Surprising Coordination Chemistry of Cu+ Cations in Zeolites: FTIR

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Surprising Coordination Chemistry of Cu Cations in Zeolites: FTIR Study of Adsorption and Coadsorption of CO, NO, N and HO on Cu–ZSM-5 2

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Videlina Zdravkova, Nikola Drenchev, Elena Ivanova, Mihail Yordanov Mihaylov, and Konstantin Ivanov Hadjiivanov J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 11 Jun 2015 Downloaded from http://pubs.acs.org on June 12, 2015

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The Journal of Physical Chemistry

Surprising Coordination Chemistry of Cu+ Cations in Zeolites: FTIR Study of Adsorption and Coadsorption of CO, NO, N2 and H2O on 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. Tel. +35929793598; E-mail: [email protected] Keywords: Adsorption; Coadsorption, Cu-ZSM-5; CO, 15N2; NO; FTIR spectroscopy

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 N2 molecule can be coordinated to such sites. Although mixed aqua-carbonyl and aqua-dinitrogen complexes are formed, no mixed carbonyl-nitrosyl, carbonyl-dinitrogen or nitrosyl-dinitrogen species can be produced. Thus, adsorption of NO on CO precovered sample results in segregation of the CO adsorption layer according to the reaction: 2 Cu+–CO + 2 NO → Cu+(CO)2 + Cu+(NO)2. Adsorption of N2 on NO precovered sample leads to a similar process: 2

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Cu+–NO + N2 → Cu+(NO)2 + Cu+–N2. No carbonyl-dinitrogen complexes are produced during CO – N2 coadsorption. The role of the ligand and the nature of the bond on 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, experimental1-34 and theoretical34-38 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 in order 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 CO1-8 or N27-9,35 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 in order geminal species to be formed.12,13 In these cases


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the change of the ligand does not affect the process and consequently mixed ligand species, e.g. Na+(CO)(N2) 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 of all, the socalled "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, e.g. selective catalytic reduction of NOx with hydrocarbons,40,41 decomposition of NO42 and N2O,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. In 1994 Zechina et al.18 reported that CO adsorption at room temperature on CuI–ZSM-5 resulted in formation of Cu+(CO)2 dicarbonyl species characterized by two IR bands: νs at 2178 and νas 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 dicarbonyls were converted into tricarbonyls (2190, 2164 and 2140 cm-1). Later on, the results were confirmed by other authors21,22,26 and similar situation was also


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found with Cu+ ions in other zeolites24,27-30,33,34 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+(H2O)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 authors23,24 and different numbers of water molecules were proposed in the aqua-carbonyl complexes. Mixed carbonyl-NH323 and carbonyl-acetone25 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 (1827-23 and 1735-29 cm-1).16-18 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 to N2 adsorption on copper containing zeolites is associated mainly with the formation of relatively stable Cu+–N2 species.42,49-53 The Cu+–N2 complexes in Cu–ZSM-5 are observed even at ambient temperature at ca. 2295 cm-1. The possibility of existence of IR invisible nitrogen bridging two cationic sites has also been pointed out.52,54 Despite of the similarity of CO and N2 as ligands, no data of geminal dinitrogen species of Cu+ in zeolites are available.


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Although there are many studies on the formation of mixed carbonyl complexes of the Cu+(CO)(L) type (L = H2O, NH3, CH3COCH3) with different copper-containing zeolites, there are only 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 carbonyl-nitrosyls complexes of Cu+ are formed. Studies of CO + N2 coadsorption have revealed that CO blocks the adsorption sites25,46 and no data on mixed ligand species are available. Despite of 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+(L1)(L2) mixed ligand species where L = CO, NO, N2 and H2O. The four ligands are of different nature: water is electrostatic base and is coordinated to Cu+ by electrostatic forces, while significant π-bonding occurs with the other ligands. NO is radical molecule and coupling of electron might play an important role in the formation of the complexes. CO and N2 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 N2 and discuss the possible reasons for this surprising coordination chemistry of Cu+ cations.

2. EXPERIMENTAL The starting zeolite material was prepared by calcination of NH4ZSM-5 (Zeolist, Si/Al = 25) at 823 K for 2 h. The Cu–ZSM-5 sample was synthesized by ion exchange from 0.006 mol L−1 solution of copper acetate and the final copper concentration in the sample was 1.01 wt %. Other


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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 a 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. In order to ensure higher concentration of Cu+ sites, the sample was reduced by CO (5 kPa, 15 min, 473 K) and then evacuated at 293-473 K (to produce CO precovered sample) or at 673 K (to produce CO-free sample). Carbon monoxide (>99.5% purity) was supplied by Merck.

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C-labeled CO (>99.0) and NO

(>99.0% purity) were obtained from Messer Griesheim GmbH. Labeled nitrogen (15N2, isotopic purity of 98 at. %) was provided by Aldrich. Before adsorption, CO and

15

N2 were additionally

purified by passing through a liquid nitrogen trap.

3. EXPERIMENTAL RESULTS The adsorption of the individual adsorbates (CO, NO, N2 and H2O) 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.


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3.1. Background Spectra and Adsorption of Individual Adsorbates: H2O, CO, NO and N2

3.1.1 Background Spectra The background spectrum of our material, registered at ambient temperature, is consistent with the literature data56 and shows, in the hydroxyl region, bands at 3745 cm-1 (silanol groups), 3664 cm-1 (aluminol groups formed with extra-framework 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. 3450 cm-1, respectively (Figure S1 from the Supporting Information, spectrum b).

3.1.2 Adsorption of H2O The spectrum registered after adsorption of H2O on the reduced Cu–ZSM-5 sample, followed by evacuation at ambient temperature to remove weakly adsorbed water is presented on Figure S1 from the Supporting Information, spectrum c. It is seen that water adsorption hardly affect 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 to 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-558,59 and assigned in different ways. It seems that


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the band corresponds (in addition to the absorbance caused by the aluminol groups) to OH(H2O)2 adducts. The low-frequency shoulder could be due to Cu2+−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 H2O adsorbed on c.u.s. sites can shift to lower frequencies.60 Although the spectra do not give a clear evidence of water coordinated to Cu+ sites, the coadsorption results suggested existence of some Cu+(H2O) species (vide infra).

