Generation of Reactive Oxygen Species via Electroprotic Interaction of H2O2 with ZrO2 Gel: Ionic Sponge Effect and pH-Switchable Peroxidase- and Catalase-Like Activity

Article abstract of DOI:10.1021/acscatal.7b00189

Formation of reactive oxygen species (ROS) is of vital importance in catalytic oxidation chemistry. In this paper we have shown that a nonredox system such as amorphous zirconium dioxide (a-ZrO₂) is highly active in ROS formation via H₂O₂ decomposition. Interaction between a-ZrO₂ and H₂O₂ in aqueous solution was investigated by means of EPR, HYSCORE, Raman, and UV-vis, along with auxiliary FTIR, TG-MS, and XPS techniques, in a broad range of pH values and H₂O₂ concentrations. Various reaction intermediates such as superoxide (O₂^?-) and hydroxyl (^?OH) radicals as well as peroxide (O₂^2-) species were identified. At pH <5.3 the superoxide and hydroxyl radicals were generated simultaneously in large amounts with the peak concentration being reached around the isoelectric point of the gel catalyst. In this pH region, the ZrO₂ gel exhibited peroxidase-type activity, quantified by an o-phenylenediamine assay. At pH >5.3 formation of O₂^2- is accompanied by a substantial release of O₂ due to the pronounced catalase-like activity of a-ZrO₂. The role of electroprotic processes (an interfacial proton transfer coupled with an intermolecular electron transfer) in H₂O₂ decomposition and ROS formation was elucidated, and a plausible mechanism of this reaction, ≡Zr⁺-HO₂^-_(surf) + H₂O_2(aq) → ^?OH_(aq) + ≡Zr⁺-O₂^?-_(surf) + H₂O, was proposed. The surface of a-ZrO₂ covered with hydroxyl groups plays a role of an ionic sponge, which controls the electroprotic equilibrium by capturing the charged reaction intermediates. Unlike the amorphous gel, crystalline zirconia exhibits only weak activity in the production of the O₂^?- and ^?OH radicals, and a different mechanism is involved. It is worth mentioning that the activity of the zirconia gel catalyst in ROS generation, as gauged by the Michaelis-Menten constant, is comparable (ca. 40%) to that of the Fenton-type oxides (Fe₃O₄, Co₃O₄).

Full text of DOI:10.1021/acscatal.7b00189

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Article
Generation of reactive oxygen species via electroprotic
2
2
2
interaction of HO with ZrO gel – ionic sponge effect and
pH-switchable peroxidase- and catalase-like activity
Kamila Soba#ska, Piotr Pietrzyk, and Zbigniew Sojka
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00189 • Publication Date (Web): 14 Mar 2017
Downloaded from http://pubs.acs.org on March 15, 2017
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Generation of reactive oxygen species via
electroprotic interaction of H2O2 with ZrO2 gel –
ionic sponge effect and pHꢀswitchable peroxidaseꢀ
and catalaseꢀlike activity
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Kamila Sobańska, Piotr Pietrzyk,* Zbigniew Sojka
Faculty of Chemistry, Jagiellonian University, ul. R. Ingardena 3, 30ꢀ060 Krakow, Poland
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ABSTRACT
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Formation of reactive oxygen species (ROS) is of vital importance in catalytic oxidation
chemistry. In this paper we have shown that a nonꢀredox system such as amorphous zirconium
dioxide (aꢀZrO2) is highly active in ROS formation via H2O2 decomposition. Interaction between
aꢀZrO2 and H2O2 in aqueous solution was investigated by means of EPR, HYSCORE, Raman,
UVꢀVis, along with auxiliary FTIR, TGꢀMS, and XPS techniques in a broad range of pH values
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–•
and H2O2 concentrations. Various reaction intermediates such as superoxide (O2 ) and hydroxyl
2–
(•OH) radicals as well as peroxide (O2 ) species were identified. At pH below 5.3 the superoxide
and hydroxyl radicals were generated simultaneously in large amounts with the peak
concentration reached around the isoelectric point of the gel catalyst. In this pH region, the ZrO2
gel exhibited the peroxidaseꢀtype activity, quantified by oꢀphenylenediamine assay. At pH > 5.3
2–
formation of O2 is accompanied by a substantial release of O2 due to the pronounced catalaseꢀ
like activity of aꢀZrO2. The role of electroprotic processes (an interfacial proton transfer coupled
with an intermolecular electron transfer) in H2O2 decomposition and ROS formation was
elucidated, and a plausible mechanism of this reaction, ≡Zr+–HO2 (surf) + H2O2(aq) → OH(aq)
+
–
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–•
≡Zr+–O2 (surf) + H2O, was proposed. The surface of aꢀZrO2 covered with hydroxyl groups plays
a role of an ionic sponge, which controls the electroprotic equilibrium by capturing the charged
reaction intermediates. Unlike amorphous gel, crystalline zirconia exhibits only weak activity in
–•
•
production of the O2 and OH radicals, and different mechanism is involved. It is worth
mentioning that the activity of the zirconia gel catalyst in ROS generation, gauged by the
MichaelisꢀMenten constant, is comparable (ca. 40%) to that of the Fentonꢀtype oxides (Fe3O4,
Co3O4).
