Dinuclear uranium(vi) salen coordination compound: An efficient visible-light-active catalyst for selective reduction of CO2to methanol

Article abstract of DOI:10.1039/d0dt02620d

A new dinuclear uranyl salen coordination compound, [(UO2)2(L)2]·2MeCN [L = 6,6′-((1E,1′E)-((2,2-dimethylpropane-1,3-diyl)bis(azaneylylidene))-bis(methaneylylidene))bis(2-methoxyphenol)], was synthesized using a multifunctional salen ligand to harvest visible light for the selective photocatalytic reduction of CO2 to MeOH. The assembling of the two U centers into one coordination moiety via a chelating-bridging doubly deprotonated tetradentate ligand allowed the formation of U centers with distorted pentagonal bipyramid geometry. Such construction of compounds leads to excellent activity for the photocatalytic reduction of CO2, permitting a production rate of 1.29 mmol g-1 h-1 of MeOH with an apparent quantum yield of 18%. Triethanolamine (TEOA) was used as a sacrificial electron donor to carry out the photocatalytic reduction of CO2. The selective methanol formation was purely a photocatalytic phenomenon and confirmed using isotopically labeled 13CO2 and product analysis by 13C-NMR spectroscopy. The spectroscopic studies also confirmed the interaction of CO2 with the molecule of the title complex. The results of these efforts made it possible to understand the reaction mechanism using ESI-mass spectrometry.

Full text of DOI:10.1039/d0dt02620d

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Dinuclear uranium(VI) salen coordination
compound: an efficient visible-light-active catalyst
Cite this: DOI: 10.1039/d0dt02620d
for selective reduction of CO to methanol†
2
a
b
b
Mohammad Azam, * Umesh Kumar, * Joshua O. Olowoyo,
a
c
c
Saud I. Al-Resayes, Agata Trzesowska-Kruszynska, Rafal Kruszynski,
Mohammad Shahidul Islam,
Mohammad Rafique Siddiqui,
Saikh Mohammad Wabaidur, Masoom Raza Siddiqui, Mohammed Rafi Shaik,
a
a
a
Mohammad Rizwan Khan,
Fahad Ahmed Al-Harthi,
S. F. Adil,
Abdul Karim Alinzi,
a
a
a
a
a
a
b
d
e
Suman L. Jain, M. Amin Farkhondehfal
and Simelys Hernàndez
A new dinuclear uranyl salen coordination compound, [(UO
methylpropane-1,3-diyl)bis(azaneylylidene))-bis(methaneylylidene))bis(2-methoxyphenol)], was synthesized
using a multifunctional salen ligand to harvest visible light for the selective photocatalytic reduction of CO
2 2 2
) (L) ]·2MeCN [L = 6,6’-((1E,1’E)-((2,2-di-
2
to MeOH. The assembling of the two U centers into one coordination moiety via a chelating–bridging
doubly deprotonated tetradentate ligand allowed the formation of U centers with distorted pentagonal
bipyramid geometry. Such construction of compounds leads to excellent activity for the photocatalytic
, permitting a production rate of 1.29 mmol g− h−1 of MeOH with an apparent quantum
1
reduction of CO
2
yield of 18%. Triethanolamine (TEOA) was used as a sacrificial electron donor to carry out the photocatalytic
Received 24th July 2020,
Accepted 25th October 2020
reduction of CO . The selective methanol formation was purely a photocatalytic phenomenon and
2
13
13
confirmed using isotopically labeled CO
2
and product analysis by C-NMR spectroscopy. The spectro-
with the molecule of the title complex. The results of
DOI: 10.1039/d0dt02620d
scopic studies also confirmed the interaction of CO
2
rsc.li/dalton
these efforts made it possible to understand the reaction mechanism using ESI–mass spectrometry.
to global warming and the future depletion of fossil energy
Introduction
3
,4
resources.
Various semiconductor photocatalysts, such as
The depletion of fossil fuel reserves and the continuous oxide-based semiconductors, have been used for the conver-
5
–9
increase in the CO
2
concentration in the atmosphere have led sion of CO
2
into useful compounds.
