Dismutation of Hg22+ catalyzed by colloidal gold

Article abstract of DOI:10.1016/j.ica.2004.07.057

Colloidal gold in acetonitrile catalyzes the disproportionation of Hg ₂²⁺ to Hg²⁺ and Hg⁰. This catalysis is apparently initiated by the amalgamation of the gold particles, which is indicated by a concomitant change of the plasmon absorption of colloidal gold. Colloidal gold in acetonitrile catalyzes the disproportionation of Hg ₂²⁺ to Hg²⁺ and Hg⁰. This catalysis is apparently initiated by the amalgamation of the gold particles which is indicated by a concomitant change of the plasmon absorption of colloidal gold.

Full text of DOI:10.1016/j.ica.2004.07.057

Inorganica Chimica Acta 358 (2005) 429–430
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Note
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Dismutation of Hg2 catalyzed by colloidal gold
*
Horst Kunkely, Arnd Vogler
Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, Universitaetsstrasse 31, D-93040 Regensburg, Germany
Received 28 June 2004; accepted 13 July 2004
Available online 6 October 2004
Abstract
Colloidal gold in acetonitrile catalyzes the disproportionation of Hg2 to Hg2+ and Hg0. This catalysis is apparently initiated by
the amalgamation of the gold particles which is indicated by a concomitant change of the plasmon absorption of colloidal gold.
ꢀ 2004 Elsevier B.V. All rights reserved.
2þ
Keywords: Electronic spectra; Mercury; Catalysis; Colloidal gold; Amalgam
1. Introduction
2. Experimental
Generally, as the noblest of all metals gold is chem-
ically very inert. While this is certainly true for the bulk
metal, colloidal gold which occurs in nano-sized parti-
cles may be quite reactive. In particular, its catalytic
properties have attracted much recent interest [1–4].
Important processes such as the water-gas shift reac-
tion or the oxidation of carbon monoxide are catalyzed
by colloidal gold which is attached to solid supports.
The catalytic activity seems to depend on various fac-
tors such as particle size, the nature of the support
and the method of preparation. Unfortunately, the
interplay of these influences is not well understood.
Here, we report about a new type of catalysis by gold
nano-particles, which apparently proceeds by a rela-
tively simple mechanism. We describe the disproportio-
Mercury(I) perchlorate tetrahydrate was commer-
cially available from Strem and used without further
purification. The salt Bu4N[AuIII(N3)4] was prepared
according to a literature procedure [5]. The photochem-
ical generation of colloidal gold in CH3CN was carried
out in analogy to [6] but modified in order to obtain a
solution of colloidal gold which was stable for at least
30 h. The irradiation was performed with a HBO 200
W/2 lamp from Osram and a Schott WG 320 cut-off fil-
ter. After a photolysis for about 20 min, a stable colloid
was formed as indicated by the invariance of the plas-
mon absorption at kmax = 525 nm.
Absorption spectra were measured with a Varian
Cary 50 or a Uvikon 860 spectrophotometer. For spect-
roscopic measurements acetonitrile of spectrograde
quality (‘‘Uvasol’’) from Merck were used.
2þ
nation of Hg2 induced by colloidal gold. The key
step of this dismutation seems to be the amalgamation
of the elemental gold.
3. Results and discussion
Colloidal gold is generated by the exhaustive photol-
ysis of Bu4N[AuIII(N3)4] in acetonitrile [6]. The photore-
action proceeds in two steps according to the scheme:
*
Corresponding author. Tel.: +49 941 9434485; fax: +49 941
9434488.
E-mail address: arnd.vogler@chemie.uni-regensburg.de (A.
hv
½AuIIIðN3Þ ꢀꢁ !½AuIðN3Þ ꢀꢁ þ 3N2
ð1Þ
Vogler).
4
2
0020-1693/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.07.057