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Keywords
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hydrogen peroxide, superoxide, hydroxyl radicals, oxidation catalyst, amorphous zirconia,
decomposition mechanisms, HYSCORE
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1. Introduction
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Due to the increasing use of hydrogen peroxide (H2O2) as a cheap and environmentally benign
oxidant in many catalytic reactions, extensive studies into mechanisms of H2O2 activation
(decomposition), and search for new catalysts active in this reaction have recently received a
great deal of attention.1,2,3,4,5,6,7 Hydrogen peroxide is a simple source of reactive oxygen species
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(ROS) such as •OH (hydroxyl radicals), O2–• (superoxide radicals), HO2 (hydroperoxyl radicals)
2–
or O2 (peroxide anions) of various oxidizing ability.2,5,8 They can be used in advanced
oxidation processes (AOP),5,9 epoxidation10 and oxidation11 of olefins, as well as for catalytic
waste water treatment, including removal of organic pollutants exemplified by phenol
derivatives.12 On the other hand, hydroxyl radicals are undesirable species in electrochemical
processes such as oxygen reduction reaction (ORR),13 and for proton conductive fuel cells where
they cause chemical degradation of the polymer membranes.14,15
•
The reactive oxygen species (especially OH radicals) are involved in many biological
processes being harmful for living organisms. Because of their kinetic instability, high oxidation
potential, and chemical nonselectivity they lead to molecular degradation of cells16 resulting in
extended inflammations. Degradation of protective polymer coatings of medical metal implants
by hydroxyl radicals generated upon reaction with transitionꢀmetal ions released from the
implant surface provides another example of the ROS destructive action.17
In catalytic applications, H2O2ꢀbased oxidation reactions rely on conversion of H2O2 into
hydroxyl1,5,12 or superoxide radicals,18 either by interaction with transitionꢀmetal catalysts such
as iron and manganese complexes19 or during photocatalytic processes.12c Formation of ROS
involves 1ꢀelectron reduction of hydrogen peroxide, which in the case of transitionꢀmetal
catalysts (molecular complexes or oxide surfaces) proceeds according to the Fentonꢀlike
mechanism or through processes analogous to those appearing in the HaberꢀWeiss scheme.12b
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Other materials active in the 1ꢀelectron H2O2 reduction are zeolites containing transitionꢀmetals
and titaniumꢀbearing molecular sieves.20a In particular, metallozeolites have been used for partial
oxidation of ethane21 and oxidation of methane to methanol,22,23 while titanosilicateꢀ1 has been
shown to catalyze epoxidation of propylene,10d or oxidation of benzene to phenol.24 Owing to
their wellꢀestablished performance for heterogeneous oxidation of organic substrates, the Tiꢀ
based materials may also serve as useful model systems for mechanistic studies.20aꢀd Another
route of H2O2 activation, but without interfacial electron transfer (nonꢀFenton pathway) takes
place on the surface of d0 transitionꢀmetal ion oxides (e.g. ZrO2, Nb2O5, Ta2O5) which possess a
nonꢀredox character.5 Such oxides exhibit high activity in glycerol oxidation by H2O2.25
In particular, the nonꢀFenton type of H2O2 activation has been intensively investigated over
crystalline ZrO2 of the micrometric grain size.26,27,28 The postulated reaction mechanism consists
of three stages
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H2O2 → 2•OH,
(1)
(2)
(3)
•OH + H2O2 → HO2 + H2O,
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2HO2 → H2O2 + O2,
and does not involve any charged intermediates. The reaction is initiated by the homolytic
splitting of H2O2 on the surface, leading to the formation of the hydroxyl radicals (Equation 1).
The latter interacting with H2O2 produce hydroperoxyl species (Equation 2). Accumulation of
•
HO2 radicals in solution leads to their bimolecular decomposition into H2O2 and dioxygen
(Equation 3). Similar mechanism has been proposed for aqua complexes of M3+ ions (M = Ga,
•
In, Sc, Y, La),29 for which formation of OH is again associated with their ability to the
homolytic cleavage of the HO–OH bond.
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Enzymatic abatement of undesired hydrogen peroxide in biological systems occurs along two
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basic pathways referred to as activity of peroxidase,
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H2O2 + e– + H+ → •OH + H2O followed by •OH + e– + H+ → H2O;
and activity of catalase, where hydrogen peroxide is “directly” decomposed into dioxygen
H2O2 → H2O + ½ O2. (5)
(4)
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In the case of the peroxidase activity the necessary electrons and protons are provided by a donor
molecule (RH), giving rise to the following overall reaction: H2O2 + 2R–H → 2H2O + R–R. The
characteristic feature of the peroxidase pathway is formation of hydroxyl radical intermediates in
the course of the reaction, in contrast to the catalaseꢀlike activity, which does not involve explicit
ROS formation.30 The systems capable of peroxidaseꢀlike activity can also be used as
electrochemical sensors for detection of H2O2 with great sensitivity, and an example is provided
by ZrO2ꢀgrafted collagen scaffold with adsorbed horseradish peroxidase.31 There are several
inorganic compounds (often referred to as inorganic mimetics of catalase and peroxidase)
exhibiting similar reactivity patterns as those of the enzymes. Examples of the peroxidase
mimetics are provided by TiO2 nanotubes,32 Co3O4 nanoparticles,33 Fe3O4,1,34 V2O5 nanowires,35
CeO2,36 or FeS.37 They produce ROS detectable by means of specific substrates such as TMB
(3,3’,5,5’ꢀtetramethylbenzidine), OPD (orthoꢀphenylenediamine) or ABTS (2,2’ꢀazinoꢀbis(3ꢀ
ethylꢀbenzthiazolineꢀ6ꢀsulfonic acid), which selectively react with the •OH radicals.38
–•
Superoxide radicals (O2 ), in turn, can be measured with nitro blue tetrazolium (NBT) substrate
and UVꢀvis detection.39 Another technique, which is helpful in observation of both •OH and O2
–•
radicals, is electron paramagnetic resonance spectroscopy (EPR) alone and in tandem with using
5,5ꢀdimethylꢀ1ꢀpyrrolineꢀNꢀoxide (DMPO) as a spin trap.40
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Unlike the previous studies concerning only a crystalline state of ZrO2, in this work we
describe application of amorphous zirconia gel for profuse generation of ROS via H2O2
decomposition, by exploiting its pHꢀtunable peroxidaseꢀ and catalaseꢀlike reactivity. The
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•
–•
observed simultaneous appearance of OH and O2 species, which cannot be accounted for
within the previously proposed radical mechanisms, indicates that the respective reaction steps
reported in literature have to be carefully revisited while explaining the activity of the gel
catalyst. For this purpose, the influence of the isoelectric point, pH of the reaction mixture, and
the crystallinity degree of the zirconia catalyst on ROS formation and speciation was examined
by means of in situ EPR/HYSCORE, UVꢀvis, Raman, TGꢀMS, and XPS spectroscopies,
corroborated by spin trapping and dissolved O2 measurements, as well as the test reactions for
peroxidaseꢀlike activity with OPD and NBT substrates. The results were accounted for by a new
electroprotic mechanism of H2O2 activation, where an ionic sponge comportment of the zirconia
gel plays a crucial role. Because the peroxidaseꢀtype activity of the zirconia gel may be regarded
•
as a measure of generation of OH radicals that can be used for the oxidation processes, and the
catalaseꢀlike activity corresponds to that part of H2O2 which is virtually lost by immediate
decomposition into O2, a diagram showing preferable conditions for the catalytic oxidation
processes involving H2O2 was proposed. To our knowledge, this is the first example of a
mechanistic study of such type of the interfacial chemistry between hydrogen peroxide and d0
metal oxide gels.
