A quantum-chemical study of the CO dissociation mechanism on low-index Miller planes of ϴ-Fe 3 C

Robin J.P. Broos, Bart Klumpers, Bart Zijlstra, Ivo A.W. Filot, Emiel J.M. 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-Fe 3 C 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-Fe 3 C. 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 (01¯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-Fe 3 C. 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-Fe 3 C 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
Early online date1 Jan 2019
DOIs
StateE-pub ahead of print - 1 Jan 2019

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

Keywords

  • CO dissociation
  • density functional theory
  • Fischer-Tropsch synthesis
  • mechanism
  • theta-Fe C

Cite this

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title = "A quantum-chemical study of the CO dissociation mechanism on low-index Miller planes of ϴ-Fe 3 C",
abstract = "Spin-polarized density functional theory was employed to determine the preferred CO bond dissociation mechanism on low-index Miller surfaces of ϴ-Fe 3 C 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-Fe 3 C. 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 (01¯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-Fe 3 C. 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-Fe 3 C 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.",
keywords = "CO dissociation, density functional theory, Fischer-Tropsch synthesis, mechanism, theta-Fe C",
author = "Broos, {Robin J.P.} and Bart Klumpers and Bart Zijlstra and Filot, {Ivo A.W.} and Hensen, {Emiel J.M.}",
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T1 - A quantum-chemical study of the CO dissociation mechanism on low-index Miller planes of ϴ-Fe 3 C

AU - Broos,Robin J.P.

AU - Klumpers,Bart

AU - Zijlstra,Bart

AU - Filot,Ivo A.W.

AU - Hensen,Emiel J.M.

PY - 2019/1/1

Y1 - 2019/1/1

N2 - Spin-polarized density functional theory was employed to determine the preferred CO bond dissociation mechanism on low-index Miller surfaces of ϴ-Fe 3 C 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-Fe 3 C. 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 (01¯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-Fe 3 C. 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-Fe 3 C 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-Fe 3 C 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-Fe 3 C. 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 (01¯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-Fe 3 C. 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-Fe 3 C 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.

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