CO dimerization in CO2 electroreductionRobert B. Sandberg, Xin Hong, Joseph H. Montoya, Chuan Shi, Karen Chan, Jens K. Nørskov

Summary

CENTER FOR INTERFACE SCIENCE AND CATALYSIS

Motivations and backgroundWe present DFT studies that investigate CO dimerization on Cu in CO2 electroreduction. CO dimers can be stabilized by a charged water layer on Cu(111), Cu(100), and Cu(211). Our calculations predict a lower barrier on Cu(100), which is consistent with Cu(100)’s higher activity for C-C coupling at low overpotentials. Our calculations suggest that straining the Cu(111) lattice can alter the activity and reactivity for CO dimerization. We also explore the stability of subsurface oxygen on Cu, and show that such surfaces are highly unstable under reducing conditions.

Field and solvent effects

•CO2 reduction represents a potential route to carbon-neutral fuels and renewable chemicals

•Koper et al. detect C2H4 at low overpotentials on Cu(100) surfaces in base [1]

•Li, Kanan et al. have shown that C2s form at low overpotentials on oxide-derived copper [2]

•Strained grain boundaries, step sites, and subsurface oxygen have been proposed to stabilize *CO and facilitate C-C bond formations.

Insights from a thermochemical perspective

Conclusions and Outlook•CO dimerization is feasible on Cu(100), (111) and (211) in the presence of ions at the interface and should vary with an absolute, not relative potential scale

•Straining the Cu(111) lattice may alter the activity and reactivity for CO dimerization

•Subsurface oxygen is very unstable and cannot account for the higher reactivity of OD-Cu

•CO, H binding energy may play role in structure sensitivity for C-C coupling•Future work: large scale grain boundary simulations with an EMT potential and CO reduction kinetic modeling

SHE!

RH

E!

*CH

O!

*OC

CO!

RH

E!

*CH

O!

pH = 7!

pH = 13!

Subsurface oxygen stability

•Solvation and field effects simulated by applying them independently [5]

•Static electric field results in 0.3-0.4 eV stabilization of CO dimer

•Presence of uncharged H2O layer stabilizes CO dimer by 0.5-0.6 eV

TPD data and calculated CO* binding energies

Acknowledgements NSF GFRP, MURI Collaboration (AFOSR)

Barriers to CO dimerization

Strained Cu(100) and (111) facets

• Barriers on 100 (0.45 eV) lower than 111 (0.7 eV) and 211 (0.7 eV), but all are feasible at room temperature

Barrier reduced by 0.2

•All states (IS, TS, FS) scale with one another for Cu(100); barrier is independent of strain

•On Cu(111), barrier scaling varies with strain

•Cu(111) barrier change may be due to strain, field, and coordination effects

•CO hydrogenation is potential limiting step for C1 hydrocarbons [3]

•C-C coupling occurs with lower barriers on protonated CO [4]

O-Oct

Octahedral Site, Below the Surface 2.30 eVΔG

O-FCC

FCC Site, On the Surface

0.96 eVΔG

OH-FCC

FCC Site, On the Surface

-0.06 eVΔG

SHE

Slab

OH-FCC

O-FCC

Cu(111)

Octahedral Site, Below the Surface

2.22 eVΔG

HCP Site, On the Surface

0.68 eVΔG

SHE

Slab

OH*

O-Oct

On the Surface

-0.53 eVΔG Strain does not stabilize the subsurface oxygen or significantly change the Pourbaix diagram.

Temperature programmed desorption on OD-Cu shows a new adsorption peak at 275K with OD-Cu, [6], which suggests the presence of stepped/kink sites

References [1] Schouten, Gallent, and Koper, ACS Catalysis (2013).[2] Kanan et al, J. Am. Chem. Soc. 2015, 137, 9808–9811.[3] Peterson et al. Energy Environ Sci. (2010)[4] Montoya, Peterson, Nørskov , ChemCatChem (2013).[5] Montoya, Shi, Chan, Nørskov, JPCL, 2015[6] Chorkendorff et al., JACS, 2015

100:-0.66eV

111:-0.65eV

211:-0.78eV

211:-0.83eV

Cu(211)