Beyond Hydrogen Evolution: Solar-Driven, Water-Donating Transfer Hydrogenation over Platinum/Carbon Nitride

Article abstract of DOI:10.1021/acscatal.0c01932

Hydrogen-rich organic molecules such as alcohols are widely used as hydrogen donors in transfer hydrogenation. Nevertheless, water as a more abundant and ecofriendly hydrogen source has hardly been used due to the high difficulty in splitting water molecules. Herein, we designed a photocatalytic water-donating transfer hydrogenation (PWDTH) technique, in which hydrogen was extracted from water under light illumination and then in situ added to different unsaturated bonds (C-C, C-O, N-O) for chemical synthesis. Platinum-loaded carbon nitride (Pt/CN) was used as the model catalyst for this cascade reaction, which is beyond its normal applications for water splitting. This approach was highly accessible to efficiency optimization, either by modifying CN for extended light absorption and enhanced charge transfer or by alloying Pt with another metal for better catalytic activities. Remarkably, a quantum efficiency of up to 21.8% was achieved for nitrobenzene hydrogenation under 380 nm irradiation, which is 3 times higher than that obtained in the single water-splitting reaction, indicating that the PWDTH can be more rewarding than hydrogen evolution for solar energy harvesting. Deep insights into the underlying mechanism were provided by detailed measurements and interpretations of femtosecond transient absorption spectra, action spectra (quantum efficiency as a function of excitation wavelength), and reaction kinetic profiles under varied conditions including the variation of light intensities, temperatures, and water isotopes. The mild reaction conditions, simple processing, and broad substituent group tolerance endow this approach with a high potential toward a general solar-to-chemical conversion technique.

Full text of DOI:10.1021/acscatal.0c01932

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Article
Beyond Hydrogen Evolution: Solar Driven Water-Donating
Transfer Hydrogenation over Platinum/Carbon Nitride
Chenhui Han, Lili Du, Muxina Konarova, Dong-Chen Qi, David Lee Phillips, and Jingsan Xu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c01932 • Publication Date (Web): 21 Jul 2020
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Beyond Hydrogen Evolution: Solar Driven Water-Donating
Transfer Hydrogenation over Platinum/Carbon Nitride
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Chenhui Han†, Lili Du‡ǁ, Muxina Konarova⊦, Dong-Chen Qi†§, David Lee Phillips‡, and Jingsan Xu*,†§
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† School of Chemistry and Physics, Science and Engineering Faculty, Queensland University of Technology, Brisbane,
QLD 4000, Australia
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‡ Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, 999077, China
⊦Australian Institute of Bioengineering and Nanotechnology (AIBN), University of Queensland, Brisbane, QLD 4072,
Australia
ǁ Institute of Life Sciences, Jiangsu University, Zhenjiang 212013, China.
§ Centre for Materials Science, Queensland University of Technology, Brisbane, QLD 4000, Australia
Keywords: carbon nitride, solar chemicals, photocatalytic hydrogen evolution, transfer hydrogenation,
eco-friendly hydrogen source
Abstract: Hydrogen-rich organic molecules such as alcohols are widely used as hydrogen donors in
transfer hydrogenation. Nevertheless, water as a more abundant and eco-friendly hydrogen source
has hardly been used due to the high difficulty in splitting water molecules. Herein, we designed a
photocatalytic water-donating transfer hydrogenation (PWDTH) technique, in which hydrogen was
extracted from water under light illumination and then in situ added to different unsaturated bonds
(C=C, C=O, N=O) for chemical synthesis. Platinum loaded carbon nitride (Pt/CN) was used as the
model catalyst for this cascade reaction, which is beyond its normal applications for water splitting.
This approach was highly accessible to efficiency optimization, either by modifying CN for extended
light absorption and enhanced charge transfer, or by alloying Pt with another metal for better catalytic
activities. Remarkably, a quantum efficiency up to 21.8% was achieved for nitrobenzene
hydrogenation under 380 nm irradiation which is 3 times higher than that obtained in a single water
splitting reaction, indicating the PWDTH can be more rewarding than hydrogen evolution for solar
energy harvesting. Deep insights into the underlying mechanism was provided by detailed
measurements and interpretations of femtosecond transient absorption spectra, action spectra
(quantum efficiency as a function of excitation wavelength) and reaction kinetic profiles under varied
conditions including the variation of light intensities, temperatures and water isotopes. The mild
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reaction conditions, simple processing and broad substituent group tolerance endow this approach a
high potential toward a general solar to chemical conversion technique.
