Solar Reforming of Biomass with Homogeneous Carbon Dots

Article abstract of DOI:10.1002/anie.202008217

A sunlight-powered process is reported that employs carbon dots (CDs) as light absorbers for the conversion of lignocellulose into sustainable H₂ fuel and organics. This photocatalytic system operates in pure and untreated sea water at benign pH (2–8) and ambient temperature and pressure. The CDs can be produced in a scalable synthesis directly from biomass itself and their solubility allows for good interactions with the insoluble biomass substrates. They also display excellent photophysical properties with a high fraction of long-lived charge carriers and the availability of a reductive and an oxidative quenching pathway. The presented CD-based biomass photoconversion system opens new avenues for sustainable, practical, and renewable fuel production through biomass valorization.

Full text of DOI:10.1002/anie.202008217

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Accepted Article
Title: Solar Reforming of Biomass with Homogeneous Carbon Dots
Authors: Demetra Achilleos, Wenxing Yang, Hatice Kasap, Aleksandr
Savateev, Yevheniia Markushyna, James Durrant, and Erwin
Reisner
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202008217
Link to VoR: https://doi.org/10.1002/anie.202008217

10.1002/anie.202008217
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Solar Reforming of Biomass with Homogeneous Carbon Dots
Demetra S. Achilleos,[a] Wenxing Yang,[b] Hatice Kasap,[a] Aleksandr Savateev,[c] Yevheniia
Markushyna,[c] James R. Durrant,*[b] and Erwin Reisner*[a]
[a]
Dr. D. S. Achilleos, Dr. H. Kasap, Prof. E. Reisner
Christian Doppler Laboratory for Sustainable SynGas Chemistry, Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2
1EW, UK; E-mail: reisner@ch.cam.ac.uk
[b]
[c]
Dr. W. Yang, Prof. J. R. Durrant
Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, UK. E-mail: j.durrant@imperial.ac.uk
Dr. A. Savateev, Y. Markushyna
Department of Colloid Chemistry, Max-Planck Institute of Colloids and Interfaces, Research Campus Golm, 14424 Potsdam, Germany
Supporting information for this article is given via a link at the end of the document.
Abstract: A sunlight-powered process is reported that employs
carbon dots (CDs) as light absorber for the conversion of
lignocellulose into sustainable H2 fuel and organics. This
photocatalytic system operates in pure and untreated sea water using
a benign pH (2-8) at ambient temperature and pressure. The CDs can
be produced in a scalable synthesis directly from biomass itself and
their solubility allows for good interactions with the insoluble biomass
substrates. They also display excellent photophysical properties with
a high fraction of long-lived charge carriers and the availability of a
reductive and an oxidative quenching pathway. The presented CD-
based biomass photoconversion system opens new avenues for
sustainable, practical, and renewable fuel production through
biomass valorization.
Figure 1. (a) Chemical structures of lignocellulosic components used as EDs.
(b) CDs are synthesized from biomass (a-cellulose) or commercial precursors
(citric, aspartic acid) and used with NiP as cocatalyst in PR of biomass to
coproduce H2 and oxidized organics.
Photocatalysis allows for the utilization of solar energy to produce
renewable H2, but most reported systems still require precious-
metal components, purified water or an expensive sacrificial
electron donor (ED).[1] Photoreforming (PR) can use sunlight to
convert biomass waste into H2 and organic chemicals.[2] Instead
of oxidizing water as in classical artificial photosynthesis,[3] PR
employs preferentially abundant and inedible lignocellulose as an
ED to quench holes (h+) in a photoexcited photocatalyst, leaving
behind low-potential electrons to drive proton reduction.[4]
PR commonly relies on UV-absorbing TiO2 colloids with noble
metal cocatalysts (Pt, RuO2),[5] and toxic CdS in organic solvents
(CH3CN)[6] or alkaline conditions (pH >14).[7] Carbon nitride (CNx)
has been shown for visible-light driven PR of biomass under
benign aqueous pH,[8] but the heterogeneous nature of CNx
restricts effective substrate/photocatalyst interactions to occur.[2b,
6] Previous PR systems have also shown conversion yields ≤ 22%
(under strongly alkaline conditions) and required purified water,[5-
8] which limit their utility, sustainability and economics.
