Utilizing hydrogen underpotential deposition in CO reduction for highly selective formaldehyde production under ambient conditions

Article abstract of DOI:10.1039/d0gc01412e

Formaldehyde is an essential building block for hundreds of chemicals and a promising liquid organic hydrogen carrier (LOHC), yet its indirect energy-intensive synthesis process prohibits it from playing a more significant role. Here we report a direct CO reduction to formaldehyde (CORTF) process that utilizes hydrogen underpotential deposition to overcome the thermodynamic barrier and the scaling relationship restriction. This is the first time that this reaction has been realized under ambient conditions. Using molybdenum phosphide as a catalyst, formaldehyde was produced with nearly a 100% faradaic efficiency in aqueous KOH solution, with its formation rate being one order of magnitude higher compared with the state-of-the-art thermal catalysis approach. Simultaneous tuning of the current density and reaction temperature led to a more selective and productive formaldehyde synthesis, indicating the electrochemical and thermal duality of this reaction. DFT calculations revealed that the desorption of the ?H₂CO intermediate likely served as the rate-limiting step, and the participation of H₂O made the reaction thermodynamically favorable. Furthermore, a full-cell reaction set-up was demonstrated with CO hydrogenation to HCHO achieved without any energy input, which fully realized the spontaneous potential of the reaction. Our study shows the feasibility of combining thermal and electrochemical approaches for realizing the thermodynamics and for scaling relationship-confined reactions, which could serve as a new strategy in future reaction design.

Full text of DOI:10.1039/d0gc01412e

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ARTICLE
Utilizing Hydrogen Underpotential Deposition in CO Reduction for
Highly Selective Formaldehyde Production under Ambient
Condition
Received 00th January 20xx,
Accepted 00th January 20xx
Libo Yao,† Yanbo Pan,† Xiaochen Shen, Dezhen Wu, Abdulaziz Bentalib and Zhenmeng Peng a*
DOI: 10.1039/x0xx00000x
Formaldehyde is an essential building block for hundreds of chemicals and a promising liquid organic hydrogen carrier
(LOHC), yet its indirect energy-intensive synthesis process prohibits it from playing more significant role. Here we report a
direct CO reduction to formaldehyde (CORTF) process that utilizes hydrogen underpotential deposition to overcome
thermodynamic barrier and scaling relationship restriction. This is the first time that this reaction is realized under ambient
condition. Using molybdenum phosphide as catalyst, formaldehyde can be produced with nearly 100% Faradaic efficiency
in aqueous KOH solution, with its formation rate being one order of magnitude higher compared with state-of-the-art
thermal catalysis approach. Simultaneous tuning of current density and reaction temperature leads to more selective and
productive formaldehyde synthesis, indicating electrochemical and thermal duality of this reaction. DFT calculations reveal
that desorption of *H2CO intermediate likely serves as rate-limiting step, and the participation of H2O turns the reaction into
thermodynamically favorable. Furthermore, a full-cell reaction set-up was demonstrated with CO hydrogenation to HCHO
being achieved without any energy input, which fully realizes the spontaneous potential of the reaction. Our study shows
the feasibility of combination of thermal and electrochemical approach in realizing thermodynamics and scaling relationship
confined reaction, which could serve as a new strategy in future reaction design.
demands annual HCHO production of over 50 million tons.19-21
New, cost-effective method for HCHO synthesis would not only
INTRODUCTION
decrease the production cost for its many uses but also enable
its application as LOHC for hydrogen storage research.
