Surprising Coordination Chemistry of Cu + Cations in Zeolites: FTIR Study of Adsorption and Coadsorption of CO, NO, N 2, and H 2 Oon Cu ZSM 5

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1 This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. pubs.acs.org/jpcc Surprising Coordination Chemistry of Cu + Cations in Zeolites: FTIR Study of Adsorption and Coadsorption of CO, NO, N 2, and H 2 Oon Cu ZSM 5 Videlina Zdravkova, Nikola Drenchev, Elena Ivanova, Mihail Mihaylov, and Konstantin Hadjiivanov* Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria *S Supporting Information Downloaded via on October 8, 2018 at 16:31:25 (UTC). See for options on how to legitimately share published articles. ABSTRACT: Cations exchanged in zeolites are generally characterized by a low coordination number and can thus attach simultaneously more than one small guest molecule. For instance, Cu + ions in ZSM-5 can accept, at low temperature, up to three CO and up to two NO molecules. However, only one N 2 molecule can be coordinated to such sites. Although mixed aqua-carbonyl and aqua-dinitrogen complexes are formed, no mixed carbonyl-nitrosyl, carbonyl-dinitrogen, or nitrosyldinitrogen species can be produced. Thus, adsorption of NO on CO precovered sample results in segregation of the CO adsorption layer according to the reaction: 2Cu + CO + 2NO Cu + (CO) 2 + Cu + (NO) 2. Adsorption of N 2 on NO precovered sample leads to a similar process: 2Cu + NO + N 2 Cu + (NO) 2 +Cu + N 2. No carbonyl-dinitrogen complexes are produced during CO N 2 coadsorption. The role of the ligand and the nature of the bond in the formation of geminal and mixed-ligand complexes are discussed. 1. INTRODUCTION The coordination state of cations exchanged in zeolites provokes an immerse interest because it is believed to be one of the main reasons for the unique catalytic and adsorption properties of the related materials. Both experimental 1 34 and theoretical studies have revealed that, in general, these cations are characterized by low coordination number and each of them can coordinate simultaneously two or more small molecules. This is particular important for catalysis because in many cases the reactant molecules should be in close proximity to form an intermediate complex. The problem has been extensively investigated during the past two decades, and many geminal adsorption complexes have been isolated and described. IR spectroscopy is by far the most convenient technique for this purpose because in most cases it allows distinguishing between complexes with different number of ligands. A series of model IR investigations with zeolites exchanged with alkali- and alkaline-earth cations have revealed the possibilities of simultaneous bonding of two or three CO 1 8 or N 7 9,35 2 molecules to one cation. The driving force of the formation of these geminal complexes is the low coordination number of the exchanged cation, and the complexes formed are called site-specified geminal species. 10,11 These investigations demonstrate that the adsorption enthalpy is not decisive for the maximal number of molecules attached to one site. It has also been reported that each cationic position is characterized by a critical cationic radius of hosted cations for geminal species to be formed. 12,13 In these cases, the change of the ligand does not affect the process, and consequently mixed ligand species, for example, Na + (CO)(N 2 ) in NaY, have been isolated. 2 For catalysis, the formation of geminal ad-species with transition metal cations in zeolites is more important. However, in these cases, the situation could be complicated. 11 First, the so-called complex-specified geminal species can be produced with particular cation adsorbate systems. For instance, with Rh + cations CO forms Rh + (CO) 2 dicarbonyls irrespective of the support. These species do not produce monocarbonyls upon decomposition. 14,15,39 In this case, the driving force of the formation of the complexes is the achievement of a stable electron configuration. However, even in these cases, the cation coordination is important. Thus, while only dicarbonyls are formed with oxide-supported rhodium, Rh + (CO) 3 and Rh + (CO) 4 species are produced with Rh + sites in zeolites. 14,15 Copper-exchanged zeolites are subjected to a continued interest. They are effective catalysts in many reactions, for example, selective catalytic reduction of NO x with hydrocarbons, 40,41 decomposition of NO 42 and N 2 O, 43 oxidation of methane to methanol, 44 oxidative carbonylation of methanol to dimethyl carbonate, 45 etc. That is why the coordination state of copper in zeolites has attracted the interest of many researchers. In what follows, we shall concentrate on Cu + sites because they demonstrate fascinating coordination chemistry. Received: April 2, 2015 Revised: June 9, 2015 Published: June 11, American Chemical Society 15292

2 In 1994, Zechina et al. 18 reported that CO adsorption at room temperature on Cu I ZSM-5 resulted in the formation of Cu + (CO) 2 dicarbonyl species characterized by two IR bands: ν s at 2178 and ν as at 2151 cm 1. Decrease of the CO equilibrium pressure led to loss of one of the CO ligands and conversion of these species into monocarbonyls (2157 cm 1 ). In contrast, at low temperature, a large part of the dicarbonyls was converted into tricarbonyls (2190, 2164, and 2140 cm 1 ). Later, the results were confirmed by other authors, 21,22,26 and a similar situation was also found with Cu + ions in other zeolites 24,27 30,33,34 and porous materials. 46 Note that when CO is adsorbed on oxide supported copper, the Cu + CO species formed are characterized by an IR band around 2130 cm 1, which is associated with the higher coordination number of Cu + in these cases. 19,47 Although there are some reports on the formation of dicarbonyls at low temperature, 20,48 the Cu + :CO stoichiometry is generally considered to be 1:1. Thus, CO can be used to detect the number of effective coordinative vacancies of particular family of Cu + sites. It is also found that coadsorption of CO and water on Cu ZSM-5 leads to formation of mixed ligand complexes, Cu + (H 2 O)CO, with a C O frequency around 2135 cm 1. 