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Green Chemistry
hydrogen electrode was employed, changing the proton–elec- (HS-GC-MS) was used to detect DMC, dimethyl oxalate (DMO),
tron chemical potential by −eU:34,35
dimethoxy methane (DMM), molecular methyl formate (mMF)
and dimethyl ether (DME) in the electrolyte solution. Product
identification was achieved by mass spectra and retention time
matches of reference solutions. Quantification was performed
by external calibration for DMC, DMO, mMF and DMM.
Faradaic efficiency (FE) is calculated by the equation below for
DMC and DMO:
1
2
μ
ꢀÞðUÞ ¼ GH ꢀ eU
ð5Þ
ðHþþe
2
thus being able to account for the effect of the applied poten-
tial on the energy of potential dependent steps.
When working with the BEEF-vdW exchange–correlation
functional the total energy calculation yields an ensemble of
2000 energies. The thermodynamic corrections and adsorption
energy calculations described from eqn (2)–(4) are applied
element-wise on the ensemble.36 This results in 2000 adsorp-
tion free energies for each adsorbate on each metal. The mean
value of the ensemble corresponds to the BEEF-vdW calculated
value while the standard deviation of the ensemble represents
the uncertainty of the calculation.
The uncertainty of the calculation holds information on
how much the calculated adsorption energy§ depends on the-
choice of functional at the generalized gradient approximation
level of accuracy. All structures with total energies are avail-
english/research/theoretical-electrocatalysis/katladb/dmc_
electrosynthesis/”.
F
Q
FEi ¼ 2ni ꢁ
ð6Þ
where i is DMC or DMO, n is quantity in mol, F is Faraday’s
constant in C mol−1 and Q is the charge in C.
The potential used for synthesis (vs. SCE) should not be
compared directly to the values shown in the DFT work (vs.
RHE). Different reference electrode potentials have been used
for practical purposes and conversion between the two presents a
challenge when working with a non-aqueous electrolyte solution.
3 Results and discussion
3.1 Mechanisms and surface species
As the first step in our theoretical analysis of the synthesis of
DMC, two simple reaction mechanism are suggested. One
leads to the formation of DMC:
2.2 Experimental
The electrochemical tests were conducted in a custom made
glass H-type cell based on the design of Funakawa et al.13 The
working and counter compartments were separated with a
glass frit. A saturated calomel reference electrode (SCE) poten-
tial was placed in a separate compartment and connected to
the working electrode compartment and close to the working
electrode by the use of a Luggin capillary. The working elec-
trode consisted of a gold wire (Premion® 99.9985% Alfa Aesar)
and was stabilised with a holder made of PFA tubing. The geo-
metric surface area of the gold wire was 0.186 cm2. A Nordic
EC ECi-200 potentiostat/galvanostat was used to apply poten-
tial and record current.
All measurements were performed at room temperature. For
the electrochemical tests the counter and working electrode
compartments each contained 20 mL of electrolyte solution of
0.1 M NaClO4·H2O dissolved in methanol (ultrapure, spectro-
photometric grade 99.8% Alfa Aesar). The electrolyte solution in
the counter compartment was purged with argon gas continu-
ously. The electrolyte was chosen based on previous studies.14,25
The electrolyte solution in the working compartment was
initially purged with argon gas for 5–10 minutes to create an
inert atmosphere. Carbon monoxide at atmospheric pressure
was subsequently bubbled into the electrolyte to saturate the
solution prior to the electrochemical synthesis tests.
CO þ * Ð CO*
CH3OH þ * Ð CH3O * þ ðHþ þ eꢀÞ
ð7Þ
methoxy
2CH3O* þ CO* Ð CH3 OCOOCH3 þ 3*
DMC
and the other leads to the co-product, DMO:
CO þ * Ð CO*
CH3OH þ CO* Ð CH3OCO* þðHþ þ eꢀÞ
ð8Þ
methyl formate
2CH3OCO* Ð CH3 OCOCOO CH3 þ 2*
DMO
The initial step is assumed to be the adsorption of carbon
monoxide which is a chemical step, followed by an electro-
chemical step in which methanol is activated. Methanol can
be activated either by direct adsorption on the surface as
methoxy as shown in eqn (7), or by co-adsorbing with carbon
monoxide to form methyl formate as shown in eqn (8). Both of
these steps are electrochemical as they are accompanied by the
exchange of a proton–electron pair. As a consequence it is
possible to lower the free energy of these electrochemical steps
by applying potential. In the suggested mechanisms DMC
forms when two methoxy species meet one carbon monoxide
species while DMO is formed when two methyl formate
species meet. The difference in the reaction mechanisms
leading to DMC and DMO is further summarized in Fig. 1.
It can be seen from the reaction mechanisms that in order
to describe the energetics of all the steps it is necessary to
work with three variables that differ from catalyst to catalyst,
namely the adsorption free energies of carbon monoxide,
Constant potential electrolysis tests were conducted each
for 20 minutes. Between tests, all components of the cell were
rinsed with methanol. A headspace sampler connecting to a
gas chromatograph with
a mass spectrometer detector
§The uncertainty is estimated only from the 0 K total energy calculation,
without the zero-point energy and thermal contributions.
6202 | Green Chem., 2019, 21, 6200–6209
This journal is © The Royal Society of Chemistry 2019