α-cel-CDs (diameter: 9±3 nm) and g-N-CDs (3±1 nm) are
graphitic with (100) intralayer spacings of 3.0 and 2.4 Å,
10b]
respectively.[9,
Powder XRD also suggests nanocrystalline,
low defect graphitic structures for α-cel-CDs (27.6o 2θ) and g-N-
CDs (27.0 2θ), in agreement with Raman (graphitic content, G
band, 1570-1580 cm–1 and defects, D band, 1331-1340 cm–1) and
13C NMR spectroscopy (predominant sp2 environments, δ = 110-
180 ppm, no sp3 centers).[9, 10b] g-CDs (4±1 nm) are graphitic,
whereas a-CDs (7±2 nm) are amorphous.[10]
Photocatalysis with the CDs (0.03−2.8 mg) and NiP (50 nmol)
was first performed using ethylenediaminetetraacetic acid (EDTA,
0.1 M, pH 6) as the sacrificial ED in purified water (3 mL; Figure
2a, S3). All systems were irradiated with simulated solar light (AM
1.5G, 100 mW cm–2) under an inert atmosphere at 25 °C and the
Here, we introduce homogeneous carbon dots (CDs, Figure
1) produced from controlled, scalable calcination of cellulose (α-
cel-CDs at 320 oC, Figure S1),[9] or commercial precursors such
as citric acid (resulting in amorphous CDs, a-CDs at 180, and g-
CDs at 320 oC),[10] and aspartic acid (resulting in graphitic N-
headspace gas was analyzed by gas chromatography. H2 yields
–1
(in µmol, Figure 2a) and specific activities (µmol H2 (gCDs
)
h–1,
Figure S3, Tables S1-S4) were optimized by varying the amounts
of CDs. α-cel-CDs showed consistently the highest H2 yields with
their best performance at 2.2 mg (15.6±0.7 µmol H2, 24 h, Figure
2a). The α-cel-CDs/NiP system was also photocatalytically active
under visible-light only irradiation (λ > 400 nm, NiP), albeit with a
lower H2 yield (28%). CDs have sufficient driving force for proton
reduction (CB at approximately –0.5 V vs. RHE)[13], however, the
accurate determination of their band levels is crucial for their
future development as photocatalysts.
o
doped CDs at 320 C, g-N-CDs; see SI)[10b, 11] for biomass PR.
The non-toxic, biocompatible CDs are employed as light
absorbers, together with a Ni bis(diphosphine) H2 evolution
cocatalyst (NiP,[12] Figure S2), to produce H2 and organics in
purified and untreated water (Figure 1b) under benign conditions.
Transient absorption spectroscopy provides insight into the
electron transfer dynamics of the PR systems.
1
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The α-cel-CD/NiP system provides a benchmark activity of
suggest the formation of 2,3,4,5,6-pentahydroxyhexanoate along
with other oligosaccharides after PR of uniformly 13C-labeled
cellulose. PR of xylan produced hydroferulic acid
C10H12O4/C11H14O4 derivatives and other depolymerization
products. PR of galactose/glucose resulted in C6H12O6 and
C6H10O5 isomers.
–1
13,450 µmol H2 (gCDs
)
h–1 (Figure S3), which is the highest
reported so far for carbonaceous photoabsorbers using
comparable conditions (Table S5).[10-11, 14] The α-cel-CDs display
a maximum internal quantum efficiency (IQE at λ = 360 nm, I =
4.05 mW cm-2) of 11.4%, which compares favorably with g-N-CDs
(5.3%) and a-CDs (1.4 %).[10b] Future improvements in the
development of the CDs should focus on high IQEs in the visible
region. The photo-stability of the CD/NiP systems is currently
limited by the fragile ligand framework of NiP, which degrades
after a few hours of operation due to formation of radicals from
EDTA oxidation or ligand displacement from the Ni center.[13] 4-
methylbenzyl alcohol (30 µmol) instead of EDTA produced
3.7±0.2 µmol H2 after 6 h irradiation with α-cel-CD/NiP (Figure
S4, Table S6).
