CO2 Methanation Mechanism Research Articles

Solar Gas-Phase CO2 Hydrogenation by Multifunctional UiO-66 Photocatalysts.

Solar-assisted CO2 conversion into fuels and chemical products involves a range of technologies aimed at driving industrial decarbonization methods. In this work, we report on the development of a series of multifunctional metal-organic frameworks (MOFs) based on nitro- or amino-functionalized UiO-66(M) (M: Zr or Zr/Ti) supported RuOx NPs as photocatalysts, having different energy band level diagrams, for CO2 hydrogenation under simulated concentrated sunlight irradiation. RuOx(1 wt %; 2.2 ± 0.9 nm)@UiO-66(Zr/Ti)-NO2 was found to be a reusable photocatalyst, to be selective for CO2 methanation (5.03 mmol g-1 after 22 h;, apparent quantum yield at 350, 400, and 600 nm of 1.67, 0.25, and 0.01%, respectively), and to show about 3-6 times activity compared with previous investigations. The photocatalysts were characterized by advanced spectroscopic techniques like femto- and nanosecond transient absorption, spin electron resonance, and photoluminescence spectroscopies together with (photo)electrochemical measurements. The photocatalytic CO2 methanation mechanism was assessed by operando FTIR spectroscopy. The results indicate that the most active photocatalyst operates under a dual photochemical and photothermal mechanism. This investigation shows the potential of multifunctional MOFs as photocatalysts for solar-driven CO2 recycling.

Research Progress of Non-Noble Metal Catalysts for Carbon Dioxide Methanation.

The extensive utilization of fossil fuels has led to a rapid increase in atmospheric CO2 concentration, resulting in various environmental issues. To reduce reliance on fossil fuels and mitigate CO2 emissions, it is important to explore alternative methods of utilizing CO2 and H2 as raw materials to obtain high-value-added chemicals or fuels. One such method is CO2 methanation, which converts CO2 and H2 into methane (CH4), a valuable fuel and raw material for other chemicals. However, CO2 methanation faces challenges in terms of kinetics and thermodynamics. The reaction rate, CO2 conversion, and CH4 yield need to be improved to make the process more efficient. To overcome these challenges, the development of suitable catalysts is essential. Non-noble metal catalysts have gained significant attention due to their high catalytic activity and relatively low cost. In this paper, the thermodynamics and kinetics of the CO2 methanation reaction are discussed. The focus is primarily on reviewing Ni-based, Co-based, and other commonly used catalysts such as Fe-based. The effects of catalyst supports, preparation methods, and promoters on the catalytic performance of the methanation reaction are highlighted. Additionally, the paper summarizes the impact of reaction conditions such as temperature, pressure, space velocity, and H2/CO2 ratio on the catalyst performance. The mechanism of CO2 methanation is also summarized to provide a comprehensive understanding of the process. The objective of this paper is to deepen the understanding of non-noble metal catalysts in CO2 methanation reactions and provide insights for improving catalyst performance. By addressing the limitations of CO2 methanation and exploring the factors influencing catalyst effectiveness, researchers can develop more efficient and cost-effective catalysts for this reaction.

Zonal activation of molecular carbon dioxide and hydrogen over dual sites Ni-Co-MgO catalyst for CO2 methanation: Synergistic catalysis of Ni and Co species

An in-depth mechanism in zonal activation of CO2 and H2 molecular over dual-active sites has not been revealed yet. Here, Ni-Co-MgO was rationally constructed to elucidate the CO2 methanation mechanism. The abundant surface nickel and cobalt components as active sites led to strong Ni-Co interaction with charge transfer from nickel to cobalt. Notably, electron-enriched Coδ− species participated in efficient chemisorption and activation of CO2 to generate monodentate carbonate. Simultaneously, plentiful available Ni0 sites facilitated H2 dissociation, thus CO2 and H2 were smoothly activated at zones of Coδ− species and Ni0, respectively. Detailed in situ DRIFTS, quasi situ XPS, TPSR, and DFT calculations substantiated a new formate evolution mechanism via monodentate carbonate instead of traditional bidentate carbonate based on synergistic catalysis of Coδ− species and Ni0. The zonal activation of CO2 and H2 by tuning electron behaviors of double-center catalysts can boost heterogeneous catalytic hydrogenation performance.

