Introduction

Nowadays, demands on natural gas have been increased as the second largest worldwide energy (Khan 2015; Dong et al. 2017; Curry 1981). Natural gas is the most essential source in daily usage as it promises less harmful by-products into the air and considered as the cleanest and safest to the environment (Solarin and Ozturk 2016; Kakaee et al. 2014; Aleman-Nava et al. 2014). However, the quality of natural gas is disputed due to the presence of unwanted compounds such as non-associated gases including carbon dioxide (CO2). According to Speight (2007), these impurities formed corrosive compound, for example carbonic acid in the presence of water, and thus corroded the pipeline of natural gas, reducing the quality and value of natural gas due to its sour properties in worldwide markets as well as causing difficulties for its distribution to the market.

On the other hand, air pollution is also being affected by the high CO2 content released into the atmosphere. Some effects of the escalating concentration in greenhouse gases (GHGs) within the atmosphere are global warming, which seems to be an invisible threat to humans. Hence, the establishment of the United Nations Framework Convention on Climate Change (UNFCCC) emphasised on stabilising GHG concentration at permissible level so as to protect the climatic stance from deleterious factors (Strakey et al. 1975). In fact, the term ‘carbon credit’ was implemented to control the commercial and industrial sectors to cut the release of excessive CO2 or GHGs into the environment. As a consequence, the necessity of green technology is unquestionable to minimise GHG emission, particularly involving fossil fuels.

Among technologies used, catalytic methanation reaction is widely explored to convert CO2 in natural gas into methane (CH4) for the effective and complete combustion. Some of potential catalysts, i.e. nickel, cobalt, iron and copper, expressed a good performance towards methanation reaction (Bakar et al. 2012; Bakar et al. 2012; Zamani et al. 2015a, b). However, among catalysts prepared for this particular purpose, lanthanide oxides (LOs) are noteworthy candidates to convert CO2 gas and are promising higher selectivity towards methane species. Up to date, a comprehensive study in natural gas to simultaneously convert CO2 to CH4 in industry by reducing emission of impurities by-products has not been conducted using lanthanide oxide. Lanthanide oxide, the unique properties of this metal oxide to among other elements, was particularly in oxidation state. LOs that existed in their trivalent state possessed higher-degree stabilisation in 4f, 5d and 4s orbitals upon ionisation and provided basic catalyst, which took them to stand out as the most versatile materials for methanation reaction. Other than that, the number of unpaired electron could be seven, whereby enhancing a large magnetic moment in chemical bond (Curry 1981). Taking these into account, therefore, this review paper aims to provide an idea on the advantage of LOs in catalytic methanation to produce higher-quality natural gas. In addition, this paper also focused on the compilation of a wide range of lanthanide oxide elements and their functions as a based promoter, co-catalyst and support. It is also purposely to grab the attention of researchers to explore on other lanthanide oxides through reasonable and systematic method catalytic methanation. Table 1 summarises the benefits and drawbacks of lanthanide oxide.

Table 1 Some of the benefits and drawbacks of lanthanide oxide
Full size table

Herein, this paper has fulfilled the gap by discussing more on the lanthanide oxides, i.e. lanthanum (La), cerium (Ce), praseodymium (Pr) and samarium (Sm) catalysts, to convert CO2 into methane gas. This article is organised as follows: the section “Current technologies in methanation reaction” reports the overview of technologies used in natural gas, while the section “Methanation catalysts” reviews the existing literature on CO2 methanation catalysts as well as in-depth deal with the performance of lanthanide oxide catalysts. The section “Physicochemical properties of lanthanide oxide catalysts through characterisation analyses” presents the physicochemical properties of lanthanide oxide catalysts, while the section “Mechanism of catalytic CO2 methanation reaction” discusses the mechanistic study of methanation catalyst; finally, the section “Conclusion and future aspect” presents concluding remarks and a perspective outlook for future works.

Current technologies in methanation reaction

Increment of standard and purified natural gas production appears to be liaised with its escalating demand. It is absolutely necessary to discard CO2 and hydrogen sulphide in generating sweet natural gas to ascertain clean burning and safety for the environment. The pressure swing adsorption (PSA) measure was commonly employed to purify gases by pushing the pressure to the down level. This particular process can be applied to discard CO2 from streams of highly pressured gases based on species molecular and affinity for an adsorbent material (Haldor Topsøe 2005; Riboldi and Bolland 2017). Additionally, the process of amine sweetening can also eliminate the pollutants to generate clean gases that were fitted for transportation purposes (William and David 2005).

Moreover, the gas scrubber (Michael et al. 2010; Stookey et al. 1986) and the water scrubber (Biernat and Gisw 2012) technologies were currently used by applying the Prism membrane to remove CO2 in order to adhere to the specification of pipelines and enhanced the value of heating for gas. The membrane separated CO2 efficiently from hydrocarbon vapours at a lower temperature, mainly because of its solubility aspect higher than methane. Upon dissolving CO2 in water, the concentration of methane within the gas phase was increased at the scrubber column (Biernat and Gisw 2012). This process of separating membrane was carried out to permeate selected gases, such as CO2, H2 and H2S, which appear to be part of the permeated gas in the membrane, thus diffused through the material of the membrane (Zuo et al. 2014). The membrane module has been reported to escalate the process of permeation in components with higher permeation rates (Aasberg-Petersen et al. 2011), in comparison to those components that suggested permeation at a lower rate, for instance N2, Cl, C2H6 and heavier hydrocarbons.

Another method that has been widely used in industry to treat sour gas referred to the iron sponge process (Anerousis and Whitman 1984), which was a simple yet effective approach in applications related to systems with low pressure, natural gas with high pressure as well as sewerage gas from sewer sludge as a result of anaerobic digestion (Abdulrahman and Sebastine 2012). However, all of the mentioned technologies spoiled a few shortcomings such as not easily adapted to continuous operating cycle, incurred high fuel cost and dependent on operating conditions of the feed gas.