The thriving of hydrogen economy and fuel cell technologies
have stimulated extensive research efforts in the associated
fields,1 among which hydrogen storage remains a challenging
topic.2-6 With a hydrogen storage capacity of 8.3 wt.% in its
hydrated form, formaldehyde (HCHO) holds a theoretical
promise as an excellent liquid organic hydrogen carrier (LOHC)
that releases H2 via decomposition and can be regenerated via
hydrogenation.7-8 It is also advantageous, comparing to other
LOHC candidates like methanol and formic acid,9-11 in terms of
its more favorable reaction thermodynamics.12 However, HCHO
was the least investigated among the three chemicals for LOHC
application in previous studies, which suffered from the harsh
reaction conditions required for HCHO synthesis that caused
high energy cost.13-15 In practice, HCHO is produced by
methanol as precursor following the so-called Formox
process,16 which partially oxidizes methanol to HCHO above 400
°C.17 Considering that methanol is typically synthesized from
syngas at elevated temperature and high pressure,18 the overall
HCHO production process is highly energy-consuming which
restricts its use for LOHC application. Besides, HCHO is an
important precursor for synthesizing many more complex
chemicals, like resins, 1,4-butanediol, polyols and so on, which
From thermodynamic point of view, gaseous HCHO can be
directly synthesized from syngas via CO hydrogenation, which
would skip the energy intensive methanol synthesis and the
Formox steps. The reaction has a positive ΔG0 value in gas phase
(Eq. 1), making it barely possible to take place under standard
condition.22 However, the ΔG0 would turn slightly negative
when the reaction occurs in an aqueous phase (Eq. 2),
benefiting from the contribution of heat released during HCHO
solvation. This offers an opportunity to realize aqueous HCHO
synthesis from direct CO hydrogenation.11
푪푶(품) + 푯ퟐ(품)→푯푪푯푶(품) ∆푮ퟎ = +ퟑퟒ.ퟔ 풌푱/풎풐풍
(ퟏ)
푪푶(품) + 푯ퟐ(품) + 푯ퟐ푶(풂풒)→푯푪푯푶 ∙ 푯ퟐ푶(풂풒) ∆푮ퟎ = ―ퟕ.ퟏ 풌푱/풎풐풍 (ퟐ)
As a matter of fact, Tanksale et al. demonstrated success by
thermally catalyzing CO hydrogenation using Pd-Ni and Ru-
Ni/Al2O3 catalysts in aqueous solution,22-23 with a small amount
of HCHO being obtained that proved the viability of this route.
However, the achieved HCHO generation rate was low, being
less than 3 mg/(gcat h) even using the best performed catalyst
and under a harsh condition of high pressure (20-120 bars),
which is far from being satisfactory in terms of potential
industrial application. The slow reaction kinetics can be
explained under scope of the discovered scaling relationship in
catalysis.24-26 CO molecules chemisorb more strongly to catalyst
a. Department of Chemical and Biomolecular Engineering, The University of Akron,
Akron, OH 44325, United States.
† These authors contributed equally to this article.
* Correspondence author: zpeng@uakron.edu.
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Journal Name
than H2 does in a generic trend. This leads to the active sites holding at this temperature for 2 h. The sample was cooled
View Article Online
DOI: 10.1039/D0GC01412E
being dominantly occupied by CO with few active sites available down to room temperature and purged with Ar for another 1h
for H2 molecules to adsorb and dissociate, which consequently before being collected.
inhibits the reaction. From this perspective, it would be
Catalyst Characterization.
challenging to realize efficient CO hydrogenation to aqueous
The as-prepared and spent MoP samples were characterized by
HCHO, unless there are means to break the scaling relationship
Transmission Electron Microscopy (TEM) using a JEOL JEM-1230
to achieve more balanced H2 adsorption.
