Catalytic Performance of B-Site-Doped LaMnO₃ Perovskite in Toluene Oxidation

Abstract

The catalytic removal of toluene, a representative aromatic volatile organic compound (VOC), requires efficient and stable catalysts. This study systematically investigated the effect of B-site doping with transition metals (Fe, Cu, and Ni) on the catalytic performance of LaMnO₃ perovskite for toluene oxidation. The LaMn_0.5X_0.5O₃ catalysts were synthesized via a sol–gel method and evaluated. The LaMn_0.5Ni_0.5O₃ catalysts exhibited the optimal catalytic performance, achieving toluene conversion temperatures of 243 °C at 50% conversion (T₅₀) and 296 °C at 90% conversion (T₉₀). Comprehensive characterization revealed that Ni doping effectively refined the catalyst’s microstructure (grain size decreased to 19.21 nm), increased the concentration of surface-active oxygen species (142.7%), elevated the Mn⁴⁺/Mn³⁺ ratio to 0.65, and enhanced lattice oxygen mobility. These modifications collectively contributed to its outstanding catalytic activity. The findings demonstrate that targeted B-site doping, particularly with Ni, is a promising strategy for engineering efficient perovskite catalysts for VOC abatement.

1. Introduction

Volatile organic compounds (VOCs) are precursors of secondary particulate matter and O₃. They drive atmospheric pollution and harm human health [1,2]. Aromatic VOCs are widely present in daily life and industrial production environments. In relatively enclosed spaces, these compounds can cause irritation to the eyes, nasal passages, and throat and may lead to symptoms such as dizziness, headaches, impaired memory and vision, and, in severe cases, even death [3]. Further studies have shown that aromatic VOCs can undergo photochemical reactions with nitrogen oxides (NO_x), generating various secondary pollutants, including tropospheric ozone and photochemical smog [4]. Aromatic VOCs possess stable benzene rings. These rings resist degradation and enhance toxicity. Efficient removal of aromatic VOCs remains a significant challenge.

Among the treatments for aromatic VOCs, catalytic oxidation is widely used [5,6]. It is efficient, energy-saving, and eco-friendly. The required temperature is substantially lower than that of direct combustion. Dioxins and other harmful by-products are suppressed [7]. Perovskites are regarded as one of the promising catalysts or catalyst supports due to their high catalytic activity, low cost, and excellent thermal stability.

Perovskite is a classic ABO₃ mixed oxide [8]. The A-site ion stabilizes the framework via twelve-fold cuboctahedral coordination with oxygen atoms. The B-site cation sits in an octahedral site with six oxygen atoms. This unique coordination activates organic molecules and tunes oxygen species [9,10,11]. Álvarez-Galván et al. [12] tested LaBO₃ (B = Ni, Co, Cr, Mn) perovskites in methyl ethyl ketone oxidation. It was found that B-site substitution clearly boosts activity. Mn and Co are the preferred B-site cations in perovskite oxides for VOCs removal. Weng et al. [13] studied the phosphoric acid etching of La₃Mn₂O_7+δ for the catalytic oxidation of chlorinated organics. This etching enhanced lattice oxygen mobility, increased proton donor availability, and enriched Mn(IV) species. These effects led to superior catalytic activity toward dichloromethane degradation. Pan et al. [14] synthesized LaFe_1−xCo_xO₃ (x = 0–0.3) perovskite catalysts and systematically investigated the effects of cobalt doping on the structure, redox properties, and total oxidation activity of LaFeO₃ for propylene. It was found that the introduction of cobalt, while slightly reducing the specific surface area, significantly enhanced the reducibility, oxygen mobility, and quantity of surface-active oxygen species of the material. Ansari et al. [15] prepared Ce³⁺-doped LaMnO₃ perovskite catalysts. Their study revealed that Ce³⁺ doping effectively modulates the Mn⁴⁺/Mn³⁺ ratio and increases the oxygen vacancy concentration. Furthermore, it enhances oxygen species migration, which collectively leads to a significant improvement in catalytic performance for benzyl alcohol oxidation. Li et al. [16] reported that the La_0.5Sr_0.5Co_0.8Fe_0.2O₃ perovskite, through Sr/Fe co-doping, optimizes the Co⁴⁺ valence state and oxygen vacancy concentration. This catalyst exhibits excellent activity in toluene catalytic combustion, following the Mars–van Krevelen (MVK) mechanism, in which lattice oxygen migration plays a key role. Zhang et al. [17] discovered that partially substituting the B-site Mn in LaMnO₃ perovskite with Co, Ni, or Fe significantly enhanced its catalytic activity for vinyl chloride oxidation. Through a combination of multiple characterization techniques, it was found that this activity improvement is primarily attributed to the enhanced low-temperature reducibility of the B-site, the increased quantity of surface-adsorbed oxygen species, and the formation of oxygen vacancies. The unique structure of perovskite allows flexible ion substitution and doping while maintaining the crystal structure intact [18]. Partial substitution at the A- or B-site exploits valence and radius differences to tune physicochemical properties and enhance catalytic activity [19].

