Citric Acid Sol Gel Research Articles

Metals doping effect of perovskite–type La-Mn oxides for NH3-SCR reaction at low temperature

Doping of perovskites with the heteroatoms has emerged as an additional lever, a dimension beyond structural perfection and compositional distinction, for the alteration of many properties of perovskites. In this work, a series of different metals were doped via citric acid sol–gel method to investigate their influences on the structure and catalytic performance of La-Mn perovskite oxides for NH3-SCR reaction. It is found that Zr doped La-Mn oxides (LMZr) displays better low-temperature deNOx performance (T90 = 118 °C) than other metal doped catalysts. Such a result is mainly attributed to the increase of Lewis acid sites, surface Mn4+ and adsorbed oxygen species. Besides, La-Mn perovskite doped with Sr, Ce, Y and Al can also obviously improve low-temperature deNOx activity and N2 selectivity. These doped metals mainly change the electron transfer between different cations, thus changing the valence distribution of Mn on the surface, meanwhile, metal doping changes the crystalline size of the catalysts, thereby affecting the adsorption and activation of NH3 and O2 involved in the removal of NO. Although other metal doping alters the tolerance of the La-Mn catalyst to SO2 and/or H2O, SO2 is inevitably oxidized to S6+ and deposits on the catalyst surface. Zr or Ce doping significantly enhances the tolerance to SO2 and/or H2O, but they have a different mechanism. Zr reduces the deposition rate of sulfate while Ce reacts with SO2 to form Ce2(SOx)3 to protect Mn active species. This paper highlights the importance of Lewis acid, Mn4+ content and adsorbed oxygen in the NH3-SCR reaction.

Boosting low-temperature NH3-SCR performance via mixed-valence cobalt dual active sites

Nitrogen oxides (NOx) have become a critical environmental concern, necessitating the development of highly efficient catalysts for temperature which is lower than 300 °C. Selective catalytic reduction with NH3 (NH3-SCR) is a process where ammonia is the reducing agent for selective catalytic reduction of nitrogen oxides in exhaust gases. The mechanism-property relationship between catalytic active sites and low-temperature denitrification efficiency, however, remains unclear. In this study, we synthesized a series of cobalt-modified nanoparticle catalysts using a scalable citric acid sol-gel method and conducted comprehensive characterizations. Our findings indicate that an optimal cobalt doping rate significantly enhances catalytic activity due to the synergistic effects of reduced nanoparticle size and the dual active sites of Co0/Co2+. Specifically, at a cobalt doping ratio of 1.0, the catalyst demonstrates peak SCR activity, achieving 85.5% NOₓ conversion at 120 °C and complete conversion at 150 °C. These cobalt-modified nanoparticle catalysts feature high deposition of doped Co ions on their surfaces, predominantly as metallic cobalt and CoOx nanoclusters. Compared to undoped samples, the cobalt-doped nanoparticle catalysts exhibit a greater specific surface area, increased acidity, enhanced chemisorbed oxygen, and improved redox properties, all contributing to superior denitrification efficacy. Additionally, Density Functional Theory (DFT) calculations provide assertive evidence into the reaction mechanism, showing that Co2+ sites effectively adsorb and activate NH₃, while Co0 sites are crucial for NO adsorption and activation. The synergistic interaction between Co2+ and Co0 facilitates NN coupling, enhancing overall reaction efficiency. These findings underscore the potential of cobalt-modified catalysts for practical industrial applications, offering robust performance over a wide temperature range and meeting stringent environmental regulations.

Effect of A-site cation doping on the catalytic oxidation of toluene over mullite catalyst GdMn2O5

Manganese-based mullite (AMn2O5) catalysts are promising for the catalytic oxidation of VOCs, with their oxidation reaction performance being tunable by modifying the composition of the A-site elements. In this work, various manganese-based mullite catalysts (Gd0.9X0.1Mn2O5, X =Sr, Ce, or Ca) with different A-site elements doping were prepared by the citric acid sol–gel method and applied to the catalytic oxidation of toluene. The activity tests indicated that the enhancement of catalytic performance by doping elements was in the order of Sr > Ce > Ca. Notably, the T90 of the Gd0.9Sr0.1Mn2O5 catalyst was 235.6 °C, a lower 37.8 °C than that of the GdMn2O5. Comprehensive characterization and density functional theory (DFT) simulations revealed that Sr doping facilitates the reduction of Mn4+ to Mn3+, increases the electron occupancy of the eg orbitals in octahedral Mn sites, and elevates the Mn-d band center, thus facilitating the adsorption and activation of O2. Additionally, the raised O-p band center caused by Sr doping enhances lattice oxygen mobility. In situ DRIFTS analysis indicated that the introduction of Sr at the A-site optimizes the toluene reaction pathway and accelerates the reaction rate. This work reveals that a simple A-site cation doping strategy can effectively regulate manganese-based mullite catalysts’ electronic structure and surface reaction activity, providing a feasible method for the large-scale preparation of manganese-based mullite catalysts with excellent catalytic performance.

