Electrochemical CO₂ Reduction to HCOOH Catalyzed by Ag_n(NO₃)_n+1 Clusters prepared by Laser Ablation at The Air-Liquid Interface

Nanomaterials generated via laser ablation in liquid have been developed because of the capability to generate nanoparticles without chemical reagents. However, it is difficult to obtain monodispersed and small (&lt; 2 nm) nanoparticles. We have reported laser ablation at the air-liquid interface to obtain monodispersed and small (ca. 5 nm) nanoparticles [1]. Here, we report laser ablation at the air-liquid interface using colloidal nanoparticles as a target to obtain smaller clusters [2]. Colloidal target was prepared by laser ablation of Ag powder precipitated on the bottom of the flask. Pulsed laser was irradiated through the bottom of the flask. After laser irradiation for 60 min, Ag nanoparticles stably dispersed in pure water was obtained. This colloid exhibited a large absorption peak at 400 nm. This colloidal solution was agitated in a measuring flask during laser ablation at the air-liquid interface. After laser ablation at the air-liquid interface, we obtained colorless and transparent solution, indicating the drastic size reduction. UV-Vis absorption spectra exhibited peaks in UV region. It indicated small cluster formation. The size was assessed by electro-spray ionization-mass spectrometry. It revealed Ag<i>_n</i>(NO₃)<i>_n+1</i> cluster generation. To investigate the electrocatalytic activity, Ag<i>_n</i>(NO₃)<i>_n+1</i> clusters were deposited on carbon paper substrate. Electrocatalytic activity for CO₂ reduction was assessed in CO₂ saturated aqueous 0.1 M KHCO₃ solution at an applied potential of -1.2 V vs RHE with Ag/AgCl reference electrode and Pt wire counter electrode. As well-known, micro and nano-sized Ag catalyst as comparisons generated CO via electrochemical CO₂ reduction. In contrast, Ag<i>_n</i>(NO₃)<i>_n+1</i> clusters generated HCOOH with a Faraday efficiency of 33%. The results indicated a drastic change in the selectivity induced by laser ablation at the air-liquid interface. Because the unique selectivity, carbon source of HCOOH was determined by isotope tracer analyses using ¹³CO₂ and KH¹³CO₃ together with ion chromatography-mass spectrometry. Because only H¹³COO^- peak was confirmed, it can be concluded that HCOOH was generated from CO₂. Both the catalyst size and modification by NO₃ potentially explain the results.<br/><br/>[1] T. Nishi et al., J. Nanopart. Res., 2013, 15, 1569.<br/>[2] T. Nishi., Jpn. J. Appl. Phys., 2015, 5,095002.<br/>[3] T. Nishi., Chem. Lett., 2021, 50, 1941-1944.