2.2. Thermodynamic Equilibrium Conversion and Selectivity
Several thermodynamic studies have examined the effects of reaction parameters on CO
2 methanation
[34,36,48,49,50,51][33][36][37][38][39][40].
Understanding the CO
2 methanation reaction significantly relies on thermodynamic equilibrium. The CO
2 methanation reaction is a combination of the deceleration of exothermic (CO methanation) and endothermic (water gas shift) reactions, as shown in Equations (1)–(3)
[52,53][41][42].
Lower temperatures are more favorable for the methanation reaction, leading to an increase in CO
2 conversion and an improvement in CH
4 selectivity
[46,54][43][44]. The CO
2 methanation process exhibits the highest productivity at low temperatures, resulting in nearly 100% CO
2 conversion and CH
4 selectivity
[55][45].
In CO
2 methanation, the number of molecules decreases from five for reactants to three for products; therefore, the rise in pressure has a positive impact on CO
2 conversion, mainly when operating within temperature ranges of 200–500 °C
[57][46]. High pressure is also favorable for CH
4 selectivity
[46][43].
3. The Catalytic Performance of Noble Metal Catalysts in CO2 Methanation
3.1. Ruthenium-Based Catalysts
The catalytic performance of alumina-supported ruthenium (Ru/Al
2O
3) was investigated by Garbarino et al.
[62][47], who observed that supported Ru catalysts are more active and stable than other metal-based catalysts. The supported Ru catalysts show excellent activity, especially at low temperatures, while the selection of the best catalyst preparation method and activation need to be carefully managed
[63][48]. Ru/Al
2O
3 demonstrates high catalytic activity and good stability at 375 °C for 100 h, exhibiting 91% conversion and standing at 91% CH
4 selectivity. The superior activity and stability were attributed to the high dispersion of Ru nanoparticles (NPs) and interactions between Ru NPs and the Al
2O
3 support. The consistent and stable high activity observed after different shutdown and start-up sequences suggests that the catalyst retains its effectiveness even under intermittent operation
[62][47].
Ru doped with ceria is one of the most promising methanation catalysts owing to the essential nature of the O
2 vacancies on the surface of CeO
2, which can activate CO
2. For Ru/Al
3O
2, adding ceria can improve the activity
[64][49]. For Ce-doped Ru/Al
3O
2 catalysts, initial Ru NPs can be redispersed during oxidative pretreatment into atomically distributed RuOx species owing to interactions between Ru and ceria.
Besides CeO
2 and Al
2O
3, TiO
2 is also a promising support for loading Ru
[66][50]. Researchers have demonstrated that TiO
2-supported Ru NP catalysts possess good stability, which is attributable to the unique interaction between metal Ru and support TiO
2. The CO
2 conversion can maintain stability for 34 h running, with 68% of CO
2 conversion and 98% selectivity to CH
4 at 290 °C and 1 atm. For Ru/TiO
2 catalysts, the catalytic performance relies on metal–support interactions, the loading amount of Ru, and the particle size of Ru NPs
[67,68,69,70,71][51][52][53][54][55].
3.2. Rhodium-Based Catalysts
For Rh-based catalysts, the best performance is shown on Rh/TiO
2 with 1 wt.% of Rh loading, which exhibits 90% CO
2 conversion and 96% selectivity to CH
4 with only 3 h of stability running under reaction conditions of a GHSV/WHSV of 12,000 h
−1 (mL.g
−1h
−1), 370 °C, and 2 bar
[79][56]. Considering the high reaction temperature and pressure, the activity of Rh is inferior to that of Ru; furthermore, investigations on the stability of Rh-based catalysts for CO
2 methanation have not been extensively covered in the literature.
Moreover, the activity of Rh-based catalysts has been improved by adding other active metals, such as Ru-Rh/Al
2O
3, showing better activity than mono Rh
[85][57], and Ni-Rh/Al
2O
3 also exhibits better activity than Rh/Al
2O
3 [86][58].
Catalytic performance may be affected by the O
2 presence; O
2 in a low percentage boosts the catalyst’s performance, whereas a higher concentration leads to a negative effect
[56,92][59][60].
TiO
2 is one of the best options for supporting Rh catalysts. The effects of Rh/TiO
2 on catalytic performance at low temperatures were investigated by Alejandro et al.