10,19 Consequently, it was concluded that the water ligand simulates a high coordinative saturation of the Cu + site (similar to that of oxide-supported copper), and thus the CO stretching frequency is observed at lower wavenumbers. More recently, the formation of mixed ligand complexes was confirmed by other authors, 23,24 and different numbers of water molecules were proposed in the aqua-carbonyl complexes. Mixed carbonyl-nh 23 3 and carbonyl-acetone 25 species were also isolated. When NO is adsorbed at low temperature on Cu + sites in Cu ZSM-5, it produces mononitrosyls (1810 cm 1 ), which are in equilibrium with dinitrosyls ( and cm 1 ) The Cu + NO species can be observed even at room temperature but are much less stable than the carbonyl complexes. No data on aqua-nitrosyl complexes are available. The interest in N 2 adsorption on copper containing zeolites is associated mainly with the formation of relatively stable Cu + N 2 species. 42,49 53 The Cu + N 2 complexes in Cu ZSM- 5 are observed even at ambient temperature at ca cm 1. The possibility of existence of IR invisible nitrogen bridging two cationic sites has also been pointed out. 52,54 Despite the similarity of CO and N 2 as ligands, no data of geminal dinitrogen species of Cu + in zeolites are available. Although there are many studies on the formation of mixed carbonyl complexes of the Cu + (CO)(L) type (L = H 2 O, NH 3, CH 3 COCH 3 ) with different copper-containing zeolites, there are only a few and contradictory reports on CO + NO coadsorption. 16,55 Formation of Cu + (CO)(NO) species with characteristic IR bands at 2137 cm 1 (C O modes) and 1890 cm 1 (N O modes) was suggested. 55 Indeed, this could be expected on the basis of the low coordination of the Cu + cations. However, according to Tortorelli et al., 16 no carbonylnitrosyls complexes of Cu + are formed. Studies of CO + N 2 coadsorption have revealed that CO blocks the adsorption sites 25,46 and no data on mixed ligand species are available. Despite the very large number of studies on the coordination chemistry of Cu + cations in zeolites, and in particular in ZSM-5, there are still many unclear points concerning mainly the simultaneous coordination of different guest molecules, which is important for catalysis. In this work, we investigate the possibility of formation of Cu + (L 1 )(L 2 ) mixed ligand species where L = CO, NO, N 2, and H 2 O. The four ligands are of different nature: water is an electrostatic base and is coordinated to Cu + by electrostatic forces, while significant π- bonding occurs with the other ligands. NO is a radical molecule, and coupling of electron might play an important role in the formation of the complexes. CO and N 2 are similarly bonded, but the enthalpy of CO adsorption is significantly higher. We show that no mixed ligand species are produced when L = CO, NO, and N 2 and discuss the possible reasons for this surprising coordination chemistry of Cu + cations. 2. EXPERIMENTAL SECTION The starting zeolite material was prepared by calcination of NH 4 ZSM-5 (Zeolist, Si/Al = 25) at 823 K for 2 h. The Cu ZSM-5 sample was synthesized by ion exchange from a mol L 1 solution of copper acetate, and the final copper concentration in the sample was 1.01 wt %. Other Cu ZSM-5 samples were also investigated and showed very similar results. For brevity, they will not be considered here. FTIR spectra were recorded with Nicolet 6700 and Nicolet Avatar 360 spectrometers accumulating up to 128 scans at a spectral resolution of 2 cm 1. Self-supporting pellets (ca. 10 mg cm 2 ) were prepared from the powdered samples and treated directly in a purpose-made IR cell allowing measurements at ambient and low temperatures. The cell was connected to a vacuum-adsorption apparatus with a residual pressure below 10 4 Pa. Prior to the adsorption experiments, the samples were activated by 1 h calcinations at 673 K and 1 h evacuation at the same temperature. To ensure higher concentration of Cu + sites, the sample was reduced by CO (5 kpa, 15 min, 473 K) and then evacuated at K (to produce CO precovered sample) or at 673 K (to produce CO-free sample). Carbon monoxide (>99.5% purity) was supplied by Merck. 13 C-labeled CO (>99.0) and NO (>99.0% purity) were obtained from Messer Griesheim GmbH. Labeled nitrogen ( 15 N 2, isotopic purity of 98 at. %) was provided by Aldrich. Before adsorption, CO and 15 N 2 were additionally purified by passing through a liquid nitrogen trap. 3. EXPERIMENTAL RESULTS The adsorption of the individual adsorbates (CO, NO, N 2, and H 2 O) on Cu ZSM-5 is well studied. That is why here we will only briefly present the main results directly related to this study and shall concentrate on some new observations Background Spectra and Adsorption of Individual Adsorbates: H 2 O, CO, NO, and N Background Spectra. The background spectrum of our material, registered at ambient temperature, is consistent with the literature data 56 and shows, in the hydroxyl region, bands at 3745 cm 1 (silanol groups), 3664 cm 1 (aluminol groups formed with extraframework alumina species), 3612 cm 1 (bridging zeolite hydroxyls), and a broad feature around 3480 cm 1 (H-bonded hydroxyls) (see Figure S1 from the Supporting Information, spectrum a). At 100 K, the bands are slightly shifted, and the maxima are set at 3747, 3666, 3616, and ca cm 1, respectively (Figure S1 from the Supporting Information, spectrum b) Adsorption of H 2 O. The spectrum registered after adsorption of H 2 O on the reduced Cu ZSM-5 sample, followed by evacuation at ambient temperature to remove weakly adsorbed water, is presented in Figure S1 in the Supporting Information, spectrum c. It is seen that water 15293

