Relevance of the Nitrite Route in the NOx Adsorption Mechanism over Pt–Ba/Al2O3 NOx Storage Reduction Catalysts Investigated by using Operando FTIR Spectroscopy

Nowadays, severe regulations are in force in the industrialized countries to limit the emission of pollutants from the exhausts of passenger cars and trucks. Today, post-treatment catalytic technologies are necessary to meet the current most strict or the forthcoming emission limits.1

In traditional gasoline-fueled stoichiometric engines, three-way catalysts are used for the abatement of NO_x, unburned hydrocarbons, and CO. However, the need to reduce fuel consumption and the corresponding CO₂ emissions has led in the past to an impressive spread in the market of lean-burn engines, such as diesel or direct-injection (DI) gasoline engines. These engines operate in the presence of excess oxygen, in which the current three-way catalysts, optimized for exhausts that fluctuate around oxygen-free conditions, do not ensure acceptable emission levels of NO_x.

The NH₃ selective catalytic reduction (SCR) and the NO_x storage–reduction (NSR) or lean NO_x trap systems are at present the top contenders for reducing NO_x concentration in the exhaust from diesel and lean-burn gasoline direct-injection vehicles. Although the NH₃-SCR technology, which accomplishes NO_x reduction by injecting urea (a precursor of NH₃) in the exhaust gases and requires an onboard urea tank, is preferred for heavy-duty vehicles and minivans, lean NO_x traps have been specifically developed for small engines.1 This technology is based on the use of a suitable catalytic material that consists of an alumina carrier on which alkali- and/or alkali-earth metal compounds (e.g., K and Ba) and noble metals (e.g., Pt) are deposited. These catalysts operate under cyclic conditions, which alternate long periods in the presence of excess oxygen during which NO_x species are stored on the alkali- and/or alkali-earth metal compounds with short rich phases during which the adsorbed NO_x species are reduced to nitrogen on Pt.2

Although the NSR technology is presently being used on a commercial scale, an agreement and understanding of the mechanistic aspects of the storage of NO_x species and of their reduction is still lacking.3 Regarding the storage phase, many authors suggest that NO is at first oxidized by Pt to NO₂, which is then stored onto the alkali- or alkali-earth metal compound (e.g., BaO) in the form of nitrates according to the overall reactions in Equations (1) and (2):4((1)), ((2))

• ((1))

• ((2))

in which O⁼ is a lattice oxygen and NO₃⁻ is an adsorbed nitrate species.

In this pathway, hereafter called the nitrate route, Pt and the alkali- or alkali-earth metal compounds catalyze the NO oxidation and the NO₂ storage reactions, respectively. Also, the reaction in Equation (2) occurs via the preliminary disproportionation of NO₂ to nitrite+nitrate intermediates [Eq. (3)], followed by the oxidation of nitrite to nitrate species [Eq. (4)]:((4))

• ((3))

• ((4))

in which NO₂⁻ is an adsorbed nitrite species. On the basis of spectroscopic evidence, both nitrite and nitrate ad-species have been simultaneously observed at the preliminary stage of storage from NO₂; nitrites eventually oxidize to nitrates, and so only nitrates are detected on the catalyst surface after a long storage period.5, 6 Notably, according to the suggested pathway for the nitrate route [Eqs. (3) and (4)], the surface concentration of nitrates is expected to be equal to (or higher than) that of nitrites. However, some of us have shown that in the storage from NO₂ or NO₂/O₂ mixtures, nitrites are not observed even at the very beginning of the storage, likely due to fast nitrite oxidation by NO₂.7, 8

In contrast, on the basis of in situ FT-IR spectroscopy and pulse reactor experiments performed over model Ba/Al₂O₃ and Pt–Ba/Al₂O₃ catalysts, some of us have indicated the existence of another pathway for the storage from NO/O₂, which leads to the formation of nitrites as stable intermediates.7, 8 This route, hereafter called the nitrite route, is based on the oxidation and adsorption of NO at Pt–BaO border, which leads to the formation of nitrite ad-species according to the stoichiometry given in Eq. (5):

• ((5))

In this case a cooperative effect operates between the noble metal and the nearby storage component, and so NO is oxidized in a stepwise manner and is adsorbed as NO₂⁻ on the Ba site, which prevents the over-oxidation of NO to NO₂. Notably, in contrast to the nitrate route, the nitrite route accomplishes the storage of NO_x only in the form of nitrites. However, the stored nitrites might be further oxidized to nitrates by O₂, and so the formation of nitrates is also expected to be a consecutive reaction.