This study employs a sol–gel method to achieve controlled B-site doping in LaMnO₃, synthesizing LaMn_0.5X_0.5O₃ (X = Fe, Cu, Ni) based on the ionic radius differences in transition metals. The evolution of physicochemical properties in the modified catalysts was systematically investigated using multiple characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and oxygen temperature-programmed desorption (O₂-TPD). The research aims to establish structure-activity relationships between doped perovskite and toluene removal performance, thereby elucidating the synergistic catalytic oxidation mechanism of multiple metal sites for toluene elimination.

2. Results and Discussion

2.1. Evaluation of the Catalytic Performance

As shown in Figure 1a, the performance curves of toluene catalytic oxidation over the prepared LaMn_0.5X_0.5O₃ (X = Fe, Cu, Ni) are presented. The temperature at 50% conversion (T₅₀) and that at 90% conversion (T₉₀) are summarized in

Table 1. Characteristic parameters of catalytic oxidation of toluene with LaMn_0.5X_0.5O₃ (X = Fe, Cu, Ni).

Catalyst	Characteristic Parameters
	T50/°C	T90/°C
LaMnO3	247	305
LaMn0.5Ni0.5O3	243	296
LaMn0.5Fe0.5O3	260	309
LaMn0.5Cu0.5O3	262	326

. The results indicate that not all metal ion doping at the B-site of LaMnO₃ enhance the catalytic performance for toluene oxidation. Among them, the LaMn_0.5Cu_0.5O₃ exhibited the poorest performance (T₅₀ = 262 °C, T₉₀ = 326 °C). Similarly, the LaMn_0.5Fe_0.5O₃ also showed relatively low activity (T₅₀ = 260 °C, T₉₀ = 309 °C). However, the catalytic performance of LaMn_0.5Ni_0.5O₃ improved to some extent (T₅₀ = 243 °C, T₉₀ = 296 °C). Compared to LaMnO₃, the T₅₀ was lowered by 4 °C and the T₉₀ by 9 °C.

As shown in Figure 1b, the toluene conversion rates of the LaMn_0.5X_0.5O₃ (X = Fe, Cu, Ni) are presented at 300 °C. The doping of different metal ions at the B-site exerted varying effects on catalytic performance. Specifically, LaMn_0.5Cu_0.5O₃ and LaMn_0.5Fe_0.5O₃ exhibited inhibitory effects. In contrast, LaMn_0.5Ni_0.5O₃ exhibited the highest toluene conversion rate of 92.7%, which was 5% higher than that of LaMnO₃.

In summary, B-site Ni doping is beneficial for enhancing catalytic activity. The toluene oxidation performance of the prepared catalysts, ranked from lowest to highest, follows the order: LaMn_0.5Cu_0.5O₃ < LaMn_0.5Fe_0.5O₃ < LaMnO₃ < LaMn_0.5Ni_0.5O₃.

