Kinetic studies of the stability of Pt for NO oxidation: Effect of sulfur and long-term aging

The stability of Pt catalysts for NO oxidation was analyzed by observing the effect of pre-adsorbed sulfur on the reaction kinetics using a series of Pt/SBA-15 catalysts with varying Pt particle sizes (ca 2–9 nm). Our results indicate that sulfur addition did not influence catalyst deactivation of any of the Pt catalysts, resulting in unchanged turnover rates (TOR) and reaction kinetics. The presence of sulfur on Pt was confirmed by X-ray absorption fine structure spectroscopy (EXAFS) under reducing environments. However, exposure of the catalyst to NO oxidation conditions displaced sulfur from the first coordination shell of Pt, yielding Pt–O bonds instead. Re-reduction fully recovered the Pt–S backscattering, implying that sulfur remained near the Pt under oxidizing conditions. X-ray photoelectron spectroscopy (XPS) and chemisorption measurements confirmed the presence of sulfur near platinum. The invariance of the NO oxidation reaction to sulfur poisoning is explained by sulfur displacement to interfacial sites and/or sulfur binding on kinetically irrelevant sites. Formation of Pt oxides remains as the main source of catalyst deactivation as observed by kinetic and X-ray absorption spectroscopy (XAS) measurements.

Two pathways are suggested during NO oxidation: (A) sulfur migration to interfacial support sites driven by the oxidation of surface Pt and (B) formation of Pt–OSOx followed by Pt oxidation. In both cases, the number of Pt sites that overcome sulfidation and/or oxidation is the same as for the S-free catalyst.Figure optionsDownload high-quality image (80 K)Download as PowerPoint slideHighlights
► Sulfided Pt/SBA-15 catalysts show the same TOR for NO oxidation as unsulfided ones.
► EXAFS confirms sulfur remained in the vicinity of Pt during reaction.
► Chemisorption methods allowed an independent and consistent quantification of sulfur coverage.
► In situ XANES suggests that the oxidation of surface Pt is the main source of deactivation.
► Changes in TOR and Ea occurred over long-term exposures on Pt/SBA-15 and Pt/Al2O3 catalysts.