Promising Catalytic Systems for CO2 Hydrogenation into CH4: A Review of Recent Studies

Low-temperature methanation of fermentation gas with Ni-based catalysts in a multicomponent system

A large amount of greenhouse gases, such as carbon dioxide and methane, are released during the production process of bioethanol and biogas. Converting CO2 into methane is a promising way of capturing CO2 and generating high-value gas. At present, CO2 methanation technology is still in the early stage. It requires high temperature (300-400 ℃) and pressure (> 1 MPa), leading to high cost and energy consumption. In this study, a new catalyst, Ni-Fe/Al-Ti, was developed. Compared with the activity of the common Ni/Al2O3 catalyst, that of the new catalyst was increased by 1/3, and its activation temperature was reduced by 100℃. The selectivity of methane was increased to 99%. In the experiment using simulated fermentation gas, the catalyst showed good catalytic activity and durability at a low temperature and atmospheric pressure. Based on the characterization of catalysts and the study of reaction mechanisms, this article innovatively proposed a Ni-Fe/Al-Ti quaternary catalytic system. Catalytic process was realized through the synergism of Al-Ti composite support and Ni-Fe promotion. The oxygen vacancies on the surface of the composite carrier and the higher activity metals and alloys promoted by Fe accelerate the capture and reduction of CO2. Compared with the existing catalysts, the new Ni-Fe/Al-Ti catalyst can significantly improve the methanation efficiency and has great practical application potential.

Niobium Modification of CeO2 Tuning Electron Density of Nickel-Ceria Interfacial Sites for Enhanced CO2 Methanation

CO2 methanation has attracted considerable attention as a promising strategy for recycling CO2 and generating valuable methane. This study presents a niobium-doped CeO2-supported Ni catalyst (Ni/NbCe), which demonstrates remarkable performance in terms of CO2 conversion and CH4 selectivity, even when operating at a low temperature of 250 °C. Structural analysis reveals the incorporation of Nb species into the CeO2 lattice, resulting in the formation of a Nb-Ce-O solid solution. Compared with the Ni/CeO2 catalyst, this solid solution demonstrates an improved spatial distribution. To comprehend the impact of the Nb-Ce-O solid solution on refining the electronic properties of the Ni-Ce interfacial sites, facilitating H2 activation, and accelerating the hydrogenation of CO2* into HCOO*, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis and density functional theory (DFT) calculations were conducted. These investigations shed light on the mechanism through which the activity of CO2 methanation is enhanced, which differs from the commonly observed CO* pathway triggered by oxygen vacancies (OV). Consequently, this study provides a comprehensive understanding of the intricate interplay between the electronic properties of the catalyst's active sites and the reaction pathway in CO2 methanation over Ni-based catalysts.

Unraveling Temperature-Dependent Plasma-Catalyzed CO2 Hydrogenation

Hydrogenation of carbon dioxide to value-added chemicals and fuels has recently gained increasing attention as a promising route for utilizing carbon dioxide to achieve a sustainable society. In this study, we investigated the hydrogenation of CO2 over M/SiO2 and M/Al2O3 (M = Co, Ni) catalysts in a dielectric barrier discharge system at different temperatures. We compared three different reaction modes: plasma alone, thermal catalysis, and plasma catalysis. The coupling of catalysts with plasma demonstrated synergy at different reaction temperatures, surpassing the thermal catalysis and plasma alone modes. The highest CO2 conversions under plasma-catalytic conditions at reaction temperatures of 350 and 500 °C were achieved with a Co/SiO2 catalyst (66%) and a Ni/Al2O3 catalyst (68%), respectively. Extensive characterizations were used to analyze the physiochemical characteristics of the catalysts. The results show that plasma power was more efficient than heating power at the same temperature for the CO2 hydrogenation. This demonstrates that the performance of CO2 hydrogenation can be significantly improved in the presence of plasma at lower temperatures.

Different reactor configurations for enhancement of CO2 methanation

Greenhouse gas emissions are a massive concern for scientists to minimize the effect of global warming in the environment. In this study, packed bed, coated wall, and membrane reactors were investigated using three novel nickel catalysts for the methanation of CO2. CFD modelling methodologies were implemented to develop 2D models. The validity of the model was investigated in a previous study where experimental and simulated results in a packed bed reactor were in a good agreement. It was observed that the coated wall reactor had poorer performance compared to the packed bed, approximately 30% difference between the results, as the residence time of the former was lower. In addition, two membrane configurations were proposed, including a membrane packed bed and membrane coated wall reactor. Additional studies were performed in the coated wall reactor revealing that lower flow rates lead to higher conversion values. As for the bed thickness the optimum layer was found to be 1 mm. In both membrane reactor configurations, the effect of the thickness of M1 membrane, which indicates the membrane for the removal of H2O, didn't show difference while the reduction of the thickness of M2 membrane, which indicates the membrane for the removal of CO2, H2 and H2O, showed better results in terms of conversion.

