Mechanistic switch in methane combustion on PdOₓ catalysts mediated by oxygen vacancies

doi:10.1016/S1872-2067(26)65197-2

Abstract

Abstract:

The superior activity of PdO_x catalysts for methane combustion is intimately linked to oxygen vacancies. However, a molecular-level understanding of how these defects govern the reaction under working conditions remains elusive. Here, we develop a novel “two site-pairs” microkinetic strategy integrated with density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations to unravel their role. AIMD simulations reveal the structural evolution from Pd(100) to a PdO(100) film and finally to the PdO(101) facet. DFT calculations identify Pd atoms adjacent to oxygen vacancies (Pdₒᵥ) as the primary active centers for facile CH₄ C-H bond cleavage on both PdOᵥ(101) and PdOᵥ(100) surfaces, while neighboring lattice oxygen atoms promote subsequent oxidation. This cooperative dual-site mechanism is central to the Mars-van Krevelen (MvK) pathway. Crucially, microkinetic modeling captures a temperature-driven switch in the dominant reaction mechanism: from the Langmuir-Hinshelwood mechanism prevalent on pristine PdO(101) to the Pdₒᵥ-mediated MvK mechanism that dominates on oxygen-deficient surfaces. The high activity of Pd_ov sites originates from their accessible frontier orbitals and the formation of covalent Pd_ov-C bonds in the transition states. Collectively, our work establishes a defect-mediated catalytic cycle, providing a fundamental basis for designing high-performance catalysts via oxygen vacancy engineering.

Key words: Methane oxidation, Pd-based catalysts, Oxygen vacancies, Density functional theory, Microkinetic modeling

Yanyan Chen, Sen Wang, Shanhui Zhu, He Wang, Mei Dong, Zhangfeng Qin, Jianguo Wang, Weibin Fan. Mechanistic switch in methane combustion on PdOₓ catalysts mediated by oxygen vacancies[J]. Chinese Journal of Catalysis, 2027, 92: 387-396.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)65197-2

https://www.cjcatal.com/EN/Y2027/V92/I1/387

Figures/Tables 7

Fig. 1. Surface evolution of Pd(100) → PdO(100) → PdO(101) transitions. Structural evolution of 16 oxygen atoms (1 ML) adsorbed on the Pd(100) surface, including the initial configuration before optimization (1 ML O*-Pd(100)), optimized structure showing the formation of 1 ML PdO(100), and the AIMD simulation snapshot after 46 ps at 1000 K, revealing the emergence of O3f-Pd-O4f-like structural motifs (yellow areas). Atomic colors: Pd (blue), O (red). Surface Pd and O atoms are labelled as 3f or 4f, indicating 3- or 4-fold coordination, respectively.

Fig. 2. High reactivity of PdOᵥ surfaces. (a) Top-view structures of the PdO(101), PdOᵥ(101), PdO(100) and PdOᵥ(100) facets. (b) Energy barriers for the C-H bond activation of methane. (c) d-PDOS of a surface Pd3f or Pd4f and Pdov atoms. (d) ELF map of the transition state (C-H-Pd) plane on the four surfaces. d-PDOS plots are referenced to the Fermi energy. In the ELF map, the isosurface color represents the degree of electron localization; blue and red indicate that the electron is randomly delocalized and strongly localized, respectively. Atomic color scheme: Pd (blue), O (red).

Table 1 Energy barriers (Ea, kJ/mol) and reaction energies (∆Er, kJ/mol) for CH4 complete oxidation via CHx and CHxO paths on PdO(101) (Pd3f-O3f as the active site) and PdOv(101) (Pdov as the active site).

Reactions	PdO(101): (Pd_3f-O_3f site)		PdO_v(101): (Pd_ov site)
CH_x path	E_a	∆E_r	E_a	∆E_r
(1) CH₄ → CH₃+ H	66	-41	47	-3
(2) CH₃ → CH₂ + H	122	47	49	10
(3) CH₂ → CH + H	95	71	45	-26
(4) CH-O_(lat) → CO_(lat) + H	11	-77	27 (Pd_ov-O_3f)	-8
CH_xO path
(5) CH₄ + O → CH₃ + OH	64	-151	102	-87
(6) CH₃ + O → CH₂OH	83	-165	139	-113
(7) CH₂OH → CH₂O +H	26	2	13	-14
(8) CH₂O + O → CHO + OH	90	-151	68	-187
(9) CHO + OH → CO + H₂O	2	-125	69	-88

Fig. 3. Structures of the initial states (IS), transition states (TS), and final states (FS) as well as the energy barriers (Ea) and reaction energies (∆Er) of CO oxidation on PdO(101) and PdOv(101).

Fig. 4. The complete catalytic mechanism of methane combustion on PdOx surface.

Table 2 Energy barriers (Ea, kJ/mol) and reaction energies (∆Er, kJ/mol) for CH4 complete oxidation on PdOv(100).

Reactions on PdO_v(100)	E_a	∆E_r
(1) CH₄ → CH₃ + H	53	-4
(2) CH₃ → CH₂ + H	65	-7
(3) CH₂ → CH + H	16	-78
(4) CH-O_(lat) → CO_(lat) + H	44 (Pd_ov-O_3f)	-131
(5) CO + O_(lat) → CO₂	82 (Pd_ov-O_3f)	-38

Fig. 5. Temperature-dependent kinetics and surface species evolution during methane oxidation. (a) Ratios of turnover frequencies on Pd3f (Pd*) and Pdov (Pd^) and the relative abundance of active sites (Pdov and Pd3f). (b) Coverages of relevant adsorbates as a function of temperature. The reaction conditions were 0.01 bar CH4 and 0.1 bar O2.

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