Perovskite-type catalysts, oxidative activity

In the case of H2 oxidation the two investigated classes of catalysts show different behaviors. Again perovskite type catalysts calcined at 973 K show higher combustion activity than hexaaluminates calcined at 1573 K, but characteristic values of parent activation energy (5-7 Kcal/mole) have been calculated for perovskite catalysts that are markedly lower than... 

The perovskite-type catalysts (ref.l), other non noble metal complex oxides catalysts (ref.2), and mixed metal oxides catalysts (ref.3) have been studied in our laboratory. The various preparation techniques of catalysts (ref.4 and 5), the adsorption and thermal desorption of CO, C2H5 and O2 (ref.6 and 7), the reactivity of lattice oxygen (ref.8), the electric conductance of catalysts (ref.9), the pattern of poisoning by SO2 (ref. 10 and 11), the improvement of crushing strength of support (ref. 12) and determination of the activated surface of complex metal oxides (ref. 13) have also been reported. 

Some of the many different types of catalysts which have good catalytic properties for the OCM reaction qualify as membrane materials. Membrane reactors for OCM were designed and tested by Nozaki et al. (1992). Three kinds of reactors were developed the first one consisted of a porous membrane covered with a thin film of catalyst (type I) the second one, a dense ionic-conducting membrane (non porous) with catalytic layer (type II) and the third one was a membrane made of perovskite-type mixed oxides which was active for OCM (type III). Figure 11 presents the diagram for the membrane reactor system and table 13 shows the different materials used for supports and coated catalysts. 

The catalytic combustion of methane over perovskite-type catalysts has been investigated by Arai and co-workers (66). Methane is the most stable alkane, and it is relatively difficult to combust by virtue of the high strength of the C—H bond that must be activated. Studies were performed using relatively high space velocities in the range 45,000-50,000 h , with a 2% methane feed in air. The catal3Ttic activity, expressed as the temperature required for 50% conversion, is shown in Table 13 for a series of unsubstituted perovskite-type oxides. 

The oxidative activity of Perovskite-type catalysts has been investigated using the DTA... 

The experimental results indicate that the order of the oxidation activities of the three perovskite-type catalysts containing rare earths is La 7Sr 3Co03 > LaCoOj > > LaMnO,. This order agrees well with the evaluation using a catalytic reactor [22,23]. 

Potassium- and strontium-substituted praseodymium manganate Pro,7Sro,2Ko,iMn03 perovskite-type catalysts were explored in supported form on ceramic foam filters for soot oxidation in typical conditions of a diesel engine [55]. Compared to bare soot oxidation reaction, the soot oxidation in the presence of these catalysts corresponded to a decrease of the Tinitiai value by 150 °C and Tfinai value by 100 °C. Besides the activity, another advantage of the system is the very good thermal stability. 

Highly active Lai K Co03 perovskite-type complex oxide catalysts for the simultaneous removal of diesel soot and nitrogen oxides under loose contact conditions. Catal. Lett., 124, 91-99. 

As already mentioned, one of the principal aims of using perovskite-type oxides as oxidation catalysts is to avoid the use of expensive and low thermally stable noble-metal-supported catalysts (e.g., Pt/Al203). Thus, by modifying the perovskite composition using alkaline earth metal or metal transition cations (A- and/ or B-substituted materials) and by improving their textural and defective properties during the synthesis process, it is possible to obtain perovskite-based catalysts as active as noble-metal-supported ones for the low-temperature CO oxidation reaction. Nevertheless, some authors have recently claimed the... 

Taking into account the Mars-van Krevelen mechanism, any enhanced oxygen vacancy densities can improve the oxidation activities of an oxide-based catalyst Perovskite-based materials can act as catalysts for NO SCR by H2 or hydrocarbons [72,73] or simultaneously reduce NO in the presence of PM under lean conditions [74-77]. The major drawback of such high-temperature crystal oxides is their low surface area, for example, <2-3 m /g. However, over the last years improved preparation methods and compositional control had a significant effect on materials features and performance of the perovskite-type catalysts [75,78-85] as illustrated in Figure 26.3. However, the performance of perovskite-based catalysts becomes remarkable when noble metal coexist, either as supported or as dopant or even in the form of a solid solution [15,17,81-86]. 

