Structural Evolution of Bimetallic PtPd/CeO₂ Methane Oxidation Catalysts Prepared by Dry Milling

Abstract

Bimetallic Pt–Pd catalysts supported on ceria have been prepared by mechanochemical synthesis and tested for lean methane oxidation in dry and wet atmosphere. Results show that the addition of platinum has a negative effect on transient light-off activity, but for Pd/Pt molar ratios between 1:1 and 8:1 an improvement during time-on-stream experiments in wet conditions is observed. The bimetallic samples undergo a complex restructuring during operation, starting from the alloying of Pt and Pd and resulting in the formation of unprecedented “mushroom-like” structures consisting of PdO bases with Pt heads as revealed by high-resolution transmission electron microscopy (HRTEM) analysis. On milled samples, these structures are well-defined and observed at the interface between palladium and ceria, whereas those on the impregnated catalyst appear less ordered and are located randomly on the surface of ceria and of large PdPt clusters. The milled catalyst prepared by first milling Pd metal and ceria followed by the addition of Pt shows better performances compared to a conventional impregnated sample and also to a sample obtained by inverting the Pd–Pt milling order. This has been ascribed to the intimate contact between Pd and CeO2 generated at the nanoscale during the milling process.

Investigation of the evolution of Pd-Pt supported on ceria for dry and wet methane oxidation

“…2f ), demonstrating that the milled catalyst does not deactivate in the presence of steam. This is in accordance with the catalytic performance reported in our previous work, where the sample milled with metallic Pd and, subsequently, metallic Pt exhibited simultaneously improved catalytic activity and stability under prolonged reaction exposure 34 . Here, in situ SXRD analysis further reveals that the increase in methane conversion parallels the increase of PdO concentration, thus allowing to correlate the progressive formation of PdO with the improvement of the catalytic performance.…”

“…The mechanochemical synthesis is a facile, eco-friendly, and readily scalable method where no solvents are used to prepare the catalysts 32 . Four catalysts were synthesized by mechanical milling (denoted as MM): (1) a monometallic 1.5 wt% Pd-CeO 2 catalyst; (2) a monometallic 1.5 wt% Pt-CeO 2 catalyst; (3) a PtPd-CeO 2 catalyst where 1.5 wt% Pd was first milled with CeO 2 and this was subsequently milled with 1.5 wt% Pt; and (4) a PdPt-CeO 2 catalyst where the milling order was inverted (1.5 wt% Pt was first milled with CeO 2 and next 1.5 wt% Pd was added and subsequently milled; note the difference in the order of the metals in the nomenclature) 34 . Additionally, a PdPt/CeO 2 catalyst prepared by traditional incipient wetness impregnation (denoted as IWI) was also synthesized as reference material (see Methods), where Pd and Pt were co-impregnated, and it was calcined at 1173 K.…”

“…To further investigate the gradual increase of PdO for PtPd-CeO 2 MM, which was the best performing catalyst in the wet atmosphere among the ones considered 34 , a PtPd-CeO 2 MM catalyst treated in the laboratory for 24 h under TOS wet at 723 K was studied under in situ conditions. The in situ measurements started directly dosing the wet reaction mixture at RT and ramping it to 723 K (10 K min –1 ).…”

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

“…The similar synergistic effect is observed for the Pd-Ni catalysts in the oxidation of CO [27] and for Pt-Pd/Al 2 O 3 -MnO x catalysts in the oxidation of CO, CH x [28]. It has been shown that the formation of a Pd-Pt alloy could significantly increase the stability of the catalysts in the total oxidation of methane [29][30][31][32][33].…”

Deep Insight into the Mechanism of Catalytic Combustion of CO and CH4 over SrTi1–xBxO3 (B = Co, Fe, Mn, Ni, and Cu) Perovskite via Flame Spray Pyrolysis

“…Until recently, supported noble-metals have been considered as the most active catalysts and have been extensively investigated in catalytic oxidation . The widely used noble metal catalysts, however, are still facing challenges of high cost, inadequate thermal stability, and being prone to deactivation. , To address these issues, great efforts have been devoted to the synthesis of alternative inexpensive and stable catalysts. Perovskites, possessing the merits of tunable catalytic activity and thermal stability, are recognized as potentially accessible substitutes for noble metals .…”