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The work in this paper evidences the viability of producing synthetic natural gas (SNG) via the methanation reaction tackling two fundamental challenges on methanation catalysis (i) the development of advanced catalysts able to achieve high CO 2 conversion and high methane yields and (ii) the unexplored effect of residual methane on the methanation stream. Both challenges have been successfully addressed using Ni/CeO 2 -ZrO 2 catalysts promoted with Mn and Co. Mn does not seem to be a good promoter while Co prevents carbon deposition and secondary reactions. In fact, our Co-doped sample reached high levels of CO 2 conversion and CH 4 selectivity, especially at low reaction temperatures. In addition, this catalyst exhibits excellent catalytic behaviour when methane is introduced into the gas mixture, indicating its feasibility for further study to be conducted in realistic flue gases environments and methanation units with recycling loops. Furthermore, when methane is introduced in the reactant mixture, the Ni-Co/CeO 2 -ZrO 2 sample is very stable maintaining high levels of conversion and selectivity. Overall our Co-doped catalyst can deliver high purity synthetic natural gas for long-term runs, promising results for gas-phase CO 2 conversion units. • Synthetic natural gas production over highly effective bimetallic Ni-Co catalysts. • First report on CO 2 methanation in the presence of methane. • Chemical CO 2 recycling via CO 2 methanation. • Design of methanation units with recycling loops. [ABSTRACT FROM AUTHOR]

The utilization of biomass and bio-waste, particularly through hydrothermal processes, has shown promise as a technology for converting these materials into valuable products. While most research has traditionally focused on the solid and liquid byproducts of these hydrothermal treatments, the gaseous phase has often been overlooked. This study specifically investigates the conversion of off-gases produced during hydrothermal carbonation (HTC) into synthetic natural gas, offering a readily marketable product with economic potential. Although the methanation of conventional flue gases has been extensively studied, dealing with non-standard off-gases from processes like HTC presents challenges due to the presence of minor impurities like CO and CH 4. This novel research seeks to experimentally evaluate the methanation of HTC off-gases using nickel-based catalysts and analyze how these impurities affect the catalytic performance. The studied catalysts include nickel supported by ceria and alumina, as well as alumina supported nickel-cobalt systems. The results demonstrate that these catalysts exhibit high CO 2 conversion and CH 4 selectivity under ideal gas conditions. However, when real gas compositions with impurities are considered, CO 2 conversion decreases at lower temperatures (ca. 20% lower conversion for real gas vs. ideal), probably due to side reactions such as CH 4 cracking. This difference becomes less pronounced at higher temperatures. Nevertheless, the catalysts perform satisfactorily, especially at temperatures exceeding 350 °C. In conclusion, this study sheds light on the methanation of HTC off-gases and underscores the significance of understanding how impurities in real gases impact the process, providing potential directions for future research. • Catalytic upgrading of HTC flue gases. • CO 2 conversion to methane under complex gaseous mixtures. • Cobalt and Ceria promoted Ni-based catalysts for CO 2 methanation. • A circular economy strategy for HTC and CO 2 conversion processes. [ABSTRACT FROM AUTHOR]

The water-gas shift (WGS) reaction over structured Cu, Ni, and bimetallic Cu-Ni supported on active carbon (AC) catalysts was investigated. The structured catalysts were prepared in pellets form and applied in the medium range WGS reaction. A good activity in the 180–350 °C temperature range was registered being the bimetallic Cu-Ni:2-1/AC catalyst the best catalyst. The presence of Cu mitigates the methanation activity of Ni favoring the shift process. In addition the active carbon gasification reaction was not observed for the Cu-containing catalyst converting the active carbon in a very convenient support for the WGS reaction. The stability of the bimetallic Cu-Ni:2-1/AC catalyst under continuous operation conditions, as well as its tolerance towards start/stop cycles was also evaluated. [ABSTRACT FROM AUTHOR]

