Formation of the silver hydride ion AgH+ upon the reduction of silver ions by H· in aqueous solution. A pulse radiolysis study

Article abstract of DOI:10.1524/zpch.2007.221.3.403

The reaction of hydrogen atoms and of alcohol radicals with monovalent silver ions is studied in aqueous solution by means of pulse radiolysis. In acidic aqueous solutions containing silver ions and an alcohol, hydrogen atoms react with silver ions to form AgH⁺ with a rate constant of k = 1.5 × 10¹⁰ dm³ mol^-1 s^_1. This complex dissociates into free silver atoms and hydrogen ions with a first order rate constant of 2 × 10⁷ s^-1. A reaction of alcohol radicals with silver ions could not be observed in aqueous solutions containing either methanol, ethanol or 2-propanol during times of up to 15 ms; the rate constant would therefore have to be smaller than 1 × 10⁴ dm³ mol^-1 s^-1. by Oldenbourg Wissenschaftsverlag.

Full text of DOI:10.1524/zpch.2007.221.3.403

Z. Phys. Chem. 221 (2007) 403–413 / DOI 10.1524/zpch.2007.221.3.403
© by Oldenbourg Wissenschaftsverlag, München
Formation of the Silver Hydride Ion AgH+
upon the Reduction of Silver Ions
•
by H in Aqueous Solution.
A Pulse Radiolysis Study
∗
∗∗
By A. Sarkar and E. Janata
Hahn-Meitner-Institut Berlin GmbH, Bereich Solarenergieforschung,
Glienicker Str. 100, D-14109 Berlin, Germany
Dedicated to Prof. Dr. Arnim Henglein on the occasion
of his 80th birthday
(Received May 10, 2006; accepted September 27, 2006)
Silver Hydride Ion / Silver Ion / Hydrogen Atom / Alkyl Radical /
Pulse Radiolysis
The reaction of hydrogen atoms and of alcohol radicals with monovalent silver ions is
studied in aqueous solution by means of pulse radiolysis. In acidic aqueous solutions
containing silver ions and an alcohol, hydrogen atoms react with silver ions to form
AgH+ with a rate constant of k = 1 5×1010 dm3 mol−1 s−1. This complex dissociates
into free silver atoms and hydrogen ions with a first order rate constant of 2×107 s−1
.
A reaction of alcohol radicals with silver ions could not be observed in aqueous solutions
containing either methanol, ethanol or 2-propanol during times of up to 15 ms; the rate
constant would therefore have to be smaller than 1×104 dm3 mol−1 s−1
.
1. Introduction
The comprehension of fundamental reactions in the reduction of metal ions
is essential for the understanding of subsequent colloid formation. In this pa-
per we address the reduction of silver ions by hydrogen atoms in aqueous
solution, and, because it could serve as an additional source of reduced sil-
ver, the reaction of silver ions with alcohol radicals, as well. There are three
*
On leave of absence from the Radiation and Photochemistry Division, Bhabha
Atomic Research Centre (BARC), Mumbai 400 085, India
Corresponding author. E-mail: janata@hmi.de
**
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A. Sarkar and E. Janata
reports [1, 2] available which deal with the reduction of silver ions by hy-
drogen atoms and in which rate constants of 1 2×1010 dm3 mol−1 s−1 [3],
2 8×1010 dm3 mol−1 s−1 [4] or 1×1010 dm3 mol−1 s−1 [5] were given. The
reduction of silver ions by alcohol radicals has led to contradictory re-
sults in the past. For pulse radiolysis experiments, some reports state that
this reaction does not occur at all [6, 7], while others include this reaction
in their reaction mechanisms [8, 9]. For gamma irradiation, this reduction
has obviously been observed [10, 11]. But no value for the rate constant is
given.
In order to gain more insight into the reduction of silver ions by hydrogen
atoms in aqueous solutions we have re-investigated this reaction, in the course
of which a short-lived complex of silver hydride ions is evidently being formed.
The reduction of silver ions by alcohol radicals was also investigated with great
care. No trace of such a reaction was found in pulse radiolysis experiments dur-
ing the period the alcohol radicals are present in solution before they dimerize
to dihydroxy alcohols.
