Surface Reactions of H2O2, H2, and O2 in Aqueous Systems

Article pubs.acs.org/JPCC

Surface Reactions of H2O2, H2, and O2 in Aqueous Systems Containing ZrO2 Alexandre Barreiro Fidalgo, Björn Dahlgren, Tore Brinck, and Mats Jonsson* School of Chemical Science and Engineering, Applied Physical Chemistry, KTH Royal Institute of Technology, SE − 100 44 Stockholm, Sweden ABSTRACT: In radiolysis of water, three molecular products are formed (H2O2, O2, and H2). It has previously been shown that aqueous hydrogen peroxide is catalytically decomposed on many oxide surfaces and that the decomposition proceeds via the formation of surface-bound hydroxyl radicals. In this work, we have investigated the behavior of aqueous H2 and O2 in contact with ZrO2. Experiments were carried out in an autoclave with high H2 pressure and low O2 pressure (40 and 0.2 bar, respectively). In the experiments the concentration of H-abstracting radicals was monitored as a function of time using tris(hydroxymethyl)aminomethane (Tris) as scavenger and the subsequent formation of formaldehyde to probe radical formation. The plausible formation of H2O2 was also monitored in the experiments. In addition, density functional theory (employing the hybrid PBE0 functional) was used to search for reaction pathways. The results from the experiments show that hydrogen-abstracting radicals are formed in the aqueous H2-/O2-system in contact with solid ZrO2. Formation of H2O2 is also detected, and the timedependent production of hydrogen-abstracting radicals follows the time-dependent H2O2 concentration, strongly indicating that the radicals are produced upon catalytic decomposition of H2O2. The DFT study implies that H2O2 formation proceeds via a pathway where HO2• is a key intermediate. It is interesting to note that all the stable molecular products from aqueous radiolysis are precursors of quite intriguing radical reactions at water/oxide interfaces.

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INTRODUCTION Surface reactions of radicals are important in numerous areas such as atmospheric chemistry, photocatalysis, heterogeneous catalysis, environmental chemistry, and radiation chemistry.1−4 In many systems, free radicals display high affinity to solid surfaces and are consequently stabilized upon adsorption.5−10 This influences the reactivity of the radicals toward other species in solution or species adsorbed to the surface. Despite the importance of surface reactions in the areas mentioned above, very little is known about the interfacial reactivity of radicals in general. Radiolysis of water is of key importance in most existing nuclear technological applications but also in many other areas such as radiation biology, radiochemistry, nuclear medicine, etc. The radiation chemistry of water has been studied for more than a century and is today considered to be quite well understood.11,12 In nuclear technological applications, one of the main concerns is corrosion of containment materials and other vital components such as nuclear fuel or fuel cladding. Corrosion in nuclear technological systems such as nuclear reactors, reprocessing plants, and repositories for radioactive waste influences the lifetime of the facility and can pose a potential safety hazard. The surfaces in contact with water are usually oxides or metals covered with an oxide layer. Hence, radiation-induced processes at the interface between water and a solid oxide would appear to be crucial.13−18 Nevertheless, very little is known about these processes. Radiolysis of water primarily yields HO•, H•, eaq−, H2O2, H2, and H3O+.11 Hence, both oxidants and reductants are produced. In secondary reactions, HO2• and O2 are formed. © 2016 American Chemical Society

The radicals formed are highly reactive, while the molecular products display considerably lower reactivity.19 For this reason, the latter can usually be accumulated in the system. While the hydroxyl radical is by far the most reactive oxidant in the system, the concentrations are very low, and the relative impact in surface reactions is in general quite low. In some systems it has been shown that H2O2 is the major oxidant involved in surface reactions.20 While H2O2 is capable of oxidizing many materials, it can also undergo catalytic decomposition on most oxide surfaces.7,21,22 Quite recently, it was shown that the surface-catalyzed decomposition of H2O2 follows the mechanism depicted below.23 H 2O2 (ads) → 2HO•(ads)

(1)

H 2O2 (ads) + HO•(ads) → H 2O(ads) + HO2•(ads)

(2)

•

2HO2 (ads) → H 2O2 + O2

(3)

