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首页> 外文期刊>Catalysis science & technology >Surface reactions of ammonia on ruthenium nanoparticles revealed by N-15 and C-13 solid-state NMR
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Surface reactions of ammonia on ruthenium nanoparticles revealed by N-15 and C-13 solid-state NMR

机译：钌表面反应的氨气纳米颗粒显示c15和c13固态核磁共振

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摘要

Ruthenium nanoparticles (Ru NPs) stabilized by bis-diphenylphosphinobutane (dppb) and surface-saturated with hydrogen have been exposed to gaseous (NH3)-N-15 and studied using solid-state N-15 CP MAS NMR. Three signals have been observed at 24.5, -12 and -42 ppm (reference external liquid ammonia) which are assigned to chemisorbed ammonia species RuNHx. Sample exposure to vacuum or aging leads to conversion of the 24.5 ppm species into the other ones, a process which is reversed by re-exposure to hydrogen gas. Exposure to a mixture of (NH3)-N-15 and (CO)-C-13 leads to the formation of surface bound urea as demonstrated by N-15 and C-13 CP MAS NMR. To understand the surface reactions of ammonia and the N-15 NMR results, quantum chemical calculations of the structures, energies and N-15 chemical shifts of ammonia species on Ru-6 and Ru-55 model clusters have been performed. The calculations indicate that under the experimental conditions applied, the fractions of RuNH3 and RuNH2 species are similar, independent of the H-2 pressure. No RuN and RuNH species are formed which are calculated to resonate at a lower field than the signals observed experimentally. However, the N-15 chemical shifts of RuNH2 depend on the number of neighboring surface hydrogens and hence on the H-2 pressure. Thus, the signal at 24.5 ppm is assigned to RuNH2 in a neighborhood rich in surface hydrogens. RuNH2 depleted in neighboring surface hydrogens and RuNH3 resonated both in a similar chemical shift range to which the signals at -12 and -42 belong. A change of the hydrogen pressure then leads to interconversion of hydrogen-rich and hydrogen-poor neighborhoods of RuNH2 but does not alter the fractions of RuNH3 and RuNH2 according to the calculated stability diagram. Nevertheless, dissociation of RuNH3 into RuNH2 and surface hydrogen is expected to take place during the initial ammonia adsorption process and at low H-2 pressures and high temperatures. Finally, some preliminary quantum chemical calculations suggest stepwise binding of two NH2 groups to adsorbed CO leading to surface bound urea where the oxygen is coordinated to Ru.
机译：钌纳米粒子(俄文NPs)稳定bis-diphenylphosphinobutane (dppb)和与氢surface-saturated已经暴露出来气体(NH3) -N-15和研究使用固态c15 CP MAS NMR。被观察到在24.5,-12和-42 ppm(参考分配给外部液氨)化学吸附氨RuNHx物种。暴露在真空或老化导致转换24.5 ppm的其他的物种,一个过程是由他们再次暴露在逆转氢气。和(CO) -C-13导致表面的形成尿素,表现出c15以及c13 CPMAS NMR。氨和c15核磁共振结果,量子化学计算的结构,能量和c15氨物种的化学变化Ru-6 Ru-55模型簇执行。应用,实验条件分数RuNH3 RuNH2物种是相似的,独立于2的压力。物种形成的计算产生共鸣的领域低于信号观察实验。化学变化RuNH2取决于数量邻近表面氢,因此在2压力。分配给RuNH2社区丰富的表面氢。表面氢和RuNH3都引起了共鸣类似的化学位移范围的信号在-12年和-42年。压力导致互变现象研究人员和贫氢的社区RuNH2但不改变RuNH3的分数并根据计算RuNH2稳定图。预计RuNH2和表面氢在初始氨吸附过程和在低2压力和高温度。化学计算表明逐步绑定的两个氨基组吸附CO导致表面绑定尿素的氧气是俄文的协调。

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