Evidence of a Cu2+âAlkane Interaction in Cu-Zeolite Catalysts Crucial

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Evidence of a Cu2+-Alkane Interaction in Cu-Zeolite catalysts crucial for the Selective Catalytic Reduction of NOx with Hydrocarbon Marta Moreno-González, Antonio Eduardo Palomares, Mario Chiesa, Mercedes Boronat, Elio GIAMELLO, and Teresa Blasco ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03473 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017

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Evidence of a Cu2+-Alkane Interaction in Cu-Zeolite Catalysts Crucial for the Selective Catalytic Reduction of NOx with Hydrocarbons M. Moreno-González,†,§ A. E. Palomares,† M. Chiesa,‡ M. Boronat,† E. Giamello‡ and T. Blasco*,† †

Instituto de Tecnología Química, Universitat Politècnica de València-Consejo Superior de

Investigaciones Científicas (UPV-CSIC), Avda. de los Naranjos s/n, 46022 Valencia (Spain). ‡

Dipartimento di Chimica, Università di Torino, Via Giuria, 7, 10125 Torino (Italia).

ABSTRACT: The selective catalytic reduction of NOx with hydrocarbons (HC-SCR-NOx) on Cu-zeolites is an alternative for the depletion of NOx present in the exhaust gas of diesel engines. The role played by copper and the general reaction pathway are investigated here using propane as reductant (C3H8-SCR-NOx) and Cu-TNU-9 and Cu-Y zeolites as catalysts, giving 80 % and 10 % NO conversion, respectively, at 573 K. In situ EPR shows that, even at room temperature, Cu2+NO3- is formed in the two Cu-zeolites and that there exists an interaction of Cu2+ with C3H8 in Cu-TNU-9. The occurrence of a C3H8---Cu2+ interplay in Cu-Y is confirmed by HYSCORE EPR experiments and theoretical calculations. Cu2+ is reduced to Cu+ after heating with the

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reaction mixture to an extent that is related with the catalyst activity. In situ NMR spectra have allowed identifying nitrile and isocyanate as reaction intermediates which are partly hydrolyzed to carboxylic acid and ammonia. The results allows concluding that the C3H8-SCR-NOx reaction requires the activation of the alkane at accessible isolated Cu2+, and that this generates ammonia that acts as the reductant and reacts with Cu2+NO3-. The low activity of Cu-Y zeolite for the C3H8-SCR-NOx reaction proves that C3H8 in the gas phase does not react with Cu2+NO3- being mandatory the activation of C3H8 on accessible Cu2+ sites. A general reaction scheme for the HC-SCR-NOx involving Cu2+/Cu+ redox cycles and the formation of ammonia that act as reductant of NO is proposed.

KEYWORDS: SCR-NOx, hydrocarbons, propane, EPR, NMR, HYSCORE, Cu-zeolite

1. INTRODUCTION Nitric oxides emitted by stationary power plants and automotive sources are one of the main air pollutants, contributing to different phenomena such as acid rains, photochemical smog and ozone depletion, with detrimental effects on both the environment and human health. Half of the total atmospheric NOx comes from mobile sources and in particular from diesel vehicles, due to the low efficiency of the traditional three ways catalyst (TWC) used in gasoline vehicles to eliminate NOx under lean-burn conditions, i.e., at high oxygen/fuel ratios. In the last years, there has been a progressive tightening of regulations on maximum NOx emission levels, which has prompted a very intense research activity to develop new technologies able to fulfill the new emission standards.1-9


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Most efforts for the abatement of nitric oxides have focused on the selective catalytic reduction of NOx (SCR-NOx) with ammonia (NH3-SCR-NOx), generated on board by the hydrolysis of urea (urea-SCR-NOx). This technology was boosted by the discovery of a high efficiency Cuzeolite catalyst with the chabazite structure, which has been commercialized and is now the general choice for applications in lean-burn engine vehicles.3,5-7,10 However, this method suffers from some drawbacks such as the slip of harmful ammonia to the atmosphere and the catalyst poisoning produced by incomplete hydrolysis of urea or by the unburnt hydrocarbons present at in the exhaust gas, especially during the cold startup period.2,4 Moreover, difficulties in dosing the urea for a good NOx conversion while minimizing the ammonia slip make necessary the use of complicated urea-delivery systems.4 Therefore, despite the great success of the new zeolite based catalysts for the NH3-SCR-NOx reaction, further research is required for the development of new efficient technologies for automotive NOx reduction. An attractive and promising alternative to control NOx emissions in the exhaust gas of leanburn diesel vehicles is the use of hydrocarbons as reductants (HC-SCR-NOx), taking advantage of the on board availability of fuel.1,11-13 The main limitation of the large variety of solids, including Cu-zeolites, tested as catalysts for automotive applications is its very low activity at temperature below about 573 K.1,13 However, the interest on the HC-SCR-NOx reaction has increased since it was reported that the presence of small amounts of hydrogen or oxygenated hydrocarbons in the feed broaden the operating temperature window of supported catalysts.14-17 Fundamental knowledge on the HC-SCR-NOx is therefore mandatory for future development of efficient catalytic systems. Despite the advances in the understanding of the NH3-SCR-NOx reaction mechanism during the last years, little is known about the HC-SCR-NOx reaction. The various mechanisms


