Journal of The Surface Finishing Society of Japan

Hydriding characteristics of LaNi_4.7Al_0.3 alloys with surfaces modified by high-purity anhydrous HF were evaluated after long-term preservation in air. Affected by poisoning species in air, untreated alloy hydriding activity was enormously decreased. HF-treated alloys with a modified surface layer containing metal fluoride NiF₂, LaF₃ and AlF₃ maintained relatively high activity. In high-humidity air, metal fluoride NiF₂ reacted with moisture to form hydrous Ni(OH)₂, and permeation of poisonous species such as O₂, CO and CO₂ was markedly inhibited by moisture-condensed hydrate screens.

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Hydrogen storage alloys exhibit reversible high-rate hydrogen absorption and desorption reactions. The hydrogen storage density of the alloys is higher than that liquid H₂. The exposure of the alloys to impurity gases such as O₂, CO, CO₂, and H₂O readily reduces reactivity with H₂ gas. In this study, the surface of LaNi_5-xAl_x alloy was treated using anhydrous hydrogen fluoride to examine the effect of CO on fluorinated alloy H₂ reactivity. This HF treatment effectively reduced alloy surface poisoning by CO. Compared with the initial transfer, the HF-treated alloy still exhibited 80% hydrogen transfer after 100-cycle hydriding-dehydriding reactions in a H₂ gas including 1000ppm CO, while the transfer became zero for untreated alloy after five hydriding-dehydriding cycles. AES analysis of HF-treated samples showed that the HF treatment induced the surface segregation of Ni and F atoms where almost no La atoms was detected. The concentration of La atoms was found to increase with depth, in which a mixed region of La, Ni, Al, F, and O atoms was formed. This fluorinated surface layer seems to function both to prevent surface CO poisoning and to act as a catalyst.

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Electrodes modified using aggregated cobalt-porphyrin/polymer ligand systems were prepared and electro-catalytic activity for oxygen (O₂) reduction was investigated physicochemically. The maximum value of water yield due to O₂ four-electron reduction was 56%. The apparent number of electrons involved in O₂ reduction was 2.7 to 31. Two reactions due to O₂ two- and four-electron reduction occurred on modified electrodes. Modification (adsorption) stability of the electrocatalyst system was over a few hours under hydrodynamic conditions.

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A titanium dioxide photocatalyst was electrochemically fixed onto an alumite surface in a (NH₄)₂ TiO (C₂O₄)₂ solution. The photocatalytic activity of alumite fixed with TiO₂ was evaluated from the CH₃CHO photodecomposition. It was found that TiO₂ has photocatalytic activity. The photocatalytic mechanism is discussed based on the photoresponse of the rest potential. Alumite fixed with TiO₂ has potential practical applications.

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