Theoretical Insights into the Structure and Activity of Cobalt Modulated by Surface and Subsurface Carbon in Operando Conditions

Abstract

To understand the effect of the in situ carbon deposition and permeation process on cobalt-catalyzed heterogeneous reactions, the thermodynamics and kinetics of the carburization process of ten different surfaces of hexagonal close-packed (hcp) and face-centered cubic (fcc) Co and the resulting evolution of crystalline morphology, electronic structure, and barriers of typical surface reactions are explored theoretically using density functional theory (DFT) and atomistic thermodynamics methods. The exposed facets of hcp-Co and fcc-Co crystallites showed distinct thermodynamic and kinetic sensitivity to the permeation of carbon into subsurface positions. The formation of surface carbides depends on both the working condition and the type of crystalline facets. Two types of carbon permeation mechanisms were discovered with distinct diffusion paths and barriers. The morphology of cobalt catalysts was found to be greatly modulated by carbon deposition under operando conditions, favoring (0001) of the hcp phase and (111) of the fcc phase at high carbon chemical potentials. As the carbon coverage increases, the d-band centers of the carburized Co surface approach the values of bulk cobalt carbides. Evaluation of a representative Fischer–Tropsch side reaction (CH3–H coupling in methane formation) shows that the barrier is highly influenced by the degree of carbide formation on the cobalt surfaces. Our study indicates that it is crucial to consider the chemical potential of gas-phase carbon to understand the surface carbon adsorption and permeation phenomenon on the skin layer of the catalyst, the overall cobalt morphology, and the observed catalytic performance of cobalt catalysts in syngas conversion.

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

“…The C permeation on Co surfaces shows a clear difference from that on Fe . Although C permeation is also thermodynamically viable on most of the exposed Co surfaces (hcp-Co, (10–10), (10–11), (10–12), (11–20), (11–21); fcc-Co, (100), (110), (311)), the close-packed hcp-Co (0001) and fcc-Co (111) facets do not prefer C permeation into subsurface.…”

“…101 The C permeation on Co surfaces shows a clear difference from that on Fe. 102 fcc-Co, ( 100), ( 110), ( 311)), the close-packed hcp-Co (0001) and fcc-Co (111) facets do not prefer C permeation into subsurface. Instead, the C atoms accumulate on these surfaces and form graphene-like structures (Figure 13).…”

“…The first-layer Co atoms are shown in light blue, the second-layer atoms in dark blue, and the third-layer atoms in cyan. Adapted with permission from ref . Copyright 2020 American Chemical Society.…”

“…(c) Average partial charges of the Co atoms in the hcp-Co (11–21) and (0001) and fcc-Co (100) and (111) surfaces as a function of the number of C atoms. Adapted with permission from ref . Copyright 2020 American Chemical Society.…”

“…(e, f) Energy profiles of carbon atom permeation from the surface (1L) to the second layer (2L) with a transition state of TS1 and from the second layer (2L) to the third layer (3L) with a transition state of TS2 on the hcp and fcc Co surfaces. Adapted with permission from ref . Copyright 2020 American Chemical Society.…”