We then studied various insoluble biomass (α-cellulose, xylan
and lignin; Figure 1a) and soluble biomass model substrates and
alcohols of industrial relevance (ethanol, glycerol; Figure S5). PR
in aqueous phosphate solution (KPi; pH 6 and 25 oC) with the CDs
showed activity under benign conditions (Figures 2b, S6; Tables
S6-S9), with the α-cel-CDs showing again the best activity
(Figures 2b).
The highest H2 yields after 24 h were observed with galactose
(8.8±0.2 µmol) and glycerol (8.5±0.1 µmol), which correspond to
turnover numbers of NiP (TONNiP) of 177±4 and 170±2,
respectively. Control experiments without ED, CDs or NiP
showed negligible or no H2 evolution (Figure S7 and Table S7).
The lowest H2 yields were observed for lignin (0.03 µmol) due to
its strong light absorption and robust cross-linked polyphenolic
structure.[15] However, a much higher H2 yield (7.8 ± 0.5 µmol,
Table S6) was observed at lower lignin quantities (0.5 mg) due to
improved light penetration through the CD solution (Figure 2b,
empty bar). PR of α-cellulose and xylan produced 5.0±0.2 and
3.6±0.3 µmol H2, respectively, similar to a heterogeneous
CNx/NiP system.[8a] However, in contrast to heterogeneous
systems that show substrate-dependent H2 yields, homogeneous
CDs photoreform soluble and insoluble biomass with a similar
efficiency.
PR of α-cellulose with the α-cel-CD/NiP system was
subsequently studied in KPi (pH 4.5, 6 and 8), H2SO4 (pH 2) and
10 M KOH (~pH 15) (Figure S8). The highest H2 yields after 24 h
were observed at pH 6 (5.0±0.2 µmol H2) and pH 8 (3.6±0.2 µmol
H2). The efficiency was decreased approximately four times
(1.2±0.1 µmol H2) in strong acid (pH 2), and PR did not proceed
under extremely basic conditions (10 M KOH) due to chemical
instability of NiP (Figure S8 and Table S10).[13]
The biomass conversion yield (CY, %) was determined in KPi
pH 6 with α-cel-CD/NiP at various α-cellulose loadings (0.8−1.65
mg, Figure S9, Table S11). A CY of 13.4% was achieved at 0.8
mg α-cellulose (12 hrs), whereas re-additions of NiP (50 nmol) to
repair the PR system in situ allowed a CY of 34.1% (48 h, Figure
S9).[13] This is higher than CYs reported for CdS/CdOx (9.7%)[7]
and CNx/Pt (22%)[8a] under strongly alkaline conditions.
The oxidation products were determined by High
Performance Liquid Chromatography Mass Spectrometry
(HPLC/MS) and 1H, 13C NMR spectroscopy after PR of α-
cellulose, xylan, glucose and galactose with α-cel-CDs (2.2 mg)
and NiP (50 nmol) in KPi (pH 6; see Figures S10-S17 for detailed
analysis). In brief, the main products of α-cellulose PR are
C6H12O6 and C6H10O5 compounds (e.g., 2,5-anhydro-D-
mannofuranose isomers). HPLC/MS and 13C NMR spectroscopy
Figure 2. (a) Photo-H2 evolution using α-cel-CDs, g-N-CDs, g-CDs and a-CDs
(0.03−2.8 mg) and EDTA (0.1 M, pH 6, 3 mL) as sacrificial ED. (b) Photo-H2
evolution with α-cel-CDs (2.2 mg), g-N-CDs (0.5 mg) and a-CDs (10 mg) using
pure lignocellulosic components and soluble substrates (100 mg, solid bars) in
purified water (KPi,pH 6). The empty bar shows the result using 0.5 mg of lignin.