microscope with an accelerating voltage of 120 kV. High-
Electrocatalysis has received significant attention in reducing
resolution TEM (HRTEM) images were taken using a FEI Tecnai
CO2 to energy fuels in aqueous phase under mild condition. In
G2 F20 microscope operated at 200 kV. The powder X-Ray
comparison, CO electrochemical reduction was less intensively
diffraction (PXRD) patterns were recorded on a Rigaku Ultima IV
multipurpose X-ray diffraction system with CuKα radiation
source. The XPS spectra were obtained with PHI 5000
Versaprobe II X-Ray Photoelectron Spectrometer. SEM image
investigated.27-29 Instead, more research focus was put on the
tandem reaction starting from CO2, with the aim to establish a
CO2-CO-chemical network.30-32 Similar to CO2 electrochemical
reduction, Cu-based materials were mainly used in CO
and EDX mapping were obtained on a Tescan LYRA3 machine
electrochemical reduction, that led to generation of different
with a working voltage of 10 kV for SEM and 20 kV for the
products including methane,30,33 hydrocarbons,34,35 alcohols29,36
mapping mode. In situ diffuse reflectance infrared Fourier
and acetates.37-39 To the best of our knowledge, no direct HCHO
transform spectroscopy (DRIFTS) experiments were conducted
synthesis via CO electrochemical reduction has been achieved,
with a Nicolet 6700 FTIR spectrometer equipped with Harrick
which indicates that the catalysis mechanism of Cu-based
Praying Mantis DRIFTS accessory. The spectra were recorded by
materials doesn’t favor HCHO formation. New catalysis
collecting 64 scans at a resolution of 4 cm-1.
mechanism would be required to realize electrochemical
Electrode Preparation and Electrochemical Measurements.
synthesis of aqueous HCHO.
The electrochemical measurements were conducted by a
Herein, we report a new synthesis strategy that combines
Gamry Reference 600+ potentiostat in an undivided
thermal CO reduction catalysis with hydrogen underpotential
electrochemical cell which includes a 3-electrode configuration.
deposition, which is a pre-step for hydrogen evolution reaction
MoP deposited on carbon cloth (MoP/CC) was used as working
(HER), to reduce CO to aqueous HCHO. Direct, selective CO
electrode (WE), Ag/AgCl and Pt wire were used as reference
reduction to formaldehyde (CORTF) is achieved under ambient
electrode (RE) and counter electrode (CE), respectively. For the
condition using molybdenum phosphide (MoP) catalyst, which
preparation of WE, MoP ink was first made by dispersing 1 mg
has been previously studied as an efficient catalyst in hydrogen
evolution reaction (HER) but has poor CO adsorption
MoP powder in 0.5 mL isopropanol which contains 2 μL Nafion
as binding agent. After sonification, the ink was uniformly
capability.40 We report that formaldehyde can be generated
dispersed on a 2 cm × 1 cm hydrophilic carbon cloth. In most
with 100% selectivity at room temperature via combinational
experiments, 0.5 M KOH was used as default electrolyte unless
utilization of electrochemical and thermal reaction
otherwise noted. All potentials in this work were converted to
mechanisms. The CORTF process improves formaldehyde
those in the scale of reversible hydrogen electrode (RHE) by
formation rate to one order of magnitude higher than the
calibration, in 0.5 M KOH the transformation is shown in eq. 3:
previous thermal catalysis method.22-23 We further
푬(푹푯푬) = 푬(푨품/푨품푪풍) +ퟎ.ퟗퟎퟔ
(ퟑ)
demonstrate a spontaneous, selective HCHO synthesis at room
temperature in a full cell, with CO fed to one electrode for
CORTF and H2 fed to the other for hydrogen oxidation reaction.
Effective aqueous HCHO production is realized without any
energy input, confirming a great potential of this new CORTF
method.
Polarization curves were obtained under linear sweep
voltammetry with 10 mV/s scan rate. Electrochemical
impedance spectroscopy (EIS) was carried out in the frequency
range of 100000 to 0.01 Hz under the amplitude of 10 mV.
The half-cell CO electrochemical reduction test was performed
with continuous feeding of CO gas. Prior to each run, CO was
bubbled to the solution for 1 h with the flow rate of 20 ml/min
for saturation, and it was continuously fed at the same rate
during the measurement.
EXPERIMENTAL
Catalyst Preparation.
