The effect of nickel additive on Pd/TiO2 catalyst for selective catalytic reduction of NO with H-2 in presence of O-2 was investigated. Synergistic effects between palladium and nickel were found on the catalytic activity and N-2 selectivity for the H-2-SCR reaction. Hydrogen adsorption isotherm and transient reaction experiment results showed that the nickel additive enhanced the distance and rate of hydrogen spillover. Operando diffuse reflectance infrared spectroscopy (DRIFTS) with transient experiments indicated that addition of nickel to Pd/TiO2 led to the formation of extra active monodentate nitrates, and enhanced the activities of the bidentate nitrates. These positive effects on the formation of spiltover hydrogen and active adsorbed NOx species led to the promotional effect on the extra reaction intermediate NHx species, which was crucial for the enhancement of H-2-SCR activity. All these results, from a view of reaction mechanism, provided a promising way for rational preparation of high-performance H-2-SCR catalysts. (C) 2019 Elsevier Inc. All rights reserved.