Co-based materials usually undergo in-situ surface reconstruction during oxidation reactions, forming high-valent Co³⁺/Co⁴⁺ species as the true active sites. However, conventional bulk structures of Co-based materials hinder deep phase transformation, limiting the utilization of internal Co sites and suppressing catalytic efficiency. Here, we report the hollow engineering of cobalt phosphide (CoP) to facilitate exposure of Co sites and promote in-situ transformation to Co³⁺/Co⁴⁺ active species for enhanced oxidation activity. Hollow CoP (H-CoP) is derived from ZIF-67 via controlled etching and phosphorization, with electrochemically active surface area 2.1 times that of conventional solid CoP (S-CoP). H-CoP achieves a current density of 10 mA·cm⁻² at a lower potential (1.26 V vs. reversible hydrogen electrode (RHE)) in 5-hydroxymethylfurfural oxidation reaction (HMFOR), with a HMF conversion of 99.5%, 2,5-furandicarboxylic acid yield of 98.6%, and Faraday efficiency of 97.5% at 1.45 V (vs. RHE), much superior to S-CoP. When applied as a bifunctional catalyst in the HMFOR coupled with hydrogen evolution reaction (HER) electrolyzer, H-CoP requires an ultralow voltage of 1.64 V to reach 10 mA·cm⁻², with the cell voltage reduced by 190 mV compared to the conventional oxygen evolution reaction coupled with HER water splitting system.

Nitrogen-rich zeolitic imidazolate frameworks (ZIFs) are ideal precursors for the synthesis of metal single atoms anchored on N-doped carbon. However, the microporous structures of conventional ZIFs lead to low mass transfer efficiency and low metal utilization of their derivatives. Here, we construct a composite of Co single atoms anchored on nitrogen-doped carbon with a three-dimensional ordered macroporous structure (Co-SA/3DOM-NC) by two-step pyrolysis of ordered macro/microporous ZnCo-ZIF. Co-SA/3DOM-NC shows high activity in the oxidative esterification of furfural, achieving a 99% yield of methyl 2-furoate under mild reaction conditions, which is significantly superior to the microporous and the Co-nanoparticle counterparts. The high activity of Co-SA/3DOM-NC should be attributed to the CoN₄ centers with high intrinsic activity and the ordered macroporous structure, promoting the mass transfer of reactants and accessibility of active sites.

The preparation of supported high-density metal nanoparticles (NPs) is of great importance to boost the performance in heterogeneous catalysis. Thermal transformation of metal-organic frameworks (MOFs) has been demonstrated as a promising route for the synthesis of supported metal NPs with high metal loadings, but it is challenge to achieve uniform metal dispersion. Here we report a strategy of “spatial isolation and dopant anchoring” to resist metal aggregation in the pyrolysis of MOFs through converting a bulk MOF into dual-heteroatom-containing flower-like MOF sheets (B/N-MOF-S). This approach can spatially isolate metal ions and increase the number of anchoring sites, thus efficiently building physical and/or chemical barriers to cooperatively prevent metal NPs from aggregation in the high-temperature transformation process. After thermolysis at 1,000 °C, the B/N-MOF-S affords B,N co-doped carbon-supported Co NPs (Co/BNC) with uniform dispersion and a high Co loading of 37.3 wt.%, while untreated bulk MOFs yield much larger sizes and uneven distribution of Co NPs. The as-obtained Co/BNC exhibits excellent electrocatalytic activities in both hydrogen evolution and hydrazine oxidation reactions, and only a voltage of 0.617 V at a high current density of 100 mA·cm⁻² is required when applied to a two-electrode overall hydrazine splitting electrolyzer.