Metal nitrides are promising materials because of their prominent properties originating from metal-nitrogen bonds and noble-metal-like electron features, such as thermal stability, mechanical hardness, superconductivity, and high catalytic performance in hydrocarbon conversion.[1] More importantly, their bulk and surface properties are significantly associated with the nitridation levels, which can be adjusted as such. Taking tantalum (oxy)nitrides as the current example, introducing N atoms into Ta 2 O 5 brings a remarkably narrowed and tunable band-gap energy, thus enabling novel visible-light photocatalysts.[2] Meanwhile, the catalytic properties of Ta in tantalum (oxy)nitrides for nonphotocatalytic reactions are also expected to be improved because of the tailored nitridation and easier electron transfer from N (vs. O) to Ta centers. However, there are still no reports on these effects to the best of our knowledge. Significantly, such modulation of material properties by controlled nitridation provides a new strategy for designing novel catalysts. [2a, 3] The controllable synthesis of TaON and Ta 3 N 5 nanoparticles (NPs) with tailored chemical composition is therefore an attractive goal, [2e, 3a] which is, however, difficult to achieve by current strategies because of easy overreactions of nitridation.[4] For example, by-products of Ta 3 N 5 , Ta 4 N 5 , and even TaN will be easily generated from further nitridation of TaON during calcination. The traditional method using NH 3 is limited for the danger associated with using NH 3 at high temperature and the complexity related to the rigid control of gas composition, flow rate, and pressure. [4, 5] Moreover, although urea and cyanamide have been reported as safe nitrogen sources for preparing tantalum nitrides, only impure and deeply reduced TaN could be obtained, as an effective control on reactions is absent. [6] Recently, we discovered that the controlled release of N source from urea is significant to control metal nitridation. [7] As a further controlled version of urea pyrolysis, we present herein a new SiO 2 -surface-assisted strategy for the controlled fabrication of TaON and Ta 3 N 5 NPs, which successfully avoids the disadvantage mentioned above. Assisted by SiO 2 , the production of TaON over Ta 3 N 5 NPs with tailored composition was achieved through calcination of Ta-urea (TaU) gels with suitable urea/Ta ratios (R U/Ta ). A mechanism is proposed that urea is converted into carbon-nitride (CN x ) species on SiO 2 -surface at mild temperature, [8] which further acts as a slow release N source for controlled nitridation. The electronic properties of Ta are tuned by the different nitridation levels in TaON and Ta 3 N 5 NPs, which significantly improves the activity for alkene epoxidation, as compared to Ta 2 O 5 NPs. This is the first time to discover that introducing N into Ta 2 O 5 can remarkably improve epoxidation activity because of the easier electron transfer from N (vs. O) to Ta. Furthermore, the controlled nitridation endows catalys...
