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Photocatalytic Reaction NO + CO + h# # CO + 1/2 N Activated on ZnO in UV-Vis Region 2

1-X

Ilya V. Blashkov, Lev A. Basov, and Andrey A. Lisachenko J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10143 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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The Journal of Physical Chemistry

Photocatalytic Reaction NO + CO + hν → CO2 + 1/2 N2 Activated on ZnO1-x in UV-Vis Region Ilya V. Blashkov, Lev L. Basov, Andrey A. Lisachenko* Department of Physics, Saint-Petersburg State University, Ul’yanovskaya str., 1, SaintPetersburg 198504, Russia. *Corresponding author: Andrey A. Lisachenko, Saint-Petersburg State University, V.A. Fock Institute of Physics, Ul’yanovskaya str., 1. Saint-Petersburg, 198504, Russia. E-mail: [email protected]

Abstract It is shown for the first time that the photocatalytic reaction of NO reduction by CO into N2 can occur on self-sensitized ZnO1-x catalysts upon visible light irradiation (λ > 400 nm) at room temperature with the selectivity reaching 95%. The reaction proceeds in two stages through the intermediate product N2O. The formed CO2 remains on the surface and can be completely desorbed after completion of the photoreaction by heating ZnO1-x up to 820 K. The spectral dependencies of the first and of the second stages repeat the optical absorption spectra of the ZnO1-x intrinsic color centers: the oxygen vacancies having captured one or two electrons (F+ and F centers), the Zn+ ions, the Zn vacancies having -

trapped the hole (V centers). High values of quantum yields in the VIS region are caused by large (up to 103 s) lifetimes of photoexcited centers.The reaction mechanism is ACS Paragon Plus Environment

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proposed, it includes the following stages: (a) the NO photoadsorption (PA) on the F-type -

-

and on V- type centers producing the adsorbed NO and NO2 species respectively; (b) the -

-

reduction of NO and NO2 into N2O and further into N2 by CO which regenerates the donor centers.

1. Introduction Due to a set of its unique properties ZnO has become one of the most studied materials of the 21st century, its applications are the development of photo cells for converting solar energy into chemical and electrical energy, and of photocatalysts for the environmental purification, as functional optoelectronic elements1,2. In the energy structure of ZnO, the reduction potential of the electron at the bottom of the conduction band lies above the level of H+/H2, and the oxidation potential of the hole lies substantially below the level of O2/H2O. Therefore, the e-/h+ pair in the UV irradiated ZnO has one of the highest redox potentials among the photocatalysts, allowing one to use it in photochemical cells to produce hydrogen and purify the environment. The photocatalysis on ZnO is effective in degrading a wide range of organic and inorganic pollutants into biodegradable or less toxic organic compounds3,4. ZnO is widely used as a photocatalyst for wastewater treatment5-7. -

The radiation active for e /h+ pair photogeneration in ZnO lies in the UV region at λ ≤ 385 nm which covers no more than 5% of the solar energy. In order to sensitize ZnO to visible light containing about 40% of solar energy different approaches were developed.

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ZnO is sensitized with dyes or doped by metal and nonmetal ions1,8-10, ZnO composites with narrow-gap semiconductors1,11,12 and plasmonic photocatalysts with nanostructured Au, Ag, Cu deposited on ZnO are created13-14. The efficiency of photovoltaic solar energy converters based on sensitized ZnO of at least 12% has been reached15]. Meanwhile, the absorption in the visible region can be created using optically active intrinsic defects16-18. Using the photoadsorption of oxygen and hydrogen as test reactions, one can trace the mechanisms of interaction of photogenerated carriers with adsorbate at the stages of reduction and oxidation19. The analysis of the kinetics of the photoactivated oxygen isotope exchange with an oxide allows one to trace the complete cycle of the redox reaction20. Earlier21, an ecologically important reaction hν

