Trapping materials prevent greenhouse gases from being released before a vehicle’s catalytic converter activates during start-up. A major consideration for any trapping candidate material is the adsorption capacity of the material for cold (below the catalyst operating temperature) gases emitted from the engine during startup before the gases sufficiently heat to the catalyst operating temperature.1 The trapping candidate material chosen for evaluation in this work is the zeolite SSZ-13 ion exchanged with Ag (Figure 1). Ethylene and water, two components present in vehicle exhaust which compete for adsorption sites are studied from density functional theory (DFT) calculations. The BEEF-vdw2 functional is used in DFT calculations, which provides an estimate of DFT binding energy uncertainty. Previously, DFT models in the literature have been limited to energies and not adsorption capacity predictions. Adsorption capacities are key to the experimental testing of candidate trapping materials. This uncertainty is propagated through two adsorption capacity models. The two models are competitive Langmuir adsorption and a mean-field microkinetic model. Both models give qualitatively similar results.