FTIR and TPD Analysis of Surface Species on a TiO2 Photocatalyst

Nov 3, 2009 - FTIR spectra show that NO− produced by the photoinduced adsorption ... In most cases, titanium dioxide is used as a photocatalyst owin...
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J. Phys. Chem. C 2009, 113, 20381–20387

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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 Ruslan V. Mikhaylov,† Andrei A. Lisachenko,† Boris N. Shelimov,*,‡ Vladimir B. Kazansky,‡ Gianmario Martra,§ Gabriele Alberto,§ 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, and Department of IPM Chemistry & Interdipartimental NIS Centre of Excellence, UniVersity of Torino, Via P. Giuria 7, 10125 Torino, Italy ReceiVed: July 1, 2009; ReVised Manuscript ReceiVed: October 15, 2009

As a continuation of our previous work on the kinetics of photocatalytic reduction of NO by CO on titanium dioxide, interaction of a Degussa P-25 TiO2 photocatalyst with NO, CO, and NO-CO mixtures at ambient temperature has been studied by FTIR and TPD. Only reversible weak adsorption of NO on surface Ti4+ ions is found to occur in the dark. UV-vis irradiation greatly enhances the NO adsorption on Ti4+ and yields N2O, NO-, NO2-, and NO3- surface species. After irradiation of TiO2 in a CO atmosphere, IR bands of surface CO2- and CO3- species appear in addition to a continuous IR absorption tail towards lower wavenumbers due to free carriers in the reduced semiconductor. When TiO2 is exposed to a equimolar NO-CO mixture, N2O and CO2- are formed without irradiation supposedly by the reaction 2 NO + 2 CO + O2- f 2 CO2- + N2O. Subsequent light irradiation is accompanied by the accumulation of NO- and Ti4+...NO complexes. No TiO2 reduction occurs in this case. FTIR spectra show that NO- produced by the photoinduced hν

adsorption of NO can be eliminated by the following reaction: NO- + CO 98 CO2- + (1/2) N2. It is believed that this reaction is a key step in the nitrogen production by the photocatalytic process. The data obtained enable us to refine the earlier proposed reaction mechanism and to directly prove some of its key steps. Introduction Photocatalytic removal of organic and inorganic pollutants from indoor air is being intensively investigated. In most cases, titanium dioxide is used as a photocatalyst owing to its high activity, versatility, nontoxicity, and low cost.1–3 Recently we have reported4 on environmentally important photoassisted selective reduction of NO by CO on Degussa P-25 TiO2 at room hν

temperature: NO + CO 98 (1/2) N2 + CO2. It was found that the photocatalytic reduction of NO can occur on TiO2 catalysts upon both UV and visible light (λ > 380 nm) irradiation, in the latter case owing to the presence of electron-donor centers (Ti3+, F, and F+ centers) in nonstoichiometric TiO2-x capable of absorbing visible light. The selectivity of the photoreaction to N2 production attained 90-95%, N2O being a minor product. Mass-spectrometric analysis of the gas phase showed that the CO2 formed by the reaction does not desorb at 300 K, suggesting formation of some carbonate or carboxylate stable surface structures which could potentially be detected by IR spectroscopy. However, the CO2 produced could be quantitatively desorbed after completion of the photoreaction by heating TiO2 to ∼500 K. A multistage reaction mechanism was proposed in ref 4 based on a combination of mass-spectroscopic kinetic analysis of gaseous reaction products with the studies of active centers in * Corresponding author. E-mail: [email protected]. † St.-Petersburg State University. ‡ Russian Academy of Sciences. § University of Torino.