Previous in situ FTIR experiments7 that aimed at investigating the relevance of the nitrite and nitrate routes in the storage of NO_x showed that the nitrite route operates at low temperatures (423–523 K) because only bands of nitrites are observed during the initial storage, with bands of nitrates apparent only after prolonged contact times. In contrast, at higher temperatures (623 K) the nitrate formation is most important, with nitrites being seen only in the preliminary stage of the storage. Note that in these experiments the occurrence of a spurious gas-phase NO oxidation to NO₂ in the FTIR cell under static conditions and at low temperatures cannot be excluded, although it seems to be of minor importance because nitrate formation is observed only at long contact times. Besides, no quantitative indications on the relevance of the suggested routes for NO_x storage could be obtained.

In this work, the storage of NO_x over a model Pt–Ba/Al₂O₃ catalyst sample has been investigated in the temperature range 423–623 K by combining FTIR spectroscopy of the adsorbed species under operando conditions with the online analysis of the gaseous species. In contrast to the previous data, this allowed the simultaneous analysis of the adsorbed and gaseous species involved in the process both at the qualitative level and at the quantitative level. Also, experimental conditions have been selected with particular care to avoid spurious phenomena such as the homogeneous NO oxidation to NO₂. Hence, more precise indications on the relevance of the nitrite and nitrate routes in the storage of NO_x at low temperatures could be obtained.

Figure 1 shows the evolution of NO, NO₂, and NO_x (NO+NO₂) with time upon stepwise exposure of the Pt–Ba/Al₂O₃ sample to a NO/O₂ mixture at different temperatures, in the range 423–623 K. Prior to NO/O₂ exposure, the catalyst sample has been conditioned by several NO_x adsorption and reduction cycles with H₂ at 623 K. During this conditioning process, the BaCO₃ phase present on the fresh catalyst is transformed into BaO and/or Ba(OH)₂, which is the actual NO_x adsorption site.9 Upon the NO/O₂ addition step at 423 K (Figure 1a), NO is immediately detected at the reactor outlet; the NO concentration increases with time and reaches the inlet value (900 ppm) after approximately 1500 s. The total amount of NO_x stored during NO/O₂ exposure up to the end of the experiment (2000 s) is near 0.35 mmol NO_x g_cat⁻¹, which is calculated by the difference between the amounts of NO fed and exiting the reactor. Notably, no NO₂ has been observed at the reactor outlet, which points out that NO oxidation to NO₂ is negligible at this temperature. Hence, NO_x uptake occurs in the absence of NO₂ in the gas phase.

Figure 1. Evolution of NO, NO₂, and NO_x (NO+NO₂) upon exposure of the model Pt–Ba/Al₂O₃ sample to a stream of NO/O₂ (1000 ppm in He+5 % v/v O₂) at a) 423 K, b) 473 K, c) 523 K, and d) 623 K.

The FTIR spectra recorded simultaneously (Figure 2) showed the initial formation of chelating nitrites on the catalyst surface [ν_sym(NO₂) at 1350 cm⁻¹ and ν_asym(NO₂) at 1217 cm⁻¹]. The bands of nitrites continue to increase with the NO/O₂ exposure time; after 10 min, a weak band is also apparent near =1544 cm⁻¹, which is associated with bidentate nitrates, likely formed through the oxidation of nitrites by O₂, which, however, is very slow at this temperature.

Figure 2. Results of operando FTIR analysis upon NO/O₂ exposure (1000 ppm in He+5 % v/v O₂) at 423 K of the model Pt–Ba/Al₂O₃ sample. Spectra from 1 to 2 are recorded every 30 s and spectra from 2 to 3 every 300 s.

This result clearly shows that the nitrite route is very effective in the storage of NO_x. At 423 K, a significant amount of NO_x species (0.35 mmol NO_x g_cat⁻¹) is stored on the catalyst surface only in the form of nitrites, as expected in the case of the nitrite route. Notably, the lack of detectable amount of NO₂ at the reactor outlet (and also of surface nitrates if one neglects the small peak observed near =1544 cm⁻¹ only at high exposure times) demonstrates that the nitrate route does not play a significant role in the NO_x storage at this temperature during the entire NO_x storage period. Furthermore, this result shows that at this temperature the oxygen is unable to oxidize significantly the nitrites to nitrates.

