Page 9 of 11
Green Chemistry
Green Chemis
Please do not adjust mt r ya rgins
DOI: 10.1039/C6GC03422E
ARTICLE
circuit and the protons reaching the cathode from the Nafion® production of acetic acid can be ascribed to the high
membrane in direct contact to the electrocatalyst. concentration of CO , with the subsequent high formation of
At this point, the –CH OH species may desorb (by a proton CO on the cathode, which can adsorb at the electrode
2
•-
2
2
attack to the carbon atom from the aqueous solution) to form surface and react with the adsorbed reduced species –CH
methanol, or proceed the reduction at the electrode surface. In the giving a molecule of acetate.
last option, the second C-O bond is broken with loss of another
3
water molecule, with the formation of the adsorbed –CH
3
species.
Acknowledgements
This intermediate can be considered as the precursor for the
•-
formation of acetic acid. Due to the high concentration of CO
operate with 100 % CO flow), the adsorbed –CH species can
undergo a nucleophilic attack from the not-adsorbed CO
subsequent formation of acetic acid. Alternatively, the adsorbed –
2
(we
The financial support of the PRIN 2015 project “Solar driven
chemistry: new materials for photo- and electro-catalysis -
2
3
•-
2
, with the
2
015K7FZLH” is gratefully acknowledged.
•
-
CH
3
species can combine with CO
2
adsorbed at a close catalytic
-
site (–COO ) with subsequent formation of acetic acid.
References
This tentative mechanistic pathway is able to explain most
experimental observations that we obtained in our electrocatalytic
tests. The introduction of formic acid does not produce an increase
in the other reduction products, thus confirming that its formation
is separate from the specific half-reactions occurring on the
electrode surface for the formation of methanol and acetic acid.
The reaction in presence of formaldehyde, however, leads to a
strong increase in the production of both methanol and formic acid,
but not acetic acid. The formaldehyde is not a stable molecule and
tends to adsorb very easily on the electrocatalytic sites. We can
suggest that the adsorption of the formaldehyde is preceded by the
1
(a) R. Schlögl R, Chemical Energy Storage, De Gruyter, Berlin -
Germany, 2013. (b) R. Schlögl, Green, 2013, , 233.
(a) G. Centi, S. Perathoner, Green Carbon Dioxide: Advances
in CO Utilization, Wiley & Sons, NY 2014. (b) S. Abate, G.
Centi, S. Perathoner, Green, 2015, , 43.
(a) G. Centi, E.A. Quadrelli, S. Perathoner, Energy & Env.
Science, 2013, , 1711. (b) C. Ampelli, S. Perathoner, G. Centi,
Phil. Trans. Royal Society of London A, 2015, 373, 20140177.
(c) S. Perathoner, G. Centi, ChemSusChem, 2014, , 1274.
2
2
2
5
3
4
6
7
EuCheMS (The European Federation of Catalysis Societies),
White Paper on Solar-driven Chemistry, EuCheMS, Bruxelles
2016. Accessible at: http://www.euchems.eu/solar-driven-
chemistry.
•-
•-
formation of an intermediate species H
2
CO (similar to CO
for the adsorption at the
electrode surface. The more favourable adsorption of formaldehyde
2
) that is
5
6
Y. Hori, In: Modern Aspects of Electrochemistry, C.G.
Vayenas, R.E. White, M. E. Gamboa-Aldeco (Ed.s), Springer
NY 2008, 42, 89.
•-
unstable and compete with CO
2
•-
(a) J.-P. Jones, G.K.S. Prakash, G.A. Olah, Israel J. Chem.,
2014, 54, 1451. (b) W.-H. Wang, Y. Himeda, J.T. Muckerman,
G.F. Manbeck, E. Fujita, Etsuko, Chem. Rev., 2015, 115
leads to the formation of methanol, while CO
2
does not adsorb at
the electrode surface and can only react in liquid phase with
protons to produce a higher quantity of formic acid with respect to
the test in standard conditions. For the same reason, the reaction in
presence of formaldehyde proceeds better towards the formation
of methanol instead of acetic acid because all the catalytic sites are
occupied with formaldehyde and there are no available sites for the
,
2936. (c) B. Kumar, J.P. Brian, V. Atla, S. Kumari, K.A.