Molybdenum phosphide (MoP) was prepared by a temperature
programmed reduction (TPR) method.41 The molybdenum
The full-cell CO reduction was conducted in a H-type 2-
compartment cell with a 2-electrode configuration in 0.5 M KOH
electrolyte. A FUMASEP-FAB-130 anion exchange membrane
was placed between the two compartments. The MoP/CC
electrode was placed in cathodic compartment with 20 ml/min
CO feed, while the Pt electrode was placed in the annodic one
with 20 ml/min H2 being continuously flowed. The cathodic,
anodic and overall reactions are shown in the following:
precursor,
ammonium
molybdate
tetrahydrate,
(NH4)6Mo7O24∙4H2O, and the phosphorus source, ammonium
phosphate dibasic, (NH4)2HPO4, were dissolved in DI water
equimolarly in terms of Mo and P. The mixture was stirred and
dried overnight at 80 °C, followed by calcination at 500 °C for 4
h. The resultant sample then underwent TPR with flowing H2 (50
ml/min). The procedures included ramping to 350 °C at a rate of
5 °C/min, followed by increasing to 650 °C at 2 °C/min and
푪풂풕풉풐풅풆: 푪푶 + ퟑ푯ퟐ푶 + ퟐ풆― →푯푪푯푶 ∙ 푯ퟐ푶 + ퟐ푶푯―
(ퟒ)
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푨풏풐풅풆: 푯ퟐ +ퟐ푶푯― →ퟐ푯ퟐ푶 + ퟐ풆―
(ퟓ)
RESULTS AND DISCUSSIONS
Characterization of MoP Catalyst.
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DOI: 10.1039/D0GC01412E
푶풗풆풓풂풍풍: 푪푶 + 푯ퟐ + 푯ퟐ푶→푯푪푯푶 ∙ 푯ퟐ푶
(ퟔ)
Product Analysis and Quantification.
The MoP synthesis follows a widely adopted temperature
programmed reduction method in previous studies.41 The XRD
pattern indicates a pure hexagonal structure (ICDD 03-065-
Gas products of the reaction were analyzed through an Agilent
6890 GC-MSD, and liquid products were analyzed by H-NMR
using a Bruker AVIII 750 MHz NMR spectrometer. Typically, 500
1
6
6487) with a space group belonging to P m2 (Figure S5).
HRTEM characterization exhibits well-arrayed (001) and (101)
planes with d-spacing of 0.32 and 0.21 nm, respectively (Figure
S6a). A thin amorphous layer is observable on MoP surface,
which suggests slight surface oxidation caused by sample
exposure to atmosphere.45 SEM imaging and EDX mapping
show an even distribution of Mo and P elements throughout the
sample (Figure S6b-d), which was in consistence with the XRD
data and confirmed a uniform MoP phase. XPS spectroscopy of
Mo 3d and P 2p bands show dominant signals assignable to MoP
phase (Figure S7). The Mo 3d spectra exhibit two minor
deconvoluted peaks at 231.4 and 234.3 eV, which are assignable
to Mo6+ as a result of slight surface oxidation due to exposure
to air, which is in good agreement with previous reports.
μL of collected electrolyte after reaction was mixed with 100 μL
D2O. The H spectrum was measured with water suppression
1
using a pre-saturation method. As the spectra from the GC-MS
1
and H-NMR (Figure S1 and S2) have shown, no carbonaceous
gaseous products or liquid products other than formaldehyde
have been detected under our reaction condition (50 μA cm-2).
Therefore, the products can be considered as to include only H2
and HCHO, and the calculation of Faradaic efficiency (FE) can be
performed based on formaldehyde formation. Formaldehyde is
quantified by an adapted acetylacetone method.42 To start
with, 0.25% (v/v) acetylacetone solution was prepared by
dissolving 25 g ammonium acetate, 3 ml acetic acid and 0.25 ml
acetylacetone in DI water and fix the volume of solution to be
100 ml. Then a series of formaldehyde standard solutions (0, Experimental Evaluation of CO Reduction to Formaldehyde.
