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標題: 利用Polyoxometalate催化零價鋁還原六價鉻
Application of polyoxometalate as a catalyst for Cr(VI) reduction by zero-valent aluminum.
作者: 林潔如
Lin, Chieh-Ju
關鍵字: Cr(VI);六價鉻;Zero-valent aluminum;Polyoxometalate;零價鋁;Polyoxometalate
出版社: 土壤環境科學系所
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摘要: 
Polyoxometalates(POMs)是一聚合金屬陰離子,其具有催化能力,在UV光照射之催化系統中,能同時降解有機污染物並還原重金屬,但在無光照之環境下,也能由溶液中之電子捐贈者(如零價鐵)上攫取電子,生成還原態之POMs(e-),然而,這些POMs(e-)卻會進一步與溶氧反應生成H2O2,使鐵離子與H2O2進行Fenton作用生成OH∙,讓系統維持氧化能力,但是,若此系統中還存在其它電子接受者,如高毒性之六價鉻時,六價鉻是否會阻礙POMs(e-)與溶氧之反應,使系統保持還原之能力,而能將六價鉻還原成三價鉻,仍屬未知範疇,因此,本論文將以零價鋁(ZVAl)取代生成氧化環境為主之零價鐵為無光照系統下之電子來源,探討ZVAl對六價鉻之還原反應,並加入POM(HNa2PW12O40)來催化此反應之進行,同時了解通氣條件對六價鉻還原之影響,並評估POM回收與再利用的可行性。
研究結果顯示,在無光照之環境下,當POM和ZVAl同時存在時,於通入N2和Ar之情況下,POM會催化氧化ZVAl生成藍色的POM(e-)([PW12O40]4-、[PW12O40]5-和[HPW12O40]6-)。經由計量結果發現,一電子還原的[PW12O40]4-為最初還原六價鉻的主要物種,但隨著反應的進行,還原六價鉻的主要物種轉變為三電子還原的[HPW12O40]6-。若系統改通入O2,生成的POM(e-)則會被氧化而形成H2O2,在低pH值的環境中能將六價鉻完全還原。而在六價鉻完全還原後,則將反應過之POM溶液的pH值提升至6.0,沉澱分離出溶液中的三價鉻與三價鋁,而此時之POM會因為pH値的上升而降解成不同之型態,但在溶液再次酸化後,POM會快速回復成[PW12O40]3-之型態,只有部份POM會以催化能力略低於[PW12O40]3-的型態([PW11O39]7−)存在,故回收後之POM溶液催化ZVAl還原六價鉻之速率會略微降低。

Polyoxometalates (POMs) is an anionic cluster and an efficient catalyst. Upon UV light absorption, POMs could induce the degradation of organic contaminants accompanied with the reduction of toxic metals. In the absence of light, POMs could be reduced by zero-valent iron (ZVI), and this reduced form of POM would react further with dissolved O2, forming H2O2 followed by the production of OH radical through a Fenton-type reaction. That is, POM would lead to an oxidizing environment in the absence of light with ZVI. Nonetheless, we speculate that the pathway of OH radical production would be inhibited in the presence of Cr(VI) because Cr(VI) scavenges rapidly reduced POM and H2O2. Therefore, the objective of the present study is to evaluate Cr(VI) reduction on zerovalent Al (ZVAl) as catalyzed by HNa2PW12O40 in an acidic solution, i.e., pH 1.0, under O2, N2 or Ar atmosphere. The ZVAl reductant instead of ZVI was used due to the effect of Fe ions on colorimetric measurement of Cr(VI).In addition, the recycle and reuse of POM is also investigated.
The results show that ZVAl would donate electrons to POM, forming blue-colored reduced POM(e-), such as [PW12O40]4-, [PW12O40]5-, and [HPW12O40]6-, under N2 or Ar atmosphere in the dark. These reduced POM could convert extremely rapid Cr(VI) to Cr(III) upon their formations. Stoichiometric calculation found that one-electron reduction of [PW12O40]4- was responsible for Cr(VI) reduction in the beginning of reaction; however, three-electron reduction of [HPW12O40]6- dominated Cr(VI) reduction afterward. In the presence of O2, reduced POM would be oxidized, leading to the production of H2O2, which could reduce Cr(VI) in an acid solution. Following Cr(VI) reduction, the solution containing Cr(III), Al(III), and POM was then adjusted to pH 6.0 to precipitate of Cr(III) and Al(III). In the meantime, POM was decomposed to low molecular weight anionic cluster such as [PW11O39]7-. These decomposed POM species would convert back to the form of [PW12O40]3- while acidifying the solution to pH 1.0. However, a portion of POM would still present as the species of [PW11O39]7-, which may result in a lower reductive efficiency of POM.
URI: http://hdl.handle.net/11455/28211
其他識別: U0005-2906200919115800
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