FTIR and TPD Study of the Room Temperature Interaction of a NO

Article pubs.acs.org/JPCC

FTIR and TPD Study of the Room Temperature Interaction of a NO− Oxygen Mixture and of NO2 with Titanium Dioxide Ruslan V. Mikhaylov,† Andrei A. Lisachenko,† Boris N. Shelimov,*,‡ Vladimir B. Kazansky,‡ Gianmario Martra,§ and Salvatore Coluccia§ †

St. Petersburg State University, Ul’yanovskaya St. 1, 198504 St. Petersburg, Russia Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninskii Pr. 47, 119991 Moscow, Russia § Department of Chemistry & Interdepartmental NIS Centre of Excellence, University of Torino, Via P. Giuria 7, 10125 Torino, Italy ‡

ABSTRACT: In the present paper, kinetics and mechanism of NO and oxygen coadsorption on TiO2 at room temperature, which is the key step of the catalytic removal of NOx pollutants from air, were studied. NO adsorption on TiO2 in the absence of oxygen is weak and reversible, but it is found to strongly increase in the presence of oxygen. The ratio between the amount of adsorbed NO and O2 in the course of adsorption is constant and close to three. A FTIR spectroscopic study reveals that the amount and composition of N-containing species on the TiO2 surface strongly depend on the contact time with the initial NO−O2 mixture and on its composition. At relatively small exposures, IR bands assigned to NO− and nitrosyl complexes Tin+−NO (n = 3−4) are predominant in the spectra. With increasing contact time, NO− disappears, and IR bands of NO3− and possibly NO2− appear and grow. The thermal stability of surface nitrates and nitrites correlates with their structure. IR spectra observed upon NO2 adsorption are similar to those after exposure to NO−O2 mixtures. Exposure of the sample with preadsorbed 14NO2 to gaseous 15 NO results in a change in the IR spectra that suggests isotopic replacement of 14N with 15 N in the adsorbed species. In the TPD profiles, after adsorption of NO−O2 and NO2, desorption peaks of NO and NO2 dominate which presumably arise from the thermal decomposition of NO3− (NO2−) and nitrosyls Tin+−NO. A multistep scheme for the interaction of NO and O2 with TiO2 is suggested which accounts for the results of both techniques applied. NO− + CO (hv) → CO2− + 1/2N2. We believe that this reaction is the key step in nitrogen production by the photocatalytic process. The data obtained allow us to refine the reaction mechanism proposed earlier and directly prove some of its key steps. The present work was aimed at elucidating the mechanism of NO interaction with TiO2 at room temperature in the presence of oxygen, i.e., under the conditions characteristic of the real air purification process. The NO and O2 coadsorption on TiO2 was earlier studied by FTIR.8−10 In these studies, coadsorption was carried out either by exposure of a TiO2 sample to a NO− oxygen mixture, or by oxygen adsorption on the sample which has been preexposed to NO alone. No significant difference was found for these two coadsorption modes. In contrast to the adsorption of “pure” NO on TiO2, where UV-light irradiation is required to reach a noticeable chemisorption, the NO−O2 coadsorption occurs rapidly without light irradiation, and, according to the FTIR data, is accompanied by the appearance of intense IR absorption bands of various adsorbed nitrogencontaining species. Nitrosyl complexes, NO+, and nitrates NO3− of different structures were identified. An IR band at 1170 cm−1 was also reported in ref 10 and assigned to NO−.

1. INTRODUCTION Nitrogen oxides (NO and NO2, NOx) are rather common in polluted air in concentrations that are harmful to human health. Because of that, removal of these pollutants attracted much attention in recent years and a number of adsorption and catalytic techniques have been employed for this purpose (see, for example, reviews1,2). In particular, photocatalytic purification of indoor air has been intensively studied during the past decades.3−6 In most cases, titanium dioxide is used as a photocatalyst, as it is highly active, versatile, nontoxic, easily available, and relatively inexpensive. In our earlier study,7 the interaction of NO, CO, and NO− CO mixtures with a Degussa P-25 photocatalyst at room temperature was explored by IR spectroscopy and temperature programmed desorption (TPD). In the absence of light irradiation, only weak and reversible NO adsorption on surface Ti4+ ions was observed. UV−vis irradiation greatly enhances the NO adsorption on Ti4+ and, according to the IR spectra, yields N2O, NO−, NO2−, and NO3− surface species. When TiO2 is exposed to an equimolar NO−CO mixture, N2O and CO2− are formed without irradiation, presumably by a 2NO + 2CO + O2− → 2CO2− + N2O reaction. Subsequent irradiation with light is accompanied by accumulation of NO− and Ti4+···NO complexes. No reduction of TiO2 occurs in this case. FTIR spectra show that NO− produced by the photoinduced adsorption of NO can be eliminated by the following reaction: © 2013 American Chemical Society

