Effect of Partial Co and Fe Substitution on LaFeO₃@C, LaCoO₃@C Catalysts in the Oxidation of Furfural

Abstract

Pure LaFeO₃@C and LaCoO₃@C and substituted LaFe_1-xCo_xO₃ and LaCo_1-xFe_xO₃ perovskites (x = 0.10; 0.30) were used as catalysts for the liquid-phase oxidation of furfural at 150 °C and 30 bar of O₂ pressure. The perovskites were characterized by XRD, H₂-TPR, N₂ physisorption, TPR-MeOH, and XPS. The carbon in situ incorporation (@C) increases the surface area, favoring oxygen mobility leading to LaFeO₃@C stabilizing the redox pair Fe³⁺/Fe²⁺. In contrast, no evidence of the formation of a LaCoO₃@C perovskite structure through @C incorporation was observed. The gradual substitution of Fe with Co (10 and 30%) in LaFeO₃@C decreases the crystallinity, redox and basic properties, and surface area. For LaCoO₃@C, after the substitution of Co with 10 and 30% of Fe, only metal (La, Fe, Co) oxides as segregated phases were observed. The highest catalytic activity and selectivity to maleic acid of LaFeO₃@C is attributed to the higher surface area, crystalline structure, and surface-reducible Fe³⁺ species, favoring oxygen mobility and promoting their more oxidizing capacity. The lower catalytic activity of LaCoO₃@C, the Co- and Fe-substituted LaFeO₃@C and LaCoO₃@C catalysts, is attributed to the smaller surface area, and the similar selectivity towards maleic acid, 5-hydroxy-2(5H) and furanone indicates that the active site type is not modified in comparison to LaFeO₃@C.

1. Introduction

Furfural is a colorless liquid, obtained from the dehydration of D-xylose or arabinose present in hemicellulose [1], and can be catalytically transformed in various ways such as oxidation, hydrogenation, electrocatalysis, and selective catalytic reduction [2]. One of these treatments of great interest is the partial oxidation of furfural, which allows the production of chemical compounds such as furoic acid, maleic acid, maleic anhydride, and others [3,4]. Regarding metal noble catalysts, Ferraz et al. [5] reported the high furfural oxidation to furoic acid via an Au-supported catalyst to the presence of a significant amount of Au^δ+ species on the surface of the catalyst. Despite the high catalytic activity displayed by noble-metal catalysts, they are expensive and, in some cases, can leach out of the catalyst over time and are, therefore, unattractive from an industrial point of view. In contrast, heterogeneous systems based on metal-oxide catalysts have been proposed to perform the gas- and liquid-oxidation of biomass-derived furans.

Perovskite-type oxides are found to have different uses such as in electrochemical reactions and heterogeneous reactions where oxidation processes stand out [6,7]. In the literature, perovskites have not been reported for furfural oxidation; however, their use in furfural hydrogenation [8] and VOC oxidation [9] has been reported, with high conversions and selectivity. The wide range of possible cation substitutions in a perovskite-type structure generates great flexibility in terms of structure, allowing for adequate adjustment to gain the desired catalytic process. In general, the B cation plays a fundamental role in the electronic structure of the perovskite, modifying its behavior in the catalytic process.

In stoichiometric LaCoO₃, cobalt is mainly present as Co³⁺; however, the Co³⁺/Co⁴⁺ redox pair may arise in the presence of oxygen non-stoichiometry or lattice defects, which can enhance redox properties and contribute to the superior catalytic performance observed in gas-phase oxidation reactions [10] as well as hydrogenation reactions in the batch reactor [11]. Yang et al. [12] reported the enhanced redox properties of LaCoO₃ in the conversion of biomass derivative molecules such as glucose, xylose, and cellulose to lactic acid. Regarding the thermal stability of LaCoO₃ in a reducing atmosphere, Huang et al. [13] reported two peaks in the TPR profile of LaCoO₃ perovskites. These peaks are associated with the reduction of Co³⁺ species to metallic Co⁰, suggesting that different Co³⁺ species are present within the crystallite structure and are reduced at different temperatures, and also indicating that Co³⁺ species in LaCoO₃ can be relatively easily reduced to metallic Co⁰ under a hydrogen atmosphere. Pure LaFeO₃, an almost non-reducible perovskite, has been reported as an active catalyst only for oxidation reactions, enhancing the catalytic performance of the partial substitution with calcium and strontium in the A site [14]. For the aerobic oxidation of lignin using LaFe_xCu_1-xO₃ perovskite, Li et al. [15] report that the extent of Cu-doping and reaction temperature can modify the activity and selectivity towards syringaldehyde.

