1) The document reports on an investigation of the mechanistic pathways during electroreduction of CO2 in the presence of CO using a Cu-tandem catalyst. 2) Operando experiments and isotope labeling showed that a cross-coupling CO2-CO reaction pathway made the dominant contribution to enhanced ethylene production. This pathway involved the dimerization of two adsorbed CO molecules from different origins. 3) A bifunctional NiNC-CuOx tandem catalyst was proposed that could internally generate CO near CuOx sites for CO2 reduction, mimicking a CO co-feed without needing external CO gas.

1. Literature Presentation
Debabrata Bagchi
Ph. D. Student
17/12/2019
Cu-tandem catalyst for CO2 Electroreduction

3. Novelty of the Work
 Industrially produced CO2 is always
contaminated with some amount of CO
 To date catalyst development and
mechanistic investigation based on either
pure CO2 or pure CO
 This article investigate the mechanistic
pathways during electroreduction of CO2
in presence of CO
 Co-feeding (CO added externally)
 Self-feeding (CO generated locally on tandem catalyst)

4. Structural characterization of CuOx Catalyst
20 nm

5. CO2 to C2H4 by Electroreduction
 Dimerization step is reported as the rate-determining step towards
C2 products*
 Higher *CO coverages enhancing the C2H4 formation rate.
 Increase the bulk and local CO concentration by increasing the
partial pressure of CO
*J. Phys. Chem. Lett. 2015, 6, 20, 4073-4082
Henry's law: the amount of dissolved gas in a liquid is proportional to its
partial pressure above the liquid.
The molar amounts of dissolved COx (CO and CO2) in the
electrolyte were calculated for each feed using Henry’s Law

6. Major Product yields under mixed CO2/CO feeds
CH4
C2H4
H2
CO > CO2/CO (co-feeds) > CO2
 Possible direct dependence of CH4
formation on the reactant redox state,
the available surface *H coverage and
local proton concentration

7. Liquid product analysis at different CO2/CO feeds

8. Formate only forms in CO2-involving feeds
 Dimerization in a proton-decoupled electron transfer pathway to C2H4
J. Phys. Chem. Lett. 2015, 6, 20, 4073-4082
J. Am. Chem. Soc. 2014, 136, 13319−13325
 Proton-coupled electron transfer pathways to CH4

9.  With increasing CO in the co-feeds, the HCOO- pathway decreased,
while the hydrocarbon yield sharply increased by about 50%,
 CO feeds favoured proton accessibility and C2H4 formed via a
hydrogenated dimer (*CO–COH)
Comparison of Hydrocarbon Production Rate

10. Hydrocarbon yield in 1:1 CO2/CO co-feeds at different potential
 In pure CO feeds, the C2H4 yields remained
low, suggesting a CO mass-transport
limitation consistent with low CO solubility
and local CO concentration at the interface

11. Post Electrochemical Characterization
 Cu phase in both reduction process,
accompanied by a very similar particle
agglomeration for all feeds.
 Observed differences in the catalytic
ethylene yields are from kinetic
mechanistic roots not from catalyst-
related chemical factors.

12. Quantitative deconvolution of C2H4 formation mechanism
 Origin of two carbon of ethylene in CO2/CO feeds
*CO-*CO dimerization
CO2-CO2
pathways
CO2-CO
pathways
CO-CO
pathways
Operando Differential Electrochemical Mass
Spectrommetry (DEMS):
 To know the atomic origin of two individual carbon atoms in the ethylene
 This was achieved by using isotope-labelled 13CO in the feeds
 DEMS analysis was used to verify the significantly enhanced C2H4
yields during cyclic voltammetric electrode potential scans

13. DEMS setup
Capillary electrochemical flow cell
Feed gas control system at DEMS setup

14.  The mixed feed gives enhanced
C2H4 ion mass currents (detected via
the molecular fragment (M–H+)
throughout)
 C2H4 ion mass currents on the
cathodic and anodic sweep
branches appeared to be
asymmetric, suggesting local CO
depletion due to diffusion limitation
at the largest overpotential.
DEMS ion mass current (iMS) as function of time
1:1

