LaCoO catalyst

Figure 3.5 Temperature dependence of methane combustion on (a) mesoporous LaCoOs after removal of silica, (b) mesoporous LaCoOs-silica hybrid material, and (c) conventional LaCoOs and (d) used mesoporous LaCoOs catalyst [23].

Seldne et al. [78] investigated the WGS reaction at low temperature (300 °C) over Pt and Pd catalysts supported on several LaBOs (B = Cr, Mn, Fe, Co, and Ni) perovskite oxides prepared using the Pechini method. They observed that perovsldte oxides without active metal showed no activity for the WGS reaction, while those loaded with Pt and Pd exhibited good WGS activity. Interaction between Pt or Pd and the support promotes the WGS reaction. They found that both Pt/LaCoOs and Pd/LaCoOs catalysts have high catalytic activity, although Pt/LaCoOs catalyst deactivated immediately, and Pd/LaCoOs, although initially less active, exhibited superior stability. The cause of deactivation of Pt/LaCoOs was attributed to the reduction of Co and Pt cations. 

According to Mul et al. [66], the CO oxidation is favored when compared to the H20xidation and much more favored than methanation (d) and water gas shift reaction (WGSR), because the sequence of values of the equilibrium cmistants is CO oxidation > H2 oxidation 3> CO methanation > WGSR. That shows that selective CO oxidation is practical from the thermodynamic perspective, since the equilibrium constant of CO oxidation is larger than the side reactions (b), (c), and (d). These reactions can be analyzed simultaneously through TPRS experiments, whose profiles are illustrated in Fig. 6.37 for the LaCoOs catalyst. 

Bialobok, B. Trawczynski, J. Mista, W. et al. Ethanol combustion over strontium-and cerium-doped LaCoOs catalysts. Appl. Catal. B Environ. 2007, 72, 395-403. 

Figure 37. SEM photographs of the LaCoOs catalysts obtained after calcination (a) SS (b) COP (c) err and (d) COPRG. (e) Steady-state conversions obtained for the oxidation reaction of 0.25% CH4 as a function of the reaction temperature over LaCoOsi ( ) COPRG, ( ) RG, (A) Cff, ( ) COP, and ( ) SS[258].

Three LaCoOs samples (1,11, and 111) with different specific surface areas were prepared by reactive grinding. In the case of LaCoOs (1), only one step of grinding was performed. This step allowed us to obtain a erystalline LaCoOs phase. LaCoOs (11) and LaCoOs (111) were prepared in two grinding steps a first step to obtain perovskite crystallization and a second step with additive to enhanee speeific surface area. The obtained compounds (perovskite + additive) were washed repeatedly (with water or solvent) to free samples from any traee of additive. The physical properties of the three catalysts are presented in Table 10. LaCoOs (1) was designed to present a very low specific surface area for comparison purposes. NaCl used as the additive in the case of LaCoOs (11) led to a lower surface area than ZnO used for LaCoOs (111), even if the crystallite size calculated with the Sherrer equation led to similar values for the three catalysts. The three catalysts prepared were perovskites having specific surface areas between 4.2, 10.9 and 17.2 m /g after calcination at 550 °C. A second milling step was performed in the presence of an additive, yielding an enhanced specific surface area. 

Li et al. developed a solid-state reaction process to synthesize perov-skite-type LaCoOs NCs with grain diameters of 15 0 run (Li et al., 2002). In the first step of the preparation, 5 run composite hydroxide NPs were s)mthesized by grinding metal nitrates liquid paste and mixing with KOH. Then the composite powders were calcined at 800 °C, yielding a single-phase oxide. Tien-Thao et al. prepared LaCo Cui J.O3 x < 0.3) by mechano-synthesis (Tien-Thao et al., 2008). The sample has various distinct Co " " ions in the perovskite lattice, which are more reducible. The reduced catalyst surface comprising cobalt and copper atoms is very selective for the hydrogenation of CO. 

Among the cobalt containing perovskites GdCoO, SmCoO, NdCoO, PrCoO, and LaCoO, tested as catalyst precursors for the partial oxidation of methane the Gd-Co-0 system showed exceptionally better performance for synthesis gas formation (Figs. 6A-6C). At 1009 K a steady-state methane conversion of 73% with selectivities of 79 and 81% for CO and H., respectively, is observed for the catalyst Gd-Co-O. The catalysts Sm-Co-O and Nd-Co-0, of lower activity, show similar steady-state methane conversions in the temperature range studied. On the other hand, the H, and CO selectivities are much higher over Sm-Co-O. 