2.2. XRD Analysis

To determine the crystal structure of the LaMn_0.5X_0.5O₃ catalysts synthesized by the sol–gel method, XRD analysis was performed on the catalyst powders.

As shown in the XRD pattern in Figure 2, the diffraction peaks of LaMnO₃ appear at 22.83°, 32.36°, 32.66°, 39.97°, 46.64°, and 57.90°, which match the characteristic peaks of the LaMnO₃ phase recorded in the Joint Committee on Powder Diffraction Standards (JCPDS) database (PDF#82-1152). These diffraction peaks correspond to the crystal planes (012), (110), (104), (202), (024), and (214), respectively. The characteristic diffraction peaks of the LaMn_0.5X_0.5O₃ series appear at 22.902°, 32.611°, 40.224°, 46.788°, 52.708°, 58.194°, 68.321°, 73.108°, 77.775°, 82.362°, and 86.899°, consistent with an orthorhombic perovskite structure (JCPDS#75-0440). The corresponding crystal planes are (100), (110), (111), (200), (210), (211), (220), (300), (310), (311), and (222), respectively.

The introduction of Cu, Ni, and Fe elements at the B-site is likely to result in slight local structural distortions in the LaMnO₃ perovskite crystal structure [20,21,22]. However, all catalyst samples maintain a single-phase pure perovskite structure, with no detectable impurity diffraction peaks. This indicates that the incorporation of these three elements does not disrupt the perovskite crystal structure or generate new crystal phases.

Table 2. Grain size of LaMnO₃ doped with different metal ions at the B-site.

Catalysts	Grain Size (nm)
LaMnO3	24.7
LaMn0.5Ni0.5O3	19.21
LaMn0.5Fe0.5O3	21.93
LaMn0.5Cu0.5O3	20.96

presents the grain size of the prepared catalysts, calculated using the Scherrer equation [23]. As shown in the table, compared with LaMnO₃, the introduction of a second metal element at the B-site leads to a reduction in grain size for all catalysts. This phenomenon occurs because the incorporation of a second metal component at the B-site causes perovskite grains to segregate at grain boundary regions during growth [24]. This segregation forms local energy barriers that inhibit grain boundary migration and consequently hinder grain growth [25].

The LaMn_0.5Ni_0.5O₃ exhibits the smallest grain size of 19.21 nm. The metal-doped perovskite exhibits a smaller grain size compared to LaMnO₃. This reduction in grain size contributes to an increased specific surface area. Based on previous studies [26,27,28], an increased specific surface area is generally considered conducive to promoting the exposure of active sites, which may be one of the factors contributing to the observed enhancement in catalytic performance.

2.3. SEM Analysis

The surface morphology of a series of prepared LaMn_0.5X_0.5O₃ was investigated using SEM. As presented in Figure 3, the LaMnO₃ exhibits a porous structure with relatively large grain sizes, along with particle agglomeration and disordered arrangement.

In contrast, the B-site-doped perovskites demonstrate significantly increased pore density. The catalyst particles appear quasi-spherical and are well-dispersed in an ordered manner. This is attributed to the introduction of a second metal ion at the B-site, which promotes crystallite refinement and leads to smaller particle sizes.

Furthermore, the SEM images reveal that the surface of the undoped perovskite catalyst is relatively smooth. In contrast, the B-site-doped catalysts exhibit rougher surfaces with more uniform pores. This porous structure is a result of thermal decomposition during the high-temperature calcination step in the catalyst preparation process. It originates from the release of gases from nitrates and citric acid, which generate abundant pore channels.

