Structural Feature and Catalytic Performance of Cu Species Distributed over TiO₂ Nanotubes

Copper oxide was deposited on tubular TiO₂ via Cu²⁺ introduction into a titanate nanotube aggregate followed by calcination. The titanate has a layered structure allowing Cu intercalation and can readily transform into anatase TiO₂ via calcination for condensation of the constituting layers. The activity of the tubular catalysts, with a Cu content of 2 wt %, in selective NO reduction with NH₃ was compared with those of other 2 wt % Cu/TiO₂ catalysts using TiO₂ nanoparticles as the support. The Cu species supported on the nanotubes showed a higher activity than those supported on the nanoparticles. X-ray absorption near-edge structure (XANES) analysis showed that the Cu species on all the TiO₂ supports are in the +2 state. Extended X-ray absorption fine structure (EXAFS) investigations of these catalysts reflected higher degrees of CuO dispersion and Cu²⁺ dissolution into the TiO₂ lattice for the tubular Cu/TiO₂ catalysts. Absence of CuO bulk detection by a temperature-programmed reduction analysis for the tubular catalysts confirmed the high CuO-dispersion feature of the tubular catalysts. The dissolution of Cu²⁺ to form a Cu_xTi_1-xO₂ type of solid solution was improved by using an in-situ ion-intercalation method for Cu deposition on the nanotubes. A fraction as high as 40% for Cu²⁺ dissolution was obtained for the tubular catalysts while only 20% was obtained for the particulate catalysts. The Cu_xTi_1-xO₂ species were considered one form of the active sites on the Cu/TiO₂ catalysts.