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Introduction
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Transfer hydrogenation is an extremely important tool in the fundamental transformations found in
organic chemistry and is widely applied in current pharmaceutical and fine chemical industries.1
Generally, hydrogen-rich molecules such as alcohols, alkanes and hydrazines are used as the
hydrogen donor toward hydrogenation of various unsaturated bonds.2-7 On the other hand, water as
the most widespread solvent and hydrogen source on earth, holds overwhelming advantages such
as being non-toxic and low cost, and therefore can be a potentially ideal alternative for organic
hydrogen donors.8 Nevertheless, the H2O molecule is too stable to be activated and dehydrogenized
under normal conditions (∆G0 = +237 kJ/mol) and has barely been used for transfer hydrogenation.
Recently, some researchers employed H2O as the hydrogen source toward hydrogenation of olefin
and nitro compounds.9-11 However, highly active metal powders such as magnesium and iron had to
be used to reduce H2O to hydrogen, which was not able to be done in a controlled manner owing to
the violent reactions between the reductive metals and H2O molecules.
Hydrogen evolution by water splitting over photocatalysts is considered as one promising pathway
for converting solar energy into chemical fuels. As a metal-free semiconductor, polymeric carbon
nitride (CN) has been extensively studied in the last decade for photocatalytic water splitting owing to
its visible-light response (Eg = 2.7 eV), low cost and high accessibility to property modifications.12-19
Since CN possesses a graphite-like 2D structure, it can be readily exfoliated via liquid-phase
sonication or post thermal treatment, resulting in nanosheets with more exposed active sites and
improved performance.20-23 Noble metal nanoparticles, typically Pt, are usually loaded as cocatalysts
to boost the photocatalytic hydrogen evolution. H2 is generated from Pt sites due to more favorable
hydrogen elimination from intermediate Pt-H species, in comparison to the more difficult covalent
elimination from a hydrogenated CN surface.18 Pt is also an excellent hydrogenation catalyst for a
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variety of unsaturated organic compounds, such as carbon-carbon multiple bonds, nitro- and nitrile
groups, carbonyl compounds, and also in general aromatics.24-26
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In this context, a feasible approach for achieving water transfer hydrogenation is to couple
photochemical hydrogen evolution with a hydrogenation reaction, using e.g. Pt/CN as the catalyst. To
our surprise, although Pt/CN has been well studied for water splitting and significant progress has
been made,27-30 it has been barely used for channeling the hydrogen from water to organic molecules.
Previous works have reported several materials (palladium/CN, Bi2MoO6) for photocatalytic
hydrogenation in water,31-32 however, some problems remain unsolved in these works: the dearth of
key measurements including action spectra and reaction kinetics for understanding the photochemical
processes, the limited type of organic substrate and the lack of direct evidence of water being the
hydrogen donor. Xiong and co-workers used PdPt/TiO2 for photocatalytic alkyne semihydrogenation
with hydrogen sourced from water, nevertheless this photocatalyst was only UV light active due to the
large bandgap of TiO2.33
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Results and Discussion
Herein, we have developed a tandem photocatalytic water-donating transfer hydrogenation (PWDTH)
using Pt/CN as the catalyst, beyond its conventional utilizations for just single water splitting. Our
design is exhibited in Scheme 1: CN acts as the light absorber and upon light illumination, the induced
electrons transfer from CN to the Pt cocatalyst (Step 1), where water is reduced to hydrogen (H0, Step
2); the holes were captured by sacrificial agent (triethanolamine) to form oxidation products.
Subsequently, H0 as a Pt activated speciesis then in situ added to different unsaturated double bonds
(C=C, N=O and C=O) over the catalytic Pt nanoparticle (Step 3), realizing the direct use of water as
a hydrogen donor for hydrogenations. Action spectra, femtosecond transient absorption spectra
reaction and kinetic profiles were measured to shed light on the reaction mechanism. Isotope-labelling
confirmed water being the hydrogen donor rather than any other hydrogen containing species. Much
higher quantum efficiencies (up to 21.8%) were achieved than in conventional water splitting reactions.
