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Catalytic Fixation of Nitrogen by the Photocatalytic CdSPtRuO2 Particulate System in the Presence of Aqueous [Ru(Hedta)N2] Complex.

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late system CdS/Pt/Ru02 brings about the photodecomposition of
In the absence of CdS/Pt/Ru02,
there was no reduction of dinitrogen to NH3. The catalyst
1 was recovered unchanged from the reaction cell after 24
hours. This was confirmed by the elemental analysis, the
electronic and vibrational spectra of the complex, and the
cyclic voltammograms before and after the nitrogen fixation experiments.
[Ru(Hedta)NJQ 1
Hedta = trianion of ethylenediaminetetraacetic acid
Catalytic Fixation of Nitrogen by the Photocatalytic
CdS/Pt/RuO, Particulate System in the Presence
of Aqueous IRu(Hedta)N21QComplex
By Mirza M . Taqui Khan,* Ramesh C. Bhardwaj. and
C. Bhardwaj
The reduction of coordinated dinitrogen to ammonia
under ambient conditions has been the subject of active
research in recent
In every case, the reduction of
dinitrogen to NH3 o r NzH4 was achieved either in a stoichiometric manner o r at the expense of large quantities
of reducing agents such as sodium dihydr~naphthylide,[~]
metallic Al,['] Ti(OH)2, V", Mo"', Nb"', and Ta'"[6"1
o r was carried out electrolytically.[81 Recently, small quantities of ammonia and hydrazine were
by a
protonatiodelectrolysis cycle involving the complex
[W(N2)2(Ph,PCH2CH2PPhz)2]. Catalytic fixation of nitrogen to NH3 by ruthenium on active carbon with 32 wt%
metallic potassium at ambient conditions has also been reported."'] The amount of NH3 obtained was 0.011 mmol
per gram of catalyst per day.["] Photoreduction of nitrogen
to NH3 has been achieved using TiOz or iron-doped
Ti0,"21 o r a gallium p h ~ s p h i d e " ~photocathode
]
and an
oxidizable anode. In all these photoreductions, N, was reduced to NH3 through a complicated, poorly understood
mechanism with the expenditure of large quantities of the
anode materials.
A truly catalytic fixation of dinitrogen at room temperature and atmospheric pressure, however, has not been realized so far. An obvious obstacle has been the nonavailability of redox electrons at the desired potential for the reduction of coordinated dinitrogen. These electrons have only
been obtained at the expense of large inputs of energy
(electrolytic) o r costly reducing agents.12] A possible solution of the problem seems to be the selection of a proper
photocatalytic system which could generate photoelectrons
for the reduction of coordinated dinitrogen.
Herein we report the photochemical fixation of N, to
NH3 in aqueous solution under ambient conditions (30°C
and 1 atm of N,) in the presence of a semiconductor particulate system, CdS/Pt/Ru02, excited by visible light
(A= 505 nm) and catalyzed by the RU(II) dinitrogen complex 1. The yield of ammonia was 6.8 moles of NH3 per
mole of catalyst per hour. In the absence of 1 , the particu-
['i Prof. Dr. M. M. Taqui Khan, R. C. Bhardwaj, C. Bhardwaj
Coordination Chemistry and Catalysis Group
Central Salt & Marine Chemicals Research Institute
Bhavnagar-364002 (India)
Angew. Chem. Int. Ed. Engl. 27 (1988) No. 7
The experiments on the photocatalytic reduction of N2
to NH3 were conducted in a water-jacketed glass cell in
aqueous solution at 30°C. About 100 mg of CdS/Pt/Ru02
powder was suspended in 25 mL of a n aqueous solution of
1 in a glass cell of 100-mL capacity and the system was
irradiated at 505 nm, which corresponds to the band gap of
2.2 eV for the particulate system, with a 250-W xenon lamp
equipped with a 505-nm filter. Nitrogen gas freed from
carbon dioxide was bubbled through the system at a constant rate of 1 5 0 m L min-', fast enough to ensure complete saturation of the solution with respect to dissolved
nitrogen at any instant. During the reaction N2 was dispersed uniformly through the solution by stirring. Most of
the ammonia produced was dissolved before escaping into
HCI traps. The total amount of ammonia produced was
calculated both from the traps and the cell by Nessler's
spectrophotometric method. The rate of ammonia production versus time for different concentrations of dinitrogen
complex 1 is shown in Figure 1. The yield of ammonia
t lhl
-
Fig. 1. Amount of NH3 produced vs. time for different concentrations of
[Ru(Hedta)NJ' 1: (a) 7.82 x lo-' mol L-',(b) 3.91 x lo-' rnol L - ' , and ( c )
1 . 9 6 ~lo-' mol L - ' . For conditions see text.
