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首页> 外文期刊>ACS catalysis >Mechanistic Insights into pH-Controlled Nitrite Reduction to Ammonia and Hydrazine over Rhodium
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Mechanistic Insights into pH-Controlled Nitrite Reduction to Ammonia and Hydrazine over Rhodium

机译：将pH控制的亚硝酸盐还原成氨和肼的机械洞察力

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摘要

An unintended consequence of industrial nitrogen fixation through the Haber-Bosch process is nitrate (NO3-) and nitrite (NO2-) contamination of ocean, ground, and surface waters from fertilizer runoff. Transition-metal catalysts, particularly those based on Pd, are effective in removing NO3-/NO2- through reduction to N-2 or NH4+. Pd is regarded as the most effective metal for NO3-/NO2- reduction, and as such, few studies have thoroughly explored the performance of other transition metals as a function of varying reaction conditions. In this work, we investigated the NO2- reduction properties of alumina-supported Rh using Pd as a benchmark, where we varied the bulk solution pH to probe the effect of reaction conditions on the catalytic chemistry. Pd expectedly showed a high reduction activity (289 L/g-surface-metal/min) and a high N2 selectivity (>99% at 20% conversion) at low pH and near inactivity at high pH. Surprisingly, the Rh catalyst, while inactive at low pH, showed moderate activity (22 L/g-surface-metal/min) and high NH4+ selectivity (>90% at 20% conversion) at high pH. Hydrazine (N2H4) was also detected as a reaction intermediate when NH4+ was formed. Microkinetic models built with energetics from density functional theory reveal that Rh catalysts are poisoned by NO* at low pH because of the rapid dissociative adsorption of protonated nitrite (HNO2) under acidic conditions, which was confirmed by in aqua surface-enhanced Raman spectroscopy. NO* poisoning of the Rh surface lessens at increased solution pH because NO2- does not dissociate as readily compared to HNO2, which explains why Rh exhibits higher activity in basic solutions. The microkinetic models further elucidate the competition between N2H4 and NH3/NH4+ formation as a function of pH, where we find that hydrogen surface coverage dictates product selectivity. These results update the common view that only Pd-based catalysts are effective for NO2- reduction and suggest unexplored avenues for nitrogen chemistry.
机译：通过Haber-Bosch方法的工业氮固定的意外后果是来自肥料径流的海洋，地面和表面水域的硝酸盐（NO 3-）和亚硝酸盐（NO2-）污染。过渡金属催化剂，特别是基于Pd的催化剂，可有效地除去NO 3- / NO 2-2-2或NH 4 +。 PD被认为是No3 / No2减少的最有效的金属，因此，很少有研究已经彻底探索了其他过渡金属的性能作为不同反应条件的函数。在这项工作中，我们使用PD作为基准调查了氧化铝支持的RH的NO2降低特性，在那里我们改变了散装溶液pH以探测反应条件对催化化学的影响。 PD预期在低pH下显示出高降低活性（289L / g-表面 - 金属/分钟）和高N 2选择性（> 99％在20％的转化率下），在高pH下接近不活动。令人惊讶的是，在低pH下无活性的rh催化剂显示出中等的活性（22L / g-表面 - 金属/分钟）和高pH值的高NH 4 +选择性（> 90％在20％的转化率下）。当形成NH 4 +时，还检测到肼（N2H4）作为反应中间体。具有来自密度函数理论的能量学建造的微芯室模型表明，由于在酸性条件下的酸性亚硝酸盐（HNO2）的快速解离吸附，rh催化剂在低pH下被效干，这是通过在Aqua表面增强的拉曼光谱中证实的质子化亚硝酸盐（HNO2）。 NO * RH表面的中毒在增加的溶液pH下减少，因为NO2-与HNO2相比，NO 2与HNO 2一起解释了为什么RH在基本溶液中表现出更高的活性。微因模型进一步阐明N2H4和NH3 / NH4 +作为pH的函数之间的竞争，在那里我们发现氢表面覆盖率决定了产品选择性。这些结果更新了仅Pd的催化剂对No2减少有效的公共视图，并提出了氮化学的未开发途径。

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