However, the poor
1
,2
to various environment-related issues. Therefore, the photo- visible light absorbance, low conversion degree, and high
catalytic reduction of CO could be a sustainable technology charge recombination have limited the usage of these
2
8
,9
for reducing CO
and fuels, subsequently providing environmental remediation system with correct molecular structure, molecular catalysts
such as dinuclear rhenium–bipyridine molecular
assemblies, cobalt–porphyrin compounds, mononuclear
iridium hetero-ligand-based molecules (e.g., terpyridine/2-phe-
2
emissions and producing useful chemicals systems. However, after tuning the properties of the catalytic
1
0,11
12
a
Department of Chemistry, College of Science, King Saud University, P. O. Box 2455,
1
3
14
Riyadh 11451, Kingdom of Saudi Arabia. E-mail: azam_res@yahoo.com,
mhashim@ksu.edu.sa; Tel: +966-1-4675982
nylpyridine), iron(0)–porphyrin compounds, cobalt–amino-
pyridine compounds, and manganese and iron-based coordi-
nation polymers with pyridyl-salen ligand can be effective in
1
5
b
Chemical Science Division, CSIR-Indian Institute of Petroleum Dehradun,
India-248005. E-mail: umesh_kumar@iip.res.in
the photocatalytic reduction of CO , especially in CO gene-
c
2
Institute of General and Ecological Chemistry, Lodz University of Technology,
1
6
ration. Similarly, the trinuclear ruthenium polyazine-GO-
Zeromskiego 116, 90-924 Lodz, Poland
1
7
d
Center for Sustainable Future Technologies @POLITO, Instituto Italiano di
phen compound,
blies, and Mn(I) hydridocarbonyl PNP pincer-type complexes
have been developed as photocatalysts for the conversion of
2
CO to MeOH.
Uranium-containing compounds have been used exten-
sively in various catalytic reactions, i.e. the alcoholysis of
ruthenium trisphenanthroline assem-
1
8
Tecnologia, Via Livorno 60, 10144 Turin, Italy
e
Department of Applied Science and Technology, Politecnico di Torino,
C.so Ducadegli Abruzzi 24, 10129 Turin, Italy
19
†
Electronic supplementary information (ESI) available. CCDC 1567193. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0dt02620d
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1
3
esters, the Michael addition, Diels–Alder reactions and poly- 12H, –C–(CH
3
)
2
;
C NMR (CDCl
3
): δ (ppm) = 165.7(–CHvN),
2
0
21
22
merizations, photochemical reduction, thermocatalysis,
24.2 (–C(CH ) ), 56.0 (–C(CH3)2), 67.3 (–C–(CH2)2), 36.1
3 2
(–OCH ), 152.0 (Ar–C–O–).
3
2
3
24
photocatalysis, and axial uranium–oxygen bond activation.
2
+
The uranyl ion (dioxouranium(VI) ion, UO
2
), a highly stable
Crystal structure determination
and dominant functional species, plays a key role in the
2
5
coordination chemistry of uranium.
The X-ray intensity data for the orange-colored prism crystal
Furthermore, the uranyl ion absorbs visible radiation, and were recorded on a Rigaku Synergy Dualflex automatic diffract-
produces a strong emission of photons for specific excitation ometer equipped with Pilatus 300 K detector with mirror
α
wavelengths. The high photocatalytic reactivity of uranyl-con- monochromated CuK (λ = 1.54184 Å, micro-focus sealed
taining coordination compounds is attributed to their high Photon Jet X-ray tube) radiation at a temperature of 100.0(1)
quantum efficiency up to 1 with efficient electron transfer K. The unit cell parameters were obtained from 36 344 stron-
2
+
26,27
between the photoexcited *UO
2
and organic molecules.
gest reflections. Details concerning crystal data and refinement
The rich and diverse photochemistry, together with the high are given in the ESI Table S1.† Anisotropic parameters were
3
3
oxidation potential, makes the uranyl ion a promising candi- applied to refine all non-hydrogen atoms. The SHELXS,
date in the synthesis of new photocatalysts.