CO + NO → 1/2N 2 ↑ +CO 2 ads

(1)

which can be photocatalyzed by a wide-bandgap TiO2-x semiconductor in the visible region of intrinsic colored defects was discovered and studied in our laboratory. This reaction is attractive for fundamental research. This way one can obtain extensive information on the mechanism of this multistage reaction combining the monitoring of products in the gas phase by a mass spectrometric (MS) method with the analysis of intermediates in the adsorbed phase by FTIR and TDS methods. The reaction (1) allows one to investigate the physical photoexcitation stages as well as the relaxation ones. The channels of transfer of the excitation energy of the electronic subsystem to the reactants, and the subsequent reactions of electronically excited precursors are manifested21,22. Note that neither the NO nor CO molecules do not absorb the UV–Vis ACS Paragon Plus Environment

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activating radiation and consequently they do not distort the investigated photoactivation ZnO spectrum. Thus the reaction (1) can be used as a test in the development and comparison of highly effective photocatalysts. Although the photocatalytic parameters of TiO2 and of ZnO are close to each other, the latter has a number of advantages. Unlike TiO2, ZnO is a direct-gap semiconductor, and so its absorption in the near-threshold region is stronger than that of TiO2. The electron mobility in ZnO is two orders of magnitude higher than that in TiO2 23,24, that sharply increases the efficiency of the photogenerated carriers transport to the surface reaction centers. The binding energy of excitons in ZnO is 60 meV, ensuring their stability at room temperature (kT=27 meV). This allows one to consider the possibility of an exciton mechanism for ZnO excitation. The aim of this work was to investigate the possibility of ZnO self-sensitization by intrinsic defects for its photoactivation in both the UV and Vis spectral regions. The reaction (1) was chosen as the test one. In order to analyze the active centers and to construct the reaction mechanism we studied the spectra of its photoactivation, the specific kinetic parameters of the reaction, the composition of the intermediates and the final products of the reaction. 2. Experimental technique and methods The reaction (1) was investigated by photomanometry, by kinetic photo-mass spectrometry in the gas phase, and by thermal desorption spectroscopy (TDS) with mass analysis of the adsorbed phase. The experimental setup is shown in Figure 1. The form of the reactor (a Dewar cylinder with a poured sample) allows us to increase the efficiency of


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the photons use in the region of weak absorption due to multiple reflections from the sample surface. The composition of the gas phase and of the desorption products was monitored using an APDM-1 monopole mass spectrometer (MS). The reactor is equipped with a heater allowing one a linear heating for measurement of TD spectra and for heat treatment of the sample. The residual vacuum in the reactor was not worse than P=10−7 Torr. The use of an absolute manometer for calibration allowed us to obtain the results in absolute units [molecules cm-2] for coatings and [molecules s-1 cm-2] for flux values.

Figure 1. Experimental system (MPi - manometer Pirani, Mabs – absolute manometer, M-S - massspectrometer, V2 - volume to prepare a gas mixture).

A highly dispersed powder ZnO «OSCH 12-2» with the mass of 5.6 g and the main substance content not less than 99.99% was investigated. The following methods were


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used to characterize the samples: X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), Brunauer–Emmett– Teller method (BET), diffuse reflectance spectroscopy (DRS). In order to exclude in our work the dependence of the type of the NO adsorption on the degree of surface hydration, we removed water molecules and other native impurities by preliminarily heating the ZnO in a stream of pure (99.99%) oxygen (P ≈ 0,2 Torr) at 823 K for two hours until the traces of oxidized contaminants completely disappear in the massspectra of the output flow. We used 12C16O of the natural isotopic composition and the isotope-enriched NO (95% of 15N16O). The following illuminators were used to irradiate the samples: a mercury lamp DRSh-250, a xenon lamp (Osram XBO 150W/4). Separate spectral lines were singled out with standard glass filters produced by the LOMO company (St.-Petersburg) using a thermal water filter. The intensity of the incident monochromatic radiation was of the order of ≈1015 [photons cm-2 s-1]. Figure 2 shows the absorption spectra of the planar and the cylindrical cells filled with ZnO, as well as the efficiency factor χ =

(Abs ZnO )cylinder . Thus, at the incident light with (Abs ZnO )flat

λ=546 nm the number of absorbed photons for the Dewar cylinder is five times greater than that for a planar reactor. In the UV region, the efficiency of the cylinder does not exceed the factor of 2.