3.1.3 Adsorption of CO The results on CO adsorption on our sample (including

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C18O 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 (νs at 2180 and νas at 2151 cm-1). In 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 νs 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


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cationic positions.18,19,47 For convenience, the observed CO stretching frequencies are summarized in Table 1.

Table 1. Spectral characteristics of the monoligand adsorption complexes formed with Cu+ and Cu+(H2O) sites in Cu–ZSM-5. The ligands are CO, 15N2 and NO. Site

Description

ν(CO), cm-1

ν(15N2), ν(NO), cm-1 cm-1

Note

I

Cu+ in cationic positions

2166

2217 1812 (2295)*

most electrophilic Cu+ sites; preferentially covered by water

II

Cu+ in cationic positions

2158

2220

1810

principal Cu+ sites

III

Cu+ from oxidelike phase

2141

2225 (2303)

1870 ?

Cu+ sites are more coordinated than sites I and II

IV

Cu+(H2O) sites

2139

2208

-

sites expected to be heterogeneous

(2298)

(2285) * The calculated 14N–14N frequencies are presented in brackets

3.1.4 Adsorption of NO Figure 1 presents the spectra registered after successive adsorption of small doses of NO at 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)16-18 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 suggest some additional factors


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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 Cu2+-NO species.61

1900

1850

1800

1750

1700

2239 - 2231 0.01

j

0.1

k

2250 2220 - 1894

- 1684

+

OH -NO

k

k

1786 -

B

j

A 2+

Cu -NO

+

Cu (NO)2

- 1731

h - 1824

Absorbance

+

Cu -NO - 1913

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g

- 1809

1812

1900

0.02

1850

1800

a 1750

Wavenumber, cm

1700 -1

Figure 1. FTIR spectra of small doses of NO successively adsorbed at 100 K on reduced Cu– ZSM-5 (a-k). The band at 1812-1809 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 N2 and N2O. All spectra are background corrected.


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At higher NO coverage (Figure 1B) a series of other bands are observed. The bands associated with cooper (1960-1850 cm-1) are due to mono- and dinitrosyl species of Cun+ sites (n > 1).16 The other bands are observed also with H–ZSM-5 and are assigned as follows:62 1786 cm-1, to trans(N2O2); 1894 cm-1, to OH-NO species; 2205 and 1684 cm-1, to the symmetric and antisymmetric modes, respectively, of [N2O2]+ adducts. Evacuation leads to loss of one of the NO ligands form the Cu+(NO)2 species, i.e. they are converted into mononitrosyls. 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 1800 - 1745 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.

3.1.5 Adsorption of 15N2 The results on 15N2 adsorption on our sample are generally consistent with previous reports on 14

N2 adsorption. As already mentioned, a Cu+–N2 band at 2295 cm-1 is observed after low

temperature

14

N2 adsorption on Cu–ZSM-5.49-53,63 In our experiments we used the

15

N2

isotopologue in order to avoid any hindrance from the spectrum of CO2 in the air. Based on the theoretical isotopic shift factor, 1.035,64 the Cu+–15N2 band is expected around 2217 cm-1. The spectra registered after successive adsorption of small doses of 15N2 on our sample at 100 K show initial development of a Cu+-15N2 band at 2117 cm-1 (Figure 2A, spectrum a). With increase of amount of 15N2 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,


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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. 2240

2230

2220

2210

2220

A

2200

+ 15

Cu - N2

0.01 h e

Absorbance

a 2220

B 0.005 h

2217

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e

a 2240

2230

2220

2210

Wavenumber, cm Figure 2. FTIR spectra of

15

N2 (panel A) and 1 : 1

14

2200

-1

N2 +

15

N2 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.


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The Cu+−15N2 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 15N2 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 15N2 polarized by the zeolite bridging hydroxyls.56 Indeed, the original OH band at 3611 cm-1 was shifted to 3496 cm-1 (∆νOH = -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–15N2 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+(N2)2 adducts were recently proven by

14

N2 and

15

N2 coadsorption experiments.32 In order to confirm/reject the

possibility of formation of geminal dinitrogen adspecies in Cu–ZSM-5 we have studied the adsorption of a 1 : 1 registered in the

15

14

N2 +

15

N2 isotopic mixture at 100 K. Figure 2B presents the spectra

N–15N stretching region. It is evident that they coincide very well with the

spectra obtained after 15N2 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+–N2. It is difficult to give definite conclusions on the stability of the Cu+(H2O) complexes, but is seems (see below) they are slightly more stable than the Cu+–NO species. Based on the stability, we studied the adsorption of

15

N2 on CO and NO precovered

samples and adsorption of NO on a CO precovered sample. However, in order to allow measurements at low temperature, the water coadsorption experiments were performed with H2O precovered sample.


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3.2. Adsorption of CO, NO and 15N2 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).

3.2.1 Adsorption of CO It is well established that water adsorption on CO precovered Cu–ZSM-5 leads to formation of Cu+(CO)(H2O)n complexes.10,19,23,24 This is demonstrated on Figure S4 from the Supporting Information. Introduction of water at ambient temperature to the CO-precovered sample leads to shift of the carbonyl band from 2158 to 2132 cm-1 (Figure S4, spectrum a). This shifted band is attributed to Cu+(H2O)2CO species. Decrease of the equilibrium H2O pressure leads to 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+(H2O)2CO 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 already reported.19 The results show that the aqua-carbonyl complexes easily loose water ligand(s) but are fully destructed only after prolonged (30 min) evacuation. Therefore, one can expect that a part of the Cu+ sites on water precovered sample should have adsorbed water molecules. In order to check whether Cu+(H2O) 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 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


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Cu+(CO)(H2O) 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 hold 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 νs dicarbonyl band. Therefore, water has been preferentially adsorbed on the respective sites, i.e. they are characterized by a higher electrophilicity than the type II Cu+ sites.