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Scheme 1. Schematic illustration of the PWDTH process over the Pt/CN catalyst. Step 1: CN is light-
excited with the generated electrons transferring to the loaded Pt nanoparticles and holes being
captured by scavenger (triethanolamine). Step 2: H2O is reduced to hydrogen (H0) over the Pt
nanoparticles. Step 3: Organic molecules containing unsaturated double bonds (C=C, N=O and C=O)
are in situ hydrogenized at the Pt sites.
CN nanosheets were first prepared by water-assisted exfoliation of bulk CN, which was
synthesized by heating the cyanuric acid-melamine precursor. Then Pt nanoparticles were loaded
onto the CN nanosheets using a thermal reduction method (see Supporting Information for details).
The Pt nanoparticles were uniformly dispersed (Figure 1a) and exposed a narrow size distribution in
the range of 2-6 nm (Figure 1b). A high-resolution TEM (HRTEM) image (Figure 1c) shows that Pt
nanoparticles are tightly attached to the CN nanosheets. The fringe spacing of 0.226 nm can be
ascribed to the (111) planes of face-centered cubic platinum. X-ray diffraction patterns show that the
diffraction peaks from Pt become intensified with increased loading mass (Figure S1). The chemical
state of the Pt nanoparticles was studied by X-ray photoelectron spectroscopy (XPS). As shown in
the XPS spectrum (Figure 1d), the deconvoluted peaks at 71.1 and 74.2 eV were assigned to the 4f7/2
and 4f5/2 orbitals of Pt0, respectively.34 The shoulder peaks centered at 72.4 and 76.2 eV most likely
indicate the presence of Pt in an oxidized state.35 Moreover, to extend light absorption and improve
the charge separation and migration, a carbon-rich molecule, barbituric acid, was co-polymerized with
the cyanuric acid-melamine precursor,36-38 resulting in a modified photocatalyst denoted Pt/CNB (3
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wt% Pt loading, unless otherwise specified). The TEM observation and IR spectra are shown in Figure
S2 and S3.
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Figure 1. The sample of 3 wt% Pt/CN: (a) TEM image, (b) size distribution of Pt nanoparticles, (c)
HRTEM image of Pt nanoparticles and (d) Pt 4f XPS spectrum.
The energy band structures of CN and CNB were determined by electrochemical Mott-Schottky
plots and photoluminescence (PL) spectra. As shown in Figure S4a, the positive slope of the linear
plots indicated the typical n-type characteristic of samples and the derived flat-band potential for CN
and CNB were -1.43 and -1.15 V versus Ag/AgCl, respectively, suggesting that both materials are
thermodynamically suitable for photocatalytic reduction of water (H+/H2: − 0.59 V vs. Ag/AgCl at pH
6.6).[37] The band gaps (Eg) of CN and CNB were calculated according to their PL spectra as 2.68 eV
and 2.43 eV, respectively (Figure S4b). Thus, the valence band top of CN and CNB was then
determined as 1.25 and 1.28 V vs. Ag/AgCl (Figure S4b inset). As shown in the ultraviolet
photoelectron spectroscopy (Figure S5), the binding energy cut-off edge for CN and CNB was 17.17
and 17.01 eV, respectively, from which the work function of the two materials was calculated to be
4.05 and 4.21 eV, lower than that of metal Pt (Φ=5.64 eV) and Pd (Φ=5.22 eV).[39]
The interaction between CN/CNB and metal nanoparticles under equilibrium (no light irradiation)
was investigated by XPS. After the deposition of metal nanoparticles on CN/CNB, electron migration
occurred from the CN/CNB supports to the metal nanoparticles until the Fermi levels are aligned.[40]
This conclusion was verified by the XPS result: the Pt 4f peak in Pt/CN, Pt/CNB, and PtPd/CNB shifted
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to lower binding energy (up to Δ= 0.36 eV) compared to that of metallic Pt (Figure S6a).
Photoluminescence (PL) spectroscopy was employed to study the charge transfer upon light
illumination. As shown in Figure S6b and c, the PL intensity of CN and CNB significantly decreased
after the deposition of metal nanoparticles, suggesting the transfer of photo-generated electrons from
the carbon nitride substrate to the metal nanoparticles.
