Adsorption of Nitrogen Oxides on TiO2 Surface as a Function of NO2

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage 2

Adsorption of nitrogen oxides on TiO surface as a function of NO and NO fraction in the gas phase 2

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Kalev Erme, Jüri Raud, and Indrek Jogi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03864 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Adsorption of nitrogen oxides on TiO2 surface as a function of NO2 and N2O5 fraction in the gas phase Kalev Erme, Jüri Raud, Indrek Jõgi* Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia E-mail: [email protected]

Keywords: NOx removal, ozone, TiO2 catalyst, adsorbent

Abstract Present study was devoted to the investigation of adsorption of nitrogen oxides on TiO2, with the focus on the effect of NOx concentration, composition and flow rate. The inlet NO with concentration of 200-800 ppm in pure N2 was mixed with ozone, produced from pure oxygen, and directed to a reactor with catalytic TiO2 powder. The oxidation of NO by ozone allowed to prepare mixtures with variable concentrations of NO and NO2 or NO2 and N2O5 which were adsorbed on the catalyst surface during the oxidation phase and were desorbed when only NO was flowing through the reactor. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) studies showed NO3- as the main adsorbed nitrogen oxide specimen on the surface. The amount of adsorbed nitrogen oxide species increased with the increasing fraction of NO2 in the gas phase and were inversely proportional with gas phase NO concentration. An important finding was the abrupt increase in the nitrogen oxide adsorption capacity of TiO2 when the inlet concentration of ozone became sufficiently large to oxidize NOx to N2O5. Based on the results, a model of 1 ACS Paragon Plus Environment

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surface processes is proposed, involving the production of NO3 and N2O5 on the surface of TiO2.

Introduction Fossil fuel burning in power stations, boilers and engines results in the emission of considerable amount of NOx species (NO and NO2) 1. After reaching the atmosphere and reacting with water vapor, the NOx species form nitrogen acids and cause the acid rain, smog, etc. As a consequence, the removal of NOx from the exhaust is essential to preserve the environment and public health. Most important end-of-pipe NOx removal methods are scrubbing, selective catalytic reduction (SCR) and NOx storage reduction (NSR) technology

2,3

. In the case of

scrubbing, NOx species are absorbed in water or other liquids where they form nitrogen acids or salts. In the case of SCR, the species of NOx are continuously reduced on the catalyst surfaces by suitable reducing agents while in the case of NSR, the species of NOx are cyclically adsorbed during the lean conditions and reduced during a short fuel rich pulse. All listed methods suffer from poor absorption and adsorption of NO, which is the most abundant specimen in exhaust whereas the oxidation of NO to NO2 or N2O5 makes the removal considerably easier. It is possible to oxidize NO by precious metal catalyst which are often present in SCR and NSR processes but the oxidation requires relatively high temperatures – above 200 °C – to proceed with sufficient efficiency

4–6

. Low

temperature oxidation can be carried out by O and OH radicals and ozone, which are efficiently produced by non-equilibrium plasmas. However, the back-reaction of NO2 to 2 ACS Paragon Plus Environment

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NO limits the maximum amount of NO which can be oxidized directly by plasma

7,8

.

Considerably higher amounts of NO can be oxidized and subsequently removed by absorption or adsorption based methods when plasma is used separately for ozone production and the produced ozone oxidizes NO 9–15. Nevertheless, the oxidation of NO2 to N2O5, which is especially useful in the case of scrubbing 1, is still slow and the process needs further improvements for economic feasibility 9. Plasma-catalytic and ozone-catalytic systems have been successfully used for the oxidation of Volatile Organic Compounds (VOC), where the inclusion of catalyst has shown to considerably increase the oxidation efficiency

16,17

. Our recent study showed

that cheap and abundant TiO2 catalyst may also improve the efficiency of ozone-assisted oxidation of NO2 to N2O5

18

. In addition to the catalytic effect, we observed the

adsorption of nitrogen oxides on TiO2 surface which was markedly different when the gas contained N2O5 in addition to NO2. The latter effect was not studied in detail while more detailed investigation of the adsorption processes of nitrogen oxides might give supplementary information about the catalytic processes. From the practical point of view, the knowledge of the ozone enhanced adsorption processes of nitrogen oxides is also applicable for non-thermal plasma assisted adsorption complemented with subsequent plasma-assisted decomposition at reducing conditions