Molecular elucidation of CO2 methanation over a highly active, selective and stable LaNiO3/CeO2-derived catalyst by in situ FTIR and NAP-XPS

The CO2 methanation mechanism over the highly active (TOF=75.1 h−1), selective (>92%) and stable 10% LaNiO3/CeO2-derived catalyst is still unresolved. The surface of the catalyst is monitored under hydrogenation (H2), oxidizing (CO2) and CO2 methanation (H2 +CO2) conditions by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) using synchrotron radiation. Meanwhile, the main reaction intermediates are identified by in situ FTIR analysis. NAP-XPS experiments confirm that LaNiO3 perovskite reduction leads to the ex-solution of Ni0 nanoparticles and Ni2+CeO2−x and Ni2+La2O3 interfaces conformation, favouring the CO2 adsorption and the H2 dissociation/transfer. In situ FTIR experiments combined with the C1s spectra (NAP-XPS) suggest that the CO2 activation occurs on CeO2−x (oxygen vacancies and OH–) at low temperatures, in the form of bicarbonates; whereas, mono-/bidentate carbonates are formed on different strength La2O3 sites at increasing temperatures. These species are consecutively reduced to formates, as the main reaction intermediate, and methane by the H spilled from Ni0 nanoparticles near to NiOCeO2−x and NiOLa2O3 interfaces.

Promotional effects of Ru and Fe on Ni/ZrO2 catalyst during CO2 methanation: A comparative evaluation of the mechanism

Ni-based catalysts are widely investigated non-noble metal-based systems for CO2 methanation. However, their industrial application is still limited due to lower activity at low-temperature and catalyst deactivation. Incorporating a second metal such as Ru and Fe is considered as a successful strategy to overcome these challenges through alloy formation or the synergies provided by the interplay of two adjacent metallic sites. Nonetheless, their promotional effect on the CO2 methanation mechanism under similar conditions has not been reported yet. In this work, Fe and Ru-promoted Ni/ZrO2 catalysts were investigated to evaluate their promotional effect on the mechanism. The Ni/Fe ratio was first optimized and a CO2 conversion rate of 37.7 mmolCO2/(molNi+Fe s) and 96.3% CH4 selectivity was obtained over the Ni0.8Fe0.2/ZrO2 catalyst. In comparison with Ni0.8Fe0.2/ZrO2, Ni0.8Ru0.2/ZrO2 prepared with the same composition showed higher activity and stability in CO2 methanation. Characterization results indicate alloys formation and H spillover for Ni0.8Ru0.2/ZrO2 to be responsible for promotion. Besides, in situ DRIFTS studies evidenced the occurrence of both CO2 dissociative and associative pathways over Ni0.8Ru0.2/ZrO2 catalyst, while solely the CO2 associative pathway occurred for Ni0.8Fe0.2/ZrO2.

Experimental and DFT study on single atom solution for carbon dioxide methanation

The single atom solution is proposed for the CO2 methanation. The catalytic mechanism of single atom solution on the methanation of CO2 is studied. The catalytic performance of single atom solution with different solution proportions was experimentally determined. It was found that each gram of metal atom afforded to produce 294.50 g of methane and 9.24 g of hydrogen. The stability of catalytic performance and good cycling performance of the CO2 conversion reaction were verified. A DFT model was developed to analyze the microscopic mechanism of CO2 methanation by single atom solution. It was found that the reaction path formed by COOH* intermediates is the more easily realized reaction path. It is calculated that the conversion of CO* to HCO* was the rate-limiting step of the optimal reaction path, which corresponded to a limiting potential of 0.85 V. The conversion of CO2 by single atom solution provides a new direction.

