Pt/TiO_2催化剂上的氢—氧作用 Ⅰ.氢、氧的吸咐—脱附行为以及相互促进作用

Interaction of Hydrogen and Oxygen on Pt/TiO_2 Catalyst Ⅰ. Temperature Programmed Desorption of Hydrogen and Oxygen and Their Mutual Effect

本文采用程序升温脱附(TPD)技术研究了光沉积方法制备的Pt/TiO_2催化剂经过氧化、还原后氧、氢的脱附行为.光沉积过程中,Pt/TiO_2表面上可以生成大量的吸咐氢,在TPD中脱附;同时Pt/TiO_2表面上化学吸附的水在TPD过程中也可以分解释氢.氧化处理的Pt/TiO_2在TPD过程中于550～750K温区出现氧脱附峰,随着氧化温度升高,脱附峰位向高温移动,经实验证明,这种可脱附活泼氧物种的生成是由样品前身中留存氢引起的.还原处理的Pt/TiO_2在TPD过程中分别在300～600和大于600K出现两个氢脱附峰,认为是由于表面羟基和钛—氢(Ti^(4+)—H^-)物种的分解释氢引起的Pt/TiO_2上活泼氧物种的存在,增加了样品在室温条件下的吸氢量;在中温(473～573K)这种活泼氧物种则和氢发生反应,减少了TPD过程中的脱氢量;Pt/TiO_2在大于673K温度还原,可以消除活泼氧物种的影响.

Desorption of hydrogen and oxygen of Pt/TiO2 were studied by temperature programmed desorption (TPD) technique. In the TPD procedure, hydrogen desorption was observed on the fresh Pt/ iO2 catalyst prepared by photo-deposited method, which was found to be arisen from both the adsorbed hydrogen formed during catalyst preparation and dissociation of the chemically adsorbed water. For the Pt/TiO2samples oxidized in oxygen at 573,673,773 K respectively, one kind of labile oxygen species was detected to give oxygen desorption in the temperature range 550-750 K. With the increase of oxidation temperature, the peak of oxygen desorption shifted to higher temperature. Furthermore it was proved that the formation of the labile oxygen species depended on the residual hydrogen of Pt/TiO2. The influence of the labile oxygen species on hydrogen adsorption-desorption was investigated using two different kinds of Pt/TiO2catalysts, i. e. , PA sample with the presence of labile oxygen and PB sample on which the labile oxygen species had been removed. From the comparison of the TPD patterns of PA and PB catalysts reduced in the temperature range 300-573 K, it was observed that existence of the labile oxygen species gave a positive effect on hydrogen adsorption on Pt/TiO2 at room temperature, and reaction of the labile oxygen species with hydrogen occured at above 473 K to decrease the hydrogen desorption in TPD. After they were reduced in hydrogen at above 673 K, PA and PB samples showed similar TPD patterns, two peaks of hydrogen desorption appeared in the temperature regions of 300 - 600 K and above 600 K, which are presumably contributed by dissociation of surface hydroxyl groups and the titanium hydride(Ti4+-H-) respectively.