Kinetics of the selective catalytic reduction of NO by NH3 on a Cu-faujasite catalyst

Applied Catalysis B: Environmental 52 (2004) 251–257

Kinetics of the selective catalytic reduction of NO by NH3 on a Cu-faujasite catalyst Gérard Delahay, Stéphane Kieger, Nathalie Tanchoux, Philippe Trens, Bernard Coq∗ Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 ENSCM-CNRS, 8, Rue de l’Ecole Normale, 34296 Montpellier, France Received 5 February 2004; received in revised form 18 April 2004; accepted 20 April 2004 Available online 2 June 2004

Abstract The kinetics of the selective catalytic reduction (SCR) of NO by NH3 in the presence of O2 has been studied on a 5.5% Cu-faujasite (Cu-FAU) catalyst. Cu-FAU was composed of cationic and oxocationic Cu species. The SCR was studied in a gas phase-flowing reactor operating at atmospheric pressure. The reaction conditions explored were: 458 < TR < 513 K, 250 < NO (ppm) < 3000, 1000 < NH3 (ppm) < 4000, 1 < O2 (%) < 4. The kinetic orders were 0.8–1 with respect to NO, 0.5–1 with respect to O2 , and essentially 0 with respect to NH3 . Based on these kinetic partial orders of reactions and elementary chemistry, a wide variety of mechanisms were explored, and different rate laws were derived. The best fit between the measured and calculated rates for the SCR of NO by NH3 was obtained with a rate law derived from a redox Mars and van Krevelen mechanism. The catalytic cycle is described by a sequence of three reactions: (i) CuI is oxidized by O2 to “CuII -oxo”, (ii) “CuII -oxo” reacts with NO to yield “CuII -Nx Oy ”, and (iii) finally “CuII -Nx Oy ” is reduced by NH3 to give N2 , H2 O, and the regeneration of CuI (closing of the catalytic cycle). The rate constants of the three steps have been determined at 458, 483, and 513 K. It is shown that CuI or “CuII -oxo” species constitute the rate-determining active center. © 2004 Elsevier B.V. All rights reserved. Keywords: Nitric oxide; Ammonia; Selective catalytic reduction; Copper; Zeolite; Kinetics

1. Introduction The selective catalytic reduction (SCR) by NH3 is the most efficient control technology to remove noxious environmental NOx emissions from chemical plants and stationnary power sources. A great variety of catalysts formulations have been claimed for this process, including supported noble metals, the mixed oxide V2 O5 –TiO2 system, and zeolite-based materials [1]. The generally accepted stoichiometry for this reaction involves the combination of equimolecular amounts of NO and NH3 in the presence of oxygen to produce water and nitrogen according to 4NO + 4NH3 + O2 = 4N2 + 6H2 O

(1)

Mo- or W-promoted vanadia-titania catalysts are the most widely used industrially, and have given rise to basic studies regarding mechanisms and kinetics [2–5]. There is ∗ Corresponding author. Tel.: +33-4-67-14-43-95; fax: +33-4-67-14-43-49. E-mail address: [email protected] (B. Coq).

0926-3373/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.04.008

a general agreement for proposing a dual-site mechanism on neighboring VIV and VV sites bearing NH3 and NO, respectively, and oxygen taking part to the catalytic cycle by oxidation of VIV to VV . It is postulated that the kinetics obeys an Eley-Rideal formalism between gaseous NO and adsorbed NH3 , or better a Langmuir–Hinshelwood formalism with weakly adsorbed NO species. The order with respect to (w.r.t.) NO and O2 is one, and −0.3 to –0.6 w.r.t. NH3 . Albeit V-Ti catalysts yet remain the most widely used, zeolite-based materials attract more and more attention, and there has been a significant interest in developing zeolite-based catalysts [6–8]. Zeolites indeed offer a number of advantages over V-Ti catalysts; they are active and selective over a wider temperature range, they are more thermally stable, and the spent catalyst is easier to dispose of. Moreover, zeolite-based catalysts exhibit very low selectivity to N2 O, with regard to N2 , even at high temperature. However, studies regarding mechanism and kinetics have attracted much less attention than those devoted to V–Ti materials. On protonic zeolites, the kinetics dependence w.r.t. NO, NH3 , and O2 is very similar with that observed on V-Ti

