Homogeneous: Vs. heterogeneous catalysis for hydrogen evolution by a nickel(ii) bis(diphosphine) complex

Article abstract of DOI:10.1039/c9dt02846c

Nickel(ii) bis(diphosphine) complex 1 bearing carboxylic acid functional groups at the periphery has been prepared and characterized. Its catalytic ability towards the hydrogen evolution reaction (HER) has been evaluated under different experimental conditions: (i) under homogeneous electrocatalysis in acetonitrile using trifluoroacetic acid as the proton source, (ii) under homogeneous photocatalysis in aqueous solution using Ru(bpy)₃²⁺ (where bpy = 2,2′-bipyridine) as the sensitizer and ascorbic acid as the sacrificial electron donor, and (iii) under heterogeneous electrochemical conditions upon grafting onto mesoporous TiO₂. The results show that complex 1 is a competent and efficient catalyst for the HER under both homogeneous electro- and photochemical conditions, while undergoes rapid deactivation once attached onto the TiO₂ surface. This is attributed to both catalyst binding mode and structural rigidity imparted by covalent grafting, which highlights how catalyst design has to be considered for the direct transposition of molecular catalysis on (photo)electrode surfaces.

Full text of DOI:10.1039/c9dt02846c

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DOI: 10.1039/C9DT02846C
ARTICLE
Homogeneous vs. heterogeneous catalysis for hydrogen evolution
by a nickel(II) bis(diphosphine) complex
Giovanni Bergamini a and Mirco Natali *a
Received 00th January 20xx,
Accepted 00th January 20xx
The nickel(II) bis(diphosphine) complex 1 bearing carboxylic acid functional groups at the periphery has been prepared and
characterized. Its catalytic ability towards the hydrogen evolution reaction (HER) has been evaluated under different
experimental conditions: i) under homogeneous electrocatalysis in acetonitrile using trifluoroacetic acid as the proton
DOI: 10.1039/x0xx00000x
2+
source, ii) under homogeneous photocatalysis in aqueous solution using Ru(bpy)3 (where bpy = 2,2’-bipyridine) as the
sensitizer and ascorbic acid as the sacrificial electron donor, and iii) under heterogeneous electrochemical conditions upon
grafting onto mesoporous TiO2. The results show that complex 1 is a competent and efficient catalyst for the HER under
homogeneous conditions, both electro- and photochemical ones, while undergoes rapid deactivation once attached onto
the TiO2 surface. This is attributed to both catalyst binding mode and structural rigidity imparted by the covalent grafting
which highlights how catalyst design has to be considered for direct transposition of molecular catalysis onto
(photo)electrode surfaces.
Nickel(II) bis(diphosphine) complexes, known as ‘DuBois’
catalysts, have received considerable attention in the last years.
Introduction
R
R’
These compounds, with the general formula [NiII(P2 N2 )2](BF4)2
(where R, R’ are aliphatic and/or aromatic groups), display
pendant amines that functions as proton relays and assist the
protonation steps during the hydrogen evolution mechanism,24
providing small overpotentials for catalysing proton reduction.
Their characteristic and favourable properties towards
hydrogen evolution catalysis have stimulated many interests on
both a fundamental and applicative standpoint. Many
molecular analogues, based on different R and R’ functionalities
and/or diverse ligand architectures,25-29 have been indeed
prepared and studied in terms of their electrocatalytic
properties showing TOFs above 100,000 s-1.29 At the same time,
other nickel(II) bis(diphosphine) complexes have been
synthesized and used in combination with diverse materials for
the construction of both electro- and photo-active systems.30-37
In spite of the apparent versatility of the class of the nickel(II)
‘DuBois’ catalysts, direct comparisons of the catalytic properties
of the same compound under different experimental conditions
are rare.35 In this work we have prepared the complex
The increasing of the energy demand due to population growth
and technological development requires a scientific challenge
based on the implementation of renewable energy sources.
Direct storage of energy into chemical bonds with the
production of “chemical fuels” represents a flexible approach to
overcome the intrinsic inhomogeneity of natural sources such
as solar energy.1,2 The simplest chemical reaction that allows for
a direct interconversion between external energy and chemical
energy is represented by the formation of a hydrogen molecule
via proton reduction (eq 1).