2. Experimental
The pulse radiolysis apparatus [12, 13] and the computer software [14, 15] of
the 4 MeV van de Graff accelerator facility ELBENA at the Hahn-Meitner-
Institut were described earlier. The optical detection system in the UV range
consists of an intensified XBO 450 lamp [16], several lenses and mirrors, a UV
band-pass filter in front of the monochromator (Bausch&Lomb, 33-86-75) [17]
and an R7400U photomultiplier tube (Hamamatsu) in a nine-stage configura-
tion [18]. The high transmission UV band-pass filters (UV-R types, Schott)
are centered every 30 nm from 220 nm to 340 nm. They are inserted in order
to avoid stray-light effects. The photo detector was connected to the digital
recorder (Tektronix TDS7104) via a baseline compensation circuit [19] em-
ploying a special adapter [20] in order to keep the time constant of the detection
electronics as small as possible. The charge per pulse was measured by inte-
grating Cherenkov emission in a “foil” of fibers [21]. Further details of the
pulse radiolysis apparatus can be found elsewhere [22].
Nitrous oxide and argon were of ultra high purity. Solutions were prepared
daily using HQ-water (Millipore). After purging two liters of this water with
argon for two hours, the reactants were added. After that, the solution was
purged further either with argon or nitrous oxide for another hour. The pH of
the HQ-water was measured to be 6.2 before bubbling and 6.8 after bubbling
and pH will be referenced here as “natural”. HClO4 was used to adjust pH.
Experiments were carried out at 23 ◦C.
Some of the experiments were carried out at low pH. At this pH the kinetic
salt effect need to be considered. The rate constants given here were those for
natural pH.
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Formation of the Silver Hydride Ion AgH+ upon the Reduction of Silver Ions
405
Fig. 1. Kinetic curves at 360 nm. Solutions: 1 ×10−4 mol dm−3 AgClO4, 0 01 mol dm−3
HClO4, purged with Ar and containing 0 1 mol dm−3 MeOH (curve a), 0 01 mol dm−3
EtOH (curve b), or 0 01 mol dm−3 2-PrOH (curve c). Irradiation dose corresponds to
−
2 5×10−6 mol dm−3 of eaq (G = 2 6), pulse duration 15 ns (curve a) or 100 ns (curves b
and c). Each curve represents the average of 50 experiments. Curve for reaction (0) is also
added (dashed line).
3. Results and discussion
The reduction of silver ions by hydrogen atoms is generally assumed to occur
according to
H• +Ag+ → Ag0 +H+
(0)
This reaction is studied here in acidic solutions where hydrated electrons
of the water radiolysis are converted into hydrogen atoms. The solutions also
contain an aliphatic alcohol for scavenging hydroxyl radicals. Figure 1 shows
the increase in absorption vs. time after irradiation with a short electron pulse,
the irradiated solution contains silver ions and either methanol, ethanol or
2-propanol. On a shorter time scale, the build-up in absorption is also shown
in Fig. 2 for the same silver ion concentration (curve b) and for a concen-
tration five times higher (curve a). Clearly, a delay in absorption build-up
can be seen in both figures, the calculated absorption as predicted by re-
action (0) is included as the dashed lines for the sake of comparison. As
shown in Fig. 1, the initial increase in absorption is the same, for all three
alcohols. Also, the initial increase does not change on this time scale when
the hydrogen ion concentration is varied from 0.01 to 0 05 mol dm−3. In
Fig. 3 the absorption vs. time curves are shown for a silver solution con-
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Fig. 2. Kinetic curves at 360 nm and analysis of curve b. Solutions: 5 ×10−4 mol dm−3
(curve a) or 1×10−4 mol dm−3 (curve b) of AgClO4, 0 05 mol dm−3 HClO4, 0 01 mol dm−3
−
2-PrOH, purged with Ar. Dose corresponds to 5×10−6 mol dm−3 of eaq (G = 2 6), pulse
duration 5 ns. Each curve represents the average of 50 experiments. Calculated contribu-
+
tions of Ag0 (dotted line), AgH+ (dash-dotted line) and Ag2 (dash-dotted-dotted line),
and their sum (solid line). Curve for reaction (0) is also added (dashed line).