attains a maximum after 12 hours and the reaction continues with only slight change u p to 24 hours. After the absorption of NH3 in the traps the gases contain a mixture of
O2 and excess N2. The gases were allowed to collect in a
column for an hour at the peak of NH3 production (8th
hour). The amount of oxygen in the mixture was determined by absorption in a known volume of alkaline pyrogallol. The amount of 0, absorbed was about 0.7 mole per
mole of NH3 produced, confirming the stoichiornetry of
the reaction:
0 VCH Verlagsgesellschajl mbH, 0-6940 Weinheirn. 1988
0570-0833/88/0707-0923 $ 02.50/0
923
Nz+ 3 H 2 0
+2
NH,
+ 3/2 O2
The rate of ammonia production for different concentrations of the dinitrogen complex 1 is shown in Figure 2.
One observes a first-order dependence of the rate of NH,
production on the catalyst concentration at lower concentration of the catalyst. The pseudo-first-order rate constant
of 2.8 x l o p 3s - ' was calculated from the slope of the plot
in Figure 2.
-0
0
2
1
6
8
1 Imoi L-'I .lo3-
Fig. 2. Amount of NH, produced per hour vs. concentration of
[Ru(Hedta)NJ' 1. For conditions see text.
In order to confirm the source of NH, isotopic studies
were conducted with "N2 (25% enrichment) as the source
of N2. The NH3 obtained was absorbed in HNO, and the
N H 4 N 0 3 was carefully dried and subjected to mass spectrometric analysis. Peaks were obtained at rn/z 44 and 45
in the ratio of 3 : l corresponding to the formation of
I4N20 and i4N'5N0, respectively. The reaction did not
show any isotopic effect in D20.
Based on the kinetics of the reaction, a first-order dependence on the concentration of 1, the following mechanism is proposed for the photocatalytic reduction of N2 to
NH, (the photolysis of water is not shown):
The electron is transferred in a rate-determining step to
1 to form an intermediate species 3, where the extra electron can be accommodated on either the metal ion or the
dinitrogen ligand. Fast protonation and successive electron
transfer to 3 produces NH3 as the end product and species
2, which reacts with dinitrogen in the preequilibrium step
to form 1 and continue the catalytic cycle. The lack of a
deuterium isotope effect rules out a coupled proton transfer in the rate-determining step.
Electrochemical reduction of 1 in the presence of N2
in the p H range 4-7 at the controlled potential of -0.20
to -0.40V, corresponding to the redox potential of the
Ru"/Ru' couple, gave 5 x
moles of NH, in three
hours. The yield then droped rapidly.['51This confirms the
role of the metal ion in the electron transfer to N2.
A blank experiment in the absence of 1 (see above) produced only hydrogen by the photodecomposition of water.[I4l This indicates the role of 1 in the activation of N2
and its reduction to NH,. The same experiment conducted
in the presence of H4edta again results in the photodecomposition of water. This rules out the possibility of formation of NH3 from the decomposition of H,edta. This is further confirmed by the isotopic studies with "N2; the products contain the label in the same ratio as the dinitrogen
employed. The composition of the dinitrogen complex 1
did not change even after 12 hours of experiment, as confirmed by elemental analysis and spectrophotometry. The
experiments conducted in the presence of 1 and nitrogen
but in the absence of the particulate system also failed to
produce NH, even after 48 hours. These experiments indicate that the redox electron from the photoexcitation of
the particulate system is essential for the reaction. The
photocatalytic cycle is shown in Figure 3.
f
.6H"* 4 0 2
[Ru(L)H20Ie
+ NZ
[Ru(L)Nzle
2
+ H20
1
\
h0
CdS
[Ru(L)N2]'
1
+ ee
[ R U ( L ) N ~ ] '+
~ 6H'
3
3H20
+ ee + . . .
Fig. 3. Mechanism of photocatalytic ammonia production. CB = conduction
band, VB = valence band.
[Ru(L)N2IZe
3
The decrease in the yield of NH, after 12 hours can be
attributed to the photocorrosion of the semiconductor:
+ 5 e Q + H 2 0 5 2NH, + [RuL(H2O)Ie
+ e,",)
S + CdZ0
CdS 3 CdS(hfB
2
+ CdS
-
L = Hedta
2h$,
The irradiation of CdS/Pt/Ru02 in aqueous solution
excites a n electron from the valence band to the conduction band and a hole is created in the valence band. The
holes trap the electrons released in the Ru02-catalyzed
photooxidation of water to 2 H @and 1/202. The RuO, in
this system thus acts as a hole scavanger. There is a competition between He and the dinitrogen complex l for electrons in the conduction band. Because of the negative potential of the Ru"/Ru' couple (-0.46 V), the complex is a
stronger oxidizing agent than the protons and competes
successfully with the latter for the electron.