An emerging and high-performance sepiolite-supported Ni catalyst for low-temperature CO2 methanation: The critical role of hydroxyl groups

Natural clay minerals compensate for the high cost, complex synthesis and environmental unfriendliness of conventional catalytic supports. To obtain highly-effective and stable clay mineral-supported Ni catalysts for CO2 methanation, 10 wt% Ni was loaded on five common clay minerals (sepiolite (Sep), attapulgite (ATP), montmorillonite (Mnt), diatomite (Dia) and halloysite (Hly)), SiO2 and Al2O3 with a simple impregnation method. 10 wt% Ni/Sep displayed the best catalytic activity, the highest turnover frequency (TOF) and excellent stability, due to the smallest Ni nanoparticles, the strongest weak and moderate basicity, the most abundant surface hydroxyl groups, electron transfer from Sep to Ni, and good resistance to sintering and carbon deposition. Meanwhile, correlation results indicated that hydroxyl groups of clay mineral catalysts are critical for CO2 methanation. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis revealed that magnesium in Sep demonstrates an outstanding ability to activate CO2 at room temperature and the reaction follows both CO and formate routes over 10 wt% Ni/Sep, where CO2 reacted with abundant hydroxyl groups to form bicarbonate; Then, the bicarbonate was further hydrogenated into formate or decomposed into CO, and finally the generation of CH4, which also explains the excellent catalytic performance of 10 wt% Ni/Sep, and deepens the understanding of CO2 methanation mechanism over clay mineral catalysts. This work provides a novel perspective regarding the natural character of clay minerals on CO2 methanation and facilitates the industrial application of high-performance sepiolite-supported Ni catalyst in CO2 methanation.

Coal char supported Ni catalysts prepared for CO2 methanation by hydrogenation

Catalytic CO2 methanation is a potential solution for conversion of CO2 into valuable products, and the catalyst plays a crucial role on the CO2 conversion and CH4 selectivity. However, some details involved in the CO2 methanation over the carbon supported Ni catalysts are not yet fully understood. In this work, commercial coal char (CC) supported Ni catalysts were designed and prepared by two different methods (impregnation-thermal treatment method and thermal treatment-impregnation method) for CO2 methanation. Effects of the preparation conditions (including the thermal treatment temperature and time, the mass ratio of CC:Ni and the preparation method), as well as the reaction temperature of CO2 methanation, were investigated on the catalyst morphology, reducibility, structure and catalytic performance. Fibrous Ni-CC catalyst is achieved and shows high CO2 conversion (72.9%–100%) and CH4 selectivity (>99.0%) during the 600-min methanation process. Adverse changes of the catalyst surface and textural properties, reducibility, particle size and morphology are the potential factors leading to the catalyst deactivation, and possible solutions resistant to the deactivation were analyzed and discussed. The CO2 methanation mechanism with the CO route was proposed based on the oxidation-reduction cycle of Ni in this work.

Highly active sunlight-driven CO[formula omitted] methanation catalyst: Design of the interfacial structure

A kind of nickel catalyst supported on the defect-rich ZrO2 (5Ni/ZrO2-0.5R) was developed, which exhibited 66∼70% of CO2 conversion accompanied with more than 99% CH4 selectivity in the range of 300∼380 °C. Thus, the green and sustainable CO2 methanation over 5Ni/ZrO2-0.5R can be achieved smoothly in a photo-thermal transform tube that can provide temperature range of 300∼400 °C under sun. A combination of control experiments, in-situ spectroscopy analyses and characterizations provides an insight into the active sites at the interface between nickel and oxygen vacancy(Ov)-rich ZrO2. The presence of sodium and oxygen vacancies promotes the dissociative adsorption of CO2 so that change the mechanism of CO2 methanation from formate-hydrogenation pathway into CO-hydrogenation pathway. Further theoretical calculations indicate that the Zr4+-Ov structures leads to significantly improved Ni dispersion and the Na+-Ov-Ni sites drive the CO2 methanation through a CO-hydrogenation mechanism.