XRD analyses of the used catalysts Gd-Co-O and Sm-Co-O showed similar patterns to the reduced catalysts with very strong and sharp peaks for the sesquioxides Gd,0, and Sm20,. On the other hand, the XRD analysis of the La-Co-O catalyst after reaction at 1023 K for 19 h clearly showed the formation of the perovskite LaCoO,. Therefore, it is not surprising that the only reaction products observed were water and carbon dioxide. This agrees with previous works on this perovskite and other forms of cobalt oxide which have been shown to be active catalysts for methane combustion and also for CO and H, oxidation [19], The high Co/Ln surface ratio determined by XPS for the used catalyst is expected for a perovskite like surface. 

Slagten and Olsbye [ 10] studied the perovskite LaCoO, (containing some impurities of La O, and COiOj) for the partial oxidation of methane to syngas and observed the production of mainly CO,. If the catalyst was kept at 1073 K after 30 h on-stream the activity changed to give mainly CO which they assigned to the in situ reduction of cobalt. The XRD for Nd-Co-O after reaction revealed the presence of the phases Nd,0, and also the perovskite NdCoO,. For all used catalysts no clear evidence for the presence of simple cobalt oxides such as CoO, COjO, and CO3O4 could be found by XRD. 

LaMnOs and Lao.8Ago.2Mn03) no structural modifications on the samples were observed (XRD analyses) contrarily to sulphur poisoned LaCoOs perovskites which dislocated [12]. Finally, it should be noted that, the preparation method has a great influence on the catalysts behaviour toward poisoning. 

Catalytic tests showed the highest activity of Co-containing samples among the oxide catalysts (Fig. 2) that is in good accordance with the literary data. Cobalt oxide, however, has low thermostability. Much higher stability, close to conventional CuCr204, was observed for the LaCoOs sample. 

Foam Pd/Nichrome and LaCoOs/ceramics catalysts can be recommended for neutralization of gas emissions of enameled wire production. 

The pure perovskites are all more active for CO + O2 than CO + NO reactions. The best catalysts for both reactions are LaMnOs and LaCoOs. The activity of the different pervoskites for CO oxydation can be linked semiquantitatively with the ease of anionic vacancy formation in the lattice, described by the B-O bond energy (Figure 3). 

The CO + NO reaction is better explained by a redox mechanism, where CO is oxidized by the catalyst which is regenerated by the reduction of NO. Moreover, a dissociative adsorption of NO better explains the experimental results on LaCoOs and LaMnOs, while a molecular adsorption has to be supposed on LaCrOs adn LaFeOs. 

The addition of Ce to the perovskites leads to different effects depending on the nature of B ions and on the relative amount of Ce. It has to be emphasized, that Ce02 itself is also a good catalyst for CO oxidation with O2 (97 mol % CO conversion at 300°C). This activity is equal to that of LaMnOs, but it is inferior to the activity of LaCoOs. 

The following reaction scheme is adequate for the LaCoOs and LaMnOs catalysts ... 

Afterburning processes enable the removal of pollutants such as hydrocarbons and volatile organic compounds (VOCs) by treatment under thermal or catalytical conditions. Combinations of both techniques are also known. VOCs are emissions from various sources (e.g. solvents, reaction products etc. from the paint industry, enaml-ing operations, plywood manufacture, printing industry). They are mostly oxidized catalytically in the presence of Pt, Pd, Fe, Mn, Cu or Cr catalysts. The temperatures in catalytic afterburning processes are much lower than for thermal processes, so avoiding higher NOx levels. The catalysts involved are ceramic or metal honeycombs with washcoats based on cordierite, mullite or perovskites such as LaCoOs or Sr-doped LaCoOs. Conventional catalysts contain Ba-stabilized alumina plus Pt or Pd. 

The LaCoOs perovskite catalyst alone showed similar degradation compared with the stabilised platinum/alumina catalyst, whereas the addition of 0.2 mol% ruthenium to the perovskite catalyst led to stable performance. 

For applications in heterogeneous catalysis, perovskites generally comprise a lanthanide (La is the most common) in the A site and a transition metal (Mn, Co, etc.) in the B site. The efficiency of such perovskite oxides, with or without cationic substitution, is well documented for a variety of catalytic reactions [2-9]. Actually, the specific catalytic activities of perovskites were sometimes found to be comparable to that of noble metals for various oxidation reactions. Early on, Arai et al. illustrated the activity of strontium-substituted LaMnOs, which was found to be superior to that of Pt/alumina catalysts at a conversion level below 80% [5]. Several authors have also discussed the application of La-based perovskite oxides as catalysts for volatile organic compound (VOC) oxidation (see, for example. Refs [10-14]). Zhang et al. have also shown that some perovskite oxides substituted with Pd or Cu are also good catalysts for the reduction of NO by CsHg [15-18] and by CO [19,20]. More recently, Kim et al. studied the effect of Sr substitution in LaCoOs and LaMnOs perovskites for diesel oxidation (DOC) and lean NO, trap (LNT) processes [9]. The observations made by these authors clearly indicate that the perovskites used in their study could efficiently outperform Pt-based catalysts. Typically, Lai. Sr cCoOs catalysts achieved higher... 

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