Lean NOx reduction over silver-alumina catalysts - Aspects on synthesis, structure and activity

Doctoral thesis, 2010

The aim of this thesis is to increase the understanding of how to design highly active silver-alumina catalysts for selective catalytic reduction of NOx with hydrocarbons (HC-SCR) in oxygen excess at low temperatures (150-250 °C). Silver-alumina catalysts have been prepared using different methods, including a new unique preparation method developed during the thesis project. The obtained catalysts have been thoroughly characterized by a range of methods, to determine the chemical and physical properties. The catalytic performance of the samples was determined using flow reactor experiments and mechanistic aspects of the HC-SCR reaction were elucidated by kinetic modeling and in-situ diffuse reflection infra-red Fourier transformed (DRIFT) spectroscopy. The results show that silver-alumina catalysts with highly dispersed silver can be obtained by the new preparation method based on freeze-drying of the silver-alumina gel, formed by the sol-gel method, presented in this thesis. However, the flow reactor experiments showed that to achieve a high NOx reduction activity at low temperatures, both the nature of the reducing agent and the silver loading of the catalyst must be considered. A delicate balance between the oxidative and reductive properties of the catalyst is required, which in turn is dependent on both the silver loading and the preparation method. This balance varies for different reducing agents, most likely due to the different terminal C-H bond strengths in the hydrocarbons. It is suggested that these two factors determine the rate of partial oxidation, required to open up the HC-SCR reaction path. A transient kinetic model for hydrogen assisted n-octane-SCR has been developed, using the reduction of stable surface nitrates by hydrogen as a key-step. The model is able to well reproduce changes in the gas feed, especially poisoning effects by higher NO concentrations in the feed. These results, together with in-situ DRIFT results showing the reduction of surface nitrates by hydrogen, strongly indicate that one important role of hydrogen is to suppress the self-poisoning of the active sites by reduction of surface nitrates.