11202
J. Phys. Chem. C 2010, 114, 11202–11208
Kinetics, Mechanism, and Activation Energy of H2O2 Decomposition on the Surface of ZrO2 Cla´udio M. Lousada* and Mats Jonsson KTH Chemical Science and Engineering, Nuclear Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden ReceiVed: March 31, 2010; ReVised Manuscript ReceiVed: May 10, 2010
The kinetics, mechanism, and activation energy of H2O2 decomposition in ZrO2 particle suspensions were studied. The obtained first-order and second-order rate constants for the decomposition of H2O2 in the presence of ZrO2 at T ) 298.15 K produced the values k1 ) (6.15 ( 0.04) × 10-5 s-1 and k2 ) (2.39 ( 0.09) × 10-10 m · s-1, respectively. The dependency of the reaction first-order rate constant with temperature was studied; consequently, the activation energy for the reaction was obtained in the temperature interval 294.15-353.15 K having yielded the value Ea ) 33 ( 1.0 kJ · mol-1. The dependency of the zeroth-order reaction rate constant with pH was investigated and discussed. A mechanistic study encompassing the investigation of the dynamics of formation of hydroxyl radicals during the course of the reaction was performed. A version of the modified Hantzsch method was applied for this purpose, and it was verified that the dynamics of formation of hydroxyl radicals during the reaction are in good agreement with the proposed reaction mechanism. 1. Introduction Zirconium dioxide, mostly known as zirconia, is one of the most versatile ceramic materials known. Its physical and chemical properties make it suitable for a wide range of applications including nanotechnology,1 catalysis and synthesis,2-5 medicine,6 electronics and sensors,7 and the manufacture of a diversity of materials.8 The fact that its radiation stability is good,9 it has a low neutron cross section, and it has low solubility in water at high temperatures10 makes it presently a main favored candidate to be a component of the inert fuel matrix in nuclear reactors.11 The presence of zirconium dioxide in nuclear systems is not only restricted to its incorporation into the fuel matrix. The fuel material in a nuclear reactor is protected by cladding pipes made of alloyed zirconium. In contact with water near and above its critical temperature, a corrosion layer of hydrated zirconium dioxide ZrO2 · nH2O is formed and has implications on the chemistry of the reactor.12-14 One of the most important water radiolysis products to concern about in reactor chemistry is hydrogen peroxide.15 Hydrogen peroxide is an important compound that has found innumerous uses such as a bleaching agent,16 disinfectant,17 oxidizer,18 or as a nontoxic monopropellant in rocket fuel.19 Hydrogen peroxide is the main oxidizing molecular product formed during the radiolysis of water. It is formed primarily by combination reactions of HO radicals produced in the radiolytic decomposition of water. Its importance due to an increase in its concentration is augmented under conditions subjected to radiation with high linear energy transfer, as in an operating nuclear power plant,20 where ∼2% of the total fast neutrons and γ-ray energy released in the core of an operating nuclear reactor is deposited in the cooling water21 or under conditions of storage of spent nuclear fuel.22 The importance of the system ZrO2/H2O2 lead us to develop a study on the dynamics, energetics, and mechanism of the reaction between these two chemical species. It was previously reported that the reaction of H2O2 on the surface of ZrO2 in liquid water consists of the decomposition * To whom correspondence should be addressed. Tel: (46) 8 790 87 89. Fax: (46) 8 790 87 72. E-mail:
[email protected].
