Spatial-confinement induced electroreduction of CO and CO2 to diols on densely-arrayed Cu nanopyramids†

The electroreduction of carbon dioxide (CO2) and carbon monoxide (CO) to liquid alcohol is of significant research interest. This is because of a high mass-energy density, readiness for transportation and established utilization infrastructure. Current success is mainly around monohydric alcohols, such as methanol and ethanol. There exist few reports on converting CO2 or CO to higher-valued diols such as ethylene glycol (EG; (CH2OH)2). The challenge to producing diols lies in the requirement to retain two oxygen atoms in the compound. Here for the first time, we demonstrate that densely-arrayed Cu nanopyramids (Cu-DAN) are able to retain two oxygen atoms for hydroxyl formation. This results in selective electroreduction of CO2 or CO to diols. Density Functional Theory (DFT) computations highlight that the unique spatial-confinement induced by Cu-DAN is crucial to selectively generating EG through a new reaction pathway. This structure promotes C–C coupling with a decreased reaction barrier. Following C–C coupling the structure facilitates EG production by (1) retaining oxygen and promoting the *COH–CHO pathway, which is a newly identified pathway toward ethylene glycol production; and, (2) suppressing the carbon–oxygen bond breaking in intermediate *CH2OH–CH2O and boosting hydrogenation to EG. Our findings will be of immediate interest to researchers in the design of highly active and selective CO2 and CO electroreduction to diols.

purple, *C-CO pathway bifurcating to C 2 H 4 ; green, *COH-CHO pathway toward C 2 H 6 O 2 , and; brown, *COH-CHO pathway bifurcating to C 2 H 5 OH. Reaction barrier ΔG ≠ values at 0 V vs RHE appear in bolded-and shaded-font. Free energy change ΔG values appear in standard-font. Green and red values denote, respectively, exergonic and endergonic process. The solid-arrow denotes protoncoupled electron transfer (H + + e -), and dotted-arrow denotes desorption. The unit of energy is eV.
For the first mechanism Cheng et al. 1 hypothesized a pathway on planar Cu (100) facets in which *COH-CO formation is followed by hydrogenation of βO atoms. This rules out the possibility of hydrogenation of βC to *COH-CHO. We attribute this to the instability of intermediate *COH-CHO.
The structures of *COH-CHO and *COH-COH are tautomeric. The former is comparatively unstable because it is vertically adsorbed on surface via a double bond, and because it has a free radical on the C atom. We computed *COH-CHO to be 0.26 eV less stable than *COH-COH on planar Cu (100) surface.
This finding means the bulk of *COH-CHO are spontaneously tautomerized to *COH-COH, despite being formed first. Because *COH-CHO is not a stable molecular compound with a significant concentration on planar Cu (100) surface, the *COH-CHO pathway is blocked.
Additionally if the formation of *COH-CHO follows the second mechanism, it is difficult for it to proceed on a planar surface. *COH and *CHO are preferably, respectively, stabilized on Cu (111) and Cu (100) surface. 4 It is reportedly rare for those two adsorbates to simultaneously attain the kinetically reactable concentration on a homogeneous facet. This is the reason why we record a high-level barrier of 0.72 eV at 0 V vs RHE for *COH-CHO formation on planar Cu (100) surface.
We conclude therefore that on a planar Cu (100) surface *COH-CHO is not favored and the pathway is blocked.  Table S2. Summary of elementary reaction steps, hydrogen transfer model, reaction barriers at various electrode potentials and involved parameter for computation. ΔG(0V) and ΔG ≠ (0V) are the free energy change and reaction barrier without potential bias, ΔG ≠ (U 0 ) is the reaction barrier under U 0 . U 0 is the equilibrium potential for the reductive adsorption of one proton in the system, and β' is the reaction symmetry factor, as defined in the manuscript. The two hydrogen models considered were, 1) Langmuir-Hinshelwood (LH) mechanism (i.e. direct transfer of an adsorbed *H), and 2) Eley-Rideal (ER) mechanism (i.e. water molecule shuttles an adsorbed *H). Adding hydrogen to carbon species via LH mechanism always gives lower kinetic barriers, comparied with hydrogen transfer via ER mechanism. ER mechanism contributed to a lower kinetic barrier when hydrogen is added to oxygen species. 5,6 For all steps computed with H-shuttling, one H 2 O molecule was included in the computation.
a U 0 is the potential where reaction *A+H + +e -→*A+*H possess zero free energy change i.e. G(*A+*H) + eU 0 -G(*A) -1/2H 2 = 0, where *A means the system with A adsorbed on the surface. b β' is the reaction symmetry factor, which is approximated as 0.49 for all elementary steps. 7