Applying thermo-destabilization of
microemulsions as a new method for co-catalyst
loading on mesoporous polymeric carbon nitride –
towards large scale applications†
M. Schr¨
oder, K. Kailasam, S. Rudi, K. F¨
undling, J. Rieß, M. Lublow, A. Thomas,
R. Schom¨
acker and M. Schwarze*
Photocatalytic water reduction using polymeric carbon nitride is an emerging research area, but other
issues like the co-catalyst loading and the choice of sacrificial agents as hole scavengers have received
very little attention in the literature. We addressed the above issues by employing microemulsion based
ex situ co-catalyst loading on porous carbon nitrides as a superior method with about 40% higher HER
activity compared to the usual in situ co-catalyst loading used in the majority of previous studies. In
addition, for the first time, we introduced a large scale 1 m
2
reactor for H
2
production with 80 mL h
1
using this ex situ Pt loaded on porous carbon nitrides under natural sunlight conditions.
1. Introduction
Due to their enormous potential as heterogeneous, metal free
photocatalysts and their capability for water reduction, poly-
meric carbon nitrides, CN (also commonly called “g-C
3
N
4
”) have
been the subject of tremendous research activity over recent
years.
1–4
Carbon nitrides, which are prepared by pyrolysis of nitrogen-
rich precursors such as cyanamide, dicyandiamide or mela-
mine, have a low surface area (10 m
2
g
1
) when prepared
without any template. This bulk carbon nitride shows a rela-
tively low activity for photocatalytic hydrogen evolution in the
presence of sacricial oxidant.
3
Numerous attempts have been
made to improve the performance of carbon nitrides, for
example introducing porosity and higher surface areas by hard
templating which indeed largely enhanced the photocatalytic
activity.
5–9
Several other routes like chemical modication using
various organic molecules by copolymerization, blending of
conjugated moieties or by macroscopic assembly of the
precursors also increase their performance for water
reduction.
10–18
It has been recently reported that mesoporous carbon
nitrides (mp-CN) can be prepared by a one-pot synthesis using a
combined sol–gel/thermal condensation method, which
showed high activity for H
2
evolution in the presence of a
sacricial oxidant.
12,19–22
These materials are superior in activity
when compared to other porous carbon nitrides prepared by
two-step hard templating approaches with silica nanoparticles
or anodic alumina membranes.
7–9,23,24
However, beside all these structural variations on CNs the
addition of a suitable co-catalyst is essential to attain high
values for hydrogen evolution from water, in order to reduce the
overpotential for the reduction reaction.
3
Indeed, with the
carbon nitride alone just very low amounts of H
2
can be evolved
from water.
3
Platinum is used as co-catalyst in most studies,
while in some cases Ni, Co, Cu, Au, and Ag based catalysts are
used to enhance the activity for H
2
evolution.
25–30
However, Pt
co-catalysts are superior to any other metal ions/hydroxides/
oxides in the water reduction reaction.
Usually Pt and other co-catalysts are deposited on the carbon
nitride structure by an in situ photodeposition method, in
which the platinum precursor is simply added in certain
amounts (mostly about 3 wt% Pt) to the reaction solution. Pt
nanoparticles (Pt NPs) are then deposited on the surface of the
CN materials by photoreduction. Mostly the Pt precursor is
added in excess, which results in obscuring of the reaction
solution by the formation of dispersed Pt nanoparticles (see
Fig. S1†). Therefore, the photocatalyst absorbs less light,
causing decrease in activity in the long run. So far no studies
have looked into this issue (except by employing non-ionic Pt
precursor),
31
even though examination of the optimum Pt
content is highly important, especially to transfer this process
to large scale applications with low cost photocatalysts like
carbon nitrides. Thus, while a certain amount of Pt is needed to
increase the photocatalytic activity of the overall system, this is
of course accompanied by increasing costs and, as stated above,
when a certain value is reached can even be detrimental to the
Technische Universit¨
at Berlin, Institut f¨
ur Chemie, Straße des 17. Juni 124, 10623
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra10814k
Cite this: RSC Adv.,2014,4, 50017
Received 31st July 2014
Accepted 29th September 2014
DOI: 10.1039/c4ra10814k
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activity. Beside the pure Pt amount, another important point is
that small nanoparticle sizes and a good dispersion of the co-
catalyst on the photocatalyst surface have to be ensured to
achieve enhanced activity. None of these issues have been
described in detail up to now but certainly need to be consid-
ered, as they dene the number of “active catalytic centers”of
the overall catalyst.