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proposed have in common the formation of surface inorganic nitrates or nitrites, and of organonitrogen (cyanide and isocyanate) and oxygenated hydrocarbon (aldehydes and carboxylic acids) as intermediate species, but differ on the reaction pathway and the role played by the redox and acid sites.1,16,18-22 For Cu-zeolites, it has been suggested that propene reduces Cu2+ to Cu+ and forms surface allylic species (CxHy), which react with NO/O2/NO2 giving the nitrogen containing and oxygenated organic compounds.18,20 These intermediates can alternatively be formed by the reaction of nitrate/nitrite species formed at low temperatures on the Cu2+ cation with gaseous hydrocarbon.18-20 Independently on how they are formed, it seems that N2 comes from the oxidation of cyanide (-CN) and isocyanate (-NCO) by NO/NO2/O2 or from the reduction of nitrates/nitrites by oxygenates or hydrocarbons. In the presence of water, -CN and -NCO are readily hydrolyzed giving −NH2, which is recognized to be highly reactive with NO/NO2 leading to N2 and water.18-20 The active sites for the HC-SCR-NOx in Cu-zeolites is assumed to be isolated Cu2+ and the reaction involves a redox mechanism changing the oxidation state between Cu2+ and Cu+ in an oxygen rich environment.18,20,23 Although the redox cycle is not well established yet, it is known that the presence of oxygen in the reaction stream plays a key role avoiding over-reduction of copper and having mixed oxidation states of copper, which has been proposed to be mandatory for obtaining good catalytic results.24-26 Here we have investigated the mechanism of the HC-SCR-NOx using propane as reductant (C3H8-SCR-NOx) and zeolites Cu-Y and Cu-TNU-9 as catalysts. Zeolite Y is a 3D large pore zeolite composed by hexagonal prism d6r and sodalite sod units that link to form supercages of 11.2 Å diameter, accessible to molecules through smaller windows of 7.4 Å diameter. TNU-9 is a 3D medium pore zeolite whose larger channels along the b axis allow diffusion of molecules of


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5.4 Å diameter, while only molecules smaller than 4.9 Å can diffuse through the narrow channels along a and c axes. Besides topological differences in the microporous channel structure, the two zeolites considered also differ in the distribution and accessibility of the extra-framework Cu2+ cations responsible of their catalytic activity. The preferred sites for Cu2+ cations are 6-member ring 6mr units with two Si atoms substituted by Al. In the case of Cu-Y zeolite, these sites are mainly located within the d6r and in the sod cage, not accessible to reactant molecules. In CuTNU-9, with a framework topology very similar to that of MFI, such 6mr rings are mainly present in the 10-member ring channels, fully accessible to reactant molecules. The study has been carried out by using in situ EPR and NMR spectroscopies, which provide information on the evolution of the Cu2+ active sites and the organic intermediates formed in the SCR reaction, respectively. The results obtained by combining these two techniques prove that isolated Cu2+ are required to activate hydrocarbon molecules in Cu-zeolite catalysts for the HC-SCR-NOx reaction. This work reports new relevant data on the HC-SCR-NOx reaction shedding light on the reaction mechanism, which is essential for the development of novel catalytic systems efficient for the depletion of NOx.

2. EXPERIMENTAL SECTION Cu-TNU-9 (Si/Al = 14 and Cu/Al = 0.7), Cu-Y (Si/Al = 11 and Cu/Al = 0.5), Cu-ITQ-2 (Si/Al = 20 and Cu/Al = 0.6) and Cu-ZSM-5 (Si/Al = 11, and Cu/Al= 0.5) were prepared by ion exchange of Na-TNU-9, Na-Y, Na-ITQ-2 and H-ZSM-5 using an aqueous solution of Cu(CH3COO)2⋅4H2O. Further details on the preparation procedure are given in the Supporting Information.


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Catalytic tests were carried out in a fixed bed quartz tubular reactor, where 0.5 g of catalyst as 0.25 – 0.42 mm size particles were introduced, heated up to 773 K under N2 flow and kept at this temperature for 1h. Next, the desired temperature was set and the flow changed to the reaction feed, consisting of 900 ml/min of a mixture composed by 500 ppm of NO, 835 ppm of C3H8, 12% of O2 and N2 as balance gas, resulting a spatial velocity of SV=108000 mlh-1g-1. Reaction conditions for the C3H8-SCR-NOx: 500 ppm of NO, 835 ppm of C3H8, 12% O2 and N2 as balance gas, SV = 108000 ml·h-1·gcat-1. Reaction conditions for the NH3-SCR-NOx: 500 ppm of NO, 530 ppm of NH3, 10% O2 and N2 as balance gas, SV = 105000 ml·h-1·gcat-1º. The reaction was monitored by the conversion of NO, which was analyzed with a chemiluminescence detector Eco Physiscs 62c. N2O was analyzed by gas chromatography with a 5 A Molecular Sieve column used to separate oxygen, nitrogen and carbon monoxide and a Poraplot Q column for the separation of carbon dioxide and hydrocarbons. EPR spectra were recorded with a Bruker EMX-12 spectrometer operating at the X-band, with a modulation frequency of 100 KHz and amplitude of 0.1 mT. All spectra were measured at 105 K and quantitative analysis was carried out by double integration of the spectra, using CuSO4 as an external standard. Pulse EPR experiments at X-band (9.76 GHz) were performed on an ELEXYS 580 Bruker spectrometer equipped with a liquid-helium cryostat from Oxford Inc. Details on the HYSCORE experiments and the procedure used for carrying out the in situ EPR experiments are described in the Supporting Information. Solid state NMR experiments were carried out with a Bruker Advance 400 WB spectrometer using a BL4 probe and were measured with proton decoupling.

