Co-Mo bimetallic nanoparticles (NPs) with various compositions were prepared on a CeO2 support (Co-MoiCeO(2)) by sodium naphthalenide driven reduction. The Co-MoiCeO(2) catalyst exhibits much higher activity for ammonia synthesis than monometallic catalysts such as Co/CeO2 and MoiCeO(2), and the optimum activity is obtained at Co:Mo = 4:6. X-ray absorption fine structure (XAFS) analyses reveal that Co3Mo3N NPs are formed after ammonia synthesis reaction. Nitrogen temperature programmed desorption (N-2-TPD) measurements indicate that the Co-Mo/CeO2 catalyst possesses a large number of nitrogen adsorption sites with an intermediate nitrogen adsorption energy between that of Co and Mo. Furthermore, nitrogen vacancy sites, which are active sites for N-2 dissociation, are more easily formed on the Co-MoiCeO(2) catalyst than on bulk-type Co3Mo3N. As a consequence, the turnover frequency of the Co-Mo/CeO2 catalyst is much higher than that of bulk-type Co3Mo3N and is comparable to that of the efficient RuiCeO(2) catalyst. (C) 2018 Elsevier Inc. All rights reserved.