Photocatalytic activity of carbon nitride modified with a ruthenium(II) complex having carboxylic- or phosphonic acid anchoring groups for visible-light CO2 reduction

Article abstract of DOI:10.1246/cl.151061

Semiconducting C₃N₄ powder modified with a catalytic Ru(II) complex having carboxylic acid groups as anchors showed higher activity for CO₂ reduction into HCOOH under visible light (λ > 400 nm) at the initial stage of the reaction, whereas an analogue anchored with phosphonic acid groups exhibited higher stability.

Full text of DOI:10.1246/cl.151061

CL-151061
Received: November 16, 2015 | Accepted: November 27, 2015 | Web Released: December 4, 2015
Photocatalytic Activity of Carbon Nitride Modified with
a Ruthenium(II) Complex Having Carboxylic- or Phosphonic
Acid Anchoring Groups for Visible-light CO2 Reduction
Kazuhiko Maeda,* Ryo Kuriki, and Osamu Ishitani
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550
(E-mail: maedak@chem.titech.ac.jp)
Semiconducting C3N4 powder modified with a catalytic
different applications has not been systematically understood, it
is intriguing to compare such an anchoring effect on photo-
catalytic CO2 reduction using C3N4, because the surface of C3N4
is completely different from that of ordinary metal-based
semiconductors.18
Ru(II) complex having carboxylic acid groups as anchors
showed higher activity for CO2 reduction into HCOOH under
visible light (- > 400 nm) at the initial stage of the reaction,
whereas an analogue anchored with phosphonic acid groups
exhibited higher stability.
Here we compared two different Ru(II) complexes (Chart 1)
as promoters for photocatalytic CO2 reduction with C3N4 under
visible light. These complexes were adsorbed on C3N4 by
dispersing C3N4 powder in a methanol solution dissolving a
proper amount of a Ru(II) complex at room temperature for
20-24 h to allow for adsorption/desorption equilibrium. C3N4
employed in this work had a mesoporous structure (specific
surface area: 200 m2 g¹1, average pore size: 12 nm, pore volume:
0.64 cm3 g¹1).10b Then, the powder was collected by filtration,
followed by washing with methanol several times. The details of
materials preparation are included in Supporting Information.
We confirmed that the FT-IR spectra of the as-prepared materials
did not contain any peak assignable to C-H stretching bonds
from residual methanol.
Photocatalytic reduction of CO2 into energy-rich chemicals
has long been studied as an important half cycle of artificial
photosynthesis.1-10 CO2 reduction using illuminated semicon-
ductor particles as heterogeneous photocatalysts may become the
ultimate goal for large-scale applications. We have proposed
hybrid photocatalysts that consist of a metal complex and
a polymeric C3N4 semiconductor as potential heterogeneous
photocatalysts for CO2 reduction under visible light.10 For
example, C3N4 modified with trans(Cl)-Ru{(X2bpy)2(CO)2Cl2}
(bpy: 2,2¤-bipyridine; X = PO3H2 or CH2PO3H2) was capable
of photocatalytically reducing CO2 into HCOOH under visible
light. Besides, isotope tracer experiments using 13CO2 indicated
that the produced HCOOH originated not from carbon contam-
inants and/or C3N4 itself, but from CO2 gas.10a
Figure 1 shows the adsorption isotherms of Ru(II) com-
plexes on C3N4 at room temperature. The amount of adsorbed
complex increased with an increase in the initial concentration of
the complex, undergoing saturation at a given point in both cases.
In any hybrid system consisting of a semiconductor and a
metal complex, anchoring groups in the metal complex play
vital roles of determining the efficiency of electron transfer from
one side to the other. The combination of a Ru polypyridyl
complex and a metal oxide would be the best known system
among metal-complex/semiconductor hybrids such as dye-
sensitized solar cells and visible-light-driven water splitting
assemblies.11-16 It is known that a phosphonic acid group can
form stronger chemical bonding with the metal oxide surface
over a wide pH range, as compared to a carboxylic acid group.14a
However, the stronger bonding via phosphonate does not always
lead to higher performance. Actually, it has been reported that
the incident photon-to-current conversion efficiencies of dye-
sensitized solar cells using phosphonate anchoring groups were
approximately 30% lower than that of carboxylate-anchored
ones.14b Conversely, the H2 evolution photocatalytic activity of a
Ru-complex/TiO2 with phosphonate was much higher than that
of a carboxylic acid one.15 Suzuki et al. recently investigated
the effects of anchoring groups on photocatalytic CO2 reduction
using nitrogen-doped Ta2O5, a p-type semiconductor, modified
with [Ru(X2bpy)2(CO)2]2+ (X = COOH or PO3H2),3b which is
a well-known electrocatalyst for CO2 reduction.17 According
to that study, an analogue anchored by phosphonate exhibited
a higher rate of HCOOH production and improved catalytic
turnover number as compared to carboxylate anchored one.
Although the reason for the different performances achieved in
RuP
RuC
Chart 1. Structures and abbreviations of Ru(II) complexes used
in this work.
50
RuC
RuP
40
30
20
10
0
0
20
40
60
80
Amount of added complex / μmol g–1
Figure 1. Adsorption isotherms of Ru(II) complexes on C3N4 at
room temperature.
182 | Chem. Lett. 2016, 45, 182–184 | doi:10.1246/cl.151061
© 2016 The Chemical Society of Japan