TiO2 by UV photoelectron and UV-vis diffuse reflectance spectroscopy. The reaction scheme involved some N-containing surface intermediates (e.g., NO-, NO2-, and N2O) which could be the objects of an IR examination along with the abovementioned C-containing species. It was reported earlier5 that illumination of the NO + CO/TiO2 system at 200 K produced an IR band assigned to Ti-NCO species which could also be considered as a potential intermediate in the NO photoreduction. The present work was aimed at gaining a deeper insight into the mechanism of NO photoreduction with CO by FTIR and TPD. Successive reaction stages occurring in the adsorbed phase upon exposure of TiO2 to NO, CO, and NO-CO mixtures in the dark and under subsequent light irradiation were studied. Experimental Methods TiO2 samples for FTIR measurements were prepared by pressing powder Degussa P-25 (SSABET ) 50 m2 g-1; ca. 80% anatase, 20% rutile) into self-supported pellets (“optical thickness” of ca. 22-28 mg cm-2) The pellet was fixed in a gold frame and positioned in a quartz vacuum cell equipped with CaF2 windows, allowing thermal treatments, gas adsorption and desorption, and light irradiation to be carried out in situ. To remove organic contaminants, the sample was first heated under dynamic vacuum (∼10-6 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). Prior to admission onto TiO2, O2 and CO (PRAXAIR) were passed through a liquid nitrogen trap, whereas NO (PRAXAIR) was purified by fractional distillation. Gas pressures were measured

10.1021/jp906176c CCC: $40.75  2009 American Chemical Society Published on Web 11/03/2009

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Figure 1. Plot of NO coverage vs NO pressure at 293 K.

using a membrane PIEZOVAC PV20 gauge. FTIR spectra were recorded with a Vector 22 IR Fourier spectrometer (DTGS dectector, 4 cm-1 resolution). The samples were irradiated with full light of an OSRAM (500 W) mercury lamp through a pyrex + water filter to eliminate near-UV and near-infrared. No qualitative difference in the IR spectra was found after irradiation by either UV light (λ < 380 nm) or visible light (λ > 380 nm) through glass color filters (LOMO, St.-Petersburg). The only difference was much greater irradiation time in the latter case. For TPD experiments, ∼40 mg of powder TiO2 Degussa P-25 was deposited on the inner wall of a quartz cylindrical cell (d ) 35 mm, l ) 5 mm) from a suspension in bidistilled water. The sample in the cell was dried at 333 K overnight followed by calcination in air at 600 K for 10 h. The cell was then connected to a greaseless vacuum set up with a base pressure below 10-7 torr. To remove organic contaminants, the TiO2 sample was heated in flowing oxygen (0.5 torr) at 870 K for 10 h until no CO and CO2 was detected mass spectrometrically in the output flow. Pretreated TiO2 was exposed to NO or CO at room temperature and in some runs irradiated with a DRT120 mercury lamp (120 W) through a water filter to eliminate near-infrared. High-purity 16O2, 15NO, and 13CO (99.5%) were purchased from the St.-Petersburg State Institute of Applied Chemistry and purified by passing through a liquid nitrogen trap or by fractional distillation. Gas pressures in the range of 10-4-10 torr were measured by a calibrated digital Pirani gauge or by a membrane Baratron 122 gauge (MKS Instruments). A MI-1201 mass spectrometer (m/∆m ) 650) equipped with a data acquisition and processing system similar to that described earlier6 was used to analyze desorption products in the TPD experiments. The heating rate in all TPD measurements was 20 K/min. All adsorption and photoreaction measurements were performed at room temperature. Results and Discussion Dark NO Adsorption on TiO2. Figure 1 demonstrates the adsorption isotherm of NO on TiO2 at 293 K in the range of 0.02-0.3 torr. The NO coverage was determined by measuring the amount of NO adsorbed assuming that one NO molecule occupies a 10 Å2 area on the surface. The adsorption data fit well the Henry equation, Θ ) bP with b ) (5.5 ( 0.1) × 10-4 torr-1. The NO coverage in this pressure range is very low and does not exceed 2 · 10-4 monolayer (ML). The points of increasing- and decreasing-pressure branches coincide indicating

Figure 2. FTIR spectra of TiO2: (a) pretreated as described in the Experimental Methods section and (b) after exposure to 5 torr of NO for 60 min. The inset shows the difference spectrum (b - a).

