Perovskite catalysts for the removal of pollutants from new highly efficient gasoline engine exhaust

Resumen

This PhD thesis deals with the analysis of perovskite-based catalysts for the removal of soot and CO from the exhaust of highly efficient automotive gasoline engines. The study is primarily focused on the development of two series of samples obtained by the partial substitution of the Ba cation in BaMnO3 (BM) and BaMn0.7Cu0.3O3 (BMC), that is, with the general formula Ba0.9A0.1MnO3 (BM-A) and Ba0.9A0.1Mn0.7Cu0.3O3 (BMC-A), being A Ca, Ce, La, Mg or Sr. Subsequently, the activity of these samples as catalysts for the oxidation of soot and CO, in conditions simulating that found in the exhaust of automotive gasoline engines, has been estimated. Then after, in order to obtain the best composition of perovskites for soot oxidation, the optimal degree of Ba cation substitution for the two selected compositions (BM-Ce and BMC-La) has been explored by the synthesis of Ba1-xCexMnO3 (BM-Cex) and Ba1-xLaxCu0.3Mn0.7O3 (BMC-Lax) perovskite-type mixed oxides at different substitution levels (x = 0, 0.1, 0.3, 0.6). These catalysts were deeply characterized and used for GDI soot oxidation. Based on the results presented and discussed in the three published articles, corresponding to Ba0.9A0.1MnO3 (BM-A) and Ba0.9A0.1Mn0.7Cu0.3O3 (BMC-A) series, the following general conclusion have been extracted: - The hexagonal structure is preferred in the presence of A metal, as it is the main phase detected for BM-A, and, as the polytype structure found in the BMC sample (formed by distortion of the hexagonal perovskite due to the copper insertion into the lattice) is disfavored in BMC-A perovskites that present a mixture of the two structures. - On the surface of all perovskites, coexisting Mn(IV), Mn(III) and oxygen vacancies. Mn(IV) is the main oxidation state on the surface of all samples, but, in the bulk, it depends of the A metal and on the perovskite formulation: i) for BM-A series, Mn(III) is more abundant for BM-Ca and Mn(IV) is for BM-Mg, being both oxidation states in similar proportion for the other samples and ii) for BMC-A series, Mn(IV) is the main one for BMC-Ce and BMC-Mg, while Mn(III) was for BMC and BMC-La. - The partial substitution of Ba in BM and BMC: i) enhances the reducibility, being BM-La the most reducible sample from BM-A series (being the unique which evolves oxygen at intermediated temperature, ’-O2) and BMC-Ce from BMC-A series and ii) improves the lattice oxygen mobility, being Ce the most efficient A metal due to the contribution of the Ce(IV)/Ce(III) pair. - Almost all perovskites are active as catalysts for soot removal by oxidation, as most of the TPR-soot conversion profiles are shifted to lower temperatures in the presence of perovskites in the two atmospheres tested (0%and 1% O2 in He). However, the soot conversion is notably lower in the absence of O2 than in the presence of an 1% O2 in the reaction mixture, as the oxygen available for soot oxidation exclusively comes from the bulk of perovskites. In these conditions, BMC-La is the most active catalyst as presents the highest proportion of copper on the surface (as Ba-O-Cu species). In the presence of oxygen (1% O2 in He), BM-Ce is the best catalyst as it shows a high amount of oxygen surface vacancies, the highest oxygen mobility, and the best redox properties due to the additional participation of the Ce(IV)/Ce(III) pair which promotes the O2 emission from the bulk of perovskites, which seems being directly involved in the soot oxidation. - The role of copper seems to be relevant only if the oxygen used for the soot oxidation exclusively comes from the perovskite (i.e., in 100% He), as BMC-La, which presents the highest fraction of surface copper, is the most active catalyst. On the contrary, if soot is oxidized using the oxygen present in the reaction atmosphere (i.e., in 1% O2 in