Focus on the catalytic performances of Cu-functionalized hydroxyapatites in NH3-SCR reaction

Hydroxyapatite loaded with different amounts of copper (from 2 to 11 wt.%, by using different Cu-precursors) was investigated as catalytic material for the NH3-SCR reaction. The method of copper addition affected the location and distribution of the copper phase in the hydroxyapatite samples as well as the aggregation state (isolated Cu2+ or CuxOy or CuO nano-aggregates). Ionic exchange procedure gave rise to Cu2+ stabilization in the exchange sites of HAP lattice, irrespective of the amount of Cu addition. Incipient wetness impregnation procedure provided samples with copper aggregation up to formation of CuO species at the highest Cu-loading. Sample characterization was performed by N2-adsorption/desorption, acidity measurement, EDX, TGA-DTGA, XRPD, UV–vis-DRS, and EPR to study the morphological and structural aspects of the copper phase. Catalytic tests of NH3-SCR revealed significant differences between the samples prepared by ionic exchange and impregnation, with activity that was dependent on the copper concentration on hydroxyapatite. Presence of 6 wt.% Cu was associated to the most active catalysts, in any case. Use of Cu-chloride and Cu-nitrate as precursors of Cu-phase, with the ionic exchange procedure, gave copper-hydroxyapatite catalysts with high activity/selectivity. Use of Cu-acetate as Cu-precursor caused some modification of the hydroxyapatite surface with enrichment of carbonate groups that were detrimental for the SCR activity.