As the size of modern transistors is continuously scaled down, challenges rise in almost every component of a silicon device. Formation of ultra shallow junction (USJ) with high activation level is particularly important for suppressing short channel effects. However, the formation of low resistance USJ is made difficult by dopant Transient Enhanced Diffusion (TED) and clustering-induced deactivation. In this work, we proposed a novel point defect engineering solution to address the arsenic TED challenge. By overlapping arsenic doped region with silicon interstitials and vacancies, we observed enhanced and retarded arsenic diffusion upon anneal, respectively. We explain this phenomenon by arsenic interstitial diffusion mechanism. In addition, we implemented this interstitial-based mechanism into a kinetic Monte Carlo (kMC) simulator. The key role of interstitials in arsenic TED is confirmed. And we demonstrated that the simulator has an improved prediction capability for arsenic TED and deactivation. As a long time unsolved process challenge, arsenic segregation at SiO₂/Si interface was investigated using density functional theory (DFT) calculation. The segregation-induced arsenic dose loss not only increases resistance but also may induce interface states. We identified three arsenic complex configurations, [chemical formula] , [chemical formula] and [chemical formula], which are highly stabilized at SiO₂/Si interface due to the unique local bonding environments. Therefore, they could contribute to arsenic segregation as both initial stage precursors and dopant trapping sites. Our calculation indicates that arsenic atoms trapped in such interface complexes are electrically inactive. Finally, the formation and evolution dynamics of these interface arsenic-defect complexes are discussed and kMC models are constructed to describe the segregation effects. A potential problem for the p-type USJ formation is the recently found transient fast boron diffusion during solid phase epitaxial regrowth process. Using DFT calculations and molecular dynamics simulation, we identified an interstitial-based mechanism of fast boron diffusion in amorphous silicon. The activation energy for this diffusion mechanism is in good agreement with experimental results. In addition, this mechanism is consistent with the experimentally reported transient and concentration-dependent features of boron diffusion in amorphous silicon. / text

Deactivation of catalysts were investigated focusing on three industrial processes: 1) Selective Catalytic Reduction (SCR) for abatement of NOx from biomass combustion using V2O5-WO3 /TiO2 catalysts; 2) Catalytic oxidation of volatile organic compounds (VOC) from printing industries using a Pt/γ-Al2O3 catalyst; and 3) Ni and Pt/Rh catalysts used in steam reforming reaction of bio-syngas obtained from biomass gasification. The aim has been to simulate industrial conditions in laboratory experiments in order to comprehend influence of compounds affecting catalysts performance. Typical catalyst lifetimes in industrial processes are several years, which are a challenge when accelerating deactivation in laboratory scale experiments where possible exposure times are few hours or days. Catalysts can be introduced to deactivating compounds through different routes. The first method examined was gaseous exposure, which was applied to deactivate VOC oxidation catalyst through exposure of gaseous hexamethyldisiloxane. The second method involved wet impregnation and was used for impregnation of SCR catalyst with salt solutions. The third method was based on exposure and deposition of size selected particles of deactivating substances on the catalyst. The latter device was developed during this work. It was applied to monolithic SCR catalysts as well as to pellet catalysts intended for steam reforming of biomass gasification syngas. Deactivated SCR catalyst samples by size selected exposure method were verified and compared with SCR catalysts used in a commercial biomass boiler for 6 500 h. Evaluations of fresh and deactivated samples were investigated using BET surface area; chemisorption and temperature programmed desorption (TPD); surface morphology using Scanning Electron Microscopy (SEM) and poison penetration profile through SEM with an Electron Micro Probe Analyser (EMPA) also equipped with a energy dispersive spectrometer (EDS); chemical analysis of accumulation of exposed compounds by Inductively Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES); and influence on catalyst performance. The size selected generated particles of deactivating substances were characterized with respect to mean diameter and number size distribution through Scanning Mobility Particle Sizer (SMPS) and mass size distribution applying an Electric Low Pressure Impactor (ELPI). Results from catalyst characterization methods were useful tools in evaluation of catalyst deactivation routes. Understanding deactivation processes and impact on catalyst performance is vital for further optimization of catalysts with respect to performance and lifetime. Further research in this field can provide more resistant catalysts for application in industry leading to higher commercial benefits and further application of environmental catalysts in thermo-chemical conversion of biomass.

