Silvana Braun - Academia.edu (original) (raw)

Papers by Silvana Braun

J Phys Chem B, 2000

The thermal spreading of MoO 3 onto silica was studied in comparison with the thermal spreading o... more The thermal spreading of MoO 3 onto silica was studied in comparison with the thermal spreading onto alumina using different characterization techniques. X-ray diffraction results showed that MoO 3 crystals were transformed into Mo amorphous species on alumina and silica supports by thermal treatment. Laser Raman spectroscopy results also evidenced the transformation of MoO 3 bulk into small Mo clusters and/or dispersed Mo species, which are highly distorted, interacting with alumina and silica supports. X-ray photoelectron spectroscopy and diffuse reflection spectroscopy (DRS) results gave better evidence of the presence of higher amounts of Mo species at the surface of both supports when compared with the respective physical mixtures. Fourier transform infrared spectroscopy also provided good evidence that a surface reaction may have occurred between MoO 3 and hydroxyl groups on both supports. It was possible to infer that the original MoO 3 crystal lattices were destroyed by reacting with the support surface as a result of thermal treatment and transformed into small Mo clusters, dispersed Mo species, or both. Moreover, Raman spectroscopy showed nicely that the interaction of Mo species was higher on alumina than silica surfaces. Raman and DRS also provide insight into Mo species dispersion on alumina and silica samples, whereas DRS provided better evidence of the presence of dispersed Mo species on these supports. It was possible to infer that all calcined physical mixtures exhibited dispersed Mo species and small Mo clusters together with a small quantity of bulk MoO 3 that remained after thermal treatment. Therefore, similar results obtained on both supports demonstrated that the thermal spreading of MoO 3 also occurred on silica and the same mechanism was observed as on alumina. However, the Mo dispersion and some Mo species were different on the supports that can be attributed to the different surface properties of silica and alumina.

The Journal of Physical Chemistry B, 2000

The thermal spreading of MoO 3 onto silica was studied in comparison with the thermal spreading o... more The thermal spreading of MoO 3 onto silica was studied in comparison with the thermal spreading onto alumina using different characterization techniques. X-ray diffraction results showed that MoO 3 crystals were transformed into Mo amorphous species on alumina and silica supports by thermal treatment. Laser Raman spectroscopy results also evidenced the transformation of MoO 3 bulk into small Mo clusters and/or dispersed Mo species, which are highly distorted, interacting with alumina and silica supports. X-ray photoelectron spectroscopy and diffuse reflection spectroscopy (DRS) results gave better evidence of the presence of higher amounts of Mo species at the surface of both supports when compared with the respective physical mixtures. Fourier transform infrared spectroscopy also provided good evidence that a surface reaction may have occurred between MoO 3 and hydroxyl groups on both supports. It was possible to infer that the original MoO 3 crystal lattices were destroyed by reacting with the support surface as a result of thermal treatment and transformed into small Mo clusters, dispersed Mo species, or both. Moreover, Raman spectroscopy showed nicely that the interaction of Mo species was higher on alumina than silica surfaces. Raman and DRS also provide insight into Mo species dispersion on alumina and silica samples, whereas DRS provided better evidence of the presence of dispersed Mo species on these supports. It was possible to infer that all calcined physical mixtures exhibited dispersed Mo species and small Mo clusters together with a small quantity of bulk MoO 3 that remained after thermal treatment. Therefore, similar results obtained on both supports demonstrated that the thermal spreading of MoO 3 also occurred on silica and the same mechanism was observed as on alumina. However, the Mo dispersion and some Mo species were different on the supports that can be attributed to the different surface properties of silica and alumina.

Química Nova, 2004

Recebido em 17/3/03; aceito em 2/9/03 THE POLLUTION FROM DIESEL ENGINES-THE PARTICULATE MATTER CU... more Recebido em 17/3/03; aceito em 2/9/03 THE POLLUTION FROM DIESEL ENGINES-THE PARTICULATE MATTER CURRENT EXPERIENCES AND FUTURE NEEDS. The exhaust emissions of vehicles greatly contribute to environmental pollution. Diesel engines are extremely fuelefficient. However, the exhaust compounds emitted by diesel engines are both a health hazard and a nuisance to the public. This paper gives an overview of the emission control of particulates from diesel exhaust compounds. The worldwide emission standards are summarized. Possible devices for reducing diesel pollutants are discussed. It is clear that after-treatment devices are necessary. Catalytic converters that collect particulates from diesel exhaust and promote the catalytic burn-off are examined. Finally, recent trends in diesel particulate emission control by novel catalysts are presented.