2
8–31
34
35
To the best of SHELXL and SHELXTL programs were used for all the cal-
our knowledge, uranyl(VI) salen coordination compounds have culations. Atomic scattering factors were taken from
3
6
never been explored as photocatalysts for the reduction of CO2. International Tables for Crystallography.
Selected bond
These factors led us to develop a new dinuclear uranyl coordi- lengths are listed in the ESI Table S2,† and intermolecular
nation compound possessing a salen ligand and, sub- interactions are listed in the ESI Table S3.†
sequently, to the first-ever exploration of its role in the photo-
catalytic reduction of CO under visible light irradiation.
2
Physicochemical measurements
The intermediate species formed during the photocatalytic
reduction of CO were analyzed with an Agilent 6545 Q-TOF
2
mass spectrometer (M/s Agilent Technologies, USA), resolu-
tion: 30 000 (FWHM) ionization: electrospray ionization, posi-
tive ion mode sample introduction: flow injection (mobile
Experimental
Materials and methods
O 80 : 20 v/v), 0.3 mL min− sample injection
1
TEOA was purchased from Merck, HPLC grade MeCN from SD
Fine-Chem Limited, CO and methane (CH ) (99.9995%) from
Sigma Gases, India. HPLC grade water was used in all experi-
ments. Uranyl acetate was obtained from BDH. All other
materials used in the synthesis of the title complex were
bought from Sigma Chemical Co. and used as received. The
phase MeCN : H
volume: 1 µl, scan range: m/z 50–1700.
2
2
4
1
13
H- and C-NMR spectra were recorded on a Bruker Avance
III – 500 MHz NMR spectrometer. 200 µL of the sample was
1
dissolved in 100 µL of D
2
O. In H-NMR analysis, the number
of scans and the delay time were 32 and 10 s, respectively,
3
2
1
3
salen ligand was synthesized as described in the literature.
The new uranyl complex was characterized at room tempera-
while in C-NMR analysis, these values were 8000 and 5 s,
respectively.
1
13
ture using H and C NMR spectroscopy with the use of JEOL
instrument at 400 MHz and 100 MHz, respectively. The infra-
red spectrum was recorded with a PerkinElmer 621 spectro-
Photocatalytic reduction of CO2
The photocatalytic reduction of carbon dioxide was performed
−
1
3
meter (spectral range: 400 to 4000 cm , sample as KBr
pellets). Elemental analysis was performed with the usage of
an Elementar vario EL elemental analyzer. Electronic spectra
in a closed quartz reactor with gas recirculation and 50 cm
internal volume. The reactor was equipped with a gas inlet,
gas outlet, gas sampling port, and liquid sampling system.
TM
were recorded with an Evolution 300 UV-vis spectrometer.
5
.0 mg of the photocatalyst was suspended in the 20 mL
mixture of MeCN + H O (18 : 2 v/v in mL) or MeCN + TEOA +
O (16 : 2 : 2 v/v in mL) and ultra-sonicated for 10 min to
2
Synthesis of the uranyl compound
H
2
−
3
UO (OAc) ·2H O [0.172 g, 0.405 × 10 mol] was added to obtain a homogeneous dispersion. TEOA was used as a sacrifi-
cial electron donor for the reduction of CO . To remove the
2
mol]. The reaction mixture was vigorously stirred until its dissolved gases, the reaction mixture was evacuated and sub-
2
2
2
−
3
2
0 mL of EtOH solution of the ligand [0.150 g, 0.405 × 10
32
color turned orange, and the uranyl compound was precipi- sequently purged with N 10 times. In the next step, the reac-
2
tated out. After continuous stirring for 5 h, the formed precipi- tion mixture was purged and saturated with CO
2
for 30 min,
tate was filtered, washed in turn with diethyl ether and hexane, and then pressurized with CO at 1.1 bar. The photocatalytic
2
and dried at room temperature. The freshly prepared com- system was illuminated by a 20 W visible light LED lamp
pound was recrystallized from MeCN at room temperature, (Syska, cold white LED, 20 µW cm− ) to drive the reaction. The
2
and the orange crystals grew after a few days.
intensity of incident light intensity inside the photoreactor
Yield: 68.5%. Empirical formula C H N O U , Elemental was measured using a UVP 97-0015-02/UVX digital radiometer.
