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. 2026 Oct 9. Online ahead of print. doi: 10.1039/d6ra06388h

Influence of aliovalent doping on the La2−xSrxNi1−yCuyO4 for catalytic dry reforming of methane

Aathira Bhaskaran a, Chandan Kumar Mandal b, Arup Gayen b, Satyapaul A Singh c,d, Sounak Roy a,d,✉
PMCID: PMC13651111  PMID: 42857589

Abstract

Aliovalent A- and B-site co-doping of the Ruddlesden–Popper layered perovskite La2NiO4 is demonstrated as a rational strategy to tune redox properties and enhance catalytic performance in dry reforming of methane. Sr2+ substitution at the A-site of La1.6Sr0.4Ni1−yCuyO4 induces Ni2+ → Ni3+ oxidation and changes in the oxygen-deficient surface environment, enhancing surface basicity and CO2 chemisorption. Progressive Cu2+ incorporation at the B-site lowers the thermodynamic threshold for Ni reduction forming bimetallic NiCu alloy phases at substantially lower temperatures than the pure Ni exsolution. Among the catalysts, La1.6Sr0.4Ni0.7Cu0.3O4 exhibited superior performance with negligible coke deposition. In situ FTIR analysis, together with XPS results, suggested the formation of carbonate, CHx, CHxO, formate, and COOH surface intermediates, indicating the involvement of diverse surface species during DRM that may involve reactive surface oxygen species. This work establishes synergistic Sr/Cu co-doping of La2NiO4 as an effective, scalable approach for active, selective, and coke-resistant catalysts for syngas production via dry reforming of methane.

Aliovalent A- and B-site co-doping of the Ruddlesden–Popper layered perovskite La2NiO4 is demonstrated as a rational strategy to tune redox properties and enhance catalytic performance in dry reforming of methane.graphic file with name d6ra06388h-ga.webp

1. Introduction

Natural gas, primarily composed of methane (CH4), is one of the most abundant and cost-effective C1 feedstocks, yet logistical barriers at remote sites often result in its flaring or venting, thereby exacerbating greenhouse gas emissions.1 A strategically compelling approach to mitigate these emissions while simultaneously generating economic value lies in the catalytic conversion of CH4 into synthesis gas (syngas; H2/CO mixtures), a versatile platform intermediate in C1 chemistry. Among the established methane reforming technologies, Dry Reforming of Methane (DRM) has emerged as a particularly promising dual-purpose strategy. By co-utilizing the two most climatically significant greenhouse gases CH4 and CO2 as co-reactants, DRM offers an integrated pathway for emissions remediation coupled with syngas production.2 DRM is also an attractive route for biogas valorization, as biogas primarily consists of CH4 and CO2, which can be directly converted into syngas through this process.3 The syngas thus obtained serves as a critical precursor for the downstream synthesis of a broad spectrum of high-value chemicals and fuels. Beyond its well-established application in conventional Fischer–Tropsch synthesis and methanol production, syngas can be further upgraded via higher alcohol synthesis routes to yield oxygenated compounds, which are of considerable industrial significance.4

However, the commercialization of DRM is hindered by several technical drawbacks, most notably the rapid deactivation of catalysts due to coke deposition and the thermal sintering of active metal sites due to high operating temperature5 leading to a blockage of active surface area and a decline in the H2/CO product ratio. The noble metals (such as Ru, Pt, and Rh)6,7 are renowned for their high catalytic efficiency and minimal coking, their scarcity and prohibitive cost render them impractical for large-scale industrial applications.8–10 As a result, transition-metal-based oxides have attracted considerable attention as cost-effective catalytic materials for a range of catalytic applications including DRM.11–14 In the context of DRM, Ni-based catalysts, in particular, are favoured for their high availability and competitive performance. Specifically, the use of structured oxides such as defective fluorite,15 perovskites,16,17 and layered perovskites18 has shown great promise due to their high oxygen mobility and redox flexibility, properties considered favourable for facilitating the redox cycling required in reactions such as DRM. In a prior investigation of DRM over the layered perovskite La2NiO4, comprehensive structural characterization combined with surface-sensitive probing techniques revealed that the inherent structural robustness of this material arising from its Ruddlesden–Popper (RP) phase architecture, which integrates both the LaNiO3 perovskite sublattice and the LaO rock-salt sublattice effectively suppresses the formation of metallic Ni0 species under DRM operating conditions. The absence of appreciable Ni0 exsolution was identified as a critical limiting factor, as the generation of catalytically active metallic nickel sites is considered essential for DRM activity.19 This observation is consistent with the broader mechanistic understanding established in the literature, wherein controlled reductive exsolution from perovskite-type lattices can generate finely dispersed Ni nanoparticles that remain strongly anchored to the parent oxide matrix. Such metal–support interactions can enhance the resistance of Ni species to sintering under the harsh thermal conditions of DRM, while maintaining accessible catalytic sites.14 Motivated by these mechanistic insights, a rational co-doping strategy targeting both the A-site and B-site of the perovskite lattice was pursued in the present work. This approach was designed to systematically lower the thermodynamic threshold for Ni exsolution by tuning the reducibility of the B-site cation environment and introducing lattice strain, while concurrently optimizing intrinsic catalytic properties such as oxygen-deficient surface species concentration, surface basicity, and metal–support interactions.