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Figure 2. ZnO absorption spectra: 1– planar sample, 2 – cylindrical sample, 3 – efficiency rate.

3. Results 3.1 Irradiation of ZnO in the mixture of CO+NO It was found that the UV-Vis illumination of ZnO in a mixture of NO + CO leads to a sharp drop in the NO pressure with the release of N2O into the gas phase, which is then reduced to N2 (Figure 3 (a)). The emission of N2 starts after the completion of photoadsorption (PA) of NO. In the absence of CO, no emission of N2 into the gas phase was detected. The increase of the CO pressure at the initial stage of the irradiation with full light (Figure 3 (a)) is caused by two factors: on the one hand, a slight heating of the sample by the irradiation (from 300 K to 307 K) results in the desorption of weakly bound CO molecules; on the other hand, a partial replacement of CO molecules by the products of PA of NO is possible. That means that at the initial stage the reaction occurs mainly due to the previously adsorbed CO. Note


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that after the illumination is turned on, the rate of NO absorption is much greater than that of CO, and the C, N, O mass balance in the gas phase is violated. That means that the adsorbed initial, the intermediate and the final products play an important role throughout the whole process. The kinetic curves of CO and NO are typical for the initial products, while those of N2 are typical for the final products. The second final product CO2 is not detected in the gas phase, it is accumulated in the adsorbed state. The kinetic curve of N 2O emission has a maximum in the middle part, that is typical for the intermediate products. A satisfactory (within the error range) nitrogen mass-balance in the gas phase is observed: – ∆n (NO)≈1.8×1017 molecules compared to 2[(∆n (N2O) + ∆n (N2) ≈ 0.72×1017 molecules]. That means that N-containing forms do not remain adsorbed at the end of the reaction in appreciable amounts. At the final stage, an insignificant amount of N2O (no more than 10% of N2) remains in the gas phase, since the complete processing of N2O was not aimed in these experiments. It is known that N2O is photocatalytically decomposed on ZnO25. The kinetic features of the reaction (1) are similar to those observed earlier on TiO2 Degussa P-2521.

(a)

(b)


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Figure 3. Typical kinetics of the reaction (1) on ZnO under the irradiation by (a) – full spectral

range, (b) – visible light (λ> 390 nm).

In order to analyze the composition and the binding energy of the adsorbed intermediate and final products, a TDS with a mass spectrometric analysis of the desorbed phase was used. The method applies the Wigner-Polanyi equation for the desorption rate: E

− a dθ − = kθ n = Ae RT θ n dt

(3.1.1)

where θ is the surface concentration of the adsorbed molecules or the degree of the surface coverage, k is the desorption rate constant, A is the pre-exponential factor, Ea is the activation energy, R is the universal gas constant, T is the thermodynamic temperature, n is the order of the process (1st or 2nd). In the TDS method, the rate is measured with a linear increase in temperature: T = To + βt, where β is the heating rate (deg/min).

dt =

dT β

Substituting it in the initial equation (3.1.1) we obtain: E

dθ A − RTa n − = e θ . dT β

(3.1.2)

In the experiment, the desorption rate is determined in the mass spectrometry by the flow of molecules from the sample using the flow reactor regime26. The analysis of the TDS of the adsorbed final reaction products (Figure 4 (a, b)) allowed us to establish that the only desorption product is CO2, which is released upon


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heating up to 823 K. The TD spectra consist of a main maximum (in the 750-800 K range) and low-temperature shoulders in the range T> 500K (Figure 4 (a) and T> 600K (Figure 4 (b)). When modeling, we assumed Edes(θ) = const to reveal qualitatively the main features of the TD spectrum. The analysis of the spectra for the cases Edes(Θ(t)) ≠ const was given in our paper27. In the present work the TD spectra are modeled by a sum of monopics with the parameters ν = 1012 s-1, β = 20 K/min. For the first-order TD spectra modeled below, the presence of several monopics in the desorbed CO2 spectrum indicates the energy inhomogeneity of the surface with respect to CO2 adsorption. The analysis of the differences in the TD spectra obtained under UV-Vis and Vis illuminations (compare (a) and (b) in Figure 4) is beyond the scope of this paper.