Conditions: AM 1.5G (100 mW cm−2) irradiation, with NiP (50 nmol) for 24 h and
25 °C.
PR of α-cel-CDs (2.2 mg) with biomass substrates (100 mg)
was then studied in untreated sea water (adjusted pH = 6;
Figures S18-S19, Tables S12-S14). The H2 yields are
comparable to purified water as reaction medium, suggesting that
impurities/background organics do not hinder photocatalysis as
observed for TiO2-based systems, but rather act as EDs.[16] The
highest H2 yields were again achieved with galactose (8.4 ± 0.1
µmol, 24 h). The g-N-CDs showed 2-7 times lower H2 yields in
sea water compared to purified water (≤ 2.3 ± 0.1 µmol, 24 h),
presumably due to N-doping which may provide adsorption sites
for contaminants from the impurity-rich water.[16a] a-CDs in sea
water show low H2 yields (≤ 0.3 µmol), comparable to purified
water. Thus, undoped CDs maintain good photocatalytic
performances under real-world conditions.[16a]
TA spectroscopy was employed to study the photophysics
and charge transfer properties of α-cel-CDs, on fs–ns (fs-TA) or
µs–s (µs-TA) timescales. fs-TA spectra (355 nm excitation, under
2
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Ar) resulted in a broad absorption feature in the visible region
(Figure S20), which decays ~2 fold faster upon adding EDTA,
with the decay halftime changing from ~20 to 40 ps (Figure 3a).
This indicates that the absorption contains a partial contribution
from photoinduced h+ that are scavenged by EDTA (~0.1 ns),[8c,
16c] most likely by pre-adsorbed ED species.
conditions. In the latter cases, NiP– can only be formed with
EDTA,[10b] most likely due to differences in energy of the trapped
charges between these samples. For α-cel-CDs, the appearance
of the NiP– signal at 500 nm at long times (Figures S22e) is
indicative of reasonably efficient photoinduced NiP reduction
(Figure 4).
On µs–s timescales, a blue-shifted, long-lived signal is
observed in the absence of EDTA (Figure S21), which is
effectively quenched by O2 and thus originates primarily from
electrons. These are long-lived, trapped charge carriers with
residual signals (~100 ms) even without EDTA, similar to previous
reports for C3N4,[17] and metal oxide photocatalysts.[18] Addition of
NiP as electron scavenger for α-cel-CDs resulted in (i) quenching
of the electron signal (~0.5 ms) and (ii) appearance of a negative
signal, assigned to the ground-state bleach of NiP due to its
reduction by CDs, at 500 nm (Figures 3b, S22).[10b, 12, 19] This
suggests the direct electron transfer from CDs* to NiP, even
without EDTA, therefore demonstrating an oxidative quenching
mechanism. Titration of CDs with NiP (Figure S23) revealed a
linear relationship between the electron decay rates (at 500 nm)
and NiP concentration, and an oxidative quenching rate of
1.09±0.04 × 108 M–1 s–1. This mechanism will have a low overall
yield, as without EDTA most electrons recombine on faster
timescales (<< 100 ms), consistent with negligible H2 production
(Table S7). Nevertheless, the ability of long-lived trapped
electrons to reduce NiP indicates that they retain reactivity, with
trap energies above the NiP reduction potential.
Previous studies on g-N-CDs showed a bimolecular
recombination lifetime of t50% = 9 ps, with a residual 6% of long-
lived carriers (5 ns) to drive H2 production.[10b] Herein, using
similar excitation fluence/buffer conditions, the α-cel-CD
bimolecular recombination lifetime is t50% = 45±5 ps (i.e., 5 times
slower), with the proportion of long-lived (> 5ns) carriers being
about 15–20% (Figure 3a). We can thus propose two reasons for
improved photocatalysis with α-cel-CDs: (i) existence of both
oxidative and reductive quenching mechanisms and (ii) α-cel-CDs
show slower bimolecular recombination processes and higher
yields of long-lived carriers, which enable higher H2 yields both
under model (Figure 2a) and real-world conditions (Figure S18).