• Customised H 2 -rich syngas production from CO 2 /CH 4 mixtures. • Ni stabilised on complex inorganic mixed oxides as versatile catalysts for reforming reactions. • Effect of water, temperature and space velocity on H 2 /CO ratio. • Overcoming coke deposition and sintering by introducing Ni in a pyrochlore-perovskite structure. The bi-reforming of methane (BRM) has the advantage of utilising greenhouse gases and producing H 2 rich syngas. In this work Ni stabilised in a pyrochlore-double perovskite structure is reported as a viable catalyst for both Dry Reforming of Methane (DRM) and BRM. A 10 wt.% Ni-doped La 2 Zr 2 O 7 pyrochlore catalyst was synthesised, characterised and tested under both reaction conditions and its performance was compared to a supported Ni/La 2 Zr 2 O 7. In particular the effect of steam addition is investigated revealing that steam increases the H 2 content in the syngas but limits reactants conversions. The effect of temperature, space velocity and time on stream was studied under BRM conditions and brought out the performance of the material in terms of activity and stability. No deactivation was observed, in fact the addition of steam helped to mitigate carbon deposition. Small and well dispersed Ni clusters, possibly resulting from the progressive exsolution of Ni from the mixed oxide structure could explain the enhanced performance of the catalyst. [ABSTRACT FROM AUTHOR]

Biogas methanation emerges as a prominent technology for converting biogas into biomethane in a single step. Furthermore, this technology can be implemented at biogas plant locations, supporting local economies and reducing dependence on large energy producers. However, there is a lack of comprehensive studies on biogas methanation, particularly regarding the technical optimization of operational parameters and the profitability analysis of the overall process. To address this gap, our study represents a seminal work on the technical optimization of biogas methanation obtaining an empirical model to predict the performance of biogas methanation. We investigate the influence of operational parameters, such as reaction temperature, H 2 /CO 2 ratio, space velocity, and CO 2 share in the biogas stream through an experimental design. Based on previous research we selected a nickel supported on ceria-alumina catalyst; being nickel a benchmark system for methanation process such selection permits a reliable data extrapolation to commercial units. We showcase the remarkable impact of studied key operation parameters, being the temperature, the most critical factor affecting the reaction performance (ca. 2 to 5 times higher than the second most influencing parameter). The impact of the H 2 /CO 2 ratio is also noticeable. The response surfaces and contour maps suggest that a temperature between 350 and 450 °C and an H 2 /CO 2 ratio between 2.5 and 3.2 optimize the reaction performance. Further experimental tests were performed for model validation and optimization leading to a reliable predictive model. Overall, this study provides validated equations for technology scaling-up and techno-economic analysis, thus representing a step ahead towards real-world applications for bio-methane production. • An empirical model to predict the performance of biogas methanation was built. • The influence of the main operational parameters was studied. • The temperature is the most critical factor affecting the reaction performance. • The impact of the H 2 /CO 2 ratio is also noticeable. • Temperature of 350–450 °C and H 2 /CO 2 ratio of 2.5–3.2 optimize the reaction. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ni 12 P 5 -CeAl, Ni 12 P 5 -Al 2 O 3 and Ni 2 P/SiO 2 were developed and tested active for CO 2 hydrogenation. • Ni 2 P/SiO 2 catalyst is a highly selective catalyst for reverse water–gas shift (RWGS) reaction, producing over 80% CO in the full temperature range. • Ni 12 P 5 -CeAl and Ni 12 P 5 -Al 2 O 3 catalysts show activity for methanation at low temperatures with a switchover to RWGS at higher temperatures (reaching or approaching thermodynamic equilibrium behaviour). We report for the first time the support dependent activity and selectivity of Ni-rich nickel phosphide catalysts for CO 2 hydrogenation. New catalysts for CO 2 hydrogenation are needed to commercialise the reverse water–gas shift reaction (RWGS) which can feed captured carbon as feedstock for traditionally fossil fuel-based processes, as well as to develop flexible power-to-gas schemes that can synthesise chemicals on demand using surplus renewable energy and captured CO 2. Here we show that Ni 2 P/SiO 2 is a highly selective catalyst for RWGS, producing over 80% CO in the full temperature range of 350–750 °C. This indicates a high degree of suppression of the methanation reaction by phosphide formation, as Ni catalysts are known for their high methanation activity. This is shown to not simply be a site blocking effect, but to arise from the formation of a new more active site for RWGS. When supported on Al 2 O 3 or CeAl, the dominant phase of as synthesized catalysts is Ni 12 P 5. These Ni 12 P 5 catalysts behave very differently compared to Ni 2 P/SiO 2 , and show activity for methanation at low temperatures with a switchover to RWGS at higher temperatures (reaching or approaching thermodynamic equilibrium behaviour). This switchable activity is interesting for applications where flexibility in distributed chemicals production from captured CO 2 can be desirable. Both Ni 12 P 5 /Al 2 O 3 and Ni 12 P 5 /CeAl show excellent stability over 100 h on stream, where they switch between methanation and RWGS reactions at 50–70% conversion. Catalysts are characterized before and after reactions via X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), temperature-programmed reduction and oxidation (TPR, TPO), Transmission Electron Microscopy (TEM), and BET surface area measurement. After reaction, Ni 2 P/SiO 2 shows the emergence of a crystalline Ni 12 P 5 phase while Ni 12 P 5 /Al 2 O 3 and Ni 12 P 5 /CeAl both show the crystalline Ni 3 P phase. While stable activity of the latter catalysts is demonstrated via extended testing, this Ni enrichment in all phosphide catalysts shows the dynamic nature of the catalysts during operation. Moreover, it demonstrates that both the support and the phosphide phase play a key role in determining selectivity towards CO or CH 4. [ABSTRACT FROM AUTHOR]