A
B
RuC
RuP
80
60
40
20
0
100
200
RuC
RuP
RuC
RuP
80
60
40
20
0
150
100
50
0
-1.2 -1.4 -1.6 -1.8 -2.0 -2.2
Applied potential / V vs. Ag/AgNO3
0
10
20
30
40
0
1
2
3
4
5
Amount of adsorbed Ru complex / μmol g–1
Reaction time / h
Figure 3. Cyclic voltammograms of Ru(II) complexes in 4:1 v/v
DMF/TEOA solution containing 0.1 M Et4NBF4 as a supporting
electrolyte under a CO2 atmosphere.
Figure 2. (A) Time courses of HCOOH production over C3N4
modified with RuC and RuP (7.8 ¯mol g each) under visible
light. (B) Changes in the corresponding turnover frequencies as
¹1
a function of the amount of loaded Ru(II) complex. Reaction
Ru(II) because of the similar electron-withdrawing properties
between the carboxylic acid and phosphonic acid groups. Under
a CO2 atmosphere, a clear catalytic current due to CO2 reduction
was generated, indicating that both RuC and RuP possessed the
ability to reduce CO2. The conduction band minimum of C3N4
used in this work is estimated to be around ¹1.65 V vs. Ag/
AgNO3.10a At this potential, RuC showed an enhanced current
compared to RuP under a CO2 atmosphere (Figure 3). The
mechanism of CO2 reduction on a metal complex is very
complicated, and has not fully been clarified even in a
homogeneous system.9b Previous studies have suggested that
the coordination of CO2 to the metal center and the formation
of hydride species are important steps in CO2-to-HCOOH
conversion on a metal complex catalyst.22 While RuC might
be superior to RuP for promoting these steps, the detailed
mechanism still remains unclear.
One of the authors has previously reported that C3N4
photocatalyzes water reduction in the presence of TEOA as
an electron donor under visible light, producing H2 without
noticeable degradation for as long as 3 days.23 Therefore, the
oxidation reaction in the present system with TEOA would
stably occur, and the deactivation is likely to result from the
reduction side. Our Ru-complex/C3N4 hybrid system works
efficiently when Ru complexes are adsorbed on the surface of
C3N4; on the other hand, electron transfer from the conduction
band of C3N4 to free Ru complexes in solution is very slow.10c
In order to investigate this, the amounts of Ru complexes (RuC
and RuP) desorbed from C3N4 after stirring in MeCN (or a
mixed solution of MeCN and TEOA) were determined by
measuring the absorbance of the supernatant solution.
Table 1 summarizes the result of the desorption experiments
in the dark.24 No desorption could be identified when RuP/C3N4
or RuC/C3N4 was dispersed in MeCN (Entries 1 and 2).
However, 37% of RuP, initially adsorbed on C3N4, was
desorbed in the presence of TEOA even after 20 min of stirring
(Entry 3). In the case of RuC, the desorption behavior was more
pronounced (Entry 4). After 1 h of stirring, the ratios of RuP and
RuC desorbed were increased to 68% and 85%, respectively
(Entries 5 and 6). Nevertheless, no further desorption occurred
even after extended periods of stirring (ca. 20 h). These results
indicate that the binding of RuP on C3N4 is stronger than that
of RuC. At the same time, it is suggested that there are at least
two types of adsorption sites: one can strongly adsorb the
Ru(II) complexes, while the other cannot, although we cannot
distinguish them clearly.
conditions: Ru-complex/C3N4 (8.0 mg) in
a MeCN/TEOA
mixture (4:1, v/v; 4 mL). A Pyrex test tube with a septum
(11 mL capacity) was used as the reaction vessel and a 400 W high-
pressure Hg lamp with a NaNO2 solution filter as the light source.
