Electrochemical reduction of formic acid through its decarbonylation in phosphoric acid solution

Article abstract of DOI:10.1016/j.electacta.2016.05.163

The electrochemical reduction of formic acid on a copper cathode in 85% w/w H₃PO₄ electrolyte at 70 °C was studied. In this electrolyte formic acid is partially decomposed to carbon monoxide and water. The experimental results showed that the formation of the products is strongly related to the presence of carbon monoxide in the solution and this suggests that CO is the key intermediate for the formation of the detected producs. At -0.45 V vs. Ag/AgCl the main products were CH₃OH and CH₃CHO with %Current Efficiency (%CE) of 27.6 and 25.8% respectively, whereas a part of the produced methanol(about 50%) was converted to methyl dihydrogen phosphate (25%) and methyl formate (21.8%). At more negative potentials than -0.5 V ethanol was produced by a maximum %CE of 7.9% at -0.75 V. The electrochemical reduction of a CO saturated solution under the same conditions gave the same products albeit the %CE of methanol was lower.

Full text of DOI:10.1016/j.electacta.2016.05.163

Accepted Manuscript
Title: Electrochemical reduction of formic acid through its
decarbonylation in phosphoric acid solution
Author: A. Schizodimou I. Kotoulas G. Kyriacou
PII:
S0013-4686(16)31241-5
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http://dx.doi.org/doi:10.1016/j.electacta.2016.05.163
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Please cite this article as: A.Schizodimou, I.Kotoulas, G.Kyriacou, Electrochemical
reduction of formic acid through its decarbonylation in phosphoric acid solution,
Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.05.163
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Electrochemical reduction of formic acid through its decarbonylation in phosphoric acid
solution
A. Schizodimou, I. Kotoulas and G. Kyriacou*
Department of Chemical Engineering, Laboratory of Inorganic Chemistry,
Aristotle University of Thessaloniki, Thessaloniki 541 24, Greece
Research Highlights


Conversion of HCOOH to low molecular weight organics by a new pathway
High current efficiency to methanol and acetaldehyde
Abstract
The electrochemical reduction of formic acid on a copper cathode in 85% w/w H3PO4
electrolyte at 70 0C was studied. In this electrolyte formic acid is partially decomposed to
carbon monoxide and water. The experimental results showed that the formation of the
products is strongly related to the presence of carbon monoxide in the solution and this
suggests that CO is the key intermediate for the formation of the detected producs. At -0.45 V
vs. Ag/AgCl the main products were CH3OH and CH3CHO with %Current Efficiency (%CE)
of 27.6 and 25.8% respectively, whereas a part of the produced methanol(about 50%) was
converted to methyl dihydrogen phosphate (25%) and methyl formate (21.8%). At more
negative potentials than -0.5 V ethanol was produced by a maximum %CE of 7.9% at -0.75V.
The electrochemical reduction of a CO saturated solution under the same conditions gave the
same products albeit the %CE of methanol was lower.
Keywords: Formic acid, decarbonylation, copper, electroreduction
1. Introduction
Formic acid is one of the main products of the electrochemical reduction of CO2 on various
electrodes. Its current efficiency (%CE) is usually between10-90% depending on the metal
cathode and the other electrolysis conditions [1-3]. In addition, formic acid is one of the main
products of the treatment of biomass under hydrothermal conditions [4]. Thus, the conversion
of formic acid into low molecular weight organics that have higher energy density and are
consumed in large amounts is of great importance.
The reduction of formic acid is thermodynamically favorable [5] since its standard potential
for the reduction to formaldehyde is only 0.056 V, but kinetically difficult. Literature studies
since the beginning of the 20th century have shown that the rate of the reduction on many
metals such as Pb, Ni, Fe, Co and Pt is very low [6]. The reduction on electrodeposited tin
and on liquid Cerrolow136 [7] gives a %CE in the range 7.4-13.7% but the effective current
density was only 140 μΑ/cm2.
In 1977 Russell et al [8] achieved %CEs 11.9 and 99% on Pb and Sn respectively but the
current density was less than 4 μΑ cm-2. They found that the adsorption of HCOOH on the
electrode which takes place in a narrow potential region near the potential of zero charge
(pzc) may be the rate determining step. Kapusta and Hackerman [9] attributed the low rate of
the reduction on tin to the formation of organometallic complexes on the tin cathode which
accelerate the hydrogen evolution reaction. Cyclic voltammetry studies on many metal
cathodes and TiO2 [10, 11] showed that the current increased linearly with the concentration
of formic acid but this increase was not due to the reduction of formic acid but to the
acceleration of the hydrogen evolution reaction in the presence of formic acid. In a work from
our laboratory [12] it was found that formic acid is reduced in acidic solution (2 M HCl) on a
Cu(88)Sn(6)Pb(6) cathode and that the main products were methanol and ethanol having