4
6
54 6 12 2
analysis: calculated: C, 40.65, H, 4.00; N, 6.18; found C, 40.63, Small aliquots of gas and liquid samples were taken out
1
H, 3.95; N, 6.15%; H NMR (CDCl
3
): δ (ppm) 8.31(s, 4H, periodically after every 2 h of irradiation, and analyzed with a
–
CHvN), 6.79–6.99 (m, 12H, Ar–H), 3.48 (s, 8H, –CH ), 1.05 (s, gas chromatograph (GC) (PerkinElmer Clarus 680 equipped
2
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with FID and TCD detectors, plot-Q and ShinCarbon
columns).
A small aliquot of the liquid phase was withdrawn with the
help of a long needle, filtered through the membrane filter
and 1 µL of it was injected into the GC using an autosampler.
3
The product formed in liquid phase (CH OH) was analyzed by
−
1
−1
2
GC-FID (carrier gas flow rate 20 mL min , H : 35 mL min ,
air: 350 mL min− , makeup flow: 27 mL min , injector temp-
erature: 250 °C, and FID detector temperature: 275 °C). The
gaseous product was identified by withdrawing 20 μL sample
and analyzed with GC-TCD using a ShinCarbon packed
1
−1
−
1
column (reference flow: 45 mL min , argon flow: 2 mL
−
1
min ), injector temperature: 220 °C, TCD detector tempera-
ture: 200 °C.
To ascertain the products obtained from the photo-
catalytic reduction of CO
were performed in a preconditioned reactor under the fol-
lowing conditions: (a) in the MeCN + H O mixture without
the photocatalyst; (b) in the MeCN + H O + TEOA mixture
without the photocatalyst; (c) in the MeCN + H O + TEOA
2
, a series of blank experiments
2
2
2
mixture, with the photocatalyst and saturated with N
the absence of CO ; (d) in the MeCN + H O + TEOA
mixture, with the photocatalyst and saturated with CO2,
kept under dark conditions (absence of light). No carbon- Fig. 1 The molecular structure of [(UO ) (L) ]·2MeCN, drawn with 50%
2
, in
2
2
2
2
2
aceous product was detected in the above blank tests, which probability of displacement ellipsoids of non-hydrogen atoms. The
hydrogen atoms have been omitted for clarity.
confirmed that the reaction was solely visible light assisted,
and the synthesized methanol resulted from the reduction
of CO , and not from the degradation of any other organic
2
molecule present in the reaction mixture. The system con-
taining 0.5 mg of the catalyst was pre-conditioned by repeat-
edly applying vacuum and purging with N to remove the
2
Results and discussion
Description of the solid-state structure of the new uranyl
complex from XRD
other dissolved gases before purging with CO and applying
2
2 2 2
Single crystals of [(UO ) (L) ]·2MeCN [Fig. 1] were obtained
from the solutions of uranyl complex crystallized in MeCN,
and the structure was solved and refined in the triclinic space
group P ˉ1 . Details of the structure refinement and solution are
given in the ESI Table S1.† The compound contains two depro-
tonated O^N^N^O 6,6′-((1E,1′E)-((2,2-dimethylpropane-1,3-diyl)
bis(azaneylylidene))-bis-(methaneylylidene))bis(2-methoxyphenol)
visible light irradiation.
1
3
The isotopic CO
2
(99.99% purity, Cambridge Isotope
Laboratories) was also used to confirm whether the photo-
catalytic reduction product formed is solely from the reduction
1
3
13
2 3
of CO . The formed CH OH was analyzed by C NMR. The
quantum yield for the production of methanol was calculated
by the equation given below:
6
ꢀ moles of product ꢀ electrons required for reduction
ΦMEOH; % ¼
ꢀ 100:
moles of the incident photon
2
+
Electrochemical studies
ligands and two UO
2
units alongside with two co-crystallized
MeCN molecules. Uranyl complexes with this type of bis(salicy-
lidene)propanediamine ligand were previously reported with
The electrochemical tests were carried out with a Rotating
Ring Disk Electrode Apparatus (RRDE-3A from Biologic). The
prepared ink contained 5 mg of the catalyst added to
37–45
mononuclear or polynuclear coordination entities.