Therefore, years by years, catalytic chemical conversion has become a more promising technique to remove CO2 and H2S gases (Wan Abu Bakar et al. 2015). This method has the ability to enhance the purity of the natural gas by simultaneously converting them to methane gas using heterogeneous catalysts (Habazaki et al. 1998; Silva et al. 2012; Hoekman et al. 2010; Liu et al. 2012a, b; Munik et al. 2014; Brooks et al. 2007; Beuls et al. 2012; Djinovic et al. 2011; Abe et al. 2009; Zamani et al. 2014). Catalytic methanation reaction has received great attention due to its economical approach as these catalysts may be regenerated, reused and safe for the environment because no harmful gas was emitted into the environment at the reaction time. However, the most challenging factor for this technology was to achieve this reaction under mild conditions due to kinetic barriers. Consequently, the development of proper catalysts to lower the activation energy of methanation reaction was critical as how they worked in reaction as will be discussed in the section “Methanation catalysts”. Table 2 summarises the current technologies used in processing natural gas.

Table 2 Summarisation of current technologies in processing natural gas
Full size table

Methanation catalysts

In general, the methanation reaction is the catalytic reactions that convert CO2 or carbon monoxide into methane and water products when reacting with hydrogen as described in Eqs. (1) and (2). This reaction is also called a Sabatier reaction with moderately exothermic value of ΔH = − 165 kJ mol−1 and is dependent on the interfaces of active sites and the reactions through adsorption-desorption at the catalyst surface (Saluko 2005).

$$ {\mathrm{CO}}_{2(g)}+{4\mathrm{H}}_{2(g)}\to {\mathrm{CH}}_{4(g)}+{2\mathrm{H}}_2{\mathrm{O}}_{(l)} $$
(1)
$$ {\mathrm{CO}}_{(g)}+{3\mathrm{H}}_{2(g)}\to {\mathrm{CH}}_{4(g)}+{\mathrm{H}}_2{\mathrm{O}}_{(l)} $$
(2)

Typically, the selection of active catalyst applied in CO2 methanation considered higher basicity in accordance with CO2 acidity, as well as higher surface area with dispersion of smaller-sized particles upon the surface of the catalyst. It has been found that metal oxides were the most significant and commonly employed catalysts for its exceptional ability in transferring electron and proton, besides generating redox reactions with the assistance of catalyst support dispersion which stabilises the catalytic aspect of the active species (Seiyama 1992). For instance, nickel, an element in group VIII metals, was a conventional catalyst applied in the industrial sector (Bakar et al. 2009) in the form of metal oxide. Although it generated an enhanced reaction, its drawback lied in the severe carbon deposition at higher reaction temperature. On the other hand, for ruthenium and rhodium, both exhibited greater results with higher CO2 conversion but still were costly and scarcely available for a giant scale of operation (Akin et al. 2002).

Second drawback related to high CO2 methanation was unable to deal with sole metal oxides. However, the improvement in stability and activity can be accounted by an electronic modification which resulted from the direct bonding or from structural change induced by one metal upon the other via the contribution of promoter or co-catalyst metal oxides (Tang et al. 2006). It was also fair to state that the development of mixed metal oxide as a catalyst was much slower due to its multiple oxidation states, variable local coordination, coexisting bulk and varied surface termination functionalities (Ertesva et al. 2005). There were a number of research studies previously reported in gaining excellent catalyst that can contribute higher activity of CO2 methanation reaction. Several methanation catalysts had been evidently patented and are tabulated in Table 3.

Table 3 Patented methanation catalyst
Full size table

Overall, the lack of report about lanthanide oxide catalysts in methanation reaction was found. Under the circumstances, the most applied LO elements in methanation reaction were Ce and La.

Ideally, lanthanide oxide catalysts mostly would affect the chemical modification of catalysts as well as catalytic activity and stability in CO2 hydrogenation (Feng et al. 2014). Generally, lanthanide series that were used as catalysts was lanthanum, cerium, samarium and praseodymium. In pioneering research studies by Song et al. (2010), La2O3 commonly acted as an electronic modifier in Ni catalysts due to its good dispersion of hexagonal crystalline structure on the surface for charge trapping. The 10% Ni/La2O3 showed an excellent activity towards CO2 methanation and displayed an important activity at low temperature (280 °C) to produce CH4. Meanwhile, a significant improvement in the stability and high-temperature steam resistance was also observed over a Ni/Al2O3 catalyst doped with lanthanum oxide (Su and Guo 1999; Wierzbicki et al. 2016). The addition of rare-earth oxides has suppressed the growth of Ni particle, the oxidation of active component Ni and the formation of NiAl2O4 species, thus significantly reducing the catalyst deactivation. On the other hand, lanthanum oxide was also used as a dopant in Ru impregnated on alumina. The results obtained showed that the reaction temperature was approximately lowered by 30 °C compared to pure Ru/Al2O3 (Chen et al. 2007). The CO conversion and selectivity for Ru-La2O3/Al2O3 was above 99% in the presence of hydrogen-rich gas at temperature above 200 °C. Meanwhile, the incorporation of optimum 5 wt% Ru into Co/La/Al2O3 also has been investigated by Rosid et al. (2015b) which gave almost 100% of CO2 conversion with 43.40% of CH4 formation when the reaction temperature achieved was 300 °C.

Cerium, a commercial lanthanide element has been vastly examined due to its stronger redox ability and larger surface area (62.6 nm) (Fred 2008; Sims et al. 2019). The basicity of cerium could escalate the adsorption of CO2 at the surface of the catalyst, while its exceptional redox ability derives from the oxygen vacancies formed at its surface due to the rapidly decreasing Ce4+/Ce3+ (Rao and Mishra 2003; Centi et al. 2013; Boaro et al. 2019). Cerium oxide catalyst was also applied in NO and SO2 reduction reaction due to its stability without being deactivated by these gases. Meanwhile, the performance of bimetallic Co/Ce oxide catalyst was investigated by Arsalanfar et al. (2012) using a co-precipitation method for Fischer-Tropsch synthesis. The 60 wt% of ceria (Co/Ce, 40:60) was the optimum catalyst ratio for CO methanation reaction whereby it gave 90% conversion and 45% methane selectivity. By reducing the content of Ce to 50 wt% (Co/Ce, 50:50), the conversion obtained was similar as 60 wt% but slightly lower in their methane selectivity (35%) due to the increment of Co3O4 species (Xu et al. 2006). Furthermore, Sharma et al. (2011) also have investigated that Ru-doped ceria, Ce0.95Ru0.05O2, showed higher CO2 conversion and CH4 selectivity with 55% and 99%, respectively, at 450 °C by the ratio CO2:H2:Ar (13:54:33) at the gas hourly space velocity (GHSV) of 10,000 h−1.

