Photocatalytic Oxidation of Aqueous Ammonia over Ni, Pd, Pt-modified Titania Nanotubes
|關鍵字:||氨氮;金屬負載;光催化;氧化鈦奈米管;二氧化鈦;ammonia;metal-loaded;photolysis;titania nanotubes;titanium dioxide||公開日期:||2008||摘要:||利用光觸媒對氨氮進行光催化降解實驗，已被證實是一具可行性之處理方式，效率高且極具經濟性。因此本研究嘗試利用具ㄧ維結構與離子交換能力之氧化鈦奈米管作為光觸媒載體，選定Ni、Pd、Pt對其進行改質，評估金屬的不同屬性與負載比例改變對水中氨氮光催化降解效率之影響，以及產物之分布影響，後續並探討其反應機制。 當以Pd/TNTs與Pt/TNTs為光觸媒時，負載比例的提升，可有效增加光催化降解效率，且未見遮蔽效應之產生；除了顯示Pd與Pt適合作為氧化鈦奈米管之負載金屬外，無遮蔽效應的產生可歸因於氧化鈦奈米管獨特之離子交換特性。而當以Ni/TNTs為光觸媒時，負載比例的提升僅會使氨氮降解速率下降；推測Ni並無法有效抑制電子電洞對的再結合，反而因其本身佔據氧化鈦奈米管之活性位置，導致離子交換與吸附能力的下降與遮蔽效應的主導。 在產物分佈方面，Pd/TNTs傾向於生成亞硝酸鹽與硝酸鹽，當負載比例提升至某一程度，則會產生部分氮氣，但此時亞硝酸鹽與硝酸鹽仍為主要生成產物。Pt/TNTs相對於其它金屬負載型氧化鈦奈米管而言，在高負載比例情況下對氨氮有非常好的離子交換與吸附能力，此可能為Pt/TNTs高度選擇性光催化水中氨氮為氮氣之原因，以30% Pt/TNTs為例，有高達87.8%的氮氣產率，但確實之反應機制仍需藉由分析修飾觸媒之表面特性來加以確認。
The photocatalytic oxidation technology applied on the removal of ammonia in water has been proved to be an effective and economical treatment process. Therefore in this research we chose titania nanotubes (TNTs) which have one-dimensional structure and ion exchangeability as photocatalyst. Meanwhile nickel, palladium and platinum were chose for modifying its surface. The effects of kind of metal, loading amount on degradation efficiency of ammonia and distribution of products were examined. Finally, we discussed the reaction mechanism. When Pd/TNTs and Pt/TNTs were used as photocatalysts, the degradation efficiency of ammonia increased with the increase of loaded amount, indicating palladium and platinum are applicable to modify titania nanotubes surface. Also, there were no shielding effect occurred in this two case. It can be ascribed to the ion exchangeability of titania nanotubes. When Ni/TNTs were used as photocatalyst, the degradation efficiency of ammonia decreased with the increase of loaded amount. We supposed nickel loaded on titania nanotubes could not suppress the recombination of photo-holes and photo-electrons, but occupied active sites of titania nanotubes instead to result in the decrease of ion exchangeability and adsorption capacity. So inhibiting effect dominated the overall reaction in the case of Ni/TNTs. Pd/TNTs prefer the formation of nitrite and nitrate regarding the distribution of products. Moreover, the loaded amount increased to a certain extent would lead to a few yield of nitrogen gas, yet nitrite and nitrate were still their primary products. Pt/TNTs with high loaded amount have good ion exchangeability and adsorption capacity for ammonia in comparison with the other two metal-loaded titania nanotubes. It may be the reason that Pt/TNTs are highly selective to produce nitrogen gas in the photocatalytic oxidation of ammonia. Take 30% Pt/TNTs for example, its yield of nitrogen gas was up to 87.8%. However, the actual reaction mechanism should be confirmed by analyzing surface property of metal-loaded titania nanotubes.
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