3.2.2 Adsorption of 15N2 The similarity of CO and N2 as ligands suggests the possibility of formation of mixed Cu+(N2)(H2O) species. However, no such species have been reported in the literature. In order to check for their existence, we studied 15N2 adsorption at 100 K on water precovered sample (see Figure 3). It is seen that at high 15N2 coverages a band at 2208 cm-1 is detected. This band was not observed with the activated sample and is therefore attributed to Cu+(15N2)(H2O) species. The similarity with the CO adsorption results confirms the assignment. The results indicate a very low stability of the aqua-dinitrogen complexes. Note that in this case the described above correlation between the stability of the dinitrogen complexes and the 15

N–15N 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

N2 adsorption.

However, the band position was at 2217 cm-1 (shifting to lower wavenumber by less than 1 cm-1 at high coverages), i.e. a frequency typical of sites of type I (see Table 1). In contrast, the CO adsorption experiments revealed existence of type II Cu+ sites on "wet" surface. This seemingly contradiction could be explained by the effect of relatively large amount of presorbed water


15


(mainly at OH groups). Donating electrons to the sample, H2O slightly decreases the electrofilicity of the cationic sites of type II thus affecting the stretching frequency of adsorbed 15

N2. The high intensity of the band (much higher that that of the 2217 cm-1 band detected with

activated sample) is in line with this hypothesis.

+ 15

Cu - N2

0.01

2217 -

Absorbance

+ 15

Cu ( N2)(H2O)

- 2208

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 43

f

2230

a

2220

2210

Wavenumber, cm Figure 3. FTIR spectra of

15

2200 -1

N2 adsorbed at 100 K on water precovered reduced Cu–ZSM-5

sample (for details see text). Equilibrium

15

N2 pressure of 2 mbar (a) followed by progressive

evacuation at 100 K (b-f). All spectra are background corrected.

In order to obtain additional support on this hypothesis, we have studied low-temperature 15N2 adsorption on a sample pre-evacuated at 323 K. After this pretreatment the amount of adsorbed


16

Page 17 of 43

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water slightly decreased. The spectra of

15

N2 adsorbed on the two samples are compared on

Figure S6 from the Supporting Information. It is well seen that the maximum of the principal 15

N2 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.

3.2.3 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 Pd-ZSM5 sample where the stretching frequency of the Pd2+(NO) and Pd2+(NO)(H2O) species were reported at 1881 and 1839 cm-1, respectively.65,66 Adsorption on NO at 100 K on water precovered Cu–ZSM-5 results in formation of Cu+(NO)2 (1825 and 1731 cm-1) and Cu2+(NO) (1894 cm-1) species (Figure S7 from 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 formation of mixed ligand species. Most probably the shift is chemical and caused by the large amount of water on sample, as already discussed with 15N2. Thus, the results indicate that no Cu+(H2O)(NO) species are produced on Cu–ZSM-5. 3.3. Coadsorption Studies Involving CO, NO and 15N2

3.3.1 Adsorption of NO on 12CO + 13CO Precovered Sample Recently we have reported16 that adsorption of NO on an over-exchanged Cu–ZSM-5 sample led to formation of dicarbonyls and dinitrosyls according to the reaction: 2 Cu+–CO + 2 NO → Cu+(CO)2 + Cu+(NO)2


(1).

17


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Page 18 of 43

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 exist that mixed carbonyl-nitrosyl species are formed on Cu–ZSM-5.55 In order 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 12CO and

13

CO. After adsorption of the isotopic mixture the

sample was evacuated at ambient temperature in order to destroy any dicarbonylic species and then cooled down to 100 K. Two bands, at 2158 (Cu+–12CO) and 2108 (Cu+–13CO) cm-1, dominated in the spectra of the sample thus treated (Figure 4A, spectrum a). Subsequent NO dosage led first to development of Cu2+–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 dicarbonilic species are produced after adsorption of a 1 : 1

12

CO +

13

CO isotopic mixture

they should posses the following distribution: Cu+(12CO)2, 25 %; Cu+(12CO)(13CO), 50 %; and Cu+(13CO)2, 25 %. Also, on the basis of the experimentally observed frequencies of the Cu+(12CO)2 dicarbonyls and using 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+(12CO)(13CO) species should manifest bands at 2169 and 2113 cm-1, and the Cu+(13CO)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.


18

-2158

0.1

2112

a,b

- 1822

- 1914 - 1893

B

- 2094

d

- 2169

- 2180

CO 13 CO

- 2131

12

2103

A

2100 -2108

2150

i

+

Cu (NO)2

- 1726

2200

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


- 2151 - 2142

Page 19 of 43

0.1

i

b

1950 1900 1850 1800 1750 1700 1650

Wavenumber, cm

-1

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 12CO and 13CO (a-j). Panel A: carbonyl (12CO and 13CO) region. Panel B: nitrosyl region. All spectra are background corrected.

3.3.2 Adsorption of 15N2 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 Cu2+ 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


19


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Page 20 of 43

to 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 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

N2 were then 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–15N stretching region a Cu+–15N2 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+–15N2 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–15N2 interaction. In addition, a weak band at 2240-38 cm-1 was also detected and attributed to small amount of N2O: 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: 2 Cu+–NO +

15

N2 → Cu+(NO)2 + Cu+–15N2


(2).

20

2240 2238

Cu -NO

2254

- 1811

0.02 +

h

a

Absorbance

b 2250

2220

- 1727

1823

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


- 2220

Page 21 of 43

0.05

h +

Cu (NO)2

1850

1800

1750

Wavenumber, cm

-1

Figure 5. FTIR spectra (nitrosyl region) registered after adsorption of

15

N2 on NO precovered

Cu–ZSM-5. Spectrum of NO precovered sample (a) and after successive adsorption of small doses of 15N2 at 100 K (b-h). The inset shows the changes in the 15N–15N stretching region.

