**4. Conclusions**

In this contribution, an industrially relevant experimental approach was used to upgrade UCO to diesel-like hydrocarbons in order to compare the performance of Ni catalysts promoted with Cu, Fe or Pt. Results indicate that all catalysts tested display and retain the ability to fully deoxygenate the feed to hydrocarbons through the entirety of the time period investigated. However, the cracking activity of Ni-Cu is relatively low and stable throughout, that of Ni-Fe drops with TOS, and that of Ni-Pt is higher, variable, and is still high at the end of a 76 h run. Analysis of the fresh and spent catalysts helps explain these trends and their underlying structure-activity relationships. In the case of the Ni-Pt catalyst, neither coking, fouling, and metal particle sintering—nor changes in the bulk or surface composition of the metal particles—reduce the cracking activity of this formulation in the time period investigated. In contrast, the cracking activity of both the Ni-Cu and the Ni-Fe catalysts decrease with TOS, this decrease being more pronounced for the Fe-promoted formulation. Based on TEM-EDS data, this reduction in cracking activity can be attributed to an increased degree of alloying between Ni and Cu or Fe, the formation of Ni-Cu and Ni-Fe alloys disrupting the adjacency of Ni atoms required for C–C hydrogenolysis. In short, results sugges<sup>t</sup> that Cu- and Fe-promoted catalysts are preferable to Ni-Pt formulations, which are rendered disadvantageous by the fact that their higher price and cracking activity would reduce the cost and carbon e fficiency of a process designed to convert UCO to diesel-like hydrocarbons.

**Author Contributions:** Individual contributions are as follows: conceptualization—E.S.-J. and M.C.; methodology—G.C.R.S., E.S.-J., and M.C.; validation—G.C.R.S.; formal analysis—G.C.R.S., and O.H.; investigation—G.C.R.S., O.H., D.Q., and R.P.; resources—E.S.-J., M.C., and O.H.; data curation—O.H.; writing—original draft preparation—E.S.-J., and G.C.R.S.; writing—review and editing—E.S.-J., and M.C..; visualization—G.C.R.S., and E.S.-J.; supervision—M.C.; project administration—G.C.R.S., E.S.-J., and M.C.; funding acquisition—G.C.R.S., E.S.-J., M.C., and G.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported in part by the National Science Foundation (grant No. 1437604); by the J. William Fulbright Foreign Scholarship Board and the Bureau of Educational and Cultural A ffairs of the United States Department of State through a Fulbright Scholarship awarded to G.S; and by the French National Center for Scientific Research (CNRS) Graduate School EIPHI (contract ANR-17-EURE-0002) and through a CNRS visiting researcher position granted to E.S.-J.

**Acknowledgments:** Sarah Cummins and the Redwood Cooperative School in Lexington, Kentucky, as well as Jennifer Wyatt and personnel from the Lexington-Fayette Urban Country Government, are thanked for collecting and delivering the used cooking oil used in this study. Tonya Morgan is thanked for her help with the GC-MS analysis of liquid reaction products.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