(a)

(b)

Figure 4. TD spectrum of the products of the reaction (1) under the irradiation by (a) – full spectral range, (b) – visible light (λ> 390 nm).

The individual stages of the process were studied to reveal the reaction mechanism.


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3.2 Dark and photoinduced CO adsorption The CO admission into the reactor at room temperature is accompanied by a rapid reversible adsorption of CO. The coating at the initial pressure in the reactor P0 = 0.07 Torr does not exceed 10-4 ML. The CO adsorption is accompanied by the appearance in the ZnO IR spectrum of a band in the region 2168–2192 cm-1, see28-33, this band disappears with subsequent exhaust at room temperature, i.e. the adsorption is reversible28,29. The band is usually attributed to the CO molecule adsorbed on Zn2+ ions 28, 30-32. The illumination leads to an additional adsorption of CO. According to the MS data, CO2 is not released into the gaseous phase either at the dark adsorption or under the irradiation in CO. We have previously shown that the irradiation of TiO2 in CO induces the absorption in the IR-Vis region due to the conduction electrons, and the bands of monodentate (1450–1350 cm-1) and bidentate (1570–1315 cm-1) carbonates22. Similarly, -

under the irradiation of ZnO in CO the bands of carboxylate CO2 (1620 cm-1 band) and -

carbonate structures CO3 (1570–1315 and 1450–1350 cm-1 bands) appear34,35. The formation of negatively charged complexes with the associated positions and relative intensities of IR bands is strongly influenced by a preliminary oxidation-reduction treatment of zinc oxide28.


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Figure 5. TD spectra of the products of dark and photo– adsorption of CO (λ>390 nm) on ZnO (P0 (CO) = 0.07 Torr is identical to P0 (CO) in the mixture of NO+CO (Figure 3 (a, b)).

After the adsorption of CO (Figure 5) CO2 is present in the subsequent TD spectrum, that indicates the oxidation of CO by the structural oxygen of the sample surface, previously oxidized at 823 K. High-temperature TD peaks have an asymmetric shape, it indicates the first order mechanism of molecular desorption, i.e. breaking the bond of the CO2 molecule as a whole with the adsorption center. Therefore, in order to simulate the TD spectrum the value n in the formula 3.1.2 is taken equal to 1. One can see from Figure 5 that the simulating monopics do not fill the experimental TDS. This indicates the energy inhomogeneity of the surface, similarly as in the case of Figure 4. The analysis of TD spectra of dark and PA CO (Figure 5) allows us to conclude that the illumination promotes the formation of tightly bound CO2 structures (peaks in the


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region of 500-800 K). A small difference in the areas of low-temperature (345-360 K) CO adsorption peaks in the dark and in the light is explained by the fact that a significant part of weakly bound CO is desorbed during evacuation before recording the TD spectrum.

3.3 Dark and photoinduced NO adsorption NO is weakly adsorbed on a non-illuminated sample. At T = 300 K and P0 = 0.013 Torr, the coverage is ≈ 0.8×10-5 ML. A visible light irradiation of ZnO in NO alone gives strongly adsorbed species, which cannot be removed by evacuation at room temperature. NO photoadsorption, with the consumption of donor species, involves the following sequence of reactions:

NO ( g ) → NO ( a )

(3.3.1)

NO ( a ) + e − → NO (−a )

(3.3.2)

The illumination of ZnO in the NO atmosphere reduces the electrical conductivity36 as a result of capture of a d-electron by chemisorbed NO. The capture of a quasi-free electron by a NO molecule is typical for n-type oxides. The rate of PA of NO is linearly dependent on the intensity of the incident radiation. However, the kinetics of PA is not described by first- or second-order curves, that indicates the coexistence of several mechanisms of NO PA. In order to obtain the information on the nature of photoactivated centers, the spectral characteristics of NO PA were analyzed.