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Further, Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the C K-edge and
N K-edge was performed and the spectra were compared for CN and CNB samples with and without
Pt loading. The N K-edge spectrum of CN (Figure 2a) shows two characteristic * resonances at
399.4 eV and 402.3 eV which have been previously assigned to antibonding (unoccupied) orbitals
associated with the aromatic N (N-1) in the tris-s-triazine moieties and the bridging N (N-2)
coordinated by three tris-s-triazine moieties, respectively.[41] The N K-edge spectra for Pt/CN, CNB
and Pt/CNB exhibit very similar features, suggesting that the carbon nitride matrix is not disrupted by
the metal loading and barbituric acid co-polymerisation, and the bonding environment of N remains
largely intact. Figure 2b shows the C K-edge NEXAFS spectra. The sharp * resonance at 287.9 eV
for CN is associated with the characteristic N-C-N coordination in carbon nitride matrix. In comparison,
the *C=C resonance at around 285 eV is relatively weak without the appearance of the resonance
associated with structural defects (~284 eV) which were commonly reported for carbon nitride,[41, 42]
confirming the structural quality of our sample. A shoulder at around 287.3 eV can be attributed to
interlayer C-N bonds which ensures a high interconnectivity between CN layers facilitating fast
electron/hole transport across the layers. The additional shoulder at 288.4 eV has been previously
associated with C-N species in a N-doped carbon-rich lattice.[42] However, it could also contain
contribution from the *states of C=O bonds as remnant of the cyanuric acid precursors. The loading
of Pt (Pt/CN) barely modifies the spectral features at C K-edge, similar to that at N K-edge. The co-
polymerisation with barbituric acid in the formation of CNB, on the other hand, introduces a noticeable
enhancement of both *C-N (interlayer) and the *C=O/*C-N resonances (see inset of Figure 2b). The
increase of former suggests an increased electrical connectivity between CN layers. The increase of
latter which can be attributed mainly to an increase of C=O bonds is also not surprising owing to the
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chemical structure of barbituric acid. Intriguingly, this resonance further enhanced after Pt loading in
Pt/CNB. The C K-edge NEXAFS spectrum, to the first approximation, is a representation of the density
of unoccupied states projected to the carbon atoms in relevance. The enhancement of the *C=O/*C-
N resonance suggesting additional electron migration from C=O orbitals to Pt, which will strength the
Schottky junction between Pt and the CNB and thus should enhance the photocatalytic activities.
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Figure 2. NEXAFS spectra at (a) N K-edge and (b) C K-edge for CN and CNB without and with Pt
loading. Inset in (b): normalized C K-edge spectra.
Next, the performance of the PWDTH method was first evaluated toward C=C bond hydrogenation,
and styrene was used as a model compound (the reaction scope will be discussed later). In a typical
procedure, the Pt/CN powder, styrene and triethanolamine were dissolved in a mixed solvent (2 mL
water/3 mL 1,4-dioxane), and then this mixture was irradiated under LED light (420 nm) in a N2
atmosphere. The yield of the hydrogenation product, ethylbenzene, was determined by gas
chromatography (GC) analysis (Figure 3a). As a control, no ethylbenzene was detected when bare
CN was used. After the deposition of 1 wt% Pt nanoparticles on CN, an ethylbenzene yield of 33%
was obtained, indicating Pt nanoparticles are the catalytic sites. The yield improved to 65% with 3
wt% Pt loading. Nevertheless, the yield dropped to 50% when Pt was overloaded (5%), most likely
owing to the aggregation of excess Pt nanoparticles acting as recombination sites for the charge
carriers.43-45 Pt/CNB (3 wt% Pt) showed the optimal performance for the PWDTH process, achieving
99% ethylbenzene yield, agreeing with the above characterizations.
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To elucidate the hydrogen source for the hydrogenation, isotope-labeling experiments were
conducted by replacing H2O with D2O (deuterium oxide). The corresponding ethylbenzene products
were analyzed by mass spectroscopy (Figure 3b). When H2O was used, the m/z of 91.0 and 106.1
were assigned to the fragmentation ion [ph-CH2]+ and the molecular ion of ethylbenzene, respectively.
