Bhattacharyya K, Varma S, Kumar D, Tripathi A K, Gupta N M
Applied Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India.
J Nanosci Nanotechnol. 2005 May;5(5):797-805. doi: 10.1166/jnn.2005.114.
We report in this paper on the ultraviolet-assisted vapor-phase oxidation of methanol at room temperature, with the help of nano-size clusters of titanium dioxide dispersed in an MCM-41 silicate matrix. The surface species formed during the adsorption/oxidation of methanol and the transformation that they undergo as a result of ultraviolet irradiation were monitored using in-situ Fourier transform infrared and thermal desorption spectroscopy techniques. Parallel experiments conducted on TiO2/MCM, bulk titania, and pristine MCM-41 samples helped in identifying the individual role of titanium dioxide and host matrix in these processes. The photo-catalytic oxidation of methanol, at concentrations of 0.1 to 1.1 mol% in air, gave rise to formation of CO2 and H2O as products, for both the TiO2/MCM and bulk TiO2 samples. No such reaction occurred on titania-free MCM. Furthermore, the rate of reaction depended upon the TiO2 content of a sample and also on the concentration of methanol in reaction mixture. Thus, the rate of conversion increased progressively with the increase in TiO2 loading from 5 to 21 wt% in TiO2/MCM samples, particularly for the experiments with high concentration of methanol. For low methanol concentration (0.1 mol%) in air, the effect of titania content in MCM was very small. The specific activity (per g of titania) of a sample, on the other hand, showed an inverse relationship with the loading of titanium dioxide in a sample. Infrared and temperature-programmed desorption results revealed that the mode of CH3OH adsorption and the reactivity of the transient species formed during the oxidation process were independent of the size of dispersed titania particles. Thus, the particles of approximately 2-6 nm size, present in TiO2/MCM, exhibited a chemisorption behavior similar to that of the bulk titania. The results of the present study provide strong evidence that the hydroxyl groups, both on the host matrix and at the titania sites, participate independently in the formation of methoxyl groups and at the same time promote the heterogeneous photo-catalytic oxidation of methanol molecules via formation of transient formate groups. Our results also show that the effect of titania crystallite size in the photo-catalytic properties relate mainly to the larger surface area and hence to the enhanced number of chemisorption sites, rather than to the changes in electronic properties.
我们在本文中报道了在室温下借助分散于MCM - 41硅酸盐基质中的纳米尺寸二氧化钛团簇对甲醇进行紫外线辅助气相氧化的研究。利用原位傅里叶变换红外光谱和热脱附光谱技术监测了甲醇吸附/氧化过程中形成的表面物种以及它们因紫外线照射而发生的转变。在TiO₂/MCM、块状二氧化钛和原始MCM - 41样品上进行的平行实验有助于确定二氧化钛和主体基质在这些过程中的各自作用。对于TiO₂/MCM和块状TiO₂样品，在空气中甲醇浓度为0.1至1.1 mol%时的光催化氧化产生了CO₂和H₂O作为产物。在不含二氧化钛的MCM上未发生此类反应。此外，反应速率取决于样品的TiO₂含量以及反应混合物中甲醇的浓度。因此，在TiO₂/MCM样品中，随着TiO₂负载量从5 wt%增加到21 wt%，转化率逐渐增加，特别是对于高浓度甲醇的实验。对于空气中低甲醇浓度（0.1 mol%），MCM中二氧化钛含量的影响非常小。另一方面，样品的比活性（每克二氧化钛）与样品中二氧化钛的负载量呈反比关系。红外光谱和程序升温脱附结果表明，CH₃OH的吸附模式以及氧化过程中形成的瞬态物种的反应活性与分散的二氧化钛颗粒大小无关。因此，存在于TiO₂/MCM中的约2 - 6 nm大小的颗粒表现出与块状二氧化钛类似的化学吸附行为。本研究结果提供了有力证据，表明主体基质和二氧化钛位点上的羟基独立参与甲氧基的形成，同时通过形成瞬态甲酸根基团促进甲醇分子的多相光催化氧化。我们的结果还表明，二氧化钛微晶尺寸对光催化性能的影响主要与更大的表面积相关，因此与化学吸附位点数量的增加有关，而不是与电子性质的变化有关。
