A review of the indispensable role of oxygen vacancies for enhanced CO2 methanation activity over CeO2-based catalysts: Uncovering,influencing, and tuning strategies

It is generally accepted that carbon dioxide (CO₂) is one of the most important greenhouse gases and a primary factor in the acceleration of climate change. Methanation, which involves converting CO₂ to methane in the presence of renewable hydrogen, is a plausible technique for achieving net-zero atmospheric CO₂ concentrations. Nevertheless, the development of effective catalytic systems continues to be a significant barrier tor CO₂ methanation. CeO₂-based catalysts for CO₂ methanation could benefit from tuning their oxygen vacancies (OVs) to increase catalytic performance. This review examines and discusses in depth a number of different characterization methodologies for measuring OVs. Additionally, this review focuses specifically on the role that OVs play in various CeO₂-based catalysts as well as the numerous tuning strategies that may be used to increase the number of OVs in these catalysts. This study could give scientists new ideas for how to improve catalytic CO₂ methanation.     

Experimental optimization analysis on operating conditions of CO removal process from hydrogen-rich reformate

The catalytic effects of CO preferential oxidation and methanation catalysts for deep CO removal under different operating conditions (temperature, space velocity, water content, etc.) are systematically studied from the aspects of CO content, CO selectivity, and hydrogen loss index. Results indicate that the 3 wt% Ru/Al₂O₃ preferential oxidation catalysts reduce CO content to below 10 ppm with a high hydrogen consumption of 11.6–15.7%. And methanation catalysts with 0.7 wt% Ru/Al₂O₃ also exhibit excellent CO removal performance at 220–240 °C without hydrogen loss. Besides, NiCl_x/CeO₂ methanation catalysts possess the characteristics of high space velocity, high activity, and high water-gas resistance, and can maintain the CO content at close to 20 ppm. Based on these experimental results, the coupling scheme of combining NiCl_x/CeO₂ methanation catalysts (low cost and high reaction space velocity) with 0.7 wt% Ru/Al₂O₃ methanation catalysts (high activity) to reduce CO content to below10 ppm is proposed.     

The Ni/ZrO2 catalyst and the methanation of CO and CO2

The Ni/ZrO₂ catalyst is one of the most active systems for the methanation of CO to be employed in the hydrogen purification for PEMFC. This contribution aims to study the effect of ZrO₂ on the methanation of CO and CO₂. The catalytic behavior of Ni/ZrO₂, Ni/SiO₂, a physical mixture comprising Ni and ZrO_2, and a double-bed reactor were evaluated. The TPD of CO and CO₂, TPSR and the cyclohexane dehydrogenation reaction were carried out to describe the catalysts and the reactions. The high activity of Ni/ZrO₂ catalyst toward the methanation of CO is related to the presence of active sites on the ZrO₂ surface. The methanation of CO occurs on ZrO₂ due to its ability to adsorb CO and also because of the hydrogen spillover phenomenon. Apparently, the effect of ZrO₂ is less relevant for the methanation of CO₂. Ni/ZrO₂ is a very promising system for the purification of hydrogen.     

The role of oxygen vacancies in the CO2 methanation employing Ni/ZrO2 doped with Ca

The Ni/ZrO₂ catalyst doped with Ca and Ni/ZrO₂ were employed in the CO₂ methanation, a reaction which will possibly be used for storing intermittent energy in the future. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS, reduction in situ), X-ray diffraction (XRD, reduction in situ and Rietveld refinement), electron paramagnetic resonance (EPR), temperature-programmed surface reaction, cyclohexane dehydrogenation model reaction, temperature-programmed desorption of CO₂ and chemical analysis. The catalytic behavior of these catalysts in the CO₂ methanation was analyzed employing a conventional catalytic test. Adding Ca to Ni/ZrO₂, the metallic surface area did not change whereas the CO₂ consumption rate almost tripled. The XRD, XPS and EPR analyses showed that Ca⁺² but also some Ni²⁺ are on the ZrO₂ surface lattice of the Ni/CaZrO₂ catalyst. These cations form pairs which are composed of oxygen vacancies and coordinatively unsaturated sites (cus). By increasing the number of these pairs, the CO₂ methanation rate increases. Moreover, the number of active sites of the CO₂ methanation rate limiting step (CO and/or formate species decomposition, rls) is enhanced as well, showing that the rls occurs on the vacancies-cus sites pairs.     

