奈米鈦管光催化去除氮氧化物之研究

Abstract

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In this study, titania nanotubes (TNTs) were synthesized by hydrothermal method using commercial Degussa TiO2 (P25) as a precursor. The properties of TNTs were modified by controlling of different synthesis parameters, including calcination temperature, washing pH value, and doping with various metals. The materials were then characterized by BET, SEM, TEM, XRD, TPD, ICP, XPS, UV-Vis absorption, and FTIR analyses. The photocatalytic reactions of NO and NO2 under UVA irradiation were performed to evaluate the activity of the TNTs materials. For the effect of calcination temperature on TNTs properties and activity, results showed that the highest total NOx removal efficiency was achieved by TNTs calcined at 500℃ (T-500). The high activity of T-500 in both NO and NO2 photocatalytic reactions could be attributed to its high anatase crystallinity and high surface area. These two factors affect primarily on the conversion of NO2 to nitrate, which was the rate-limiting step for photocatalytic removal of NOx. The high anatase crystallinity could be responsible for the high efficiency at the beginning, while the high surface area could be accounted for retaining this high efficiency from nitric acid poisoning during the test period. This study reports for the first time on the role of contaminant sodium to neutralize the acidic reaction products during the NOx photocatalytic reaction and prevent surface deactivation of TNTs washed at different pH values. In the photocatalytic removals of NO and NO2, it revealed that both of the Na content and the structure of TNTs materials play important roles on the NOx removal pathway, initial efficiency and decay rate. The highest efficiencies were achieved by TNTs washed at pH 3 to 5 (T-3~5), which may be due to their high amount of crystalline anatase for photocatalytic reaction and high sodium content for neutralization of acidic products. The mechanism based on the neutralization of the HNO3 resultant from the NOx photo-oxidation was also proposed. In the inter-effect of NO and NO2 during the photocatalytic oxidation process, separated and simultaneous reactions of NO and NO2 were performed and compared using different NO/NOx ratios of 0, 25, 50, 75, and 100%. With the increase of NO/NOx ratio, the conversion of NO was stable with high efficiencies. On the other hand, NO2 conversion efficiency declined significantly while NOx removal efficiency decreased slightly. The reactions of NO and NO2 was assumed to be independent and the separated reaction results were employed for calculation of the simultaneous reactions results. When the actual results of simultaneous reactions were compared with their expected results, the presence of NO2 did not show any effect on the NO conversion. In contrast, the presence of NO considerably inhibited the conversion of NO2 and therefore the NOx removal. In the photocatalytic reduction tests using Mo-doped TNTs, Mo could be successfully doped by precipitation and impregnation methods, but not by the hydrothermal method. The NO2 photocatalytic reaction results showed that the doping of Mo sharply declined the oxidation ability of TNTs while enhanced its reduction ability. Moreover, Mo-doped TNTs prepared by the precipitation method provided the highest reduction ability, which may be due to its chemical oxidation states of Mo4+ and Mo5+. Additionally, washing pH has a strong effect on the properties and activity of the Mo-doped TNTs materials. Mo-doped TNTs washed at pH 3 had the highest total NOx removal efficiency while the one washed at pH 1 provided the highest ability for NO2 reduction. It is likely that the effect of washing pH on the activity and the selectivity of Mo-doped TNTs is via the Mo and Na contents of the materials. The high Mo but low Na content would promote the reduction reaction. On the other hand, the low Mo but high Na contents would enhance the oxidation while inhibit the reduction reaction. This study also reports for the first time the doping of metal as a novel and facile method for controlling the oxidation and reduction activities of TNTs for photocatalytic removal of NO2. Doping of Fe, Sr, and Zn enhanced the intrinsic oxidation activity of TNTs. Doping of Al, Co, La, Mg, Sn, W, and Zr showed little effect while doping of Ag, Ce, Mn, Ni, Sb inhibited the oxidation activity. Particularly, although the doping of Cr, Cu, Mo, and V inhibited the oxidation activity, it surprisingly provided TNTs with high reduction ability, which successfully reduced NO2 to NO.