Here, the saturated amount of RuP (ca. 40 ¯mol g¹1) was higher
than that of RuC (ca. 30 ¯mol g¹1). Note that a similar Ru(II)
complex having no anchoring group did not undergo adsorption
on C3N4.10c This fact clearly indicates that anchoring groups such
as -COOH or -PO3H2 are indispensable for adsorption onto the
C3N4 surface. As discussed in our previous paper,10b it is likely
that hydrogen bonding is formed between the hydrogen atoms in
the NH2 groups on C3N4 and the lone pair electrons existing on
the oxygen atoms in the -COOH (and -PO3H2) groups.
Typical time courses of HCOOH production using C3N4
modified with RuC and RuP in a MeCN (acetonitrile)/TEOA
(triethanolamine) mixed solution under visible light (- >
400 nm) are shown in Figure 2A. No reaction took place in
the absence of the Ru(II) complex, C3N4, CO2, or visible
light.10a Both RuC and RuP achieved functionality as promoters
for HCOOH production under visible light, judging from the
fact that the turnover numbers (TONs) with respect to the
loading amount of Ru far exceeded 1, indicative of the catalytic
cycle of these reactions.19,20 Interestingly, RuC exhibited higher
performance at the initial stage of the reaction, but degraded
more quickly with time. Higher initial performance of RuC was
also observed, regardless of the loading amount (Figure 2B). On
the other hand, RuP showed higher stability than RuC, although
the initial activity was lower. Thus, the photocatalytic perform-
ance of C3N4 for HCOOH production differed with respect to the
loaded Ru complex.
In order to investigate the reason for the different perform-
ances, electrochemical measurements were conducted. Figure S3
shows cyclic voltammograms of RuC and RuP in a DMF (N,N-
dimethylformamide)/TEOA mixed solution containing Et4NBF4
(0.1 M) as a supporting electrolyte under an Ar or CO2
atmosphere.21 The first irreversible wave, attributed to reduction
of the diimine (X2bpy) ligand, started to generate at ca. ¹1.5 V in
both cases. This means that both complexes undergo reduction
by accepting an electron to a similar extent; in other words, the
driving force for electron injection from C3N4 is almost the same
for both of the complexes. This is also supported by the FT-IR
spectra of the Ru(II) complexes (Figure S4); ¯CO peaks were
very similar between the complexes, which indicates that both
X2bpy ligands gave similar electronic effects on the central
Chem. Lett. 2016, 45, 182–184 | doi:10.1246/cl.151061
© 2016 The Chemical Society of Japan | 183

Table 1. The ratio of desorbed Ru(II) complex from the C3N4
surface after stirring for different durations in the dark
Supporting Information is available on http://dx.doi.org/
10.1246/cl.151061.
Ru(II)
complex
Desorption
ratio/%
Entry
Solution
Time
References and Notes
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The authors acknowledge The Noguchi Institute, The Murata
Science Foundation, the PRESTO/JST program “Chemical
Conversion of Light Energy” and a Grant-in-Aid for Young
Scientists (A) (Project No. 25709078). This work was partially
supported by the Photon and Quantum Basic Research Coordi-
nated Development Program (MEXT, Japan).
24 We tried to conduct similar desorption experiments under visible light
irradiation. Due to a significant change in the metal complex structure,
however, we could not quantify the desorption amount.
184 | Chem. Lett. 2016, 45, 182–184 | doi:10.1246/cl.151061
© 2016 The Chemical Society of Japan