La nitruration gazeuse de pièces en acier de construction est un procédé thermochimique permettant de générer un gradient de propriétés mécaniques à la surface des pièces par diffusion d’atomes d’azote. Les pièces traitées présentent un fort gradient de contraintes résiduelles et de dureté, optimales pour les applications de transmission de puissance utilisées dans les domaines de l’aéronautique ou l’automobile de compétition. L’enrichissement en azote est issu de la décomposition catalytique de l’ammoniac à la surface des pièces en acier. Cependant, le procédé reste extrêmement sensible à l’état de surface des pièces, qui inclue les caractéristiques géométriques, mécaniques mais également chimique, pouvant inhiber les propriétés de décomposition catalytique ainsi que la diffusion de l’azote. Cette désactivation peut avoir différentes origines, comme l’adsorption d’un élément poison de la réaction ou l’encrassement de la surface des pièces. Le phénomène de désactivation catalytique est étudié depuis quelques dizaines d’années. Cependant, peu de publications sont disponibles au sujet de son application dans le cas particulier de la nitruration d’aciers de construction. Comprendre l’inhibition de la nitruration permettrait d’améliorer la fiabilité industrielle de ce procédé. Afin de répondre à cette demande, une démarche expérimentale, basée sur des études de cas industriels et sur une étude bibliographique, a été mis en place. Une étude de pollution issue de résidus d’huile de coupe, de dépôt de carbone et d’empoisonnement au soufre est proposée. De plus, une démarche expérimentale visant à déterminer l’impact de pollutions mécaniques (état mécanique initial) sur le procédé de nitruration est proposée. Des analyses thermogravimétriques couplées à des observations et à la détermination des propriétés mécaniques générées par nitruration permettent d’approfondir la compréhension de ces phénomènes de désactivation des surfaces nitrurées. Suite à cette étude, la capacité d’activation de surface de trois éléments de préparation in-situ, que sont l’oxydation, l’urée et le chlorure d’ammonium, a été déterminé sur les pollutions précédemment étudiées. / The gaseous nitriding of steel parts is a thermochemical process that generates a mechanical properties gradient at the surface by nitrogen diffusion. The treated parts indicate an important residual stress and hardness gradient, optimized for power transmission applications used in aeronautics and motor sports. The increase of nitrogen concentration is due to the catalytic decomposition of ammonia at the iron-based surface of parts. However, the process is extremely sensitive to the surface state of parts that includes geometrical, mechanical but also chemical properties. This state can inhibit the catalytic decomposition properties and the nitrogen diffusion. This deactivation can have different origins, such as the adsorption of a poisoning element or the fouling of the surface of the parts. The catalytic deactivation phenomenon is studied since decades. However, few publications are available in the case of its application in the specific case of the gaseous nitriding of steel. A better understanding of the nitriding inhibition would improve the reliability of this industrial process. In order to answer this demand, an experimental approach, based on industrial case and bibliographical study, is proposed. A pollution study obtained from machining oil residues, carbon deposit, or sulfur poisoning is proposed. Moreover, an experimental approach, aiming to determine the impact of initial mechanical properties on the nitriding process is proposed. Thermogravimetric analyses coupled to observations and mechanical properties analyses enable to develop the knowledge of the nitrided surfaces deactivation processes. After this study, the surface activation capacity of three in-situ processes, namely oxidation, urea and ammonium chloride, over the previously studied contaminations is determined.

