Catalytic formation of ammonia from nitric oxide

Abstract: The emission of nitrogen oxides (NOx) from automobiles results in severe environmental and biological effects. Passive selective catalytic reduction (passive SCR) of NOx with ammonia (NH3) is a proposed method for abatement of NOx emissions in net lean-burn gasoline passenger vehicles. The principle idea of this technique is to selectively reduce NOx to nitrogen and water in the presence of excess oxygen by utilising a supply of NH3 that has been formed onboard the vehicle over an ammonia formation catalyst (AFC). The aim of this thesis work is to deepen the understanding of how ammonia can be formed from nitric oxide over noble metal based catalysts.

A bottom-up methodology has been employed where model catalysts with increasing complexity have been synthesised and characterised with several methods (BET, ICP-OES, XRD and TEM). The catalytic activity and selectivity for NH3 formation have been studied using steady-state and transient experiments in chemical flow-reactors. Also, a number of in situ and operando experimental techniques, including infrared and X-ray absorption spectroscopy, have been used to reveal catalyst structure-function relationships.

This thesis shows that Pd/Ce/Al2O3 is a promising base for AFC formulations. This AFC exhibits high activity for NH3 formation in the presence of model feed gas mixtures using either a direct source of hydrogen or hydrogen formed via the water-gas-shift reaction. It also has higher NH3 formation activity than a three-way catalyst when using complex feed compositions containing reactants relevant to those one may encounter onboard lean-burn gasoline vehicles.

The suppression of ammonia formation in net-oxidizing feeds is also investigated. In addition to oxidation of the reductant, a rapid growth in metal-oxides occurs as the feed becomes net-oxidizing. When oxidized, weak interaction between the noble metal phase and NO occurs, resulting in negligible NO dissociation. As dissociative adsorption of NO is a prerequisite step for NH3 formation, this is identified as the limiting step for NH3 formation from NO and H2 in net-oxidizing conditions.

Further, low-temperature ammonia formation is proposed as an important future area of research and the expansion towards utilisation of renewable fuels (alcohols) may open for even more efficient NH3 formation over suitable catalysts.