taining alcohol and purged with N2O and for various hydrogen ion concen-
trations between natural pH and pH 2. At natural pH, all hydrated electrons
are scavenged by nitrous oxide so that only the hydrogen atoms generated
during water radiolysis react with silver. At lower pH, however, more and
more hydrated electrons escape scavenging and react with hydrogen ions
to form additional hydrogen atoms, i.e., the amplitude of absorption in-
creases with lower pH. As can be seen from Fig. 3, the shape of all curves
is independent of pH, i.e., the absorption maximum occurs at about one
microsecond, followed by a decay. In all cases, a certain amount of hy-
drogen atoms are lost for the reaction with silver because they react also
with the alcohol. The alcohol radicals which were generated either by the
reaction of hydrogen atoms or by hydroxyl radicals with the alcohol con-
tribute to the absorption as well, see Fig. 3. The shape of the spectrum
obtained at about one microsecond after irradiation (not shown) meas-
ured in solutions of Figs. 1 or 2 resembles strongly to that of free silver
atoms [23], although slightly lower in amplitude. Moreover, the subsequent
formation of the dimer and trimer silver clusters is observed as the ab-
sorption at 360 nm fades away. In order to explain the induction period,
we propose here that the reaction of hydrogen atoms with silver ions oc-
curs via the formation of an intermediate complex of silver hydride ions
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Formation of the Silver Hydride Ion AgH+ upon the Reduction of Silver Ions
407
Fig. 3. Absorption vs. time curves at 360 nm at various pH for a solution containing 1×
10−4 mol dm−3 AgClO4, 0 01 mol dm−3 2-PrOH and purged with N2O. Irradiation dose
corresponds to 2 5×10−6 mol dm−3 of eaq−. Each curve represents the average of 50 ex-
periments.
according to
H• +Ag+ → AgH+
(1)
Because the decay of the 360 nm absorption at times longer than one mi-
crosecond as well as the simultaneous formation of the silver dimer and trimer
clusters follow a pseudo first-order law, it is further suggested that the silver
hydride ions subsequently dissociate yielding free silver atoms and hydrogen
ions
AgH+ → Ag0 +H+
(2)
In acidic solution, hydrated electrons generated in water radiolysis are con-
verted into hydrogen atoms via
eaq − +H+ → H•
(3)
thus enhancing the amount of hydrogen atoms in these experiments. Hydrated
electrons can also react directly with silver ions
eaq − +Ag+ → Ag0
(4)
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408
A. Sarkar and E. Janata
While the reaction of hydrogen atoms with alcohol
H• +RH → R• +H2
(5)
competes with reaction (1), scavenging of hydroxyl radicals by the alcohol
•OH+RH → R• +H2O
(6)
(7)
prevents oxidation of silver ions via
•OH+Ag+ → AgOH+
Silver atoms created according to reactions (1) and (2) react with silver ions
in solution as reported earlier [23]
Ag0 +Ag+ → Ag2
(8)
(9)
+
Ag2+ +Ag+ → Ag3
2+
A reaction of alkyl radicals with silver ions according to
R• +Ag+ → Ag0 +R+
(10)
would create additional silver atoms. Such a reaction was, however, not ob-
served as discussed below in detail.
A direct dissociation of silver hydride ions according to reaction (2) is
assumed here, although, if this species exhibits hydride characteristics, one
would expect an additional reaction with hydrogen ions to yield Ag2+ and H2.
This assumption is made because in the experiments described above, i) no in-
fluence of the hydrogen ion concentration on the initial build-up of absorption
can be seen, ii) the spectrum observed at about one microsecond can clearly
be assigned to that of the free silver atom, and, iii) the formation of the sil-
ver dimer and trimer colloids can be observed and identified by their sequential
build-up at 310 nm and 260 nm.
Silver hydride ions as the intermediates in the reaction of hydrogen atoms
with silver ions were postulated several years ago [24–26, 32].
