Selective Plasmon-Induced CO₂ Reduction Using AuPd Alloy Nanoparticle Catalysts

Photocatalytic CO₂ reduction – artificial photosynthesis – is an attractive method of sustainably using solar power to convert captured CO₂ to recycled fuels or longer-lived organic chemicals. However, a highly active and selective catalyst has yet to be developed. Plasmonic nanostructures are promising as photocatalysts for both efficiently driving CO₂ reduction reactions as well as offering tunable selectivity by altering the chemical mechanisms with injections of optically-excited hot carriers. Recently, plasmonic metals have been combined with more traditionally catalytic metals in bimetallic systems that exhibit both the desired optical properties of the plasmonic metal and the molecular interactions of the catalytic metal.<br/>Here, we tuned the composition of AuPd alloy nanoparticles and demonstrated the effect of Au concentration on its photocatalytic behavior for CO₂ reduction. Au was chosen for its strong, tunable, localized surface plasmon resonance (LSPR) in the visible, where it can efficiently absorb incident light and through decay of its LSPR convert that energy into excited charge carriers that then drive CO₂ chemistry. Au also has a favorably moderate CO adsorption energy, strong enough to bind chemically-related CO₂ molecules and catalyze its reduction but weak enough to not to poison its surface with CO or other reaction products. Pd was chosen for its strong hydrogenation ability in order to easily study selectivity differences when using H₂ as a reductant. Small spherical bimetallic AuPd nanoparticles (~5-10 nm) were colloidally synthesized with Au concentrations ranging from 80-99%. After loading the nanoparticles (~4 wt%) onto an optically inactive 180-mesh Al₂O₃ support, we inserted the combined catalyst particles into a photoreactor consisting of a Harrick Raman high temperature reaction chamber coupled with gas flow and tunable laser light. To isolate the activation of the rate-determining step of CO₂ reduction, we flowed CO₂ only through a packed bed of supported AuPd nanoparticles to directly convert CO₂ to CO without addition of a chemical reductant. We compared the turnover frequency (TOF) of catalysts as they increased in Au content and exhibited a stronger LSPR and subsequent response to illumination at the plasmon resonance energy. At higher Au concentrations, we expect higher rates of thermal-only conversion from CO₂ to CO without a reductant, which is further increased with illumination that peaks at the LSPR of 520 nm. We then added H₂ as a reductant to demonstrate tunable optical control of selectivity between CO and CH₄ production in Au-rich catalysts based on the illumination power and wavelength. Upon addition of H₂, we expect to see a similar behavior in the form of increased selectivity towards CO production with increased Au, also enhanced with LSPR excitation. However, with higher Pd concentrations, where the nanoparticle has a broad absorption across the entire visible range, we have observed that illumination only causes photothermal heating, increasing overall activity and the selectivity towards endothermic CO production slightly, to the same degree as heating the catalyst bed. This indicates that the change in selectivity observed with highly-plasmonic alloys is likely a photochemical effect induced by the strong LSPR of the Au component. With these results, we hope to inform future CO₂ reduction catalyst design by demonstrating the ability to tune the selectivity of bimetallic catalysts by incorporating plasmonic metals and optical excitation.