2,3,19

. Furthermore, the

adsorption of nitrogen oxides may potentially influence the VOC oxidation by plasma 20. There is already considerable amount of knowledge about the NO2 adsorption on the surface of TiO2

21–25

but there is still lack of studies concerning the adsorption of

N2O5. The adsorption of NO2 without NO, which is relevant in the studies of atmospheric chemistry, has been carried out either by bath reactors without flow

21–23

or flow-type 3

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reactors

24,26

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and the main conclusion was the adsorption of NO2 dominantly in the form

of nitrates accompanied with the production of gas phase NO. The adsorption of NO2 in exhaust gas treatment takes place in the presence of variable fraction of NO and as a consequence the amount of adsorbed nitrogen oxides expectedly decreases because NO may also reacts with surface nitrates. Present study was carried out to investigate the adsorption process of nitrogen oxides on TiO2 surface in flow conditions at varying residence times and inlet NO concentrations. The injection of certain amount of ozone to NO/N2 flow allowed to investigate the adsorption of nitrogen oxides with different proportion of NO/NO2 or NO2/N2O5 which has not been studied previously. The use of flow-type reactor was a prerequisite for the investigation of N2O5 adsorption which has relatively small life-time, especially at elevated temperatures. The experiments were made at 100°C where our earlier experiments have demonstrated ozone decomposition on TiO2 surface and catalytic oxidation of NO2 to N2O5 on TiO2

18

. At even higher temperatures, ozone and

N2O5 start to decompose also in the gas phase. The inlet concentrations of NO were varied between 200 and 800 ppm which are found in real exhaust gases

14,27–29

whereas

other usual exhaust gas constituents, H2O, CO2 and SO2 were missing to reduce the number of possible surface processes.

Experimental Section The experimental setup includes two reactors of similar build – one for the purpose of ozone production and the other for the storage of the catalyst 18. The ozone generator is based on a coaxial dielectric barrier discharge device. A stainless steel tube of 14 mm

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outer diameter was used as the inner electrode. A quartz tube with 16.3 mm inner diameter functions as the dielectric barrier and the outer electrode is a steel mesh around the tube. The length of the active zone of the reactor was 85 mm resulting in the volume of 4.7 cm3. The inner electrode was powered over a transformer. Voltage generator HQ Power DVM20FGCN connected with commercial power amplifier Industrial Test Equipment Co, Inc “Powertron” A 500 RF produced a sinusoidal voltage with frequencies from 100 Hz to 5 kHz. The voltage waveforms were recorded by a capacitive voltage divider. The input power was measured by the method of Lissajous’ figures

30

with the use of an

additional capacitor connected in series with the reactor. Finally, specific input energy, SIE, in the units of Joule per liter of gas, was obtained by dividing the input power with the flow rate. Ozone was produced

from pure O2 directed through the discharge device.

The production of ozone was a linear function of SIE within the working range of the reactor in our experiments. The estimated uncertainty of ozone concentration was 10%, which was mostly determined by the uncertainty of the measurement of SIE. Commercial Degussa P25 powder with 80% of TiO2 in anatase and 20% in rutile was used as the catalyst with the BET surface area of 50 m2/g and average size of anatase and rutile particles 25 nm and 85 nm

31,32

. The powder was pressed on the inner surface

of the catalytic reactor and the coating formed a thin layer (about 0.1-0.2 mm). The mass of the catalyst was approximately 300 mg. The temperature in the catalytic reactor was controlled by an electrically heated oven. The experiments were carried out at 100°C. Various inlet gases were mixed from gas cylinders with the use of flow-controllers from Alicat Scientific. The total gas flow rate, Fr, through the catalytic reactor was varied 5 ACS Paragon Plus Environment

