Sol–gel-entrapped nano silver catalysts-correlation between active silver species and catalytic behavior

The target reaction is the reduction of NO to nitrogen which competes with the oxidation reaction under lean conditions (excess of oxygen) that may considerably alter the selectivity. Silver colloids prepared by reducing AgNO 3 in aqueous solution are embedded in alumina leading to stable nano-catalysts. The activity of the catalysts is influenced by the preparation procedure. Silver colloids prepared by reducing AgNO 3 in aqueous solution with sodium citrate were embedded in alumina following two different preparation procedures resulting in samples containing 3 and 5 wt.% silver. Characterization of these materials using TEM, XPS, XAES, CP/MAS NMR, XRD, and adsorption–desorption isotherms of nitrogen showed that embedding the pre-prepared silver colloids into the alumina via the sol–gel procedure preserved the particle size of silver. However, as XAES demonstrates, the catalysts prepared in a sol–gel with a lower amount of water led to embedded colloids with a higher population of Ag + species. The catalytic behaviors of the resultant catalysts were well correlated with the concentration of these species. Thus, the active silver species of the catalysts containing more Ag + species selectively converts NO to N 2. However, subsequent thermal aging leads to an enhancement of the conversion of NO parallel to slight alteration of the selectivity with the appearance of low amounts of N 2O despite an increase of Ag + species. Accordingly, an optimal surface Ag 0/Ag + ratio is probably needed, independently of the size of silver particles. It was found that this optimal ratio strongly depends on the operating conditions during the synthesis route.