In-situ incorporation of binder during sol-gel preparation of Pd-based sulfated zirconia for reduction of nitrogen oxides under lean-burn conditions: Effect on activity and wash-coating characteristics

We have developed a dual-catalyst aftertreatment system for reducing nitrogen oxides (NO_x) in the exhaust stream of natural gas-fired lean-burn engines by utilizing the unburned hydrocarbons present in the engine exhaust. The dual-catalyst bed consists of a physical mixture of a reduction catalyst (Pd/SZ) and an oxidation catalyst (Co/CeO₂). In order to make this dual-catalyst system viable for practical use in the real aftertreatment units, it is necessary to develop a catalytically active washcoat for loading it onto cordierite monolith cores. To be able to handle large throughputs of gases typical of aftertreatment units, irreversible loss of activity due to separation of the wash-coat from the monolith walls can be reduced by adding binders to improve the adhesive properties of the wash-coat. However, addition of such binders may affect the catalytic activity of the resulting dual-catalyst bed. In our study, we have incorporated binders such as alumina, boehmite, bentonite or silica to the Pd/SZ catalyst using a novel technique in which the binders were added in situ during the sol-gel synthesis rather than the conventional method of adding binders (ex-situ) in catalyst slurry. Surface area analysis, X-ray diffraction (XRD) and diffuse reflectance Fourier transform spectroscopy (DRIFTS) with pyridine show that in-situ addition of binder affects the textural properties, crystal phase of zirconia and the acidic properties of the resulting Pd/SZ catalyst respectively. Electron Paramagnetic Resonance (EPR) indicated the presence of Pd⁺, Pd³⁺ and Zr³⁺ species in the binder-free and in situ alumina-incorporated Pd/SZ catalysts. Furthermore, steady-state activity tests on the modified dual-catalyst bed with the alumina-incorporated Pd/SZ exhibit the best performance amongst the other binder-incorporated catalysts in the dual-catalyst bed. In-situ incorporation of binder during sol-gel synthesis yielded catalysts with far superior catalytic activity for NO_x reduction than conventional ex-situ binder addition to the catalyst slurry for wash-coating. Time-on-stream experiments show that the alumina-incorporated Pd/SZ catalyst maintains the hydrothermal stability of the binder-free Pd/SZ in the mixed bed. Several parameters such as pH of the binder-incorporated catalyst slurry and calcination temperature of the wash-coat have also been optimized to develop a catalytically active wash-coat and its adhesivity and uniformity were tested using ultrasonication, cyclic thermal shock and SEM imaging.