CO₂ methanation offers a promising route for converting CO₂ into valuable chemicals and energy fuels at the same time as hydrogen is stored in methane, so the development of suitable catalysts is crucial. In this review, the performance of catalysts for CO₂ methanation is presented and discussed, including noble metal-based catalysts and non-noble metal-based catalysts. Among the noble metal-based catalysts (Ru, Rh, and Pd), Ru-based catalysts show the best catalytic performance. In the non-noble metal catalysts, Ni-based catalysts are the best among Ni-, Co-, and Fe-based catalysts. The factors predominantly affecting catalytic performance are the dispersion of the active metal; the synergy of the active metal with support; and the addition of dopants. Further comprehensive investigations into (i) catalytic performance under industrial conditions, (ii) stability over a much longer period and (iii) activity enhancement at low reaction temperatures are anticipated to meet the industrial applications of CO₂ methanation.

In this work, an integrated system of “Pressure swirl / inclined plate sedimentation separation pretreatment (P/I) combined with ultrafiltration / reverse osmosis two stages membrane separation (UF/RO)” was developed. The application of this system achieves full-scale separation and valorization for anaerobic digestate of livestock manure, completes anaerobic digestate separation, biogas residue recovery, biogas slurry concentration and water recovery simultaneously. The novel P/I solid-liquid separation pretreatment technology, developed independently by our team, reduces the SS and COD levels of the biogas slurry entering the membrane separation unit greatly. Compared with the centrifugal pretreatment, the particle size of the biogas slurry is reduced significantly via P/I pretreatment (94.8% of them are in the range of 1-50 μm), the liquid recovery ratio is increased to 94.0% (which is about 3 times to that via the centrifugal pretreatment), this leads to the substantial increase of the entire system nutrients recovery ratio directly. What`s even more valuable is the permeate flux of UF membrane is increased for 2.5 times via P/I pretreatment. The results showed that P/I pretreatment had much better effects in solid-liquid separation of biogas slurry, which reduced the load of the subsequent membrane separation unit and enhanced the UF membrane permeation flux greatly. That prolonged the cleaning cycle and service life of the UF membrane. In this work, the efficiency of “P/I + UF/RO” system was validated according to the nutrients recovery ratios, the biogas slurry concentration efficiency, the recovery of water and the pollutants levels of discharged water. The results showed that the total nutrients recovery ratio was about 100% (losses were negligible), the concentration ratios of biogas slurry were more than 3.7 (primary concentrated via UF) and 6.6 (secondary concentrated via RO), total water recovery ratio was 46.9% (ratio of the discharged water volume to the digestate volume). the COD removal ratio of the discharged water was 99.9%, the contents of COD, TP and TN were 43.6 mg / L，0.2 mg/L and 1.2 mg/L, which reached the nation integrated wastewater discharged standard class Ⅰ.

The multiplicity of catalyst structures and modification strategies makes possible different reaction pathways and selectivity of CO₂ methanation, and the reaction mechanism is also controversial.

Layered manganese dioxide (δ-MnO₂) exhibits potential in the hydrogen evolution reaction (HER) due to its multivalent redox properties and high specific surface area. However, its poor intrinsic conductivity and insufficient active sites limit the electron transfer efficiency and adsorption/desorption of *H intermediates. Herein, we constructed an X–MO/NF (X = Ni, Cu, Zn) electrocatalytic system by in situ growth of δ-MnO₂ on the surface of nickel foam (NF) via a one-step hydrothermal method and introduction of transition metal atoms. It has been shown that the transition metals trigger the charge compensation mechanism by changing the O coordination environment, inducing the formation of oxygen vacancies and unsaturated Mn³⁺ sites in MnO₂ lattice, thus balancing the charge distribution and optimizing the electronic structure. Ni–MO/NF, Cu–MO/NF, and Zn–MO/NF exhibit low overpotentials of 145 mV, 131 mV, and 115 mV at 10 mA cm⁻² in 1 M KOH, respectively. Theoretical calculations confirmed that via Ni, Cu, and Zn doping, the hydrogen adsorption-free energy could be reduced and the surface reaction kinetics accelerated. Nevertheless, the diversity of their orbital contributions to the electronic structure of MnO₂ led to different redistributions of the electron cloud density, thus showing variation in the degree of enhancement of the catalytic performance. This work reveals the regulation rule of transition metal atoms on the coordination environment of δ-MnO₂, which provides a new perspective and theoretical guidance for optimizing the catalytic performance of transition metal oxides.

La₄(NiCoAlCu)O_x achieves >95% CO selectivity for 1000 h at 550 °C via tripartite active sites (Ni–Co–Cu/La₂O₃/oxygen vacancies). Entropy engineering overcomes stability–activity trade-offs in CO₂ hydrogenation.

Improving the anti-carbon deposition and anti-sintering ability under the premise of maintaining high catalytic activity is the core issue of Ni-based catalysts applied in CO methanation reactions. To address this issue, a La_0.75A_0.25NiO₃/SiO₂ (A = Ce, Sr, Sm, and Ca) catalyst is prepared via a citric acid complexation method. XRD results show that the substituted elements (Sr, Sm, and Ca) enter the LaNiO₃ lattice and partially replace the A-site element La. The reduced Ni⁰ is beneficial to improve the medium temperature activity of the catalyst. The substitution of different elements produces different electronic effects that significantly affect the size of the Ni particles and the interaction between Ni and La₂O₃. The catalyst with doped Ca²⁺ as the A-site substituted element demonstrates better adsorption, storage, and migration capabilities for oxygen due to the lattice distortion that easily produces oxygen vacancies. Catalysts doped with Sr, Sm, and Ca as the A-site substituted element produce La₂O₂CO₃ after the reactions, which plays a role in eliminating carbon deposits.