Identification of the mechanism of NO reduction with ammonia (SCR) on zeolite catalysts

Cu/zeolites efficiently catalyze selective reduction of environmentally harmful nitric oxide with ammonia. Despite over a decade of research, the exact NO reduction steps remain unknown. Herein, using a combined spectroscopic, catalytic and DFT approach, we show that nitrosyl ions (NO+) in zeolitic micropores are the key intermediates for NO reduction. Remarkably, they react with ammonia even below room temperature producing molecular nitrogen (the reaction central to turning the NO pollutant to benign nitrogen) through the intermediacy of the diazo N2H+ cation. Experiments with isotopically labeled N-compounds confirm our proposed reaction path. No copper is required for N2 formation to occur during this step. However, at temperatures below 100 °C, when NO+ reacts with NH3, the bare Brønsted acid site becomes occupied by NH3 to form strongly bound NH4+, and consequently, this stops the catalytic cycle, because NO+ cannot form on NH4-zeolites when their H+ sites are already occupied by NH4+. On the other hand, we show that the reaction becomes catalytic on H–zeolites at temperatures when some ammonia desorption can occur (>120 °C). We suggest that the role of Cu(ii) ions in Cu/zeolite catalysts for low-temperature NO reduction is to produce abundant NO+ by the reaction: Cu(ii) + NO → Cu(i)⋯NO+. NO+ then reacts with ammonia to produce nitrogen and water. Furthermore, when Cu(i) gets re-oxidized, the catalytic cycle can then continue. Our findings provide novel understanding of the hitherto unknown steps of the SCR mechanism pertinent to N–N coupling. The observed chemistry of Cu ions in zeolites bears striking resemblance to the copper-containing denitrification and annamox enzymes, which catalyze transformation of NOx species to N2, via di-azo compounds.

a combined spectroscopic, catalytic and DFT approach, we show that nitrosyl ions (NO + ) in zeolitic micropores are the key intermediates for NO reduction. Remarkably, they react with ammonia even below room temperature producing molecular nitrogen (the reaction central to turning the NO pollutant to benign nitrogen) through the intermediacy of the diazo N 2 H + cation. Experiments with isotopically labeled N-compounds confirm our proposed reaction path. No copper is required for N 2 formation to occur during this step. However, at temperatures below 100 C, when NO + reacts with NH 3 , the bare Brønsted acid site becomes occupied by NH 3 to form strongly bound NH 4 + , and consequently, this stops the catalytic cycle, because NO + cannot form on NH 4 -zeolites when their H + sites are already occupied by NH 4 + . On the other hand, we show that the reaction becomes catalytic on H-zeolites at temperatures when some ammonia desorption can occur (>120 C). We suggest that the role of Cu(II) ions in Cu/zeolite catalysts for low-temperature NO reduction is to produce abundant NO + by the reaction: Cu(II) + NO / Cu(I)/NO + . NO + then reacts with ammonia to produce nitrogen and water. Furthermore, when Cu(I) gets re-oxidized, the catalytic cycle can then continue. Our findings provide novel understanding of the hitherto unknown steps of the SCR mechanism pertinent to N-N coupling. The observed chemistry of Cu ions in zeolites bears striking resemblance to the copper-containing denitrification and annamox enzymes, which catalyze transformation of NO x species to N 2 , via di-azo compounds.
Air pollution is one of the main health and environmental concerns in our (post)industrial society. [1][2][3] Worsened air quality during industrial expansion is attributed to a great extent to toxic nitric oxide (NO) gas, where nearly 55% of the global emissions are due to transportation exhaust. The ability of Cu/ zeolites to turn NO to N 2 in the presence of ammonia was rst discovered in Japan in the 1960s 4 for the Cu/FAU zeolite. However, the FAU framework is less stable than the corresponding SSZ-13 and BEA zeolites. 5 Within the last decade, the ammonia selective catalytic reduction (SCR) technology was implemented on the large scale [5][6][7][8][9][10][11][12][13][14][15][16][17][18] for diesel engines on the basis of Cu(Fe) in BEA and SSZ-13 zeolites.
