**1. Introduction**

Oxidation of nitric oxide (Equation (1)) is one of the few known third order reactions. The reaction is unusual, as the rate of reaction increases with a decrease in the temperature [1]

$$2\text{NO} + \text{O}\_2 \leftrightharpoons \text{NO}\_2 \qquad \Delta\_\text{r}\text{H}\_{298} = -113.8 \text{ kJ/mol} \tag{1}$$

NO oxidation is a key reaction in lean NOx abatement technologies and in the Ostwald process for nitric acid production. In Ostwald's process, NO oxidation is carried out as a non-catalytic process and the forward reaction is favored by the removal of heat and by providing sufficient residence time. Typical gas stream concentrations are 10% NO, 6% O2 and 15% H2O [2]. Using a catalyst for NO oxidation may lead to significant process intensification of the nitric acid plant. In addition to speeding up the oxidation process, it may reduce capital costs and increase heat recovery. Efforts have been made to find a catalyst effective under industrial conditions; but success so far has not been achieved [2]. To date, the process is carried out as a homogenous process in modern nitric acid plants. To the best of the authors knowledge, apart from an earlier patent [3], only two recent studies [4,5] report catalytic oxidation of NO at nitric acid plant conditions. However, catalytic oxidation of NO has been extensively studied with regards to lean NOx abatement technologies and reviewed by Russel and Epling [6] and Hong et al. [7]. In these studies, oxidation of NO is carried out at very lean concentrations of NO ranging from 100–1000 ppm of NO [8–10]. The huge difference in NO concentration between NOx abatement and nitric acid production makes the extrapolation of these findings to nitric acid plant conditions, questionable.

Although it has been demonstrated that platinum has significant catalytic activity for oxidation of NO to NO2 at nitric acid plant conditions [4,5] the high-cost and scarcity of platinum motivates the search for potential non-noble metal based catalysts.

Perovskites have gained particular interest as catalytic materials due to their high thermal stability, ease of synthesis and good catalytic activity [11]. Perovskites are represented by a general formula ABO3, where A represents a rare earth or alkaline earth cation and B represents a transition metal cation. The activity of perovskites can be tuned by partial substitution of A and/or B site cations with another element to obtain the desired properties [11]. The lanthanum-based perovskite *LaBO3* (B = Co, Mn, Ni) have been found to be active for NO oxidation at lean NO conditions [12,13]. It has been demonstrated for LaCoO3, that partial doping of the A-site with strontium or cerium or doping of the B-site with manganese or nickel enhances NO oxidation activity of the perovskite [13–16].

In this work, lanthanum-based perovskites with cobalt, nickel and manganese (LaCoO3, LaNiO3 and LaMnO3) were synthesized by the sol-gel method using citric acid. Catalytic tests were performed using a dry feed (10% NO, 6% O2) at atmospheric pressure; partially simulating nitric acid plant conditions. The effect of B-site substitution was studied by preparing a series of LaCo1−xMnxO3 and LaCo1−yNiyO3 catalysts. The catalysts were characterized using N2 adsorption, X-ray diffraction (XRD), Scanning electron microscopy (SEM) and temperature programmed reduction (TPR). Catalyst structure during pretreatment and oxidation of NO was monitored using in situ XRD.

#### **2. Results and Discussion**

#### *2.1. Catalyst Characterisation*

Specific surface areas are summarized in Table 1. All samples have a relatively low surface area, in the range of 8–12 m2/g, which is a typical of perovskites prepared by the citrate method [14,17,18]. The surface area remains unaffected by the degree of substitution of Mn (*x*). However, an irregular change in surface area is observed with the degree of substitution of Ni (*y*). For *y* = *0.25* and *0.75*, the surface area increased by 25% (from 8 to 10 m2/g) and 50% (from 8 to 12 m2/g) respectively. However, for *y* = *0.50* the surface area remains the same as for LaCoO3.


**Table 1.** Brunauer–Emmett–Teller (BET) surface area and crystallite size (d) calculated using the Scherrer equation.

*Catalysts* **2019**, *9*, 429

The XRD patterns shown in Figure 1 reveal the formation of phase-pure perovskite structure for all samples, and no peaks characteristic of the metallic oxides or carbonates were seen. The main characteristic peak at 2θ = 33◦ for LaCoO3 slightly shifts towards lower 2θ values with an increase in *x* and *y*, indicating an increase in the lattice parameters confirming previous findings [13,19,20]. This expansion can be explained by comparing the ionic radii of the species in the perovskite structure. The higher ionic radii of Ni3<sup>+</sup> (0.56 Å) than Co3<sup>+</sup> (0.52 Å) responsible for lattice expansion in LaCo1−yNiyO3 [20]. In case of LaCo1−xMnxO3, the average trivalent metal site is conserved by adjusting the ratio between Mn4+/Mn3<sup>+</sup> and Co<sup>3</sup>+/Co2<sup>+</sup> [21]. Therefore, the relative amount of Mn4<sup>+</sup> (0.52 Å) present, Mn3<sup>+</sup> (0.645 Å), Co2<sup>+</sup> (0.82 Å), Co3<sup>+</sup> (0.52 Å) dictates the overall lattice parameters.

