Insights into the mechanisms of NH₃ inhibition on Cu-CHA SCR catalysts

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Nature Communications volume 17 , Article number: 7421 ( 2026 ) Cite this article

Ammonia (NH 3 ) inhibition during the selective catalytic reduction (SCR) of nitrogen oxides (NO x ) over Cu-exchanged chabazite (Cu-CHA) catalysts limits low-temperature performance, yet its molecular origin remains unclear. Here we show that excess NH 3 selectively suppresses the oxidation half-cycle of the SCR reaction while leaving the reduction half-cycle largely unaffected. By combining kinetic measurements, operando electron paramagnetic resonance spectroscopy, and density functional theory calculations, we identify hindered mobility of Cu + ions as the key factor. Specifically, NH 3 coordination increases the energy barrier for Cu + diffusion, preventing the formation of reactive Cu 2+ -oxo dimer intermediates required for efficient oxidation. Spectroscopic measurements further reveal that the extent of inhibition depends strongly on temperature and copper loading. These insights provide a mechanistic basis for mitigating NH 3 inhibition, suggesting that improved catalyst design and optimized operating conditions can enhance low-temperature SCR performance in practical emission control systems.

The selective catalytic reduction (SCR) of nitrogen oxides (NO x ) with ammonia (NH 3 ) over small-pore Cu-zeolites (e.g., Cu-CHA, Cu-AEI, Cu-LTA) is a cornerstone technology for controlling NO x emissions from lean-burn engines 1 , 2 . These materials offer high activity, selectivity, and hydrothermal stability, rendering them indispensable in advanced emissions control systems. In Cu-CHA, the primary active sites are isolated Cu II ions that exist in two distinct coordination environments: ZCu II OH, where Cu II is coordinated to a single framework Al and a hydroxyl group, and Z₂Cu II , where Cu II is stabilized by paired Al sites in the framework 3 , 4 , 5 .

The NH 3 -SCR reaction (4NO + 4NH 3 + O 2 → 4N 2 + 6H 2 O) proceeds via a redox mechanism involving a reduction half-cycle (RHC) and an oxidation half-cycle (OHC), with isolated Cu sites cycling between Cu II and Cu I states 4 , 5 , 6 , 7 , 8 . The RHC is achieved through the reduction of Cu II to Cu I by NO and NH 3 , forming linear [Cu I (NH 3 ) 2 ] + complexes. The solvation effect of NH 3 on Cu I sites decreases their electrostatic interaction with the negatively charged zeolite framework, granting mobility to these complexes 9 , 10 , 11 . The OHC involves the activation of O 2 on Cu I sites. Since O 2 activation requires four electrons, and Cu I can provide only one, the process is more facile when two Cu I sites are involved which leads to the formation of O 2 -bridged dimeric Cu II species 4 , 5 , 12 , 13 , 14 . The mobility of [Cu I (NH 3 ) 2 ] + complexes is crucial for the OHC as they need to diffuse between cages to co-locate with another [Cu I (NH 3 ) 2 ] + to form Cu dimers. This quasi-homogeneous process dominates below ~250 °C, while at higher temperatures Cu(I) ions become immobilized, and SCR proceeds via more heterogeneous pathways involving mononuclear Cu-nitrate/nitrite intermediates 4 , 15 , 16 , 17 , 18 .

The mobility of Cu ions, especially at low temperatures, is influenced by several physicochemical properties, including zeolite topology, Si/Al ratio, Al siting, Cu loading, and Cu speciation 5 , 9 , 19 , 20 , 21 . For example, a higher Si/Al ratio reduces Brønsted acidity and Cu exchange capacity, impacting both reactivity and diffusion 3 . Spatial inhomogeneities in Al distribution can further constrain Cu mobility due to the difficulty of intercage migration between Al-rich and Al-lean regions 9 . Cu loading also strongly affects Cu-Cu pairing probability and, thus, OHC kinetics: higher loadings promote dimer formation, while lower loadings reduce Cu ion encounters and amplify diffusion limitations 20 .