3.3.3 Adsorption of 15N2 on CO Precovered Sample Addition of small 15N2 doses at 100 K to CO precovered Cu–ZSM-5 sample did not affect the carbonyl band which evidences that, at relatively low

15

N2 equilibrium pressures, no mixed

species are formed (spectra not shown). In these experiments the total CO coverage was slightly


21


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Page 22 of 43

lower (the sample was evacuated at 473 K) in order to avoid any formation of dicarbonyls by entering of negligible amounts of CO to the system together with 15N2. Figure 6, spectrum b, shows the effect of addition of

15

N2 at 100 K under some equilibrium

pressure on the Cu+-CO band. This leads to a red shift of the maximum of the carbonyl band by 2.5 cm-1 (from 2157 to 2154.5 cm-1). Although the shift is not important, it points out some kind of interaction. Upon decrease of the amount of

15

N2 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 7 (spectra a-e) presents the spectra shown in Figure 6 but in the 15N–15N region. Three bands were registered, at 2254, 2247 and 2220 cm-1. The band at 2254 and 2247 cm-1 were already attributed to interaction of 15N2 with OH groups. The weak band at 2220 cm-1 is due to Cu+–15N2 species formed with some free Cu+ sites. No

15

N–15N band assignable to mixed

carbonyl-dinitrogen species was observed in the spectra. This is well seen from the difference (ae) spectrum from Figure 7. The conclusions are further supported by comparing the spectra with a spectrum registered after 15N2 adsorption on a CO-free sample (Figure 7, spectrum f). Here all bands detected with the CO precovered sample were observed and, as expected, the Cu+–15N2 band was registered with strongly enhanced intensity. The spectra in the 15N-15N stretching region cannot totally rule out the possibility of 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

N2

molecule to Cu+–CO species should lead to appearance of a new carbonyl band. Therefore, conversion of one band into another should be observed but not the gradual shift which was detected.


22

Page 23 of 43

d a

b

e

0.1

- 2154.5

2150 - 2157

2160

f

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c d b

a

2170

2160

2150

Wavenumber, cm

2140

-1

Figure 6. FTIR spectra (carbonyl stretching region) registered after adsorption of 15N2 at 100 K on CO precovered Cu–ZSM-5. Spectrum of the CO precovered sample (a), under equilibrium 15

N2 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 possible reason of the small shift of the Cu+–CO band in presence of 15N2 is the formation of physisorbed

15

N2 in the zeolite cages. In order 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-546 but the material is mesoporous. Detailed description of the spectra is beyond the aim of this study. We note, however, that in this case CO adsorption


23


on the sample leads to appearance of a Cu+–CO band at 2160 cm-1 (Figure S8 from the Supporting Information, spectrum a) which is in agreement with the reported results.46 Subsequent addition of

15

N2 to the system at 100 K leads to a red shift of the band maximum

(Figure S8 from 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–ZSM5 detected in presence of

15

N2 at 100 K is due to the effect of

15

N2 molecules trapped in the

zeolite cages. + 15

OH - N2

+ 15

Cu - N2

- 2280

- 2247

2220 -

- 2254

0.01

f

e

- 2220

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 43

a

a-e 2250

Wavenumber, cm

2200 -1

Figure 7. FTIR spectra (15N–15N stretching region) registered after adsorption of 15N2 at 100 K on CO precovered Cu–ZSM-5 (sample evacuated at 473 K). Equilibrium pressure of 1 mbar 15N2


24

Page 25 of 43

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(a) and during progressive evacuation of the sample (b-e). Spectrum (f) is registered after

15

N2

adsorption (1 mbar) on a CO-free sample. In conclusion, the results demonstrated that no mixed-ligand species are observed on Cu+ sites when the ligands are CO, N2 and NO.

4. DISCUSSION 4.1. Formation of Cu+–L and Cu+(H2O)L Complexes (L = CO, N2 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 σcomponent 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 increase of ν(CO) while formation of back π-bond causes the opposite effect, i.e. decrease of ν(CO).47 Note, however, that electrostatic interaction also leads to increase of the CO stretching frequency47 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 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.


25


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Page 26 of 43

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 that the stability of the principal carbonyls of type II (see Figure S1). This suggests a slightly weaker back π-donation. At this stage we cannot give 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+(H2O) 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+(H2O)(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 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+(H2O)(CO) species the electrostatic bonding is weaker at the σ-bond is stronger as compared to the carbonyls of the type III Cu+ ions. Consider now the situation with N2. Dinitrogen is isoelectronic with CO but, as 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 15N2. After adsorption in an end-on mode the symmetry of the N2 molecule is lowered: this activates the N–N stretching modes in the IR spectrum. CO and N2 have similar properties as ligands. However, the electron donor abilities of N2 are much weaker and are connected with non-bonding 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 formation of σ-bond should lead, similarly to the case of CO, to increase of the


26

Page 27 of 43

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stretching frequency. However, the effect should be weaker as compared to CO. It is also without any doubt that electrostatic interaction also leads to increase of ν(N–N).7-9,35 In contrast, πdonation is accompanied by significant decrease of the stretching frequency. In any case, N2 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+–N2 species demonstrate stability which is remarkable for cation-dinitrogen complexes. This stability and the low N–N stretching frequency indicate an important back πdonation. 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+–N2 bond hardly affects the position of the N–N stretching modes. Consider now the Cu+(H2O) sites. Water is known to donate effectively electrons to cationic sites thus decreasing their effective charge. In this case the frequency of adsorbed

15

N2 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−15N stretching frequencies in this case is the strongly suppressed electrostatic interaction.

