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

Abstract

Pd catalysts supported on CeO 2 (Pd/CeO 2 ) are highly active towards lean methane combustion, and the catalytic behavior is substantially affected by the chemical property of surface palladium species. In this work, various Pd catalysts supported on octahedral-CeO 2 (Pd/O-CeO 2 ) were successfully prepared by redox interation-engaged strategy. The oxidation state of palladium species was controlled by varying the calcination condition. X-ray absorption characterization coupled with lightoff curves confirmed that the catalytic activity of relevant catalysts was closely related with the proportion of the metallic Pd and PdO species. Within limits, increasing Pd 0 /PdO ratio can bring superior catalytic behavior and the approximately equal ratio of interfacial hybrid Pd 0 /PdO phase exhibits the optimal catalytic activity. Importantly, in situ DRIFTS results demonstrated that the hybrid Pd 0 /PdO phase was recognized as the active site pair for lean methane oxidation, which is attributed to the fact that the metallic Pd 0 species can adsorb and dissociate methane molecules and then the resulting intermediate species further react with the active O* of PdO, which thereby promotes CH 4 activation.

Single‐Step Oxidation of Low‐Concentration Methane to Methanol in the Gaseous Phase Using Ceria‐Based Iridium‐Copper Catalysts

“…Regarding previous research, [5] the peak located at 529.3 eV can be identified as the lattice oxygen (O α , e.g, O 2− ), the binding energy situated at 531.1 eV can be attributed to the adsorbed oxygen components (O β , e.g, O 2 2− , O 2 − or O − ), and the binding energy at 533–534 eV can be caused by the weak‐bonded oxygen species (O γ ) [2b] . As compiled in Table 2, the O β /(O α +O β ) ratio is a measurement to appraise the ratio of reactive oxygen components [40] . The proportion of O β /(O α +O β ) for CeO 2 , 5 %Cu@CeO 2 , 0.5 %Ir@CeO 2 , 2Cu0.5Ir@CeO 2 , 5Cu0.5Ir@CeO 2 and 10Cu 0.5Ir@CeO 2 samples were approximately 22.5 %, 24.3 %, 34.8 %, 35.1 %, 45.6 % and 36.6 %, respectively.…”

“…[2b] As compiled in Table 2, the O β /(O α + O β ) ratio is a measurement to appraise the ratio of reactive oxygen components. [40] The proportion of O β /(O α + O β ) for CeO 2 , 5 %Cu@CeO 2 , 0.5 %Ir@CeO 2 , 2Cu0.5Ir@CeO 2 , 5Cu0.5Ir@CeO 2 and 10Cu 0.5Ir@CeO 2 samples were approximately 22.5 %, 24.3 %, 34.8 %, 35.1 %, 45.6 % and 36.6 %, respectively. Therefore, the O 1s and Ce 3d XPS spectra prove that the CuÀ Ir@CeO 2 catalysts can release more adsorbed oxygen and oxygen vacancies, which is responsible for the catalytic performance of methane.…”

Current Progress on Methods and Technologies for Catalytic Methane Activation at Low Temperatures

“…The T 90 CH 4 conversion for static air calcinated samples (Pd/CeO 2 S-x, where x stands for calcination time) were 415, 370, 349, 368, and 390 °C for Pd/CeO 2 S-0.5, Pd/CeO 2 S-1, Pd/CeO 2 S-2, Pd/CeO 2 S-3, and Pd/CeO 2 S-5, respectively. [74] Thus, within limits, increasing Pd 0 /PdO ratio can decrease the activation temperature of CH 4 . Jian et al reported similar results, using the reduction-deposition and impregnation method (IMP) to make 3 Pd/CeO 2 catalysts with various chemical and electronic states of Pd species, using hydrazine hydrate (HHA) and formaldehyde (FA) as reductants, respectively.…”

“…The T 90 CH 4 conversion for static air calcinated samples (Pd/CeO 2 S‐ x , where x stands for calcination time) were 415, 370, 349, 368, and 390 °C for Pd/CeO 2 S‐0.5, Pd/CeO 2 S‐1, Pd/CeO 2 S‐2, Pd/CeO 2 S‐3, and Pd/CeO 2 S‐5, respectively. [ 74 ] Thus, within limits, increasing Pd 0 /PdO ratio can decrease the activation temperature of CH 4 . Jian et al.…”

“…[ 72 , 78 ] Furthermore, in situ FTIRS data show that the Mars–van Krevelen mechanism governs the CH 4 oxidation reaction on PdO x /CeO 2 . [ 71 , 74 ] Moreover, Song et al.’s theoretical investigation revealed that the tetrahedral structure of CH 4 was destroyed on the surface, resulting in the production of a —CH 3 species attached to the Pd atom, which assists in the orbital hybridization between the C and Pd atoms. [ 79 ] Moreover, the high CH 4 conversion activity of PdO x /CeO 2 was suggested to arise from emergent mixed oxide chemistry at the cluster/support interface.…”