This reaction occurs both on oxides in their highest oxidation state and on oxides that can be oxidized to higher oxidation states. In the latter case, oxidation and catalytic decomposition are competing reactions. The initial step in the catalytic decomposition is the homolysis of adsorbed H2O2 to produce adsorbed hydroxyl radicals. The formation of hydroxyl radicals has been probed using radical scavengers such as tris(hydroxymethyl)aminomethane (Tris) Received: October 26, 2015 Revised: December 22, 2015 Published: January 6, 2016 1609

DOI: 10.1021/acs.jpcc.5b10491 J. Phys. Chem. C 2016, 120, 1609−1614

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The Journal of Physical Chemistry C and methanol.23−30 Both scavengers yield formaldehyde upon hydrogen abstraction. The surface reactivity of surface-bound hydroxyl radicals was recently studied.31 H2 is the other major primary aqueous molecular radiolysis product. It has been reported that H2 can also undergo homolytic cleavage on dry oxide surfaces.32−34 The hydrogen atoms thus formed can be reactive toward other solutes and thereby influence the chemistry at the oxide surface. In this work, we investigate the possible catalytic effect of ZrO2 powder on the reaction between H2 and O2 dissolved in water. Experiments were performed on aqueous ZrO2 suspensions in the presence of H2 and a mixture of H2 and O2. Tris is used as a probe for surface-bound radicals capable of hydrogen abstraction, and the possible formation of H2O2 in systems containing H2 and O2 is also monitored. A plausible reaction pathway was elucidated using density functional theory.

air. The formation of H2O2 and formaldehyde was measured as described above. When avoiding the presence of O2 in the system, the solutions were purged with N2 for 1−1.5 h. Once inside the autoclave, it was purged for an extra 1.5 h to remove all the remaining oxygen from the autoclave and the solution. The system was pressurized afterward with the gas of interest. For oxygen-containing systems, the solution was saturated in air without performing a purge of the autoclave with N2. Outside the autoclave, parallel experiments were run in nitrogen atmosphere and in air-saturated solutions to detect the influence of O2 in the formation of formaldehyde in the absence of H2, if any, in the system. All solutions were protected from light. 2.3. Computational Details. Density functional theory (employing the hybrid PBE0 functional)38−40 was used to search for reaction pathways yielding H2O2 from the surfacecatalyzed reaction between H2 and O2 on ZrO2. A relatively small ZrO2 cluster was employed (Zr2O4 truncated by dissociatively adsorbed water), motivated by its good performance for reproducing experimental barrier heights for H2O2 decomposition.7 The initial search for the lowest-lying reaction path was conducted through geometry optimizations using Gaussian09 program suite rev c.01.41 The basis set used for this initial screening was the LANL2DZ42 basis set for Zr and the D95V basis set for oxygen and hydrogen.43 The transition state (TS) structures found in the survey were subsequently used in intrinsic reaction coordinate scans to verify that the TS led to the expected reactants and products. The stationary points of the lowest energy path were subsequently optimized in ORCA v3.0.244 using the larger def2-TZVP basis set,45,46 the COSMO47 continuum solvation model with parameters for water, and empirical dispersion corrections (Grimme’s D3 corrections with Becke−Johnson damping).48,49 Vibrational frequencies were calculated throughout to characterize the stationary points and to calculate the vibrational contribution to the free energy.