%CEs of 30.3 and 37.6% respectively at -0.8 V vs. Ag/AgCl. In concentrated solutions of
formic acid a noticeable amount of CH3CHO (%CE 17.1) was also detected.
The reduction of formic acid by chemical methods is also difficult since strong reducing
agents such as Cr+2 that can reduce dinitrogen [13] and carbon monoxide [14] are not capable
to reduce formic acid. The only efficient reducing agent for formic acid is metallic
magnesium in strongly acidic solution which reduces it to formaldehyde and this is a good
analytical method for its determination [15].
It is well known that formic acid undergoes decarbonylation or decarboxylation under various
conditions through the reactions:
HCOOH⇄CO +H2O (1)
HCOOH⇄CO2 + H2
(2)
It has been proved that in the presence of some inorganic acids such as orthophosphoric and
pyrophoshporic at low temperature (20-70 0C) the decarbonylation pathway is the dominant
one [16].
The present work aims to the reduction of the produced CO from the decarbonylation reaction
(1) instead of that of formic acid which is much more difficult.
2. Experimental
Formic acid (purity 98%) and H3PO4 85% w/w was supplied from Baker and the other
chemicals from Fluka. Copper foil 99.999% was supplied from Sigma-Aldrich. The solutions
were prepared using ultra pure water from a Sation9000 apparatus and the electrolyte (85%
w/w H3PO4) was pre-electrolyzed for 4 h at −1.2 V using a carbon cloth as cathode in order to
remove trace metallic impurities. The electrolyte was freshly prepared in order to avoid the
decomposition of formic acid from phosphoric acid.
A Teflon cell having a total volume of 24 mL divided in two equal volume compartments by
a Nafion 117 (H+ form) cation exchange membrane was used in all electrolytic experiments.
A detailed description of this cell has been given in a previous work [17].The copper cathode
had a geometrical area of 1.76 cm2. A Pt foil (Alpha Metal) having an area of 7 cm2 was used
as anode.The cell was placed in a thermostated water bath until the required temperature was
achieved.
The potential was controlled by a Wenking POS 73(Bank Elektronik) potentiostat and the
reference was the saturated Ag/AgCl electrode. The copper electrode was electrolytically
polished by anodization in 85% H3PO4 and subsequently rinsed with ultra–pure water. A
stream of He having a flow rate of 10 mL min−1 was used to withdraw the gaseous products
and a part of the produced organic liquids such as methanol ethanol and acetaldehyde from
the cell during the electrolysis. The escaped liquids from the cell by the gaseous stream were
collected in three tubes containing cold water.
No significant volume loss of the catholyte was observed at the end of electrolysis since the
liquid sample was only 1 μL.
A gas chromatograph (GC) supplied by a Plot Q 30 m, 0.530 mm and a Molecular Sieve 5A,
30 m, 0.530 mm connected in series by a three way valve and a TCD detector was used for
the determination of H2, CO and CO2. A second gas chromatograph supplied by a Pora Plot Q
25 m, 0.53 mm column and FID detector was used for the analysis of the low molecular
weight organics and hydrocarbons. The samples from the catholyte were neutralized by
NaOH 5M before their injection to GC. Tests for the possible production of formaldehyde
were performed by the chromotropic acid method [15].The detection limit for methane and
methanol was 1 ppm and the reproducibility of the experimental results was established to be
within ±5%.
The %CEs of the products were calculated by assuming that the number of electrons is 4 for
CH3OH, 8 for CH3CH2OH, 6 for CH3CHO and CH4 and 10 for C2H6.

