Stepwise assembly of thiacalix[4]arene-protected Ag/Ti bimetallic nanoclusters: accurate identification of catalytic Ag sites in CO2 electroreduction

Abstract

The accurate identification of catalytic sites in heterogeneous catalysts poses a significant challenge due to the intricate nature of controlling interfacial chemistry at the molecular level. In this study, we introduce a novel strategy to address this issue by utilizing a thiacalix[4]arene (TC4A)-protected Ti-oxo core as a template for loading Ag¹⁺ ions, leading to the successful synthesis of a unique Ag/Ti bimetallic nanocluster denoted as Ti₈Ag₈. This nanocluster exhibits multiple surface-exposed Ag sites and possesses a distinctive "core-shell" structure, consisting of a {Ti₄@Ag₈(TC4A)₄} core housing a {Ti₂O₂@Ag₄(TC4A)₂} motif and two {Ti@Ag₂(TC4A)} motifs. To enable a comprehensive analysis, we also prepared a Ti₂Ag₄ cluster with the same {Ti₂O₂@Ag₄(TC4A)₂} structure found within Ti₈Ag₈. The structural disparities between Ti₈Ag₈ and Ti₂Ag₄ provide an excellent platform for a comparison of catalytic activity at different Ag sites. Remarkably, Ti₈Ag₈ exhibits exceptional performance in the electroreduction of CO₂ (eCO₂RR), showcasing a CO faradaic efficiency (FE_CO) of 92.33% at -0.9 V vs. RHE, surpassing the FE_CO of Ti₂Ag₄ (69.87% at -0.9 V vs. RHE) by a significant margin. Through density functional theory (DFT) calculations, we unveil the catalytic mechanism and further discover that Ag active sites located at {Ti@Ag₂(TC4A)} possess a higher ε_d value compared to those at {Ti₂O₂@Ag₄(TC4A)₂}, enhancing the stabilization of the *COOH intermediate during the eCO₂RR. This study provides valuable insights into the accurate identification of catalytic sites in bimetallic nanoclusters and opens up promising avenues for efficient CO₂ reduction catalyst design.