Effect of Ag, Pd and Co promoters on the Selective Catalytic Reduction (SCR) of NOx by ethanol over sulfated ceria-zirconia catalysts

In this study, our purpose was to investigate to what extent the deposition of some metal promoters such as Pd, Co and Ag may alter the performances of EtOH-SCR sulfated ceria-zirconia catalysts and the complex network of reactions pertaining to the reaction mechanism. In the case of Pd and Co, the characterization of metal-promoting species by X-Ray Diffraction (XRD), thermogravimetry (TGA) and DRIFTS of adsorbed CO revealed the presence of highly-dispersed cationic species coexisting with small clusters (< 1–2 nm), whereas for Ag an unstable bulk Ag2SO4 phase was also detected before the SCR test. Specific temperature-programmed surface reactions (TPSR) and steady-state profiles of a dozen of gaseous species monitored under 1920 ppm NO + 3020 ppm C2H5OH + 5% O2 conditions were confronted to the temperature profiles of adsorbed species obtained by in situ DRIFTS. Under isothermal conditions, DeNOx yields of 40, 37 30 and 30% at 350–400 °C were obtained respectively for the sulfated support (SCZ28) and the Co(1.03wt%)/SCZ28, Ag(3.00wt%)/SCZ28 and Pd(0.24wt%)/SCZ28 catalysts. TPSR data were then deeply discussed according to the nature of the metal deposited. This allowed us to reveal that the promoter effect can be sometimes either positive or negative depending on the temperature range investigated and the occurrence of deactivation processes.

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► Cex+ acidic species on the sulfated ceria-zirconia support are the most active sites for NOx-SCR by ethanol.
► Among the different organics formed under EtOH-SCR conditions, acetaldehyde, and to a lesser extent ethene, are directly involved in some pathways leading to N2, some of the final steps being close to those occurring in NH3-SCR.
► Ethylnitrite is a major product at low temperatures and is a secondary source for acetaldehyde.
► Supported Pd species have mostly an inhibiting effect on SCR activity and N2 selectivity due to the promotion of non-selective HC oxidation reactions.
► Supported Ag+ species, which are rather active for EtOH-SCR, deactivate to metallic silver under reaction conditions.