具磁性可回收之可見光應答光觸媒於光催化分解水產氫之研究

Magnetically recyclable visible-light responsive photocatalysts for hydrogen production through photocatalytic water splitting

余宗軒

指導教授 ： 呂世源

國立清華大學/工學院/化學工程學系/碩士(2012年)

摘要

ZnFe2O4是一具有磁性且可見光應答的光觸媒材料，但過去在光催化水分解產氫方面的應用很少被探討。本研究以ZnFe2O4為觸媒材料，初次嘗試利用氣凝膠結構以及與CdS結合成複合材料作為可見光光觸媒，探討光觸媒在可見光下催化產氫能力的變化。由於氣凝膠擁有以下特性：1.連續的3-D立體網狀結構，使光生電荷載子易於表面移動；2.高比表面積有利於表面反應進行；3.中孔洞結構(孔洞半徑：2~50 nm)有助於反應物及產物進出觸媒，提升效率。研究中對ZnFe2O4氣凝膠在不同鍛燒溫與持溫時間的實驗條件下的產氫能力做比較。在複合材料方面，本研究利用ZnFe2O4具有磁性的優點與能階位置和CdS相互匹配的關係，探討作為一功能性可見光應答光觸媒的可行性，並利用觸媒物性分析解釋實驗結果。
在ZnFe2O4氣凝膠的光催化反應中，使用的研究系統為甲醇水溶液，其中甲醇為系統中的犧牲劑，於400W的高壓汞燈照射下進行反應。在全光譜的照射下，ZnFe2O4氣凝膠在500℃鍛燒10小時的光催化產氫效率為9840μmol/g•hr，結果顯示ZnFe2O4的產氫速率隨著結晶性的改善而增加。而用光沉積法的實驗中發現負載上貴金屬鉑對ZnFe2O4的產氫能力沒有顯著的效益。可見光產氫的部分，利用亞硝酸鈉作為濾光溶液，在持續光照6小時下發現產氫速率有趨於平緩的現象，而在鍛燒溫度350℃、持溫10小時的ZnFe2O4氣凝膠因具有不錯的晶相與高比表面積，有利於光生電子在生成後的傳遞以及光觸媒與水溶液的接觸，使產氫速率可以達到7.4μmol/g•hr。
在ZnFe2O4與CdS的複合材在可見光催化產氫能力的表現上，所使用的研究系統為硫化鈉-亞硫酸鈉水溶液，以硫化鈉與亞硫酸鈉作為犧牲試劑。在以ZnFe2O4氣凝膠為主體負載CdS的光觸媒方面，ZnFe2O4氣凝膠提供了高比表面積與適當的孔洞大小讓CdS承載在上面，於可見光照下產氫速率為25.8μmol/g•hr，在12小時的長效性測試也保持不錯的穩定性。而以CdS為主體，鑲嵌上ZnFe2O4奈米粒子的光觸媒藉由XRD、BET、XPS、SEM與TEM鑑定其晶相，比表面積，表面鍵結元素與形貌。結果證實CdS以奈米棒的形式上面負載的確實為ZnFe2O4奈米粒子，不僅使得光觸媒在受到磁力吸引時在水溶液中具有分離的效果，在產氫測試中，由於ZnFe2O4和CdS能階的相對位置關係，光生電子會趨向從ZnFe2O4傳到CdS的導帶位置並進行氫氣還原，電洞則從CdS 的價帶能階傳到ZnFe2O4的價帶，再與水溶液發生氧化反應，因而達到電子電洞分離的效果，使產氫速率上有所提升；在長達30小時的穩定性測試中，ZnFe2O4的負載不但抑制了CdS的光腐蝕現象，甚至有活性增加，產氫速率逐漸提高的現象，相較於單純CdS產氫速率的1400μmol/g•hr，最高產氫速率達到2812μmol/g•hr。

關鍵字

光觸媒 ； 產氫 ； 可見光 ； 磁性 ； 回收 ； 光催化 ； 鋅鐵氧化物 ； 硫化鎘 ； 氣凝膠

並列摘要

ZnFe2O4 is a magnetic and visible light response photocatalyst material, but was rarely studied in photocatalytic water splitting for hydrogen production. In this study, we first used ZnFe2O4 aerogels and ZnFe2O4/CdS composites as the photocatalyst for photocatalytic water splitting under visible light irradiation. The high surface area, 3-D connected pore structure, and large porosity are the advantageous for aerogels to serve as the photocatalyst. We prepared ZnFe2O4 aerogels under different annealing conditions and tried to find out the relationship between the hydrogen production rate and annealing condition. In the part of composite material, we utilized the magnetic property of ZnFe2O4 and the compatible energy band structure of ZnFe2O4 and CdS for our photocatalytic water splitting purposes. We also used varius catalyst material analyses to interpret the experimental results.
The photocatalytic water splitting reaction of ZnFe2O4 aerogels was carried out in a methanol solution under 400W high pressure Hg light source. The methanol served as the sacrificial agent. Under the full spectrum of radiation, the hydrogen evolution rate of ZnFe2O4 aerogels, calcined at 500 ° C for 10 hours, was 9840μmol / g • hr. The data showed that the rates of hydrogen evolution increased as the crystallinity improved. In our experiments, a co-catalyst, Pt, was loaded via a photodeposition method to give a Pt/ZnFe2O4 aerogels photocatalyst (1 wt %), but it did not show significant improvements in the rate of hydrogen evolution. In the part of visible light irradiation, 1M NaNO2 was circulated through the reactor jacket to filter out the UV light (λ<400 nm). After 6 hours of continuous light irradiation, the hydrogen evolution became saturated. The best performance was achieved by the ZnFe2O4 aerogels calcined at 500 ° C and held for 10 hours, and the hydrogen evolution rate was 7.43μmol/g•hr.
Another part of this research focused on the performance of CdS/ZnFe2O4 composite materials in photocatalytic water splitting. In this part, we used a Na2S /Na2SO3 solution as the sacrificial reagent. About 20wt.% of CdS was loaded onto the ZnFe2O4 aerogel. Here, aerogels provided high specific surface area and appropriate pore sizes for loading CdS, leading to 25.8μmol/g•hr in hydrogen evolution rate under visible light irradiation, and showed good stability in the cycle experiment. The photocatalyst of 20wt.% ZnFe2O4 decorated on CdS nanorods were characterized by XRD, BET, XPS, SEM, PL, and TEM. The result proved that ZnFe2O4 was successfully decorated on CdS nanorods, and made the photocatalyst magnetic for easy recycle. The conduction band of ZnFe2O4 is more negative than that of CdS, and thus the electron can move from ZnFe2O4 to CdS for hydrogen reduction. The valence band of ZnFe2O4 is less positive than that of CdS, so that the holes generated in CdS can move to ZnFe2O4 to protect CdS from photocorrosion. In the long term experiment, the ZnFe2O4 decoration not only hampered the photocorrosion of CdS, but also improved the activity of hydrogen evolution. The highest hydrogen production rate of 20wt.% ZnFe2O4 decorating on CdS nanorod was 2811μmol/g•hr.

並列關鍵字

hydrogen ； photocatalyst ； zinc ferrite ； cds ； visible light ； magnetic ； aerogel ； nanorod ； cadmium sulfide

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