Alkali Promotion in the Formation of CH 4 from CO 2 and Renewably Produced H 2 over Supported Ni Catalysts

K-guided selective regulation mechanism for CO2 hydrogenation over Ni/CeO2 catalyst

Regulating the selectivity between CO and CH4 during CO2 hydrogenation is a challenging research topic. Previous research has indicated that potassium (K) modification can adjust the product selectivity by regulating the adsorption strength of formate/CO* intermediates. Going beyond the regulation mechanism described above, this study proposes a K-guided selectivity control method based on the regulation of key intermediates HCO*/H3CO* for Ni catalysts supported on reducible carrier CeO2. By incorporating K, the CO selectivity of CO2 hydrogenation shifts from around 25.4% for Ni/CeO2 to approximately 93.8% for Ni/CeO2-K. This can be attributed to K modification causes electron aggregation in the bonding regions of HCO* and H3CO* intermediates, thus enhancing their adsorption strength. Consequently, the reaction pathway from HCO*/H3CO* to CH4 is limited, favoring the decomposition of formates to CO products. Moreover, the addition of K leads to a moderate decrease in CO2 conversion from 55.2% to 48.6%, which still surpasses values reported in most other studies. This reduction is associated with a decline in reducible Ni species and oxygen vacancy concentration in Ni/CeO2-K. As a result, the adsorption capacity for CO2 and H2 reduces, ultimately reducing CO2 hydrogenation activity.

Flexible NiRu Systems for CO2 Methanation: From Efficient Catalysts to Advanced Dual-Function Materials

CO₂ emissions in the atmosphere have been increasing rapidly in recent years, causing global warming. CO₂ methanation reaction is deemed to be a way to combat these emissions by converting CO₂ into synthetic natural gas, i.e., CH₄. NiRu/CeAl and NiRu/CeZr both demonstrated favourable activity for CO₂ methanation, with NiRu/CeAl approaching equilibrium conversion at 350 °C with 100% CH₄ selectivity. Its stability under high space velocity (400 L·g^-1·h^-1) was also commendable. By adding an adsorbent, potassium, the CO₂ adsorption capability of NiRu/CeAl was boosted, allowing it to function as a dual-function material (DFM) for integrated CO₂ capture and utilisation, producing 0.264 mol of CH₄/kg of sample from captured CO₂. Furthermore, time-resolved operando DRIFTS-MS measurements were performed to gain insights into the process mechanism. The obtained results demonstrate that CO₂ was captured on basic sites and was also dissociated on metallic sites in such a way that during the reduction step, methane was produced by two different pathways. This study reveals that by adding an adsorbent to the formulation of an effective NiRu methanation catalyst, advanced dual-function materials can be designed.

Promotion of Ru or Ni on Alumina Catalysts with a Basic Metal for CO2 Hydrogenation: Effect of the Type of Metal (Na, K, Ba)

Ru and Ni on alumina catalysts have been promoted with a 10 wt% of alkali metal (K or Na) or alkaline earth metal (Ba) and tested in CO₂ methanation. For the catalyst consisting of Ni and Ba, the variation of Ba loading while keeping Ni loading constant was studied. The promotion in terms of enhanced CH₄ yield was found only for the addition of barium to 15 wt% Ni/Al₂O₃. In contrast, K and Na addition increased the selectivity to CO while decreasing conversion. For the Ru-based catalyst series, no enhancement in conversion or CH₄ yield was attained by any of the alkaline metals. CO₂ temperature-programed desorption (CO₂-TPD) revealed that the amount of chemisorbed CO₂ increased significantly after the addition of the base metal. The reactivity of CO_x ad-species for each catalyst was assessed by temperature-programed surface reaction (TPSR). The characterization revealed that the performance in the Sabatier reaction was a result of the interplay between the amount of chemisorbed CO₂ and the reactivity of the COx ad-species, which was maximized for the (10%Ba)15%Ni/Al₂O₃ catalyst.