Thus, N2 appears to be more suitable probe than CO to distinguish sites III and IV from Table 1. Consider now the mononitrosyl species. Due to 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 in a significant increase of ν(NO). For instance, NO+ in ZSM-5 is observed at 2133 cm-1.57


27


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Page 28 of 43

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 N2, 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+(N2) adducts because the energies of the antibonding orbitals of NO are closer to the energies of the d orbitals of the transitional metals. 4.2. Difference in the Formation of Cu+(L)n Geminal Complexes (L = CO, N2 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 N2 molecules. Consider now the adsorption of dinitrogen. Taking into account the similarity of CO and N2 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 N2 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+–15N2 species is reported at 2177 cm-1

32

compared to the Cu+–N2 species. When a second

which indicates more effective π-donation as

15

N2 molecule is adsorbed at the same site the

geminal complexes formed are characterized by νs at 2212 and νas at 2194 cm-1. Note that both frequencies are at higher wavenumber as compared to the monoligand species. Also the 15N–15N frequency of the Ni+(14N2)(15N2) species is observed at 2201 cm-1. All this indicates a strong weakening of the π-bonding in the geminal complexes, i.e. an essential competition for donated


28

Page 29 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrons. Most probably the adsorption geometry contributes to this phenomenon, i.e. no effective electron transfer could occur when two molecules are attached to the same site. The π-donation in the Cu+–N2 species is more restricted, as compared to Ni+–N2, 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. To 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 adspecies10 have 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. 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 ½ of the concentration of the geminal species at saturation. Consequently, the maximal intensity of the IR bands of the monoligand species will be ¼ of the intensity of the bands due to geminal complexes at saturation. If this maximal intensity is lower than ¼, this is an indication that the adsorption of the second molecule is energetically more favored.


29


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Page 30 of 43

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 that ¼ 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, namely coupling of single electrons. Note that this conclusion concern the low-temperature 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 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 formation of trinitrosyls, similarly to the case of geminal dinitrogen complexes. Note that the above considerations concern the Cu+ cations which have a d10 electron configuration and cannot be automatically spread to other systems. For instance, it is well documented that Co2+ ions (electron configuration d7) form exclusively dinitrosyl complexes.61 Also, Fe2+ sites (d6 configuration) are supposed to form trinitrosyl species.70 4.3. Formation of Mixed Ligand Species The Cu+–CO complexes can easily accommodate one (or more) H2O or NH3 molecules. These molecules are electrostatic bases. However, no N2 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)(N2) species are formed and are characterized by a (corresponding

15

14

N–14N band at 2305 cm-1

N–15N modes at ca. 2227 cm-1). This means that bonding of CO to Ni+–N2

leads to a substantial decrease of the π-bonding between to Ni+ and N2, much more pronounced


30

Page 31 of 43

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than bonding of a second N2 molecule. In the case of Cu+ sites this decrease is enough in order to prevent insertion of an N2 molecule to 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 co-presence 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 N2 but in this case the stable state includes coexisting Cu+(NO)2 and Cu+–N2 species (Eq. 2). Here again, the considerations should be restricted to Cu+ cation. For instance, mixed carbonyl-nitrosyls complexes are reported with Ni2+ and Pd2+ ions (electron configuration d8) 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 dorbitals 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


31


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Page 32 of 43

these species the back π-donation is strongly reduced. At low temperature tricarbonyls are formed. •

One or two H2O molecules can be attached to a Cu+ cation from Cu+–CO species to form Cu+(H2O)nCO 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 H2O ligands.

•

Dinitrogen is weaker σ-donor and π-acceptor than CO. As a result, the Cu+–N2 species are much less stable than Cu+–CO. Due to the strong competition for d-electrons, neither geminal dinitrogen species nor mixed Cu+(CO)(N2) species are produced. However, Cu+(H2O)N2 complexes can be formed because water hardly affects the π-component of the Cu+–N2 bond.

•

Cu+–NO species are less stable than Cu+–CO, but more stable than Cu+–N2. No evidence of aqua-nitrosyl species was found. Due to 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 formation of coexisting Cu+(CO)2 and Cu+(NO)2 species rather than mixed-ligand Cu+(CO)(NO) complexes. Similarly, Cu+–N2 and Cu+(NO)2 species are formed upon NO and N2 coadsorption.

Acknowledgments: The authors are indebted to the National Science Fund of Bulgaria (Contract No DFNI T02/20). V.Z. acknowledges the support from the project BG051PO001– 3.3.06–0050, financed by the operational programme "Human Resources Development".


32

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Supporting Information Available. Figures S1−S8 with FTIR spectra. This information is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES (1)

Paukshtis, E. A.; Soltanov, R. I.; Yurchenko, E. N. IR Spectroscopic Studies of Low-

Temperature CO Adsorption on CaNaY Zeolite. React. Kinet. Catal. Lett. 1983, 22, 147−151. (2)

Hadjiivanov, K.; Knözinger, H. FTIR Study of the Low-Temperature Adsorption and Co-

adsorption of CO and N2 on NaY Zeolite: Evidence of Simultaneous Coordination of Two Molecules to One Na+ Site. Chem. Phys. Lett. 1999, 303, 513−520. (3)

Cairon, O.; Guesmi, H. How Does CO Capture Process on Microporous NaY Zeolites? A

FTIR and DFT Combined Study. Phys. Chem. Chem. Phys. 2011, 13, 11430−11437. (4)

Bulánek, R.; Voleská, I.; Ivanova, E.; Hadjiivanov, K.; Nachtigall, P. Localization and

Coordination of Mg2+ Cations in Ferrierite: Combined FTIR Spectroscopic and Computation Investigation of CO Adsorption Complexes. J. Phys. Chem. C 2009, 113, 11066−11076. (5)

Hadjiivanov, K.; Knözinger, H. Formation of Ca2+(CO)3 Complexes During Low-

Temperature CO Adsorption on CaNaY Zeolite. J. Phys. Chem. B 2001, 105, 4531−4534. (6)

Voleská, I.; Nachtigall, P.; Ivanova, E.; Hadjiivanov, K.; Bulánek, R. Theoretical and

Experimental Study of CO Adsorption on Ca-FER Zeolite. Catal. Today 2015, 243, 53−61. (7)

Zecchina, A.; Otero Areán, C.; Turnes Palomino, G.; Geobaldo, F.; Lamberti, C.; Spoto,

G.; Bordiga, S. The Vibrational Spectroscopy of H2, N2, CO and NO Adsorbed on the Titanosilicate Molecular Sieve ETS-10. Phys. Chem. Chem. Phys. 1999, 1, 1649−1657.


