Mechanistic assessments of NO oxidation turnover rates and active site densities on WO3-promoted CeO2 catalysts

Abstract

The effects of NO, NO2 and O2 pressures on NO oxidation rates and UV-visible spectra are used here to assess the elementary steps and the number and type of redox-active sites involved in NO oxidation on CeO2 promoted by contact with WO3 domains. The reversible chemisorption of O2 on vacancies (∗) and the subsequent dissociation of O2∗ assisted by NO to form O∗ and NO2 are the kinetically-relevant steps on surfaces with O∗ coverage set by NO−NO2 equilibration. O2p→Ce4f ligand-to-metal charge transfer (LMCT) bands probe the rate constants for O2∗ formation and desorption at catalytic conditions; their comparison with those derived from rate data confirms the mechanistic conclusions and the involvement of CeO2 surfaces promoted by contact with WO3 domains. These data allow an accurate assessment of the number and type of redox-active sites, thus allowing reactivity comparisons among catalysts based on turnover rates. The number of redox-active sites increased with increasing W surface density (2.1–9.5W/nm2), but NO oxidation turnover rates were essentially unchanged. These elementary steps and active structures differ markedly from those that mediate NO oxidation on Pt, PdO, RhO2 and Co3O4 catalysts. Turnover rates are similar on WO3/CeO2 and Pt-based catalysts at practical temperatures of diesel exhaust treatment (∼500K), but WO3/CeO2 catalysts exhibit much higher rates based on catalyst mass (>10-fold), thus rendering useful as less costly and more resilient alternatives to noble metals. These findings illustrate a method to probe the number and type of redox-active sites and conceptual insights into the pathways that mediate the chemisorption and activation of O2 by isolated vacancies and the subsequent dissociation of OO bonds by assistance from co-reactants.