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Electrochemical Reduction Of Carbon Dioxide In The Presence Of [NiII–5,7,12,14-Tetramethyldinaphtho [b,i] [1,4,8,11] Tetra Aza[14]Annulene]++ Cation.
A. Rios-Escudero1, M. Isaacs1, M. Villagrán1, J. Zagal1 and J. Costamagna1.
1Faculty of Chemistry and Biology, Universidad de Santiago de Chile
Av. B. O’Higgins 3363, P.O. Box 40, Santiago 33 Chile.
FAX: (56-2) 681-2108,
Email: jcostama@lauca.usach.cl
Received
January 06, 2004. In final form March 19, 2004
Dedicated
to Professor Dr. P. J. Aymonino on occasion of his 75thAnniversary
Abstract
Cyclic
Voltammograms of [Ni(II)5,7,12,14-tetrametyldinaphtho[b,i]
[1,4,8,11]tetraaza[14]annulene]++ cation complex ([Ni(tmdnTAA)]2+ hereafter) present two irreversible reduction peaks at -0.70 V y -1.50 V vs.
Ag/AgCl in N,N´-dimethylformamide (DMF), while in acetonitrile (MeCN) these peaks are at -0.93V. and -1.80 V vs. Ag/AgCl. When DMF/H2O
or MeCN/H2O are used as solvents, the peaks are also shifted with
respect to those in pure DMF as solvent. For example, in DMF/ H2O
they are -0.76 and -1.04 V respectively. Cyclic voltammograms of the cation
complex in CO2 saturated solutions exhibited the catalytic wave of
the CO2 reduction, Ecp, at ~ -1.50 V in DMF/H2O
and at ~ -1.80 V in MeCN/H2O. Under these
conditions, the only product of the bulk electrolysis was hydrogen. Due to the
formation of a stable product, the catalytic current for the CO2 reduction of the second voltammetric cycle is 30% less intense than the
current of the first cycle. The stable adduct, [Ni(II)tmdnTAA-CO]++,
formed on the electrode was characterizated by spectroeletrochemistry and FT-IR
spectroscopy.
Resumen
Voltamogramas cíclicos del catión
complejo macrocíclico [Ni(II)-1,7,12,14-tetrametildinafto[b,i] [1,4,8,11]
tetraaza[14]anuleno]++, ([Ni(II) tmdnTAA]++), presenta
dos picos de reducción irreversibles a -0.70 V y -1.50 V vs. Ag/AgCl en
solución de N,N dimetilformamida (DMF), mientras que en solución de
acetonitrilo(MeCN) los potenciales de pico se desplazan a -0.93 V y -1.80 V
respectivamente. La voltametría cíclica del complejo en una solución saturada
con CO2 muestra una onda catalítica de reducción del CO2
cercana a -1.50V para la mezcla DMF/H2O y a -1.80V para la mezcla de
MeCN/H2O. En todos lo experimentos de electrólisis a potencial
controlado solamente fue detectado hidrógeno como producto. Además, la
corriente de pico catalítica para la reducción de CO2 se reduce en
un 30% al segundo ciclo voltamétrico lo cual indica el envenenamiento de la superficie
del electrodo por formación de un aducto estable del tipo [Ni(II)tmdnTAA-CO]++.
Este aducto fue caracterizado por espectroelectroquímica y medidas de FT-IR.
Introduction
There has been considerable interest in the design of electrocatalysts for carbon dioxide reduction to useful fuel products [1-5]. A number of transition metal complexes in both homogeneous solutions or confined on electrode surfaces have been shown to be effective in the electrocatalytic reduction of carbon dioxide [1-13]. The electroreduction of CO2 involves not only multiple electron transfers but also complex chemical steps which may involve protonation [1,3,14,15,16]. Furthermore, there can be multiple and competing reaction pathways giving rise to a variety of reaction products and some of them can deactivate the catalyst. In the mechanism of electrode poisoning when the reduction of CO2 is catalyzed by [Ni(CRH)]2+ (CR= 2,12- dimethyl-3,7,11,17-tetraazabicyclo-[11,3,1]heptadeca-1(17),2,11,13,15-pentaene) or [Ni(cyclam)]2+ (cyclam= 1,4,8,11-tetraazacyclotetradecane), the CO formed reacts with Ni(I) to form a carbonyl adduct. The adsorbed Ni(I) carbonyl is further reduced to the catalytically inactive Ni(0) carbonyl [14]. On the basis of these precedents, we have investigated the electroreduction of CO2 on glassy carbon electrodes in the presence of [Ni(II)-5,7,12,14-tetramethyldinaphtho[b,i][1,4,8,11]tetraaza[14]annulene]2+, [Ni(II)(tmdnTAA)]++, shown in Figure 1.