Applied Surface Science, 2007

This study focused on preparation of tungsten oxide supported on zirconia by thermal spreading. T... more This study focused on preparation of tungsten oxide supported on zirconia by thermal spreading. The prepared samples were characterized by infrared spectroscopy, UV–vis diffuse reflection spectroscopy, X-ray diffraction, and also by methanol dehydration reaction. It was observed that isolated octahedral tungsten dispersed species and dispersed polytungstate were formed on zirconia surface, although some WO3 that remained after the thermal treatment

Applied Surface Science, 2002

Mo/SiO 2 and Mo/TiO 2 catalysts with three different molybdenum contents were prepared using non-... more Mo/SiO 2 and Mo/TiO 2 catalysts with three different molybdenum contents were prepared using non-porous supports and the thermal spreading method for the combustion of a particulate material (PM). The results of scanning electron microscopy (SEM) and N 2 adsorption/desorption techniques showed that the thermal spreading preparation method does not induce relevant textural changes on the supports. X-ray diffraction (XRD) results showed the occurrence of thermal spreading of MoO 3 onto silica and titania supports. Diffuse reflection spectroscopy (DRS) results provided clear evidence of different Mo species on these systems: highly dispersed species on the silica catalysts and polymolybdates on the titania catalysts. It may be inferred that when prepared by the thermal spreading method the nature of the support determines the kind of molybdenum species formed in these catalysts, irrespective of the Mo content. The reactive data were evaluated by differential scanning calorimetry (DSC), using a physical mixture of PM and the catalysts. The silica-supported catalysts showed higher reactivity for PM combustion than the titania-supported ones, being the most active the systems with the Mo monolayer. The results suggested that the dispersed species are far more active than the polymolybdates or MoO 3 itself.

Applied Catalysis A: General, 2006

The effect of sodium addition on the physicochemical properties of Mo/SnO 2 catalysts was studied... more The effect of sodium addition on the physicochemical properties of Mo/SnO 2 catalysts was studied. The catalysts were prepared by precipitation using aqueous solution of ammonium heptamolybdate in two Mo amounts (5.5 and 14.5% (wt)); afterwards they were impregnated with sodium ion (1% wt). The solids were characterized by nitrogen adsorption, X-ray diffraction, UV-vis diffuse reflectance spectroscopy, FTIR, and FTIR of adsorbed pyridine in order to evaluate changes on Mo species and textural and acidic properties. The results have shown that Mo/SnO 2 samples present dispersed isolated tetrahedral Mo species. The addition of sodium ion to Mo/SnO 2 catalysts promotes the transformation of dispersed isolated tetrahedral Mo species into dispersed two-dimensional polymolybdates and also causes changes in textural and acidic properties of these Mo/SnO 2 catalysts.

ABSTRACT The exhaust emissions of vehicles greatly contribute to environmental pollution. Diesel ... more ABSTRACT The exhaust emissions of vehicles greatly contribute to environmental pollution. Diesel engines are extremely fuel-efficient. However, the exhaust compounds emitted by diesel engines are both a health hazard and a nuisance to the public. This paper gives an overview of the emission control of particulates from diesel exhaust compounds. The worldwide emission standards are summarized. Possible devices for reducing diesel pollutants are discussed. It is clear that after-treatment devices are necessary. Catalytic converters that collect particulates from diesel exhaust and promote the catalytic burn-off are examined. Finally, recent trends in diesel particulate emission control by novel catalysts are presented.

Applied Catalysis B Environmental, 2008

The Journal of Physical Chemistry B, 2000

Química Nova, 2004

Recebido em 17/3/03; aceito em 2/9/03 THE POLLUTION FROM DIESEL ENGINES -THE PARTICULATE MATTER C... more Recebido em 17/3/03; aceito em 2/9/03 THE POLLUTION FROM DIESEL ENGINES -THE PARTICULATE MATTER CURRENT EXPERIENCES AND FUTURE NEEDS. The exhaust emissions of vehicles greatly contribute to environmental pollution. Diesel engines are extremely fuelefficient. However, the exhaust compounds emitted by diesel engines are both a health hazard and a nuisance to the public. This paper gives an overview of the emission control of particulates from diesel exhaust compounds. The worldwide emission standards are summarized. Possible devices for reducing diesel pollutants are discussed. It is clear that after-treatment devices are necessary. Catalytic converters that collect particulates from diesel exhaust and promote the catalytic burn-off are examined. Finally, recent trends in diesel particulate emission control by novel catalysts are presented.

