A Surface Analytical Study on Poisoning of LaNi_4.7AL_0.3 by CO

In this paper, studies have been made concerning the poisoning mechanism. The processes of poisoning of LaNi₄₇Al_0.3 alloy are analyzed in detail by means of X-ray photoelectron spectroscopy (XPS), second ion mass spectroscopy (SIMS), Auger-energy spectroscopy (AES) and X-ray diffraction (XRD). The changes of the valence and the concentration distribution of the elements of the alloy LaNi_4.7Al_0.3 poisoned by CO are studied. The process and the mechanism of CO's poisoning of alloy LaNi₄₇Al_0.3 are proposed as follows: CO is absorbed on the surface of alloy, part of which reacts with La forming LaC₂ and La₂O₃, or reacts with Ni forming NiO and C in the surface layer, the rest of the CO is decomposed into C and O, which diffuse into the bulk to react with La, Ni and Al. These results in phase-split reaction in surface layer of the particle, and enrichment of La and impoverishment Ni on the surface have taken place. The poisoning effect decreases with a increase of depth. The diffusion depth of C is within 600 Å in the surface layer, and that of O is within 1000 Å.The oxide film and carbonizing film prevent the H-storage alloys from further absorbing hydrogen, which leads to a deceleration of the H-storage capability. Moreover, The formation of a new phase with poor H-absorption capability is caused by the phase split reactions, which is one of reasons for the decrease of H-absorption property of the H-storage alloys.