The transportation sector continues to be a primary source of greenhouse gas (GHG) emissions, contributing more than any other sector in the United States in 2017. Medium-duty and heavy-duty trucks trail only passenger cars as the largest GHG contributor in this sector [1]. The intense operating requirements of these vehicles create a reliance on the diesel engine that is projected to last for many decades. Therefore, it is vital that the efficiency and environmental sustainability of diesel engines continue to be advanced.<br><br>Cylinder deactivation (CDA) is a promising technology to improve diesel engine fuel efficiency and aftertreatment thermal management for emissions reduction. This work presents original experimental results demonstrating fuel efficiency improvements of CDA implemented on a modern engine at idle operating conditions through testing of various CDA configurations. Idle calibration optimizations result in up to 28% fuel consumption reduction at steady-state unloaded idle operation and 0.7% fuel consumption reduction over HD-FTP drive cycles at equivalent emissions levels. The low-load thermal management performance of CDA is also investigated through creep and extended idle transient cycles, during which CDA is shown to reduce fuel consumption by up to 40% with similar thermal management performance and reduced NOx and soot emissions. <br><br>Variants of CDA implementation are explored through an experimental comparison of deactivation strategies. The effort described here compares charge trapping strategies through examination of in-cylinder pressures following deactivation because: (1) choice of trapping strategy dictates the in-cylinder pressure characteristics of the deactivated cylinders, and (2) deactivated cylinders can affect torque, oil consumption, and emissions upon reactivation. Results discussed here suggest no significant differences between the strategies. As an example, the in-cylinder pressures of both trapping strategies are shown to converge as quickly as 0.8 seconds after deactivation.<br><br>Finally, the NVH effects of CDA are characterized through studies of torsional vibration, linear vibration, and acoustics. CDA causes frequency content at reduced frequencies compared to conventional operation, which has effects on all aspects of NVH. This creates possible constraints on achievable fuel efficiency and thermal management performance by restricting CDA usage. An alternate form of CDA, dynamic cylinder activation (DCA), is explored as a possible option of avoiding undesirable frequency output while maintaining the desired engine performance. <br>

Em termos ambientais, os catalisadores automotivos se destacam pelos resultados altamente significativos alcançados após seu uso obrigatório em veículos leves. No entanto, as condições térmicas em que eles operam podem levar a um processo de perda de atividade significativa, após certo tempo de operação. Dentro desse contexto, este trabalho estudou o efeito da temperatura na desativação térmica de catalisadores automotivo modelo. Foram preparados catalisadores baseados em óxido misto de cério e zircônio na proporção 50% em mol de cério e zircônio (CZ). A partir dele foram produzidos os catalisadores Pd-CZ e Pd-CZ-LaAl. O catalisador Pd-CZ foi produzido pela impregnação do CZ com Pd na concentração de 0,5% m/m de CZ. O catalisador Pd-CZ-LaAl foi produzido a partir de uma mistura física do Pd-CZ com o suporte LaAl (alumina dopada com La na concentração de 1,9 % m/m de Al2O3), seguida de calcinação a 500&#730;C. Foram realizados envelhecimentos a 900C e 1200C em mufla com atmosfera oxidante por 12 e 36h. Os catalisadores foram caracterizados por um conjunto de técnicas físico-químicas. Foram realizadas análises de fisissorção de N2 para a medição da área específica e o estudo da evolução do diâmetro e volume de poros das amostras novas e envelhecidas. Análises de difração de raios X (DRX) foram feitas de forma a acompanhar possíveis transições de fases após o envelhecimento das amostras. Foi realizada análise química para validar a composição das amostras e ensaios de análise térmica para o catalisador CZ visando identificar a temperatura onde ocorre o fenômeno de segregação de fases. Realizaram-se ensaios de redução a temperatura programada (RTP) visando quantificar o consumo de hidrogênio e associá-lo à evolução da redutibilidade das amostras após o envelhecimento térmico. Finalmente, a avaliação catalítica foi realizada com base nas reações de oxidação do CO e do propano e de redução do NO pelo CO, através da obtenção de curvas de lightoff. As análises de DRX mostraram que o envelhecimento a 900C ocasionou alterações de fases da alumina, mas não foi verificada segregação de fases no CZ. Já a 1200C observou-se a referida segregação de fases, que coincide com a drástica queda na área específica das amostras, em alguns casos observando-se o colapso das propriedades texturais do catalisador. As análises de RTP mostraram que, em determinadas condições, o envelhecimento térmico promove a redutibilidade do sistema CZ e a introdução de Pd torna o catalisador mais facilmente redutível o que é evidenciado pelo deslocamento dos picos de redução para temperaturas mais baixas em comparação ao CZ puro. Os testes catalíticos mostraram que a introdução do Pd é um fator fundamental para a conversão do propano. Os catalisadores contendo Pd também converteram melhor o CO. Para os catalisadores envelhecidos a 1200C, o único resultado positivo foi no caso do Pd-CZ-LaAl que apesar deste tratamento térmico, ainda converteu o CO, propano e NO. Desta forma o catalisador Pd-CZ-LaAl apresentou resultados mais satisfatórios e isto evidencia que a mistura com LaAl melhora o desempenho e a estabilidade térmica do catalisador em altas temperaturas (acima de 300C). / The automotive catalyst has significantly contributed to environmental protection since its use in light vehicles became mandatory. However, it is constantly subjected to severe thermal conditions which may cause a significant decrease in catalytic activity. Thus, this work studied the effect of temperature in the thermal deactivation of model automotive catalysts. All the catalysts were prepared with cerium and zirconium mixed oxides containing 50 mol % of cerium and zirconium (CZ). The Pd-CZ and Pd-CZ-LaAl catalysts were prepared by impregnation of CZ with Pd to obtain 0.5% of the noble metal in the catalyst. The Pd-CZ-LaAl catalyst was prepared from a physical mixture of Pd-CZ and the LaAl support (alumina doped with 1,9 wt% of La), followed by calcination at 500C. The samples were subjected to aging at 900C and 1200C in oxidizing atmosphere for 12 and 36h. The catalysts were characterized by a set of physicochemical techniques. Analyses of N2 physisorption for the determination of the surface area and the study of the evolution of the pore volume and diameter for fresh and aged samples were carried out. X-ray diffraction (XRD) experiments were done to follow possible phase transitions after aging. Chemical analysis was carried out to validate the composition of the samples. The CZ samples were subjected to thermal analysis to identify the temperature in which phase segregation occurs. Temperature programmed reduction (TPR) experiments were carried out to quantify the amount of hydrogen necessary to the catalyst reduction and to associate it to the samples reducibility after thermal aging. Finally, catalytic evaluation was based on the lightoff curves obtained for CO and C3H8 oxidation and reduction of NO by CO. XRD analyses evidenced alumina phase transitions after aging at 900C, but not for CZ. However, after aging at 1200C, there was phase segregation, which is consistent with the drastic decrease in surface area. Some catalysts presented collapse of the textural properties. TPR analyses showed that, in determined conditions, thermal aging promotes the reduction of CZ system and the introduction of Pd promotes the catalyst reduction. This is evidenced by the shift of the reduction peaks to lower temperatures observed in PdCZ profiles compared to pure CZ profiles. Catalytic tests showed that Pd introduction is crucial for C3H8 conversion. Pd catalysts also achieved higher CO conversions. For the catalysts aged at 1200C, the only positive result was obtained with the Pd-CZ-LaAl catalyst, which still converted CO, C3H8 and NO. Thus, the Pd-CZ-LaAL catalysts presented the best results, indicating that the impregnation of Pd-CZ with LaAl improves the performance and the stability of the catalyst at high temperatures (above 300C).

