Elucidating the Significance of Copper and Nitrate Speciation in Cu-SSZ-13 for N₂O Formation during NH₃-SCR

Abstract

Unwanted N2O formation is a problem that has been noted in selective catalytic reduction (SCR) where copper zeolite catalysts are utilized. With its immense global warming potential and long-term stability, elevated atmospheric N2O has already been identified as a future challenge in the war on climate change. This paper explores the phenomenon of N2O formation during NH3-SCR over Cu-SSZ-13 catalysts, which are currently commercialized in automotive emissions control systems, and proposes a link between N2O production and the local copper environment found within the zeolite. To achieve this, a comparison is made between two Cu-SSZ-13 samples with different copper co-ordinations produced via different synthesis methods. A combination of synchrotron X-ray absorption near-edge spectroscopy, UV–vis, Raman, and density functional theory (DFT) is used to characterize the nature of copper species present within each sample. Synchrotron IR microspectroscopy is then used to compare their behavior during SCR under operando conditions and monitor the evolution of nitrate intermediates, which, along with further DFT, informs a mechanistic model for nitrate decomposition pathways. Increased N2O production is seen in the Cu-SSZ-13 sample postulated to contain a linear Cu species, providing an important correlation between the catalytic behavior of Cu-zeolites and the nature of their metal ion loading and speciation.

A [Cu2O2]2+ Core in Cu/ZSM-5 for Efficient Selective Catalytic Oxidation of Nitrogen-Containing VOCs and NH3

“…The intensity of the peaks at 35 300 and 22 700 cm –1 dropped to the background line, whereas the peak at 14 500 cm –1 remained unchanged after 60 min (Figure b). According to previous reports, the peak at 14 500 cm –1 , 22 700 cm –1 , and 35 300 cm –1 corresponded to isolated Cu 2+ , ,− the [Cu 2 O 2 ] 2+ core, − and mono-(μ-oxo) dicopper ([Cu 2 O] 2+ ) core, ,, respectively. Noteworthily, there is a complementary and reversible intensity shift of the peak at 35 300 and 22 700 cm –1 during introduction into DMF and oxygen, indicating there is an evolution from the [Cu 2 O 2 ] 2+ core to the [Cu 2 O] 2+ core upon DMF absorption and reverse evolution upon oxygen attack.…”

Dynamic Binuclear CuII Sites in the Reduction Half-Cycle of Low-Temperature NH3–SCR over Cu-CHA Catalysts

“…In addition to these discussions on SCR reactions that convert NO to N 2 , another unwanted side product, N 2 O, has now garnered more and more attention due to its high global warming potential. It is suggested that the low-temperature N 2 O formation proceeds in the RHC and is favored by high Cu-CHA oxidation states, − but its mechanism is still unclear and remains debated. While further efforts are needed to resolve all the mechanistic puzzles of the RHC in LT NH 3 –SCR, we believe that the present discussions may contribute a consistent framework to such endeavors.…”

Multiscale QM/MM modelling of catalytic systems with ChemShell

“…ChemShell provides both covalent and ionic embedded cluster models for heterogeneous catalysis applications. The covalent embedding model is extensively used for studies of porous catalytic materials, with a number of recent ChemShell investigations targeting microporous zeolites [202][203][204][205] and mesoporous MCM-41. 206,207 Nastase et al used QM/MM simulations of the zeolites H-ZSM-5 and H-Y to study a crucial step in the conversion of methanol to hydrocarbons where the intermediate dimethyl ether is formed.…”