In spite of the high performance and thermal stability of perovskite oxides in catalytic reactions, a general drawback with their use is that those oxides tend to display low surface areas caused by the high calcination temperatures and hence expose only a limited number of active sites. In this context, the role of the carbon surface chemistry in the enhancement of catalytic activity in biomass-derived furan conversion has been reported, as graphitic species are key active sites for the catalytic oxidation of furfural [16,17]. Xiao et al. [18] studied the effect of in situ incorporation of carbon in LaFeO₃ perovskites, comparing their activity in hydrogenation by catalytic transfer, demonstrating the superior performance of the perovskites after in situ carbon incorporation.

Perovskites such as LaFeO₃ and LaCoO₃ are highly relevant catalytic materials, and B-site cation substitution offers an effective strategy to tune their redox behavior and oxygen mobility. Because Fe and Co differ in oxidation state stability and vacancy formation, partial substitution allows controlled modification of active sites and a clearer understanding of structure–property relationships in furfural oxidation. Additionally, the in situ carbon incorporation (@C) in LaFeO₃@C and LaCoO₃@C enhances oxygen mobility and catalytic performance. Therefore, this study evaluates the effect of Fe and Co substitution (10 and 30%) and in situ carbon incorporation (@C) on the partial oxidation of furfural.

2. Materials and Methods

2.1. Materials

Lanthanum (III) nitrate-hexahydrate (La(NO₃)₃·6H₂O, >99% purity), cobalt(II)nitratehexahydrate (Co(NO₃)₂·6H₂O, >99% purity), Iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O, >99% purity), citric acid (C₆H₈O₇·H₂O, >99% purity), n-dodecane (>99% purity), n-hexadecane(>99% purity), furfural (98% purity), N, N-dimethylformamide (>99% purity) were purchased from Sigma-Aldrich (St. Louis, MO, USA). All chemicals were used without further purification.

2.2. Synthesis of Catalysts

Two pure LaFeO₃@C, LaCoO₃@C perovskites and four substituted LaFe_1-xCo_xO₃ and LaCo_1-xFe_xO₃ perovskites (x = 0.10; 0.30) were prepared by the citrate method [19]. The citrate method is based on the chelation and subsequent polymerization of metal ions to ensure homogeneous distribution prior to calcination. In this procedure, the precursor salts of the A and B cations of the perovskite (typically nitrates) are first solved in water. Citric acid is then added as a chelating agent to form stable citrate–metal complexes with each ion.

To synthesize the perovskites, stoichiometric amounts of aqueous solutions of the nitrates of the corresponding metals (La(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O and Co(NO₃)₂·6H₂O) as precursors salts were added to an aqueous solution of citric acid under stirring for 15 min at room temperature. The resulting solution was slowly evaporated at 70 °C for 1 h under vacuum in a rotary evaporator. The obtained red gel was dried in a vacuum oven, increasing the temperature to 150 °C for 12 h, and finally, each material was pulverized and pyrolyzed under 50 mL min⁻¹ of N₂ flow at 700 °C during 4 h at a heating ratio of 3 °C min⁻¹, to obtain the in situ @C formed in the materials. The in situ carbon (@C) present in the samples originates from the thermal decomposition of the organic precursor used during the synthesis process. The characterization results indicate that the perovskite structure was not always formed, even though, to simplify the nomenclature, the pure samples were denoted as LaFeO₃@C and LaCoO₃@C, and the substituted samples as LaFeO₃ + 10Co@C, LaFeO₃ + 30Co@C, LaCoO₃ + 10Fe@C, and LaCoO₃ + 30Fe@C.

