The influence of H-2 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 H-2 was made based on differences in time scales of transient responses. H-2 was proposed to retard Pt oxide formation mainly at low temperatures (ca. <200 degrees C), whereas, at higher temperatures Pt oxide formation was unhindered by H-2. As a result, a temporal enhancement in NO2 yield due to H-2 was obtained during temperature ramp experiments. In C3H6 containing mixtures, it was evident that the promotional role of H-2 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 H-2 on surface chemistry and/or reactions. H-2 decreased the blocking effect of adsorbed C3H6 species and prevented the reaction of C(3)H(6)with NO2 forming NO. For a complete DOC feed mixture (containing NO, CO, C3H6), an addition of ca. 250 ppm of H-2 appeared to be optimal, while higher H-2 concentrations were disadvantageous due to NO2 consumption by H-2. (C) 2015 Elsevier B.V. All rights reserved.