Building on this rational design framework, the present study systematically examines how A and B-site co-doping influences the structural, redox, and catalytic properties of La1.6Sr0.4Ni1−yCuyO4 (y = 0, 0.1, 0.3). Partial substitution of La3+ by Sr2+ in La1.6Sr0.4NiO4 induces charge compensation through Ni2+ → Ni3+ oxidation accompanied by the formation of oxygen vacancy.20 The presence of Ni3+ and oxygen vacancy enhances reducibility facilitating CH4 activation. Crucially, Sr2+ incorporation amplifies the Lewis basicity of the surface, strengthening CO2 chemisorption in DRM.21 Cu2+ reduces to metallic Cu0 at lower temperatures than either Ni3+ or Ni2+, and the resulting Cu0 crystallites promote the reduction of neighbouring nickel cations.22 At the B-site, incorporation of Cu2+ takes advantage of its lower reduction potential compared to Ni, as evidenced by individual reported H2-TPR profiles that display a distinct shift to lower reduction temperatures.23,24 This behaviour is consistent with a nucleation-mediated co-reduction mechanism, in which in situ-formed Cu0 nuclei seed the reduction of adjacent Ni2+/Ni3+ species via hydrogen spillover based on previous literature.25 Owing to the complete structural miscibility of Cu and Ni, this process culminates in spontaneous Ni–Cu alloy formation at substantially lower temperatures compared to pure Ni exsolution.26 The catalysts here synthesized by the facile single-step solution combustion method, were evaluated for DRM without any ex situ pre-reduction, rendering the exsolution process to proceed operando. Structural evolution, lattice defect chemistry, and oxidation state distributions were probed using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Redox characteristics were investigated by H2-temperature programmed reduction (H2-TPR), while surface basicity and CO2 adsorption behaviour were assessed via CO2-temperature programmed desorption (CO2-TPD). In situ Diffuse reflectance infrared Fourier transform spectroscopy was employed to identify surface intermediates and to elucidate pathways of carbon deposition and reforming, revealing the mechanistic insights. Collectively, this multi-technique approach establishes clear correlations between dopant identity, exsolution dynamics, and surface chemistry, and demonstrates how these factors govern catalytic activity, and resistance to carbon accumulation under DRM conditions.

2. Materials and methods

La2−xSrxNi1−yCuyO4 (x = 0.4; y = 0, 0.1, and 0.3) catalysts were synthesized via a single-step solution combustion technique.27–30 In this method, water-soluble metal nitrates served as oxidizing agents, while glycine (C2H5NO2) was used as the fuel. The oxidizer-to-fuel ratio was determined based on propellant chemistry principles to ensure a stoichiometric redox reaction. Appropriate amounts of La(NO3)3·6H2O (SRL Chemicals, 99%), Ni(NO3)2·6H2O (SD Fine, 99%), Cu(NO3)2. 3H2O (SD Fine, 99%), Sr(NO3)2 (Sigma-Aldrich, 99%) and glycine (Sigma-Aldrich, 99%) corresponding to the desired oxide composition were placed in a 400 mL crystalline dish. The precursors were dissolved in 40 mL of deionized water to form a clear homogeneous solution. This solution was then transferred to a muffle furnace preheated to 450 °C. Upon heating, the solution initially boiled and foamed, followed by gradual evaporation of water. Subsequent to dehydration triggered a self-sustained combustion reaction, characterized by surface flames and rapid gas evolution, ultimately yielding a voluminous, fluffy solid product. The generalized combustion reaction can be expressed as follows:

1.6La(NO3)3·6H2O + 0.4Sr(NO3)2 + (1 − y) Ni(NO3)2·6H2O + yCu(NO3)2·3H2O + 4.18C2H5NO2 → La1.6Sr0.4Ni1−yCuyO4 + 8.36CO2 + [26.04 − 3y]H2O + 5.89N2 1

After naturally cooling to room temperature, the as-combusted material was ground into fine powder. Finally, the powder was calcined at 1000 °C for 5 hours to eliminate residual species and obtain phase-pure oxide materials.

The crystalline structure of the synthesized materials was examined using powder XRD on a Rigaku Ultima IV diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å). Diffraction patterns were recorded with a step size of 0.01° and a scanning speed of 0.33° min−1. The average crystallite size (D) was estimated using the Debye–Scherrer equation: D = 0.9λ/(B cos θ), where λ represents the X-ray wavelength, B denotes the full width at half maximum of the diffraction peak, and θ is the Bragg angle. Surface chemical composition and oxidation states were analyzed by XPS using a Thermo Scientific K-Alpha surface analysis system with Al Kα radiation (1486.6 eV) as the excitation source. The binding energy values were calibrated using the C 1s peak at 284.8 eV as a reference. Morphological features and surface characteristics were investigated using field-emission scanning electron microscopy (FE-SEM) on an FEI Apreo S instrument. The reducibility of the catalysts was studied by H2 temperature-programmed reduction (H2-TPR) using an Altamira Instruments (AMI) system equipped with a thermal conductivity detector (TCD). In a typical experiment, approximately 45 mg of catalyst was initially pretreated under Ar flow at 150 °C for 1 h to remove adsorbed species. After cooling to room temperature, a 10% H2/Ar mixture was introduced at a total flow rate of 30 mL min−1. The temperature was then increased from room temperature to 900 °C at a ramp rate of 10 °C min−1, while the H2 consumption was continuously monitored using the TCD. The TCD response was calibrated by pulse experiments using a known quantity of H2, and the calibrated TCD signal was integrated to quantify the total H2 consumption during the reduction process. The same apparatus was used to perform CO2-TPD measurements. Approximately 45 mg of catalyst was pretreated at 150 °C for 1 hour under helium flow. After cooling to room temperature, adsorption was carried out using a 10% CO2/He mixture at 30 mL min−1 for 1 hour to achieve stabilization. Subsequently, the CO2 supply was stopped, and He gas was passed at 100 mL min−1 to remove physisorbed CO2. The temperature was then ramped to 750 °C at 10 °C min−1, and the CO2 desorption profiles were recorded. Additionally, in situ Fourier-transform infrared (FTIR) spectroscopy was conducted using a PerkinElmer Spectrum 3 FTIR spectrometer. The catalyst was loaded into the reaction chamber and enclosed with a ZnSe-windowed dome equipped with a Praying Mantis attachment and a PID temperature controller (Harrick, USA). The catalyst was heated to 400 °C under N2, and a background spectrum was recorded. The reaction gases were then introduced, and FTIR spectra were acquired from 400 to 650 °C with a temperature difference of 50 °C and also collected a time dependent spectra at 650 °C.