The experiments of Figs. 1 and 2 are carried out in acidic silver solution
in which the hydrogen ion concentration is large enough so that hydrated elec-
trons react predominantly with hydrogen ions to form hydrogen atoms. Their
formation time is short compared to the time scale of the experiments. Adding
various aliphatic alcohols, such as methanol, ethanol or 2-propanol to the so-
lution shows practically no influence on the initial increase in absorption of
free silver atoms, as shown in Figs. 1 and 2. Their concentration is, however,
critical because they should ideally scavenge all hydroxyl radicals without re-
acting with the hydrogen atoms. Tert.-butyl alcohol was not used because of its
instability in acidic solutions [27].
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Formation of the Silver Hydride Ion AgH+ upon the Reduction of Silver Ions
409
reaction
rate constant k
ref.
H•
+
Ag+
→
→
→
→
→
→
→
→
→
→
→
AgH+
Ag0
H•
1 5×1010 dm3 mol−1 s−1
2×107 s−1
X
X
[1]
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
AgH+
+ H+
−
eaq
eaq
H•
•OH
•OH
Ag0
+
+
+
+
+
+
+
+
+
+
+
+
H+
2 3×1010 dm3 mol−1 s−1
4 8×1010 dm3 mol−1 s−1
−
Ag+
RH
RH
Ag+
Ag+
Ag+
Ag+
H•
Ag0
R•
[23]
+ H2
R•
AgOH+
+ H2O
1 2×1010 dm3 mol−1 s−1
8 5×109 dm3 mol−1 s−1
2×109 dm3 mol−1 s−1
1×104 dm3 mol−1 s−1
7 75×109 dm3 mol−1 s−1
5 5×109 dm3 mol−1 s−1
5 5×109 dm3 mol−1 s−1
[31]
[23]
[23]
X
[1] (11)
[1] (12)
[1] (13)
(14)
+
Ag2
+
2+
Ag2
Ag3
R•
Ag0
H2
+ R+
H•
•OH
•OH → H2O2
−
−
eaq
R•
eaq
R•
→
→
H2
product
+ 2OH−
(X): this work
Rate constants for reaction−(15) [28,29]
k5 = 1 1×106 dm3 mol−1
k5 = 5 3×107 dm3 mol−1 s−1 for 2-propanol
s
for methanol
k5 = 1 8×107 dm3 mol−1 s−1 for ethanol
Rate constants for reaction (6) [28,30]
k6 = 9×108 dm3 mol−1 s−1 for methanol
k6 = 2 2×109 dm3 mol−1 s−1 for ethanol
k6 = 2×109 dm3 mol−1 s−1 for 2-propanol
Rate constants for reaction (14) [28,29]
2k13 = 2 4×109 dm3 mol−1 s−1 for methanol
2k13 = 1 5×109 dm3 mol−1 s−1 for ethanol
2k13 = 1 1×107 dm3 mol−1 s−1 for 2-propanol
Scheme 1. Reaction mechanism used in the computer simulation.
The kinetic curves for the formation of silver hydride ions and free silver
atoms could easily be fitted by computer simulation using the reaction mech-
anism of Scheme 1. Computer simulation could be carried out unambiguously
because the molar absorbtivities for the silver components [23], alcohol radi-
cals [28] and radiolysis products as well as the rate constants, except for those
of reactions (1) and (2), are well known. Also, the shape and the amplitude of
the calculated absorption curves depend sensitively on the rate constants for re-
actions (1) and (2) thus allowing their accurate determination. From fitting all
kinetic curves, i.e., the experimental data of Figs. 1, 2 and 3 as well as those
used for obtaining the spectrum of Fig. 4, the values for the rate constants for
reactions (1) and (2) were determined to be k1 = 1 5×1010 dm3 mol−1 s−1 and
k2 = 2×107 s−1.
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A. Sarkar and E. Janata
Fig. 4. Calculated concentration vs. time curves for curve+b of Fig. 2: H• (thick dotted
−
line), AgH+ (thick solid line), Ag0 (thick dashed line), Ag2 (thick dash-dotted line), eaq
•
(thin solid line), OH (thin dotted line), R• (thin dashed line).
