First-Principles Thermodynamic Background of the Comprehensive Reaction Network of NO Oxidation over CuSSZ-13 Catalysts─Influence of Copper Speciation and Interpretation of TPD and TPSR Profiles

Abstract

A thorough molecular DFT modeling coupled with first-principles thermodynamic (FPT), spectroscopic (EPR/IR), and catalytic investigations into a complex network of reactions involved in the interaction of NO and O₂ with a comprehensive variety of active centers present in the CuSSZ-13 zeolites (Cu²⁺, Cu⁺, Cu²⁺–OH^–, Cu²⁺–O^2––Cu²⁺, Cu²⁺–O₂^2––Cu²⁺, and segregated CuO) were carried out. The molecular structure, energetics, and electronic and magnetic properties of the identified profuse adspecies and intermediates were ascertained. Their thermal stability and reactivity at a wide range of experimental conditions were interpreted by using the constructed thermodynamic ΔG(p,T) diagrams. The course of selective catalytic oxidation of NO (NO–SCO) with ¹⁶O₂ or ¹⁸O₂ was examined by a temperature-programmed surface reaction (TPSR) using two types of CuSSZ-13 catalysts of intentionally diverse copper speciation. The results obtained, supported by the corroborative IR and EPR measurements, revealed multiple molecular pathways of the NO and O₂ interactions with the single (Cu²⁺, Cu⁺, Cu²⁺–OH^–) and dual (Cu²⁺–O^2––Cu²⁺, Cu²⁺–O₂^2––Cu²⁺) copper centers of the 6MR and 8MR topologies and with segregated CuO. The complex reaction network and temperature behavior of the critical intermediates (HONO, nitrate, and nitrite), and their evolvement routes into NO₂, were rationalized using the calculated FPT thermodynamic profiles. The unraveled reactions were classified into metal (cationic) redox, ligand (anionic) redox, and HONO redox cycles. The Cu²⁺–OH^– species were identified as prime active centers for the formation of NO₂ via the HONO pathway. The elusive HONO intermediates allow for chemical communication between the individual redox cycles. Depending on the actual reaction conditions, HONO can act as a reduction agent for Cu²⁺ with the electroprotic formation of NO₂, a source of nitrites upon deprotonation, or as an oxidant of Cu⁺ with the formation of H₂O and NO. For the metal redox pathway, a significant difference in the reactivity between the Cu²⁺ cations accommodated in the 6MRs and 8MRs was observed, with the Cu²⁺/6MR being spectators and the Cu²⁺/8MR active species. Dimeric copper centers with bridging oxo and peroxy moieties can produce a variety of nitrates and nitrites via ligand redox mechanisms. Segregated CuO nanocrystals contribute to NO oxidation only at high temperatures (T > 400 °C), leading to the isotopic scrambling of ¹⁸O-labeled oxygen and nitric oxide. The established complex reaction network was successfully used to clarify the temperature dependence of the experimental NO–SCO profiles, also providing a suitable mechanistic background for interpreting the nature of the oxidative half-cycle of the selective catalytic reduction of NO over Cu-SSZ-13 catalysts.