2 H+ + 2 e− ↔ H2
(1)
In nature, microalgae and bacteria are capable of performing
such a reversible chemical transformation by means of natural
enzymes called hydrogenases featuring active sites based on
iron and/or nickel clusters.3,4 Many artificial molecular
analogues have been synthesized and characterized which can
mimic the natural functions and produce hydrogen via proton
reduction under both electrochemical and light-driven
conditions.5,6 Some examples include hydrogenase mimics,7,8
cobaloximes and cobalt diimine complexes,9-11 cobalt and iron
porphyrins,12-16 polypyridine complexes based on iron, cobalt,
and nickel.17-23
[NiII(P2PhN2PhCH COOH)2](BF4)2 (1, Scheme 1A) and studied its
2
behaviour as a hydrogen evolving catalyst (HEC) under three
different conditions: i) under homogenous electrocatalysis in
acetonitrile as the solvent using trifluoroacetic acid (TFA) as the
proton source, ii) under homogenous photocatalysis in aqueous
2+
solution using Ru(bpy)3 (where bpy = 2,2’-bipyridine) as the
sensitizer and ascorbic acid as the sacrificial electron donor, and
iii) under heterogeneous electrocatalysis in aqueous solution by
grafting the nickel complex onto the surface of mesoporous
TiO2 (1@TiO2, Scheme 1B). The results show that the present
compound displays good catalytic properties under
homogenous conditions (conditions i and ii), while shows a
a. Department of Chemical and Pharmaceutical Sciences, University of Ferrara, Via
L. Borsari 46, 44121, Ferrara, Italy. E-mail: mirco.natali@unife.it
Electronic Supplementary Information (ESI) available: characterization of the ligand
and complex 1, details of the electrocatalytic characterization, details of the
photocatalytic experiments, details of the characterization of 1 onto TiO2.
See DOI: 10.1039/x0xx00000x
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rapid deactivation leading to inefficient catalysis under sample was focused onto the entrance slit of a 300 mmViewfoAcrtaiclleleOnnglitnhe
DOI: 10.1039/C9DT02846C
heterogeneous ones (condition iii). The data reported herein Acton SpectraPro 2300i triple grating, flat field, double exit
thus point out how a suitable catalyst design is important in monochromator equipped with
order to apply a certain system under specific experimental (Hamamatsu R3896). Signals from the photomultiplier (kinetic
conditions.
a
photomultiplier detector
traces) were processed by means of a TeledyneLeCroy 604Zi (400
MHz, 20 GS/s) digital oscilloscope. Before all the measurements the
solutions were purged with nitrogen for 10 minutes. Electrochemical
measurements were carried out with a PC-interfaced Eco Chemie
Autolab/Pgstat 30 Potentiostat. In cyclic voltammetry experiments,
a conventional three-electrode cell assembly was adopted: a glassy
carbon (GC, 7 mm2 surface area) and a platinum electrode were used
as the working and counter electrodes, respectively. A bare silver
wire was used as a quasi-reference, potentials were then referenced
to ferrocene, added as an internal standard. All experiments were
performed in nitrogen-purged acetonitrile solutions using TBAPF6 as
the supporting electrolyte. TBAPF6 powder was dried overnight in an
oven at 60°C before preparation of the electrolyte solution. The GC
electrode was accurately polished with alumina slurry before every
scan. Potential controlled electrolysis experiments were performed
in a gas-tight custom-made electrochemical cell using a large surface
area carbon rod as the working electrode, a platinum grid as the
counter electrode (separated from the test solution by a frit) and a
saturated calomel electrode (SCE, Amel) as the reference electrode.
The headspace of the cell was connected to the gas chromatography
apparatus described below for the determination and quantification
of hydrogen. Electrochemical experiments in aqueous solutions were
performed in the same cell by replacing the GC working electrode
with the functionalized glass slide and using a SCE as the reference
electrode. The photochemical hydrogen evolution experiments were
carried out upon continuous visible-light irradiation of a reactor
containing the solution (a 10 mm pathlength pyrex glass cuvette with
A
COOH
HOOC
N
N
Ph Ph
N
P
P
N
NiII
COOH
HOOC
P
P
Ph
Ph
1
B
O
COOH
O
N
N
Ph Ph
O
N
P
P
P
N
NiII
COOH
P
O
Ph
Ph
1@TiO2
TiO2
Scheme 1. (A) Molecular structure of complex 1; (B) schematic representation of the
functionalization of mesoporous TiO2 thin films with 1.
Experimental section
Materials and methods.
