On the Promoting Effect of Water during NO x Removal over Single-Site Copper in Hydrophobic Silica APD-Aerogels

Single-site copper cations incorporated into hydrophobic silica APD-aerogels (2–8 wt %) are highly active for the Selective Catalytic Reduction of NO x with C3H6 as reducing agent (SCR-HC-deNO x ) in the range 300–450 °C, reaching conversions up to 67% at 450 °C. In contrast to reported behavior of zeolite type matrixes, water in the feed has a promoting effect on the deNO x activity of the hydrophobic Cu-aerogels, making these systems promising commercial candidates in the 300–450 °C activity windows under realistic conditions. The Cu-aerogels were compared to Cu-xerogel, a hydrophilic denser gel analogue, and Cu-ZSM-5, an established deNO x catalyst featuring poor hydrothermal stability. This study aims to elucidate the origins of deNO x activity and the effect of water by correlating copper speciation with surface species and gel nature at different reaction stages in dry and wet feed. X-ray Absorption Spectroscopy (XAS) and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) measurements have been performed in situ, while monitoring reactor effluents during the experiments. Reversibility of the Cu2+/Cu+ redox pair was confirmed in the Cu-aerogel during and after wet redox cycling. This was not the case for the Cu-xerogel, or the Cu-ZSM-5, where the Cu2+/Cu+ redox pair and surroundings were reversibly and irreversibly affected by wet feed, respectively. Exclusive to Cu-aerogels was the formation of Brønsted acidic silanol cluster surrounding copper by multihydroxyl interaction with water. These silanol clusters are capable of storing reaction components and key intermediates, which we believe is responsible for enhancing the catalytic removal of NO x in Cu-aerogels.