| Summary: | 博士 === 中興大學 === 材料工程學系所 === 95 === Due to excellent properties such as high specific strength, excellent vibration damping property and good EMI (electromagnetic interference), Mg alloys has an increasingly number of uses in transportation vehicle (e.g., door frame, engineering covering, oil pan, wheel, etc.) and the outer shell of 3C electronic products (e.g., the upper cover and base seat of notebook computer, the outer covering of personal mobile communication tool, etc.). Recycling of Mg scraps (i.e., post-consumed or end-of-life Mg products) has become increasingly important. This study proposes a new method for generating H2 gas in aqueous NaCl by the hydrolysis of Mg scraps. The experimental findings of this study not only indicate a method for generating hydrogen but also promote the recycling of the Mg scraps.
Pt-coated Ti net (i.e., ~ 2.5 μm of platinum film being electroplated on the surface of Ti net) was adopted as a catalyst to promote the hydrolytic reaction of an Mg sample in aqueous NaCl (5 wt.%) to generate H2 without extra supply of power. Two experiments were conducted and the volumes of H2 generated were compared. In one of the experiments, the Pt-coated Ti net was statically loaded on the top surface of the Mg sample, with a loading force of 6 ± 0.5 kg. In the other, the Pt-coated Ti net was ground against the surface of the Mg sample (6 ± 0.5 kg, 8 ± 1 rpm). When a Pt-coated Ti net was statically loaded (6 ± 0.5 kg) on the Mg sample, the average H2 generation rate of about 302.3 ml min-1(g of catalyst)-1 was measured. The curve of cumulative volume of generated H2 reached a plateau after the hydrolysis reaction proceeded a certain time. Mg(OH)2 passive layer which prevented contact between the catalyst (Pt/Ti net) and the Mg sample was the major reason leading to the plateau. When the Pt-coated Ti net was ground (6 ± 0.5 kg, 8 ± 1 rpm) onto the Mg sample surface, more H2 gas was produced than was generated by static loading. The cumulative volume of H2 gas generated was almost linearly proportional to the reaction time. The average H2 generation rate was calculated to be 432.4 ml min-1(g of catalyst)-1. No Mg(OH)2 passive layer formed on the Mg sample surface, because the Pt-coated Ti net ground against the sample, removing the Mg(OH)2 layer. A little of the Pt had been consumed (~ 0.0278 g). The generated gas was analyzed by GC (gas chromatography). Only hydrogen and water vapor were detected. The purity of the hydrogen was analyzed to be around ~ 97.2 % mole fraction. Additionally, the maximum volume of generated H2 was observed in 3.5 wt.% aqueous NaCl.
In order to improve the H2 generation efficiency of the Mg scraps, platinum-coated Ti (Pt-Ti) net and AISI 304 stainless steel (S.S.) net were dipped in the semi-solid Mg scraps. The average cumulative volume of generated H2 in 3.5 wt.% NaCl solution at 25 ˚C using Mg scraps/Pt-Ti samples and Mg scraps/S.S. samples was about 28.2 ± 5.7 liter and 16.1 ± 7.8 liter. The generated H2 volume per one gram of Mg scraps consumption was 1.0 ± 0.1 liters from Mg scraps/Pt-Ti samples, which was similar to the result by using Mg scraps/S.S. samples (1.1 ± 0.1 liters). In addition, both of these two metallic net catalysts showed a good durability of more than 5 experimental cycles to be re-used for H2 generation. The generated H2 in this study was converted into the electrical energy by fuel cell. A set of used Pt-coated Ti nets (20 pieces, the 4th time of use) was applied to be the catalyst metal, the cumulative H2 volume was about 11.3 liters in 50 min. The power generated was about 1.79 MJ (498.1 Wh) during the H2 generation. New AISI 304 stainless steel nets (20 pieces) were used as the catalyst. The cumulative H2 volume within 50 min was about 10.4 liters. The generation of power was about 1.53 MJ (424.4 Wh).
The systematic analytical method of life cycle assessment (LCA) was carried out to investigate the environmental impacts of proposed H2 production process. The H2 production processes from metallic materials (i.e., Al powders, Mg powders) and the recycling process of Mg scraps were considered for comparison with LCA method. The energy requirement using Mg scraps/Pt-Ti couples to generate 1 kg of H2 was about 35.1 MJ. Around 34.9 MJ was needed by applying Mg scraps/S.S. couples to produce 1 kg of H2. The main energy consumption resulted from the preparation of Mg scraps/Pt-Ti and Mg scraps/S.S. couples. Moreover, the energy requirement of 1 kg H2 production by Al and Mg powders was about 11804 MJ and 6240 MJ. The energy requirements for generating 1 kg of H2 are mainly because of the energy being consumed to prepare the metallic powders. In addition, the energy requirement for producing 1 kg H2 by electrolysis water method (235 MJ or 243 MJ) was much higher than that of present study. Also, the H2 production process by this method, the air pollutants such as CO2, SO2 and NOx was released. From the viewpoint of recycling process of Mg scraps, the energy requirement and pollutant released of present study and conventional process were compared. The proposed process in present study for the recycle of 1 kg Mg scraps was about 2.9 MJ. The energy requirement for recycling 1 kg Mg scraps is about 151 MJ or 164 MJ in conventional processes.
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