These signals shifted to 92.0 and 108.1 with the use of D2O, verifying that water was the hydrogen
source rather than any other hydrogen-containing agents (triethanolamine, organic solvent, edge
hydrogen from CN, etc).
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Action spectra, i.e. the apparent quantum efficiencies (AQE) as a function of different excitation
wavelengths can provide greater insights into photochemical processes. In PWDTH, specifically, the
AQE was calculated according to the following equation,
퐴푄퐸 = n × (mol of hydrogenation product) × 100%
,
mol of incident photons
where, n = 2 for C=C and C=O hydrogenation, and n = 6 for N=O hydrogenation, according to the
reaction equations in Scheme 1. As shown in Figure 3c, the Pt/CNB catalyst demonstrated enhanced
AQEs than Pt/CN across the whole UV-vis range, in accordance with the results of the hydrogenation
yield. The enhanced photocatalytic activity for Pt/CNB partly resulted from the increased light
absorption of the CNB support, by which more photons could be harvested and used in the hydrogen
evolution step. Besides, a higher AQE was achieved at lower wavelengths for both catalysts, and an
AQE up to 6.2% was achieved under 380 nm illumination. Notably, the AQEs matched better with the
absorption spectrum of the CN support than that of the whole Pt/CN hybrid, indicating that although
a strong absorption tail up to 650 nm (dashed red line) was enabled by the Pt nanoparticles, Pt only
played a role in the chemical catalysis (Step 2 and 3, Scheme 1). The overall light-harvesting
efficiency of the synergistic catalyst is determined by the CN support, which executes photon-to-
electron conversion, the initial step of the whole PWDTH process. Besides, the impact of light intensity
on the catalytic activity was also investigated (Figure 3d). As a reference, no hydrogenation product
was detected in the dark. Higher reaction rates were observed when the light intensity was increased
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from 10 to 40 mW/cm2. The positive correlation between the reaction rate and the light intensity further
verifies the light-driven nature of the catalytic hydrogenation reactions.
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Wavelength (nm)
o
Figure 3. (a) The yields of styrene hydrogenation reaction catalyzed by different catalysts at 35 C
under the illumination of 420 nm LED. (b) MS spectra of the hydrogenation product (ethylbenzene)
from using H2O or D2O. (c) Action spectra for styrene hydrogenation catalyzed by Pt/CN and Pt/CNB
at 35 C; light intensity: 20 mW/cm2 for each wavelength. (d) Time-course yield of styrene
o
hydrogenation under different light intensity catalyzed by Pt/CNB at 35 oC in a mixed solvent (2 mL
water/3 mL 1,4-dioxane); light source: 420 nm LED.
Femtosecond transient absorption (fs-TA) spectroscopy was employed to study the dynamics of
the excited states and to better understand the performances of the above-mentioned materials. The
3D contour plots and global fitting results46 from the fs-TA spectra of the Pt/CN sample are displayed
in Figure 4a,b. The ground state bleaching signal (a negative absorption signal) of Pt/CN was
observed in the range of 430 nm – 600 nm (Figure 4a). The positive absorption feature in the visible
and NIR regions (> 650 nm) was primarily assigned to photogenerated electrons in the carbon
nitride.47 The global fitting results displayed in Figure 4b and Figure S7a provide three time constants
of 8.7 ps, 270 ps and > 3 ns, which can be assigned to the electron quenching by recombination with
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photogenerated holes,48 shallow electron trap state and deep trapping states with relatively long
lifetimes, respectively.47 For the Pt/CNB (Figure 4c,d and Figure S7b), similar results were obtained,
while the time constant of the recombination process is two times higher (15.6 ps) than that of Pt/CN,
indicating much slower photogenerated electron-hole recombination in the Pt/CNB. The shallow
electron trap state may be favourable to the charge transfer process due to its ability to temporarily
trap electrons,49 and we noted that the time constants of this state were quite close for Pt/CN and
Pt/CNB (270 ns and 240 ns). Therefore, the significantly improved catalytic efficiency of Pt/CNB
compared to Pt/CN should mainly result from the hampered charge carrier recombination in the
modified material.