All reagents were of reagent grade quality and purchased from head space obtained from a round-bottom flask). A solar simulator
commercial suppliers (Sigma Aldrich, Alfa Aesar). Anhydrous (Abet Technologies) was used as the light source and a cut-off filter
acetonitrile was used for the electrochemical characterization. at  < 400 nm was employed. The gas phase of the reaction vessel
MilliQ ultrapure water was used for electrochemical and was analysed on an Agilent Technologies 490 microGC equipped with
photochemical studies in aqueous solutions. FTO-covered glass a 5 Å molecular sieve column (10 m), a thermal conductivity detector,
slides (20 cm × 20 cm, TEC 8, 8 /cm) were purchased from and using argon as carrier gas. Additional details of the setup and
Pilkington. A commercial TiO2 paste (18NR-T, GreatCell-Solar) procedures used for the hydrogen evolution experiments can be
was used for the preparation of the TiO2 thin films.
found in previous reports.11
Apparatus and procedures.
Synthetic procedures.
NMR spectra were collected on a Bruker Avance 300 MHz
spectrometer. Diffuse reflectance IR spectra were obtained using a
Bruker Vertex 10 FT-IR spectrometer. UV-Vis absorption spectra
were recorded on a Jasco V-570 UV/Vis/NIR spectrophotometer.
Emission spectra were taken on an Edinburgh Instrument
spectrofluorometer equipped with a 900 W xenon arc lamp as
excitation source, a photomultiplier tube, and an InGaAs detector for
the visible and the NIR detection, respectively. Nanosecond transient
absorption measurements were performed with a custom laser
spectrometer comprised of a Continuum Surelite II Nd:YAG laser
(FWHM = 8 ns) with frequency doubled (532 nm, 330 mJ) option, an
Applied Photophysics xenon light source including a mod. 720 150 W
lamp housing, a mod. 620 power-controlled lamp supply and a mod.
03 - 102 arc lamp pulser. Laser excitation was provided at 90° with
respect to the white light probe beam. Light transmitted by the
The syntheses of the nickel complexes were performed under
nitrogen using the Schlenk technique according to literature
procedures.26,33,38
PhCH COOH 33a
Synthesis of ligand P2PhN2
.
In 25 mL degassed
2
ethanol 180 mg (6.0 mmol) paraformaldehyde and
phenylphosphine (10% in hexane, 3.3 mL, 3.0 mmol) were
added. The mixture was stirred at 80°C until the solution
became transparent. Then, 453 mg (3.0 mmol) 4-
(aminophenyl)acetic acid were added and the solution was
refluxed overnight under continuous stirring. Subsequently, the
solution was cooled down to room temperature. The white
precipitate was collected on a frit and washed with cold
ethanol. Yield: 50%. 1H-NMR (300 MHz, DMSO-d6): 3.30 (s, 4H,
Ph-CH2-COOH), 4.07-4.11 (t, 4H, P-CH2-N), 4.48-4.52 (dd, 4H, P-
2 | J. Name., 2012, 00, 1-3
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CH2-N), 6.57-6.59 (d, 4H, NC6H4), 7.02 (d, 4H, NC6H4), 7.45-7.49 and Ni(I)/Ni(0), respectively, by analogy with the processes
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(m, 6H, PC6H5), 7.51-7.67 (m, 4H, PC6H5), 12.05 ppm (s, 2H, observed in similar nickel(II) bis(diphosDpOhIi:n1e0).10c3o9m/Cp9lDeTx0e2s8.2466,2C7
COOH); 31P-NMR (300 MHz, DMSO-d6): −50.15 ppm. FT-IR (KBr The irreversible nature of the reduction processes is, however,
pellet): 3000 cm-1, 1730 cm-1.
peculiar of compound 1 and can be attributed either to the
Synthesis of the Ni(ACN)6(BF4)2 precursor.38 In 30 mL coupling of some chemical step likely promoted by the presence
acetonitrile 0.5 g (0.85 mmol) Ni powder and 2.0 g (17 mmol) of the acid functionalities or to some adsorption phenomena
NOBF4 were added. The mixture was left under stirring for 2 resulting in electrode passivation. The failure to observe any
hours until the solution became deep blue coloured. The anodic process during the return scan, however, prompts us to
mixture was filtered and diethyl ether was added to the solution favour the latter hypothesis over the first one.
to favour precipitation. The nickel salt was collected on a frit
Homogeneous electrocatalysis in acetonitrile.
Synthesis of complex NiII(P2PhN2PhCH COOH)(BF4)2 (1).26 In 5 mL Addition of trifluoroacetic acid (pKa = 12.7 in acetonitrile)39 in
acetonitrile 128 mg (0.2 mmol) ligand P2PhN2PhCH COOH and 50 mg the acetonitrile solution containing complex 1 is followed by the
and washed with diethyl ether.
