Boosting the activity of silver catalysts for soot combustion by modification of the alumina support

Abstract

Catalysts with 15 wt% Ag supported on alumina and alumina-based solid solutions were synthesized and characterized using a variety of techniques such as SEM-EDX, XRD, N₂-physisorption, TPR, CO₂-TPD, FTIR, ToF-SIMS and Hammett indicator tests. Their activity in tight-contact soot combustion was evaluated. The aim of the studies was to determine how the surface and structure of the support influence the activity of the supported Ag catalysts. A series of alumina supports calcined at temperatures ranging between 550 °C and 1300 °C were obtained and used as supports for silver to investigate the Ag-support interactions. XRD studies revealed that the supports calcined below 1000 °C contain low-temperature alumina polymorphs, but those calcined at or above this temperature contain α-alumina. Catalysts supported on α-alumina were more active. ToF-SIMS measurement results indicate that this step-wise change can be attributed to differences in the surface of the undoped alumina and Hammett indicator tests revealed that the acidic sites in supports calcined above 1000 °C require lower regeneration temperatures. The modification of the alumina support with ions such as cobalt, cerium and chromium ions prior to silver deposition can improve catalytic activity in soot oxidation. The catalysts with doped-alumina supports exhibit a similar activity and surfaces properties, including acidity, to Ag/α-alumina catalysts. These studies show that the same effect can obtained by two different methods, one of them being doping the support.