*Article* **Hydrothermal CO2 Reduction by Glucose as Reducing Agent and Metals and Metal Oxides as Catalysts**

**Maira I. Chinchilla , Fidel A. Mato, Ángel Martín and María D. Bermejo \***

High Pressure Process Group, Department of Chemical Engineering and Environmental Technology, BioEcoUva Research Institute on Bioeconomy, Universidad de Valladolid, 47011 Valladolid, Spain; mairaivette.chinchilla@alumnos.uva.es (M.I.C.); fidel@iq.uva.es (F.A.M.); mamaan@iq.uva.es (Á.M.) **\*** Correspondence: mdbermejo@iq.uva.es

**Abstract:** High-temperature water reactions to reduce carbon dioxide were carried out by using an organic reductant and a series of metals and metal oxides as catalysts, as well as activated carbon (C). As CO2 source, sodium bicarbonate and ammonium carbamate were used. Glucose was the reductant. Cu, Ni, Pd/C 5%, Ru/C 5%, C, Fe2O3 and Fe3O4 were the catalysts tested. The products of CO2 reduction were formic acid and other subproducts from sugar hydrolysis such as acetic acid and lactic acid. Reactions with sodium bicarbonate reached higher yields of formic acid in comparison to ammonium carbamate reactions. Higher yields of formic acid (53% and 52%) were obtained by using C and Fe3O4 as catalysts and sodium bicarbonate as carbon source. Reactions with ammonium carbamate achieved a yield of formic acid up to 25% by using Fe3O4 as catalyst. The origin of the carbon that forms formic acid was investigated by using NaH13CO3 as carbon source. Depending on the catalyst, the fraction of formic acid coming from the reduction of the isotope of sodium bicarbonate varied from 32 to 81%. This fraction decreased in the following order: Pd/C 5% > Ru/C 5% > Ni > Cu > C ≈ Fe2O3 > Fe3O4.

**Keywords:** hydrothermal reaction; CO2 conversion; glucose; metal catalysts; metal oxide catalysts