of H2O2 to produce water and oxygen. Given that the zirconium in ZrO2 is in its highest oxidation state, no redox reactions are involved in the process. This metal-oxide-catalyzed reaction is proposed to follow the scheme23
H2O2 + M f 2HO · + M ·
HO + H2O2 f 2HO2·
HO2·
+ H2O
f H2O2 + O2
(R1) (R2) (R3)
where M represents an undefined site located at the oxide surface. The decomposition of hydrogen peroxide on a solid surface is a spontaneous process at temperatures that range from room temperature to 286 °C, and its reported activation energy ranges from 20.93 to 96.30 kJ · mol-1, depending on the surface type and on factors such as the oxidation state of the metal, among others.24 Reaction R3 corresponds to the chain termination and occurs via the disproportionation of two hydroperoxyl radicals as represented. When reaction R3 occurs with pure water as a solvent, the activation energy is 25.0 kJ · mol-1 in the temperature range 274-316 K.25 The evaluation of the effect of zirconium dioxide on the kinetics and energetics of decomposition of hydrogen peroxide has to be done at a temperature and pH range where the spontaneous uncatalyzed decomposition of hydrogen peroxide is negligible when compared with the rate of decomposition of hydrogen peroxide on the surface of the oxide itself. In neutral water, from the species involved in the reactions mentioned above, only dissociation of HO2 · needs to be considered because the pKa values for H2O2, HO · , and HO2 · are 11.8, 11.9, and 4.88, respectively.26 The HO2 · hydroperoxyl radical is a weak acid and is also the protonated form of the superoxide radical anion which is easily formed and sorbed at the surface of the zirconium dioxide according to the following reaction27 ZrO2
· HO2· 98 H+ + O2
(R4)
The superoxide anion radical is stabilized by adsorption on the surface of the zirconium dioxide and has been used
10.1021/jp1028933 2010 American Chemical Society Published on Web 06/04/2010
H2O2 Decomposition on the Surface of ZrO2
J. Phys. Chem. C, Vol. 114, No. 25, 2010 11203
previously as a probe for surface cationic fields. The superoxide radical adsorbs to the surface exclusively by coordination with exposed Zr4+ surface sites.27 The superoxide radical anion is in many cases a precursor of the highly reactive hydroxyl radical following a Fenton-type mechanism in systems where the metal cation can undergo further stages in oxidation.28 The formed superoxide anion radical is an active reductive species and can reduce ions in the higher valence state, for example25 · 3+ Of O2 + Fe2+ 2 + Fe
(R5)
The rate constant for the reaction of the hydroxyl radical with organic molecules ranges from ∼4 × 106 to 2 × 1011 M-1 · s-1.29 The surface of the oxide is capable of stabilizing the intermediary radical species formed during the decomposition of H2O2 by means of interactions of the formed radicals with the oxide lattice.27,30 As shown above, the proposed mechanism of the reaction of decomposition of hydrogen peroxide on the surface of an oxide that cannot undergo further oxidation is rather complex and has not been clearly elucidated. In this work, the reaction of H2O2 in the presence of ZrO2 was studied at different temperatures and pH values. A mechanistic study was also performed and consisted of developing a method to study the dynamics of formation of the intermediary radical species during the decomposition of H2O2 at the surface of ZrO2. A study involving water radiolysis in the presence of tris/HCl buffer allowed us to calibrate the method cited above and consequently to quantify the rate of formation of HO radicals during the reaction of H2O2 in the presence of ZrO2. 2. Experimental Details Instrumentation. Specific surface areas of the powders were determined using the BET method of isothermal adsorption and desorption of a gaseous mixture consisting of 30% N2 and 70% He on a Micrometrics Flowsorb II 2300 instrument. γ-Irradiation was performed using a MDS Nordion 1000 Elite Cs-137 γ-source with a dose rate of 0.15 Gy · s-1; this value was determined by Fricke dosimetry.31 X-ray powder diffractograms (XRD) were obtained at 293 K using Cu KR radiation, on a PANanalytical X’pert instrument. Powders were encapsulated on Lindemann capillaries. The data was collected over the range 3 e 2θ e 80° with a step size of 0.033° (2θ). Data evaluation was done using The High Score Plus software package, and the PDF-2 database was used for matching the experimentally obtained diffractograms. The samples were weighted to (10-5 g, in a Mettler Toledo AT261 Delta Range microbalance. The reactions were performed under an inert atmosphere with a constant flux of N2 gas (AGA Gas AB) with a flow rate of 0.21 L · min-1 that was also used for stirring the solutions. We kept the temperature constant throughout the experiments by using a Huber CC1 or a Lauda E100 thermostat calibrated against a Therma 1 thermometer coupled to a submersible K-type (NiCrNi) temperature probe with a precision of (0.1 K. UV/ vis spectra were collected using a WPA Lightwave S2000 or a WPA Biowave II UV/vis spectrophotometer. Trace elemental analysis were performed using the technique of inductively coupled plasma spectroscopy on a Thermo Scientific iCAP 6000 series ICP spectrometer. The analysis for Zr was performed at the wavelength of 343.823 nm. Reagents and Experiments. All solutions used in this study were prepared using water from a Millipore Milli-Q system. Zirconium dioxide (CAS[1314-23-4], Aldrich 99%, particle size 98%) in the presence of ammonium acetate (CAS[631-61-8], Lancaster 98%) to form a dihydropyridine derivative, which has the maximum absorption wavelength at 368 nm. A calibration curve plotting the absorbance of the dihydropyridine derivative as a function of formaldehyde concentration was obtained at 368 nm, giving a linear correlation between absorbance and concentration in the concentration range 0.15 µM to 1 mM in formaldehyde. The plotting of the calibration curve for formaldehyde required the preparation of several solutions of CH2O with different rigorously known concentrations in the concentration range mentioned above. It was then necessary to proceed to the accurate determination of the concentration of formaldehyde in the solution used initially (CAS[50-00-0]), Aldrich 37 wt % in H2O) using the iodometric method.37 The solutions and respective standardizations necessary to follow the iodometric method procedure were prepared as stated in the cited paper37 and as described elsewhere.38 The error associated with the determination of the concentration of formaldehyde in the initial solution was
