Pt/Al₂O₃@Ce/ZrO₂-S bifunctional catalysts prepared by mechanically milling for selective catalytic oxidation of...

The selective catalytic oxidation (SCO) is an effective method for removing slipped high-concentration ammonia from NH₃-fueled engine exhaust gas. Herein a novel bifunctional catalyst was synthesized by mechanically mixing sulfated Ce/ZrO₂ (Ce/ZrO₂-S) with a small fraction of Pt/Al₂O₃ (Pt 0.1 wt.%) for SCO of NH₃. As expected, the introduction of a small amount of Pt/Al₂O₃ significantly improved NH₃ conversion ability of Ce/ZrO₂-S catalysts toward low-temperature direction. When the mass ratio of Pt/Al₂O₃ to Ce/ZrO₂-S was 7.5% (the corresponding mixed catalyst was denoted as P@CZS-7.5), T₉₀ temperature was 312 °C. More importantly, P@CZS-7.5 catalyst exhibited a much better N₂ selectivity (> 96%) in a wide temperature range (320 ~ 450 °C). H₂-TPR results revealed that the addition of a trace amount of Pt/Al₂O₃ significantly led to a distinct shift of reduction temperature peak toward low-temperature direction, thereby greatly improved the low-temperature redox performance of mixed catalysts. Furthermore, NH₃-TPD and BET results showed that P@CZS-7.5 catalyst exhibited a similar NH₃ adsorption capacity to Ce/ZrO₂-S catalyst, while the former had a relatively higher specific surface area than the latter. It was considered as a crucial factor for P@CZS-7.5 catalyst maintaining superior N₂ selectivity in high-concentration NH₃ (5000 ppm) removal processes. In situ DRIFTS results indicated that P@CZS-7.5 catalyst followed the internal selective catalytic reduction (i-SCR) mechanism in NH₃-SCO reactions.