Effect of preparation method on the performance of silver-zirconia catalysts for soot oxidation in diesel engine exhaust

This paper presents a study on the effect of the preparation method on silver-zirconia (Ag/ZrO2) catalysts' physicochemical characteristics and their activity for soot oxidation in diesel engine exhaust. Two series of Ag/ZrO2 catalysts with different molar fractions of Ag (5-30 mol%) were prepared by incipient wetness impregnation (IM) and citric acid-assisted sol-gel (SG) methods The catalysts were characterized using Brunauer-Emmett-Teller (BET) surface area analysis, X-ray diffraction (XRD), O-2 chemisorption, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and O-2 temperature-programmed desorption (O-2-TPD) techniques. The characterization results showed that silver was present mainly in the metallic state demonstrating heterogeneity in the Ag particle size distribution. XRD experiments indicated the presence of a tetragonal zirconia phase in Ag/ZrO2-SG catalysts. Soot oxidation activity of the silver-zirconia catalysts was evaluated by temperature-programmed oxidation (TPO) in presence of 10 vol% oxygen using carbon black (CB) as model soot. Catalytic activity was shown to be dependent on the method of preparation and silver loading. The synergetic effect of metal and support on the reaction was higher for the SG catalysts. The SG catalyst containing 30 mol% Ag exhibited the best catalytic performance for CB oxidation during TPO experiments. Adsorption properties of catalysts were evaluated with the O-2-TPD. The amounts of desorbed oxygen correlated very well with the values of T-max obtained in TPO runs indicating that adsorbed atomic oxygen played a major role in the mechanism of CB oxidation. Testing of the best catalyst under simulated diesel exhaust conditions revealed improved CB oxidation activity in the presence of water and no detrimental effect from SO2. These experiments showed that the new Ag/ZrO2-SG catalyst enabled effective soot combustion under conditions that do not require NOx. Soot removal over silver-zirconia catalysts can be an alternative method to NO-based technologies, which uses expensive Pt catalyst.