Nanotubos de carbeto de silício (SiC) foram preparados a partir da reação gás-sólido entre vapor de SiO e nanotubos de carbono. A fase ativa NiS2 suportada em nanotubos de SiC apresentou maior atividade catalítica e maior capacidade de armazenamento de enxofre na reação de oxidação seletiva do H2S a enxofre elementar, em reator do tipo trickle-bed, que a mesma fase ativa suportada em grãos de SiC. O melhor desempenho deste catalisador foi explicado pela ocorrência do efeito de confinamento no interior dos nanotubos de SiC; ou seja, pelo aumento artificial da pressão parcial de H2S no interior dos nanotubos em relação à mesma pressão fora dos nanotubos, resultando assim em um aumento na taxa da oxidação, uma vez que a velocidade de reação do H2S é de primeira ordem. O catalisador suportado em nanotubos de SiC apresentou elevada resistência ao carregamento de enxofre devido a uma peculiar evacuação do enxofre pelo vapor de água condensado, permitindo assim uma limpeza contínua dos sítios ativos. A maior capacidade de armazenamento do enxofre sólido no suporte a base de nanotubos de SiC foi atribuído ao seu maior volume específico disponível para o armazenamento do enxofre do que o do suporte a base de grãos de SiC.

Silicon carbide nanotubes were prepared via a gas-solid reaction between SiO vapor and carbon nanotubes. The NiS2 active phase on this support displayed both a high catalytic activity and high solid sulfur storage capacity in the trickle-bed selective oxidation of H2S into elemental sulfur as compared to the grain-based SiC catalyst. The hypothesis of a confinement effect inside the SiC nanotubes has been put forward to explain the catalytic results. An artificial increase in the H2S partial pressure inside the tubes when compared to the H2S partial pressure outside the tubes would lead to an increase in the oxidation rate, due to the first order reaction rate toward H2S. The SiC nanotube supported catalyst displayed very high resistance to the sulfur loading, due to a peculiar mode of sulfur evacuation by condensed steam which allows the continuous cleaning of the active site. The high solid sulfur storage capacity was due to a much larger void volume between each SiC nanotube available for the sulfur storage, than the void volume of SiC support with a grain size morphology.

As fases ativas NiS2 e Fe2O3 suportadas em beta-carbeto de silício (SiC) com média área específica mostraram alta atividade, seletividade e estabilidade na oxidação direta do H2S a enxofre elementar. Os catalisadores foram testados a temperaturas que variaram da temperatura ambiente, no caso do Ni em reator de leito gotejante, até temperaturas superiores à do ponto de orvalho do enxofre, no caso do Fe em reator de leito fixo. Para ambos os casos, foi proposta a formação de uma fase bastante ativa de oxisulfeto de Ni ou de Fe, formada pela oxidação do NiS2 e pela sulfuração do Fe2O3. A ausência de microporosidade no suporte contribuiu à alta seletividade do catalisador. A grande estabilidade ao carregamento de enxofre sólido, apresentada pelos catalisadores suportados em SiC em temperaturas inferiores a 100 ºC, foi explicada pela maneira especial da deposição do enxofre, a qual depende do papel da água presente na reação e do caráter heterogêneo (hidrofílico e hidrofóbico) da superfície do suporte.

The NiS2 and Fe2O3 active phases supported on medium surface area beta-silicon carbide (SiC) showed high activity, selectivity and stability in the direct oxidation of H2S into elemental sulfur. The catalysts were tested under a large range of temperatures, from room temperature using Ni (trickle-bed reactor) to over-dewpoint temperatures using Fe (fixed bed reactor). For both cases, the formation on stream of a Ni and Fe oxysulfide high active phase was proposed by oxidation of NiS2 and sulfidation of Fe2O3, respectively. The absence of microporosity in the support contributed to the high selectivity into sulfur. At low temperatures (below 100 ºC), the high stability of beta-SiC supported catalysts towards the solid sulfur loading was explained by a specific mode of sulfur deposition, involving the role of water in the feed and the heterogeneous nature of the SiC surface, being partly hydrophilic and hydrophobic.

Today satellites propulsion is based on the use of monopropellant and/or bipropellant chemical systems. The maneuvering of satellite is based on the hydrazine decomposition micropropulsors catalyzed by metallic iridium supported on g-alumina. This reaction is a surface reaction and is strongly exothermic and implies that the operation of the micropropulsor is controlled by the mass and heat diffusions. For this reason and for the fact that the propulsor operation is frequently in pulsed regime, the catalyst should support high pressure and temperature variations within a short time period. The performance and the durability of the commercial catalyst are jeopardized by the low thermal conductivity of the alumina. The low thermal conductivity of the alumina support restricts the heat diffusion and leads to the formation of hot spots on the catalyst surface causing the metal sintering and/or fractures of the support, resulting in loss of the activity and catalyst destruction. This work presents the synthesis and characterization of new carbon composite support for the active element iridium, in substitution of the commercial catalysts alumina based support. These supports are constituted of carbon nanofibers (30 to 40 nm diameter) supported on a macroscopic carbon felt. These materials present high thermal conductivity and mechanical resistance, as well as the easiness to be shaped with different macroscopic shapes. The mechanical stability and the performance of the iridium supported on the carbon composite support, evaluated in a laboratory scale test in hydrazine decomposition reaction, are superior compared to the commercial catalyst.