1
Bertram, R.T. White, J.M. Spurgeon, Catal. Today, 2016, 270
,
9. (d) Q. Lu, F. Jiao, Nano Energy, 2016, Ahead of Print. DOI:
1
j.nanoen.2016.04.009. (e) R. Kortlever, J. Shen, K.s J. P.
Schouten, F. Calle-Vallejo, M- T. M. Koper, J. Phys. Chem.
Lett. 2015, 6, 4073. (f) M. Gattrell, N. Gupta, A.A. Co, J.
Electroanal. Chem. 2006, 594, 1.
•-
adsorption of CO
2
. This can confirm that the production of acetic
acid occurs for the combination of two adjacent adsorbed species –
7
8
European Research Institute of Catalysis (ERIC), Science and
Technology Roadmap of Catalysis for Europe. A Path to
create a Sustainable Future, ERIC Pub.: 2016 (ISBN 979-12-
-
CH
acetic acid might be the reaction of methanol carbonylation (CO +
CH OH) but i) the catalytic systems for this reaction are well
3
and –COO . An alternative pathway toward the formation of
2
00-1453-3).
3
(a) K.P. Kuhl, T. Hatsukade, E.R. Cave, D.N. Abram, J.
Kibsgaard, T.F. Jaramillo, J. Am. Chem. Soc. 2014, 136, 14107.
(b) F.S. Roberts, K.P. Kuhl KP, A. Nilsson, Angew Chem Int Ed
Engl., 2015, 54, 5179. (c) Y. Hori, K. Kikuchi, S. Suzuki, Chem.
Lett. 1985, 11, 1695. (d) Y. Hori, A. Murata, R. Takahashi, S.
Suzuki, J. Am. Chem. Soc. 1987, 109, 5022. (e) K.P. Kuhl, E.R.
different, ii) we did not observe CO production from the outlet gas
stream and iii) in the experiments with formaldehyde in presence of
CO we did not obtain an increased production of acetic acid.
Furthermore, the electrocatalytic tests without CO
2
produce
can be formed only in small
concentration due to the presence of the electrolyte KHCO (in
equilibrium with CO , see Eq. 2). If KHCO is replaced with KCl,
•-
much less acetic acid, as CO
2
Cave, D.N. Abram, T.F. Jaramillo, Energy Environ. Sci. 2012,
7
5,
050. (f) Y. Hori, A. Murata, R. Takahashi, J. Chem. Soc.,
3
Faraday Trans. 1 1989, 85, 2309. (g) R.L. Cook, R.C. MacDuff,
A.F. Sammells, J. Electrochem. Soc. 1989, 136, 1982.
Y. Hori, R. Takahashi, Y. Yoshinami, A. Murata, J. Phys. Chem.
B 1997, 101, 7075.
2
3
acetic acid formation becomes close to zero, confirming our
hypothesis.
9
1
The production of acetic acid can be related to the
0 A.A. Peterson, F. Abild-Pedersen, F. Studt, J. Rossmeisl, J.K.
2
concentration of CO in the cathode compartment. Koleli et
Norskov, Energy Environ. Sci. 2010,
al. reported a high Faradaic efficiency in the formation of 11 J.H. Montoya, A.A. Peterson, J.K. Norskov, ChemCatChem
013, , 737.
2 K. J. P. Schouten, Y. Kwon, C. J. M. van der Ham, Z. Qin, M. T.
M. Koper, Chem. Sci. 2011, , 1902.
3, 1311.
3
4
2
5
acetic acid on polyaniline electrode in a membrane cell for the
electrochemical reduction of CO , but they operated under
high pressure to increase the solubility of CO . We operate at
ambient pressure, but we used a pure flow of CO (100 %),
1
1
2
2
2
3 X. Nie, M.R. Esopi, M.J. Janik, A. Asthagiri, Angew. Chem., Int.
Ed. 2013, 52, 2459.
2
except for the tests in presence of reaction intermediates. The
This journal is © The Royal Society of Chemistry 2016
Green Chemistry | 9
Please do not adjust margins