• Advanced catalysts for Chemical CO 2 recycling. • Opening the possibility to couple a RWGS unit with a syngas upgrading reactor. • Cu as an excellent promoter to improve the activity/selectivity of FeOx-based catalysts. • Remarkable RWGS activity at low medium-low temperatures with very high selectivity. The RWGS reaction represents a direct approach for gas-phase CO 2 upgrading. This work showcases the efficiency of Fe/CeO 2 -Al 2 O 3 catalysts for this process, and the effect of selected transition metal promoters such as Cu, Ni and Mo. Our results demonstrated that both Ni and Cu remarkably improved the performance of the monometallic Fe-catalyst. The competition Reverse Water-Gas Shift (RWGS) reaction/CO 2 methanation reaction was evident particularly for the Ni-catalyst, which displayed high selectivity to methane in the low-temperature range. Among the studied catalysts the Cu promoted sample represented the best choice, exhibiting the best activity/selectivity balance. In addition, the Cu-doped catalyst was very stable for long-term runs – an essential requisite for its implementation in flue gas upgrading units. This material can effectively catalyse the RWGS reaction at medium-low temperatures, providing the possibility to couple the RWGS reactor with a syngas conversion reaction. Such an integrated unit opens the horizons for a direct CO 2 to fuels/chemicals approach. [ABSTRACT FROM AUTHOR]

• Economically viable catalysts for gas phase CO 2 upgrading. • Ni promoted active phases supported on saponite clay as nature inspired catalysts. • Advantageous bimetallic Ni-Cu formulation to suppress methanation. • Remarkable selectivity for low temperature RWGS using Ni-Cu/Saponite. Chemical CO 2 upgrading via reverse water gas shift (RWGS) represents an interesting route for gas phase CO 2 conversion. Herein, nature inspired clay-based catalysts are used to design highly effective materials, which could make this route viable for practical applications. Ni and transition metal promoted Ni saponite clays has been developed as highly effective catalysts for the RWGS. Saponite supported NiCu catalyst displayed a remarkable preference for the formation of CO over CH 4 across the entire temperature range compared to the saponite supported NiCo and Ni catalysts. The NiCu sample is also highly stable maintaining ∼ 55% CO 2 conversion and ∼ 80% selectivity for CO for long terms runs. Very importantly, when compared with reference catalysts our materials display significantly higher levels of CO 2 conversion and CO selectivity. This confirmed the suitability of these catalysts to upgrade CO 2 -rich streams under continuous operation conditions. [ABSTRACT FROM AUTHOR]