Thus the photocorrosion takes place only when the rate
of the electron transfer is inadequate for the number of
holes generated in the system. This may be partly due to
the inefficient scavanging of the holes by R u 0 2 particles
loaded on the semiconductor. The main cause, however, is
presumably the poisoning of the CdS by O2 which is generated in the
In conclusion, we feel that efficient reduction of N2 to
NH3 at ambient conditions is possible if a proper photocatalyst is used and dinitrogen is activated by coordinating to
a suitable metal center.
924
0 VCH Verlagsgesellsch~~
mbH, 0-6940 Weinheim. 1988
0570-0833/88/0707-0924 S 02.50/0
Angew. Chem. Inf. Ed. Engl. 27 (1988) No. 7
Experimental Procedure
1 : [Ru"'(Hedta)CI]" was reduced to (Ru"(Hedta)(H20)JB 2 using hydrogen
over platinum black under an atmosphere of hydrogen."" The photocatalyst
CdS/Pt/RuO> was prepared by the published procedure [IS]. Complex 2 exhibits LMCT peaks at 283, 332, and 296 nm. On passing Nz through dilute
M), the peaks corresponding to 2 decrease with the
solutions of 2 ( =
appearance or a new peak at 221 nm, which was taken as a marker peak for
the calculation of the stability constant of 1. The equilibrium constant for the
formation of 1 is given by the following expression:
+
[ R ~ ( H e d t a ) ( H , o ) ] ~ N,
[Ru(Hedta)(N2)le
2
+ H20
1
From the solubility of N 2 in water at 25" (7.81 x
mol L-') the equilibrium constant for the formation of 1 at 25" was calculated as
log K , = 2.90_+0.02.
Complex 1 exhibits the v(M-N,) band at 2040 cm-'. Differential pulse polarography (DPP) of l gives a single peak at -0.48 V corresponding to the
redox couple Ru"/Ru'. The cyclic voltammogram of 1 gives reversible peaks
at -0.24 and - 1.0 V for the Ru"'/Ru" and H"/H potentials, respectively.
The peak corresponding to Ru"/Ru' cannot be observed because of the
crossover of the cathodic-potential and anodic-potential curves in this region.
Received: January 7, 1987;
revised: November 27, 1987 and April 7, 1988 [Z 2040 1EJ
German version: Angew. Chem. 100 (1988) 1000
CAS Registry numbers:
NH,, 7664-41-7; N2, 7727-37-9; CdS, 1306-23-6; F't, 7440-06-4; RuO,, 1203610-1 ; [Ru( Hedta)( N2)]'. 76058- 13-4.
[ I ] M. M. Taqui Khan, A. E. Martell: Homogeneous Catalysis by Meral
Complexes. Vol. I , Academic Press, New York 1974.
[2] M. E. Volpin, V. B. Shur: New Trends in the Chemistry ofNtirogen Fixafron. Academic Press, New York 1980.
[3] J. Chatt, J. R. Dilworth, R. L. Richards, Chem. Reu. 78 (1978) 589.
[4] E. E. van Tamelen, Acc. Chem. Res. 3 (1970) 361.
[S] E. E. van Tamelen, B. J. Akermark, J. Am. Chem. SOC.90 (1968) 4492;
N. H. Liu, N. Shanpach, J G. Palmer, G . N. Schrauzer, Inorg. Chem. 23
(1984) 2772.
161 N. T. Denisov, E. M. Burbo, N. 1. Shuvalova, A. E. Shilov, Kinet. Karal.
23 (1982) 874; E. M. Burbo, M. I. Lebedeva, N. T. Denisov, ibid. 27
(1986) 1504.
171 N. T. Denisov, N. I. Shuvalova, A. E. Shilov, Kinet. Katal. 26 (1985)
1493.
(81 C. R. Dickson, A. J. Nozik, J. Am. Chem. SOC.I10 (1978) 8007.
[9] C. J. Pickett, J. Talarmin, Nature (London) 317 (1985) 651.
[lo] C. J. Pickett, K. S . Ryder, J. Talarmin, J. Chem. SOC.Dalton Trans. 1986,
1453.
[l 11 K. Aika, Angew. Chem. 98 (1986) 556; Angew. Chem. h i . Ed. Engl. 25
(1986) 558.
[I21 G. N. Schrauzer, T. D. Guth, J. Am. Chem. SOC.99 (1977) 7189.
1131 M. Koizumi, H. Yoneyama, H. Tamura, Bull. Chem. SOC.Jpn. 54 (1981)
1682.