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between 0.5 and 2 L/min. The flow of pure oxygen (purity 5.0) was directed through the ozone generator to avoid the production of NOx by the discharge. The outlet of the ozone generator with variable amount of O3 was mixed with NO in pure nitrogen (purity class 5.0) flow to produce a gas mixture with 200-800 ppm NO. The system had a slight over pressure and it was heated and purged for several hours before each experimental series to reduce the effect of possible residual water. The gas composition at the outlet of the catalytic reactor was identified by optical absorption spectroscopy, where the setup consisted of a Hamamatsu L7296-50 deuterium lamp, a 20 cm absorption cell and an Ocean Optics 4000 spectrometer. The latter has a 200-850 nm wavelength range and a 1.5-2.3 nm optical resolution. This method allows measuring the concentrations of NO, NO2, NO3, N2O5 and O3, as described in our previous papers

33,34

. The absorption cross-sections of the nitrogen-oxide species were

obtained from MPI-Mainz UV/VIS spectral atlas

35

with the exception of NO. The

absorption coefficient of NO was obtained by calibration procedure using mixtures of NO and N2 with different concentrations of NO. Since the spectrum of N2O5 around 200 nm overlaps with the spectra of NO2 and O3, it was necessary to determine the signal around 200 nm caused by a certain concentration of NO2 and O3 in separate experiments, where only NO2 or O3 were produced. After subtraction of the absorption caused by known values of NO2 and O3 (determined at wavelengths 252 and 400 nm), the shape of absorption band near 200 nm corresponded to N2O5. It was additionally verified that the absorption signal was not due to HNO3 by determining the spectra after intentionally including 1% H2O to the mixture. Finally, the sum of concentrations of NO, NO2 and 2 times N2O5 corresponded to the 6 ACS Paragon Plus Environment

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inlet concentration of NO, which further confirms the assignment of the absorption at 200 nm to N2O5. The absorption features of NO3 were not observed in the absorption spectra which suggests that the concentration remains below 1 ppm. The estimated uncertainty for the outlet concentration was 15 ppm for NO, 20 ppm for NO2 and 30 ppm for N2O5. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) studies were carried out with the Interspec 2020 spectrometer supplied with Pike Technologies DiffusIRTM Diffuce Reflectance Accessory and a homemade reaction chamber with powder sample holder and ZnSe window. The P25 powder with the mass of approximately 30 mg was pressed into the sample holder which was then positioned inside of the heatable reaction chamber. The experiments were carried out at 100°C. The spectra were recorded with varying NO and O3 inlet compositions similarly to other experiments and reference spectra were obtained with only NO in the gas. 100 scans were accumulated for one spectra and spectral resolution was 4 cm-1.

Results and Discussion Time-dependence of outlet concentrations with and without TiO2 The time-dependence of the outlet concentrations of NO, NO2 and N2O5 with and without the TiO2 catalyst is shown in figure 1. It is possible to distinguish between two regimes. In the first regime the inlet concentration of O3 was smaller than the inlet concentration of NO. Thus all of the inlet ozone was consumed and only part of the NO was oxidized to NO2, while N2O5 and O3 were missing in the outlet. In the second regime, the inlet concentration of O3 was higher than the inlet concentration of NO and all NO was oxidized to NO2, while part of the NO2 was further oxidized to N2O5

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. In 7


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addition, there appeared unreacted O3 in the outlet. The first regime will be referred to as NO2 production regime and the second regime as N2O5 production regime. In both regimes, the concentrations of NOx and N2O5 varied in time when the TiO2 catalyst was introduced to the reactor (Figure 1c and 1d). The data shown on these and subsequent figures was obtained after a first cycle was carried out with fresh catalyst where 20 minutes in NO2 production regime was followed by 20 minutes of NO flow. In subsequent cycles, the time-dependences were reproducible in the limits of the uncertainty.

Figure 1. Time-dependent outlet concentrations of NO, NO2, N2O5 and O3 at different inlet O3 concentrations: (a), (c) – only NO and NO2 were present in the mixture; (b), (d) – N2O5 was also formed and NO was completely oxidized. Results (a) and (b) were obtained without TiO2 and (c) and (d) with TiO2. In all cases the inlet concentration of 8 ACS Paragon Plus Environment