A simple technique of impregnation was applied for the preparation of Ru/cerium oxide (CeO2)/Al2O3, with Ru loading of 2% by weight with Ce as a based catalyst (Tada et al. 2014). As expected, the Ru/CeO2/Al2O3 catalyst showed higher catalytic activity of methanation than CeO2/Al2O3 and Ru/Al2O3. Selectivity methane formation of Ru/CeO2/Al2O3 was greater than that of Ru/CeO2. Cerium oxide could also be recognised as a promoter (Jenewein et al. 2003) for CO2 hydrogenation. The addition of 2 wt% Ce towards a Ni-based catalyst by impregnation method was done by Liu et al. (2012a, b). They found that the incorporating of Ce had promoted CO2 adsorption on the Ni-2% CeO2/Al2O3 catalyst surface and caused CO2 conversion increased to 71% with 99% selectivity to methane gas. Meanwhile, the Ni/Al2O3 catalyst just achieved 45% CO2 conversion with 99% CH4 selectivity. Both catalysts were tested at a reaction temperature of 300 °C, with a ratio of 4:1 (CO2:H2) and GHSV of 15,000 mL g−1 h−1.

In addition, Xavier et al. (1999) also investigated the influence of CeO2 as the catalyst. As a result, under decreasing settings, the CeO2 catalyst applied with Ni/Al2O3 exhibited the highest activity as the electronic interaction was imparted by the dopant. The methanation activity that incorporated catalyst-doped 1.5 wt% CeO2 displayed the highest conversion for CO and CO2, which was 86.34% at 3.674 mol s−1. In fact, inclusion of CeO2 in Ni/γ-Al2O3 catalysts appeared to weaken the bond of alumina with nickel, thus increasing the dispersion of nickel on the surface of the catalyst during the catalytic testing. This exemplified that the addition of CeO2 did not only reduce the deposition of carbon but also managed to accelerate the reaction between steam and the adsorbed species upon the surface of the nickel (Wang and Lu 1998).

In another report, Rynkowski et al. (2000) examined the catalysts in CO2 methanation, whereby conversion of CO2 appeared to be maximised for Ru/CeO2/Al2O3 and Ru/Al2O3 catalysts at 76% and 72%, respectively. In fact, when the temperature exceeded 200 °C, the active catalysts displayed higher stability and higher selectivity towards methane, especially after 340 h. At temperatures that ranged between 200 and 227 °C, selectivity towards methane was near 100%. As such, high temperatures seemed to have a positive impact in reducing catalytic performance, mainly due to the existence of ceria within the catalyst. This also supported by Rosid et al. (2019a) who studied the effect loading of cerium-based catalytic methanation reaction at a calcination temperature of 1000 °C. The results showed that when the Ce content was increased to 60%, the conversion of CO2 was also increased to 100% at 400 °C reaction temperature; however, a further increase of Ce content up to 85% caused the catalytic performance to decline.

The catalytic activity exhibited by α-Al2O3-ZrO2-TiO2-CeO2 composite oxide, along with the support of Ni-based catalyst, was examined with the composite ratio Al2O3/ZrO2/TiO2/CeO2 (55:15:15:15), respectively. The best catalytic activity was achieved at 300 °C with 81.4% conversion of CO2 to CH4 which might be due to improvement in the reducibility nature of the catalysts by the addition of ceria (Abate et al. 2016). The use of ratio-based loadings also has been employed in methanation reaction which gave 100% CO2 conversion and 80% CH4 formation at 400 °C reaction temperature. The results showed that the trimetallic oxide catalyst Ru/Mn/Ce (5:35:60)/Al2O3 would convert 92.92% of CO2 gas at 400 °C calcination temperature. Upon increment of the calcination temperature to 700 °C, the catalytic performance reached the maximum 100% CO2 conversion (Rosid et al. 2015a). Catalytic activity by various ratio-based loadings also has been investigated, and it showed that the performance decreased as the amount of ceria-based catalyst increased which might be due to a blockage of active site by an excess amount of metal oxide (Vicente et al. 2004).

Revisiting the work of Sharma et al. (2016), Ru-substituted CeO2 catalyst was prepared using a solution combustion method. The findings showed that at 270 °C, the peak of methane appeared, and when it reached the maximum temperature at 450 °C, the highest methane yield with 55% CO2 conversion and 99% selectivity for methane was obtained. Meanwhile, the CO2 conversion was also higher on Mn/Ce-75/Al2O3 as compared to Ni/Ce-75/Al2O3, Cu/Ce-75/Al2O3 and Ce-100/Al2O3 over the whole range of studied temperatures. The maximum CO2 conversion achieved was 69.44% with 27.35% of methane at 300 °C (Toemen et al. 2014).

The Ce0.72Zr0.28O2 was used as a support to the research study by Ocampo et al. (2009), which was prepared with 5 wt% nickel-based catalyst via a pseudo-sol-gel technique. The catalyst revealed higher catalytic activity with 71.5% CO2 conversion by 98.5% CH4 selectivity at 350 °C. The operating condition used was 1 atm pressure and a CO2/H2/N2 ratio of 36:9:10 with a total gas flow of 55 mL min−1. However, the catalyst started to deactivate after 150 h on stream with 41.1% CO2 conversion by 94.7% CH4 selectivity.