Journal of Catalysis, 2004

We studied the reactivity of Mo/Al2O3 and V/Al2O3 catalysts for soot combustion, and the influenc... more We studied the reactivity of Mo/Al2O3 and V/Al2O3 catalysts for soot combustion, and the influence of their volatile (MoO3 and V2O5) and superficial dispersed species. Differential exploratory calorimetry and temperature-programmed oxidation (TPO) experiments with catalyst and soot mixtures showed that lower combustion temperatures were obtained with increasing loading of Mo or V. TPO experiments performed on separate layers of soot

Catalysis Communications, 2005

Applied Surface Science, 2007

This study focused on preparation of tungsten oxide supported on zirconia by thermal spreading. T... more This study focused on preparation of tungsten oxide supported on zirconia by thermal spreading. The prepared samples were characterized by infrared spectroscopy, UV–vis diffuse reflection spectroscopy, X-ray diffraction, and also by methanol dehydration reaction. It was observed that isolated octahedral tungsten dispersed species and dispersed polytungstate were formed on zirconia surface, although some WO3 that remained after the thermal treatment

Applied Surface Science, 2002

Mo/Si02 and Mo/Ti02 catalysts with three different molybdenum contents were prepared using non-po... more Mo/Si02 and Mo/Ti02 catalysts with three different molybdenum contents were prepared using non-porous supports and the thermal spreading method for the combustion of a particulate material (PM). The results of scanning electron microscopy (SEM) and N2 adsorption/desorption techniques showed that the thermal spreading preparation method does not induce relevant textural changes on the supports. X-ray diffraction (XRD) results showed the occurrence of thermal spreading of Mo03 onto silica and titania supports. Diffuse reflection spectroscopy (DRS) results provided clear evidence of different Mo species on these systems: highly dispersed species on the silica catalysts and polymolybdates on the titania catalysts. It may be inferred that when prepared by the thermal spreading method the nature of the support determincs the kind of molybdenum species formed in these catalysts, irrespective of the Mo content. The reactive data were evaluated by differential scanning calorimetry (DSe), using a physical mixture of PM and the catalysts. The silica-supported catalysts showed higher reactivity for PM combustion than the titania-supported ones, being the most active the systems with the Mo monolayer. The results suggested that the dispersed species are far more active than the polvmolybdates or Mo03 itself.

Applied Catalysis B: Environmental, 2008

Applied Catalysis A: General, 2006

The effect of sodium addition on the physicochemical properties of Mo/SnO 2 catalysts was studied... more The effect of sodium addition on the physicochemical properties of Mo/SnO 2 catalysts was studied. The catalysts were prepared by precipitation using aqueous solution of ammonium heptamolybdate in two Mo amounts (5.5 and 14.5% (wt)); afterwards they were impregnated with sodium ion (1% wt). The solids were characterized by nitrogen adsorption, X-ray diffraction, UV-vis diffuse reflectance spectroscopy, FTIR, and FTIR of adsorbed pyridine in order to evaluate changes on Mo species and textural and acidic properties. The results have shown that Mo/SnO 2 samples present dispersed isolated tetrahedral Mo species. The addition of sodium ion to Mo/SnO 2 catalysts promotes the transformation of dispersed isolated tetrahedral Mo species into dispersed two-dimensional polymolybdates and also causes changes in textural and acidic properties of these Mo/SnO 2 catalysts. #

British Ceramic Transactions, 1999

ABSTRACT The thermal stability of two commercial aluminas before and after dry impregnation treat... more ABSTRACT The thermal stability of two commercial aluminas before and after dry impregnation treatment was studied. One alumina was mesoporous and the other, a spherical material, was non-porous. Characterisation of these materials before and after thermal treatment was performed by XRD, SEM, and measurements of surface area and pore size distribution. Spherical alumina was found to exhibit greater thermal stability when compared with the conventional porous alumina. The stability of spherical alumina was reduced by dry impregnation. It was shown that the morphology of these materials plays an important role in their thermal stability.

