A quantum-chemical study of CO dissociation mechanism on low-index Miller planes of ϴ-Fe3C

Robin Broos, B. Klumpers, Bart Zijlstra, Ivo Filot, Emiel Hensen (Corresponding author)

Research output: Contribution to journalArticleAcademicpeer-review

Abstract

Spin-polarized density functional theory was employed to determine the preferred CO bond dissociation mechanism on low-index Miller surfaces of ϴ-Fe3C in the context of Fischer-Tropsch synthesis. Compared to the most reactive (111) surface of bcc-Fe on which CO binds in a 7-fold coordination, CO prefers to locate in 3-fold or 4-fold sites on the carburized surfaces due to the presence of interstitial C atoms at or below the surface. An important finding is that the lowest activation energies for direct CO bond dissociation are associated with the presence of step-like sites, similar to the case of metallic surfaces. We could identify such sites for 3 out of the 9 investigated surfaces, namely the (111),
11¯1
, and (010) terminations of ϴ-Fe3C. On the other hand, H-assisted CO dissociation is preferred on the
01¯1
, (001), and (100) surfaces. The other (011), (110), and (101) surfaces are inert with CO dissociation barriers close to or exceeding the CO adsorption energy. A kinetic analysis shows that the (111) surface (direct CO dissociation) and the (0

1) surface (H-assisted CO dissociation via HCO) display comparable CO bond dissociation rates, much higher than the rates computed for the other surfaces. Together these two surfaces make up ca. 28% of the surface enclosing a Wulff nanoparticle of ϴ-Fe3C. Using an atomic population analysis, we show that the activation barrier for C-O bond dissociation correlates well with the bond order of adsorbed CO. This implies that pre-activation of CO is important for lowering the overall activation barrier. The present work demonstrates that the high-temperature ϴ-Fe3C phase is highly active towards CO bond dissociation, which is the essential first step in the Fischer-Tropsch reaction. Several of the exposed surfaces present lower overall CO dissociation barriers than α-Fe (known to be unstable under Fischer-Tropsch conditions) and the χ-carbide of Fe (usually assumed to be the most stable phase of Fe-carbide under Fischer-Tropsch conditions). Notably, the activity of the (111) surface is higher than that of a stepped cobalt surface.
LanguageEnglish
JournalCatalysis Today
DOIs
StateAccepted/In press - 2019

Fingerprint

Carbon Monoxide
Chemical activation
Carbides
Fischer-Tropsch synthesis
Cobalt
Density functional theory

Cite this

@article{bb9a3f858e064c2999cc364efd293c37,
title = "A quantum-chemical study of CO dissociation mechanism on low-index Miller planes of ϴ-Fe3C",
abstract = "Spin-polarized density functional theory was employed to determine the preferred CO bond dissociation mechanism on low-index Miller surfaces of ϴ-Fe3C in the context of Fischer-Tropsch synthesis. Compared to the most reactive (111) surface of bcc-Fe on which CO binds in a 7-fold coordination, CO prefers to locate in 3-fold or 4-fold sites on the carburized surfaces due to the presence of interstitial C atoms at or below the surface. An important finding is that the lowest activation energies for direct CO bond dissociation are associated with the presence of step-like sites, similar to the case of metallic surfaces. We could identify such sites for 3 out of the 9 investigated surfaces, namely the (111), 11¯1, and (010) terminations of ϴ-Fe3C. On the other hand, H-assisted CO dissociation is preferred on the 01¯1, (001), and (100) surfaces. The other (011), (110), and (101) surfaces are inert with CO dissociation barriers close to or exceeding the CO adsorption energy. A kinetic analysis shows that the (111) surface (direct CO dissociation) and the (0 1¯ 1) surface (H-assisted CO dissociation via HCO) display comparable CO bond dissociation rates, much higher than the rates computed for the other surfaces. Together these two surfaces make up ca. 28{\%} of the surface enclosing a Wulff nanoparticle of ϴ-Fe3C. Using an atomic population analysis, we show that the activation barrier for C-O bond dissociation correlates well with the bond order of adsorbed CO. This implies that pre-activation of CO is important for lowering the overall activation barrier. The present work demonstrates that the high-temperature ϴ-Fe3C phase is highly active towards CO bond dissociation, which is the essential first step in the Fischer-Tropsch reaction. Several of the exposed surfaces present lower overall CO dissociation barriers than α-Fe (known to be unstable under Fischer-Tropsch conditions) and the χ-carbide of Fe (usually assumed to be the most stable phase of Fe-carbide under Fischer-Tropsch conditions). Notably, the activity of the (111) surface is higher than that of a stepped cobalt surface.",
author = "Robin Broos and B. Klumpers and Bart Zijlstra and Ivo Filot and Emiel Hensen",
year = "2019",
doi = "10.1016/j.cattod.2019.02.015",
language = "English",
journal = "Catalysis Today",
issn = "0920-5861",
publisher = "Elsevier",

}

TY - JOUR

T1 - A quantum-chemical study of CO dissociation mechanism on low-index Miller planes of ϴ-Fe3C

AU - Broos,Robin

AU - Klumpers,B.

