The Journal of
Physical Chemistry
Q Copyright, 1992, by the American Chemical Society
VOLUME 96,NUMBER 21 OCTOBER IS, 1992
LETTERS Electrochemistry of Mixed Metal Clusters in Aqueous Solution: Reduction of Ag+ by the Lead Atom Anton Fojtik, Amim Henglein,* and Eberhard Janata Hahn-Meitner-Institut GmbH, Bereich S, 1000 Berlin 39, FRG (Received: May 18, 1992; In Final Form: July 30, 1992)
The electron transfer between lead and silver particles in the micro size range, where conventional electrode-potential considerations are not applicable, is studied by pulse radiolysis. In deaerated solutions containing 1 X 10-* M Pb(C104)2, 2.5 X 1C5-2.0X 10-4 M AgCIO,, and 1 X 1W2M NaC02H,Pb+ is the first and metallic silver the fml product of reduction, and no metallic Pb particles are formed as intermediates. A mechanism is proposed in which the crossover from reduced lead to reduced silver occurs via the mixed leadsilver cluster (PbAg)+,which absorbs weakly at 460 nm. In the reaction of this cluster with Ag+, the Pb component is oxidized and dimeric silver is formed that starts an agglomeration chain leading to larger metallic Ag particles. During these agglomeration steps, intermediate oligomeric silver species, which absorb at 330 and 365 nm, are detected. The results are discussed in the framework of our limited knowledge about the redox potentials of small nonmetallic lead particles.
Introduction
Whereas the electrochemistry of metals has been well established since the beginning of this century, the electrochemical behavior of small clusters of metals in the atom-temetal transition range (with inQeasing particle size) has not yet been fully explored. Pulse radiolysis is a method for generating metal atoms in aqueous solution, which form clusters and then finally colloidal particles of metallic character. During the growth, the optical and electrochemical properties of the particles An important question, for example, is at which particle size does the typical metal absarption spectrum first appear? Typical studies have been made with silver3and lead4 solutions. In the present studies, a mixed system, Le., a system containing both lead and silver, is investigated. To describe the problem, let us first consider a convenrional electrochemical experiment: silver ions in the form of AgC10, are added to the solution of colloidal lead particles. The lead particles, which have a diameter of about 6 nm and have metallic properties, possess a narrow
absorption band at 215 nm! This band disappears immediately upon the addition of the Ag+ ions, and the typical 380-nm absorption band of colloidal silver is produced. The reaction Pb,
+ 2nAg+
-
nPb2+
+ Ag2,
(1)
takes place, as the Ag+/Ag redox potential is much greater than that of the Pbz+/Pb system. The question now arises whether reaction 1 m r s when n is very small, for example, equal to one. Will the Pb atom or a small Pb cluster be able to undergo reaction 1, or will they grow by association until they have metallic character, enabling the trivial reaction of macroscopic electrochemistry to take place? The pulse radiolysis experiment is performed in the following manner: a deaerated solution containing both Pbz+and Ag+ ions as well as sodium formate is exposed to a 0.5-r(s pulse of highenergy radiation. The reducing species which arc generated during the pulse are hydrated electrons and carboxyl radical anions. Both Pb2+and Ag+ react rapidly with these reductants. However,
0022-3654f 92f 2096-8203SO3.00f 0 0 1992 American Chemical Society
8204 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992
Letters
L
0
VI
n
m2 0
20
40
60
80
Ills1
f
Figure 1. Kinetic traces at 660 nm for solutions containing various concentrations of Ag+. [Pb(CIO,),]: 1 X lo4 M; [NaCO,H]: 1 X lo-, M reducing radicals (ea; plus COT) were M, pH = 4. 1.2 X produced in the pulse.