Experimental and theoretical insights into an enhanced CO2 methanation mechanism over a Ru-based catalyst

Converting excessive CO2 emissions from industrial operation into methane (CH4) is critical for the remediation of global warming. However, this reaction requires a high temperature to overcome the high thermodynamic stability of CO2. The development of efficient catalysts for the CO2 methanation reaction at low temperature remains challenging. Here we report that when a TiO2 photothermal catalyst is loaded with Ru nanoparticles, the CO2 to CH4 conversion efficiency is markedly enhanced under light illumination. In fact, the CO2 performance was enhanced fourfold and twofold from those of the traditional thermal catalytic reaction at 200 °C and 250 °C, respectively. In-situ DRIFTS analysis revealed that light irradiation accelerates the transition of bicarbonate to formates via light-induced charge carriers. Moreover, theoretical calculations revealed that photo-induced electron transfer effectively promotes H2 dissociation and CO2 activation on the Ru atoms. This work provides new theoretical insights into CO2 activation and methanation under light illumination.

Ni-Based Catalyst for Carbon Dioxide Methanation: A Review on Performance and Progress

Catalytic conversion of CO2 into methane is an attractive method because it can alleviate global warming and provide a solution for the energy depletion crisis. Nickel-based catalysts were commonly employed in such conversions due to their high performance over cost ratio. However, the major challenges are that Ni tends to agglomerate and cause carbon deposition during the high-temperature reaction. In the past decades, extensive works have been carried out to design and synthesize more active nickel-based catalysts to achieve high CO2 conversion and CH4 selectivity. This review critically discusses the recent application of Ni-based catalyst for CO2 methanation, including the progress on the effect of supporting material, promoters, and catalyst composition. The thermodynamics, kinetics, and mechanism of CO2 methanation are also briefly addressed.

Monitoring by in situ NAP-XPS of active sites for CO2 methanation on a Ni/CeO2 catalyst

Ni/CeO2 catalysts are very active and selective for total hydrogenation of CO2 to methane, but the nature of the active sites is still unclear. The surface of a Ni/CeO2 catalyst has been monitored under CO2 methanation conditions by Near Ambient Pressure-XPS (NAP-XPS) using synchrotron radiation, and has been concluded that the species involved in the redox processes taking place during the CO2 methanation mechanism are the Ni2+-CeO2/Ni0 and Ce4+/Ce3+ pairs. In addition, a small fraction of nickel is present on the catalyst surface forming NiO and Ni2+-carbonates/hydroxyls (around 20% of the total surface nickel), but these species do not participate in the redox processes of the methanation mechanism.Under CO2 methanation conditions the H2 reduction rate of the Ni2+-CeO2/Ni0 and Ce4+/Ce3+ couples is much faster than their CO2 reoxidation rate (2 times faster, at least, at 300ºC), but a certain proportion of nickel always remains oxidized under reaction conditions. The high activity of Ni/CeO2 catalysts for CO2 methanation is tentatively attributed to the simultaneous presence of Ni2+-CeO2 and Ni0 active sites where CO2 and H2 are expected to be efficiently dissociated, respectively.

Research Progress and Reaction Mechanism of CO2 Methanation over Ni-Based Catalysts at Low Temperature: A Review

The combustion of fossil fuels has led to a large amount of carbon dioxide emissions and increased greenhouse effect. Methanation of carbon dioxide can not only mitigate the greenhouse effect, but also utilize the hydrogen generated by renewable electricity such as wind, solar, tidal energy, and others, which could ameliorate the energy crisis to some extent. Highly efficient catalysts and processes are important to make CO2 methanation practical. Although noble metal catalysts exhibit higher catalytic activity and CH4 selectivity at low temperature, their large-scale industrial applications are limited by the high costs. Ni-based catalysts have attracted extensive attention due to their high activity, low cost, and abundance. At the same time, it is of great importance to study the mechanism of CO2 methanation on Ni-based catalysts in designing high-activity and stability catalysts. Herein, the present review focused on the recent progress of CO2 methanation and the key parameters of catalysts including the essential nature of nickel active sites, supports, promoters, and preparation methods, and elucidated the reaction mechanism on Ni-based catalysts. The design and preparation of catalysts with high activity and stability at low temperature as well as the investigation of the reaction mechanism are important areas that deserve further study.

