Oxidation of C3H8, iso-C5H12 and C3H6 under near-stoichiometric and fuel-lean conditions over aged Pt–Pd/Al2O3 catalysts with different Pt:Pd ratios

• Published in: Applied catalysis. B, Environmental Vol. 251; pp. 283 - 294
• Main Authors: Kim, Jonghyun, Kim, Yongwoo, Wiebenga, Michelle H., Oh, Se H., Kim, Do Heui
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 15.08.2019; Elsevier BV
• Subjects: Adsorption; Alloying effects; Aluminum oxide; Catalysis; Catalysts; Chemisorption; Cold starts; Emissions; Exhaust pipes; Fully lean condition; High temperature; Hydrocarbon oxidation; Hydrocarbons; Lean-rich cycling; Low temperature; Near-stoichiometric condition; Noble metals; Organic chemistry; Oxidation; Oxygen; Palladium; Pentane; Platinum; Propane; Propylene; Pt–Pd; Surface chemistry; Near-stoichiometric condition; Lean-rich cycling; Hydrocarbon oxidation; Fully lean condition; Pt–Pd
• ISSN: 0926-3373; 1873-3883
• DOI: 10.1016/j.apcatb.2019.04.001

[Display omitted] •Hydrocarbon (HC) oxidations were tested over Pd-only, Pt-only, and Pt–Pd catalysts.•Activity tests were performed under both near-stoichiometric and fully lean conditions.•Pd was oxidized under fully lean condition, while Pt preserved its oxidation state.•Synergistic effect of bimetallic Pt–Pd was observed under fully lean condition. Effective control of hydrocarbon (HC) emissions at low temperatures is critical for emission compliance, since a large fraction (typically 70–80%) of tailpipe HC emissions occurs during the first couple of minutes after an engine cold-start. The oxidation of propane, iso-pentane and propylene was investigated over Pd-only, Pt-only and Pt–Pd catalysts aged under lean-rich cycling conditions. Various characterizations such as XRD, XANES, STEM, CO chemisorption and H2-TPR were performed over the initial catalysts. Moreover, before and after the reaction, the states of the catalysts were characterized by XRD, XANES and STEM. The oxidation activity tests were carried out under both near-stoichiometric and fully lean conditions in the feedstream containing HC, O2, CO, H2, NO and H2O. Under the near-stoichiometric condition, the Pt/Al2O3 catalyst showed much higher activity for propane oxidation than the Pd/Al2O3 and Pt–Pd/Al2O3 catalysts. For propylene oxidation, on the other hand, the Pd/Al2O3 catalyst was the most active, and the Pt/Al2O3 catalyst exhibited the lowest activity due to the fact that the co-presence of CO and propylene on the Pt surface leads to their competition for a limited amount of adsorbed oxygen. Under the fully lean condition, metallic Pd in the Pd/Al2O3 catalyst was gradually oxidized to PdO and consequently poisoned by H2O, whereas there was little change in the state of the Pt/Al2O3 catalyst, indicating higher stability of metallic Pt than Pd. However, a drastic decrease in the propane oxidation activity was observed over Pt/Al2O3 in the presence of excess oxygen in the fully lean feed since preferential adsorption of oxygen on the Pt surface inhibited propane adsorption and thus the oxidation reaction. In addition, the propylene oxidation activity under the fully lean condition was greatly enhanced over all three catalysts compared to that observed under the near-stoichiometric condition, with Pd/Al2O3 still being more active than Pt/Al2O3. The catalyst activity for iso-pentane oxidation was affected only to a relatively small extent by the high oxygen concentration, so that the same activity order among the catalysts (Pt > Pt–Pd > Pd) was observed under both the slightly lean and fully lean conditions. These results indicate that both hydrocarbon type and oxygen concentration level are important factors determining the relative oxidation activities of the various noble metal catalysts. Relatively small beneficial effects of Pt–Pd alloying were observed during iso-pentane oxidation at low temperatures under the near-stoichiometric conditions and during propane oxidation at high temperatures under the fully lean condition. However, alloy formation was found to be detrimental for the propane oxidation under the slightly lean condition.