Abstract
The influence of H2 on NO oxidation over Pt/Al2O3 as a model DOC catalyst was evaluated with various DOC feed mixtures. Discrimination of surface chemistry and exothermal effects due to addition of H2 was made based on differences in time scales of transient responses. H2 was proposed to retard Pt oxide formation mainly at low temperatures (ca. <200°C), whereas, at higher temperatures Pt oxide formation was unhindered by H2. As a result, a temporal enhancement in NO2 yield due to H2 was obtained during temperature ramp experiments. In C3H6 containing mixtures, it was evident that the promotional role of H2 was to weaken the inhibition effect of C3H6 by lowering the light-off temperature for C3H6 oxidation. The results from transient experiments clearly showed an increase in NO2 yield that resulted from effects of H2 on surface chemistry and/or reactions. H2 decreased the blocking effect of adsorbed C3H6 species and prevented the reaction of C3H6 with NO2 forming NO. For a complete DOC feed mixture (containing NO, CO, C3H6), an addition of ca. 250ppm of H2 appeared to be optimal, while higher H2 concentrations were disadvantageous due to NO2 consumption by H2.
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