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catalysts, namely a positive order (0.6–1) w.r.t. NO and O2 , and a negative one (−0.3 to −0.6) w.r.t. NH3 [9,10]. The rate-determing step (r.d.s.) was the oxidation of NO by O2 to yield NO+ and/or NO2 , which may take place on Brønsted or extra-framework Al Lewis sites. The activity of zeolite materials definitively increases upon introduction of transition metal ions (TMI), e.g. Cu, Fe, . . . [8]. This is precisely the case for Cu-faujasite (Cu-FAU) based catalysts, which are industrially developed in nitric acid plants and gas turbine applications worldwide [11–13]. In spite of the real interest of TMI-zeolite catalysts for deNOxing, scarce studies adressed the kinetics of the reaction [14–17]. On Fe-FAU [14], the SCR was found to be of first order w.r.t. NO and essentially zero order w.r.t. NH3 at temperature up to 545 K. At higher temperatures, the order w.r.t. NH3 became slightly positive. It was proposed that the rate-determining step in the SCR would be the reaction between NH3 and gaseous or weakly adsorbed NO. On Cu-mordenite, Choi et al. [15] concluded to a positive order w.r.t. NO and NH3 ; the rate law proposed obeyed to a Langmuir–Hinshelwood formalism without competitive adsorption between NO and NH3 ; this was a dual-site mechanism with the surface reaction occurring between NH3 and NO adsorbed on different sites with the help of oxygen. On Cu-ZSM-5, Komatsu et al. [16] reported orders of one w.r.t. NO, 0.5 w.r.t. O2 , and zero w.r.t. NH3 . They suggested a mechanism involving [CuOCu]2+ dimer species as the active sites; the rate-determining step would be the reaction of NO on the active site to yield a bridge-bonded nitrato species on the copper atoms. Oxygen-bonded dimer species were also proposed to constitute the most active sites for NO decomposition [18,19], and deNOx on Cu-FAU, -MOR and ZSM-5 [20]. The catalytic oxidation by O2 of NO to NO2 was also proposed as rate-determining step in a kinetic study on deNOx on Fe-ZSM-5 [17]; the rate was zeroth order w.r.t. NH3 , nearly half order w.r.t. O2 , and first order w.r.t. NO. On Cu-zeolite catalysts, there are experimental evidences that the couple Cu+ /Cu2+ is an important matter for the SCR [15,20–25]. We have indeed proposed that the catalytic cycle involves the oxidation of neighboring Cu+ by O2 or NO + O2 to [CuOCu]2+ or [Cu(Nx Oy )Cu]2+ , which are in turn reduced back to Cu+ by NH3 + NO or NH3 [20,25]. The aim of the present work was to carry out a kinetic study of the SCR on a well-defined Cu-FAU catalyst to identify the best fit for a rate law based on a redox Mars and van Krevelen-like mechanism.

2. Experimental 2.1. Catalysts The preparation and characterization of the sample Cu-FAU were given in detail in a previous work [20]. Briefly, Cu-FAU was prepared by ion exchange of Na-FAU (Süd

Chemie, Si/Al = 2.7, specific surface area = 700 m2 g−1 ) with Cu(NO3 )2 at pH ∼ 5. The dried solid was calcined at 773 K. It contains 5.5 wt.% Cu corresponding to a nominal exchange degree of 56% (Cu/Al × 200, molmol−1 ). It was characterized by N2 sorption at 77 K, X-ray diffraction, temperature programmed reduction and oxidation, and Fourier transform infra-red and UV-Vis spectroscopies. The copper species yielded by ion exchange were found to be mainly isolated Cu2+ species and Cu oxocations, after calcination [20]. 2.2. Activity tests The SCR of NO by NH3 was studied in a down flow reactor operating at atmospheric pressure. Catalysts aliquot (20–60 mg) were activated in situ at 773 K in helium (ramp: 10 K min−1 , flow: 50 cm3 min−1 ) and cooled down to the reaction temperature. The reaction gas, a mixture of NO, NH3 , and O2 in helium, was fed to the reactor. The space velocity varied from 80,000 to 300,000 h−1 to maintain the NO conversion lower than 30%. The effluent composition was monitored continuously by sampling on line to a quadrupole mass spectrometer Balzer QMS 421 equipped with Faraday and Channeltron detectors (0–200 amu). Nine masses characteristic of NO (30), NO2 (30,46), N2 O (28,30,44), N2 (28), NH3 (17,18), H2 O (17,18), O2 (16,32), and He (4) were followed. The intensities of NH3 (17), H2 O(18), and NO (30) were determined by solving a linear system of equations. The concentrations were derived from intensities by using prior standardization procedures, with the actual fragmentation factors determined periodically on the diluted pure gases. The SCR was carried out at 458, 483, and 513 K for various compositions of the feed, with NO from 250 to 3000 ppm, NH3 from 1000 to 4000 ppm, and O2 from 1 to 4%. For each set of feed composition, the SCR was carried out for 1 h and the averaged NO conversion was calculated from the last 15 min. The stability of the catalytic activity was excellent. A blank test showed that no reaction occurs on the Cu-free Na-FAU sample. Previous works on a slightly more active Cu-FAU demonstrated the absence of mass transfer limitations in the same set up, reaction conditions, and grain size of the catalyst [20]. 2.3. Parameter estimation The effect of deviation from pure differential conversions were accounted for according to the formula proposed by Stevenson et al. [9] in the SCR on H-ZSM-5:
 2rinlet rtrue = rmeas (2) (rinlet + rexit ) where rtrue is the actual reaction rate at the inlet conditions, rmeas is the experimentally measured rate, rinlet is the rate calculated from the kinetic expression using the inlet NO