2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. Materials and Sample Preparation. Millipore Milli-Q water was used to prepare all solutions. ZrO2 monoclinic powder (CAS[1314-23-4], Aldrich 99%) with a surface area of 5.0 ± 0.2 m2·g−1 was used in all experiments. The surface area was determined by using the BET method of isothermal adsorption and desorption of a gaseous mixture consisting of 30% N2 and 70% He in a Micrometrics Flowsorb II 2300 instrument. Scavenging of the Radicals. Scavenging of radicals formed on the surface was performed using tris(hydroxymethyl)aminomethane (CAS[77-86-1], Sigma-Aldrich, ≥99.9%). Before its use, pH was adjusted to 7.5 with HCl and Tris. The formaldehyde produced was quantified spectrophotometrically at 368 nm, by using a modified version of the Hantzsch method.35 The formaldehyde formed due to the radical scavenging is reacted with ammonium acetate 4 M (CAS[631-61-8], Sigma-Aldrich ≥98%) and acetoacetanilide 0.2 M (CAS[102-01-2], Sigma-Aldrich ≥98%) to form a dihydropyridine derivate, which was measured at its maximum absorption wavelength, 368 nm. The error associated with the determination of the formaldehyde is less than 2%.22 Concentration of H2O2. The concentration of H2O2 was indirectly measured at 350 nm by UV/visible spectroscopy using the Ghormley method.36,37 The autoclave used is equipped with inlet and outlet tubes for pressurizing and sampling, as well as a stirrer. The outlet tube is connected to a valve which, due to overpressure, makes it possible to take a sample. The total volume of the reaction mixture was 100 mL. After extraction from the reaction vessel, approximately 3 mL of sample was filtered through a 0.20 μm cellulose acetate syringe filter to avoid the presence of particles during the measurements. From those 3 mL, 1.5 mL was used for the modified Hantzsch method, mixing them with 2.5 mL of 4 M ammonium acetate and 1 mL of 0.2 M acetoacetanilide, letting the reaction take place in a water bath at 40 °C. Finally, 200 μL of sample was mixed with 1.6 mL of mQ water, 100 μL of 1 M KI, and 100 μL of the 1 M NaAc/HAc/catalyst buffer mixture to determine the H2O2 concentration. 2.2. H2/O2 Experiments. Aqueous suspensions of ZrO2 powder containing either 20 mM Tris or pure water were exposed to three different gas mixtures in separate experiments using a glass vessel inside an autoclave. These gas mixtures were: 5 bar of N2, 40 bar of H2 or 40 bar of H2 with 1 atm of

3. RESULTS AND DISCUSSION 3.1. Experimental Results. As mentioned above, hydrogen abstraction from Tris yields formaldehyde. Consequently, hydrogen-abstracting radicals formed on the surface of ZrO2 should yield formaldehyde in the presence of Tris. In the initial autoclave experiments, we monitored the formaldehyde production in aqueous solution containing ZrO2, Tris, and H2 (40 bar) to have a reference for the experiments where a mixture of H2 and O2 was used. The results are shown in Figure 1. As can be seen, the formaldehyde concentration increases slightly with reaction time. However, in a reference experiment performed on the same systems with N2 instead of H2, formaldehyde formation is also observed. The apparent formation of formaldehyde in the reference experiments with H2 and N2 is attributed to aging of the reagent solutions used in the modified Hantzsch method for formaldehyde quantification. It is worth noting that several similar experiments were performed. In a series of preliminary experiments the deoxygenation by N2 purging prior to pressurization with H2 was not as efficient. Interestingly, it was shown that in experiments where deoxygenation was less efficient the rate of formaldehyde production was significantly higher, which suggested that the combination of H2 and O2 produced hydrogen-abstracting radicals. This led us to perform a second 1610

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Figure 1. Formaldehyde concentration as a function of time in aqueous solution containing ZrO2 and Tris under N2 (□) (5 bar) and H2 (●) (40 bar). The experiments were performed inside an autoclave. The errors correspond to the uncertainty in the analytical method and are taken as twice the detection limit of CH2O.

Figure 3. Formaldehyde concentration as a function of reaction time for aqueous solution containing ZrO2 and Tris. The solution was saturated with air and then exposed to H2 (40 bar). The autoclave was opened and exposed to air after ca. 1.8 × 106 s and then closed and pressurized with H2 again. The errors correspond to the uncertainty in the analytical method and are taken as twice the detection limit of CH2O.

type of reference experiment, where we investigated the effect of O2 alone before studying the combination of H2 and O2 using a controlled amount of O2. This new knowledge also led us to purge all samples supposed to be O2 free extensively. Aqueous solutions containing ZrO2 and Tris were exposed to air and N2, respectively. The formaldehyde production in the two systems is plotted as a function of reaction time in Figure 2.

