Influence of the Ce–Zr promoter on Pd behaviour under dynamic CO/NO cycling conditions: a structural and chemical approach

Keys Unlocking Redispersion of Reactive PdOx Nanoclusters on Ce-Functionalized Perovskite Oxides for Methane Activation

Nowadays, trace CH₄ emitted from vehicle exhausts severely threaten the balance of the ecology system of our earth. Thereby, the development of active and stable catalysts capable of methane conversion under mild conditions is critical. Here, we present a convenient method to redisperse catalytically inert PdO nanoparticles (NPs) (>10 nm) into reactive PdO_x nanoclusters (∼2 nm) anchored on a Ce-doped LaFeO₃ parent. Isothermally activated in an N₂ flow, the redispersed catalyst achieved a CH₄ conversion of 90% at 400 °C, which is significantly higher than the fresh and H₂- and O₂-treated counterparts (625, 616, and 641 °C, respectively), indicating the importance of the gas atmosphere in the redispersion of PdO NPs. In addition, the comprehensive catalyst characterizations demonstrated that the isolated Ce ions in the perovskite lattice play an irreplaceable role in the redispersion of reactive sites and the reduction of the energy barrier for C-H scission. More importantly, the Ce additive helps to stabilize the PdO_x species by reducing overoxidation, resulting in significant lifetime extension. Through a thorough understanding of structural manipulation, this study sheds light on the design of highly performing supported catalysts for methane oxidation.

Effect of oxygen storage materials on the performance of Pt-based three-way catalysts

Pt supported on oxygen storage materials (CeO2 and CeO2–ZrO2) as effective three-way catalysts.

Thermal Deactivation of Pd/CeO2-ZrO2 Three-Way Catalysts during Real Engine Aging: Analysis by a Surface plus Peripheral Site Model

The thermal deactivation of Pd/CeO₂-ZrO₂ (Pd/CZ) three-way catalysts was studied via nanoscale structural characterization and catalytic kinetic analysis to obtain a fundamental modeling concept for predicting the real catalyst lifetime. The catalysts were engine-aged at 600-1100 °C and used for chassis dynamometer driving test cycles. Observations using an electron microscope and chemisorption experiments showed that the Pd particle size significantly changed in the range of 10-550 nm as a function of aging temperatures. The deactivated catalyst structure was modeled using different-sized hemispherical Pd particles that were in intimate contact with the support surface. Therefore, Pd/CZ contained two types of surface Pd sites residing on the surface of a hemisphere (Pd_s) and circular periphery of the Pd/CZ interface (Pd_b), whereas a reference catalyst, Pd/Al₂O₃, contained only Pd_s. In all Pd particle sizes investigated herein, Pd/CZ exhibited higher reaction rates than Pd/Al₂O₃, which nonlinearly increased with increasing slope as the weight-based number of surface-exposed Pd atoms ([Pd_s] + [Pd_b]) increased. This finding contrasted with that of Pd/Al₂O₃, where the reaction rate linearly increased with [Pd_s]. When the Pd_s sites in both catalysts were equivalent in terms of their specific activities, the activity difference between Pd/CZ and Pd/Al₂O₃ corresponded to the contribution from Pd_b, where oxygen storage/release to/from CZ played a key role. This contribution linearly increased with [Pd_b] and therefore decreased with Pd sintering. Although both Pd_s and Pd_b sites showed nearly constant turnover frequencies despite the difference in the Pd particle size, the values for Pd_b were more than 2 orders of magnitude greater than those for Pd_s when assuming a single-atom width one-dimensional Pd_b row model. These results suggest that the thermal deterioration of the three-phase boundary site, where Pd, CZ, and the gas phase meet, determines the activity under surface-controlled conditions.

DRIFTS study of the role of alkaline earths in promoting the catalytic activity of HC and NOx conversion over Pd-only three-way catalysts

Modification with alkaline earths, especially Ba, promotes NO_x storage efficiency and NO dissociation, as well as the deep oxidation of HC. Thus doped catalysts present improved catalytic activity.