because a large Pb2+/Ag+concentration ratio is used, practically only Pb2+ ions are reduced:
-
+
eaq- Pb2+ C02-
+ Pb2+
+
Pb+
(2)
Pb+ + C02
(3)
These reduction reactions also occur during the pulse under the concentration conditions chosen. Thus, a well-known concentration of Pb+ is instantaneously generated at zero time. It is known that the Pb+ ions-in the absence of Ag+-react after the pulse with each other to form PbO atoms which then agglomerate until metallic colloidal particles are formede4 In the presence of Ag+ ions, the final product is certainly colloidal silver. When does the transfer of the electrons, which were first deposited on lead ions, to the silver ions occur? The question is not trivial, if one remembers that the standard reduction potential Eo of the redox couple Ag+[Ago in aqueous solution is as negative as -1.8 V'*2.596 and that Pb is "more noble" with Eo = -0.93 V.* These potentials for the free atoms were calculated from the known standard reduction potentials of the compact electrodes and the free enthalpies of sublimation of the metals; the free enthalpies of hydration of the atoms were assumed to be negligible in these calculations. In the case of the silver atom, it was found experimentally that it is a strong electron donor in aqueous solution.5 The pulse radiolysis equipment has been described previously' (0.5-ps pulses; 3.8 MeV).
Results The Pb+ ions which are formed during the pulse react rapidly with each other to yield PbO atoms: 2Pb+
-
Pbo
+ Pb2+
(4)
(2k = 8.2 X lo9 M-l s-')! As Pbo has an absorption band at 660 nm, the bimolecular reaction of the Pb+ ions can be monitored at this wavelength. After reaching a maximum, the 660-nm absorption decreases, as the Pbo atoms disappear via
-
2Pb0 Pb2 (5) (2k = 1.5 X lo9 M-I s-')? Curve 0 in Figure 1 shows the 660-nm absorption as a function of time for a solution in which silver ions were not present. The other curves were obtained with solutions containing Ag+ at different concentrations. The buildup of the 660-nm absorption also occurs in the presence of Ag+, although the maximum is lower and is reached at a shorter time than in the absence of Ag+. Whereas the decay of the 660-nm absorption occurs via second-order kinetics in the absence of Ag+, it follows pseudo-first-order kinetics at higher Ag+ concentrations, the half-life being reciprocally proportional to the concentration of Ag+. It is concluded that Pbo is able to react with Ag+. Reasons are given below for the cluster (PbAg') being the product of this reaction. The rate constant for the reaction Pbo + Ag+ (PbAg)+ (6)
-
400
I
I
500
600
700
A Inml Figure 2. Long-wavelength spectrum of a solution containing 5 X M Ag+ ions at various times after the pulse. Other concentrations as in Figure 1.
1 300 400
520
X lnml Figure 3. Absorption spectrum of a solution at various times after the pulse. 1 X lo4 M Pb(CIO,),; 5 X M AgCIO,; 1 X to-, M NaHCOz. Radical concentration as in Figure 1.
is 8.2 X lo8 M-' s-I as obtained from a computer fit to the curva in Figure 1. The long-wavelength spectrum at different times after the pulse is shown in Figure 2 for a solution containing 5 X M Ag+ ions. It can be seen how the 660-nm absorption of Pbodisappears. A new absorption at 460 nm is also built up. This absorption is not produced in the absence of Ag+ ions; it is therefore attributed to the (PbAg)+ cluster formed in reaction 1. As can be seen, the 460-nm absorption is not long-lived, which indicates that the cluster undergoes further reaction in the 10% range. The spectrum of the solution at shorter wavelengths is shown in Figure 3. Note that the absorptions in the UV range are much stronger than those in the visible wavelength range in Figures 1 and 2. At 25 ps after the pulse, the spectrum contains a maximum at 305 nm. At this time, reactions 5 and 6 are not yet complete; thus, some Pb+ (maximum at 320 nm4) and some Pbo (maximum at 290 nm4) are still present. Under the concentration and pulse dose conditions in F w e 3, reaction 5 can practically be neglected. Within 160 ps after the pulse, a strong absorption at 260 nm is built up. At 400 ps, the 260-nm absorption has decreased a little and the absorption in the 300-400-nm range has increased. Most noteworthy are the absorption bands at 330 and 365 nm, which are superimposed on the 300-400-nm absorption underground. These bands have previously been observed in the pulse radiolysisa and in the y-radioly~is~+'~ of silver salt solutions; they were attributed to intermediate oligomeric clusters of silver. It can thus be said that the transfer of the electrons from lead to silver has occurred rather rapidly in the experiment of Figure 3, i.e., at a time shorter than 400 ps. At longer times, the bands of the oligomeric species disappear and the plasmon absorption band in the 380-400-nm range of larger metallic silver particles is built UP. The build up of the absorption at 260 nm at times shorter than 400 M S is rather complex as can be seen from Figure 4 where
The Journal of Physical Chemistry, Vol. 96, No. 21, I992 8205
Letters
+ Ago F? Ag+ + e-
Pb+ $ Pb2+ e-
0 1 ' 0
'
I
100
'
I
200
'
'
300
'
' 400
t Ips1 Figure 4. Kinetic traces at 260 nm for various doses and Ag+ wnccntrations. The doses are expressed as the concentrations of the reducing radicals generated.