2. Materials and Methods
2.1. Materials. Amorphous zirconia (aꢀZrO2) powder was prepared from analytically pure
ZrOCl2ꢁ8H2O that was dissolved in distilled water. Subsequently, 25% ammonia solution was
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added dropꢀwise under vigorous stirring. The precipitate was washed with distilled water until a
negative reaction for Cl– ions, and dried at room temperature. For the spectroscopic
characterization of solid samples (EPR, Raman, XPS, TGꢀMS) typically 0.1 g of the catalyst was
treated with 1.35 mL of hydrogen peroxide (Sigma Aldrich) aqueous solution of various
concentrations (5 – 30%). This treatment was performed at room temperature by gradual addition
of H2O2 solution. The pH level was adjusted using 0.1M NH3 or 0.1M HCl aqueous solutions.
The resulting samples were dried at room temperature. As a reference sample, a commercial
crystalline mꢀZrO2 (Sigma Aldrich) of the monoclinic structure with the grain size of ~5 ꢀm was
used.
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2.2. Reactions. The peroxidaseꢀlike activity of the amorphous zirconia was investigated by
•
measuring absorbance of the oxidation product of the peroxidase substrate, OPD, with OH
radicals. The experiments were carried out using suspensions composed of 1.3 mg/mL of aꢀZrO2
with 1.2 mmol/L OPD as a substrate, and 0.3 mol/L of H2O2. Whereas for colorimetric detection
of the produced superoxide radicals NBT was applied as a specific reagent (4 mmol/L). The
reactions were followed by means of a Cary 60 UVꢀVis spectrophotometer (Agilent
Technologies), and the spectra were recorded with elapsing time in the range of 200 – 800 nm.
The hydroxyl and superoxide radicals generated in the liquid phase were additionally detected
by applying the spin trapping technique. DMPO (Sigma Aldrich) was chosen as a suitable spin
trap because of its well documented high trapping ability and selectivity toward oxygenꢀcentered
radicals.40 In the spinꢀtrapping experiments a small amount of ZrO2 was contacted with a mixture
of 1 mL of H2O2/H2O and 10 ꢀL of DMPO. The liquid sample was next transferred into a quartz
capillary, and the EPR spectra were recorded at room temperature.
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2.3. Physicochemical Methods. Continuous wave EPR spectra were recorded with an Xꢀband
Bruker ELEXSYSꢀII E580 spectrometer, operating at 100 kHz field modulation with the 0.1 –
0.5 mT modulation amplitude. The spectra of the gel part of the samples were measured at 77 K,
whereas the liquid component was measured at ambient temperature. Prior to the measurements,
the samples (10 mg) were sealed in quartz tubes and outgassed in a vacuum line (p < 10–3 mbar)
for 2h at room temperature. The EPR parameters of the resultant paramagnetic species were
determined by computer simulation of the experimental spectra using the EPRsim32 package.41
Pulse EPR experiments were performed at the microwave frequency of 9.68 GHz at 40 K
(Oxford Instruments LHe cryostat). Electron spin echo (ESE) detected EPR experiments were
carried out based on the pulse sequence of π/2–τ–π–τ–echo, with the microwave pulse lengths
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tπ/2 = 16 ns, tπ = 32 ns, and the
τ value of 200 ns. Hyperfine sublevel correlation (HYSCORE)
measurements were carried out using the pulse sequence of π/2–τ–π/2–t1–π–t2–π/2–τ–echo, with
the microwave pulse lengths tπ/2 = 16 ns and tπ = 16 ns. In order to avoid blind spot effects,
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different τ values were used, as specified in the figure captions. In order to enhance the H
superhyperfine signal and the resulting correlation ridges due to the specifically interacting
surface protons, the matched HYSCORE experiments were performed with highꢀturning angle
(HTA) pulses incorporated into the pulse sequence π/2–τ–HTA–t1–π–t2–HTA–τ–echo. The
optimal length of the HTA pulse of 80 ns was determined experimentally. More details about
pulse experiments can be found in Supporting Information. The obtained correlation spectra
were simulated with the EasySpin software.42
Diamagnetic ROS varieties were detected by Raman spectroscopy. The measurements were
performed at ambient conditions with a Renishaw InVia dispersive spectrometer quipped with a
CCD detector and integrated with a Leica DMLM confocal microscope. Two laser lines (785 nm
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and 514 nm) were used depending on the observed signal intensity. The Raman scattered light
was collected with a 50x Olympus objective in the spectral range of 100 – 1500 cm–1. Nine scans
were accumulated to assure a good signalꢀtoꢀnoise ratio.