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Figure 6. Spectral dependence of the quantum yield of NO PA on ZnO (T = 300 K, P0 (NO) = 0.013 Torr).

Figure 6 shows the spectral dependence of the quantum yield of NO PA. The experiments at all selected wavelengths were performed under identical initial conditions (T = 300 K and P0 (NO) = 0.013 Torr). The quantum yield can be calculated from the initial velocity in the linear region: Y( λ ) =

N (NO ads ) × 100% , where N λ ,abs is the number of photons N λ ,abs

absorbed by ZnO. It is established that the quantum yield value has its maxima at hν=3.07 eV (λ=404 nm) and hν=2.6 eV (λ=480 nm) (Figure 6). Earlier we identified basing on the diffuse reflection spectra (DR), the absorption band hν= 3.1 eV, related to the oxygen vacancy that captured a single electron VO+ (F+ center) and the band 2.4 eV related to F centers (an oxygen vacancy that captured two electrons VO0, see 37). In38 the absorption spectrum of ZnO oxide in the range of 2.2–3.4 eV is decomposed into individual bands corresponding


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to the defects: the first peak coincides with the absorption of the F+–center, the peaks 2.6 -

eV and 2.5 eV are attributed respectively to a Zn vacancy which captured one hole VZn , -

(V –center) and to F–centers. These peaks give the main contribution to the observed effect of NO PA. Taking into account the real resolution of the PA excitation spectrum of NO, the coincidence of the PA excitation and the ZnO absorption spectra should be considered as satisfactory. It was shown earlier39 that these centers are the most active in photoadsorption and photocatalytic reactions on the wide-gap oxides. As it is seen in Figure 6, an insignificant contribution to the total effect is also provided by the interstitial -

-

oxygen ions Oi , and the zinc vacancies which trapped two holes, VZn 2 , (V0 –centers). The sum of these bands allows us to approximate the spectral dependence of NO PA with the accuracy of about 97% (Figure 6).

Figure 7. TD spectra of the products of the dark and photo– adsorption of NO (λ>390 nm) on ZnO (P0 (NO) = 0.07 Torr is identical to P0 (NO) in the mixture NO+CO (Figure 3 (a, b))


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Figure 7 shows the TD spectra of the products of dark adsorption and dark adsorption + photoadsorption of pure NO on ZnO at equal initial NO pressures. A significant part of weakly bound adsorbed forms of NO is removed when the reactor is evacuated at room temperature. Three types of adsorbed species with the desorption maximums at T1≈400 К (Edes=1.07 eV), T2≈650 К (Edes=1.79 eV) and T3≈760 К (Edes=2.08 eV) appear in the spectra. In the first two peaks NO is desorbed. However, the third peak of NO is accompanied by a molecular oxygen peak identical in shape and maximum position. This indicates a common source of both peaks. It can be a dissociative desorption of NO2 nitrites or NO3 nitrates, possibly negatively charged. Note that both high-temperature forms are specific just for photoadsorption of NO. The TD spectra allow us to discriminate the adsorbed forms by their surface binding energies and are an important addition to the analysis of adsorbed species by the FTIR method.

3.4 Dark adsorption of NO+CO The admission of the mixture CO+NO onto the sample leads to a rapid adsorption of -

CO and NO with the formation of weakly bound forms NO , CO-Zn+, CO-Zn2+ described above. When studying the products of dark adsorption of NO + CO by the TDS method (Fig. 8) we revealed a CO2 desorption with a peak maximum at 771 K, a CO desorption at 367 K, a NO desorption with maxima at about 378 K, 690 K, 730 K. The presence of a significant amount of products in the TD spectrum indicates an incomplete reversibility of the dark adsorption of the gas mixture. ACS Paragon Plus Environment

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Figure 8. TD spectrum of the products of dark adsorption of the mixture NO+CO on ZnO (the mixture parameters are identical to those presented in Figure 3 and Figure 7).