15. 1:3
 lower 12CO2 partial pressure to
highlight the kinetic effects of
CO in the feed.
 Dimerization of CO2-derived
surface *CO in the cathodic
sweep direction. The self-feeding
supply of CO by CO2-to-CO is
sufficient to maintain ethylene
generation.
 12CO2/12CO co-feed generates
comparable amounts of ethylene
(purple curve) proving the
suppression of local CO-
depletion in anodic voltage
sweep for pure CO2
 Contribution of each mechanistic
pathway is a complex function of
the applied electrode potential
and scanning direction
Deconvolution of DEMS ion mass current (iMS)
12 12
12
Im/z=29
Im/z=27
Im/z=28
 Evaluate the kinetic onset potentials of ethylene
for each mechanism separately

16.  Time-resolved, transient kinetic study to date where reaction
mechanisms with kinetic onset potentials
 Analysis revealed that the C2H4 onset potentials (E) shifted
anodically by about 140 mV RHE when operating with pure
CO compared to pure CO2 feeds (ECO2=−0.81 VRHE)
Kinetic Onset Potential

17. The reaction ratio for the three mechanisms
 A high production ratio over a large potential range is observed for
the CO2–CO mechanism, demonstrating a continuous contribution

18.  The length of the purple bars shows the
enhanced ethylene production under
CO2/CO co-feed conditions compared to
the orange and cyan bars of the pure CO2
and CO feeds
 Comparison of the DEMS-
derived cyclic mass ion current
sweep of ethylene under
CO2/CO (1:3) co-feeding, plotted
in the potential domain
Decomposition of the three pathways for the co-feed
Two-thirds (67%) of the total ethylene yield can be directly kinetically attributed
reactivity of gaseous CO in the reactant feed during COx electroreduction.

19. Tafel slopes of each pathway of ethylene formation
Lowest Tafel slope was found for the cross coupling CO2-CO
pathway, underlining the facile reaction kinetics of the dimerization of
two CO molecules from distinct origins

20. CO2–CO–C2H4 reaction pathways
 Dimerization of two adsorbed*CO molecules to form a OC–CO* which is
further reduced to ethylene
 Cross-coupling L–H type mechanism with the reactive *CO coming from
both CO2 and CO is the dominant dimerization pathway
 The existence of two distinct reactant-specific adsorption sites in atomic
proximity on the Cu nanocatalyst
 Spatially inhomogeneous distributions of reactant-specific *12CO and *13CO
sites on the catalyst surface should yield products

21. From external CO2/CO co-feeds to internal co-feeds on tandem catalysts
Another active site for CO production in micrometre or nanometre proximity
to Cu-based CO2-reducing sites may substitute for external CO gas
Bifunctional non-metallic/metallic tandem catalyst
Ni–N-functionalized carbon (NiNC) acts as selective CO producer and
is the support material for CuOx NPs
 SEM images of the as-prepared CuOx–NiNC tandem catalyst
illustrates the location of the two distinct components
 The neighbouring CuOx NPs and NiNC are spaced on nanometre and
micrometre scales, respectively, in part overlapping with each other.

22. Catalytic C2H4 performance of CuOx–NiNC tandem catalysts
 CuOx–NiNC (1:2) catalysts yield twice the C2H4 production rate.
 At overpotentials of −0.84 V RHE, CuOx–NiNC tandem catalysts produce
considerably less free gaseous CO compared to pure NiNC, indicating some
of the internally generated CO is immediately consumed by the tandem
catalysts.

23. Summary
 Using operando DEMS, time-resolved isotope-labelling experiments were carried
out in a newly designed CO2 capillary cell
 The mechanistic pathways were quantitatively deconvoluted
 Enhanced C2H4 production originated mainly from a cross-coupling CO2–CO
reaction pathway
 Co-fed CO does not compete with CO2 for adsorption sites, which implies the
existence of separate reactant specific adsorption sites for CO2 and CO
 Proposing the concept of tandem catalyst for the utilisation of internal co-feeding.