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(a)
(b)
0.5
0.0
-0.5
-1.0
0.5
8.7 ps
270 ps
> 3 ns
450 500 550 600 650 700 750
Wavelength (nm)
(d)
(c)
(e)
(d)
0.0
-0.5
-1.0
15.6 ps
240 ps
> 3 ns
450 500 550 600 650 700 750
Wavelength (nm)
Figure 4. Fs-TA study of the Pt/CN (a,b) and Pt/CNB (c,d) materials in water: (a,c) contour plots of
the time-resolved absorption spectroscopic responses, (b,d) decay-associated difference spectra
(DAS) and corresponding concentrations of the individual time constants according to the kinetic
models.
The PWDTH approach was then performed to N=O and C=O bonds, using nitrobenzene and
benzaldehyde as the model substrates. As shown in Figure 5a, a 99% yield was achieved for
nitrobenzene hydrogenation, with an AQE of up to 21.8% under 380 nm illumination, much higher
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than many carbon nitride-based photocatalysts for water splitting (Table S1). The corresponding
action spectra are shown in Figure 5b, featuring similar behaviour as that of styrene (C=C)
hydrogenation. However, the yield for benzaldehyde hydrogenation was only 17% under the same
condition being applied. The action spectrum (Figure 5c) illustrates that the AQE was poor under all
wavelengths (below 0.5%). These results can be interpreted that although Pt is able to catalyze a
range of organic transformations, yet it is not an ideal catalyst for C=O reduction.50 To solve this
problem, palladium, a well-recognized catalyst for C=O reduction,51 but less active in oxidation, was
alloyed into Pt nanoparticles (Figure S8) to enhance the catalytic activity for hydrogenation. The
resulting PtPd/CNB catalyst led to a 99% yield of the benzyl alcohol. Significantly improved AQEs
were reached across the wavelength range (10.4% at 380 nm) and matched with the absorption
spectrum of CNB (Figure 5c). Interestingly, alloying with Pd resulted in slightly improved activity for
C=C hydrogenation, and in contrast, resulted in decreased activity for C=O hydrogenation (Figure
5a).
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OH
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PtPd/CNB
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450
500
550
600
650
C=C
N=O
C=O
Wavelength (nm)
(PtPd, 3h)
(Pt, 5h)
(Pt, 6h)
Figure 5. (a) The yield and AQE (on columns) of the hydrogenation of styrene (C=C), nitrobenzene
(N=O) and benzaldehyde (C=O) catalyzed by Pt/CNB and PtPd/CNB; light source: 380 nm LED, 20
mW/cm2. Action spectra for (b) nitrobenzene hydrogenation catalyzed by Pt/CNB and (c)
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benzaldehyde hydrogenation catalyzed by Pt/CNB (red dots) and PtPd/CNB (blue dots); light
intensity: 20 mW/cm2 for each wavelength. (d) Comparison of hydrogen (H2/2Ho) amounts generated
from isolated water splitting (without organic substrates) and the PWDTH process (with organic
substrates); catalysts: Pt/CNB for C=C and N=O hydrogenation, PtPd/CNB for C=O hydrogenation
(unless otherwise specified); solvent: water/1,4-dioxane mixture (2mL/3mL); light source: 420 nm LED,
40 mW/cm2. Reaction temperature for all cases: 35 oC.
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Since H0 is an intermediate product in the PWDTH process, monitoring the generation of H0 will
be very helpful to understand the reaction mechanism. To this end, the photocatalytic water splitting
into H2 gas was isolated by not adding the organic reactants. The amount of produced H2 gas was
measured and compared with that of the H0 formed in the PWDTH process (converted into H2,
calculated according to the amount of hydrogenation products). As shown in Figure 5d, more
hydrogen was generated in the presence of styrene, nitrobenzene or benzaldehyde. This
phenomenon is particularly pronounced for nitrobenzene, where the hydrogen reduction equivalents
towards organic chemistry are 3 times higher than the realized hydrogen yield from isolated water
splitting. The enhancement of hydrogen evolution in the presence of organic reactants should result
from the below effects: (i) higher separation efficiency of photoinduced electron-hole pairs with the
organic reactants adsorbed onto the photocatalysts. As shown in the PL spectra (Figure S9), the
emission intensity of Pt/CNB and PtPd/CNB decreased after the adsorption of organic compounds,
implying enhanced photo-induced charge carrier separation, which is beneficial to the proton
reduction in the hydrogen evolution reaction. (ii) The reduced protons (H0) could be immediately
captured by the adjacent organic reactants and consumed in the coupled hydrogenation reaction,
avoiding the elimination process at metal surfaces to form H2 molecules. Therefore, the presence of
organic substrates should be kinetically beneficial to the formation of H0, which was the critical
intermediate of the PWDTH process. A similar enhancement was reported by Jiang et al, in which
hydrogen generation from ammonia borane was greatly accelerated when coupling to the reduction
of nitro compounds.52 The present observation is inspiring and highly important for the solar-to-fuel
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discussion, as the efficiency is easily improved by a factor of 3 when moving from hydrogen as an
energy carrier to more rewarding organic chemicals.
