Synergistic Upcycling of NH 3 and CO 2 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 RhCrOx 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 RhCrOx-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.