We exposed the NO + /zeolite to 15 N-labeled ammonia at room temperature ($20 C) ( Fig. 2A) (Fig. 2A). The analysis of the gas-phase product by mass-spectrometry reveals a major peak at 29 amu per charge, corresponding to 14 N-15 N molecules (Fig. 2B). Thus, NO + reacts with ammonia to form molecular nitrogen at room temperature on H-BEA. Eqn (2) and (3) can be used to describe this process. Note that NO + does not necessarily have to be bound to zeolite in the presence of NH 3 and can initially become solvated by NH 3 (it is wellknown that ammonia has propensity to solvate cationic species in and outside zeolites; 16,17,24 our DFT calculations in Table 1 further conrm strong favorability of NO + solvation by ammonia with an energy gain of 95 kJ mol À1 ); the designation zeolite-NO + is just a representation of the NO + /H-zeolite system In situ FTIR during the first sequential NO adsorption (0.1 Torr equilibrium pressure), followed by sequential O 2 addition (0.17 Torr total eq. pressurewhen O 2 is added, NO + begins to develop in significant amounts); T ¼ 20 C. (B, right spectrum) FTIR spectra collected after pulling high vacuum 10 À8 Torr from time ¼ 0 to time ¼ 10 minutes at 150 C. Spectra were recorded after cooling back to 20 C. in which NO + may be solvated or semi-solvated by ammonia molecules.

Zeolite
The hydronium diazonium cation (N 2 H + ) is extremely unstable. Its immediate decomposition to N 2 and H-zeolite drives the reaction forward. Despite this, we nd that at sufficiently high molecular N 2 pressure in the cell ($15 Torr), we can observe a small N-N stretch of the -HN 2 + complex at 2334 cm À1 even at room temperature (Fig. 3A), which was previously shown to form at low temperatures upon N 2 interaction with Brønsted acid sites (Fig. 3B). [25][26][27] To provide proof that the reaction proceeds through a diazo compound, we chose aniline PhNH 2 , an equivalent of the ammonia molecule but with 1 hydrogen atom substituted by a phenyl group, and it reacted with NO + in the BEA zeolite. The phenyl group stabilizes the formation of PhN 2 + salts (phenyl diazonium salts) through the mesomeric effect, and unlike alkyl diazonium salts, aryl diazonium compounds are stable and characterized by N-N stretches in the $2250-2300 cm À1 region, more specically $2270 cm À1 for phenyl diazonium in solution. 28 Indeed, we monitored the reaction of NO + and PhNH 2 , spectroscopically: the intensity of the NO + band diminished, and a new N-N stretch appeared at $2270 cm À1 , corresponding to the N-N vibration of the Ph-N 2 + fragment (Fig. 4). The reaction of NO + with NH 3 is more sluggish than with NH 3 , No direct participation of the chabazite at some reaction steps consistent with the higher activation barrier calculated for PhN 2 + formation (Table 1). This is the rst observation of a diazo-salt stabilized on a solid support (zeolite). NO + in H-SSZ-13 (with Si/Al $ 12; typical HAADF-STEM images of this sample are shown in Fig. S7 †) reacts similarly with NH 3 (Fig. 5), with concomitant N 2 evolution (conrmed by mass spectrometry): As such, NO + is the critical intermediate species in the conversion of NO in these zeolites. Copper is not required to observe the NO + reactivity with NH 3 . Moreover, we reacted inorganic nitrosyl salt NO + with 15 NH 3 and observed 14 N-15 N in the gas-phase (consistent with our ndings for NO + in zeolites; this reaction takes place vigorously even at a temperature as low À50 C) (eqn (4) This reaction (eqn (4)) most likely proceeds through the N 2 H + intermediate as well.
For NO + in the zeolite, once it reacts with NH 3 with the release of N 2 , the Brønsted acid is free and immediately interacts with ammonia to produce NH 4 + . This latter process "kills" the reactivity as the NO + species can no longer be produced due to the necessity of Brønsted acid sites, as evidenced by FTIR (Fig. S4 †); indeed, no NO + evolves above trace amounts upon the NO + O 2 reaction on the NH 4 -zeolite (Fig. S4 †). Only at elevated temperatures, when some NH 3 can desorb and free up a portion of Brønsted acid sites to re-form NO + , can the reaction proceed In situ sequential FTIR during 14 NH 3 adsorption (total equilibrium pressure $ 0.15 Torr) on the NO + / H-SSZ-13 sample at 20 C. Ammonia reacts with NO + as evidenced by the disappearance of the NO + stretch. The intense band at $2110 cm À1 is due to zeolite interactions with NH 3 . Pulling vacuum on the sample at 120 C produces spectra showing no NO + stretches ( Fig. S3 †), consistent with the complete reaction of NO + .  catalytically which we show in Fig. S5. † The bare H-BEA zeolite is catalytically active for NO reduction with NH 3 in dry streams (see Fig. S5 † for reactivity at 200 and 150 C with time-onstream) (measurable activity is observed). NO + is formed in zeolite through eqn (1) (see the earlier discussion in the manuscript).