2.3. Catalyst Characterization

X-ray powder diffraction analysis was carried out using a Rigaku 3700 powder diffractometer (radiation source CuKα λ = 1.5406 Å, Concepción, Chile), operated at 30 kV/10 mA, and a solid-state LinxEye detector. Temperature-programmed reduction (H₂-TPR) was obtained using Micromeritics TPD/TPR 2900 (Concepción, Chile) equipment equipped with a thermal conductivity detector. In each experiment, 0.050 g of the sample was heated under 5% H₂/Ar with a flow of 50 mL min⁻¹. The sample was heated at a rate of 10 °C min⁻¹ from room temperature to 900 °C. The BET surface area (S_BET) and textural properties of the perovskite catalysts were determined by N₂ physisorption at 77 K using a Micromeritics Tristar II 3020 instrument (Concepción, Chile). TPRe-MeOH analysis was carried out using 3Flex Micromeritics (Concepción, Chile) coupled to mass spectrometry (MS): 0.050 g of sample was pre-treated under a He flow of 50 mL min⁻¹ at 300 °C at a heating rate of 10 °C min⁻¹. Adsorption was then carried out by saturating methanol with He flow at room temperature for 0.5 h. The solid was then purged with He until the TCD baseline remained stable before the start of the temperature-programmed reaction. The products formed were analyzed by MS and the active sites were classified according to the reaction product formed: acid sites produce DME, basic sites form CO and CO₂, and redox sites produce CH₂O [20]. X-ray photoelectron spectroscopy (XPS) of the calcined catalysts was recorded on a VG Escalab 200R electron spectrometer (Concepción, Chile) using a Mg Kα (1253.6 eV) photon source. The binding energies (BE) were referenced to the C 1s level of carbon support at 284.8 eV. An estimated error of ±0.1 eV can be assumed for all measurements. The intensities of the peaks were calculated from the respective peak areas after background subtraction and spectrum fitting by the standard computer-based statistical analysis which included fitting the experimental spectra.

2.4. Catalytic Tests

The catalytic oxidation of furfural was carried out in a Batch Parr 4841 reactor. In each experiment, 125 µL of furfural (0.232 mol L⁻¹) in N, N-dimethylformamide (50.0 mL) with 100 µL of n-hexadecane as an internal standard was added to the vessel reactor. After adding the catalysts (0.050 g), the reactor was purged with the He flow for 15 min to remove the oxygen content inside the reactor. Still, under the atmosphere, the reactor was heated up to the reaction temperature of 150 °C under stirring at 600 rpm to minimize internal and external mass transfer limitations [21]. O₂ was adjusted to 30 bars of pressure, which was kept constant during the experiment, and then aliquots were periodically withdrawn during the reaction. The reaction products were analyzed by gas chromatography (Nexis GC 2023, Shimadzu, Concepcion, Chile) with a flame-ionization detector (FID) and an RTX-5B column (Perkin Elmer, 30 m × 0.32 mm × 1.0 μm film thickness, Concepción, Chile). The products were also identified by their column retention time in comparison with available standards. The compounds were identified using a GC system coupled to a mass spectrometer (Perkin Elmer Clarus 680 GC–MS SQ8T, Concepción, Chile). The conversion of furfural and yield of the product is defined as follows in Equations (1) and (2):

$$\mathit{F}\mathit{u}\mathit{r}\mathit{f}\mathit{u}\mathit{r}\mathit{a}\mathit{l} \mathit{c}\mathit{o}\mathit{n}\mathit{v}\mathit{e}\mathit{r}\mathit{s}\mathit{i}\mathit{o}\mathit{n}\left({\mathit{X}}_{\mathit{T}}\right)={\displaystyle \frac{\mathit{r}\mathit{e}\mathit{a}\mathit{c}\mathit{t}\mathit{e}\mathit{d} \mathit{m}\mathit{o}\mathit{l} \mathit{o}\mathit{f} \mathit{f}\mathit{u}\mathit{r}\mathit{f}\mathit{u}\mathit{r}\mathit{a}\mathit{l}}{\mathit{i}\mathit{n}\mathit{i}\mathit{t}\mathit{i}\mathit{a}\mathit{l} \mathit{m}\mathit{o}\mathit{l} \mathit{o}\mathit{f} \mathit{f}\mathit{u}\mathit{r}\mathit{f}\mathit{u}\mathit{r}\mathit{a}\mathit{l}}}\times 100$$

