Upgrading CO ₂ and H ₂ O to Analytical-Grade Propanal via an Electrocatalysis–Thermal Catalysis Relay on Single-Atom Catalysts

Sustainable synthesis of analytical-grade propanal from CO₂ and H₂O via an electro-thermal cascade process is highly attractive but remains challenging due to the limited selectivity of CO₂ electroreduction to gaseous products (CO/C₂H₄) and the sluggish kinetics of the subsequent thermal catalytic step at ambient pressure. In this work, we demonstrate a new pathway for the direct synthesis of purification-free analytical-grade propanal via electroreduction-hydroformylation cascade conversion of CO₂ and H₂O over rationally designed single-atom catalysts (SACs). The Sn₁Cu single-atom alloy (SAA) catalyst exhibits an exceptional potential-dependent CO₂ electroreduction selectivity toward C₂H₄ and CO, with the C₂H₄ to CO ratio increasing by 2 orders of magnitude in the potential range from -0.6 to -2.3 V (vs RHE). Results from in situ/operando characterizations and density functional theory (DFT) calculations reveal that the enhanced ethylene selectivity over Sn₁Cu SAA arises from the high *CO coverage generated over a single-Sn-atom-modified Cu site, which promotes the symmetric *CO-*CO coupling, thereby significantly enhancing the electrochemical CO₂ reduction to ethylene. The resulting C₂H₄/CO/H₂ mixture is directly converted in a fixed-bed hydroformylation reactor over a triphenylphosphine-modified Rh SAC (PPh₃-Rh₁/ZnO), achieving an optimized ethylene-to-propanal selectivity of up to 98%. Analytical-grade propanal (∼99%) is obtained without further purification, and stable production was maintained for 200 h with a maximum C₃H₆O rate of 3.8 mg h^-1 cm^-2 under ambient pressure. This work establishes a general framework for integrating electrochemical and thermal catalysis to convert CO₂ and H₂O into value-added aldehydes, offering a sustainable route for synthesizing value-added chemicals from basic feedstocks.