NO _x Storage and High Temperature Soot Oxidation on Pt–Sr/ZrO₂ Catalyst

For the storage–release of NO _x FTIR Thermo Nicolet Nexus spectrometer using Spectratech diffuse reflectance accessory equipped with a high temperature cell was used. OMNIC software was used to collect and process the spectra. Pfeiffer ThermoStar Mass spectrometer was coupled to the FTIR unit. In the standard experiment approximately 50 mg of sample was placed into the high-temperature ceramic cell. Further temperature was increased from 200 to 600 °C with a 100 °C steps in a flow of 20% O₂ in He (30 mL/min). With the increase of the temperature, bulk nitrate decomposes with the release of NO _x from the sample. The MS signal of evolving NO is used to calibrate MS signal, since the exact amount of nitrates in the starting sample is known.

The sample obtained after the previous calcination step was treated at 200 °C with a flow of 800 ppm of NO + 20% O₂ in He (30 mL/min) and NO is adsorbed on the catalyst with the formation of nitrates and nitrites. At the same time FTIR spectra were collected every 2–5 min until no changes were observed in the spectra. Afterwards the flow of NO was switched off and the temperature was increased from 200 °C until 600 °C with 100 °C step in 20% O₂ in He mixture (30 mL/min). The decomposition of the nitrates was monitored by MS. The amount of the released NO _x can be used to estimate the share of the storage component involved into the NO _x storage using following formula:

Soot oxidation is studied by temperature programmed reaction method. Soot is mixed with catalyst and silicium carbide as dilutant in proportion of 1:20:120 with a spatula in order to obtain a loose contact mode. A total of 200 mg of sample is packed between two quartz wool plugs in a tubular quartz reactor (5 mm internal diameter). Reactor is heated from 40 to 900 °C with a heating rate of 5 °C/min in a flow of 20% O₂ in He (50 mL/min). Reaction products are analyzed by Balzer mass spectrometer. Soot conversion at each temperature and selectivity of soot oxidation to CO₂ can be quantified using the areas under CO- and CO₂-signals and assuming complete soot combustion at 900 °C:where S_CO(T) and S_CO2(T) represent areas under the CO and CO₂ curves at selected temperature T and \( {\text{S}}_{\text{CO}}^{\text{tot}} \) and\( {\text{S}}_{{{\text{CO}}_{ 2} }}^{\text{tot}} \) represent areas under the CO and CO₂ curves at 900 °C

During the storage stage NO is adsorbed by the alkaline earth oxides from the mixture of 800 ppm of NO + 20% O₂ in He at 200 °C, while the FTIR spectra are recorded. The evolution of the spectra with the increase of the adsorption time for pure ZrO₂ and two supported samples (Sr/ZrO₂ and Pt–Sr/ZrO₂ obtained after decomposition of corresponding precursors in air at 600 °C) are shown on Fig. 1a–c. The desorption spectra for Pt–Sr/ZrO₂ sample after saturation with NO _x is shown on Fig. 1d.

It is possible to observe that already after few minutes bands, characteristic to nitrite species appear in the region 1000–1800 cm⁻¹ [20, 21]. Further these bands progress and at higher exposure times new adsorption bands grow at expenses of nitrite bands, which can be assigned to nitrates in a monodentate, chelating, and bridging configurations until saturation is reached. Noteworthy, according to the FTIR results most of the NO _x is adsorbed within the first 20–30 min. For pure ZrO₂ pronounced bands at 1630, 1230, and 1010 cm⁻¹ can be observed. These bands and a minor band at around 1300 cm⁻¹ can be associated with the formation of bidentate nitrites and nitrates (chelating or bridging configurations) [17, 20]. For Sr containing sample (Fig. 1b) additional bands at 1450, 1360 and 1055 cm⁻¹ appear which can be assigned to the ionic nitrates. Weak band at 1790 cm⁻¹ can be assigned to the formation of nitrosyl M–ON⁻ or M–NO⁻ species and can be correlated to the presence of Sr on the surface, since the intensity of this band increases with increasing of the Sr concentration (spectra for samples with different Sr concentrations are not shown here for the sake of brevity). With addition of platinum (Fig. 1c) the intensities of the bands at 1450 and 1360 cm⁻¹ become somewhat higher indicating the enhancement of the formation of ionic nitrate species.

The decomposition and desorption of stored nitrates can be also observed in FTIR spectra (Fig. 1d). With the increase of the temperature above 300 °C the intensity of the bands decreases significantly and at 600 °C most of the bands disappears, indicating almost complete decomposition of stored nitrates. However, some minor bands characteristic for bidentate or ionic nitrate species remain even at 600 °C. The results of NO _x storage–release for different systems are summarized in Table 1. Noteworthy, the amount of desorbed NO _x and storage capacity increase significantly with increasing surface area of the support. This can be related with a better distribution of Sr and its higher availability for the NO _x storage on the support with higher surface area and, therefore, storage capacity increases. Addition of Pt has no effect on the amount of stored NO _x and according to FTIR data only promotes the formation of ionic nitrate species on the expense of bridged and bidentate nitrates. One might find such a result as rather unexpected, since, for example, in the works of Nova [22] significant positive effect of Pt on the NO _x storage capacity in Ba/Al₂O₃ system was reported. This contradiction can, however, be explained by the difference in the storage conditions. In the works of Nova NO _x storage was performed at 350 °C, where thermodynamic equilibrium between NO and NO₂ is strongly shifted towards the formation of nitrous monoxide. Therefore, the addition of Pt is essential for converting of NO to NO₂ and formation of nitrates. At the temperature used in this study (200 °C) NO₂ is still present in significant amounts in the equilibrium with NO. Therefore, Pt is not that essential for the formation of nitrates. It should also be pointed out that in the works of Nova et al. [22] the adsorption experiments were performed with the mixture of NO in He with no or very small amounts of oxygen (NO/O₂ ratio of 1:4), while in our case experiments are conducted with a large excess of oxygen, which also favors the formation of NO₂.

With the increase of the temperature the nitrates start to decompose with the release of nitrogen dioxide that reacts with the soot forming CO, CO₂ and back to nitrogen monoxide. The results of the soot oxidation for different catalysts are summarized in Table 2, where temperature of 20% soot conversion as well as amount of soot converted up to 500 °C are given to provide the comparison between different systems. Selectivities of soot conversion to CO₂ are also shown. For the un-catalyzed soot oxidation (no stored nitrates present in the system) the temperature of 20% soot conversion is quite high, around 580 °C. After the introduction of even small amounts of stored nitrates into the system the temperature of the soot oxidation decreases quite significantly (~33 °C for sample containing 2.5% Sr(NO₃)₂). With further increase of the concentration of nitrates the temperature decreases even further and for the most active sample 20%Sr(NO₃)₂/ZrO₂ it is 508 °C, which is 72 °C lower than that for the un-catalyzed reaction. The amount of soot converted before 500 °C also increases gradually with the increase in the Sr concentration and reach 16.7% for catalyst with 20%Sr(NO₃)₂ versus 6.5% for pure ZrO₂.

Platinum alone also does not improve the soot oxidation (Table 2, sample 1%Pt/ZrO₂) and only increases the selectivity of the soot conversion to CO₂ by oxidizing the formed CO. However, a significant improvement in terms of catalytic activity as well as selectivity was observed when Pt was added to the storage component. Combination of both storage component and Pt lower the temperature of soot oxidation ~100 °C and the amount of soot removed before 500 °C increases from 6.5 to 32%.