With these new data, we can now further explain the possible role of Cu in zeolites for SCR. As is well-established in the literature, Cu(II) ions are required for the continuous catalytic reaction to proceed. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] In the presence of NO, Cu(II) ions can produce NO + via 1-electron reduction of Cu(II) (analogous chemistry is observed, for example, for Pd(II) in zeolites 30 where Pd(II) was shown to reduce to Pd(I) by NO pulses with the concomitant formation of NO + ) (eqn (5) We have previously been able to conrm by solid-state 15 N NMR studies that NO indeed is capable of reducing Cu(II) to Cu(I) in SSZ-13: the resulting complex had Cu(I) and NO + in proximity to each other, with NO + located side-on towards a copper ion. 19 Similar chemistry is observed for Pd(II) in zeolites. 29 The mechanism of NO + -zeolite formation from the H +zeolite and NO and/or NO 2 has been investigated experimentally 22,34 and theoretically. 35 It was shown 35 that the barrier for NO + formation on H + -CHA from NO 2 is only 15 kJ mol À1 , while the process is endothermic by only 2 kJ mol À1 . Our novel experimental ndings for NO + in zeolites prompted us to investigate the proposed reaction steps with density functional theory (DFT) calculations. First, we investigated two pathways for selective catalytic reduction of NO by the ammonia via formation of the NO + species in the zeolite: (1) with the direct (Fig. 7, Table 1) participation of the zeolite and (2) without (Fig. 8, Table 1) the direct zeolite participation at some reaction steps. Both pathways start with adsorption of NH 3 to NO + /Zeo with a binding energy of ammonia of $97 kJ mol À1 and subsequent formation of nitrosamine via transfer of one of the H atoms from the ammonia to an O center from the AlO 4 tetrahedron.
When the reaction step occurs via direct H transfer, the energy barrier is only 5 kJ mol À1 (Fig. 7B, Table 1). We also found another transition state (TS) structure with elongated N-H distance (Fig. 8, Table 1), and it is less stable by more than 120 kJ mol À1 than the previous one, despite the fact that the zeolite participates in the reaction step since one of the H atoms from NH 3 is transferred to the zeolite as a proton. Further, one of the H atoms of the NH 2 group of the nitrosamine should migrate to the O center of the same molecule thus forming a HON]NH molecule. This can be done via H transfer occurring in the gas phase without the participation of the zeolite. In this case however, the reaction step is endothermic by 41 kJ mol À1 , while the barrier is as high as 148 kJ mol À1 . Alternatively, the process can occur stepwise with the direct participation of the zeolite support. In this case initially the molecule of nitrosamine reorientates so that two hydrogen bonds are formed between the guest molecule and support: the N-H fragment with the O center from a zeolite and a zeolite proton with the O center from the nitrosamine. The new conguration is more stable by 31 kJ mol À1 than the previous one.
Further, via a synchronous transition state structure, the H atom from the NH fragment moves to the zeolite O center, and simultaneously, the zeolite proton migrates to the O atom from the guest molecule. The energy barrier is only 13 kJ mol À1 , while the reaction step is only slightly endothermic by 12 kJ mol À1 . Similar energetics were also reported previously. 36 In the next step, the HON]NH molecule should be converted into the nal products: H 2 O and N 2 . This can be done again via direct H transfer in the gas phase from the N to the O atom with no direct participation of the zeolite support. Based on the previous knowledge, it is expected that the reaction step will have a high barrier. Alternatively, we considered the process with the participation of the zeolite, as two reaction pathways were considered: concerted and stepwise pathways, as in both cases, initially, the guest molecule changes its position with respect to the zeolite support as one of the hydrogen bonds, NH/O, breaks. The new structures are less stable by 18 and 29 kJ mol À1 than the nal state structure of the previous reaction step. In the concerted mechanism, in one step, the zeolite proton moves to the OH group from the HON]NH species, thus forming a water molecule, while the H bound to the N atom moves to an O zeolite center forming a N 2 molecule. The reaction step is strongly exothermic, À225 kJ mol À1 , with a very low barrier, 8 kJ mol À1 . The high exothermicity 36,37 and the low barrier 36 were also inferred in previous theoretical studies. Alternatively, in the stepwise mechanism, H 2 O and NNH + can be formed rst, overcoming a low barrier of 14 kJ mol À1 as the reaction step is slightly exothermic, À8 kJ mol À1 . Next, NNH + reorients so that a hydrogen bond is formed with the zeolite O center. In the nal step, H moves to the zeolite O center; this bridging OH group and N 2 molecule are formed. This step is essentially barrierless and strongly exothermic, À207 kJ mol À1 . These DFT results conrm favorability of NO + interaction with NH 3 to form N 2 basically with little to no barriers, consistent with the experimentally observed low-temperature reactivity of these species. Similar mechanisms of NH 2 NO decomposition were reported 36,37 as they considered additional steps of transfer of the zeolite proton from one basic O center to another assisted by the O or N atom in the HNNOH, which seems to be the highest ones according to the latter study. According to us, such steps can be omitted as shown by the mechanism proposed by us.