kinetic traces are shown for two silver concentrations and various doses in the pulse. To compare the shape of the curves at different doses, the entity c is plotted on the ordinate scale. t is obtained by dividing the observed absorbance signal by the concentration of radicals (ea; plus COZ-) produced in the pulse and by the thickness of the optical cell (1 -5 cm). Immediately after the pulse, the absorption is increased as Pb+ is formed during the pulse. We first regard the traces for [AB+] = 6.0 X lW5M. At the highest dose applied, Le., the production of 2 X l e 5 M radicals in the pulse, reaction 4 has a first half life of only 6.3 p. Reaction 6 occurs with a half life of 14 p. Thus, after roughly 30 p, (PbAg)' formation is 75% complete. During this time, the 260-nm absorption increases which is attributed to (PbAg)+ absorbing more strongly than Pb+ at 260 nm. However, the absorption continues to increase substantially until a maximum is reached at 100 p. This additional increase follows another rate law as can be seen from the initially convex shape of the 260-nm traces. The additional increase becomes faster with increasing Ag+ concentration. This indicates that the fmt product, (PbAg)+, which is essentially formed during 30 p, undergoes reaction with Ag+ ions. The following process is postulated: (PbAg)'
+ Ag+
+
+
Pb2+ Ag2
(7)
Reaction 7 is the process in which the transfer of the reduction equivalents from lead to silver occurs. Ag2 is the particle that possesses the strong 260-nm absorption (see 160-ps curve in Figure 3). Upon its agglomeration to yield oligomeric silver clusters and finally metallic particles, the 260-nm absorption decreases. At the lower pulse doses in Figure 4, reaction 4 is slowed down substantially: the first half-life of reaction 4 amounts to 63 and 126 pa at the respective doses of 2 X 10-6 and 1 X 10-6 M radicals, which is now much longer than those of the pseudo-first-order processes 6 and 7. Thus, the rate of reaction 4 determines the rate of the increase in absorption due to Ag, formation and a change in Ag+ concentration has little influence on the rate of the Ag, buildup.
Mscrrclsion The detailed discussion of redox reactions of metal clusters requires knowledge of their thermodynamic properties. These data are known to only a limited extent as pointed out in a recent consideration of the redox potentials of silver clusterse6 Most of the information stems from gas-phase studies, such as mass spectrometry of metal vapors,11J2electron exchange as studied by cylotron rcsonance,I3 molecular beam experiments,14 and theoretical calculati~ns.~~ When using gas-phase data to describe clusters in solution, one also needs the hydration free enthalpies. No data arc available here. However, as pointed out previously, as long as neutral clusters are considered, the hydration energies may be neglected, as they are expected to be ~ m a l l . ~In. ~some cases,known dissociation energies are used instead of free enthalpies of dissociation; again, the neglect of the accompanying entropy effects seems admissible. Pb+, the first reduction product, cannot reduce a silver ion as is seen from the standard potentials of the redox systems involved:16
Eo = -1.0 VI7
(8)
Eo = -1.8 V5*6
(9)
The transfer of an electron from Pb+ to Ag+ would be endoergic by 0.8 eV. In fact, the Pb+ ion reacts with Pb+ to yield Pbo within some s after the pulse as in the absence of silver ions. However, the 660-nm absorption of Pbo disappears much more rapidly in the presence of AB+, the rate being proportional to the Ag+ concentration (Figure 1). It is therefore concluded that Pbo reacts with Ag+. However, the transfer of an electron to form Ago is not possible as the standard potential of the system
+
