Theoretical exploration of intrinsic facet-dependent CH4 and C2 formation on Fe5C2 particle

Yin, Junqing; Liu, Xingchen; Liu, Xing-Wu; Wang, He; Wan, Hongliu; Wang, Shuyuan; Zhang, Wei; Zhou, Xiong; Teng, Botao; Yang, Yong; Li, Yongwang; Cao, Zhi; Wen, Xiaodong

doi:10.1016/j.apcatb.2020.119308

Cited by 36 publications

(22 citation statements)

References 72 publications

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“…Additionally, the contribution of (4̅11) 0.188 reaches the maximum (12%) at a μ O of −7.64 eV, and the contribution of (001) 0.00 is the minimum (6%) at −7.91 eV. According to the DFT studies by Yin et al , the effective free energy barrier for the overall CH 4 formation on (510) 0.00 , (4̅11) 0.188 , (010) 0.25 , and (111̅) 0.00 is relatively higher than surface (111) 0.602 , (110) 0.573 , and (100) 0.402 . Experimentally, Ma et al also showed that Fe 5 C 2 catalysts with high exposure of the (510) facets exhibit higher activity and selectivity for C 5+ hydrocarbon than those synthesized from conventional methods.…”

Section: Resultsmentioning

confidence: 99%

“…Specifically, Fischer–Tropsch synthesis (FTS), as a sustainable way to generate contaminant-free transportation fuels (sulfur-, nitrogen-, and aromatics-free) and essential chemical building blocks from syngas, has drew much attention over the past decades. It is the distinguishing feature, such as low cost and wider operation conditions, of iron-based catalysts that makes them attractive in practical industries. − In catalytic FTS reactions, iron carbides have been recognized as the active phase and are responsible for observed activity and selectivity. − Until now, a great deal of endeavor has been committed to design iron-based catalysts with outstanding performance, ,,− together with comprehension of specific reaction mechanisms. − Meanwhile, understanding upon the deactivation behavior of iron-based catalysts is rather significant for us to enhance the stability of catalysts and furthermore realize the balance between cost and performance in industrial applications …”

Section: Introductionmentioning

confidence: 99%

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Oxygen Adsorption-Induced Morphological Evolution of Hägg Iron Carbide at High Oxygen Chemical Potentials

et al. 2021

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Comprehension upon the mechanism of oxidative deactivation of carbides is of significance for extending their applications, in particular, for developing Fe-based catalysts with outstanding performance in Fischer−Tropsch synthesis (FTS). Here, combining density functional theory, ab initio atomistic thermodynamics, and Wulff construction, we have investigated the interaction between oxygen and nine χ-Fe 5 C 2 surfaces, as well as the morphology change of the theoretically established Fe 5 C 2 model at varied oxygen chemical potential under FTS-related reaction conditions. The calculations suggest that the surface oxygen preferentially bonds to Fe sites than to C (Cs) sites. A linear relationship was discovered between the C/Fe ratio of the surface and its O coverage under typical FTS conditions. The weaker bonding strength and lower O coverage at the same oxygen chemical potential on surfaces with a higher C/Fe ratio such as (110) 0.573 , (111) 0.602 , and (100) 0.402 mean that C-rich surfaces are resistant to oxidation. Elevating temperature and reducing partial pressure ratio of H 2 O/H 2 will destabilize the O adsorption and at the same time lead to morphological change of the particle with the antioxidant facets contributing more to the total area, which is beneficial to keep the catalyst stable. Our atomistic insights shed light on the process of deactivation caused by oxidation.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Oxygen Adsorption-Induced Morphological Evolution of Hägg Iron Carbide at High Oxygen Chemical Potentials

et al. 2021

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“…As for the FTS mechanism on FeC x catalysts, most theoretical studies ,,− were performed on simplified surface models, e.g., small unit cells without considering the possible surface reconstruction or neglecting the in situ coverage effects of adsorbates (e.g., carbon and hydrogen atoms) that are important to FTS kinetics. , This has led to diverse mechanisms for CO activation. Broos et al investigated the direct, H-assisted and C-assisted CO dissociation on the bulk-truncated surfaces of χ-Fe 5 C 2 and found the direct dissociation pathway on the Fe-B 5 site of χ-Fe 5 C 2 (11) has the lowest barrier of 1.22 eV.…”

