Vibrational study of CO, O<sub>2</sub>, and H<sub>2</sub> Adsorbed\non the CoCrFeNi (110) High Entropy Alloy Surface

The vibrational properties\nof CO, O<sub>2</sub>, and H<sub>2</sub> molecularly or dissociatively\nadsorbed on a CoCrFeNi(110) surface\nhave been probed using high-resolution energy loss spectroscopy (HREELS)\nand modeled using density functional theory (DFT) calculations. Large\n(∼20 mm<sup>3</sup>) single-crystal, quaternary face-centered\ncubic CoCrFeNi was synthesized via a modified Czochralski technique.\nWe show strong evidence that CO adsorbs primarily on bridge and on-top\nsites in compositionally varied local environments, which reflect\nthe random, multielemental surface composition inherent in a high\nentropy alloy. A variation of adsorption sites is also found with\noxygen, which exhibits two broad groups of modes. Comparison to previous\nphotoemission and theoretical studies suggests that the higher energy\nmodes consist primarily of local CrO<sub><i>x</i></sub> species,\nwhile the lower energy modes are due to oxygen atoms adsorbed on other\nmetal sites. Unlike CO and O<sub>2</sub>, HREELS upon H<sub>2</sub> adsorption shows only two much narrower modes and is consistent\nwith atomic adsorption on 3-fold hollow sites. The hypothesized adsorption\nsites for all three species are directly corroborated by our DFT calculations.