CO, volatile organic compounds, and polyaromatics areubiquitous air pollutants that give rise to deleterious healthand environmental effects. Such compounds are emitted, forinstance, by the combustion of wood, particularly fromsmall-scale heating appliances. Total catalytic oxidation isconsidered to be an effective approach in controlling theseemissions, however, some problems remain such as thenon-availability of catalysts with low-cost, high activity andstability in prevailing conditions. Hence, this thesis aims atthe development of oxidation catalysts and improvedunderstanding of their behaviour. The catalytic activity was evaluated for the oxidation of amixture of CO, naphthalene (or ethylene), and methane inpresence of carbon dioxide, water, oxygen and nitrogen. Variouscharacterisation techniques, including Temperature-ProgrammedReduction and Oxidation, BET-Surface Area Analysis, X-RayDiffraction, X-Ray Photoelectron Spectroscopy, RamanSpectroscopy and Scanning and Transmission Electron Microscopywere used. In the first part of this thesis, catalysts based on metaloxides (MnOx, CuO) and/or a low amount of noble metals (Pt, Pd)supported on alumina washcoat were selected. It was shown thatPt and Pd possessed a superior catalytic activity to that ofCuO and MnOxfor the oxidation of CO, C10H8and C2H4, while for the oxidation of CH4, CuO was largely more active than noble metals,and MnOxas active as Pd and Pt. Some mixed metaloxide-noble metal catalysts showed decreased activity comparedto that of noble metals, however, a higher noble metal loadingor a successive impregnation with noble metals led to positivesynergetic effects for oxidation. Deactivation of the catalysts by thermal damage and sulphurpoisoning is addressed in the second part of the dissertation.An alumina washcoat was found to be well anchored to themetallic support after thermal treatment at 900°C due tothe growth of alumina whiskers. The sintering of the washcoatwas accelerated after high temperature treatments in thepresence of metal catalysts. In addition, alumina was found toreact with CuO, particularly in presence of noble metals at900°C, to form inactive CuAl2O4. However, MnOxcatalyst benefits from the more active Mn3O4phase at high temperature, which makes it asuitable active catalyst for the difficult oxidation of CH4. Pt sintering was delayed when mixed with CuO,thus giving more thermally resistant catalyst. The mixed metaloxide-noble metal catalysts showed higher activity afterpre-sulphation of the catalysts with 1000 ppm SO2in air at 600°C or during activitymeasurement in presence of 20 ppm SO2in the gas mixture, compared to single componentcatalysts. In some cases, the activities of the mixed catalystswere promoted by pre-sulphation due to the presence of sulphatespecies. Thermal stabilisation of the catalytic componentsand thealumina by promotion of La in the washcoat is discussed in thethird section. The stabilising effect of La at high temperatureis also compared to that of Ce added in the catalysts for otherpurposes. Due to its better dispersion, La contributed to thethermal stabilisation of the alumina washcoat and its activecomponents to a higher extent than Ce did. La provided a betterdispersion and a higher saturation of metal oxides in thealumina support, and at the same time stabilised the activityof the catalysts by preventing undesirable solid-phasereactions between metal oxide and alumina. In addition, La wasfound to enhance the dispersion and the oxygen mobility of CeO2. Cu-Ce interactions were found to promotesubstantially the CO oxidation due to an increase of thestability and reducibility of Cu species. Synergetic effectswere also found between Ce and La in the washcoat of CuO-Ptcatalyst, which facilitated the formation of reduced Pt and CeO2, thus enhancing significantly the catalyticactivity compared to that of a Pt only catalyst. The last part was an attempt to demonstrate the potential ofa catalyst equipped with a pre-heating device in a full-scalewood-fired boiler for minimising the high emissions during thestart-up phase. During the first ten minutes of the burningcycle a significant reduction of CO and hydrocarbons wereachieved. <b>Keywords</b>: wood combustion, catalysts, total oxidation,manganese, copper, platinum, palladium, lanthanum, cerium, CO,VOC, methane, deactivation, thermal stability, sulphurdioxide.