[93][61], who found that large Rh particle sizes have more active sites and weak CO intermediates, which also affects the order of CO
2 methanation reaction and activation energies. Notably, a slight variation in the activation energy of CO dissociation with Rh particle size was observed. Higher response orders in H
2 were also seen for smaller particles, indicative of reduced H
2 coverage. CH
4 selectivity gradually increased with a change in particle size (from 2 nm to 7 nm), while, beyond this particle size, there was no discernible difference
[93][61].
It has been observed that Rh/TiO
2 catalysts have higher CH
4 selectivity as compared with Rh/Al
2O
3 and Rh/SiO
2. The literature suggests that the breakdown of the C-O bond could be aided by electron interactions between metal and supports or interactions between CO absorbed by a catalyst and Ti 3
+ ions positioned at the border metal support
[79,93][56][61].
Research on Rh/TiO
2 by Martnez et al.
[68][52] showed that changes in activity depend on interactions between Rh NPs and the support. The average particle size after H
2 reduction at a high temperature is substantially smaller than the size of calcined Ru in the presence of synthetic air and then further heated at 300 °C, which was confirmed with TEM and XRD
[68][52]. The interaction can make Rh re-disperse, which leads to the high dispersion state of Rh NPs, resulting in higher CO
2 conversion.
3.3. Palladium-Based Catalysts
For Pd-based catalysts, the best catalytic performance was shown on Pd@FeO with a Pd loading amount of 5.2 wt.%, showing 98% CO
2 conversion and 100% selectivity to CH
4 at a low temperature of 180 °C, and the catalytic performance was maintained for 20 rounds of cyclic running
[91][62]. The high activity was attributed to the face-centered tetragonal structure of the Pd-Fe intermetallic nanocrystal, and it was proposed that Pd-Fe intermetallic nanocrystals aided in maintaining metallic Fe species during CO
2 methanation via a reversible oxidation-reduction mechanism; thus, adding metallic Fe facilitated the direct conversion of CO
2. This study was efficient on a laboratory scale, but for practical applications, the loading amount of Pd is too high, and the stability needs to be investigated.
The bimetallic catalysts of Ni-Pd supported on Al
2O
3 show better activity with 0.5 wt.% and 10 wt.% loadings of Pd and Ni, respectively. In one study, a CO
2 conversion of 91% and 99% CH
4 selectivity was achieved under reaction conditions of a GHSV/WHSV of 57,000 (mL.g
−1h
−1), 300 °C, and 1 atm, and it remained stable for 4 h
[86][58].
3.4. Summary of Performance of Noble Metal Catalysts
Studies on noble metal catalysts for CO
2 methanation have mainly concentrated on Ru-, Rh-, and Pd-based catalysts, among them Ru based catalysts exhibited the best catalytic performance. For all catalysts, selectivity to CH
4 can be very good, especially at low reaction temperatures, so the challenge is to enhance the activity at low temperatures. When the loading amount of noble metal is as high as 3–6 wt.%, much higher CO
2 conversion can be achieved, although the high price means it has promise in practical applications. Ru-based catalysts have shown comparatively good stability in maintaining stability for 300 h running, but stability investigations of Rh- and Pd-based catalysts are needed.
Research on noble metal catalysts for CO
2 methanation has predominantly focused on Ru-, Rh-, and Pd-based catalysts; among them, Ru-based catalysts demonstrate superior catalytic performance. These catalysts generally exhibit high CH
4 selectivity, particularly at low reaction temperatures. However, enhancing activity at low temperatures remains a challenge.
4. The Catalytic Performance of Non-Noble Metal Catalysts in CO2 Methanation
4.1. Nickel-Based Catalysts
Ni-based catalysts have gained significant attention in CO
2 methanation. The research emphasis has been on enhancing its activity in low reaction temperatures and improving the sintering resistance of Ni NPs because of the inert nature of CO
2 and the strong exothermic activity of the methanation reaction. In designing a promising catalyst for industrial applications, the critical factors of catalytic performance are the properties of Ni NPs, including dispersion and its chemical state, metal–support interactions, and additive materials. Supports such as Al
2O
3 [62,100][47][63], SiO
2 [64[49][64],
101], TiO
2 [102[65][66],
103], ZrO
2 [104[67][68],
105], CeO
2 [106[69][70],
107], and the solid solution Ce-Zr-O
[108][71] are predominantly utilized for loading Ni NPs. The additives mostly employed are alkaline earth metals, such as La, Y, and Ce; the basic element Mg; the transition element Mn; and so on. Catalyst synthesis methods are diverse, with the principal objectives being to increase Ni NP dispersion and/or adjust interactions between Ni NPs and the support or additives.