Each
salen ligand binds to one uranyl via two imine N atoms, one
terminal alkoxide and one bridging alkoxide ligand making up
a seven-coordinated U(IV) atom in a pentagonal bipyramidal
geometry. The salicylidene-1-methaneaminato moieties are
distorted from planarity (the endocyclic CC–CvN dihedral
4
5 microliters of Nafion 117 5% solution in a mixture of
lower aliphatic alcohols, H O and 1.5 mL of C OH. The
prepared ink was sonicated for 4 hours. A glassy carbon rotat-
ing disk electrode, which was dropcast with 6 µl of the ink
2
2 5
H
4
6
−
2
mentioned above (1.91 mg cm of the catalyst), was used for
the tests. The rotation speed of the disk was set to 900 rpm. angles lie in range 15.7(2)–32.8°) as a result of the large size of
6
+
The reference electrode used for these experiments was Ag/
AgNO
in 0.1 M tetrabutylammonium perchlorate (TBAP) in between the coordinating atoms. For smaller cations and
U
cation and the consequent increase of the distance
3
MeCN solution.
similar pure ligands, the salicylidene-1-methaneaminato
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7–49
moieties are almost planar.
The mutual arrangement of reaction. The addition of 1.0 mL and 2.0 mL water to the reac-
the salicylidene-1-methaneaminato moieties in both ligand tion system led to MeOH production rate of 1.08 and
molecules is different, and the dihedral angle between all non- 1.29 mmol g−
1
h , respectively. However, further increment in
−1
hydrogen atoms of these moieties are 27.7(2) and 32.3(2)°. The water concentration, i.e. 3.0 mL and 4.0 mL of water in the
analysis of C–N bond lengths shows that the double bonds are reaction system resulted in a marginal increase in the metha-
−
1
−1
fully localised within benzylideneamino moieties (ESI nol formation at the rate of 1.31 and 1.35 mmol g
Table S2†). respectively [Fig. 2a]. Therefore, the optimum reaction
The weak C–H⋯O intramolecular hydrogen bonds (ESI medium for the photocatalytic reduction of CO to CH OH was
h ,
4
8
2
3
Table S3†) provide some linkage between the complex and selected to contain 16 mL MeCN, 2 mL H
2
O and 2 mL TEOA.
solvent MeCN molecules. The possibility of π⋯π stacking, Similarly, the effect of TEOA, keeping the amount of water con-
5
0
which has been observed in related complexes, is probably stant, was monitored and 2 mL TEOA was found optimum to
impeded in this structure by the large dihedral angle between get the best possible rate of methanol production i.e.
1.29 mmol g−
1
h
−1
[Fig. 2b].
Taking into account the optimum reaction medium, the
and oxygen was also observed [Table 1 and
centered at 425 nm (ε = 1420 M cm ). This band can be Fig. 3]. The formation of O is possibly due to the oxidation of
assigned to a ligand (multidentate hydroxyl groups)-to-metal water by holes, and the presence of O2 was also quantified
planes of neighboring aromatic rings (larger than 32°).
The UV-vis absorption spectrum of the compound is
characterized in the visible range by a broad absorption band formation of CH
4
−
1
−1
2
(
uranyl ions) charge transfer in line with previous reports of using GC (the method included corrections for the presence of
5
1,52
similar complexes.
3
uranyl cation.
Bands of higher intensity were found at air in the sample and the use of Ar as the carrier gas).
−
1
−1
30 nm and 300 nm, that can be assigned to LMCT within the Specifically, 0.01 mmol g of CH , 30.84 mmol g of MeOH
4
5
1,52
−1
The intense bands at 380 and 220 nm domi- and 22.12 mmol g of O
2
were produced during the first 24 h
nating the UV range were assigned to p–p* transitions within [Table 1 and Fig. 3]. The rate of production of MeOH from the
5
1,52
the organic ligand.