The influence of Ce-Zr ratio was further investigated by Ocampo et al. (2011). The catalytic activity of 5Ni/Ce-Zr catalysts with a cerium content of 20 wt%, 60 wt% and 80 wt% was studied at a reaction temperature of 350 °C. The CO2 conversion for 5Ni/(80:20), 5Ni/(20:80) and 5Ni/(60:40) reached 71.5%, 73% and 79.7% with 98.5%, 99% and 99.3% selectivity to CH4, respectively. The higher conversion obtained was due to incorporation of Ni2+ cations into the Ce-Zr structure. Below than 150 °C, no conversion was detected until 200 °C with lower CO2 conversion (0.6%). The dosage of catalyst was 150 mg with fixed GHSV at 43,000 h−1.

Recently, Aldana et al. (2013) investigated the catalytic activity of 5Ni/Ce-Zr catalyst at different preparation methods which were pseudo-sol-gel method and wet impregnation method. The CO2 conversion was higher on 5Ni/Ce-Zrsol-gel than on 5Ni/Ce-Zrimp over the whole range of studied temperature. At a reaction temperature of 350 °C, the 5Ni/Ce-Zrsol-gel catalyst showed 79.7% conversion with 99.3% selectivity towards methane while 59.8% conversion with 97.3% methane selectivity over the 5Ni/Ce-Zrimp catalyst. They found that the species on impregnated catalyst was more difficult to be reduced due to the stronger interaction between Ni and support material (Ce-Zr). However, the performance of 5Ni/Ce-Zrimp catalyst was better than that of the Ni/SiO2imp catalyst with 35% CO2 conversion and 88.3% selectivity towards CH4. This showed that ceria-zirconia was a better support for Ni compared to silica in the methanation reaction.

In addition, mixed oxide of ZrxCe30 − xAl70O∂ has also been studied as the supported material for Ni catalyst in auto-thermal reforming of methane to hydrogen (Puduki and Yaakob 2014). Ni/ZrxCe30 − xAl70O∂ catalyst showed the highest activity at 650 °C with 75% conversion. Metallic catalyst supported with cerium was remarkable due to its properties which can affect the mechanical and thermal resistance of support, catalytic performance, carbon reduction and metallic dispersion on the catalyst surface (Luisetto et al. 2012). The influence of CeO2 as a promoter on CO2 methanation activity and CH4 selectivity was also studied by Tada et al. (2012). At 300 °C reaction temperature, ceria-supported Ni catalyst exhibited high CO2 conversion of 90% with 100% CH4 selectivity compared to Ni/Al2O3 catalyst which only gave 20% conversion with 90% selectivity towards methane. This was due to the basic oxide character of CeO2 support which can adsorb a sufficient amount of CO2 and reduced it on CeO2 support owing to its oxygen vacancies. The testing was conducted under a GHSV of 10,000 h−1 with the ratio of CO2:H2 = 1:4.

Similar observation was found by Ramaroson et al. (1992) over Ni/CeO2 and Ni/SiO2 catalysts. At 260 °C reaction temperature with the H2:CO ratio of 3:1, the Ni/CeO2 catalyst achieved 11.6% CO conversion and 30.5% selectivity towards methane gas compared to the Ni/SiO2 catalyst which managed to convert CO with only 2% and 96.4% CH4 selectivity. Therefore, the use of ceria as a support catalyst gave better result in CO conversion than SiO2-supported catalyst. The reason was due to the presence of oxygen vacancies on these catalysts which released free electrons and easily ionise. A reversible migration of electrons in the nickel metal can occur, leading to an ease of CO dissociation over the Ni/CeO2 catalyst.

On the other hand, samarium oxide also has a potential use as a dopant in the methanation process (Chunhui et al. 2010). The addition of Sm to Ba-Ru-K/activated carbon (AC) significantly improved the activity and stability of catalyst. This showed that samarium has enhanced the adsorption of hydrogen on the catalyst surface which leads to higher catalytic performance. Another research has also been conducted on amorphous Ni-Zr-Sm catalysts for CO2 methanation (Michiaki et al. 1999). This catalyst showed an excellent catalytic activity compared to Ni-Zr catalyst due to the increase number of active nickel sites and stabilisation of tetragonal zirconia by the addition of samarium. This reason has led to the enhancement of catalytic activity. As for zirconia, samarium oxide is regarded as a good support for nickel oxide catalyst in CO2 methanation.

The performance of samarium as a based catalyst in methanation reaction also has been done by Rosid et al. (2013). The potential catalyst Ru/Mn/Sm (5:35:60)/Al2O3 which calcined at 1000 °C gave more than 95% CO2 conversion with 93.46% CH4 formation at 300 °C reaction temperature. This showed that samarium has enhanced the performance of catalyst by increasing the rate adsorption of hydrogen onto the surface of catalyst as has been discussed by Michiaki et al. (1999).

Other than that, praseodymium and neodymium also have been studied as based catalysts in the methanation reaction (Rosid et al. 2015c, 2019b). Both catalysts gave > 96% CO2 conversion at 400 °C reaction temperature with Ru/Mn/Nd (5:20:75)/Al2O3 calcined at 1000 °C and Ru/Mn/Pr (5:30:65)/Al2O3 calcined at 800 °C. The methane formation for Ru/Mn/Nd (5:20:75)/Al2O3 and Ru/Mn/Pr (5:30:65)/Al2O3 catalysts was 40% and 41%, respectively. There are no other reports regarding praseodymium and neodymium as a catalyst in methanation reaction have been found recently.

Physicochemical properties of lanthanide oxide catalysts through characterisation analyses

Physicochemical properties of catalyst are another critical part in the methanation reaction. Different metal oxides demonstrated different structural, active species, morphology, textural particle size, pore distribution and desorption of acidity and basicity of catalysts in order to achieve higher catalytic performance.