[I41 M. M . Taqui Khan, R. C. Bhardwaj, C. Bhardwaj, J . Chem. SOC.Chem.
Commun. 1988, 1690.
[IS] M. M. Taqui Khan, G. Ramachandraiah, unpublished.
[I61 K. Kalayanasundaram, E. Borgarello, D. Duonghong, M. Gratzel, Angew. Chem. 93 (1981) 1012: Angew. Chem. Int. Ed. Engl. 20 (1981) 987.
1171 A. A. Diamantis, J. V. Dubrawski, Inorg. Chem. 20 (1981) 1142.
[IS] J.-M. Lehn, J. P. Sauvage, R. Ziessel, K. Hilaire, Isr. J . Chem. 22 (1982)
168.
Tetrahalodiborane(4) molecules BzX4 are transformed
upon heating into trihaloboranes and short-lived BX speciesfZ1or into stable cage molecules B,X, (X = C1, Br;
n = 8- 12).I3]Copyrolysis of halogenated hydrocarbons and
B2X4 results in the formation of perhalogenated carboranes;c41 little is known about the reaction course. The investigation described here was motivated by the question
whether other heteroatoms could be incorporated into halogen-substituted boron clusters.
Pyrolysis of a mixture of B2C14and PCl, at 330°C resulted in the formation of closo-3,4,5,6-tetrachloro-I
,Z-diphosphahexaborane(4) 1 as a hygroscopic, colorless, crystalline solid, in addition to small amounts of side products.
B2Cl, is apparently not only the source of "BCl" units but
acts at the same time as a reducing agent. The idealized
formulation of the reaction is given in Equation (a). In the
mass spectrum of the crude product, additional signals are
observed which have to be assigned to a molecule having
the composition PZB5C15.
7 B2CI.q
+ 2 PC13 * P,B,CI, + 10BCI,
1
The composition and structure of the phosphaborane 1
are derived from the spectroscopic data (Table 1). Since
the two phosphorus atoms can each contribute three electrons to the framework bonds, a closo-hexaborane is to be
expected according to the Wade rules. Its structure should
be derivable from an octahedron. The structure of the cluster is derived from the values for the chemical shift of the
boron atoms, and the cis arrangement of the phosphorus
atoms from the fact that two "B-NMR signals (1 : 1 ratio)
are observed. The X-ray structure analysis confirms the interpretation of the spectra (Fig.
Table I. Spectroscopic data for 1
"B-NMR (relative to BF3.0Et2, 25.67 MHz, C6D,, room temperature (RT));
61=2.5,62=22.1 (intensity ratio 1 : 1, halfwidth 128 Hz).--"P-NMR(re1ative
t o 85% H3P04, 32.2 MHz, C6D6, RT): 6 = - 187.-MS (70 ev): calculated
and measured pattern of '5C1/37CI/"B/'"B isotope distribution are in agreement): M e (rel. intensity 84%), B,Clf (7), B3CIY (7), [M-BCI,]" (100). Pf
(30), PBzClt (27), P,B2Cla (12), BzCl? (S), BCl? (17)
The First closo-Diphosphahexaborane P,B,CI,**
By Woygang Haubold,* Willi Keller, and Gisela Sawitzki
Dedicated to Professor Heinrich Noth
on the occasion of his 60th birthday
Higher boranes have often been prepared by pyrolysis
of smaller boranes. If appropriate mixtures of boranes and
hydrocarbons are used, carboranes are obtained."' Both
synthetic principles can be extended to haloboranes.
[*I
['*I
Prof. Dr. W. Haubold, Dr. W. Keller, Dr. G. Sawitzki
lnstitut fur Chemie der Universitat Hohenheim
Garbenstrasse 30, D-7000 Stuttgart 70 (FRG)
This work was supported by the Deutsche Forschungsgerneinschaft and
the Fonds der Chemischen Industrie.
Angew. Chem. Inl. Ed. Engl. 27 (1988) No. 7
Fig. 1. Molecular structure of 1. Selected distances [pm] and angles ["I: P1-P2
222.2(3), B2-B4 167.8(12), other B-B(average) 173, B-P(average) 200, B-Cl(average) 176; Bl-PI-63 76.8(4), BI-P2-B3 77.6(4), PI-P2-B2 81.9(3), P2-Pl-B4
82.3(3), P2-82-84 98.2(5), BI-B2-B3 94.0(6), PI-B4-B2 97.6(5), BI-B4-B3
92.3(6).
0 VCH Verlagsgesellschaji mbH. 0-6940 Weinheim. 1988
0870-0833/88/0707-0928 $ 02.50/0
925
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