0.083, 0.333, 0.833, 1.667, 2.5 and 2.917 μg/ml) were prepared.
Next, mix each of 5 ml standard solution with 1 ml 0.25%
The first step in achieving CO reduction to formaldehyde (CORTF)
would be finding an appropriate electrolyte. Figure 1a shows the
acetylacetone solution and put in boiling water bath for 3 min.
linear sweep voltammetry (LSV) curves of MoP in CO-saturated acidic
After cooled down to room temperature, the solution was
(0.5 M H2SO4, pH=0.6), neutral (0.5 M Na2SO4, pH=6.7), and alkaline
transferred to Shimadzu UV-2600 spectrophotometer for the
(0.5 M KOH, pH=13.2) media. The currents were corrected with the
ohmic resistance (Rs, Table S1) which were fitted with equivalent
circuit obtained from electrochemical impedance spectroscopy
(Figure S9). For comparison purpose, LSV curves were also obtained
under CO absence condition, which corresponded to HER (Figure
S10). In both cases, the catalytic activity of MoP represented by
current density follows a same order of H2SO4>KOH>>Na2SO4. Tafel
plots were subsequently derived from the LSV curves (Figure S11).
detection of absorbance from 800-190 nm wavelength. A
working curve (Figure S4) was constructed by building the
relationship between the absorbance at 413 nm (Figure S3) and
concentration for standard solutions, and it turned out to be
perfectly linear. The curve was therefore used for the
quantification of formaldehyde for all the samples obtained in
this work. The Faradaic efficiency of HCHO (FEHCHO) and can be
nFxV)
FEHCHO = (
(I × t) × 100
calculated using
, where n is the
Nearly identical values and Tafel slopes, 92.0 mV dec-1 in presence of
CO versus 93.5 mV dec-1 in absence of CO, were obtained in H2SO4,
suggesting the activity accounts predominantly for HER but little CO
reduction. Interestingly in KOH electrolyte, the Tafel slope changed
significantly from 94.4 to 119.3 mV dec-1 after CO was introduced,
suggesting an alteration in reaction kinetics in presence of CO and
implying possible CORTF occurrence. Similar phenomenon was also
observed in neutral electrolyte, however the overall activity was too
low to be considered. The CORTF reaction was confirmed by
analyzing the liquid and gas products collected at -5 mA cm-2
with GC-MS and 1H-NMR (Figure S1 and S2), which found HCHO
as the only product besides H2. Quantitative analyses show little
HCHO being produced in acidic electrolyte but considerable
HCHO formation in alkaline solution (Figure 1b). The finding
indicates HCHO could indeed be produced via CORTF in
appropriate electrolyte even under ambient condition. It needs
noted that HCHO Faradaic efficiency (FEHCHO) was calculated to
be only 0.52% under the specific testing condition, suggesting a
need for performance optimization.
number of electrons being transferred, F is Faraday’s constant,
x is the concentration of HCHO, V is the volume of electrolyte, I
is the current and t is the reaction time.
DFT Calculations.
DFT calculations were performed using the Quantum ESPRESSO
package.43 Generalized gradient approximation (GGA) and the
Perdew-Burke-Ernzerhof (PBE) with projector-augmented wave
(PAW) sets from PSlibrary 0.3.1 were used to carry out the
structure relaxation and energy calculation.44 The bulk lattice of
MoP was optimized prior to the cleavage of the (001) surface.
