**4. Conclusions**

The race for the development of low-cost and high-performing CO2 methanation catalysts stems from the need to efficiently convert excess electricity and H2 generated from renewables, as well as CO2 captured from flue gases, into a reliable energy carrier. Ni is the standard option to be used in CO2 methanation catalysts, due to its high activity and low cost. However, insufficient low-temperature activity and the degradation of Ni catalysts over time due to oxidation and sintering creates the need for the employment of specific metal additives to counter such drawbacks. These additives can fall in two generalized categories: other transition metals (including Fe and Co) and noble metals (including Ru, Rh, Pt, Pd and Re).

The transition metals Fe and Co offer the obvious advantage of being cheap like Ni and their similar size and electronic properties allow for their intricate interaction with the Ni primary phase and their easy dissolution into the Ni lattice, forming NiFe and NiCo alloys, respectively. The composition of the formed alloy is of grea<sup>t</sup> importance, since only specific bimetallic combinations can lead to an optimal CO2 methanation performance, especially in the case of NiFe alloys. The combined bimetallic catalysts can also offer additional advantages, such as higher stability, as well as resistance towards oxidation and sulphur poisoning.

Noble metals generally increase the reducibility and dispersion of the Ni primary phase and they can also participate in the reaction as active CO2 methanation phases. Stand-alone Ru catalysts are highly active for low-temperature CO2 methanation and the presence of Ru in bimetallic Ni catalysts as a separate monometallic phase also boosts catalytic activity. Additionally, the cost-effectiveness of Ru compared to other noble metals renders the bimetallic NiRu combinations quite popular in the field of heterogeneous catalysis. Rh and Pt can also greatly enhance the catalytic activity for CO2 methanation when dissolved or deposited upon Ni in small quantities. Lastly, Pd and Re have been also tested as potential promoters in Ni-based catalysts.

The assumed trade-off between cost and catalytic activity for CO2 methanation catalysts can be potentially overcome via the development of bimetallic Ni-containing catalysts with an optimised Ni–dopant metal synergy. Recent advances in operando spectroscopic techniques can shed light on how the reaction mechanism differs between Ni-based alloys or Ni–dopant metal interfaces and monometallic Ni, allowing for the development of catalysts with the lowest possible cost and highest possible performance.

**Author Contributions:** Conceptualization, A.I.T.; Data curation, A.I.T.; Formal analysis, A.I.T.; Funding acquisition, I.V.Y., M.A.G.; Investigation, Methodology, M.A.G.; Project administration, M.A.G., I.V.Y.; Project coordination, I.V.Y.; Resources, M.A.G.; Supervision, N.D.C.; Writing—original draft, A.I.T., N.D.C.; Writing—review and editing, N.D.C., I.V.Y. and M.A.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research has been co-financed by the European Union and Greek national funds through the operational program "Regional Excellence" and the operational program Competitiveness, Entrepreneurship and Innovation, under the call Research—Create—Innovate (Project code: T1EDK-00782).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing is not applicable to this article as no new data were created or analyzed in this study.

**Conflicts of Interest:** The authors declare no conflict of interest.