Complete removal of carbon monoxide in hydrogen-rich gas stream through methanation over supported metal catalysts

Complete removal of CO by methanation in H₂-rich gas stream was performed over different metal catalysts. Ni/ZrO₂ and Ru/TiO₂ were the most effective catalysts for complete removal of CO through the methanation. These catalysts can decrease a concentration of CO from 0.5% to $\text{20}\text{ppm}$ in the gases formed by the steam reforming of methane with a significantly low conversion of CO₂ into methane. Catalytic activities of supported Ni and Ru strongly depended on the type of supports, i.e. ZrO₂ for Ni and TiO₂ for Ru are suitable supports for the methanation of CO. The effect of catalytic supports on methanation of CO could be explained by particles sizes of Ni and Ru metal. Catalytic activity of supported Ru catalysts for the complete removal of CO through methanation became higher as particle sizes of Ru metal became smaller, while Ni metal particles with relatively larger diameters were effective for the reaction.     

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.     

Phosphate-modified Al2O3–ZrO2 supported Ni catalysts for efficient selective methanation of CO in H2-rich reformate gas

CO selective methanation can remove the CO in H₂-rich reformate gas to prevent the poisoning of Pt anode electrode in proton exchange membrane fuel cell. However, the methanation of CO₂ in H₂-rich gas consumes a lot of hydrogen, which greatly reduces the energy efficiency. In order to inhibit CO₂ methanation, mesostructured Al₂O₃–ZrO₂ was modified by different amounts of phosphate, and then was as Ni support. The structures and surface properties of Ni/Al₂O₃–ZrO₂ catalyst modified by phosphate were studied to reveal the effect of phosphate-modification on CO conversion and selectivity for CO methanation. It was found that the phosphate-modification inhibited the adsorption of CO₂, which increased the selective for CO methanation. But the modification with excess phosphate lessened active sites of Ni and weakened the adsorption of H₂ and CO, which decreased the activity of CO methanation.     

Unsupported cobalt nanoparticles as catalysts: Effect of preparation method on catalytic activity in CO2 methanation and ethanol steam reforming

Cobalt nanoparticles (10–50 nm) have been prepared by different procedures. Materials produced by reduction of cobalt chloride and nitrate by NaBH₄ contain B impurities as borates or borides. They are very active in ethanol steam reforming at 673–773 K with up to 85% hydrogen yield at 773 K. B-free samples obtained by thermal decomposition of Co₂(CO)₈ is slightly less selective to hydrogen, due to its activity in ethanol cracking to methane which is probably poisoned by boron impurities on the other catalysts. B-containing samples are inactive in CO₂ methanation and have weak activity in the reverse water gas shift (RWGS) reaction to CO. B-free nanoparticles have high activity in both CO₂ methanation and RWGS. However, methanation activity is reduced fast by growth of encapsulating carbon species. These particles however also show quite stable activity in RWGS to CO, attributed to CoO impurities.     

CO deep removal with a method of two-stage methanation

CO methanation experiments showed that it was difficult to reach both goals of CO removal depth of below 10 ppm and CO₂ conversion rate of below 5% by using a single catalyst in this paper. A two-stage methanation method by applying two kinds of catalysts is proposed, that is, one catalyst with relatively low activity and high selectivity for the first stage at higher temperatures, and another one with relatively high activity for the second stage at lower temperatures. CO can be removed from 1% to below 0.1% at the first stage and to below 10 ppm at the second stage with CO₂ conversion rate below 1% and below 4% at each stage respectively. In addition, results also showed that the reverse water-gas shift (RWGS) reaction at the second stage was the dominant factor of CO removal depth. Temperature programmed reduction (TPR) and H₂ chemisorption were applied to characterize the catalysts.     

Solid-state synthesis method for the preparation of cobalt doped Ni–Al2O3 mesoporous catalysts for CO2 methanation

In this study, a simple solid-state synthesis method was employed for the preparation of the Ni–Co–Al₂O₃ catalysts with various Co loadings, and the prepared catalysts were used in CO₂ methanation reaction. The results demonstrated that the incorporation of cobalt in nickel-based catalysts enhanced the activity of the catalyst. The results showed that the 15 wt%Ni-12.5 wt%Co–Al₂O₃ sample with a specific surface area of 129.96 m²/g possessed the highest catalytic performance in CO₂ methanation (76.2% CO₂ conversion and 96.39% CH₄ selectivity at 400 °C) and this catalyst presented high stability over 10 h time-on-stream. Also, CO methanation was investigated and the results showed a complete CO conversion at 300 °C.     