As it is seen from the TD spectra of the dark reaction (Figure 8), a significant part of the weakly bound adsorbed forms of NO and CO is removed when the reactor is evacuated at room temperature. At equal initial pressures of CO and of NO in the gas phase, the amount of adsorbed NO molecules is smaller than when pure NO is adsorbed, though it is ~ 2.5 times higher than the number of CO molecules. The difference in the values of the coating is explained by blocking the hole NO adsorption centers by CO molecules. A comparison of the obtained TD spectra of the dark adsorption products for NO+CO mixture (Figure 8) and for pure NO (Figure 7) reveals the peak shape of the adsorbed NO formed in the mixture (peak 4, Figure 7) with Edes=1.94 eV. A comparison of Figure 7, 8 with Figure 4 (a, b) proves that by the end of the reaction (1) NO is completely reduced to N2, without leaving intermediates in the adsorbed phase. The activation energies of the low-temperature CO form (Edes ≈ 1 eV) and the high-temperature CO2 form (Edes ≈ 2.1 eV) remain unchanged (Figure 5, 8) in the presence of NO.


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3.5. First stage of the reaction (1): 2NO + CO + hν → CO2 + N2O At the first stage of the reaction (1), which ends by the time of completion of the NO PA, no emission of N2 molecules into the adsorbed phase was observed. The TD spectrum of the adsorbed products in the first stage after evacuation of the gas phase is shown in Figure 9.

Figure 9. TD spectrum after the end of NO PA in the mixture NO+CO+hν (thν=10min) (the mixture parameters are identical to those presented in Figure 3, 8).

The amount of reduced NO is found from the nitrogen material balance: ∆n (NOphotoads) = ∆n (NOads) + 2∆n (N2Ogas). The amount of NO emitted at temperatures of 320-400 K is small, the NO molecules have formed strong bonds with ZnO and are revealed in the TD spectrum only at high temperatures (T> 670 K). The data of TD spectra of the products in corresponding reactions (Figure 8, 9) confirm the increase in the number of adsorbed molecules at PA. Three forms of adsorbed ACS Paragon Plus Environment

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NO are observed: a low-temperature form at about 370-380 K (Edes ≈ 1 eV) and two hightemperature forms. The first one is revealed as a peak with Tmax ≈ 680 K (Edes ≈ 1.84 eV), for the second species the Edes values differ for the dark and for the photoactivated species: Edes ≈ 1.94 eV (740 K) and Edes ≈ 2.08 eV (760 K), respectively. Thus the irradiation slightly increases the binding energy of the adsorbed NO species. The species with Edes ≈ 1.84 eV is apparently the precursor for the second stage.

3.6 Second stage of the reaction (1): N2O + CO + hν → N2 + CO2 At the first stage of the reaction (1), the carbon containing species: Zn+-CO, Zn2+-CO, -

-

-

-

CO2 , CO3 and the nitrogen containing ones, Zn2+-NO, NO , NO , NO+, are formed on the surface. N2O is released into the gas phase. The second stage is the reduction of N2O to N2 -

and the discharge of CO2 . Figure 10 shows the spectral dependence of the quantum yield of N2 at ZnO. The maxima (404 nm and 490 nm) repeat the maxima of the quantum yield of PA for pure NO (Figure 6), but the value of the quantum yield of N2 is more than 20 times lower than that of NO.


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Figure 10. Spectral dependence of the quantum yield of N2 in the mixture NO+CO on ZnO.

As it follows from the reaction kinetics (Figure 3), molecular nitrogen is formed through the reduction of the intermediate N2O. Note that the emission of N2 starts only after the completion of NO photoadsorption. It means that the capture of an electron from -

the electron-donor centers (V0+(F+-center), VZn (Zn+) and V00 (F-center)) by a NO molecule is more effective than its dissociative capture by a N2O molecule. The rates of N2O adsorption and of N2 release are comparable. It means that the reaction in the adsorbed phase is not the limiting one. Note that the quantum yield of N2 in the visible region, up to 530 nm, is higher than in the UV.