For catalytic performance evaluation, the synthesized powders were pelletized, crushed, and sieved to 60/80 mesh before being loaded into the fixed-bed reactor. Approximately 200 mg of the catalyst was loaded into a fixed-bed quartz tubular reactor (4 mm internal diameter and 1 cm catalyst bed length). The catalyst bed was held in position using ceramic wool. The reactor was placed inside a tubular furnace, and the reaction temperature was monitored and controlled using a temperature controller coupled with a K-type thermocouple. The total inlet gas flow was maintained at 100 mL min−1, corresponding to a gas hourly space velocity (GHSV) of 47 770 h−1. The feed composition consisted of CH4 and CO2 supplied at 10 mL min−1 each, with nitrogen used as the balance gas. All gases were procured from CHEMIX (Bangalore). Individual gas lines were equipped with pressure regulators and needle valves, and the gases were mixed through a manifold system before entering the reactor. The flow rates were verified using a bubble flow meter. Catalytic activity measurements were conducted at temperatures ranging from 600 to 900 °C. At each temperature, the system was allowed to reach a stable temperature, typically for approximately 20–30 min, before the reactor effluent was sampled for GC analysis. The reactor effluent was periodically sampled using a Hamilton gas-tight syringe (maximum capacity 300 µL) and manually injected into a Nucon 5765 gas chromatograph. Product analysis was carried out using a flame ionization detector (FID) for CH4, CO, and CO2, with a methanizer installed upstream of the FID for CO and CO2 detection, and a thermal conductivity detector (TCD) for H2 detection. Quantification of reactants and products was achieved through calibration using standard gas mixtures with known concentrations. Calibration curves were constructed by plotting peak area against concentration, and the corresponding response factors were obtained from the slopes. The conversion of the reactant gases was calculated using the following equations:

2. 2
2. 3
2. 4

Thermogravimetric analysis (TGA) was employed to quantify carbon deposition on the spent catalysts. The measurements were performed using a Shimadzu DTG-60 thermal analyser, where the exhausted catalyst samples were heated from 30 to 900 °C under an oxygen atmosphere at a heating rate of 10 °C min−1.

The coke formation rate is calculated as

2. 5

where Wt. loss is the weight difference of the used catalyst calculated from TGA under O2 atmosphere, w is the weight of the catalyst taken, and t is the duration of time on stream reaction.

3. Results and discussion

As synthesised catalysts, La1.6Sr0.4NiO4, La1.6Sr0.4Ni0.9Cu0.1O4 and La1.6Sr0.4Ni0.7Cu0.3O4 were evaluated for dry reforming of methane over a temperature range of 600–900 °C as presented in Fig. 1. La1.6Sr0.4NiO4, with Ni as the sole active site, exhibited no appreciable reactivity at lower temperatures (Fig. 1a). A measurable rate of CO2 conversion was observed only from 750 °C; however, CH4 conversion remained negligible at this temperature, which may be attributed to the alkaline nature of Sr facilitating preferential CO2 adsorption and consumption at this stage. As a result, the H2/CO ratio over La1.6Sr0.4NiO4 at 750 °C was only 0.3 (Fig. 1d). Significant conversions of both CO2 and CH4 were observed only from 800 °C onwards. At 850 °C and 900 °C, CH4 conversion reached ∼54%, while CO2 conversion reached ∼83% with a H2/CO ratio 0.7. Upon partial substitution of Ni with Cu (La1.6Sr0.4Ni0.9Cu0.1O4), a notable enhancement in catalytic activity was observed, with measurable conversions of both CO2 and CH4 recorded from 750 °C onwards (Fig. 1b). At elevated temperatures of 850 °C and 900 °C, CH4 and CO2 conversions increased to 65% and 90%, respectively, accompanied by an improved H2/CO ratio of 0.8. This enhancement suggests that Cu incorporation at the B-site contributes to enhanced catalytic activity toward DRM. Notably, the H2/CO ratio declines from 850 °C onwards for La1.6Sr0.4NiO4, and 900 °C for La1.6Sr0.4Ni0.9Cu0.1O4, along with minimal/reduced conversion of CH4 likely reflecting thermally induced sintering of the Ni-rich phase and consequent loss of accessible metallic Ni sites. Further increasing the Cu content at the B-site (La1.6Sr0.4Ni0.7Cu0.3O4) resulted in a substantial improvement in catalytic performance across the entire temperature range (Fig. 1c). At 750 °C, CH4 and CO2 conversions reached 52% and 68%, respectively, with a H2/CO ratio of 0.83, indicating a markedly more balanced activation of both reactants compared to the Cu-free counterpart. At higher temperatures of 850 °C and 900 °C, CH4 and CO2 conversions further increased to 86% and 92%, respectively, with the H2/CO ratio approaching unity, suggesting a reaction stoichiometry closer to the theoretical DRM equilibrium. Overall, the progressive incorporation of Cu at the B-site of the Ruddlesden–Popper structured catalysts led to a systematic enhancement in DRM activity, with La1.6Sr0.4Ni0.7Cu0.3O4 demonstrating the highest catalytic performance in terms of both reactant conversion and H2/CO ratio across the entire temperature range, making it the most promising candidate among the series evaluated.

Fig. 1. Steady-state dry reforming of methane over the as-synthesized (a) La1.6Sr0.4NiO4, (b) La1.6Sr0.4Ni0.9Cu0.1O4, (c) La1.6Sr0.4Ni0.7Cu0.3O4, and (d) H2/CO ratio over a wide temperature range of 600–900 °C (reaction conditions: 200 mg loading, 1 cm bed length, gas space velocity 47 770 h−1, 1 atm).