The observed absorption of the UV-vis photocatalytic reduction of CO was found to be 1.29 mmol
2
−
1
−1
light excites electrons from the valence band (VB) to the con-
duction band (CB), leaving “positive holes” in the VB. The 3977.57 ± 5.60 μmol gcat from the photocatalytic reduction of
g
h . Kumar and co-workers reported a MeOH yield of
−
1
photogenerated electrons and positive holes migrate to the CO after 48 h of illumination using a ruthenium trinuclear
2
1
7
surface of the photocatalyst during which some of the elec- polyazine complex as a photocatalyst. Li et al. reported a
trons and holes recombine and release energy in the form of copper(II)imidazolate framework for the photocatalytic
−
1
emitted light of longer wavelength to harvest solar light radi- reduction of CO to MeOH with a 1712.7 µmol g turnover in
2
5
3
ation, thus confirming the photocatalytic potential of the com- 5 h. Comparison of the results shows that the title complex
pound [Fig. S1†].
is an effective homogeneous catalyst for the photocatalytic
reduction of CO2.
Photocatalytic reduction of CO2
The reaction progress was monitored up to 48 h, and the
was performed in an product formation rate remained consistent, yielding
2
The photocatalytic reduction of CO
−
1
−1
MeCN–H O system with TEOA as the sacrificial agent in a pre- 0.03 mmol g of methane, 46.86 mmol g of methanol and
2
conditioned reactor. The rate of methanol formation was 29.53 mmol g− of O
1
(Table 1). The amount of O
in the MeCN–TEOA system. than the stoichiometric amount may originate from compe-
However, the rate of formation of methanol increased to tition between TEOA and H O as electron donors and the solu-
in the reaction mixture. After 48 h, the reaction
2
2
being lower
−
1
−1
observed to be 0.038 mmol g
h
2
.80 mmol g−
1
h
−1
with the addition of 0.5 mL water to the bility of O
2
0
2 2 2
Fig. 2 Effect of the TEOA : H O : MeCN ratio on the MeOH production. (a) Variation of H O and (b) variation of TEOA. CO pressure 1.1 bar, reaction
mixture volume 20 mL with 5.0 mg catalyst under visible light irradiation.
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Table 1 Photocatalytic reduction of CO
2
to CH
4
, MeOH, and formation of O
2
(from H
2
O) by the catalyst under visible light
(mmol g−1
CH
4
yield (mmol g−
1
)
MeOH (mmol g−1
)
O
2
)
Time (h)
MeCN + H
2
O
2
MeCN + H O + TEOA
MeCN + H
2
O
MeCN + H
2
O + TEOA
MeCN + H
2
O
2
MeCN + H O + TEOA
2
4
4
8
0.025
0.043
0.01
0.03
11.76
13.59
30.84
46.86
12.93
16.85
22.12
29.53
After 48 h, the reaction samples were purged with CO
2
a
72
0.031
0.02
23.81
74.33
14.29
27.84
a
500 µL of TEOA was further added to the reaction mixture.
Fig. 3 Time-dependent effect of the MeCN : H
2 2
O : catalyst : visible light. (a) Selective formation of MeOH and (b) formation of O . Reaction con-
ditions: CO pressure 1.1 bar, reaction mixture volume 20 mL with 5.0 mg catalyst under visible light irradiation.
2
mixture was again purged with CO , and the reaction contin- amounts of methanol (10, 100 and 1000 µL) added into the
2
ued for the next 24 h (72 h of total reaction time). Further MeCN + H
2
O + TEOA mixture were performed for 24 h. During
increases in MeOH production to 23.81 mmol g in the case these experiments, CH was produced [Fig. S3†], and its for-
of the MeCN + H O mixture and 74.33 mmol g in the case of mation rate increased with an increase in the initial amount of
−
1
4
−
1
2
the MeCN + H
2
O + TEOA mixture (initially containing 500 µL MeOH. This result indicates that higher concentrations of
of TEOA) were observed. On this basis, it can be concluded MeOH result in the production of more CH
4
.
1
3
that the catalyst is stable, i.e., there is no activity loss during
the 48 h reaction time. The turnover number (TON) was calcu- The MeCN + H
lated to be 39.36 after 24 h, while the turnover frequency (TOF) and saturated with CO
In addition, C labeling was used to study the reaction.