He), the presence of copper in the perovskite composition is not significant, as the most active catalyst is BM-Ce because it shows a higher fraction of surface Ce(IV) than BMC-Ce and, consequently, a better redox performance. - All BM-A and BMC-A perovskites are catalytically active for the oxidation of CO under all the reaction conditions tested, being more active in the gaseous mixtures with low CO/O2 ratios and showing the highest activity in 0.1% CO and 10% O2. - The addition of A metal increased the catalytic activity for the oxidation of CO at T < 500 °C with respect to BM and BMC, but BMC-A samples show the highest efficiency as catalysts for CO oxidation due to the presence of copper. For BM-A series, BM-La is the most effective to improve the catalytic performance as it this the most reducible and because generates ά-O2. For BMC-A series, BMC-Ce is the most active catalyst as it combines the presence of surface copper, oxygen vacancies, a high proportion of bulk and surface Mn(IV), and the contribution of the Ce(IV)/Ce(III) pair. - BMC-Ce perovskites presents at 200 °C, and using the 0.1% CO + 10%O2 gas mixture, a CO conversion very similar than the Pt-Al reference catalyst. From the un published results obtained for Ba0.9Ce0.1MnO3 (BM-Cex) and Ba0.9La0.1Cu0.3Mn0.7O3 (BMC-Lax) perovskites (with different substitution levels x = 0, 0.1, 0.3, 0.6), the following conclusions are proposed: - The characterization of BM-Cex series reveals that: i) as the percentage of Ce increases, the hexagonal perovskite structure is progressively replaced by CeO2 crystalline phase, which is the main one for BM-Ce0.6 , ii) Mn(IV) is the main oxidation state on surface for BM and BM-Ce0.1, but it is Mn(III) for BM-Ce0.3, while for BM-Ce0.6, an almost similar amount of Mn(III) and Mn(IV) are present, iii) Ce(III) and Ce(IV) coexist on the surface of all BM-Cex samples, and a considerable increase in the surface Ce(IV) proportion is detected from BM-Ce0.1 to BM-Ce0.6, iv) after doping with Ce, the reduction of Mn/Ce takes place at lower temperatures due to the synergetic effect between Mn and Ce and, finally, v) the oxygen mobility through the perovskite lattice increases in the presence of Ce (due to the contribution of Ce(IV)/Ce(III) pair) and all samples evolve -O2, but only BM-Ce0.1 generates a low amount of α´-O2. - The characterization of BMC-Lax series indicates that: i) as the percentage of lanthanum increases, the intensity of XRD peaks corresponding to BaMnO3 polytype structure decreases in favor of an increase in the intensity of the peaks corresponding to hexagonal 2H-BaMnO3 and trigonal La0.93MnO3 perovskite structures, being the latter the main phase for BMC-La0.6, ii) the amount of surface oxygen vacancies seems not to be sensible to the increase in the La amount, iii) Mn (III) and Mn (IV) coexist on the surface and in the bulk, but, on the surface Mn(III) increases with the La content, while in the bulk Mn(IV) is favored as La content is higher, iv) the accumulation of Cu (II) on the surface increases with the amount of La, v) an increase in the reducibility of BMC-La0.3 and BMC-La0.6 samples respect BMC and BMC-La0.1 is found, and, finally, vi) the oxygen mobility increases with the percentage of La. - The analysis of the catalytic performance for soot oxidation in the two conditions tested suggests that: i) in the absence of oxygen in the reaction atmosphere (100 % He), BMC-La0.1 is the best catalyst as copper is also able to catalyze the soot oxidation, ii) if oxygen is present in the reaction atmosphere (1 % O2 /He), BM-Ce0.1 is the most active catalyst as it presents a higher proportion of Mn(IV) than BMC-La0.1. - The addition of an amount of Ce or La higher than the corresponding to x=0.1 in Ba1-xCexMnO3 and Ba1-xLaxCu0.3Mn0.7O3 does not allow improving the catalytic performance of BM-Ce0.1 and BMC-La0.1 for soot oxidation in the tested conditions.