Surfaces LaMnOs

Giannakas, AE Vaimakis, TC Ladavos, AK Trikalitis, PN Pomonis, PJ. Variation of surface properties and textural features of spinel ZnAl204 and perovskite LaMnOs nanoparticles prepared via CTAB-butanol-octane-nitrate salt microemulsions in the reverse and bicontinuous states. Journal of Colloid end Interface Science, 2003, Volume 259, Issue 2, 244-253. 

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]. 

At complete filling, n = 1, in the case of undoped LaMnOs, the Fermi surfaces become straight lines, flat . Also, at n = 1, there is a perfect nesting. The energies of two bands coincide, ei (p -l- Q) = 2 (p), with a shift by the wave vector Q = (jT, n). In three dimensions (3D), for the ferromagnetic case, a similar nesting takes place with the 3D-vector, Q = it, it, n). 

The LaMnOs and Lao.sAgo.2Mn03 samples, prepared under microwaves irradiation at atmospheric pressure (MW) or synthesised by microwaves-assisted hydrothermal accelerated solid state synthesis (MWhyd), exhibited higher specific surface areas (19 m /g for MW LaMnOs, 16 m /g for MW Lao,8Ago,2Mn03 29 m /g for for MWhyd LaMnOs, and 25 m /g for MW Lao,8Ago,2Mn03) than the same samples prepared by conventional routes [8]. 

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. 

As follows from numerical analysis of ag(t) and /34(f) responses observed for LaMnOs, in the initial period of time after the switch oxygen exchange on the surface is the rate-determining step, while diffusion in the bulk becomes more significant with time. A complete hetero-molecular exchange between the O2 molecule and two oxygen atoms of the solid (i mechanism) provides a more accurate description of/34(f) response, which is in agreement with the literature [30]. 

Slab Models of SrTiOs, SrZrOs and LaMnOs Surfaces. 507... 

In Table 11.3 are given in brackets the atomic planes charges for (001) surface in SrTiOs and LaMnOs crystals calculated for the bufk crystals with the use of Wannier-type atomic functions in the population analysis, see Chap. 9 and reference [736], As the Ti-0 bond in SrTiOs presents a non-negligible part of covalent character the actual charges of atomic planes SrO and Ti02 are nonzero. This means that SrTiOs (001) should be considered as a polar (type-3) surface. 

Slab Models of SrTiOs, SrZrOg and LaMnOs Surfaces 515... 

The first ab-initio calculation of cubic LaMnOs surface properties [851] has been made by the HF LCAO method for (110) LMO surface in the single-slab model. The extension of these calculations to the (001) LMO surface was made in [852]. The bulk cubic rmit cell atoms are distributed over atomic planes (normal to the surface direction) in the following way for the (110) surface - 02-LaMnO-02-LaMnO - - - for the (001) surface - La0-Mn02-La0-Mn02 (see Fig. 11.4). In both cases the surfaces are polar. 

In LMO (110) surface calculations [851] the 02-terminated slab consisting of seven planes was taken, i.e. four O2 planes and three LaMnO planes. Such a symmetrical slab is nonstoichiometric, i.e. it does not consist of an integer number of formula units. To restore the stoichiometry of the 7-plane slab, one oxygen atom has been removed from both 02-planes terminating the slab, i.e. the slab of three bulk primitive unit cells with periodically repeated surface oxygen vacancies was used. Such an approach is justified since it is weU known that the polar surfaces are stabilized by surface defects and surface-atom relaxation. The smface energy Eg (per surface unit cell) for such a slab equals ... 

Table 11.22. The effective atomic charges Q( e ) in four unxelaxed top layers of the LMO (110) surface, both stoichiometric O-terminated (Qi), and nonstoichiometric O2- and LaMnO-terminated (Q2 and Qs), as well as the relevant deviations of plane charges, AQ e ) from those in the bulk (Q(La)=2.56 Q(Mn)=2.09 (5(0)=-1.55)...

In terms of application, the high specific surface area and the nano-size obtained from FH provide catalytic materials with improved resistance to high temperature treatment As an example, the methane combustion was used as a probe reaction. Although LaMnOs and LaCoOs+g prepared by the amorphous citrate method had lower temperature of methane oxidation, the corresponding catalysts prepared by FH demonstrated higher resistance to sintering after forced exposiu-e to reaction conditions at 800 °C for 24 h [76]. The catalysts prepared by the citrate method possess small particles obtained at low calcination temperature, for example, 600-700 °C. Thermal resistance is obtained only after calcination at 850-900 °C but at the expense of the specific surface area, whereas an ideal compromise is obtained by FH. 

Pd-doped catalysts have been produced by USS [82]. The fingerprint of Pd adopting the octahedral coordination of Fe in LaFeo,95Pdo,o503 has been observed in the XANES spectra of the material prepared by spray synthesis (27m /g) similarly to the preparation by the amorphous citrate method (14m /g) [17,82]. In contrast, the flame-made material of the same composition (22m /g) exposed metallic Pd particles on LaFeOs similarly to preparation by solution combustion. The different nature of the Pd species obtained by changing the synthesis method dramatically influences their catalytic performance, since PdO nanoparticles exposed at the surface of the mixed oxide exhibit catalytic activity, whereas Pd—O species in the bulk of the mixed oxide are inactive, at least in the case of methane oxidation [27]. In contrast to LaFeOs, LaMnOs did not allow Pd to adopt the octahedral coordination irrespective of synthesis method. Therefore, the coordination of Pd strongly depends on both the composition of the perovskite-type oxide and the synthesis method. 

From XPS analyses, the coexistence of Mn and Mn species was evidenced in LaMnOs material, while the LaCoOs one showed the presence of Co and Co " at the same time. Additionally, these authors established a higher Oads/Oiatt molar ratio on LaCoOs perovskite than that observed for the LaMnOs one. This phenomenon, observed at the surface of the material, improved the low-temperature reducibility of LaCoOs material, where the catalyst s properties claimed to be responsible for the remarkable catalytic performance of the cobalt-based perovskite in the CO oxidation reaction. Other authors have also remarked the importance of the presence of Co " species as active sites for the adsorption of CO [37-39]. For instance, by using DRIFT spectroscopy, Natile et al. [37]... 

Recently, Rajesh et al. [79] studied the oxidation state of Pt incorporated in perovskites LaBOs (B = Mn, Co, Fe) for WGS activity. They observed that Pt-doped Mn perovskite showed very poor activity likely attributed to the sintering of Pt particles on the surface, while Pt-doped Co and Fe perovskite catalysts showed almost 90% CO conversion above 300 °C. XPS measurements indicate that Pt could be stabilized in ionic form in Co and Fe perovskites but not in LaMnOs one. This result suggests a possible key influence of the oxidation state... 

It must be noted, however, that efforts have been spent by several research groups with the aim of combining the favourable features of monolithic catalysts operated under autothermal conditions (thus favouring the onset in the gas-phase volumes of selective oxidation paths of the alkane fuel), with the development of intrinsically active and selective formulations able to contribute directly or indirectly to the selective production of olefins. A specific mention is deserved by rare earth oxides-based catalysts wherein contributions of the catalyst surface to the formation of ethyl species were reported. " Interestingly, through a detailed comparison of the observed performances of Pt-based and LaMnOs-based monoliths in ethane ODH experiments, Donsi et observed that the use of the perovskite-coated monolith yielded an improvement in ethylene yield. An overview of their results is reported in Fig. 28.6. 

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