a very low amount of NO remaining on the surface (coverage is less than 10-5 ML) after sample evacuation. Despite the limited extent of the adsorption, the use of IR to monitor the interaction of NO with the surface was successful. After outgassing and reoxidation at 873 K, the TiO2 spectrum in the MIR range (limited toward the low-frequency side by an intense absorption due to lattice modes) (Figure 2a) exhibited very weak bands in the 3750-3550 cm-1 range only, due to residual hydroxyl groups (the feature labeled with the asterisk is due to CO2 present in the instrument in slightly different amount when the reference and the sample spectra were taken; similar features in the following IR spectra will be indicated as due to “uncompensated CO2”). Admission of 5 torr of NO produces extremely weak signals in the 2000-1800 cm-1 range that does not evolve during the time of contact (Figure 2b, spectrum taken after 60 min). By extrapolating the NO coverage dependence of Figure 1 to NO pressure of 5 torr, one can assess the upper limit of NO coverage at this pressure as Θ ) 2.75 · 10-3 ML. Such signals are better observed after subtraction of the initial spectrum prior to NO adsorption (Figure 2, inset). They appear as a set of two bands at 1915 and 1900 cm-1 superimposed to the rotovibrational profile of gaseous NO and can be assigned to weakly bonded Ti-NO nitrosyl complexes.7–11 Outgassing at room temperature results in the disappearance of the nitrosyl bands (not shown in Figure 2). It is noteworthy that, contrary to previous studies on the IR of Degussa P-25 TiO2 in the presence of gaseous NO,8–10 we do not observe a set of strong IR bands in the 1650-1500 and 1300-1200 cm-1 region which is characteristic of surface nitrate and nitrite species. We believe that these bands may arise if oxygen and/or NO2 admixtures are present in the initial NO. Indeed, our experiments show that when even a minor admixture of O2 is present in NO, the IR bands of nitrates sharply increase. It is therefore concluded that strong precautions have to be taken to eliminate these admixtures in the starting NO, in particular, if the IR spectra are taken at relatively high NO pressures (10-100 torr) as in the studies of refs 8–10. After evacuation of TiO2 exposed to NO at room temperature, a TPD spectrum shown in Figure 3 is obtained. It displays a weak broad NO peak in the 350-700 K temperature interval. The amount of NO desorbed corresponds to a 10-5 ML coverage. Note that there is no NO2 desorption peak in the TPD spectrum.

Analysis of TiO2 Exposed to NO, CO, and NO-CO

Figure 3. TPD spectra after TiO2 exposure to 0.1 torr of NO for 10 min and after photoadsorption of NO (irradiation with full light of DRT120 for 10 min, 0.056 torr of NO adsorbed, 0.008 torr of N2O produced).

Figure 4. FTIR spectra of TiO2 irradiation for 30, 60, 90, and 120 min in the presence of 15 torr of NO.

Photoinduced Adsorption of NO on TiO2. As we reported earlier,4 upon UV-vis irradiation of TiO2 in the presence of gaseous NO, the NO pressure considerably decreases and simultaneously small amounts of N2O and N2 appear in the gas phase (5-10% of the amount of NO photoadsorbed). As a result of the photoadsorption, the NO coverage significantly increases (up to 10-2-10-1 ML), and a NO2 peak appears in the TPD spectrum along with the NO peak (Figure 3). No N2O is found in the TPD demonstrating that N2O is weakly bonded to the surface and can be easily removed upon evacuation at room temperature. The position of maxima (414 K) and the shape of NO and NO2 desorption curves are similar suggesting a common surface precursor that contributes to the desorption spectrum. The IR spectra recorded after the performance of a similar experiment with the sample in the IR cell are shown in Figure 4 (the initial spectrum taken in the presence of 5 torr of NO before irradiation has been subtracted). A number of upward bands appear and develop in a parallel way in the 2300-1000 cm-1 region, due to the newly produced surface species, while the downward feature with a main minimum at 1915 and a shoulder at 1900 cm-1 (corresponding to the main component of the spectrum in the inset of Figure 2) indicates the consumption of NO molecules initially adsorbed on the surface.

