Co–Ti Bimetallic Complex-Induced Phase Modulation of Co@Black TiO2 for Catalytic Hydrogenation of Cinnamaldehyde

Transition-metal-doped black titania, primarily in the anatase phase, shows promise for redox reactions, water splitting, hydrogen generation, and organic pollutant removal, but exploring other titania phases for broader catalytic applications is underexplored. This study introduces a synthetic approach using a Co-Ti bimetallic complex bridged by a 1,10-phenanthroline-5,6-dione ligand as a precursor for the synthesis of cobalt-doped black titania [Co@L2N@b-TiO₂]. The synthesis involves precise control of pyrolysis conditions, yielding a distinct structure dominated by the rutile phase over anatase, with active cobalt encapsulated within a nitrogen-doped graphitic layer, primarily as Co⁰ rather than Co^II and Co^III. The synthesized material is employed for the selective hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol (COL) under industrially viable conditions. The efficiency and selectivity of Co@L2N@b-TiO₂ was compared with other catalysts, including cobalt-doped rutile TiO₂ (Co@r-TiO₂), anatase TiO₂ (Co@a-TiO₂), and black titania (Co@b-TiO₂) as well as materials pyrolyzed under different atmospheres and temperatures, materials with phenanthroline ligands, and materials lacking any ligands. The superior performance of Co@L₂N@b-TiO₂ is attributed to its high surface area, stable Co⁰ within the nitrogen-doped graphitic layer, and composition of rutile and anatase phases of TiO₂ and Ti₂O₃ (referred to as RAT), along with the synergistic interaction between RAT and Co⁰. These factors significantly influence the efficiency and selectivity of COL over hydrocinnamaldehyde (HCAL) and hydrocinnamyl alcohol (HCOL), indicating potential for broader applications beyond catalysis, particularly in designing of black titania-based materials.