Pt-Ba/alumina NOx storage-reduction catalysts: Influence of Ba loading on NOx storage behavior
Introduction
Reducing the NOx emission of exhaust gases from HC combustion under lean conditions has been one of the key targets in environmental catalysis research in the past years. One of the most promising strategies towards this aim is the use of NOx storage-reduction (NSR) catalysts [1]. The widely used reference catalyst consists of Pt-Ba/Al2O3 with a Pt loading of ca. 1 wt.% and a Ba loading between 8 and 20 wt.%. During the NOx storage process the trapping sites are the Ba-containing species, whereas Pt activates the NO oxidation, as NO2 appears to play a major role in the NOx storage [2], [3], [4], [5], [6], [7], [8]. Experimental conditions strongly influence the storage behavior, giving rise to different interpretation of the phenomenon even in simplified model system where the gas feed contains NO, O2 and NO2, only. The real exhaust gases contain additionally CO2, H2O and SOx in various concentrations, what makes the interpretation of the storage-reduction mechanism even more difficult.
The actual state of the art of the DeNOx process has recently been reviewed by Epling et al. [9]. This review clearly indicates that a thorough characterization of the active phases and their interplay in the storage process are indispensable for gaining a better understanding of the functioning of NSR catalysts.
NOx adsorption on Ba-containing sites has been extensively studied by various groups using different approaches (see [9] and references therein). Olsson et al. [10], [11], [12] developed a kinetic model that assumes a single BaO storage site. According to their work, the formation of Ba(NO3)2 is achieved via progressive NO2 adsorption on BaO2 formed by reaction of adsorbed NO2 with BaO. Albeit the model works well under the specific conditions set by the authors, it revealed some limits when oxidation of NO to NO2 and NOx storage have to be taken into account simultaneously during the adsorption of NO in the presence of oxygen over Pt-Ba/Al2O3 catalyst [13]. Further studies indicate that a single site model is inadequate to describe the complex set of reactions occurring during the NOx storage process. Although the finale state of NOx stored species seems to be in the form of barium nitrate [14], the adsorption mechanism is still not fully understood. Schmitz et al. in their XPS studies on NO and NO2 adsorption over an evaporated BaO/aluminum film, propose a different adsorption pathway for NO and NO2: the former adsorbing mainly in the form of nitrite the latter in the form of nitrate [15]. Sedlmair et al. reported a detailed FTIR study of NO and NO2 adsorption at 50 °C on BaO/BaCO3 containing catalyst, showing different nitrate and nitrite species forming both over BaO and BaCO3 sites [16]. In an other FTIR study performed at 350 °C over standard Ba/Al2O3 and Pt-Ba/Al2O3 samples, Nova et al. show that nitrates can form either via nitrite from NO/O2 adsorption or NO2 disproportionation over BaO sites [8], [17]. The participation of nitrites as intermediates in the storage process is proposed in several other studies, e.g. [18], [19].
A NOx storage mechanism accounting for a multiple trapping sites model seems better fit with the recent results. The presence of different types of Ba-containing species in the catalyst is widely accepted [20], [21], [22], [23], [24]. On the other hand, the role that each phase can play in the storage is still debated. Lietti et al. proposed a reactivity order for Ba-containing species with NOx [20] in which BaO sites possess higher NOx storage activity, followed by other Ba-containing species such as Ba(OH)2 and BaCO3, in line with the basic character of these species.
Further evidence that different Ba-containing phases are involved in the storage process emerges from the typical NOx breakthrough profiles over a standard Pt-Ba/Al2O3 NSR catalyst reported by several authors, e.g. [20], [23]. Three different stages were identified: the first was characterized by a complete NOx uptake, the second by a rapid one and the final stage by a slow, but still significant NOx uptake.
The role of surface and bulk active species is still unresolved. Recently, shrinking core-type models have been proposed by Tuttlies et al. [24], Muncrief et al. [25] and Olsson [26]. In these models more than just the surface layer of the particle is assumed to be reactive towards nitrate formation and diffusional limitations hinder the inner Ba-containing species from participating in the storage process.
In a preceding paper [27] we have studied systematically the build-up and thermal stability of barium containing phases in differently Ba loaded Pt-Ba/Al2O3 catalysts. After calcination of these catalysts at 500 °C, three Ba-containing phases of different thermal stability were identified: BaO, low temperature barium carbonate (LT-BaCO3), and high temperature barium carbonate (HT-BaCO3). The BaO phase was formed during catalyst preparation via decomposition of BaCO3 being in intimate contact with the alumina support. The build-up of the highly dispersed BaO phase reached completion at a Ba loading of about 13 wt.%. A more stable, but still well-dispersed, BaCO3 which decomposes at low temperature (LT-BaCO3), was formed upon further increase of the Ba loading. Finally barium carbonate possessing a bulk-like stability (HT-BaCO3) was formed at Ba loadings higher than 17 wt.%.
The aim of the present study is to assess the activity of the different Ba-containing species in the storage process and to explore how the storage is affected by the Ba loading in the range from 4.5 to 28 wt.% Ba. The NOx storage process was investigated by means of pulse thermal analysis combined with mass spectroscopy and X-ray diffractometer (XRD).
Section snippets
Experimental
NOx storage catalysts, with the standard formulation Pt-Ba/Al2O3, were prepared by means of the incipient wetness technique using Pt(NH3)2(NO2)2 and Ba(CH3COO)2 as precursors, and commercial γ-Al2O3 (OMG, BET surface area 210 m2/g) as support. The preparation procedure has been described in detail in a preceding study [27]. In brief, Pt/Al2O3 (Pt content of 1 wt.%) was prepared by the incipient wetness method using a Pt(NH3)2(NO2)2 solution. As prepared samples were dried for 12 h at 80 °C and then
Results
In order to investigate the NOx storage behavior of the Pt-Ba/Al2O3 catalysts they were calcined in air at 500 °C and then exposed to a series of NO pulses injected into an oxygen containing carrier gas stream (5 vol.% O2/He). The thermogravimetric (TG) curves presented in Fig. 1 show the mass uptake of the catalysts due to NOx storage. NO pulses were repeated till nearly saturation of the Ba-containing sites by NOx was observed and the TG curve did not show appreciable further mass uptake. The
Discussion
The present systematic study of the influence of the Ba loading on the NOx storage behavior of Pt-Ba/Al2O3 shows that the Ba species existing on these catalysts possess different thermal stability and activity for NOx storage and that their relative abundance is affected by the Ba loading. In the following we discuss: (A) the activity of the different Ba-containing species in the storage process; (B) the role of the newly discovered VLT-BaCO3 phase which forms during the storage process; and
Conclusion
The NOx storage efficiency of Ba-containing phases in Pt-Ba/Al2O3 catalysts depends on the Ba loading, showing a maximum around 17 wt.% in conventionally prepared catalysts. This behavior is traced to the fact that different Ba-containing phases with different reactivity towards reaction with NOx are present and their relative proportion depends on the Ba loading. Based on TPRD measurements the following Ba-containing phases involved in the storage process could be distinguished: BaO, LT-BaCO3,
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