Electrocatalytic conversion of carbon dioxide to formic acid over nanosized Cu 6 Sn 5 intermetallic compounds with a SnO 2 shell layer

A novel ordered intermetallic compound of carbon-black-supported Cu 6 Sn 5 nanoparticles (Cu 6 Sn 5 NP/CB) in which Cu 6 Sn 5 has a NiAs-type structure was successfully prepared through a wet chemical method using lithium triethylborohydride as a reducing agent. The prepared ordered intermetallic compound was characterized using X-ray diffraction (XRD), transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and X-ray absorption fine structure spectroscopy (XAFS). The XRD measurements confirm the formation of the NiAs-type ordered intermetallic Cu 6 Sn 5 . XPS and STEM-X-ray energy dispersive spectroscopy measurements allowed us to confirm the Cu 6 Sn 5 structure. The surface of the intermetallic Cu 6 Sn 5 was found to be covered by SnO 2 , indicating that a core–shell structured intermetallic compound ( i.e. , Cu 6 Sn 5 core/SnO 2 shell) had formed. The Cu 6 Sn 5 NP/CB material exhibited a faradaic efficiency of 65.3% at −0.6 V for HCOO − formation via electrochemical CO 2 reduction, which is superior to those of the non-intermetallic Cu NP/CB and Sn NP/CB samples. From the XAFS measurements, we determined the Sn–Sn distance in the SnO 2 on the surface of the Cu 6 Sn 5 NPs, and the key factor affecting the high selectivity was found to be the 4.9% compressive strain of the SnO 2 shell layers on the Cu 6 Sn 5 compared to that of the Sn NP/CB sample.