3 Table 1. Spectral Characteristics of the Monoligand Adsorption Complexes Formed with Cu + and Cu + (H 2 O) Sites in Cu ZSM- 5 a site description ν(co), cm 1 ν( 15 N 2 ), cm 1 ν(no), cm 1 note I Cu + in cationic positions (2295) b 1812 most electrophilic Cu + sites; preferentially covered by water II Cu + in cationic positions (2298) 1810 principal Cu + sites III Cu + from oxide-like phase (2303) 1870? Cu + sites are more coordinated than sites I and II IV Cu + (H 2 O) sites (2285) sites expected to be heterogeneous a The ligands are CO, 15 N 2, and NO. b The calculated 14 N 14 N frequencies are presented in parentheses. adsorption hardly affects the silanol groups. In contrast, the band characterizing bridging hydroxyls has disappeared from the spectrum, evidencing that these OH groups are involved in hydrogen bonding with water molecules. Indeed, a broad absorbance (not shown) due to the OH stretching modes of H- bonded hydroxyls was detected. It is split into two bands with maxima at 2917 and 2467 cm 1 (AB structure of H-bonded hydroxyls) as a result of Fermi resonance. 56,57 The spectral features of adsorbed water molecule are at 3700 and 3525 cm 1 (OH stretching modes) and at 1626 cm 1 (water deformation modes). 56 It is also seen that the band at 3664 cm 1 has strongly increased in intensity, and its maximum was set at 3670 cm 1. A band around 3660 cm 1 has already been observed after adsorption of water on H ZSM-5 58,59 and assigned in different ways. It seems that the band corresponds (in addition to the absorbance caused by the aluminol groups) to OH (H 2 O) 2 adducts. The low-frequency shoulder could be due to Cu 2+ OH groups (reported at 3657 cm 1 ). 33 It is also possible water adsorbed on cationic sites to contribute to the band. Indeed, the symmetric mode of H 2 O adsorbed on c.u.s. sites can shift to lower frequencies. 60 Although the spectra do not give clear evidence of water coordinated to Cu + sites, the coadsorption results suggested the existence of some Cu + (H 2 O) species (vide infra) Adsorption of CO. The results on CO adsorption on our sample (including 13 C 18 O isotopic studies) have been recently reported. 26 CO adsorption at 100 K at increasing coverages leads first to formation of Cu + CO species (2157 cm 1 ), which are further converted into Cu + (CO) 2 dicarbonyls (ν s at 2180 and ν as at 2151 cm 1 ). In the presence of gas-phase CO, a large part of the dicarbonyls can be transformed into Cu + (CO) 3 tricarbonyls with specific IR bands at 2191 and 2167 cm 1. At these conditions, bands due to CO attached to the bridging zeolite hydroxyls (2175 cm 1 ) and physically adsorbed CO (2138 cm 1 ) are also observed. Evacuation at 100 K leads to destruction of the tricarbonyls, while the dicarbonyls are stable at this temperature. A careful analysis of the spectra registered at low CO coverages (spectra registered at ambient temperature) reveals some heterogeneity of the Cu + sites (Figure S2 from the Supporting Information). The principal carbonyl band (2158 cm 1 ) has a shoulder at 2166 cm 1, which is attributed to another family of Cu + sites in cationic positions. This conclusion is supported by the appearance of a second component (at 2182 cm 1 ) of the ν s modes of the dicarbonyl species. At higher coverages (Figure S2 from the Supporting Information, spectra a,b) another band at 2141 cm 1 is discernible as a shoulder and is assigned to carbonyls formed with Cu + ions that are not in cationic positions. 18,19,47 For convenience, the observed CO stretching frequencies are summarized in Table Adsorption of NO. Figure 1 presents the spectra registered after successive adsorption of small doses of NO at Figure 1. FTIR spectra of small doses of NO successively adsorbed at 100 K on reduced Cu ZSM-5 (a k). The band at cm 1 increases in intensity in the set of spectra presented in panel A and decreases in the spectra shown in panel B. The inset in panel B shows the N N stretching region of N 2 and N 2 O. All spectra are background corrected. 100 K on our sample. Initially, mononitrosyls of Cu + (1812 cm 1 ) are produced (Figure 1A, spectrum a) and start to be converted into dinitrosyls (1824 and 1731 cm 1 ) far before the occupation of all Cu + sites (Figure 1A, spectrum b). This shows that the stability of mono- and dinitrosyls is comparable (contrary to the case of mono and dicarbonyls) and suggests some additional factors stabilizing the dinitrosyl structures. At the same time, a band at 1913 cm 1 also develops (Figure 1A, spectra c g) and is assigned to Cu 2+ NO species. 61 At higher NO coverage (Figure 1B), a series of other bands are observed. The bands associated with copper ( cm 1 ) are due to mono- and dinitrosyl species of Cu n+ sites (n > 1). 16 The other bands are observed also with H ZSM-5 and are assigned as follows: cm 1,totrans-(N 2 O 2 ); 1894 cm 1, to OH NO species; 2205 and 1684 cm 1, to the symmetric and antisymmetric modes, respectively, of [N 2 O 2 ] + adducts. Evacuation leads to loss of one of the NO ligands from the Cu + (NO) 2 species; that is, they are converted into mononitrosyls

4 We were not able to resolve a nitrosyl band corresponding to Cu + sites that are not in cationic positions. Such a band is expected in the cm 1 region. 61 A possible candidate is a weak feature at 1780 cm 1 detected when the mononitrosyls were practically converted into dinitrosyls (Figure 1B, spectrum h). However, at higher coverages, it is masked by the strong band at 1786 cm 1, which makes the assignment only tentative Adsorption of 15 N 2. The results on 15 N 2 adsorption on our sample are generally consistent with previous reports on 14 N 2 adsorption. As was already mentioned, a Cu + N 2 band at 2295 cm 1 is observed after low temperature 14 N 2 adsorption on Cu ZSM ,63 In our experiments, we used the 15 N 2 isotopologue to avoid any hindrance from the spectrum of CO 2 in the air. On the basis of the theoretical isotopic shift factor, 1.035, 64 the Cu + 15 N 2 band is expected around 2217 cm 1. The spectra registered after successive adsorption of small doses of 15 N 2 on our sample at 100 K show initial development of a Cu + 15 N 2 band at 2117 cm 1 (Figure 2A, spectrum a). Figure 2. FTIR spectra of 15 N 2 (panel A) and 1:1 14 N N 2 isotopic mixture (panel B) adsorbed at 100 K on reduced Cu ZSM-5 sample. Spectra a h correspond to increasing amounts of dinitrogen introduced to the system up to 1 mbar equilibrium pressure. The spectra are background corrected. With increase of amount of 15 N 2 added to the system, the band grows in intensity, and another band at 2220 cm 1 develops (Figure 2A, spectra b h). This band strongly rises in intensity and becomes the principal band in the region. Second derivatives of the spectra indicate development, at high coverage, of a weak band at 2225 cm 1 (see Figure S3 from the Supporting Information, spectrum h). Simultaneously, the maximum of the principal N N band is slightly (by 0.5 cm 1 ) red-shifted. The results indicate the existence of three families of Cu + sites on the sample, which is consistent with the CO adsorption results (see Table 1). However, in this case, the opposite dependence between the frequency and stability is observed. The Cu + 15 N 2 bands decrease in intensity during evacuation at 100 K and easily disappear from the spectrum at higher temperature. These bands are also observed at ambient temperature when some 15 N 2 equilibrium pressure is maintained in the IR cell. In addition, two more bands were detected at high coverages. A band at 2254 cm 1 is attributed to 15 N 2 polarized by the zeolite bridging hydroxyls. 