<p>CO, volatile organic compounds, and polyaromatics areubiquitous air pollutants that give rise to deleterious healthand environmental effects. Such compounds are emitted, forinstance, by the combustion of wood, particularly fromsmall-scale heating appliances. Total catalytic oxidation isconsidered to be an effective approach in controlling theseemissions, however, some problems remain such as thenon-availability of catalysts with low-cost, high activity andstability in prevailing conditions. Hence, this thesis aims atthe development of oxidation catalysts and improvedunderstanding of their behaviour.</p><p>The catalytic activity was evaluated for the oxidation of amixture of CO, naphthalene (or ethylene), and methane inpresence of carbon dioxide, water, oxygen and nitrogen. Variouscharacterisation techniques, including Temperature-ProgrammedReduction and Oxidation, BET-Surface Area Analysis, X-RayDiffraction, X-Ray Photoelectron Spectroscopy, RamanSpectroscopy and Scanning and Transmission Electron Microscopywere used.</p><p>In the first part of this thesis, catalysts based on metaloxides (MnO_x, CuO) and/or a low amount of noble metals (Pt, Pd)supported on alumina washcoat were selected. It was shown thatPt and Pd possessed a superior catalytic activity to that ofCuO and MnO_xfor the oxidation of CO, C₁₀H₈and C₂H₄, while for the oxidation of CH₄, CuO was largely more active than noble metals,and MnO_xas active as Pd and Pt. Some mixed metaloxide-noble metal catalysts showed decreased activity comparedto that of noble metals, however, a higher noble metal loadingor a successive impregnation with noble metals led to positivesynergetic effects for oxidation.</p><p>Deactivation of the catalysts by thermal damage and sulphurpoisoning is addressed in the second part of the dissertation.An alumina washcoat was found to be well anchored to themetallic support after thermal treatment at 900°C due tothe growth of alumina whiskers. The sintering of the washcoatwas accelerated after high temperature treatments in thepresence of metal catalysts. In addition, alumina was found toreact with CuO, particularly in presence of noble metals at900°C, to form inactive CuAl₂O₄. However, MnO_xcatalyst benefits from the more active Mn₃O₄phase at high temperature, which makes it asuitable active catalyst for the difficult oxidation of CH₄. Pt sintering was delayed when mixed with CuO,thus giving more thermally resistant catalyst. The mixed metaloxide-noble metal catalysts showed higher activity afterpre-sulphation of the catalysts with 1000 ppm SO₂in air at 600°C or during activitymeasurement in presence of 20 ppm SO₂in the gas mixture, compared to single componentcatalysts. In some cases, the activities of the mixed catalystswere promoted by pre-sulphation due to the presence of sulphatespecies.</p><p>Thermal stabilisation of the catalytic componentsand thealumina by promotion of La in the washcoat is discussed in thethird section. The stabilising effect of La at high temperatureis also compared to that of Ce added in the catalysts for otherpurposes. Due to its better dispersion, La contributed to thethermal stabilisation of the alumina washcoat and its activecomponents to a higher extent than Ce did. La provided a betterdispersion and a higher saturation of metal oxides in thealumina support, and at the same time stabilised the activityof the catalysts by preventing undesirable solid-phasereactions between metal oxide and alumina. In addition, La wasfound to enhance the dispersion and the oxygen mobility of CeO₂. Cu-Ce interactions were found to promotesubstantially the CO oxidation due to an increase of thestability and reducibility of Cu species. Synergetic effectswere also found between Ce and La in the washcoat of CuO-Ptcatalyst, which facilitated the formation of reduced Pt and CeO₂, thus enhancing significantly the catalyticactivity compared to that of a Pt only catalyst.</p><p>The last part was an attempt to demonstrate the potential ofa catalyst equipped with a pre-heating device in a full-scalewood-fired boiler for minimising the high emissions during thestart-up phase. During the first ten minutes of the burningcycle a significant reduction of CO and hydrocarbons wereachieved.</p><p><b>Keywords</b>: wood combustion, catalysts, total oxidation,manganese, copper, platinum, palladium, lanthanum, cerium, CO,VOC, methane, deactivation, thermal stability, sulphurdioxide.</p>