Al
2O
3 is the most frequently applied catalyst support for CO
2 methanation
[109,110][72][73]. Al
2O
3 is extensively used owing to its high surface area, adjustable porous structure, and complex chemistry properties
[99,111,112][74][75][76]. Therefore, Al
2O
3 is not suitable for loading Ni NPs for CO
2 methanation, but reports on Ni/Al
2O
3 for CO
2 methanation are significant
[113,114][77][78]. An issue with using Al
2O
3 as a support for the methanation reaction is that it tends to sinter when exposed to water (a byproduct of the process) at high temperatures
[99][74].
The activity of Ni/Al
2O
3 is not good enough; with a loading of 12.5 wt.% Ni on Al
2O
3, CO
2 conversion could reach 71% at a high temperature of 500 °C
[115][79], and the reason for the poor activity can likely be attributed to the poor CO
2 activation ability of Al
2O
3. When adding an additive to activate CO
2, such as ceria, the activity could be improved effectively over Ni/CeO
2-Al
2O
3; 71% CO
2 conversion and 99% selectivity to CH
4 could be achieved at a low temperature of 350 °C, at reaction conditions of a GHSV/WHSV of 15,000 (mL.g
−1h
−1) and 1 atm
[116][80].
Silica (SiO
2) is another popular choice since it has a large surface area and may adjust its pore diameter to suit a given application
[117,118,119][81][82][83]. Ni and SiO
2 form metal–support interactions, which are antagonistic to the growth of Ni carbide, which leads to the improved catalyst’s resistance to coke production and Ni sintering.
[120,121][84][85]. In the literature, CO
2 methanation with a SiO
2-supported catalyst has shown a CO
2 conversion efficiency of only about 60% to 90%
[90[86][87][88][89][90],
122,123,124,125], which may be due to the inertness of SiO
2; therefore, the additive addition or surface modification of the support needs to be further investigated
[126,127][91][92].
Similar to studies that used alumina-supported Ni catalysts, Li et al. used Mg as a promoter for Ni/SiO
2; the resultant catalyst showed obviously improved activity with 82% CO
2 conversion and 99% selectivity to CH
4 under reaction conditions of a GHSV/WHSV of 60,000 (mL.g
−1h
−1), 250 °C, and 1 atm
[129][93]. SiO
2 with a high surface area and a mesoporosity of MCM-41 is sufficient support for CO
2 methanation. A high surface area favors the high dispersion of Ni NPs, and mesoporosity is beneficial for reactant transfer
[110,130][73][94].
The distinctive catalytic performance of SiO
2 with a high surface area loaded with Ni NPs has naturally inspired researchers to investigate zeolite as a support for CO
2 methanation, as it is known that a key characteristic of silica zeolite is a high surface area. Chen et al.
[143][95] used a conventional Ni/SiO
2 as a precursor-prepared core–shell catalyst for Ni@HZSM-5 via the hydrothermal method. Compared with traditionally prepared Ni/SiO
2 and Ni/HZSM-5 catalysts, Ni@HZSM-5 exhibited superior performance, preserving its Ni content and the structure of the active Ni after a 40 h CO
2 methanation reaction. A key feature of this catalyst is the interaction between the Ni active phase and zeolite, with the former donating more electrons to the latter, thus preventing sintering and enhancing the activity of Ni. At 400 °C, the Ni@HZSM-5 catalyst demonstrated a CO
2 conversion of 64% and near 100% CH
4 selectivity under reaction conditions of a GHSV/WHSV of 36,000 (mL.g
−1h
−1), 400 °C, and 1 atm
[131][96].
4.2. Cobalt-Based and Iron-Based Catalysts
Co-based catalysts are the most well-studied active component of Fischer–Tropsch synthesis (FTS), which generates hydrocarbons from syngas (a gas mixture of CO and H
2), as CO
2 can be converted into CO via reverse water gas shift reactions; therefore, studying Co-based catalysts for CO methanation is expected
[191,192][97][98].