J. Phys. Chem. C, Vol. 113, No. 47, 2009 20383 The very weak component at 2240 cm-1, partly overlapped to the signal due to uncompensated CO2, is characteristic of ν(NO) in N2O weakly bonded to the surface and disappeared after evacuation at room temperature (not shown). The more intense band with a maximum at 1830 cm-1 can be assigned to Ti4+-NO complexes.7–11 The broadness and complex profile of the band suggest the formation of different kinds of nytrosyl species apparently linked to Ti4+ ions in different surroundings. The component at 1700 cm-1 can be attributed to a Ti3+-NO complex.9,10 In contrast to the Ti-NO species formed by dark NO adsorption (Figure 2), the bands in the 2000-1700 cm-1 interval produced by NO photoadsorption cannot be completely removed by sample evacuation at room temperature (not shown). The higher stability of these nitrosyl species is consistent with the hypothesis by Hadjiivanov and Knozinger9 who assumed enhancement of the Lewis acidity of some Ti4+ cations adjacent to negatively charged N-containing oxo species formed during NO adsorption (Vide infra). Note that a part of the broad band at 1830 cm-1 is located at higher frequency than the ν(NO) mode of gaseous NO (1876 cm-1), suggesting an enhancement of the σ-component of the Ti4+-NO bond. Conversely, the part of the band below such reference position, actually the main one, should be due to nitrosyl species with a considerable π-electron contribution of the adsorption center to the Ti-NO bond. The bands in the 1650-1050 cm-1 region are typical of surface nitrate and nitrite species.12,13 A schematic diagram for the band positions (labeled with letters from A to G) of different types of NO3- and NO2- is reported in Figure 5. The 1650-1520 cm-1 interval is characteristic of NO3- species only (Figure 5c), and the bands at 1648, 1572, and 1545 cm-1 in Figure 4 and Figure 5b can therefore be assigned to bridging (F) and/or to bidentate nitrates (E). The IR bands that fall into the lower wavenumbers interval (1520-1330 cm-1) are typical of surface nitrites. Thus, the bands at 1450, 1350, and 1330 cm-1 can be most likely attributed to a monodentate nitrite (A) and/or to a nitro compound (B). The most intense band at 1165 cm-1 in Figure 4 is to be assigned to NO-.9,10 Note that this band is markedly broader than the other bands of the spectrum. Its broadening may result from overlapping of lower frequency components of the NO3- and NO2- spectra falling into the 1300-1170 cm-1 gap. It appears however that a more comprehensive identification of the IR bands in this spectral region is not possible on the basis of exclusively IR data, and information from other techniques would be of great help to refine the assignment. The formation of NO3- and NO2- in the course of NO photoadsorption is also consistent with the appearance of the NO2 desorption peak in the TPD spectrum (Figure 3). One may suggest that NO2 results from a thermal decomposition of the oxo-nitrogen species. The following sequence of reactions to describe NO photoadsorption on TiO2, which substantially conforms to the earlier proposed mechanism,4 can be considered. Following absorption of a UV photon by TiO2, an electron donor (Ti3+)-hole (O-) pair is created: hν

Ti4+ + O2- {\} Ti3+ + O-

(1)

Ti3+ and O- react with NO to yield NO- and NO2-, respectively:

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NO + Ti3+ a NO- + Ti4+

(2)

Ti4+ + O- + NO f Ti3+ + NO2

O- + NO f NO2-

(3)

which in combination with reaction 1 gives the overall reaction of TiO2 reduction by NO earlier suggested by Ramis et al.:8

Subsequent reactions of NO- and NO2- can then produce N2O and NO3-:

2Ti4+ + NO + O2- f 2Ti3+ + NO2

(7)

(8)

NO- + NO f N2O + O-

(4)

Nitrous oxide is supposed to be produced by the interaction of NO with Ti3+:8

2Ti4+ + O2- + NO2- f 2Ti3+ + NO3-

(5)

