NO oxidation catalyzed by high surface area perovskites

Abstract

The production of nitric acid is crucial for the production of fertilizer which is necessary to meet the world s demand for food. NO oxidation is one of three chemical steps in the nitric acid production, also called the Ostwald process. The oxidation of NO is currently performed homogeneously, and the introduction of a catalyst will be beneficial with respect to heat recovery and investment cost.

Noble metals as catalysts have the downside of being really expensive in addition to being scarce. Perovskites like LaCoO3 has shown great activity towards the oxidation of NO and it also circumvents the use of noble metals. One obvious downside with respect to catalysis is that perovskites typically has a low surface area. It is believed that an increase in the surface area can further increase the activity of the catalyst. Nanocasting of perovskites is the approach chosen in the attempt to increase the surface area which is believed to increase the activity. Mesoporous silica and carbon were used as templates for the casting procedure. Lanthanum was found to possibly interact with the silica template, making it really difficult to remove. Yttrium was used as an A-site substitution in the perovskite to possibly relieve this interaction.

The synthesis of the perovskites is based on the sol-gel method in addition to an impregnation technique of the solid templates by the use of a rotary evaporator. XRF was mainly used to detect residual silica in the samples. X-ray diffraction was used to confirm which phases were present in the samples in addition to calculating the particle size. Nitrogen adsorption was performed to reveal any increase in surface area. SEM was used to compare the samples relative to each other. Finally, the activity of the catalysts was tested both in the absence and presence of water, which is known to strongly inhibit the catalytic oxidation of NO.

The results show that nanocasting using silica gave a surface area increase from 1.9 to almost 96m2/g for LaCoO3 and from 1.2 to 11m2/g for YCoO3, even though it was thought to be unsuccessful for the yttrium-based perovskite due to the formation of silicates. When carbon was used the surface area increased from 1.9 to 7.5m2/g for the lanthanum-based perovskite while the yttrium-based perovskite showed a decrease in surface area from 1.2 to 1m2/g for the carbon casted sample. With respect to catalytic activity, the best performing catalyst was the silica cast LaCoO3 which gave a maximum conversion exceeding 40% at dry conditions and 27% at wet conditions.