Methane Combustion over Pd/Al₂O₃ Catalysts in the Presence of Water: Effects of Pd Particle Size and Alumina Crystalline Phase

Abstract

The effects of the Pd particle size and the Al2O3 crystalline phase of Pd/Al2O3 catalysts on the CH4 combustion in the presence of H2O were investigated. According to X-ray absorption fine structure (XAFS) measurements, it was revealed that, during the CH4 combustion, the Pd nanoparticles existed on the Al2O3 support as a PdO phase, while the crystallinity of PdO depended on the Pd particle size. On the basis of X-ray diffraction (XRD), amorphous PdO particles with a size of <7 nm exhibited low activity for CH4 combustion. In contrast, as the Pd particle size increased, larger crystalline PdO particles (>7 nm) were formed, which were highly active for CH4 combustion. Comparing the effects of the different Al2O3 crystalline phases, Pd/α-Al2O3 was proved to be more resistant to deactivation by H2O than Pd/γ-Al2O3 and Pd/θ-Al2O3. Moreover, according to H2O temperature-programmed desorption (TPD) and infrared (IR) measurements, since α-Al2O3 was relatively hydrophobic compared to γ-Al2O3 and θ-Al2O3, a faster and reversible adsorption/desorption of OH/H2O species was achieved, while the H2O poisoning on PdO species in the vicinity was limited.

Atomic Structure of Pd-, Pt-, and PdPt-Based Catalysts of Total Oxidation of Methane: In Situ EXAFS Study

“…Chen et al found a decrease in activity with an increase in the Pd particle size [18]. The increase in catalytic activity with particle size was reported by Stakheev et al [24] and Murata et al [25].…”

Penta‐coordinated Al3+ Stabilized Defect‐Rich Ceria on Al2O3 Supported Palladium Catalysts for Lean Methane Oxidation

“…However, Murata et al. reported that the Pd nano‐particles over the bare γ−Al 2 O 3 exposed an amorphous‐like surface with a high fraction of corner sites, resulting in low activities and hydrothermal durability for lean methane combustion [6] . Thus, improving the potential of γ−Al 2 O 3 supported nano‐catalysts requires a meticulous analysis of the effects of active dopants at the atomic scale on the formation of active Pd sites through electronic and steric interactions.…”

Elucidation of the Active Phase in Pd‐Based Catalysts Supporting on Octahedral CeO2 for Low‐Temperature Methane Oxidation

“…For decades, a lot of efforts have been put into development of palladium catalysts for lean CH 4 oxidation, finding that the activity of the CH 4 oxidation is closely related to the shape and size of Pd phase, and even with reaction gas compositions. [5] Besides, numerous research studies have been devoted to unravel the chemical nature for the remarkable activity of Pd species, whereas there are still some controversies in reported literature concerning which is the most active state of the Pd-based catalysts, and the nature and identity of the active phase in Pd catalysts also remain a subject of active dispute. [6] Notably, it is generally reported that metallic Pd, [7] PdO, [8] reduced PdO x [9] and the synergy among disparate species [10] have been supposed as the possible active phase for methane oxidation.…”