2
2
(0.1 mmol) Ni(ACN)6(BF4)2 precursor were added. The solution appearance of a catalytic wave at potentials close to the
was left under continuous stirring for 3 hours. The red solution Ni(I)/Ni(0) process, whose current value (icat) is dependent on
was then concentrated under vacuum and diethyl ether was the concentration of added acid (Figure 1A).
added to allow precipitation of the desired complex. The solid
was collected on a frit and washed with diethyl ether. Yield:
85%. 1H-NMR (300 MHz, ACN-d3): 3.65 (s, 8H, Ph-CH2- COOH), 3.98
0.5
(d, 8H, PCH2N), 4.20 (d, 8H, PCH2N), 7.18 (d, 12H, NC6H4), 7.38 (d,
12H, NC6H4), 7.46 (m, 4H, PC6H5), 9.20 ppm (s, br, 4H, COOH); 31P-
NMR (300 MHz, ACN-d3): 3.47 ppm. UV-Vis: 479 nm. FT-IR (KBr
pellet): 3000 cm-1, 1730 cm-1.
0.5 M
0.4 M
0.3 M
0.2 M
0.1 M
0.05 M
0.02 M
no TFA
A
0.4
0.3
0.2
0.1
0.0
-0.1
Preparation of TiO2 electrodes.
Mesoporous TiO2 thin films (1 cm2 surface area) were prepared
by doctor-blading of the TiO2 paste onto FTO glass slides using
a Scotch tape mask, followed by calcination at 450°C for 30
minutes. Prior to deposition of the TiO2 paste the FTO film was
polished using Alconox detergent and isopropyl alcohol. Direct
functionalization with complex 1 (1@TiO2, Scheme 1B) was
achieved at room temperature by leaving the TiO2 electrodes
overnight into a 1 mM solution of 1 in acetonitrile. The
functionalized electrodes 1@TiO2 were then washed with
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
E, V vs. Fc/Fc+
N
N
B
P
NiII P
N
N
acetonitrile and dried under
a stream of air. The
P
P
+ e
H

2
functionalization was confirmed by diffuse reflectance IR
spectroscopy of samples obtained by mixing scraped 1@TiO2
films with KBr. The coverage was quantified using a reported
procedure involving immersion of the electrodes into a 0.1 M
NaOH solution followed by spectrophotometric analysis of the
resultant solution.31a The value obtained is an average of three
different electrode tested.
N
H
N
P
P
N
H
P
P
NiI
P
P
N
N
NiII P
N
P
N
N
+ e
+ H
k2
N
N
H
NiII P
P
P
N
N
Ni0 P
N
N
P
P
P
P
Results and discussion
N
N
k1
+ H
H
N
Synthesis and characterization.
P
P
N
Ni0 P
N
Compounds 1 has been prepared by adopting literature
protocols26,33,38 and characterized by 1H-NMR, 31P-NMR, UV-Vis
spectroscopy, and IR spectroscopy (Figure S1-S5). The results
are fully consistent with those obtained in previous reports.33
Cyclic voltammetry (CV) experiments have been performed to
characterize the electrochemical properties of complex 1 in
acetonitrile. Upon cathodic scan, complex 1 displays two
irreversible cathodic processes with peak potentials at −0.89 V
and −1.05 V vs. Fc/Fc+ (Figure S6) which can be attributed to
consecutive reductions at the metal centre, namely Ni(II)/Ni(I)
P
N
Figure 1. (A) CV of 1 mM 1 in acetonitrile (0.1 M TBAPF6 as supporting electrolyte) at a
scan rate of v = 100 mV/s in the presence of 0-0.5 M TFA; (B) EECC catalytic mechanism
for hydrogen evolution catalysed by 1 in acetonitrile with TFA as the proton source.
This electrochemical response can be ascribed to
electrocatalytic proton reduction promoted by the nickel(II)
complex. This attribution is further confirmed by bulk
electrolysis experiments conducted at −1.20 V vs. Fc/Fc+ which
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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show the progressive formation of hydrogen without molecular electrocatalysts reported in the literature (Figure 2).
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appreciable degradation of the sample during a 2-hour Although high catalysis rates (TOFs) canDnOotI: b10e.10a3c9h/iCe9vDeTd02w8h4e6Cn
experiment. A Faradaic efficiency (FE) of 77% has been obtained compared to other catalysts such as cobaloximes and iron
under these conditions (Figure S7). An overpotential of  = porphyrins, complex
1
displays an appreciably low
0.32(±0.03) can be estimated from the comparison of the mid- overpotential. Interestingly, its electrocatalytic behaviour is
wave potential (−0.93 V vs. Fc/Fc+) and the thermodynamic comparable to that recorded for a nickel(II) bis(diphosphine)
potential for the reduction of TFA in acetonitrile (−0.61 V vs. analogue bearing phosphonic ester groups (green traces in
Fc/Fc+) estimated using literature protocols.40 Interestingly, the Figure 2).
mid-wave potential is appreciably constant upon increasing the
concentration of TFA up to 0.5 M and positive than the
Homogeneous photocatalysis in aqueous solution.
