Photocatalytic H2O2 Synthesis Over Au9 Clusters and Au101 Nanoparticles: Does A Smaller Co-catalyst Always Perform Better?

Photocatalytic H2O2 production is a green and sustainable alternative to the commercial anthraquinone process that requires large energy input and causes high CO2 emission. Gold (Au) co-catalysts are often employed as catalytic active sites in photocatalytic material designs. However, little is known about the effect of Au particle size, particularly in different size domains (cluster vs nanoparticle), on photocatalytic H2O2 synthesis. Here we employed Au9 clusters and Au101 nanoparticles supported on sulfur-doped graphitic carbon nitride (SCN) to study the size effects on the photocatalytic performance in H2O2 synthesis. Au101/SCN exhibits the highest photoactivity with H2O2 production rate 19.5 mM h-1 g-1 followed by SCN (16.2 mM h-1 g-1) and Au9/SCN (13.6 mM h-1 g-1). Transient photocurrent response and photoluminescence spectroscopy indicate that gold loaded SCN (both Au9/SCN and Au101/SCN) has higher efficiency of charge carrier generation and separation compared to pure SCN. A kinetic analysis revealed that Au9/SCN catalyzes the fastest H2O2 decomposition (high kd) leading to a reduced overall H2O2 yield despite having higher charge generation and separation efficiency than SCN. The findings from this work show that better charge carrier generation, separation and transfer alone is not enough to warrant higher photoactivity in H2O2 synthesis because the rate of H2O2 decomposition supersedes the charge carrier properties. This work offers valuable insights into photocatalyst designs with superior activity.