Ruthenium Nanoparticles Mediated Electrocatalytic Reduction of

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Ruthenium Nanoparticles Mediated Electrocatalytic Reduction of UO22+ Ions for Its Rapid and Sensitive Detection in Natural Waters Ruma Gupta, Mahesh Sundararajan, and Jayashree S. Gamare Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 24 Jun 2017 Downloaded from http://pubs.acs.org on June 24, 2017

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Analytical Chemistry

Ruthenium Nanoparticles Mediated Electrocatalytic Reduction of UO22+ Ions for Its Rapid and Sensitive Detection in Natural Waters Ruma Gupta,*† Mahesh Sundararajan*# and Jayashree S. Gamare† †Fuel Chemistry Division, #Theoretical Chemistry Section, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India ABSTRACT. Reduction of UO22+ ions to U4+ ions is difficult due to involvement of two axially bonded oxygen atoms, and often requires a catalyst to lower the activation barrier. The noble metal nanoparticles (NPs) exhibit high electrocatalytic activity, and could be employed for the sensitive and rapid quantifications of U022+ ions in the aqueous matrix. Therefore, the Pd, Ru and Rh NPs decorated glassy carbon electrode were examined for their efficacy toward electrocatalytic reduction of UO22+ ions, and observed that Ru NPs mediate efficiently the electro-reduction of UO22+ ions.

The mechanism of the electroreduction of UO22+ by the

RuNPs/GC was studied using density functional theory calculations which pointed different approach of 5f metal ions electroreduction unlike 4p metal ions such as As(III). RuNP decorated on the glassy carbon would be hydrated which in turn assist to adsorb the uranyl sulfates through hydrogen bonding thus facilitated electro-reduction. Differential pulse voltammetric (DPV) technique, was used for rapid and sensitive quantification of UO22+ ions. The RuNPs/GC based DPV technique could be used to determine the concentration of uranyl in a few minutes with a detection limit of 1.95 ppb. The RuNPs/GC based DPV was evaluated for its analytical performance using seawater as well lake water and ground water spiked with known amounts of UO22+.

ACS Paragon Plus Environment


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INTRODUCTION Metal nanoparticles are one of the most widely used materials for heterogeneous and electrochemical catalysis

[1-7].

For these catalyst materials, the ability to control the dispersion

and the durability of catalytic metal nanoparticles is essential for better performance and economical feasibility. Ruthenium nanoparticles (RuNPs) have attracted considerable scientific attention as they have remarkable catalytic properties. They have been widely used for practical applications in many fields such as catalysis, sensors, capacitors and electrochemical reactions [8 -14].

Lot of attention has been given to RuNPs due to their strong affinity towards carbene π

bonds and are being studied for different applications.[15-17] In our previous work, we have reported the covalent interactions of RuNPs with arsenite wherein As(III) selectively adsorbed on surface of RuNPs having conducting core[18]. Nuclear power provides a clean energy source which is accounted for approximately 13% electricity generation worldwide without emitting greenhouse gases or other environmental contaminants [19]. Uranium is one of the very important actinides in view of its use as the primary fuel in nuclear industry. For fabricating high quality nuclear fuels, chemical and physical quality control of the starting, intermediate and final product of the fuel is essential. Also, uranium is one of the most dangerous heavy metals in the environment, due to its long half-life, high radioactivity, and biological toxicity. High uranium concentration in the environment is known to pose serious health hazard to mankind as it can cause genetic defects and cancer (affecting kidney, brain, liver, heart etc)

[20,21].

The WHO have regulated the maximum

concentration level of uranium in both drinking and environmental waters

[22].

Hence, it is

of great importance to monitor the trace amount of uranium in water systems, which is of particularly

public

concern. Various

analytical

techniques


have

been used

for

the

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determination of uranium in natural water systems. X-ray fluorescence spectrometry , laserinduced fluorescence, and inductively coupled plasma mass spectrometry are most suitable for routine analysis due to better sensitivities

[23-30]

. However, these methods normally

require expensive and complex systems with high cost of operation and maintenance, which are hardly available in small laboratories as well as for the on-site environmental monitoring. Electrochemical analysis has always been considered as a powerful approach for the detection of sub-ppb levels of metal ions since it combines the advantages of high precision, excellent

sensitivity,

reproducibility and stable current response with simple

experimental set up and instruments.