The presence of smaller CeO2 and RuO2 species with the size range of 78.2 nm to 127 nm successfully improved the catalytic activity of CO2/H2 methanation with 100% CO2 conversion and 80% CH4 formation (Rosid et al. 2015a). Temperature-programmed reduction (TPR) profile showed the amount of H2 consumed increased with ceria catalyst which suppressed the metal-support interaction, thus increasing the reducibility of RuO2 on the catalyst surface at lower temperature as shown in Fig. 1 (Rosid et al. 2018).

Fig. 1
figure 1

TPR profile of the Ru/Mn/Ce (5:35:60)/Al2O3 catalyst at various calcination temperatures (Rosid et al. 2018)

Full size image

Razzaq et al. (2013) also found that the CO2 desorption peak of Al2O3-CeO2-supported catalyst was dominant at low temperature which was below 100 °C compared to the Al2O3-ZrO2. This region was ascribed to weak basicity related with the weakly adsorbed CO2 on the catalyst surface. More formation of active sites (CeO2) observed through XPS analysis led to higher catalytic activity due to transfer of oxygen vacancies from bulk to surface. The formation of oxygen vacancies under reduced Ce4+/Ce3+ valance ratio also restrained the coke formation.

The reducibility of Ni/Al2O3 catalyst also increased when doped with CeO2 as well as lowered the reaction temperature (Xavier et al. 1999). The oxygen atom (strong base) from the chemisorbed CO molecule created a strong interaction with the Ce3+ site Lewis acid. This interaction was weakening the C–O bond and easily hydrogenated to form methane. Jinghuan et al. (2011) reported that incorporation of 5% cerium in cerium-substituted cobalt chromite catalyst deforms the spinel structure and consequently increases the crystal defect by redistribution of metal ion with different oxidation states, thus performing 90% of CH4 conversion at 465 °C.

Inclusion of ceria investigated by Toemen et al. (2016) appeared to enlarge the surface area and exemplified the presence of mesopores that optimised the size of the pores to promote reactant gas adsorption. Additionally, this particular catalyst showcased type IV that reflected pores in slit shape, along with shapes and size that seemed to be non-uniform. This notion was supported with the HRTEM that showed mixtures of shapes that resembled platelets and rods, in which ceria could have been represented by the platelet shape. On top of that, the HRTEM illustrated the presence of lattice fringes in multiple directions, hence signifying the presence of more than a compound on the surface of the catalyst. The outcome showed that the 0.321 lattice did fit rather well with the related CeO2 phase.

XRD results expressed that highly spread La2O3 during reaction encourages the scattering of NiO on SiC support which enhanced the catalytic activity besides constraining the production of Ni particles (Zhi et al. 2011). TPR results recommended that La2O3 could improve the metal-support interaction, hence inhibited the production and agglomeration of metallic particles during the reduction and reaction processes. The XPS result suggested that La2O3 in Ni-La/SiC catalyst would increase the adsorption and dissociation of CO2 molecule by donation of d-electron from nickel atoms to vacant anti-bonding π* orbit of CO2 molecules. TEM results showed that La2O3 encourages the scattering of NiO on the support surface and also can prevent the aggregation and migration of Ni particles as shown in Fig. 2.

Fig. 2
figure 2

TEM images of fresh a Ni/SiC and b Ni-La/SiC and used c Ni/SiC and d Ni-La/SiC catalysts. Adopted from Zhi et al. (2011)

Full size image

As reported by Gao et al. (2009), the LaNiO3 perovskite catalysts activated at 400–700 °C shows a CH4 selectivity of 98.7–99.2% due to the formation of dispersed small metallic Ni particles which were liable for the high catalytic performance and stability even at high temperature (400–500 °C). H2-TPD-MS profiles of La2O2CO3 under citrate method recommended that hexagonal formation (La2O2CO3) also supports the methanation reaction.

The influence of catalytic activation condition by the addition of lanthanum oxide on γ-alumina-supported Co catalyst was also investigated (Kok et al. 2011; Schaper et al. 1985; Rosid et al. 2015b). The presence of this metal oxide has shifted the reduction peak of Co3O4 to CoO to a slightly lower temperature around 200 °C which can be beneficial for CO2 conversion of the catalyst. The pore size distribution was designated to mesoporous structure with an increase in surface area with the addition of lanthanum. Similar investigation was conducted by Rosid et al. (2015b) on cobalt-lanthanum-based catalyst supported on Al2O3. As shown in Fig. 3, the catalyst displayed amorphous phase which was suppressed by the presence of lanthanum oxide. The low degree of crystallinity was attributed to the larger surface area of catalyst and enhanced the performance of catalyst.

Fig. 3
figure 3

XRD diffractograms of the Ru/Co/La(5:35:60)/Al2O3 catalyst calcined at various temperatures (900 °C. 1000 °C and 1100 °C for 5 h) (Rosid et al. 2015b)

Full size image

Nickel-based catalysts doped with transition elements such as Mg, Zr, Mo, Mn, Fe, Co and Cu were favourably prepared via sol-gel method (Buang et al. 2008). Pr was selected as a co-dopant among rare-earth metals for all catalysts as it was the most active oxide that has various stable oxidation states (Buang et al. 2008; Holden and Coplen 2004). Ni/Co/Pr was found to be more potential among all catalysts, as it produced highest CH4 at 350 °C, almost 100% of CO2 elimination as XRD analysis showed that Co3O4 which was a mixture of cubic CoO and Co2O3 was the active site for the higher catalytic activity of Ni/Co/Pr. The larger surface area of 59.86 m2 g−1 for Ni/Co/Pr and Ni/Co showed 38.50 m2 g−1, indicating that the addition of Pr would enhance the surface area of the catalyst (Buang et al. 2008). Praseodymium was also investigated, and the stability test of Ru/Mn/Pr (5:30:65)/Al2O3 was carried out consistently up to 7 h through 96% CO2 conversion. The low degree of crystallinity was patterned in XRD due to the praseodymium with small particle size as supported with FESEM analysis. RuO2 was assigned as an active species in this catalyst and supported by EDX with abundance of Ru mass ratio (Rosid et al. 2015c).