Grätzel cells or Dye-Sensitized Solar Cells (DSSCs) are considered one of the most promising methods to convert the sun's energy into electricity due to their low cost and simple technology of production. The Grätzel cell is based on a photosensitizer adsorbed on a low band gap semiconductor. The photosensitizer can be a metal complex or an organic dye. Organic dyes can be produced on a large scale resulting in cheaper dyes than complexes based on rare elements. However, the performance of Grätzel cells based on metal-free, organic dyes is not high enough yet. The dye's performance depends primarily on the electron dynamics. The electron dynamics in Grätzel cells includes electron injection, recombination, and regeneration. Different deactivation processes affect the electron dynamics and the cells’ performance. In this thesis, the electron dynamics was explored by various time-resolved spectroscopic techniques, namely time-correlated single photon counting, streak camera, and femtosecond transient absorption. Using these techniques, new deactivation processes for organic dyes used in DSSCs were uncovered. These processes include photoisomerization, and quenching through complexation with the electrolyte. These deactivation processes affect the performance of organic dyes in Grätzel cells, and should be avoided. For instance, the photoisomerization can compete with the electron injection and produce isomers with unknown performance. Photoisomerization as a general phenomenon in DSSC dyes has not been shown before, but is shown to occur in several organic dyes, among them D149, D102, L0 and L0Br. In addition, D149 forms ground state complexes with the standard iodide/triiodide electrolyte, which directly affect the electron dynamics on TiO2. Also, new dyes were designed with the aim of using ferrocene(s) as intramolecular regenerators, and their dynamics was studied by transient absorption. This thesis provides deeper insights into some deactivation processes of organic dyes used in DSSCs. New rules for the design of organic dyes, based on these insights, can further improve the efficiency of DSSCs.

The objective of the thesis was to analyze the ageing processes and the modifications of an Ir/CeO2catalyst for steam reforming of ethanol. Over a model Ir/CeO2 catalyst, the initial and fast deactivationwas ascribed to ceria surface restructuring and the build-up of intermediates monolayer (acetate,carbonate and hydroxyl groups). In parallel, a progressive and slow deactivation was found to come fromthe structural changes at the ceria/Ir interface linked to Ir sintering and ceria restructuring. Theencapsulating carbon, coming from C2 intermediates polymerization, did not seem too detrimental to theactivity in the investigated operating conditions. By doping ceria with PrOx, the oxygen storage capacityand thermal stability were greatly promoted, resulting in the enhanced activity and stability. The Ir/CeO2catalyst was then modified by changing the shape of ceria. It was found that the shape and therefore thestructure of ceria influenced the activity and stability significantly. A simplified modeling of theseprocesses has contributed to support the new proposals of this work.

Em termos ambientais, os catalisadores automotivos se destacam pelos resultados altamente significativos alcançados após seu uso obrigatório em veículos leves. No entanto, as condições térmicas em que eles operam podem levar a um processo de perda de atividade significativa, após certo tempo de operação. Dentro desse contexto, este trabalho estudou o efeito da temperatura na desativação térmica de catalisadores automotivo modelo. Foram preparados catalisadores baseados em óxido misto de cério e zircônio na proporção 50% em mol de cério e zircônio (CZ). A partir dele foram produzidos os catalisadores Pd-CZ e Pd-CZ-LaAl. O catalisador Pd-CZ foi produzido pela impregnação do CZ com Pd na concentração de 0,5% m/m de CZ. O catalisador Pd-CZ-LaAl foi produzido a partir de uma mistura física do Pd-CZ com o suporte LaAl (alumina dopada com La na concentração de 1,9 % m/m de Al2O3), seguida de calcinação a 500&#730;C. Foram realizados envelhecimentos a 900C e 1200C em mufla com atmosfera oxidante por 12 e 36h. Os catalisadores foram caracterizados por um conjunto de técnicas físico-químicas. Foram realizadas análises de fisissorção de N2 para a medição da área específica e o estudo da evolução do diâmetro e volume de poros das amostras novas e envelhecidas. Análises de difração de raios X (DRX) foram feitas de forma a acompanhar possíveis transições de fases após o envelhecimento das amostras. Foi realizada