Article pubs.acs.org/JPCC
Kinetics and Mechanism of the Reaction between H2O2 and Tungsten Powder in Water Miao Yang,† Xian Zhang,‡ Alex Grosjean,† Inna Soroka,† and Mats Jonsson*,† †
School of Chemical Science and Engineering, Applied Physical Chemistry, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden ‡ School of Chemical Science and Engineering, Division of Surface & Corrosion Science, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden S Supporting Information *
ABSTRACT: In this work, the reaction between H2O2 and tungsten powder in the presence of Tris(hydroxymethyl) aminomethane was studied experimentally. The production of hydroxyl radicals can be quantified indirectly by quantifying the scavenging product formaldehyde (CH2O). XRD, XPS, and SEM analysis shows that no significant structural or compositional changes occur after reaction. We compared H2O2 consumption and CH2O formation in both heterogeneous W(s)/H2O2/Tris system and homogeneous W(aq)/H2O2/Tris system. Increasing the amount of W powder leads to the increase in dissolution rate of W species, insignificant increase of H2O2 consumption rate and the decrease of final CH2O production. By contrast, the consumption rate of H2O2 increases as increasing the concentration of dissolved W species. Based on the experimental results, a mechanism of H2O2 reacting with W powder in the presence of Tris is proposed. The mechanism well explained the relationship between surface reactions and homogeneous Haber−Weiss peroxide chain breakdown.
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and W(V).16,20,21 H2O2 can react with W(VI) forming peroxocomplexes which are efficient catalysts for decomposition of H2O222,23 and the complexation reaction is reversible.12 [W2O3(O2)4]2− is one typical peroxocomplex for which the structure has been determined.12,24 The tetrahexylammonium (THA) salt of the peroxocomplex [W2O3(O2)4]2− was used as a catalyst for oxidation of organic compounds with H2O2.12 Polytungstates (W6O20(OH)5−, W12O396−, and W12O4110−) can exist up to pH 8 if the dissolved tungsten concentration is sufficiently high.9,25 However, W(VI) is the predominating oxidation state of tungsten species in solution under basic condition according to the reported Eh-pH diagram of the W− H2O system.9 Haber−Weiss peroxide chain breakdown (R1 and R2) is known for H2O2 reacting with metal ions via single-electron transfer producing HO•.26−29
INTRODUCTION Tungsten (W) is an important metal with many technological applications such as metallurgy,1 nuclear power,2 catalysis,3 solar energy,4 photocatalysis,5,6 optics/electronics,7 and semiconductors.8 Due to the diverse applications, numerous studies regarding the complicated solution chemistry, pH sensitivity, oxidation, and dissolution have been performed.9−12 H2O2 has been widely used as an oxidant to study the oxidation behavior, catalytic performance and dissolution kinetics of W(s) in water.10−16 The heterogeneous system with H2O2 and W(s) is fairly complex and may involve oxidative dissolution, homogeneous reaction and surface reaction. To the best of our knowledge, no comprehensive study regarding H2O2 and W covering all aspects has been conducted before. H2O2 can react with metal/metal oxide surfaces both via redox reaction and catalytic decomposition.17−19 It has been shown that metallic tungsten is readily dissolved in aqueous H2O2 solution through a redox reaction.10,14 This is a fairly complex oxidative dissolution process involving stepwise oxidation from metallic state to W(VI) via W(III), W(IV) © 2015 American Chemical Society
Received: July 20, 2015 Revised: September 3, 2015 Published: September 8, 2015 22560
DOI: 10.1021/acs.jpcc.5b07012 J. Phys. Chem. C 2015, 119, 22560−22569
Article
The Journal of Physical Chemistry C Mox + H 2O2 → M red + HO2• + H+
(1)
M red + H 2O2 → Mox + HO• + HO−
(2)