The slab model of MoP was constructed using a p (3 × 3) unit
cell with six layers of close-packed (001) surfaces which were
separated by a vacuum layer of 20 Å in the direction
perpendicular to the surface. A plane wave basis set was used
with a cutoff energy of 450 eV with the density cutoff being
4500 eV and a Monkhorst-Pack k-point mesh being 5 × 5 × 1,
and the Fermi-level smearing was set at 0.1 eV. The bottom
four layers of atoms were held fixed in their bulk positions,
while the top two layers of atoms and all adsorbate degrees of By carefully comparing with the LSV curve obtained in Ar-
freedom were allowed to relax.
protected 0.5 M KOH, we observed the current density was
lower below about -0.2 V but higher above this potential when
CO was present (Figure 1c). It revealed the CORTF contribution
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Figure 1. Electrocatalytic properties of MoP with and without CO presence. (a) Polarization curves in CO-saturated acidic (0.5 M
H2SO4, pH=0.6), neutral (0.5 M Na2SO4, pH=6.7) and alkaline (0.5 M KOH, pH=13.2) electrolytes. (b) Effect of electrolyte on
formaldehyde formation at -5 mA cm-2. (c) Comparison of the polarization curves and the zoom-in curves from -0.2 to 0.1 V (inset)
in 0.5 M KOH electrolyte with and without CO. (d) Tafel plots obtained from polarization curves in c.
to the overall current density is more significant before the As shown in our in situ DRIFTS experiment (Figure S12) as well as in
onset potential, where the hydrogen underpotential deposition previous studies,41 it is known that MoP barely chemisorbs CO.
(HUPD) occurs. This is a good indication that the deposited CORTF is likely resulted from reaction between dissolved CO and in
hydrogen is combined with CO for formaldehyde production. situ generated *H species on MoP active sites. In other words, there
Figure 1d shows the difference between the two Tafel plots to would exist a competition relationship between CORTF and HER
yield more insights. The Tafel plot with CO shows two linear (Figure 2). Sharing a same hydrogen deposition step that generates
sections, which intersected at about -0.2 V in potential and *H species, the preference of product is dependent on potential, as
about -0.5 in log |j| that corresponds to a low current density higher overpotential explicitly triggers HER, while CORTF is more
of -0.3 mA cm-2. The plot exhibits an abnormally high Tafel slope significant within underpotential range (<-0.2 V). The HCHO
of 895 mV dec-1 in the upper potential range, i.e., the lower generation rate (rHCHO) and FEHCHO as well as their dependence on the
current density range. This value is distinct from 119 mV dec-1 testing condition were thus carefully examined in the lower current
in the lower potential range where HER dominates, implying density range. There is a clear trend that with a decrease in current
possible dominance by the CORTF reaction in its regime.
density, rHCHO decreases monotonously while FEHCHO keeps increasing
(Figure 3a), with FEHCHO reaching 96.6% at -5 μA cm-2. The finding
reveals that HCHO can be more selectively produced at a lower
current density, i.e., at a more positive potential. The alteration in
product distribution between HCHO and H2, considering the HER
kinetics is purely potential dependent, implies the CORTF kinetics
does not simply rely on the electrochemical potential but also some
other factors. This was confirmed by examination of the temperature
effect on the reaction kinetics and selectivity by keeping the
electrode potential unchanged (E=-30 mV, Figure 3b). Both the rHCHO
and FEHCHO increase with temperature, suggesting CORTF as a
“hybrid” reaction with the kinetics being influenced by not only
electrochemical potential but also thermochemistry. An apparent
activation energy of 17.1 kJ/mol was determined for CORTF from the
Arrhenius plot (Figure S13). The temperature effect shows a same
trend when the current density was fixed at -250 and -50 μA cm-2
(Figure S14). The exception appeared at -5 μA cm-2, where both rHCHO
and FEHCHO became nearly independent on temperature with FEHCHO
approaching 100%. At this low current density, the CORTF rate would
Figure 2. Schematic representation of competitive CORTF and HER
pathways.
