Influence of CO2 and H2O co-feeding in the NOx abatement by SCR over an efficient Cu-CHA catalyst

Abstract

Cu-SAPO-34 materials with 2 wt% of copper synthesized in one pot with all the copper inside the framework are hydrothermally stable. Copper oxide species were not detected by XRD after thermal and hydrothermal aging at 750 °C. CO2 and H2O molecules are retained on Cu-SAPO catalyst and are related to CO2 adsorption aided by water involving the Cu-species. Copper species coordination is modified by the reaction atmosphere and the species are directly involved in the SCR-process. A decrease in the ammonia storage capacity in the presence of water has been justified due to the cooperative competition between the ammonia and water adsorption and/or CO2 carbonation over the same sites. Higher coordinated Cu species localized inside of CHA-framework, up to a loading close to 4% with this methodology, are highly efficient in the removal of NOx by reaction with ammonia, in the presence of oxygen, water (1.5%) and CO2 (0.3%) as reaction atmosphere. The SCR mechanism is essentially the same as in dry conditions because the Cu2+/Cu+ ions, in equilibrium with H2O + CO2, are the active species in the SCR-reaction. Hydrated-carbonated Cu(II) and hydrated-carbonated Cu(I) species are associated with the redox-chemistry of NOx abatement as responsible for the improvement in the standard SCR; favoring the hindering of ammonia oxidation and nitrate decomposition. In the SCR temperature range, the conversion values registered for this catalyst are higher than 80% with total selectivity to N2.