2Ti3+ + 2NO f 2Ti4+ + O2- + N2O

Reactions 1-5 can be grouped into the overall reaction which explains well the IR data discussed above: hν

Actually, reaction 9 results from the summation of reactions 2 and the following reaction:

2NO- f N2O + O2-

6NO + 2O2- 98 N2O + 2NO- + NO2- + NO3-

(9)

(10)

(TiO2)

(6) An alternative mechanism for the NO photoadsorption suggests an intermediate formation of NO2 via reaction

Ramis et al. suggested8 that NO3- can be formed in the following process:

Ti4+ + O2- + NO2 f Ti3+ + NO3-

(11)

Note that this reaction can be presented as a combination of reactions 1 and 12:

O- + NO2 f NO3-

(12)

One may assume that NO2- species are produced by the following reaction:

2NO + O2- f NO- + NO2-

Figure 5. IR band positions of (a) surface nitrites and (c) surface nitrates according to refs 9, 10, and 12–14. A, monodentate nitrite; B, nitro compound; C, bidentate nitrite; D, bridging nitrite; E, bidentate nitrate; F, bridging nitrate; G, monodentate nitrate. (b) Schematic diagram of the IR spectrum of TiO2 with photoadsorbed NO (Figure 4) in the 1700-1000 cm-1 interval.

(13)

which is a combination of reactions 1-3. Summation of reactions 2, 8, 9, 11, and 13 results in the overall reaction 6. It is important to underline that NO2 is not present in the final eq 6, although it supposedly participates in intermediate steps of the process. Thus, we can conclude that further studies are necessary to discriminate between the two proposed schemes of the NO photoadsorption on TiO2. Adsorption of CO and Light Irradiation. CO admission onto the initial TiO2 is accompanied by a rapid CO adsorption (within tens of seconds) and by the appearance of two IR bands at 2205 and 2189 cm-1 (Figure 6, curve a and inset) which disappear under evacuation at room temperature. The former is generally attributed to CO molecules adsorbed on R-Ti4+ ions, i.e., on coordinatively unsaturated four-coordinated Ti4+, while the band at 2189 cm-1 is typical of CO adsorbed on β-Ti4+ ions, i.e., on five-coordinated Ti4+.14,15 An estimate of CO coverage at P(CO) ) 18 torr gives a value not exceeding 0.1 ML. After irradiation in CO for 5 h and subsequent evacuation, the FTIR spectrum (Figure 6, curve b) exhibits an absorption tail at lower wavenumbers due to free carriers in TiO2 and bands at 1620, 1570, 1450, and 1315 (with a shoulder at ca. 1350) cm-1. The tail can be completely removed upon admission of NO and O2 onto the reduced TiO2.

Analysis of TiO2 Exposed to NO, CO, and NO-CO

J. Phys. Chem. C, Vol. 113, No. 47, 2009 20385 hν

Figure 6. Change in the IR absorbance of TiO2 caused by (a) admission of CO (PCO ) 18 torr) and (b) subsequent UV-vis irradiation in CO for 5 h and outgassing at room temperature.

SCHEME 1: Structures of Surface Carbonate and Carboxylate Species

According to literature data,14 irreversible (reactive) CO adsorption on TiO2 is possible only above 250 °C. On the other hand, carboxylates and carbonates can be formed on TiO2 in the presence of gaseous CO2 at ambient temperature. Such interaction has been the subject of a number of IR investigations reviewed in refs 16 and 17 and is still attracting interest.18,19 Basically, it can be concluded that position and relative intensity of the IR bands of the negatively charged species possibly resulting from the adsorption of CO2 are highly sensitive to the conditions of the titania pretreatment. However, on the basis of the various results reported in the quoted references and of the widely accepted correlation between carbonates structure and splitting of their ν3 mode,16 it can be proposed that the components at 1570-1315 and 1450-1350 cm-1 arise from bidentate (Scheme 1A) and monodentate (Scheme 1B) carbonates, respectively. The third band of this species (νC-O), which is expected at 1080-1040 cm-1, is not seen apparently because of its low intensity and the presence of the low-frequency absorption tail in the reduced TiO2. In an IR study of the photoinduced activation of CO2 on TiO2,20 the formation of carboxylate species (Scheme 1C) favored by the partial reduction of titania was related to the appearance of a pair of bands at 1630-1640 and 1218 (much weaker) cm-1. On such a basis the weak component at 1620 cm-1 could monitor the formation of such species also in the present case, the partner mode at lower frequency being likely too weak to be observed. Production of CO2- and CO3- can be presented as a sequence of the following reactions:

Ti4+ + O2- {\} Ti3+ + O-

(1)

O- + CO f CO2-

(14)

CO2- + Ti4+ f CO2 + Ti3+

(15)

CO2 + O- f CO3-

(16)

TiO2 in Contact with a CO-NO Mixture without and under Light Irradiation. Figure 7a depicts the IR spectrum after exposure of TiO2 to a CO-NO mixture for 280 min at room temperature. It exhibits the N2O band at 2235 cm-1, the bands at 2205, 2189 cm-1 due to CO adsorbed on coordinatively unsaturated Ti4+, and the bands due to Ti4+-NO adducts in the 2000-1700 cm-1 range. All these components disappear upon evacuation (not shown). In contrast, the bands at 1576, 1447, 1338-1315, and 1080 cm-1 related to surface CO3species14,16,17 remain almost unchanged upon evacuation. Subsequent light irradiation of TiO2 in the presence of the NO-CO mixture results in a marked increase in the intensity of the Ti4+-NO bands and in the appearance of a strong NO- band at 1170 cm-1 (Figure 7b). The bands at 1576 and 1447 cm-1 slightly increase upon irradiation. It is of interest to notice that the irradiation of TiO2 in the presence of NO and CO could also result in the formation of adsorbed isocyanate species, expected to exhibit a band at ca. 2210 cm-1 (antisymmetric NCO stretching).5 However, such species should be produced in trace amount (if any) under irradiation at room temperature (likely because of the very small amount of adsorbed NO),5 and actually such a band was not observed in the present case even after careful inspection of the spectrum. In our previous study,4 we suggested that NO reacts with the TiO2 surface according to the following equilibrium:

Ti4+ + O2- + NO a Ti3+ + NO2-

(17)

The forward reaction results in the reduction of Ti4+ to Ti3+ with concomitant oxidation of NO to NO2-. N2O is supposed to be produced by the reaction involving oxidation of Ti3+ to Ti4+ and a parallel disproportionation of NO to N2O and NO2-:

Ti3+ + 3NO f Ti4+ + N2O + NO2-

(18)

It is reasonable to assume that NO2- may further interact with CO to yield CO2-:

NO2- + CO f NO + CO2-

(19)

An alternative route for the N2O formation was proposed by Ramis et al.:8

2Ti3+ + 2NO f 2Ti4+ + N2O + O2-

(20)

No matter whether reaction 18 or reaction 20 actually occurs, the overall process of NO interaction with CO on TiO2 in the absence of light irradiation can be represented by the following equation:

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2NO + 2CO + O2- f 2CO2- + N2O

Mikhaylov et al.

(21)