G. Delahay et al. / Applied Catalysis B: Environmental 52 (2004) 251–257 9 8

3

7

-1

-1

r meas (mol g h ) x 10

and NH3 concentrations, and rexit is the rate calculated from the kinetic expression using the NO and NH3 concentrations measured at the reactor exit. This expression, while not exact, allows us a good approximation for conversions less than 50%, especially for net reaction orders of one and less. The apparent rate parameters were estimated by non-linear least-square methods minimizing the sum of squares of the residual (=rtrue − rinlet ) NO conversion rate. Convergence during minimization was pretty fast and a few local minima were found yielding almost identical values of k1 , k2 , and k3 (difference smaller than 0.1%). Initial guesses of the three parameters k1 , k2 , and k3 were chosen according to both their relative importance in the different proposed mechanisms and also to their expected values for the experimental conditions described earlier. The tool was implemented using EXCEL software.

253

6 5 4 3 2 1 0 0

1000

2000 3000 NH3 (ppm)

4000

Fig. 2. Experimental measured SCR rate (rmeas ) as a function of NH3 concentration at 458 (䊊), 483 (), and 513 K (䊐).

3. Results and discussion On the same series of Cu-faujasite catalysts in previous studies we have shown that: (1) The direct oxidation of NH3 by O2 to N2 , N2 O and/or NO becomes significant when large CuO aggregates are present in the materials, and above 550 K [26]. On Cu-faujasite composed only of cationic and/or oxocationic Cu species, such as the present Cu-FAU, the direct ammonia oxidation by O2 is initiated above 600 K. (2) In the SCR of NO on the current Cu-FAU (5.5%Cu-FAU), the selectivity of the NO reduction to N2 was close to 100%. Trace amounts of N2 O (50 ppm, 5% selectivity) only appears at 700 K [20]. These observations were confirmed in this kinetic study on Cu-FAU, where N2 O was not identified in the effluent, and the NO and NH3 conversions were very close.

Figs. 1–3 present the dependence of rmeas as a function of NO, NH3 , and O2 concentrations, respectively. These plots demonstrate kinetic dependencies perfectly in line with previous reports about the SCR on TMI-zeolite [14–17]. The apparent orders are 0.5–1 w.r.t. NO and O2 , and essentially 0 w.r.t. NH3 , but becoming slightly positive at low NH3 concentration. This provides evidence of a large surface coverage in ammonia. Several catalytic cycles have then been considered for the SCR reaction based on (1) the involvement of the redox couple CuI /“CuII -oxo”. (2) O2 or O2 + NO as oxidant of CuI . (3) NH3 or NH3 + NO as reductant of “CuII -oxo”. 9 8

10

r meas (mol g-1 h-1) x 103

7

6

-1

-1

r meas (mol g h ) x 10

3

8

4

6 5 4 3 2

2

1 0

0 0

500

1000

1500

2000

2500

3000

3500

NO (ppm)

Fig. 1. Experimental measured SCR rate (rmeas ) as a function of NO concentration at 458 (䊊), 483 (), and 513 K (䊐).

