(465d) Single-Atom Catalysts for CO2 Electroreduction with Significant Activity and Selectivity Improvements

Single-atom
catalyst (SAC) has an electronic structure that is very different from its bulk
counterparts, and has shown unexpectedly high specific activity with a
significant reduction of noble metal usages although physical origins of such
performance enhancements are still poorly understood. Herein, by means of
density functional theory calculations, we for the first time investigate the
great potential of single atom catalyst for CO₂electroreduction
applications. In particular, we study a single transition metal atom anchored
on the defective graphene with single or double
vacancies, denoted as M@sv-Gr or M@dv-Gr, where M are various transition metals, as a CO₂
reduction catalyst. Many SACs are indeed shown to be highly selective for CO₂
reduction reaction over a competitive H₂ evolution reaction due to a
favorable adsorption of carboxyl or formate over
hydrogen on the catalysts. On the basis of free energies, we found that the Pt@dv-Gr catalyst shows a remarkable reduction in the
limiting potential for CH₃OH production compared to any existing
catalysts, synthesized or predicted (Figure 1).

We focus
on the Pt@dv-Gr to investigate the origin of
improvement on the SACs compared to the transition metal. As shown in Figure 1,
with the Pt@dv-Gr catalyst, all the 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. We also observe that
with SACs, the conventional scaling relation between the *CO binding and *CHO
binding established for the bulk transition metal significantly deviates from
linearity (Figure 2).

We discuss
features of SACs which contribute to the breakdown of scaling relation between
*CO and *CHO, namely, the lack of atomic ensemble for adsorbates
binding and the metal-support interactions that lead to the electronic
structures conducive to the catalysis.

Atomic
Ensemble:

The
optimized geometries of the bare catalysts are shown for Pt@dv-Gr
and Pt (211) in Figure 3. One can see that, for Pt (211), two surface Pt atoms
are involved in the *CO bonding while only one Pt
atom is bonding with *CHO, leading to a large destabilization in the relative
free energies when going from *CO to *CHO. On the other hand, for Pt@dv-Gr, only one Pt atom is
utilized for both *CO and *CHO binding, resulting in a much more moderate
destabilization in relative free energies compared to Pt
(211). Thus, the lack of Pt ensemble in the Pt@dv-Gr is responsible for a significantly weaker binding
of *CO on the Pt@dv-Gr compared to the Pt (211) surface.

Electronic
Structure:

The
strong metal-support interaction affects the electronic structure of a metal
atom in the SACs greatly. In Figure 4A, the Pt 5d
density of states (DOS) in Pt@dv-Gr shows a
significant orbital overlap with the C 2p orbitals of the graphene.
The electron density isosurfaces (Figure 4B)
illustrate that electron clouds of four carbon atoms surrounding the Pt atom are significantly hybridized with the Pt atom. The differential charge density map (Figure 4C)
between the defective graphene and Pt@dv-Gr also
suggests that the Pt atom is
positively charged by electron transfer from the Pt
atom to the defective graphene support.

In
this paper, we investigated the single atom catalysts (SACs) as a promising CO₂electroreduction catalyst using DFT calculations. The
main findings of this work are as follows.

(i) By comparing the free energies of the initial
protonation steps for the CRR and HER, we found that all SACs can selectively
reduce CO₂ rather than producing H₂. In particular, the
predicted limiting potential for Pt@dv-Gr (-0.27 V)
for CH₃OH production is considerably less negative than the
conventional transition metal catalysts.

(ii)
To understand the origin of the improvements in SACs, we investigated two
aspects of the Pt@dv-Gr that affect the relative
stability of *CO vs. *CHO. A one-fold bonding of *CO on Pt@dv-Gr
due to a lack of atomic ensemble, as compared to the two-fold *CO bonding on
the Pt (211) is responsible for the significant
weakening of *CO binding on the Pt@dv-Gr.

(iii)
We investigated the electronic structure of Pt atom
in the SAC to find the origin of the deviation of SACs from the conventional
scaling relation of transition metals, which arises from the d-band center
theory. We suggest that the strong electronic interaction between the d-orbital
of metal atom and the p-orbital of graphene is
responsible for the different behavior from the transition metal surfaces, as
evidenced by the electron transfer and the overlap in the DOS.