Hence

with

low

costs

for

experiments and

maintenance, there is a possibility to establish useful and portable equipments for on-site environmental monitoring. Table S1 (Supporting Information, S.I.) tabulates the recent literature available for uranium determination employing various different techniques and compares the advantage of the present method with the other available methods. From our previous experience of excellent catalytic activity of RuNPs for arsenite determinations [18]

, we wanted to explore the possibility for uranyl electrocatalysis and hence its quantification

employing RuNPs. UO22+ is highly stable oxocation due to the presence of trans-dioxo bonds around the uranium therefore activation barrier of the electron transfer reaction is quite high. RuNPs catalyses the electron transfer reaction by lowering down the height of the activation barrier. Here also RuNPs exhibited excellent sensitivity and electrocatalytic activity towards uranyl ion but the mechanism of electrocatalysis is different from that of Ru-Arsenite. There is no such covalent bond formation unlike in case Ru-Arsenite[18]. A detailed mechanistic study has been carried out to understand the mechanism of electrocatalysis using density functional theory (DFT) calculations. An understanding of the electron transfer properties is of critical importance



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in the optimization of their applications in these diverse electrocatalytic reaction processes. Hence, in the present work, Ru NPs decorated on the glassy carbon have been studied to explore possibility of electrocatalysis and electroanalysis of UO22+. RuNPs have been never subjected for electrocatalysis of uranyl ion and its determination. For this work, the Ru particles have been electrosynthesised on the glassy carbon electrode (Ru NPs/GC) by controlled electrodeposition as explained in our previous work

[18]

. To understand the insight of electrocatalysis in this case

DFT calculations were performed. Electrochemical impedance spectroscopy (EIS) have been carried out to study the change in charge transfer resistance of RuNPs decorated glassy carbon electrode for uranyl ion as compared to the one observed at bare glassy carbon electrode. Finally, RuNps/GC based differential pulse voltammetric (DPV) technique have been used to quantify UO22+ at sub-ppb concentrations in seawater as well spiked in lake water and ground water respectively. EXPERIMENTAL SECTION All chemicals, namely, ruthenium trichloride hydrate (RuCl3.xH2O) (Aldrich), potassium tetrachloropalladate (K2PdCl4) (Aldrich), Rhodium (III) chloride hydrate (RhCl3.xH2O) (Aldrich), H2SO4 (Merck) and HNO3 (Merck) were used as received. All the solutions were prepared in Millipore Milli-Q water (~ 18 MΩ cm-1). Electrodeposition of RuNPs, cyclic voltammetry and differential pulse voltammetry were performed using CHI 760D electrochemical workstation with a three electrode voltammetric cell having glassy carbon disk working electrode (area, A = 0.07 cm2), platinum wire counter electrode and Ag/AgCl reference electrode. All the potentials were quoted with respect to Ag/AgCl (3 M KCl) reference electrode. The experiments were carried out at room temperature (25 ± 10C) and the solutions were deoxygenated using high purity nitrogen prior to electrochemical experiments. Each


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measurement was repeated thrice and the average numerical value of each parameter was quoted for discussion (relative error < ± 0.1%). Uranyl nitrate stock solution was prepared by dissolving U3O8 in dilute nitric acid. The electrochemical impedance spectroscopy (EIS) measurements were done at open circuit potential employing ac perturbation amplitude of 5 mV for 48 different frequencies ranging between 0.01 Hz to 106 Hz. The RuNPs /GC electrode was used as a working electrode in a solution containing uranyl nitrate in 1M H2SO4 and then subjected to the EIS measurements. The analytical performance of RuNPs/GC was evaluated by differential pulse voltammetry (DPV) using solution having known concentration of UO22+ in 1M H2SO4. For inspection of the efficiency of RuNPs/GC, the determination of UO22+ in the real water samples collected from seawater from Trombay area, Mumbai, Lake of Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, and ground water from West Bengal was done. Since UO22+ in the later two samples were not detected, a precise measured quantity of UO22+ was spiked in these sample and adjusted to the pH of 1M H2SO4 which was taken as a blank and subtracted from the sample data. DPV of each sample were recorded and the concentrations of UO22+ were evaluated by extrapolation of the straight line to the negative x-axis intercept (i.e. considering the y-axis value as 0) from the calibration curve. The mechanism for the conversion of U(VI) to U(IV) is computed at the density functional theory (DFT) level. The structures are optimized with BP86 functional [31-32] in conjunction with TZVP basis sets [33] for all other atoms except uranium. For uranyl, a def-TZVP basis set is used to describe the valence electrons, whereas the core electrons are modeled through small core effective core pseudo potential (SC-ECP). The energetics are computed using all electron def2TZVP basis sets. The solvation and scalar relativistic effects are incorporated using COSMO model with water dielectric constant and with ZORA Hamiltonian. Geometry optimizations and