2
O mixture containing 0.5 mg of the catalyst
1
3
2
was studied under visible light
was found to be 1.64 h . The light intensity reaching the irradiation. After the photocatalytic reaction, the product was
−
1
−
2
1
13
interior of the reactor was 3.6 mW cm of visible light at analyzed by H and C NMR. The signal at approximately
1
4
50 nm. Consequently, the calculated quantum yield (QY) of 3.7 ppm [Fig. S4†] is associated with the H of methanol. The
1
the catalyst under visible light was 18%. The TON and shift in H of methanol is caused by the influence of the
1
3
quantum yield obtained in the present study are also compar- nuclear magnetic moment of the
C on the hydrogen
5
9
13
able to the recently reported homogeneous complex system for nucleus. The C signal at approximately 49.5 ppm [Fig. S5†]
5
4–59
13
the photocatalytic reduction of CO to fuels.
Next, the for- arose due to the presence of CH OH. These results con-
3
2
mation of hydrogen during the photocatalytic reduction of firmed that methanol was produced directly from the photo-
1
CO
were produced in 24 h and 48 h, respectively [Fig. S2†]. The photodissociation of the carbon-containing catalyst or solvent.
formation of H in the presence of CO could arise from the UV-vis absorption spectra were recorded during the photo-
2
was examined, and 242.82 and 282.60 µmol g− of H
2
2
catalytic reduction of CO , and it was not produced from the
2
2
splitting of formic acid that was produced during the photo- catalytic process. The initial spectrum containing the catalyst
catalytic reduction of CO2 with H O, as well as from the in a MeCN : H O mixture of 18 : 2 (v/v) showed absorption
2
2
decomposition of water to H
2
and O
2
(determined in the maxima at 367, 270 and 255 nm (Fig. 4).
gaseous products of the reaction). The formation of formic
acid is confirmed using electrospray ionization-mass spec- spectrum of this system was recorded. This spectrum was con-
troscopy (ESI–MS) (discussed later in this manuscript). siderably different from the spectrum of the initial solution;
2
Next, the solution was saturated with CO , and the UV-vis
To explain why the methanol production almost reached i.e., new absorption maxima at 341 and 263 nm were observed,
equilibrium at 24 h, a series of experiments with different and the maxima at 225 nm had a shoulder, suggesting a clea-
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The effect of the presence of water in the electrolyte was
investigated by linear sweep voltammetry (LSV) tests under
both N
2
and CO
2
bubbling. The studied compound is fairly
in the absence of water in the
active for the reduction of CO
2
electrolyte [Fig. 5b]. However, the higher amount of water
increased the current densities, but the current densities
recorded under N bubbling exceeded those under CO bub-
2 2
bling. This increase in the total current densities can be justi-
fied by an increase in the hydrogen evolution reaction (HER),
which in the presence of water and at higher potentials tends
6
0
to be dominant,
and consequently lowers the faradaic
efficiency of the electrochemical reduction of CO
2
.
To investigate the selectivity of the studied catalyst under
CO electrochemical reduction conditions, two electrodes were
2
prepared (over titanium foil and carbon paper supports) with
−
2
Fig. 4 UV-vis absorption spectra of the catalyst in the presence of CO
2
.
the same catalyst loading (1.91 mg cm ) but with a higher
The spectra were recorded every 30 min after illumination with visible
light. Inset: (A) Compound + MeCN + H O, (B) compound + MeCN +
O + CO + 30 min light irradiation, (C) compound + MeCN + H O +
CO + 60 min light irradiation, (D) compound + MeCN + H O + CO
0 min light irradiation.
2
electroactive area (1 cm ) to allow the quantification of pro-
ducts. In chronoamperometric tests under different applied
potentials, the current densities ≤1 mA cm were measured;
the only detected liquid product was methanol, and the
2
H
2
2
2
2
2
2
2
+
9
gaseous product was mainly
H
2
.