British Ceramic Transactions, 1999

ABSTRACT The effect of additions of lanthanide ions (La and Ce) and of the cerium ion concentrati... more ABSTRACT The effect of additions of lanthanide ions (La and Ce) and of the cerium ion concentration on the thermal stability of a ? alumina was studied. Ln//Al2O3 compositions were treated at high temperature (1100°C) for different calcination times (1, 5, and 24 h) and the resultant materials characterised by X-ray diffraction (XRD), BET surface area, X-ray photoelectron spectroscopy (XPS), and temperature programmed reduction (TPR). It was observed that cerium ion stabilisation was dependent on concentration with 6 wt-%Ce being the most efficient. It is suggested that the stabilisation is due to occupation of vacant cation sites in the spinel structure by Ce3+, and that stabilisation breaks down on prolonged exposure at high temperatures because of oxidation of Ce3+ to CeO2 . The same stabilisation mechanism is considered to operate for the lanthanum ion but, unlike Ce3+, it is not subject to oxidation, making La a more effective stabiliser than Ce.

1-INTRODUÇÃO Os motores de combustão interna utilizados atualmente dividem-se em duas classes: os... more 1-INTRODUÇÃO Os motores de combustão interna utilizados atualmente dividem-se em duas classes: os do tipo Diesel e Otto. Os motores a Diesel são utilizados, de modo geral, em veículos de transporte coletivo, de carga, e, em alguns países, também em veículos leves. Esses motores apresentam algumas características especificas: utilizam combustíveis de alto peso molecular (C9-C20), em alguns casos com elevado teor de S; operam em condições pobres, ou seja, com excesso de O 2 , apresentando temperaturas médias de exaustão em torno de 300 o C. Essas especificidades resultam num perfil de emissões bastante diferenciado em relação ao gerado pelos motores de ciclo Otto, conforme pode-se observar na tabela 1 1 . Tabela 1-Emissões típicas de processos de combustão Otto e Diesel (g /Kg de combustível). Os dados listados são valores médios que dependem fortemente do processo de combustão, da operação do veículo e da especificação do combustível 1 .

J Phys Chem B, 2000

The thermal spreading of MoO 3 onto silica was studied in comparison with the thermal spreading o... more The thermal spreading of MoO 3 onto silica was studied in comparison with the thermal spreading onto alumina using different characterization techniques. X-ray diffraction results showed that MoO 3 crystals were transformed into Mo amorphous species on alumina and silica supports by thermal treatment. Laser Raman spectroscopy results also evidenced the transformation of MoO 3 bulk into small Mo clusters and/or dispersed Mo species, which are highly distorted, interacting with alumina and silica supports. X-ray photoelectron spectroscopy and diffuse reflection spectroscopy (DRS) results gave better evidence of the presence of higher amounts of Mo species at the surface of both supports when compared with the respective physical mixtures. Fourier transform infrared spectroscopy also provided good evidence that a surface reaction may have occurred between MoO 3 and hydroxyl groups on both supports. It was possible to infer that the original MoO 3 crystal lattices were destroyed by reacting with the support surface as a result of thermal treatment and transformed into small Mo clusters, dispersed Mo species, or both. Moreover, Raman spectroscopy showed nicely that the interaction of Mo species was higher on alumina than silica surfaces. Raman and DRS also provide insight into Mo species dispersion on alumina and silica samples, whereas DRS provided better evidence of the presence of dispersed Mo species on these supports. It was possible to infer that all calcined physical mixtures exhibited dispersed Mo species and small Mo clusters together with a small quantity of bulk MoO 3 that remained after thermal treatment. Therefore, similar results obtained on both supports demonstrated that the thermal spreading of MoO 3 also occurred on silica and the same mechanism was observed as on alumina. However, the Mo dispersion and some Mo species were different on the supports that can be attributed to the different surface properties of silica and alumina.