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NO was 800 ppm, the gas flow rate was 1 slm and temperature was 100 °C. The beginning and end of ozone production are indicated with arrows. In NO2 production regime (Fig. 1c), the outlet concentrations of NO and NO2 stabilized after several minutes of the ozone injection with the presence of TiO2. The stabilization time was longer during the first cycle with a fresh catalyst but subsequent cycles showed repeatable results. During the first minutes of ozone injection the outlet concentration of NO exceeded the final stable concentration by a certain value ∆[NO] whereas NO2 concentration remained smaller than the stable concentration by the value of ∆[NO2]. Immediately after the ozone injection stopped, the concentration of NO2 slightly increased and then started to diminish whereas NO outlet concentration remained initially below the value of inlet concentration. The ratio of ∆NO to ∆NO2 remained between 2.5 and 3.0 at each time-instance during the stabilization period following the beginning or finishing of the ozone injection. In N2O5 production regime, both NO2 and N2O5 initially increased and reached stable concentration after a few minutes. After switching off the O3 reactor, the concentration of NO2 sharply increased, while N2O5 concentration decreased. When N2O5 concentration diminished to zero, NO appeared in the outlet and NO2 concentration started to slowly decrease. Similarly to NO2 production regime, the ratio of ∆NO to ∆NO2 was approximately 3.0 in the latter phase where only NO and NO2 were observable in the outlet. The steady state concentrations at various inlet NO and O3 concentrations obtained after 5-10 minutes after the start of ozone oxidation have been published in our earlier paper 18. In the NO2 production regime, all O3 was consumed for the oxidation of NO to 9 ACS Paragon Plus Environment

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NO2 and the produced NO2 concentration was equal with the inlet O3 concentration regardless of the experimental conditions. In the N2O5 production regime, there was certain amount of unreacted O3 present whereas the concentration of outlet O3 concentration was higher at increasing inlet O3 and NO concentration and without the presence of TiO2. Furthermore, some O3 decomposed whereas the amount of decomposed O3 increased in the presence of TiO2. The latter effect can also be deduced from the figures 1c and 1d.

Time dependence of the total concentration of nitrogen oxide species The time-dependent outlet concentration of the sum of the nitrogen oxide species, [NxOy]=[NO]+[NO2]+2[N2O5], is shown in figure 2 for various inlet ozone concentrations. Without the effect of surface processes, the outlet concentration of [NxOy] would remain at the value of the inlet concentration of NO, [NO]in, even during the ozone injection. This was also observed when catalyst was not present in the reactor. In the presence of catalyst however, [NxOy] remained smaller than [NO]in at the beginning of ozone injection while at the end of ozone injection [NxOy] became larger than [NO]in. The difference between [NxOy] and [NO]in, denoted as ∆[NxOy], can be explained by the net adsorption of nitrogen oxide species on the TiO2 catalyst when [NxOy][NO]in. Increasing inlet ozone concentrations resulted in the increase of ∆[NxOy] while at certain ozone concentration there was no further increase of ∆[NxOy] and the curves obtained at higher inlet O3 concentration coincided.


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Figure 2. The time-dependence of the sum of concentrations of all nitrogen oxide species at different inlet O3 concentrations. The results were obtained at the inlet concentration of NO 800 ppm, gas flow rate 1 slm and temperature 100 ºC. The time-dependence of [NxOy] obtained when the missing amount of [NxOy] was highest is shown in figure 3 for different inlet NO concentrations and flow rates. At all conditions, practically all nitrogen oxides were removed when the ozone injection started. The time which was necessary to reach stable conditions decreased with the growing inlet NO concentration and gas flow rate. When the ozone injection was stopped, the increase of [NxOy] above the value of [NO]in was larger at higher NO inlet concentration and the maximum value of [NxOy] was 2.5 to 3 times larger than [NO]in. The maximum [NxOy] value depended also on the gas flow rate.


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Figure 3. Time-dependence of the total concentration of nitrogen oxides after the start and stop of ozone injection at different inlet NO concentrations (flow rate of 1 slm) and gas flow rates (800 ppm NO). The inlet ozone concentrations were chosen at sufficiently high values and any further increase didn’t change the time dependency of [NxOy]. The temperature was 100°C.


