The effect of H2O on the H2-SCR of NOx over Pt/HZSM-5

•The influence of H2O on H2-SCR over Pt/HZSM-5 was determined by temperature.•H2 activation is the rate-determining step of the reaction at low temperature.•H2O favors H2 activation by suppressing NOx adsorption at lower temperature.•H2O is unfavorable for H2-SCR by suppressing NOx adsorption at high temperature.•Appropriate NOx adsorption capacity is account for the high NOx conversion.

The influence and corresponding mechanism of H2O on catalytic performance of Pt/HZSM-5 in selective catalytic reduction of NOx by hydrogen (H2-SCR) was investigated. Positive effect was detected on H2 conversion, whereas divergent effects were observed on NOx conversion. NOx conversion was promoted below 120 °C, while that at high temperature was inhibited by H2O. FTIR study and H2–O2 reaction results reveal that H2 activation is rate-determining step of H2-SCR, competitive adsorption of NOx with H2 on the active sites (Pt0) 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 H2 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 H2 combustion, which explains why NOx conversion at high temperature was decreased with the addition of H2O.

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