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energetics are carried out with TURBOMOLE

[34]

and ORCA electronic structure packages

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[35].

The bio-reduction and surface based reduction mechanism using models such as uranyl acetate, uranyl aqua and uranyl hydroxo species are earlier probed by us [36-38]. However, the experiments are carried out with uranyl sulfates whose reduction mechanism was never attempted before. RESULTS AND DISCUSSION Nanoparticles of Pd, Ru and Rh were electrodeposited at glassy carbon electrode under optimized condition from the simple palladium salt, ruthenium salt and rhodium salt respectively. Figure 1a, S2a and S2b (Supporting Information, S.I.) shows the SEM image of Ru, Pd and Rh nanoparticles respectively. It can be seen that highly monodispersed and homogeneous nanoparticles were deposited under condition optimized to attain maximum electrocatalytic activity. Figure S3 (S.I.), shows the cyclic Voltammogram of 10 mM UO22+ in 1 M H2SO4 at PdNPs/GC, RuNPs/GC and RhNPs/GC respectively. The three voltammograms are overlaid in the same scale for the sake of comparison. It can be clearly seen that RuNPs/GC exhibit maximum electrocatalytic activity for UO22+ reduction as compared to Pd and Rh nanoparticles. Hence RuNPs/GC was chosen for carrying out further studies. Figure 1(b) shows the Cyclic Voltammogram of 10 mM UO22+ in 1 M H2SO4 on bare GC and RuNPs/GC at 20 mVs-1 scan rates. A broad cathodic peak of U(VI) on GC was observed at ~ -0.45 V. The cathodic peak potential was shifted anodically to -0.15 V on RuNPs/GC giving a sharp peak. The peak potential is having an anodic shift of 300 mV at RuNPs/GC electrode with respect to the bare GC electrode. The enhancement of the redox peak current along with the significant anodic shift i.e. decrease in overpotential suggests electrocatalytic action of the RuNPs/GC for the electrochemical reduction of U(VI) in 1 M H2SO4. Further, the significant enhancement of the cathodic peak current of U(VI) at RuNPs/GC draws analytical importance for the quantitative


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measurement of U(VI). Figure 1(c) shows the variation of cathodic peak current with scan rate. The Ipc of U(VI) reduction showed linear dependence on the square root of scan rates (ν0.5) at RuNPs/GC (Inset of figure 1(c)). The cathodic current is proportional to the square root of scan rate for diffusion controlled electron transfer reaction.

Figure1(a)SEM image showing NPs formed on the glassy carbon electrode after electrodeposition of Ru (b) Comparison of cyclic voltammograms of bare GC and RuNPs/GC electrodes recorded for 1 mM U(VI) in 1M H2SO4 (c) Cyclic voltammograms of RuNPs/GC electrode recorded at different scan rates in 10mM U(VI) in in 1M H2SO4.Inset shows the



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variation of cathodic peak current with square root of scan rate (d) Variation of the scan ratenormalized current (Ip/ν1/2) with scan rate. The cathodic peak current function (ipc/ν0.5) vs scan rate plot of the U(VI) reduction at RuNPs/GC (figure 1(d)) exhibits the characteristic shape typical of an electrochemical-chemical (EC) mechanism