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Figure 4 plots the time interval between the start of ozone production and the moment when [NxOy]=0.5[NO]in, i.e. the characteristic time of adsorption (t1/2), as a function of the inlet flow rate of NO (FNO), determined by the total gas flow rate and the inlet concentration of NO. Similar dependences were obtained for 20°C and 100°C. The t1/2 was inversely proportional to FNO: t1/2=A/FNO. The constant A has the units of moles and would correspond to the total number of adsorbed surface species when the timedependences of [NxOy] shown on figure 3 would be sharp step-function. The value of A can give an approximate value of total number of adsorbed species even when the timedependence of [NxOy] is not a step function. The value of A was approximately 40 µmol (100 µmol/g). Single value of A characterizing different flow rates and inlet concentrations of NO suggests that the number of adsorbed surface species was practically independent on the FNO and [NO]in.

Figure 4. Characteristic time of adsorption at different values on inlet NO flow rate. The data points correspond to time dependencies presented on Figure 3. The theoretical dependence t1/2=A/FNO, where A = 39.4 µmol, is shown as the dashed curve.


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Total amounts of adsorbed substances The amount of adsorbed nitrogen oxide species during the ozone injection can be obtained by integrating the missing amount of nitrogen oxide ∆[NxOy] over the time of ozone injection:

∆ ′. The net adsorption of nitrogen oxide species stops when [NxOy] reaches the value of [NO]in and the A(t) at this time instant is the value of total amount of adsorbed species. It is also necessary to emphasize, that the following results were obtained from catalyst which were initially treated with a cycle in NO2 production regime and the adsorption corresponds to reversible processes. After the first treatment cycle, the total amount of adsorbed and subsequently desorbed nitrogen oxide species were equal in the limits of uncertainty.

Figure 5. Integrated amount of adsorbed species as a function of inlet concentration of O3, expressed in units of µmol per gram of TiO2. The inset on (a) shows a typical timedependence of the total outlet concentration of nitrogen oxides. The temperature in all 14 ACS Paragon Plus Environment

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cases was 100 ºC. On (a) the flow rate was 1 slm and on (b) the inlet concentration of NO was 800 ppm. Figure 5 shows the total amount of adsorbed species as a function of inlet concentration of ozone, [O3]in, at 100°C. Both graphs indicate a linear dependence of adsorption on the [O3]in at NO2 production regime. Sharp increases can be seen, when the inlet ozone concentration exceeds the inlet concentration of NO, coinciding with transitions from the NO2 production regime to the N2O5 production regime. The total amounts of adsorbed species after the end of ozone production, calculated by the same method, exhibited similar tendencies. The total amount of adsorbed species obtained by integration was somewhat larger than found from the data shown in figure 4. This difference can be attributed to the non-symmetric shape of the time-dependence of [NxOy]. While the total amount of adsorbed species were independent of the total gas flow rate, as seen on Figure 5b, the results presented on Figure 5a allow us to estimate the effect of gas stream composition on the total amount of adsorbed species. Due to reactions that take place in the gas phase

18

, a mixture of both NO and NO2 reaches the

surface of TiO2 in the NO2 production regime. Comparison of results with different inlet NO concentrations and the same inlet O3 concentration, i.e. the same concentration of NO2, indicates that the total amount of adsorbed species depends both on the concentration of NO and NO2 in the gas stream reaching the surface. The adsorbed NOx is approximately inversely proportional to the concentration of NO.

DRIFTS measurements 15 ACS Paragon Plus Environment

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The DRIFTS spectra obtained for 430 ppm NO inlet concentration at NO2 and N2O5 production regimes are shown in figure 6. The spectra are the average of 100 spectrum collected after the stable conditions were reached in the outlet. The collection of the spectra took approximately 20 minutes with the used setup and this prevented the measurement of time-dependent changes of surface species. The most prominent peak in NO2 production regime was at 1640 cm-1 whereas small features were also detectable at 1538, 1500, 1724 and 1300 cm-1. The feature at 1640 cm-1 can be assigned to the bridging NO3-

22,23,25,36

and its intensity increased with the increase of NO2 fraction in the inlet

mixture (Fig. 6b). The increase of the intensity was inversely dependent on NO inlet concentration similarly to the amount of adsorbed NOx (Fig. 5a). The bands disappeared during the time when only NO was present in the mixture.

