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| 9. Formation Mechanism of Mesoporous CrN. The mechanism of formation of mesoporous CrN is similar to the case of mesoporous VN. 35 Under ammonia fl ow at 700 °C, the formation of KOH or KNH2 from potassium oxide is thermodynamically possible. Other compounds such as mixed hydroxides and amides or imides may also be produced during the heating and cooling processes. In any case, these byproducts are either solids or liquids at the synthesis temperature and apparently sublime slowly during the reaction or wash out using DI water after the reaction. The observed mesoporosity in CrN results from phase separation of the nitride from the other byproducts. We hypothesize that the surface energies and diffusion constants of the relevant phases allow such separation to proceed at the mesoscale. In addition, the sintering rate of the nitride is slow enough that the porosity is not eliminated during the heating period, because the diffusion rates in nitrides are so low. This mechanism is similar to that seen in the formation of porous Vycor, where spinodal decomposition of aborosilicate glass forms two phases that are separated on the nanoscale. 42 An ordered mesoporous CrN, made using a hard silica template, with comparable high surface area (78 m 2 /g) was previously reported by Shi et al. 33 In their work, a wide range of pore-size distribution was observed (from 5 to 25 nm), which is also comparable to our CrN product. However, the present method does not require the use of a template and its subsequent removal. 10. Mesoporous CrN as Catalyst Support. Pt particles are deposited on mesoporous CrN and carbon black through a polyol process. Figure 1a shows the XRD patterns of the Pt/ CrN and Pt/C electrocatalysts in addition to the XRD patterns of the mesoporous CrN that was synthesized and used as the catalyst support. All of the di ff raction peaks of mesoporous CrN are also observed in the Pt/CrN catalyst. The Pt particles crystallize in the space group Fm3̅m with re fi ned lattice parameter a = 3.9311(4) Å. The di ff raction peaks of Pt in the Pt/C system are almost identical to those of Pt on CrN. The weak diffraction peak observed at 2Θ ≈ 24° for Pt/C is attributed to the carbon black. The size and morphology of the Pt/CrN catalyst can be observed in Figure 2c and d. The average size of the Pt particles in the Pt/CrN catalyst is 4.5 nm, and the average size of the Pt particles in the Pt/C catalyst that was used for comparison was 4.2 nm. The difference in average Pt particle size between the two catalysts was not very large, indicating that average Pt particle size was relatively unaffected by which catalyst support was used. Pt particles are uniformly dispersed on mesoporous CrN with little obvious aggregation and have a narrow size distribution. The cyclic voltammograms of the mesoporous CrN seen in Figure 4 show that no redox peaks associated with CrN are observed when cycling between −0.8 and 1.2 V (20 mV/s vs SCE) for 10 cycles, indicating that the CrN is electrochemically stable, at least on the minutes time scale. |
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