Received: November 25, 2012 Revised: March 20, 2013 Published: May 9, 2013 10345

dx.doi.org/10.1021/jp311593s | J. Phys. Chem. C 2013, 117, 10345−10352

The Journal of Physical Chemistry C

Article

Figure 1. Kinetics of NO−O2 chemisorption over TiO2. (a) NO and O2 pressure vs time; (b) amount of chemisorbed NO [Δ(NO)] and chemisorbed oxygen [Δ(O2)] and Δ(NO)/Δ(O2) as a function of time.

Previous FTIR studies8−10 were confined by monitoring spectrum variations as a function of time, whereas compositions of the gas phase in the course of adsorption were not controlled. In the present study, FTIR data were correlated with kinetic and temperature-programmed desorption (TPD) experiments, and the stoichiometry of the NO−O2 chemisorption has been determined. Furthermore, since it was known from the literature9−11 that NO2 adsorption on TiO2 resulted in the production of surface species similar to those produced upon NO−O2 coadsorption, it appeared reasonable to perform some FTIR, TPD, and mass-spectroscopic measurements on NO2 adsorption. As a result, a multistep scheme for the interaction of NO−O2 and NO2 with TiO2 is proposed which is consistent with the IR data and the chemisorption stoichiometry. Finally, some new and rather unexpected results have been obtained, in particular on 14N ⇆ 15N isotopic exchange between 14N-contining surface species produced by 14 NO2 adsorption with gaseous 15NO.

providing the high-temperature treatment of the sample and a linear temperature ramp-up at a 20 K min−1 rate in the TPD experiments. 2.2. FTIR Measurements. TiO2 samples were prepared by pressing powder Degussa P-25 into self-supporting pellets with a density of 20−30 mg cm−2. They were fixed in a gold frame and positioned in a quartz optical cell with CaF2 windows allowing thermal treatment and connected to a vacuum line. To remove organic contaminants, the sample was first heated under dynamic vacuum (∼10−5 Torr) at 870 K for 30 min and then in oxygen at 0.5−1 Torr for 60 min. A liquid nitrogen trap was used to freeze out oxidation products (mainly CO2). Gas adsorption on the pretreated samples was carried out at room temperature. FTIR spectra were recorded with a Vector 22 (Bruker) IR Fourier spectrometer at a beam temperature (DTGS detector, 4 cm−1 resolution). To monitor the transformation of adsorbed species, IR spectra were taken every 2 min during gas adsorption and sample outgassing. The IR spectra reported in this paper were obtained by subtraction of the initial IR spectrum of TiO2 recorded prior to gas adsorption.

2. EXPERIMENTAL METHODS 2.1. Sample Preparation and Pretreatment for Adsorption and TPD Runs. Powder TiO2 Degussa P-25 (ca. 80% anatase and 20% rutile, SSA = 50 m2 g−1) was deposited on the inner wall of a quartz flat reactor from a suspension in bidistilled water followed by heating the reactor at 330 K overnight. The reactor was then attached to a greaseless vacuum line and heated in flowing oxygen (0.5 Torr) at 870 K for at least 10 h to remove surface organic contaminants until no CO and CO2 was detected by mass spectroscopy at the reactor outlet. Such treatment resulted in a reduction of the SSA to approximately 40 m2 g−1, whereas the anatase-to-rutile ratio remained unchanged.12 High purity 16O2, 14NO, 15NO, and 14NO2 used in this study in the TPD runs were purchased from the St-Petersburg State Institute of Applied Chemistry, and were further purified by passing through a liquid nitrogen trap or by low-temperature fractional distillation. Gas pressures in the range of 10−4−10 Torr were measured by a calibrated digital Pirani gauge and by a membrane Baratron 122 or a PIEZOVAC PV20 manometer. A MI-1201 mass spectrometer equipped with a computer-based data acquisition and processing system was used to control the composition of the gas phase in the TPD and isotopic exchange runs. The reactor was supplied with an electrical heater