Next, we modeled the reduction process using aniline PhNH 2 interacting with the NO + species positioned as a chargecompensating cation in chabazite (Fig. 9, Table 1).
The adsorption energy of aniline to NO + species is À186 kJ mol À1 . Further, one of the H atoms of the amino groups migrates to the O center from the zeolite, thus forming ONNHPh coordinated to the zeolite proton. This reaction step is slightly endothermic by 5 kJ mol À1 , and its energy barrier is Fig. 8 Energy diagram and optimized models of reaction steps with no direct participation of the chabazite at some reaction steps. The models are visualized with the VESTA program. 33 31 kJ mol À1 . From the latter structure, one can obtain a benzenediazonium cation, PhNN + , via two alternative pathways: -In the more plausible pathway, the HONNPh structure is formed via a synchronous TS structure in which a zeolite proton migrates onto the O atom from the organic molecule, while the H atom from the NH fragment moves to a zeolite O center. The reaction step is essentially barrierless and very slightly exothermic, À4 kJ mol À1 . Further, the zeolite proton interacts with the OH group from the organic molecule which leads to formation of PhNN + and a water molecule. The barrier for this reaction step is 23 kJ mol À1 , and it is exothermic by 57 kJ mol À1 .
-The alternative pathway requires overcoming of higher barriers than in the rst one. In the rst step, the zeolite proton migrates to the O center from the organic molecule as PhNH] NOH + is formed. The process is essentially barrierless and exothermic by 49 kJ mol À1 . At the nal stage, a PhNN + cation should be formed, as we considered two possible reaction pathways. In the concerted mechanism, in one step, the O center forms bonds with both H centers which are initially positioned at the zeolite O center and the N atom from the organic molecule. The second H transfer is in the gas phase without direct participation of the zeolite which leads to an unstable transition state structure, and thus, the barrier is 158 kJ mol À1 , and reaction energy is only slightly exothermic by 12 kJ mol À1 . In the stepwise mechanism, a H atom bound to the N center is transferred to an O zeolite center forming a HON]NPh molecule. This reaction step is endothermic by 57 kJ mol À1 , and the barrier is 65 kJ mol À1 . Finally, the zeolite proton interacts with the OH group which leads to formation of H 2 O and a PhNN + cation. This step requires overcoming of a very low barrier, 11 kJ mol À1 , and it is exothermic by 69 kJ mol À1 .
Furthermore, because we observed that the NO + reaction with ammonia is not exclusive to zeolite and that inorganic nitrosyl salts (such as NO[BF 4 ]) react with ammonia to form molecular nitrogen at a temperature as low as À50 C, two pathways for the SCR reaction with ammonia on BF 4 À /NO + were investigated: (1) with and (2) without the direct participation of the boron uoride complex (Fig. 10, Table 1). Both schemes start with adsorption of ammonia onto the BF 4 À /NO + substrate forming the H 3 NNO + species, as the binding energy is À91 kJ mol À1 . In the rst step, one of the H atoms of H 3 NNO + moves to the BF 4 À anion. The reaction step is slightly endothermic by 15 kJ mol À1 and requires overcoming a barrier of 16 kJ mol À1 . In the nal structure, the nitrosamine and hydrogen uoride are produced and bound by a hydrogen bond between H from the BF 3 -HF complex and the N from the amino group. Further, a rearrangement of the complex occurs, so that the O atom interacts with the proton from the BF 3 -HF complex. This complex is more stable by 25 kJ mol À1 than the previous one, while if a second hydrogen bond is formed, a further  is the most probable as it is essentially barrierless and highly exothermic by 221 kJ mol À1 . In order to assess the favorability of Cu(II) reduction by NO in SSZ-13, we considered the reduction of Cu 2+ located in six and eight membered rings of a CHA type zeolite structure containing two Al centers by NO (the structures are shown in Fig. 11). First, we modeled formation of a Cu 2+ (NO) complex in the zeolite, which is exothermic by À145 and À203 kJ mol À1 , respectively. In the second step, the complex converts to two cationic species (Cu + and NO + ); each of them compensates for one Al center. This step is endothermic by 31 and 43 kJ mol À1 . Thus, the overall exothermicity of the reduction of Cu 2+ by NO to Cu + and NO + is À114 and À160 kJ mol À1 , respectively. The second reaction step seems to be also kinetically feasible, as the calculated barrier for the case of Cu 2+ located in the six membered ring of CHA is 62 kJ mol À1 . These DFT results are fully consistent with our experimental ndings.