(1)

$${(\mathit{Y}\mathit{i}\mathit{e}\mathit{l}\mathit{d} \mathit{o}\mathit{f} \mathit{p}\mathit{r}\mathit{o}\mathit{d}\mathit{u}\mathit{c}\mathit{t})}_{\mathit{i}}={\displaystyle \frac{{(\mathit{m}\mathit{o}\mathit{l} \mathit{o}\mathit{f} \mathit{p}\mathit{r}\mathit{o}\mathit{d}\mathit{u}\mathit{c}\mathit{t})}_{\mathit{i}}}{(\mathit{i}\mathit{n}\mathit{i}\mathit{t}\mathit{i}\mathit{a}\mathit{l} \mathit{m}\mathit{o}\mathit{l} \mathit{o}\mathit{f} \mathit{f}\mathit{u}\mathit{r}\mathit{f}\mathit{u}\mathit{r}\mathit{a}\mathit{l})}}\times 100$$

(2)

The specific rate, r_s (mol g_cat⁻¹ s⁻¹), was calculated from the initial slope of the plot of furfural conversion as a function of time, according to Equation (3):

$${\mathit{r}}_{\mathit{S}}={\displaystyle \frac{(\mathit{b}\times \mathit{n})}{\mathit{m}}}$$

(3)

where b is the initial slope of conversion vs. time plot (s⁻¹), n is the initial moles of the furfural in the solution (mol), and m is the mass of catalysts (g).

The selectivity (%) was determined at 20% of furfural conversion, according to Equation (4):

$$\mathit{S}\left(\%\right)={\displaystyle \frac{{\mathit{X}}_{\mathit{i}}}{{\mathit{X}}_{\mathit{T}}}}\times 100$$

(4)

where X_i is the percentage of product formation i.

3. Results

3.1. Characterization Results

X-ray diffraction analyses of LaFeO₃@C, LaCoO₃@C, LaFeO₃ + xCo@C and LaCoO₃ + xFe@C (X = 10, 30%) were performed to reveal structural transformations and crystallinity, and their results are presented in Figure 1a,b. Figure 1a shows that pure LaFeO₃@C perovskite presents a well-defined crystalline structure and diffraction peaks at 2θ = 22°, 32°, 39°, 45°, 58° and 67° corresponding to the perovskite structure with orthorhombic geometry (JCPDS No. 37-1493) [22] and an overlapping signal at 2θ = 25.9°and 42.7° corresponding to the (002) and (100) diffraction planes of the graphitic carbon base structure (ICDDfileno. 01-075-0444), respectively. As Fe is replaced by 10% and 30% of Co, the perovskite structure is maintained with a drastic decrease in crystallinity. On the contrary, for LaCoO₃@C (Figure 1b), the absence of the diffraction peaks corresponding to the rhombohedral crystalline structure of LaCoO₃ indicates no formation of the perovskite structure. This result indicates that the presence of in situ @C inhibits the LaCoO₃ perovskite structure formation [23] with the appearance of crystalline Co₂O₃ at 2θ = 44° [24].

The respective H₂-TPR profiles are shown in Figure 2. In Figure 2a the pure LaFeO₃@C perovskite presents a broad signal between 350 °C and 550 °C indicative of more than one reduction process. This unexpected result is attributed to the partial reduction of Fe³⁺ to Fe²⁺ species [25], not usually present in LaFeO₃ which is well known as a non-reducible perovskite [26].

H₂-TPR measurements show that the non-reducible LaFeO₃ perovskite exhibits a partial reduction peak of Fe³⁺ species to Fe²⁺ in LaFeO₃@C (Figure S1) and confirm that in situ @C deposition significantly modifies the thermal stability in a reducing atmosphere. In the reduction profiles of the partially substituted perovskites (Figure 2a) the broad signal assigned to the reduction of Fe³⁺ to Fe²⁺ shifts towards lower temperature with Co overlapping the signal of Co³⁺ to Co²⁺. The modification of the reducibility of the LaFeO₃@C perovskite by a partial substitution with Co is attributed to the partial loss of the perovskite structure due to the presence of the Co³⁺/Co²⁺ redox couple. For pure LaCoO₃@C, in Figure 2b the well-defined signal at a lower reduction temperature between 220 °C and 330 °C represents a reduction of Co³⁺ species to Co²⁺ in Co₂O₃ oxide [27]. A shift towards higher temperatures maintaining the well-defined reduction peak of Co³⁺ to Co²⁺ in the presence of iron is attributed to the higher strength of La-Fe-O₃ than La-Co-O₃ disfavoring the reduction of Co³⁺ species [28]. This behavior can be explained by two combinate effects: (i) according to XRD results the LaCoO₃ perovskite is not formed, and the reduction process is due to Co₂O₃ segregating species on the surface, and (ii) the shift to a higher reduction temperature as the Fe substitution increases can be associated with the higher strength of La-Fe-O₃ than La-Co-O₃, which disfavors the reduction of Co³⁺ species.

