DFT: graphene SACs break CO2 scaling relations
Single metal atoms in graphene vacancies are predicted to reduce CO2 to methanol or methane at milder limiting potentials than metal terraces by weakening *CO more than *CHO.
Source
Single-atom catalysts for CO<sub>2</sub> electroreduction with significant activity and selectivity improvements
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DFT: graphene SACs break CO2 scaling relations
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Metal (211) limiting potentials for CH4 are –0.7 to –0.8 V
For example, the theoretical limiting potentials for CH 4 production on Ni (211), Pt (211) and Cu (211) surfaces are –0.7 to –0.8 V vs.
*CO is destabilized by 0.98 eV versus 0.42 eV for *CHO on Pt@dv-Gr vs Pt(211)
In , it is visually clear that, with the Pt@dv-Gr catalyst, all reaction intermediates are destabilized compared to those on Pt (211), but most importantly, the destabilization of *CO (0.98 eV) is much more noticeable than that of *CHO (0.42 eV), leading to a 0.49 V reduction in the limiting potential.
What they did
They ran PBE plane-wave DFT on M@sv-Gr and M@dv-Gr, computed *COOH/*OCHO versus *H, then full pathways to HCOOH, CH3OH, and CH4 with harmonic free-energy and solvation corrections. Binding, Bader charges, and DOS compared Pt@dv-Gr to Pt(211).
What they found
All candidate SACs prefer initial CO2 protonation over HER. Pt@dv-Gr UL = −0.27 V toward CH3OH; Ni@dv-Gr −0.41 V (CH3OH); Os and Ru@dv-Gr −0.52 V (CH4)—versus −0.7 to −0.8 V on Ni/Pt/Cu(211). *CO is destabilized 0.98 eV versus *CHO 0.42 eV on Pt@dv-Gr vs Pt(211). Isolated sites force onefold *CO. Filled d8 Ag/Au/Cu SACs weaken *CHO via antibonding occupation.
The limits
What it doesn't show
These are 0 K computational limiting potentials, not measured electrolyzer FEs. Coverage effects are argued to be absent but not proven in situ. PBE plus empirical solvation can mis-order *COOH vs *OCHO. No new catalyst was synthesized in this paper.
Key terms
- Single-atom catalyst (SAC)
- Isolated metal atoms anchored on a support, here a graphene vacancy.
- Limiting potential UL
- UL = −ΔGMAX/e for the hardest electrochemical step; less negative is better.
- Scaling relation
- Linear correlation of *CO and *CHO binding on metals that caps CO2RR activity.
- Atomic ensemble
- Multiple metal atoms needed for bridging *CO; SACs have only one.
- dv-Gr / sv-Gr
- Double- or single-vacancy graphene sites that bind the metal atom.
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The most favorable predicted CH3OH limiting potential is
Common questions
Why can SACs beat metal (211) facets?
*CO must bind onefold, so it is weakened more than *CHO, shrinking the *CO→*CHO barrier.
Best predicted methanol catalyst here?
Pt@dv-Gr with UL = −0.27 V.
Do they make H2 instead?
Computed first protonation favors *COOH/*OCHO over *H on all candidates.
Why are Ag/Cu SACs worse for *CHO?
Filled d8 character occupies antibonding *CHO levels and weakens that intermediate.
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