In another study, Co/ZrO
2 showed good catalytic performance with 92% CO
2 conversion and 99% selectivity to CH
4, and this performance was maintained for 300 h under reaction conditions of a GHSV/WHSV of 36,000 (mL.g
−1h
−1), 400 °C, and 30 atm
[170][99]. Considering the low space velocity, high pressure, and comparatively high temperature in these reaction conditions, the activity is very good but not excellent compared with Ni-based catalysts, although the stability seems very good.
In a study by Moghaddam et al.
[173][100], Ni/Al
2O
3 catalysts were prepared using the one-pot sol–gel method, with small amounts of additional elements such as Fe, Co, Cu, Zr, and La added. The catalyst containing Fe demonstrated exceptional performance, achieving 71% CO
2 conversion and nearly 99% selectivity for CH
4 at 350 °C, a GHSV/WHSV of 9000 (mL.g
−1h
−1), and 1 atm. This improvement can be attributed to the presence of a Ni-Fe alloy, which enhanced the adsorption of H
2 and the dissociation of CO
2. Interestingly, increasing the Fe content from 5 to 7 wt.% resulted in enhanced activity at lower temperatures and maintained stability over a 10 h period.
4.3. Summary of Performance of Non-Noble Catalysts
Non-noble catalysts for CO
2 methanation include Ni-, Co-, and Fe-based catalysts; among all catalyst systems, Ni-based catalysts show the best catalytic performance. The selectivity to CH
4 is generally high (close to 100%), especially at low reaction temperatures (lower than 400 °C), and they have revealed very good activity. Excellent Ni-based catalysts can reach or come close to equilibrium conversion at around 350 °C and 1 atm with a WHSV of 10,000 mL.g
−1h
−1 or even higher, and the stability generally extends to several hundred hours. For Co- and Fe-based catalysts, some show very high activity at low temperatures, though the stability and selectivity compared with the corresponding catalyst is possibly not good enough. Co- and Fe-based catalysts are well known for being used as FTS catalysts, which means that hydrocarbons besides CH
4 can be easily generated, thus leading to decreased selectivity to CH
4.
In this study, the catalytic performance of noble metal catalysts was found to be dependent on metal dispersion, metal–support interactions, and additive doping. Various catalyst configurations, including supports made of single-oxide supports (Al
2O
3, TiO
2, ZrO
2, and CeO
2), composite oxide supports (Ce-Zr-O), basic oxide promoters, and bimetallic systems, were investigated to determine their impact on catalytic activity and stability. These results suggest that Ni-based catalysts are the most promising.
5. Conclusions
In combatting the challenges posed by CO
2 emissions, using renewable energy to produce green hydrogen and then hydrogenating CO
2 into valuable chemicals is a promising route, including through the production of CH
4 via CO
2 methanation. Given the thermodynamic equilibrium of the reaction, to convert CO
2 completely into CH
4, low reaction temperatures and high pressure are favorable, and meeting the requirements of developing catalysts is critical. The catalysts that have been extensively reported include noble metal-based catalysts and non-noble metal-based catalysts.
Among the noble metal catalysts, Ru shows the best catalytic performance. In excellent Ru-based catalysts, CO
2 conversion can be achieved close to equilibrium (98%) and with 100% selectivity to CH
4, with 300 h of stability obtained at 24,000 mL.g
−1h
−1, 230 °C, and 1 atm. Excellent noble metal-based catalysts feature the high dispersion of the noble metal, the strong metal-support interaction with noble metal NPs, and doped with a transitional metal: Ni, Co, or Fe. The disadvantages of noble metal-based catalysts are the high price and high loading, and the stability needs to be improved.
Non-noble metal catalysts include Ni-, Co-, and Fe-based catalysts. Ni-based catalysts show the best catalytic performance and are well studied, while studies on Co- and Fe-based catalysts are much fewer, possibly because other hydrocarbons besides CH
4 can be formed over Co and Fe catalysts. With excellent Ni-based catalysts, CO
2 conversion close to equilibrium (around 90%) and 100% selectivity to CH
4 can be obtained at around 350 °C a WHSV of 10,000 (mL.g
−1h
−1) or higher, and 1 atm with good stability. Excellent Ni-based catalysts feature high Ni NP dispersion; a support and/or additive to activate or help activate CO
2; and interactions between Ni NPs and the support that resist sintering. Bimetals (Ni-Fe, Ni-Co) are favorable for improving catalytic performance.