One way to make better use of the Pt and to avoid large
amounts of unsupported, inactive Pt NPs in solution is to use an
ex situ process. Photodeposition in certain solvents and wet-
impregnation are ex situ methods oen used for the preparation
of supported catalysts, but the dispersion of the nanoparticles is
oen inhomogeneous and combined with a high degree of
particle agglomeration. Therefore, a microemulsion technique
followed by photoreduction to achieve uniform Pt dispersion
with small Pt NPs on the surface of porous CN materials was used
instead. Microemulsions are excellent media for the synthesis of
metal nanoparticles and allow for size and shape control of Pt
NPs.
32
The rst formed NPs are then impregnated onto the
support directly from the microemulsion via thermo-destabili-
zation, a new method which was recently developed for the
deposition of Pt nanoparticles on alumina surfaces.
33,34
In this
report, we employed this technique to prepare Pt NPs and also
other noble metal NPs such as Rh and Pd on the porous carbon
nitride networks. For comparison also the standard in situ pho-
todeposition method was used to prepare the Pt co-catalyst. Most
of the earlier reports on photocatalysis using CN materials use
high power Xe-lamps as light sources, but for future commercial
applications it is crucial that real sunlight can be used. In this
report, we show that photocatalytic activity can be observed using
natural sunlight as light source.
Another important task for future applications is to transform
the lab-scale photocatalytic process to a pilot scale process. In
this work, a large-scale dispersion setup using porous carbon
nitrides for large scale H
2
production in the presence of natural
sunlight is described which can pave the way for possible future
technologies to target growing energy demands.
2. Experimental part
2.1. Chemicals for thermo-destabilization of
microemulsions
In the thermo-destabilization of microemulsions the following
chemicals were used without further purication: cyclohexane
($99.5%, Carl-Roth) as the oil phase, distilled water, Triton X-
100 (100%, Sigma-Aldrich) as the surfactant, 1-pentanol ($98%,
Carl-Roth) as the co-surfactant, chloroplatinic acid solution (8
wt% in H
2
O, Sigma-Aldrich) as precursor for platinum nano-
particles as co-catalyst, ascorbic acid (>99%, Alfa Aesar) as the
reducing agent and acetone ($99.8%, Carl Roth) as the washing
agent. Mesoporous carbon nitride (mp-CN) (see paragraph 2.3)
was used as Photocatalyst.
2.2. Chemicals for photocatalytic water reduction
For photocatalytic water reduction experiments, triethanol-
amine (TEOA, $99% purity, Sigma-Aldrich) was used as
sacricial agent and distilled water was used as hydrogen
source. All chemicals were used without any further
purication.
2.3. Thermo-destabilization of microemulsions
2.3.1. Setup for pretreatment of mp-CN with co-catalyst.
The reactor used was a 200 mL double-wall glass reactor
equipped with a three parallel two-blade impeller powered by an
IKA stirrer (Janke & Kunkel IKA-Werk, RW20 DZM), an outlet
valve at the bottom and two inlets at the top of the reactor. In
addition to the glass reactor, the setup consisted of a micro-
pump (Ismatec, MV-Z) and a thermostat (Lauda, RE207).
2.3.2. Procedure for the ex situ deposition of metal nano-
particles. For the deposition of the nanoparticles on the carbon
nitride, two microemulsions were used, one containing the
metal precursor and one containing the ascorbic acid as
reducing agent. The ascorbic acid was used with a molar excess
of 50. The microemulsions can be characterized by the
following parameters: the oil fraction a, the surfactant fraction
gand the co-surfactant fraction d. The parameters are dened
by the following equations:
a¼m
oil
/(m
oil
+m
water
)
g¼(m
surfactant
+m
co-surfactant
)/
(m
oil
+m
water
+m
surfactant
+m
co-surfactant
)
d¼m
co-surfactant
/(m
surfactant
+m
co-surfactant
)
In our standard experiment, a,gand dwere equal to 0.75, 0.4
and 0.5, respectively.
For the nanoparticle formation, the ascorbic acid containing
microemulsion was added by a micropump (0.2 mL s
1
) to the
glass reactor, which already contained the microemulsion with
the metal precursor. The reaction mixture was stirred at 720
rpm for 1 h at 298 K to form the nanoparticles. Subsequently,
200 mg carbon nitride was added and the microemulsion was
heated up to 318 K and stirred for 2 h. During this time, the
microemulsion turned into a two-phase system, releasing the
nanoparticles to the carbon nitride. It must be mentioned that
for Pt no nanoparticles were deposited on the carbon nitride
surface due to the negative zeta potentials of both the carbon
nitride and the Pt NPs as described in more detail in Section 3.2.
For Pt as co-catalyst, only the precursor was adsorbed on the
carbon nitride. Aer the deposition process or rather for Pt the
adsorption process, the reactor was cooled down to 298 K, the
co-catalyst loaded carbon nitride was separated from the reac-
tion mixture by centrifugation at 8500 rpm for 15 min, the
catalyst was washed three times with acetone and dried under
vacuum at 353 K for 24 h. A more detailed description is given in
the publication of Parapat et al.
