Table 2.

Summary of some typical bimetallic Ni-based catalysts promoted with noble metals (Ru, Rh, Pt, Pd and Re) for the methanation of CO₂.

Second Metal	Catalyst Composition	Preparation Method	Conditions	Performance	Comments	Ref.
Ru	10% Ni and 0.5–5% Ru/Al2O3	Wet impregnation (sequential and co-impregnation)	GHSV = 9000 h−1 H2/CO2 = 4/1	XCO2 = 82.7%, SCH4 = 100% at 400 °C (10% Ni and 1% Ru/Al2O3)	Co-impregnation of Ni and Ru precursor salts could provide more active sites in the catalyst. The best results were achieved using 1% Ru loading. Ru and Ni were found to form separate phases and it was suggested that CO2 and H2 activation took place at Ru and Ni sites, respectively.	[30]
Ru	10% Ni and 1% Ru/2% CaO—ordered mesoporous alumina (OMA)	Evaporaton-induced self-assembly (EISA)	WHSV = 30,000 mL g−1 h−1 H2/CO2 = 4/1	XCO2 = 83.8%, SCH4 = 100% at 380 °C (10% Ni and 1% Ru/2% CaO—OMA)	A step increase in the CO2 conversion and CH4 selectivity was observed after the addition of the CaO basic promoter and the Ru second metal. Catalysts stable at 550 °C for 109 h due to the confinement effect. Ni and Ru synergy could reduce the activation energy for CO2 methanation.	[97]
Ru	11.1–12.7% Ni and 0.9–4.8% Ru/Al2O3-washcoated cordierite	Equilibrium adsorption (Ni) and wet impregnation (Ru)	GHSV = 104,000 h−1 H2/CO2 = 4/1	XCO2 = 55%, SCH4 ≈ 100% at 350 °C (12.7% Ni and 0.9% Ru/Al2O3-washcoated cordierite)	Ni was homogeneously dispersed over the structured support as small nanoparticles (2–4 nm) via equilibrium adsorption, while Ru was atomically dispersed via wet impregnation. The structured catalyst on Al2O3-washcoated monolith provided stable performance with low pressure drop under high space velocities.	[99]
Ru	30% Ni and 0–5% Ru/CeO2-ZrO2	One-pot hydrolysis of metal nitrates with (NH4)2CO3	GHSV = 2400 h−1 H2/CO2 = 4/1	XCO2 = 98.2%, SCH4 = 100% at 230 °C (30% Ni and 3% Ru/Ce0.9Zr0.1O2)	Ru addition on Ni/CeO2-ZrO2 could increase surface basicity and promote Ni dispersion. Thus, the formation of a NiRu bimetallic catalyst enhanced the low-temperature catalytic activity for CO2 methanation.	[107]
Ru	10% Ni 0.5–3% Ru/CeO2-ZrO2	Wet impregnation with different Ru precursor salts	WHSV = 60,000 mL g−1 h−1 H2/CO2 = 4/1	XCO2 ≈ 80%, SCH4 ≈ 100% at 350 °C (10% Ni and 0.5% Ru/CeO2-ZrO2)	Adding Ru in 10% Ni/CeO2-ZrO2 increased the CO2 methanation performance. When ruthenium acetylacetonate was used instead of ruthenium chloride as the precursor salt, the metal dispersion and catalytic activity were enhanced due to the templating effect of the precursor salt molecule.	[108]
Ru	15% Ni and 1% Ru/CeO2-ZrO2	Wet impregnation	WHSV = 24,000 mL g−1 h−1 H2/CO2 = 4/1	XCO2 = 53%, SCH4 = 93% at 350 °C (10% Ni and 0.5% Ru/CeO2-ZrO2)	The introduction of 1% Ru in 15% Ni/CeO2-ZrO2 improved the dispersion of Ni and the intrinsic activity for CO2 reduction. The catalyst was active for both CO2 methanation at 350 °C and reverse water–gas shift (RWGS) at 700 °C.	[70]
Ru	1.5% Ru/Ni	Ni deposition on Ru/SiO2 intermediate and then silica etching	Flow rate = 3000 mL h−1 H2/CO2 = 4/1	XCO2 ≈ 100%, SCH4 ≈ 100% at 200 °C (1.5% Ru/Ni)	This novel synthesis method, using an intermediate silica carrier to disperse Ru, yielded fine Ru nanoparticles supported on Ni grains. The 1.5% Ru/Ni catalyst had an oxide passivation layer and a very high low-temperature catalytic activity.	[109]
Ru	1.5% Ru/Ni nanowires (NWs)	Ni NW deposition on Ru/SiO2 intermediate and then silica etching	Flow rate = 3000 mL h−1 H2/CO2 = 4/1	XCO2 ≈ 100%, SCH4 ≈ 100% at 179 °C (1.5% Ru/Ni NWs)	When Ni nanowires were used instead of Ni powder, the higher surface area of the 1D nanostructure led to a higher CO2 methanation catalytic activity, with 100% CO2 conversion being reached at just 179 °C.	[110]
Rh, Ru	5% Ni and 0.5% Rh, Ru/CeO2-ZrO2	Pseudo sol-gel in propionic acid	GHSV = 43,000 h−1 H2/CO2 = 4/1	XCO2 = 77.8%, SCH4 = 99.2% at 350 °C (0.5% Rh and 5% Ni/Ce0.72Zr0.28O2)	Noble metal addition (Rh and Ru) increased the dispersion of Ni and the catalytic activity for CO2 methanation. Rh addition led to slightly better results compared to Ru.	[106]