15

N MAS NMR spectra were

acquired with the sample spinning at 5 kHz using a 90° pulse of 6 µs and a recycle delay of 10 s and the 1H/15N CP MAS NMR spectra were measured using 90º pulses for 1H of 4.5 µs and a


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contact time of 1 ms. 13C MAS NMR spectra were measured at spinning rates of 10 kHz using a 90° pulse of 3 µs and a recycle delay of 3 s, while the 1H/13C cross polarization NMR spectra were acquired with a pulse of 3.5 µs and a contact time of 1 ms. δ 15N are referred to CH3NO2 (δ 15

N = 0 ppm) using (15NH4)2(SO4) as secondary reference (δ 15N = -365.6 ppm CH3NO2) and δ

13

C are referred to TMS (δ

13

C = 0 ppm). The method used for the preparation of the in situ

NMR experiments is described in the Supporting Information. Density functional calculations were performed using the Perdew-Wang (PW91) exchangecorrelation functional within the generalized gradient approach (GGA)27,28 as implemented in the VASP code.29,30 The valence density was expanded in a plane wave basis set with a kinetic energy cutoff of 500 eV, and the effect of the core electrons in the valence density was taken into account by means of the projected augmented wave (PAW) formalism.31 The cluster models were cut out from the periodic structures of FAU (lattice parameters a = b = c = 24.345 Å, α = β = γ = 90º) and TUN (lattice parameters a = 27.845 Å, b = 20.015 Å, c = 19.596 Å, α = γ = 90º, β = 93.2º), and the dangling bonds were saturated with H atoms. Two Si atoms were replaced by Al atoms and a Cu2+ cation was added to neutralize the charge. During geometry optimizations the positions of the terminal H atoms were kept fixed while other atoms were allowed to relax without restrictions

3. RESULTS AND DISCUSSION 3.1 Catalytic tests Figure 1 shows the NO conversion in the SCR-NOx reaction using ammonia and propane as reductants for Cu-TNU-9 and Cu-Y zeolites as a function of the reaction temperature. They exhibit very high activity in the whole temperature range 523 K - 723 K when ammonia is used


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as reducing agent. As it is known, higher reaction temperatures are required for achieving good catalytic activity when hydrocarbon are used as reductant, as shown in Figure 1 for Cu-TNU-9. However, Cu-Y zeolite display a much lower activity than Cu-TNU-9 in the temperature range 523 K – 673 K, giving 10 % and 80 % NO conversion at 623 K, respectively. The differences in the catalytic behavior of Cu-Y and Cu-TNU-9 for C3H8-SCR-NOx cannot be explained by restrictions in propane diffusion inside the zeolites channels, since the pores of Cu-Y (large size, 7.4 Å) are larger than those of the most active zeolite Cu-TNU-9 (medium size, 5.4 Å).32 Likewise, a first screening of the catalysts by H2-TPR reveals that the reducibility in H2 of the Cu hosted in the two zeolites is very similar (see Figure S1). The differences in the catalytic activity of the two Cu-zeolites can be thought to due to the aggregation state of copper. Isolated copper is deemed to be the active site for the SCR reaction, whereas CuO clusters have been reported to be less active and selective to N2, giving undesired N2O when hydrocarbon are used.1,11,13,23,33,34 However, no N2O was detected in the reaction products indicating that Cu-Y and Cu-TNU-9 are selective catalysts to N2, as expected for the copper exchange level and the reaction temperature range.34,35 An alternative explanation for the catalytic results of Figure 1 is that the nature or the distribution of copper sites is different in CuY and Cu-TNU-9 so that they are non-active for the C3H8-SCR-NOx in Cu-Y.34,35


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Figure 1. NO conversion profiles in the C3H8-SCR-NOx (straight line) and NH3-SCR-NOx (dotted line) reactions as a function of temperature for Cu-TNU-9 and Cu-Y. 3.2 Copper species The occurrence of isolated Cu2+ in Cu-Y and Cu-TNU-9 zeolites, their redox properties and coordination environment under different atmospheres were investigated by EPR spectroscopy, taking advantage of the paramagnetic properties of Cu2+, which possesses a d9 electronic configuration and then an unpaired electron. The two naturally abundant copper isotopes possess a nuclear spin I = 3/2, splitting the EPR lines into four hyperfine coupled components. As shown in Figure 2, the EPR signals of Cu2+ species in Cu-zeolites are usually axially symmetric with well resolved hyperfine coupling in the parallel region. The EPR spectra of Cu-Y and Cu-TNU-9 under ambient conditions (not shown) consist of a signal of isolated Cu2+(H2O)x accounting for about 50 % of total copper in both cases. These complexes are similar to those present in aqueous solutions and are created by the detachment of isolated Cu2+ bonded to framework oxygen in the calcined Cu-zeolite by the coordination to water molecules. The other 50 % copper must be as EPR-silent Cu2+ in the form of CuxOy aggregates. The amount of isolated Cu2+ observed by EPR decreases to about 15 % of total copper after dehydration at 773 K indicating the thermo-reduction of Cu2+ to diamagnetic Cu+. Indeed, the formation of Cu2+ EPR silent 9 ACS Paragon Plus Environment