Zeoð2Al_6ringÞ
Cu 2þ þ NOðgÞ ! To summarize the DFT results, we investigated two pathways for the NO reduction reaction with ammonia on both substrates (zeolite/NO + and BF 4 À /NO + ): (1) with and (2) without the direct participation of the substrate. When the reaction occurs on the zeolite/NO + system with the participation of the zeolite (Fig. 7), the barriers are very low (all of them are below 20 kJ mol À1 ), manifesting that the reaction can occur at very low temperatures, in line with our experimental results. However, if the zeolite does not participate in the catalytic process directly, the barriers are >130 kJ mol À1 (Fig. 8). According to our calculations, nitrosamine (NH 2 NO) can be formed as an intermediate (see Fig. 7), but the barrier for its transformation is very low. Similarly, the NNH + species can be formed as well (Fig. 7), which we prove experimentally through studies with the phenyl diazonium cation (that is more stable than NNH + shown in Fig. 4). The results with the other substrate, BF 4 À /NO + , are similar (Fig. 10). When NH 3 is decomposed with the participation of the HF-BF 3 substrate, the barriers are very low, while if the substrate does not participate in the reaction directly, some of the barriers become higher than 120 kJ mol À1 . Similar results are also found for the aniline reduction ( Fig. 9), where PhNN + Fig. 11 Optimized structures of selected models. The models are visualized with the VESTA program. 33 can be formed via the catalytic role of the zeolite as the highest barrier in the initial rate-limiting step is only 31 kJ mol À1 . Based on these combined theoretical and experimental data, we can suggest that the role of copper is to promote NO + formation since Brønsted acid sites get occupied by NH 4 + aer NO + reacts with NH 3 on the bare H-zeolite and cannot contribute to NO + formation as we show herein. The resulting NO + reacts with ammonia to reform N 2 . The Cu(I)(NH 3 ) 2 complex, in turn, gets re-oxidized back to Cu(II) with oxygen. 16,17 Notably, this bears striking resemblance to the recent advances in enzymatic chemistry of Cu-containing enzymes for denitri-cation and annamox (anaerobic ammonia oxidation to nitrogen) processes. Only in recent decades, the pioneering studies of Murphy and co-workers (for denitrication) 31 and Kartal, Strous, and co-workers (for annamox) 32 revealed the central role of the Cu(I)/NO + intermediate and diazocompounds in these processes. Our data point to the presence of a similar active site for the Cu-zeolite system (used industrially in vehicles) and Cu-enzymes, occurring in nature. We note that our ndings represent the rst observation of the potential intermediates of NO reduction with ammonia (SCR) responsible for N-N bond formation for zeolites and copper-zeolite systems and lay out the strategy to investigate vehicle-relevant SCR under more complex gas feed conditions (that include water and hydrocarbons, in addition to oxygen, NO and ammonia); under these complex vehicle-relevant gas conditions, additional mechanistic pathways, leading to N-N coupling, may also be operative.

Data availability
The data are available in the main text and the ESI. †

Author contributions
KK conceived the project, performed most synthesis, catalytic, infra-red experiments, analyzed data and wrote the manuscript, KK and NRJ obtained funding through the QuickStarter LDRD program at PNNL. NRJ, JHK, MAD, JS performed synthesis and spectroscopy experiments, analyzed data and contributed valuable discussion. HAA, IZK, GNV perfromed all DFT calculations, discussed the data and co-wrote the DFT section of the manuscript. KK, MAD, NRJ, JHK, IZK, HAA, GNV, YW and JS discussed all the data and the nal manuscript.

Conflicts of interestn
The authors have no conicts to declare.