Rh	Rh-Ni/3DOM-LaAlO3 (≈2% Ni and 1% Rh)	Rh wet impregnation and Ni exsolution from LaAl0.92Ni0.08O3	WHSV = 48,000 mL g−1 h−1 H2/CO2 = 4/1	XCO2 ≈ 93%, SCH4 high at 308 °C (Rh-Ni/3DOM-LaAlO3)	Rh-Ni/3DOM-LaAlO3 with bimetallic NiRh alloy nanoparticles was highly efficient for CO2 methanation. 3DOM LaAl0.92Ni0.08O3 perovskite was prepared via PMMA colloidal crystal templating and Rh was added via wet impregnation. Ni exsolution and NiRh alloy formation followed after reduction treatment.	[115]
Rh	1.56–1.9% Ni 0.69–1.18% Rh/Al2O3	Galvanic replacement (GR) and chemical reduction (CR)	WHSV = 48,000 mL g−1 h−1 H2/CO2 = 4/1	XCO2 ≈ 97%, SCH4 ≈ 100%, at 300 °C (1.56% Ni and 1.08% Rh/Al2O3 (GR))	RhNi/Al2O3 catalysts prepared by galvanic replacement exhibited superior CO2 methanation performance at low temperatures. Galvanic replacement led to Ni nanoparticles encapsulated by an atomically thin RhOx shell, while chemical reduction led to a higher degree of Ni and Rh intermixing.	[117]
Pt, Ru, Rh	10% Ni and 0.5–3% Pt, Ru and Rh/CeO2	Wet impregnation (sequential)	WHSV = 60,000 mL g−1 h−1 H2/CO2 = 4/1	XCO2 = 82%, SCH4 ≈ 100%, at 325 °C (10% Ni and 0.5% Pt/CeO2)	Promotion of 10% Ni/CeO2 with Pt and Ru further increased the catalytic activity, while Rh led to the worst performance. The optimal Pt loading was 0.5%, while for Ru it was 1%. Pt enhanced the dissociation of CO2 into intermediate CO, while Ru provided additional methanation sites.	[33]
Pt	Ni100-xPtx/Al2O3, (1 mmol Ni + Pt metal gcat−1)	Wet impregnation	WHSV = 30,000 mL g−1 h−1 H2/CO2 = 4/1	XCO2 = 70%, SCH4 = 97%, at 427 °C (Ni95Pt5/Al2O3)	Single atom alloy catalysts (SAAC) with Pt atoms dissolved into the lattice of Ni nanoparticles supported on Al2O3 were highly active for CO2 methanation. A Pt/(Ni + Pt) molar ratio of 5% was optimal. Isolated Pt atoms adjacent to Ni enhanced the adsorption of intermediate CO, while weakening the C-O bond energy and thus favoured the further conversion to CH4.	[31]
Pd, Pt and Rh	10% Ni and 0.5% Pd, Pt and Rh/Al2O3	Incipient wetness impregnation	GHSV = 5700 h−1 H2/CO2 = 4/1	XCO2 = 90.6%, SCH4 = 97%, at 300 °C (10% Ni and 0.5% Pd/Al2O3)	Pd and Pt addition improved the catalytic activity of 10% Ni/Al2O3, while Rh addition led to worse performance. Pd and Pt increased the dispersion and reducibility of NiO and provided active sites for H2 chemisorption and activation. The Pd-promoted catalyst was slightly better than the Pt-promoted one.	[119]
Pd	Ni1-xPdx/SBA-15	NiPd nanoparticle synthesis in oil amine and impregnation on SBA-15 with hexane	WHSV = 6000 mL g−1 h−1 H2/CO2 = 4/1	XCO2 = 96.1%, SCH4 = 97.5%, at 430 °C (Ni0.75Pd0.25/SBA-15)	The synergy between Ni and Pd metals led to active NiPd alloy catalysts for CO2 methanation supported on mesoporous SBA-15 support. All bimetallic catalysts were better than the monometallic ones. A Ni/(Ni + Pd) ratio of 3 (Ni0.75Pd0.25) led to the best performing catalyst.	[32]
Pd	30% NiOTPd-TMOS/acid-treated CNTs (Pd/Ni molar ratio = 1.5)	Ni wet impregnation on acid-treated CNTs, Pd addition with NaBH4 and TMOS decoration	WHSV = 100,000 mL g−1 h−1 H2/CO2 = 3/1	Methane yield: YCH4 = 1905.1 μmol CH4 gcat−1, at 300 °C (NiOTPd-TMOS/acid-treated CNTs)	The catalyst consisted of Pd nanoclusters adjacent to NiO with tetrahedral symmetry, supported on acid-treated CNTs and decorated with a layer of tetramethyl orthosilicate (TMOS). The maximum amount of CH4 was produced due to the Ni-Pd synergy at their interface, with H and CO being adsorbed over interfacial Ni and Pd sites, respectively.	[125]
Re	15% Ni and 1% Re/CCFA (coal combustion fly ash)	Wet impregnation (co-impregnation)	GHSV = 2000 h−1 H2/CO2 = 4/1	XCO2 = 99.6%, SCH4 = 70.3%, at 400 °C (NiRe/CCFA)	NiRe bimetallic catalysts were supported on coal combustion fly ash (CCFA) from industrial waste. Re addition improved Ni dispersion and the catalyst resistance towards sintering and coking, leading to better performance for CO2 methanation.	[130]