High Yield Single-Walled Carbon Nanotube Growth with Platinum-Group Metal Based High Entropy Alloy Catalysts

High-entropy alloys (HEAs), which are solid solutions consisting of five or more elements in an approximately equal atomic ratio [1-3], have attracted tremendous attention in various fields because of their specific properties such as high hardness and strength, and high thermal and chemical stabilities. In addition, HEAs exhibit unique surface and electronic structures because all surface atoms have different configurations with specific local electronic structures. Such a unique surface structure contains diverse active sites for various catalytic reactions. In the past few years, the catalytic properties of HEA nanoparticles (NPs) have been extensively investigated, demonstrating their high activities in NH₃ oxidation, oxygen reduction reaction, and ethanol oxidation [4-6].<br/>In this study, we investigated the catalytic activity of HEA NPs in growth of single-walled carbon nanotubes (SWCNTs). Using HEA NPs composed of five platinum-group metals (5PGM), Ru, Rh, Pd, Ir, and Pt, as catalysts, we succeeded in growing SWCNT via chemical vapor deposition (CVD) with C₂H₂ feedstock. After CVD growth at 750°C for 10 min, high-density SWCNTs with lengths of 1 mm or more were grown from 5PGM HEA NPs. Transmission electron microscopy (TEM) and Raman spectra showed that the diameters of most SWCNTs grown from 5PGM HEA NPs were less than 1.0 nm, exhibiting the growth of small-diameter SWCNTs. The SWCNT yield with the 5PGM HEA NP catalysts was much higher than those with monometal PGM catalysts (Ru, Pd, Ir, and Pt). In addition, the SWCNT yield was compatible with those grown from Fe and Co catalysts, which are the most common catalysts for obtaining high-yield SWCNTs. Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) mapping showed that, even after SWCNT growth, each element was homogeneously distributed in the 5PGM HEA NPs and that their crystal structure was preserved, confirming their robustness in the chemical reaction during SWCNT growth. Our results demonstrate that the 5PGM HEA NPs act as highly active catalysts for obtaining small-diameter SWCNTs, which was realized by unique properties of HEA NPs.<br/><br/><b>References</b><br/>[1] K. Kusada, D. We and H. Kitagawa, Chem. Eur. J. <b>26</b>, 5105 (2020).<br/>[2] K. Kusada, H. Kitagawa et al., J. Phys. Chem. C <b>125</b>, 458 (2021).<br/>[3] D. Wu, K. Kusada, H. Kitagawa et al., J. Am. Chem. Soc. <b>144</b>, 3365 (2022).<br/>[4] D. Wu, K. Kusada, H. Kitagawa et al., J. Am. Chem. Soc. <b>142</b>, 13833 (2020).<br/>[5] Y. Yao et al. Science <b>359</b>, 1489 (2018).<br/>[6] T. Löffler et al. Energy Mater. <b>8</b>, 1802259 (2018).