Perovskites activity

The general formula of perovskites is ABO3, and soot combustion perovskite catalysts with many different cations have been reported for both A and B positions. The most studied formulations include transition metals that are able to accomplish redox cycles, such as Mn, Ni, Cu, Co, and Fe among others. The partial substitution of one of the cations usually has a positive effect on the perovskite activity, and alkali metals seem to be the most studied dopants. 

Although supported Pd catalysts have been the most extensively studied for butadiene hydrogenation, a number of other catalysts have also been the object of research studies. Some examples are Pd film catalysts, molybdenum sulfide, metal catalysts containing Fe, Co, Ni, Ru, Rh, Os, Ir, Pt, Cu, MgO, HCo(CN) on supports, and LaCoC Perovskite. There are many others (79—85). Studies on the weU-characteri2ed Mo(II) monomer and Mo(II) dimer on siUca carrier catalysts have shown wide variations not only in catalyst performance, but also of activation energies (86). 

The relatively high cost and lack of domestic supply of noble metals has spurred considerable efforts toward the development of nonnoble metal catalysts for automobile exhaust control. A very large number of base metal oxides and mixtures of oxides have been considered, especially the transition metals, such as copper, chromium, nickel, manganese, cobalt vanadium, and iron. Particularly prominent are the copper chromites, which are mixtures of the oxides of copper and chromium, with various promoters added. These materials are active in the oxidation of CO and hydrocarbons, as well as in the reduction of NO in the presence of CO (55-59). Rare earth oxides, such as lanthanum cobaltate and lanthanum lead manganite with Perovskite structure, have been investigated for CO oxidation, but have not been tested and shown to be sufficiently active under realistic and demanding conditions (60-63). Hopcalities are out-... 

Although several metals, such as Pt and Ag, can also act as electrocatalysts for reaction (3.7) the most efficient electrocatalysts known so far are perovskites such as Lai-xSrxMn03. These materials are mixed conductors, i.e., they exhibit both anionic (O2 ) and electronic conductivity. This, in principle, can extend the electrocatalytically active zone to include not only the three-phase-boundaries but also the entire gas-exposed electrode surface. 

F can he suppressed hy the high site symmetry of the central atom In many perovskite-like structures of the ABO3 type the lone pair of the B-cat-ion leads not to a structural distortion. In CsPbF3 under ambient conditions no lone-pair activity observed [27], but upon cooling a phase transition is observed that leads to less symmetrical surrounding of Pb by fluoride [28]. 

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]. 

In this paper, we prepared LaMnOa perovskite-type oxides using the malic acid method and investigated their physical properties. It has been also investigated the effect of partial substitution of metal iorrs into La and Mn sites and the reaction conditions on the activity for the combustion of soot particulates. 

In the Lai.,CsxMn03 catalyst, the T decreases with an increase of x value and shows an almost constant value upon substitution of x>0.3. It is thought that the oxygen vacancy sites of perovskite oxide increase with an increase of amount of Cs and the oxidation activity also increases. This result is also verified by the TPR result of these catalysts(Fig. 3). As shown in Fig. 3, the reduction peak appears at low temperature with an increase of x value and no change is shown at more than x=0.3. It can thus be concluded that the catalytic performance of these oxides increases as the amount of Cs in the crystal lattice increases. However, the substitution of Cs to more than x=0.3 leads to excess Cs, which is present on the surface of mixed oxides might have no effect on the catalytic activity... 

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. 

With respect to CO oxidation an activity order similar to that described above for CH4 combustion has been obtained. A specific activity enhancement is observed for Lai Co 1-973 that has provided a 10% conversion of CO already at 393 K, 60 K below the temperature required by LalMnl-973. This behavior is in line with literature reports on CO oxidation over lanthanum metallates with perovskite structures [17] indicating LaCoOs as the most active system. As in the case of CH4 combustion, calcination at 1373 K of LalMnl has resulted in a significant decrease of the catalytic activity. Indeed the activity of LalMnl-1373 is similar to those of Mn-substituted hexaaluminates calcined at 1573 K. Dififerently from the results of CH4 combustion tests no stability problems have been evidenced under reaction conditions for LalMnl-1373 possibly due to the low temperature range of CO oxidation experiments. Similar apparent activation energies have been calculated for all the investigated systems, ranging from 13 to 15 Kcal/mole, i.e almost 10 Kcal/mole lower than those calculated for CH4 oxidation. 

