Lean NOx trap catalyst formulation for selective ammonia synthesis from captured NOx

Abstract

In the Lean NOx Trap (LNT) technology, ammonia (NH3) occurs as reaction intermediate in conversion of trapped nitrogen oxides (NOx) to nitrogen gas (N2) using hydrogen gas (H2) reductant. The use of an LNT for synthesizing NH3 from NOx is potentially an attractive option for small-scale green NH3 production by converting locally produced NOx pollutant to an important chemical. In this work, it is investigated to what extent the product selectivity towards NH3 on the standard model catalyst Pt/BaO/Al2O3 can be maximized depending on reaction conditions for trapping and reducing NOx with H2 in the temperature range of 75–200 °C. The LNT catalyst preparation was varied by using Pt, Pd and bimetallic combinations, as well as different support materials, namely Al2O3, TiO2, ZrO2 and MgO. NOx adsorption and selectivity of captured NOx reduction to ammonia is discussed in terms of texture and the chemical nature of the support. The NH3 selectivity is influenced by the water vapor content in the gas feed. Storage of NOx in absence of water followed by reduction with hydrogen gas in presence of water vapor was found to be the most favorable operation mode of the LNT for NH3 formation. The NH3 selectivity reached 82% at 150 °C. Explanations for the role of water and in this catalytic chemistry are offered and its consequences for catalyst preparation are discussed. The optimum water content of the reacting gases is predicted to be strongly dependent on catalyst preparation.