430
H. Kunkely, A. Vogler / Inorganica Chimica Acta 358 (2005) 429–430
hv
½AuIðN3Þ ꢀꢁ ! Au0 þ 1:5N2 þ N
ð2Þ
ꢁ
3
amalgamation of the colloidal gold and an additional
deposition of a layer of elemental mercury on the gold
particles [12].
2
Azide functions as an intramolecular reductant. It is
oxidized to molecular nitrogen. The elemental gold is
formed as red colloid, which is characterized by its plas-
mon absorption at kmax = 525 nm (Fig. 1). In addition to
colloidal gold only azide and [Au(CN)2]ꢁ are present in
Henglein and Gierig [12] reported similar observa-
2þ
tions when they radiolyzed Hg2 in the presence of col-
loidal gold in aqueous solution. However, without
2þ
radiolysis Hg2 did not react with colloidal gold in dis-
tinction to our observation. The origin of this discrep-
ancy is not quite clear but is probably associated with
the photolyzed solution [6,7]. However, these species do
2þ
not react with Hg2
.
2þ
Upon addition of Hg2(ClO4)2 in CH3CN to the
solution which contains colloidal gold, the solution
immediately becomes black and turbid. Finally, ele-
mental mercury separates as small droplets. The solu-
tion which is separated from elemental mercury by
centrifugation contains apparently Hg2+. Upon addi-
tion of dithizone, Hg2+ is converted to the well known
complex Hg(dithizonate)2 (kmax = 485 nm, e = 70,500
the different solvent properties. Although Hg2 is rela-
tively stable in aqueous solution, the dismutation be-
comes more favorable in acetonitrile [13,14]. However,
it is also conceivable that other influences such as the
method of preparation of colloidal gold contribute to
the different dismutation behavior in both cases. Why
2þ
does colloidal gold catalyze the dismutation of Hg2
2þ
?
Generally, the disproportionation of Hg2 takes place
ꢁ1 cmꢁ 1), which is used for the spectrophotometric
identification and determination of Hg2+ [8,9]. The
analysis shows that roughly one half of the added
when a ligand is present which stabilizes Hg2+. This lig-
M
and extracts Hg2+ from Hg2
[15,16]:
leaving behind Hg0
2þ
2þ
Hg2 is recovered as Hg(dithizonate)2. It follows that
2þ
2þ
2þ
under our reaction conditions Hg2
according to the equation:
dismutates
Hg22þ þ nL ! ½HgI–HgILnꢀ ! Hg0 þ ½HgIILnꢀ
ð4Þ
2þ
In contrast, colloidal gold pulls out Hg0 from Hg2
by amalgamation and releasing Hg2+
:
Hg2 ! Hg0 þ Hg2þ
In the absence of colloidal gold, the solution of
Hg2(ClO4)2 in acetonitrile remains stable as indicated
by the invariance of the absorption spectrum
ð3Þ
2þ
2þ
I
I
0
0
Hg22þ þ Aun ! ½Hg –Hg ꢂ ꢂ ꢂ Aun ꢀ
! Hg2þ þ Hg0Aun
ð5Þ
0
2þ
ðHg2 : kmax ¼ 232 nm; e ¼ 27; 600Þ [10,11]. When
2þ
In conclusion, the dismutation of Hg2 in CH3CN is
catalyzed by gold nano-particles. The first step of this
catalysis is apparently induced by the extraction of
only small amounts of Hg2(ClO4)2 are added to the
solution of colloidal gold, elemental mercury is also
formed but remains suspended as a colloid. It is recog-
nized by an absorption which extends from the visible
to the UV spectral region and increases towards short-
er wavelength (Fig. 1) owing to light scattering by the
colloidal particles. Moreover, the plasmon absorption
of colloidal gold undergoes a blue shift from 525 to
495 nm and a new maximum appears at approximately
370 nm. These observations can be attributed to the
Hg0 from Hg2 by gold. Amalgam formation seems
2þ
thus to be the driving force of this catalysis.
References
[1] M. Haruta, Chem. Rec. 3 (2003) 75.
[2] R.J. Davis, Science 301 (2003) 926.
[3] M.B. Cortie, E. van der Lingen, Mater. Forum. 26 (2002) 1.
[4] M. Jacoby, Chem. Eng. News July 5 (1999) 33.
[5] W. Beck, W.P. Fehlhammer, P. Po¨llmann, E. Schuierer, F. Feldl,
Chem. Ber. 100 (1967) 2335.
[6] A. Vogler, C. Quett, H. Kunkely, Ber. Bunsenges. Phys. Chem. 92
(1988) 1486.
[7] H. Kunkely, A. Vogler, Inorg. Chem. Commun.
553.
6 (2003)
[8] L.S. Meriwether, E.C. Breitner, N.B. Colthup, J. Am. Chem. Soc.
87 (1965) 4448.
[9] B. Borderie, D. Lavabre, G. Levy, J.C. Michean, J. Chem. Soc.
Educ. 67 (1990) 459, and ref cited therein.
[10] M.R. Mason, Inorg. Chem. 22 (1983) 147.
[11] A. Vogler, H. Kunkely, Inorg. Chim. Acta 162 (1989) 169.
[12] A. Henglein, M. Giersig, J. Phys. Chem. B 104 (2000) 5056.
[13] J.F. Coetzee, J.J. Campion, D.R. Liberman, Anal. Chem. 45
(1973) 343.
Fig. 1. Electronic absorption spectrum of colloidal gold (a) obtained
by the photolysis of 2.9 · 10ꢁ4 M Bu4N[Au(N3)4] in CH3CN (room
temperature, 1-cm cell) and the spectrum of this solution (b) in the
presence of 2.9 · 10ꢁ4 M Hg2(ClO4)2 · 4H2O.
[14] L.N. Balyatinskaya, A.P. Kreshkov, Talanta 29 (1982) 153.
[15] R.A. Potts, A.L. Allred, Inorg. Chem. 5 (1966) 1066.
[16] I. Sanemasa, Inorg. Chem. 16 (1977) 2786.