Catalytic combustion of diesel soot particulates over LaMnOs perovskite-type oxides prepared by malic acid method has been studied. In the LaMn03 catalyst, the partial substitution of alkali metal ions into A site enhanced the catalytic activity in the combustion of diesel soot particulates and the activity was shown in following order Cs>K>Na. In the LarxCs MnOj catalyst, the catalytic activity increased with an increase of X value and showed constant activity at the substitution of x>0.3... 

Several researchers have focused their attention on the application of oxide materials to lower the oxidation temperature of soot particulates. It was reported that active soot oxidation catalysts are PbO, C03O4, V2O5, M0O3, CuO, and perovskite type oxides[3]. 

Fig. 2 shows the temperature as a function of irradiation time of Cu based material under microwave irradiation. CuO reached 792 K, whereas La2Cu04, CuTa20e and Cu-MOR gave only 325, 299 and 312 K, respectively. The performances of the perovskite type oxides were not very significant compared to the expectation from the paper reported by Will et al. [5]. This is probably because we used a single mode microwave oven whereas Will et al. employed multi-mode one. The multi-mode microwave oven is sometimes not very sensitive to sample s physical properties, such as electronic conductivity, crystal sizes. From the results by electric fixmace heating in Fig. 1, at least 400 K is necessary for NH3 removal. So, CuO was employed in the further experiments although other materials still reserve the possibility as active catalysts when we employ a multi-mode microwave oven. 

The Incentive to modify our existing continuous-flow microunit to incorporate the square pulse capability was provided by our work on perovskite-type oxides as oxidation-reduction catalysts. In earlier work, it had been inferred that oxygen vacancies in the perovskite structure played an important role in catalytic activity (3). Pursuing this idea with perovskites of the type Lai-xSrxFeg 51 10 503, our experiments were hampered by hysteresis effects which we assumed to be due to the response of the catalyst s oxygen stoichiometry to the reaction conditions. 

The activities of the perovskite-type oxides are strongly dependent on pretreatment in reducing or oxidizing atmospheres at a.600°C. This was found for other perovskite catalysts as well (1). Reducing pretreatments lead to more active catalysts (Figures 5 and 6). The reason for this is not known, but better binding of CO to the reduced surface is a possible explanation. 

A combustion catalyst must thus simultaneously fulfill requirements of high activity at combustor inlet conditions and high stability at the maximum temperatures occurring in the catalytic combustor. Unfortunately, these are contradictory demands This was demonstrated by McCarty and Wise [551. Figure 6, taken from their study, shows the relationship between methane oxidation activity and the stability of various perovskite-type materials (LaM03) The trade-off between activity and stability is clear. 

Manganese oxides have long been known to be catalysts for a variety of gas clean-up reactions. Manganese/copper mbced oxide (Hopcalite) is the catalytically active component in gas mask filters for CO CO is converted to CO2 at room temperature [4]. Further applications of manganese oxide catalysts are the NH3 oxidation to N2 [5], the combustion of VOC [6,7] and methane [8], the oxidation of methanol [7], the O3 decomposition [9] and the NOx reduction [14]. Perovskite-type oxide catalysts (e.g. LaMnOs) have been proven to be effective catalysts for the total oxidation of chlorinated hydrocarbons [10]. Several studies have shown that besides preparation method and calcination temperature the kind... 

Zr) for NjO decomposition. The actual oxidation states of the metals are investigated with XPS [21]. The possible active sites for the reaction may also be related to lattice oxygen anion defects formed after cation incorporation [22]. For high Pb Zr ratio catalysts, such as in the case of Pb Zr = 1 1, the catalytic activity is substantially reduced. This can be attributed to change of ZrOj structure to perovskite type and thus to demolishment of cation pairs with multiple oxidation states that are essential for facilitating the decomposition of NjO gas [23-25]. 

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