As summary, the study of physicochemical properties of each catalyst is a main point to attain higher catalytic performance. The best physicochemical properties of lanthanide oxide are higher surface area, reduced active species at lower temperature, presence of oxygen vacancies and basic properties to comply with acidity of CO2 gas.

Mechanism of catalytic CO2 methanation reaction

The mechanism of heterogeneous catalyst commonly follows the Langmuir-Hinshelwood (LH) mechanism in which both reactants are adsorbed on the surface, collide and form products (Eq. (3)). It is composed of various steps on the surface of the catalyst which include the adsorption, surface reaction and desorption of metal species. However, the formation of intermediate from the reaction depends on the nature and mode of adsorption of reactants and surface of reaction on the adsorbed species. In other words, it relies on the reactiveness of metal oxide species towards CO2 and H2 molecules. Therefore, insight into the mechanism of chemical reaction is important to optimise the methanation reaction and improves the performance.

$$ {\displaystyle \begin{array}{c}{\mathrm{A}}_{\left(\mathrm{reactant}\right)}\rightleftharpoons {\mathrm{A}}_{\left(\mathrm{absorbed}\right)},{\mathrm{B}}_{\left(\mathrm{reactant}\right)}\rightleftharpoons {\mathrm{B}}_{\left(\mathrm{adsorbed}\right)}\\ {}{\mathrm{A}}_{\left(\mathrm{absorbed}\right)}+{\mathrm{B}}_{\left(\mathrm{adsorbed}\right)}\rightleftharpoons {\mathrm{C}}_{\left(\mathrm{adsorbed}\right)}\\ {}{\mathrm{C}}_{\left(\mathrm{adsorbed}\right)}\rightleftharpoons {\mathrm{C}}_{\left(\mathrm{product}\right)}\end{array}} $$
(3)

Several studies have stated that the mechanism of methanation comprised direct detachment of CO2 to CO(ads) and O(ads) on the catalyst surface, with CO(ads) being subsequently hydrogenated to CH4 (Xu et al. 2006; Razzaq et al. 2013; Beuls et al. 2011; Marwood et al. 1997; Kopyscinski et al. 2011; Sehested et al. 2004). At initial, CO2 and H2 molecules reacted with the catalyst surface (S) by chemisorption to create an active species that adsorbed onto the catalyst surface (Eqs. (4) and (5)). Both adsorbed species then reacted to each other (Eq. (6)) and, finally, released products of methane and water (Eqs. (7) and (8)).

$$ {\mathrm{CO}}_2+\mathrm{S}\rightleftarrows {\mathrm{CO}}_{2\left(\mathrm{ads}\right)} $$
(4)
$$ {\mathrm{H}}_2+\mathrm{S}\rightleftarrows {\mathrm{H}}_{2\left(\mathrm{ads}\right)} $$
(5)
$$ {\mathrm{CO}}_{2\left(\mathrm{ads}\right)}+{\mathrm{H}}_{2\left(\mathrm{ads}\right)}\rightleftarrows {\mathrm{CH}}_{4\left(\mathrm{ads}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{ads}\right)} $$
(6)
$$ {\mathrm{CH}}_{4\left(\mathrm{ads}\right)}\rightleftarrows {\mathrm{CH}}_{4\left(\mathrm{desorp}\right)}+\mathrm{S} $$
(7)
$$ {\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{ads}\right)}\rightleftarrows {\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{desorp}\right)}+\mathrm{S} $$
(8)

where S is the catalyst surface, (ads) is the adsorption of molecule on the catalyst surface and (desorp) is the desorption of molecule on the catalyst surface.

In fact, this particular concept was in line with Solymosi et al. (1981), who asserted the process of adsorption and dissociation of CO2 with transition metals took place to transfer electrons in order to generate radical species in anion structure. Hence, adsorbed species decreased energy to minimise obstacle in activation between species at gas phase and support adsorption. Next, the H atom species that was active got attached to O atom in order to generate water as its by-product. This process was continuous until all the free carbon atoms were completely attached with H atoms in order to generate CH4.

Different mechanistic perspectives were found by Miao et al. (2016). The methanation process involved two categories which were associative and dissociative. In the associative scheme, the hydrogenation process took place in the C–O bond-breaking process while the dissociative scheme did not involve hydrogenation for breaking C–O bond. Then, the hydrogen atom was then attracted to free carbide to form methane molecule. Referring to Hu et al. (2012), they claimed that metal-based oxide and dopant played different roles in the hydrogenation reaction. Metal-based oxide initiated the reaction by binding with CO2 molecule, whereas dopant activated the reaction by providing H atoms that were needed for further hydrogenation.

The finding of Kim et al. (2010) confirmed the mechanism of Pd-MgO/SiO2 catalyst was started with initiation of MgO binding together with CO2 molecule to form magnesium carbonate. The Pd was essential to dissociate H2 molecule and supplied H atoms to carbonates on MgO, for hydrogenation of oxygen species, and produced two molecules of water. A free C on MgO was hydrogenated to CH4 product. The similar concept of mechanism of CO2 methanation was also reported by Jacquemin et al. (2010). The first step could be the chemisorption of CO2 on reduced Rh/γ-Al2O3 catalyst followed by the bond breaking of CO2 to form CO(ads) and O(ads) species. The formation of CO(ads) was supported by the presence of peaks belonging to linear Rh-CO (2048 cm−1), Rh3+-CO (2123 cm−1) and Rh-(CO)2 (2024 cm−1 and 2092 cm−1). The adsorbed Rh-(CO)2 was the most reactive species with hydrogen. The CO(ads) was then detached into C and O on the surface, and further hydrogenation of C took place to produce CH4. No CO2 conversion was observed over unreduced Rh/γ-Al2O3 catalyst compared to 0.63% conversion (99% CH4 selectivity) of reduced catalyst at room temperature.