The NO_x uptake at higher temperatures presents different features. On increasing the temperature, a complete uptake of NO_x is initially observed above 523 K (Figure 1); then the NO_x concentration at the reactor outlet increases with time, but more rapidly than that observed at 423 K. As a result, the amount of NO_x stored at the end of NO/O₂ exposure (2000 s) is only slightly different from that measured at 423 K, being in the range 0.35–0.40 mmol g_cat⁻¹. Notably, the formation of NO₂ is also observed upon increasing the temperature: at 623 K, about 170 ppm of NO₂ is detected at the reactor outlet at the end of NO/O₂ exposure (after 2000 s).

These changes in the gas-phase composition are accompanied by changes in the FTIR spectra recorded simultaneously. At 623 K (Figure 3), nitrite species (bands at =1217 and 1350 cm⁻¹) are observed only at a very short exposure time (1 min); bands near =1420 and 1320 cm⁻¹ (strong) and =1027 cm⁻¹ (weak), characteristic of ionic nitrates, develop. A shoulder near =1550 cm⁻¹, associated with bidentate nitrates, is also observed. Notably, the bands of nitrite species increase in intensity at the very beginning of the storage and then decrease so that after 10 min of exposure only the features of adsorbed nitrate species are apparent in the spectra. Nitrites that are formed at the beginning of the storage are oxidized into nitrates: hence, nitrites exhibit the typical behavior of the intermediate surface species, as indicated by the presence of an isosbestic point in the FTIR spectra in Figure 3.

Figure 3. Results of operando FTIR analysis upon NO/O₂ exposure (1000 ppm in He+5 % v/v O₂) at 623 K of the model Pt–Ba/Al₂O₃ sample. Spectra from 1 to 2 are recorded every 30 s and spectra from 2 to 3 every 300 s.

By combining the results of operando FTIR spectroscopy with those of the synchronous quantitative analysis of the gaseous species, the molar absorption coefficient of the bands related to nitrite and nitrate vibration modes could be evaluated. For this purpose, we have considered, for nitrite species, the linear intensity of the band at =1217 cm⁻¹ (I_1217cm−1) under steady-state conditions (i.e., after exposure for 2000 s) in the spectrum obtained at 423 K. For nitrates we have instead considered the linear intensity at steady-state conditions of the band at =1320 cm⁻¹ (I_1320cm−1) in the spectrum obtained at 623 K. Note that the choice of considering linear intensities, rather than integrated intensities, was dictated by the strong superposition of the examined IR bands. Also, note that in the case of nitrates, we have chosen the mode at =1320 cm⁻¹ rather than that at =1420 cm⁻¹ attributed to the higher heterogeneity of the latter.

The amount of stored nitrites at steady-state conditions at 423 K was near 0.35 mmol NO_x g_cat⁻¹, whereas that of stored nitrates was 0.40 mmol NO_x g_cat⁻¹. A value of 1.51 [I_1217cm−1 (mmol_nitrites g_cat⁻¹)] was thus estimated for the linear molar absorption coefficient of the nitrite band at =1217 cm⁻¹, whereas that for nitrates was 2.63 [I_1320cm−1 (mmol_nitrates g_cat⁻¹)]. Notably, the linear molar adsorption coefficient for nitrates is almost twice that for nitrites.

On this basis, the surface concentration of nitrite and nitrate species could be estimated as a function of time, as shown in Figure 4. When nitrites and nitrates are present simultaneously, the intensity of the band at =1320 cm⁻¹ attributed to nitrates was corrected by subtracting the contribution of the overlapping band at =1350 cm⁻¹ related to nitrites.

Figure 4. Evolution versus time of the main bands for different surface species detected by using FTIR spectroscopy under operando conditions upon NO/O₂ exposure (1000 ppm in He+5 % v/v O₂) at 423 and 623 K of the model Pt–Ba/Al₂O₃ sample.

Inspection of Figure 4 provides valuable quantitative information on the importance of the pathways that operate during the NO_x adsorption over Pt–Ba/Al₂O₃. At the lowest investigated temperature (423 K), NO_x species are stored on the catalyst surface in the form of nitrites only, with traces of nitrates being observed only at high exposure times. The storage capacity at this low temperature is notable, being similar to that observed at high temperatures (0.35 mmol NO_x g_cat⁻¹ at 423 K vs. 0.4 mmol NO_x g_cat⁻¹ at 623 K). As already discussed, the lack of detectable amount of NO₂ in the gas phase at 423 K and of negligible nitrate ad-species points out that at such a low temperature the storage of NO_x involves only the nitrite route, that is, the NO oxidation to surface nitrites, during the entire NO_x storage period.

