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

• Published in: Applied catalysis. B, Environmental Environmental, 1997-09, Vol.13 (1), p.59-67
• Main Authors: Hoost, T.E., Kudla, R.J., Collins, K.M., Chattha, M.S.
• Format: Article
• Language: English
• Subjects: Hydrogen-oxygen titration; Nitric oxide reduction; Oxygen adsorption; Silver on alumina
• ISSN: 0926-3373; 1873-3883
• DOI: 10.1016/S0926-3373(96)00090-2

Ag supported on a nonporous γ-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. O2 adsorption and H2O2 titration were measured at 170°C after catalyst reduction at 250°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. O2 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°C) diminished the extent of the metal-support interaction. A 2 wt%Ag/γ-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.