Figure 6. DRIFTS absorbance spectra obtained for NO2 and N2O5 production regime with 430 ppm NO inlet concentration. The integrated absorbance intensity of the peak between 1600 and 1720 cm-1 as a function of ozone concentration for different inlet NO concentrations. The temperature was 100 ºC. In the N2O5 production regime, the intensity of this band further increased and shifted towards higher wavenumbers. There appeared also additional bands in the 16 ACS Paragon Plus Environment

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absorbance spectra whereas at wavenumbers around 1500 cm-1 the absorption became negative (Figure 6a). Based on the earlier studies 22,25,36, we assign the band at 1210 cm-1 also to bridged NO3-, the band at 1558 and 1270 cm-1 to bidentate NO3- while nitrites may also partially give the features in this wavenumber region

25,36

. The negative absorption

feature at 1500 and 1300 cm-1 could be connected to monodentate NO3- or nitro NO2compounds 36. The species connected with these bands have been found to be less stable on the surface

22,36

and could be either desorbed or converted to some other specimens

e.g. bridging NO3- in the N2O5 production regime. The band at 2184 cm-1 could be related to NO+ 22,36 and the band at 1186 cm-1 to NO- 22 The bands between 2000-1700 cm-1 have been attributed to Ti3+-NO and Ti4+-NO nitrosyl complexes NO+

23

22

but also to N2O3

22,36

or

. The band at 2360 cm-1 is partially overlapping with the CO2 bands but

corresponds clearly to a separate specimen related to nitrogen oxides.

Surface processes in NO2 production regime We start the discussion of surface processes with the model that applies to surface processes on TiO2 in the case of the NO2 production regime ([O3]in < [NO]in). Several authors have proposed surface reaction models, according to which two adsorbed NO2 molecules give rise to the production of a surface-bound NO3 and a gas phase NO

21,37

.

This model determines the ratio between consumed NO2 and produced NO to be 2:1. However, as described above, our experimental data suggests that the ratio lies between 2.5:1 and 3:1. This ratio is better explained by an NO2 adsorption mechanism which was proposed for TiO2 by Sivachandiran et al

23,24

and has also been proposed for Al2O3

38

and Cu-ZSM-5 39. The mechanism involves the following reactions:


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2NO2(s) ↔ NO3-(s) + NO+(s)

(1)

NO+(s) + O2-(s) ↔ NO2-(s)

(2)

NO2-(s) + NO2(s) ↔ NO3-(s) + NO(g).

(3)

In reaction (1) two adsorbed NO2 molecules undergo a disproportionation reaction to produce a nitrate ion NO3- and a nitrosonium ion NO+. The latter is highly reactive and reacts with a TiO2 surface lattice oxygen O2- to produce a nitrite ion NO2- in reaction (2). The nitrite ion is consumed in reaction (3) with an adsorbed NO2 molecule, producing another nitrate ion and an NO molecule, which desorbs from the surface. As a net result, three NO2 molecules are involved in two consecutive disproportionation reactions to produce two NO3- species and one NO in gas phase, and these reactions determine the theoretical ratio between consumed NO2 and produced NO to be 3:1. The DRIFTS data confirmed that NO3- was the prominent surface specimen during stable conditions while the absorbance due to other nitrogen oxide species remained below the noise level. Unfortunately, due to the small mass of catalyst which could be introduced into the DRIFTS cells, the time-changes of nitrogen-oxide concentrations in the outlet of DRIFTS cells could not be determined with sufficient accuracy. This prevented the determination of total amount of adsorbed nitrogen oxides and correlation of the data with the intensities of absorption bands of DRIFTS spectra. At increasing NO2 fraction in the gas phase the balance of reaction (3) shifts towards the production of NO3- while at higher gas phase NO fraction the balance shifts towards the removal of NO3-. This explains the dependence of NO3- production on the NO concentration and the appearance of additional NO2 in the outlet after the ozone production was stopped. The amount of adsorbed NOx and produced NO3- appears to be 18 ACS Paragon Plus Environment

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the same regardless of the maximum concentration of gas phase NO2 (Fig. 5a and 6b) which suggests that there exists a maximum level of NO3- species which can be bound to the surface in NO/NO2 mixtures. The model proposed for the explanation of experimental findings suggests that the surface processes in the NO2 production regime involve only nitrogen oxides. The ozone is quickly consumed by the oxidation of NO to NO2 in gas phase ( [NO]in), we have proposed that the process is enhanced by the catalytic decomposition of ozone, which is enabled by the existence of surplus ozone from the fast gas-phase oxidation of NO to NO2

18

. The remaining amount of ozone participates partly in the

slower gas-phase oxidation of NO2 to N2O5 and partly in a decomposition reaction on the surface of TiO2. The latter process produces a supply of reactive oxygen on the surface 41,42

: TiO2 + O3 → O-TiO2 + O2.