33
2.4. Hydrogen evolution experiments
2.4.1. Setup for photocatalytic activity measurements. In
our investigations we used a gas tight photoreactor equipped
with vacuum and argon lines, wherein the photocatalytic
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activity was monitored by a pressure sensor (Type-P30, WIKA
Alexander Wiegand SE & Co. KG, DP¼0.1%). The pressure
was measured as a function of time and aer photocatalytic
measurements the increased pressure data were converted into
volume applying the ideal gas law. To obtain the reaction rates
with units of L m
2
h
1
, the evolved volume was related to the
irradiation area (3.54 cm 3.54 cm). The photocatalytic reac-
tion chamber was an inlay made of polytetrauoroethylene
(PTFE) with a total volume of 38 mL. To enable photocatalytic
measurements with efficient irradiation and little reection
(about 5%), the reactor front was closed by planar quartz glass
with a total thickness of 6 mm. In addition to the photoreactor,
the entire device consisted of a solar simulator as light source
(L.O.T. QuantumDesign, Germany, 1000 W m
2
), a magnetic
stirrer (IKA, RH basic 2) and a thermostat (Lauda, ECO Gold RE
630) connected to a PC, which allowed measurement and
control of a constant temperature inside the photoreactor. The
photoreactor is described in more detailed in the publication of
Schwarze et al.
19,20
2.4.2. Reaction procedure for photocatalytic reaction. To
remove all oxygen from reaction solutions, Millipore water was
sonicated under reduced pressure for 1 h. Subsequently, the
Millipore water and the sacricial agent triethanolamine were
ushed by argon for 1 h. The magnetic stirring bar and the co-
catalyst loaded carbon nitride (1.3 g L
1
) were placed into the
reaction chamber and the photoreactor was closed. Aerwards,
oxygen was removed by applying vacuum and ushing the
reactor with argon 3 times. The reaction mixture, consisting of
water and triethanolamine with a total volume of 38 mL, was
added over a dosing valve to the reaction chamber under argon
counter ow. Subsequently, the dosing valve was closed, the
thermostat was adjusted to the desired temperature (298 K) and
the magnetic stirrer was started. Finally, the solar simulator
(positioned at a 10 cm distance to the reactor window) and the
online measurement soware were started.
In this investigation the initial co-catalyst concentration was
varied in a range from 0 to 5 wt% Pt (0–11 wt% H
2
PtCl
6
) and the
TEOA concentration was varied in a range from 2.5 to 60 vol%.
2.4.3. Large-scale hydrogen evolution. For large-scale
hydrogen evolution a new setup was constructed which is
shown in Fig. 6. It consists of a reservoir (Fig. 6a) with a
maximum volume of 11 L for the photocatalyst dispersion
placed on a stirring plate (IKA, RH basic 2) to keep the catalyst
dispersed, a peristaltic pump (Ismatec, ISM1079B) to circulate
the catalyst dispersion (Fig. 6b), and the photoreactor (Fig. 6c),
which was designed similar to a solar panel. The window of the
photoreactor is made of extruded polycarbonate (Quinn Plas-
tics) with a thickness of 8 mm and an irradiation area of about
1m
2
, which corresponds to a scale-up factor of approximately
800 compared to the lab-scale photoreactor. A further valve
allows the connection of an argon bottle to purge the free gas
volume prior to the photocatalytic experiments. To test the
carbon nitride photocatalyst in this new setup for hydrogen
evolution under real sunlight, 6.8 g Pt@mp-CN prepared using
the ex situ method as described above was placed into the
reservoir together with water and the sacricial agent TEOA
(10 vol%). The solution was stirred and circulated between the
photoreactor and the reservoir. The setup was ushed with
argon and then hydrogen evolution was carried out with natural
sunlight. The hydrogen produced was collected in the reservoir
where samples of the gas-phase were taken over time and
analyzed via gas chromatography for hydrogen content.
2.5. Analysis of photocatalyst, reaction mixture and evolved
gases
Porous CN powders were xed to XPS sample holders by
pressing them onto adhesive and UHV-stable carbon tape. XPS
was carried out at pass energies of 50 eV to increase the detec-
tion sensitivity for the minute Pt concentrations. Thereby,
broadening of the energy dissipative curves occurred. A Mg
K(alpha) X-ray source (1253.6 eV) was used for probing the
chemical state of the surfaces. The X-ray instability of the
powders was mitigated by reducing the power of the X-ray
source down to 100 W (compared to typical power values of 250
W). Evaluation of the oxygen Auger line (KL23–KL23) at 745 eV
suggests charging of the powders during XPS analysis, resulting
in an apparent shiof the binding energies by about 0.5–0.8 eV.
