The effect of Si content in SAPO-34 on NO selective catalytic reduction over Cu/SAPO-34 catalyst was investigated. Three fresh catalysts with different Si contents, prepared by the same procedure, showed different NO conversions from 120 degrees C to 600 degrees C. These catalysts were characterized in detail by various techniques (ICP, XRD, SEM, NMR, NH3-TPD, H-2-TPR and EPR). The NH3-TPD results indicate that the number of acid sites in Cu/SAPO-34 catalyst increases with increasing Si contents and the SCR activities of the samples correlate well with the number of acidic sites at low temperatures (200 degrees C). The EPR and H-2-TPR results show that both the Si and the Al contents affect the number of isolated Cu2+ ions in Cu/SAPO-34 catalysts. The catalysts aged at 750 degrees C for 12 h showed higher SCR activities than those of the untreated samples, consistent with the higher numbers of the isolated Cu2+ ions. (C) 2012 Elsevier B.V. All rights reserved.