Characterizing Adsorption Sites on Ag/SSZ-13 Zeolites: Experimental Observations and Bayesian Inference

Three potential adsorption sites within the Ag/SSZ-13 zeolite are compared for their ethylene and water adsorption capacity. Ethylene acts as a model hydrocarbon molecule in this work to stand in for vehicle exhaust, as Ag/SSZ-13 is a candidate material for trapping vehicle emissions during cold-start. Water is also present in vehicle exhaust and competes for adsorption sites with ethylene. The three active sites studied are the intended Ag ion-exchanged with an H in the zeolite framework, an H site (known as a Brønsted acid site) and Ag2O which may form as a nonzeolite adsorption site during the Ag ion-exchange synthesis. Density functional theory (DFT) calculations are conducted using the BEEF-vdw functional for up to two molecules of ethylene and/or water adsorbed per site. A microkinetic model parametrized by the DFT predicts the ethylene adsorption capacity for shifting ethylene feed gas concentrations at 100 °C in the presence of 6% water. Experimental observations are taken at matching conditions as microkinetic model simulations. The DFT energies and their uncertainties for each adsorption site are updated from experiments using a Bayesian statistical framework. The Ag ion and Ag2O sites adsorb more ethylene relative to the Brønsted acid site. The Ag ion and Brønsted acid sites adsorb more water relative to Ag2O.