Figure 3 shows the adsorption–desorption isotherms at 77 K of LaFeO₃@C, LaCoO₃@C, LaFeO₃ + xCo@C and LaCoO₃ + xFe@C (X = 10, 30%). According to the IUPAC classification, all materials display an IV-type isotherm, corresponding to mesoporous solids with a larger H3 hysteresis loop attributed to structural or textural defects produced by the addition of @C [29]. The surface area (S_BET) and pore volume (Vp) are summarized in

Table 1. Textural properties of LaFeO₃@C, LaCoO₃@C, LaFeO₃ + xCo@C and LaCoO₃ + xFe@C (X = 10, 30%).

	SBET (m2 g−1)	Vp (cm3 g−1)
LaFeO3@C	111	0.1
LaFeO3 + 10Co@C	96	0.09
LaFeO3 + 30Co@C	84	0.07
LaCoO3@C	49	0.11
LaCoO3 + 10Fe@C	49	0.10
LaCoO3 + 30Fe@C	36	0.03

. It is noticeable that the large surface area of LaFeO₃@C is almost 10 times higher than the one commonly reported for LaFeO₃, between 5 and 12 m²/g [30,31]. This behavior is similar to that reported by Xiao et al. [12], associated with in situ carbon incorporation, which favors the higher porosity. With the further substitution with 10% and 30% of Co, the slight decrease in S_BET agrees with the partial loss of crystalline structure observed by XRD. The lower values for LaCoO₃@C (Figure 3b) and the Fe-substituted solids were attributed to the segregated Co and Fe species not being incorporated in the structure, in agreement with XRD and H₂-TPR results.

Figure 4 shows the TPRe-MeOH profiles for LaFeO₃@C substituted with Co (Figure 4a) and LaCoO₃@C substituted with Fe (Figure 4b), respectively.

This technique has been used as an indirect method to determine the nature of surface-active sites. The product distribution from the temperature-programmed reaction of methanol (TPRe-OH) can be associated with the presence of: (i) acid sites when dimethyl ether (DME) is formed; (ii) basic sites when carbon monoxide and/or carbon dioxide are formed (CO and CO₂); and (iii) redox sites when formaldehyde (CH₂O) is formed. In addition, the maximum temperature of the desorption peak indicates the strength of the acid/basic/redox sites [32]. For the studied solids, Figure 4 shows that DME (blue signal) was not detected in all catalysts, indicating the absence of acid sites or weak acid sites, not strong enough to transform MeOH to DME. For CH₂O (red signal), a similar signal is detected in Figure 4a,b, indicating that redox properties do not change with Fe or Co substitution in the perovskite-type structure or mixed oxide. Finally, differences in the formation of CO and CO₂ (gray and black signals) indicatives of basic sites are detected. The CO signal in the temperature range from 200 °C to 300 °C shows a similar desorption peak for LaFeO₃@C, LaFeO₃ + 10Co@C, and LaFeO₃ + 30Co@C, while for LaCoO₃@C, LaCoO₃ + 10Fe@C, and LaCoO₃ + 30Fe@C a drastic decrease or disappearance is seen. Regarding CO₂, only LaFeO₃@C presents a desorption peak between 200 °C and 300 °C associated with the large strength of the basic sites associated with the presence of the perovskite structure [33]. For LaCoO₃@C, LaFeO₃ + 10Co@C, LaFeO₃ + 30Co@C LaCoO₃ + 10Fe@C, and LaCoO₃ + 30Fe@C, a drastic decrease in CO₂ at lower temperatures is observed. Similar behavior was reported by Leal et al. [34] for Ru catalysts supported on SrZrO₃ and BaZrO₃, associated with a decrease in the lattice oxygen species in the perovskite structure [35,36] and a loss of the perovskite structure after Co substitution in LaFeO₃@C (XRD results). The second CO₂ desorption peak starting at 300 °C is attributed to the easier carbonation of lanthanum [34]. The TPRe-MeOH results indicate that the catalysts exhibit negligible acidity and similar redox properties, while the main difference among them lies in their basicity, which strongly depends on the presence and stability of the perovskite structure.