Recent Advances on CO2 Mitigation Technologies: On the Role of Hydrogenation Route via Green H2

The increasing trend in global energy demand has led to an extensive use of fossil fuels and subsequently in a marked increase in atmospheric CO2 content, which is the main culprit for the greenhouse effect. In order to successfully reverse this trend, many schemes for CO2 mitigation have been proposed, taking into consideration that large-scale decarbonization is still infeasible. At the same time, the projected increase in the share of variable renewables in the future energy mix will necessitate large-scale curtailment of excess energy. Collectively, the above crucial problems can be addressed by the general scheme of CO2 hydrogenation. This refers to the conversion of both captured CO2 and green H2 produced by RES-powered water electrolysis for the production of added-value chemicals and fuels, which are a great alternative to CO2 sequestration and the use of green H2 as a standalone fuel. Indeed, direct utilization of both CO2 and H2 via CO2 hydrogenation offers, on the one hand, the advantage of CO2 valorization instead of its permanent storage, and the direct transformation of otherwise curtailed excess electricity to stable and reliable carriers such as methane and methanol on the other, thereby bypassing the inherent complexities associated with the transformation towards a H2-based economy. In light of the above, herein an overview of the two main CO2 abatement schemes, Carbon Capture and Storage (CCS) and Carbon Capture and Utilization (CCU), is firstly presented, focusing on the route of CO2 hydrogenation by green electrolytic hydrogen. Next, the integration of large-scale RES-based H2 production with CO2 capture units on-site industrial point sources for the production of added-value chemicals and energy carriers is contextualized and highlighted. In this regard, a specific reference is made to the so-called Power-to-X schemes, exemplified by the production of synthetic natural gas via the Power-to-Gas route. Lastly, several outlooks towards the future of CO2 hydrogenation are presented.

Cerium d-Block Element (Co, Ni) Bimetallic Oxides as Catalysts for the Methanation of CO2: Effect of Pressure

Nickel– and cobalt–cerium bimetallic oxides were used as catalysts for the methanation of CO2 under pressure. The catalysts’ activity increases with pressure and an increase of just 10 bar is enough to double the yield of methane and to significantly improve the selectivity. The best results were those obtained over nickel–cerium bimetallic oxides, but the effect of pressure was particularly relevant over cobalt–cerium bimetallic oxides, which yield to methane increases from almost zero at atmospheric pressure to 50–60% at 30 bar. Both catalyst types are remarkably competitive, especially those containing nickel, which were always more active than a commercial rhodium catalyst used as a reference (5wt.% Rh/Al2O3) and tested under the same conditions. For the cobalt–cerium bimetallic oxides, the existence of a synergetic interaction between Co and CoO and the formation of cobalt carbides seems to play an important role in their catalytic behavior. Correlation between experimental reaction rates and simulated data confirms that the catalysts’ behavior follows the Langmuir–Hinshelwood–Hougen–Watson kinetic model, but Le Chatelier’s principle is also important to understand the catalysts’ behavior under pressure. A catalyst recycle study was also performed. The results obtained after five cycles using a nickel–cerium catalyst show insignificant variations in activity and selectivity, which are important for any type of practical application.

Alternative Clinker Technologies for Reducing Carbon Emissions in Cement Industry: A Critical Review

Currently, the production of one ton of ordinary Portland cement (OPC) releases considerable amounts of CO₂ into the atmosphere. As the need and demand for this material grows exponentially, it has become a challenge to increase its production at a time when climate-related problems represent a major global concern. The two main CO₂ contributors in this process are fossil fuel combustion to heat the rotary kiln and the chemical reaction associated with the calcination process, in the production of the clinker, the main component of OPC. The current paper presents a critical review of the existent alternative clinker technologies (ACTs) that are under an investigation trial phase or under restricted use for niche applications and that lead to reduced emissions of CO₂. Also, the possibility of transition of clinker production from traditional rotary kilns based on fuel combustion processes to electrification is discussed, since this may lead to the partial or even complete elimination of the CO₂ combustion-related emissions, arising from the heating of the clinker kiln.

CO2 Methanation: Nickel-Alumina Catalyst Prepared by Solid-State Combustion

The development of solvent-free methods for the synthesis of catalysts is one of the main tasks of green chemistry. A nickel–alumina catalyst for CO₂ methanation was synthesized by solid-state combustion method using hexakis-(imidazole) nickel (II) nitrate complex. Using X-ray Powder Diffraction (XRD), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Hydrogen temperature-programmed reduction (H₂-TPR), it was shown that the synthesized catalyst is characterized by the localization of easily reduced nickel oxide on alumina surface. This provided low-temperature activation of the catalyst in the reaction mixture containing 4 vol% CO₂. In addition, the synthesized catalyst had higher activity in low-temperature CO₂ methanation compared to industrial NIAP-07-01 catalyst, which contained almost three times more hard-to-reduce nickel–aluminum spinel. Thus, the proposed approaches to the synthesis and activation of the catalyst make it possible to simplify the catalyst preparation procedure and to abandon the use of solvents, which must be disposed of later on.

