Synergistic Upcycling of NH₃ and CO₂ Gases via an Integrated Tandem Photocatalytic System

Coupled NH₃ and CO₂ upcycling represents a promising strategy for the treatment of NH₃- and CO₂-containing gas streams. Photocatalysis delivering high-potential redox charges enables NH₃ oxidation to N₂ and CO₂ reduction to CO, yet competing pathways and active-site interference usually limit overall performance. Here, we develop a tandem photocatalytic system comprising two spatially separated barium tetratitanate-based modules for NH₃ oxidation coupled with CO₂ reduction. The Ag or RhCrO_x sites on the photocatalysts complementarily regulate competitive CO and H₂ formation and, crucially, do not promote and instead partially suppress the generation of reactive oxygen species responsible for NH₃ overoxidation, while hole-driven NH₃ deprotonation promotes proton-coupled electron transfer for the progression of H₂-evolution and CO₂-reduction intermediates. By harnessing reaction-specific contributions of Ag- and RhCrO_x-modified photocatalyst modules, the tandem system accomplishes effective gas-phase NH₃ removal with ≥92% NH₃ conversion to near-exclusive N₂ and sustains stable CO/H₂ production, outperforming most temperature- and concentration-dependent thermocatalytic and photocatalytic NH₃ oxidation to N₂ processes. The reaction integration and catalyst system design provide a process-intensified and resource-efficient route toward the unified control of pollutant- and CO₂-containing gas streams.