3. Results and Discussion
3.1 Cyclic voltammetry
The cyclic voltammograms (Fig. 1) in 85% H3PO4 solution showed that at potential more
negative than -0.43 V the current in presence of carbon monoxide was lower than that of the
background electrolyte and this is in accordance with previous works [18,19]. The decrease in
the current was attributed to the chemisorption of CO on the cathode resulting in the
inhibition of the hydrogen evolution reaction. On the contrary, in the presence of HCOOH
the current was higher than that of the background electrolyte. A similar behavior was
observed on other cathodes such as Sn [9], Pt [10] and TiO2, Ti, In, Ag [11] and was assigned
to the acceleration of the hydrogen evolution reaction in the presence of HCOOH rather than
to its reduction [11].
3.2.
Influence of the potential. In open circuit the analysis of the outgoing from the cell
He stream showed that it contained 1330 ppm of CO, 80ppm of CO2 and 75 ppm of H2. The
ratio of the concentrations of CO2 and H2 was about one as it is expected by the stoichiometry
of the reaction (2). This amount of hydrogen which was formed by the non electrochemical
reaction (2) was not taken into account in the reported %CE of H2. As it is shown in Table 1
the main products of the reduction in formic acid containing solutions at -0.45 V were
CH3OH and CH3CHO with %CEs 27.6 and 25.8 respectively. At more negative potentials
than -0.50 V ethanol was detected with a maximum %CE of 7.9% at -0.75 V.
The % CEs of the hydrocarbons which were the main products of the reduction of CO [18-20]
in our conditions were very low and reached only 2.6% at -1.05 V. During these experiments
a part of CH3OH (about 10%) passed through the membrane in the anodic compartment of the
cell and after its oxidation on the anode gave traces of HCHO. A better view of the
distribution of the products at various potentials is shown in Fig.2.
The observed maximum at about -0.75 V in the %CE of ethanol vs. E and the corresponding
decrease in the %CE of acetaldehyde is characteristic for the intermediate product of a
consecutive reaction mechanism [21] and implies that ethanol is produced by the reduction of
acetaldehyde.
The calculated total %CEs deviates significantly from 100% in the experiments performed
between -0.45 and -0.55 V. Under the conditions of the electrolysis the partial esterification of
H3PO4 and HCOOH by the produced CH3OH was possible. In order to find the decree of
esterification, a 10 mL mixture of 85%H3PO4, 1% HCOOH and 320 ppm of CH3OH was left
to react at 70 0C for 2 h. The analysis of the solution showed that the unreacted methanol was
170 ppm (53%) and that 70 ppm of CH3OH (21.8%) were converted to HCOOCH3. The mass
balance showed that 80 ppm of CH3OH (25%) were converted to CH3H2PO4. By taking into
account these results it can be concluded that the %CE of CH3OH shown in Table 1
represents about 50% of the total CH3OH production. These findings can explain the
deviation of the total %CEs from 100%. Attempts to detect other organic products by proton
and 13C NMR and LC-MS (positive electrospray ionization) were not conclusive due to their
small concentrations. However, the presence of CH3H2PO4 was verified by LC-MS analysis
and that of HCOOCH3 by direct injection of a sample from the outgoing from the cell gaseous
stream to MS-electron ionization (molecular ion m/z=60 and its main fragments 31 and 29).
As it is shown in Fig.3 the current density was almost constant during the electrolysis and this
means that that the copper electrode was not deactivated by adsorbed organic species as it
was observed in previous works on the reduction of HCOOH where a black coating
completely covered the surface of the copper [22, 23]. A second finding showing that
deactivation does not occur is that the amount of all the products were in a linear relationship
with the passed charge as it is expected by the Faraday’s law (Fig.4).
3.3 The effect of the electrolysis medium. In order to clarify which of the CO, CO2 or HCOOH
was the precursor of the obtained products some additional electrolysis experiments were
performed at 70 0C (Table 2).As the concentration of H3PO4 decreases from 85% to 20% the
concentration of CO in the outgoing gaseous stream decreased from 1330 to 320 ppm. In
parallel, the %CE of CH3OH was decreased from 27.6% to 10.5% and that of acetaldehyde
from 25.8 to 11.8%. The decrease of the %CEs was balanced by an increase in the hydrogen
%CE from 10 to 44%. This means that there is a relationship between the concentration of