The Journal of Physical Chemistry B, 2000

The thermal spreading of MoO 3 onto silica was studied in comparison with the thermal spreading o... more The thermal spreading of MoO 3 onto silica was studied in comparison with the thermal spreading onto alumina using different characterization techniques. X-ray diffraction results showed that MoO 3 crystals were transformed into Mo amorphous species on alumina and silica supports by thermal treatment. Laser Raman spectroscopy results also evidenced the transformation of MoO 3 bulk into small Mo clusters and/or dispersed Mo species, which are highly distorted, interacting with alumina and silica supports. X-ray photoelectron spectroscopy and diffuse reflection spectroscopy (DRS) results gave better evidence of the presence of higher amounts of Mo species at the surface of both supports when compared with the respective physical mixtures. Fourier transform infrared spectroscopy also provided good evidence that a surface reaction may have occurred between MoO 3 and hydroxyl groups on both supports. It was possible to infer that the original MoO 3 crystal lattices were destroyed by reacting with the support surface as a result of thermal treatment and transformed into small Mo clusters, dispersed Mo species, or both. Moreover, Raman spectroscopy showed nicely that the interaction of Mo species was higher on alumina than silica surfaces. Raman and DRS also provide insight into Mo species dispersion on alumina and silica samples, whereas DRS provided better evidence of the presence of dispersed Mo species on these supports. It was possible to infer that all calcined physical mixtures exhibited dispersed Mo species and small Mo clusters together with a small quantity of bulk MoO 3 that remained after thermal treatment. Therefore, similar results obtained on both supports demonstrated that the thermal spreading of MoO 3 also occurred on silica and the same mechanism was observed as on alumina. However, the Mo dispersion and some Mo species were different on the supports that can be attributed to the different surface properties of silica and alumina.

Química Nova, 2004

Recebido em 17/3/03; aceito em 2/9/03 THE POLLUTION FROM DIESEL ENGINES-THE PARTICULATE MATTER CU... more Recebido em 17/3/03; aceito em 2/9/03 THE POLLUTION FROM DIESEL ENGINES-THE PARTICULATE MATTER CURRENT EXPERIENCES AND FUTURE NEEDS. The exhaust emissions of vehicles greatly contribute to environmental pollution. Diesel engines are extremely fuelefficient. However, the exhaust compounds emitted by diesel engines are both a health hazard and a nuisance to the public. This paper gives an overview of the emission control of particulates from diesel exhaust compounds. The worldwide emission standards are summarized. Possible devices for reducing diesel pollutants are discussed. It is clear that after-treatment devices are necessary. Catalytic converters that collect particulates from diesel exhaust and promote the catalytic burn-off are examined. Finally, recent trends in diesel particulate emission control by novel catalysts are presented.

Applied Surface Science, 2007

This study focused on preparation of tungsten oxide supported on zirconia by thermal spreading. T... more This study focused on preparation of tungsten oxide supported on zirconia by thermal spreading. The prepared samples were characterized by infrared spectroscopy, UV–vis diffuse reflection spectroscopy, X-ray diffraction, and also by methanol dehydration reaction. It was observed that isolated octahedral tungsten dispersed species and dispersed polytungstate were formed on zirconia surface, although some WO3 that remained after the thermal treatment

Applied Surface Science, 2002

Mo/SiO 2 and Mo/TiO 2 catalysts with three different molybdenum contents were prepared using non-... more Mo/SiO 2 and Mo/TiO 2 catalysts with three different molybdenum contents were prepared using non-porous supports and the thermal spreading method for the combustion of a particulate material (PM). The results of scanning electron microscopy (SEM) and N 2 adsorption/desorption techniques showed that the thermal spreading preparation method does not induce relevant textural changes on the supports. X-ray diffraction (XRD) results showed the occurrence of thermal spreading of MoO 3 onto silica and titania supports. Diffuse reflection spectroscopy (DRS) results provided clear evidence of different Mo species on these systems: highly dispersed species on the silica catalysts and polymolybdates on the titania catalysts. It may be inferred that when prepared by the thermal spreading method the nature of the support determines the kind of molybdenum species formed in these catalysts, irrespective of the Mo content. The reactive data were evaluated by differential scanning calorimetry (DSC), using a physical mixture of PM and the catalysts. The silica-supported catalysts showed higher reactivity for PM combustion than the titania-supported ones, being the most active the systems with the Mo monolayer. The results suggested that the dispersed species are far more active than the polymolybdates or MoO 3 itself.

