Impact of Zeolite Beta on Hydrocarbon Trapping and Light-Off Behavior on Pt/Pd/BEA/Al_2O_3 Monolith Catalysts

摘要

The impact of large-pore zeolite beta (BEA) on the performance of model monolithic Pt/Pd/Al_2O_3 diesel oxida-tion catalyst is investigated in terms of hydrocarbon (HC) storage capacity, light-off, and oxidation. Dodecane (C12) is employed as the model HC to elucidate BEA loading impact on catalyst performance during temperature-programmed ox-idation of CO and C12 and mixtures in the absence of feed water. A C12 pre-storage experiment protocol quantifies stor-age at room temperature, the effect of C12 storage on light-off temperature, and the extent of release and conversion of C12. The temperature ramp experiments indicate a negligible im-pact of the zeolite on the individual reactant (CO and C12) light-offs. Co-feed experiments (HC + CO simultaneous feed) reveal light-off inhibition by CO and C12 in addition to that by pre-stored C12. This trend is consistent for all zeolite load-ing levels. During both co-feed and C12 pre-storage experi-ments, an intermediate zeolite loading results in a decreased CO light-off temperature when compared to the zeolite-free catalyst. This beneficial effect is attributed to the zeolite pro-viding alternative storage sites for the HC, thereby mitigating the inhibition on the CO light-off by freeing up precious metal sites for CO oxidation. However, the highest zeolite loading catalyst has an increased CO light-off temperature in C12 pre-storage experiments compared to the catalyst with intermedi-ate loading. This trend reversal is attributed to the kinetic and/ or transport inhibitory effect of stored C12 which overcomes the enhancement at lower loadings. The data indicate that the highest zeolite loading yields the highest percentage of stored C12 converted to CO_2, a finding relevant for HC trap catalyst design. The study findings suggest a need to optimize the zeolite loading for the desired operating conditions to achieve desired light-off and HC trapping and conversion. The impact of feed water will be investigated in future work to evaluate the generality of these findings.