The catalytic properties of physical mixtures of Ni particles (100-200 nm) with nanoparticles of anatase TiO 2 (TiO 2 -A), ZrO 2 , Al 2 O 3 , rutile TiO 2 (TiO 2 -R), and CeO 2 were investigated for the hydrodeoxygenation (HDO) of guaiacol. High selectivities to phenolics were obtained only for Ni mixed with anatase TiO 2 (Ni and TiO 2 -A), while saturated hydrocarbons were the main products for the mixtures with other supports. By thermal treatment in hydrogen gas only at 300 °C or higher and subsequently separating the large Ni particles from the TiO 2 -A particles with a magnet, it was further discovered that there was migration of TiO 2 from TiO 2 -A onto the large Ni particles, resulting in an amorphous TiO 2 overlayer on the Ni particles as evidenced by high-resolution TEM, and vice versa, migration of Ni onto TiO 2 -A. The TiO 2 overlayer rendered the Ni particles completely inactive as a hydrogenation/hydrodeoxygenation catalyst. Conversely, the small amounts of Ni (<1.5 wt %) migrated onto TiO 2 -A formed highly dispersed Ni, undetectable by high-resolution TEM (<2 nm), that were remarkably highly active for HDO of guaiacol, producing selectively phenolics. Such highly selective HDO catalysts could also be formed by incipient wetness impregnation of Ni in loadings above 2 wt % onto the TiO 2 -A, but it was essential to pretreat the sample in H 2 at 300 °C or higher. Pretreatment in H 2 at 200 °C generated catalysts that produced saturated ring products. The activity of the impregnated catalysts, as measured by guaiacol conversion, increased linearly with Ni loading below 0.5 wt %. The activity continued to increase with Ni loading but more slowly up to 2 wt %, beyond which there was little further change. The results suggested that two types of Ni species existed on the TiO 2 -A surface. One type consisted of a cluster of Ni atoms that were dominant on larger Ni particles that were active in aromatic ring hydrogenation and hydrodeoxygenation. They were readily covered by reducible TiO 2 -A at 300 °C or higher due to the traditional strong metal support interaction (SMSI) effect and became inactive. Another type was clusters of a very small number of Ni atoms, perhaps one atom, that were present as highly dispersed Ni clusters interacting strongly with the defect sites of TiO 2 -A. The strong interaction of this type of Ni with the TiO 2 defect deterred TiO x migration allowing surface exposed Ni atoms to catalyze the HDO of guaiacol with very high selectivities that were not characteristic of typical Ni particles.
- guaiacol hydrodeoxygenation
- SMSI or strong metal-support interaction
- titania and nickel
ASJC Scopus subject areas