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species such as Cu2+OH-, or diamagnetic Cu2+O- pairs, can be ruled out as they are favored by heating at low temperatures and under oxidizing atmosphere.36-41 Therefore, Cu-Y and Cu-TNU9 zeolites dehydrated at 773 K contain the same proportion of isolated and oligomeric species, that is, about 15 % as isolated Cu2+, 35 % as isolated Cu+ and about 50 % as oligomeric CuxOy. The small difference in the isolated Cu2+ content in Cu-Y in Cu-TNU-9 due to the distinct level of copper exchange (0.50 and 0.62 wt.% of isolated Cu2+, respectively) cannot explain the discrepancy in the catalytic activity for the C3H8-SCR-NOX shown in Figure 1, as can be inferred from previous results on Cu-zeolites, or by the high activity of Cu-Y for the SCR with ammonia, as this reaction is also catalyzed by isolated copper.34,35 Figure 2 shows the EPR spectra of Cu-Y and Cu-TNU-9 zeolites dehydrated at 773 K, in the presence of the reaction mixture (213C3H8/1,215NO/6-8O2/Cu) and after subsequent heating at 623 K. This temperature was chosen because of the different NO conversion in the C3H8-SCRNOx for Cu-TNU-9 (80 %) and Cu-Y (10 %) zeolites (see Figure 1). The spectrum of the dehydrated Cu-Y (Figure 2, black line) consists of two axially symmetric signals assigned to Cu2+ at different exchange positions of the FAU type zeolite, the most intense at site SI within the d6r (gII = 2.330 and AII = 15.7 mT) and the weaker at site SI’ at the six member ring within the sod cage (gII = 2.380 and AII = 12.7 mT).42-44 This attribution is supported by previous IR results which indicated that about 80 % of total copper in this sample is non accessible to CO.45 This molecule, with a kinetic diameter of 3.76 Å, is too bulky to diffuse through the six (2.6 Å) or the four member rings to enter the d6r and sod cages where most isolated copper must be placed (see inset in Figure 2, left). Indeed, the EPR spectrum of the dehydrated Cu-Y zeolite does not appreciably change upon the admission of propane (kinetic diameter 4.30 Å) into the sample tube, and very little after the adsorption of NO (kinetic diameter 3.2 Å) (see Figure S2


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and Table S1). The spectrum of zeolite Cu-Y with the reaction mixture, displayed in Figure 2 (red line), shows the presence of some Cu2+ at site SI and a new signal recently assigned to Cu2+NO3- formed by reaction of NO and O2 with Cu+.46-48 The spectrum recorded after heating Cu-Y zeolite at 623 K with the reaction mixture (Figure 2, green line) is quite similar to that of the dehydrated Cu-Y broadened because of the presence of O2 in the sample tube. This result suggests that nitrates decompose at 623 K.

Figure 2. EPR spectra recorded at 105 K (left) and magnification of the low-field hyperfine structure (right side) of zeolites Cu-Y and Cu-TNU-9: dehydrated at 773 K (black lines), followed by adsorption of the reaction mixture C3H8/NO/O2 (red lines) and subsequent heating at 623 K (green lines).

Because of steric constraints, Cu2+NO3- must be formed in the super-cage of zeolite Cu-Y, what in principle seems to be in contradiction with the location of Cu2+ inside the d6r and sod cages. However, it must be noticed that Cu2+NO3- species are formed from Cu+, and this cation 11 ACS Paragon Plus Environment

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can be located in the super-cage or migrate to the super-cage to form stable species. The most interesting conclusions from the results depicted for zeolite Cu-Y in Figure 2 is the formation of Cu2+NO3- at room temperature and its decomposition at 623 K. Somehow different results are obtained for Cu-TNU-9 zeolite. The spectrum of the dehydrated Cu-TNU-9, shown in Figure 2 (black line) consists of two axially symmetric signals, attributed to isolated Cu2+ in square-pyramidal (gII = 2.303 and AII = 16.3 mT) and square-planar (gII = 2.282 and AII = 17.1 mT) coordination.42 Although the location of Cu2+ at specific zeolite exchange positions in TNU-9 is unknown, the similarity between the TUN and MFI frameworks allows us to assume that, as in the case of Cu-ZSM-5, Cu2+ cations in Cu-TNU-9 preferentially occupy positions in 6mr units facing the 10-member ring channel system, as depicted in the inset of Figure 2.7 The spectrum recorded in the presence of the reaction mixture (red line) shows, as for Cu-Y, a signal of Cu2+NO3- and another one that must be originated by the proximity of propane to Cu2+ (Cu2+---C3H8) since this is also observed when only propane is adsorbed (see Figure S3 and Table S2). The interaction of propane with Cu2+ is further supported by 13C-NMR spectroscopy (see Figure S4). Heating Cu-TNU-9 with the reactant mixture at 623 K (green line) provokes a strong intensity decrease of the EPR signals, what suggests the almost complete reduction of Cu2+ to Cu+. Yet, it is still possible to identify signals of Cu2+ coordinated to H2O molecules,42 of Cu2+ interacting with some carbonaceous intermediate and a very sharp signal at gII=2.002 of carbonaceous radicals.49 The changes observed upon heating Cu-TNU-9 zeolite with the reaction mixture differ again with those obtained for Cu-Y and must be associated to the higher NO conversion (80 %) on Cu-TNU-9 at 623 K (see Figure 1). In order to check if the reduction of Cu2+ after heating Cu-zeolites with the reaction mixture observed by EPR is related to the SCR activity, we expanded the study to Cu-ITQ-2 and Cu-