2.4. XPS Analysis

Figure 4 presents the XPS O 1s envelopes of B-site Ni-, Fe-, and Cu-doped perovskites. Each spectrum is deconvoluted into two peaks: lattice oxygen (~529 eV) and surface chemisorbed oxygen (~531 eV) [29,30]. Deconvolution analysis of the XPS O 1s spectra reveals that B-site doping effectively reduces the binding energy of oxygen species in the catalyst. Compared to the oxygen species in LaMnO₃, a chemical shift of approximately 0.4–0.8 eV was observed. Among them, the LaMn_0.5Ni_0.5O₃ exhibits the lowest oxygen species binding energy. The decreased binding energy of surface chemisorbed oxygen facilitates its desorption from the perovskite surface, thereby promoting participation in toluene oxidation. Meanwhile, the reduction in lattice oxygen binding energy enhances the mobility of lattice oxygen toward surface active sites [31].

In catalytic reactions, active oxygen species participate in the oxidation of reactants. Therefore, a higher concentration of active oxygen in the catalyst can effectively enhance its activity. As shown in

Table 3. Surface oxygen content of LaMnO₃ doped with different metal ions at the B-site.

Catalyst	B.E (eV)	Oads/Olat (%)
LaMnO3	530.7	128.3
LaMn0.5Ni0.5O3	529.9	142.7
LaMn0.5Fe0.5O3	530.2	112.8
LaMn0.5Cu0.5O3	530.4	153.8

, LaMn_0.5Cu_0.5O₃ exhibits a higher surface oxygen content (153.8%) compared to that of LaMnO₃ (128.3%). In toluene catalytic oxidation, surface chemisorbed oxygen species act as the key active oxygen, and their concentration exhibits a positive correlation with catalytic activity [32,33]. This indicates that B-site doping effectively increases both the surface chemisorbed oxygen content and catalytic activity. Doping was shown to increase oxygen vacancy concentration in perovskites, thereby enriching surface chemisorbed oxygen and improving catalytic performance [34].

Figure 5 displays the asymmetric Mn 2p XPS spectra of the catalysts. As shown, the Mn 2p_3/2 peak was deconvoluted into two sub-peaks corresponding to Mn³⁺ (~640.1–641.1 eV) and Mn⁴⁺ (~642.0–643.0 eV) [34,35]. As listed in

Table 4. Mn content in LaMnO₃ doped with different metal ions at the B-site.

Catalyst	Type of Mn
	Mn3+ (%)	Mn4+ (%)	Mn4+/Mn3+
LaMnO3	68.5	31.5	0.46
LaMn0.5Ni0.5O3	60.6	39.4	0.65
LaMn0.5Fe0.5O3	54.3	45.7	0.84
LaMn0.5Cu0.5O3	63.7	36.3	0.57

, B-site transition metal doping promotes the formation of higher Mn oxidation states. This confirms that B-site modification alters the ionic valence distribution in perovskites and favors higher oxidation states.

After B-site doping, the elevated Mn⁴⁺ content enhances electron attraction and acceptance capabilities, thereby improving the catalyst’s oxidative properties and charge transfer efficiency. Concurrently, the higher Mn⁴⁺/Mn³⁺ ratio suggests increased oxygen vacancy concentration on the catalyst surface. However, only optimized doping elements and stoichiometry can simultaneously achieve both high Mn⁴⁺/Mn³⁺ ratios and superior toluene oxidation activity. This explains why LaMn_0.5Ni_0.5O₃ exhibits superior catalytic performance.

As shown in Figure 6, two main peaks corresponding to Ni 2p_3/2 (845–860 eV) and Ni 2p_1/2 (∼870 eV) are observed in the range of 846–890 eV for LaMn_0.5Ni_0.5O₃. Deconvolution analysis of the Ni 2p_3/2 peak reveals two characteristic peaks corresponding to Ni³⁺ (~853 eV) and Ni²⁺ (~850 eV) [36]. The satellite peak of Ni 2p_3/2 is observed in the 856–869 eV range. The coexistence of Ni³⁺ and Ni²⁺ in the Ni 2p spectrum indicates the presence of defects in Ni²⁺ ions, which can generate oxygen vacancies [37].