An Agilent 7890A gas chromatograph was used to determine the
hydrogen content in the headspace of the reactor. The GC was
equipped with a Carboxen-1000 column and a thermal
conductivity detector (TCD). The carrier gas was argon (30 mL
min
1
). The contents of platinum on the catalysts were
measured by inductive coupled plasma emission spectroscopy
(ICP-OES) using an ICP-OES 517 (Varian Inc., USA). Prior to ICP-
OES analysis the photocatalysts were pretreated by microwave
(Discover SP-D, CEM, USA) digestion to dissolve the platinum
nanoparticles. For calibration of the set-up, commercial stan-
dard solutions of platinum (1000 mg L
1
, Aldrich) were diluted
with distilled water to obtain concentrations of 2, 4.5, and
7.5 mg L
1
. Platinum was analyzed at 203.464, 214.424, 224.552
and 265.945 nm. Transmission Electron Microscopy (TEM)
measurements were operated by a FEI Tecnai G2 20 S-TWIN
transmission electron microscope equipped with a LaB
6
-source
at 200 kV acceleration voltages. For TEM-investigations a small
amount of the sample powder was placed on a TEM-grid
(carbon lm on 300 mesh Cu-grid, Plano GmbH, Germany) and
was sputtered with carbon. Nitrogen sorption analyses were
carried out at the temperature of liquid nitrogen on a Quad-
rasorb instrument (Quantachrome) aer evacuating the
samples at 150 C overnight. The surface areas and pore size
distribution analysis were Brunauer–Emmett–Teller (BET) and
Barrett–Joyner–Halenda (BJH) methods, respectively. The pore
volume was calculated at the relative pressure 0.99. For UV/Vis
measurements a Lambda 35 UV/Vis spectrometer from Perkin
Elmer (USA) was used. The surface morphology of the mp-CN
was analyzed by scanning electron microscope (JEOL 7401F) at
an acceleration voltage of 10 kV and at a working distance of
9 mm.
3. Results and discussion
The photocatalytic activity of semiconducting materials
strongly depends on the properties of the photocatalyst and on
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the amount, distribution and properties of the loaded co-cata-
lyst. For photocatalytic hydrogen evolution in most cases, the
co-catalyst is loaded by in situ photodeposition. However, it has
been shown for mesoporous carbon nitrides (mp-CN) that the in
situ photoreduction is strongly affected by the chosen reaction
conditions and yields agglomerated nanoparticles and inho-
mogeneous nanoparticle distributions.
22
Therefore, ex situ
methods like wet impregnation
35
or chemical reduction of
metal precursors
36
seems to be more appropriate for co-catalyst
deposition. Here, we investigated a new ex situ approach based
on microemulsion derived Pt NPs for the synthesis of highly
distributed and well dened nanoparticles, functioning as co-
catalyst on mp-CN.
3.1. Synthesis and characterization of mp-CN photocatalysts
Mesoporous graphitic carbon nitride, mp-CN, was prepared
using a published procedure.
7
In short, mp-CN was prepared by
a sol–gel/thermal condensation approach, where the carbon
nitride precursor cyanamide (CA) was added to a HCl–ethanol
solution. Aer dissolving the cyanamide, the pH was adjusted to
2 and tetraethylorthosilicate (TEOS) was added, and stirred for
one hour (1 : 6 molar ratio of TEOS : CA). Aer the removal of
the solvents, the composite gel was aged at 80 C, thermally
treated at 550 C to obtain a carbon nitride-silica composite.
Finally carbon nitride, mp-CN, is obtained by the removal of
silica using NH
4
HF
2
solution.
The porous carbon nitride mp-CN was characterized using
different techniques, showing the same features as reported
before,
7
i.e. a peak at 27.4in the powder XRD (Fig. S2a†) con-
rming the layered structure of the material and a broad
absorption starting at 450 nm in the UV/Vis spectrum
(Fig. 2Sb†). Nitrogen sorption measurements show type IV
isotherm typical for mesoporous materials with surface area of
about 149 m
2
g
1
(Fig. S2c†), pore diameter of 38 ˚
A (Fig. S1d†)
and total pore volume of 0.28 cm
3
g
1
.
3.2. Synthesis and characterization of co-catalyst loaded
carbon nitride
In recent years it has been shown that the Schottky effect plays
an important role in capturing excited electrons on the surface
of photocatalytic active materials.
22,37
Therefore, the deposition
of a dened amount of co-catalyst on the semiconductors
surface can improve the photocatalytic activity signicantly. For
photocatalytic hydrogen evolution with carbon nitrides, several
precious metals have been used as co-catalyst which clearly
improved the photocatalytic performance.