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(8)

Page 34 of 43

Hadjiivanov, K.; Massiani, P.; Knözinger, H. Low-Temperature CO and 15N2 Adsorption

and Co-adsorption on Alkali Cation Exchanged EMT Zeolites: an FTIR Study. Phys. Chem. Chem. Phys. 1999, 1, 3831−3838. (9)

Hadjiivanov, K.; Knözinger, H. FTIR Spectroscopic Evidence of Formation of Geminal

Dinitrogen Species During the Low-Temperature N2 Adsorption on NaY Zeolites. Catal. Lett. 1999, 58, 21−26. (10) Hadjiivanov, K.; Ivanova, E.; Klissurski, D. Site-Specified and Complex-Specified Formation of Geminal Species during Adsorption of Small Molecules on Cationic Sites. Catal. Today 2001, 70, 73−82. (11) Hadjiivanov, K.; Ivanova, E.; Mihaylov, M.; Chakarova, K. Adsorption of Two or More Molecules on One Cationic Center in Porous Materials. in Comprehensive Guide for Mesoporous Materials, Vol. 2: Analysis and Functionalization, Nova Sci. Publ., New York, USA, 2015, 269-288. (12) Hadjiivanov, K.; Ivanova, E.; Kantcheva, M.; Cifitikli, E.; Klissurski, D.; Dimitrov, L.; Knözinger, H. FTIR Study of Low-Temperature CO Adsorption on Mn-ZSM-5 and MnY Zeolites. Effect of the Zeolite Matrix on the Formation of Mn2+(CO)2 Geminal Species. Catal. Commun. 2002, 3, 313−319. (13) Bae, M. N.; Song, M. K.; Kim, Y.; Seff, K. Crystal Structure of Mn46Si100Al92O384 . 89H2S, a Hydrogen Sulfide Sorption Complex of Fully Dehydrated Mn2+-Exchanged Zeolite X. Microporous Mesoporous Mater. 2003, 63, 21−31.


34

Page 35 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Ivanova, E.; Hadjiivanov, K. Polycarbonyls of Rh+ Formed after Interaction of CO with Rh–MFI: an FTIR Spectroscopic Study. Phys. Chem. Chem. Phys. 2003, 5, 655−661. (15) Miessner, H.; Burkhardt, I.; Gutschick, D.; Zecchina, A.; Morterra, C.; Spoto, G. Surface Chemistry of Rh Carbonyls and Nitrosyls on Dealuminated Y-zeolite at Low Temperature. J. Chem. Soc., Faraday Trans. 1990, 86, 2321−2327. (16) Tortorelli, M.; Chakarova, K.; Lisi, L.; Hadjiivanov, K. Disproportionation of Associated Cu2+ Sites in Cu-ZSM-5 to Cu+ and Cu3+ and FTIR Detection of Cu3+(NO)x ( x = 1, 2) Species J. Catal. 2014, 309, 376−385. (17) Spoto, G.; Bordiga, S.; Scarano, D.; Zecchina, A. Well Defined CuI(NO), CuI(NO)2 and CuII(NO)X (X= O− and/or NO2−) Complexes in CuI-ZSM5 Prepared by Interaction of H-ZSM5 with Gaseous CuCl. Catal. Lett. 1992, 13, 39−44. (18) Spoto, G.; Zecchina, A.; Bordiga, S.; Ricchiardi, G.; Martra, G.; Leofanti, G.; Petrini, G. Cu(I)-ZSM-5 Zeolites Prepared by Reaction of H-ZSM-5 with Gaseous CuCl: Spectroscopic Characterization and Reactivity Towards Carbon Monoxide and Nitric Oxide. Appl. Catal. B: Environ. 1994, 3, 151−172. (19) Hadjiivanov, K.; Kantcheva, M.; Klissurski, D. IR Study of CO Adsorption on Cu-ZSM5 and CuO/SiO2 Catalysts: σ- and π-Components of the Cu+-CO Bond. J. Chem. Soc., Faraday Trans. 1996, 92, 4595−4600. (20) Scarano, D.; Bordiga, S.; Lamberti, C.; Spoto, G.; Richiardi, G.; Zecchina, A.; Otero Areán, C. FTIR Study of the Interaction of CO with Pure and Silica-Supported Copper(I) Oxide. Surf. Sci. 1998, 411, 272−285.


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 43

(21) Pieplu, T.; Poignant, F.; Vallet, A.; Saussey, J.; Lavalley, J.-C. Oxidation State of Copper during the Reduction of NOx with Propane on H-Cu-ZSM-5 in Excess Oxygen. Stud. Surf. Sci. Catal. 1995, 96, 619−629. (22) Konduru, M. V.; Chuang, S. S. C. Active and Spectator Adsorbates during NO Decomposition over Cu–ZSM-5: Transient IR, Site-Poisoning, and Site-Promotion Studies. J. Catal. 1999, 187, 436−452. (23) Zecchina, A.; Bordiga, S.; Turnes Palomino, G.; Scarano, D.; Lamberti, C.; Salvalaggio, M. Mono-, Di-, and Tricarbonylic Species in Copper(I)-Exchanged Zeolite ZSM-5: Comparison with Homogeneous Copper(I) Carbonylic Structures. J. Phys. Chem. B 1999, 103, 3833−3844. (24) Miessner, H.; Landmesser, H.; Jaeger, N.; Richter, K. Surface Carbonyl Species of Copper Supported on Dealuminated Y Zeolite. J. Chem. Soc., Faraday Trans. 1997, 93, 3417−3422. (25) Szanyi, J.; Paffett, M.T. Co-adsorption of Acetone and CO on Cu-ZSM-5: an In-situ FTIR Study. Catal. Lett. 1997, 43, 37−44. (26) Drenchev, N.; Georgiev, P. A.; Hadjiivanov, K. FTIR Study of

12 16

C O and

13 18

C O

Coadsorption on Cu–ZSM-5. J. Mol. Catal. A: Chem. 2011, 341, 7−13. (27) Turnes Palomino, G.; Bordiga, S.; Zecchina, A.; Marra, G. L.; Lamberti, C. XRD, XAS, and IR Characterization of Copper-Exchanged Y Zeolite. J. Phys. Chem. B 2000, 104, 8641−8651.