As can be seen, a significant amount of formaldehyde is formed under the present conditions. This clearly indicates that the species responsible for formaldehyde formation from Tris is only formed when H2 and O2 are simultaneously present in the system. The species responsible for formaldehyde production must consequently be either HO2• or HO• (the involvement of adsorbed H• is ruled out on the basis of no CH2O being observed in the O2-free experiments; furthermore H2 is not expected to dissociate on hydrated ZrO2).33,50,51 The HO• is produced upon catalytic decomposition of H2O2 (formed from the disproportionation of HO2•). In aqueous solution, HO2• is not expected to be able to abstract hydrogen from Tris on the basis of the relative bond dissociation enthalpies. However, the formation of HO2• and H2O2 implies initial splitting of H2. To further investigate the nature of the species responsible for formaldehyde production, we measured the concentration of H2O2 as a function of time using the Ghormley triiodide method. It should be noted that while this method is not specific to H2O2 the other stable oxidant in the system (O2) does not interfere with the analysis. The results of the analysis are presented in Figure 4a. Clearly, H2O2 is produced in the H2/O2/ZrO2 system. The H2O2 concentration increases quite rapidly initially and reaches a maximum after which it slowly decreases. It appears to follow the rate of formaldehyde formation. In Figure 4b, the rate of formaldehyde formation is plotted as a function of reaction time. As can be seen, the rate of formaldehyde production does indeed parallel the transient H2O2 concentration, indicating that formaldehyde formation in the H2/O2/ZrO2 system is attributed to H2O2 formation followed by hydroxyl radical production due to catalytic decomposition of H2O2 on ZrO2. It is interesting to note that direct H2O2 synthesis from H2 and O2 is known to be catalyzed by noble metals.52 This is to our knowledge the first report of catalytic activity of a metal oxide for the direct formation of H2O2. In addition, H2 and O2 are known to react on the ZrO2 surface under UV irradiation.53 The authors conclude that the reaction at the ZrO2/gas interface only occurs in the presence of light. However, our results show that for aqueous H2 and O2 the reaction also occurs in the dark.

Figure 2. Formaldehyde concentration as a function of reaction time for aqueous solutions containing ZrO2 and Tris under air (×) and N2 (□) atmosphere, respectively. The experiments were run at ambient pressure, stirred, and protected from light outside an autoclave. The errors correspond to the uncertainty in the analytical method and are taken as twice the detection limit of CH2O.

As can be seen, the formaldehyde production in both systems is virtually identical and very low. Hence, we cannot conclude that O2 alone is responsible for the formaldehyde production in the H2 experiments where the deoxygenation was less efficient. The obvious conclusion is then that the species responsible for formaldehyde production is formed when combining H2, O2, and ZrO2. To verify this, we performed experiments where the autoclave was exposed to air before being closed and pressurized with H2. The formaldehyde concentration was measured as a function of time after H2 pressurization. The result is shown in Figure 3. 1611

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Figure 5. H2O2 concentration as a function of time for an air-saturated aqueous solution containing ZrO2 pressurized with H2 (40 bar). The autoclave was opened and exposed to air after ca. 3 × 106 s and then closed and pressurized with H2 again. The errors correspond to the uncertainty in the analytical method and are taken as twice the detection limit of H2O2.

Figure 4. (a) H2O2 concentration as a function of time for an airsaturated aqueous solution containing ZrO2 and Tris pressurized with H2 (40 bar). The autoclave was opened and exposed to air after ca. 1.8 × 106 s and then closed and pressurized with H2 again. The errors correspond to the uncertainty in the analytical method and are taken as twice the detection limit of H2O2. (b) Rate of formaldehyde production plotted as a function of reaction time (based on data in Figure 3).

decomposed by adsorbed HO2• (hydroperoxyl radical). A small (ZrO2)2 dimer cluster was found to favorably react with three H2O molecules, one O2 molecule, and one H2 molecule, forming a hydroxylated and hydrated cluster with a surfacebound HO2• in bridge position. This structure was able to facilitate a catalytic cycle for H2O2 formation (see Scheme 1).

