The effects of nitrogen-doping on photocatalytic mineralization of TiO2 nanocatalyst against formaldehyde in ambient air

Abstract

A series of proto-type photocatalytic air purifier (AP (Nx-Cy)) systems are built with a nitrogen-doped TiO2 (N-TiO2)-impregnated honeycomb (HC) filter for photocatalytic decomposition of 0.5–5 ppm formaldehyde (FA: CH2O) vapor under varying conditions and UV-LED light (1 watt). The binary codes of Nx and Cy in AP systems are used as the composition identifiers to represent N/Ti molar ratios (0 to 20) and N-TiO2 concentration (2 to 20 mg mL–1), respectively. The AP (N10-C10) is found as an optimum unit with the highest capability to boost the catalytic conversion of CH2O to CO2 (yield = 89.2% over 10th cycles and the clean air delivery rate (CADR) of 9.45 L min–1 in dry air). The superior charge carrier lifetime (τa: 1.70 ns) of N10-C10 over others (e.g., 1.37 ns for pure TiO2) should indicate the influential role of N-defects (No) in reducing the bandgap (3.10 eV) and in creating defect-related oxygen vacancy (OVs-Ti3+) states as predicted by the density functional theory (DFT) simulation. The photocatalytic oxidation pathway of CH2O, when assessed by diverse approaches (e.g., in-situ diffuse reflectance infrared Fourier transform, electron paramagnetic resonance, and DFT analyses), is found to involve several energetically favorable intermediate steps (such as exothermic covalent adsorption of CH2O to bridged O/OH groups on TiO2-OV {110} surface in the form of CH2O2 followed by catalytic dehydrogenation/oxidation reactions to yield CO2 through direct route: CH2O2/HCOO– + •OH → H2O + CO2). These steps are supported by the calculated density of states (DOS) for chemically active Ti-atom on {101} surface with N-impurity. The presence of No-defects and OVs is expected to influence the reaction energetics and intermediates for efficient mineralization in humidified conditions by lowering the activation barriers. This study offers valuable insights into the design and construction of an advanced photocatalytic system for efficient mineralization of aldehyde VOCs in ambient air.

通过氮掺杂TiO2 (N-TiO2)浸渍蜂窝过滤器构建了一系列原型光催化空气净化器(AP(Nx-Cy))系统, 并用于在UV-LED 光(1 W)照射条件下光催化分解0.5–5 ppm甲醛(CH2O)蒸汽. 在上述催化过滤器系统中, Nx和Cy分别代表N/Ti摩尔比(0‒20) 和N-TiO2浓度(2‒20 mg mL‒1 ). 光催化分解实验结果表明, AP(N10-C10)的性能最好, 其催化CH2O转化为CO2的转化率最高, 循环反应10次后CO2产率仍达到89.2%, 在干燥空气中的清洁空气输送速率为9.45 L min‒1 . N10-C10的电荷载流子寿命(τa:1.70 ns)优于其他样品(如纯TiO2的电荷载流子寿命为1.37 ns), 这表明N缺陷(No)有助于降低带隙(3.10 eV)和产生氧空位 OVs-Ti3+, 这与密度泛函理论(DFT)模拟结果一致. 采用原位漫反射红外傅里叶变换、电子顺磁共振和DFT分析等多种方法研究了CH2O的光催化氧化途径. 结果表明, 氧化过程涉及多个能量有利的中间步骤(例如CH2O以CH2O2的形式在TiO2-OV {110} 表面的桥连O/OH基团上发生放热共价吸附, 随后通过催化脱氢/氧化反应直接生成CO2: CH2O2/HCOO‒ + •OH → H2O + CO2）. 这些步骤与具有N杂质的{101}表面上化学活性Ti原子的态密度计算结果一致. 预计No缺陷和OVs的存在将通过降低活化能垒来影响反应能量和中间产物, 从而在加湿条件下实现有效的矿化. 综上, 本文为设计和构建先进的光催化系统, 并用于环境空气中醛类挥发性有机物(VOCs)的有效矿化提供了新思路.