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FTIR spectra were recorded with Nicolet 6700, Thermo Scientific, spectrophotometer using a
single reflection ATR device with a diamond crystal. Powder Xꢀray diffraction (XRD)
measurements were performed by means of a Rigaku MiniFlex 600 diffractometer equipped with
a D/teX high speed silicon strip detector and a 600 W Xꢀray source (Cu Kα1 radiation).
The zeta potential as a function of pH, and the isoelectric point (IEP) were measured by means
of a Zetasizer Nano ZS apparatus (Malvern), equipped with a 633 nm laser and combined with a
MPTꢀ2 autoꢀtitrator. The suspension was titrated with either 0.1 M NaOH or HCl. The zeta
potential value was calculated from the electrophoretic mobility by applying the Smoluchowski
equation.
The concentration of dissolved oxygen was monitored in situ with an inoLab Multi 9430 IDS
setup, equipped with an optical sensor FDO 925 (WTW). The sensor was immersed in the aꢀ
ZrO2/H2O2 suspension, and the concentration of the evolving O2 was monitored as a function of
time and pH of the solution, while keeping the temperature constant at T = 295 K.
The Xꢀray photoelectron spectra (XPS) were recorded on a Prevac photoelectron spectrometer
equipped with a hemispherical VG SCIENTA R3000 analyzer. The spectra were measured using
a monochromatized aluminum AlKα source (E = 1486.6 eV). A low energy electron flood gun
(FS40AꢀPS) was used to compensate the surface charge. The background pressure in the
analytical chamber during the measurements was equal to 5×10−9 mbar. The spectra were
recorded with constant pass energy of 100 eV. The binding energies were referenced to the C 1s
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core level (Eb = 285.0 eV). Fitting of the O 1s and Zr 3d spectra was performed with the
CasaXPS software.
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Thermogravimetric analysis combined with mass spectrometry detection (TGꢀMS) was
performed using a Mettler Toledo TGA/SDTA 851e apparatus. The evolving gas products were
identified using a ThermoStar GSD300T Balzers quadruple mass spectrometer (QMS) operated
in the selectedꢀion monitoring (SIM) mode. To monitor O2, H2O and H2O2 (and their
fragmentation products) the following masses (m/z) were recorded: 17, 18, 32, 33 and 34. The
measurements were carried out in a flow of argon (80 cm3⋅min–1) in the temperature range 290 –
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1073 K, at a constant heating rate of 3 K⋅min–1.
3. Results and Discussion
Interaction of an aqueous solution of H2O2 with amorphous zirconia led to the formation of
both the liquidꢀphase and the surfaceꢀtrapped reactive oxygen species. The surface ROS were
analyzed by XPS, Raman, EPR, and TGꢀMS techniques, whereas liquidꢀphase radicals were
detected by EPR spectroscopy, applied in tandem with the spin trapping technique, and UVꢀvis
colorimetric measurements using OPD substrate. The peroxidaseꢀ or catalaseꢀlike activity of the
amorphous ZrO2 was next evaluated as a function of pH by measuring the amount of the
hydroxyl radicals (OPD substrate assay) and dissolved oxygen, respectively.
3.1. Oxygen species trapped on a-ZrO2 surface
Accumulation of the oxygen species on the surface upon interaction of H2O2 aqueous solution
with aꢀZrO2 was revealed by XPS spectra, recorded in the 542 – 525 eV region, before (Figure
1a) and after (Figure 1b) the reaction. The observed multicomponent O 1s band was disentangled
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by a leastꢀsquares fitting, assuming the contributions from four distinct signals corresponding to
adsorbed H2O (536 eV), adsorbed oxygen species (534 eV), surface Zr–OH groups (531.5 eV)
and lattice O2– anions (530.3 eV).43 The results indicate clearly a substantial increase in the
amount of the adsorbed oxygen species (yellowꢀshaded signal in Figure 1) upon the interaction
with H2O2, as the area of the 534 eV peak increased from 10% to 18% of the total intensity of the
O 1s band. At the same time the component due to the surface hydroxyls (green area in Figure 1)
augmented from 37% to 42%. In the Zr 3d region (Figure S1) virtually no changes in the XPS
spectra were observed for the pristine and the H2O2ꢀtreated samples.
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Figure 1. XPS spectra recorded in the O 1s region for aꢀZrO2 (a) before and (b) after the reaction
with 30% H2O2, along with decomposition of the oxygen band into the individual component
signals (yellow area – adsorbed oxygen species, green area – surface hydroxyl groups).
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The XPS results show that after the reaction with hydrogen peroxide the surfaces of the dried
amorphous zirconia gel is covered simultaneously by the adoxygen, hydroxyls, and adsorbed
water. The chemical nature of the adoxygen species unraveled by XPS was next examined by
Raman and EPR measurements.
Figure 2. Raman spectra of amorphous ZrO2 treated with H2O2 at various pH. The shadowed
2–
region around 840 cm–1 is diagnostic of the surface peroxo (O2 ) species.
Raman spectra of the H2O2ꢀtreated zirconia gel at pH ranging from 1 to 11 (Figure 2) show a
band located at 841 cm–1 assigned to the O–O vibration of surface peroxo species44 (a similar
band was observed for peroxo species bound to zirconiaꢀbased oxoclusters).45 Such attribution is
in agreement with the observed changes in the intensity of this band with respect to pH of the
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reaction mixture. It increases significantly with the increasing pH (upon addition of NH3(aq)),
which can be accounted for by forced dissociation of H2O2 into peroxo species that are next
captured on the zirconia surface (see discussion below).