56 Indeed, the original OH band at 3611 cm 1 was shifted to 3496 cm 1 (Δν OH = 115 cm 1 )in parallel with the development/disappearance of the 2254 cm 1 band. Another band, at 2247 cm 1, is often assigned to SiOH 15 N 2 interaction. 56 In any case, the spectra are rather complex and do not exclude a priori the existence of geminal species. Moreover, similar species could be expected (i) on the basis of the low coordination of the Cu + sites and (ii) by analogy with Ni ZSM-5 where Ni + (N 2 ) 2 adducts were recently proven by 14 N 2 and 15 N 2 coadsorption experiments. 32 To confirm/reject the possibility of formation of geminal dinitrogen adspecies in Cu ZSM-5, we have studied the adsorption of a 1:1 14 N N 2 isotopic mixture at 100 K. Figure 2B presents the spectra registered in the 15 N 15 N stretching region. It is evident that they coincide very well with the spectra obtained after 15 N 2 adsorption only. Therefore, there is no vibrational coupling between the adsorbed molecules, which proves that no geminal species are produced. In conclusion, we underline that stability of the monoligand species decreases in the order Cu + CO Cu + NO > Cu + N 2.Itisdifficult to give definite conclusions on the stability of the Cu + (H 2 O) complexes, but it seems (see below) they are slightly more stable than the Cu + NO species. On the basis of the stability, we studied the adsorption of 15 N 2 on CO and NO precovered samples and adsorption of NO on a CO precovered sample. However, to allow measurements at low temperature, the water coadsorption experiments were performed with H 2 O precovered sample Adsorption of CO, NO, and 15 N 2 on Water- Precovered Sample. Before the adsorption of the individual gases, water was introduced to the sample and then evacuated at ambient temperature (see Figure S1 from the Supporting Information, spectrum c) Adsorption of CO. It is well established that water adsorption on CO precovered Cu ZSM-5 leads to the formation of Cu + (CO)(H 2 O) n complexes. 10,19,23,24 This is demonstrated in Figure S4 of the Supporting Information. Introduction of water at ambient temperature to the COprecovered sample leads to a shift of the carbonyl band from 2158 to 2132 cm 1 (Figure S4, spectrum a). This shifted band is attributed to Cu + (H 2 O) 2 CO species. Decrease of the equilibrium H 2 O pressure leads to a decrease of the 2132 cm 1 band in intensity. Simultaneously, two bands, at 2139 and 2158 cm 1, developed at its expense. The band at 2139 cm 1 appears first and is assigned to Cu + (H 2 O) 2 CO species. The band at 2158 cm 1 (Cu + CO species) is the only carbonyl band detected after prolonged evacuation. The integral intensity of the carbonyl band is hardly affected by water, which indicates that the π-component of the Cu + CO bond remains practically the same. Therefore, the decrease of the CO stretching frequency is due to weakening of the σ- and the electrostatic bonds between copper and CO, as was already reported. 19 The results show that the aqua-carbonyl complexes easily lose water ligand(s) but are fully destructed only after 15295

5 prolonged (30 min) evacuation. Therefore, one can expect that a part of the Cu + sites on water precovered sample should have adsorbed water molecules. To check whether Cu + (H 2 O) species indeed existed on the water-precovered Cu ZSM-5, we studied CO adsorption on this sample. Successive adsorption at 100 K on small doses of CO led to the development first of a Cu + CO band at 2157 cm 1 (Figure S5 from the Supporting Information, spectrum a) and then of another band at 2139 cm 1, which was already attributed to Cu + (CO)(H 2 O) species (Figure S5 from the Supporting Information, spectra b d). At higher CO coverages, a process of conversion of mono- to dicarbonyls of Cu + (2178 and 2151 cm 1 ) starts to take place (Figure S5 from the Supporting Information, spectrum e). The results obtained imply that a small part of the Cu + sites on water precovered sample indeed holds water molecules. It is also to be noted that the carbonyl band at 2166 cm 1 was not observed in the spectra of CO adsorbed on water precovered sample, and this also accounts for the 2182 cm 1 shoulder of the ν s dicarbonyl band. Therefore, water has been preferentially adsorbed on the respective sites; that is, they are characterized by a higher electrophilicity than the type II Cu + sites Adsorption of 15 N 2. The similarity of CO and N 2 as ligands suggests the possibility of formation of mixed Cu + (N 2 )(H 2 O) species. However, no such species have been reported in the literature. To check for their existence, we studied 15 N 2 adsorption at 100 K on water precovered sample (see Figure 3). It is seen that at high 15 N 2 coverages, a band at Figure 3. FTIR spectra of 15 N 2 adsorbed at 100 K on water precovered reduced Cu ZSM-5 sample (for details see text). Equilibrium 15 N 2 pressure of 2 mbar (a) followed by progressive evacuation at 100 K (b f). All spectra are background corrected cm 1 is detected. This band was not observed with the activated sample and is therefore attributed to Cu + ( 15 N 2 )(H 2 O) species. The similarity with the CO adsorption results confirms the assignment. The results indicate a very low stability of the aquadinitrogen complexes. Note that in this case the described above correlation between the stability of the dinitrogen complexes and the 15 N 15 N stretching frequency is not fulfilled. This observation will be discussed below. As with CO adsorption, only one family of dry Cu + sites was detected by 15 N 2 adsorption. However, the band position was at 2217 cm 1 (shifting to lower wavenumber by less than 1 cm 1 at high coverages), that is, a frequency typical of sites of type I (see Table 1). In contrast, the CO adsorption experiments revealed the existence of type II Cu + sites on wet surface. This seemingly contradiction could be explained by the effect of the relatively large amount of presorbed water (mainly at OH groups). Donating electrons to the sample, H 2 O slightly decreases