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The isotope effect on the reaction rate of the PWDTH process was investigated using H2O and
D2O as the water source, respectively. In terms of the C=C and N=O hydrogenations, the reaction
rate was almost the same using either H2O or D2O (Figure 6a, b). For C=O reduction, on the contrary,
the rate was lower when D2O was used (Figure 6c). In combination with the above analysis on the
action spectra, we can conclude that the overall reaction rates of C=C and N=O hydrogenation are
rate-independent of hydrogen (proton) associated steps. Rather, the reaction rate is predominantly
determined by the photo-induced charge separation and transfer within the CN.53 For C=O
hydrogenation, the catalytic rate is on the contrary limited by the hydrogen involved steps over the
PtPd sites, such as bonds cleavage (H-Pd) and bonds formation (H-C, H-O) upon the metal
nanoparticles.
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(a)100
(c)100
100
80
60
40
20
0
(b)
NO2
NH2
OH
O
H
D
2
2
O
O
H
D
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H
D
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0
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Reaction time (h)
Reaction time (h)
Reaction time (h)
Figure 6. Time-dependent yields for hydrogenation of (a) styrene, (b) nitrobenzene and (c)
benzaldehyde using H2O/D2O as the hydrogen source. Water (H2O or D2O)/1,4-dioxane (2mL/3mL)
mixed solvent, 420 nm LED, 40 mW/cm2, 35 oC.
The effect of temperature on the PWDTH rates was studied and shown in Figure 7. The yields of
C=C and C=O hydrogenations decreased from 99% at 35 oC to around 80% at 50 oC and then below
o
50% at 65 C (Figure 7a, c). In a thermocatalytic reaction, the yields of organic transformations
normally increase with temperature.54 However, in a solar driven catalysis process, the separation
and transport of the charge carriers can be inhibited due to more recombination at elevated
temperatures.45 Therefore, the C=C and C=O hydrogenations by the PWDTH approach suffered at
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higher reaction temperatures, resulting in decreased yields. On the contrary, the yields of N=O
hydrogenation remained the same even at higher temperatures (Figure 7b). This performance should
result from the strong coupling effect between nitro group and Pt nanoparticle after adsorption (Figure
S10 and S11),55 which endowed the catalyst capability to efficiently extract photo-induced electrons
from CN (prior to recombination) even at elevated temperatures.
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(c)100
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OH
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O
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0
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Reaction time (h)
Reaction time (h)
Reaction time (h)
Figure 7. Time-dependent yields for hydrogenation of (a) styrene, (b) nitrobenzene and (c)
benzaldehyde at 35, 50 and 65 oC, respectively. Solvent: water/1,4-dioxane mixture (2mL/3mL); Light
source: 420 nm LED, 40 mW/cm2.
The PWDTH system shows wide substrate compatibility. As listed in Table 1, different types of
alkenes, and nitroaromatic compounds, as well as aromatic aldehydes with various substitution
electron-withdrawing/donating groups, were hydrogenated in excellent conversion and selectivity
(>95% in most cases) under optimized conditions. For halide-substituted substrates (entry 5, 9, and
10), the reduction of halide groups (dehalogenation) was observed during the PWDTH process. As a
result, the selectivity for these reactions was declined in comparison to that of using H2 as the
hydrogen source (the values in parentheses). Furthermore, the recyclability of the catalysts was
evaluated. After simple washing and drying, the photocatalysts were reused in the next reaction cycles
and only slight activity degradation was observed after five reaction runs (Figure S12), which might
be owing to the loss or aggregation of the metal nanoparticles during reactions (Figure S13).
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Table 1. The reaction scope of the light-driven hydrogenation system.
