鍛燒溫度對釩離子摻雜二氧化鈦光觸媒物化與光催化還原二氧化碳特性研究

Abstract

近年來，利用光觸媒進行光還原二氧化碳產生燃料之議題備受矚目，其中，有很多研究著重於參雜不純物至光觸媒二氧化鈦裡，藉由不純物減少電子電洞再結合的速率，有效提升二氧化鈦進行光還原二氧化碳的效率。本研究利用溶膠-凝膠法(sol-gel)製備摻雜釩離子的二氧化鈦，探討不同鍛燒溫度(200 °C-700 °C)與不同釩離子濃度(0.01 wt%與1.00 wt%)對二氧化鈦光觸媒結構以及光還原二氧化碳的影響。研究結果顯示TiO2經300 °C鍛燒後呈現銳礦鈦的結構，鍛燒溫度超過600 °C會產生金紅石，而在V/Ti比例高於1 %時，會使金紅石相在500 °C時，提早產生，另外，當鍛燒溫度高於塔曼溫度時，釩會往表面遷移而形成V2O5晶相，SIMS實驗發現，高濃度釩在鍛燒600 °C時表面釩的濃度為鍛燒200 °C時的7.6倍，由EPR及XAS發現，在鍛燒溫度較低的樣品中，內部有三價與四價的釩參雜在二氧化鈦晶格中，而隨著鍛燒溫度增加，釩的價態會逐漸轉變為五價的型態，當鍛燒溫度增加到600 °C，可由GI-XRD觀察到明顯的表面V2O5晶相產生，UV-VIS光譜可發現參雜高濃度釩的樣品可明顯降低TiO2能隙至1.6 eV。在300-500 牵C鍛燒溫度下，觸媒對0.01 mM Rhodamine B光降解活性依次為0.01 at.% V-doped TiO2> pure TiO2 > 1.00 at.% V-doped TiO2。在光還原二氧化碳的實驗裡，500 °C鍛燒的樣品有最高還原活性，甲烷為還原反應中唯一可測得產物，單純TiO2在反應第一小時有最高CH4量子產率 2.98 %，其次為1.00 at.% V-doped TiO2 (2.65 %) 與0.01at.% V-doped TiO2 (2.44 %)，然而，於8小時反應後，各觸媒產生甲烷的量子產率依序為1.00 at.% V-doped TiO2 (0.66 %) >單純TiO2 (0.39 %)~0.01 at.% V-doped TiO2 (0.39 %)。EPR光譜發現，表面電荷於CO2與H2O分子間的轉移迅速，因此具低還原電位的中間產物是限制光催化還原效率的關鍵，表面缺陷造成傳導帶下低還原能力的能階決定釩摻雜二氧化鈦低初始還原活性，而V2O5與TiO2間的異質界面則抑制甲烷再氧化速率。

In this study, the physicochemical properties and photoreduction behavior of the TiO2 samples doped with 0.01 and 1.00 at.% V ions and calined at different temperatures were investigated. The pure TiO2 exhibited anatase phase at 300 °C and underwent phase transition to rutile one at 600 °C. Incorporation of V ions decreased the transition temperature to 500 °C. Calcination greatly increased the surface V/Ti ratio of the doped TiO2 by 7.6 times as the temperature increased from 200 to 600 牵C. The increase in the surface concentration of the V ions also led to the formation of V2O5 moiety. Doping 1.00 at.% V ions dereduced the bandgap energy of the TiO2 from 3.1-3.3 to 1.6 eV. For oxidation of Rhodamine B, the photocatalysts exhibited the activity in the order of 0.01 at% V-doped TiO2 > pure TiO2 > 1.00 at. % V-doped TiO2. The samples calcined at 500 牵C showed the highest activity for CO2 reduction over other temperatures. CH4 was the only detectable product in the reduction systems. After 1 hr irradiation, the pure TiO2 had the highest quantum efficiency (2.98 %) for CH4 generation, followed by 1.00 at.% V-doped TiO2 (2.65 %) and 0.01 at.% V-doped TiO2 (2.44 %). However, the quantum efficiency of the photocatalysts for CH4 yield after 8 hr irradiation was in the order of 1.00 at.% V-doped TiO2 (0.66 %) > pureTiO2 (0.39 %)~0.01 at.% V-doped TiO2 (0.39 %). The EPR results showed that interfacial charge transfer from the photocatalysts to the adsorbed CO2 and H2O is efficient. Thus, the reduced intermediates determined the low reduction efficiency of CO2 to CH4. The impurity levels locating below the conduction band result in slow reduction kinetics., and the presence of V2O5 moiety at the surface inhibited the reoxidation of CH4.