Above 423 K, the formation of nitrate ad-species is also observed, along with nitrites; the surface concentrations of nitrites and nitrates show very different temporal evolutions with temperature. Although at 423 and 473 K nitrites increase monotonically with time, at higher temperatures (523 and 623 K) the surface concentration of these ad-species shows a maximum with time. In contrast, the surface concentration of nitrates, which is negligible at 423 K, grows monotonically with time: the rate of growth is small at low temperatures (473 K) but increases markedly with temperature.

Figure 4 also shows that the surface concentration of nitrites is greater than that of nitrates during the entire NO_x storage period at 423 and 473 K and also at the beginning of the NO_x uptake at 523 K (t<14 min). Even at 623 K, nitrites are more abundant than nitrates at the very beginning of the NO_x storage, and the surface concentration of nitrates shows a horizontal tangent at t=0 min (see the inset in Figure 4 d). These observations demonstrate at the quantitative level that in the temperature range 423–623 K the nitrite route is always faster than the nitrate route and it leads to the formation of nitrites as precursors in the formation of nitrates, which is in line with previous suggestions by some of us on the basis of in situ FTIR experiments.7, 8 The nitrite and nitrate surface concentration profiles versus time shown in Figure 4 are typical of a consecutive reaction scheme in which nitrites are the primary product in the formation of nitrates.

The nitrate route might also play a role in the formation of nitrate ad-species, owing to the presence of NO₂ (upon NO oxidation) at temperatures above 473 K (Figure 1). However, the faster growth in the surface concentration of nitrites at the beginning of the NO_x storage with respect to nitrates (and the horizontal tangent observed in the surface concentration of nitrates as well) is not consistent with the occurrence of reactions (3) and (4) from which equimolecular amounts of nitrites and nitrates are expected, and it supports the predominance of the nitrite route in the storage of NO_x, at least during the early phase, that is, during the time generally used for the duration of the lean phase in the catalytic devices.

In summary, a novel approach based on the simultaneous and quantitative analysis of adsorbed and gas-phase species monitored at different temperatures during the uptake of NO_x over a model Pt–Ba/Al₂O₃ NSR catalyst is presented herein. Through the analysis of nitrite and nitrate adsorbed species and of their evolution with exposure time at various temperatures, the relevance of the different routes for the NO_x storage could be derived. The nitrite route is the unique pathway responsible for the storage of NO_x at 423 K over BaO: only nitrites are stored at this temperature, whereas NO₂ and nitrate ad-species are not detected. The nitrite route dominates at higher temperatures as well, but the so-formed nitrites are oxidized, partly or totally, to nitrate ad-species as the storage proceeds. Besides, above 473 K and at long exposure times at which the NO oxidation to NO₂ is observed, the storage of NO_x also likely involves the participation of NO₂ through the nitrate route.

Finally, this study was done under “clean” conditions, that is, in the absence of CO₂ and water, which, on the contrary, are always present in the exhausts. Studies are ongoing to establish the role of CO₂+H₂O in the suggested pathways for NO_x storage, and results will be discussed in a forthcoming paper. The preliminary results indicate that the pathways responsible for NO_x storage are not significantly affected by the presence of CO₂ and water; that is, the nitrite route dominates in NO_x storage under real operating conditions as well.

Experimental Section

A homemade Pt–Ba/Al₂O₃ (1/20/100 w/w) catalyst was considered in this study.9 The NO_x storage phase was investigated by transient experiments in which rectangular step feeds of NO (900 ppm) and 3 % v/v O₂ in Ar had been admitted to an IR reactor cell (Aabspec CX cell) directly connected to an FTIR Nicolet 6700 instrument for the analysis of the surface species as well as to online detectors for the quantitative analysis of the gases exiting the reactor (MS Pfeiffer Omnistar, IR gas analyzer, chemiluminescence ThermoScience 42i-HL).10 For IR analysis, a powdered sample (15 mg) was compressed in self-supporting discs (diameter=13 mm, thickness=0.1 mm).