0

1

2

3

4

5

O2 (%)

Fig. 3. Experimental measured SCR rate (rmeas ) as a function of O2 concentration at 458 (䊊), 483 (), and 513 K (䊐).

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G. Delahay et al. / Applied Catalysis B: Environmental 52 (2004) 251–257

(NH3)CuI

N 2 , H 2O

- oxidation by O2+NO - reduction byNH 3

k2

r=

k2

R2 = 0.950

NH3+NO II

(NH3)"Cu -NxOy"

(a)

N2, H2O

(NH3)Cu

k2

NH3

k1

(NH3)"CuII-oxo"

(b)

(NH3)CuI

I

K

O2

- oxidation by O2 - Langmuir adsorption of NO r=

k1k2PO2 PNO PNH3 k1PO2 + k2PNH3PNO

r=

k1

NH3

O2

- oxidation by O 2 - reduction by NH3+NO

k1k2PO2 PNO PNH3 k1PO2 PNO + k2PNH3

R2 = 0.844

NH3 CuI

N2, H2O

k1k2 K PO2 PNO PNH3 k1PO2(1+ K PNO )+ k2 K PNH3PNO

- oxidation by O 2 +NO

k1

k2

R2 = 0.675

O2+NO

- (NH3)Cu I cannot be oxidized: inhibition by NH 3

II

II

(NH3)"Cu -oxo"

(NH3)"Cu -NxOy"

R2 = 0.800

NH3

N 2, H 2 O

k1

(NH3)"CuII-NxOy"

(d)

K

(NH3)CuI O2

k3

NH3

k1k2PO2 PNO PNH3 k1PO2 PNO + k2PNH3(1+ K PNH3)

r=

NO (c)

(NH3)CuI

N2, H2O

O2+NO

R2 = 0.977

(NH3)"CuII-oxo"

(NH3)"CuII-NxOy" (e)

k1

k2

NO

Fig. 4. Some reaction mechanisms for the SCR of NO by NH3 on Cu-FAU, with the rate law expressions derived.

(4) the possibility of a slight inhibition by NH3 as reported in other works [9,10]. (5) and obviously the kinetic dependencies observed. These propositions of mechanism are shown in Fig. 4. The rate laws were derived from classical kinetic treatment: (i) the Mars and van Krevelen formalism, (ii) steady-state approximation, (iii) Langmuir-type adsorption, and (iv) the conservation of Cu sites. This kinetic treatment is exemplified in Appendix A for the model shown in Fig. 4e. The full fit between the calculated rate and the true experimental rate (rtrue ) at the various feed compositions and the three reaction temperatures was not very good for each of the models shown in Fig. 4a–d. In contrast, the fit was excellent with the model depicted in Fig. 4e. This model involves three sequential reactions in the catalytic cycle: (i) the oxidation of CuI to “CuII -oxo” by O2 , (ii) the reaction of “CuII -oxo” with NO to yield “CuII -Nx Oy ”, and (iii) the reaction of “CuII -Nx Oy ” with NH3 to yield N2 and H2 O and the regeneration of CuI .

This reaction scheme looks similar to that proposed by Komatsu et al. [16] for the SCR of NO by NH3 on Cu-zeolite. Moreover, these authors postulated that the reaction of NO with bridge-bonded O on two Cu sites would constitute the r.d.s. The proposition of this catalytic cycle calls for several comments: (1) CuI is oxidized sequentially to “CuII -Nx Oy ” by O2 then by NO. That means that the direct oxidation of CuI by NO2 (formed from gas phase reaction between NO and O2 ) takes part for a lesser extent. (2) The reaction of NO with “CuII -oxo” can be considered as irreversible. The replacement of this reaction (Eley-Rideal-type) by a reversible Langmuir-type adsorption of NO (Fig. 4c) leads to a poorer fit. There are several examples in the literature demonstrating that the interaction of NO and O2 with CuII -zeolite leads to formation of nitrite/nitrate-like species, which further on decompose into NO, NO2 , and O2 above 623 K [27–29].

G. Delahay et al. / Applied Catalysis B: Environmental 52 (2004) 251–257

It is also worth noting that this Eley-Rideal-type interaction of NO with Cu-oxo species is very similar to the same step proposed by Komatsu et al. [16]. In contrast, it differs from the mechanism proposed by Ham et al. [30], in which the SCR proceeds from coordinated NH3 on Cu2+ and weakly adsorbed NO. (3) Finally, TPD experiments [20] and operando DRIFT studies of the SCR [25] have shown that ca. one NH3 ligand is permanently bonded to Cu in the course of the reaction. It is noteworthy that this NH3 ligand does not inhibit the process of CuI oxidation.

