Ultrafast Scavenging of the Precursor of Hâ¢ Atom, (eâ, H3O+), in

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Ultrafast Scavenging of the Precursor of H Atom, (e, HO), in Aqueous Solutions •

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Anna Balcerzyk, Uli Schmidhammer, Furong Wang, Aurélien de la Lande, and Mehran Mostafavi J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b04944 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on August 1, 2016

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The Journal of Physical Chemistry

Ultrafast Scavenging of the Precursor of H• Atom, (e-, H3O+), in Aqueous Solutions Anna Balcerzyk, Uli Schmidhammer, Furong Wang, Aurélien de la Lande and Mehran Mostafavi* Laboratoire de Chimie Physique, CNRS/Université Paris-Sud, Bâtiment 349, 91405 Orsay, France

Abstract

Picosecond pulse radiolysis measurements have been performed in several highly concentrated HClO4 and H3PO4 aqueous solutions, containing silver ions at different concentrations. Silver ion reduction is used to unravel the ultrafast reduction reactions observed at the end of 7 ps electron pulse. Solvated electron and silver atom are observed by pulse (electron beam) - probe (supercontinuum light) method. In highly acidic solutions, ultrafast reduction of the silver ion is observed, a finding that is not compatible with a reaction between H• atom and the silver ion which is known to be thermally activated. In addition, silver ion reduction is found to be even more efficient in phosphoric acid solution than in neutral solution. In the acidic solutions investigated here, the species responsible for the reduction of silver atom is considered to be the precursor of H• atom. This precursor denoted (e-, H3O+), is constituted by a pair between an electron (not fully solvated) and H3O+. Its structure differs from the pair between solvated electron and hydronium ion (es-, H3O+) which absorbs in the visible. The pair (e-, H3O+), called here pre-H• atom, undergoes an ultrafast electron transfer and can like the pre-solvated electron reduce silver ions much faster than H• atom. Moreover, it is found that with the same concentration of H3O+, in phosphoric acid solution, the reduction reaction is favored compared to that in perchloric acid solution due to less efficient electron solvation process. The kinetics show that between three reducing species, (e-, H3O+), (es-, H3O+) and H• atom, the first one is the most efficient.

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Introduction Pulse radiolysis is a substantial tool to study the reactivity of the products of aqueous solution decomposition caused by ionizing radiation. Among the products of water radiolysis, reducing and oxidizing species are found, however the fate of radicals formed under radiation at the very short time scale is still not fully understood. Reactions taking place are so fast that for a long time the information about primary events was inaccessible. The solvation in water of the electron and the proton transfer by the water hole are fully accomplished at shorter time than1 ps (reaction 1 and 2 in Table 1). But in some conditions, reactions could take place on even shorter time scales. For example, it was shown that the electron transfer reaction of water hole, H2O•+ competes with the known proton transfer reaction; the water hole can oxidize a solute when the solute is at high concentration.1-8 For the excess electron also, reactions can occur before and during its solvation process. The reactivity of pre-solvated and solvated electron is crucial for understanding the chemistry of water under irradiation. However, in spite of the well-developed technique of pulse radiolysis, direct observation of the precursors of hydrated electron is still difficult at least in water where solvation dynamics is very fast and the time resolution of pulse radiolysis facilities are not sufficient to follow kinetics in the sub-picosecond range. One indirect method to study the reactivity the presolvated electron is the scavenging method. This method has been used for more than 50 years to probe solvated electron and radicals in the nanosecond range.9,10 With the development of the picosecond pulse radiolysis studies, the reactivity of the pre-solvated electron were quantified by measuring the initial absorption of solvated electron at short time.11,12,13 Recently, it was shown that in the case of the silver ion used as a scavenger, the solvated electron and the silver atom absorption band, in red and in UV, respectively, can be observed simultaneously after a 7 ps electron pulse revealing the reactivity of the pre-solvated electron.14 It is known that silver ions are not only reduced to silver atoms by pre-solvated or 2 ACS Paragon Plus Environment

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solvated electrons, it can also be reduced by hydrogen atoms.15 The formation of silver atom through the reduction of silver ions by solvated electrons (reaction 5) and the hydrogen atom (reaction 6 and 7) has been studied several decades ago (see Table 1).16,17 The silver ion reduction was considered as a model system in order to study the formation of colloidal metal particles by following the kinetics of intermediate cluster of silver atom.18

