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Illuminating thermal catalysts with visible light is an effective strategy to reduce the thermal requirements of CO 2 methanation. In this study, we systematically varied the cobalt loading and properties of xCo/CeO 2 catalysts (between 0 and 10 wt%) to understand changes in the visible light-assisted reaction mechanism with cobalt loading. 10Co/CeO 2 had the highest CO 2 conversion of 90% at 450 °C. The light promoted the CO 2 conversion of all catalysts from 300 to 450°C, peaking for 7.5Co/CeO 2 with a 125% improvement relative to thermal conditions (300 °C) before diminishing for 10Co/CeO 2. The light facilitated the conversion of the formate intermediate adsorbed onto CeO 2. In-situ DRIFTS and DFT unveiled a particle size trade-off between maximising CO 2 adsorbed at the Co-Ce interface while minimising CO 2 adsorbed onto cobalt, which is required for the best light enhancement. These findings underscore the importance of careful deposit size optimisation to unlock the light-assisted methanation's full potential. [Display omitted] • Contribution of visible light can be maximised with an optimal deposit size. • CO 2 adsorption onto large cobalt deposits limits the contribution of visible light. • Trade-off between CO 2 and H 2 on cobalt deposits required for best photo-enhancement. • One-step flame spray pyrolysis made Co/CeO2 with controllable deposit sizes by varying Co loading. [ABSTRACT FROM AUTHOR]

The electrocatalytic recycling of waste nitrate (NO3RR) is a promising decentralized route for green ammonia synthesis. Nonetheless, it suffers from the competing hydrogen evolution reaction and the insufficient proton supply in high pH conditions. Herein, iron oxide nanoparticles anchored on ZnO is introduced as a strategy to enhance the water dissociation ability and proton transfer rate, advancing NH4+ production from alkaline NO3RR. Supported by a set of ex situ and in situ characterization, the findings reveal the reduction of iron oxides, along with improvements in charge transfer properties and proton generation from H2O. Theoretical calculations show that iron oxides reduce the kinetic barrier of the rate‐limiting step (*NO2‐to‐*NO2H) and result in a thermodynamically favorable process to hydrogenation steps, which in turn reduce the overall energy barrier of alkaline NO3RR. Optimal catalytic activity is realized with a Fe loading of 0.5 wt.%, delivering a Faradaic efficiency of ≈83% for ammonium with a NH4+ yield rate of 31 nmol s−1 cm−2 at −0.7 V versus RHE. The results pave the way for the utilization of bi‐metal interaction to tune the reaction pathway for achieving sustainable ammonium synthesis in alkaline, contributing to ongoing efforts to achieve a sustainable nitrogen cycle via N‐based electrochemistry. [ABSTRACT FROM AUTHOR]