A series of samarium-modified vanadium phosphate catalysts were prepared and studied in selective oxidation of n-butane to maleic anhydride. The catalytic evaluation showed that Sm modification significantly increased the overall n-butane conversion and intrinsic activity. N-2-adsorption, XRD, SEM, Raman, XPS, EPR and H-2-TPR techniques were used to investigate the intrinsic difference among these catalysts. The results revealed that the addition of Sm to VPO catalyst can increase the surface area of the catalyst, lead to a significant change in catalyst morphology from plate-like structure into rosette-shape clusters, and largely promote the formation of (VO)(2)P2O2. All of these were related to the different catalytic performance of Sm-doped and undoped VPO catalysts. The roles of the different VOPO4 phases and the influence of Sm were also described and discussed. (C) 2015 Elsevier B.V. All rights reserved.