Identification of electron-rich mononuclear Ni atoms on TiO2-A distinguished from Ni particles on TiO2-R in guaiacol hydrodeoxygenation pathways

Abstract

The electronic and structural differences of anatase TiO₂ (TiO₂-A) and rutile TiO₂ (TiO₂-R) supported Ni catalysts are studied to understand the distinctively different catalytic pathways in guaiacol hydrodeoxygenation over Ni/TiO₂-A and Ni/TiO₂-R catalysts. Temperature programmed reduction profiles of NiO/TiO₂-A and NiO/TiO₂-R reveal two types of metal–support interactions. NiO on TiO₂-R requires much higher temperature to reduce than NiO on TiO₂-A by hydrogen reduction. Surface TiO₂-A is partially reduced together with NiO at low temperature driven by strong interaction between reduced Ni and the reducible TiO₂-A surface. In situ X-ray photoelectron spectroscopy (XPS), high resolution TEM (HRTEM), and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy of adsorbed CO reveal two types of reduced Ni species on Ni/TiO₂-A: Ni⁰ particles and atomically dispersed electron-rich Ni^δ− atoms. While Ni⁰ particles are formed only at high Ni loadings, they are fully encapsulated by an overlayer of partially reduced TiO_x, making the surface of Ni particles totally inaccessible to CO and to reactants. The electron-rich Ni^δ− atoms are favorably formed at all Ni loadings on the TiO₂-A surface and show only linearly adsorbed CO in the DRIFT spectra. Electron transfer from Ti and O on the TiO₂-A surface to atomically dispersed Ni is indicated by the observation of dominant electron-rich Ni^δ− species in conjunction with Ti^(4+δ)+ and O^(2−δ)− on Ni/TiO₂-A following H₂ reduction. In contrast, the chemical states of metallic Ni⁰ and Ti⁴⁺ remain unchanged on Ni/TiO₂-R after H₂ reduction. The atomically dispersed Ni^δ− species, formed only on the Ni/TiO₂-A surface, is proposed as the active site responsible for the selective hydrodeoxygenation of guaiacol to phenolics. In contrast, the metallic Ni⁰ particles formed on the Ni/TiO₂-R surface catalyze the typical catalytic hydrogenation, preferentially saturating the aromatic ring.