Synergistic Upcycling of NH ₃ and CO ₂ Gases via an Integrated Tandem Photocatalytic System

Coupled NH 3 and CO 2 upcycling represents a promising strategy for the treatment of NH 3 - and CO 2 -containing gas streams. Photocatalysis delivering high-potential redox charges enables NH 3 oxidation to N 2 and CO 2 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 NH 3 oxidation coupled with CO 2 reduction. The Ag or RhCrO x sites on the photocatalysts complementarily regulate competitive CO and H 2 formation and, crucially, do not promote and instead partially suppress the generation of reactive oxygen species responsible for NH 3 overoxidation, while hole-driven NH 3 deprotonation promotes proton-coupled electron transfer for the progression of H 2 -evolution and CO 2 -reduction intermediates. By harnessing reaction-specific contributions of Ag- and RhCrO x -modified photocatalyst modules, the tandem system accomplishes effective gas-phase NH 3 removal with ≥92% NH 3 conversion to near-exclusive N 2 and sustains stable CO/H 2 production, outperforming most temperature- and concentration-dependent thermocatalytic and photocatalytic NH 3 oxidation to N 2 processes. The reaction integration and catalyst system design provide a process-intensified and resource-efficient route toward the unified control of pollutant- and CO 2 -containing gas streams.