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Figure 3. Powder EPR spectra (recorded at 77 K) of aꢀZrO2 (a) treated with hydrogen peroxide
at various pH and (b) of various H2O2 concentrations, (c) comparison of superoxide signals
observed for amorphous and crystalline ZrO2 samples. Gray dashed line represents
corresponding simulated spectrum.
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Complementary EPR measurements of the aꢀZrO2 treated with H2O2 solution at various pH
were subsequently performed. The obtained orthorhombic signal (Figure 3a), observed upon
drying of the samples, is characteristic of the surfaceꢀstabilized superoxide anions.26,46 The
intensity of the O2–• signal changes strongly with the acidity of the reaction medium (Figure 3a)
and the concentration of H2O2 as well (Figure 3b), but its symmetry and shape are essentially
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–•
preserved. Whereas for the increasing H2O2 concentration the intensity of the O2 signal grows
steadily, in the case of pHꢀdependent measurements a clearly nonꢀmonotonous variation of the
EPR intensity was observed, with a maximum appearing at pH around 3. The mechanistic
significance of these observations is discussed below.
The spinꢀHamiltonian parameters of the observed EPR signal, gxx = 2.0029, gyy = 2.0095, and
gzz = 2.0329, obtained by computer simulation, are typical of the superoxide ions stabilized
electrostatically on the surface Zr(IV) centers.26,46 On the contrary to the amorphous zirconia, for
–•
the crystalline mꢀZrO2 specimen the EPR signal due to O2 is barely visible (Figure 3c),
reaching up to 1.6% of the intensity obtained for the amorphous system, indicating that the
crystallinity of zirconia plays an important role in the interaction with H2O2 that leads to the
formation of superoxide species. Similar EPR signals were observed previously for crystalline
monoclinic zirconia.26
To unravel the role of zirconia crystallinity in the activation of hydrogen peroxide in a more
detail, we studied formation of the superoxide radicals over a series of ZrO2 samples of gradually
decreasing BET surface area (Figure 4a), obtained by calcination of the parent zirconia gel at
progressively increasing temperatures (298 – 873 K). The EPR measurements show that the
–•
intensity of the O2 signal declines sharply with the decreasing surface area of the zirconia
sample (Figure 4a). However, by plotting the normalized EPR signal intensity (IEPR) with respect
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to the surface area (SBET) as a function of the calcination temperature (Figure 4b), it can be
observed that this ratio exhibits a sigmoidal shape, falling abruptly around 500 K. Such behavior
indicates that not only a mere surface area itself, but also the concomitant changes in the
chemical nature of the surface induced by the calcination are of crucial importance for the
observed depletion of the trapped superoxide species.
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Figure 4. (a) Influence of calcination temperature of aꢀZrO2 sample on EPR signal intensity
(IEPR) of superoxide species and BET surface area (SBET). (b) Variation of the IEPR/SBEA ratio with
increasing calcination temperature.
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The implied chemical changes involve dehydroxylation and dehydration of the surface, as
shown by the IR spectra of the OH vibration region (Figure 5a), followed by phase
transformation from an amorphous gel into a mixture of the monoclinic and tetragonal
polymorphs of zirconia, determined by XRD measurements (Figure 5b). The obtained results
show the importance of the Zr–OH groups for seizing the superoxide anions on the surface of the
zirconia gel in large amounts.
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Figure 5. (a) FTIR ATR spectra and (b) powder XRD patterns of amorphous and crystalline
ZrO2 samples (calcination at 773 K in air). t indicates a tetragonal phase, and m a monoclinic
phase of ZrO2.
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To probe the chemical environment (surface OH groups) of the superoxide species adsorbed
on the aꢀZrO2 surface in more detail, pulse EPR measurements were carried out. At the
beginning, an electron spin echo (ESE) detected EPR spectrum was recorded (Figure S2a,
Supporting Information), the first derivative of which closely resembles the corresponding CWꢀ
EPR spectrum shown in Figure 3. The ESE spectrum was used for selection of the magnetic field
observer positions (B1, B2, and B3), corresponding to the principal orientations of the g tensor at
which the HYSCORE experiments were then performed.
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Figure 6. HYSCORE spectra taken at magnetic field of B2 = 344.2 mT with τ = 128 and 176 ns.
The red patterns represent computer simulation results, where (a) the matrix effect resulting from
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remote protons due to the surface Zr–OH and H2O adspecies was included along with the
specifically interacting protons, (b) only one type of the protons coupled via superhyperfine
interaction with the superoxide species was considered.
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An exemplary 2D matched HYSCORE spectrum recorded for the B2 observer position is
shown in Figure 6. The spectrum is featured by a singular correlation peak with the major
1
intensity at (14.6, 14.6) MHz, corresponding to the H Larmor frequency (νH) at 344.2 mT.47
This type of signal (present for all the observer positions B1, B2 and B3 shown in Figure S2b,c,d,
Supporting Information) originates from the abundant remote protons due to the surface Zr–OH
and H2O adspecies, forming a matrix effect (see simulation in Figure 6a). It tends to suppress the
signal originating from the presence of the proximal protons interacting with the superoxide
species explicitly, revealed by the simulation (Figure 6b). The superhyperfine interaction
(gauged by the Aiso + [–T, –T, 2T] tensor) with the proximal protons (S = 1/2 and I (1H) = 1/2)
gives rise to the appearance of ridges (extended correlation peaks due to powder nature of the
samples), clearly seen in the (+,+) quadrant of the HYSCORE spectrum. This is indicative of a
weak coupling regime |Aiso| < 2νH, and the shape of the ridge suggests that |Aiso| > |T|.48
Additionally, the presence of a signal near 2νH frequency (29 MHz), stemming from the doubleꢀ
quantum nuclear coherence,47,49 indicates that there are at least two proximal protons with
moderate couplings to the superoxide species. The upper limit of the superhyperfine coupling
parameters, Aiso = 16 ± 2 MHz and T = –1 ± 0.3 MHz, was assessed by extensive computer
simulations. Within the dipole model approximation, the obtained T value corresponds to the
distance of 4.3 ± 0.5 Å, where the protons around the O2–• species can be located. We may then
conclude that the superoxide radical is trapped on the surface Zr(IV) centers and interacts with
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the surface hydroxyls located in the vicinity of the adsorption center. There are also abundant
protons situated beyond the estimated distance limit. A plausible structure of such superoxide
environment is shown in Scheme 1.
