Selective Hydrodeoxygenation of Guaiacol to Phenolics by Ni/Anatase TiO ₂ Catalyst Formed by Cross-Surface Migration of Ni and TiO ₂

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.