TiO2 and ZrO2 supported Ru catalysts for CO mitigation following the water-gas shift reaction

CO oxidation and methanation over Ru-TiO₂ and Ru-ZrO₂ catalysts were investigated for CO removal for applications in proton exchange membrane fuel cells. The catalysts were synthesised by the deposition precipitation method at a pH of 7–7.5 for better interactions between the support and the active Ru metal. Various characterization experiments such as TPR, XPS, FTIR-CO, CO chemisorption and HRTEM were conducted to better understand the physio-chemical properties of Ru on the supports. Both catalysts showed excellent activity for the total oxidation of CO, however, with the addition of H₂, the catalysts activity to CO oxidation decreased significantly. Higher temperatures for the preferential oxidation reaction indicated that the Ru catalysts not only oxidize CO, but hydrogenate it as well. Furthermore, H₂ oxidation was favoured over the catalysts. Hydrogenation of CO over these catalysts gave high CO conversion and selectivity towards CH₄. Both the catalysts showed similar activity across the temperature range screened and gave maximum CO conversions of 99.9% from 240 °C onwards, with 99.9% selectivity towards CH₄. The catalysts also showed good stability in the reaction and the similarities in the catalytic activity of these were attributed to the well-dispersed Ru metal over the supports. The Ru catalysts effectively reduced CO concentrations in the reformate gas to less than 10 ppm, as is required for practical applications.     

Thermal management of CO2 methanation with axial staging of active metal concentration in Ni-YSZ tubular catalysts

CO₂ methanation has attracted considerable interests as a promising approach to productively utilizing CO₂ and reducing emissions to realize a low-carbon society. One major difficulty with packed bed reactors for catalyzed CO₂ methanation is maintaining an optimal reactor temperature distribution. Although a high temperature increases the catalytic activity, it also leads to the formation of an inlet hotspot, which causes thermal runaway, unfavorable equilibrium products, and catalyst degradation. To address this, in this study, we proposed an approach to manage the temperature profile in CO₂ methanation reactors by increasing catalytic activity along the reactor length using different Ni composition catalysts (gradient-distributed Ni-YSZ catalyst). Ni-based tubular catalysts with different Ni compositions were prepared and stacked in order of ascending Ni content from the inlet to the outlet. The effect of gradient Ni compositions on the temperature profile was investigated based on both numerical simulations and experimental observations. The gradient-distributed Ni catalyst could successfully prevent hotspot formation at the inlet of the reactor compared to the highly active uniform catalysts. The use of the catalyst caused a small difference in the reactor temperature (of ~70 °C) and afforded a high CH₄ yield (~90%). The proposed approach using gradient-distributed catalysts could be a potential method to manage CO₂ methanation reactor temperature and to achieve high CO₂ conversion in compact reactors.     

The influence of CO,CO2 and H2O on selective CO methanation over Ni(Cl)/CeO2 catalyst: On the way to formic acid derived CO-free hydrogen

For the first time the influence of CO, CO₂ and H₂O content on the performance of chlorinated NiCeO₂ catalyst in selective or preferential CO methanation was studied systematically. It was shown that the rate of CO methanation over Ni(Cl)/CeO₂ increases with the increasing H₂ concentration, is independent of CO₂ concentration and decreases with increasing CO and H₂O concentrations; the rate of CO₂ methanation is weakly sensitive to H₂ and CO₂ concentrations and decreases with increasing CO and H₂O concentrations. High catalyst selectivity was attributed to Ni surface blockage by strongly adsorbed CO molecules and ceria surface blockage by Cl, which both inhibit CO₂ hydrogenation.For the first time, selective CO methanation over Ni(Cl)/CeO₂ was studied for deep CO removal from formic acid derived hydrogen-rich gases characterized by high CO₂ (40–50 vol%), low CO (30–1000 ppm) content and trace amounts of water. Composite Ni(Cl)/CeO₂-η-Al₂O₃/FeCrAl wire mesh catalyst was demonstrated to be effective for this process at temperatures of 180–220°С, selectivity 30–70%, WHSV up to 200 L (STP)/(g∙h). The catalyst provides high process productivity, low pressure drop, uniform temperature distribution, and appears highly promising for the development of a compact CO cleanup reactor. Selective CO methanation was concluded to be a convenient way to CO-free hydrogen produced by formic acid decomposition.     