36

Page 37 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Lamberti, C.; Bordiga, S.; Zecchina, A.; Salvalaggio, M.; Geobaldo, F.; Otero Areán, C. XANES, EXAFS and FTIR Characterization of Copper-Exchanged Mordenite. J. Chem. Soc., Faraday Trans. 1998, 94, 1519−1525. (29) Bordiga, S.; Pazé, C.; Berlier, G.; Scarano, D.; Spoto, G.; Zecchina, A.; Lamberti, C. Interaction of N2, CO and NO with Cu-Exchanged ETS-10: a Compared FTIR Study with other Cu-Zeolites and with Dispersed Cu2O. Catal. Today 2001, 70, 91−105. (30) Giordanino, F.; Vennestrøm, P. N. R. Lundegaard, L. F. Stappen, F. N.; Mossin, S.; Beato, P.; Bordiga, S.; Lamberti, C. Characterization of Cu-Exchanged SSZ-13: a Comparative FTIR, UV-Vis, and EPR Study with Cu-ZSM-5 and Cu-β with Similar Si/Al and Cu/Al Ratios. Dalton Trans. 2013, 42, 12741−12761. (31) Hadjiivanov, K.; Knözinger, H.; Mihaylov, M. FTIR Study of CO Adsorption on NiZSM-5. J. Phys. Chem. B 2002, 106, 2618−2624. (32) Zdravkova, V.; Mihaylov, M.; Hadjiivanov, K. Coordination of Two N2 Molecules to One Ni+ Site in Ni-ZSM-5: An FTIR Spectroscopy Study. J. Phys. Chem. C 2012, 116, 12706−12711. (33) Borfecchia, E.; Lomachenko, K. A.; Giordanino, F.; Falsig, H.; Beato, P.; Soldatov, A. V.; Bordiga, S.; Lamberti, C. Revisiting the Nature of Cu Sites in the Activated Cu-SSZ-13 Catalyst for SCR Reaction. Chem. Sci. 2015, 6, 548-563. (34) Bulánek, R.; Drobná, H.; Nachtigall, P.; Rubeš, M.; Bludský, O. On the Site-Specificity of Polycarbonyl Complexes in Cu/Zeolites: Combined Experimental and DFT Study. Phys. Chem. Chem. Phys. 2006, 8, 5535−5542.


37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 43

(35) Vayssilov, G.; Hu, A.; Birkenheuer, U.; Rösch, N. Dinitrogen as Probe Molecule of Alkali-Exchanged Zeolites. A Density Functional Study. J. Mol. Catal. A: Chem. 2000, 162, 135−145. (36) Jiang, S.; Huang, S.; Tu, W.; Zhu, J. Infrared Spectra and Stability of CO and H2O Sorption over Ag-Exchanged ZSM-5 Zeolite: DFT Study. Appl. Surf. Sci. 2009, 255, 5764−5769. (37) Aleksandrov, H. A.; Petkov, P. St.; Vayssilov, G. N. Computational Evaluation of the Capability of Transition Metal Exchanged Zeolites for Complete Purification of Hydrogen for Fuel Cell Applications: the Cheapest Performs the Best. Energy & Environ. Sci. 2011, 4, 1879−1885. (38) Vityuk, A.; Aleksandrov, H. A.; Vayssilov, G. N.; Ma, S.; Alexeev, O. S.; Amiridis, M. D.; Effect of Si/Al Ratio on the Nature and Reactivity of HY Zeolite-Supported Rhodium Dicarbonyl Complexes, J. Phys. Chem. C 2014, 118, 26772−26788. (39) Basu, P.; Panayotov, D.; Yates, J. T. Jr. Rhodium - Carbon Monoxide Surface Chemistry: the Involvement of Surface Hydroxyl Groups on Alumina and Silica Supports. J. Am. Chem. Soc. 1988, 110, 2074−2081. (40) Shelef, M. Selective Catalytic Reduction of NOx with N-Free Reductants. Chem. Rev. 1995, 95, 209−225. (41) Iwamoto, M.; Hamada, H. Removal of Nitrogen Monoxide from Exhaust Gases through Novel Catalytic Processes. Catal. Today 1991, 10, 57−71.


38

Page 39 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(42) Iwamoto, M.; Yahiro, H.; Tanda, K.; Mizuno, N.; Mine, Y.; Kagawa, S. Removal of Nitrogen Monoxide through a Novel Catalytic Process. 1. Decomposition on Excessively Copper-Ion-Exchanged ZSM-5 Zeolites. J. Phys. Chem. 1991, 95, 3727−3730. (43) Kapteijn, F.; Marbán, G.; Rodriguez-Mirasol, J.; Moulijn, J. A. Kinetic Analysis of the Decomposition of Nitrous Oxide over ZSM-5 Catalysts. J. Catal. 1997, 167, 256−265. (44) Woertink, J. S.; Smeets, P. J.; Groothaert, M. H.; Vance, M. A.; Sels, B. F.; Schoonheydt, R. A.; Solomona, E. I. A [Cu2O]2+ Core in Cu-ZSM-5, the Active Site in the Oxidation of Methane to Methanol. Proc. Natl. Acad. Sci. USA 2009, 106, 18908−18913. (45) Zhang, Y.; Drake, I. J.; Briggs, D. N.; Bell, A. T. Synthesis of Dimethyl Carbonate and Dimethoxy Methane over Cu-ZSM-5. J. Catal. 2006, 244, 219−229. (46) Chakarova, K.; Petrova, G.; Dimitrov, M.; Dimitrov, L.; Vayssilov, G.; Tsoncheva, T.; Hadjiivanov, K. Coordination State of Cu+ Ions in Cu-[Al]MCM-41. Appl. Catal. B: Environ. 2011, 106, 186−194. (47) Hadjiivanov, K.; Vayssilov, G. Characterization of Oxide Surfaces and Zeolites by Carbon Monoxide as an IR Probe Molecule, Adv. Catal. 2002, 47, 307–511. (48) Venkov, T.; Hadjiivanov, K. FTIR Study of CO Interaction with Cu/TiO2. Catal. Commun. 2003, 4, 209−213. (49) Kuroda, Y.; Konno, S.; Morimoto, K; Yoshikawa, Y. Strong Adsorption of Dinitrogen on Copper-Ion-Exchanged Mordenite at Room Temperature J. Chem. Soc., Chem. Commun. 1993, 18−20.