PbO F? Pb+ eEo = -0.92 V2 (10) is not negative enough. It must be concluded that PbO forms an adduct with Ag+ (eq 6). This product reacts further with Ag+ to build up the absorption of Ag2within about 100-200 p after the pulse, depending on dose rate and silver ion concentration. The overall reaction taking place is the sum of reactions 6 and 7: Pbo + 2Ag+
-
+
Pb2+ Ag,
(1 1) To examine the energetics of this overall reaction, the following cycle of reactions is considered:
- -+ + - +
PbO 2e-
net:
Pbo
Pb2+ 2e-
+ 2Ag+
2Ag0
AG = -1.92 eV2 AG = +3.6 eV5v6 AG = -1.65 eVII
2Ag0
Ag,
2Ag+
Pb2+ Ag2
(12) (13) (14)
AG = +0.03 eV (11)
Thus, reaction 11 is practically thermoneutral. However, as it is rather fast, it must have a negative value of AG. One must conclude that the redox potentials used in the above calculation are not known well enough to predict the energetics of a reaction with a small AG value. The free hydration enthalpies of the free atoms have to be taken into consideration, which is not possible at the present time. For example, if the free silver atom had a slightly negative value of AGhydr,say -0.1 eV, or if the free enthalpy of hydration of Ag, were more negative by 0.03 eV than that of PbO, reaction 11 would already be exoergic. Another source of error could lie in the values for the free energies of sublimation, which were used to calculate the standard potentials of the redox systems of eqs 12 and 13. One also has to conclude that the binding energy of the (PbAg)+ cluster must be very low, as otherwise reaction 7 would become too endoergic to occur rapidly. The thermodynamics of the overall reaction 11 would also become more favorable, if the dimeric silver molecule formed would be complexed by Ag+. In fact, in the previous studies on silver agglomeration processes, the formation of complexed particles such as Ag3+and Ah2+in solutions containing excess silver ions have been discussed.18 To summarize the results of the experiments and of the potential considerations, it can be said that the transfer of the electrons from the lead to the silver component of the solution occurs neither in the very first step, Le., via interaction of Pb+ with Ag+, nor via the normal electrochemical reaction through the formation of larger metallic lead particles. The transfer occurs through PbO, Le., still at a very early stage, although the thermodynamic driving force is certainly very small.
Acknowledgment. The authors thank Dr. Paul Mulvaney for valuable discussions and reading the manuscript.
References and Notes (1) (2) (3) (4) (5) (6) (7) (8) (9)
Henglein, A. Top. Curr. Chem. 1988, 143, 113. Henglein, A. Chem. Rev. 1989, 89, 1861. Henglein, A.; Tausch-Trend,R. J . Colloid Znrerfoce Sd. 1981,80,84. Henglein, A.; Janata, E.; Fojtik, A. J . Phys. Chem. 1992, 96, 4734. Henglein, A. Ber. Bunsen-Ces. Phys. Chem. 1977,81, 556. Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 600. Kumar, A.; Janata, E.; Henglein, A. J . Phys. Chem. 1988.92, 2587. Mulvaney, P.; Henglein, A. Chem. Phys. Lett. 1990, 168, 391. Henglein, A. Chem. Phys. b i t . 1989, 154, 473.
J . Phys. Chem. 1992,96, 8206-8209
8206
(10) Mostafavi. M.; Keghouche, N.; Delcourt, M.-0.; Belloni, J. Chem. Phys. Lett. 1990, 167, 193. (1 1) Hilpert, K.; Gingerich, K. A. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 739. (12) Gingerich. K. A.; Cccke, D. L.; Miller, F. J . Chem. Phys. 1976,64, 4027. (13) Chceacman, M. A,; Eyler, J. R. J . Phys. Chem. 1992, 96, 1082. (14) GantefBr, G.; Gausa, M.; Meiwcs-Brocr, K.-H.; Lutz, H. 0.J . Chem. Soc., Faraday Trans. 1990,86, 2483.
(15) Bauschlicher, C. W.;Langhoff, S. R.;Partridge, H. J . Chem. Phys.
1990, 93, 8133.
(16) In writing the redox equations (electron, e-, on the right side, and using the standard reduction potential without change in sign) we follow the method of: Pourbaix, M. Atlas of Electrochemical Equniibrla; Pergamon Press: Oxford, 1966. (17) Breitenkamp, M.; Henglein, A.; Lilie, J. Ber. Bunsen-Ges. Phys. Chem. 1976,80, 973. (18) Mulvaney, P.; Henglein, A. J . Phys. Chem. 1990, 94, 4182.