Reaction 21 agrees well with the IR data of Figure 7a, proving the formation of N2O and carboxylates as the reaction products. When TiO2 is exposed to NO-CO under light irradiation, the mobile holes created by reaction 1 are readily captured by CO to yield CO2- by reaction 14, whereas the electron-donor Ti3+ ions are oxidized to Ti4+ by NO molecules to produce NO(reaction 3). The NO- thus obtained further reacts with NO to form N2O (reaction 4). This reaction sequence qualitatively agrees with the IR data of Figure 7b, except for the carboxylate bands at 1576 and 1447 cm-1 growing slower than expected. It is assumed that this effect may arise from the photodecomposition of CO2- which limits the concentration of this species. Interaction of CO with TiO2 Irradiated in the Presence of NO. As a complementary experiment, CO was admitted on TiO2 previously irradiated in the presence of NO (see above Photoadsorption of NO section), and then the system was again irraddiated. The IR spectra recorded after each step of the experiment are reported in Figure 8, after subtraction of the spectrum of TiO2 irradiated in NO as background. By dosing CO, the peak at 2189 cm-1 with the shoulder at 2205 cm-1 due to carbonyl adducts on Ti4+ sites appears, while no significant change occurs in the 2000-1000 cm-1 range (Figure 8, dashed line). After subsequent light irradiation for 1 and 3 h, the band intensities of Ti4+-NO (2000-1700 cm-1) and NO- (1168 cm-1) considerably decrease (negative ∆ absorbance), and, in parallel, the CO2- and CO3- bands at 1576, 1450, and 1312 cm-1 markedly grow (positive ∆ absorbance, Figure 8, dotted and solid lines). The described spectral behavior suggests that under irradiation CO can interact with NO- to form CO2- as follows

Figure 8. Change in the IR absorbance of TiO2 with preliminary photoadsorbed NO after CO admission (dashed line), light irradiation for 1 h (dotted line), and light irradiation for 3 h and evacuation (solid line).

hν

NO- + CO 98 CO2- + (1/2)N2

(22)

The occurrence of this reaction is supported by our previous kinetic data4 showing that, after the accomplishment of NO photoadsorption, a further irradiation in the presence of CO resulted in a continuous evolution of gaseous N2 and in a decrease in the CO pressure.

Figure 7. (a) FTIR spectra of TiO2 exposed to CO + NO (10 + 10 torr) for 280 min; (b) after light irradiation for 300 min.

Figure 9. FTIR spectra of CO and N2O on TiO2. Solid line, CO admission on TiO2 with preliminary photoadsorbed NO; dashed line, after light irradiation for 2 h in CO; dotted line, CO adsorbed on initial TiO2.

Finally, Figure 9 shows a zoomed view of the above IR spectra in the 2275-2150 cm-1 range, where for the sake of comparison the spectrum of CO adsorbed on initial TiO2 is also reported. It can be observed that the intensity ratio of the 2205 and 2189 cm-1 components depends on TiO2 pretreatment. In particular, the former one appears significantly less intense for TiO2 with prephotoadsorbed NO (solid line) suggesting that most of the R-Ti4+ sites have been occupied by adsorbed NO and other nitrous species in the course of preliminary photoadsorption. Following subsequent irradiation in CO, the intensity of the 2205 cm-1 band increases (dashed line), and the I(2205)/ I(2187) ratio becomes nearly the same as for the initial TiO2 (dotted line), indicating that the R-Ti4+ sites are now completely accessible for CO molecules. Simultaneously, a weak N2O band develops at 2235 cm-1 in agreement with kinetic data4 evidencing that some N2O evolves into the gas phase. In conclusion, it should be noted that here we report on some new features of the interaction of NO, CO, and NO-CO mixtures with Degussa P-25 TiO2 both in the absence and in the presence of UV-vis irradiation. In contrast to what has been earlier reported in the literature,7–10 we did not observe develop-

