Ceria nanoparticles (average particle size: 8 nm) have been obtained by the calcination of alginate aerogel beads that were precipitated from aqueous solutions of (NH4)2Ce(NO3)6. These nanoparticles were considerably more active as a catalyst for CO2-insertion into aliphatic α,ω-diamines than the analogous commercial CeO2 with larger particle size (40 nm). CeO2 that was obtained by templating with the natural alginate biopolymer afforded the cyclic urea of ethylenediamine in EtOH solvent at 160 °C in 37 % yield. This yield is remarkable for a process that involves CO2 as a feedstock. Other α,ω-diamines, such as diethylenetriamine, N,N′-dimethylethylenediamine, N-(2-aminoethyl)acetamide, and 1,4-diaminobutane, also formed their corresponding cyclic ureas in 4–36 % yield. The catalyst lost activity upon reuse, thereby leading to severe deactivation that was only partially recovered by washing with aqueous acidic solutions.