The effect of trivalent framework heteroatoms in Cu-CHA on the Selective Catalytic Reduction of NO

Abstract The effect of the trivalent framework atoms (Si4+/M3+ = 13-105) in Cu exchanged CHA-type zeolites was investigated for the selective catalytic reduction of NO with NH3. While the increased hydrophobicity of B-CHA compared to Al-CHA could make it a good candidate for low temperature SCR, the material was too unstable under reaction conditions and the exchanged Cu2+ did not remain isolated. Consequently a lower activity and selectivity than for Al-CHA were observed. Ga-CHA on the other hand showed a similar Cu2+ speciation as Al-CHA at similar Si4+/M3+ ratios. The difference in catalytic activity was remarkably small. While Ga-CHA was less stable upon hydrothermal aging, this was markedly improved by increasing the Si/Ga ratio and by inserting the Cu2+ during the synthesis of the zeolite. Remarkably, the Cu-loaded Ga-CHA catalysts demonstrated a consistently lower selectivity to N2O by-product formation compared to Al-CHA because of a lower tendency for NO oxidation.