Alkali and Alkali-Earth Metals Incorporation to Ni/USY Catalysts for CO2 Methanation: The Effect of the Metal Nature

CO2 methanation is typically carried out using Ni-supported catalysts containing promoters such as alkali or alkali-earth metals to improve their properties. In this work, bimetallic Ni-based USY zeolite catalysts containing alkali (Li, K and Cs) and alkali-earth (Mg, Ca) metal compounds were prepared using the same conditions (15 wt% of metals; co-impregnation), characterized by N2 sorption, XRD, TGA, CO2 adsorption–desorption, DRS UV-Vis and H2-TPR, and finally applied in CO2 methanation reaction (86,100 mL h−1 g−1, PCO2 = 0.16 bar, H2:CO2 = 4:1). For each group, the effects of the second metal nature on the properties and performances were assessed. Alkali metals incorporation induced considerably low catalytic performances (CH4 yields < 26%), attributed to their negative impact on zeolite structure preservation. On the contrary, alkali-earth metal-containing catalysts exhibited lower structural damage. However, the formation of Ni-Mg mixed oxides in Ni-Mg/USY catalyst and CaCO3 during the reaction in Ni-Ca/USY sample could explain their performances, similar or lower than those obtained for Ni/USY catalyst. Among the studied metals, calcium was identified as the most interesting (CH4 yield of 65% at 415 °C), which was ascribed to the slight improvement of the Ni0 dispersion.

New insight into the mechanism of carbon dioxide activation on copper-based catalysts: A theoretical study

A combination of Artificial Bee Colony algorithm, eXtended Tight Binding and Density functional theory methods were performed to study the activation process of carbon dioxide (CO₂) over copper (Cu₄ cluster) based catalytic systems. The findings revealed that the activation of the C-O bond resulted from the electron transfer to σ*, π* - MO of CO₂. The more the electrons are transferred to CO₂, the more the C-O bond is activated and elongated. The suitability of several metal oxide supports (Fe₂O₃, Al₂O₃, MgO, ZnO) is estimated using calculated electronic parameters (global electrophilicity index, vertical ionization potential and vertical electron affinity). Aside from demonstrating the appropriateness of Al₂O₃ and ZnO, a thorough examination of MgO revealed that, due to the formation of stable carbonate products, this oxide is not really appropriate as a support for copper-based catalysts in CO₂ conversion. Our studies have also shown that the electron enrichment of copper atoms plays a key role in the activation of C-O bonds. Alkali metal doping (Li, K, Cs) significantly improves the catalytic efficiency of the Cu₄ cluster. Based on the results of electron transfer to the CO₂ molecule, the effect of doping alkali metal atoms may be organized in the following order: Cs > K > Li. A new core/shell catalytic system with potassium atoms in the core and copper atoms in the shell has been proposed and has proven to be a promising, efficient catalytic system in the CO₂ adsorption and activation.

Natural Kaolin-Based Ni Catalysts for CO2 Methanation: On the Effect of Ce Enhancement and Microwave-Assisted Hydrothermal Synthesis

Natural kaolin-based Ni catalysts have been developed for low-temperature CO2 methanation. The catalysts were prepared via a one-step co-impregnation of Ni and Ce onto a natural kaolin-derived metakaolin using a microwave-assisted hydrothermal method as an acid-/base-free synthesis method. The influences of microwave irradiation and Ce promotion on the catalytic enhancement including the CO2 conversion, CH4 selectivity, and CH4 yield were experimentally investigated by a catalytic test of as-prepared catalysts in a fixed-bed tubular reactor. The relationship between the catalyst properties and its methanation activities was revealed by various characterization techniques including X-ray fluorescence, X-ray diffraction, Brunauer-Emmett-Teller, scanning electron microscopy, selected area electron diffraction, transmission electron microscopy, elemental mapping, H2 temperature-programmed reduction, and X-ray absorption near-edge structure analyses. Among the two enhancement methods, microwave and Ce promotion, the microwave-assisted synthesis could produce a catalyst containing highly dispersed Ni particles with a smaller Ni crystallite size and higher catalyst reducibility, resulting in a higher CO2 conversion from 1.6 to 7.5% and a better CH4 selectivity from 76.3 to 79.9% at 300 °C. Meanwhile, the enhancement by Ce addition exhibited a great improvement on the catalyst activities. It was experimentally found that the CO2 conversion increased approximately 7-fold from 7.5 to 52.9%, while the CH4 selectivity significantly improved from 79.9 to 98.0% at 300 °C. Though the microwave-assisted synthesis could further improve the catalyst activities of Ce-promoted catalysts, the Ce addition exhibited a more prominent impact than the microwave enhancement. Cerium oxide (CeO2) improved the catalyst activities through mechanisms of higher CO2 adsorption capacity with its basic sites and the unique structure of CeO2 with a reversible valence change of Ce4+ and Ce3+ and high oxygen vacancies. However, it was found that the catalyst prepared by microwave-assisted synthesis and Ce promotion proved to be the optimum catalyst in this study. Therefore, the present work demonstrated the potential to synthesize a nickel-based catalyst with improved catalytic activities by adding a small amount of Ce as a catalytic promoter and employing microwave irradiation for improving the Ni dispersion.