Characterization of Ag/gamma-Al2O3 catalysts and their lean-NOx properties

Ag supported on a nonporous gamma-alumina was shown to have an optimal metal loading of about 2 wt% Ag for NOx conversion of simulated lean-NOx, automotive exhaust. Ag was primarily in an oxidized state during reaction. O-2 adsorption and H-2-O-2 titration were measured at 170 degrees C after catalyst reduction at 250 degrees C to determine metal dispersion. The results indicate an unusual increase in apparent metal dispersion with increased metal loading below 3 wt% Ag. Above 3 wt%, the apparent dispersion decreased with Ag loading. The observed trend for apparent dispersion was attributed to a metal-support interaction. No XRD pattern could be detected for the proposed phase which would suggest a surface rather than bulk phenomenon. Comparison of the adsorption data to a simple model for dispersed metal particles also indicated the likelihood for a metal-support interaction. The model also suggests that apparent dispersions may be significantly different from true metal dispersions even for fairly large particles. O-2 TPD and lean-NOx reaction results indicate that the proposed metal-support interaction is relatively stable under inert and net-oxidizing conditions. Extended reduction at mild temperature (170 degrees C) diminished the extent of the metal-support interaction. A 2 wt% Ag/gamma-Al2O3 catalyst prepared by multiple impregnation was more active for lean-NOx reaction than one prepared using a single impregnation step. This would suggest that Ag alumination results in more active catalysts for lean-NOx applications.