Hägglund Carl, Kasemo Bengt, Osterlund Lars
Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden.
J Phys Chem B. 2005 Jun 2;109(21):10886-95. doi: 10.1021/jp0442448.
The photocatalytic oxidation (PCO) of trace amounts of propane (500 ppm) on nanocrystalline anatase TiO2 has been investigated in situ as a function of temperature (T = 318-473 K), humidity (C(H2O) = 0-4%), and time by means of mass spectrometry and diffuse reflectance Fourier transform infrared spectroscopy (DRIFT). Propane adsorbs associatively on TiO2 at 318 K in dry air, while at 473 K small amounts of thermal dissociation products appear on the surface. In agreement with previous studies, propane is found primarily to be converted to acetone by reactions with photogenerated oxygen radicals. Various successive reaction paths exist, where the branching depends on the temperature and hydroxylation state of the surface. Under dry conditions at 318 K, acetone oxidation is initially kinetically hindered, while, above 400 K, acetone readily decomposes. The thermally assisted reaction channel leads to detrimental bonding of surface species and inhibition of the catalytic activity. It is manifested by a coloration of the sample and suggested to be coupled to surface reduction. Under humidified conditions, there is an optimum of the PCO in C(H2O) and T space, which is estimated to correspond to an equilibrium coverage of one monolayer of H2O (or bilayer). The latter reaction condition also corresponds to sustained high propane conversion and is characterized by rapid establishment of steady state rates. The optimum PCO is discussed in terms of a balance between (i) sustaining enough of a photoactive water monolayer to avoid detrimental bonding of surface species, (ii) allowing reactants to adsorb and access bulk TiO2 photoexcitations, and at the same time (iii) maximizing the thermally assisted decomposition of intermediates.
通过质谱和漫反射傅里叶变换红外光谱（DRIFT），对纳米晶锐钛矿型TiO₂上痕量丙烷（500 ppm）的光催化氧化（PCO）进行了原位研究，考察了其随温度（T = 318 - 473 K）、湿度（C(H₂O) = 0 - 4%）和时间的变化。在干燥空气中，丙烷于318 K时在TiO₂上发生缔合吸附，而在473 K时，表面会出现少量热解离产物。与先前的研究一致，发现丙烷主要通过与光生氧自由基反应转化为丙酮。存在各种连续的反应路径，其分支取决于表面的温度和羟基化状态。在318 K的干燥条件下，丙酮氧化最初在动力学上受到阻碍，而在400 K以上，丙酮容易分解。热辅助反应通道导致表面物种的有害键合并抑制催化活性。这表现为样品变色，并表明与表面还原有关。在加湿条件下，在C(H₂O)和T空间中存在PCO的最佳值，估计其对应于单层H₂O（或双层）的平衡覆盖率。后一种反应条件还对应于持续的高丙烷转化率，其特征是稳态速率迅速建立。从以下几个方面讨论了最佳PCO：（i）维持足够的光活性水单层以避免表面物种的有害键合；（ii）允许反应物吸附并接触本体TiO₂光激发；同时（iii）使中间体的热辅助分解最大化。
