| Hydrogen has an atomic radius of 0.25～2.54 A, which is much smaller than that of other metallic atoms. It makes hydrogen easily move in metals. When Hydrogen atoms diffuse into metals, which will destroy the original steady structure of metals, bring hydrogen induced cracking of metal equipment and even give rise to severe accident. In order to monitor the hydrogen diffusion in metal devices and ensure the safe state of equipment operation, it is urgently necessary to develop online practical hydrogen sensors.Based on Devanathan-Stachurski (DS) principle of double electrolytic cell, the coatings of catalytic oxidation in sensors was studied systematically in this work. The research mainly focused on plating processes of both palladium and nickel, their catalytic activity on hydrogen oxidation, respectively. The optimum plating processes and the proper oxidation potential range on the coatings of palladium and nickel were determined. The catalytic activity of palladium and nickel on hydrogen oxidation was compared. Finally, the catalytic mechanism of the both coatings was explained sketchily. The results were shown as follows:1. The optimized palladium electroplating process on iron is:Pd(NH3)2Cl2 35 g·dm-3, NH4Cl 23 g·dm-3, NH4OH(25%) 50 g·dm-3, pH 9～10, room temperature (18±1℃), current density 0.333 A·dm-2, plating time 8min. The optimum nickel plating process on iron is:NiSO4·7H2O 215 g·dm-3, NaCl 11 g·dm-3, H3BO3 33 g·dm-3, Na2SO4 25 g·dm-3, MgSO4 35 g·dm-3, pH 5～5.5, room temperature (18±1℃), current density 1.00 A·dm-2.2. The coatings of palladium and nickel have wide steady potential windows in 0.2 mol·dm-3 KOH alkaline solution, which is 0.20～0.50 VNi for Palladium coating between 0.20 and 0.02 VNi for nickel coating. The same steady permeation current on palladium and nickel coating is obtained in the oxidizing potential 0.30～0.40 VNi·3. In steady permeation state, nickel coating needs higher hydrogen over-potential than palladium coating to oxidize hydrogen entirely. So hydrogen oxidation on nickel coating is more difficult than on palladium. To obtain identical steady permeation current on two metal coatings, it is necessary to enhance hydrogen overpotential on nickel. Hydrogen atoms are oxidized on nickel coating by applying more than 0.3VNi potential. 4. Catalytic mechanisms of hydrogen oxidation on two coatings are explained: after oxidation potential is applied, it makes the iron substrate without coatings oxidize and form Fe(OH)2 passive coating, which hinders the reaction of hydrogen oxidation. After nickel is plated on iron substrate, it can be oxidized to form NiOOH and Ni(OH)2 at the oxidizing potential. The NiOOH and Ni(OH)2 has positive catalytical effect on the oxidation of hydrogen. The d Orbits of on palladium with some unpaired electrons make hydrogen be chemically adsorbed on palladium surface, which significantly reduce the activation energy of hydrogen oxidation and is apt to oxidize the diffused hydrogen atoms on palladium coating completely. |
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