4. Discussion


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The obtained kinetic data agree with the previously proposed two-step redox mechanism of the reaction (1)21. The first stage, i.e. the reduction of NO to N2O, is described by the reaction: hν

CO + 2 NO → CO 2 + N 2 O

(4.1)

where N2O is an intermediate product, as indicated by a specific shape of its kinetic curve passing through the maximum. At the second stage N2O is reduced to N2: hν

CO + N 2 O → CO 2 + N 2

(4.2)

The results of the above decomposition (Figure 6) show that the main contribution to -

-

the absorption is given by the bands corresponding to vacancies V0+(F+-center), VZn (V center) and V00 (F-center). In order to describe the kinetics of NO PA with the formation of various adsorbed species, the following chain of reactions can be proposed: electron-hole pairs are formed at the short-wave edge of the efficiency curve hν1

Zn 2 + + O 2 − → Zn + + O −

(4.3)

The following process is possible in the 404 nm band40: hν1

F + + O 2− → F* + O − → F + + O − + e −

(4.4)

here F* is the excited state of the F center. In the long-wavelength band, a photogeneration of an electron and a hole occurs: +

hν 2

Zn → Zn 2 + + e −

(4.5)


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F + hν ( 2.5eV) → F + + e −

(4.6)

V − + hν(2.6eV) → V 2− + h +

(4.7)

The reactions of NO with photogenerated carriers follow:

NO ads + Zn + → NO − + Zn 2+

(4.8)

O − + NO ( a ) → NO 2− ( a )

(4.9)

Secondary reactions in the adsorbed phase are:

NO − + NO → N 2 O + O −

(4.10)

2− N 2 O + Zn + ( F + , e) → N 2 ↑ + Zn 2 + + O − (O lattice )

(4.11)

Zn 2+ + O − + NO → Zn + + NO 2− + F +

(4.12)

Zn 2 + + O 2− + NO −2 → Zn + NO 3−

(4.13)

One can see that in the processes 4.3–4.6 the electron-donor centers are generated, -

-

-

yielding the species NO , NO2 , NO3 . In addition, when NO is admitted, weakly bound nitrosyl Zn-NO complexes are formed, giving the IR bands at 1915 and 1900 cm-1. These bands are removed by evacuation at room temperature. The analysis of the isotope exchange 14N ↔ 15N and 16O ↔ 18O by FTIR and MS methods reveals the presence on TiO2 surface of adsorbed dimers N2O4, N2O3 in neutral and negatively charged forms41. Here the formation of positively charged NO+ is also shown. It has been established by the FTIR method42 that the oxidized ZnO, previously thoroughly purified from surface contamination, is highly transparent in the IR range of


22

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4000–1100 cm-1. Only some groups of residual hydroxyls absorb in the range of 3700– 3400 cm-1. –

–

The formation of CO2 and CO3 on the surface during irradiation can also be –

represented by the interaction of CO with a hole center O

:

O − + CO → CO 2−

(4.14)

O − + CO → CO 2 ads + VO ( F + )

(4.15)

–

A recharge of CO2 is possible:

CO 2− + Zn 2 + → Zn + + CO 2 ads

(4.16)

In its turn, a carboxylate CO2– complex can be formed during the interaction with Zn+, –

or a carbonate complex CO3 , see31,32, during the interaction with the hole center:

(CO 2 ) адс + Zn + ↔ CO 2− + Zn 2 +

(4.17)

(CO 2 ) адс + O − → CO 3−

(4.18)

The above mentioned structures of carbon oxides are strongly bound to the surface and are removed only when being heated to 750-800 K. At the same time, the TD spectra demonstrate on the illuminated surface weakly bound carbonyl groups Zn+-CO removable by evacuation at room temperature. Namely, these are the structures that react at the first stage. The separate steps of the second stage require, in their turn, a photoactivation. The photoactivation channels are discussed above. The interaction of CO with the hole center O– leads to the formation of a F + -centre (vacancy VO ).


