Redox mechanisms have been theorized for the selective catalytic reduction (SCR) of NO_x over small-pore Cu-zeolites. These mechanisms generally rely on NH₃ solvation of active copper sites as the initial step in the reduction half cycle for standard SCR. In this work, we demonstrate that this pre-requisite for NH₃ solvation of active copper sites is inconsistent with experimental data for initial NO consumption under SCR conditions. Reactor data under standard SCR and reduction half cycle (no O₂) conditions shows instantaneous consumption of NO upon introduction of NO and NH₃. However, detailed and global kinetic models relying on a sequential adsorption–conversion mechanism predict an initial delay in NO consumption associated with the relatively slow macroscopic coverage dynamics. These model discrepancies at low NH₃ coverage are quantitatively resolved by relaxing the pre-requisite of NH₃ storage for NO_x conversion, via an alternate SCR pathway utilizing gas-phase NH₃. Addition of this standard SCR reaction via gas-phase NH₃ leads to predicted NH₃ reaction orders >1 at low NH₃ coverage, confirmed subsequently by experiments. The results shown here are consistent with an energetically feasible mechanistic pathway involving the reduction of copper sites by NO to generate mobile HONO intermediates, which can potentially react with gas-phase NH₃ in the zeolite cages.

中文翻译：

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含氨的铜沸石上标准SCR的替代途径

氧化还原机制已经被理论化的选择性催化还原（SCR）的NO _X在小孔的Cu-沸石。这些机制通常依赖于活性铜位点的NH ₃溶剂化，作为标准SCR还原半循环的第一步。在这项工作中，我们证明了活性铜位点NH ₃溶剂化的先决条件与SCR条件下初始NO消耗的实验数据不一致。在标准SCR和还原半循环（无O ₂）条件下的反应器数据显示，引入NO和NH ₃后瞬时消耗NO。然而，依赖于顺序吸附-转化机制的详细的整体动力学模型预测，与相对较慢的宏观覆盖动力学有关的NO消耗会出现初始延迟。在低NH ₃覆盖率下，这些模型差异可通过使用气相NH ₃的备用SCR途径，通过放宽NH ₃储存进行NO _x转化的先决条件来定量解决。通过气相NH ₃添加此标准SCR反应可导致在低NH _3下预测的NH ₃反应阶数> 1覆盖率，随后通过实验确认。此处显示的结果与在能量上可行的机制途径一致，该途径涉及通过NO还原铜位以生成可移动的HONO中间体，该中间体可能与沸石笼中的气相NH ₃反应。