Plasmonic effects on CO2 reduction over bimetallic Ni-Au catalysts

Abstract

The anthropogenic rise of carbon emissions has led to increased interest in the innovative utilisation of CO2. CO2 reduction via the Sabatier reaction presents a thermodynamically favourable pathway to reduce CO2; however, to date, there has been a lack of understanding of how plasmonic enhancement may be utilised for this reaction particularly with respect to the role of bimetallic catalysts. In this study, the effect of laser light illumination on Ni-Au/SiO2, at 520 nm and 660 nm, was studied for CO2 reduction over a temperature range of 150 °C to 450 °C, with the catalytic performance being assessed by CO2 conversion along with CH4 and CO selectivity. Under laser light illumination, at 520 nm and 450 °C, CO2 conversion for Ni–Au/SiO2 increased by 79% whilst simultaneously lowering CH4 selectivity, which was attributed to localised surface plasmon resonance effects and promotion of the reverse water-gas shift reaction by the Au. Laser light illumination at 660 nm also introduced a mild conversion increase, however not to the same extent as for 520 nm. XPS analysis of Ni-Au/SiO2 implied that the observed photo-enhancement could be due to plasmonic mediated electron charge transfer from the Au deposits to the Ni deposits. Monometallic Ni and Au samples were examined and compared with the bimetallic system to elucidate the origin of the photo enhancement and to understand the plasmonic and catalytic interactions. The Au/SiO2 and Ni/SiO2 samples demonstrated that the photo-enhancement arose from the presence of the Au, with the increase in CO2 conversion at 520 nm increasing with increasing Au content; Au (∼110%) > Ni-Au (∼80%) > Ni (∼21%). However, the overall better performance of the Ni/SiO2 sample, facilitating high CO2 conversion and CH4 selectivity emphasises that, while plasmonic metal inclusion can induce synergistic effects, the benefits can be muted if it is unable to promote the target reaction.