Studies on Lean NOx Reduction with Ammonia. Catalysts and Sensors

Doctoral thesis, 2005

Selective catalytic reduction of NOx with ammonia, NH3-SCR, is a promising technique to reduce NOx under lean conditions, e.g. in diesel engine exhausts. The main objective of this thesis was to gain further understanding of key parameters affecting the performance of catalysts and sensors for NH3-SCR. The work primarily comprises flow-reactor and in situ FTIR spectroscopy studies along with electronic structure calculations performed to support experimental results. In addition, catalyst screening and novel preparation techniques were explored. Several TiO2 supported metal and metal oxide catalysts were evaluated for the NH3-SCR reaction in a high-throughput screening reactor. In this survey, Cr-, Mn-, Fe- and Rh-containing catalysts showed the most promising catalytic properties with respect to NOx reduction activity and selectivity. It was further shown that wider operating temperature windows were obtained with combinations of these metals and there was a general trend towards increased low-temperature NH3-SCR activity when rhodium was included. The same trend was observed for Rh-containing mixed metal oxide catalysts with crystalline perovskite structures. The perovskite catalysts were synthesized using microemulsions, which lowered the required temperature to form the perovskite phase. Zeolite H-ZSM-5 was investigated for the NH3-SCR reaction and very high NOx reduction activity was observed with high amounts of NO2 in the feed. The rate-limiting step for this catalyst was found to be NO oxidation to NO2. In situ FTIR spectroscopy showed that ammonia formed NH4+ at the Brønsted acid sites in the zeolite, which participated in the SCR reaction. NOx was observed to adsorb at the Brønsted acid sites and NH3 present in the zeolite was found to suppress the NO oxidation. When NH3 was removed from the feed, the NO reduction activity increased, as NO oxidation sites became available. It was further shown that a transient supply of ammonia enhanced the total NO reduction up to five times compared to a continuous ammonia supply. The sensing layer of an ammonia sensor was investigated by studying a model system consisting of Pt nanoparticles supported on silica. In situ FTIR spectroscopy provided evidence for NH2 species when the Pt/SiO2 samples were exposed to NH3, indicating NH3 dissociation. At high temperatures, formation of OH groups was observed on the silica surface, indicative of H diffusion from Pt to silica. The same OH stretching modes were observed when the Pt/SiO2 was exposed to H2. It was further shown that these OH groups were formed at the Pt-silica interface. Electronic structure calculations indicated that charge transfer might occur at the Pt-silica interface during OH group formation.