Fig. 1

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The catalytic results prompted us to evaluate the structure of the catalysts before and after the reaction. The powder XRD patterns of the as-synthesized samples in Fig. 2a confirm the formation of well-defined tetragonal Ruddlesden–Popper crystalline phases with the I4/mmm space group (JCPDS no. 72–0247).20 With partial substitution of B-site of Ni by Cu in La1.6Sr0.4Ni0.9Cu0.1O4 and La1.6Sr0.4Ni0.7Cu0.3O4 shifts the diffraction reflections progressively toward lower 2θ values relative to La1.6Sr0.4NiO4, consistent with systematic lattice expansion (inset Fig. 2a). This behaviour could be attributed to the larger ionic radius of Cu2+ (0.73 Å, CN = 6) compared to Ni2+ (0.69 Å, CN = 6). Crystallite size, estimated from the most intense (103) reflection, was calculated to be 54.8, 46.7, and 47.4 nm for La1.6Sr0.4NiO4, La1.6Sr0.4Ni0.9Cu0.1O4, and La1.6Sr0.4Ni0.7Cu0.3O4, respectively. The XRD patterns of the exhausted catalysts in Fig. 2b reveal that initial Ruddlesden–Popper phases are retained very minimum after the DRM reaction. All three exhausted samples exhibit phase segregation, with reflections corresponding to La2O3, NiO, and CuO, alongside metallic Ni and Ni–Cu alloy phases.31 The reflections around 43–45° may involve overlapping contributions from the NiO (200) and metallic Ni (111) planes. Notably, the metallic FCC Ni peak observed at 2θ = 44.5° in La1.6Sr0.4NiO4 shifts to lower angles, with increasing Cu content (Fig. 2b), appearing at 44.02° in La1.6Sr0.4Ni0.7Cu0.3O4 consistent with the formation of a Ni–Cu alloy and Cu phase in which the larger Cu atoms expand the metallic lattice.32,33 Under DRM conditions, the elevated reaction temperature (750–900 °C) induces in situ reduction and exsolution of Ni species from the Ruddlesden–Popper lattice, accompanied by the formation of La2O3, which plays a crucial role in CO2 activation through the intermediate formation of La2O2CO3.19 The exsolved metallic Ni species derived from the perovskite lattice can provide active sites for CH4 activation during DRM, while Ni–Cu species have also been reported to facilitate CH4 activation in Ni–Cu-based catalysts.16,34 The FE-SEM images of the fresh catalysts reveal a porous but agglomerated morphology with an average particle size of approximately 80–100 nm (Fig. 2c). However, the FE-SEM images of the spent La1.6Sr0.4NiO4, La1.6Sr0.4Ni0.9Cu0.1O4, and La1.6Sr0.4Ni0.7Cu0.3O4 catalysts exhibit a more agglomerated morphology without visible filamentous carbon, suggesting sintering of the active metal phase with minimal coke deposition under DRM conditions (Fig. 2d). EDX and elemental mapping of the fresh La1.6Sr0.4NiO4, La1.6Sr0.4Ni0.9Cu0.1O4, and La1.6Sr0.4Ni0.7Cu0.3O4 catalysts displayed a uniform dispersion (Fig. S1). Whereas, elemental mapping of the La1.6Sr0.4Ni0.9Cu0.1O4, and La1.6Sr0.4Ni0.7Cu0.3O4 catalysts after reaction exhibits a slight variation in elemental dispersion indicating possibility of exsolution as shown in Fig. S2.

Fig. 2. XRD patterns and SEM images of (a and c) as-prepared and (b and d) exhausted catalytic materials La1.6Sr0.4NiO4, La1.6Sr0.4Ni0.9Cu0.1O4 and La1.6Sr0.4Ni0.7Cu0.3O4.

Fig. 2

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To investigate the surface chemical states and local bonding environments arising from the compositional modifications, XPS analysis was performed on the as-prepared as well as the exhausted La1.6Sr0.4NiO4, La1.6Sr0.4Ni0.9Cu0.1O4, and La1.6Sr0.4Ni0.7Cu0.3O4 catalysts. Due to the overlap between La 3d and Ni 2p signals, the Ni 3p region was selected for quantitative XPS analysis. The Ni 3p spectra of the fresh catalysts in Fig. 3a exhibit contributions from both Ni2+ and Ni3+ species, deconvoluted into their respective spin–orbit doublets. La1.6Sr0.4NiO4 exhibits a Ni2+/Ni3+ ratio of 3.45. Upon Cu substitution at the B-site, the Ni2+/Ni3+ ratio was found to be to 4.10 and 4.31 in La1.6Sr0.4Ni0.9Cu0.1O4, and La1.6Sr0.4Ni0.7Cu0.3O4, respectively. Under DRM conditions, Ni3+ species are reduced to Ni2+ and further to metallic Ni0 (Fig. 3b), as evidenced by a shift of approximately 0.2 eV in the Ni 3p binding energy toward lower values.19 The increase in the Ni2+ fraction after reaction is attributed to partial reduction of Ni3+ during CH4 activation, while the emergence of Ni0 confirms reductive restructuring of the catalyst surface. The Ni2+/Ni3+ ratios increased to 5.40, and 5.91 in the spent La1.6Sr0.4Ni0.9Cu0.1O4, and La1.6Sr0.4Ni0.7Cu0.3O4, respectively. The corresponding Ni0/Ni2+ ratios were 0.16, 0.43, and 0.58 for La1.6Sr0.4NiO4, La1.6Sr0.4Ni0.9Cu0.1O4, and La1.6Sr0.4Ni0.7Cu0.3O4, respectively. The progressive increase in the Ni0/Ni2+ ratio with Cu substitution indicates a higher relative contribution of metallic Ni0 species. For the Cu-containing compositions, the Cu 2p core-level spectra were analysed to determine the oxidation state of Cu before and after reaction. The fresh catalysts (Fig. 3c) display characteristic Cu 2p3/2 and Cu 2p1/2 peaks at approximately 933.2 eV accompanied by strong shake-up satellite features, confirming the incorporation of Cu2+ species within the lattice.35,36 After DRM (Fig. 3d), a new component appears at approximately 932.6 eV without associated satellite features, characteristic of metallic Cu0, alongside a decrease in satellite intensity and slight binding energy shifts indicative of partial reduction of Cu2+ to Cu+/Cu0 species. The Cu0 fraction increases with increasing Cu content from 0.1 to 0.3, suggesting enhanced reducibility and progressive surface segregation of Cu at higher doping levels. The concurrent exsolution of Ni and reduction of Cu indicates the possible formation of Ni–Cu metallic ensembles that contribute synergistically to CH4 activation during DRM, consistent with the Ni–Cu alloy phase identified in the post-reaction XRD analysis.

