Structural Feature and Catalytic Performance of Cu Species Distributed over TiO<sub>2</sub> Nanotubes

Copper oxide was deposited on tubular TiO<sub>2</sub> via Cu<sup>2+</sup> introduction into a titanate nanotube aggregate followed by calcination. The titanate has a layered structure allowing Cu intercalation and can readily transform into anatase TiO<sub>2</sub> 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 NH<sub>3</sub> was compared with those of other 2 wt % Cu/TiO<sub>2</sub> catalysts using TiO<sub>2</sub> 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 TiO<sub>2</sub> supports are in the +2 state. Extended X-ray absorption fine structure (EXAFS) investigations of these catalysts reflected higher degrees of CuO dispersion and Cu<sup>2+</sup> dissolution into the TiO<sub>2</sub> lattice for the tubular Cu/TiO<sub>2</sub> 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 Cu<sup>2+</sup> to form a Cu<i><sub>x</sub></i>Ti<sub>1-</sub><i><sub>x</sub></i>O<sub>2</sub> 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 Cu<sup>2+</sup> dissolution was obtained for the tubular catalysts while only 20% was obtained for the particulate catalysts. The Cu<i><sub>x</sub></i>Ti<sub>1-</sub><i><sub>x</sub></i>O<sub>2</sub> species were considered one form of the active sites on the Cu/TiO<sub>2</sub> catalysts.