. In acidic solution, UO2+

[39-40]

disproportionates to UO22+ and U4+ [41-42] and this disproportionantion reaction occurs so fast in 1 M H2SO4 medium that in the cyclic voltammogram we do not see the corresponding peaks but only one peak is seen corresponding to UO22+ reduced to U4+ at the electrode surface. Since, U4+is stable in 1 M H2SO4 and the oxidation of UO22+involved the formation of two U=O bonds, which occurs much slowly with respect to the time frame of the electrochemical potential scan at the electrode. Hence, UO22+ / U4+ shows irreversible redox behaviour and no oxidation peak of uranium (IV) was observed in reverse scan direction. RuNPs/GC was equilibrated with solution containing UO22+ and subjected to EDS and XPS analyses for understanding Ru NPs affinity towards UO22+ Figure S4 and S5(a,b) (S.I.). The studies show that there is no such covalent bond formation of UO22+ with Ru NPs as was seen in case of As(III). To gain further insight into the mechanism of electrocatalysis and to support results obtained from cyclic voltammogram DFT calculations were carried out. In 1 M H2SO4, the possible speciation of uranyl is [UO2SO4(H2O)3] is proposed by Vallet and Grenthe[43]. Our optimized structure of this U(VI) species is in line with the experimental EXAFS data of Moll et al[44] which gives confidence in our computational strategy (figure S6) [45-48].


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Figure 2. Disproportionation mechanism of uranyl sulfate proposed through electronic structure calculations. Values in brackets are the spin populations on uranium centers in different species. The role of RuNPs coated to the glassy electrode is very important. As shown in figure 1 the reduction of uranyl is faster only in the presence of RuNPs. Further, we note that no direct binding between uranyl and RuNPs is noted from XPS, which suggest the binding is rather weak. Thus, we propose that RuNPs is solvated by water molecules which can anchor the uranyl (VI) sulfate through hydrogen bonding. These weak adsorptions are indeed noted in our previous studies on reduction of uranyl and chromates in iron containing mineral surfaces.[38] The [UO2SO4(H2O)3] species can be absorbed on the surface of RuNPs/GC electrode and undergoes reduction. The possible reduction pathway is shown in figure 2.



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The reduction pathway can be initiated through the electron transfer from the electrode to U(VI) species to form the reduced [UO2SO4(H2O)3]1- species. Upon reduction, the U=O bond length is elongated from 1.80 (in U(VI)) to 1.88 Å. Alternatively, the [UO2SO4(H2O)3] species can dimerize through the formation of T-shaped complex (denoted as U(VI)-U(VI). In the Tcomplex, one of the uranyl is coordinated to the second uranium through the displacement of one water molecule. Although the formation of such dimeric species is slightly unfavorable (by 4.77 kcal mol-1), this weak complex can be anchored by two strong hydrogen bonding which can reduce to form U(VI)-U(V) species. The formation of this species is favorable by -22.9 k cal mol-1. Of the two uranium centers, the added electron is delocalized between the two centers. As compared to U(VI)-U(VI) species, the reduced U(VI)-U(V) species is strongly bound which are firmly held through hydrogen bonding interactions. The U(VI)-U(V) can

further reduced

to

U(V)-U(V) species

which

can

undergo

disproportionation which can be formed in two different pathways. In the first case, the two individually reduced U(V) species can form a T-shaped complex which is -32 kcal mol-1 more favourable as compared to the two individual U(V) species. In this case, each uranium center has one unpaired electrons (~1.10e-) which are firmly held by hydrogen bonding interactions. However, in the second case, the reduced U(V) species can form a T-shaped complex with the U(VI) species, thus forming U(VI)-U(V) species. Finally, due to the acidic pH condition, protonation at the 'yl' oxygen is indeed feasible which can trigger the electron transfer from first uranyl center to the second center leading to the formation of U(VI) and U(IV) species (U(VI)U(IV)). The proton affinity of U(V)-U(V) species is favorable by more than 300 kcal mol-1 is approximately more than 40 kcal mol-1 as compared to the hydration free energy of the proton(-(-


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265 kcal mol-1)[49]. Thus, the disproportionation mechanism is spontaneous which are consistent with our earlier proposed bio-reduction mechanism of uranyl and chromate [36-38] Further, the electrochemical impedance spectroscopy of U(VI) was performed in 1M H2SO4 employing RuNPs/GC and bare GC electrode respectively. Nyquist curves thus obtained are shown in figure 3. The diameters of the semicircles on the x-axis of the plot correspond to the interfacial charge transfer resistances (Rct). It is evident from Figure 3 that the charge transfer resistance of RuNPs/GC electrode is much lower than that at bare GC. This could be attributed to the faster charge transfer kinetics and thus electrocatalysis at RuNPs modified electrode.

