Nitration Poisoning of Silver on Al2O3 for the Catalytic Oxidation of Soot in NOx-Containing Atmospheres

Silver-based catalysts are promising candidates for soot oxidation because of their good catalytic performances and low costs compared with platinum catalysts. NO2 acts as a strong agent for the oxidation of soot over Pt/Al2O3. However, an abnormal inhibition effect of NOx was observed over the Ag/Al2O3 catalyst during the processes of soot temperature-programmed oxidation and transient reaction analysis. The formation of silver nitrates was confirmed by temperature-programmed decomposition of NOx, ultraviolet–visible spectroscopy, and temperature-programmed reduction with H2. The intrinsic reactivity of silver nitrate was found to be much lower than that of metallic silver in ″loose″ and ″tight″ contacts between soot and catalyst. Moreover, the formed silver nitrate was relatively thermally stable, with the decomposition completed at around 450 °C. At higher temperatures, the catalyst recovered and the nitrate poisoning effect no longer worked.