AU - Zijlstra,Bart

AU - Filot,Ivo

AU - Hensen,Emiel

PY - 2019

Y1 - 2019

N2 - Spin-polarized density functional theory was employed to determine the preferred CO bond dissociation mechanism on low-index Miller surfaces of ϴ-Fe3C in the context of Fischer-Tropsch synthesis. Compared to the most reactive (111) surface of bcc-Fe on which CO binds in a 7-fold coordination, CO prefers to locate in 3-fold or 4-fold sites on the carburized surfaces due to the presence of interstitial C atoms at or below the surface. An important finding is that the lowest activation energies for direct CO bond dissociation are associated with the presence of step-like sites, similar to the case of metallic surfaces. We could identify such sites for 3 out of the 9 investigated surfaces, namely the (111), 11¯1, and (010) terminations of ϴ-Fe3C. On the other hand, H-assisted CO dissociation is preferred on the 01¯1, (001), and (100) surfaces. The other (011), (110), and (101) surfaces are inert with CO dissociation barriers close to or exceeding the CO adsorption energy. A kinetic analysis shows that the (111) surface (direct CO dissociation) and the (0 1¯ 1) surface (H-assisted CO dissociation via HCO) display comparable CO bond dissociation rates, much higher than the rates computed for the other surfaces. Together these two surfaces make up ca. 28% of the surface enclosing a Wulff nanoparticle of ϴ-Fe3C. Using an atomic population analysis, we show that the activation barrier for C-O bond dissociation correlates well with the bond order of adsorbed CO. This implies that pre-activation of CO is important for lowering the overall activation barrier. The present work demonstrates that the high-temperature ϴ-Fe3C phase is highly active towards CO bond dissociation, which is the essential first step in the Fischer-Tropsch reaction. Several of the exposed surfaces present lower overall CO dissociation barriers than α-Fe (known to be unstable under Fischer-Tropsch conditions) and the χ-carbide of Fe (usually assumed to be the most stable phase of Fe-carbide under Fischer-Tropsch conditions). Notably, the activity of the (111) surface is higher than that of a stepped cobalt surface.

AB - Spin-polarized density functional theory was employed to determine the preferred CO bond dissociation mechanism on low-index Miller surfaces of ϴ-Fe3C in the context of Fischer-Tropsch synthesis. Compared to the most reactive (111) surface of bcc-Fe on which CO binds in a 7-fold coordination, CO prefers to locate in 3-fold or 4-fold sites on the carburized surfaces due to the presence of interstitial C atoms at or below the surface. An important finding is that the lowest activation energies for direct CO bond dissociation are associated with the presence of step-like sites, similar to the case of metallic surfaces. We could identify such sites for 3 out of the 9 investigated surfaces, namely the (111), 11¯1, and (010) terminations of ϴ-Fe3C. On the other hand, H-assisted CO dissociation is preferred on the 01¯1, (001), and (100) surfaces. The other (011), (110), and (101) surfaces are inert with CO dissociation barriers close to or exceeding the CO adsorption energy. A kinetic analysis shows that the (111) surface (direct CO dissociation) and the (0 1¯ 1) surface (H-assisted CO dissociation via HCO) display comparable CO bond dissociation rates, much higher than the rates computed for the other surfaces. Together these two surfaces make up ca. 28% of the surface enclosing a Wulff nanoparticle of ϴ-Fe3C. Using an atomic population analysis, we show that the activation barrier for C-O bond dissociation correlates well with the bond order of adsorbed CO. This implies that pre-activation of CO is important for lowering the overall activation barrier. The present work demonstrates that the high-temperature ϴ-Fe3C phase is highly active towards CO bond dissociation, which is the essential first step in the Fischer-Tropsch reaction. Several of the exposed surfaces present lower overall CO dissociation barriers than α-Fe (known to be unstable under Fischer-Tropsch conditions) and the χ-carbide of Fe (usually assumed to be the most stable phase of Fe-carbide under Fischer-Tropsch conditions). Notably, the activity of the (111) surface is higher than that of a stepped cobalt surface.

U2 - 10.1016/j.cattod.2019.02.015

DO - 10.1016/j.cattod.2019.02.015

M3 - Article

JO - Catalysis Today

T2 - Catalysis Today

JF - Catalysis Today

SN - 0920-5861

ER -