Figure 1: -Molecular structure for: [Ni(II)-5,7,12,14-tetramethyldinaphtho[b,i][1,4,8,11]tetraaza[14]annulene]++, [Ni(tmdnTAA)]++.
Experimental
The
synthesis, spectroscopic characterization, acid-base properties, and
photochemistry behavior of [Ni(tmdnTAA)]++, and [Cu(tmdnTAA)]++ macrocyclic cation complexes have been previously reported [17].
Electrochemical measurements were carried out with a PAR-173 potentiostat
coupled to a PAR-175 universal programmer, and a X-Y Omnigraphic recorder. All
potentials are referred to the Ag/AgCl electrode without correction for the
liquid junction. Deaerated solutions 10-4 M in [Ni(tmdnTAA)]Cl2 and 0.01M in tetraethylammoniumn perclorate (TEAP), were placed in a three
electrode cell with a glassy carbon working electrode (GC), platinum wire
counter electrode and a Ag/AgCl reference electrode. All cyclic voltammetry
experiments were performed at 200 mV/s.
Electrolysis
experiments in saturated CO2 atmosphere were carried out in the same
configuration used for cyclic voltammetry. A three electrode airtight cell with
a Duocel reticulated glassy carbon(RVC) 10PPI working electrode
was used in these experiments. A homemade thin layer cell similar to one
described by C.M.Duff and G.A.Heath [18] was used. An optical
transparent Pt mesh electrode was the working electrode, a platinum wire was
the counter electrode, and Ag/AgCl was the reference electrode. This thin layer
cell configuration was placed inside a Varian Cary 1E spectrometer. In a
typical spectro-electrochemical experiment, the spectra were recorded at
three minutes intervals until constant absorbance. In the CO
saturated atmosphere experiment the same configuration was used. Reaction
products were analyzed by a Varian 3400 Gas Chromatograph equipped with TCD and
FID double detectors, respectively. The capillar column used with the TCD
detector was Molesiv 115-3632, 30m length, and 0.53 mm I.D. The
capillary column used with the FID was a DB-1.
The
products formaldehyde and formic acid were investigated using the chromotropic
acid spot test [19]. A chromatographic column of 30 cm x 1.5 cm was packed with
silica gel, and then the electrolysis products were separated using a 1:1 (v\v)
chloroform-methanol mixture as eluent.
FT-IR
spectra for bulk electrolysis products were measured in a Bruker model Vector
22 apparatus using a film sample over a KBr window.
Results And Discussion
Voltammetric behavior
of [Ni(tmdnTAA)]++ cation.
In general, the complex investigated was electrochemically well behaved and exhibited both metal-based as well as ligand-based irreversible redox processes under nitrogen, carbon dioxide and carbon monoxide atmospheres. Under nitrogen, the first electron reduces the metal center [20] to generate the [Ni(I)(tmdnTAA)]+ species. This process is followed by the addition of another electron to generate the ligand-reduced
Figure 2.- Voltammograms under nitrogen(¾¾) and carbon dioxide(- - - -) atmospheres for the macrocycle studied in N,N’-dimethyl formamide/tetraethylammonium perchlorate (TEAP) at v=200 mV/s.
Table I. Redox potentials for [Ni(II)-5,7,12,14-tertramethyl-dinaphtho [b,i][1,4,8,11]tetraaza[14] annuleneH]++ at a glassy carbon electrode in 0,01M tetraethylammonium perchlorate, Under N2 and CO2 atmosphere.
Solvents |
Metal-based(a) |
Ligand-based(a) |
Ered (b ) |
DMF |
-0.70 |
-1.50 |
-1.75 |
MeCN |
-0.93 |
- 1.80 |
- 1.80 |
DMF/H2O |
-0.76 |
-1.04 |
-1.50 |
MeCN/H2O |
-1.10 |
-1.80 |
-1.80 |
(a) Potential in Volts, vs. Ag/AgCl, in N2.