Fig. 3. Ni 3p core level spectra (a and b), and Cu 2p core level spectra (c & d) for the fresh and used catalytic materials La1.6Sr0.4NiO4, La1.6Sr0.4Ni0.9Cu0.1O4 and La1.6Sr0.4Ni0.7Cu0.3O4.

Fig. 3

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The O 1s spectra of all fresh catalysts in Fig. 4a exhibit three primary components centred at approximately 529.0 eV, 531.4 eV and 533.1 eV attributed to lattice oxygen (Oα), surface hydroxyl groups (Oβ), and adsorbed surface oxygen species (Oγ), respectively.37 In the fresh catalyst the relative Oβ contribution with respect to the total oxygen signal (Oβ/Ot) progressively increased from 0.59 to 0.62 to 0.64 with Cu incorporation from x = 0 to 0.3, indicating a gradual modification of the surface oxygen environment. After DRM, the Oα contribution decreases across all compositions, suggesting changes in the lattice-oxygen environment and is consistent with a possible contribution from lattice-oxygen participation in carbon oxidation, as similarly proposed for related lattice-oxygen-mediated DRM systems (Fig. 4b).38,39 These changes in the lattice-oxygen contribution may be associated with surface hydroxyl groups, which could facilitate CO2 adsorption and activation. The slight increase in the surface oxygen (Oγ) component after DRM is attributed to the formation of surface carbonate species arising from CO2 adsorption on defect-rich La–O sites, consistent with the La2O2CO3 phase identified in the post-reaction XRD patterns.39,40

Fig. 4. O 1s core level spectra for the (a) fresh and (b) used catalytic materials La1.6Sr0.4NiO4, La1.6Sr0.4Ni0.9Cu0.1O4 and La1.6Sr0.4Ni0.7Cu0.3O4.

Fig. 4

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The exsolution of metallic Ni and Ni–Cu alloy species observed after the DRM reaction and changes in the oxygen environment prompted us to evaluate the catalytic performance of the spent catalysts in a second dry reforming cycle. Fig. 5a shows that for La1.6Sr0.4NiO4, the catalytic behaviour in the second cycle increased from the first cycle, with measurable conversions of both CH4 and CO2 observed from 600 °C onwards. At 650 °C, La1.6Sr0.4NiO4 exhibited higher CH4 and CO2 conversions than the Cu-substituted catalysts, reflecting its already-established Ni0 inventory from the first cycle. The CH4 conversion was 45%, CO2 conversion was 58%, and the H2/CO ratio was 0.68. At 750 °C, the conversions were 75 and 50% for CO2 and CH4, with an H2/CO ratio of 0.75. At 850 °C and 900 °C, the conversions reached approximately 52% for CH4 and 86% for CO2, with an H2/CO ratio of 0.7. The limited further increase in CH4 conversion with rising temperature in the second cycle likely due to deactivation of catalysts. For La1.6Sr0.4Ni0.9Cu0.1O4, both reactant conversion and the H2/CO ratio were significantly enhanced relative to the first cycle (Fig. 5b), also exhibited slightly higher CH4 conversion compared to La1.6Sr0.4Ni0.7Cu0.3O4 (Fig. 5c) at higher temperatures. The superior catalytic performance observed in the second cycle indicates progressive activation of the catalyst under DRM conditions, consistent with the continued exsolution of active Ni species from the Ruddlesden–Popper lattice during the initial reaction period. At 700 °C, CH4 and CO2 conversions reached 65% and 72%, respectively, with an H2/CO ratio of 0.8. At 850 °C and 900 °C, CH4 and CO2 conversions increased to 80% and 93%, respectively, accompanied by an H2/CO ratio of 0.86. Upon increasing the Cu content in La1.6Sr0.4Ni0.7Cu0.3O4, the catalytic performance was observed across the temperature range, particularly at 800–900 °C, with CH4 and CO2 conversions reaching 83% and 93% at 900 °C, with an H2/CO ratio of 0.9 (Fig. 5c and d). Unlike the other two catalysts, conversions above 750 °C for La1.6Sr0.4Ni0.7Cu0.3O4 are lower than in the first cycle, possibly reflecting thermal coarsening of its more extensive exsolved Ni–Cu phase. The H2/CO ratio for La1.6Sr0.4NiO4 also exceeds that of La1.6Sr0.4Ni0.7Cu0.3O4 in this cycle, reversing the first-cycle trend, consistent with an increased RWGS contribution over the Cu-richest catalyst.

Fig. 5. Second cycle of dry reforming of methane over (a) La1.6Sr0.4NiO4, (b) La1.6Sr0.4Ni0.9Cu0.1O4, (c) La1.6Sr0.4Ni0.7Cu0.3O4, and (d) H2/CO ratio over a wide temperature range of 600–900 °C (reaction conditions: 200 mg loading, 1 cm bed length, gas space velocity 47 770 h−1, 1 atm).