the electrophilicity of the cationic sites of type II, thus affecting the stretching frequency of adsorbed 15 N 2. The high intensity of the band (much higher that that of the 2217 cm 1 band detected with the activated sample) is in line with this hypothesis. To obtain additional support on this hypothesis, we have studied low-temperature 15 N 2 adsorption on a sample preevacuated at 323 K. After this pretreatment, the amount of adsorbed water slightly decreased. The spectra of 15 N 2 adsorbed on the two samples are compared in Figure S6 of the Supporting Information. It is well seen that the maximum of the principal 15 N 2 band registered with the sample containing less water is slightly shifted to higher frequencies. Although the shift amounts to ca. 0.5 cm 1, it confirms our supposition Adsorption of NO. The possibility of formation of aqua-carbonyl and aqua-dinitrogen complexes provoked us to search for aqua-nitrosyl species. Moreover, similar adducts were already reported with the Pd-ZSM-5 sample where the stretching frequency of the Pd 2+ (NO) and Pd 2+ (NO)(H 2 O) species was reported at 1881 and 1839 cm 1, respectively. 65,66 Adsorption on NO at 100 K on water precovered Cu ZSM- 5 results in the formation of Cu + (NO) 2 (1825 and 1731 cm 1 ) and Cu 2+ (NO) (1894 cm 1 ) species (Figure S7 of the Supporting Information, spectrum a). Note the homogeneity of the dinitrosyls in this case, which is consistent with the CO adsorption experiments. During evacuation, the dinitrosyl species are converted into mononitrosyls. The latter appear at slightly lower wavenumbers as compared to the activated sample. However, this difference is too small to be attributed to the formation of mixed ligand species. Most probably the shift is chemical and caused by the large amount of water on sample, as was already discussed with 15 N 2. Thus, the results indicate that no Cu + (H 2 O)(NO) species are produced on Cu ZSM Coadsorption Studies Involving CO, NO, and 15 N Adsorption of NO on 12 CO + 13 CO Precovered Sample. Recently, we have reported 16 that adsorption of NO on an overexchanged Cu ZSM-5 sample led to the formation of dicarbonyls and dinitrosyls according to the reaction: Cu CO + 2NO Cu (CO) 2 + Cu (NO) 2 (1) This conclusion was made on the basis of simultaneous development of dicarbonyl and dinitrosyl bands upon NO dosage at 100 K on a CO-precovered sample. With our sample, we obtained essentially the same results. However, an opinion exists that mixed carbonyl-nitrosyl species are formed on Cu ZSM To prove unambiguously the formation of dicarbonyls, we studied the successive adsorption of small NO doses at 100 K on a sample precovered with equal amounts of 12 CO and 13 CO. After adsorption of the isotopic mixture, the sample was evacuated at ambient temperature to destroy any dicarbonylic species and then cooled to 100 K. Two bands, at 15296

6 2158 (Cu + 12 CO) and 2108 (Cu + 13 CO) cm 1, dominated in the spectra of the sample thus treated (Figure 4A, spectrum a). precovered sample because of the oxidative action of NO at ambient temperature. Subsequent cooling to 100 K hardly affected the spectrum (Figure 5, spectrum a). Small doses of 15 N 2 were then Figure 4. FTIR spectra registered after successive adsorption of small doses of NO at 100 K on reduced Cu ZSM-5 preliminary precovered with equal amounts of 12 CO and 13 CO (a j). Panel A: carbonyl ( 12 CO and 13 CO) region. Panel B: nitrosyl region. All spectra are background corrected. Figure 5. FTIR spectra (nitrosyl region) registered after adsorption of 15 N 2 on NO precovered Cu ZSM-5. Spectrum of NO precovered sample (a) and after successive adsorption of small doses of 15 N 2 at 100 K (b h). The inset shows the changes in the 15 N 15 N stretching region. Subsequent NO dosage led first to the development of Cu 2+ NO band at 1914 cm 1 (Figure 4B, spectrum b) and then to progressive erosion of the two carbonyl bands with a simultaneous development of dinitrosyl bands at 1822 and 1726 cm 1 (Figure 4A, spectra c i). At the same time, new carbonyl bands developed: at 2180, 2169, 2151, 2131, 2112, and 2103 cm 1. If dicarbonylic species are produced after adsorption of a 1:1 12 CO + 13 CO isotopic mixture, they should possess the following distribution: Cu + ( 12 CO) 2, 25%; Cu + ( 12 CO)( 13 CO), 50%; and Cu + ( 13 CO) 2, 25%. Also, on the basis of the experimentally observed frequencies of the Cu + ( 12 CO) 2 dicarbonyls and using the approximate force field model, 67 it is easy to calculate the frequencies of the species containing one or two 13 CO ligands. The calculation shows the Cu + ( 12 CO)- ( 13 CO) species should manifest bands at 2169 and 2113 cm 1, and the Cu + ( 13 CO) 2 species at 2131 and 2103 cm 1. The excellent coincidence between the expected and observed bands unambiguously proves the formation of dicarbonyls in our experiments Adsorption of 15 N 2 on NO Precovered Sample. Nitrogen monoxide (100 Pa equilibrium pressure) was adsorbed at ambient temperature on the Cu ZSM-5 sample and then evacuated until destruction of all dinitrosyls. As a result, only nononitrosyl species of Cu 2+ and Cu + and some amount of NO + (2134 cm 1 ) were observed on the sample. Because at ambient temperature the mononitrosyl dinitrosyl equilibrium is shifted to the left, we achieved a high concentration of the mononitrosyl species. Note also that the sample is expected to be more oxidized as compared to the CO successively added to the NO precovered sample. This caused erosion of the Cu + NO band at 1811 cm 1 and simultaneous development of dinitrosyl bands (1823 and 1727 cm 1 ) (Figure 5, spectra b h). In the 15 N 15 N stretching region, a Cu + 15 N 2 band at 2222 cm 1 raised in intensity and shifted to 2220 cm 1 (see the inset in Figure 5). When all mononitrosyls were practically converted into dinitrosyls, the Cu + 15 N 2 band reached its maximal intensity. Note that the intensity of the dinitrogen band was lower than the intensity of the same band observed with the NO-free sample. Consider the other bands in the region. A band at 2254 cm 1 was already assigned to OH 15 N 2 interaction. In addition, a weak band at cm 1 was also detected and attributed to a small amount of N 2 O: the same band was observed during NO adsorption experiments (see the inset in Figure 1). It should be also noted that the heterogeneity of the Cu + sites was well detected in this case (see the high-frequency shoulders of the dinitrosyl bands). The results obtained indicate that the following reaction proceeds: Cu NO + N2 Cu (NO) 2 + Cu N2 (2) Adsorption of 15 N 2 on CO Precovered Sample. Addition of small 15 N 2 doses at 100 K to CO precovered Cu ZSM-5 sample did not affect the carbonyl band, which evidences that, at relatively low 15 N 2 equilibrium pressures, no mixed species are formed (spectra not shown). In these experiments, the total CO coverage was slightly lower (the sample was evacuated at 473 K) to avoid any formation of

