1660, 1425 and 1220 cm⁻¹ are more intense and a band at 1540 cm⁻¹ corresponding to R–NO₂ appears. Thus, the toluene oxidation is observed by the strong bands for aromatic substitutes, especially for the experiment with NO_x.