Preliminary reduction strategy: activating electron-rich polyoxometalates precursors for superior hydrodesulfurization.

The precise construction of abundant CoMoS active phases with a high degree of sulfidation is essential to enhance the efficiency of hydrodesulfurization (HDS). Herein, the ε-Keggin-type reduced-state polyoxometalates (POMs) have been utilized as precursors to obtain small-sized (about 3 nm) CoMoS catalysts in hollow mesoporous carbon spheres (HMCS) by a "preliminary reduction" strategy (pre-reduction strategy) (S-Co4Mo12/HMCS). Among them, the Mo(V) produced by the POMs through pre-reduction facilitate more sufficient sulfidation of Mo, thereby enhancing the intrinsic activity of the catalyst. The high electronegativity of POMs promotes the coordination of more Co2+ to produce [MoV12O28(OH)12{Co(H2O)3}4] (Co4Mo12) with high Co/Mo ratio, which promotes the bimetallic synergistic to generate the abundant CoMoS active phases and improves the apparent activity of the catalyst. Meanwhile, the moderate metal-support interactions between the active components and the HMCS support could effectively regulate the sulfidation process and significantly enhance the active phase formation of the catalyst. The deeply reduced POMs are highly dispersed by the pore confinement effect of HMCS, producing highly dispersed CoMoS active phase structure with short slabs length to sufficiently expose the "corner" sites and promoting the desulfurization of macromolecular sulfur-containing compounds. In the subsequent performance tests, the S-Co4Mo12/HMCS catalyst exhibited excellent performance with 99.6% and 87.6% conversion of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT), respectively. Therefore, the "pre-reduction" strategy regulates the electronic structure of reduced POMs to produce atomically optimized CoMoS catalyst, which establishes a novel approach for the development of efficient HDS catalysts.