From Figure 7 and the standard XPS spectra of iron, it can be observed that the LaMn_0.5Fe_0.5O₃ contains iron ions in both Fe²⁺ (peak corresponding to the Fe 2p_3/2 orbital) and Fe³⁺ (peak corresponding to the Fe 2p_1/2 orbital) valence states [38]. The peak of the Fe 2p_3/2 orbital can be deconvoluted into two components corresponding to Fe³⁺ (~710 eV) and Fe⁴⁺ (~711 eV). Similarly, the peak of the Fe 2p_1/2 orbital can be deconvoluted into two components attributed to Fe³⁺ (~724 eV) and Fe⁴⁺ (~725 eV). Additionally, a satellite peak of Fe 2p_3/2 is observed at 718.8 eV [39].

Figure 8 displays the XPS Cu 2p spectrum of the LaMn_0.5Cu_0.5O₃. The peak corresponding to Cu 2p_3/2 can be deconvoluted into two components, attributable to the reduced state Cu⁺ (931.7 eV) and the oxidized state Cu²⁺ (933.5 eV) [40,41]. The spectral features between 936–946 eV and 956–965 eV are identified as satellite peaks of Cu 2p_3/2 and Cu 2p_1/2, respectively [42].

2.5. O₂-TPD Analysis

Perovskite catalysts exhibit exceptional oxygen storage capacity and oxygen mobility due to their unique crystal structure. The concentration of oxygen vacancies in perovskites can be modulated by varying the type and content of B-site metal ions [43]. The oxygen desorption profile of perovskite catalysts typically comprises two distinct features: α-oxygen and β-oxygen [44]. The α-oxygen desorption peak (below 500 °C) corresponds to surface-adsorbed oxygen species at vacancy sites, while the β-oxygen peak (600–800 °C) arises from the release of bulk lattice oxygen. As shown in Figure 9, Ni doping at the B-site lowers the desorption temperatures of both α-oxygen and β-oxygen. This indicates that the LaMn_0.5Ni_0.5O₃ possesses a higher oxygen vacancy concentration and enhanced oxygen mobility at lower temperatures. Ni doping enhances the content of chemically adsorbed oxygen on the catalyst surface. This further increases the oxygen vacancy concentration. Chemically adsorbed surface oxygen has a weaker binding energy. This makes it more readily adsorbed and desorbed, thereby improving catalytic activity. Thus, Ni doping facilitates the formation of surface oxygen, demonstrating higher reactivity of surface-adsorbed oxygen.

In summary, the LaMn_0.5Ni_0.5O₃ exhibits significantly enhanced catalytic activity compared to the LaMnO₃, which aligns with the toluene catalytic oxidation performance tests.

In summary, LaMn_0.5Ni_0.5O₃ exhibits the most favorable overall characteristics: it possesses an appropriate Mn⁴⁺/Mn³⁺ ratio, a high content of surface-adsorbed oxygen, the smallest grain size, and relatively low desorption temperatures for both lattice oxygen and adsorbed oxygen. In contrast, although LaMn_0.5Cu_0.5O₃ shows the highest surface oxygen content, its higher binding energy hinders the migration of lattice oxygen to surface active sites. Meanwhile, LaMn_0.5Fe_0.5O₃ has the highest Mn⁴⁺/Mn³⁺ ratio, but the lowest surface oxygen content. Consequently, LaMn_0.5Ni_0.5O₃ achieves the optimal balance among microstructure, surface chemistry, and oxygen mobility, which contributes to its highest catalytic activity in toluene oxidation.

3. Materials and Methods

3.1. Preparation of Perovskite Samples

In this study, LaMnO₃ and a series of LaMn_0.5X_0.5O₃ (X = Fe, Cu, Ni) were synthesized via the sol–gel method, with citric acid as the complexing agent. The chemical reagents used in the preparation of the perovskite samples are listed in

Table 5. Chemical reagents used for the synthesis of perovskite samples.