(4)

This reaction is less significant at room temperature, but occurs to a considerable extent at 100 °C

18

. The atomic oxygen stored on the surface may react with adsorbed NO2 to

produce surface-bound NO3 molecules: 19 ACS Paragon Plus Environment

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NO2(s) + O-TiO2 → NO3-TiO2.

(5)

The work of Karagulian and Rossi on atmospheric mineral dust surrogates suggests that surface NO3 may undergo a heterogeneous recombination reaction, in which adsorbed NO3 reacts with NO2 from the gas phase via the Eley-Rideal mechanism, producing N2O5. Once N2O5 has been formed on the surface, it may desorb into gas phase 43,44

. Similar mechanism can be proposed for the formation of N2O5 on TiO2, along with

its subsequent desorption: NO3(s) + NO2(g) → N2O5(s)

(6)

N2O5(s) → N2O5(g)

(7)

According to the DRIFTS spectra, the amount of NO3- species increases considerably in the N2O5 production regime. This suggest that additional surface sites are involved in the NOx adsorption in N2O5 production regime. Furthermore, gas phase N2O5 is not detected in the NO2 production regime, which suggests that O3 decomposition results in the production of different type of surface adsorbed NO3- when compared to the disproportionation reactions (1)-(3). In the latter case, at least one NO3- specimen becomes connected to the O2- site of TiO2 surface lattice

20

. At the same time, the

decomposition of O3 produces O atoms which are attached to strong Lewis acid sites related to certain five-coordinated Ti4+

45

41

. The shift of the absorption band at 1640 cm-1

towards larger wavenumbers may also indicate the appearance of different type of bridged NO3- specimen on the TiO2 surface. It is possible, that the reaction (6) proceeds also backwards and surface-bound N2O5 decomposes back to NO3 and NO2. During ozone injection in N2O5 production regime,


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NO2 dominantly reaches the surface of TiO2, shifting the balance of reaction (6) towards the formation of N2O5. After the ozone production stops, NO replaces NO2 as the gasphase reactant reaching the surface. In this case, the production of NO2 from surface bound N2O5 by backward reaction (6) should dominate and result in slowly decreasing concentration of NO2. However, NO2 concentration actually increases immediately after the end of ozone production and peaks at the concentrations which are approximately two times higher than the inlet NO concentration. The following rate of decrease of NO2 concentration is approximately three times larger than the increase of NO concentration. These results are explainable by the reactions (1)-(3) proceeding backwards similarly as in experiments in NO2 production regime. Higher peak concentration of NO2 in N2O5 production regime can be explained by higher concentration of surface bound NO3observed also by DRIFTS. Another interesting finding was the appearance of NO in the outlet only after N2O5 became absent in the outlet. One possible explanation for this observation is the presence of surface bound O atoms which react both with NO2 and NO. The reaction with NO2 results in the production of N2O5 according to reactions (5)-(7) whereas reaction with NO could produce additional NO2. The decrease of surface bound O atoms due to these reactions results in the slowly decreasing outlet concentration of N2O5 and the absence of NO until all O atoms are removed from the surface. The high concentration of NO2 produced from NO through the reactions (1)-(3) also shifts the balance of reaction (6) towards additional production of N2O5 even when the ozone production stopped.


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Summary and conclusions The aim of this study was to investigate the adsorption processes on the surface of TiO2 depending on the concentration of different nitrogen oxides and on the gas flow rate. The study was carried out at 100°C. The mixtures of NO/NO2 or NO2/N2O5 with varying composition of constituents were prepared by the introduction of certain amount of ozone to the NO:O2:N2 mixture. The main findings of the study are the following. •

Most of the adsorbed nitrogen oxides remained on the surface of TiO2 during cyclic introduction of ozone whereas certain amount of nitrogen oxides which were adsorbed during the time when NO2 and N2O5 were present in the gas mixture could be removed during the time when the gas contained only NO. This reversible adsorption of nitrogen oxides was repeatable after the first cycle with ozone injection.