Section: Introductionmentioning

confidence: 99%

In Situ Active Site for CO Activation in Fe-Catalyzed Fischer–Tropsch Synthesis from Machine Learning

2021

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In situ-formed iron carbides (FeC x ) are the key components responsible for Fischer–Tropsch synthesis (FTS, CO + H2 → long-chain hydrocarbons) on Fe-based catalysts in industry. The true active site is, however, highly controversial despite more than a century of study, which is largely due to the combined complexity in both FeC x structures and mechanism of CO hydrogenation. Herein powered by machine learning simulation, millions of structure candidates for FeC x bulk and surfaces are explored under FTS conditions, which leads to resolving the active site for CO activation. This is achieved without a priori input from experiment by first constructing the thermodynamics convex hull of bulk phases, followed by identifying the low surface energy surfaces and evaluating the adsorption ability of CO and H, and finally determining the lowest energy reaction pathway of CO activation. Rich information on FeC x structures and CO hydrogenation pathways is gleaned: (i) Fe5C2, Fe7C3, and Fe2C are the three stable bulk phases under FTS in producing olefins, where Fe7C3 and Fe2C have multiple energetically nearly degenerate bulk crystal phases; (ii) only three low surface energy surfaces of these bulk phases, namely, χ-Fe5C2(510), χ-Fe5C2(111), and η-Fe2C(111), expose the Fe sites that can adsorb H atoms exothermically, where the surface Fe:C ratio is 2, 1.75, and 2, respectively; (iii) CO activation via direct dissociation can occur at the surface C vacancies (e.g., with a barrier of 1.1 eV) that are created dynamically via hydrogenation. These atomic-level understandings facilitate the building of the structure–activity correlation and designing better FT catalysts.

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“…After evaluating the CH 4 /C 2+ selectivity of the most exposed facets of Fe 5 C 2 with DFT and microkinetic analysis using simplified kinetic models, we found that (111) and (10–1) are kinetically more viable for C 2 formation, whereas CH 4 formation is kinetically more facile on (010), (110) and (11–1). The last three surfaces together account for only 23.5% of the total exposed surface area of an Fe 5 C 2 model particle, yet they are predicted to be responsible for remarkable kinetic competitiveness in the CH 4 formation (96.4%) . Pham et al , also showed that the CH 4 /C 2+ selectivity depends highly on the exposed surface of Fe 5 C 2 .…”

Section: When Carbon Meets Ironmentioning

confidence: 99%

Theoretical Perspectives on the Modulation of Carbon on Transition-Metal Catalysts for Conversion of Carbon-Containing Resources

Liu

Yang

et al. 2021

ACS Catal.

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At present, the catalytic conversion of carbon-containing resources remains a major challenge faced during the economic growth and energy mix adjustment around the world. The development of catalysts with high efficiency for the conversion of carbon-containing resources is one of the major solutions to the energy and environmental problems. During these conversion processes, carbon plays an essential role in the sense that it is not only the key element in the reactions but may also cause the modification of the chemical nature of the catalysts. Notably, comprehension of the structure−performance relationship of catalytic materials is the basis for catalyst development. In particular, the modulating role of carbon on the catalyst structures during the conversion of carbon-containing resources has attracted increasing attention. In the past five years, we have systematically studied the modulation of Fe-, Co-, Ni-, and Mo-based catalysts by carbon using theoretical approaches. In this review, with a focus on the active phases, morphologies, surface structures, electronic properties, and catalytic performances of transition-metal catalysts (Fe, Co, Ni, Mo, and other transition metals), the modulation of these catalysts by carbon will be summarized to shed light on the question of how to tune the behavior of carbon in terms of catalyst carburization and carbon-related surface reactions. This review provides systematic and fundamental information for the further design and development of catalysts for the conversion of carbon-containing resources.

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Theoretical exploration of intrinsic facet-dependent CH4 and C2 formation on Fe5C2 particle

Cited by 36 publications

References 72 publications

Oxygen Adsorption-Induced Morphological Evolution of Hägg Iron Carbide at High Oxygen Chemical Potentials

Oxygen Adsorption-Induced Morphological Evolution of Hägg Iron Carbide at High Oxygen Chemical Potentials

In Situ Active Site for CO Activation in Fe-Catalyzed Fischer–Tropsch Synthesis from Machine Learning

Theoretical Perspectives on the Modulation of Carbon on Transition-Metal Catalysts for Conversion of Carbon-Containing Resources

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