The XP spectra of the O 1s, Fe 2p_3/2, and Co 2p_3/2 core-level spectra for the materials are shown in Figure 5 and Figure 6. The C 1s and La 3d_5/2 (Figures S2 and S3), and the BE values of all elements are shown in

Table 2. BE (eV) of Fe 2p_3/2, Co 2p_3/2 and O 1s for LaFeO₃@C, LaCoO₃@C, LaFeO₃ + xCo@C and LaCoO₃ + xFe@C (X = 10, 30%).

	Fe 2p3/2		Co 2p3/2	O 1s
	Fe2+	Fe3+	Co3+	OL	OHC	OW
LaFeO3@C	710.0 (66)	711.8 (34)	---	529.2 (18)	531.8 (67)	533.6 (15)
LaFeO3 + 10Co@C	710.3 (64)	712.1 (36)	779.8 (100)	529.4 (24)	531.7 (69)	533.9 (7)
LaFeO3 + 30Co@C	710.5 (63)	712.4 (37)	780.6 (100)	529.4 (10)	531.6 (84)	533.1 (6)
LaCoO3@C	---	---	780.8 (100)	---	531.2 (79)	532.7 (21)
LaCoO3 + 10Fe@C	710.6 (41)	713.5 (59)	780.9 (100)	---	531.2 (81)	532.7 (19)
LaCoO3 + 30Fe@C	710.6 (61)	712.9 (39)	780.8 (100)	---	531.3 (97)	533.7 (3)

. Figure 5a shows the O 1s core-level spectra of LaFeO₃@C and the substitution with Co (10% and 30%), and Figure 5b LaCoO₃@C and the substitution with Fe (10% and 30%). The O 1s signal for perovskite-type oxides can be deconvoluted into three peaks: (i) lattice oxygen (O_L) of the O²⁻ at 530 eV; (ii) surface O₂²⁻ ads and O-ads (O_HC) at 531 eV; and (iii) weakly bonded O²⁻ (O_W) species at 532 eV [37]. The lattice and surface oxygens are always present, and the weakly bonded O²⁻ (O_W) species is present only when surface molecular water was not removed before the XPS measurement. In Figure 5a, the three expected peaks appear only for LaFeO₃@C, whereas for LaCoO₃@C the lattice oxygens species does not appear (Figure 5b). This result is in line with XRD, indicative that in the presence of @C the perovskite structure in the LaFeO₃@C catalyst is formed and not formed in LaCoO₃@C. Considering that LaFeO₃ is a well-known stoichiometric perovskite with large thermal stability in oxidant and reduction atmospheres [38,39], it can be concluded that the presence of in situ @C does not affect the formation of the perovskite structure. Conversely, the characteristic Co²⁺/Co³⁺ surface atomic ratio is around 0.6, similar to previously reported [40,41]. For the non-stoichiometric LaCoO₃ perovskite is not observed, indicating that the presence of in situ @C inhibits the formation of the perovskite structure in LaCoO₃@C. In Figure 5a, a monotonic decrease in network oxygens (O_L) is observed with 10% and 30% Co substitution in LaFeO₃@C. This trend indicates the formation of a less stoichiometric structure, associated with the presence of the Co³⁺/Co²⁺ redox couple and higher adsorbed electrophilic oxygen species (O_HC) [42]. Whereas, in line with XRD, in Figure 5b, the substitution of 10% and 30% of Fe, in LaCoO₃@C, does not allow the appearance of the lattice oxygen O_L species of a perovskite structure.

