Effects induced by interaction of the Pt/CeOx/ZrOx/γ-Al2O3 ternary mixed oxide DeNOx catalyst with hydrogen

Abstract

Effects of H2/D2 adsorption on the surface chemistry of Pt/CeOx-ZrOx/γ-Al2O3 DeNOx catalyst were investigated. In-situ FTIR spectroscopy and NOx-TPD techniques were utilized to monitor changes in the surface chemistry of studied materials. Adsorption studies of CO and O2 revealed that the Pt/Ce-Zr/Al sample, initially reduced with H2 at 723 K, is characterized by the presence of oxygen vacancies in close vicinity of Ce3+ centres and metallic Pt sites. Adsorption of O2 occurred through the formation of superoxide (O2−)ads species and oxidation of Ce3+ to Ce4+ ions. The ability of the catalyst to activate molecular O2 originates from its relatively high population of oxygen vacancies located on/near the surface. Interaction of Pt/Ce-Zr/Al system with H2 or D2 takes place through heterolytic dissociation at ambient temperature. D2 adsorption leads to the reduction of Ce4+ to Ce3+ ions and formation of adsorbed molecular heavy water and gradual D/H exchange with the existing surface hydroxyl groups. Generated D2O interacts with isolated hydroxyls/deuteroxyls through H-bonding and this provokes the formation of H-bonded OH/OD groups. These later species are relatively stable and gradually vanish with increasing temperatures above 523 K, leaving behind only isolated hydroxyls. Surfaces enriched with H-bonded hydroxyls are characterized with an enhanced NOx storage ability revealing their significant role in low-temperature NOx adsorption mechanism.