10

R2 = 0.977

6

-1

-1

-1

r calc (mol.g .atm .h )

8

With the help of the rate law (Eq. (A.9), Appendix A) and Eq. (2), the actual reaction rate at the inlet conditions (rtrue or rcalc ) can be determined from the measured rate (rmeas ), and compared with the calculated one (rinlet ). These values are given in Table 1 for each experimental conditions set. Fig. 5 presents a comparison of the actual and calculated rates of NO reduction by NH3 over the ranges of NO, NH3 , Table 1 Measured, experimental, and calculated (according to Eq. (A.9)) NO conversion at various SCR conditions

255

4

2

0

0

2

4

6 -1

-1

8

10

-1

r true (mol.g .atm .h )

Fig. 5. Comparison of the experimentally true rate (rtrue ) with the calculated rate (rcalc ) at various SCR conditions and according to the mechanism featured in Fig. 4e (data from Table 1); (䊊) 458 K, () 483 K, (䊐) 513 K.

Rate (mol g−1 atm−1 h−1 ) × 103

SCR conditions T (K)

NO (ppm)

NH3 (ppm)

O2 (%)

Measured

True

Calculated

458 458 458 458 458 458 458 458 458 458 458 483 483 483 483 483 483 483 483 483 483 483 513 513 513 513 513 513 513 513 513 513 513 513 513

1000 1500 2000 2500 3000 2000 2000 2000 2000 2000 2000 1000 1500 2000 2500 3000 2000 2000 2000 2000 2000 2000 250 500 1000 1500 2000 2500 3000 2000 2000 2000 2000 2000 2000

2000 2000 2000 2000 2000 2000 2000 2000 1000 3000 4000 2000 2000 2000 2000 2000 2000 2000 2000 1000 3000 4000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 1000 3000 4000

3 3 3 3 3 1 2 4 3 3 3 3 3 3 3 3 1 2 4 3 3 3 3 3 3 3 3 3 3 1 2 4 3 3 3

1.89 2.19 2.92 3.38 3.41 1.84 2.49 3.14 2.16 2.92 3.03 2.84 3.29 4.38 5.07 5.11 2.76 3.73 4.71 3.25 4.38 4.54 1.87 3.08 5.03 6.33 6.49 8.11 8.27 4.22 5.52 7.79 5.19 7.14 7.14

2.24 2.54 3.35 3.86 3.83 1.97 2.81 3.65 2.47 3.36 3.49 3.31 3.61 4.82 5.53 5.53 2.83 4.03 5.36 3.49 4.92 5.05 2.18 3.56 5.68 6.93 7.02 8.70 8.76 4.36 5.82 8.47 5.53 7.73 7.71

2.17 2.79 3.25 3.61 3.90 2.02 2.82 3.52 2.71 3.48 3.61 3.31 4.12 4.70 5.13 5.46 2.76 4.00 5.15 3.82 5.08 5.30 1.90 3.32 5.28 6.57 7.49 8.18 8.71 4.30 6.32 8.26 6.19 8.06 8.37

and O2 concentrations and temperatures studied. The agreement between the experimental and predicted rates is seen as satisfactory. The values of the rate constants k1 , k2 , and k3 for the three steps of the catalytic cycle are given in Table 2 at the three temperatures, as well as the estimated activation energies of the corresponding steps derived from log ki . On the other hand, the value of the apparent activation energy, derived from the global SCR process (log(rate)),was found to be 29 kJ mol−1 with NO = NH3 = 2000 ppm and 3% O2 . This value can be compared with apparent activation energy of ca. 35 kJ mol−1 on Cu-MOR [31], and of 40–42 kJ mol−1 on Cu-ZSM-5 [16]. The lowest rate constant k1 corresponds to the oxidation of CuI to “CuII -oxo”. This oxidation step was indeed suggested as r.d.s. in some previous studies on SCR [20,32], whereas Komatsu et al. [16] proposed the reaction of [CuOCu]2+ (the “CuII -oxo”-like species in our model) with NO as r.d.s. on Cu-ZSM-5. However, in a closed sequence where all steps are irreversible (Fig. 4e and Appendix A), there can be no question in this case of a r.d.s., but it may exist as a rate-determining active center [33], e.g. CuI , “CuII -oxo”, or “CuII -Nx Oy ”. They are characterized by their relative surface abundance, θ 1 , θ 2 , and θ 3 (Appendix A), which can for Table 2 Kinetic parameters calculated for the catalytic cycle described in Fig. 4e Ea (kJ mol−1 )