4
However, until now
platinum shows the highest activity in photocatalytic hydrogen
evolution applying carbon nitride materials as catalyst. It is
expected that the hydrogen elimination from Pt–H surface
bonds is favored in comparison to the covalent elimination
from the CN surface.
3
For platinum deposition, in most of the
previous studies the in situ photoreduction is used. Therein, a
metal precursor, usually hexachloroplatinic acid (H
2
PtCl
6
), is
mixed with the photocatalyst in the reaction solution and due to
irradiation the platinum ions get photochemically reduced and
deposited on the carbon nitride surface. The advantages of this
method are the easy experimental feasibility and that no further
preparations are required. However, the in situ photoreduction
for Pt loaded carbon nitride results in inhomogeneous distri-
bution and highly agglomerated nanoparticles on the CN
surface (Fig. 1a), with an average particle size between 10 to
30 nm. Due to the broad size distribution and high agglomer-
ation it can be expected that the number of active sites on the
surface of the photocatalyst and thereby the photocatalytic
activity is reduced. Moreover, the inhomogeneous catalyst
dispersion affects the photocatalytic activity to a large extent.
Apart from limitations in photocatalytic activity, the difficult
realization of a scale-up is another disadvantage. A more
detailed description of the inuences of the in situ photore-
duction on the photocatalytic activity with carbon nitride
materials was recently described by Schr¨
oder et al.
22
Therefore, to obtain well dened and highly distributed
platinum nanoparticles on mp-CN photocatalysts with the
possibility for application on a large scale, a new ex situ
microemulsion method was applied. This method was rst
reported by Parapat et al. for the preparation of supported Pt
catalysts (on a-Al
2
O
3
, SiO
2
and SBA-15) but has so far not been
reported for applications in photocatalysis.
38
For nanoparticle
deposition by this method, rst a metal precursor is reduced by
a reduction agent in a reverse micellar system and then the
formed nanoparticles are deposited onto the support material,
here mesoporous carbon nitrides (mp-CN), aer heating the
solution above the phase transition temperature (for details see
the Experimental part). In the beginning of our investigations
we applied this method only for platinum as co-catalyst, with
H
2
PtCl
6
as metal precursor and ascorbic acid as reducing agent.
Aer the rst synthesis, the catalyst was analyzed using TEM
and the result is shown in Fig. 1c. As can be seen from the
image, no nanoparticles were observed, while EDX-measure-
ments (Fig. S3†) clearly indicates Pt located on the surface of the
carbon nitride support. However, aer calcination (Fig. 1e) or
aer photocatalytic reaction (Fig. 1d) Pt nanoparticles appear
on the surface of the ex situ treated photocatalyst.
It should be noted that the zeta-potentials of the catalyst
surfaces and metal nanoparticles were found to be crucial
factors for the successful loading of the nanoparticles onto the
support material.
22
Both the Pt NPs and carbon nitride have
negative zeta-potentials, resulting in repulsions during the
deposition procedure. This results in a lower amount of Pt NPs
observed on the surface than expected from the initial loading
of Pt. In contrast, Pd nanoparticles have a positive zeta-potential
resulting in higher amount of nanoparticles on the surface due
to stronger attraction to the carbon nitride surface (shown in
Fig. 1b).
To investigate the formation of nanoparticles on the CN
surface, we analyzed the support material, aer applying the ex
situ microemulsion approach, in X-ray photoelectron spectros-
copy (XPS). The result is shown in Fig. 2. Aer ex situ prepara-
tion, the XPS shows a double-peak at binding energy values of
74 and 78 eV, which corresponds to Pt(IV). No Pt(0) formation
was observed and together with the signal for Cl (not shown), it
was claried that only adsorbed H
2
PtCl
6
are deposited on the
surface. Aer testing this catalyst in HER, Pt(0) signal was
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obtained in XPS (double peak at about 71.5 and 75 eV) due to
photoreduction of the adsorbed precursor. It should be
mentioned that a similar double-peak is observed for in situ co-
catalyst loaded Pt. These results are supported by earlier studies
on Pt@TiO
2
which shows a double-peak at 70.9 and 74.3 eV.
39
Aer photocatalytic reaction highly dispersed nanoparticles
with an average diameter smaller than 5 nm are obtained (see
Fig. 1d) compared to the in situ loaded catalyst where larger and
agglomerated nanoparticles are observed (Fig. 1a). This obser-
vation on the in situ and the ex situ loaded mp-CN was reected
from their photocatalytic activity. To compare the photo-
catalytic activity and the optimal Pt loading of the in situ and the
ex situ treated mp-CN, the hydrogen evolution rate as function
of the initial Pt concentration is shown in Fig. 3.