Ni(I)/Ni(0) half-wave potential of E1/2 = −1.00 V vs. Fc/Fc+ (as The ability of complex 1 to promote catalytic hydrogen
possibly extracted from the second cathodic wave of 1 in the evolution has been then assessed under homogeneous
absence of TFA, when the current is half the peak value). These photochemical conditions by using a three-component system
results are compatible with hydrogen evolution catalysis involving Ru(bpy)32+ as the photosensitizer, ascorbic acid as the
occurring via an EECC mechanism (Figure 1B) in which the sacrificial donor, and 1 as the catalyst. We first checked the
second protonation step (k2) is rate-limiting.41 This result is in effect of the pH on hydrogen evolution by irradiating 5 mL of
line with those reported earlier by DuBois and co-workers on aqueous solutions containing 50 M 1, 0.5 mM Ru(bpy)32+, and
similar nickel bis(diphosphine) complexes using protonated 0.5 M ascorbic acid and changing the pH between 3 and 6 upon
DMF (pKa = 6.1 in acetonitrile)39 as the acid.26-28 In those cases, addition of small aliquots of a concentrated NaOH solution.
however, an ECEC pathway was also found to be competitive Consistently with the results obtained previously for a water-
with the EECC mechanism most likely as the result of the soluble nickel analogue,46 the optimum pH has been found at 5
stronger acidity of the proton source used (DMFH+ vs. TFA). (Figure S9).
Foot-of-the-wave analysis (FOWA)42 has been then used to
extract kinetic information from the cyclic voltammetry data. A
rate-constant for the first protonation step of k1 = 1.4(±0.2)×105
M-1s-1 can be estimated from the linear fitting of the CV
response near the foot of the wave, while a rate-constant for
the second protonation step of k2 = 7.8(±0.8)×102 M-1s-1 can be
obtained from the difference between the mid-wave potential
and the half-wave potential of the Ni(I)/Ni(0) redox couple in 1
(see ESI for details).41 Interestingly, the value obtained is
comparable with the one measured by a stopped-flow
technique for the protonation of a related nickel(II)-hydride
species with acids of moderate strength.43
10 M
25 M
50 M
100 M
150 M
25
20
15
10
5
A
0
0
1
2
3
4
5
6
7
Time, h
6
4
2
0
300
250
200
150
100
50
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
B
-2
Co(dmgH)2Py
-4
FeTPP
Ni(P2PhN2PhR)(BF4)2
-6
1
-8
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0
20 40 60 80 100 120 140 160
, V
[1], M
Figure 2. Tafel plot analysis of the electrocatalytic property of complex 1 in acetonitrile
using TFA (1 M) as the proton source (blue) and comparison with reference literature
systems: (black) cobaloxime in DMF, Et3NH+ (1 M) as the acid;45 (red) iron
tetraphenylporphyrin in DMF, Et3NH+ (1 M) as the acid;15 (green) NiII(P2PhN2PhR)(BF4)2 in
acetonitrile (R = -CH2P(O)(OEt)2), DMFH+ (1 M) as the acid.26
Figure 3. (A) Kinetics of photoinduced hydrogen evolution from 5 mL aqueous solutions
containing 10-150 M 1, 0.5 mM Ru(bpy)3Cl2·6H2O, and 0.5 M ascorbic acid at pH 5; (B)
relevant photocatalytic data at pH 5 as a function of catalyst concentration.
We then explored the photocatalytic activity towards hydrogen
evolution at pH 5 by changing the catalyst concentration within
the range 10-150 M. The kinetic traces are reported in Figure
Tafel plot analysis44 has been then used to benchmark the
present system with a parent nickel complex and other
4 | J. Name., 2012, 00, 1-3
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3A, while relevant photocatalytic data are collected in Figure consistent with the bleaching of the ground-state absorption of
View Article Online
3B. Hydrogen formation levels off after 6-7 hour of continuous 1.46
DOI: 10.1039/C9DT02846C
irradiation with visible light with the best results in terms of
both amount of hydrogen evolved and production rate attained
at 100 M concentration of 1 (see ESI for additional details).
0.10
Interestingly, remarkable maximum TONs up to 272 with
maximum TOFs up to 1.14 min-1 can be estimated for the
present photochemical system at the lowest concentration of 1.