Applied Catalysis A: General, 2006

The effect of sodium addition on the physicochemical properties of Mo/SnO 2 catalysts was studied... more The effect of sodium addition on the physicochemical properties of Mo/SnO 2 catalysts was studied. The catalysts were prepared by precipitation using aqueous solution of ammonium heptamolybdate in two Mo amounts (5.5 and 14.5% (wt)); afterwards they were impregnated with sodium ion (1% wt). The solids were characterized by nitrogen adsorption, X-ray diffraction, UV-vis diffuse reflectance spectroscopy, FTIR, and FTIR of adsorbed pyridine in order to evaluate changes on Mo species and textural and acidic properties. The results have shown that Mo/SnO 2 samples present dispersed isolated tetrahedral Mo species. The addition of sodium ion to Mo/SnO 2 catalysts promotes the transformation of dispersed isolated tetrahedral Mo species into dispersed two-dimensional polymolybdates and also causes changes in textural and acidic properties of these Mo/SnO 2 catalysts.

ABSTRACT The exhaust emissions of vehicles greatly contribute to environmental pollution. Diesel ... more ABSTRACT The exhaust emissions of vehicles greatly contribute to environmental pollution. Diesel engines are extremely fuel-efficient. However, the exhaust compounds emitted by diesel engines are both a health hazard and a nuisance to the public. This paper gives an overview of the emission control of particulates from diesel exhaust compounds. The worldwide emission standards are summarized. Possible devices for reducing diesel pollutants are discussed. It is clear that after-treatment devices are necessary. Catalytic converters that collect particulates from diesel exhaust and promote the catalytic burn-off are examined. Finally, recent trends in diesel particulate emission control by novel catalysts are presented.

Applied Catalysis B Environmental, 2008

The Journal of Physical Chemistry B, 2000

Química Nova, 2004

Recebido em 17/3/03; aceito em 2/9/03 THE POLLUTION FROM DIESEL ENGINES -THE PARTICULATE MATTER C... more Recebido em 17/3/03; aceito em 2/9/03 THE POLLUTION FROM DIESEL ENGINES -THE PARTICULATE MATTER CURRENT EXPERIENCES AND FUTURE NEEDS. The exhaust emissions of vehicles greatly contribute to environmental pollution. Diesel engines are extremely fuelefficient. However, the exhaust compounds emitted by diesel engines are both a health hazard and a nuisance to the public. This paper gives an overview of the emission control of particulates from diesel exhaust compounds. The worldwide emission standards are summarized. Possible devices for reducing diesel pollutants are discussed. It is clear that after-treatment devices are necessary. Catalytic converters that collect particulates from diesel exhaust and promote the catalytic burn-off are examined. Finally, recent trends in diesel particulate emission control by novel catalysts are presented.

Journal of Catalysis, 2004

We studied the reactivity of Mo/Al2O3 and V/Al2O3 catalysts for soot combustion, and the influenc... more We studied the reactivity of Mo/Al2O3 and V/Al2O3 catalysts for soot combustion, and the influence of their volatile (MoO3 and V2O5) and superficial dispersed species. Differential exploratory calorimetry and temperature-programmed oxidation (TPO) experiments with catalyst and soot mixtures showed that lower combustion temperatures were obtained with increasing loading of Mo or V. TPO experiments performed on separate layers of soot

Catalysis Communications, 2005

Applied Surface Science, 2007

This study focused on preparation of tungsten oxide supported on zirconia by thermal spreading. T... more This study focused on preparation of tungsten oxide supported on zirconia by thermal spreading. The prepared samples were characterized by infrared spectroscopy, UV–vis diffuse reflection spectroscopy, X-ray diffraction, and also by methanol dehydration reaction. It was observed that isolated octahedral tungsten dispersed species and dispersed polytungstate were formed on zirconia surface, although some WO3 that remained after the thermal treatment

Applied Surface Science, 2002

Mo/Si02 and Mo/Ti02 catalysts with three different molybdenum contents were prepared using non-po... more Mo/Si02 and Mo/Ti02 catalysts with three different molybdenum contents were prepared using non-porous supports and the thermal spreading method for the combustion of a particulate material (PM). The results of scanning electron microscopy (SEM) and N2 adsorption/desorption techniques showed that the thermal spreading preparation method does not induce relevant textural changes on the supports. X-ray diffraction (XRD) results showed the occurrence of thermal spreading of Mo03 onto silica and titania supports. Diffuse reflection spectroscopy (DRS) results provided clear evidence of different Mo species on these systems: highly dispersed species on the silica catalysts and polymolybdates on the titania catalysts. It may be inferred that when prepared by the thermal spreading method the nature of the support determincs the kind of molybdenum species formed in these catalysts, irrespective of the Mo content. The reactive data were evaluated by differential scanning calorimetry (DSe), using a physical mixture of PM and the catalysts. The silica-supported catalysts showed higher reactivity for PM combustion than the titania-supported ones, being the most active the systems with the Mo monolayer. The results suggested that the dispersed species are far more active than the polvmolybdates or Mo03 itself.

