The influence and corresponding mechanism of H2O on catalytic performance of Pt/HZSM-5 in selective catalytic reduction of NO by hydrogen (H-2-SCR) was investigated. Positive effect was detected on H-2 conversion, whereas divergent effects were observed on NO conversion. NOx conversion was promoted below 120 degrees C, while that at high temperature was inhibited by H2O. FTIR study and H-2-O-2 reaction results reveal that H-2 activation is rate-determining step of H-2-SCR, competitive adsorption of NOx with H-2 on the active sites (Pt-0) is suppressed by H2O. It explains the corresponding enhancement of H2O on the NOx conversion at low temperature. On the other hand, the lack of reductant due to H-2 combustion is the reason for the decline of NOx conversion at high temperature. H2O suppresses surface nitrous species formation that has the function of inhibiting H-2 combustion, which explains why NOx conversion at high temperature was decreased with the addition of H2O. (C) 2015 Published by Elsevier B.V.