Self‐Anticoking of a Cobalt Surface by Subsurface Oxygen in the Fischer–Tropsch Synthesis

Abstract

Understanding the fundamental processes taking place on Co surfaces during the Fischer-Tropsch (FT) synthesis is of great interest and importance. We herein report a self-anticoking mechanism of a cobalt surface by subsurface oxygen. The active carbidic carbon species for FT synthesis tends to transform into the inactive graphitic carbon species on clean Co(0001) and poisons the Co surface. Subsurface atomic oxygen on Co(0001) can stabilize the active carbidic carbon species and quench the transformation process. These results reveal, to the best of our knowledge, for the first time the reactivity of various surface species on Co surfaces that dynamically maintain a delicate balance to enhance the long-term stability of Co catalysts during FT synthesis.

Near Ambient-Pressure X-ray Photoelectron Spectroscopy Study of CO Activation and Hydrogenation on Co(0001)

“…A new C 1s feature at 283.8 eV emerges at 425 K, and its binding energy gradually shifts to 284.2 eV as the temperature increases. At 550 K, another new C 1s feature appears at 284.8 eV, which can be assigned to the graphitic carbon. , Meanwhile, the CO­(g) signals were observed to weaken with the increasing temperature, and its C 1s and O 1s binding energy shift from 290.5 to 291.3 eV and from 537.0 to 537.9 eV as the temperature increases from 520 to 630 K, while the Co 2p binding energy does not shift.…”

“…We previously observed that the atomic carbon species on Co(0001) could transform to the graphitic carbon species at 550 K and above, but without the formation of polymeric carbon species. 27,32 Thus, the formation of polymeric carbon species should be related to the CH species or even the CO(a) species, which needs further investigations.…”

“…Figure panels a–f show C 1s, O 1s, and Co 2p NAP-XPS spectra of Co(0001) surfaces at P CO = 0.5 mbar at different temperatures. The starting Co(0001) surface shows tiny residual signals of atomic carbon at 283.2 eV and O adatoms at 530.3 eV. ,, At 300 K, atop-adsorbed CO­(a) with the C 1s and O 1s binding energies respectively at 286.1 and 532.2 eV, bridgingly adsorbed CO­(a) with the C 1s and O 1s binding energies respectively at 285.4 and 531.3 eV, and CO­(g) with the C 1s and O 1s binding energies respectively at 290.5 and 537.0 eV appear. At 400 K, the signals of atomic (carbidic) carbon at 283.2 eV and O adatoms at 530.3 eV obviously grow, suggesting the occurrence of CO dissociation.…”

“…Fundamental understanding of complex heterogeneous catalysis is of great importance and interest, and the use of model catalysts with well-defined surface structures has been demonstrated as an effective approach. − Reaction mechanisms and structure–activity relation of Co-catalyzed FTS have been extensively explored using Co single crystals both experimentally and theoretically, ranging from adsorption and elementary surface reactions of reactants, products, and surface intermediates under ultrahigh vacuum (UHV) conditions − to (quasi) in situ characterizations under near ambient and high pressures. − A key issue, CO activation, was found to proceed via two mechanisms: one is the direct dissociation of CO, ,− the other is H-assisted activation of CO via the HCO intermediate. ,,,− DFT calculation results suggested that the HCP-Co and FCC-Co catalysts should prefer the direct dissociation and H-assisted routes, respectively, and that CO activation should proceed via the H-assisted pathway on the Co(0001) surface but proceed via both direct dissociation and H-assisted pathways on Co(10–12) and (11–20) surfaces. , Another key issue, carbon chain growth, was observed to proceed via three mechanisms: one is the self-coupling reactions of CH x intermediates, − , the second is the coupling reactions of CH x intermediates with CO or HCO intermediates followed by hydrogenation reactions, , and the third is coupling reactions of C 2 H x intermediates. , Although various elementary surface reactions were observed, identification of their contributions under the working FTS reaction have remained as an extremely challenging task.…”

“…Near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) has been widely used to characterize working catalysts under mbar-level pressures . We have comprehensively studied elementary surface reactions on Co(0001) surface relevant to the FTS reaction under UHV conditions ,,,,,, and characterized Co(0001) surfaces treated under CO and H 2 with pressures up to 4 bar and temperatures up to 490 K without exposures to air with XPS . In this paper, we have comparatively characterized Co(0001) surfaces under CO and CO + H 2 atmospheres using in situ NAP-XPS.…”