Atom-dispersed Au combined with nano-Au on halloysite nanotubes with closo-dodecaborate promotes synergistic effects for enhanced photocatalysis

Abstract

The maximized metal dispersity provided by single-atom (SA) catalysts offers high catalytic activity and selectivity, and maximum metal utilization. However, the lack of adjacent metal atoms in these systems prevents further improvements in intrinsic activity. Here, halloysite nanotubes (HNTs), with their unique charge distribution, are ingeniously combined with the dodecahydro-closo-dodecaborate anion [closo-B₁₂H₁₂]²⁻. Au SAs were synthesized via the reducibility of [closo-B₁₂H₁₂]²⁻and active hydroxyl coordination on the HNT surface. This newly designed material (HNT@B₁₂H₁₂@Au) avoids the normally used high-temperature calcination by preparing Au SAs accompanied by Au NPs while keeping an ∼100% Au utilization through the reduction of [closo-B₁₂H₁₂]²⁻ and coordination stabilization of HNTs. HNT@B₁₂H₁₂@Au showed excellent photocatalytic selective reduction activity for nitrobenzene compounds. Up to 23 kinds of nitrobenzene substrates could be selectively reduced efficiently to corresponding azobenzenes within 2.5 h at room temperature. According to the electron paramagnetic resonance results and theoretical calculations, the photocatalytic properties were further considered at the atomic level, and the underlying mechanism was discussed accordingly. It is concluded that Au NPs and Au SAs have excellent synergistic catalytic effects on photocatalysis precisely because the existence of Au–O and Au–B bonds in HNT@B₁₂H₁₂@Au benefits the reaction kinetics.