Tandem catalyst for CO2 reduction

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Cu-tandem catalyst for CO2 Electroreduction
Literature Presentation Debabrata Bagchi Ph. D.
Student 17/12/2019
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(No Transcript)
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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)

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Structural characterization of CuOx Catalyst
20 nm
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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

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 Henrys Law
J. Phys. Chem. Lett. 2015, 6, 20, 4073-4082
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Major Product yields under mixed CO2/CO feeds
CH4
C2H4
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Liquid product analysis at different CO2/CO feeds
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Formate only forms in CO2-involving feeds
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Comparison of Hydrocarbon Production Rate

• 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 (COCOH)

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Hydrocarbon yield in 11 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

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Post Electrochemical Characterization

• Cu phase in both reduction process, accompanied
  by a very similar particleagglomeration for all
  feeds.
• Observed differences in the catalytic ethylene
  yields are from kinetic mechanistic roots not
  from catalyst-related chemical factors.

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Quantitative deconvolution of C2H4 formation
mechanism

• Origin of two carbon of ethylene in CO2/CO feeds

CO-CO dimerization

• 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

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DEMS setup

• Capillary electrochemical flow cell

Feed gas control system at DEMS setup
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DEMS ion mass current (iMS) as function of time
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• The mixed feed gives enhanced C2H4 ion mass
  currents (detected via the molecular fragment
  (MH) 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.

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Deconvolution of DEMS ion mass current (iMS)

• 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

12 12
13
12
Im/z28
Im/z27
Im/z29

• Evaluate the kinetic onset potentials of ethylene
  for each mechanism separately

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Kinetic Onset Potential

• 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)

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The reaction ratio for the three mechanisms

• A high production ratio over a large potential
  range is observed for the CO2CO mechanism,
  demonstrating a continuous contribution

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Decomposition of the three pathways for the
co-feed

• Comparison of the DEMS-derived cyclic mass ion
  current sweep of ethylene under CO2/CO (13)
  co-feeding, plotted in the potential domain

• 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

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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
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CO2COC2H4 reaction pathways

• Dimerization of two adsorbedCO molecules to form
  a OCCO which is further reduced to ethylene
• Cross-coupling LH 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

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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
NiN-functionalized carbon (NiNC) acts as
selective CO producer and is the support material
for CuOx NPs

• SEM images of the as-prepared CuOxNiNC 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.

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Catalytic C2H4 performance of CuOxNiNC tandem
catalysts

• CuOxNiNC (12) catalysts yield twice the C2H4
  production rate.
• At overpotentials of -0.84 V RHE, CuOxNiNC
  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.

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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 CO2CO 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.