氧化鈦奈米管負載銅鈀異相催化水中硝酸鹽

Abstract

硝酸鹽氮為自然界中常見污染源，主要因農業過度施肥而入滲至土壤造成地下水之污染，一旦攝入過多硝酸鹽，硝酸鹽氮將還原成亞硝酸鹽氮，而阻礙血液氧氣的輸送。而本研究利用氧化鈦奈米管(TNTs)作為批覆鈀、銅雙金屬之載體，並同時將此雙金屬批覆於二氧化鈦(TiO2)上，作為比較兩者間作為載體時，用以降解硝酸鹽之優劣。 因酸洗程序為TNTs合成之重要因子，故將雙金屬批覆於三種不同酸洗程序之TNTs上，其中以鹽酸酸洗最為有助於TNTs之成形。而批覆20%之銅鈀雙金屬具最佳之硝酸降解速率及氮氣選擇性。 而為了解觸媒之持續利用性，故進行多次加藥試驗。Pd-Cu/TNTs系統中，氮氣選擇性於第六循環反應中才明顯減少；而Pd-Cu/TiO2系統之氮氣選擇性則隨著循環數增加而遞減。利用表面化學元素分析儀(ECSA)分析各循環反應後觸媒之鈦、銅及鈀之鍵能變化可知，TiO2表面之Pd及Cu反應後，其鍵能明顯偏移至較高鍵能，而因TNTs為電子提供者，故表面金屬無明顯氧化現象。換言之，TNTs可視為一犧牲形材料而避免金屬受到氧化。 空氣中之氧化物質將使觸媒表面之金屬受到嚴重氧化而失去活性，而因TNTs為管狀結構，致使管內金屬不易與空氣接觸而無法被氧化，因此置於空氣15天後之Pd-Cu/TNTs，其表面上只有部份金屬受到氧化，而TiO2表面金屬則因直接接觸空氣而完全受到氧化，因此TNTs作為載體時，可避免其表面金屬於空氣中迅速失去活性。 雖含鈀觸媒具良好催化硝酸之能力，然而因水體中往往含其它陰陽離子而影響其反應性，故本研究利用不同濃度硫化鈉進行積垢試驗，了解觸媒受毒化之敏感性。相較於Pd-Cu/TiO2，Pd-Cu/TNTs較不易受到硫離子之毒化。而最後分別利用氧化還原等程序再生Pd-Cu/TNTs之，尋求有效之再生方法。而以硼氫化鈉作為再生劑可有效回復硝酸降解速率；反之，利用次氯酸鈉作為還原試劑反而更加減緩觸媒之反應性。

Nitrate contamination is widespread due to over-fertilization in agriculture. Once taken into the body, nitrates are converted into nitrites, which can interfere with the oxygen-carrying capacity of the blood. This study examined the removal of aqueous nitrates (NO3-) by catalytic hydrogenation over microwave-induced titanate nanotubes supported bimetallic Pd-Cu catalysts (Pd-Cu/TNTs), and used Pd-Cu/TiO2 to be the contrast. Because acid-treating step are the key factor affecting the physical property of TNTs, three different acid reagents were used to treat TNTs, and the effect of loading amount of bimetallic metals was also studied. In terms of the selectivity to N2 and the degradation kinetic of nitrate, the optimum loading amount located at 20% w.t., and HCl was the best reagent to assist in TNT synthesis. With an intention to explore the sustainability of Pd-Cu/TNTs, a multi-spiking test was also carried out and the selectivity to N2 decreased only at the 6th cycle. But for Cu-Pd/TiO2, the selectivity to N2 yield decreased with each cycle. Based on the Cu 2p and Pd 3d spectra analyzed by Electron Spectroscopy for Chemical Analysis (ESCA), a relatively more shift to higher binding energy was observed in the case of Cu-Pd/TiO2. And TNTs were the electron donor so that a trivial shift to higher binding energy was observed after reactions. That means almost no oxidation process occurred for bimetals supported TNTs after reaction. In other words, TNTs can be considered as a sacrifice material to prevent the supported bimetallic catalysts from being oxidized. Catalyst can be deactivated due to the oxidant in the air, but only partial metals on the TNTs’ surface were oxidized. This result is probably owing to its tube shape so that metals inside TNTs could not contact with air. In the contrary, metals of Pd-Cu/TiO2 were oxidized more. Therefore, TNTs can prevent metals from oxidation in the air. Although Pd-based catalysts provide efficient reduction, it requires successful approaches for catalyst regeneration in terms of fouling by constituents in nature water. Therefore, this study also focused on the effect of sulfide-fouled catalysts and found applicable regeneration process. Catalysts were severely deactivated after sulfide poisoning. Nevertheless, Pd-Cu/TNTs were deactivated slighter than Pd-Cu/TiO2. Sulfide-fouled catalysts were studied with oxidative and reductive regenerative conditions. However, only heated air along with sodium borohydride provided efficient recovery of nitrate degradation. On the contrary, Pd-Cu/TNTs were deactivated more after oxidative regeneration.