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According to

numerous studies, the rate constant of hydrated electron with silver ion (reaction 5) is found to be 4 × 1010 M-1 s-1. In contrast to this diffusion controlled reaction rate, the reaction of silver ion with hydrogen atom has a two times lower rate constant and is thermally activated.26 The formation of a pair between silver ion and H• atom, before the intra-pair electron transfer with forming silver atom was demonstrated (reaction 6 and 7).27 In the present study, reduction of silver ion is used as a probe to follow the reduction mechanism in acidic condition where all electrons are scavenged by H3O+. Perchlorate and phosphoric acid solutions are used for picosecond pulse radiolysis studies. In highly acidic solutions, unexpected results are obtained that cannot be explained by the thermally controlled reduction of silver ion by H• atom: an ultrafast silver atom formation is observed and measured within the 7 ps electron pulse. The efficiency of this reaction can be larger than in the case of neutral solutions where the pre-hydrated electron reduces silver ion.14 Based on the comparison of the yield of the silver atom at the end of the 7 ps electron pulse obtained in perchloric acid and in phosphoric acid solutions, the nature of the reducing species in these conditions is discussed. Experimental method The present work was performed at the experimental area EA-1 of the picosecond electron pulse facility ELYSE that is based on the radiofrequency (RF) photogun technology.28, 29 The silver ion reduction was revealed by picosecond radiolysis measurements with a broadband pulse-probe setup whose principle and geometrical configuration at the sample was already 3 ACS Paragon Plus Environment


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described elsewhere.30 In brief, the transient absorption pulse-probe setup uses the time

synchronization between electron pump and optical probe arising from the laser triggered electron generation on a photocathode. The main part of a femtosecond Ti:Sapphire laser output is frequency tripled and used to produce the electrons that are accelerated by the RF fields. A part of the laser source is split off to generate the optical probe pulse that can be delayed relative to the electron bunch by a mechanical translation stage. About 1 µJ of the laser source is focused into a 6 mm thick CaF2 disk to generate a supercontinuum used as the optical probe. A reference signal is split off from the broadband probe before the fused silica optical flow cell (FSOFC). Probe and reference beams were each coupled into an optical fiber, transmitted to a spectrometer, and dispersed onto a CCD. The combination of the broadband probe and the multichannel detector allows recording directly the entire transient spectra, independently of the shot-to-shot fluctuations and possible long-term drifts of the electron source. Optical measurements were carried out in a FSOFC cell with a 5 mm optical path in the solution. To neglect the signal induced in fused silica a thin window of 200 µm is used for the FSOFC.30 The probe beam was collinear with the electron pulse propagation. The electron pulses were of ~4 nC, with an electron energy of 6-8 MeV, delivered at repetition frequency of 10 Hz. Dose delivered by pulse measured in pure water was 49 Gy. The density of each solution is considered for the absorbed dose. Measurements were performed in ambient temperature. To avoid the accumulation of radiolytic products and particularly the formation of silver clusters, a peristaltic pump is used to refresh the solution in the optical cell during the pump-probe experiments. In the current study, the chemical reagents, silver perchlorate, perchloric acid and phosphoric acid, for which the purity is higher than 99% were purchased from Sigma Aldrich. Aqueous


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solutions of silver perchlorate were prepared in pure water provided by a Millipore Milli-Q system. The viscosity of solution was measured in ambient temperature. Different aqueous solutions of silver perchlorate were studied in three different series, with respect to two different acids, 14.6 M phosphoric acid and 5.7 and 10 M of perchloric acid Table 2. Each solution contained a different concentration of silver ion.

Results and discussion For each series of experiment, transient absorption spectra were recorded. In our detection window that covers the UVA and the visible, the transient absorbing species can be solvated electron with its broad absorption band increasing towards the NIR, silver atom peaking at 360 nm and the radical issued form the radiolysis of counter-ion: the phosphate H2PO4• (for phosphoric acid solutions) or perchlorate ClO4• (for perchloric acid solutions) radicals. The latter are formed by the direct effect of the radiation on these solutes and by the electron transfer reaction between the water hole, H₂O•+, and the anions (H2PO4- and that of perchlorate ClO4-). In the present case, we are not interested on these oxidation pathways. For acidic HClO4 solution, when the concentration of hydronium ions is large (10 mol L-1), the electron is already scavenged within the electron pulse by hydronium ion and the spectral signature of the solvated electron is not observed in the recorded spectrum (Figure 1). It is worth noting that recently, we found from the analysis of the kinetics of solvated electron in the ps range, that C37 ~3.5 mol L-1 (defined by the concentration that reduces the initial yield of solvated electrons to 1/e) for scavenging of the pre-solvated electron by H3O+.31 This value is much larger than that for efficient scavengers of pre-solvated electrons such as cadmium, selenite, bipyridine or nitrate.12 However, it is clear that in highly acidic solutions, the presolvated electrons are scavenged by H3O+. Therefore, in a solution containing 10 mol L-1