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Scheme 1. Structural model of the intimate environment of a superoxide radical anion trapped on
the surface of aꢀZrO2. The model was generated with Materials Studio (Biovia), structural
–
parameters were taken from ref. [50]: average Zr–O distance 2.2 Å, Zr–O2 distance 1.9 Å, Zr–
–•
OH 2.08 Å, O–H 1 Å. The arrows show the shortest and the longest O2 –HO distance calculated
based on HYSCORE data.
Thermal stability of the oxygen species accumulated on the aꢀZrO2 surface upon interaction
with hydrogen peroxide at autogenous pH = 3 (the equilibrium value obtained after mixing up
the gel and the H2O2 solution) was examined by TG measurements combined with QMS
identification of the evolving gas products. The corresponding mass loss profile is shown in
Figure 7a. In the temperature range of 300 – 1100 K the observed 29.5% mass loss with the
maximum around 350 K (see the m/z = 17 signal in Figure 7b), following our previous paper,51
results mainly from cooperative processes of dehydration ([Zr4O(8ꢀx)(OH)2x⋅yH2O]n → [Zr4O(8ꢀ
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x)(OH)2x]n + ynH2O) and dehydroxylation ([Zr4O(8ꢀx)(OH)2x]n → 4n(m/t–ZrO2) + xnH2O) of the
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amorphous zirconia.
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Figure 7. Thermal decomposition of aꢀZrO2 after treatment with H2O2 at pH = 3. (a) TG profile
and (b) QMS signals of H2O (m/z = 17) and O2 (m/z = 32) evolving into gas phase during
decomposition along with parallel evolution of the EPR signal due to surface superoxide species.
At temperatures around 373 K the surface trapped ROS produced upon interaction with H2O2
are decomposed, leading to a release of gaseous O2 (see the m/z = 32 signal in Figure 7b). The
temperature variation of the O2 evolution matches quite well the corresponding intensity changes
in the EPR signal due to the surface superoxide species. Interestingly, it also correlates with the
m/z = 17 profile of the desorbed water, indicating that ROS recombination into O2 may be
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associated with the surface H2O discharge. As a result, the concerted changes in the intensities of
both the QMS and EPR signals indicate that the surface superoxides formed initially during the
reaction between H2O2 and aꢀZrO2 at ambient temperature become unstable while heating above
–
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330 – 350 K. Large amounts of desorbed O2 indicates that the charged oxygen species (O2 and
2–
O2 ) can only be accommodated on the surface in the presence of charge balancing protons
+
attached to the hydroxyl groups (≡Zr–OH2 ). The observed concomitant desorption of H2O and
–•
–•
O2 implies a mechanism involving superoxide disproportionation, O2
+ O2
→ O2(g)
+
(surf)
(surf)
2–
+
2–
O2 (surf), coupled with thermal dehydration of the surface, 2(≡Zr–OH2 (surf)) + O2
→
(surf)
≡Zr–O–Zr≡(surf) + ½O2(g) +2H2O. The implied superoxide disproportionation and O2 release has
been observed previously for, e.g., MoOx/SiO2 system.52 Thus, large amounts of negatively
charged O2 and O22– species accommodated on the gel surface, revealed by thermal desorption,
–
XPS, EPR, and RS investigations are made possible by simultaneous protonation of the surface
(–OH(surf) + H+ → –OH2 (surf) and –O–
+ H+ → –OH(surf)), in order to preserve overall
+
(aq)
(surf)
(aq)
charge neutrality within the interfacial layer of the dried gel.
3.2. ROS detected in aqueous phase – hydroxyl radicals
Apart from identification of ROS accumulated on the aꢀZrO2 gel upon H2O2 treatment, the
radicals generated in the aqueous phase of the suspension were determined in situ by applying
DMPO spinꢀtrap technique and the EPR measurements. After addition of aꢀZrO2 gel to the
H2O2/H2O mixture, and subsequent separation of the solid phase, an isotropic quartet EPR
•
spectrum characteristic of the DMPO adducts with OH radicals was recorded (Figure 8a). The
spinꢀHamiltonian parameters of the DMPO–•OH adduct obtained by computer simulation (giso
=
2.0057, aN = 1.51 mT, aH = 1.45 mT) remain in a good agreement with the literature data,40
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confirming definitely the presence of OH radicals in the liquid phase. Blank experiments,
carried out at the same conditions but without aꢀZrO2 added, showed that no EPR signal was
formed (green line in Figure 8a). Parallel EPR measurements of the separated gel revealed a
signal that is characteristic of the already observed superoxide species with gxx = 2.0029, gyy =
2.0095, and gzz = 2.0329 (Figure 8b). It was contaminated by a triplet signal due to the nitrosyl
radicals, arising from partial degradation of DMPO trap. For the slurry sample, where both the
solid aꢀZrO2 and the liquid (H2O2/H2O) phases were present together, the EPR signals
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–
characteristic of the surface superoxide species (≡Zr(IV)–O2 (surf)) and the liquid phase hydroxyl
radicals (DMPO–•OH(aq) adduct) were observed simultaneously (Figure 8c).
