2.2.1. Catalysts Activity

Catalytic performances of materials were evaluated in the toluene total oxidation reaction. Conversion versus reaction temperature curves are shown in Figure 7. As first remark, reactions performed with different LaFe samples lead to the production of only CO2 and water as products, while benzene traces (not quantifiable) appear as a byproduct during reactions over LaMn samples, for high conversion value (*X* > 50%). Consequently, conversions plotted in Figure 7 refer to selective conversion into CO2.

**Figure 7.** Toluene conversion curves obtained for LaMnO3.15 (**A**) and LaFeO3 (**B**) samples: (a) SSR, solid state reaction; (b) HEBM, high-energy ball milling; (c) LEBM, low-energy ball milling. Conditions: 200 mg of catalyst, 100 mL·min−<sup>1</sup> of 1000 ppmv C7H8 in synthetic air (20% O2 in N2).

LaMn\_SSR and LaFe\_SSR (straight lines) show low performances for the toluene oxidation reaction with 39% and 13% toluene converted in CO2 at T = 330 ◦C, respectively. After the high-energy ball milling process, far better performances were obtained with a shift of the light-off curves toward the lower temperature by −39 ◦C (LaMn\_HEBM) and −52 ◦C (LaFe\_HEBM). Samples from the second step of grinding at low energy showed even better performances with additional shifts toward the lower temperature by −12 ◦C for LaMn\_LEBM, and by −40 ◦C for LaFe\_LEBM. T10, T50 and T90 values, reflecting the catalytic performances of each materials are given in Table 4. The following ranking of activity was obtained:

**Table 4.** T50 values and kinetics data obtain from light-off curves of LaMn and LaFe samples.


<sup>1</sup> determined at T = 250 ◦C; <sup>2</sup> determined in the range *X*(%) < 20%; <sup>3</sup> extrapolated using the Ea average value.

LaFe\_LEBM > LaMn\_LEBM > LaMn\_HEBM > LaFe\_HEBM > LaMn\_SSR > LaFe\_SSR

Then, and as easily observed in Figure 7, the most active materials are LEBM-derived solids. It is interesting to note that LaFe\_HEBM is little more active than LaMn\_HEBM, while in the literature, a reverse order is generally reported even at low temperature (CO [11,15]) and at high temperature (CH4 [15,40]). However, when analyzing the normalized activities (per surface unit), the activity ranking becomes:

LaMn\_HEBM > LaMn\_LEBM > LaFe\_LEBM > LaFe\_HEBM >> LaMn\_SSR > LaFe\_SSR

This result clearly demonstrates that the Mn-containing formulation are little more active than the Fe-containing ones, in line with the more important reducibility of manganese as determined by TPR (Figure 4) and demonstrating a lower temperature of activation for the Mn(+IV)/Mn(+III) redox couple than for the Fe(+III)/Fe(+II) redox couple.

Figure 8 shows the Arrhenius plots drawn from conversion curves at X < 20%, assuming a first order toward toluene concentration and a zero order toward oxygen. Activation energies, estimated from Arrhenius plots slopes, are reported in Table 4. Calculated activation energies for the toluene oxidation over LaMn samples show values comprised between 126 and 146 kJ·mol−1. Calculated activation energies for the reaction over LaFe samples oscillated from 137 to 164 kJ·mol−<sup>1</sup> respectively. Considering the precision of the measure, i.e., light off curve measurement, whole materials display comparable Ea, at an average value of 145 kJ·mol−<sup>1</sup> suggesting a comparable oxidation mechanism for LaFe- and LaMn-containing formulation and whatever the synthesis step (SSR, HEBM, LEBM). Similar activation energy values had been reported for La1−xCaxBO3 (B = Fe, Ni) systems by Pecchi et al. [41].

**Figure 8.** Arrhenius plots obtained at X < 20% for the toluene oxidation reaction over LaMnO3.15 (**A**) and LaFeO3 (**B**) samples: (a) SSR, solid state reaction; (b) HEBM, high-energy ball milling; (c) LEBM, low-energy ball milling.

Assuming an average Ea of 145 kJ·mol<sup>−</sup>1, corrected pre-exponential factor (A0 cor) is re-calculated, and obtained values are listed in Table 4. A0 cor factors, obtained at constant Ea for all materials, reflect the evolution in active sites on the catalysts, and when plotted as a function of the surface area of the solid (Figure 9), this allows us to determine the active site surface density. From Figure 9, it seems evident that a roughly linear correlation was obtained between A0 cor and materials surface area. Consequently, the active sites in materials evolves linearly with the surface area. Nonetheless, LaMn samples showed a slightly higher active site surface density than their LaFe equivalent.

**Figure 9.** Evolution of corrected pre-exponential factors with catalyst surface areas. Open symbols: LaMn-compositions; Full symbols: LaFe-compositions.

#### 2.2.2. Stability Tests

The toluene conversion into CO2 evolution as a function of time are presented in Figure 10 for the HEBM and LEBM catalysts (T = 285 ◦C; 70 h). Both LaMn based catalysts show a rapid deactivation in the five first hours before to slightly linearly deactivate with time. The activity coefficient a285 (see experimental part) are rather similar for both catalysts amounting to 0.66 and 0.64 for LaTM\_HEBM and LaTM\_LEBM catalysts, respectively. This likely indicates that the LEBM treatment has no effect on the stability of the catalyst on stream. By opposition, a good stability for the LaFe-based catalysts was found over time in terms of toluene conversion into CO2. It should be noted that, for all catalysts, it is found neither phase transformations nor crystallite size increase by XRD. Furthermore, the SSA keeps unchanged after the stability test considering the margin of uncertainties.

**Figure 10.** Stability experiments of LaMnO3.15 (**A**) and LaFeO3 (**B**) samples. Conditions are: 200 mg of catalyst exposed to 1000 ppmv C7H8 in 100 mL·min−<sup>1</sup> of synthetic air (20% O2 in N2) for 70 h at a constant temperature of 285 ◦C. HEBM, high-energy ball milling; LEBM, low-energy ball milling.

Behar et al. have shown that the total oxidation of toluene can be easily described by a Mars–van Krevelen model when Cu-Mn mixed oxide is used as catalyst [42]. In this model the toluene is oxidized by the catalyst and not directly by the gaseous oxygen, and the role of gaseous oxygen was in restoring and maintaining the oxidized state of the catalyst. The oxidized state of the LaMn\_LEBM after the stability experiment was studied by means of H2-TPR and XPS analyses. Figure 11A shows the comparison between the two H2-TPR profiles obtained on the fresh and used LaMn\_LEBM catalysts. It can be clearly seen that, for the used LaMn\_LEBM sample, the shoulder at low temperature (280 ◦C) disappeared, suggesting that some Mn species in a high oxidation state (Mn(+IV)) are reduced during the stability test. Similarly, the analysis of the Mn 3s photopic (Figure 11B) shows an increase in the peak splitting ΔE(Mn 3s) value, from 4.64 eV for fresh LaMn\_LEBM to 4.91 eV for the used one, which demonstrates a decrease in the Mn AOS value (from 3.7 to 3.4) after the stability experiment. Therefore, the deactivation observed for LaMn\_LEBM catalyst could be related to the decrease in Mn AOS, the step of reoxidizing the reduced catalyst with gaseous oxygen being most likely the rate-determining step.

**Figure 11.** Comparison of (**A**) H2-TPR profiles and (**B**) XPS high resolution spectra of Mn3s obtained for the fresh and used LaMn\_LEBM catalysts.
