Nano-Oxide Surface Coatings on Cu-SSZ-13 for High Temperature Selectivity SCR Applications
David Shepard, Jeremy Madynski, Ashwin Sankaran, Geng Zhang
Pacific Industrial Development Corporation
Cu-SSZ-13 has been one of the dominant zeolites utilized in NH3 SCR applications in the past decade. The material is utilized in both mobile and stationary applications across a broad temperature range. The performance of Cu-SSZ-13 has been heavily investigated regarding SCR activity versus Cu content. The literature has shown a compromise between low temperature and high temperature selectivity towards NOx reduction as Cu levels increase, we see an increase in low temperature activity but a decrease in high temperature selectivity and vice versa for lower Cu levels.
The goal of this research was to investigate if different nano-oxide surface coatings can maintain low temperature activity while also maintaining selectivity of NOx reduction above 550°C. The investigation was conducted on low and high Cu-exchanged SSZ-13 material that had been loaded with different oxides (Al and Ce), varied weight loadings, and crystallite sizes of the nano-oxide. The zeolites were then prepared into catalysts and evaluated for SCR activity both fresh and after hydrothermal aging.
A slurry containing water and Cu-exchanged SSZ-13 material at 20% solids was placed under high-shear mixing while boehmite was dispersed in a separate container with water. Upon dispersion the boehmite was added to the zeolite slurry which was then high-shear mixed for 20 minutes. The slurry was then dried in an oven at 120°C for 12 hours and passed through a 300 μm sieve.
The resulting powder was re-slurried and applied as a wash coat to a 13 cc cordierite honeycomb core, with a target catalyst loading of 180 g/L. The wash coated cores were dried at 120°C for two hours and calcined at 600°C for two hours. Evaluation took place on a synthetic gas bench including a Parr Instruments furnace, MKS mass flow controllers, and an MKS MultiGas 2000 FTIR under the following conditions: 40,000 SV1 including 450 ppm NO, 550 ppm NH3, 8~10% each of CO2, O2, and steam with nitrogen balance. The SCR measurements were taken via steady state and specific temperature points from 175°C to 650°C, and the ammonia storage protocol was done at 200°C.
The addition of nano-Al2O3 to the surface of the Cu-SSZ-13 provided an improvement in high temperature NO reduction, while the low temperature performance was negatively impacted by higher weight % loadings. The NH3 break-through time also an improvement with over 75 seconds longer time to saturation.
The findings here can potentially lead to broader usage of Cu-SSZ-13 in applications which require selectivity at both low and high temperature windows. This could potentially be done now with a single modified zeolite versus a combination of Cu/Fe zeolites.