Overall, the performances are comparable with those of known
molecular catalysts such as cobaloximes and cobalt
polypyridine complexes measured under the same
experimental conditions.11,21,47,48 This observation thus
confirms that the present nickel bis(diphosphine) complex 1 is
effective in promoting efficient hydrogen evolution under
homogeneous photochemical conditions, in which main
limitations to the achievement of high turnover numbers are
determined by degradation of the ruthenium sensitizer.18
The origin of this remarkable photochemical performance has
been also investigated by means of steady-state and time-
resolved optical spectroscopy. The luminescence of the
A
no 1
[1] = 50 M
[1] = 100 M
[1] = 150 M
0.08
0.06
0.04
0.02
0.00
-0.02
0
10 20 30 40 50 60 70 80
Time, s
2.5
2.0
1.5
1.0
0.5
0.0
B
2+
Ru(bpy)3 sensitizer in water is progressively quenched upon
addition of increasing amounts of 1 (Figure S10) with yields
between 15-60% within the concentration range 25-150 M.‡
On the other hand, the emission of Ru(bpy)32+ is more efficiently
quenched in the presence of 0.5 M ascorbic acid at pH 5
(quenching yield of ~90%, Figure S11).§ These results thus
suggest that the primary photoinduced process within the
1/Ru(bpy)32+/ascorbate three-component system is the
reductive quenching of the excited sensitizer by the sacrificial
donor. Although reductive quenching is the dominating
photochemical pathway, opposing effects from the competing
quenching by complex 1 are indeed apparent in the hydrogen
evolution experiments (Figure 3A). As a matter of fact, the
hydrogen evolution rates show an appreciable first-order
dependence at low catalyst concentration (i.e., [1] < 100 M),
then reach a maximum value at [1] = 100 M, and finally
decrease at larger concentrations (Figure S12). Not only, a
progressive increase in the lag-phase to attain steady-state (i.e.,
linear) hydrogen evolution is observed with increasing catalyst
concentration from 50 M to 150 M 1 (Figure 3A). Both these
results are consistent with an improved efficiency from the
competing excited state quenching by 1 with increasing
concentration of the nickel complex, as expected on the basis
of luminescence experiments (Figure S10), and suggest that this
process, regardless of its nature (energy or electron transfer), is
detrimental towards light-driven hydrogen evolution.
k = 1.5(0.1) x 109 M-1s-1
0
20 40 60 80 100 120 140 160
[1], M
Figure 4. (A) Kinetic traces at 510 nm (with related single-exponential fitting) measured
by laser flash photolysis (excitation at 355 nm) on aqueous solutions containing 0-150
M 1, 0.07 mM Ru(bpy)3Cl2·6H2O, and 0.5 M ascorbic acid at pH 5; (B) plot of the
observed rate (kobs) vs. [1] for the estimation of the bimolecular rate constant.
+
Under pseudo-first order conditions ([1]>>[Ru(bpy)3 ]), the
bimolecular rate constant for the electron transfer process from
the reduced chromophore to catalyst 1 can be estimated
yielding a value of k = 1.5(±0.1)×109 M-1s-1 (Figure 4B). This value
is closed to the diffusion-controlled regime and strongly support
the hypothesis that catalyst 1 is fast in promoting consecutive
electron transfer processes from the photogenerated
+
Ru(bpy)3 . This is indeed expected based on the efficient
photocatalytic activity experimentally observed, particularly at
low catalyst concentrations. The decrease in the light-driven
hydrogen evolution with increasing concentration of 1 above
100 M can be rationalized considering that, although the rate
of the electron transfer process from the photogenerated
Transient absorption experiments have been performed to
monitor the subsequent electron transfer from the reduced
sensitizer to the nickel complex 1 (Figure 4).11,18,21 The transient
+
Ru(bpy)3 to 1 increases, as expected for a bimolecular process,
the concomitant increase in the competitive excited state
quenching of *Ru(bpy)3 by 1 (see above) may potentially
+
signal at 510 nm, characteristic of the reduced Ru(bpy)3
2+
species,48 decays with kinetics which are dependent on the
concentration of the nickel complex (Figure 4A), thus pointing
to the occurrence of a bimolecular electron transfer process
introduce additional, unfavourable channels leading to a
decrease of the overall hydrogen yield. Accordingly, the best
results attained at 100 M 1 very likely arise from a balance
between favourable and detrimental effects on photocatalysis
induced by the concentration of the nickel complex.