To investigate the extent of catalysis in this process, we performed two consecutive experiments on the same aqueous solution containing ZrO2 and Tris. After reaching the maximum formaldehyde concentration at the point where all H2O2 was consumed, we opened the autoclave and exposed the solution to air for 24 h and then closed the autoclave and pressurized with H2 again. We could then observe a small increase in formaldehyde concentration (Figure 3, from 2 × 106 s) and a very low transient H2O2 concentration (Figure 4a, from 2 × 106 s). It is clear that the system behavior is different the second time the experiment is performed even though the initial conditions are expected to be the same. This may be attributable to reactions of Tris producing organic surface deposits that block the catalytically active sites. To elucidate the role of Tris, an identical series of experiments was performed in the absence of Tris. For obvious reasons, formaldehyde was not produced in this system. The time-dependent H2O2 concentration is presented in Figure 5. As can be seen here, the maximum H2O2 concentration in the second experiment (after exposing the system to air again) is very close to the maximum H2O2 concentration in the first experiment, and the dynamics of the two transients are quite similar. This implies that the reactions producing and consuming H2O2 are purely catalytic. It also strengthens the assumption that the nonreproducibility observed in the Triscontaining system is attributed to Tris-derived reaction products. The reason for the transient behavior of H2O2 is the limited amount of O2 present in the system. 3.2. Computational Results: Mechanism for H2O2 Production. In the computational study, a large number of small ZrO2 clusters of different sizes and with different extents of hydroxylation/hydration were investigated for their ability to catalyze reactions between H2 and O2. The energetically most favorable reaction path yielding H2O2 did not involve any adsorbed hydrogen atoms. Instead, H2 was found to be

Scheme 1. Reaction Scheme for the Catalytic Cyclea

a

The upper structure corresponds to B/D in Figure 6, and the lower structure corresponds to C. Both of these reaction steps are occurring on the singlet surface.

Structures with fewer water molecules or lacking O2/H2 were prone to dissociate H2O2. The catalytic cycle then consists of two steps H 2O(Zr) + HO2 (Zr) + H 2 + O2 → OH(Zr) + H 2O2 (Zr) + H 2O2 OH(Zr) + H 2O2 (Zr) → H 2O(Zr) + HO2 (Zr)

where Zr within parentheses denotes adsorbed species. The first (rate-limiting) step produces one hydrogen peroxide molecule in solution from H2 and O2. In that process, the surface-bound hydroperoxyl, i.e., HO2(Zr), activates H2 which is split between O2 and HO2(Zr) forming HO2 not bound to 1612

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The Journal of Physical Chemistry C the cluster and H2O2(Zr). The newly formed HO2 then abstracts a hydrogen atom (without an energy barrier) from a surface-bound water molecule, forming a free H2O2 molecule, and leaving a surface-bound hydroxyl group behind. The second step is the reformation of the surface-bound hydroperoxyl and water by hydrogen transfer from H2O2(Zr) to OH(Zr). The free energy profile of the reaction is shown in Figure 6, and the corresponding thermochemistry data are presented in Table 1.

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• On the basis of DFT calculations the mechanism most likely involves a surface-bound hydroperoxyl radical which catalyzes hydrogen atom transfer from molecular hydrogen to molecular oxygen.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +46-8-790 9123. Fax: +46-8-21 26 26. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Swedish Nuclear Fuel and Waste Management Co. (SKB) and The Royal Institute of Technology (KTH) are gratefully acknowledged for financial support.