Rate constants (mol g−1 h−1 ) × 103

T (K) 458

483

513

k1 k2 k3

2.0 73.0 184.0

2.5 125 230

3.8 200 400

22.6 35.8 27.7

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G. Delahay et al. / Applied Catalysis B: Environmental 52 (2004) 251–257

instance be calculated at 513 K under classical SCR conditions, i.e. 1000 < NO = NH3 (ppm) < 2000 and 3% O2 . These values are 0.26–0.41, 0.51–0.40, and 0.23–0.18 for θ 1 , θ 2 , and θ 3 , respectively. These values demonstrate that “CuII -oxo” is generally the most abundant active center, but with a significant fraction of CuI . In this respect, the fraction of “CuII -Nx Oy ” stands low, which provides evidence of its high reactivity with NH3 on Cu-FAU. Such species were indeed difficult to identify during SCR under operando DRIFT observation [25]. This kinetic study provides good arguments that there is not a definite rate-determining active center because the population of the two candidates, “CuII -oxo” and “CuI ” are well balanced in many situations.

Step 3.

4. Conclusions

r = r2 = r3 = 4r1

The best fit between the measured and calculated rates for the SCR of NO by NH3 is obtained with a rate law derived from a redox Mars and van Krevelen mechanism. The catalytic cycle is described by a sequence of three reactions: (i) CuI is oxidized by O2 to “CuII -oxo”, (ii) “CuII -oxo” reacts with NO to yield “CuII -Nx Oy ”, and (iii) finally “CuII -Nx Oy ” is reduced by NH3 to give N2 , H2 O, and the regeneration of CuI (closing of the catalytic cycle). The rate constants of the three steps have been determined at 458, 483, and 513 K. Further calculations demonstrate that at 513 K, the most abundant surface species are CuI and “CuII -oxo”, which provides evidence that there is not a clear rate-determining active center.

The second assumption is the Cu site balance: 1 = θ1 + θ2 + θ3 . Eqs. (A.4–A.7) allow then to determine the proportion of active centers, e.g.:

“CuII -Nx Oy ” + NH3 → CuI + N2 + H2 O (k3 )

(A.3)

Given [(NH3 )CuI ]/ Cu = θ 1 , [(NH3 )“CuII -oxo”]/ Cu = θ 2 , and [(NH3 )“CuII -Nx Oy ”]/ Cu = θ 3 , the rate expression of the three steps of the catalytic cycle are: r1 = k1 θ1 PO2

(A.4)

r2 = k2 θ2 PNO

(A.5)

r3 = k3 θ3 PNH3

(A.6)

Since Steps 2 and 3 must proceed four times per overall reaction and Step 1 once (Eq. (1)), the stoichiometric numbers of the steps are four, four, and one, respectively, and the overall net rate with steady state assumption is:

θ2 =

k2 k3 PNO PNH3

(A.7)

4k1 k3 PO2 PNH3 + 4k1 k3 PO2 PNH3 + 4k1 k2 PO2 PNO (A.8)

then r = k2 θ2 PNO ×

4k1 k2 k3 PO2 PNO PNH3 k2 k3 PNO PNH3 + 4k1 k3 PO2 PNH3 + 4k1 k2 PO2 PNO (A.9)

Acknowledgements References The authors thank the Grande Paroisse Company (TOTAL Group) for supporting this study.

Appendix A The derivation of rate expressions was carried out according to the general principles presented by Boudart [33] and Kapteijn and Moulijn [34]. Because the exact nature of CuI , “CuII -oxo”, and “CuII -Nx Oy ” remains a matter of debate, one may consider that the three steps in the closed catalytic sequence featured by Fig. 4e are elementary steps. This is obviously a simplification, but which allows to derive a rate law expression. Step 1. CuI + O2 → “CuII -oxo” (k1 )

(A.1)

Step 2. “CuII -oxo” + NO → “CuII -Nx Oy ” (k2 )

(A.2)

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