3. RESULTS 3.1. Kinetics of NO and O2 Coadsorption on TiO2. Nitrogen oxide adsorbs on TiO2 at room temperature weakly and reversibly.7 The NO coverage does not exceed 2 × 10−4 ML in the range of 0.02−0.3 Torr. The situation radically changes upon exposure of TiO2 to a NO−O2 mixture. Figure 1a depicts NO and O2 pressure as a function of exposure time when a fixed amount of a NO−O2 mixture was brought into contact with TiO2 in a closed system. In Figure 1b, the same data are presented as a plot of the amounts of NO and O2 consumed (Δ(NO) and Δ(O2), respectively) and Δ(NO)/ Δ(O2) vs exposure time. It is quite remarkable that, except for several initial points, the Δ(NO)/Δ(O2) ratio is constant and very close to three in the course of the whole run. These data cannot be explained by NO oxidation via the reactions 2NO + O2 → 2NO2 or 4NO + O2 → 2N2O3, because in these cases Δ(NO)/Δ(O2) would be equal to two or four, respectively. In addition, our control experiments show that, in the absence of TiO2, NO does not react with oxygen at a pressure of ∼0.1 Torr as used in these runs. Hence, the mechanism of NO interaction with oxygen on the TiO2 surface requires a 10346



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Figure 2. Time evolution of IR spectra upon coadsorption of 1 Torr NO−1 Torr O2 on TiO2. The inset shows the 4000−2400 cm−1 interval. Adsorption time: (1) 1, (2) 16, (3) 25, (4) 60, and (5) 140 min.

thorough analysis with the use of IR spectroscopy and other techniques. 3.2. FTIR Study of Interaction of NO−O2 and NO2 with Titanium Dioxide. 3.2.1. NO−O2 Coadsorption. Figure 2 demonstrates time evolution of the FTIR spectra after the admission of an equimolar 1 Torr NO−1 Torr O2 mixture on TiO2. During the first 15−25 min, an intense broad IR band at 1168 cm−1 and a series of IR bands in the 2000−1700 cm−1 interval with maxima at 1710, 1834−1846, and 1905 cm−1 and a shoulder at 1980 cm−1 dominate in the spectrum. The growth of these IR bands is accompanied by the appearance of weak IR bands in the 1650−1200 cm−1 interval. After 20−25 min (Figure 2, curve 3), the intensities of the 1168, 1846, and 1905 cm−1 IR bands reach a maximum and then gradually decrease. Simultaneously, a series of intense IR bands in the 1650−1450 cm−1 range (at 1628, 1616, 1581, 1554, 1536, and 1505 cm−1) and in the 1350−1200 cm−1 interval (at 1315, 1290, 1241, and 1225 cm−1) build up. In the OH group stretching region (see inset in Figure 2), a broad IR band with a maximum at ∼3450 cm−1 grows monotonically. In approximately 2 h, the system reaches a steady state at which the IR spectra remain unchanged. The band at 1168 cm−1 can be attributed to NO−, and the series in the 2000−1700 cm−1 interval can be assigned to stretching vibrations ν(NO) in Tin+−NO nitrosyl complexes (n = 3 or 4) with different coordination of the Tin+ ions.7,10 In particular, the lowest frequency band at 1710 cm−1 can be attributed to Ti3+−NO.10 Furthermore, adsorbed N2O3 may also be present in the system, as its stretching mode ν(NO) falls into the same spectral region (1930−1880 cm−1 , ref 13). The presence of NO2 appears to be less probable, because it transforms very rapidly into other species on the TiO2 surface. According to ref 9, the broad band at ∼3450 cm−1 can be assigned to N−OH groups. A series of intense IR bands in the 1650−1200 cm−1 interval are more difficult to identify. Generally these IR bands are attributed to N−O stretching vibrations of surface nitrate (NO3−) and nitrite (NO2−) species of different structures (see Scheme 1). However, an accurate assignment of these IR bands only on the basis of their IR spectra is hardly possible, as the ν(N−O) of different NO3− and NO2− species fall in the same

Scheme 1. Structures of Surface Nitrate and Nitrite Complexes According to References 13−15

1700−1000 cm−1 interval and their IR bands strongly overlap (Scheme 2). Furthermore, recent quantum chemical calculations16 demonstrated that the N−O stretching frequencies of surface NO3− are determined not only by their structure, but also by geometry of the adsorption site. Nevertheless, a comparison of the IR-band positions observed in the IR spectrum (Scheme 2b) with those of surface nitrates (Scheme 2c) suggests that, most likely, the IR bands of surface bridging (1628 and 1225 cm−1), bidentate (1581, 1536, and 1225 cm−1), and monodentate (1505 and 1290 cm−1) nitrates are present in the spectrum of Figure 2. One should also bear in mind that at least a part of these bands may be attributed to surface nitrites (cf. Scheme 2, parts a and b), most likely to nitro- and bridging nitro-nitrito structures. Formation of monodentate nitrites is less probable, because no low-frequency IR band in the 1200−1050 cm−1 region characteristic of these species is observed. The sample exposed to the NO−O2 mixture for 140 min was then pumped off for 30 min at room temperature. Such treatment leads to an appreciable change in the IR spectrum (Figure 3, curve 2). The intensity of the 2200−1700 cm−1 10347