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ZSM-5 zeolites (see Table S3) and the reaction temperature range to T = 523 K -723 K. The results are reported in Figure 3, which represents the % NO conversion from the catalytic test at a given temperature T versus the quantity (wt. % of total copper) of isolated Cu2+ reduced. This quantity is determined by in situ EPR spectroscopy from the amount of Cu2+ in the dehydrated Cu-zeolite minus that after being heated in situ with the reaction mixture. The graph of Figure 3 shows that, as a general trend, the higher the catalytic activity of the Cu-zeolite the higher the amount of Cu2+ reduced, proving the involvement of isolated Cu2+ in the SCR-NOx reaction

Figure 3. Graphical representation of the % NO conversion in the C3H8-SCR-NOx in the temperature range 523 K – 723 K versus the quantity of Cu2+ reduced to Cu+ (see the text).

The nature of the Cu2+---C3H8 species observed by EPR in Cu-TNU-9 zeolite was investigated by adsorbing propane and applying advanced pulsed EPR HYSCORE, which is a 2D experiment that allows detecting the NMR transitions of magnetically active nuclei (I ≠ 0) interacting with the electron spin.50 The HYSCORE spectrum of Cu-TNU-9 zeolite dehydrated at 773 K, displayed in Figure 4 a, is characterized by cross peaks centered at (3.47, 3.47) MHz corresponding to the Larmor frequency of the 27Al nuclei. The cross peaks are originated by the 13 ACS Paragon Plus Environment

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interaction of the unpaired electron of isolated Cu2+ with framework aluminum atoms in its second coordination sphere (see Figure S5 and Table S4). The spectrum of Cu-TNU-9 in the presence of propane, displayed in Figure 4b, shows an extended ridge signal centered at (νH, νH) absent for the dehydrated zeolite (Figure 4a) that must arise from the hyperfine interaction of isolated Cu2+ with the 1H atoms of the C3H8 molecules. Meanwhile, the spectrum of zeolite Cu-Y recorded in the presence of C3H8, shown in Figure 4c, displays a spot at (νH, νH) but no hyperfine interaction indicating the that Cu2+---C3H8 species are not present. Inspection of the proton ridge in the spectrum of zeolite Cu-TNU-9 with propane, shown in detail in Figure 4d reveals two signals denoted as 1HA and 1HB. Since the CW-EPR spectrum shows a sole signal of Cu2+---C3H8 (see Figure S6), the peaks 1HA and 1HB must come from two different type of protons interacting with the same Cu2+ atom, rather than from two different Cu2+ species. Analysis of the data of Figure 4d allows calculating the Cu2+- 1H distances, which are r = 2.6 Å for the strongly coupled proton 1HA, and r = 3.4 Å for 1HB. Detailed description of the HYSCORE experiments is given in the Supporting Information (Figures S5 and S6 and Table S4).


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Figure 4. HYSCORE spectra taken at 20 K at the observer position B0 = 336.2 mT (g = 2.06) of (a) Cu-TNU-9 dehydrated at 773 K, (b) Cu-TNU-9 with 2C3H8/Cu adsorbed, (c) Cu-Y with 2C3H8/Cu adsorbed, and (d) magnification and simulation of the proton signal of (b). The dotted anti-diagonal indicates the 1H Larmor frequency. The HYSCORE spectra were recorded at different τ values (96, 136,176, 278 ns) and summed together after Fourier transform, in order to avoid blind spot effects. The lack of interaction of C3H8 with Cu2+ in Cu-Y and the adsorption mode of this molecule on Cu-TNU-9 derived from the EPR experiments was confirmed by theoretical calculations. Two zeolite models containing a Cu2+ cation compensating the negative charge generated by two Al atoms in a 6mr unit were cut out from the periodic structures of FAU and TUN frameworks, as displayed in Figure 5, and a propane molecule was placed nearby. The model for Cu-Y zeolite is composed by one sod cage and one d6r (30T units) and contains a Cu2+ cation in the SI’ position, that is, in the 6mr of the sod cage. The model for Cu-TNU-9 is composed by 32T units accessible from the 10-member ring channel, and contains a Cu2+ cation in a 6mr ring facing the


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channel. After geometry optimization, propane approaches the Al atom facing the cavity in Cu-Y model, but cannot directly interact with the Cu2+ cation inside the sod cage. In contrast, propane binds to the Cu2+ cation in Cu-TNU-9, with two hydrogen atoms of a methyl group at only 2.04 and 2.58 Å from Cu2+, while all other hydrogen atoms are at distances longer than 3.55 Å, in agreement with experiment. Altogether, the same overall picture for the adsorption of a propane molecule on a Cu-zeolite is obtained experimentally by 2D HYSCORE experiments and by theoretical calculations

Figure 5. DFT optimized geometry of propane interacting with a Cu2+ center in a) Cu-Y and b) Cu-TNU-9 zeolites. The top image shows the location in the framework of the clusters actually


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used, depicted in the bottom image. Si, O, Al, Cu, C and H atoms are displayed in yellow, red, purple, green, orange and white, respectively. Summarizing, EPR results indicate that Cu+ in Cu-zeolites reacts with NO and O2 to produce Cu2+NO3- species as illustrated here for Cu-Y and Cu-TNU-9. The sensitively higher catalytic activity for the C3H8-SCR-NOx of Cu-TNU-9 zeolite compared with Cu-Y (see Figure 1) can be attributed to the interaction of propane with Cu2+. Meanwhile, Cu2+---C3H8 does not takes place in Cu-Y, probably because Cu2+ occupies non-accessible sites within d6r and sod cages of the FAU-type structure. The C3H8-SCR-NOx reaction is accompanied by the reduction of Cu2+ to Cu+ to an extent that increases with the catalytic activity. The results of Figure 3 prove that isolated Cu2+ participates in the HC-SCR-NOx reaction, being the Cu+ re-oxidized by the excess of O2 under real SCR flow condition.