7 dicarbonyls by entering of negligible amounts of CO to the system together with 15 N 2. Figure 6b shows the effect of addition of 15 N 2 at 100 K under some equilibrium pressure on the Cu + CO band. This leads to Figure 6. FTIR spectra (carbonyl stretching region) registered after adsorption of 15 N 2 at 100 K on CO precovered Cu ZSM-5. Spectrum of the CO precovered sample (a), under equilibrium 15 N 2 pressure of 2 mbar (b), and during progressive evacuation of the sample (b e). Second derivatives of selected spectra are presented in the inset. a red shift of the maximum of the carbonyl band by 2.5 cm 1 (from 2157 to cm 1 ). Although the shift is not important, it points out some kind of interaction. Upon a decrease of the amount of 15 N 2 by evacuation (Figure 6, spectra c f), the band is gradually shifted to its initial position. The gradual shift is also evidenced by the second derivatives of the spectra (see the inset in Figure 6). Figure 7a e presents the spectra shown in Figure 6 but in the 15 N 15 N region. Three bands were registered, at 2254, 2247, and 2220 cm 1. The bands at 2254 and 2247 cm 1 were already attributed to interaction of 15 N 2 with OH groups. The weak band at 2220 cm 1 is due to Cu + 15 N 2 species formed with some free Cu + sites. No 15 N 15 N band assignable to mixed carbonyl-dinitrogen species was observed in the spectra. This is well seen from the difference (a e) spectrum from Figure 7. The conclusions are further supported by comparing the spectra with a spectrum registered after 15 N 2 adsorption on a CO-free sample (Figure 7f). Here, all bands detected with the CO precovered sample were observed, and, as expected, the Cu + 15 N 2 band was registered with strongly enhanced intensity. The spectra in the 15 N 15 N stretching region cannot totally rule out the possibility of the formation of some kind of mixed ligand species containing IR invisible dinitrogen. However, analysis of the carbonyl stretching region strongly impeaches this possibility. Indeed, insertion of a 15 N 2 molecule to Cu + CO species should lead to the appearance of a new carbonyl band. Therefore, conversion of one band into another should be observed but not the gradual shift that was detected. Figure 7. FTIR spectra ( 15 N 15 N stretching region) registered after adsorption of 15 N 2 at 100 K on CO precovered Cu ZSM-5 (sample evacuated at 473 K). Equilibrium pressure of 1 mbar 15 N 2 (a) and during progressive evacuation of the sample (b e). Spectrum f is registered after 15 N 2 adsorption (1 mbar) on a CO-free sample. A possible reason for the small shift of the Cu + CO band in the presence of 15 N 2 is the formation of physisorbed 15 N 2 in the zeolite cages. To verify this hypothesis, we have performed analogous experiments with a Cu MCM-41 sample where the coordination chemistry of Cu + sites is very similar to that in Cu ZSM-5 46 but the material is mesoporous. A detailed description of the spectra is beyond the aim of this study. We note, however, that in this case CO adsorption on the sample leads to the appearance of a Cu + CO band at 2160 cm 1 (Figure S8 of the Supporting Information, spectrum a), which is in agreement with the reported results. 46 Subsequent addition of 15 N 2 to the system at 100 K leads to a red shift of the band maximum (Figure S8 of the Supporting Information, spectrum b). However, this shift is negligible, around 0.3 cm 1. This strongly indicates that the small red shift of the carbonyl band in Cu ZSM-5 detected in the presence of 15 N 2 at 100 K is due to the effect of 15 N 2 molecules trapped in the zeolite cages. In conclusion, the results demonstrated that no mixed-ligand species are observed on Cu + sites when the ligands are CO, N 2, and NO. 4. DISCUSSION 4.1. Formation of Cu + L and Cu + (H 2 O)L Complexes (L = CO, N 2, and NO). It is reported that the higher CO stretching frequency of Cu + CO species formed with Cu + sites in cationic zeolite positions, as compared to oxide-supported Cu +, is due to enhanced σ-component of the Cu + CO bond. This effect arises from the low coordination number and the resulting high electrophilicity of the sites. 19 Indeed, the formation of σ-bond leads to an increase of ν(co), while the formation of the back π-bond causes the opposite effect, that is, a decrease of ν(co). 47 Note, however, that electrostatic interaction also leads to an increase of the CO stretching 15298

8 frequency 47 and should also affect the band position. In fact, it is difficult to distinguish between the effect of the σ-bonding and the Stark effect because they are favored by the same factors and have a similar effect on the CO frequency. However, in contrast to the σ-bond, there is no synergistic effect between the electrostatic and π-bonds. In most cases, the Cu + sites on Cu-ZSM-5 detected by CO are homogeneous. In this work, we observed a new site (site I from Table 1) monitored by CO at 2166 cm 1. The concentration of these sites is low and the respective carbonyl band is observed only as a weak shoulder. Although not specially reported, a high-frequency shoulder of the carbonyl band at 2158 cm 1 has been observed in some cases. 26,68 A possible reason for the detection of these sites in our experiments is the sample pretreatment including reduction with CO. It was found that the type I Cu + sites are preferentially covered by water, which indicates they are more electrophilic than the principal Cu + sites (type II). This accounts for the higher stretching frequency of the respective monocarbonyls as a result of the enhanced electrostatic interaction. However, the stability of the 2166 cm 1 carbonyls is even slightly lower than the stability of the principal carbonyls of type II (see Supporting Information Figure S1). This suggests a slightly weaker back π-donation. At this stage, we cannot give a definite conclusion on the exact location of these sites. It is also seen from Table 1 that the stretching frequency of CO adsorbed on type III of Cu + sites and on Cu + (H 2 O) sites (type IV) almost coincides. This effect has been discussed and attributed to the similar coordination state of the Cu + cation in the two cases. 