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a distinct decrease in the 1510 and 1290 cm−1 band intensity of monodentate nitrates is clearly seen. After the heat treatment, the intensity of bridging and bidentate nitrates changes only insignificantly indicating that the nitrate thermal stability is determined by their structure. Figure 4 depicts the IR spectra after TiO2 exposure to a NO−O2 mixture with a higher NO/O2 = 3 ratio, followed by

Scheme 2. IR Band Positions of (a) Surface Nitrites and (c) Surface Nitrates According to References 13−15a and (b) Schematic Diagram of the 1700−1000 cm−1 Range after Exposure of TiO2 to 1 Torr NO−1 Torr O2 for 140 min (Figure 2)

Figure 4. IR spectra after exposure of TiO2 to 3.9 Torr NO−1.3 Torr O2 for (1) 1 and (2) 60 min, (3) evacuation at 300 K for 30 min, and (4) heating at 600 K for 5 min.

evacuation at room temperature and subsequent heating under vacuum at 600 K. At a short exposure (1 min), the 1169 cm−1 band of NO−, the 1705, 1844, and 1905 cm−1 bands of nitrosyl complexes are observed (Figure 4, curve 1) which are similar to the case of the NO/O2 = 1 mixture adsorption. However, at higher exposures (60 min), the IR spectra for the mixtures of different compositions clearly differ. There is an intense 2198 cm−1 band in the spectrum of Figure 4, curve 2, which can be assigned to NO+,11,13 as well as a relatively high-frequency band at 1957 cm−1, which can be attributed to nitrosyl complex Tin+−NO or to a ν(NO) vibration of adsorbed N2O3 molecules. According to ref 13, the latter exhibit IR bands in the 1930−1880 cm−1 interval. Similar to the NO/O2 = 1 mixture, a series of intense IR bands at 1634, 1581, 1509 1280, and 1225 cm−1 of NO3− and possibly NO2− gradually grow. After evacuation at 300 K (Figure 4, curve 3), the intensity of all IR bands in the 2200−1700 cm−1 region noticeably diminishes. In contrast to the equimolar NO−O2 mixture, the IR spectrum distinctly changes in the 1650−1200 cm−1 region as well: the 1634 cm−1 band increases, while the 1509 and 1280 cm−1 bands considerably decrease. These effects may be related to the decay or reconstruction of monodentate NO3−, which is unlikely, or to N2O3 desorption. The νas(NO2) and νs(NO2) bands of the latter fall into the 1590−1550 and 1305−1290 cm−1 intervals, respectively.11 After a short heat treatment at 600 K (Figure 4,curve 4), the IR bands in the nitrosyl complex region almost completely disappear, and, in the N−O stretching region, only IR bands at 1634, 1581, and 1212 cm−1 remain. 3.2.2. Interaction of TiO2 with 14NO2 and 15NO. Nitrogen dioxide adsorbs on TiO2 very rapidly. Figure 5, curve 1, shows the IR spectrum obtained after exposure to NO2 for 20 min (initial pressure 1.2 Torr) and subsequent evacuation at room temperature. The similarity between this spectrum and the one after TiO2 exposure to the NO/O2 = 3 mixture for 60 min

a A − monodentate nitrite; B − nitrocompound; C − bridging bidentate nitrite; D − bridging nitro-nitrite; E − bidentate nitrate; F − bridging nitrate; G − bidentate nitrate.

Figure 3. IR spectra after (1) exposure of TiO2 to 1 Torr NO−1 Torr O2 for 140 min (spectrum 5 in Figure 2), (2) evacuation for 30 min at 300 K, and (3) heating at 600 K for 5 min.

bands considerably decreases suggesting the destruction of weak Tin+−NO complexes and N2O3 desorption. The 1650− 1200 cm−1 region changes insignificantly, and the intensity of N−OH groups remains practically constant. After the heat treatment under vacuum at 600 K for 5 min, the IR bands of nitrosyl complexes completely disappear, whereas the ∼3450 cm−1 band of N−OH partly survives (Figure.3, curve 3). In the region of nitrate and nitrite ν(N−O), 10348



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1258, and 1196 cm−1) markedly diminish. Since desorption or destruction of NO3− or NO2−, as charged species, induced by a simple evacuation is unlikely, one may suggest that the decrease in the intensities of these bands is associated with the desorption of N2O3, whose IR bands fall in this wavenumber interval.11 The occurrence of isotopic exchange of gaseous 15NO with the adsorbed 14N-containing surface species formed by the preliminary 14NO2 adsorption is supported by the massspectroscopic analysis. Figure 6 (curve 3) demonstrates a

Figure 5. IR spectra after (1) exposure of TiO2 to 1.2 Torr 14NO2 for 20 min; after subsequent evacuation and exposure to 1.9 Torr 15NO for (2) 2 and (3) 60 min; and (4) after evacuation at 300 K.