Fig. 5

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Further, to evaluate catalytic stability, 20 h time-on-stream experiments were performed for both La1.6Sr0.4Ni0.9Cu0.1O4 and La1.6Sr0.4Ni0.7Cu0.3O4 catalysts at the optimized temperature of 750 °C as exhibited in Fig. 6. It should be noted that, the lower conversions of La1.6Sr0.4Ni0.7Cu0.3O4 above 750 °C in the second cycle (Fig. 5), relative to its steady-state performance in Fig. 6, are attributed to thermally induced sintering/coarsening of its extensively exsolved Ni–Cu phase during prior exposure to 900 °C in the first cycle. Under the isothermal 750 °C protocol of Fig. 6, this catalyst is never exposed to such temperatures, allowing its higher intrinsic reducibility to be reflected directly in its steady-state activity. The conversions of CH4 and CO2, along with the H2/CO ratio over La1.6Sr0.4Ni0.9Cu0.1O4 and La1.6Sr0.4Ni0.7Cu0.3O4 are presented in Fig. 6a and b over the entire 20 h period, while the corresponding Space Time Yield (STY) of H2 and CO is shown in Fig. 6c. Notably, La1.6Sr0.4Ni0.7Cu0.3O4 exhibited more than 40% CH4 conversion within the first hour of the reaction, indicating rapid activation of the fresh catalyst associated with the in situ generation of the active reduced Ni/Ni–Cu species. La1.6Sr0.4Ni0.9Cu0.1O4 exhibited CH4 and CO2 conversions of 75% and 89%, respectively, with a H2/CO ratio of 0.8. The STY values for H2 and CO were approximately 0.67 and 0.82 mol gcat−1 h−1, respectively. The comparatively lower H2 yield relative to CO accounts for the H2/CO ratio of less than unity. In contrast, La1.6Sr0.4Ni0.7Cu0.3O4 demonstrated significantly higher catalytic activity, achieving 85% CH4 conversion and 89% CO2 conversion, with an H2/CO ratio of approximately 0.97. The corresponding STY values for H2 and CO were ∼0.74 and ∼0.77 mol gcat−1 h−1, respectively. Thermogravimetric analysis was performed on the spent catalysts after 20 h of time-on-stream to evaluate coke deposition and quantify its extent as exhibited in Fig. 6d and the Fig. S3 represent the weight loss observed from TGA. La1.6Sr0.4Ni0.9Cu0.1O4 exhibited a minimal coke formation rate of 1.31 µgC gcat−1 h−1 with La1.6Sr0.4Ni0.7Cu0.3O4 exhibiting 1.01 µgC gcat−1 h−1. This might be due to the involvement of lattice oxygen in the reaction mechanism.38,41 The diluted feed likely results in lower coke formation than a less-diluted or stoichiometric CH4 : CO2 feed; thus, the reported rates are best compared relatively under identical conditions.

Fig. 6. 20 h time on-stream stability study at 750 °C over (a) La1.6Sr0.4Ni0.9Cu0.1O4 and (b) La1.6Sr0.4Ni0.7Cu0.3O4 with conversion of CO2 and CH4 and H2/CO ratio (c) space-time yield of H2 and CO of the catalysts over time (reaction conditions: 200 mg loading, 1 cm bed length, gas space velocity 47 770 h−1, 1 atm). (d) Coke formation rate over each catalyst after 20 h time on-stream stability study.

Fig. 6

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As summarized in Table 1, within the broader family of La2NiO4 type and Ruddlesden–Popper Ni-based oxide catalysts, Cu-modified catalysts generally exhibit improved DRM activity compared to the parent La2NiO4, although the extent of enhancement strongly depends on the dopant composition. Compared with previously reported Ni-based perovskite and Ruddlesden–Popper catalysts, the Sr–Cu co-substituted catalysts investigated in this work demonstrate a more favourable balance between CH4 and CO2 conversions, H2/CO ratio, and carbon resistance under comparable operating conditions. While several literature catalysts achieve reasonable methane conversion, they often suffer from significant coke deposition or require higher reaction temperatures.19,31,42–46 Direct comparison with previously reported La2NiO4 based Ruddlesden–Popper catalysts are challenging because of differences in reaction conditions, including GHSV, feed composition, and time-on-stream. Nevertheless, the present catalysts exhibit high CH4 and CO2 conversions and H2/CO ratios within the range reported for doped La2NiO4-based systems. More importantly, under the same reaction conditions used in our previous study.19 Overall, the present catalysts maintain high catalytic activity while exhibiting markedly lower carbon formation, highlighting the beneficial synergistic effect of Sr and Cu substitution in enhancing catalyst stability without compromising reforming performance.

Table 1. Comparison table of different layered perovskite catalysts for DRM.

Catalysts Reaction conditions (time on stream) % CH4 conversion; % CO2 conversion; H2/CO ratio STY of H2 and CO (mol gcat−1 h−1) Coke deposition rate Ref.
Mg0.80Ni0.20Al2O4 13 h @ 700 °C, GHSV = 18 000 mL g−1 72.6%; 80.7%; 0.6 NR NR 42
La2NiO4 without pre-reduction 0.5 h @ 800 °C, GSHV = 60 000 mL gcat h−1 30%; 35%; 1 NR NR 43
Non-reduced La2NiO4 15 h @ 700 °C, GHSV = 3005 mL g−1 h−1 82%; 88%; 0.90 NR NR 44
La1.8Ba0.2Ni0.9Cu0.1O4 3 h @ 800 °C, 6237 h−1 80%; 70%; 1.1 NR NR 31
La2Ni0.8Fe0.2O4 5 h @ 750 °C, GHSV = 12 000 mL g−1 h−1) 41%; 59%; 0.65 NR 1 mgc gcat−1 h−1 45
La2Ni0.8Mn0.2O4 5 h @ 750 °C, GHSV = 12 000 mL g−1 h−1) 38%; 58%; 0.55 NR 1 mgc gcat−1 h−1 45
La2(Ni0.8Cu0.2)O4 (reduced) 750 °C, CH4/CO2 = 1, GHSV = 1.8 × 104 mL g−1 h−1 73%; 80%; 0.9 NR 0.4 gc gcat−1 h−1 46
La2NiO4 15 h @ 750 °C, GHSV = 47 770 h−1 40%; 75%; 0.3 0.1, 0.5 4.5 µgC gcat−1 h−1 19
La1.6Sr0.4Ni0.7Cu0.3O4 750 °C, CH4/CO2 = 1, GHSV = 47 770 h−1 85%; 89%; 0.9 0.6, 0.69 1.01 µgC gcat−1 h−1 This work
La1.6Sr0.4Ni0.9Cu0.1O4 750 °C, CH4/CO2 = 1, GHSV = 47 770 h−1 78%; 89%; 0.8 0.67, 0.83 1.01 µgC gcat−1 h−1 This work
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To correlate catalytic performance with redox properties, H2-TPR measurements were performed as presented in Fig. 7a. As La3+ is non-reducible, the observed reduction features are attributed to the stepwise reduction of Ni3+/Ni2+, Cu2+/Cu1+ and associated lattice oxygen removal. La1.6Sr0.4NiO4 exhibited a total H2 consumption of 7.4 mmolg−1 with reduction peaks at 385 °C and 620 °C corresponding to partial surface Ni3+ reduction and bulk Ni2+ → Ni0 transformation, respectively. Cu incorporation further promoted the reduction of the catalyst, indicating that Cu substitution facilitates the generation of reduced Ni species under reaction conditions. La1.6Sr0.4Ni0.9Cu0.1O4 showed a higher H2 consumption of 8.1 mmolg−1 with a more pronounced low-temperature peak (331 °C), suggesting facilitated Ni reduction due to weakened Ni–O interactions.47 The Cu-rich sample, La1.6Sr0.4Ni0.7Cu0.3O4, displayed a markedly increased total consumption of 11.4 mmolg−1 with multiple reduction events (283–319, 397–440 °C and 697 °C), where the low-temperature peaks are attributed to the stepwise reduction of Cu2+ → Cu+ and Ni3+ species, while the broad intermediate feature preceding the final peak corresponds to Cu+ → Cu0 reduction. The sharp high-temperature peak at ∼697 °C is assigned to the spillover-assisted Ni2+ → Ni0 population. Overall, H2 consumption follows the order: La1.6Sr0.4NiO4 < La1.6Sr0.4Ni0.9Cu0.1O4 < La1.6Sr0.4Ni0.7Cu0.3O4, demonstrating progressively improved reducibility with Cu incorporation. The higher hydrogen uptake and lower reduction temperatures indicate facilitated Ni3+/Ni2+ reduction to active Ni0 species and enhanced lattice oxygen mobility. The substantially higher H2 consumption for the Cu-rich sample confirms the promotional role of Cu in accelerating Ni0 formation, which directly contributes to superior DRM activity. The comparison of the parent La2NiO4 exhibited comparatively lower H2 consumed of <60 µmol to further prove this.