Stability and photoluminescence of Bi doped (La,Y)2O3 phosphor powders

La2-yYyO3 phosphor powders were prepared via the citric acid sol-gel combustion method and their stability was checked after four months of storage in the atmosphere using X-ray diffraction. The material, like La2O3, was prone to hydroxylation unless a high proportion of Y was added, which also caused changes in the phases present. Based on its stability, the composition La0.5Y1.5O3 was selected for further study with Bi doping. The luminescence from this material was not as intense as that from Y2-xO3:Bix=0.005, but there was a significant shift in colour as a result of the added La which meant that emission from the Bi3+ ions at the S6 sites of the alloy was close to that of unstable La2O3 on the colour chart.

Modification of SmMn2O5 Catalyst with Silver for Soot Oxidation: Ag Loading and Metal–Support Interactions

A series of Ag-modified manganese-mullite (SmMn2O5) catalysts with different Ag contents (1, 3, and 6 wt.%) were prepared via a citric acid sol–gel method for catalytic soot oxidation. The catalysts were characterized by powder X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), Raman spectroscopy, transmission electron microscopy (TEM), high-resolution transmission electron microscopy analysis (HRTEM), X-ray photoelectron spectroscopy (XPS), and H2 temperature-programmed reduction (H2-TPR). The soot oxidation activity of the mullite was significantly promoted by the addition of silver and affected by the loading amount of the metal. Herein, the influences of silver loading on the metal size distribution and its interactions with the mullite were studied. Based on these characterizations, a possible soot oxidation reaction mechanism was proposed for silver-modified SmMn2O5.

Substitution effects on Y-type hexaferrites’ magnetic characteristics

The effect is discussed of the partial replacement of the Me2+ in the Y-type hexaferrite crystal structure with non-magnetic (Mg2+) and magnetic (Ni2+) cations on the observed magnetic characteristics of Ba0.5Sr1.5Me2Fe12O22. Y-type hexaferrite powder material was prepared by citric acid sol-gel self-ignition and sintering at 1170 °C in air to reach Ba0.5Sr1.5MgxNiyFe12O22. To determine the magnetic phase transitions, the ZFC/FC magnetisation was followed in magnetic fields of 50 Oe and 100 Oe between 4.2 K and 300 K; this was complemented with hysteresis loops recorded at the end temperatures. The results show that combining the magnetic Ni2+ with the nonmagnetic Mg2+ in Ba0.5Sr1.5 Me2Fe12O22 can be effective in controlling the magnetic interactions to regulate the temperature of the magnetic transition from helical to ferrimagnetic arrangement above room temperature.

Fabrication of fine-grained and high thermal properties Y2O3 transparent ceramics without sintering aids

Infrared transparent pure Y2O3 nanoceramics were fabricated from citric acid sol-gel combustion synthesized nanopowder and through the combination of hot pressing and hot isostatic pressing. The influence of calcination temperature on the morphology, particle size and agglomeration state of the nanopowders during powder synthesis process was investigated. Powder with a particle size of 15.5 nm and agglomeration factor of 3.3 was obtained with the calcination temperature at 700 °C for 4 h. Green compact with homogenous microstructure and improved sintering activity was consolidated by cold isostatic pressing after dry pressing. Y2O3 transparent ceramic was achieved with hot pre-sintering at 1400 °C without holding plus hot isostatic pressing at 1450 °C for 2 h under 200 MPa pressure. As a result, the ceramic prepared with 1.0 mm thickness got the transmission of 80.4 %–83.9 % at 2.5 μm–6.0 μm wavelength with an average grain size of 413 nm. The sample's thermal conductivity was 12.84 W m−1 K−1 and the phonon energy was 592 cm−1, making it a promising material for IR transparent windows and host material for efficient infrared solid-state laser. The sintering mechanism was proposed and discussed.