(Figure 4, curve 2) is quite evident. In the spectrum of Figure 5, curve 1, there are IR bands of N−OH group at 3580 and 3405 cm−1, NO+ at 2206 cm−1, broad IR bands of nitrosyl complexes Tin+−NO or adsorbed N2O3 at 1985 and 1920−1900 cm−1, as well as the bands of NO3− and NO2− at 1634, 1581, 1556, 1536, 1523, 1279, and 1212 cm−1. Subsequent exposure of the sample to 15NO results in a considerable change in the spectrum (Figure 5, curves 2 and 3). The intensity of some bands changes, and they shift toward lower wavenumbers due to the isotopic substitution of 14N by 15 N (see Table 1). The ratio of wavenumbers ν(15N)/ν(14N) is

Figure 6. Isotopic exchange of gaseous 15NO with 14NO2 preadsorbed on TiO2. 5.8 × 1016 molecules of NO2 adsorbed, 5.8 × 1016 molecule of NO contacting TiO2. (1, 2) partial pressure of 15NO and 14NO, respectively; (3) α(15N)−15N fraction in gaseous NO; and (4) total pressure 15NO + 14NO. The vertical arrow indicates the onset of the exchange reaction.

Table 1. IR Absorption-Band Position for 14NO2/TiO2 and 14 NO2/TiO2 + 15NO band position (cm−1) species +

NO Tin+−NO N2O3 NO3− (NO2−)

14

NO2/TiO2 2206 1985 1920 1634 1581 1279 1212

NO2/TiO2 + 15NO

14

2155 not observed 1900 1599 1543 1258 1196

gradual decrease in the 15NO fraction (α) in the gas phase with time, while the total pressure of 15NO + 14NO remains almost constant in the course of the isotopic exchange (curve 4). Since the number of adsorbed 14NO2 and 15NO molecules in the gas phase in this run is equal, one may expect that, under the equilibrium conditions, α would be one-half of the initial value, i.e., 0.45. However, the final α value is about 0.62. This inconsistency may be explained either by the inability of some adsorbed species to exchange with 15NO, or by desorption of a certain amount of exchangeable 14N-containing species upon evacuation of the sample prior to the 15NO admission. Our estimates show that this “loss” can be as high as 50%. Figure 7 shows the composition of the desorbed gas in a TPD experiment after the completion of the run described above (Figure 6). A distinct two-peak plot is obtained, NO being the dominant component. Maxima of the low- and hightemperature peaks for NO and NO2 desorption differ in temperature insignificantly. In the high-temperature region, a small oxygen peak appears. The closeness of the temperature maxima for the NO and NO2 desorption peaks suggests that they have a common precursor. As seen in Figure 7, the 15NO fraction in the gas phase (α) decreases with increasing temperature.

ν(15N)/ν(14N) 0.98 0.99 0.98 0.98 0.98 0.99

close to the theoretical value [μ(14N−16O)/μ(15N−16O)]1/2 = 0.98, where μ(14N−16O) = 7.47 and μ(15N−16O)] = 7.74 are the reduced masses of the N−O bonds. 14 NO+ exchanges rapidly with 15NO: already after a 2-min exposure, the 2206 cm−1 band vanishes completely, and a new band at 2155 cm−1 appears (Figure 5, curve 2). In contrast, 14 NO3− exchanges with 15NO more slowly, but after 60 min all IR bands of the initial 14NO3− disappear, and only the IR bands of 15NO3− (15NO2−) remain in the spectrum (Figure 5, curve 3). The behavior of the Tin+−NO IR bands or adsorbed N2O3 (1985 and 1920−1900 cm−1) when contacting 15NO is more complex: the 1985 cm−1 band disappears, and the broad 1920− 1900 cm−1 band grows considerably and shifts to 1900 cm−1 (Figure 5, curves 2 and 3). Evacuation of the sample leads to a sharp decrease in the intensity of the NO+, nitrosyl, and N2O3 IR bands (Figure 5, curve 4). It should be noted that in parallel the intensities of some IR bands in the N−O stretch region (at 1543, 1470,