+
involving reduction of 1 by Ru(bpy)3 . This attribution is further
confirmed by the residual negative absorption at 510 nm left
+
after the decay of the Ru(bpy)3 transient (Figure 4A) which is
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J. Name., 2013, 00, 1-3 | 5
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ARTICLE
Journal Name
Heterogeneous catalysis of functionalized TiO2 electrodes.
carbon nanotube-based electrodes (1.5 nmol/cm2)V3i3eawaAnrtdicleglOanslisnye
carbon electrodes (0.55 nmol/cm2)35 prepared via covalent
DOI: 10.1039/C9DT02846C
The nickel(II) bis(diphosphine) complex 1 has been grafted onto
mesoporous TiO2 by adsorption from a 1 mM solution of 1 in
acetonitrile to achieve 1@TiO2 (Scheme 1B). After overnight
soaking and subsequent rinsing with acetonitrile the TiO2
electrodes display a pale red colour, indicative of grafting of
complex 1. However, direct identification and quantification by
UV-Vis spectroscopy is prevented by the small absorptivity of
the functionalized TiO2 film (molar extinction coefficient of 
~1500 M-1cm-1 at 480 nm for compound 1)31a,33 and the
concomitant rise of the TiO2 band-gap absorption at shorter
wavelength. This notwithstanding, anchoring of complex 1 onto
TiO2 has been confirmed by the IR spectrum of 1@TiO2 which
displays the characteristic signal of 1 at around 3000 cm-1,
absent in the bare TiO2 (Figure S13). Also, the IR spectra of
1@TiO2 (Figure 5) shows the shift of the C=O stretching mode
towards lower frequencies (1650 cm-1) with respect to the same
mode in 1 (1730 cm-1), consistent with the direct binding of the
carboxylic function to the TiO2 surface.49 The presence of some
residual signal at higher wavenumbers (shoulder with relative
maximum at 1720 cm-1), compatible with that of pristine
complex 1, clearly points out that covalent grafting of 1 onto the
TiO2 surface does not involve all the carboxylic acid groups
amongst the four available.
functionalization with a nickel bis(diphosphine) complex. In
order to characterize the electrocatalytic performance of
1@TiO2, linear sweep voltammetry has been performed at
three different pH values 2, 3, and 4 on 1@TiO2 electrodes and,
for comparison, on blank electrodes (bare TiO2). The data
obtained at pH 3 are reported in Figure 6A, while those at pH 2
and 4 are reported in Figure S15. Overall, under cathodic scan
the functionalized electrodes display larger current densities
than the bare TiO2 which can be attributed to the presence of
the nickel complex on the surface that favours the electron
transfer processes at the semiconductor/electrolyte interface
resulting in hydrogen formation. An overpotential for the
hydrogen evolution reaction (HER) of  = 0.36(±0.05) V can be
estimated for 1@TiO2 at a current density value of J = 1 mA. This
value is appreciably comparable with the one measured in
homogeneous electrocatalysis by 1 in acetonitrile solution using
TFA as the proton source.
TiO2
A
1.5
1@TiO2
1.0
1.0
0.9
0.8
0.7
0.6
0.5
0.0
0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6
E, V vs. RHE
1.5
B
1.0
2000
1800
1600
1400
1200
Wavenumber, cm-1
Figure 5. Comparison of the diffuse reflectance spectra (KBr pellet) of 1 (powder, black
trace) and 1@TiO2 (red trace).
0.5
0.0
In order to quantify the amount of nickel complex present on
the surface of the mesoporous TiO2 film, we have then exploited
a procedure recently reported by Reisner and co-workers.31a
The electrodes have been immersed into an aqueous 0.1 M
NaOH solution to irreversibly detach the nickel catalysts from
the TiO2 surface. This process provides a way for the direct
quantification of the catalyst loading by monitoring the
featuring absorption at 257 nm of the dissolved complex in
solution.31a This analysis (Figure S14) yields a degree of coverage
of 32.5(±0.2) nmol/cm2 for 1@TiO2. Interestingly, this value is
comparable yet larger than the coverage of mesoporous TiO2 by
0
5
10
300 600 900 1200
Time, s
Figure 6. (A) J-V curves of bare TiO2 and 1@TiO2 in 0.1 M Na2SO4 (pH 3), scan rate v =
100 mV/s; (B) chronoamperometry of 1@TiO2 in 0.1 M Na2SO4 (pH 3) at −0.5 V vs RHE.