Applied Catalysis B: Environmental, 2008

Applied Catalysis A: General, 2006

The effect of sodium addition on the physicochemical properties of Mo/SnO 2 catalysts was studied... more The effect of sodium addition on the physicochemical properties of Mo/SnO 2 catalysts was studied. The catalysts were prepared by precipitation using aqueous solution of ammonium heptamolybdate in two Mo amounts (5.5 and 14.5% (wt)); afterwards they were impregnated with sodium ion (1% wt). The solids were characterized by nitrogen adsorption, X-ray diffraction, UV-vis diffuse reflectance spectroscopy, FTIR, and FTIR of adsorbed pyridine in order to evaluate changes on Mo species and textural and acidic properties. The results have shown that Mo/SnO 2 samples present dispersed isolated tetrahedral Mo species. The addition of sodium ion to Mo/SnO 2 catalysts promotes the transformation of dispersed isolated tetrahedral Mo species into dispersed two-dimensional polymolybdates and also causes changes in textural and acidic properties of these Mo/SnO 2 catalysts. #

British Ceramic Transactions, 1999

ABSTRACT The thermal stability of two commercial aluminas before and after dry impregnation treat... more ABSTRACT The thermal stability of two commercial aluminas before and after dry impregnation treatment was studied. One alumina was mesoporous and the other, a spherical material, was non-porous. Characterisation of these materials before and after thermal treatment was performed by XRD, SEM, and measurements of surface area and pore size distribution. Spherical alumina was found to exhibit greater thermal stability when compared with the conventional porous alumina. The stability of spherical alumina was reduced by dry impregnation. It was shown that the morphology of these materials plays an important role in their thermal stability.

British Ceramic Transactions, 1999

ABSTRACT The effect of additions of lanthanide ions (La and Ce) and of the cerium ion concentrati... more ABSTRACT The effect of additions of lanthanide ions (La and Ce) and of the cerium ion concentration on the thermal stability of a ? alumina was studied. Ln//Al2O3 compositions were treated at high temperature (1100°C) for different calcination times (1, 5, and 24 h) and the resultant materials characterised by X-ray diffraction (XRD), BET surface area, X-ray photoelectron spectroscopy (XPS), and temperature programmed reduction (TPR). It was observed that cerium ion stabilisation was dependent on concentration with 6 wt-%Ce being the most efficient. It is suggested that the stabilisation is due to occupation of vacant cation sites in the spinel structure by Ce3+, and that stabilisation breaks down on prolonged exposure at high temperatures because of oxidation of Ce3+ to CeO2 . The same stabilisation mechanism is considered to operate for the lanthanum ion but, unlike Ce3+, it is not subject to oxidation, making La a more effective stabiliser than Ce.

1-INTRODUÇÃO Os motores de combustão interna utilizados atualmente dividem-se em duas classes: os... more 1-INTRODUÇÃO Os motores de combustão interna utilizados atualmente dividem-se em duas classes: os do tipo Diesel e Otto. Os motores a Diesel são utilizados, de modo geral, em veículos de transporte coletivo, de carga, e, em alguns países, também em veículos leves. Esses motores apresentam algumas características especificas: utilizam combustíveis de alto peso molecular (C9-C20), em alguns casos com elevado teor de S; operam em condições pobres, ou seja, com excesso de O 2 , apresentando temperaturas médias de exaustão em torno de 300 o C. Essas especificidades resultam num perfil de emissões bastante diferenciado em relação ao gerado pelos motores de ciclo Otto, conforme pode-se observar na tabela 1 1 . Tabela 1-Emissões típicas de processos de combustão Otto e Diesel (g /Kg de combustível). Os dados listados são valores médios que dependem fortemente do processo de combustão, da operação do veículo e da especificação do combustível 1 .