4. DISCUSSION 4.1. Interaction of NO−O2 with TiO2. As follows from the IR data, this interaction occurs via several sequential steps. At the first step, at a relatively short contact time (up to 15−25 min), Tin+−NO complexes and NO− anions are predominant 10349



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The Ti3+ are oxidized with NO2 Ti 3 + + NO2 → Ti4 + + NO2−

(10)

and the overall process can be written as 2NO2 + O2 − → NO3− + NO2−

One may further suggest that reaction proposed in ref 19:

(11)

NO2−

decomposes by the

2NO2− → NO2 + NO + O2 −

(12)

Summation of reactions 11 and 12 with appropriate coefficients results in 3NO2 + O2 − → 2NO3− + NO

Reactions 11 and 13 were considered in ref 18 when discussing possible mechanisms of NO3− and NO2− formation upon NO2 interaction with surface adsorption centers of δ- and γ-Al2O3 at elevated temperatures. Finally, one may assume that heterolytic NO2− decomposition may be an alternative to reaction 12:

Figure 7. TPD profiles after the completion of the run shown in Figure 6 followed by the evacuation at room temperature. (1) NO, (2) NO2, (3) O2, (4) α(15N), 15N fraction in the products of thermal desorption.

on the surface, and the Δ(NO)/Δ(O2) ratio is close to three. The presence of N2O3 molecules on the surface cannot be ruled out. Supposedly the interaction of NO with TiO2 occurs via the following reaction which was first suggested by Ramis et al.:17 2Ti4 + + NO + O2 − ⇄ 2Ti 3 + + NO2 4+

NO2− + □ → NO+ + O2 −

Ti

4+

(1)

2NO2 ⇄ [N2O4 ] ⇄ NO+ + NO3−

3+

+ NO → Ti

−

Ti 3 + + O2 → Ti4 + + O2−

Reaction 15 also explains the appearance of NO in the IR spectra when TiO2 is exposed to the NO−O2 mixture with the higher NO content (Figure 4, curve 2). Thus, various N-containing species can be formed in the NO2−TiO2 system by the reactions discussed above, namely, NO+ (reaction 15), nitrosyl complexes Tin+−NO (reaction 13), NO3− (reactions 11, 13, and 15) and NO2− (reaction 11). It is worth noting that, in addition to the above-mentioned species, N2O3 (a product of NO and NO2 interaction), which is hard to identify by IR, may also be present in the NO2−TiO2 system. 4.3. Isotopic Exchange with 15NO. The change in the IR spectra observed upon exposure of TiO2 with chemisorbed 14 NO2 to 15NO (Figure 5 and Table 1) evidence isotopic exchange of nitrogen atoms of the surface 14NO3− (14NO2−) and 14NO+ species with 15NO molecules from the gas phase. When discussing possible reactions responsible for the isotopic exchange, one should bear in mind that the total pressure 15NO + 14NO does not change significantly in the course of the interaction (Figure 6, curve 4). It is quite evident that the exchange 14N ⇆ 15N can occur only via intermediate dimeric forms of nitrogen oxides, such as N2O4, N2O3, N2O4−, N2O3−, etc. The exchange of 14NO3− with 15 NO can be described as follows:

(3)

Thus, the resulting reaction at this step can be presented as a sum of reactions 1−3: 2NO + O2 + O2 − → NO2 + NO− + O2−

(4)

Taking into account possible interaction of NO2 with NO NO + NO2 ⇄ N2O3

we get 3NO + O2 + O2 − → N2O3 + NO− + O2−

(5)

Reaction 5 is in accord with the IR spectra observed at the earlier stages of the NO + O2 adsorption (Figure 2), and its stoichiometry corresponds to Δ(NO)/Δ(O2) = 3 (Figure 1b). At longer exposures, the IR bands of NO− disappear and those of NO3− and possibly NO2− appear and grow. It is likely that there are several paths to form NO3−: NO− + O2 → NO3−

(6)

NO + O2− → NO3−

(7)

15

Note that a combination of reactions 4 and 7 also leads to the stoichiometry Δ(NO)/Δ(O2) =3: 2−

3NO + O2 + O

−

→ NO + NO2 +

NO3−

NO + 14 NO3− ⇄ [O15 N − O − 14 N(O)2 ]− I

⇄ [(O)2 − N − O − 14 NO]− 15

II

(8)

⇄

4.2. Interaction of NO2 with TiO2. The first reactive step of the NO2 interaction with TiO2 was suggested to give rise to NO3− and Ti3+:17 Ti4 + + NO2 + O2 − ⇄ Ti 3 + + NO3−

(15) +

(2)

