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.
Highlights
The widespread diffusion of natural gas fueled vehicles (NGVs) has led to the enforcement of strict regulation on CH4 emissions due to the 100 year time horizon global warming potential of methane, which is 28 times that of CO2.1,2 The emission limits coupled with the low reactivity of methane molecule constitute a serious challenge for methane abatement catalysts, which should attain high activity at low temperature, thermal stability, and resistance to poisoning by water and sulfur.[3]
Palladium-based catalysts are generally recognized as the most active for methane oxidation,[4,5] but their performances are severely affected by thermal aging and the presence of steam and SO2 in the exhaust gases.[6−8] Several attempts have been made to improve the resistance against deactivation, in many cases by engineering the catalyst structure[9−11] or by the addition of co-metals and promoters.[12−18] an effective way to enhance the stability and durability of Pd-based catalysts is the introduction of platinum in the catalytic formulation,[19−23] with bimetallic PtPd-based catalysts being the state-of-the-art catalysts for lean burn NGVs.[3]
We have recently reported the outstanding activity for methane oxidation observed on Pd/CeO2 catalysts prepared by mechanochemical synthesis, which has been attributed to the strong palladium−ceria interface interaction obtained at the Received: March 17, 2021
Summary
The widespread diffusion of natural gas fueled vehicles (NGVs) has led to the enforcement of strict regulation on CH4 emissions due to the 100 year time horizon global warming potential of methane, which is 28 times that of CO2.1,2 The emission limits coupled with the low reactivity of methane molecule (least reactive among all hydrocarbons) constitute a serious challenge for methane abatement catalysts, which should attain high activity at low temperature, thermal stability, and resistance to poisoning by water and sulfur.[3]. The significant amount of research on PtPd bimetallic systems has clearly established the beneficial role of Pt introduction on the stability of Pd-based catalysts during timeon-stream operation and against poisoning,[12,19,23−26] whereas the effect on the transient activity during light-off experiments is still debated, with some authors observing an improvement for specific Pt/Pd ratios[27−31] and others concluding that the presence of Pt is generally detrimental for transient operation.[12,19,32] The discrepancies often arise from different experimental conditions and/or catalyst pretreatment before testing. The results unambiguously show that a deep restructuring takes place between Pd and Pt during the operation and at the same time prove the effectiveness of the mechanical milling to prepare highly stable bimetallic PtPd/CeO2 catalysts with unique interface characteristics
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