INCAPE   05401
Unidad Ejecutora - UE
capítulos de libros
The Nature of Cobalt Species in Co-Zeolite Used for the Selective Catalytic Reduction of NOx with Hydrocarbons
Cobalt: Occurrence, Uses and Properties
Nova Publishers
Año: 2013; p. 135 - 166
The Selective Catalytic Reduction of NOx (SCR-NOx) in excess of oxygen and using a hydrocarbon (HC) as a reducing agent represents an alternative technology for the NOx removal from stationary and mobile sources. Cobalt-exchanged zeolites (ZSM5, MOR, BEA) are active and selective catalysts to reduce NOx by hydrocarbons under dry conditions. However, a key factor that has limited their use is the deleterious effect of water upon the selectivity to N2 production. This effect is particularly evident when methane is used as reductant. Extensive efforts have been made to improve the NOx reduction activity of Co-based catalysts and more important still, their durability under real operation conditions. In this sense, the addition of noble metals (Pt, Pd or Ag) to Co-zeolites resulted in a better water tolerance due to the cooperative effect between different active centers. Moreover, some effects of water are revealed by comparing the catalytic performances of SCR with various reductants under dry and wet conditions. This work presents a more detailed study of the nature of cobalt species in the catalysts based on cobalt exchanged in zeolites of different structure (MOR, ZSM5 and FER). It also analyzes the effect of the addition of Pt or Ag on the active sites for the SCR-NOx. The selective reduction of NOx was performed with various reductants (methane, butane or toluene) under dry and wet conditions. Moreover, a thorough characterization of catalysts before and after catalytic reaction was carried out using a battery of techniques such as TPR, Laser-Raman, UV-Vis diffuse reflectance and XPS. Different cobalt species were present in the active and selective Co-zeolite catalysts. Through TPR, Laser Raman, XPS and UV-Vis, it was possible to identify the presence of a small fraction of highly dispersed Co3O4 species with Co2+ ions located in α, β and γ exchange sites. The addition of Pt promoted the NO to N2 conversion on the SCR with CH4. The catalysts obtained proved to be stable under dry and wet conditions due to the synergic effect of Pt incorporated to Co-zeolites. Furthermore, the catalytic behavior of Co-exchanged mordenite using butane or toluene as reducing agent was evaluated as well as the effect of silver on the activity and selectivity of catalysts. The catalytic results showed that the presence of water in the stream produced a positive effect on the conversion to N2 with both hydrocarbons, in contrast with the catalytic performance observed using methane. Through temperature-programmed oxidation (TPO) and XPS of the samples used in wet and dry conditions with butane, it was observed that the water vapor helped to maintain the surface clean of carbonaceous deposits. Moreover, the results obtained by TPR, XPS and UV-Vis on the used samples indicated that the Co2+ ion with high interaction with the structure migrated towards less stable sites.