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H3O+, even in the presence of silver atom for which C37 is 0.15 mol L-1, almost all electrons are scavenged by H3O+. When the concentration of hydronium ions is lower (5.7 mol L-1 HClO4), a small contribution of the absorption due to the solvated electron is observed (Figure 1). Regardless of the concentration of hydronium ion, it is necessary to consider the absorption due to the formation of ClO4• or H2PO4• radicals that is located partially in the same spectral range as that for the silver atom. Thus, the spectra of these radicals were recorded in acid solutions in the absence of silver ion (Red color in Figure 1) and then the contributions of these radicals were subtracted from the spectrum recorded in acidic solution containing silver ion. Also the contribution of the solvated electron has to be subtracted from the recorded transient absorption (Figure 1) in order to find the absorbance only due to silver atom. Figures 1 a, b, c, d show the absorption spectrum due to the silver atom (Black color). H2PO4• and ClO4• radicals are formed by direct effect and by electron scavenging from the radical cation of water.6,7,32 The absorption band of these radicals in the visible is not intense. The maximum of the absorption band of the radical H2PO4• is located at 520 nm with a molar extinction coefficient of 1850 L mol-1 cm-1 and that of the radical ClO4• is located at 450 nm with 1250 L mol-1 cm-1. The

absorption spectra of the solutions, without silver ion, present the contribution of these radicals (Figure 1, b, c, d, red color) and that of the solvated electron for neutral solution recorded just after the 7 ps electron pulse. For all solutions with silver ion, the presence of the absorption band with a maximum located at 360 nm shows that the silver atom is formed within the 7 ps electron pulse. It is important to mention that in case of perchloric acid solution, the concentration of hydronim ion is the same as a concentration of acid, however in case of 14.6 mol L-1 of phosphoric acid, that is triprotic acid, the concentration of hydronium ion after taking into account its three dissociation reactions is only 5.7 mol L-1. The kinetics at 370 nm obtained in


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acidic solutions are presented on Figure 2 and 3. All kinetics show the same tendency, the higher concentration of silver ion the higher initial amplitude of the silver atom. However, the initial amplitude of silver atom for the same concentration of silver ions is not the same for different concentration of acid solutions. The first difference is the shape of kinetics which is related to the diffusion varying with the viscosity of the solution. The silver atoms can disappear after the pulse following processes of coalescence until they are transformed into Ag2+ and even Ag42+ (reaction 16 and 17).33 The viscosity of 14.6 mol L-1 of H3PO4 (48.4 cP) solution is almost 12 times higher than the viscosity of 10 mol L-1 HClO4 (4.06 cP). Consequently, the reaction 16 and 17 are slowed down in the phosphoric acid solution. For very viscous solutions such as phosphoric acid within the first 100 ps, we do not observe the decay of silver atom in our ultrafast detection window even for the highest concentration 0.12 mol L-1. When decreasing the viscosity of the solution, in the case of 5.7 M HClO4, we observe a slight decrease of the silver atom concentration at short time scale (Figure 3). However, the decay is still slow and partial and the initial absorbance can be considered to not be affected by the reactions. Here, we are interested in the initial yield of silver atom observed just after the electron pulse. Extinction coefficient of silver atom at 370 nm is 14 500 L mol-1 cm-1.22 For the solution containing 0.08 mol L-1 silver ion, the initial absorbance is 0.05 in the case of 10 mol L-1 HClO4, whereas for the 5.7 mol L-1 HClO4 and 14.6 mol L-1 H3PO4, it is 0.062 and 0.138, respectively. This result shows that there is significant dependence of the silver atom concentration on the hydronium ion concentration and composition of the solvent as well. In spite of the different concentration of acid, 14.6 mol L-1 H3PO4 and the 5.7 mol L-1 HClO4, the concentration of hydronium ions H3O+ is in both cases the same. The factor that plays an important role in silver atom formation is the composition of the solvent, particular the molar concentration of water molecules in the solution. The solvation of primary electrons is