(b)Potentials in Volts, vs. Ag/AgCl, in CO2. Ered values represents the potential of the wave at which the current enhancement
due to electrocatalytic reduction of carbon dioxide was observed for the macrocycle studied.
species [Ni(I)(tmdnTAA)]. Typical voltammograms for these processes are shown in Figure
2. Table 1 contains peak potential values for the irreversible processes under
nitrogen and carbon dioxide atmospheres in N,N’-dimethylformamide (DMF),
Acetonitrile (MeCN), DMF/H2O, and MeCN/ H2O solvents.
The
last column of Table I lists the potentials at the peak of the wave at which the
current enhancement due to electrocatalytic reduction of carbon dioxide was
observed in the solvents studied. The electrocatalytic activity of the Ni(II)
macrocycle complex toward reduction of carbon dioxide was evaluated by
comparing its voltammetric response under nitrogen and carbon dioxide
atmospheres as it can be seen in Fig. 2. A clear electrocatalytic current was
detected in the presence of the Ni(II) macrocycle complex. The
reduction of carbon dioxide observed at –1.8V (vs. Ag/AgCl) in the absence of
the complex takes place at a potential more negative than –2.0 V(11). On the
other hand, [Ni(tmdnTAA)]++ has always shown electrocatalytic
current after the second reduction process as it is shown in Figure 2. Cyclic
voltammograms in protic media exhibit an anodic feature at –0.05 V when the
limiting potential was more negative than the potential where the hydrogen
evolution began, Figure 3. We propose that the feature corresponds to the
oxidation of a nickel hydride formed on the electrode surface,
eqns (1) and (2).
[Ni(II)(tmdnTAA)]+++e-→[Ni(I)(tmdnTAA))]+ads | (1) |
[Ni(I)(tmdnTAA)]+ads + H+ →[HNi(II)(tmdnTAA)]++ ads | (2) |
Similar
electrochemical processes were reported by Aramata et al. for the reduction of
carbon dioxide on a glassy carbon electrode chemically modified by
aza-macrocyclic-cobalt(II) complexes [21,22].
In our
eletrochemical experiments with the Ni(II) complex in protic media, an
enhancement of the carbon dioxide electrocatalytic current was evidenced in the
voltammograms and also hydrogen evolution occurred in DMF and
MeCN wet solvents. Furthermore, the catalytic current of the second cycle,
e.g., in DMF/ H2O, is ~30% less than the current of the first cycle.
At the same time, shifts to a more positive catalytic potential were observed in DMF/H2O respect to dry DMF solvent (see
Table 1), and the hydrogen evolution is also increased. These
results suggest a possible competition between eq. (5) and eq. (6).
[Ni(II)(tmdnTAA)]+++ e-+ CO2→ [N(I)(tmdnTAA) CO2]+ | (3) |
[Ni(I)(tmdnTAA)CO2]+ + H →[Ni(I)(tmdnTAA) COOH ]++ | (4) |
[Ni(I)(tmdnTAA)COOH]+++ e-→ [Ni(II)(tmdnTAA)CO]+++OH- | (5) |
[HNi(II)(tmdnTAA)]++ads+H++e-→[Ni(II)(tmdnTAA)]++ads+H2 | (6) |
Figure 3.- Voltammogram of [Ni(tmdnTAA)]++ in N,N’-dimethylformamide after addition of water (showing the anodic hump at –0.05 V), under nitrogen (¾¾) and carbon dioxide (× × × ×) at v=200 mV/s.
The
reaction mechanism proposed by Sauvage et al. [15] and Anson et al. [16] for the electrocatalytic reduction of carbon dioxide with [NiCyclam]++ over a mercury electrode can be applied to the reduction catalyzed with
[Ni(tmdnTAA)]++ . In case of the latter, the nickel(II) carbonylated
complex shown in equation (5) is very stable, and this may explain the low
efficiency of the electrocatalytic reduction with [Ni(tmdnTAA)]++
cation complex.
In
MeCN solutions only the second cathodic peak electrocatalyzed the carbon
dioxide reduction at –1.80V (see Table I). If protons were added to the media,
e.g. as a wet solvent, the voltammogram of N2-saturated
solutions exhibited the anodic hump of the hydride oxidation also observed in
wet DMF solutions (see Figure 3). A very strong enhancement of the
electrocatalytic peak and the absence of the hydride-related anodic hump
resulted in wet MeCN solutions saturated with carbon dioxide. This experimental
observations suggest that in acetonitrile and in protic media the reaction
corresponding to equation 6 would not take place (at ca. –1.80V).