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REFERENCES

(1) George, I. J.; Abbatt, J. P. D. Heterogeneous Oxidation of Atmospheric Aerosol Particles by Gas-Phase Radicals. Nat. Chem. 2010, 2, 713−722. (2) Caër, S. L. Water Radiolysis: Influence of Oxide Surfaces on H2 Production under Ionizing Radiation. Water 2011, 3, 235−253. (3) Ibhadon, A.; Fitzpatrick, P. Heterogeneous Photocatalysis: Recent Advances and Applications. Catalysts 2013, 3, 189−218. (4) Argyle, M.; Bartholomew, C. Heterogeneous Catalyst Deactivation and Regeneration: A Review. Catalysts 2015, 5, 145−269. (5) Anpo, M.; Che, M.; Fubini, B.; Garrone, E.; Giamello, E.; Paganini, M. C. Generation of Superoxide Ions at Oxide Surfaces. Top. Catal. 1999, 8, 189−198. (6) Das, A. K.; Saha, B.; Islam, M. Micellar Effect on the Catalytic Co-Oxidation of Dimethyl Sulfoxide and Oxalic Acid by Chromium(VI) in Aqueous Acid Media: A Kinetic Study. Prog. React. Kinet. Mech. 2005, 30, 215−226. (7) Lousada, C. M.; Johansson, A. J.; Brinck, T.; Jonsson, M. Reactivity of Metal Oxide Clusters with Hydrogen Peroxide and Water - a DFT Study Evaluating the Performance of Different ExchangeCorrelation Functionals. Phys. Chem. Chem. Phys. 2013, 15, 5539− 5552. (8) Kitajima, N.; Fukuzumi, S.; Ono, Y. Formation of Superoxide Ion During the Decomposition of Hydrogen Peroxide on Supported Metal Oxides. J. Phys. Chem. 1978, 82, 1505−1509. (9) Amorelli, A.; Evans, J. C.; Rowlands, C. C. An Electron Spin Resonance Study of the Superoxide Radical Anion in Polycrystalline Magnesium Oxide and Titanium Dioxide Powders. J. Chem. Soc., Faraday Trans. 1 1988, 84, 1723−1728. (10) Giamello, E.; Calosso, L.; Fubini, B.; Geobaldo, F. Evidence of Stable Hydroxyl Radicals and Other Oxygen Radical Species Generated by Interaction of Hydrogen Peroxide with Magnesium Oxide. J. Phys. Chem. 1993, 97, 5735−5740. (11) Spinks, J. W. T.; Woods, R. J. An Introduction to Radiation Chemistry; Wiley: New York, 1990. (12) Buxton, G. V. Radiation Chemistry. Principles and Applications; Weinheim: Germany, 1987.

Figure 6. Calculated free energy profile for the reaction path. A is a minimum on the triplet surface; and MECP is the minimum energy crossing point between the singlet and triplet surface. B, TS1, C, TS2, and D are all stationary points on the singlet surface.

If we estimate a rate coefficient from barrier height of TS1 using the Eyring equation we need to exclude the triplet surface to be in quantitative agreement with the experimental data. This does not necessarily mean that the mechanism is incorrect but that the model is unable to quantitatively reproduce the experimental rate. The proposed mechanism may still be the energetically most favorable path. The appearance of the intermediate radical species HO2• is not too surprisingit is often invoked as an intermediate in proposed mechanisms of the cathodic oxygen reduction reaction on noble metals54 and on zirconia HO2• is stable enough to be experimentally observable even at room temperature using ESR spectroscopy.55 It is interesting to note that all the stable molecular products from aqueous radiolysis are precursors of quite intriguing radical reactions at water/oxide interfaces

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CONCLUSIONS From the results presented above we can draw the following conclusions: • ZrO2 catalyzes the reaction between H2 and O2 dissolved in water. The surface-catalyzed reaction produces H2O2. H2O2 is subsequently decomposed to hydroxyl radicals on the ZrO2 surface.

Table 1. Calculated Relative Energies (with Respect to B) of the Stationary Pointsa ΔEel/μEh ΔZPE/μEh ΔH/kJ·mol−1 ΔG/kJ·mol−1

A

MECP

B

TS1

C

TS2

D

−57 0.1 −152 −150

5 0.8 15 16

0 0.0 0 0

20 0.0 42 60

−116 9.0 −286 −311

−104 5.3 −267 −288

−119 9.6 −292 −317

a ΔEel is the electronic energy; ΔZPE is the zero-point vibrational energy correction; ΔH is the enthalpy; and ΔG is the standard Gibb’s free energy. Energies were calculated at the PBE0-D3/def2-TZVP (COSMO) level of theory, and the enthalpies and standard free energies (T = 298.15 K, c⊖ = 1 mol·dm−3) include finite temperature contributions from translational, rotational, and vibrational degrees of freedom.

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DOI: 10.1021/acs.jpcc.5b10491 J. Phys. Chem. C 2016, 120, 1609−1614