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strongly dependent on the water molecule concentration. The environment of the primary electron influences therefore their fate between solvation and the competing pathways of the reduction of monovalent silver ion. In the case of silver aqueous solution prepared in 5.7 mol L-1 HClO4, the concentration of water is around 40.5 mol L-1. Such high concentration of water compared to the concentration of water molecule (14.1 mol L-1) in 14.6 mol L-1 H3PO4, favors the solvation process of electron that is in competition with pair formation (H+, e-) (reaction 11) and reduction of silver ion by pre-solvated electron, (reaction 10). The absence of any absorbance due to the solvated electron just after the electron pulse in 14.6 mol L-1 H3PO4 solution can also be explained by the deficiency of water molecule to favor the solvation of excess electron. By femtosecond laser photolysis measurements, the scavenging of the pre-solvated electron by H3O+ (11) in highly concentrated acid solutions was reported in sub-picosecond time range by Gauduel et al. 34 It was proposed that an ‘ion pair’, (e-, H3O+), is formed between the presolvated electron and H3O+, exhibiting an absorption band around 900 nm. In the absence of any electron acceptor in the solution this pair forms H• atom. Considering the very recent ultrafast radiolytic measurements in solutions containing acid phosphoric and acid perchloric,31 we can state that in our systems pairs between the pre-solvated electron and hydronium ion are formed. Note that this pair is different from that formed between solvated electron and H3O+. The pair formation strictly depends on the concentration of hydronium ion,31 the higher the concentration of H3O+, the more efficient is the pair formation. However, although having the same concentration of H3O+ in the system, the formation of the pair between pre-solvated electron and hydronium ion can be in competition with the electron solvation when the concentration of water is quite high like in case of 5.7 mol L-1 HClO4. This competition reaction is very important for silver atom formation since pre-solvated electron can follow the pair formation that reduces silver ion (reaction 13), or can reduce silver ion 8 ACS Paragon Plus Environment

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directly (reaction 10). Taking into account the absorbed dose in each solution, the yield of silver atom after the pulse is calculated based on the value of absorbance reported in Figure 2 and 3. For the same concentration of H3O+ and the same concentration of Ag+, 0.08 mol L-1 the yield of the silver atom is 1.46 × 10-7 mol J-1 and 2.4 × 10-7 mol J-1 in 5.7 mol L-1 HClO4 and 14.6 mol L-1 H3PO4, respectively. The higher yield obtained in case of phosphoric acid is due to the fact that excess electron undergoes reaction of silver ion reduction and pair’s formation that leads to silver atom formation as well. In 14.6 mol L-1 acid phosphoric solutions the amount of free water molecule is low for efficient solvation because the concentration of water is low and also a large amount of water is related to the H2PO42-. It is worth noting that for the same concentration of H3O+ in HClO4 and H3PO4 solutions, the signal of solvated electron is not observed in the case of phosphoric acid (Figure 1). Assuming that the maximum of the yield of pre-solvated electron is around 4.5 × 10-7 mol J-1, in phosphoric acid solution, more than half of pre-solvated electrons are finally converted into silver atom by passing through the pair (e-, H3O+). The value of 4.5 × 10-7 mol J-1 was obtained also by simulations in different conditions.35 Our previous measurements performed in neutral solution allowed us to follow the kinetics of solvated electron depending on the silver concentration recorded at 700 nm where silver atom does not absorb. The higher concentration of silver ion the lower amplitude of solvated electron since its precursor is scavenged within the pulse. According to our recent results the C37 for neutral silver aqueous solution is found to be around 0.14 mol L-1 which is very close to the literature value.31 In neutral water solution, the yield of silver atom varies from 0.24-2.9 ×10-7 mol J-1 for the silver ion concentration between 0.01 to 0.15 mol L-1. The sum of the yield of solvated electron and silver atom at the end of 7 ps electron pulse for 0.15 mol L-1 Ag+ is 4.5 ×10-7 mol J-1 that is slightly higher than the value of the yield of the solvated electron at 7 ps in neutral solution. Figure 4 presents the comparison of our results. It shows



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the concentration of silver atom observed at the end of 7 ps electron pulse in function of silver ion concentration for different concentration of acid and for neutral solution. In 14.6 mol L-1 H3PO4, the yield of silver atom at the end of the pulse is the highest compared to other acids. The yield of silver atom is ranging from 0.47 to 2.8 ×10-7 mol J-1 for the concentration ranging from 0.01 to 0.12 M. These radiolytic values are much higher than the value of radiolytic yield obtained in 5.7 mol L-1 HClO4. For example, for the concentration of silver ion of 0.05 M mol L-1, the yield of silver atom formation is 1.6 and 0.95 ×10-7 mol J-1 for 14.6 mol L-1 H3PO4 and 5.7 mol L-1 HClO4. Such important difference can be explained by the fact that in case of perchloric acid the concentration of free water is higher than that for phosphoric acid. That privileges the hydration reaction of the electron, instead of the pair formation that leads to the silver atom Ag0 within the pulse. We can also see that formation of Ag0 observed at the end of the electron pulse is less efficient in 10 mol L-1 HClO4. For this solution the concentration of H3O+ is 10 mol L-1, thus the pre-solvated electron is scavenged by hydronium ion and the pair (e-, H3O+) is formed even before it starts reacting with silver ion. Hydrogen atom formed through the scavenging of pre-solvated electron participates in reduction of Ag+, however this reaction is not diffusion controlled and it cannot be seen within the electron pulse. For 5.7 mol L-1 HClO4 the yield of Ag0 is higher than yield observed in 10 mol L-1 HClO4. This can be explained by immediate reduction of silver ion by pre-solvated electron. This path of silver atom formation is more efficient than formation by the reaction of Ag+ with the pair (e-, H3O+). This pair formed undergoes two reactions within the pulse: hydrogen atom formation and reaction leading to the formation of Ag0. The driving energy for the reducing reaction of silver ion is schematically presented in Figure 5. The energy levels are based on the value of the reduction potential of the couples (E°(H2O/es-), E(H+/H•) and E(Ag+/Ag°) which are known) and the maximum of the absorption band (e- and (H3O+ + e-)). The redox potential of the solvated electron is -2.87 V