•

Nitrogen oxides were mainly adsorbed in the form of NO3- whereas the total amount of adsorbed nitrogen oxides depended on the inlet NO and ozone concentration. The total amount of reversibly adsorbed nitrogen oxides was proportional with the fraction of NO2 in the NO/NO2 mixture. Additional abrupt increase in the adsorption capacity of TiO2 was observed when N2O5 appeared in the mixture. The absorption capacity in the presence of N2O5 further increased until all NOx was oxidized to N2O5.

•

Based on the results of this and earlier studies, possible reaction mechanisms were proposed to take place on the surface of TiO2, depending on the composition of gas mixture. The reversibility of the nitrogen adsorption and the dependence of


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total amount of reversibly adsorbed nitrogen-oxide species on the inlet ozone concentration can be explained by the dynamic balance between forward and backward processes of reactions (1)-(3) and (6)-(7). It has been previously shown that the oxidation of NO to N2O5 increases the absorption of NOx into the liquids. The results of present study demonstrate that the adsorption of NOx onto the catalyst surfaces can also be increased by oxidizing NO2 to N2O5 when sufficient amount of ozone is used. However, the experiments in this study were conducted in dry air conditions which is different from real exhausts. Further research is therefore required in order to investigate the effect of water vapor and sulfur oxides on the adsorption on TiO2.

Acknowledgement The study was partially financed by the Estonian Science Foundation grant no. 9310 and Estonian Research Council grant Nr. 585.

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Table of Contents/Abstract Graphic


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Figure 1. Time-dependent outlet concentrations of NO, NO2, N2O5 and O3 at different inlet O3 concentrations: (a), (c) – only NO and NO2 were present in the mixture; (b), (d) – N2O5 was also formed and NO was completely oxidized. Results (a) and (b) were obtained without TiO2 and (c) and (d) with TiO2. In all cases the inlet concentration of NO was 800 ppm, the gas flow rate was 1 slm and temperature was 100 °C. The beginning and end of ozone production are indicated with arrows. 529x370mm (96 x 96 DPI)


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Figure 2. The time-dependence of the sum of concentrations of all nitrogen oxide species at different inlet O3 concentrations. The results were obtained at the inlet concentration of NO 800 ppm, gas flow rate 1 slm and temperature 100 ºC. 529x185mm (96 x 96 DPI)


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Figure 3. Time-dependence of the total concentration of nitrogen oxides after the start and stop of ozone injection at different inlet NO concentrations (flow rate of 1 slm) and gas flow rates (800 ppm NO). The inlet ozone concentrations were chosen at sufficiently high values and any further increase didn’t change the time dependency of [NxOy]. The temperature was 100°C. 529x188mm (96 x 96 DPI)


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Figure 4. Characteristic time of adsorption at different values on inlet NO flow rate. The data points correspond to time dependencies presented on Figure 3. The theoretical dependence t1/2=A/FNO, where A = 39.4 µmol, is shown as the dashed curve. 529x193mm (96 x 96 DPI)


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Figure 5. Integrated amount of adsorbed species as a function of inlet concentration of O3, expressed in units of µmol per gram of TiO2. The inset on (a) shows a typical time-dependence of the total outlet concentration of nitrogen oxides. The temperature in all cases was 100 ºC. On (a) the flow rate was 1 slm and on (b) the inlet concentration of NO was 800 ppm. 529x187mm (96 x 96 DPI)


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Figure 6. DRIFTS absorbance spectra obtained for NO2 and N2O5 production regime with 430 ppm NO inlet concentration. The integrated absorbance intensity of the peak between 1600 and 1720 cm-1 as a function of ozone concentration for different inlet NO concentrations. The temperature was 100 ºC. 529x185mm (96 x 96 DPI)


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Graphical abstract 227x166mm (96 x 96 DPI)


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Adsorption of Nitrogen Oxides on TiO2 Surface as a Function of NO2

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