Poisoning of Copper Chabazite Catalyst by Biodiesel Metal Contaminants: Effect of Alkali and Alkaline Earth Metals

Biodiesel is an environmentally friendly fuel that may partially replace petroleum diesel in transportation. One possible disadvantage of biodiesel is that alkali and alkaline earth metals can significantly affect Cu speciation and the elemental composition of copper chabazite (Cu-CHA) catalysts used in vehicle emission control units. To investigate this, we perform atomic-scale characterization of Cu-SSZ-13 zeolites contaminated by Mg, K, and Na using inductively coupled plasma-optical emission spectroscopy and Cu K-edge X-ray absorption spectroscopy (XAS) coupled with hydrogen temperature-programmed reduction. The experimental data decomposed by a multivariate curve resolution algorithm are interpreted using theoretical XAS spectra and reactive molecular dynamics. We find that Mg poisoning leads to a more severe dealumination compared to K or Na poisoning. Poisoning of catalysts also leads to the aggregation of isolated Cu ions into CuO_x clusters, while alkali and alkaline earth metal ions replace Cu ions in ion-exchange sites. The site preferences of Mg, K, and Na ions in Cu-CHA are thoroughly explored using density functional theory (DFT) calculations, suggesting an explanation of results from elemental analysis. Our study provides important theoretical and experimental insights into the poisoning of Cu-CHA catalysts by biodiesel metal contaminants.