For the ex situ treated carbon nitride the optimal initial
platinum concentration is between 1.9 and 3.6 wt% and peak
activity is reached at a hydrogen evolution rate of 0.53 L m
2
h
1
. The photocatalytic activity of the in situ loaded mp-CN
reaches peak activity at 0.32 L m
2
h
1
. By comparing the
activities of both approaches, Fig. 3 illustrates that the highest
hydrogen evolution rate of the ex situ treated mp-CN is about
Fig. 1 TEM images of (a) Pt@mp-CN (1.2 wt% Pt) for in situ photodeposition, (b) Pd@mp-CN (1.1 wt% Pd) after ex situ microemulsion approach,
(c) Pt@mp-CN (1.4 wt% Pt) after ex situ microemulsion approach, (d) after photocatalytic reaction and (e) after calcination at 573 K.
Fig. 2 XPS of ex situ prepared Pt@mp-CN (top), after applying ex situ
Pt@mp-CN in HER (middle) and in situ prepared Pt@mp-CN.
Fig. 3 H
2
evolution rate and Pt loading as a function of the initial Pt
concentration for in situ and ex situ treated mp-CN (1.3 g L
1
mp-CN,
10 vol% TEOA, 298 K, solar simulator 1000 W m
2
).
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40% (10%) higher. This led to the assumption that the better
distribution of the nanoparticles by the microemulsion
approach leads to increased photocatalytic activity. The
increase in activity between ex situ and in situ was conrmed by
using different carbon nitride batches.
The optimal initial Pt loading for achieving maximum pho-
tocatalytic activity of the in situ loaded mp-CN is at 1.2 wt%.
However, it was shown before that the optimal initial weight
fraction of Pt for in situ photoreduction also depends on the
batch of the mesoporous carbon nitride and is in a range
between 1 and 5 wt%.
22
This is corroborated by studies of Wang
et al. showing that the optimal initial Pt concentration for
graphitic carbon nitride is between 2 and 4 wt%.
3
Therefore, the
ex situ pathway requires nearly the same initial Pt concentra-
tion. To obtain the effective Pt loading on the surface of the
carbon nitride support, we analyzed the Pt loaded photo-
catalysts aer photocatalytic reaction by ICP-OES. It is shown
that the effective platinum loading strongly differs from the
initial Pt amount, due to the similar negative zeta potentials for
both carbon nitride and Pt NPs as mentioned above. Therefore,
only low loading of Pt was obtained on the carbon nitride.
However, Fig. 3 illustrates that for the same initial weight
fraction, the effective Pt loading for the ex situ treated carbon
nitride is about ten times higher. Probably, this effect is caused
by the protective barrier of the micelles, which build an elec-
trostatic shield and weaken the rejection between the nano-
particles and the CN material resulting in higher loading of
nanoparticles. For comparison we prepared one catalyst using
the impregnation method with 2 wt% Pt loading, but activity of
this catalyst was about 40% (10%) lower than the activity of
the catalyst prepared via the microemulsion approach.
Beside photoreduction during photocatalytic reaction,
calcination is an alternative pathway to reduce the adsorbed
platinum precursor on the carbon nitride surface. Aer calci-
nation, the nanoparticles are even smaller and better dispersed
(see Fig. 1d), but the photocatalytic activity is reduced as shown
in Table 1. This may be speculated to be caused by the forma-
tion of more condensed structure and changing properties of
the carbon nitride semiconductor and further investigations are
going on in our research to clarify this.
Aer the optimization of the platinum concentration on mp-
CN along with higher photocatalytic H
2
evolution by ex situ
method, the amount of photocatalyst in ex situ synthesis was
increased to obtain higher amounts of Pt-loaded carbon nitride
for large scale hydrogen production. Therefore, we varied the
amount of photocatalyst from 200 to 2000 mg and measured the
photocatalytic activity for HER. Table 2 illustrates that the
photocatalytic activity varies only in a narrow range showing the
superiority of this method when compared to the in situ
method, where the loading of Pt cannot be controlled. Thus,
this ex situ method seems to be an appropriate synthesis
strategy for Pt loaded carbon nitride which could be applied in
large scale hydrogen production. Below we present a new “large
scale setup”for hydrogen evolution under real sunlight condi-
tions and show the activity for the ex situ prepared mp-CN
photocatalyst.
For further improvement of the HER with mesoporous
carbon nitride, the microemulsion approach was applied to
load nanoparticles of various metals as co-catalyst (Pd, Ru and
Rh). It was found that although the ex situ microemulsion
approach is transferable for loading different co-catalysts on the
surface of a photocatalyst, the activity for HER is very low when
compared to Pt as co-catalyst. It would be interesting to see how
the loading of different metal precursors would affect the size,
concentration and distribution of the metal nanoparticles, but
this is beyond the scope of the present study.