23


Page 24 of 32

-

At the interaction with Zn+, NO2 turns into the weakly bound state NOads: hν NO 2− + Zn + ←→ Zn 2+ + O 2− + NOads

(4.19)

N2O is formed by the interaction of NO- and NOads (4.10) through the formation of the dimer N2O2-. In its turn, N2O is reduced to molecular nitrogen in the dissociative capture of an electron according to the scheme: − N 2 O + Zn + ( F + , e) → N 2 ↑ + Zn 2 + + O − (O 2lattice )

(4.20)

with the formation of a labile O − on the surface, or with filling a vacancy.

V − + hν → V 2− + h(O − )

(4.6)

CO 2− + F + + → CO 2 + F +

(4.21)

F+ + N 2O → N 2 + O −

(4.22)

The total process of the closed cycle, including the first and the second stages, is presented as: hν

CO + NO → 1/2N 2 ↑ +CO 2 ads

(1)

The equations (4.3–4.22) completely describe the selective reduction of NO, in the presence of CO, to molecular nitrogen with parallel oxidation of CO up to CO2 on the ZnO surface upon photoactivation of the reaction (1) in the UV-Vis region. The photoadsorbed -

-

species NO , NO2 , NO-Zn2+ are almost completely restored to N2 under additional irradiation, as seen from the nitrogen material balance in the gas phase. Note that the quantum yield of the N2 release in the visible region, up to 530 nm, is higher than that in the UV. It is explained by two factors39, 43. First, the number of intrinsic


24

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surface defects in ZnO1-x, absorbing in the visible region, does not exceed ≈1011 cm-2. This ensures a high transparency of crystallites in the visible region and, as a result, an increase of the illuminated surface compared to that in the UV region, by several orders of -

magnitude. Second, if the lifetime of a e /h+ photogenerated pair before recombination is estimated as 10–9 s, the lifetime of photoactivated defects can reach 103 s. It is the combination of these two factors that ensures a high quantum yield.

5. Conclusion hν

The ecologically important redox reaction CO + NO →1/2N 2 ↑ +CO2 ads (1) was discovered and investigated for the first time on self-sensitized zinc oxide ZnO1-x under illumination in UV-Vis spectral regions. The quantum yield of the N2 release is maximal in the visible region, the action spectrum extends to λ ≥ 530 nm and is typical for a wide-gap oxide self-sensitized by intrinsic defects such as: anion vacancies F+, F-centers; Zn+ and -

-

negatively charged cation vacancies V , V2 -centers. The quantum yield of the reaction in the visible region is higher than in the UV region, in spite of a low surface concentration of the active sites. This is explained firstly by the fact that the surface illuminated with VIS light is larger than for the UV illumination, and secondly by a longer, up to 103 s, lifetime of VIS photoactivated centers. The reaction (1) proceeds in 2 stages. The first stage is characterized by PA of initial products with the formation of CO2(ads) and the reduction of NO to N2O : hν CO + 2 NO → CO2 + N 2 O .


25


Page 26 of 32

After the completion of NO PA, the second stage starts: the reduction of the intermediate product N2O to N2: hν CO + N 2 O → CO2 + N 2

The final product CO2 remains in the adsorbed state and can be thermally extracted in the stoichiometric ratio. The structures of the intermediate adsorption complexes are proposed based on the TD spectra and the literature data on FTIR spectra. The basic features of the kinetics of the photo-activated reaction hν

CO + NO →1/2N2 ↑ +CO2 ads on ZnO1-x repeat those obtained earlier for TiO2-x (Degussa)21, however the quantum yield and the action spectrum obtained in the present work exceed these parameters for TiO2-x (Degussa).

Associated content Supporting Information Data on the characterization of the used ZnO powdered sample by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller method (BET) and on the spectral characteristics of the used lamps and light filters. Available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Author E-mail: [email protected], phone: +7 921 384 6599

Notes The authors declare no competing financial interest.


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Acknowledgements The research was supported by “Nanocomposite”, “Physical Methods of Surface Investigation”, “X-ray Diffraction Centre” and “Nanophotonics” centres of St. Petersburg State University. The work is fulfilled at the expense of the budget of St. Petersburg State University.

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2 N2 Activated on

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