From Dissociation of H2 and Co2 Toward Co2 Methanation Mechanism

From Dissociation of H2 and Co2 Toward Co2 Methanation Mechanism

In situ Ce-doped catalyst derived from NiCeAl-LDHs with enhanced low-temperature performance for CO2 methanation

In this work, a Ni-supported CeO2-Al2O3 catalyst, called NiCeAl-RMO (reduced metal-oxide), was prepared via air calcination and H2 reduction of NiCeAl-LDH (layered double hydroxide). The NiCeAl-RMO catalyst shows superior CO2 methanation activity, especially at low temperature (250 °C), and it performed 78.6% CO2 conversion with ∼ 100% CH4 selectivity. Compared with NiAl-RMO, the NiCeAl-RMO catalyst prepared via the Ce-doping strategy has significantly improved CO2 methanation activity. Characterizations revealed that Ce doping would increase the amount of active Ni0 species, cause a large number of oxygen vacancies, and generate new moderate basicity sites on the catalyst. In addition, in situ DRIFTS found that, unlike the RWGS + CO methanation mechanism over NiAl-RMO, the CO2 methanation mechanism over NiCeAl-RMO followed the formate pathway by generating bidentate carbonate species, which was revealed as a key intermediate for enhancing the low-temperature activity of CO2 methanation.

Elucidating the Role of the Metal Catalyst and Oxide Support in the Ru/CeO2-Catalyzed CO2 Methanation Mechanism.

This study addresses the yet unresolved CO2 methanation mechanism on a Ru/CeO2 catalyst by means of near-ambient-pressure X-ray photoelectron spectroscopy (NAP–XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) complemented with periodic density functional theory (DFT) calculations. NAP–XPS results show that the switch from H2 to CO2 + H2 mixture oxidizes both the Ru and CeO2 phases at low temperatures, which is explained by the CO2 adsorption modes assessed by means of DFT on each representative surface. CO2 adsorption on Ru is dissociative and moderately endergonic, leading to polybonded Ru-carbonyl groups whose hydrogenation is the rate-determining step in the overall process. Unlike on Ru metal, CO2 can be strongly adsorbed as carbonates on ceria surface oxygen sites or on the reduced ceria at oxygen vacancies as carboxylates (CO2–δ), resulting in the reoxidation of ceria. Carboxylates can then evolve as CO, which is released either via direct splitting at relatively low temperatures or through stable formate species at higher temperatures. DRIFTS confirm the great stability of formates, whose depletion relates with CO2 conversion in the reaction cell, while carbonates remain on the surface up to higher temperatures. CO generation on ceria serves as an additional reservoir of Ru-carbonyls, cooperating to the overall CO2 methanation process. Altogether, this study highlights the noninnocent role of the ceria support in the performance of Ru/CeO2 toward CO2 methanation.

Theoretical study on the synthesis of methane by CO2 hydrogenation on Ni3Fe(111) surface

Density functional theory methods are used to explore the possible mechanism of CO2 methanation on Ni3Fe(111) surface. The optimal adsorption site and adsorption energy of all species are determined. Meanwhile, for the adsorption of CO2, the obvious orbital hybridization between CO2 and Ni3Fe(111) surface indicates that CO2 is commendably activated, which is conducive to further hydrogenation reaction. Subsequently, the optimal pathway for CO2 methanation is determined through a detailed study of the activation barrier. The lower activation barrier (1.60 eV) of rate-determining step on Ni3Fe(111) surface than that on Ni(111) and Pt-doped Ni(111) surfaces suggests excellent catalytic activity of Ni3Fe(111) surface towards CO2 methanation. Moreover, the formation of CH4 is more favorable in kinetics compared with that of HCOOH and CH3OH, indicating the high selectivity of CO2 methanation on Ni3Fe(111). In addition, the calculated results also reveal that the Ni3Fe(111) surface can effectively suppress the carbon deposition compared with Ni(111) surface, which can be attributed to the obvious enhancement of the activation barrier for the reactions of surface C* formation (3.27 eV for CO* → C*, 2.23 eV for COH* → C*, and 1.70 eV for CH* → C*).

