Selective Hydrodeoxygenation of Guaiacol to Phenolics by Ni/Anatase TiO2 Catalyst Formed by Cross-Surface Migration of Ni and TiO2
The catalytic properties of physical mixtures of Ni particles (100–200 nm) with nanoparticles of anatase TiO2 (TiO2-A), ZrO2, Al2O3, rutile TiO2 (TiO2-R), and CeO2 were investigated for the hydrodeoxygenation (HDO) of guaiacol. High selectivities to phenolics were obtained only for Ni mixed with anatase TiO2 (Ni and TiO2-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 TiO2-A particles with a magnet, it was further discovered that there was migration of TiO2 from TiO2-A onto the large Ni particles, resulting in an amorphous TiO2 overlayer on the Ni particles as evidenced by high-resolution TEM, and vice versa, migration of Ni onto TiO2-A. The TiO2 overlayer rendered the Ni particles completely inactive as a hydrogenation/hydrodeoxygenation catalyst. Conversely, the small amounts of Ni (<1.5 wt %) migrated onto TiO2-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 TiO2-A, but it was essential to pretreat the sample in H2 at 300 °C or higher. Pretreatment in H2 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 TiO2-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 TiO2-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 TiO2-A. The strong interaction of this type of Ni with the TiO2 defect deterred TiOx migration allowing surface exposed Ni atoms to catalyze the HDO of guaiacol with very high selectivities that were not characteristic of typical Ni particles.
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