Study of the deactivation of Co- and Pt,Co-mordenite during the SCR of NOx with CH4

Co- and Pt,Co-mordenite catalysts were found to lose about 50% of their initial NO-reduction and CH4-oxidation activity during a long catalytic NOx-SCR test. Adsorption capacities for N2, X-ray diffraction and Raman spectroscopic results suggested that the activity loss cannot be explained by either mordenite dealumination or extensive Co3O4 formation. Infrared spectra of the adsorbed pyridine showed that fresh catalysts have strong Brönsted and Lewis acidity. The Brönsted acidity was attributed to bridging OH groups of mordenite, formed from H2O by heterolytic dissociation in the electric field around the Co2+ ions. During the catalytic reaction, a large fraction of both kinds of acid sites was lost. The drop of acidity and catalytic activity was explained by migration of the Co2+ ions to lattice positions where the ion is not accessible by pyridine and is coordinated by framework oxygen atoms only. High-pressure DRIFTS spectra of adsorbed N2 and H2-TPR results provided additional evidence of the connection between the deactivation and cation migration processes.