Promotion of CO2 methanation at low temperature over hydrotalcite-derived catalysts-effect of the tunable metal species and basicity

The CO₂ methanation is an effective strategy for making full use of waste gases and converting them into valuable chemicals. Nowadays, the key challenge is the design of efficient catalysts to enhance low-temperature catalytic performance. A series of hydrotalcite-derived catalysts with tunable metal species were prepared by hydrothermal synthesis method for CO₂ methanation at low-temperature. It was found that suitable synergistic effect between metallic Ni and basic sites in the support could be achieved by regulating the metal composition of hydrotalcite-derived catalysts. The NiMgAl catalyst exhibited the highest catalytic activity with CO₂ conversion of 91.8% at 250 °C. The in situ DRIFTS measurements and DFT calculation further revealed that the alkaline metal oxide MgO and more Ni(111) active sites in the NiMgAl catalyst could promote the activation of CO₂ and the formation of active intermediates, which actively participated in improving low-temperature activity. Therefore, the ternary mixed oxides catalyst derived from LDHs precursors shows a potential strategy in achieving excellent catalytic properties for CO₂ methanation at low-temperature.     

Effect of noble metal addition over active Ru/TiO2 catalyst for CO selective methanation from H2 rich-streams

Selective CO methanation from H₂-rich stream has been regarded as a promising route for deep removal of low CO concentration and catalytic hydrogen purification processes. This work is focused on the development of more efficient catalysts applied in practical conditions. For this purpose, we prepared a series of catalysts based on Ru supported over titania and promoted with small amounts of Rh and Pt. Characterization details revealed that Rh and Pt modify the electronic properties of Ru. The results of catalytic activity showed that Pt has a negative effect since it promotes the reverse water gas shift reaction decreasing the selectivity of methanation but Rh increases remarkably the activity and selectivity of CO methanation. The obtained results suggest that RuRh-based catalyst could become important for the treatment of industrial-volume streams.     

Techno-economic analysis of adiabatic four-stage CO2 methanation process for optimization and evaluation of power-to-gas technology

Owing to increasing demands for clean energy, caused by global warming, renewable energy sources have attracted significant attention. However, these sources can affect the reliability of electrical grids owing to their intermittency. Power-to-gas technology is expected to help address this issue. In this study, the CO₂ methanation process, which yields synthetic natural gas (SNG) via the synthesis of CO₂ and H₂ through proton exchange membrane (PEM) water electrolysis using surplus electricity generated from renewable energy, was evaluated and optimized based on techno-economic analyses. Requirements for the introduction of SNG produced through CO₂ methanation in domestic natural gas markets are presented by considering various scenarios. Results indicate that, even if the electricity costs, including system marginal price and renewable energy costs, are minimal, the costs for PEM water electrolysis and CO₂ methanation must be reduced by ~$550/kW and 25%, respectively, relative to current levels for the viable introduction of SNG in domestic markets.     

Impacts of alkali or alkaline earth metals addition on reaction intermediates formed in methanation of CO2 over cobalt catalysts

Additives affect the physiochemical properties of the catalyst as well as the evolution of the reaction intermediates produced during the reaction process such as the methanation of CO₂. In this study, Co/Al₂O₃ catalysts modified with Na, K, Mg or Ca were prepared and the reaction intermediates formed during CO₂ methanation were investigated. The results showed that Na, K or Mg species reacted with alumina, forming Al(OH)₃ or MgAl₂O₄ spinel structure, leading to the re-structure of the catalysts and a remarkable decrease of the specific surface area. The increased alkalinity of the catalyst did not promote the catalytic activity for methanation but promoted CO formation. The addition of Na or K enhanced the affinity of the catalyst to the reaction intermediates of HCOO* and CO₃²⁻, slowing down their further reduction to CH₄ and leading to the lower catalytic activity. The evolution of HCOO* and CO₃²⁻ species strongly correlated with the catalytic activity, while the direct correlation between the capability for the absorbance of CO₂* as well as the C–O functionality and the catalytic activity was not found. In addition, the addition of Na or K to Co/Al₂O₃ could also induce the formation of a significant amount of the coke species in the nanotube form.     