Fig. 7. (a) H2-TPR and (b) CO2 TPD profiles of La1.6Sr0.4NiO4, La1.6Sr0.4Ni0.9Cu0.1O4, and La1.6Sr0.4Ni0.7Cu0.3O4.

Fig. 7

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The surface basicity of the catalysts series was evaluated by CO2-TPD, and the corresponding desorption profiles revealed distinct weak, medium, and strong basic sites as shown in Fig. 7b. Desorption peaks around ∼110–200 °C are attributed to weak basic sites (surface hydroxyl groups), whereas broad features between ∼200–500 °C correspond to medium-strength basic sites associated with surface oxygen species and metal–oxygen pairs. High-temperature desorption above 500 °C is assigned to strong basic sites related to lattice oxygen and oxygen-deficient sites. La1.6Sr0.4NiO4 showed a total uptake of 25 µmol g−1, with peaks at 117 °C and 206–550 °C, indicating higher CO2 uptake than the Sr-free La2NiO4 reference, consistent with the reported influence of Sr incorporation on surface basicity.48–50 Cu-substituted samples displayed modified basic distributions. The La1.6Sr0.4Ni0.9Cu0.1O4 exhibited a total CO2 uptake of 23.3 µmol g−1 with desorption at 114 °C, 199–485 °C, and a distinct high-temperature peak at 725 °C. Similarly, La1.6Sr0.4Ni0.7Cu0.3O4 showed 22.5 µmol g−1 CO2 uptake with peaks at 113 °C, 187–550 °C, and a pronounced desorption at 740 °C. The emergence of high-temperature (>720 °C) desorption features for Cu-containing catalysts indicates the presence of stronger basic sites together with changes in lattice-oxygen environments associated with the modified oxide lattice. Overall, Sr incorporation is associated with increased CO2 uptake relative to La2NiO4, while Cu substitution at constant Sr content modifies the distribution of medium-to-strong basic sites, facilitating CO2 activation and oxygen mobility under DRM conditions and thereby enhancing catalytic stability and coke resistance. This was further compared with the Sr-free parent La2NiO4, which exhibited a total CO2 uptake of 14.6 µmol g−1 under the same CO2-TPD conditions.

Aliovalent Cu substitution in La1.6Sr0.4NiO4 enhances catalyst reducibility and surface basicity, promoting the formation of metallic Ni0/Cu0 active sites for CH4 dissociation and improving CO2 adsorption and activation. These effects facilitate the generation of reactive oxygen species that oxidize carbon intermediates and suppress coke deposition, resulting in the superior DRM activity and stability observed for La1.6Sr0.4Ni0.7Cu0.3O4. To obtain further insight into the reaction mechanism of DRM, the in situ DRIFTS analysis was carried out under DRM condition with 1 : 1 ratio of CO2 and CH4 over La1.6Sr0.4Ni0.7Cu0.3O4, and the spectra were recorded in increments of 50 °C, ranging from 400 to 650 °C. During the reaction, bands observed at 1320, 1461 and 1526 cm−1 in Fig. 8a correspond to monodentate and bidentate carbonate species, indicating initial CO2 adsorption on the basic sites of the catalyst.51 The band at 1526 cm−1 can be additionally attributed to the asymmetric stretching vibration of the CO32− group associated with lanthanum oxycarbonate species.52 The band at 1030 cm−1 indicates the CO band corresponds to the stretch of surface methoxy group. Simultaneously, the appearance of bands at 1301 cm−1 indicate the presence of bending of CH4 species indicating the presence of methane-derived surface species during the reaction.53 The formation of oxygenated intermediates is evidenced by the bands at 1420 cm−1 (CHxO*),54 1730–1660 cm−1 (*COOH),55 1347–1550 cm−1 (HCOO*)56 and 1761 cm−1 (CHO*),57 Together, these bands are consistent with a contribution from progressive, oxygen-assisted oxidation of methane-derived fragments, occurring alongside direct methane cracking.

Fig. 8. In situ FTIR spectra of La1.6Sr0.4Ni0.7Cu0.3O4 during DRM from 400–650 °C with range of (a) 1000–2200 cm−1 and (b) 2200–4000 cm−1.

