On the activation of Pt/Al2O3 catalysts in HC-SCR by sintering: determination of redox-active sites using Multitrack

A highly dispersed Pt/Al2O3 catalyst was used for the selective catalytic reduction of NOx using propene (HC-SCR). Contact with the reaction gas mixture led to a significant activation of the catalyst at temperatures above 523 K. According to CO chemisorption data and HRTEM analysis, Pt particles on the activated catalyst had sintered. The redox behavior of the fresh and sintered catalysts was investigated using Multitrack, a TAP-like pulse reactor. If Pt particles on the catalyst are highly dispersed (average size below ∼2 nm), only a small part (∼10%) of the total number of Pt surface sites as determined by CO chemisorption (Ptsurf) participates in H2/O2 redox cycles (Ptsurf,redox) in Multitrack conditions. For a sintered catalyst, with an average particle size of 2.7 nm, the number of Ptsurf and Ptsurf,redox sites are in good agreement. Similar results were obtained for both catalysts using NO as the oxidant. The low number of Ptsurf,redox sites on highly dispersed Pt/Al2O3 is explained by the presence of a kinetically more stable—probably ionic—form of PtO bonds on all surface sites of the smaller Pt particles, including corner, edge and terrace sites. When the average particle size shifts to ∼2.7 nm, the kinetic stability of all PtO bonds is collectively decreased, enabling the participation of all Pt surface sites in the redox cycles. ar correlation between the NOx conversion in HC-SCR, and the amount of Ptsurf,redox was found. This suggests that redox-active Pt sites are necessary for catalytic activity. In addition, the correlation could be significantly improved by assuming that Ptsurf,terrace sites of the particles larger than 2.7 nm are mainly responsible for HC-SCR activity in steady state conditions. Implications of these results for the pathway of HC-SCR over Pt catalysts are discussed.