Chemically bonded zirconium oxide into CaO-based high-temperature coal-fired flue gas As2O3 absorbents for enhanced anti-sintering properties: Experimental and DFT study

CaO, as a mineral oxide adsorbent, has excellent arsenic trapping performance. Still, it has a flaw that inhibits its application: at high temperatures, CaO begins to sinter and ruduces its adsorption capacity. In this work, the anti-sintering ZrO2/CaO composite adsorbent was fabricated by the citric acid sol-gel process and applied for the first time in As2O3 adsorption studies in flue gas at 500–900 ℃. A combination of experimental and theoretical analytical methodologies was adopted to explore the influence of Zr modification on the behaviour of CaO adsorption of As2O3 and the adsorption mechanism of As2O3 on ZrO2/CaO. The experimental findings demonstrate that the introduction of ZrO2 not only significantly increases the specific surface area of the adsorbent and provides more adsorption sites, but also provides a large number of free electrons for the conversion of adsorbed oxygen in the system, which effectively promotes the reaction process of the chemical oxidation of As(III) to As(V). Furthermore, the partial CaZrO3 structure formed by chemical bonding of ZrO2 with CaO significantly improves the sintering resistance of ZrO2/CaO at high temperatures. Therefore, the adsorption capacity of the prepared ZrO2/CaO for As2O3 reaches 857.7 mg/kg at 900 ℃, which is approximately 16 times greater than that of untreated CaO. Meanwhile, the microscopic mechanism of ZrO2/CaO adsorbent was further investigated by density functional theory (DFT). The analysis revealed that the Zr site is the main active adsorption site of ZrO2/CaO. During the adsorption of arsenic, Zr undergoes strong electron-orbital hybridization with the O atoms of the system and arsenic to form a stable ZrO6 octahedral structure, which promotes the strong stability of the chemisorption system. ZrO2/CaO has remarkable arsenic trapping ability and resistance to sintering, and is a composite adsorbent material with great potential for arsenic removal.

Degradation of ofloxacin by lanthanum cerate perovskite activated bisulfite

In recent years, the extensive utilization of ofloxacin (OFX) has led to elevated concentration of OFX compound in the aquatic environment. At the same time, the inherent recalcitrance of OFX has presented a formidable challenge for OFX degradation. In this study, lanthanum cerium perovskite (LaCeO3) was prepared by citric acid sol–gel method to activate bisulfite (BS) for the effective degradation of OFX. The results revealed that the LaCeO3/BS system removed 87.5% of OFX within 180 min with a pseudo-first-order kinetic constant of 3 × 10−3 min−1. Radical quenching experiments and electron paramagnetic resonance (EPR) spectroscopy confirmed that sulfate radicals (SO4•_) and hydroxyl radicals (HO•) were the primary reactive species responsible for OFX removal in LaCeO3/BS system. The reaction mechanism indicated that Ce in LaCeO3/BS system served as the primary active site for BS activation, and a regenerating cycle involving ≡Ce(III)/≡Ce(IV) was present. Additionally, possible intermediate products were identified by a liquid chromatograph-mass spectrometer (LC-MS), revealing potential degradation pathways of OFX. Cycle tests and energy dispersive spectrometer (EDS) data demonstrated the favorable reusability and stability of LaCeO3.

Influence of doping concentration and annealing temperature on the structure, particle morphology, and luminescent properties of powders and pulsed laser deposited thin films of bismuth-doped lanthanum gallate

Bi3+ doped LaGaO3 powders and thin films were grown via citric acid sol-gel combustion method and pulsed laser deposition (PLD) technique, respectively. X-ray diffraction data confirmed crystallization of the orthorhombic LaGaO3 phase. Inductive coupled plasma spectroscopy data indicated that some Bi3+ ions evaporated at temperatures exceeding 800 °C during thermal treatment. The highest photoluminescence (PL) and cathodoluminescence (CL) emissions with a peak stabilized at 370 nm were observed from La1-x GaO3:Bi3+x=0.003 powders annealed at 1200 °C. The PL and CL emission spectra of La1-x GaO3:Bi3+x=0.003 PLD thin films were similar to those of the powder samples. The effect of Bi3+ concentration and annealing temperature on the properties of powders and thin films is discussed.

