Influence of Pretreatment on the Interaction of Oxygen with Silver and the Catalytic Activity of Ag/SiO_2 Catalysts for CO Selective Oxidation in H_2

摘要

The interactions of oxygen with pre-reduced silver catalysts as well as their catalytic properties for CO selective oxidation in H_2 after oxygen pre-treatment are studied in this paper. It is found that the pretreatment exerts a strong influence on the activity and selectivity of the silver catalyst. A drop in activity and selectivity is observed after treating a pre-reduced catalyst with oxygen at low temperatures, whereas a converse result is obtained after an oxidizing treatment at high temperatures (T ≥ 350℃). O_2-TPD results show that surface oxygen species adsorbs on silver surface after the oxygen treatment at low temperatures. However, penetration of oxygen into the silver is enhanced by a high temperature treatment, meanwhile the surface oxygen species disappear. No other silver species except metallic silver are observed on all the catalysts by XRD, and the size of silver particle is not changed after the treatment with oxygen at low temperatures. The surface oxygen species formed by oxygen treatment can also be removed by hydrogen reduction. The strongly-adsorbed surface oxygen species prohibit the adsorption and diffusion of oxygen species in reaction gas on the surface of silver catalyst, causing the decrease in CO oxidation activity, in other words, it is important to obtain a clean silver surface for increasing the catalyst activity in CO removal from H_2-rich feed gas. The differences in activity and selectivity due to the oxygen pretreatment at different temperatures are discussed in terms of the changes in the surface/subsurface oxygen species of the silver particles.