Electrochemical reduction of SPR-catalysate 4,4′-dimercaptoazobenzene at [BMIm]BF4/Ag interface probed by SERS

Abstract

The surface plasmon resonance (SPR) catalyzed reaction has attracted considerable attention due to its promising application in heterogeneous catalysis. The acquirements on mechanism of the conversion reaction are still in challenge. It was found that the cyclic conversion of para-aminothiophenol (PATP) and 4,4′-dimercaptoazobenzene (DMAB) was critically dependent on the interfacial environments, such as hydrogen, oxygen and water, which were of the fundamental issues in electrochemistry. Here, the in situ surface enhanced Raman spectroscopy (SERS) combined with electrochemical control was developed to investigate the cyclic conversion of PATP and DMAB at the interface of [BMIm]BF4/Ag electrode and to probe the interfacial structure of water. The surface-catalyzed coupling reaction of PATP commonly occurred to produce DMAB in a fast rate with the illumination of laser. The results revealed that water played the vital role in the reduction of DMAB. The expected reduction of DMAB was absent in the extremely negative potential due to the lack of the free water at the interface in the “dried” [BMIm]BF4. The similar experiments were performed in the [BMIm]BF4/H2O mixtures with the water molar fraction of 0.01, 0.1, 0.3, 0.5, 0.7 and 0.9 respectively. The trace water in the [BMIm]BF4/H2O mixture (xw<0.1) was mainly mixed with RTILs through the hydrogen bonding between water and RTILs. It hindered the diffusion of trace water to interface which resulted in the absence of the reduction of DMAB. With the increase of water content, the formation of clusters network water allowed the diffusion of water to the interface to facilitate the reduction. The surface reaction behavior was similar to that in the aqueous solution as the interface was completely surrounded by larger water clusters in solutions with high water content (xw=0.7–0.9). By this strategy, the electrochemical reduction of DMAB could be served as the probe to monitor the water structure in the [BMIm]BF4/H2O mixtures, and it was believed that SERS could be developed as the powerful tool for monitoring the interfacial reaction and to resolve the interfacial structure in RTILs, particularly for the water structure.