measured by Metrohm 713 pH Meter with an accuracy of ±0.1 pH units, calibrated by standard pH reagents. XRD (X-ray diffraction) was recorded with PANalytical X’Pert PRO diffraction system using Bragg−Brentano geometry in the 2θ angle from 10° to 100° and Cu Kα irradiation (λ = 1.54 Å). SEM/EDS (scanning electron microscopy and energy dispersive spectroscopy) analysis was conducted to obtain surface morphology and elemental information. Surface analysis was performed using a FEI-XL 30 Series instrument, equipped with an EDS X-Max SDD (Silicon Drift Detector) 50 mm2 detector from Oxford Instruments. All images (secondary electrons) were obtained using an accelerating voltage of 20 kV and EDS analysis was performed using an accelerating voltage of 5 kV. ICP-OES (inductively coupled plasma optical emission spectroscopy, Thermo Scientific iCAP6000 series) were used for trace elemental analysis of all solutions. The limitation of ICP-OES is that no information regarding the oxidation state of the element can be obtained. The analysis for tungsten was performed at wavelengths of 207.9, 209.8, and 224.8 nm using ICP element standard IV from Sigma-Aldrich. XPS (X-ray photoelectron spectroscopy) spectra were recorded with a Kratos Axis Ultra electron spectrometer with a delay line detector. A monochromated Al Kα source operated at 150 W, a hybrid lens system with a magnetic lens, providing an analyzed area of 0.3 × 0.7 mm2, and a charge neutralizer were used for the measurements. The base pressure in the analysis chamber was below 3 × 10−9 Torr. The binding energy (BE) scale was referenced to the C 1s of aliphatic carbon, set at 285.0 eV. Processing of the spectra was accomplished with Kratos software using Gaussian and Lorenzian functions in the ratio of about 70% to 30%. A Shirley background was applied. The depth of analysis for metal oxides/hydroxides was about 6 nm. The element detection limit was typically 0.1 at. %. Reagents and Experiments. All the solutions used in this study were prepared using Millipore Milli-Q water. The B.E.T. specific surface area of tungsten powder (CAS[7440-33-7], purity 99.9%, 12 μm, industrially produced by Sigma-Aldrich) and tungsten trioxide (CAS[1314-35-8], purity 99.9%, industrially produced by Sigma-Aldrich) is 0.11 ± 0.01 m2 g−1 and 3.51 ± 0.01 m2 g−1, respectively. The H2O2 solutions were prepared from a 30% standard solution (Merck). Ammoniumdimolybdate ADM (Alfa Aesar, 4% w/v) and potassium iodide KI (VWR BDH Prolabo, 99.0%) were used in the Ghormley triiodide method46,47 to determine the concentration of H2O2. Produced hydroxyl radicals are scavenged by Tris(hydroxymethyl) aminomethane (CAS[7786-1], BDH Chemicals, 99%). Acetoacetanilide AAA (CAS[102-01-2], Aldrich, ≥ 98%) and ammonium acetate (CAS[631-61-8], Aldrich, ≥ 98%) was used for quantifying formaldehyde with the modified version of the Hantzsch method.41,42 Heterogeneous Reaction Study. To study the reactions between H2O2 and W powder, W powder (surface to volume ratio SA/V = 550−8800 m−1) was immersed into H2O2 solution (5 mM). The experiments were performed at ambient temperature in the dark, under N2 atmosphere and continuous magnetic stirring at a rate of 600 rpm. The pH of the suspension was adjusted to 7.5 by HCl/NaOH after mixing Tris and W powder. After 30 min premixing, H2O2 was added to the suspension to initialize the reaction. [Tris]0 and [H2O2]0 were fixed at 100 and 5 mM, respectively, which is in line with
Mox and Mred represents the oxidized and reduced forms of the one-electron redox couple in transition metal complexes (e.g., Fe(III)/Fe(II), W(VI)/W(V)). Except in the aqueous phase, the reactions (R1 and R2) can also occur in heterogeneous systems at the solid−liquid interface.30,31 The rate limiting step is R129 which can be enhanced by the addition of ligands or the formation of polyperoxometalate.12,32,33 R2 is a Fenton-like reaction.34 The Haber−Weiss peroxide chain breakdown (e.g., H2O2/Fe(III)/Fe(II)) displays the behavior of continuous production of hydroxyl radicals.35,36 The HO• generation rate is governed by the steady-state