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  • 學位論文

微波誘導製備氧化鈦奈米管於可見光催化產氫之研究

Microwave-Induced Titanate Nanotubes for Visible-Light-Driven Hydrogen Production

指導教授 : 駱尚廉
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摘要


本研究以微波誘導製備氧化鈦奈米管(TNTs),運用於紫外線和可見光下催化產氫,其中TiO2 (P25)不僅用於TNTs合成之原料,同時作為與TNTs表現進行比較之參考載體,由TNTs之UV-Vis分光光譜分析顯示,修飾共觸媒後,吸收光譜偏移至可見光區之位移大於TiO2。 本研究以產氫量作為評估光催化性能,首先以Pt分別修飾TiO2及TNTs,分別於紫外光及可見光下進行光催化甲醇產氫,實驗結果顯示,在20%甲醇溶液中,在紫外線和可見光照射下,Pt/TNTs產氫率高於Pt/TiO2。由於甲醇於光催化產氫先氧化為甲醛,再氧化為甲酸,而甲酸可能氧化為CO及H2O或CO2及H2,且由TNTs及Pt/TNTs之產氫結果顯示CO生成量分別較TiO2和Pt/TiO2高,可確認TNTs作為光觸媒載體可較Pt/TiO2提高由甲酸生成H2之光催化選擇性,此結果推測係因CO(0.38 nm)之動力學直徑大於CO2(0.33 nm)之動力學直徑,故CO於TNTs反應系統不易擴散致不易使甲酸氧化為CO及H2O。 且為進一步聚焦甲酸產氫效率及提高可見光之光催化能力,續以10%甲酸溶液進行試驗,以Pt、CdS分別修飾TiO2及TNTs,結果顯示,CdS/TNTs光催化能力高於CdS /TiO2,另以光沉積法負載Pt時,Pt(P)/ CdS/TNTs產氫率為661.1μmolg-1 h-1,產氫率為以熱含浸法披覆Pt之Pt(T)/CdS/TNTs的3.6倍,推測是因光沉積法能均勻披覆Pt納米顆粒,避免所披覆之金屬如熱含浸法之團聚,故能有效地促進電子電洞對之分離。 此外,本研究以X光光電子能譜儀(XPS)分析觸媒表面元素價態,結果顯示,由於Pt和TNTs之間強金屬作用力(strong metal-support interaction),使TNTs上Pt束縛能較零價態略低、Ti之束縛能提高之情形,且在反應3小時後,TNTs上之CdS也未顯示有氧化後產物(如S0或S6 +),因此可推測TNTs有助於避免CdS之光腐蝕,因此,以TNTs作為載體有助於提昇光催化反應。

並列摘要


Titanate nanotubes (TNTs) fabricated through microwave-assisted synthesis were examined for their ability to catalyze hydrogen production under UV and visible light irradiation. Herein, TiO2 was used not only as the raw material for TNT synthesis but also as a reference support to compare its performance with that of TNTs. The UV-Vis spectral analyses of the TNT composites showed greater shifts toward the visible region after co-catalysts loading than the spectra of TiO2. The photocatalytic performances were evaluated by measuring the hydrogen production, and Pt loaded on TiO2 and TNTs were examined for their ability to catalyze hydrogen production. The experimental results showed that Pt/TNTs exhibited higher activity than Pt/TiO2 from an aqueous solution containing 20 vol% methanol solution under UV and visible light irradiation. Furthermore, the presumed mechanism of the photocatalytic conversion of a methanol solution to H2 is the oxidation of methanol and transformation to formaldehyde and then formic acid. Then, formic acid is transformed to CO and H2O or CO2 and H2. Because bare TNTs and Pt/TNTs showed lower CO generation than bare TiO2 and Pt/TiO2, TNT composites enhanced the photocatalytic selectivity for H2 generation from formic acid to a greater extent than Pt/TiO2. Because the kinetic diameter of CO (0.38 nm) is larger than that of CO2 (0.33 nm), this result may be attributed to the inability of CO to diffuse into the pores of TNTs because of the diameter difference. To enhance the photocatalytic performances from an aqueous solution containing 10 vol% formic acid solution under visible light irradiation, the composites formed from Pt, CdS, and titanate nanotubes (TNTs) were examined. Experimental results showed that CdS/TNTs exhibited much higher activity than CdS/TiO2. And TNT composites with Pt loading by the photo-deposition method had a significantly enhanced photoactivity and exhibited a hydrogen production rate of 661.1 μmol g-1 h−1, which is approximately 3.6 times higher than that observed for Pt deposited by the thermal impregnation method. This result indicates that smaller and more uniform Pt nanoparticles efficiently promoted the separation of photogenerated charges. Furthermore, the valence states of the catalysts were determined from X-ray photoelectron spectroscopy (XPS) analysis. XPS results showed negative shifts of the Pt binding energies and positive shifts of Ti binding energies due to the strong metal-support interaction between Pt and TNTs. And the TNT composites showed no significant amount of oxidized S0 or S6+ on the catalyst surface even after 3 h of reaction. Thus, we conclude that the photocorrosion of CdS was significantly inhibited when combined with TNTs during the photocatalytic reactions. Thus, the remarkably high photocatalytic efficiency of TNT composites facilitates their application as promising photocatalysts.

參考文獻


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