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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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| 9。介孔氮化铬的形成机理。介孔氮化铬的形成机理是类似介孔氮化钒。 35 于700℃的氨流量，从氧化钾形成KOH或KNH 2热力学上可能的。其它化合物如混合氢氧化物、酰胺或酰亚胺也可在过程中加热和冷却产生。任何情况下副产品在合成温度是固体或液体，显然在反应过程中慢慢生成，反应后使用去离子水将其洗去。氮化物与其它副产物相分离产生的孔隙可在氮化铬中观察到。我们假设，表面能与有关相的扩散常数允许这种分离在中尺度继续进行。我们着重假设，因为氮化物扩散速率太低，氮化物烧结速率慢到孔隙不能在加热期间消除。这一机制与硼硅酸盐玻璃的亚稳分解在纳米尺度上形成两相分离形成多孔质玻璃类似。42先前施（Shi ）等人报道了使用硬硅胶模板产出的同比更高表面积（78米2 /克）的有序介孔氮化铬。 33他们的工作中可见到宽范围的孔隙尺寸分布（从5到25纳米），相当于我们的CrN产品。然而，现在的方法不需要使用模板，而后去除。 10。介孔氮化铬作为催化剂载体。铂颗粒和炭黑通过多元过程沉积在CrN的介孔。图1a示出了合成并用作催化剂载体的介孔CrN的X射线衍射图谱和Pt/氮化铬和Pt/ C电催化剂的X射线衍射图谱。所有的介孔CrN的衍射峰在Pt/氮化铬催化剂也观察到。Pt粒子结晶空间群Fm3m精确晶格参数为a =3.9311（4）唉。Pt在Pt/ C系统中的衍射峰几乎与Pt在CrN上相同。因为炭黑，对Pt / C在2Θ≈24°观察到的衍射峰弱。Pt/氮化铬催化剂的大小和形态如图2c和d。Pt粒子在Pt/氮化铬催化剂的平均大小为4.5 nm，相比较而言，Pt粒子在Pt / C催化剂平均大小为4.2nm。两种催化剂之间平均Pt粒子大小差异不大，表明平均Pt粒大小受催化剂载体使用影响较小。Pt粒子均匀分布于介孔CrN，无明显聚集，并具有较窄的粒径分布。图4的介孔CrN的循环伏安曲线，-0.8和1.2伏特（20毫伏/秒相对于SCE）进行10周期循环观察时，氮化铬没有相关的氧化还原峰出现。表明CrN，至少在几分钟的时间尺度，是电化学稳定的。 |
2楼2013-12-01 23:12:57