concentration of Fe(II) which is mainly controlled by the total iron concentration.35 The catalytic decomposition of H2O2 on metal/metal oxide surfaces has been studied for decades and the mechanism is elucidated both experimentally and theoretically.18,37−39 In recent years, Tris (tris(hydroxymethyl) aminomethane) has been used to investigate the decomposition since it is an efficient and stable scavenger for the hydroxyl radical―the intermediate produced during the process.40 Formaldehyde (CH2O) is an intrinsic product from the scavenging reaction and can be quantified by a modified Hantzsch method.41,42 The scavenging mechanism and factors (O2 and pH) influencing the CH2O-yield have recently been studied.43 The production of CH2O is mainly dependent on the surface concentrations of H2O2 and Tris.44 The chemistry (e.g., the consumption rate of H2O244 and the final production of O245) of the H2O2/ZrO2/ Probe system is influenced by the probe due to the competition between the probe and H2O2 on reacting with formed hydroxyl radicals. To the best of our knowledge, the catalytic decomposition of H2O2 on tungsten or tungsten oxide has not been studied yet. In this study, we investigate the interaction between H2O2 and tungsten powder in aqueous suspension in the presence of Tris. The decomposition of H2O2 and formation of CH2O are investigated and compared between a heterogeneous W(s)/ H2O2/Tris system and a homogeneous W(aq)/H2O2/Tris (filtered from the former system). In addition, the kinetics of the reactions with varied amounts of tungsten powder is evaluated. The dissolution of tungsten species during the reaction is determined by ICP-OES. Surface characterization is conducted for the tungsten powder before and after treatments via XRD, SEM, and XPS. A mechanism of the reaction between H2O2 and tungsten is proposed based on the experimental observations.
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EXPERIMENTAL SECTION
Instrumentation. The specific surface area of tungsten powder was determined by the Brunauer−Emmett−Teller (B.E.T) method based on isothermal adsorption and desorption of a gaseous mixture consisting of 30% N2 and 70% He using a Micrometrics Flowsorb II 2300 instrument. The samples were weighed using a Mettler Toledo AT261 Delta Range microbalance. The experiments were performed in aqueous solution saturated by N2 (≥99.999%, Strandmøllen A/ S) using Milipore water. The powder suspensions were stirred using a Heidolph MR3001K magnetic stirrer at 600 rpm. UV/ vis spectra were collected by using a Thermo Scientific Genesys 20 UV/vis spectrophotometer. The pH of the solutions was 22561
DOI: 10.1021/acs.jpcc.5b07012 J. Phys. Chem. C 2015, 119, 22560−22569
Article
The Journal of Physical Chemistry C
Figure 1. (a) X-ray diffraction pattern of untreated tungsten powder (purity 99.9%, industrially produced by Sigma-Aldrich); two types of crystalline structures of tungsten and their crystalline phases are labeled on the graph. SEM image of (b) untreated tungsten powder; (c) tungsten powder after heterogeneous reaction with Tris/H2O2 (5 h).
In order to further investigate the homogeneous reaction, extra H2O2 was injected to the filtered solution when all H2O2 was consumed. The concentration of H2O2 and CH2O were monitored. Besides, the amount of dissolved tungsten prior to and after addition of H2O2 was examined.
previous studies of the surface reactivity of H2O2 upon ZrO2.43,44 Samples were extracted from the reaction vessel and filtered through a 0.2 μm cellulose acetate syringe filter at selected time points. Then, the H2O2 concentration, CH2O concentration, and dissolved amount of W were measured, respectively. The concentration of H2O2 was determined by the Ghormley triiodide method.46,47 H2O2 oxidizes I− to I3− in this method. The absorbance of I 3 − was measured by a spectrophotometer at 350 nm. The uncertainty associated with the determination of the concentration of H2O2 in the initial solution is
