2.1.4. Evolution of Transition Metal Reducibility upon Grinding

Iron contamination is classically observed as a direct consequence of the grinding process [17,21,28] and has been estimated for LaMn samples (Table 1). Values staying below 3.0 at.% are reasonable even if an impact on the catalytic properties cannot be excluded, as already observed for MnOx [21].

Temperature-programmed reduction (H2-TPR) profiles of LaMnO3.15 (A) and LaFeO3 (B) materials are shown in Figure 4. Quantification of consumed hydrogen and temperatures at maximum hydrogen consumption are listed in Table 2.

**Figure 4.** Temperature-programmed reduction (H2-TPR) profiles of LaMnO3.15 (**A**) and LaFeO3 (**B**) samples after the (a) SSR, solid state reaction; (b) HEBM, high-energy ball milling; (c) LEBM, low-energy ball milling, steps.


**Table 2.** H2-TPR results for LaMnO3.15 and LaFeO3 samples.

TM: transition metal; <sup>1</sup> H2 uptake from the low temperature (LT) consumption peak domain, total uptake being into brackets; 2, calculated for LaMnO3.15 from the total H2 uptake assuming a complete reduction up to Mn(+II), and calculated for LaFeO3 from the LT H2 uptake and assuming a reduction to Fe(+II) during this LT process, assuming bulk La/MT ratio of 1.

*Reduction of LaMnO3.15:* the H2 consumption profile displays 2 main peaks for all the LaMn based catalysts. Overall, the Mn(+IV) and Mn(+III) entities initially present in the perovskite structure were reduced to Mn(+II). According to the literature, the two H2 consumption peaks can be ascribed to the consecutive Mn(+IV) → Mn(+III) and the Mn(+III) → Mn(+II) reductions [29,30]. Regarding the LaMn\_SSR sample, these peaks were located at ~510 ◦C (shoulder: ~590 ◦C) and ~880 ◦C. Comparatively, for the LaMn\_HEBM sample there was a lowering in terms of initial H2 consumption temperature as well as of the position of the peak maximum indicating an enhancement in the reducibility of the sample. Such improvements in reducibility have been already observed for the reduction of LaCoO3 [17] and have been interpreted in terms of a decrease of the crystal domain size. Finally, the reduction profile of LaMn\_LEBM is globally comparable to that for LaMn\_HEBM. However, it was now found at low temperature a new H2-consumption shoulder. This observation indicates that the LEBM plays a beneficial role in promoting the reducibility of the sample through particle deagglomeration. The Mn AOS estimated from the global H2 uptake gave a value of 3.4 (±0.1) in line with a LaMnO3.15 phase detected by XRD.

*Reduction of LaFeO3:* as for the Mn-based materials, two reduction steps were observed on the reduction profile of Fe-based samples. The hydrogen consumption profile registered over the LaFe\_SSR sample shows one first reduction peak, with a maximum consumption at 520 ◦C. A second hydrogen consumption was observed to start at T > 750 ◦C, and is not achieved at the end of the experiment, i.e., at 1000 ◦C. According to the literature [31], and knowing that iron is essentially at the +III oxidation state in LaFeO3, the two successive steps should be described as:


Quantification of hydrogen consumed in the first reduction peak, for LaFe\_SSR allowed us to calculate an AOS of 2.4 (considering that the first step is only related to the Fe(+III) to Fe(+II) reaction), that is consistent with results available in the literature [31]. As observed for the LaMnO3.15 series, the HEBM step led to a shift of the hydrogen consumption step toward the low temperatures by 30 ◦C with a small additional consumption visible at 410 ◦C. Quantification led to a comparable value of AOS (Table 2). Finally, the LEBM step does not induce a significant change in hydrogen consumed and hydrogen consumption position. The only minor modification concerns the low temperature contribution, and the increase in intensity of the consumption located at 410 ◦C. However, global consumption always leads to an AOS of 2.5 (±0.1), comparable to the value obtained for the SSR and HEBM materials. The consumption peak at 410 ◦C can originate from the reduction of surface FeOx clusters, since Faye et al. reported a comparable temperature of reduction of Fe2O3 highly dispersed on the surface of LaFeO3 (405 ◦C for the first consumption [31]).
