ELECTROCATALYTIC REDUCTION
OF CARBON DIOXIDE ON METAL
SURFACES TO SMALL MOLECULE
FUELS
Presentation based on Advances in CO2
conversion and utilization
2010 ACS – Wenzhen Li
Why CO2 appears important today?
• Increase of CO2 one of the causes of green
house effect and global warming issues
• Electro-catalytic reduction of CO2 to liquid
fuels
• Carbon balance by recycling into usable fuels
1.Carbon dioxide is a stable molecule
Produced by fossil fuel combustion and respiration
2. Returning CO2 to useful state on the same scale as its
current production rates is beyond our current scientific
and technical ability
No commercial available process for the conversion of
CO2 to fuels and chemicals – challenges are great
potential rewards enormous .Fundamental knowledge
for activation of CO2
3.Require catalysts that operate near TD potentials and
high rates
3. novel catalyst systems are required multi-site systems
C-O,C-C,C-H multi-step, multi electron, charge and atom
transfer reactions
Increase of Carbon dioxide in the
atmosphere
electro-catalytic reduction is one
possible way to mitigate the
carbon balance
No commercial process for
conversion of carbon dioxide
Understanding of the chemistry of
activation of carbon dioxide
multi-functional catalysts
C-O bond activation C-H and C-C
bond formation
energy input and reasonable
selectivity
Reverse of electrochemical reactions taking
place at anode of fuel cells
a process of converting electrical energy to
chemical energy
Gibbs free energy is always positive
overvoltage is >1 V in aqueous medium, water
reduction is a competing process – high
Hydrogen overvoltage metals like Hg suppress
H2 evolution leads to HCOO- at high overpotentials
Copper different from other metals
CO2 to HC- CH4 or C2H4- 5-10
mA/cm2
Current efficiency >69%
copper single crystals, ad-atom cu, cu
alloys, H2,CH4,C2H4 and CO hythane
combined fuel can be produced in
aqueous electrolyte
CO2 reduction in gas phase GDE or
SPE
Isopropanol and C4 oxygenates in
GDE CNT-encapsulated metal
catalysts although small amounts but
can open up new avenues for
electro-catalytic conversion to liquid
fuels
Current knowledge
metal electrodes GDE, SPE
Homogeneous catalysis is efficient
but outside the scope of this
presentation
Liquid fuels like HCOOH,
isopropanol, HC and fuel precursor
CO The equilibirum potentials are
negative with respect to hydrogen
evolution (HER) in aqueous
electrolyte solutions
Fundamental challenges
The primary reactions at pH = 7 at
298 K against NHE
CO2+H2O+2e→HCOO- + OH- (-0.43V)
CO2 +H2O+2e=CO+ 2OH-(-0.52V)
CO2+6H2O=8e=CH4+8OH-(-0.25V)
2CO2+8H2O +12e=C2H4 +12OH-(-0.34V)
2CO2+9H2O+12e= C2H5OH +12OH- (-0.33V)
3CO2+13H2O+18e=C3H7OH+18OH-(-0.32V)
2H2O+2e= 2OH- +H2 (-0.41V)
However reduction of CO2 does not
occur at equilibrium values more
negative potentials
since single electron reduction
CO2 + E = CO2- (-1.90 V) due to large
reorganizational energy between the
linear molecule and bent radical
anion first step
CO2 + e === CO2.(-1.90V)
The equilibrium potential that is
considered is dependent on pH
CO2+8H= +8e=CH4 +2H2O (+0.17V)
at pH = 0 while it changes with pH
shown in Fig.1.
What is CO2 reduction?
Assembling nuclei
formation of chemical bonds to convert the
simple molecule into more complex and
energetic molecules
kinetic control since low equilibrium
potentials
TD Methane and ethylene should occur at
less cathodic potential than hydrogen,
kinetically does not happen
The product distribution for CO2
on Cu is shown as a function of
potential in Fig.2.
1. Initially CO and HCOO at -1.12V
then hydrocarbon first ethylene and
methane form- these potential
dependent and predominates at
around -1.35 V. So both TD and
kinetics are important
HER in aqueous electrolyte
competes with CO2 reduction HER
predominates in acid and CO2
does not exist in basic and hence
most of the measurements have to
be done in neutral medium
The product selectivity depends on
many factors
concentration, electrode potential ,
temperature, electro-catalyst
material, electrolyte
product on electro-catalyst
material if other factors are remain
the same.
Four groups
st
1 group
Pb Hg, In,Sn,Cd,Tl, Bi high
hydrogen overvoltage negligible
CO adsorption high overvoltages
for CO2 to Co2radical ion weak
stabilisation of the CO2radical ion.
