Effect of boron nitride overlayers on Co@BNNSs/BN-Catalyzed aqueous phase selective hydrogenation of cinnamaldehyde

The confinement effect known as efficient strategy to enhance the heterocatalytic activity and stability, but a clear view regarding the role of encapsulated overlayers is far from convincing at present, especially, the penetration of substrates with different size. Herein, the experimental evidence about the impacts of BN overlayers on hydrogenation is obtained over Co@BN/BN model catalysts with tuned thinness, in which BN overlayers encapsuled Co particles that dispersed on the defective BN supports, fabricating by the nitridizing of ball-milled BN microplates under NH₃ atmosphere. The thinness and crystallinity of BN shells was simply tuned by controlling the pyrolyzed temperature (600-900 °C). The cinnamaldehyde (CAL) selective hydrogenation is taken as probe reaction due to (1) both larger CAL and small H₂ molecule involved simultaneously, (2) the middle CC and terminal CO bonds all involved. Combined with structural analysis, the results demonstrate that small H₂ molecule can penetrate into the metal-cover interface through the defect sites, then inner Co core dissociate it to atomic H, and the hydrogenation mainly resulting from the spillover of H atoms which occurred on the BN surface. As a result, the thickest BN shells (∼3.4 nm) with ordered-layer-lattice significantly hindered the adsorption and activation of CAL, also the longest H spillover distance led to the lowest activity of Co@BN/BN-900. Instead, Co@BN/BN-600 with merely 2 ∼ 3 overlayers presented the most efficient and highly chemoselective hydrogenation activity. The ultrathin but turbostratic BN overlayers provide more migrating sites also shortened the H spillover distance effectively, and the more favorable CO hydrogenation was also achieved driven by the steric hindrance effect of thinner BN shells. These observations provide new insights towards understanding of confined effect on catalysis process through the accurate regulation of BN shells properties.