Reaction environment and solvation further modulate Cu mobility. NH 3 and H 2 O solvation lowers mobility restrictions at low temperatures, but excess NH 3 can occupy Brønsted sites and obstruct critical diffusion pathways (e.g., 8-membered ring windows), thereby impeding Cu transport 3 . Excess reactants such as NO, H 2 O, and O 2 can further increase the energy required for [Cu I (NH 3 ) 2 ] + diffusion. 22 . Additionally, certain reactants, such as NO 2 or O 2 , can destabilize mobile Cu I species or oxidize them into framework-bound Cu II , further limiting mobility and redox cycling 23 . These observations suggest that SCR performance is highly sensitive to local reaction conditions and the coordination environment of Cu sites.

Reactant-induced inhibition is a broader catalytic phenomenon wherein excess reactants or intermediates impair activity by forming inactive complexes, competing for adsorption, or hindering diffusion 23 , 24 , 25 . Understanding these mechanisms and their impacts is essential for optimizing catalytic processes. The inhibition of SCR by NH 3 is well-documented, particularly on vanadium-based catalysts, where NH 3 competes with NO for active V 2 O 5 sites 26 . NH 3 inhibition of fast SCR reactions on Fe-zeolites, on the other hand, results from NH 3 interacting with surface nitrates, preventing their reaction with NO 27 . However, despite growing interest in Cu-CHA, NH 3 inhibition in standard SCR has received limited attention. Most existing insights derive from theoretical predictions, indirect experimental evidence, or demonstration under transient conditions 19 , 22 , 28 , 29 , 30 , 31 . The lack of fundamental studies can be attributed to the focus on high Cu-loadings, where inhibition is not easily observable in steady-state tests. In real-world applications, Cu sites deactivate with time, reducing effective loading and thereby exposing the catalyst to NH 3 inhibition, especially under onboard excess dosing strategies aimed at avoiding parasitic NH 3 oxidation losses. Furthermore, NH 3 internal combustion engine exhaust may contain up to 2, 3 times as much NH 3 as NO x , raising the potential for NH 3 inhibition even on relatively fresh Cu-CHA catalysts.

In this study, we uncover the atomic-level mechanisms of NH 3 inhibition on Cu-CHA catalysts by integrating steady-state kinetic measurements, RHC/OHC-specific modeling, operando EPR spectroscopy, and DFT calculations. We demonstrate that NH 3 inhibition emerges above a Cu- and temperature-dependent critical NH 3 :NO ratio, where excess NH₃ impairs Cu ion mobility and selectively suppresses OHC kinetics. These findings provide actionable insights into catalyst design and operating conditions for mitigating NH 3 inhibition in next-generation SCR systems.

We first demonstrate the practical relevance of NH 3 inhibition on Cu-CHA during SCR by examining the performance of commercial degreened (DG) and field-aged (FA) samples. Sample details are provided in Supplementary Table 1 . As shown in Supplementary Fig. 1a , both DG and FA samples showed a steady increase in NOₓ conversion at 200 °C with rising ammonia-to-NO x ratios (ANR), up to 1.0 for DG and 0.5 for FA. Beyond these points, conversion declines with further increases in ANR, indicating NH 3 -induced inhibition above a critical ANR that depends on catalyst state. The manifestation of inhibition at 200 °C, a temperature relevant to real-world exhaust conditions, highlights the operational significance of this phenomenon. Transient light-off and light-down tests (Supplementary Fig. 1b ) further corroborate the presence of NH 3 inhibition.

To investigate its mechanistic origin, we synthesized a series of model Cu-CHA catalysts with Cu loadings of 0.48 wt%, 1.38 wt%, and 2.48 wt%, referred to as Cu-0.5, Cu-1.4, and Cu-2.5, respectively. The elemental compositions are detailed in Supplementary Table 1 . All samples were prepared from the same parent zeolite support (Si/Al ratio of ~12). Recent studies have shown that differences in the spatial distribution of [AlO 2 ], centers in the zeolite framework can influence Cu ion mobility and SCR activity 19 . Thus, by using a single parent support, we eliminate the confounding effects of both Cu and Al ions to ensure that our observations are primarily influenced by the state of the Cu ions.