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and that of H+ /H• is -2.1 V. Therefore, the difference between the driving energy for the silver ion reduction by the solvated electron and H• atom, is almost 0.77 eV. The redox potential of Ag+/Ag° is -1.8 V which is close to that of H+/H• couple, giving a low driving energy of the reduction of silver ion by H• atom. Moreover, an activation step is present in this reduction reaction, but the energy barrier should not be too high, because the rate constant of the reaction at room temperature is not very far from that of diffusion-controlled rate constant. We assume here that the pre-solvated electron, noted here e-, is absorbing in near IR and its absorption maximum is considered to be around 1400 nm. The absorption band of the pair (H3O+ + e-) is observed at 900 nm, giving us the difference between the driving energy of silver ion reduction by (H3O+ + e-) and by e- , to be around 0.5 eV. The solvated electron is stabilized by the presence of H3O+, which is observed by the shift of the absorption band. Therefore, the difference between the driving energy of silver ion reduction by es- and the pair (H3O+, es-) could be around 0.1 eV. These considerations give us the different level of the energy reported on the diagram (Figure 5). Among the species involved in the reduction of silver ion, e-, (e-, H3O+), es-, (es-, H3O+) and H• atom, the highest driving energy is for e-, (e-, H3O+). In phosphoric acid solutions, the fastest reduction reaction occurs with (e-, H3O+) which is faster than conventional diffusion controlled reaction and the slowest one occurs with H• atom which is controlled by thermal activation. The main reason for the efficiency of the reduction reaction with (e-, H3O+) is that competing solvation reaction is considerably slowed down due to the lack of water molecules. Conclusion Silver atom formation is used as a probe in different solutions to reveal the competition for excess electron among 4 ultrafast reactions occurring in less than 1 ps: i) geminate recombination with water hole cation, ii) electron hydration process, iii) silver ion reduction, and iv) pairing with hydronium cation. The main result of this study is that in highly acidic 11 ACS Paragon Plus Environment


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solutions, when the pre-solvated electron is paired with H3O+, this pair is as the pre-solvated electron in neutral solution, a very powerful reducing agent and can reduce silver ion. The pair (e-, H3O+), called here pre-H• atom was already observed by fs laser ionizing water measurements in highly acidic solutions. It is clear that this pair, (e-, H3O+), contrary to H• atom undergoes an efficient ultrafast electron transfer and not H• atom transfer. In H3PO4 solutions, where the pre-solvated electron recombination with water hole and its solvation dynamics are perturbed by the presence of the phosphate anions H3PO42-, the pathway of reduction of silver atom by the pre-H• atom (the pair (H+, e-)) becomes very efficient and an important radiolytic yield of the silver atom formation is observed.

AUTHOR INFORMATION Corresponding Author

E-mail: [email protected] Tel: 33169157887. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the French Agence Nationale de la Recherche (Convention ANR-13-JS08-0010-01). We express our gratitude to the French ANR.


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0.10

A bsorbance after 7 ps electron pulse/0.5 cm

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Figure 1. Transient absorption spectra measured after the 7 ps electron pulse in solution containing 0.08 mol L-1 AgClO4 (Blue) or in absence of silver ion (Red) in neutral solution (a), in 14.6 mol L-1 H3PO4 solution (b), in 10 M HClO4 solution (c) and in 5.7 M HClO4 solution (d). Black curves (a, b, c, d) represent the spectrum only due to the formation of silver atom after subtraction of other contributions.


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A bsorban ce at 37 0 n m /0.5 cm


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0.2 M

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Time(ps) Figure 2. Kinetics at 370 nm of the silver atom in acid aqueous solution (10 mol L-1 HClO4) containing different concentration of silver ion; for each kinetics the concentration is given. The signal is obtained after subtracting the contribution of perchlorate radical. The absorbed dose is the same for all solutions. Solid lines are drawn to guide the eye.