Fig. 8

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Notably, as the temperature increases from 400 to 650 °C, a gradual enhancement in the intensity of these intermediate bands is observed, with the CHxO* band at 1350 cm−1 showing a particularly prominent intensification, indicating that elevated temperatures actively promote the formation and accumulation of methoxy-type surface species and thereby accelerate the oxygen-assisted oxidation of methane-derived fragments. The broad bands observed at 3318–3528 cm−1, along with the overtone at 3650 cm−1, correspond to surface hydroxyl groups (Fig. 8b), which further participate in hydrogen transfer and oxidation steps during the reaction.58,59 Notably, the decrease in lattice oxygen (Oα) after reaction, together with the oxygen-redox nature of the catalyst, suggests that lattice oxygen may contribute to the oxygen-assisted activation of methane and CO2 activation. In this pathway, methane-derived CHx species may react with lattice oxygen and/or reactive surface oxygen species to form oxygenated intermediates (CHxO), which subsequently convert into CHO and finally to CO. Meanwhile, the carbonate species formed from CO2 adsorption can also interact with surface hydroxyl species (Oβ) on the catalyst surface, where CO32− decomposes into CO while replenishing the lattice oxygen, thereby regenerating active oxygen sites. The mechanism can be explained by the following equations:

Step 1: CO2 adsorption on basic sites: CO2 + O2− → CO2−3 6
3. 7
3. 8
CHxO*/HCOO* → CO* + H* 9
2H* → H2(g)↑ 10
CO* → CO(g)↑ 11

Among the temperatures investigated, 650 °C was selected as the representative condition for detailed time-resolved in situ DRIFT analysis over a 1000 s window, as it captures a particularly informative sequence of surface events as presented in Fig. S4. At this temperature, a brief but distinct transition is observed around 800 s, where the CO2 band momentarily diminishes while the CH4 band brightens, accompanied by the appearance of a surface hydroxyl band in the 3300–3520 cm−1 region. This transient spectral signature is indicative of reverse water gas shift (RWGS) episode (CO2 + H2 ⇌ CO + H2O) after which the system promptly reverts to its dominant reaction pathway confirming that RWGS occurs as a competing route.19 This fleeting occurrence, rather than being a drawback, actually underscores the robustness of the oxygen-mediated mechanism: the cyclic participation of lattice oxygen and oxygen vacancies continuously facilitates oxidation of methane-derived intermediates and suppresses carbon deposition. The reaction therefore proceeds predominantly through an oxygen-assisted methane activation route with oxygenated intermediates, which accounts for the significantly lower coke formation observed at this condition. The 1000 s observation window at 650 °C was thus deliberately chosen to capture and highlight this brief RWGS excursion and its rapid self-correction, providing direct spectroscopic evidence of the dynamic surface chemistry governing selectivity and coking resistance.

4. Conclusion

Aliovalent Sr and Cu co-doping of the Ruddlesden–Popper La2NiO4 structure provides an effective approach to tune the redox properties, surface basicity, and formation of catalytically active reduced species for DRM. The catalytic studies revealed that La1.6Sr0.4Ni0.9Cu0.1O4, and La1.6Sr0.4Ni0.7Cu0.3O4 exhibited distinct activity trends during the dry reforming of methane. The parent La1.6Sr0.4NiO4 initially showed lower catalytic performance due to limited reducibility and the absence of readily available metallic Ni sites. XRD and XPS analyses of fresh and exhausted catalysts revealed the gradual formation of metallic Ni for La1.6Sr0.4NiO4 and Ni–Cu species for the Cu-doped catalysts during reaction, significantly influencing catalytic activity. The H2-TPR results further confirmed the improved reducibility of the Cu-doped catalysts. Furthermore, progressive Cu incorporation also enhanced the strong basic sites available for CO2 adsorption, while TGA confirmed minimal carbon deposition across the series. Among the investigated catalysts, La1.6Sr0.4Ni0.7Cu0.3O4 exhibited superior catalytic performance, achieving 85% CH4 conversion and 89% CO2 conversion with an H2/CO ratio of ∼0.97. The corresponding H2 and CO STY values were ∼0.61 and ∼0.63 mol gcat−1 h−1, respectively, along with a low coke formation rate of ∼1.01 µgC gcat−1 h−1. In comparison, La1.6Sr0.4Ni0.9Cu0.1O4 showed 75% CH4 conversion and 89% CO2 conversion with an H2/CO ratio of ∼0.8, H2 and CO STY values of ∼0.67 and ∼0.82 mol gcat−1 h−1, and a slightly higher coke formation rate of ∼1.31 µgC gcat−1 h−1. The changes in the lattice-oxygen environment observed by XPS, together with the in situ DRIFT results, provide evidence for changes in surface oxygen species during DRM. Overall, this work demonstrates that Sr and Cu co-substitution in Ruddlesden–Popper La2NiO4 provides an effective strategy to tune reducibility, facilitate the formation of active Ni/Ni–Cu species, modify surface basicity, and achieve high-performance, coke-resistant catalysts for syngas production via dry reforming of methane.

Author contributions

Aathira Bhaskaran: writing – original draft, methodology, investigation, formal analysis. Chandan Kumar Mandal: data accusation. Arup Gayen: data accusation. Satyapaul A Singh: writing – review and editing, project supervision. Sounak Roy: writing – review and editing, funding acquisition, conceptualization, supervision, project administration.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

RA-OLF-D6RA06388H-s001

Acknowledgments

Authors thank BITS Pilani Hyderabad Campus for the financial aid for this work. Authors acknowledge DST-PURSE for in situ DRIFT facility.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: EDX, elemental mapping, TGA, and in situ FTIR plots. See DOI: https://doi.org/10.1039/d6ra06388h.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-OLF-D6RA06388H-s001
RA-OLF-D6RA06388H-s001.pdf (1.8MB, pdf)

Data Availability Statement

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: EDX, elemental mapping, TGA, and in situ FTIR plots. See DOI: https://doi.org/10.1039/d6ra06388h.

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