Insights into CO2 methanation mechanism on cubic ZrO2 supported Ni catalyst via a combination of experiments and DFT calculations

The CO2 methanation mechanism was studied via combined in-situ FTIR and DFT calculations on the cubic ZrO2 supported Ni catalyst. The formation and evolution of CO2 initial adsorbed species were addressed. The bicarbonates produced from CO2 reacting with surface OH sites are transformed to the CO2* adsorbed state on the Ni/ZrO2 interface (CO2*-interface) that are hydrogenated to CH4. The CO2 adsorbed states (CO2*-Ni) generated from CO2 reacting with surface Ni sites are hydrogenated to CO* as a byproduct rather than an intermediate for methanation. The formate pathway is dominated for methanation on Ni/ZrO2, in which bidentate formates hydrogenation to H2COO* is the rate-determining step with the activation energy of 1.01 eV. The presence of ZrO2 support improves the electron mobility and reducibility of Ni, and induces H-spillover effect to promote H2O* formation. Y3+ doping into the support of Ni/ZrO2 stabilizes the c-ZrO2 phase and promote methanation activity of the catalyst.

Reaction mechanism of CO2 methanation over Rh/TiO2 catalyst

Rh/TiO2 has been regarded as a very promising catalyst for the low-temperature CO2 methanation. However, the atomic-level reaction mechanism that dictates the reactivity and selectivity of CO2 reduction over Rh/TiO2 catalyst remains elusive. The reaction mechanism governed by a delicate interplay of surface reaction chemistry and thermodynamics was systematically investigated using density functional theory calculations. Theoretical results indicate that significant charges accumulate at the perimeter of the interface between support TiO2 and Rh nanoparticle. Metal-support interface is identified as the most active site for CO2 adsorption and activation over Rh/TiO2 catalyst. Compared with the direct C–O bond cleavage pathway and formate pathway, the reverse water–gas shift (RWGS) reaction followed by CO hydrogenation is much more thermodynamically and kinetically favorable for CO2 methanation over Rh/TiO2 catalyst. The RWGS + CO hydrogenation pathway via H2COH* dissociation dominates CO2 methanation due to the relatively lower energy barrier. CO2 methanation via the RWGS + CO hydrogenation pathway prefers to proceed through the channel: CO2* → COOH* → CO* → COH* → HCOH* → H2COH* → CH3* → CH4*. H-assisted COOH* dissociation is identified as the rate-determining step of CO2 methanation over Rh/TiO2 catalyst. Finally, a reaction network is established to understand the atomic-level reaction mechanism of CO2 methanation over Rh/TiO2 catalyst. These mechanistic insights can guide the rational design of catalyst active centers to boost the activity and selectivity of CO2 reduction.

The reaction kinetics of CO2 methanation on a bifunctional Ni/MgO catalyst

A bifunctional Ni/MgO catalyst was prepared to catalyze CO2 methanation and make use of CO2 as an abundant hydrogen storage facility. The effect of Ni loading and MgO quality on the rate of methanation was tested in a temperature range of 533–648K. The Ni loading was varied between 0 to 27wt.% on MgO. To investigate the impact of matrix elements, a MgO/CaO support was tested with 21wt.% nickel loading. Further, the role of MgO in the bifunctional catalyst was proven. The reaction kinetics was modeled with a Langmuir–Hinshelwood approach considering the bifunctional character of the catalyst. Nickel provides the adsorbent capacity for hydrogen and is highly selective for methane. MgO activates CO2 through chemisorption. Increasing Ni loading of the catalyst increased the rate of CO2 conversion. According to the results, the mechanism of CO2 methanation did not change with Ni loading. The Ni/MgO catalyst acted as a robust, active and highly selective catalyst for CO2 methanation. With CO2 conversion of 87%, the selectivity to methane was ≥99%. Besides excellent catalytic activity the catalysts suffice the necessity of simple catalyst preparation, usage and recyclability for industrial applicability of CO2 methanation.