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0,175 0.12 mol L-1 in 14.6 mol L-1 H3PO4

0,150

Absorbance at 370 nm/0.5 cm

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Figure 3. Kinetics at 370 nm of the silver atom in aqueous solutions of 5.7 mol L-1 HClO4 containing different concentration of silver ion; for each kinetics the concentration is given. The results obtained in 14.6 mol L-1 H3PO4 that contains the same concentration of H3O+ are reported for comparison. The absorbed dose is the same for all solutions.



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0,20

Ag+ mol L-1

Figure 4. Absorption due to Ag0 observed at the end of 7 ps electron pulse as function of Ag+ concentration for different acidic and neutral solutions. The dose in water is 49 J L-1. Inset: Radiolytic yield of the silver atom at the end of 7 ps pulse deduced from the absorbance.


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Figure 5. Schematic driving energy of the silver ion reduction



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Reaction

Number

Rate constant/estimated reaction time

H2O → H2O•+ + e−

(1)

< 1 fs

e - → e s-

(2)

250 fs

H2O•+ + e− → H2O

(3)

A few 10 fs

H2O + H2O•+ → H3O+ + OH•

(4)

A few 10 fs

es- + Ag+ →Ag°

(5)

k = 4 × 1010

(6)

k = 2 × 1010

(7)

k = 2 × 107 s-1

Ag + + OH • → AgOH • +

(8)

k = 1.5 × 1010

AgOH + + H+ → Ag2+ + H2O

(9)

k = 1.2 × 1010

e- + Ag+ → Ag°

(10)

A few 100 fs

e- + H 3O + → ( H 3O + ,e - )

(11)

< 100 fs

(12)

< 100 fs

(13)

A few 10 fs

(14)

< 1 fs

H 2 O • + + H 3 PO 4 → H 2 PO •4 + H 3O +

(15)

A few 10 fs

Ag0 + Ag+→ Ag2+

(16)

k = 8 × 109

2Ag2+ → Ag42+

(17)

k = 4 × 108

H +A g + → A gH A gH

(H O 3

(H O 3

+

+

+

+

→ A g° + H +

+ e − ) → H 2O + H

+ e - ) + Ag + → Ag ° + H 3O +

H 3PO 4 → H 2 P O 4• + e − + H +

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Table 1. Elementary reactions involved in pulse radiolysis of aqueous solutions of neutral and concentrated phosphoric acid containing silver ions. The unit for the bimolecular rate constant reaction is M-1 s-1.

(Ag+)

Acid

H 2O

η

Sample

G(Ag°)7 ps F

mol/L-1

mol/L-1

mol/L-1

cP

107 mol J-1

1

0

0.01

≈ 55.5

≈1

≈1

0.24

2

0

0.05

≈ 55.5

≈1

≈1

1.50

3

0

0.08

≈ 55.5

≈1

≈1

2.1

4

0

0.12

≈ 55.5

≈1

≈1

2.8

5

0

0.15

≈ 55.5

≈1

≈1

2.9

6

14.6 /H3PO4

0.01

14.1

48.40

1.57

0.47

7

14.6 /H3PO4

0.05

14.1

48.40

1.57

1.68

8

14.6/H3PO4

0.08

14.1

48.40

1.57

2.4

9

14.6/ H3PO4

0.1

14.1

48.40

1.57

2.5

10

14.6/ H3PO4

0.12

14.1

48.40

1.57

2.8

11

5.7 HClO4

0.05

40.5

1.224

1.24

0.95

12

5.7 HClO4

0.08

40.5

1.224

1.24

1.46

13

5.7 HClO4

0.1

40.5

1.224

1.24

1.66

14

10 HClO4

0.01

31.4

4.06

1.46

0.22

15

10 HClO4

0.05

31.4

4.06

1.46

0.74

16

10 HClO4

0.08

31.4

4.06

1.46

1.03

17

10 HClO4

0.15

31.4

4.06

1.46

1.68

18

10 HClO4

0.2

31.4

4.06

1.46

2.0

Table 2. Chemical Composition of aqueous solutions studied by picosecond pulse radiolysis with silver and hydronium ion concentration, water concentration, density (F), viscosity, η, and radiolytic yield of silver atom formation. The neutral solutions and phosphoric acid solutions are studied in previous work and are reported here for comparison.14