+ NO

(14)

where □ stands for an anion vacancy. Apparently, this reaction is energetically favorable, because a lattice anion O2− is created filling the anion vacancy. Summation of reactions 11 and 14 gives rise to an overall process proposed by Hadjiivanov et al.:11

In this reaction, NO reduces Ti to Ti ions which can be oxidized in parallel by both NO and oxygen: 3+

(13)

14

NO + NO3− 15

(16) 14

One of the two oxygen atoms bonded to N in dimer I migrates to the 15N atom to form dimer II. Decomposition of II can yield products of the isotopic exchange. The asymmetric uncharged dimer ONO−NO2 is known to be less stable than

(9) 10350


The Journal of Physical Chemistry C symmetric dimer O2N−NO2, but the existence of the former in the condensed phase has been repeatedly proved by IR spectroscopy and quantum-chemical calculations.20−22 However, there is no literature data on whether negatively charged dimer I is formed on the oxide surface. One may assume that dimer I can decompose in a different way: 15

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REFERENCES

(1) Shelef, M. Selective Catalytic Reduction of NOx with N-Free Reductants. Chem. Rev. 1995, 95, 209−225. (2) Skalska, K.; Miller, J. S.; Ledakowicz, S. Trends in NOx Abatement: A review. Sci. Total Environ. 2010, 408, 3976−3989. (3) Hashimoto, K.; Wasada, K.; Toukai, N.; Kominami, H.; Kera, Y. Photocatalytic Oxidation of Nitrogen Monoxide Over Titanium(IV) Oxide Nanocrystals Large Size Areas. J. Photochem. Photobiol., A 2000, 136, 103−109. (4) Devahasdin, S.; Fan, C., Jr.; Li, K.; Chen, D. H. TiO2 Photocatalytic Oxidation of Nitric Oxide: Transient Behavior and Reaction Kinetics. J. Photochem. Photobiol., A 2003, 156, 161−170. (5) Ohko, Y.; Nakamura, Y.; Negishi, N.; Matsuzawa, S.; Takeuchi, K. Photocatalytic Oxidation of Nitrogen Monoxide Using TiO2 Thin Films Under Continuous UV Light Illumination. J. Photochem. Photobiol., A 2009, 205, 28−33. (6) Yu, Q. L.; Brouwers, H. J. H. Indoor Air Purification Using Heterogeneous Photocatalytic Oxidation. Part I: Experimental Study. Appl. Catal., B 2009, 92, 454−461. (7) Mikhaylov, R. V.; Lisachenko, A. A.; Shelimov, B. N.; Kazansky, V. B.; Martra, G.; Alberto, G.; Coluccia, S. FTIR and TPD Analysis of Surface Species on a TiO2 Photocatalyst Exposed to NO, CO, and NO−CO Mixtures: Effect of UV−Vis Light Irradiation. J. Phys. Chem. C 2009, 113, 20381−20387. (8) Yang, R. T.; Li, W. B.; Chen, N. Reversible Chemisorption of Nitric Oxide in the Presence of Oxygen on Titania and Titania Modified with Surface Sulfate. Appl. Catal., A 1998, 169, 215−225. (9) Kantcheva, M. Identification, Stability, and Reactivity of NOx Species Adsorbed on Titania-Supported Manganese Catalysts. J. Catal. 2001, 204, 479−494. (10) Hadjiivanov, K.; Knözinger, H. Species Formed After NO Adsorption and NO+O 2 Coadsorption on TiO 2 : an FTIR spectroscopic study. Phys. Chem. Chem. Phys. 2000, 2, 2803−2806. (11) Hadjiivanov, K.; Bushev, V.; Kantcheva, M.; Klissurski, D. Infrared Spectroscopy Study of the Species Arising During Nitrogen Dioxide Adsorption on Titania (Anatase). Langmuir 1994, 10, 464− 471. (12) Deiana, C.; Fois, E.; Coluccia, S.; Martra, G. Surface Structure of TiO2 P25 Nanoparticles: Infrared Study of Hydroxy Groups on Coordinative Defect Sites. J. Phys. Chem. C 2010, 114, 21531−21538. (13) Hadjiivanov, K. Identification of Neutral and Charged NxOy Surface Species by IR Spectroscopy. Catal. Rev.: Sci. Eng. 2000, 42, 71−144. (14) Pozdnyakov, D. V.; Filimonov, V. N. An IR Spectroscopic Study of Nitrogen Monoxide and Nitrogen Dioxide Chemisorption on Metal Oxides. Kinet. Katal. 1972, 14, 760−766. (15) Davydov, A. Molecular Spectroscopy of Oxide Catalyst Surfaces; Wiley: New York, NY; 2003. (16) Ingelsten, H. H.; Hellman, A.; Kannisto, H.; Grönbeck, H. Experimental and Theoretical Characterization of NOx Species on Ag/ α-Al2O3. J. Mol. Catal. A: Chem. 2009, 314, 102−109. (17) Ramis, G.; Busca, G.; Lorenzelli, V.; Forzatti, P. Fourier Transform Infrared Study of the Adsorption and Coadsorption of Nitric Oxide, Nitrogen Dioxide and Ammonia on TiO2−Anatase. Appl. Catal. 1990, 64, 243−257. (18) Pazé, C.; Gubitoza, G.; Giacone, O. S.; Spoto, G.; Llabrés I Xamena, F. X.; Zecchina, A. An XRD, FTIR and TPD Investigation of NO2 Surface Adsorption Sites of δ,γAl2O3 and Barium Supported δ,γAl2O3. Top. Catal. 2004, 30/31, 169−175. (19) Sadokhina, N. A. Hydrogen Promoting Effect in the Selective Reduction of NOx with Hydrocarbons over Ag/Al2O3 Catalysts. PhD Thesis, Zelinsky Institute of Organic Chemistry RAS, 2011.

