Low temperature adsorption of CO and O2 on Ni(100) and Ni(111): Evidence for precursors

Abstract

Studies of the adsorption of O2 and CO on different faces of single crystal Ni held at T≂6 K are reported which provide direct evidence for a precursor adsorption preceding chemisorption. Values of the work function were obtained using the field-emission retarding potential method as Ni surfaces held at 6 K were exposed to O2 and CO and, subsequently, as the gas covered surfaces were annealed. Comparison with results for chemisorbed surfaces formed by gas exposure at 77 K indicates stabilized precursor adsorption phases at 6 K for O/Ni(111), O/Ni(100), and CO/Ni(111). Starting at temperatures on the order of 20 K, these convert at appreciable rates to the usual chemisorption phases. Analysis of the temperature and time dependence of the reaction rates indicates activation energies EA of approximately 13, 6, and 4 meV for O/Ni(111), O/Ni(100), and CO/Ni(111), respectively. Precursor adsorption was not observed for CO/Ni(100). Presumably the activation energy, if it exists, is too small for effective stabilization at 6 K.