We show that in some cases, the metallic impurities within residual catalyst metallic nanoparticle impurities, which remain in carbon nanotubes from their synthesis even after purification, are responsible for "electrocatalytic" properties of carbon nanotubes. This is demonstrated in an example of double-walled carbon nanotubes (DWCNTs) containing cobalt residual catalyst nanoparticle impurities, which themselves contain iron-based impurities. It was found that iron-based impurities within Co residual nanoparticles present in DWCNTs are responsible for the electrochemical reduction of hydrogen peroxide. The chemical states of metallic impurities were investigated by high-resolution X-ray photoelectron spectroscopy.