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References 1

Balcerzyk, A.; Schmidhammer, U.; El Omar, A. K.; Jeunesse, P.; Larbre J. P.; Mostafavi. M. Direct and Indirect Radiolytic Effects in Highly Concentrated Aqueous Solutions of Bromide. J. Phys. Chem. A. 2011, 115, 4326−4333. 2 El Omar, A. K.; Schmidhammer, U.; Rousseau, B.; LaVerne, J.; Mostafavi, M. Competition Reactions of H2O•+ Radical in Concentrated Cl- Aqueous Solutions: Picosecond Pulse Radiolysis Study. J. Phys. Chem. A. 2012, 116, 11509−11518. 3 Balcerzyk, A.; LaVerne, J.; Mostafavi, M. Picosecond Pulse Radiolysis of Direct and Indirect Radiolytic Effects in Highly Concentrated Halide Aqueous Solutions. J. Phys. Chem. A. 2011, 115, 9151−9159. 4 Balcerzyk, A.; El Omar, A. K.; Schmidhammer, U.; Pernot, P.; Mostafavi. M. Picosecond Pulse Radiolysis Study of Highly Concentrated Nitric Acid Solutions: Formation Mechanism of NO3• Radical. J. Phys. Chem. A. 2012, 116, 7302−7307. 5 El Omar, A. K.; Schmidhammer, U.; Balcerzyk, A.; LaVerne, J.; Mostafavi, M. Spur Reactions Observed by Picosecond Pulse Radiolysis in Highly Concentrated Bromide Aqueous Solutions. J. Phys. Chem. A. 2013, 117, 2287−2293. 6 Ma, J.; Schmidhammer, U.; Pernot, P.; Mostafavi, M. Reactivity of the Strongest Oxidizing Species in Aqueous Solutions: The Short-Lived Radical Cation H2O•+. J. Phys. Chem. Lett. 2014, 5, 258−261. 7 Ma, J.; Schmidhammer, U.; Mostafavi, M. Picosecond Pulse Radiolysis of Highly Concentrated Sulfuric Acid Solutions: Evidence for the Oxidation Reactivity of Radical Cation H2O•+. J. Phys. Chem. A. 2014, 118, 4030−4037. 8 Ghalei, M.; Ma, J.; Schmidhammer, U.; Vandenborre, J.; Fattahi, M.; Mostafavi, M. Picosecond Pulse Radiolysis of Highly Concentrated Carbonate Solutions. J. Phys. Chem. B. 2016, 120, 2434−2439. 9 Thomas, J. K., Gordon, S.; Hart, E. J. The Rate of the Hydrated Electron in Aqueous Solution of Some Inorganic Compounds. J. Phys. Chem. 1964, 68, 1524−1527. 10 Baxendale, J.H.; Fielden, E. M.; Keen, J.P. The Pulse Radiolysis of Aqueous Solution of Some Inorganic Compounds. Proc. R. Soc. London, Ser. A 1965, 286, 320−336. 11 Aldrich, J. E.; Bronskill, M. J.; Wolff, R. K.; Hunt, J. W. Picosecond Pulse Radiolysis. III. Reaction Rates and Reduction in Yields of Hydrated Electrons. J. Chem. Phys. 1971, 55, 530−539. 12 Jonah, C. D.; Miller, J. R.; Matheson, M. S. The Reaction of the Precursor of the Hydrated Electron with Electron Scavengers. J. Phys. Chem. 1977, 81, 1618−1622. 13 Pimblott, S. M.; LaVerne, J. A. On the Radiation Chemical Kinetics of the Precursor to the Hydrated Electron. J. Phys. Chem. A. 1998, 102, 2967−2975. 14 Balcerzyk, A.; Schmidhammer, U.; Horne, G.; Wang, F.; Ma, J.; Pimblott, S. M.; de la Lande, A.; Mostafavi, M. Unexpected Ultrafast Silver Ion Reduction: Dynamics Driven by the Solvent Structure. J. Phys. Chem. B. 2015, 119, 10096−10101. 15 Henglein, A. The Reactivity of Silver Atoms in Aqueous Solution (a Gamma -Radiolysis Study). Ber. Bunsen-Ges. Phys.Chem. 1977, 81, 556−561. 16 Haïssinsky, M. Radiolysis of Aqueous Solutions of Silver Sulphate. Proceedings of The Third Tihany Symposium on Radiation Chemistry; Dobo, J. and Hedvig, P., Ed; Akademiai Kiado Publishing House of the Hungarian Academy of Sciences: Budapest, Hungary, 1972; pp 1353−1366. 17 Henglein, A. Reactions of Organic Free Radicals at Colloidal Silver in Aqueous Solution: Electron Pool Effect and Water Decomposition. J. Phys. Chem. 1979, 83, 2209−2216. 21 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