The XP spectra, of the B cation (Fe and Co) are shown in Figure 6a,b. In Figure 6a, the presence of surface species of Fe³⁺ and Fe²⁺ located at 710 eV and 711.8 eV respectively for LaFeO₃@C is not an expected result, due to LaFeO₃ being a well-known stoichiometric perovskite with the presence of only bulk and surface Fe(III). This result indicates that in situ @C enhances the non-stochiometric properties of surface iron in LaFeO₃@C and the further 10% and 30% Co substitution does not show significant differences (Figure 6a), maintaining the non-stochimetric characteristic of the perovskite.

The respective Co 2p_3/2 spectra (Figure S4a) indicate surface Co₂O₃ as segregate phases. With regard to LaCoO₃@C and the 10% and 30% Fe-substituted (Figure 6b) Co2p_3/2, the single peak at 779 eV, corresponding to surface Co³⁺ species, with the corresponding satellite peaks centered at ca. 790 eV, indicates only surface Co₂O₃ as a segregated phase. The absence of species associated with Co²⁺ surface species at 779 eV [43,44] indicates the effect of in situ carbon inhibits the formation of the perovskite structure for LaCoO₃@C. Therefore, in line with the XRD, the labeled LaCoO₃@C corresponds to surface Co₂O₃ on @C. The XP spectra of C1s at 289.8 eV (Figure S2) do not show differences in the @C species, and the two components of La3d_5/2 at 834.3 and 838.1 eV indicate that surface lanthanum is present as La³⁺ (Figure S3) [14,45,46]. The surface La/(La + Fe + Co) ratios shown in

Table 3. Atomic surface ratio for LaFeO₃@C, LaCoO₃@C, LaFeO₃ + xCo@C and LaCoO₃ + xFe@C (X = 10, 30%).

	La/(La + Fe + Co)	Fe/(Fe + Co)	Co/(Fe + Co)
LaFeO3@C	0.72 (0.5)	1.0 (1.0)	---
LaFeO3 + 10Co@C	0.79 (0.5)	0.90 (0.9)	---
LaFeO3 + 30Co@C	0.82 (0.5)	0.85 (0.7)	---
LaCoO3@C	0.86 (0.5)	---	1.0 (1.0)
LaCoO3 + 10Fe@C	0.91 (0.5)	---	0.59 (0.9)
LaCoO3 + 30Fe@C	0.94 (0.5)	---	0.54 (0.7)

indicate a surface enrichment of La for all catalysts. Concerning the B cation, the Fe/(Fe + Co) atomic ratio value indicates the formation of a non-stoichiometric LaFeO₃ perovskite structure with surface Fe enrichment for LaFeO₃ + 30Co@C and, the same behavior is obtained for LaCoO₃@C + (10 and 30% Fe) in the Co/(Fe + Co) surface ratio.

The XPS results confirm that the formation of the perovskite structure strongly depends on the B-site cation in the presence of in situ carbon (@C). For LaFeO₃@C, the O 1s spectra show the characteristic lattice oxygen species, confirming the preservation of the perovskite structure, in agreement with the XRD results. However, the presence of both Fe³⁺ and Fe²⁺ species suggests that the incorporation of @C promotes surface non-stoichiometry and enhances the redox character of the iron species.

In contrast, for LaCoO₃@C the absence of lattice oxygen species together with the presence of Co₂O₃ segregated phases indicates that the incorporation of @C inhibits the formation of the perovskite structure. Consequently, the material corresponds to cobalt oxide species dispersed on the carbon-containing matrix rather than a true LaCoO₃ perovskite phase.

Overall, these results demonstrate that in situ carbon incorporation preserves the LaFeO₃ perovskite framework while modifying its surface redox properties, whereas in the case of LaCoO₃ it prevents the formation of the perovskite structure, leading instead to segregated cobalt oxide phases.

3.2. Catalytic Activity

The conversion of furfural and yield of products as function of time were carried out at 150 °C and 30 bars of O₂ pressure for LaFeO₃@C substituted with Co and LaCoO₃@C substituted with Fe, at 10% and 30% (Figure S5). A blank test was performed showing that furfural was unreactive in the absence of catalysts and the pure LaFeO₃@C catalyst presents the higher conversion close to 100% at 4 h of reaction. When Fe is substituted with 10% of Co the conversion decreases to 80%, and after 30% of Co substitution, a drastic decrease ~60% was detected (

Table 4. Catalytic activity of LaFeO₃@C, LaCoO₃@C, LaFeO₃ + xCo@C and LaCoO₃ + xFe@C (X = 10, 30%).