3.3. Impact of the reaction conditions on the photocatalytic
activity
Recently we have shown that the reaction conditions have a
high impact on the in situ photoreduction and thereby on the
photocatalytic activity.
22
Therein, the triethanolamine concen-
tration and the platinum concentration were shown to have the
highest impact on the activity. The inuence of the platinum
concentration was already discussed before. Here, we want to
investigate the inuence of the TEOA concentration, where the
alcohol acts as electron donor in the photocatalytic water
reduction reaction.
Fig. 4 shows that in the absence of TEOA, no H
2
evolution is
observed for the ex situ as well as for the in situ platinum loaded
carbon nitride. With increasing TEOA concentration the
hydrogen evolution rate increases strongly up to a maximum
activity. For the in situ loaded carbon nitride the maximum rate
is obtained for a TEOA concentration of 30 vol% and beyond
this concentration the activity decreases substantially. This
Table 1 Influence of calcination temperature on the photocatalytic
activity (1.3 g L
1
ex situ Pt@mp-CN, 10 vol% TEOA, 298 K, solar
simulator 1000 W m
2
)
Calcination temperature (K) r(H
2
)(Lm
2
h
1
)
No calcinations 0.410
423.15 0.227
573.15 0.082
723.15 0.009
Table 2 Synthesis of larger amounts of Pt loaded carbon nitride
photocatalysts via microemulsion (a,gand dare equal to 0.75, 0.4 and
0.5, respectively, 50 times excess of ascorbic acid and 2.0 wt% Pt
related to mp-CN) and photocatalytic testing in HER (1.3 g L
1
ex situ
Pt@mp-CN, 10 vol% TEOA, 298 K, solar simulator 1000 W m
2
)
Entry m
MEa
(g) m
mp-CN
(mg) r(H
2
)(Lm
2
h
1
)
1 50 200 0.46
2 50 400 0.48
3 50 500 0.44
4 100 1000 0.48
5 100 2000 0.56
a
Total weight of the microemulsion solution without mp-CN
photocatalyst.
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trend is due to the strong impact of the TEOA concentration on
the in situ photoreduction process of H
2
PtCl
6
. With an
increasing TEOA concentration, the effective platinum loading
on the carbon nitride surface is reduced and thereby the
optimal platinum loading falls below a certain limit.
22
There-
fore, the hydrogen evolution rate decreases substantially with
increasing TEOA concentration aer highest photocatalytic
activity at 30 vol% TEOA concentration. For the ex situ platinum
loaded mp-CN the hydrogen evolution rate stays constant and
reaches a plateau beyond a concentration of 5 vol%. This trend
for the photocatalytic activity is in accordance with the Lang-
muir–Hinshelwood type rate law and is similar to the trend
observed by Li et al.
40
Thus, it is shown that the impact of the
higher sacricial agent concentration could be overcome by
applying the ex situ microemulsion approach.
Table 3 shows the photocatalytic activity of various sacricial
agents used in HER with the ex situ loaded carbon nitrides. The
highest rates are obtained with triethanolamine followed by
EDTA, which is probably due to strong H-bonding interaction of
the hydroxyl and carboxyl groups of TEOA and EDTA with the
nitrogen rich surface of the carbon nitride. For other alcohols
the activity is strongly reduced. However, with the in situ Pt-
loaded mp-CN, we obtained a signicant hydrogen evolution
rate only with TEOA.
3.4. Large scale hydrogen production with ex situ prepared
mp-CN
There are different types of photoreactors to perform sacricial
hydrogen evolution with light being injected from the top,
41,42
from the side
43
or even from inside.
44
It is also possible to use a
membrane reactor and perform Z-type overall water splitting
with two photocatalysts and an electron mediator.
45
However,
all these reactors are used for lab scale experiments. Prior to our
attempts to develop a large scale setup for hydrogen evolution,
for comparison, we determined the activity of Pt@mp-CN in the
lab-scale setup under optimized conditions (0.42 L h
1
m
2
)
and with a small demonstrator (Fig. S5†) it was proven that
hydrogen evolution with real sunlight is possible (Fig. 5).
Thereaer, based on our lab scale photoreactor with dened
geometry, we developed a setup for large scale hydrogen
evolution which is shown in Fig. 6, using the irradiation area as
the main scale-up criterion.
In our kinetic studies with mp-CN we found that in situ co-
catalyst loading should be avoided for a better utilization of the
expensive noble metals, e.g. Pt, and under optimized conditions
more hydrogen can be produced only by having a larger irra-
diation area. Therefore, we investigated the ex situ co-catalyst
loading via microemulsion in higher amounts (2 g scale) which
is an important aspect for the scale-up where higher amounts of
this photocatalyst are needed. Furthermore, we increased the
irradiation area from about 12 cm
2
in lab scale to about 1 m
2
for
the large scale photoreactor. This corresponds to a scale-up
factor of 800 with respect to the irradiation area and the
experiments were performed with catalyst dispersions.
