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Kinetic Model for the Reduction of Cu<sup>II</sup> Sites by NO + NH<sub>3</sub> and Reoxidation of NH<sub>3</sub>-Solvated Cu<sup>I</sup> Sites by O<sub>2</sub> and NO in Cu-SSZ-13

DOIFull Paper Access

47

Citations

57

References

2022

Year

Rohil Daya, Dylan Trandal, Unmesh Menon, Dhruba Jyoti Deka, William P. Partridge, Saurabh Y. Joshi
ACS Catalysis

Abstract

In this work, a kinetic model is developed for the reduction of Cu&lt;sup&gt;II&lt;/sup&gt; sites by NO + NH&lt;sub&gt;3&lt;/sub&gt; and the reoxidation of NH&lt;sub&gt;3&lt;/sub&gt;-solvated Cu&lt;sup&gt;I&lt;/sup&gt; sites by O&lt;sub&gt;2&lt;/sub&gt; and NO in Cu-SSZ-13. Fourier transform infrared (FTIR) spectroscopy and spatially resolved capillary inlet mass spectrometry (SpaciMS) measurements during transient reactor experiments are utilized to identify the rate parameters associated with NO + NH&lt;sub&gt;3&lt;/sub&gt; RHC (reduction half-cycle), proposed to occur via two distinct pathways involving adsorbed NH&lt;sub&gt;3&lt;/sub&gt; and gas-phase NH&lt;sub&gt;3&lt;/sub&gt;. The resulting NO + NH&lt;sub&gt;3&lt;/sub&gt; RHC model is validated using spatiotemporal N&lt;sub&gt;2&lt;/sub&gt; measurements covering a wide range of temperatures (200–450 °C) and space velocities (53,000–640,000 h&lt;sup&gt;–1&lt;/sup&gt;). N&lt;sub&gt;2&lt;/sub&gt;O formation is observed and modeled during NO + NH&lt;sub&gt;3&lt;/sub&gt; RHC, with quantitative validation under standard selective catalytic reduction (SCR) conditions. Experimentally measured enthalpic and entropic changes associated with O&lt;sub&gt;2&lt;/sub&gt; adsorption on NH&lt;sub&gt;3&lt;/sub&gt;-solvated Cu&lt;sup&gt;I&lt;/sup&gt; (ZCu(NH&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;) complexes [&lt;contrib-group&gt;&lt;span class="NLM_string-name"&gt;Kamasamudram, K.&lt;/span&gt;&lt;/contrib-group&gt; &lt;cite&gt;&lt;i&gt;Catal. Today&lt;/i&gt;&lt;/cite&gt; &lt;span class="NLM_year"&gt;2010&lt;/span&gt;, &lt;em&gt;151&lt;/em&gt;(3–4), 212-222], along with activation energies estimated computationally for the intercage diffusion of ZCu(NH&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt; complexes [&lt;contrib-group&gt;&lt;span class="NLM_string-name"&gt;Paolucci, C.&lt;/span&gt;&lt;/contrib-group&gt; &lt;cite&gt;&lt;i&gt;Science&lt;/i&gt;&lt;/cite&gt; &lt;span class="NLM_year"&gt;2017&lt;/span&gt;, &lt;em&gt;357&lt;/em&gt;(6 354), 898-903], are incorporated into a mean field kinetic model for the low-temperature oxidation half-cycle (OHC). Significant NH&lt;sub&gt;3&lt;/sub&gt; release is observed during the isothermal oxidation of Cu&lt;sup&gt;I&lt;/sup&gt; sites, attributed to desorption of NH&lt;sub&gt;3&lt;/sub&gt; ligands from NH&lt;sub&gt;3&lt;/sub&gt;-solvated Cu&lt;sup&gt;II&lt;/sup&gt; dimers (Z&lt;sub&gt;2&lt;/sub&gt;Cu&lt;sub&gt;2&lt;/sub&gt;(NH&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;4&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;). Reduction of these dimeric complexes leads to the consumption of one NO/Cu&lt;sup&gt;II&lt;/sup&gt;, contradicting the expected reduction stoichiometry. Inclusion of a global Arrhenius rate for the NO titration of Z&lt;sub&gt;2&lt;/sub&gt;Cu&lt;sub&gt;2&lt;/sub&gt;(NH&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;4&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt; complexes provides accurate representations of standard SCR on reduced and oxidized catalysts, predicting transient NO and NH&lt;sub&gt;3&lt;/sub&gt; consumption between 150 and 250 °C as a function of hydrothermal aging. Deactivation of low-temperature standard SCR by NH&lt;sub&gt;3&lt;/sub&gt; is observed at high NH&lt;sub&gt;3&lt;/sub&gt; pressures, modeled via the formation of superoxo amino (ZCu(NH&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;OO*) complexes during NH&lt;sub&gt;3&lt;/sub&gt; titration of Z&lt;sub&gt;2&lt;/sub&gt;Cu&lt;sub&gt;2&lt;/sub&gt;(NH&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;4&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt; complexes [&lt;contrib-group&gt;&lt;span class="NLM_string-name"&gt;Negri, C.&lt;/span&gt;&lt;/contrib-group&gt; &lt;cite&gt;&lt;i&gt;J. Am. Chem. Soc.&lt;/i&gt;&lt;/cite&gt; &lt;span class="NLM_year"&gt;2020&lt;/span&gt;, &lt;em&gt;142&lt;/em&gt;(37), 15884-15896]. The redox kinetic model presented here provides a foundational description of active site redox during low-temperature standard SCR, combining the recent kinetic, spectroscopic, and computational findings on the mechanism of standard SCR over Cu-SSZ-13.

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