CeO2 and Ce-Zr mixed oxides with different Ce:Zr ratios were prepared; characterised by Raman spectroscopy, XRD, TEM, N2 adsorption at -196 degrees C, and H2-TPR; and tested for soot oxidation under ...NOx/O2. Among the different mixed oxides, Ce0.76Zr0.24O2 provided the best results. Ce0.76Zr0.24O2 presented greater activity than pure CeO2 for soot oxidation by NOx/O2 when both catalysts were calcined at 500 degrees C (soot oxidation rates at 500 degrees C are 14.9 and 11.4 mcg soot/s, respectively), and the catalytic activity of CeO2 decayed significantly with calcination temperature (from 500 to 1000 degrees C), whereas Ce0.76Zr0.24O2 presented enhanced thermal stability at temperatures as high as 1000 degrees C. In addition, Ce0.76Zr0.24O2 catalysed the reduction of NOx by soot at around 500 degrees C more efficiently than CeO2, thereby contributing to the decreased NOx emission level. The catalytic activity of CeO2 and Ce0.76Zr0.24O2 for soot oxidation by NOx/O2 depended on the textural properties (BET area; crystallite size), but other properties of the oxides, such as redox behaviour and/or enhanced lattice oxygen mobility, also played a significant role. PUBLICATION ABSTRACT
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•A detailed study of the composition effect on structural and textural properties in CexPr1-xO2-δ (0 < x < 1) was conducted.•The calcination temperature on the catalysts’ synthesis ...was explored, along with their catalytic activity for soot combustion.•Ceria-praseodymia mixed oxides show improved catalytic activities for soot combustion with respect to ceria.•Ce0.5Pr0.5O2-δ-500 exhibits the highest catalytic activity in soot combustion under NOx/O2 atmosphere, among those studied.
The effect of Ce/Pr ratio on ceria-praseodymia on structural and surface properties is studied. The catalytic activity towards soot combustion under NOx/O2 and O2/N2 atmospheres is also evaluated, together with the NO oxidation activity to NO2. CexPr1-xO2-δ compositions (x = 0.8, 0.5 and 0.2) have been prepared along with ceria and praseodymia. Catalysts were prepared by co-precipitation. The Ce0.5Pr0.5O2-δ composition, was also prepared by nitrate calcination. Ceria-praseodymia mixed oxides yield reduction profiles shifted to lower temperatures, higher NO oxidation activities to NO2 and improved catalytic activities for soot combustion with respect to pure ceria. Ce0.5Pr0.5O2-δ was the most active one. Under NOx/O2, it has greater soot combustion activity if it is prepared by nitrate calcination. However, under O2/N2, the co-precipitation method is more favorable because of the better dopant insertion achieved in the ceria lattice, which seems to lead better oxygen mobility on the surface and in the bulk oxide.
Copper catalysts prepared using four supports (Mg- and Sr-modified Al2O3 and MgTiO3 and SrTiO3 perovskites) have been tested for soot oxidation by O2 and NO x /O2. Among the catalysts studied, ...Cu/SrTiO3 is the most active for soot oxidation by NO x /O2 and the support affects positively copper activity. With this catalyst, and under the experimental conditions used, the soot combustion by NO x /O2 presents a considerable rate from 500 °C (100 °C below the uncatalysed reaction). The Cu/SrTiO3 catalyst is also the most effective for NO x chemisorption around 425 °C. The best activity of Cu/SrTiO3 can be attributed to the improved redox properties of copper originated by Cu-support interactions. This seems to be related to the presence of weakly bound oxygen on this sample. The copper species present in the catalyst Cu/SrTiO3 can be reduced more easily than those in other supports, and for this reason, this catalyst seems to be the most effective to convert NO into NO2, which explains its highest activity for soot oxidation.
Commercial TiO2 (P25, from Degussa) was modified with variable amounts of La, Ce, Y, Pr, Sm (generally rare earth (RE)), by thermal treatment of physical mixtures of TiO2 and the nitrates of the ...various RE. Doping of P25 with RE, combined with calcination at 600 or 800 °C, yields materials with surface areas ranging from ∼10 to 50 m2/g, and an anatase to rutile phase ratio ranging from ∼0.03 to 0.7, as determined by evaluation of XRD data. After calcination at 600 °C, unpromoted P25 yields the highest activity in methylene blue degradation, while RE addition decreases the activity. After pretreatment of P25 at 800 °C, RE modified catalysts perform better than unpromoted P-25, La being the preferred RE. By evaluation of the DRIFT spectra of the various catalysts, a correlation between the number of a specific anatase Ti–OH group, yielding an IR absorption at 3635 cm−1, and the methylene blue degradation rate was determined. This suggests that this OH-group is an important precursor for the reactive site in aqueous phase methylene blue degradation, and a dominant factor in controlling performance of P-25 in this reaction.
RNA viruses exist as complex mixtures of genotypes, known as quasispecies, where the evolution potential resides in the whole community of related genotypes. Quasispecies structure and dynamics have ...been studied in detail for virus infecting animals and plants but remain unexplored for those infecting micro‐organisms in environmental samples. We report the first metagenomic study of RNA viruses in an Antarctic lake (Lake Limnopolar, Livingston Island). Similar to low‐latitude aquatic environments, this lake harbours an RNA virome dominated by positive single‐strand RNA viruses from the order Picornavirales probably infecting micro‐organisms. Antarctic picorna‐like virus 1 (APLV1), one of the most abundant viruses in the lake, does not incorporate any mutation in the consensus sequence from 2006 to 2010 and shows stable quasispecies with low‐complexity indexes. By contrast, APLV2‐APLV3 are detected in the lake water exclusively in summer samples and are major constituents of surrounding cyanobacterial mats. Their quasispecies exhibit low complexity in cyanobacterial mat, but their run‐off‐mediated transfer to the lake results in a remarkable increase of complexity that may reflect the convergence of different viral quasispecies from the catchment area or replication in a more diverse host community. This is the first example of viral quasispecies from natural aquatic ecosystems and points to ecological connectivity as a modulating factor of quasispecies complexity.
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