Reaction Conv./Sel. [a]
Entry
1
Reactants
Products
time (h)
(%)
6
98/99
9
2
10
7
7
6
6
9
8
3
4
3
6
85/99
97/99
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C10H21
C10H21
3
OH
OH
4
95/98(99) [b]
97/84(88) [b]
98/99
NH2
NO2
5
Br
Br
NO2
NH2
6
NO2
NH2
7
86/99
H3CO
H3CO
NO2
OH
NH2
OH
8
97/99
OH
O
O
9
97/66(90) [b]
98/75(92) [b]
99/99
Cl
Cl
OH
10
11
12
Br
Br
OH
O
O
OH
94/99
H3CO
H3CO
[a]
[b]
The selectivity was calculated from the content of target product and detected side products. The
figures in parentheses give the selectivity when using gaseous H2 as hydrogen source. Reaction
conditions: 3% Pt/g-CNB catalyst was used for all the C=C and N=O substrates; 3% PtPd/g-CNB
catalyst was used for all the C=O substrates. Light intensity: 40 mW/cm2, wavelength: 420 nm,
reaction temperature: 35oC, N2 atmosphere.
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Conclusion
In summary, we have moved forward from the extensively studied solar-driven water splitting to
photocatalytic water donating transfer hydrogenation (PWDTH), i.e. hydrogen was firstly generated
from water splitting, and then in situ added to C=C, N=O or C=O bonds for solar chemical conversion.
Pt/CN was used to catalyse the reaction, although it has been largely conceived as a standard
material for single water splitting in previous reports. Interpretations of the action spectra suggest the
overall light-harvesting efficiency was determined by the CN support, which executed photon-to-
electron conversion, while Pt played its role as a (co)catalyst for the hydrogen evolution and
hydrogenation. Isotope-labeling experiments confirm that the hydrogen originated from water, rather
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than from any organic species or contaminants. Hydrogenation yields were optimized by material
modifications, and light was shed on the dynamic photochemical processes by femtosecond transient
absorption spectroscopy. High quantum efficiencies were obtained in the PWDTH processes,
achieving up to 21.8% for nitrobenzene hydrogenation. Notably, the rate of hydrogen generation from
water was facilitated by the sequential hydrogenation reactions, as compared to the isolated single
water splitting reaction, indicating the PWDTH may be a more rewarding strategy than water splitting
for solar harvesting. The specific catalytic hydrogenation performances of the C=C, N=O and C=O
bonds were comparatively studied by action spectra and reaction kinetics, providing guidance for
hydrogenations of other unsaturated bonds. We believe that this solar-powered catalytic process high
potential to a general, eco-friendly and energy-saving technique for a wider range of hydrogenations
and solar chemical conversion.
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AUTHOR INFORMATION
Corresponding Author
*Jingsan Xu, Email: Jingsan.Xu@qut.edu.au
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
Supporting Information. Materials, methods, XRD patterns, TEM images of Pt/CNB and PtPd/CNB,
FTIR spectra, Mott–Schottky plots, XPS, UPS and PL spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
ACKNOWLEDGMENT
J.X. is grateful for the financial support from the Australian Research Council (DP190101607). D. Q.
acknowledges the support of the Australian Research Council (Grant No. FT160100207). Science
and Engineering Faculty and Central Analytical Research Facility (CARF) at QUT are greatly
acknowledged for technical assistance. Part of this research was undertaken on the Soft X-ray
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Spectroscopy beamline at the Australian Synchrotron, part of ANSTO. D.L.P. acknowledges the
support from the Hong Kong Research Grants Council (GRF 17302218, GRF 17302419, SEG
HKU/07), The University of Hong Kong Development Fund 2013–2014 project “New Ultrafast
Spectroscopy Experiments for Shared Facilities” and the Major Program of Guangdong Basic and
Applied Research (2019B030302009).
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A photocatalytic water-donating transfer hydrogenation (PWDTH) strategy is developed, in which
hydrogen is extracted from water over platinum loaded carbon nitride (Pt/CN) under light illumination,
and then directly transferred to various unsaturated double bonds. This system demonstrates a
remarkable ability to harvest solar energy through intermediary hydrogen, achieving a quantum
efficiency up to 21.8% under 380 nm irradiation.
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