3.3 Stable organic intermediates and reaction mechanism In order to get insight on the nature of the organic intermediates formed, Cu-TNU-9 and Cu-Y zeolites were investigated by in situ

15

N (Figure 6) and

13

C (Figure 7) solid state NMR

spectroscopy. The spectra of Figures 6 and 7 were recorded after heating the zeolite samples at 623 K with the reactant mixture (213C3H8/1,215NO/6O2/Cu) and subsequent degassing at 298 K.


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Figure 6. (a-b)

15

N-MAS NMR and (a’-b’) 1H/

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N CP-NMR spectra measured at 298 K of

zeolites (a,a’) Cu-Y and (b,b’) Cu-TNU-9 heated at 623 K with the reaction mixture (213C3H8/1,215NO/6O2/Cu), and then degassed at 298 K. Figure 6 shows the 15N NMR spectra of Cu-Y (Figures 6a and 6a’) and Cu-TNU-9 (Figures 6b and 6b’) zeolites. The spectrum of Cu-TNU-9, drawn in Figure 6b, shows a relatively sharp signal at δ

15

N = -363.5 ppm assigned to

15

NH4+, on the basis of the results obtained upon the

adsorption of 15NH3 on this (spectrum not shown) and other zeolites.21,22,51 The 1H/15N CP MAS spectrum, displayed in Figure 6b’, consists of a very broad resonance at δ 15N ≈ -195 ppm with shoulders at δ 15N ≈ -205 ppm and at δ 15N ≈ -185 ppm, all of them within the typical δ 15N range of nitriles (R−C≡15N).52 Meanwhile, the 15N NMR spectra of Cu-Y zeolite show no signal in the δ 15N region of nitriles (Figure 6a’) and only a very weak peak of 15NH4+ (Figure 6a). Figure 7 displays the corresponding

13

C NMR spectra of zeolites Cu-Y (Figure 7a) and Cu-

TNU-9 (Figure 7b and 7b’). All spectra show two relatively narrow peaks at δ 13C ≈18 ppm and at δ 13C = -1.8 ppm and three broad signals centered at δ 13C ≈ 89 ppm, δ 13C ≈125 ppm and δ 13

C ≈ 165 ppm. The methyl signal at δ 13C ≈18 ppm can be due to unreacted 13C3H8 (δ 13C ≈16

ppm), especially for zeolite Cu-Y, and that at δ

13

C = -1.8 ppm can be tentatively assigned to


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13

CH4 (δ

13

C = -2.4 ppm) formed during the activation of propane.53 On the basis of the δ

13

C

ranges of organic compounds, the peak at δ 13C ≈ 89 ppm can be attributed to nitro- (or nitroso-) compounds,54 which have been reported to be formed from propane and NO2 in the first stages of the SCR reaction.21,22 According to the 15N NMR spectra, the broad signal at δ 13C ≈ -125 ppm must be due to nitriles (R-13C≡N), although the presence of isocyanate (R-N13CO) cannot be ruled out as they appear in the same δ 13C region.52 The peak δ 13C ≈ 165 ppm can be originated by an amide (−13CO−NH−) or more probably by a cyanide (13CN-) anion compensating Cu+ as it is not observed under 1H/13C CP MAS conditions (see Figure 7b’).

Figure 7. (a-b) 1H decoupled 13C MAS NMR and (b’) 1H/13C CP MAS NMR spectra measured at 298 K of zeolites (a) Cu-Y and (b, b’) Cu-TNU-9 heated at 623 K with the reaction mixture (213C3H8/1,215NO/6O2/Cu) and then degassed at 298 K. Figure 7 shows a higher overall intensity of the 13C NMR spectra for Cu-TNU-9, but the most significant differences with Cu-Y are the much higher relative intensity of the signal of nitriles (δ 13

C ≈ 125 ppm) and the presence of two peaks at δ 13C = 183 ppm and δ 13C = 30 ppm. Although

the δ 13C = 183 ppm is characteristic of CO2, the intensity increase under cross polarization (see


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Figure 7b’) suggests that it can also be originated by carboxylic acid probably coming from the hydrolysis of nitriles. The peak at δ 13C ≈ 30 ppm must correspond to compounds containing a 13