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Figure 3. CORTF properties on MoP and the influencing factors. (a) Measured rHCHO and FEHCHO as function of current density. (b) Temperature
effect on rHCHO and FEHCHO obtained at E=-30 mV (vs RHE). (c) Effect of electrolyte concentration on rHCHO. (d) Stability test at 0.2 V vs RHE.
likely be restricted by the *H generation rate but not the following mechanism and properties (Figure S19). Figure 4 presents the
steps that are influenced by temperature. By simultaneously tuning calculated Gibbs free energy diagram for the two reactions on Mo-
the current and temperature, HCHO can be produced at a rate of terminated MoP (001) surface at E=0 V. Detailed structure and
over 30 mg/(gcat h) with more than 50% Faradaic efficiency (Figure energy information of surface intermediates are summarized in
S14b). The activity represents over one order of magnitude higher Figure S20 and Table S3, respectively. The two reactions share a
than state-of-the-art thermal catalysis approach (Table S2) The same initial step (step 1), i.e. the Volmer step, in which *H is
electrolyte concentration effect on the CORTF properties was also generated on MoP surface. The generated *H would follow the
examined (Figure 3c and Figure S15). An increase in the KOH Heyrovsky mechanism to further electrochemically react that leads
concentration led to rHCHO enhancement. It is well known that a to HER and thus H2 formation. Alternatively, *H would react with free
higher electrolyte concentration can substantially increase the HER molecular COaq to form *HCO (step 2), which involves no electron
intrinsic activity, leading to more positive onset potential and lower transfer and is hence a non-galvanic process. *HCO would be further
Tafel slope.46,47 Such enhancement would contribute to more hydrogenated to *H2CO (step 3), followed by desorption to form
efficient hydrogen deposition and subsequent CO reduction.
The durability test at 0.2 V vs RHE, where CORTF dominates, shows a
minor decrease of less than 20 μA cm-2 in the current density within
24 h, suggesting a good stability of the MoP catalyst (Figure 3d). The
slight reaction rate decrease could be related to reduced CO mass
transfer efficiency caused by a mismatch between the continuous
COaq consumption and under-compensation due to a low solubility
(Figure S16). Based on measurement of the accumulative HCHO
production amount after different periods of time, it can be deduced
that the CORTF reaction rate was relatively faster at the very
beginning and then remained steady till completion of the stability
test. It is worth mentioning that the measured electrode potential
became positive when the current density was below -250 μA cm-2
(Figure S17), which can be attributed to a more positive CORTF
Figure 4. Gibbs free energy diagram of DFT-simulated CORTF and
potential than HER induced by CO presence (Figure S18). XRD
HER pathways at E=0 V.
characterizations of the MoP catalyst after the stability test show no
observable difference to the fresh sample, confirming its structural
stability (Figure S8).
HCHO (step 4) and by solvation to form dissolved HCHO (step 5). Thus
the CORTF reaction pathway combines both galvanic and non-
galvanic steps, which makes it subject to the influence of both
electrochemical and thermal factors. The most energy demanding
step for CORTF is found to be *HCHO desorption, which is likely the
Proposed Reaction Pathway and DFT Calculations
Density functional theory (DFT) simulations of both CORTF and HER
pathways were conducted to gain more insights into the reaction
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rate-limiting step. Moreover, the result agrees with the reaction cathode compartment where CORTF occurs leadinVgiewtoArtsicolelvOantleinde
DOI: 10.1039/D0GC01412E
thermodynamics that direct CO reduction to gas phase HCHO has a
HCHO formation (Eq. 4), and H2 was simultaneously fed to the anode
compartment where hydrogen oxidation reaction occurs (Eq. 5).
Figure 5b shows the full cell testing results with different voltage
loads. Interestingly even under zero cell voltage condition, HCHO was
still produced at a rate of 6.1 mg/(gcat h), indicating spontaneity
characteristic of the reaction. Thus direct HCHO synthesis from CO
hydrogenation can be realized without any energy input under
ambient condition with a full electrochemical cell setup. A faster
HCHO formation rate was yielded with a more negative cell voltage,
which was in consistence with the observed potential dependency of
the CORTF activity on MoP. It needs noted that the achieved HCHO
formation rate under the full cell condition was lower than that
under the half-cell condition, which was attributed to a considerable
internal resistance of the anion exchange membrane and the simple
cell setup without optimization. The durability of the full cell for
HCHO production was evaluated at 0 V for 24 h (Figure 5c). HCHO
was continuously produced under the testing condition as evidenced
by its measured accumulated amount over reaction time. Less than
5 μA cm-2 decay in the current density was witnessed during the 24h
test, indicating good cell durability property.
positive ΔG value of 0.16 eV. The solvation effect due to HCHO
dissolution in water turns the overall reaction into a spontaneous
process (ΔG=-0.06 eV).