With the large scale setup, two experiments with real
sunlight were performed. It should be mentioned that for the
Fig. 4 H
2
evolution rate as a function of the initial TEOA-concentra-
tion for ex situ and in situ loaded mp-CN (1.3 g L
1
mp-CN, 2 wt% Pt for
ex situ and 4.8 wt% Pt for in situ related to mp-CN, 298 K, solar
simulator 1000 W m
2
).
Table 3 Influence of various sacrificial agents (SA) on the photo-
catalytic activity (1.3 g L
1
ex situ Pt@mp-CN, 2.5 wt% Pt related to mp-
CN, 10 vol% SA, 298 K, solar simulator 1000 W m
2
)
Entry Sacricial Agent (SA) Normalized rate
a
1 Triethanolamine (TEOA) 1.000
2 Ethylenediaminetetraacetic acid (EDTA) 0.830
3 Methoxybenzaldehyde (MeOBA) 0.140
4 Benzaldehyde (BA) 0.111
5 Methanol (MeOH) 0.028
6 Ethanol (EtOH) 0.007
7 Butanol (BuOH) 0.007
a
Based on hydrogen evolution rate of carbon nitride with TEOA as SA.
Fig. 5 Small scale hydrogen evolution with real sunlight using a small
glass reactor equipped with a glass burette for hydrogen quantification
(see Fig. S5†) (1.3 g L
1
ex situ Pt@mp-CN, 2.5 wt% Pt related to mp-
CN, 10 vol% SA, room temperature, real sunlight) which is shown in
figure that should run with mp-CN as sustainable photocatalyst.
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calculations only the sun hours were considered. The rst
experiment was done directly aer charging the setup with the
photocatalyst, water and TEOA, and we obtained a hydrogen
evolution rate of about 0.08 L h
1
m
2
(80 mL h
1
) for a period
of one day. The second experiment was done one month
later and we obtained a hydrogen evolution rate of about
0.04 L h
1
m
2
(40 mL h
1
) for a period of three days. In
comparison to the lab scale experiment, we produced 80 to
160 times more hydrogen, but the rate was lower. We could
identify three main reasons for the lower activity. The rst
reason is the dispersion itself. In the experiment we used a
lower catalyst concentration (0.62 g L
1
instead of 1.3 g L
1
)so
that based on our earlier investigations the rate must be lower
(approximately 0.21 L h
1
m
2
).
22
The second reason is that it
was not possible to keep the catalyst dispersed all the time. A
larger fraction of the catalyst settled down and could not be
re-dispersed by the pump. The third reason is the used
polymeric window which was made of polycarbonate
(Makrolon). The window absorbs also the UV part of the sun
spectrum (shown in Fig. S4†), which was always used in the lab
scale experiments with the sun-light simulator. Although,
mp-CN is able to absorb light in the visible range, the natural
UV fraction leads to a higher photocatalytic activity in hydrogen
evolution reaction. Nevertheless, regardless of these minor
issues, the rst runs with this large scale setup successfully
show the hydrogen evolution for mp-CN in the real sunlight
conditions. However, modications of the large scale setup are
necessary to obtain the expected rate that was lowered by a
factor of 2.5 to 5 due to catalyst settling and increased light
absorption by the polycarbonate window.
4. Conclusion
A new ex situ method based on microemulsions was investi-
gated for loading various metals as co-catalyst on porous carbon
nitrides. We found that the particle size and distribution is
optimized by applying this method for loading Pt on porous
carbon nitrides and the effective platinum loading is about 10
times higher in comparison to in situ photodeposited Pt.
Moreover, the hydrogen evolution rate was improved by about
40%, which led to the assumption that the better distribution of
the nanoparticles by the microemulsion approach leads to
increased photocatalytic activity. Furthermore, the micro-
emulsion approach allows for the synthesis of larger amounts of
co-catalyst loaded carbon nitrides, which were used for large
scale hydrogen evolution with real sunlight in a newly designed
photoreactor setup having an irradiation area of 1 m
2
.In
comparison to the lab scale experiments the total amount of
produced hydrogen was increase by a factor of 80 to 160 (40 to
80 mL h
1
). However, further modications are in progress, in
which the Pt@mp-CN catalyst is immobilized on conductive
plates without leaching for long-term use and further the
window will be exchanged for better UV transmittance.
Acknowledgements
This work was supported by the BMBF (Spitzenforschung und
Innovation in den neuen L¨
andern, FKZ 03IS2071D).
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