Preparative–scale electrolysis.
Preparative–scale electrolysis was performed in a gas-tight electrolysis cell. The current decayed slowly and the solution turned from red to yellow over a period of three hours under an applied potential of –1.80V vs. Ag/AgCl. Samples of the gas phase were collected at thirty minute intervals. Hydrogen was the only reaction product as shown in figure 4.
Figure 4.-Time-evolution of hydrogen for a bulk electrolysis experiment for [Ni(tmdnTAA)]++ under carbon dioxide in N,N’-dimethylformamide /water(1:1) solvent at -1.65V vs. Ag/AgCl.
Analytical
tests for oxalate, formic acid, and formaldehyde in the liquid phase were
negative for all the cases studied. Spectroelectrochemical observations were
made in the presence of carbon dioxide or carbon monoxide at open circuit and
-1.8 V, they revealed a new absorption band at 358 nm as shown in figure 5.
The absorption band can be attributed to a stable species [Ni(tmdnTAA)CO]++.
A similar but unstable species was detected by Anson et al. in the
electrocatalytic reduction of carbon dioxide to carbon monoxide by [Ni(cyclam)]2+
[16]. Similar experiments in a carbon monoxide atmosphere
have shown the same behavior as presented in Figure 6 (see experimental).
The
FT-IR spectra of electrolyzed and non electrolyzed [Ni(tmdnTAA)]++
solutions were recorded after chromathography on silica gel. The results are
shown in figure 7 for an electrolysis of a solution saturated with nitrogen,
for an electrolysis of a solution saturated with carbon dioxide, and non
electrolyzed [Ni(tmdnTAA) ]++ cation. The spectra from the non
electrolyzed and the electrolysis under nitrogen are similar (Fig. 7a, 7b) but
the spectrum from the electrolysis under carbon dioxide exhibited a new band at
1729 cm-1
Figure 5.- Spectroelectrochemistry for [Ni(tmdnTAA)]++ in acetonitrile under carbon dioxide saturated atmosphere at open circuit (O.C.) and for Eap.(applied potential)= -1.8 V. See inset for details.
Figure 6.- Spectroelectrochemistry for [Ni(II) (tmdnTAA)]++ at Eap = -1.8 V in acetonitrile, under carbon monoxide saturated atmosphere, showing the spectral changes each 3 minutes until constant behavior.
corresponding to the carbonyl [Ni(tmdnTAA)-CO]++ complex. Therefore, this stable adduct, detected by spectroelectrochemical and FT-IR spectroscopy, could be responsible for the catalyst poisoning when carbon dioxide is electroreduced in DMF, DMF/H2O, MeCN or MeCN/H2O solutions.
Figure 7.- FT-IR film spectra of: a) [Ni(tmdnTAA)]++; and the electrolysis products: b) under nitrogen, c) under carbon dioxide.
Conclusions
The
fact that catalytic currents were observed after the second reduction process,
suggest that probably two electrons are involved in the carbon dioxide
electroreduction. In addition, it is quite likely that more than one mechanism
can explain the behaviour observed in cyclic voltametry.
It
is also clear that the nature of the products and their distribution depend on
the amount of water present in the system for the macrocycle studied.
The [Ni(II)-5,7,12,14-tetramethyldinaphtho[b,i][1,4,8,11]tetraaza[14] annulene]++ cation complex showed electrocatalytic effect in cyclic voltammetric
experiments for carbon dioxide reduction over GC electrode in the media
studied. Electrolysis experiments at –1.8V vs. Ag/Ag/Cl showed the formation of
a stable adduct [Ni(II)(tmdnTAA)CO]++ which would be responsible of
the loss of catalytic actvity, and the absence of chromathographic detection
of carbon monoxide product in the bulk electrolysis experiments. These results
have shown that the stable Ni(II) carbonyl complex is the catalytically
inactive species in the system studied. The selectivity and efficiency for the
hydrogen evolution in the catalytic process can be modified as a function of
the amount of water added.
Acknowledgements
This work has been financed by FONDECYT (Chile), project Nº 8010006,M.I. thanks FONDECYT (Chile), project Nº 2000010. A.R.E. thanks FONDECYT (Chile) for a doctoral fellowship.
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