**Figure 1.** XRD patterns of: (**a**) LaCo1−xMnxO3; (**b**) LaCo1−yNiyO3 perovskites.

Among undoped perovskites, LaCoO3 and LaMnO3 belong to the rhombohedral phase in agreement with the results reported in literature [22,23]. LaNiO3 belongs to the cubic phase, which is consistent with previous studies [24].

Crystallite sizes calculated using the Scherrer equation are summarized in Table 1. Figure 2 shows crystallite size as a function of the degree of substitution. The highest crystallite size of 28 nm was observed for LaCoO3 and a linear decrease was observed with an increase in *x* and *y* for partially substituted samples indicating that partial substitution effectively restrains the crystal growth. However, other contributions to XRD peak broadening such as strain cannot be ruled out. Partially substituted nickel perovskites had smaller crystallite size compared to their manganese counterparts.

**Figure 2.** Crystallite size as a function of the degree of substitution for LaCo1−xMnxO3 and LaCo1−yNiyO3 perovskites.

Figure 3 shows the XRD pattern recorded in situ during pretreatment of LaCoO3 and LaMnO3. No change in structure is observed apart from lattice expansion with an increase in temperature. The structural stability of the perovskites is in accordance with the fact that they are calcined at higher temperatures in comparison with the pretreatment temperature of 500 ◦C. No change in structure was observed for LaCoO3 and LaMnO3 during steady state oxidation of NO at 350 ◦C indicating that the bulk structure of perovskite remains unaffected during the catalytic process. However, minor changes beyond the detectable range of XRD cannot be ruled out.

**Figure 3.** XRD patterns (λ = 0.49324 Å) recorded in situ during pretreatment of: (**a**) LaCoO3; (**b**) LaMnO3 perovskites.

Figure 4 shows the SEM images of LaCoO3 with varying content of Mn and pure LaNiO3. The presence of agglomerated non-spherical particles is observed for all samples. A significant change in morphology is observed with the substitution of Co with Mn along with an increase in the extent of agglomeration (Figure 4b–d).

**Figure 4.** SEM images: (**a**) LaCoO3; (**b**) LaCo0.75Mn0.25O3; (**c**) LaCo0.50Mn0.50O3; (**d**) LaMnO3; (**e**) LaNiO3.

The H2-TPR profiles of LaCo1−xMnxO3 perovskites are given in Figure 5a. Three reduction peaks at 334, 373 and 526 ◦C are observed in the TPR profile of LaCoO3. The first two peaks are attributed to the reduction of Co3<sup>+</sup> to Co2+, while the peak at 526 ◦C represents the reduction of Co2<sup>+</sup> to Co<sup>0</sup> leading to the destruction of the perovskite structure [25]. TPR of LaMnO3 shows two main reduction peaks at 383 and 818 ◦C. The first peak represents the reduction of Mn4<sup>+</sup> to Mn3<sup>+</sup>, while the reduction of Mn3<sup>+</sup> to Mn2<sup>+</sup> occurs at elevated temperatures (above 700 ◦C), forming MnO and simultaneous collapse of the perovskite structure [26]. For *x* = *0.25* and *0.5*, broad peaks overlapping reduction peaks of Co3<sup>+</sup> to Co2<sup>+</sup> and Mn4<sup>+</sup> to Mn3<sup>+</sup> are observed below 550 ◦C.

**Figure 5.** TPR profiles of: (**a**) LaCo1−xMnxO3; (**b**) LaCo1−yNiyO3 perovskites.

Figure 5b shows H2-TPR profiles of LaCo1−yNiyO3 perovskites. Reduction of LaNiO3 also proceeds via three peaks at 304 ◦C, one at 334 ◦C and one at 465 ◦C. A similar three-step reduction process for LaNiO3 has been reported [27] and in situ XRD revealed that reduction proceeds via formation of the La2NiO4 phase [27]. The highest peak is associated with the formation of Ni0 and La2O3 resulting in the destruction of the perovskite structure. Four reduction peaks were observed for *y* = *0.25*. Comparison with LaCoO3 and LaNiO3 indicates that the first two peaks at 303 and 342 ◦C match with the reduction peaks of Ni3<sup>+</sup> to Ni2<sup>+</sup> and Co3<sup>+</sup> to Co2+. Whereas the former two peaks at 459, 495 ◦C corresponds to further reduction to form the metallic phases (Ni0 and Co0). This reduction profile matches well with the previous findings [20]. In contrast to distinct reduction peaks for cobalt and nickel for LaCo0.75Ni0.25O3, only two broad peaks were observed for *y* = *0.50* and *0.75*. The first peak overlaps with the reduction of Ni3<sup>+</sup> to Ni2<sup>+</sup> and Co3<sup>+</sup> to Co2<sup>+</sup> and increases from 313 ◦C for *y* = *0.50* to 346 ◦C for *y* = *0.75*. Simultaneous reduction to metallic phases (Ni<sup>0</sup> and Co0) was observed at 418 ◦C and 488 ◦C for *y* = *0.50* and *0.75*, respectively.