39


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 43

(50) Spoto, G.; Bordiga, S.; Ricchiardi, G.; Scarano, D.; Zecchina, A; Geobaldo, F. Formation of CuI-N2 Adducts at 298 and 77 K in CuI-ZSM-5: an FTIR Investigation. J. Chem. Soc., Faraday Trans. 1995, 91, 3285−3290. (51) Kuroda, Y.; Kumashiro, R.; Yoshimoto, T.; Nagao, M. Characterization of Active Sites on Copper Ion-Exchanged ZSM-5-Type Zeolite for NO Decomposition Reaction. Phys. Chem. Chem. Phys. 1999, 1, 649−656. (52) Recchia, S.; Dossi, C.; Psaro, S.; Fusi, A.; Ugo, R.; Moretti, G. Dinitrogen Irreversible Adsorption on Overexchanged Cu-ZSM-5. J. Phys. Chem. B 2002, 106, 13326−13332. (53) Serykh, A. I.; Amiridis, M. D. DRIFTS Evidence for the Formation of Binuclear Cu(I)– Dinitrogen Complexes upon Adsorption of N2 on CuZSM-5. Microporous Mesoporous Mater. 2006, 94, 320−324. (54) Morpurgo, S.; Moretti, G.; Bossa, M. A Computational Study on N2 Adsorption in CuZSM-5. Phys. Chem. Chem. Phys. 2007, 9, 417−424. (55) Góra-Marek, K.; Datka, J. IR Studies of Coadsorption of CO and NO on Co2+ and Cu+ Sites in Zeolites. Vibr. Spectrosc. 2011, 57, 148−151. (56) Hadjiivanov, K. Identification and Characterization of Surface Hydroxyl Groups by Infrared Spectroscopy. Adv. Catal. 2014, 57, 99–318. (57) Hadjiivanov, K.; Saussey, J.; Freysz, J. L.; Lavalley, J.-C. FT-IR Study of NO + O2 Coadsorption on H-ZSM-5: Re-assignment of the 2133 cm-1 Band to NO+ Species. Catal. Lett. 1998, 52, 103−108.


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Page 41 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(58) Maache, M.; Janin, A.; Lavalley, J.-C.; Benazzi, F. FT Infrared Study of Brønsted acidity of H-Mordenites: Heterogeneity and Effect of Dealumination. Zeolites 1995, 15, 507–516. (59) Wakabayashi, F.; Kondo, J. N.; Domen, K.; Hirose, C. An Attempt to a SemiQuantitative Analysis of Lewis Acid Sites in H-ZSM-5 Zeolite Using Water as an IRSpectroscopic Titrant. Catal. Lett. 1996, 38, 15–19. (60) Vimont, A.; Leclerc, H.; Maugé, F.; Daturi, M.; Lavalley, J.-C. Surblé, S.; Serre, C.; Férey, G. Creation of Controlled Brønsted Acidity on a Zeotypic Mesoporous Chromium(III) Carboxylate by Grafting Water and Alcohol Molecules. J. Phys. Chem. C 2007, 111, 383−388. (61) Hadjiivanov, K. Identification of Neutral and Charged NOx Surface Species by IR Spectroscopy. Catal. Rev. Sci. Eng. 2000, 42, 71−144. (62) Penkova, A.; Hadjiivanov, K.; Mihaylov, M.; Daturi, M.; Saussey, J.; lavalley, J.-C. FTIR Spectroscopic Study of Low-Temperature NO Adsorption and NO + O2 Coadsorption on H-ZSM-5. Langmuir 2004, 20, 5425−541. (63) Broclawik, E.; Datka, J.; Gil, B.; Piskorz, W.; Kozyra, P. The Interaction of CO, N2 and NO with Cu Cations in ZSM-5: Quantum Chemical Description and IR Study. Top. Catal. 2000, 11/12, 335–341. (64) Hadjiivanov, K.; Mihaylov, M.; Panayotov, D.; Ivanova, E.; Chakarova, K. Isotopes in the FTIR Investigations of Solid Surfaces. Spectrosc. Prop. Inorg. Organomet. Compd. 2014, 45, 43−78.


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Page 42 of 43

(65) Descorme, F.; Gélin, P.; Lécuyer, C.; Primet, M. Catalytic Reduction of Nitric Oxide by Methane in the Presence of Oxygen on Palladium-Exchanged Mordenite Zeolites. J. Catal. 1998, 177, 352−362. (66) Chakarova, K.; Ivanova, E.; Hadjiivanov, K.; Klissurski, D.; Knozinger, H. Co-ordination Chemistry of Palladium Cations in Pd-H-ZSM-5 as Revealed by FTIR Spectra of Adsorbed and Co-adsorbed Probe Molecules (CO and NO). Phys. Chem. Chem. Phys. 2004, 6, 3702–3709. (67) Braterman, P. S. Metal Carbonyl Spectra, Academic Press, New York, 1975. (68) Dossi, C.; Fusi, A.; Moretti, G.; Recchia, S.; Psaro, P. On the Role of Carbonaceous Material in the Reduction of Cu2+ to Cu+ in Cu-ZSM-5 Catalysts. Appl. Catal. A: Gen. 1999, 188, 107–119. (69) Caulton, K. G. DeKock, R. L.; Fenskel, R. F. A Comparison of Carbon Monoxide and Nitrogen as Ligands in Transition Metal Complexes. J. Am. Chem. Soc. 1970, 92, 515–518. (70) Berlier, G.; Zecchina, A.; Spoto, G.; Ricchiardi, G.; Bordiga, S.; Lamberti, C. The Role of Al in the Structure and Reactivity of Iron Centers in Fe-ZSM-5-Based Catalysts: a Statistically Based Infrared Study. J. Catal. 2003, 215, 264–270.


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Surprising Coordination Chemistry of Cu+ Cations in Zeolites: FTIR

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