Figure 3.Nyquist plots obtained from impedance measurement for RuNps/GC and Bare GC in a solution containing 10 mM U(VI) in 1M H2SO4.electrode. Z’and –Z” are the real and imaginary components respectively of the impedance as a function of frequency



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Figure 4:Differential pulse voltammetry curves as a function of U(VI) conc. Inset shows the corresponding variation of peak current as a function of conc. of U(VI) conc. in 1M H2SO4. Differential pulse voltammetry was used to study the efficacy of the Ru NPs decorated glassy carbon electrode (RuNPs/GC) for the quantification of uranyl in water. For determination of ultratrace concentration of U(VI), differential pulse voltammetry (DPV) was employed to reduce U(VI) to U(IV). For DPV experiment, the pulse amplitude of 50 mV, pulse width 0.05 s, and potential increment of 4 mV were employed. A reproducible peak current appeared at -0.15 V and such peak current was absent in the 1M H2SO4 solution without U(VI) (Figure 4). There was a linear increase in DPV current with an increase in concentration of U(VI) with a detection limit of 1.95 ppb, at a signal to noise ratio of 3 as shown in inset of Figure 4.The value of correlation coefficient was 0.999 indicating an excellent linear fit of the experimental data. The slope of the linear plot (sensitivity) obtained for RuNPs/GC was 4.05nA ppb-1.The reproducibility of the electrochemical signal of the RuNPs/GC electrode was checked by carrying out a series of repetitive experiments (ten) for a fixed concentrationof U(VI) of 20 µM. Precise peak currents was obtained with a relative standard deviation (RSD) of 6.1% (n= 10).


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Table 1. Determination of U(VI) in spiked and unspiked real water samples. Sample

Lake water (spiked) Groundwater (spiked) Seawater (unspiked)

U(VI) expected (ppb) 5.4

Efficiency Concentration determined(ppb)* (%)** 5.1

94.4

8.1

7.7

95.1

3.3

3.1

94

*: average of three experiments, **: (Conc. determined/Conc. expected)/100. The same electrode was used for the determination of 20 µM of U(VI) for five consecutive days and reproducible peak currents were obtained. Thus, the RuNPs/GC electrode exhibited a highly precise, reproducible and stable response for the quantification of trace concentration of U(VI). The analytical performance of RuNPs/GC based DPV was studied by using real water samples such as lake water, ground water and seawater. The spiked/expected and obtained values are listed in Table 1. The obtained values given in table 1 calculated from the triplicate DPV analyses of each sample. As such, the U(VI) concentration determined by using RuNPs/GC based DPV is highly reproducible. CONCLUSIONS Successful development of low cost and reproducible sensor for ultra trace detection of uranium could enable a commercially viable process that would provide sensitive quantification of uranium in chemical quality control of nuclear fuel as well as environmental samples. In this study for the first time we have presented the suitability of Ru nanoparticles for the determination of uranyl ions by virtue of its excellent electrocatalytic action. A detailed mechanistic study has been carried out using DFT calculations in addition to electrochemical



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study to unravel the mechanism of electrocatalysis. Our results provided a very simple, fast and cheap approach for the determination of UO22+ in natural water samples without addition of any chemical reagent. Such complete studies on actinide chemistry and its electrocatalytic behaviour at RuNPs modified electrode provide an exciting and challenging road map not only for actinide researcher but also to those working in the field of catalysis and analytical chemistry. Supporting Information. SEM, EDS, XPS and Voltammetry experiments figures are given. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Tel: +91-22-25594598; E-mail: [email protected], [email protected] (RG), and Fax: +91-22-25505151; Tel: +91-22-25590300; E-mail: [email protected]. (MS), Fax: +91-2225505151. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Authors thanks Dr. Ashok K Pandey, Radiochemistry Division, BARC for valuable discussion and Dr S. Kannan, Head Fuel chemistry Division, BARC for constant support and


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encouragement during this work. RG thanks Dr. Deepak Tyagi, Chemistry Division, BARC for XPS measurements. MS thank BARC computational facilities.

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