CdS/TiO2-WO3三觸媒系統之可見光的催化產氫研究

Abstract

能源需求日益擴大的今日，開發環境友好的新能源成為人們關注的焦點。氫能源燃燒熱值高，燃燒唯一產物即為水，是極具潛力的替代能源。近年來，不少研究利用光催化分解水製氫。研發在可見光下具有高活性的光觸媒，將大大提升對光能的利用率，擴大其運用範圍。 本研究採用CdS，TiO2，WO3三種半導體，通過熱合成與物理結合方式構成三觸媒系統，藉助兩步反應，有效降低電子電洞對複合的機率，提高光催化效率。並進一步對系統進行優化，如: 探究三種半導體的較佳比例，使其發揮較好的整體效能；用微波法合成氧化鈦奈米管，擴大觸媒接觸面積；用金屬批覆法修飾WO3，延長電子電洞對分開的週期。另外，甲酸作為光催化系統的犧牲劑，能夠有效的儲存氫氣並擔任電洞捕捉劑，延緩電子、電洞對再結合。 常溫下以150 W可見光燈源 (350≤λ≤800)催化20 vol%甲酸溶液。TNTs作為載體，相較於TiO2，能批覆較高比例之CdS，28 wt%為較佳之批覆比例。0.2 g 28 wt% CdS/TNTs在可見光下的產氫效率為179.35 μmol．h-1，而單純CdS與TNTs的產氫效率僅分別為69.79 μmol．h-1與0.35 μmol．h-1。引入WO3後的三觸媒系統，得益於二步反應，產氫效率提升到212.68 μmol．h-1。其中，CdS/TNTs與WO3各0.2 g為較佳之觸媒比例。本研究之最佳產氫結果為428.43 μmol．h-1，使用之觸媒為0.2 g 28 wt% CdS/TNTs與0.2 g 0.1 wt% Pt/WO3。在批覆鉑金屬後，產氫效率提高了一倍。

Hydrogen gas is one of the most promising renewable energy nowadays as it has high energy yield and zero carbon emission. An attractive and effective method for converting solar energy to hydrogen energy is photocatalytic water splitting over semiconductors. This study investigated the photocatalytic conversion of formic acid solution to hydrogen using visible light (150 W, 350 < λ < 800 nm). The resultant materials were well characterized by high-resolution transmission electron microscope (HR-TEM), X-ray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray (SEM/EDX), and UV-Vis spectra. The study aimed at utilizing organic sacrificial agents in water, modeled by formic acid, in combination with visible light driven photocatalysts to produce hydrogen with high efficiencies. CdS/TiO2-WO3 ternary hybrid was used as photoactive composite. Microwave induced titanate nanotubes (TNTs) were used as the main carrier to incorporate with CdS for the reason that it holds higher surface area than TiO2. The optimized CdS content is 28 wt% and the production rate of 28 wt%CdS/TNTs achieved 179.35 μmol．h-1. Furthermore, WO3 was physically mixed with the optimized CdS/TNTs binary hybrid. The enhanced photocatalytic activity could be attributed to the electron transfer from CdS to TiO2 to WO3 through the interfacial potential gradient in the ternary hybrid conduction bands, which effectively reduces the chance of charge recombination compared with the binary hybrids. The hydrogen production rate reached 212.68 μmol．h-1. Coating of platinum metal onto the WO3 could further promote the reaction. Results showed that 0.2 g 0.1 wt%Pt/WO3 + 0.2 g 28 wt%CdS/TNTs had the best hydrogen production rate of 428.43 μmol．h-1 , which was more than double compared with CdS/TNTs+ WO3.