CO and the %CEs of the main products. The %CE of the main products was not affected by
further increase in the concentration of HCOOH from 1 to 10% w/v.
A comparison of the experiments performed in 85% H3PO4 solution containing 1% w/v
HCOOH and in a CO saturated solution under the same conditions showed that the % CE of
acetaldehyde was about the same (25% and 23% respectively) while the %CE of methanol
was by 10.8% lower. The decreased % CE of CH3OH was attributed to the fact that this
experiment was performed in a closed system where the produced methanol was not stripped
by the He stream resulting to an increased production of mono methyl phosphate. In a CO2
saturated solution the %CEs of CH3OH and CH3CHO were significantly lower but in this
experiment CO and formic acid having %CEs of 10.5 and 12% respectively were detected
and this was expected since they are the main products of the reduction of CO2. In this
experiment we can also assume that CO was also the intermediate for the formation of the
detected products. In a 0.1 M H2SO4 solution containing 1% HCOOH where decarbonylation
of HCOOH was not observed, since CO was not detected in the gaseous stream, reduction
products were not detected. This shows that the nature of the electrolyte plays a more
important role than pH since the pH of a 0.1 M H2SO4 solution is about the same with that of
a 85% H3PO4 solution. The conclusion which can be drawn from the above experiments is
that the products formation is strongly related with the presence of carbon monoxide in the
solution which suggests that CO is the key intermediate. In addition, the presence of CO in
the solution is necessary for the formation of acetaldehyde and ethanol according to the
pathway proposed by Hori et al [20]:
Ethanol which is one of the products detected at potentials more negative than -0.5 V is
possibly produced from the electrochemical reduction of CH3CHO by a consecutive reaction
mechanism.
The presented results show that the reduction of HCOOH through its decarbonylation is an
alternative way for its reduction because it leads to useful products such as CH3OH and
CH3CHO at relatively high % CEs. The main disadvantage is that a part of the produced
CH3OH (about 25%) is converted to mono methyl phosphate and this makes difficult its
separation.
4. Conclusion
This work shows that formic acid can be reduced through its decarbonylation route in
concentrated 85% H2PO4 at 70 0C. The main products were CH3OH and CH3OH, while at
more negative potentials CH3CH2OH was detected as a secondary of the reduction of
CH3CHO. The results showed that CO is the key intermediate for the formation of the
detected products. The total %CE of the products decreased when the concentration of
phosphoric acid was decreased, while it was not depended on the concentration of HCOOH.
A part of the produced methanol (about 50%) reacted with H3PO4 and HCOOH to give the
corresponding esters.
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List of Figures and Tables
Figure 1. Cyclic voltammograms on a copper electrode: Deaerated H3PO4 85% w/w,
after addition of HCOOH 1% w/v and a CO saturated solution at 70 0 C. Rate 5 mV s-
1, at 70 0C

Figure 2. %CE of the products vs. potential in H3PO4 85% w/w containing 1% w/v
HCOOH at 70 0 C (□) CH3OH, (▲) CH3CHO, (●) CH3CH2OH, () CH4 and (o) C2H6

Figure 3. Electrolysis current density vs. time at various potentials in H3PO4 85%
w/w containing 1% w/v HCOOH at 70 0 C

Figure 4. Mass of products vs. passed charge in H3PO4 85% w/w containing 1% w/v
HCOOH at 70 0 C and -0.55 V (□) CH3OH, (▲) CH3CHO, (●) CH3CH2OH, () CH4
and (o) C2H6

Table 1. %CEs of the reduction products at various potentials in 85% H3PO4 and 1%
HCOOH at 70 0C
63.4
77.5
87.1
94.3
97.6
97.2
Table 2. The influence of the electrolyte on the %CEs of the products at 70 0C, at -0.45 V
%CE
Total
Electrolyte
Total Charge
CH3OH
27.6
19.3
CH3CHO CH4 C2H6
H2
10
35
44
CE
63.4
69.2
66.3
/C
85% Η3PO4 + 1%HCOOH
50% Η3PO4 + 1%HCOOH
20% Η3PO4 + 1%HCOOH
25.8
14.9
11.8
-
-
-
-
-
-
6.48
5.76
5.40
10.5
85% Η3PO4 + 10%HCOOH
85% Η3PO4 CO saturated
23.8
16.8
9.7
17.2
23.0
1.1
-
-
-
-
-
-
18
16
30
59.0
55.8
6.62
7.20
5.76
85% Η3PO4 CO2 saturated at
40.8*
-0.45 V
0.1 M H2SO4+ 1% HCOOH
-
-
tr
-
98
98
6.2
*In this experiment HCOOH and CO were detected having %CEs of 10.5 and 12%
respectively