Communication
RSC Advances
Bocarsly group suggested a possible intermediate, hydrogen Table 2 Effect of electron configuration of active sites on reduction
products with the cocatalysis of pyridine
bond pyridine dimer, might be the pre-electrocatalyst in the
electrocatalytic reduction of CO to methanol, and isolated it at
2
Electrode
material
Active
site
Electron
conguration
15
higher total concentration of pyridine and pyridinium. Pyri-
dine dimer concentration reaches the maximum at pH 5.2 since
Product
1
0
0
0
0
3
15
a near equimolar mixture of pyridine and pyridinium.
CuInS
GaP
Pd
2
Cu(I)
Ga(III)
Pd(0)
Pt(0)
3d 4s
Methanol2
Methanol
1
0
3d 4s
According to this statement, the maximum yield of methanol
1
0
4
4d 5s
Methanol
3
should be obtained at pH 5.2; it is true in our study.
9
1
Pt
5d 6s
Formic acid, formaldehyde,
When CO
2
gas is introduced into the electrolyte, pyridine
5,6
methanol, hydrogen
6
0
7
dimer might react with CO2 into pyridine and the positively FeS2
Fe(II)
Cd(II)
3d 4s
Formic acid, hydrogen
Formic acid
10
0
8
charged adduct of pyridinium and CO (Scheme 1). The energy CdTe
4d 5s
2
À1
of this reaction is estimated to be À47 kcal mol , and the
À1
energy of barrier is only 2 kcal mol
.
The positively charged adduct moves to CuInS
cathode surface; unfortunately, pyridine adsorption layer at
CuInS cathode prevents them from arriving at cathode surface,
2
photo-
halide-conned Cu-mesh electrodes, ethylene is the main
18
product in acidic solutions.
Although Cd(II) has the electron conguration of 4d 5s ,
only formic acid is produced. The reason might lie in its
2
1
0
0
it is therefore difficult to go on with the subsequent reactions.
With the increase of applied bias from À450 to À590 mV,
8
smaller electronegativity (1.69). In contrast, the electronega-
tivity of Cu (1.90) and Ga (1.81) is higher. The elements of higher
electronegativity allow the lone pair electrons of carbonyl O
atom to enter their vacant s orbit, leading to the formation of
hydroxyl group; while those of smaller electronegativity do not.
pyridine coverage at CuInS surface decreases, the diffusion
2
resistance therefore becomes smaller (Fig. 3a, Table S1†),
leading to the increase of methanol concentration.
Due to its p bond in pyridine ring, the adduct can adsorb at
cathode surface with pyridine ring paralleling to electrode
surface, and the electrons at cathode surface can therefore
1
6
transfer to the adduct (Scheme 2). With the increase of Conclusions
electron cloud density of carbonyl group in the adduct, the
The crystal size and composition of CuInS
2
, coverage of pyridine
lone pair electrons of carbonyl O atom can enter the vacant 4s
orbit of Cu(I), carbonyl group can therefore be activated
at CuInS thin lm photocathode and the applied bias have
2
signicant effects on methanol yield in photoelectrochemical
(
Scheme 2), and nally be reduced into hydroxyl group. The
1
0 0
reduction of CO . The mass transfer resistance, resulting from
2
electron conguration of active sites, d s , just as that of
Cu(I), is the most striking feature for all of reported electrode
materials leading to highly selectivity to methanol with the
cocatalysis of pyridine; otherwise, other products are formed
pyridine adsorption layer on CuInS photocathode, slows down
2
the reduction rate of CO . The selectivity to methanol might
2
depend on the electron conguration and electronegativity of
1
0 0
active sites. The electron conguration of d s and higher
electronegativity of active sites are benecial to the formation of
methanol.
(
Table 2).
Without pyridine involving, even if the active site has the
1
0 0
electron conguration of d s , methanol cannot be obtained.
At hydrogenated Pd electrode, products with carbonyl group,
À
17
CO or formic acid, are obtained via a CO _ route. At cuprous
2
Acknowledgements
We gratefully acknowledge National Natural Sciences Foundation
of China (Grant no. 20776013), Beijing Natural Science Founda-
tion (Grant no. 2102034) and the Special Fund of Basic Research
in Central Universities (Grant no. JD1107) for nancial support.
Notes and references
1
2
3
4
5
B. Kumar, M. Liorente, J. Froehlich, T. Dang, A. Sathrum and
C. P. Kubiak, Annu. Rev. Phys. Chem., 2012, 63, 541–569.
E. E. Barton, D. M. Rampulla and A. B. Bocarsly, J. Am. Chem.
Soc., 2008, 130, 6342–6344.
2
Scheme 1 Activation of CO .
J. Yuan and C. Hao, Sol. Energy Mater. Sol. Cells, 2013, 108,
1
70–174.
G. Seshadri, C. Lin and A. B. Bocarsly, J. Electroanal. Chem.,
994, 372, 145–150.
1
E. B. Cole, P. S. Lakkaraju, D. M. Rampulla, A. J. Morris,
E. Abelev and A. B. Bocarsly, J. Am. Chem. Soc., 2010, 132,
11539–11551.
2
Scheme 2 Electron transfer at CuInS cathode surface.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 39435–39438 | 39437