The　addition　of　3d　transition　metal　(Fe,　Co,　Cu)　oxides　to　Pd/TiO2catalysts　was　investigated　for　the　selective　catalytic　reduction　of　NO　with　H2in　the　presence　of　O2.　It　was　found　that　the　addition　of　Fe　and　Co　resulted　in　a　promotional　effect　on　the　NOxreduction,　especially　at　low　temperatures,　compared　with　the　Pd/TiO2catalyst.　However,　the　addition　of　Cu　resulted　in　a　negative　effect　on　the　NOxreduction.　Transient　reaction　experiment　results　showed　that　the　amounts　of　stored　H2on　the　1Pd-5Fe/TiO2and　1Pd-5Co/TiO2catalysts　were　similar,　which　were　twice　that　on　the　1Pd-5Cu/TiO2catalyst,　suggesting　that　the　stored　hydrogen　alone　was　not　the　crucial　factor　for　the　effect　of　the　3d　transition　metal　additives　on　the　H2-SCR　reaction.Operandodiffuse　reflectance　infrared　spectroscopy　(DRIFTS)　results　showed　that　addition　of　3d　transition　metal　oxides　affected　the　formation　and　distribution　of　stored　NOxspecies.　Moreover,　the　bridging　nitrates,　monodentate　nitrates　and　bidentate　nitrates　played　different　roles　in　the　various　Pd-M/TiO2catalysts.　For　the　1Pd-5Cu/TiO2catalyst,　most　of　the　stored　NOxspecies　were　only　spectator　species　which　were　not　active　for　the　H2-SCR　reaction.　However,　for　the　1Pd-5Fe/TiO2and　1Pd-5Co/TiO2catalysts,　the　monodentate　nitrates　were　the　main　active　species　that　were　involved　(along　with　spiltover　hydrogen)　in　the　formation　of　intermediates,i.e.,　NHx,　which　were　crucial　for　the　enhancement　of　H2-SCR　catalytic　activity　at　low　temperatures.　The　spillover　hydrogen,　although　required　for　the　formation　of　NHxintermediates,　is　abundant　with　or　without　promoters.　This　work　shows　that　the　formation　of　monodentate　nitrates　is　the　rate　limiting　step　in　the　formation　of　the　active　NHxintermediates.　©　The　Royal　Society　of　Chemistry　2020.