Spontaneous CO Hydrogenation to Formaldehyde in Full Cell
Configuration
Based on the finding that the MoP can catalyze CORTF with high
selectivity at E > 0 V, spontaneous CO hydrogenation to HCHO was
demonstrated in a full cell configuration. Figure 5a and Figure S21
show schematic and photograph of
a simple H-type two-
compartment cell set-up, which consists of carbon cloth-supported
MoP as cathode, Pt wire as anode and aqueous KOH as electrolyte.
An anion exchange membrane was employed between the two
compartments that allows hydroxyl ion migration across the
membrane and prevents crossover of gases. CO was fed to the
CONCLUSIONS
In summary, we report a new strategy for realizing direct CO
reduction to formaldehyde (CORTF) under ambient condition by
means of combining electrochemical and thermal approaches.
For the first time formaldehyde was successfully produced at
room temperature and ambient pressure in aqueous medium.
The experimental results and DFT simulations suggest the
reaction initiates by means of “sharing” the hydrogen
underpotential deposition (HUPD) with HER, where Had is in situ
electrocatalytically generated on MoP. This breaks down the
scaling relationship restriction in thermal CO reduction
catalysis, wherein H2 dissociative adsorption to generate Had is
strongly inhibited due to always stronger CO adsorption that
poisons the catalyst. CO then reacts with Had following the
thermal catalysis pathway that leads to formaldehyde
formation. Up to 100% CORTF Faradaic efficiency was achieved
by controlling a positive catalyst potential to inhibit HER. By
tuning the electrochemical and thermal condition parameters,
HCHO production rate as high as 31 mg/(gcat h) was obtained,
which was one order of magnitude higher than the previously
reported thermal catalysis method (2~3 mg/(gcat h)). More
importantly, spontaneous CO hydrogenation to formaldehyde
was realized by demonstrating the overall reaction in a full cell.
Continuous formaldehyde production was observed by feeding
CO and H2 to the cathode and the anode without applying any
cell voltage. The concept of hybridizing electrocatalysis and
thermal catalysis for overcoming the thermodynamic barrier
not only realizes successful direct spontaneous formaldehyde
synthesis under ambient condition, but also is likely extendable
to a wide range of thermodynamically and kinetically confined
reactions.
Figure 5. Spontaneous formaldehyde production demonstration in a
full cell. (a) Graphic representation of the full-cell configuration. (b)
Formaldehyde formation rate as function of cell voltage. (c) Cell
durability test at 0 V cell voltage.
Conflicts of interest
6 | J. Name., 2012, 00, 1-3
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There are no conflicts to declare.
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Acknowledgements
We acknowledge the financial support of this work by National
Science Foundation (1665265).
The authors are grateful to Dr. Andrew Knoll at the National
Polymer Innovation Center (NPIC) in University of Akron with the
assistance of XPS. We are also grateful to Dr. Lingyan Li in National
Center for Education and Research on Corrosion and Materials
Performance (NCERAMP) at the University of Akron for the
assistance of SEM analyses, and Mr. Thomas J. Quick in
Geosciences Department at the University of Akron for the
assistance of XRD. The HRTEM test was taken at the (cryo)TEM
facility at the Liquid Crystal Institute, Kent State University,
supported by the Ohio Research Scholars Program Research
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Highly selective formaldehyde production via CO electro-reduction utilizing hydrogen under-
potential deposition under ambient condition.