ReaCSpace Imaging of Molecular Sieves Composed of Aluminum Phosphates and Their Metal-Substituted Analogues hatibha
L.Gai-Boyes,
Central Research & Development, E . I. Du Pont de Nemours Experimental Station, Wilmington, Delaware 19880-0356
John Meurig Thomas,* Paul A. Wright, Richard H. Jones, Srinivasan Natarajan, Jiesheng Chen, Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W l X 4BS
and Ruren Xu Department of Chemistry, Jilin University, Changchun, People's Republic of China (Received: June 23, 1992; In Final Form: August 12, 1992)
High-resolution electron microscopic images of representative title materials, notably JDF-2and CoALPO-5, both of which are extremely beam sensitive, have been recorded and compared with computed images. It is shown that real-space images, coupled with information derived from electron diffraction, distanceleast-squares,and energy minimization procedures, form the basis of a new method for determining the three-dimensional structure of aluminum phosphates (ALPOs) and their metal-substituted analogues (MeALPOs).
Introduction Our understanding of the structures and properties of naturally occurring and synthetic molecular sieves of the aluminosilicate (zeolitic) type has been greatly enlarged by real-space imaging using transmission electron microscopy. From the time when high-resolution elcctron " p y (HREM) was fmt successNly applied to image the structures of zeolite A' and ZSM-5,* considerable progress has been made with this technique. Thus,direct proof for the existence of an infinite family of ordered intergrowths of ZSM-5 and ZSM-11 has becn ~ b t a i n e d ;the ~ structures of several zeolites that had defied solution by X-ray methods (e.g., ZSM-23?y5 Theta- 1: ECR- 1 zeolite beta*) were all established principally through vital clues provided by HREM, and new insights have been gained into the nature of recurrent, as well as nonrecurrent, but nonrandom intergrowths in several distinct families such as zeolites ABC-69,10and ZSM-3/ZSM-20.11-14 If only such HREM studies were applied to the much larger families of ALP&, MeALPOs, and SAPOS (silicon aluminum many of which have not yet had their structures detennined-principally because they do not form crystals suitable for single-crystal X-ray analysissimilar progress could be expected in our appreciation of the properties of this additional family of molecular sieves and catalysts. The application of HREM to these interesting solids has, however, been much hampered by their extraordinary tendency to become amorphous when exposed to electron beams of the intensities required for imaging at the requisite degree of resolution. This kind of experimental difficulty was encountered in the early days of imaging of aluminosilicate zeolites. It was overcome for that class of solid by a combination of approaches: use of higher
,'
0022-3654/92/2096-8206$03.00/0
acceleratingvoltages (200,in m e cases lO00, instead of 100 keV) in the electron microscope (so as to minimize the production of inelastic collisions which are principally responsible for the structural degradation of the sample); employing a better quality vacuum in the specimen chamber, so as to reduce greatly the presence of water vapor, a key agent for triggering the loss of crystallinity; and, when possible, dduminating the aluminosilicate, i.e. replacing the AI3+ by Si4+in the framework without loss of structural integrity. (For reasons that are not well understood, A I 4 bonds are more susceptible to rupture under electron irradiation than S i 4 bonds.) The first two options for improving beam stability of ALPOs and MeALPOs have been followed hm; the third is not open to us. Even so, it transpires that ALPOs and MeALPOs are extraordinarily sensitive to electron irradiation, and this is the principal reason why no high-resolution images of these solids have been hitherto reported. Previous studied9 have underlined the magnitude of the experimental difficulties encountered in producing high-resolution, real-space images of ALPOs (contrast the situation for, say, oxide superconductorsm). Experimental Section Using accelerating voltages of 300 keV in a Philip CM30 electron microscope with a super twin (ST)objective lens, we have succeeded in imaging representative ALPOs and MeALPOs in such a manner as to encourage us to believe that, soon, progress in structural elucidation comparable to that achieved for aluminosilicates will also be achievd with ALPOs and related materials. High-precision microanalysis on a nanometer scale was carried out simultaneously using a windowless energy-dispersive X-ray (EDX) detector. The spherical aberration coefficient (C,) 0 1992 American Chemical Society