The highest faradaic
efficiency (i.e., selectivity for CO
2
reduction) was achieved at
vage in the bridging bond and formation of mononuclear −1.5 V vs. Ag/AgNO3 in both electrode supports [Fig. S6†],
complex units. After irradiation with visible light, the intensity although titanium foil was more suitable than carbon paper as
of these newly formed absorption maxima decreased slowly, the support for dropcasting this catalyst material. Hence, the
indicating the utilization of CO
2
in the reaction. The addition highest production rate achieved after 3 h of testing was
of CO to the compound results in an eight- and a six-electron 5 µmol cm−
2
−1
2
h
(over Ti foil at −1.5 V vs. Ag/AgNO ). The
3
reduction of CO to CH and MeOH.
lower performance observed with the carbon paper support
than with the Ti foil support can be explained first by the
detachment of some electrocatalyst, and second by the fragility
in of the electrode itself when used in an H-type cell (like the one
2
2
4
Electrochemical activity
Cyclic voltammetry (CV) revealed that the presence of CO
the electrolyte caused a significant increase in the current used in this work); i.e., constant bubbling of CO
density versus the CV under N
bubbling [Fig. 5a], indicating a netic stirring create a turbulent flow which causes the breaking
substantial electrochemical reduction activity in the presence of the support in some parts. Therefore, for future applications
of CO , which is a well-defined reduction process and associ- of this catalyst for the electrocatalytic reduction of CO , the
and mag-
2
2
2
2
ated with the reduction of CO
2
.
deposition on the electrode surface must be optimized.
Fig. 5 (a) Cyclic voltammograms of the uranyl complex in 0.1 M Bu
4
NPF
NPF
2 2
lines represent N bubbling, solid lines CO bubbling through the solutions.
6
/MeCN solution under argon (red trace) or CO
2
atmosphere (black trace).
(
b) Linear sweep voltammograms of the uranyl complex in 0.1 M Bu
4
6
/MeCN solution with different amounts of H O in the electrolyte. Dashed
2
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To trace complex species in the reaction solutions during and the catalyst remained stable during the photocatalytic
catalytic CO2 reduction and to describe a possible reaction reduction of CO2 [Fig. S8 and S9†]. The carbonate species
mechanism, ESI–MS(+) experiments were conducted, and the formed from CO
results were summarized in Scheme 1. The ion with m/z production of formic acid, which, upon further reduction, was
277.4394 matches exactly with the mass of the title complex converted to MeOH.
when it exists as the +1 molecular ion. Furthermore, the pres- During the photocatalytic reduction of CO
2
dissolved in water was responsible for the
1
2
, the interactions
ence of ions with m/z = 639.226 in the ESI–MS spectra of the of solvent, reactants, intermediates and products with the
MeCN solution suggests that the compound can dissociate compound are possible. The less populated ions observed in
into a mononuclear uranyl species in solution [Fig. S7†]. The ESI–mass spectra represent the adducts with solvent, reac-
presence of ions with m/z equal to 1277.43 and 639.22 in the tants, intermediates and products. Based on the ESI–mass
reaction mixture before and after the 24 h photocatalytic spectra analysis, Scheme 1 has been developed. The ion with
2 2
reduction of CO in MeCN + H O + TEOA confirms that the m/z = 680.24 originates from the interaction of MeCN with
identity of the catalyst remained intact even after the reaction, species 2 and formation of species 3. Further, species 3 upon
Scheme 1 Proposed species from ESI–MS(+) experiments and proposed mechanistic pathway for the CO
UO (L)] catalyst.
2
reduction to MeOH with a mononuclear
[
2
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2
interaction with H O can form 4 (m/z = 657.23), which sub-
Acknowledgements
sequently interacting with CO2 undergoes photocatalytic
reduction and forms formic acid. An observed ion with m/z = The authors acknowledge funding from the Research and
85.23 matches perfectly with the formic acid-attached 5. Development (R&D) Program (Research Pooling Initiative),
6
Later, 5 undergoes a photocatalytic reduction of formic acid Ministry of Education, Riyadh, Saudi Arabia, (RPI-KSU).
to methanol yielding 6 (m/z = 671.25). Upon dissociation of
methanol, 6 goes back to 2 or undergoes a further reduction
of methanol to methane (7, m/z = 655.25) followed by dis-
sociation of methane and making 2 available to carry forward
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