The species that adsorbed on the Ru/Mn/Ce-65/Al2O3 catalyst surface during reaction was examined by Toemen et al. (2016). The investigated mechanism only portrayed direct adsorption and dissociation for CO2 and H2, as illustrated in Fig. 4. Initially, all molecules of H2 and CO2 had been directed towards the catalyst surface, wherein the C atom of CO2 had been coordinated directly to the catalyst of cerium oxide in order to generate inorganic carboxylate, as step 1. Next, in step 2, the surface of the catalyst displayed the coordination of CO2 molecules that had been adsorbed (step 2). As such, the peak of adsorption noted at 2087 cm−1 could be associated to the adsorbed CO that reflected a configuration in linear manner on the Ce catalyst surface (Asedegbega-Nieto et al. 2005; Kramer et al. 2009). After that, the adsorbed H2 molecules were dissociated, hence turning into H atom species followed by dissociation of CO2 molecules (step 3). In step 4, as asserted by Kim et al. (2010), hydrogenation of oxygen species occurred in a spontaneous manner, thus releasing dual water molecules. As for the H atoms that were released from O, they formed methane after combining with C molecules.

Fig. 4
figure 4

Proposed mechanism for Ru/Mn/Ce-65/Al2O3 catalysts (Toemen et al. 2016)

Full size image

The mechanism of Ru/Mn/Sr-65/Al2O3 was extended using FTIR analysis to identify the surface species present during the reaction process. Figure 5 shows two broad O–H regions at 3434 cm−1 and 1635 cm−1 which were assigned to the stretching and bending vibration of adsorbed water (Eckle et al. 2011; Karelovic and Ruiz 2013). Meanwhile, adsorption peak at 2071 cm−1 corresponded to CO-adsorbed linear or bridged configuration (Stevens et al. 2008) on the Sr catalyst surface. When the temperature was increased to 100 °C, two new peaks around 1769 cm−1 and 1462 cm−1 were observed which were assigned to carbonyl or carbonate species. The formate species was not detected since no (C-H) peak was observed in FTIR. However, at 200 °C reaction temperature, the carbonate species was further hydrogenated by detachment hydrogen atom to form formate, leading to a decrease in carbonate intensity. The new adsorption peaks were observed at 2962 cm−1 and 2851 cm−1 which were assigned to CH3 and CH2 symmetric, respectively. These peaks were sharp and intense at 200 °C but decreased the intensity at 210 °C due to the release of methane gas. Therefore, higher CH4 gas was detected at this temperature.

Fig. 5
figure 5

FTIR spectra for Ru/Mn/Sr-65/Al2O3 catalysts (Toemen et al. 2016)

Full size image

From the FTIR spectra in Fig. 5, the mechanism channel is depicted in Fig. 6 that shows the incorporated CO2 adsorption, reflecting step 1, which suggested the formation of monodentate carbonate by attaching oxygen to the catalyst surface. Next, in step 2, two formations took place at lower surface of CO2 coverage, which were bidentate carbonate as well as bridged bidentate carbonate. In step 3, the products of steps 1 and 2, which were hydrogenated carbonate species, were attached to free H atoms to generate formate species (HCOO), apart from releasing a water molecule. This hydrogenation process was continued to generate dioxomethylene (H2COO) (step 4) and formaldehyde (CH2O) (step 5) (Sahki et al. 2011), including methoxy species (Aldana et al. 2013) that contained a methyl group attached to oxygen (–OCH3) (step 6), as well as a water molecule prior to finalising the whole process by producing methane gas.

Fig. 6
figure 6

Proposed mechanism over the Ru/Mn/Sr (5:30:65)/Al2O3 catalyst calcined at 1000 °C for 5 h (Toemen et al. 2016). M = Ru/Mn/Sr (5:30:65)/Al2O3 catalyst

Full size image

Overall, there were still arguments on reaction intermediates and methane formation involved in this process (Eckle et al. 2011; Karelovic and Ruiz 2013; Gao et al. 2015). Kudo and Komatsu (1999) presumed that the current methanation mechanism proceeds via formation of formate intermediate. The CO2 molecule was adsorbed on the surface as a metal carbonate and then reduced by the hydrogen to the surface formate. Then, the formate decomposed to a surface active carbon on the Ru catalyst and was then hydrogenated to form methane gas.

Here, the role of Ru was to activate the dissociation of hydrogen molecule to interact with formate for the methanation process. The formation of intermediate formate species over 2% Ru/TiO2 catalyst proposed by Marwood et al. (1997) was slightly different from the mechanism of Kudo and Komatsu (1999). The CO2 was firstly reacted to the hydroxyl group on the catalyst surface to form a surface-bound hydrogen carbonate species, HCO3−. It was then reduced with metal-adsorbed hydrogen to an interfacial formate species, HCOO−. The interfacial formate decomposed to adsorb CO species while regenerating the surface hydroxyl group again. Lastly, the hydrogenation of CO adsorbed species with adsorbed hydrogen in order to produce methane gas. The similar mechanism had been observed by Aldana et al. (2013) on Ni-based ceria-zirconia catalysts. They found that H2 was dissociated on Ni0 sites while CO2 was initiated on the ceria-zirconia surface to produce carbonates which could be hydrogenated into formate and further into methoxy species. This mechanism involved weak basic site for adsorption of acidic CO2 and indicates a stable metal-support interface.

Pan et al. (2014) have proposed that CO2 preferably attached at the oxygen site adjacent to Ce(III) compared with adjacent to Ce(IV)/Zr or surface hydroxyl site. The adsorption species were revealed during the mechanistic studies which were monodentate carbonate adjacent to Ce(IV), Ce(III) or Zr, bidentate carbonate adjacent to Ce(III) or Ce(IV) and hydrogen carbonates in the surface hydroxyl site. Interestingly, monodentate carbonate on Ce(III) was easier to be hydrogenated than Ce(IV). Hahn et al. (2013) also found that CO2 was preferably adsorbed in monodentate configuration to form a carbonate species with a surface oxygen atom on CeO2(III).