In order to test the catalytic ability of 1@TiO2 under operative
conditions, chronoamperometry experiments have been
conducted (Figure 6B). After application of a constant potential
of −0.94 V vs. SCE at pH 3 (−0.5 V vs. RHE) a rapid abatement of
the catalytic current is observed within the first minute
electrolysis until a current density of ~10 A/cm-2 is reached
after ca 30 minutes.
a
related nickel(II) bis(diphosphine) complex bearing
phosphonic acids as anchoring groups (14.6(±0.2) nmol/cm2).31a
Also, the observed coverage is much larger than those of both
6 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
The rapid loss of catalytic activity can be mostly related to superior to previously reported strategies, 1@TiOV2ieiswsAurtibcljeeOcntleinde
leaching of nickel complex from the TiO2 surface. As a matter of to a rapid abatement of the HER catalyDsiOs.I: T10h.e10s3e9/rCe9sDuTlt0s28th46uCs
fact, treatment of the electrodes with 0.1 M NaOH after 30 confirm how catalyst design must be tailored with respect to
minutes electrolysis shows that only traces of nickel complex 1 the conditions adopted, particularly when heterogeneous
(<1% of the initial amount) are still present on the TiO2 surface. systems are considered. Accordingly, new strategies will be
Whether catalyst loss is due to simple dissolution triggered by pursued in order to improve the stability and flexibility of
the negative applied bias§§ or is related to inactivation under nickel(II) bis(diphosphine) complexes on electroactive surfaces
catalytic conditions an open question. Regardless of the actual for application in energy conversion schemes.
reason, direct application of 1 in a heterogeneous system does
not lead to valuable results as observed in the homogeneous
phase. Similar conclusions were recently drawn for a related
complex covalently attached to a glassy carbon electrode.35 It is
Conflicts of interest
There are no conflicts to declare.
worth noting, however, that the behaviour of 1 onto a metal
oxide electrode is apparently different from that observed for a
nickel bis(diphosphine) analogue based on phosphonic acid
Acknowledgements
anchoring groups.31a To this respect, the different binding mode
Financial support from the University of Ferrara (FAR2018,
(phosphonic vs. carboxylic acid groups) very likely favours a
FAR2019) is gratefully acknowledged.
more stable binding and thus improved activity towards
hydrogen evolution in aqueous solution. Nevertheless, a
substantial current abatement was observed also in the that
Notes and references
case during continuous electrolysis within 1-2 hours.31a
‡ It should be considered that these efficiencies represent higher
Furthermore, when other types of covalent attachment were
limiting values. Indeed, due to the partial superposition of the
considered,34,36 in one case no (photo)electrochemical
absorption of 1, selective excitation of the ruthenium
characterization was performed,34 while in the other case the
chromophore cannot be performed.
§ It should be remarked that, rather than ascorbic acid, the actual
quencher is the ascorbate anion46,47 which is almost exclusively
present in aqueous solution at pH 5 (pKa = 4.1).
§§ No appreciable leaching of 1 is observed in the pH range 2-4
under open-circuit potential conditions.
studies, limited to acetonitrile as the solvent, did not look in
detail at long-term performances.36 These observations thus
suggest that the direct application of nickel bis(diphosphine)
complexes in heterogeneous conditions is not as
straightforward as in the homogeneous phase. As a matter of
fact, other aspects that must be considered when molecular
species are attached onto solid surfaces are the additional
rigidity imparted to the catalyst as well as the proximity of the
catalyst molecules. These features, induced by the surface
grafting, might considerably alter the catalytic properties,
particularly in the case of nickel bis(diphosphine) complexes, in
which substantial conformational freedom is required for the
pendant amine groups to positively assist the protonation steps
needed for enhanced hydrogen evolution catalysis.24 A careful
evaluation of the functionalization strategy33 is thus relevant in
order to prepare suitable (photo)electrodes based on nickel
bis(diphosphine) catalysts.
1
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9
Conclusions
A nickel(II) bis(diphosphine) complex bearing carboxylic acids
groups (1) has been synthesized and characterized. Its catalytic
ability towards hydrogen evolution has been tested under three
different experimental conditions, namely in homogeneous
electrocatalysis in acetonitrile with TFA as the proton source, in
homogeneous photocatalysis in aqueous solutions using
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Ru(bpy)3 and ascorbic acid as the sensitizer and electron
donor, respectively, and under heterogeneous electrochemical
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8 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9DT02846C
Table of content entry
+
Homogeneous
electrocatalysis
Homogeneous
photocatalysis


2 H+
COOH
HOOC
H2
N
N
Ph Ph
= 1
= Ru(bpy)3
= 1
HOOC
NiII
P
P
N
P
N
COOH
2+
P
Ph Ph
= AscH
1
2 H+
Heterogeneous
electrocatalysis
X
H2
= TiO2
= 1
A nickel(II) bis(diphosphine) complex bearing carboxylic acid groups has been tested as a catalyst
for hydrogen evolution under different conditions.