C−N-R bond and is tentatively attributed to isocyanate (13CH2−N=C=O) as there is a general

agreement on their participation as reaction intermediates,7,19-22 although it could also be due to amine (−13CH−NH2) or amide (−CO−NH−13CH3)54 coming from partial hydrolysis of nitrile/isocyanate or nitrile, respectively. In any case, the 13C NMR spectra of zeolite Cu-TNU-9 (see Figure 7b) are dominated by the signal of nitriles (δ 13C ≈ 125 ppm). The main differences between the two zeolites are consequence of their different catalytic activity as proved by the fact that the NMR spectra of Cu-TNU-9 zeolite heated with the reaction mixture at 523 K (spectrum not shown) instead of 623 K, are similar to those of Cu-Y zeolite, according to their comparable NO conversion (20 % for Cu-TNU-9) (see Figure 1). Therefore, NMR provides evidence on the formation of nitriles, isocyanate (or amine or amide) and their hydrolysis products, carboxylic acid and/or CO2 and ammonia (or amines) as intermediates in the C3H8-SCR-NOx reaction over Cu-zeolites, whereas aldehydes or ketones are not observed. Although NMR results do not allow the unequivocal identification of organic intermediates, it is possible to propose a general reaction pathway, which is schematically depicted in Scheme 1 by reactions (1) - (8), disregarding the stoichiometry of the process. According to our results, the admission of the reactant mixture onto Cu-Y and Cu-TNU-9 zeolites generates Cu2+NO3- species by reaction of Cu+ with NO/O2 according to reaction (1) in Scheme 1. Besides the development of nitrate, there is an additional interaction of the methyl group of the C3H8 molecule with Cu2+ depicted by step (2) in Scheme 1, allowing the activation and reaction of the alkane at higher temperatures. The Cu2+---C3H8 complex has been identified in Cu-TNU-9 zeolite and is characterized by a H-Cu2+ distance of 2.6 Å (or 2.04 Å according to


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DFT modeling). The NMR results for Cu-TNU-9 show that C3H8 reacts at intermediate temperatures (573 K – 673 K) with NO/O2 on Cu2+ giving rise to nitriles and isocyanate intermediate species (step 3). Step (3) can be speculated to be initiated by the hydrogen abstraction of the C3H8 and reaction with NO to give a nitro alkane (CH3-CHNO-CH3) (signal at δ

13

C ≈ 89 ppm) that can tautomerize to the oxime CH3-CNOH-CH3. This can give the

Beckmann rearrangement to the N-methyl acetamide (CH3-CO-NH-CH3, isocyanate group) or be oxidized to nitriles (R-CN).21, 55

In agreement with previously proposed reaction mechanisms, one possible way for generating N2 is the oxidation of nitriles and isocyanates according to the reaction (4).18-22 However, it is also generally admitted that these nitrogen containing species are easily hydrolyzed in the presence of water giving a carboxylic acid and NH2/R−NH2/NH3. The hydrolysis (step 5) is proved here by the detection of 15NH4+, formed by the protonation of NH3 on a zeolite Brønsted acid site (reaction 6, Scheme 1), being its reaction with Cu2+NO3- schematized by step (7) is the preferred way for generating N2. The carboxylic acid formed is the main source of CO2 either by reacting with Cu2+NO3- giving also N2 and H2O (step 8), or by direct oxidation, but moreover they can take part in the formation nitriles or isocyanates.18,20-22,55 The hydrolysis of the organic nitrogen compounds as depicted in Scheme 1 would involve only the water generated in the reaction. In fact all our experiments have been carried out without adding water to make the study simpler. However, water is present in the exhaust gas under real reaction conditions (2-18 %) and the effect on the SCR-NOx, depends very much on the structure, properties and characteristics of the catalyst used.11,13,18,25 In this sense, the presence of water in the feed does not practically affect the catalytic performance of Cu-TNU-9 zeolite for


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the C3H8-SCR-NOx.25 However, it can be detrimental for other Cu-zeolites and the deactivation is usually attributed to changes in the coordination, structure or position of copper active sites for the reaction, or to zeolite dealumination.11,13,18,25

Scheme 1. Proposed mechanism of the SCR-NOx reaction in Cu-zeolites using propane as reductant. Intermediate species experimentally detected either by NMR (red) or EPR (green).

Another question arising from Scheme 1 is the involvement of Cu2+NO3- in the HC-SCR-NOx reaction. The formation or Cu2+NO3- is a way of activating NO with O2 on Cu+ sites and then have been assumed to play a key role for the SCR reaction specially when there is no NO2 in the reactant mixture. In the most accepted the NH3-SCR-NOx scheme, Cu2+NO3- and NO are in equilibrium with NO2 (g) and Cu2+NO2-, and the nitrite reacts with NH3 releasing N2 and H2O.46 For the HC-SCR-NOx reaction it is generally assumed that nitrates (nitrites) are able to react with hydrocarbon in the gas phase leading to the formation of N2.18,19 However, the catalytic activity of Cu-Y in the temperature range 573 K – 673 K is very low in spite of the availability of Cu2+NO3- in the supercage, indicating that nitrate does not react with propane in the gas phase.