Wang et al. (2016) investigated the reaction of CO2 methanation via formate species in the catalyst of Ru/CeO2 through in situ DRIFT infrared spectroscopy with steady-state isotope transient kinetic analysis. CO2 methanation over Ru/Al2O3 catalyst underwent CO route due to the absence of oxygen vacancy. Next, the Ce–O bond on the surface of CeO2 was attacked by dissociated H atoms to generate vacancies at hydroxyl surface, oxygen and Ce3+. Upon usage of CO2, carbonyl peak shift was absent until the temperature was hiked to 250 °C. It was noteworthy to highlight that the CH4 peak at 3017 cm−1 was noted upon temperature hike, thus signifying 250 °C as the carbonyl activation temperature for Ru/Al2O3. Nevertheless, the carbonyl for Ru/CeO2 appeared at 150 °C, which succeeded in converting to CH4 exceeding 250 °C. Meanwhile, the activation temperature to convert Ce3+ to Ce4+ (25 °C) seemed to coincide with that temperature employed to convert CO2 to CO2−. In addition, CO2− activation temperature to formate appeared to agree rather well with the initial temperature of 25 °C used to reduce the surface of hydroxyl. This showed that formate was derived from CO2 hydrogenation and enhanced by the H atom found on the hydroxyl surface that was upon the surface of CeO2. Simultaneously, methanol bands at 1008 cm−1 and 3659 cm−1 had been observed during the reaction process involving Ru/CeO2 catalyst, which further signified the fact that the step to determine the rate referred to formate detachment from methanol. At the final step, methane was easily produced after methanol was hydrogenated.

CO2 adsorption on the ceria surface over Ni/CeO2 catalyst also has been suggested to occur through hydrocarbonate and formate intermediates to form methane (Konishcheva et al. 2016). The methanation reaction proceeded through dissociation of hydrogen from Ni particles. The presence of hydrocarbonates and formates on the ceria surface over Ni/CeO2 indicates the participation of CO2 derivative species adsorbed on ceria in the reaction. These intermediate species was converted to methane by hydrogenation reaction with hydrogen migrated from Ni surface catalyst. This mechanism was also suggested by Aldana et al. (2013) and Pan et al. (2014) who were using Ni/Zr0.5Ce0.5O2 catalyst.

Sharma et al. (2016) have reported the mechanistic study of CO2 methanation over Ru-substituted CeO2 catalyst that has been prepared by combustion. The results showed that CO2 was adsorbed on the surface of Ce0.95Ru0.05O2 and formed carbonate intermediate in the presence of H2. During dissociation, carbonates led to adsorption of CO which formed methane upon reaction with H2. The catalyst produced methane at 270 °C, reading the highest methane yield at 450 °C. Above 450 °C, the concentration of CO and H2 increased, the concentration of O2 increased slightly while the concentration of CH4 decreased. This might be due to unreacted carbonates that decomposed to O2 and CO above 450 °C. From the results, the peak of formate was not observed by FTIR analysis, which suggested that the reaction pathway was only via formation of carbonate and CO intermediates.

The interesting side product that may be produced from the reaction is methanol (CH3OH), but the production should be conducted in the presence of pressure. Borodko and Somorjai (1999) who studied on the CuO/ZnO/Al2O3 catalyst found that CO2 conversion and CH3OH selectivity increased with an increase in the pressure. The CO2 conversion was increased from 3.1 to 11.9% as well as the methanol selectivity from 5.3 to 47.1% when the pressure was incremented from 1 to 75 bar at the flow rate 2 L h−1 and 230 °C. Meanwhile, the ethanol selectivity was strongly enhanced by changes with basic additives such as alkali and alkaline earth metals (Matsuzaki et al. 1997; Chang et al. 2017).

The mechanism of CO2 on Ni(111) lattice surface catalyst was also confirmed by computational modelling (Choe et al. 2005). The elementary reaction steps consisted of two mechanisms which were carbon formation and carbon methanation. The activation energy of dissociation of CO2 to form CO intermediate and atomic oxygen was valued at 1.27 eV. The detachment of CO into C and O (step 2) was considered as rate determining based on calculated activation energy of 2.97 eV, the highest from all the elementary steps. Finally, the hydrogenation of C occurred on the surface catalyst until methane gas is produced.

From these mechanisms, it showed that there were many side products that will be produced from the CO2 methanation reaction, particularly CO gas intermediate species. All possible variables such as different metals, supports, reaction conditions and preparation methods can lead to different mechanisms and yields of products.

Conclusion and future aspect

Fossil fuels are the most demanding world’s energy supply with almost more than 98% of needs in daily activity. Unfortunately, the by-product from combustion of fossil fuels has led to emission of CO2 to the atmosphere which is one of the major sources of GHGs, especially from transportation (Sudhanshu et al. 2011). Several methods have been approached in order to reduce the production of CO2 gas and prevent the global warming from becoming worst (Niven 2005; Rahman et al. 2017). Accordingly, the raw natural gas contains impurities of CO2 gas which contributed to the annual increment in the percentage of CO2 gases in Malaysia and globally. Therefore, methanation reaction has been chosen as one of the potential technologies to convert waste (CO2) gas to wealth (CH4) gas in the combustion of natural gas in order to reduce emission of CO2 to atmosphere. A few parameters have been studied which were preparation methods, support of catalysts and promoter utilising rare-earth metal oxide which gave an excellent performance. A mechanistic study also has been investigated by many researchers to determine the pathway that contributes more methane product and minimises the production of by-products. In summary, a future aspect of research should be specified on optimisation of catalyst parameter in order to achieve excellent CO2 conversion and CH4 formation at low reaction temperature and possess long-term stability. Alternatively, response surface methodology should be useful in the methanation reaction for the optimisation of critical parameters such as calcination temperature, ratio-based loading, catalyst dosage, time of reaction, temperature for reduction condition and so on. This method has great advantage towards research in terms of time saving since only a few experimental points needed for its application (Khuri and Mukhopadhyay 2010). The application of Box-Behnken design in industrial research is widespread due to its economical design and improvement in response to chemical and processing fields (Myers et al. 1992). Other than that, a modelling study also should be conducted in order to support the actual reaction that occurs in methanation reaction. This modelling study also can give an exact kinetic energy of active species which lead to higher catalytic performance.