Analysis of TiO2 Exposed to NO, CO, and NO-CO ment of strong IR bands of Ti4+-NO, NO3-, and NO2- species after NO admission onto TiO2 without light irradiation. Correspondingly, neither noticeable manometrically detected pressure drop of NO in the dark nor NO2 desorption in the TPD runs after TiO2 exposure to NO were found to occur. In view of this, we assume that an oxygen admixture in the initial NO gas might be responsible for the growth of intense IR bands in the 2000-1000 cm-1 region as reported in the literature.7–10 Only a few studies are available on the light-assisted NO adsorption on TiO2.7,21 Our results are in general agreement with the idea about photon-induced disproportionation of NO on the TiO2 surface7 to give rise to N2O and a NO2 molecule coordinated to a surface Ti ion. However, the present study shows that the disproportionation actually occurs via a more complex route to yield N2O, NO-, NO2-, and NO3- as suggested by reaction 6. The FTIR examination of TiO2 after UV irradiation in CO at ambient temperature reveals the appearance of IR bands assigned to surface carboxylate and carbonate species. The formation of these species at low temperature is quite remarkable taking into consideration that the thermal reaction of CO with TiO2 surface proceeds only above 250 °C.14 UV irradiation of TiO2 in CO results in the reduction of the solid as well. This fact is evidenced by the development of a continuous IR absorption caused by the formation of free carriers in TiO2 and by the evolution of CO2 into gas phase in the course of light irradiation.4 When TiO2 is exposed to a NO-CO mixture, CO2- and CO3are readily formed without irradiation as supposed by reaction 21. Subsequent light irradiation results in the accumulation of NO- and Ti4+-NO complexes. It is noteworthy that UV-vis irradiation in NO-CO does not lead to reduction of TiO2 as evidenced by the IR spectra of Figure 7. Finally, the FTIR study of CO interaction with preliminary photoadsorbed NO (Figure 8) enables us to directly prove a key step (reaction 22) in the mechanism of photocatalytic NO

J. Phys. Chem. C, Vol. 113, No. 47, 2009 20387 reduction by CO on TiO2 earlier proposed in ref 4 on the basis of kinetic and TPD experiments. Acknowledgment. This work was supported by the Russian Foundation for Basic Research under grant 09-03-00795-a. G.M. and S.C. acknowledge Nanomat-ASP (Docup 2000-2006, Linea 2.4a) for financial support. References and Notes (1) Mills, A.; Le Hunte, S. J. Photochem. Photobiol., A 1997, 108, 1. (2) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735. (3) Chen, X.; Mao, S. S. Chem. ReV. 2007, 107, 2891. (4) Lisachenko, A. A.; Mikhailov, R. V.; Basov, L. L.; Shelimov, B. N.; Che, M. J. Phys. Chem. C 2007, 111, 14440. (5) Rasko´, J.; Szabo´, Z.; Ba´nsa´gi, T.; Solymosi, F. Phys. Chem. Chem. Phys. 2001, 3, 4437. (6) Lisachenko, A. A.; Chikhachev, K. S.; Zakharov, M. N.; Basov, L. L.; Shelimov, B. N.; Subbotina, I. R.; Che, M.; Coluccia, S. Top. Catal. 2002, 20, 119. (7) Alekseev, A. V.; Filimonov, V. N. Russ. J. Phys. Chem. 1984, 58, 2023. (8) Ramis, G.; Busca, G.; Lorenzelli, V.; Forzatti, P. Appl. Catal. 1990, 64, 243. (9) Hadjiivanov, K.; Kno¨zinger, H. Phys. Chem. Chem. Phys. 2000, 2, 2803. (10) Kantcheva, M. J. Catal. 2001, 204, 479. (11) Rusu, C. N.; Yates, J. T., Jr. J. Phys. Chem. B 2000, 104, 1729. (12) Hadjiivanov, K. Catal. ReV.sSci. Eng. 2000, 42, 71. (13) Pozdnyakov, D. V.; Filimonov, V. N. Kinet. Katal. 1972, 14, 760. (14) Davydov, A. IR Spectroscopy Applied to Surface Chemistry of Oxides; Nauka: Novosibirsk, 1984. (15) Hagjiivanov, K. I.; Vayssilov, G. N. AdV. Catal. 2002, 47, 307. (16) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89. (17) Solymosi, F. J. Mol. Catal. 1991, 65, 337. (18) Liao, L.-F.; Lien, C.-F.; Shih, D.-L.; Chen, M.-T.; Lin, J.-L. J. Phys. Chem. B 2002, 106, 11240. (19) Su, W.; Zhang, J.; Feng, Z.; Chen, T.; Ying, P.; Li, C. J. Phys. Chem. C 2008, 112, 7710. (20) Rasko´, J.; Solymosi, F. J. Phys. Chem. 1994, 98, 7147. (21) Wu, J. C. S.; Cheng, Y.-T. J. Catal. 2006, 237, 393.

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