Figure 1a presents the standard SCR NOₓ conversion as a function of temperature. As expected, NOₓ conversion at a given temperature increases with Cu loading. Cu-0.5 exhibits the well-known “seagull” shape behavior with a mid-temperature activity dip, which reflects the temperature-dependent shift from quasi-homogeneous catalysis (Cu I mobility enabled by NH 3 solvation) below 250 °C to heterogeneous SCR above 300 °C involving less mobile, framework-bound Cu sites 4 . The low-temperature SCR OHC involves oxygen activation on a dimeric oxygen-bridged copper species, requiring isolated Cu I ions to migrate between cages to reside in proximity with another Cu I ion. Negri and coworkers used in-situ X-ray absorption and infrared spectroscopies, along with DFT calculations, to reveal that these dimers were formed by two mobile linear [Cu I (NH 3 ) 2 ] + complexes, consisting of a side-on μ-η 2 ,η 2 -peroxo diamino copper(II) structure ([Cu II 2 (NH 3 ) 4 (O 2 )] 2+ ) 12 . At low Cu loading (Cu-0.5), the reduced likelihood of Cu, Cu encounters limits the formation of dimeric Cu-oxo species needed for efficient OHC, intensifying this kinetic bottleneck. This explains the lower activity and pronounced seagull dip in these samples. In contrast, Cu-1.4 shows only a subtle dip, while Cu-2.5 displays a monotonic increase, reflecting reduced kinetic constraints with higher Cu content.

a NO x conversion during standard SCR as a function of temperature for three Cu-CHA catalysts with different copper loadings: Cu-0.5 (0.48 wt% Cu), Cu-1.4 (1.39 wt% Cu), and Cu-2.5 (2.48 wt% Cu), measured at NH 3 :NO = 1. b NO x conversion as a function of NH 3 :NO molar ratio ( x -axis) for Cu-1.4 and Cu-0.5 at multiple temperatures: 220 °C, 200 °C, and 160 °C for Cu-1.4; and 220 °C, 200 °C, and 175 °C for Cu-0.5. Feed conditions: 350 ppm NO, 10% O 2 , 3% H 2 O and varying NH 3 levels to achieve desired NH 3 -to-NO x ratios, at 150,000/h gas hourly space velocity (GHSV).

Figure 1b presents the SCR NOₓ conversion on Cu-1.4 and Cu-0.5 as a function of ANR across temperatures ranging from 160 to 220 °C. Both samples exhibit a clear inhibition regime at elevated ANR. Two systematic trends emerge: (a) for a given catalyst, the critical ANR, defined as the ratio beyond which activity begins to decline − decreases with temperature (e.g., for Cu-1.4: 1.25 at 220 °C, 1.1 at 200 °C, 0.6 at 160 °C); and (b) at a given temperature, the critical ANR decreases with Cu loading (e.g., 1.25 for Cu-1.4 vs. 0.4 for Cu-0.5 at 220 °C). These trends highlight a strong interplay between Cu mobility and NH 3 concentration. We selected Cu-0.5 for further kinetic evaluations as it provides a copious amount of data points in the kinetically limited region (<20% NO x conversion). It is essential to note that, while we require data under kinetic limitation for reliable rate parameter estimations (i.e., negligible reactant or product mass transfer limitation), the NH 3 inhibition process is not only confined to low NO x conversions; Cu-1.4 exhibits inhibition at conversions as high as 85%, underscoring the practical importance of this effect. Further supporting this notion, the SCR light-off and light-down performance of Cu-2.5, shown in Supplementary Fig. 2 and discussed in Supplementary Note 2 , illustrates significant NH 3 inhibition below 220 °C during transient operations typical of real-world driving conditions. This sample has a high Cu loading similar to fresh co