Influence of synthetic temperature on structural and magnetic properties of Dy substituted Ni nanoferrite

ABSTRACT Dy substituted Ni ferrite nanoparticles with composition NiFe1.95Dy0.05O4 were synthesized by using the citric acid sol–gel auto combustion method. The samples were sintered at temperatures of 400°C, 700°C, and 900°C. The X-ray diffraction measurements clearly showed the formation of a cubic spinel structure along with a small orthoferrite phase for all the sintered samples. The crystallite size increases from 17.79 to 39.36 nm while the grain size increases from 50.70 nm to 83.05 nm. The lattice parameter and lattice volume decrease with increasing sintering temperature was observed. The Spinel ferrite structure of prepared samples has been confirmed from FTIR spectra. With increasing sintering temperature saturation magnetization increases from 25.80 emu/gm to 49.48 emu/gm while the coercivity value decreases from 207.95 Oe to 165.43 Oe. Increasing saturation magnetization and decreasing coercivity values with increasing sintering temperature make the synthesized nanoparticles suitable for high-density data storage devices.

B-site metal modulation of phosphate adsorption properties and mechanism of LaBO3 (B = Fe, Al and Mn) perovskites.

La-based adsorbents are widely used for controlling phosphate concentration in water bodies. In order to explore the effect of different B-site metals regulating La-based perovskites on phosphate adsorption, three La-based perovskites (LaBO3, B = Fe, Al, and Mn) were prepared using the citric acid sol-gel method. Adsorption experiments showed that LaFeO3 exhibited the highest adsorption capacity for phosphate, which was 2.7 and 5 times higher than those of LaAlO3 and LaMnO3, respectively. The characterization results demonstrated that LaFeO3 has dispersed particles exhibiting larger pore size and more pores than LaAlO3 and LaMnO3. Spectroscopy analysis and density functional theory calculation results showed that different B-positions cause a change in the type of perovskite crystals. Among them, the differences between lattice oxygen consumption ratio, zeta potential and adsorption energy are the main reasons for the differences in adsorption capacity. In addition, the adsorption of phosphate by La-based perovskites were well fitted with Langmuir isotherm and pursues the pseudo-second-order kinetic models. The maximum adsorption capacities were 33.51, 12.31 and 6.61mg/g for LaFeO3, LaAlO3 and LaMnO3, respectively. The adsorption mechanism was mainly based on inner-sphere complexation and electrostatic attraction. This study provides an explanation for the influence of different B sites on phosphate adsorption by perovskite.

Broadband electromagnetic wave absorption of Ni0.5Zn0.5Nd0.04Fe1.96O4 ferrites modified by nano-silver particles

Ferrite materials have the potential to become excellent absorbing materials due to their high magnetic loss and good impedance matching. However, the disadvantages of high density and lack of dielectric loss capability limit its application. Herein, we used the citric acid sol-gel method and the self-propagating combustion method to prepare neodymium-doped nickel-zinc ferrite (NZNF), then the target effect of Sn2+ and an improved electroless silver plating process was used to plate a layer composed of silver nanoparticles (Ag NPs) with strong dielectric loss on the NZNF, and a magnetic/dielectric composite material (NZNF@Ag) with a heterogeneous structure was prepared. The number and particle size of Ag NPs on the surface of NZNF can be precisely controlled, thereby greatly enhancing the dielectric loss capability with little impact on the magnetic loss. The huge difference in conductivity between conductors and semiconductors promotes the occurrence of polarization at the heterogeneous interface and significantly enhances the electromagnetic wave absorption ability of the composite material. In the 2–18 GHz frequency band, the best sample can obtain an effective bandwidth of 6.82 GHz when the matching thickness is 2.1 mm. Combining conductors with semiconductor materials to obtain significantly enhanced interfacial polarization provides a new idea for improving the performance of wave absorbing materials.