(17)

However, in this case, no 15NO3− and 14NO is formed and, hence, reaction 17 can be ruled out from consideration. The 14N ⇆ 15N exchange in 14NO2− with 15NO to yield 15 NO2− and 14NO could occur via intermediate dimers [O15N−O−14NO]− or [O15N−14N(O)2]−. Similarly, 14 NO+ads may exchange with 15NOgas through a dimer [O14N−15NO]+. The reactions discussed in this section account for the principal changes in the IR spectra and TPD profiles observed upon 15NO admission onto NO2/TiO2. Of course, they do not exhaust all feasible transformations of diverse nitrogencontaining species. Likewise, they do not provide a reasonable explanation of why the experimental value of 15N fraction in the gas phase (α = 0.62) deviates from the calculated value α = 0.45.

5. CONCLUSIONS It should be noted that we consider in this work some new results on the NO−O2 coadsorption on TiO2 that have not yet been known from the literature. We report here for the first time the NO/O2 = 3 stoichiometry in the course of fast chemisorption in the absence of light irradiation. Furthermore, we suggest several reaction routs that can account for such stoichiometry and agree with the IR spectroscopic data obtained. However, we believe that more experiments on coadsorption of NO−O2 mixtures with a wide variation of their composition and initial pressure would be helpful for getting a better insight into the chemisorption mechanism. Our IR data agree well with those reported earlier in the literature. A variety of adsorbed NOx species has been observed thus confirming the complexity of the chemistry of nitrogen oxides on titania surface. Another interesting finding of this work worth noting is the occurrence of isotopic exchange of 14N in adsorbed species with gaseous 15NO. We suggest that the use of nitrogen oxide labeled with 15N and/or 18O isotopes coupled with IR and mass spectroscopic analyses in the dark- and light-assisted chemisorption experiments may bring highly valuable information for elucidating the mechanism of their formation and transformation. Although several reactions are suggested in this work to account for the 14N ⇆ 15N exchange, it is clear that more effort is necessary to build up the mechanism and quantitatively describe kinetics of the exchange.

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ACKNOWLEDGMENTS

This work was supported by the Russian Foundation for Basic Research under grant 09-03-00795a. The authors would like to acknowledge the assistance of C. Deiana in editing the paper.

NO + 14 NO3− ⇄ I ⇄ 15 NO2 + 14 NO2− (or 15 NO2− + 14 NO2 )

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*Tel: +7-499-1355307. Fax: +7-499-1355328. E-mail: bns@ioc. ac.ru. Notes

The authors declare no competing financial interest. 10351



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(20) Pimentel, A. S.; Lima, F. C. A.; da Silva, A. B. F. The Isomerization of Dinitrogen Tetroxide: O2N−NO2 → ONO−NO2. J. Phys. Chem. A 2007, 111, 2913−2920. (21) Beckers, H.; Zeng, X.; Willner, H. Intermediates Involved in the Oxidation of Nitrogen Monoxide: Photochemistry of the cis-N2O2·O2 Complex and of sym-N2O4 in Solid Ne Matrices. Chem.Eur. J. 2010, 16, 1506−1520. (22) Liu, W.-G.; Goddard, W. A., III First-Principles Study of the Role of Interconversion Between NO2, N2O4, cis-ONO-NO2, and trans-ONO-NO2 in Chemical Processes. J. Am. Chem. Soc. 2012, 134, 12970−12978.

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FTIR and TPD Study of the Room Temperature Interaction of a NO

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