Chemical Reagent	Purity	Supplier
Lanthanum nitrate hexahydrate	Analytical Reagent	Aladdin
Nickel nitrate hexahydrate	Analytical Reagent	Sinopharm
Copper nitrate trihydrate	Analytical Reagent	Sinopharm
50% Manganese nitrate solution	Analytical Reagent	Sinopharm
Iron (III) nitrate nonahydrate	Analytical Reagent	Sinopharm
Citric acid monohydrate	Analytical Reagent	Sinopharm
Ethylene glycol	Analytical Reagent	Sinopharm
Ammonia solution	Analytical Reagent	Sinopharm

.

The preparation procedure is shown in Figure 10. First, stoichiometric amounts of nitrate salts were weighed and dissolved in ultrapure water. Citric acid monohydrate was weighed as the complexing agent at a molar ratio of 1:1.5 relative to the total metal cations. After being completely dissolved in the mixed solution, a small amount of ethylene glycol was added as a polymerization agent. The mixture was heated at 40 °C under stirring for 30 min. Ammonia solution was used to adjust the pH to neutral, yielding a neutral precursor solution. The solution was continuously stirred at 80 °C. As polymerization proceeded, the viscosity increased until a wet gel formed, characterized by the cessation of flow and no deformation upon container tilting. The wet gel was dried at 120 °C to obtain a dry gel. Finally, the dry gel was calcined in a muffle furnace at 800 °C for 4 h. After cooling to room temperature, the resulting catalyst was ground and sieved to obtain powdered catalyst with a particle size of less than 75 μm.

3.2. Experimental Process of the Catalytic Oxidation of Toluene

The experiment for evaluating the catalytic oxidation performance of perovskite on toluene was conducted in a fixed-bed reaction system set up in the laboratory, as illustrated in Figure 11. The experimental setup consists of four main components: the gas delivery system, gas mixing system, catalytic oxidation reaction system, and gas detection system.

The gas delivery system comprises gas cylinders, pressure-reducing valves, associated gas pipelines, and flow meters. The gas mixing system includes a syringe pump and a VOCs generator. The reaction system is composed of a vertical tube furnace and a quartz reactor with a diameter of 48 mm. For gas detection, a gas chromatograph equipped with a Flame Ionization Detector was employed for analysis.

Based on preliminary experiments, the reaction space velocity was determined to be 18,000 mL/(g·h). Before starting the experiment, the setup was assembled and tested for airtightness. A catalyst sample weighing 1.0 ± 0.001 g was weighed in advance and placed on the air distributor inside the quartz tube reactor. The toluene flow rate was adjusted to maintain a reaction concentration of 1000 ppm, and the temperature control system was set to the desired reaction temperature. The reactant mixture of toluene and air was then passed through the catalytic bed, where oxidation reactions occurred.

4. Conclusions

Through a systematic investigation of doping strategies with B-site transition metals (Fe, Cu, and Ni), the crystal structure and surface properties of LaMnO₃ were successfully modulated. By combining multiscale characterization and catalytic performance tests, this study clarifies the influence of different B-site metal dopants on the physicochemical properties of the material and its performance in the catalytic oxidation of toluene. The results indicate that the activity of LaMn_0.5X_0.5O₃ (X = Fe, Cu, and Ni) exhibits significant metal dependence, with LaMn_0.5Ni_0.5O₃ demonstrating the best catalytic performance. The toluene conversion temperatures were significantly reduced, with T₅₀ at 243 °C and T₉₀ at 296 °C. Further characterization revealed that Ni doping effectively refines the grain size, decreasing it from 24.70 nm to 19.21 nm. It also increases the surface-adsorbed oxygen content from 128.3% to 142.7%. Furthermore, it significantly enhances the Mn⁴⁺/Mn³⁺ ratio from 0.46 to 0.65. The synergistic improvement of these properties collectively contributes to the enhanced catalytic performance. In summary, LaMn_0.5Ni_0.5O₃ exhibits optimal catalytic activity for toluene oxidation, owing to its optimized microstructure and surface chemical properties.