Major product is formate
Second group
Au,Ag,Zn
medium hydrogen overvoltage,
weak CO adsorption major product
is CO C-O bond break and desorb
CO
Third group Ni, Fe,Pt,Ti low
hydrogen overvoltage strong CO
adsorption major product is H2
Fourth group
Cu Unique more reduced species
like methane ethylene Fig 2
Under potential deposition copper
-1.44 V Co selectivity is 60% while
that of Cd and Pd adatom modified
Cu is 82% and 0 respectively.
Reaction mechanism
limited from charge transfer
coefficients and reaction orders
CO2 adsorbed as CO2δpromoted by defects alkali metals and irraditions
CO2 is amphoteric - both acidic and basic
to adsorb as CO2δdepends on electrode surface carbon or oxygen
or mixed coordination anion radical is first step
where is the excess charge on C as a nucleophilic
agent Std potential -1.9vs SHE or -2.21 C vs
SCE Transfer coefficient is 0.67 in aqueous and
non aqueous solutions CO2Two main pathways to CO or formate depends
on metal Fog 4
on Hg the major product is formate
Co2 by one electron transfer to for CO2.at the negative potential of -1.6 V it will take a
proton from water H will not be bonded to
oxygen atom since the pKa I 1.4 formate radical
is reduced to formate ion subsequently The
steps CO2.- (ads) + H2O === HCOO. + OH- HCOO. + e == HCOOor directly
CO2.- + Hads=== HCOO-
The reaction scheme is suitable to
other metal electrodes like Ag, Au,
Cu and Zn. Sequence of CO
selectivity follows the electrode
potential only that stabilizes
carbon dioxide anion radical CO is
main product fig 4c weak CO
adsorption
HER side reaction for CO2 reduction in
aqueous medium
Ph dependent in acid and independent in
alkaline medium
H+ + e- → Hads
2Hads → H2
Hads + H+ + e- → H2
Hads. H+ are the hydrogen source for CO2
reduction
Pt/Fe/Ni/Ti CO is strongly adsorbed and
major product is H2
Cu Based electro-catalysts
CO2 → CH4 /C2H4/alcohols
At low over potential CO/COO- yield
appreciable at -1.1V C2H4 increases
CO/HCOO- precursors to HC/alcohols
CO linear adsorbed at -0.6 V Coverage high
heat (17.7 kcal/mol) appropriate.
So subsequent reduction Co to HC/alcohols
COads to HC
CH4 more negative potential than C2H4 (1.22 to 1.12V)
C2H4, CH4 through different reactions CO bond is
broken since alcohol is not formed
CH4 CO anion radical Cu-C bond decrease C-O bond
increase
Two Paths
Co anion radical proton and second electron transfer
CH4 formation irreversible (5b)
Co anon radical + adsorbed hydrogen C----O H addition
(5c)
C2H4 associated pair
Ch2ads two dimerise
Or CO-CH3 (Fig.6)
Crystal face (100) for copper Pi-CO two oxygen
atoms close to Cu
(111) CH4 formation more negative potential
(110) 2/3 carbon product
different over potential
Surface treatment Cu Alloy
CH3 OH intentional peroxide
Alloy Cu-Ni, Cu-Fr, hydrogen increases and CH4
C2H4 decrease
Cu-Cd CH4, ethylene
other alloys CO and formate
Cu-Au majority is CO
GDE/SPE
CO2 to fuel precursor CO
nd
group
CO2 to CO 2
Au Ag H2O to H2
CO2+H2O to CO + H2 (1:2) GDE
Au/Ag Cathode (Fig8)
Time dependent
CO2 to C1-C2 fuels
CO2 to HCOOH Pb impregnated GDE
CO2 to higher than C2 SPE
Copper catalyst Cation/anion
exchange membrane (CEM/AEM
Only 20-25% current efficienty
Product depends on CEM/AEM
CO2 long chain HC
Challenge Upto C6 Cu electrode
FT distribution
Product distribution
IPA, Acetone, Ethanol,
acetaldehyde
and methanol in Fe encapsulated
CNT
1.CO2 is stable
2.Electrocatalytic method high potential
3.Energy efficiency TD/rate
4.Mechanism limited knowledge
5.Beyond current ability
6.New methods approaches of activating
7.Novel catalysts multi-site
8.C-O bond cleavage C_C and C-H
9.Multi step, multi-electron transformations
10.Space restrictions intermediates
11.Model catalysts single crystals, adatom,
electrodeposited