**4. Conclusions**

The commercial large-scale synthesis of a well-characterized NRL iron-based catalyst and its performance in a commercial-scale modular fixed-bed reactor prototype was evaluated in this test series. A significant reduction in CO2 conversion and change in product selectivity at the large scale suggested the commercially scaled-up synthesized catalyst (Fe:M2-3) was significantly less active than previous catalysts synthesized and tested at the laboratory scale (Fe:M2-2). The reduced catalyst activity was further substantiated spectroscopically by the confirmation of lower iron loadings and differences in catalyst support characteristics used in the large-scale synthesis of the catalyst. The ability to recycle a portion of the product stream at this scale provided a solution to overcome challenges associated with the lower catalyst activity. CO2 conversion increased from 26% to as high as 69% and the product selectivity shifted from 45% CO to 9% CO in favor of C2–C5+ hydrocarbon production upon recycling the effluent product stream.

The results presented serve to highlight the potential challenges associated with scaling-up materials and processes for commercial implementation. They also suggest that the iron-based catalyst with the higher metal loading will have even better performance characteristics in the commercial-scale prototype reactor. The modeling and kinetic analysis using kinetics established at the laboratory support trends associated with reaction temperatures and FT reaction rates. The higher loading catalyst and the endothermic cooling associated with the RWGS reaction will be the subject of further evaluations as NRL pursues determination of the feasibility of producing olefin intermediates from CO2 and H2 as the first step of a two-step thermochemical process to produce operational fuel for the military and commercial applications.

**Author Contributions:** All authors assisted in the collection of the data, the data analysis, and the preparation of the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Office of Naval Research both directly and through the Naval Research Laboratory and OPNAV N45.

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