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

Silver colloids prepared by reducing AgNO₃ 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 N₂. 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 N₂O despite an increase of Ag⁺ species. Accordingly, an optimal surface Ag⁰/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.