Impacts of La addition on formation of the reaction intermediates over alumina and silica supported nickel catalysts in methanation of CO2

This study aimed to investigate impacts of Al₂O₃ and SiO₂, the supports of Ni catalysts with distinct properties, and the additive of La on catalytic behaviors and reaction intermediates formed during methanation of CO₂. The results showed that the addition of La to either Ni/Al₂O₃ or Ni/SiO₂ led to the reduced size of metallic nickel, the reduced reduction degree of nickel oxide, the increased alkalinity number and the increased activity for methanation of CO₂. Furthermore, the addition of La to the Ni/SiO₂ catalyst could suppress the formation of CO via the reverse water gas shift (RWGS) reaction. Ni/SiO₂ was much more active than the Ni/Al₂O₃, even though nickel size was much bigger. The in situ Diffuse Reflection Infrared Fourier Transform Spectroscopy (DRIFTS) studies showed that the addition of La to Ni/Al₂O₃ interfered with integration of hydroxyl group with *CO₂ species and formation of the bicarbonate and carbonate, while favored formation of the formate specie, enhancing the catalytic activity. For Ni/SiO₂, instead of formate, CO* became the main reaction intermediate. The strong absorption of CO* favored its further conversion and explained the low selectivity of the silica-based catalysts toward CO. The addition of La to Ni/SiO₂ catalyst facilitated further hydrogenation of CO* species to CH₄ and promoted the catalytic activity.     

Effect of Ni content on CO2 methanation performance with tubular-structured Ni-YSZ catalysts and optimization of catalytic activity for temperature management in the reactor

This paper presents high-performance Ni-YSZ tubular catalysts for CO₂ methanation prepared by the extrusion molding. We fabricated tubular-shaped Ni-YSZ catalysts with various Ni contents (25–100 wt% NiO) and investigated the effect of Ni content on CO₂ methanation performance under various temperatures and gas flow rates. Catalysts with Ni contents >75 wt% showed CH₄ yields >91% above 270 °C with high CH₄ selectivities (>99%). High CH₄ yields were also observed under high GHSVs at 300 °C: 93% and 92% at 8700 and 17,500 h⁻¹, respectively. Investigation of methanation with the catalysts revealed that CO₂ methanation was accelerated by a localized hotspot at the reactor inlet arising from the interaction between reaction kinetics and heat generation. Using a numerical simulation, we considered the optimum arrangement of catalytic activity in the reactor to avoid hotspot generation and realize a stable high CO₂ methanation performance. We can simultaneously achieve high CH₄ production and prevent hotspot formation by properly arranging catalysts with different activities.     

CO removal via selective methanation over the catalysts Ni/ZrO2 prepared with reduction by the wet H2-rich gas

The wet H₂-rich gas was used as reducing gas instead of the H₂/N₂ gas in the reduction step of the catalyst preparation. It is found that the selectivity for CO methanation over the catalysts 0.4Ni/ZrO₂ so-obtained was decreased in comparison to the case of the H₂/N₂ gas used as reducing gas. Even though, the samples with the different feed atomic ratios of Ni/Zr prepared by the impregnation method and the co-precipitation method, respectively, were evaluated with the wet H₂-rich gas both as reducing gas and as reactant gas. The catalysts Ni/ZrO₂-CP prepared by the co-precipitation method exhibited a high catalytic activity for CO removal at a lowered reaction temperature with increasing the Ni loading. Over the catalyst 3.0Ni/ZrO₂-CP, CO in the reactant gas could be removed to below 10 ppm at reaction temperatures of 220–260 °C with the selectivity higher than 50%. And the selectivity was kept at 100% during the 100 h test at 220 °C. The catalysts were characterized by XRD, XPS, XRF and the adsorption isotherm measurement. In addition, effect of water vapor in reactant gas was studied over the catalysts 0.4Ni/ZrO₂ with the wet H₂-rich gas and the dry H₂-rich gas as reactant gas, respectively, in the case of the H₂/N₂ gas fixed as reducing gas. It is seen that presence of water vapor in the reactant gas retarded methanation reactions of CO and CO₂ on the catalysts.