19 However, the Cu + (H 2 O)(CO) species appear to be more stable that the carbonyls of the type III Cu + cations: they are formed before starting the conversion of mono- to dicarbonyls (compare Supporting Information Figures S2 and S4). Therefore, the balance between the electrostatic, σ-, and π- bonding in the two species is different. It appears that in the Cu + (H 2 O)(CO) species the electrostatic bonding is weaker and the σ-bond is stronger as compared to the carbonyls of the type III Cu + ions. Consider now the situation with N 2. Dinitrogen is isoelectronic with CO but, as a homonuclear diatomic molecule, is IR silent. The gas-phase stretching frequency is 2330 cm 1, which corresponds to a value of 2252 cm 1 for 15 N 2. After adsorption in an end-on mode, the symmetry of the N 2 molecule is lowered: this activates the N N stretching modes in the IR spectrum. As ligands, CO and N 2 have similar properties. However, the electron donor abilities of N 2 are much weaker and are connected with nonbonding orbitals located at the nitrogen atom, which is close to the coordination site. In fact, these orbitals possess a slightly antibonding character, 69 which means that the formation of σ-bond should lead, similarly to the case of CO, to an increase of the stretching frequency. However, the effect should be weaker as compared to CO. It is also without any doubt that electrostatic interaction also leads to an increase of ν(n N). 7 9,35 In contrast, π-donation is accompanied by a significant decrease of the stretching frequency. In any case, N 2 is not a good π-acceptor because the energy of the antibonding orbitals is not similar to the energy of the transition metals d- orbitals. The Cu + N 2 species demonstrate stability, which is remarkable for cation-dinitrogen complexes. This stability and the low N N stretching frequency indicate an important back π-donation. The observed dependency between the stability of the complexes and the stretching frequency (see Table 1, rows 1 3) suggests that the σ-component of the Cu + N 2 bond hardly affects the position of the N N stretching modes. Consider now the Cu + (H 2 O) sites. Water is known to donate effectively electrons to cationic sites, thus decreasing their effective charge. In this case, the frequency of adsorbed 15 N 2 is low, which could suggest enhanced π-donation. However, if this is the only effect, the complexes should be the most stable ones, which is not the case. Evidently, the main reason for the low 15 N 15 N stretching frequencies in this case is the strongly suppressed electrostatic interaction. Thus, N 2 appears to be a more suitable probe than CO to distinguish sites III and IV from Table 1. Consider now the mononitrosyl species. Because of the low polarizability of the NO molecule, the electrostatic interaction should be not essential and the NO stretching frequency will be determined, to a high extent, on the covalent σ- and π-bonds. The properties of NO as a ligand are often described in terms of NO +, which is isoelectronic with CO. However, this approach does not account for the NO frequency. Formation of NO + via donation of the single electron situated on antibonding orbital should reflect a significant increase of ν(no). For instance, NO + in ZSM-5 is observed at 2133 cm The observed red shift of ν(no) in the Cu + NO species with respect to the gas-phase NO frequency (1876 cm 1 ) indicates, similarly to the case with N 2, a prevailing π-bonding. The fact that the mononitrosyls are definitely less stable than the monocarbonyls confirms that in this case the σ-bonding is rather weak. However, the stability of the mononitrosyls species is higher than the stability of the Cu + (N 2 ) adducts because the energies of the antibonding orbitals of NO are closer to the energies of the d orbitals of the transitional metals Difference in the Formation of Cu + (L) n Geminal Complexes (L = CO, N 2, and NO). Although a large fraction of the Cu + sites in Cu ZSM-5 possess three coordinative vacancies each and can form tricarbonyls, they can attach simultaneously only two NO and only one N 2 molecules. Consider now the adsorption of dinitrogen. Taking into account the similarity of CO and N 2 as ligands, the fact that dinitrogen is not able to form geminal species with Cu + ions is surprising. Note also that geminal dinitrogen complexes are easily formed with alkali- and alkaline earth cations in zeolites, irrespective of the very low adsorption enthalpy. 7 9,35 Evidently, when one N 2 molecule is attached to a Cu + site, it hinders in some way the adsorption of a second molecule. An analogy with Ni + ions in ZSM-5 could help in explaining the phenomenon. The frequency of the Ni + 15 N 2 species is reported at 2177 cm 1, 32 which indicates more effective π- donation as compared to the Cu + N 2 species. When a second 15 N 2 molecule is adsorbed at the same site, the geminal complexes formed are characterized by ν s at 2212 and ν as at 2194 cm 1. Note that both frequencies are at higher wavenumber as compared to the monoligand species. Also, the 15 N 15 N frequency of the Ni + ( 14 N 2 )( 15 N 2 ) species is observed at 2201 cm 1. All of this indicates a strong weakening of the π-bonding in the geminal complexes, that is, an essential competition for donated electrons. Most probably the adsorption geometry contributes to this phenomenon; that is, no effective electron transfer could occur when two molecules are attached to the same site

