LaMnO catalysts

In this study, catalytic combustion of diesel soot particulates over LaMnOj perovskite-type oxides prepared by malic acid method has been carried out. In the LaMnOs 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. 

Johnson, DWJr Gallaguer, PK Sehrey, F Rhodes, WW. Preparation of high smfaee area substituted LaMnOs catalysts, Am. Cer. Soc. Bull, Annual meeting of the Ameriean Ceramic Society Washington, DC, USA, 1976, Volume 55, Issue 5, 520-523. 

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

Y. (2011) Activity and deactivation behavior of Au/LaMnOs catalysts for CO oxidation. /. Rare Earths, 29,213-216. 

Guo, Y., Lu, G., Baylet, A., and Giroir-Fendler, A. (2013). The eflect of A-site substitution by Sr, Mg and Ce on the catalytic performance of LaMnOs catalysts for the oxidation of vinyl chloride emission. Appl Catal B Environ., 134,... 

Various metal and metal oxide nanoparticles have been prepared on polymer (sacrificial) templates, with the polymers subsequently removed. Synthesis of nanoparticles inside mesoporus materials such as MCM-41 is an illustrative template synthesis route. In this method, ions adsorbed into the pores can subsequently be oxidized or reduced to nanoparticulate materials (oxides or metals). Such composite materials are particularly attractive as supported catalysts. A classical example of the technique is deposition of 10 nm particles of NiO inside the pore structure of MCM-41 by impregnating the mesoporus material with an aqueous solution of nickel citrate followed by calicination of the composite at 450°C in air [68]. Successful synthesis of nanosized perovskites (ABO3) and spinels (AB2O4), such as LaMnOs and CuMn204, of high surface area have been demonstrated using a porous silica template [69]. 

Svensson et al. prepared catalysts with 20% LaMnOs supported on MgO via CTAB-l-butanol-iso-nitrate salt microemulsion (Svensson et al. 2006). By varying the synthetic procedure, an attempt to vary the interaction between LaMnOa and MgO was made. Compared with bulk LaMnOa, the LaMnOa supported on MgO showed a greatly improved activity. 

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... 

LaMno.976Rho.o2403+8 catalyst. Oxidation step ( ig. lA)... 

Figure 1. Step-change activity of LaMno.976Rho.o2403+8 under successive streams of CO and O2 at 400°C (100 mg catalyst, total flow rate 10 l.h" ). (A) oxidation step (B) reduction step.

Similar tests were performed on the La2Cuo.9Pdo.i04+8 catalyst (Fig. 2). The same profiles are obtained as in the case of the LaMno.976Rho.o2403+s catalyst, although the small CO2 peak due to the oxidation of carbonaceous deposits is somewhat bigger than previously (440 ppm CO2 at the maximum). The main difference with the previous catalyst is that the amount of CO2 evolved on introduction of CO on the oxidised solid is much larger (about 400 pmoles), and corresponds in this case to 0.75 to 0.8 mole CO2 per mole catalyst. 

Figure 3. Light-oflf activity for CO, NO and C3H6 conversion in cycling conditions over LaMno.976Rho.o2403+8 (A, full Ime), La2Cuo.9Pdo.i04+8 (B, dashed line) and Pt-Rh/Ce02-Al203 (C, thick line) catalysts.

The catalysts LaMnOs, Lao.gAg o,2Mn03 were prepared, in the presence of microwave radiation, via nitrates mediated-synthesis at atmospheric pressure and hydrothermal processes. Reagents La(N03)3.9H20 (98%), Mn(N03)3.4H20(98%) and Ag(N03) (RP) with 99.8% purity were used. 

Figure 1 XRD paterns of MW and MWhyd LaMnOs and Lao.gAgojMnO3 catalysts...

The TPR patterns of the MWhyd catalysts showed that the consumption of hydrogen, corresponding to the reactivity of low a-oxygen species, for both LaMnOs host structure and for Ag-substituted perovskites is 1,02 and 1,39.10 mol/gcat respectively. However, reduction temperature of Lao.8Ago.2Mn03, as with MW catalysts, was lower than that of LaMnOs. 

The insertion of Ag in the LaMnOs host structure, in both MW and MWhyd catalysts, causes a shift of TPR peak (low-temperature a oxygen species) to lower temperature. 

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. 

Table 1 shows the surface composition of the fresh and sulphur poisoned catalysts determined by using XPS. La/Mn ratio was similar for both MW and MWhyd LaMnOs (1.2 vs. 1.3 respeetively) and lower for MWhyd Lao.8Ago.2Mn03+5 (0.8). This ratio decreased to ca. 0.7 for both sulphur poisoned S-MWhyd LaMn03 and S-MWhyd Lao.8Ago.2Mn03+s. 

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. 

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