Kinetic Matching of Plastic-Derived Lactate Oxidation and Nitrogen Reduction for Selective Photocatalytic Alanine Synthesis

Abstract

Photocatalytic conversion of plastic-derived oxygenates and sustainable nitrogen sources into amino acids offers an attractive route for artificial photosynthesis and waste upcycling. However, selective C–N coupling is strongly constrained by the kinetic mismatch between rapid carbon oxidation and sluggish nitrogen reduction, which causes carbonyl intermediate accumulation and undesired side reactions. Herein, we demonstrate that this limitation can be addressed by kinetically matching lactate oxidation with nitrogen-source reduction over a Co-modified Cd-vacancy-rich CdS photocatalyst. Using CdS/NiAl-layered double hydroxides (LDHs) as a benchmark system, we first show that the excessive oxidation of plastic-derived lactate leads to pyruvate accumulation and limited alanine selectivity, whereas suppressing lactate overoxidation with formic acid markedly improves alanine formation. To develop a sacrificial-agent-free system, Co-dCdS was constructed by introducing Cd vacancies and highly dispersed Co sites. The Cd vacancies enhance charge separation and regulate carbon-oxidation kinetics, while Co sites promote nitrogen-source activation and interfacial electron transfer, thereby synchronizing the generation of pyruvate and reactive nitrogen intermediates for selective reductive amination. As a result, Co-dCdS enables alanine synthesis from lactate and N2 with a selectivity of 81.7% without any sacrificial reagent. The same kinetic-matching principle is further extended to nitrate reduction, where Co-dCdS maintains alanine selectivities above 67% over a broad lactate/nitrate concentration window and reaches 73.8% under optimized conditions. This work establishes kinetic compatibility between carbon oxidation and nitrogen reduction as a key design principle for photocatalytic amino acid synthesis from plastic-derived substrates and sustainable nitrogen sources.