	Xt (%)	Initial Rate (vo) × 104 (mol g−1 s−1)	Intrinsic Rate (Ir) × 106 (mol m−1 s−1)
LaFeO3@C	98	7.1	6.4
LaFeO3 + 10Co@C	77	4.5	4.7
LaFeO3 + 30Co@C	31	4.7	5.6
LaCoO3@C	37	5.1	10.4
LaCoO3 + 10Fe@C	40	4.9	10.0
LaCoO3 + 30Fe@C	36	4.5	12.5

). For pure and substituted LaCoO₃@C with 10 and 30% of Fe (Figure S5b), a similar furfural conversion of ~40% is observed. The larger furfural conversion obtained with LaFeO₃@C in comparison to LaCoO₃@C and the Fe- or Co-substituted catalysts is related to their physical-chemical composition, a non-stoichiometric perovskite-type structure with enhanced redox properties and high surface area favoring the oxygen mobility and diffusion of furfural on the active sites. In Table 4, the catalytic activity is presented both per gram of catalyst (initial rate, mol g⁻¹ s⁻¹) and per surface area of the materials (intrinsic rate, mol m⁻² s⁻¹). A similar trend in catalytic performance is observed when the activity is normalized per gram of catalyst, which is consistent with the conversion values obtained for the different catalysts. However, when the activity is expressed per surface area (mol m⁻² s⁻¹), a different behavior is observed. In this case, the pure LaFeO₃@C catalyst displays slightly higher activity compared to the Co-substituted samples (10 and 30%), suggesting that both textural and redox properties influence the catalytic performance. On the other hand, the LaCoO₃@C catalyst substituted with 30% Fe shows the highest catalytic activity when normalized by surface area, which indicates that the higher presence of Fe redox-active sites enhances the intrinsic catalytic activity.

Concerning product distribution, Figure 7 shows the selectivity of the perovskites calculated at 20% of furfural conversion.

It is seen that LaFeO₃@C shows the largest selectivity (>90%) towards maleic acid (MA) and for the Co- and Fe-substituted LaFeO₃@C and LaCoO₃@C catalysts, appears 5-hydroxy-2(5H) furanone as an intermediate, furoic acid, formic acid and maleic anhydride appear as secondary products. These results are associated with the oxidizing capacity of the redox Fe³⁺/Fe²⁺ species present in the LaFeO₃@C perovskite structure. In line with this, H₂-TPR shows the signal associated with the partial reduction of Fe³⁺ to Fe²⁺ attributed to in situ @C incorporation, the presence of lattice oxygens and surface Fe(II) and Fe(III) by XPS, and the higher strength of the basic sites associated with lattice oxygen corroborated by TPR-MeOH. However, for LaFeO₃ + xCo@C and LaCoO₃ + xFe@C (X = 10 and 30%) catalysts the appearance of 5-hydroxy-2(5H) furanone as an intermediate is associated with the decrease in the redox properties through the presence of surface segregated metal oxides.

4. Conclusions

The effect of the partial substitution of the B cation (Fe and Co) and carbon in situ incorporation (@C) in LaCoO₃ and LaFeO₃ on the oxidation of furfural was studied. It was found that in situ @C increased the surface area and favored oxygen mobility in LaFeO₃@C and inhibited the formation of the perovskite structure in LaCoO₃@C perovskite. The presence of the pair redox Fe³⁺/Fe²⁺ in the LaFeO₃@C perovskite structure enhances their catalytic properties. The decrease in the surface area, the Fe³⁺/Fe²⁺ redox pair, the strength of basic sites, and the appearance of a segregated phase is detected upon the partial substitution of cobalt, explaining the lower catalytic performance. For LaCoO₃@C and LaCoO₃ + xFe@C materials, only metallic (La, Co, Fe) oxides were observed as segregated phases with lower surface area and basic properties. The result of catalytic activity indicates that textural and redox properties influence the reactivity in oxidation of furfural, and the similar values of selectivity to maleic acid, 5-hydroxy-2(5H) furanone for the LaCoO₃@C and the Co- and Fe-substituted LaFeO₃@C and LaCoO₃@C catalysts indicates no modification of their active sites in comparison to LaFeO₃@C.