Finally, µs-TA spectra of α-cel-CDs with biomass were
collected to analyze their capacity to quench the photogenerated
h+. Biomass addition induced a similar oxidative quenching
mechanism as with EDTA (Figures S24), but with a 50% lower
yield of NiP– (Figure 3d). The slower h+ extraction is assigned to
the less accessible biomass compared to EDTA, which results in
increased recombination and thus fewer long-lived electrons that
to be extracted by NiP. This agrees with photocatalysis, where
twice the H2 yield was observed with EDTA compared to biomass
(Figure 2). It is also possible that long-lived, trapped h+
accumulate in CDs with biomass as ED due to the oxidative
quenching pathway by NiP (Figure 4, white panel), facilitating
oxidation of the challenging lignocellulosic substrates.
Figure 3. Normalized (a) (~1 ps) fs-TA kinetics between 500 and 520 nm, (b)
(~50 µs) µs-TA kinetics (electrons) at 500 nm, (c) (~50 µs) µs-TA kinetics
(electrons) at 700 nm of α-cel-CDs with EDTA and/or NiP. (d) Normalized (~50
µs) µs-TA kinetics (electrons) of α-cel-CDs at 500 nm with NiP and various
biomass EDs (0.1 M). Inset shows the bleach region of ΔA which corresponds
to NiP–. Conditions: KPi (pH = 6.6) with NiP (50 nmol) upon excitation at 355
Figure 4. Timescales of relaxation and possible charge transfer reactions under
photocatalytic conditions for α-cel-CDs.
nm with an energy of 1 mJ cm–2
.
In summary, we report the development of a homogeneous
PR system using CDs as light absorbers, which use the nexus of
natural resources for coupled sustainable fuel production with
biomass utilization and chemical synthesis. CDs prepared from
biomass have well-suited photophysical characteristics such as
the availability of an oxidative quenching pathway to convert
challenging substrates and a high fraction of long-lived charge
Consistent with the fast hole scavenging process (~0.1 ns),
addition of EDTA resulted in prolonged electron signals at 700 nm
(Figure 3c), indicative of reductive quenching. Signals at 500 nm
were not prolonged with EDTA, suggesting multiple electronic
states in the α-cel-CDs.[20] Nevertheless, these results show both
oxidative and reductive quenching for α-cel-CDs, which is
different from that observed for g-N-CDs and a-CDs under similar
3
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C. K. Chan, M. Elimelech, N. J. Halas, D. Villagrán, Nat.
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carriers. The cellulose-derived CDs allow solar-driven fuel
synthesis from lignocellulosic biomass under benign conditions
with the prospect to simultaneously produce valuable chemicals
in solution. Such systems, which use a noble metal-free
cocatalyst, maintain their photocatalytic activity even in untreated
sea water, which creates promising perspectives for the
development of energy self-sufficient and low-carbon economies.
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Acknowledgements
[19]
[20]
This work was financially supported by the Christian Doppler
Research Association (Austrian Federal Ministry for Digital and
Economic Affairs and the National Foundation for Research,
Technology and Development) and OMV. W.Y. acknowledges
support from the Swedish Research Council for an International
Postdoc Fellowship.
Conflict of Interest
A patent application on the reported work has been filed by
Cambridge Enterprise (international patent number: WO
2019/229255) which lists D.S.A., H.K. and E.R. as the inventors.
Keywords: biomass • photoreforming • carbon dots • hydrogen •
organics
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4
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Entry for the Table of Contents
Biomass-derived materials photoconvert biomass: A triple natural resource system is reported that uses solar energy to convert
biomass into sustainable H2 and organics from untreated water. The process is photocatalyzed by scalable carbon dots produced from
α-cellulose that act as a water-soluble light absorber to photoconvert lignocellulose with a Ni cocatalyst. The carbon dots are bestowed
with suitable photophysical properties to function under ambient conditions and benign aqueous solution.
5
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