Figure 8. Experimental and simulated EPR spectra recorded at room temperature after the
reaction of aꢀZrO2 with H2O2 in the presence of DMPO spin trap. (a) EPR spectrum of the liquid
phase separated from the catalyst, (b) spectrum of the solid aꢀZrO2 phase, (c) spectrum of the
slurry (aꢀZrO2 suspension in H2O2/H2O solution) with the features of the adsorbed superoxide
species and DMPO–•OH adducts present simultaneously. The nitroxide degradation product of
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the spin trap is indicated by gray dots. The simulated EPR signals of particular components are
shown as dotted lines.
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Apparently, among both types of the simultaneously generated radicals, O2–• and •OH, only the
neutral hydroxyl radicals were trapped with DMPO, while within the experimental detection
limit, essentially all the charged superoxide species were effectively captured at the surface of aꢀ
ZrO2 gel sponge, and additionally stabilized by the surface protons as shown by HYSCORE and
XPS measurements.
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3.3. The pH dependence of O2–• vs O22– formation
As described above, the amount of the surface peroxide (Figure 2) and superoxide anions
(Figure 3a) changes strongly with pH of the reaction mixture. This observation suggests that
ROS generation is mediated by protons and their relocation between the solution and the gel
surface. Indeed, the pH value of the suspension may influence speciation into the peroxo and
superoxo entities by controlling the protic equilibria between the surface (hydroxyls) and liquid
phase (H2O2/H2O) components of the suspension. For the pH values below the isoelectic point,
IEP = 4.1, (see Figure S3, Supporting Information), the surface of the amorphous zirconia
becomes positively charged
+
≡Zr–OH(surf) + H3O+(aq) → ≡Zr–OH2 (surf) + H2O,
(6)
where ≡Zr–OH(surf) should be regarded as surface OH– group attached to a low coordinated
≡Zr(IV)+ surface species exhibiting a formal charge of +1 with respect to the fully coordinated
Zr(IV) cation. Surface protonation (Equation 6) competes with surface condensation and
complexation reactions (owing to the wellꢀknown dissociation process ≡Zr–OH(surf) → ≡Zr+
(surf)
+ OH–(aq)),53 similar to those previously proposed for titania:54
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≡Zr–OH(surf) + H2O2(aq) → ≡Zr+–HO2 (surf) + H2O,
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(condensation reaction)
(7)
(8)
≡Zr+(surf) + ≡Zr–OH(surf) + H2O2(aq) → ≡Zr+–HO2 (surf) + ≡Zr–OH2 (surf)
.
–
+
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(complexation reaction)
For pH > 4.1 the surface of aꢀZrO2 is negatively charged due to deprotonation of the ≡Zr–
OH(surf) groups (≡Zr–OH(surf) + OH– → ≡Zr–O–
+ H2O), thus, the dissociation of H2O2 is
(surf)
(aq)
controlled by the OH–(aq) anions in the liquid phase and the ability of seizing the resultant anions
by the gel surface.
–•
Figure 9. Impact of solution pH on the formation of (a) O2 (relative intensity of the EPR
spectra) versus O22− (relative intensity of the Raman band) species and (b) OH• radicals (changes
in the UVꢀVis absorbance at 440 nm in the presence of OPD) during the interaction between aꢀ
ZrO2 and H2O2 solution.
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Figure 9 shows variation in the intensity of the spectral signatures of the superoxo and peroxo
species, produced during interaction of hydrogen peroxide with aꢀZrO2 as a function of pH in
more detail. The maximum of the intensity of the superoxide EPR signal is located in the vicinity
of the isoelectric point of aꢀZrO2 (Figure 9a). An increase or decrease of the pH value in this
region results in a rapid drop of the concentration of the surface superoxo radicals. At the same
time the concentration of the surface peroxo species increases steadily with pH and both curves
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–•
2–
(O2 and O2 ) cross slightly above the IEP value. We have recently observed the same IEPꢀ
–•
2–
dependent trends in the O2 and O2 formation for Nb2O5 and Ta2O5 gels.5 Indeed, the
–•
maximum of the EPR signal intensity for ≡Zr+–O2 is shifted toward higher pH, in agreement
with the higher value of the isoelectric point of ZrO2 as compared to Nb2O5, (IEP = 2.8 for
amorphous Nb2O5 and 4.1 for amorphous ZrO2). Thus, such behavior is more general in nature,
–•
2–
and specific shape of the ([O2 ], [O2 ]) = f(pH) dependence is characteristic of a given
amorphous oxide.
The identified ROS varieties produced during interaction of H2O2 with aꢀZrO2 disclose a
double role of the amorphous surface in this reaction. The equilibrium between the surface
–
hydroxyls and H2O2(aq) leads to the formation of surface HO2 anions (Equations (7)ꢀ(8)),
constituting the first step of H2O2 decomposition. The second role of aꢀZrO2 was revealed by
analyzing the amount of the produced hydroxyl radicals as a function of the pH value. As
implied by Figure 9, apparently it varies in a parallel way with the content of the superoxide
radicals, implying that during H2O2 decomposition these both intermediates are produced
simultaneously. This mechanistically crucial observation rules out the homolytic splitting of
H2O2 into the hydroxyl radicals (Equation 1), proposed earlier for crystalline ZrO2 oxides,26,27,28
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•
–•
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since in this mechanism HO2 (and O2 ) species are formed at the expense of the hydroxyl
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–•
radicals (Equation (2)). The observed concomitant formation of OH and O2 species with the
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same pHꢀdependency (Figure 9a,b) implies the following electroprotic reaction to be involved:
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–
–•
≡Zr+–HO2 (surf) + H2O2(aq) → •OH(aq) + ≡Zr+–O2 (surf) + H2O.
