Fe oxide nanoparticles show enhanced electrocatalytic performance in the reduction of CO2 to isopropanol when deposited on an N-functionalized carbon nanotube (CNT) support rather than on a pristine or oxidized CNT support. XRD and high-resolution TEM were used to investigate the nanostructure of the electrocatalysts, and CO2 adsorptive microcalorimetry was used to study the chemical nature of the interaction of CO2 with the surface sites. Although the particles always present the same Fe3O4 phase, their structural anisotropy and size inhomogeneity are consequences of the preparation method of the carbon surface. Two types of chemisorption sites have been determined by using microcalorimetry: irreversible sites (280 kJ mol−1) at the uncoordinated sites of the facets and reversible sites (120 kJ mol−1) at the hydrated oxide surface of the small nanoparticles. N-Functionalization of the carbon support is advantageous, as it causes the formation of small nanoparticles, which are highly populated by reversible chemisorbing sites. These characteristic features correlate with a higher electrocatalytic performance.