The calculated contributions of silver hydride ions, of silver atoms and of
dimer silver clusters are added to the experimental curve of the lower silver
concentration (curve b) in Fig. 2. The corresponding concentration vs. time
curves for various transient species are depicted in Fig. 4. As time elapses,
a little less than half of the hydrogen atoms are already consumed by react-
ing with silver ions and the alcohol; the latter reaction accounts for 30% of the
total hydrogen atom consumption. The silver dimer cluster is formed only to
a few percent under these experimental conditions. The amplitude in absorp-
tion at longer times in Fig. 1 depends on the amount of hydrogen atoms being
scavenged by the various alcohols. The maximum absorption of silver hydride
ions occurs at 125 ns with low amplitude because these species vanish quickly
according to reaction (2). The spectrum of silver hydride ions, as a result of
computer simulation is shown in Fig. 5. The data scatters considerably due to
their low contribution in absorption. Nevertheless, the curve shows a maximum
at around 350 nm and increases at shorter wavelengths.
The reaction of alcohol radicals with silver ions according to reaction (10)
is also studied with great care because this reaction would severely influence
the results obtained above by serving as an additional source of reduced silver.
An aqueous solution containing silver ions, alcohol and purged with nitrous
oxide at natural pH, was irradiated. On the microsecond time scale, a small ab-
sorption with a maximum at 360 nm is observed immediately after irradiation
(not shown). This absorption is due to the formation of free silver atoms be-
cause a fraction of hydrated electrons escapes scavenging by nitrous oxide and
reacts directly with silver ions according to reaction (4). After this absorption
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Formation of the Silver Hydride Ion AgH+ upon the Reduction of Silver Ions
411
Fig. 5. Spectrum of AgH+ from computer analysis (marker) and fit (solid line). Experi-
mental conditions as for curve b in Fig. 2.
Fig. 6. Kinetic curve at 360 nm and analysis thereof. Solution: 2×10−4 mol dm−3 AgClO4,
2 mol dm−3 2-PrOH at natural pH and N2O saturated. Dose corresponds to 6 8×10−6
•
mol dm−3 of OH (G = 5 3). Curve represents the average of 800 experiments. Calculated
contributions of R• (dashed line) and the dihydroxy alcohol (dotted line).
disappears, another small absorption remains as shown in Fig. 6. This absorp-
tion decays during 15 ms to a small constant value; this decay clearly follows
a second order reaction law when varying irradiation intensity. The absorp-
tion on the millisecond time scale is identified as that of the respective alcohol
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A. Sarkar and E. Janata
radical by its molar absorbtivity and rate of decay [28–30] as it dimerize to
dihydroxy alcohol. On this time scale no further absorption due to the initial
formation of silver compounds can be observed, i.e., no clue for the reduc-
tion of silver ions by alcohol radicals generated from methanol, ethanol, or
2-propanol can be found. Computer simulation reveals that the rate constant,
if this reaction occurs at all, must be smaller than 1×104 dm3 mol−1 s−1. This
value is clearly too low for the reduction of silver ions by these alcohol rad-
icals in pulse radiolysis experiments, but the value might be large enough to
make this reaction possible in gamma radiolysis experiments where dose rate
is extremely low. Unfortunately, measurements of the transient conductance
were not possible at our laboratory due to the high intrinsic conductance of the
solutions.
4. Conclusions
The reaction of hydrogen atoms with silver ions in aqueous solution was
studied in great detail. The product of this reaction is assigned to a short-lived
complex of silver hydride ions, which dissociates subsequently into free silver
atoms and hydrogen ions. The rate constants for the formation and disappear-
ance of silver hydride ions have been determined as well as its spectrum. Any
absorption due to the reaction of some alcohol radicals with silver ions was
not detected by the pulse radiolysis experiments described above, which means
that the rate constant of this reaction would have to be very small.
Acknowledgement
In particular, the authors would like to thank Mr. J. Porsinger for running the
accelerator.
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