Although Sn-based catalysts have recently achieved considerable improvement in selective electro-catalyzing CO₂ into HCOOH, the role of various valence Sn species is not fully understood due to the complexity and uncertainty of their evolution during the reaction process. Here, inspired by the theoretical simulations that the concomitant multivalent Sn (Sn⁰, Sn^II and Sn^IV) can significantly motivate the intrinsic activity of Sn-based catalyst, the Sn/SnO/SnO₂ nanosheets were proposed to experimentally verify the synergistic effect of multivalent Sn species on the CO₂-into-HCOOH conversion. During CO₂ reduction reaction, the Sn/SnO/SnO₂ nanosheets, which are prepared by the sequential hydrothermal reaction, calcined crystallization and low-temperature H₂ treatment, exhibit a high FE_HCOOH of 89.6% at -0.9 V_RHE as well as a large cathodic current density. Systematic experimental and theoretical results corroborate that multivalent Sn species synergistically energize the CO₂ activation, the HCOO* adsorption, and the electron transfer, which make Sn/SnO/SnO₂ favour the conversion from CO₂ into HCOOH in both thermodynamics and kinetics. This proof-of-concept study establishes a relationship between the enhanced performance and the multivalent Sn species, and also provides a practicable and scalable avenue for rational engineering high-powered electrocatalysts.

Size-controlled synthesis of two-dimensional (2D) catalysts with low stacking numbers and small nanoflake lengths is crucial for promoting the catalytic performance in diverse heterogeneous catalysis. Herein, we report a facile and general "surface curvature-confined synthesis" strategy to modulate the slab lengths and stacking numbers of 2D transition metal sulfides by controlling the strain induced by different surface curvature of supports. An efficient NiMo sulfide with shorter slab length (average 3.71 nm), less stacking number (1-2 layers) and more edge active sites is synthesized onto ZSM-5 zeolites with the average size of 100 nm, which shows superior k_HDS value of dibenzothiophene (14.05 × 10^-7 mol/(g·s)), enhanced stability up to 80 h, and high direct desulfurization selectivity (> 95%). This design concept is also proved to be generally applicable to modulate the slab lengths and stacking numbers of other 2D catalysts such as MoS₂ and WS₂ nanoflakes, which shows great potentials for developing more ultrasmall 2D catalysts with controlled sizes and excellent catalytic activities.