Direct synthesis of H2O2 over Pd–M@HCS (M = Sn, Fe, Co, or Ni): effects of non-noble metal M on the electronic state and particle size of Pd

Abstract

For the mono-metal Pd based catalysts, the O–O bond in O₂ and H₂O₂ is easily dissociated due to the higher energy sites of Pd, which decreases the selectivity and productivity of H₂O₂. To address this issue, non-noble metals M (M = Sn, Fe, Co, Ni) were doped into palladium precursors to create a series of yolk–shell structure bimetallic catalysts Pd–M@HCS (H = hollow; C = carbon; S = sphere) with Pd–M as the core and porous carbon as the shell. The results indicated that the catalyst Pd–M@HCS doped with Sn, Fe, Co, and Ni has a weak dissociation ability for O₂, and both the adsorption and dissociation ability for H₂ are improved. Of these catalysts, the catalyst Pd–Sn@HCS gives the best performance, which is benefited by the suitable Pd–Sn nanoparticle size and the best Pd²⁺/Pd⁰ ratio. The influence of the n_Pd/n_Sn ratio in the Pd–Sn@HCS system on catalyst performance was further explored, and the findings revealed that Pd–Sn@HCS had the best catalytic activity and stability when the n_Pd/n_Sn ratio was 2. The catalyst's H₂ conversion, H₂O₂ selectivity, and productivity were 35%, 97%, and 3961 mmol g_Pd⁻¹ h⁻¹, respectively. After 10 cycles, the H₂O₂ productivity of this catalyst remained at 92.1%, which was greater than the H₂O₂ productivity of the typical Pd-based catalyst Pd/C, which stayed at 37% under the same conditions.