18

Mostafavi, M.; Marignier, J.-L.; Amblard, J.; J. Belloni, J. Nucleation Dynamics of Silver Aggregates Simulation of Photographic Development Processes. Radiat. Phys. Chem. 1989, 34, 605−617. 19 Ershov, B.G.; Janata, E.; Henglein, A.; Fojtik, A. Silver Atoms and Cluster in Aqueous Solution: Absorption Spectra and the Particle Growth in the Absence of Stabilizing Ag+ Ions. J. Phys. Chem. 1993, 97, 4589−4594. 20 Janata, E; Lilie, J.; Martin, M. Instrumentation of Kinetic Spectroscopy−11. An Apparatus for A.C.-Conductivity Measurements in Laser Flash Photolysis and Pulse Radiolysis Experiments. Radiat. Phys. Chem., 1994, 43, 353−356. 21 Janata, E. Instrumentation of Kinetic Spectroscopy−12. Software for Data Acquisition in Kinetic Experiments. Radiat. Phys. Chem., 1994, 44, 449−454. 22 Janata, E.; Henglein, A.; Ershov, J. First Clusters of Ag+ Reduction in Aqueous Solution. J. Phys. Chem.; 1994, 98, 10888−10890. 23 Remita, S.; Mostafavi, M.; Delcourt, M. O. Stabilization, Growth and Reactivity of Silver Aggregates Produced by Radiolysis in the Presence of EDTA, New J. Chem., 1994, 18, 581−588. 24 Mostafavi, M.; Delcourt, M.-O.; Keghouche, N.; Picq, G. Early Steps of Formation of Silver/Polyacrylate Complexed Oligomer Clusters. A Pulse Radiolysis Study. Radiat. Phys. Chem. 1992, 40, 445−450. 25 Belloni, J.; Mostafavi, M.; Remita, H.; Marignier, J.-L.; Delcourt, M.-O. Radiation-Induced Synthesis of Mono- and Multi-Metallic Clusters and Nanocolloids. New J. Chem. 1998, 22, 1239−1255. 26 Buxton G.V., Greenstock C.L., Helman W.P., Ross A.B., 1988. Critical Review of Rate Constants for Reactions of Hydrated Electrons, Hydrogen Atoms and Hydroxyl Radicals in Aqueous Solutions. J. Phys. Chem. Ref. Data, 1988, 17, 513−886. 27 Sarkar, A.; Janata, E. Formation of the Silver Hydride Ion AgH+ Upon the Reduction of Silver Ions by H• in Aqueous Solution. A Pulse Radiolysis Study. Z. Phys. Chem. 2007, 221, 403−413. 28 Belloni, J.; Monard, H.; Gobert, F.; Larbre, J. P.; Demarque, A.; De Waele, V.; Lampre, I.; Marignier, J. L.; Mostafavi, M.; Bourdon, J. C. et al. ELYSE A Picosecond Electron Accelerator for Pulse Radiolysis Research. Nucl. Instrum. Methods. Phys. Res., Sect. A. 2005, 539, 527–539. 29 Marignier, J. L.; de Waele, V.; Monard, H.; Gobert, F.; Larbre, J. P.; Demarque, A.; Mostafavi, M.; Belloni, J. Time-Resolved Spectroscopy at the Picosecond Laser-Triggered Electron Accelerator ELYSE. Radiat. Phys. Chem. 2006, 75, 1024−1033. 30 Schmidhammer, U.; El Omar, A. K.; Balcerzyk, A.; Mostafavi, M., Transient Absorption Induced by a Picosecond Electron Pulse in the Fused Silica Windows of an Optical Cell. Radiat. Phys. Chem. 2012, 81, 1715−1719. 31 Ma, J.; Schmidhammer, U. and Mostafavi, M. Direct Evidence for Transient Pair Formation between a Solvated Electron and H3O+ Observed by Picosecond Pulse Radiolysis. J. Phys. Chem. Lett. 2014, 5, 2219−2223. 32 Ma, J.; Schmidhammer, U.; Mostafavi, M. Picosecond Pulse Radiolysis of Highly Concentrated Phosphoric Acid Solutions: Mechanism of Phosphate Radical Formation. J. Phys. Chem. B. 2015, 119, 7180−7185. 33 Henglein, A.; Tausch-Treml, R. Optical Absorption and Catalytic Activity of Subcolloidal and Colloidal Silver in Aqueous Solution: A Pulse Radiolysis Study, J. Colloid. Interface Sci. 1981, 80, 84−93. 34 Gauduel, Y.; Pommeret, S. ; Migus, A. ; Antonetii, A. Femtosecond Spectroscopy of an Encounter Pair Radical (H3O+…...e-)hyd in Concentrated Aqueous Solution J. Am. Chem. Soc. 1990, 112, 925– 2931. 35 Ma, J.; LaVerne, J. A.; Mostafavi, M. Scavenging the Water Cation in Concentrated Acidic Solutions. J. Phys. Chem. A. 2015, 119, 10629−10636.


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