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Contrarily, when ammonia instead of C3H8 is used as reductant, the catalytic activity of Cu-Y sharply increases (see Figure 1), indicating that ammonia is able to react with Cu2+NO3- and NO, while propane needs to be activated on accessible isolated Cu2+ for the SCR reaction. Indeed, the catalytic activity of Cu-TNU-9 and Cu-Y follows the same trend for the oxidation of propane42 strongly suggesting that the same sites are required for activating hydrocarbon molecules in both hydrocarbon oxidation and HC-SCR-NOx reactions in Cu-zeolites, and that the same active sites operate in the two processes. An interesting result reported here is the observation that the higher the NO conversion in the C3H8-SCR-NOx reaction, the higher is the reducibility of Cu2+ by heating under the reaction mixture (see Figure 3). Although it is not possible to stablish in which stage of the SCR reaction scheme (1)-(8) Cu2+ is reduced to Cu+, it can be speculated to occur with the formation of nitriles and isocyanates, since Cu2+ sites are required in this step (reaction (3)). At this point, the oxygen present in the reaction mixture plays a key role to oxidize Cu+ restoring the isolated Cu2+ active site required for the progress of the SCR reaction. As depicted by reaction (1) in Scheme 1, Cu+ is oxidized to Cu2+ by reaction with NO/O2 to give Cu2+NO3- that reacts with NO and ammonia according to the overall NH3-SCR-NOx reactions (steps (1) and (7) in scheme 1). The HC-SCRNOx reaction mechanism proposed here implicates the participation of Cu+ and of Cu2+ sites accessible to hydrocarbon, co-existing both oxidation states during the SCR-NOx reaction. The Scheme 1 for the HC-SCR-NOx reaction on Cu-zeolite catalysts have been thoroughly or partially proposed in previous publication on the basis of the observed reaction intermediates.1822,55

However, our work provides evidence on the essential role of Cu2+ in Cu-zeolites, which are

required for the activation of C3H8, as it does not react with Cu2+NO3- in the gas phase. Therefore, decreasing the reaction temperature for the HC-SCR-NOx needs catalysts containing


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accessible sites active for the oxidation of hydrocarbon at lower temperatures. Moreover we have observed the development of ammonia by the hydrolysis of nitrogen containing intermediate with the water generated in the reaction medium. Ammonia can act as reducing agent, which we believe is the main path for the production of N2.

4. CONCLUSIONS The investigation of the C3H8-SCR-NOx reaction over Cu-zeolites by means of in situ solid state NMR and EPR spectroscopies has shed light on the nature of the active sites for the reaction and led to a general reaction pathway involving Cu2+/Cu+ redox cycles. The results reported here indicate that Cu+ present in Cu-zeolites reacts with NO/O2 to form Cu2+NO3- even at room temperature. In Cu-TNU-9 zeolite, Cu2+ at accessible sites interacts with propane molecules (Cu2+---C3H8) being this interaction essential for the HC-SCR-NOx, since the process is initiated there by the reaction of C3H8 with NO/O2, forming nitriles and isocyanate intermediate compounds. Although several reaction pathways have been proposed, our results point out towards the hydrolysis of the nitrogen-containing intermediates with the water generated in the reaction medium to give ammonia, which acts as the actual reductant. From this point on, ammonia can react with Cu2+NO3- and NO as proposed in the NH3-SCR-NOx, being this the main path for the production of N2. This general scheme for the HC-SCR-NOx reaction is supported by the low catalytic activity of Cu-Y zeolite, where most copper cations are placed in the small d6r or sod cages of the FAU structure and Cu2+ remains non accessible to propane. In spite of this, Cu2+NO3- is formed probably because Cu+ is able to migrate to form stable species in the super-cage, but the unavailability of active sites for the activation of propane inhibits the formation of ammonia,


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giving a NO conversion of about 10 % at 623 K. Indeed, when ammonia is directly used as a reductant (NH3-SCR-NOx), it reacts with the Cu2+NO3- and Cu-Y gives a NO conversions of 99 % at 623 K. Moreover this result proves that the HC-SCR-NOx on Cu-zeolites does not take place by direct reaction of propane in the gas phase with Cu2+NO3-. Regarding the re-dox cycles, Cu+ and Cu2+ are present in Cu-zeolite dehydrated at 500 ºC. In the presence of the HC-SCR-NOx reaction mixture Cu+ is oxidized to Cu2+ by formation of Cu2+NO3- and reduced back to Cu+ during the reaction with ammonia. Meanwhile, isolated Cu2+ is reduced to Cu+ probably during the propane activation and reaction at intermediate temperatures, as supported by the observation that the higher catalytic activity the higher amount of copper reduced. This Cu+ is re-oxidized by the oxygen present in the reaction mixture.

ASSOCIATED CONTENT

AUTHOR INFORMATION Corresponding Author *[email protected] Present Addresses § Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie Curie 2, Campus de Cantoblanco, 28049 Madrid (Spain). Notes The authors declare no competing financial interests.


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SUPPORTING INFORMATION.

Experimental section including catalysts preparation, details on the H2 TPR measurements, HYSCORE experiments and the procedure used for the in situ EPR and NMR measurements; H2-TPR results; EPR spectra of Cu-Y and Cu-TNU-9 with reactants and Tables with the characteristics of the main signals; in situ 13C-NMR spectra of 13C3H8 adsorbed on Cu-Y and CuTNU-9; HYSCORE supporting data including magnification and simulation of the

27

Al signal

and calculation of Cu2+-1H distances; table of NO conversion at different temperatures of CuITQ-2 and Cu-ZSM-5 zeolites in the C3H8-SCR-NOx reaction. This material is available free of charge via the Internet at http://pubs.acs.org

ACKNOWLEDGMENT The authors acknowledge financial support by the Spanish Government-MINECO through CTQ2015-68951-C3-1-R, “Severo Ochoa” (SEV 2012-0267) and by CSIC (i-Link0821). Red Española de Supercomputación (RES) and Centre de Càlcul de la Universitat de València are gratefully acknowledged for computational facilities and technical assistance. M. M. thanks ITQ for grants.

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