Reactions on nanostructured and microporous surfaces

Doktorsavhandling, 2015

Interactions between surfaces and molecules play a central role in various fields where catalysis is one example. In order to understand different surface processes, physicochemical characterisation of the surface materials and molecules is required. The present work aims at characterisation of such processes on nanostructured and microporous surfaces with application for astrochemistry and selective NOx reduction. The astrochemical studies are motivated by the need for realistic model systems of interstellar dust grains that serve as catalysts for the formation of molecules in the interstellar medium. Experimentally, interstellar dust grains are commonly modelled by flat metal surfaces, although such structures are far from reality. This can result in inaccurate predictions of the surface processes, and thus, the molecular abundances in different regions in space. Temperature programmed desorption (TPD) with quadrupole mass spectrometry (QMS) at ultra-high vacuum conditions and optical reflection spectroscopy are used to study the thermal desorption of water from an ice-covered nanostructured graphite surface. It is found that the nanostructured surface gives rise to the presence of multiple peaks in TPD-QMS spectra, which can be assigned to water bound in two- and three-dimensional hydrogen-bonded networks, defect-bound water, and to water intercalated in the graphite structures. The diffusely reflected light from the surface measured during a TPD experiment is found to be dependent on the water ice phase and thickness, allowing the observation of water ice also in subsurface regions In emission control for automotive applications, there is a requirement for catalysts that are active for the conversion of nitrogen oxides (NOx) to harmless N2 over a broad temperature range. One of the most common approaches for heavy-duty vehicles is selective catalytic reduction with ammonia as reducing agent (NH3-SCR) over copper-exchanged zeolites. However, for cold exhausts, the catalytic activity of the copper zeolites is not sufficient and a deeper understanding of the detailed reaction mechanism and the properties of the catalytically active centres are required to further improve the technique. In this work, the properties of copper-containing chabazite (Cu-CHA) zeolites are investigated. The samples are prepared using both high-temperature solid-state ion-exchange (SSIE) in air and low-temperature SSIE performed in a reactive gas atmosphere containing NH3 and NO. The Cu-CHA samples are characterised by infrared, optical and X-ray photoelectron spectroscopy together with X-ray diffraction, electron microscopy, and TPD of NH3 and NO. In addition, the catalytic activity for the NH3-SCR reaction is investigated. It is observed that Cu ions from external CuO diffuse into the microporous structure of the zeolite under NH3-SCR conditions as well as in the presence of NH3 and NO. Materials prepared by low-temperature SSIE assisted by NH3 and NO show the highest conversion of NOx for NH3-SCR with a maintained selectivity for N2 formation. In both types of Cu-CHA materials, the presence of Cu2+ and Cu+ species is suggested as a result of the SSIE procedures.