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

The increasing utilization of renewable sources for electricity production turns CO2 methanation into a key process in the future energy context, as this reaction allows storing the temporary renewable electricity surplus in the natural gas network (Power-to-Gas). This kind of chemical reaction requires the use of a catalyst and thus it has gained the attention of many researchers thriving to achieve active, selective and stable materials in a remarkable number of studies. The existing papers published in literature in the past few years about CO2 methanation tackled the catalysts composition and their related performances and mechanisms, which served as a basis for researchers to further extend their in-depth investigations in the reported systems. In summary, the focus was mainly in the enhancement of the synthesized materials that involved the active metal phase (i.e., boosting its dispersion), the different types of solid supports, and the frequent addition of a second metal oxide (usually behaving as a promoter). The current manuscript aims in recapping a huge number of trials and is divided based on the support nature: SiO2, Al2O3, CeO2, ZrO2, MgO, hydrotalcites, carbons and zeolites, and proposes the main properties to be kept for obtaining highly efficient carbon dioxide methanation catalysts.

Spray-Dried Ni Catalysts with Tailored Properties for CO2 Methanation

A catalyst production method that enables the independent tailoring of the structural properties of the catalyst, such as pore size, metal particle size, metal loading or surface area, allows to increase the efficiency of a catalytic process. Such tailoring can help to make the valorization of CO2 into synthetic fuels on Ni catalysts competitive to conventional fossil fuel production. In this work, a new spray-drying method was used to produce Ni catalysts supported on SiO2 and Al2O3 nanoparticles with tunable properties. The influence of the primary particle size of the support, different metal loadings, and heat treatments were applied to investigate the potential to tailor the properties of catalysts. The catalysts were examined with physical and chemical characterization methods, including X-ray diffraction, temperature-programmed reduction, and chemisorption. A temperature-scanning technique was applied to screen the catalysts for CO2 methanation. With the spray-drying method presented here, well-organized porous spherical nanoparticles of highly dispersed NiO nanoparticles supported on silica with tunable properties were produced and characterized. Moreover, the pore size, metal particle size, and metal loading can be controlled independently, which allows to produce catalyst particles with the desired properties. Ni/SiO2 catalysts with surface areas of up to 40 m2 g−1 with Ni crystals in the range of 4 nm were produced, which exhibited a high activity for the CO2 methanation.

Influence of sodium-modified Ni/SiO2 catalysts on the tunable selectivity of CO2 hydrogenation: Effect of the CH4 selectivity, reaction pathway and mechanism on the catalytic reaction

CO₂ hydrogenation over Ni/SiO₂ catalysts with and without Na additives was investigated in terms of the catalytic activity, selectivity of CO₂ methanation and reaction mechanism. Na additives could cause the formation of Na₂O species that might deposit on the Ni surface of Ni/SiO₂ (NiNa_x/SiO₂). When the Ni metal is partially covered with Na₂O species, a highly positive charge on the Ni metal could occur compared to the original Ni/SiO₂ catalyst. The addition of Na to the Ni/SiO₂ catalyst could influence selectivity toward CO formation. The adsorbed formic acid is the major intermediate on the Ni/SiO₂ catalyst during CO₂ hydrogenation. The formic acid species might decompose into adsorbed CO complexes in the forms of linear CO, bridged CO and multibonded CO. CH₄ formation should be ascribed to the hydrogenation of these adsorbed CO complexes. The Ni/SiO₂ catalyst with the Na additive might have very weak ability for H₂ and CO adsorption, thus making it difficult for CO methanation to occur. The hydrogen carbonate species adsorbed on the NiNa_x/SiO₂ catalysts were proposed to be the key intermediate, and they might decompose to CO or be hydrogenated to form CH₄.

The Role of Alkali and Alkaline Earth Metals in the CO2 Methanation Reaction and the Combined Capture and Methanation of CO2

CO2 methanation has great potential for the better utilization of existing carbon resources via the transformation of spent carbon (CO2) to synthetic natural gas (CH4). Alkali and alkaline earth metals can serve both as promoters for methanation catalysts and as adsorbent phases upon the combined capture and methanation of CO2. Their promotion effect during methanation of carbon dioxide mainly relies on their ability to generate new basic sites on the surface of metal oxide supports that favour CO2 chemisorption and activation. However, suppression of methanation activity can also occur under certain conditions. Regarding the combined CO2 capture and methanation process, the development of novel dual-function materials (DFMs) that incorporate both adsorption and methanation functions has opened a new pathway towards the utilization of carbon dioxide emitted from point sources. The sorption and catalytically active phases on these types of materials are crucial parameters influencing their performance and stability and thus, great efforts have been undertaken for their optimization. In this review, we present some of the most recent works on the development of alkali and alkaline earth metal promoted CO2 methanation catalysts, as well as DFMs for the combined capture and methanation of CO2.