摘要
研究了氧在Ag/SiO2 催化剂上的超高真空程序升温脱附 .结果表明 ,脱附谱中出现了对应于表面分子氧 (Tp=340K)、体相氧 (Tp=5 70K)和次表层氧 (Tp=70 0～ 80 0K)的脱附峰 .由于催化剂在制备过程中经过高温焙烧 ,因而其表面原子氧浓度低 ,脱附谱中未出现原子氧的脱附峰 .高温焙烧还可使表面缺陷浓度增大 ,有利于原子氧向体相扩散 ,形成体相溶解氧 ,也有利于体相氧向表面扩散 ,所以对应于体相氧的 5 70K脱附峰较强 .体相氧和次表层氧向表面的扩散遵循不同的扩散机理 .
There are two adsorption modes for oxygen on silver: atomic and molecular. The results of ultra high vacuum temperature programmed desorption of oxygen show that three states of oxygen are present on the Ag/SiO 2 surface. They are surface molecular oxygen ( T p=340 K), bulk dissolved atomic oxygen ( T p=570 K), and sub surface atomic oxygen ( T p=700～800 K). No surface atomic oxygen species is detected, owing to the decrease of the concentration of the surface atomic oxygen during the high temperature treatment in the preparation process of Ag/SiO 2. The high temperature treatment also increases the defect concentration on the silver surface, which favors the diffusion of surface atomic oxygen to the bulk and to form bulk dissolved oxygen. The surface molecular oxygen desorbs at 340 K and does not react with CO. Increasing the partial pressure of gas phase oxygen can facilitate the diffusion of oxygen to the bulk, resulting in an increase in the intensity of the desorption peak of the bulk dissolved oxygen. The desorption temperature of the sub surface oxygen is higher than those of the other two oxygen species because of the high resistance for the diffusion of oxygen from the bulk to the surface. Both the bulk dissolved and the sub surface oxygen desorption peaks are formed via the diffusion of bulk dissolved oxygen to the surface but follow different diffusion mechanisms. The bulk dissolved oxygen diffuses via an interstitial mechanism at low temperature since the active energy is low, while the sub surface oxygen must get over much higher active energy to perform interstitialcy diffusion at high temperature.
出处
《催化学报》
SCIE
CAS
CSCD
北大核心
2003年第9期669-673,共5页
基金
国家自然科学基金资助项目 (2 0 0 73 0 0 9)
关键词
银
二氧化硅
超高真空程序升温脱附
扩散机理
silver, silica,ultra high vacuum temperature programmed desorption, diffusion mechanism