(9)
•
It can be regarded as the ionic equivalent of the HaberꢀWeiss reaction (HO2 + H2O2 = OH• + O2
+ H2O)55 induced by the ionic sponge effect (strong electrostatic stabilization of the charged
superoxide ions on the tetravalent zirconium ≡Zr(IV) centers of the amorphous zirconia gel).
Due to the coupled proton and electron transfer it can formally be envisaged as transfer of a
hydrogen atom (H• (p+/e–) + H2O2 = OH + H2O) released from the HO2 moiety. It is worth
mentioning that akin hydrogen transfer processes have been postulated for H2O2 decomposition
previously.56
•
–
For the analogous reaction in the liquid phase (HO2 (aq) + H2O2(aq) = •OH(aq) + O2 (aq) + H2O),
the thermodynamic calculations, based on the averaged values of the redox potentials of the
corresponding partial reactions available from literature (see Supporting Information), give the
ꢁrG° value around 15 kcal/mol. Such positive value is caused by weak dissociation constant of
H2O2 (Ka = 2×10–12), and low stability of the superoxide anion in the aqueous environment.
Thus, the unfavorable thermodynamics has to be overcome by engagement of the gel surface for
–
–•
–
–•
stabilization of the ionic intermediates (HO2 and O2 ). From the observed gzz value of the
–•
superoxide species trapped on the Zr(IV) sites, the stabilization energy of the adsorbed O2 can
be estimated,57 which amounts to about 0.9 eV, driving the above electroprotic reactions toward
formation of the superoxide and hydroxyl radicals.
At pH < IEP, surface is charged positively by protonation, favoring stabilization of the anionic
species as counter ions. Above IEP, the negative potential of the surface disfavors trapping of the
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superoxide anions making the whole process not only pHꢀdependent but also IEPꢀdependent.
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electroprotic processes. The decreasing O2 trapping efficiency of the zirconia surface with the
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increasing pH value above IEP accounts for the observed concomitant decline of the superoxide
and hydroxyl radical formation. The proposed mechanism explains also the crucial role of the
zirconia phase in the H2O2 decomposition. In contrast to crystalline ZrO2, only in the amorphous
state of zirconia, its surface area (Figure 4a) and the amount of terminal hydroxyl groups are
sufficiently large to promote effectively the reaction described by Equation (9).
It is also worth noting that analogous radical products were observed for reaction between aꢀ
ZrO2 and H2O2 carried out in a dry phase (adsorption of gaseous H2O2, produced by thermal
decomposition of CO(NH2)2⋅H2O2 substrate, on the surface of aꢀZrO2 preꢀimpregnated with
•
DMPO for OH detection by EPR). Thus, it implies that reaction (9) proceeds regardless the
presence of aqueous medium, and that the crucial role is played by the ability of H2O2 to ligate to
the gel surface:
2H2O2(aq) + ≡Zr–OH(surf) + DMPO(surf) → •OH–DMPO(surf) + ≡Zr+–O2 (surf) + 2H2O.
(10)
–•
–•
An alternative radical mechanism of O2 formation via deprotonation of the hydroperoxyl
radical,58 produced according to the reactions (1) and (2),
•
–•
HO2 (aq) → H+(aq) + O2
,
(11)
(surf)
can be disregarded as the observed concordance between the formation of O2–• and OH• (shown
•
in Figure 8) is lost, for the intermediate HO2 radicals may decay in a parallel route described by
Equation (3). Furthermore, in the low pH region the deprotonation equilibrium (11), for which
pKa = 4.8,59 in an aqueous medium will be strongly shifted to the left. Although the EPR signals
of the superoxide radicals formed on the surface of crystalline26 and amorphous zirconia samples
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are virtually identical in shape, they differ dramatically in the intensity (1:60) implying that the
involved mechanisms must be different. According to the postulated reaction steps (1), (2) and
(11), one may expect that rate of decomposition of H2O2 via homolytic HO–OH bond splitting is
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–•
proportional to concentration of O2 and reversibly proportional to [•OH]. This remains in
variance with our experimental results shown in Figure 9. It should be noted, however, that the
mechanisms based on the homolytic HO–OH bond splitting may operate in a feeble way at the
background being dominated by the electroprotic pathway. The proportion of involvement of
both mechanisms may vary with the nature of gel catalysts used for H2O2 decomposition.
In contrast to the nonꢀmonotonous profile of the O2–• formation, the surface peroxy anions are
produced steadily with the increasing pH implying a simple pathway that may involve formation
of bridging peroxo groups:
H2O2(aq) + 2(≡Zr–O–(surf)) → ≡Zr+–O2 –+Zr≡(surf) + 2OH–
,
(12)
2–
(aq)
or bidentate peroxo groups:
H2O2(aq) + ≡Zr–O–(surf) → ≡Zr+–O2 (surf) + H2O.
(13)
2–
2–
Both reactions are driven by stabilization of the resultant O2 anionic species at the surface of
the amorphous zirconia in form of the ꢀꢀperoxide or η2ꢀperoxide complexes, respectively. Such
adsorption species have been described in the zirconiaꢀbased systems45 as well other supported
systems active in H2O2 decomposition.2 The crucial role in these processes is played by high
2–
charge of the Zr(IV) surface cations and that of the O2 species as well. Due to the sizeable
2–
electrostatic interactions the dianions O2 can be trapped at the surface more effectively,
replacing the monoanionic O2 (surf) or OH–(surf) species.
–•
3.4. Peroxidase- and catalase-like activity
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