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... 

Catalysts include oxides, mixed oxides (perovskites) and zeolites [3]. The latter, transition metal ion-exchanged systems, have been shown to exhibit high activities for the decomposition reaction [4-9]. Most studies deal with Fe-zeolites [5-8,10,11], but also Co- and Cu-systems exhibit high activities [4,5]. Especially ZSM-5 catalysts are quite active [3]. Detailed kinetic studies, and those accounting for the influence of other components that may be present, like O2, H2O, NO and SO2, have hardly been reported. For Fe-zeolites mainly a first order in N2O and a zero order in O2 is reported [7,8], although also a positive influence of O2 has been found [11]. Mechanistic studies mainly concern Fe-systems, too [5,7,8,10]. Generally, the reaction can be described by an oxidation of active sites, followed by a removal of the deposited oxygen, either by N2O itself or by recombination, eqs. (2)-(4). 

Complex Base-Metal Oxides Complex oxide systems include the mixed oxides of some metals which have perovskite or spinel structure. Both the perovskites and the spinels exhibit catalytic activity toward cathodic oxygen reduction, but important differences exist in the behavior of these systems. 

An example for a compound of the perovskite type is LaNiOj. In other com-ponnds of the perovskite type, nickel may be replaced by cobalt or iron, and lan-thannm in part by alkaline-earth metals, an example being Lag 8Sro2Co03. The activity of perovskites toward cathodic oxygen reduction is low at room temperature but rises drastically with increasing temperature (particularly so above 150°C). In certain cases the activity rises so much that the equilibrium potential of the oxygen electrode is established. 

Cobaltites with spinel stractnre have compositions MC02O4, where M is a metal forming divalent cations, snch as zinc, cadminm, magnesinm, nickel, manganese, and divalent cobalt. In contrast to the perovskites, the cobaltites have a rather high catalytic activity already at room temperatnre. Experiments show that the activity increases with increasing spinel structure content (i.e., increasing number of Co ions) of the catalyst snrface. The trivalent cobalt ions promote the withdrawal of... 

Catalyst recycling. The solid Pd-doped perovskite catalysts are easily filtered from the reaction mixtnre for reuse. The activity of the recycled BaCco 95Pdo.o502 95 catalyst was investigated in the coupling of 4-bromoanisole with 4-phenylboronic acid. The results in Table 27.3 show that high activity was retained even after seven cycles of catalyst use. 

The greater rearrangement of the perovskite stmeture in the catalyst associated with the higher level of Pd-doping may be responsible for the longer indnetion period. After the onset of catalytie activity, the slopes of the two conversion vs. time eurves for X = 0.05 and x = 0.10 in Figure 27. la are very similar, demonstrating that the two catalysts produce the same soluble aetive site in similar amounts. 

Presently the effective role of sulphur additive is not well explained because sometimes activation or deactivation phenomena are observed. Such a versatile behaviour is well-illustrated over noble metal-based catalysts particularly when they are dispersed on perovskite supports [111]. The catalytic performances of a prereduced Pt/LaCo03 in H2 at 450°C are illustrated in Figure 10.13a. After preactivation in H2 subsequent bulk and surface characterisation highlighted an extensive reduction of the perovskite... 

Based on these considerations the development of perovskite-based catalysts could be an alternative due to their ability to stabilised well-dispersed active sites more resistant to deactivation and to their tolerance to poisoning effects. Consequently, those materials... 

Our recent our works show that even higher activity and stability can be demonstrated by the three-layer electrodes with nickel layer, active in the oxygen evolution, middle layer with catalyst, active in the oxygen reduction (Mn02, pyropolymer or a perovskite), and a diffusion (waterproof) layer,... 

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