Shifting nitrogen atoms by angstroms multiplies hydrogen production sixfold

Move a nitrogen atom a fraction of a nanometer, and a solar-powered hydrogen factory becomes six times more productive. That is the central finding from a team at the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences, where researchers have developed a strategy they call nitrogen-shift engineering to control exactly how platinum catalyst particles anchor inside porous organic materials.

The study, published in Angewandte Chemie International Edition, demonstrates that the precise position of nitrogen anchoring sites within covalent organic frameworks (COFs) determines whether platinum deposits as isolated single atoms, as metallic clusters, or as both. That distinction, it turns out, makes an enormous difference in how efficiently the material splits water into hydrogen fuel using sunlight.

Four frameworks, one topology, different nitrogen positions

COFs are crystalline, porous materials built from organic molecules linked together in repeating patterns. Their modular construction makes them attractive for catalysis because researchers can, in principle, tune their properties by swapping in different molecular building blocks. Prof. Zhou Xukai and his colleagues exploited this tunability by designing four COFs that shared identical hexagonal pore structures but differed in the positions of their nitrogen atoms.

Two of the frameworks used olefin (carbon-carbon double bond) linkages, designated COF-A1 and COF-A2. The other two used imine (carbon-nitrogen double bond) linkages, designated COF-I1 and COF-I2. The imine-linked versions placed nitrogen atoms at specific positions along the framework walls, while the olefin-linked versions kept nitrogen in different locations. The spatial differences between these isomers amounted to mere angstroms.

Dual active sites from a single deposition step

When the researchers deposited platinum onto these frameworks using light (a process called photodeposition), the nitrogen positions dictated the outcome. COF-A2, with its olefin linkages, primarily stabilized platinum as isolated single atoms. COF-I2, with its imine linkages, simultaneously stabilized both single platinum atoms carrying a 2+ charge and metallic platinum clusters. This dual-site arrangement emerged naturally from the framework geometry rather than requiring any special preparation.

The performance gap was stark. COF-I2 loaded with platinum produced hydrogen at 6.1 times the rate of COF-A2 with platinum. Under monochromatic 420-nanometer light, COF-I2-Pt achieved an apparent quantum efficiency of 12.1%, a strong result for an organic photocatalyst.

Why two types of platinum outperform one

Mechanistic studies revealed that the synergy between platinum clusters and single atoms was essential. The charge redistribution between the two types of platinum sites promoted the separation of photogenerated electron-hole pairs, the fundamental step in converting light energy into chemical energy. Better charge separation meant more electrons were available to reduce protons into hydrogen gas, and fewer were lost to recombination.

Femtosecond transient absorption spectroscopy, which tracks charge movement on timescales of quadrillionths of a second, confirmed that the key charge-separated state in COF-I2-Pt lasted significantly longer than in the other frameworks. This prolonged lifetime gave the electrons more time to reach the catalytic sites and do useful chemistry.

A strategy, not just a catalyst

The broader significance lies in the design principle rather than any single catalyst. By using constitutional isomers, molecules with the same atoms and bonds but different spatial arrangements, the researchers demonstrated a method for precisely controlling how metal cocatalysts interact with their supports at the atomic level. This level of control has been difficult to achieve with conventional catalyst materials, where surface heterogeneity makes it hard to predict where metal atoms will land.

The approach is not limited to hydrogen production or to platinum. The concept of nitrogen-shift engineering could be applied to other porous framework materials and other metal catalysts, potentially improving performance in carbon dioxide reduction, nitrogen fixation, or organic synthesis reactions.

From laboratory to practical energy conversion

Photocatalytic hydrogen evolution remains largely a laboratory technology. While the quantum efficiency achieved here is notable, practical solar hydrogen production would need to work with broader-spectrum sunlight, maintain stability over thousands of hours, and scale to large areas. This study does not address those engineering challenges directly. The platinum loading, while efficient, still relies on a precious metal that is expensive and geographically concentrated.

Still, understanding the atomic-level interactions between catalysts and their supports is a prerequisite for rational design of better systems. The ability to predict and control whether a metal deposits as single atoms, clusters, or nanoparticles by adjusting nitrogen positions at angstrom scale represents a meaningful step in that direction.