Boosting the Activity of Pd Single Atoms by Tuning Their Local Environment on Ceria for Methane Combustion

Abstract

Supported Pd single atom catalysts (SACs) have triggered great research interest in methane combustion yet with contradicting views on their activity and stability. Here, we show that the Pd SAs can take different electronic structure and atomic geometry on ceria support, resulting in different catalytic properties. By a simple thermal pretreatment to ceria prior to Pd deposition, a unique anchoring site is created. The Pd SA, taking this site, can be activated to Pd δ + (0 < δ < 2) that has greatly enhanced activity for methane oxidation: T 50 lowered by up to 130 °C and almost 10 times higher turnover frequency compared to the untreated catalyst. The enhanced activity of Pd δ + site is related to its oxygen-deficient local structure and elongated interacting distance with ceria, leading to enhanced capability in delivering reactive oxygen species and decomposing reaction intermediates. This work provides insights into designing highly efficient Pd SACs for oxidation reactions.

Enhancing Methane Combustion Activity by Modulating the Local Environment of Pd Single Atoms in Pd1/CeO2 Catalysts

“…When 1% CH 4 /N 2 was introduced into the IR cell over the Pd 1 /CeO 2 -TS catalyst, a significant number of the CH x species (∼2930 and ∼2840 cm –1 ), carbonate-type species (1350–1540 cm –1 ), and formate species (approximately 1574 cm –1 ) were observed during CH 4 activation, , indicating that the C–H bond can be activated without introducing O 2 . The corresponding isolated hydroxyls formed on the CeO 2 surface were detected at approximately 3653 and 3710 cm –1 , indicating that the H* released from CH 4 * reacts with the surface oxygen (Pd–O*–Ce) species to form hydroxyls . However, these intermediate species were not stable on the Pd 1 /CeO 2 -TS surface when O 2 /N 2 was introduced into the reaction cell.…”

“…Methane is one of the most significant anthropogenic greenhouse gases, with a global warming potential more than 20 times greater than that of carbon dioxide. Therefore, low-temperature methane combustion is crucial for maximizing the use of clean energy while minimizing the greenhouse effect. , Cerium dioxide (CeO 2 ) is an excellent catalyst support with high oxygen storage capacity. , When used to support Pd-based catalysts, it exhibits excellent performance. − Single-atom catalysts (SACs) have become increasingly popular due to their potential to maximize atom efficiency and achieve unique reactivities. ,, In the case of Pd 1 /CeO 2 , high dispersion of Pd is crucial for CO oxidation at low temperatures. − However, earlier research investigating Pd SACs for methane combustion has yielded contradictory results among different research groups. For example, Goodman et al investigated the deactivation mechanism of γ-Al 2 O 3 -supported Pd nanoparticles in methane combustion and found that Pd nanoparticles rapidly lose activity due to redispersion decomposition to inactive Pd single atoms (SAs) at high temperatures .…”

“…Lou et al recently discovered that Pd species with lower oxidation states promoted methane combustion activity and proposed that the Pd species in the form of [AlO 2 ]Pd (OH)-ZSM-5 are active sites, where the stronger Lewis acidity of the Pd and the basicity of oxygen from the anchored PdO x species can facilitate the activation of the C–H bond . Yang et al reported that Pd SAs synthesized by simple thermal pretreatment to ceria can be activated to Pd σ+ (0 < σ < 2), which has greatly increased activity for methane oxidation due to its local oxygen deficient structure as well as its elongated interaction distance with the ceria . These contradictory findings suggest that the catalytic activity of methane combustion may be regulated by the local atomic structure and electronic properties of Pd SAs.…”

“…19 Yang et al reported that Pd SAs synthesized by simple thermal pretreatment to ceria can be activated to Pd σ+ (0 < σ < 2), which has greatly increased activity for methane oxidation due to its local oxygen deficient structure as well as its elongated interaction distance with the ceria. 7 These contradictory findings suggest that the catalytic activity of methane combustion may be regulated by the local atomic structure and electronic properties of Pd SAs. However, a fundamental understanding of the nature of active Pd SACs for methane combustion is lacking.…”

Local structural environment of single-atom catalysts

“…37,[77][78][79][80][81][82][83][84][85] However, there is still a lack of a universal design principle that can provide a universal interpretation of the relationship between the intrinsic properties of the active sites and the catalytic activity of supported SACs, as well as a set of unified guiding principles that govern the formation of SACs. 28,[86][87][88][89][90][91][92][93][94] Thus, analyses of individual elements of SACs not only provide unified principles to understand the nature of active sites in different kinds of SAC, but also inspire deeper insights into the SAC formation mechanism and shed light on carefully rationally designing SACs with effective targeting performance.…”