Orange-red luminescence features of Eu3+ doped CaAl4O7 phosphors

A series of orange-red emitting Eu3+ doped CaAl4O7 phosphors was synthesized by the citric acid sol gel process using the metal nitrates as precursors. A study of the phosphors according to their X-ray diffraction patterns confirmed the desired phase without any intermediate or precursor compounds. The SEM imaging does not reveal the crystal structure, but it shows a variation of crystalline size within a few micrometers. In the excitation spectrum of CaAl4O7:xEu3+ (x = 0.01–0.11 mol), sharp multiple peaks at 293 nm, 464 nm, and 533 nm which are characteristics of Eu3+ transitions from 7F → 5D were observed. The photoluminescence emission spectra with an excitation wavelength of 464 nm showed characteristic peaks corresponding to the transition of 5D0 → 7F2 at 612 nm and expressed orange-red region of the electromagnetic field. The emission intensity increased as the concentration of Eu3+ increased to 0.05 mol, where it exhibited the highest emission intensity in the series. The intensity ratio results also indicate an intensity drop after a 0.05 concentration of the Eu3+ ion. The full width at half maximum (FWHM) of the synthesized phosphors is expressed as the concentration increase, which leads to a drop in sharpness. The chromaticity properties indicate high color purity of the synthesized phosphor in the orange-red region by the position of the color coordinates at the edge of the CIE color chromaticity diagram.

Facile construction of C and TiO2 surface coating to improve the cycling stability of NiMn2O4 composite electrode materials

• The rod-shaped NiMn 2 O 4 electrode materials have been synthesized by the citric acid sol–gel method. • NiMn 2 O 4 @TiO 2 and NiMn 2 O 4 @C composites demonstrate remarkable electrochemical properties. • The mechanism of surface-coating modification can be attributed to the coating layer forming a stabile SEI layer. In this paper, nickel-manganese bimetal oxide is used as the research object. The rod-shaped spinel NiMn 2 O 4 is synthesized by the citric acid sol–gel method and the subsequent high-temperature sintering process, and the surface is modified by carbon coating and titanium dioxide coating, and studied the effect of increasing the sintering temperature on the sample. Electrochemical experiments show that the C coating is beneficial to the improvement of the rate performance, while the TiO 2 -coated electrode material exhibits excellent cycle stability. The reason for the difference is that the TiO 2 coating on the surface can effectively buffer the volume expansion of the material, while the C coating can form a stable SEI layer.

Sol-gel synthesis of nano block-like ZnMn2O4 using citric acid complexing agent and electrochemical performance as anode for lithium-ion batteries

Spinel ZnMn 2 O 4 anode material has high research value as a potential alternative anode material for lithium-ion batteries due to the limited theoretical capacity of commercial graphite. This study prepared a block structured lithium-ion battery anode material ZnMn 2 O 4 using the citric acid sol-gel combustion method. The thermal decomposition process of the xerogel precursors was investigated using thermogravimetric differential thermal analysis. When the ratio of citric acid to metal cations is 4:1, the prepared ZnMn 2 O 4 material had an uneven block structure (ca. 0.3–1.3 µm). The material shows high electrochemical performance: the initial capacity is as high as 1100.2 mAh·g −1 at 0.1 C, and the capacity remains at 277.6 mAh·g −1 after 40 cycles. Finally, the first discharge and the second charge–discharge processes of ZnMn 2 O 4 were thoroughly investigated, and the electrochemical behavior in the electrochemical process was analyzed. • ZnMn 2 O 4 block-like structure are synthesized by sol-gel combined combustion method. • Smaller particle size can effectively increase the reactive sites between electrolyte and material. • The optimized sample shows good lithium storage properties.

Development of Y2O3 Dispersion-Strengthened Copper Alloy by Sol-Gel Method.

In this study, oxide dispersion-strengthened Cu alloy with a Y2O3 content of 1 wt.% was fabricated through citric acid sol-gel synthesis and spark plasma sintering (SPS). The citric acid sol-gel method provides molecular mixing for the preparation of precursor powders, which produces nanoscale and uniformly distributed Y2O3 particles in an ultrafine-grained Cu matrix. The effects of nanoscale Y2O3 particles on the microstructure, mechanical properties and thermal conductivity of the Cu-1wt.%Y2O3 alloy were investigated. The average grain size of the Cu-1wt.%Y2O3 alloy is 0.42 μm, while the average particle size of Y2O3 is 16.4 nm. The unique microstructure provides excellent mechanical properties with a tensile strength of 572 MPa and a total elongation of 6.4%. After annealing at 800 °C for 1 h, the strength of the alloy does not decrease obviously, showing excellent thermal stability. The thermal conductivity of Cu-1wt.%Y2O3 alloy is about 308 Wm−1K−1 at room temperature and it decreases with increasing temperature. The refined grain size, high strength and excellent thermal stability of Cu-1wt.%Y2O3 alloys can be ascribed to the pinning effects of nanoscale Y2O3 particles dispersed in the Cu matrix. The Cu-Y2O3 alloys with high strength and high thermal conductivity have potential applications in high thermal load components of fusion reactors.