análise química para validar a composição das amostras e ensaios de análise térmica para o catalisador CZ visando identificar a temperatura onde ocorre o fenômeno de segregação de fases. Realizaram-se ensaios de redução a temperatura programada (RTP) visando quantificar o consumo de hidrogênio e associá-lo à evolução da redutibilidade das amostras após o envelhecimento térmico. Finalmente, a avaliação catalítica foi realizada com base nas reações de oxidação do CO e do propano e de redução do NO pelo CO, através da obtenção de curvas de lightoff. As análises de DRX mostraram que o envelhecimento a 900C ocasionou alterações de fases da alumina, mas não foi verificada segregação de fases no CZ. Já a 1200C observou-se a referida segregação de fases, que coincide com a drástica queda na área específica das amostras, em alguns casos observando-se o colapso das propriedades texturais do catalisador. As análises de RTP mostraram que, em determinadas condições, o envelhecimento térmico promove a redutibilidade do sistema CZ e a introdução de Pd torna o catalisador mais facilmente redutível o que é evidenciado pelo deslocamento dos picos de redução para temperaturas mais baixas em comparação ao CZ puro. Os testes catalíticos mostraram que a introdução do Pd é um fator fundamental para a conversão do propano. Os catalisadores contendo Pd também converteram melhor o CO. Para os catalisadores envelhecidos a 1200C, o único resultado positivo foi no caso do Pd-CZ-LaAl que apesar deste tratamento térmico, ainda converteu o CO, propano e NO. Desta forma o catalisador Pd-CZ-LaAl apresentou resultados mais satisfatórios e isto evidencia que a mistura com LaAl melhora o desempenho e a estabilidade térmica do catalisador em altas temperaturas (acima de 300C). / The automotive catalyst has significantly contributed to environmental protection since its use in light vehicles became mandatory. However, it is constantly subjected to severe thermal conditions which may cause a significant decrease in catalytic activity. Thus, this work studied the effect of temperature in the thermal deactivation of model automotive catalysts. All the catalysts were prepared with cerium and zirconium mixed oxides containing 50 mol % of cerium and zirconium (CZ). The Pd-CZ and Pd-CZ-LaAl catalysts were prepared by impregnation of CZ with Pd to obtain 0.5% of the noble metal in the catalyst. The Pd-CZ-LaAl catalyst was prepared from a physical mixture of Pd-CZ and the LaAl support (alumina doped with 1,9 wt% of La), followed by calcination at 500C. The samples were subjected to aging at 900C and 1200C in oxidizing atmosphere for 12 and 36h. The catalysts were characterized by a set of physicochemical techniques. Analyses of N2 physisorption for the determination of the surface area and the study of the evolution of the pore volume and diameter for fresh and aged samples were carried out. X-ray diffraction (XRD) experiments were done to follow possible phase transitions after aging. Chemical analysis was carried out to validate the composition of the samples. The CZ samples were subjected to thermal analysis to identify the temperature in which phase segregation occurs. Temperature programmed reduction (TPR) experiments were carried out to quantify the amount of hydrogen necessary to the catalyst reduction and to associate it to the samples reducibility after thermal aging. Finally, catalytic evaluation was based on the lightoff curves obtained for CO and C3H8 oxidation and reduction of NO by CO. XRD analyses evidenced alumina phase transitions after aging at 900C, but not for CZ. However, after aging at 1200C, there was phase segregation, which is consistent with the drastic decrease in surface area. Some catalysts presented collapse of the textural properties. TPR analyses showed that, in determined conditions, thermal aging promotes the reduction of CZ system and the introduction of Pd promotes the catalyst reduction. This is evidenced by the shift of the reduction peaks to lower temperatures observed in PdCZ profiles compared to pure CZ profiles. Catalytic tests showed that Pd introduction is crucial for C3H8 conversion. Pd catalysts also achieved higher CO conversions. For the catalysts aged at 1200C, the only positive result was obtained with the Pd-CZ-LaAl catalyst, which still converted CO, C3H8 and NO. Thus, the Pd-CZ-LaAL catalysts presented the best results, indicating that the impregnation of Pd-CZ with LaAl improves the performance and the stability of the catalyst at high temperatures (above 300C).