9 The π-donation in the Cu + N 2 species is more restricted, as compared to Ni + N 2, and eventual geminal species should be characterized by extremely low stability. Thus, the enthalpy of adsorption of only one molecule could exceed the enthalpy of adsorption of two molecules. In our opinion, this is the most probable reason for the fact that no geminal dinitrogen species are formed in Cu ZSM-5. Thus, it appears that, when a covalent bond is formed, the molecular probe could fail to give information on the coordination state of the cation. Consider now the formation of Cu + (NO) 2 species. These dinitrosyls possess an even-electron structure, which could contribute to their stability. Theoretical consideration of the formation of geminal adspecies 10 has indicated that, when the adsorption of a second molecule to the same site is much weaker than the adsorption of the first molecule, the maximal observed concentrations for monoligand and geminal species should be almost equal. Because the geminal species contain two ligands, the maximal intensity of the IR bands corresponding to monoligand species will be ca. one-half of the maximal intensity of the bands characterizing geminal species (providing the extinction coefficient is not changed). The adsorption of CO on our sample is well described by this model. If the enthalpy of adsorption of the second molecule is hypothetically equal to the enthalpy of adsorption of the first molecule, the maximal concentration of the monoligand species will be 1/2 of the concentration of the geminal species at saturation. Consequently, the maximal intensity of the IR bands of the monoligand species will be 1/4 of the intensity of the bands due to geminal complexes at saturation. If this maximal intensity is lower than 1/4, this is an indication that the adsorption of the second molecule is energetically more favored. Analysis of the spectra of NO adsorbed at 100 K (see Figure 1B) shows that the maximal intensity of the Cu + NO band (1810 cm 1 ) is definitely lower than 1/4 of the maximal intensity of the dinitrosyl bands at 1824 and 1731 cm 1. This shows that the geminal structures are favored. Evidently, this is due to electronic configuration factors, coupling of single electrons. Note that this conclusion concerns the lowtemperature NO adsorption: it is known that lowering temperature shifts the equilibrium between mono- and dinitrosyls to the right. Eventual formation of Cu + (NO) 3 species should result in an odd-electron complex. The odd number of electrons seems to lead to low stability of these complexes, and consequently they are not experimentally observed even at low temperature. However, because the Cu + NO bond is highly covalent, steric factors could also hinder the formation of trinitrosyls, similarly to the case of geminal dinitrogen complexes. Note that the above considerations concern the Cu + cations, which have a d 10 electron configuration and cannot be automatically spread to other systems. For instance, it is well documented that Co 2+ ions (electron configuration d 7 ) form exclusively dinitrosyl complexes. 61 Also, Fe 2+ sites (d 6 configuration) are supposed to form trinitrosyl species Formation of Mixed Ligand Species. The Cu + CO complexes can easily accommodate one (or more) H 2 OorNH 3 molecules. These molecules are electrostatic bases. However, no N 2 molecule can be inserted in the Cu + CO complexes. The reasons are the same as those discussed for the impossibility of formation of geminal dinitrogen species. Here, the analogy with Ni ZSM-5 is again helpful. With this sample, mixed Ni + (CO)(N 2 ) species are formed and are characterized by a 14 N 14 N band at 2305 cm 1 (corresponding 15 N 15 N modes at ca cm 1 ). This means that bonding of CO to Ni + N 2 leads to a substantial decrease of the π-bonding between Ni + and N 2, much more pronounced than bonding of a second N 2 molecule. In the case of Cu + sites, this decrease is enough to prevent insertion of an N 2 molecule into the Cu + CO adducts. The reasons why no mixed carbonyl-nitrosyl complexes are produced seem to be different. At first, these species have an odd number of electrons. That is why, in the copresence of CO and NO, the system prefers to form dicarbonyl and dinitrosyl complexes instead of mixed-ligand species (eq 1). The same is the situation with NO and N 2, but in this case the stable state includes coexisting Cu + (NO) 2 and Cu + N 2 species (eq 2). Here again, the considerations should be restricted to Cu + cation. For instance, mixed carbonyl-nitrosyls complexes are reported with Ni 2+ and Pd 2+ ions (electron configuration d 8 )in ZSM-5. Copper-containing zeolites are important catalytic systems. It is considered that the low coordinative saturation of Cu + sites is a reason for their unique catalytic properties. We do not impeach this point of view, but our results indicate that coordination of ligands of different nature is a complex process and these peculiarities should be taken into account when considering possible reaction mechanisms. 5. CONCLUSIONS The Cu + ions in Cu ZSM-5, being low coordinated, are highly electrophilic. As a result, they make a strong σ-bond with adsorbed CO. On the other hand, the energetic fit between the d-orbitals of Cu + and CO antibonding orbitals is a premise for formation of a strong π-bond. The synergism between the σ- and π-bonds results in very stable carbonyl complexes, which are additionally strengthened by electrostatic interaction. Another effect of the low coordination of the Cu + sites is the formation of dicarbonyls even at ambient temperature. In these species, the back π-donation is strongly reduced. At low temperature, tricarbonyls are formed. One or two H 2 O molecules can be attached to a Cu + cation from Cu + CO species to form Cu + (H 2 O) n CO complexes (n = 1 or 2). Insertion of water hardly affects the π-bond but weakens the σ-bond and the electrostatic interaction between Cu + and CO. As a result, the CO stretching frequency decreases. The aqua-carbonyl complexes easily loose H 2 O ligands. Dinitrogen is a weaker σ-donor and π-acceptor than CO. As a result, the Cu + N 2 species are much less stable than Cu + CO. Because of the strong competition for d-electrons, neither geminal dinitrogen species nor mixed Cu + (CO)(N 2 ) species are produced. However, Cu + (H 2 O)N 2 complexes can be formed because water hardly affects the π-component of the Cu + N 2 bond. Cu + NO species are less stable than Cu + CO, but more stable than Cu + N 2. No evidence of aqua-nitrosyl species was found. Because of the existence of unpaired electron in NO, the dinitrosyl structures of Cu + are stabilized by coupling of these electrons. For this reason, coadsorption of CO and NO leads to the formation of coexisting Cu + (CO) 2 and Cu + (NO) 2 species rather than mixed-ligand Cu + (CO)(NO) complexes. Similarly, Cu + N 2 and Cu + (NO) 2 species are formed upon NO and N 2 coadsorption