Synthesis and Optimization of ZnMn2O4 Cathode Material for Zinc-Ion Battery by Citric Acid Sol-Gel Method

Zinc manganese spinel composite oxide ZnMn2O4 is an important inorganic material, which is rich in natural resources and environment-friendly. Herein, pure phase ZnMn2O4 was prepared by a simple sol-gel process as cathode material for zinc ion batteries. In order to improve the cycle performance of the material while maintaining the capacity, acetonitrile/aqueous co-solvent electrolyte (0.5 M Zn(CF3SO3)2/AN-H2O) was used as the electrolyte of the zinc ion battery. The influence of different experimental conditions on the electrochemical properties of ZnMn2O4 was studied by orthogonal experiment. In order to improve its electrochemical performance and cycling ability, the non-equivalent substitution of pure ZnMn2O4 was performed by adding Al3+. The non-equivalent substitution of Al3+ reduced the cell volume of ZnMn2O4 and the mixed manganese valence appeared. The specific capacity of initial discharge increases from 52.9 mAh·g−1 to 109.4 mAh·g−1 at 0.05C, and the cycle stability is also greatly improved. After 100 cycles, the capacity retention rate was 82.3%. This work shows that ZnMn2O4 is a promising cathode material for zinc ion batteries, which expands the application of spinel oxide in zinc ion batteries.

Different La/Fe oxide composites for efficient phosphate removal from wastewater: Properties and mechanisms

Metal oxides are widely used in the removal of oxygen-containing anions from the aqueous phase, and multi-metallic oxides were promising adsorbents for removal performance. In order to explore the effect of different La/Fe oxide composites (LF) on phosphate adsorption, three bimetallic composite oxides of La/Fe were prepared by citric acid sol-gel method at 600, 700 and 800 oC (named LF-600, LF-700 and LF-800). According to adsorption experiments, the adsorption capacity was changed obviously with the temperature increasing. The maximum adsorption capacity was 77.08, 41.17 and 14.58 mg g−1 at pH 7.0 of LF-600, LF-700 and LF-800, respectively. The specific surface area (SBET) and the crystal type are the main factors affecting the adsorption capacity. With the temperature increasing, the SBET reduced and the crystal phase was LaOCl and Fe2O3 of LF-600, LaOCl, LaFeO3 and Fe2O3 of LF-700, and LaFeO3 and LaOCl of LF-800. In addition, the adsorption capacities decreased obviously with the increase of pH and the addition of CO32−. The FTIR, XRD and XPS analysis proved that the adsorption mechanism mainly was surface precipitation, electrostatic attraction and complexation, and complexation was the main adsorption mechanism.

Performance of Zn-Al co-doped La2O3 catalysts in the oxidative coupling of methane

Zn-doped and Zn-Al co-doped La2O3 catalysts were prepared by citric acid sol-gel method and characterized by a series of in situ technologies, to investigate the structure-activity relationship of La2O3-based catalysts in the oxidative coupling of methane (OCM). The in situ XRD results reveal a thermal expansion of the La2O3 crystal along the c-axis at high temperature. The H2-TPR results show two types of oxygen species on the La2O3-based catalysts, viz., the strong-binding oxygen species and weak-binding oxygen species; in addition, the XPS results indicate that the strong-binding oxygen species is probably attributed to anion radical O−. The doping with Zn can significantly increase the number of oxygen vacancies in the Zn-doped La2O3 catalysts, which can promote the activation of oxygen and generate more strong-binding oxygen species; as a result, the Zn-doped La2O3 catalyst shows better performance in OCM in comparison with the unmodified La2O3 catalyst. Moreover, the co-doping with Al can promote the dispersion of Zn in La2O3 and further raise the number of strong-binding oxygen species in the Zn-Al co-doped La2O3 catalysts, which is beneficial to enhance the selectivity to C2+ hydrocarbons in the OCM reaction.