Selective catalytic reduction of NOx with hydrogen (H-2-SCR) in excess oxygen over Pd and Ir based catalysts with various silica supports was studied. The Pd/V2O5/TiO2/SBA-15 and Ir/SBA-15 catalysts showed the highest activities. The effects of noble metal, various silica supports, CO and SO2 on H-2-SCR over these catalysts were also studied and compared, and possible underlying mechanisms discussed. A comparison of 1 % Ir-doped on silicas with a wide range of pore sizes showed that the peak temperature (where the NO conversion maximum was located) was directly related to the pore size: larger pores of the support resulted in higher peak temperatures. This result indicates that pore diffusion limitation played a role in determining the peak temperature. In addition, a non-noble metal catalyst, Nb2O5/SiO2, was found to have considerable activity. Larger pores of the support resulted in higher peak temperatures, which indicated that pore diffusion limitation played a direct role. [GRAPHICS] .