Carbon Monoxide Sorption Monitored by IR Spectroscopy - American

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Quantification of Silver Sites in Zeolites: Carbon Monoxide Sorption Monitored by IR Spectroscopy Karolina Tarach, Kinga Góra-Marek, Magdalena Chrzan, and Stanis#aw Walas J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp506820v • Publication Date (Web): 23 Sep 2014 Downloaded from http://pubs.acs.org on September 27, 2014

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The Journal of Physical Chemistry

Quantification of Silver Sites in Zeolites: Carbon Monoxide Sorption Monitored by IR Spectroscopy Karolina Tarach*, Kinga Góra-Marek*, Magdalena Chrzan, Stanisław Walas Faculty of Chemistry, Jagiellonian University in Kraków, Ingardena 3, 30-060 Kraków, Poland

corresponding authors: Karolina Tarach: [email protected] Kinga Góra-Marek: [email protected] phone:+48 12 663 20 81

ABSTRACT This work was aimed to provide a well-established approach for the quantification of silver sites in zeolites. The experimental procedure based on carbon monoxide sorption in silver exchanged zeolites ZSM-5, Y and X was monitored by IR spectroscopy. With regard to Lambert-Beer law the values of the absorption coefficients of the IR bands of CO interacting with exchangeable cations (Ag+) and metallic species (Ago) were attained. Subsequently, the concentrations of silver species of different kinds were calculated. The values of the absorption coefficients are valid for zeolites of FAU and MFI topologies thus they are argued to be used for zeolite of other structures.

Keywords: Silver; Silver monocarbonyls; CO sorption; IR spectroscopy; Quantification

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Introduction

Zeolites are of the greatest interest by reason of their microporous structure of channels and cavities which offers possibility to stabilize the small metal clusters and other coordination complexes. The high reactivity of cations hosted in zeolites is explained in terms of a considerably high coordinative unsaturation. Ag-exchanged zeolites are concerned as effective catalysts in several catalytic and photocatalytic processes. In particular, zeolites hosting highly dispersed Ag+ cations show high activity in the selective catalytic reduction of NO by ethylene1, photocatalytic decomposition of NO2, the aromatization of alkanes, alkenes and methanol3, and the photochemical of H2O into H2 and O24. Also the formation of the active Ag clusters and their detailed structure have been discussed extensively5–12 and some reports demonstrated a specific catalytic activity of Ag clusters in zeolites in a photocatalytic degradation of malathion5, photo-dimerization of alkane6, and methane conversion into propylene in the presence of ethylene.13 Silver ions introduced to zeolite can be reduced by heating, radiation by ultraviolet rays or by the reaction with reducing molecules. Reduction of the Ag+ ion to clusters Ago can be realized by the treatment with CO, alcohols and alkylobenzenes above 350 oC.12 In turn, a very important role of the thermal treatment of the Ag-zeolites has been reported. Highly dispersed isolated Ag+ cations were found to be the most favoured at temperatures of treatment below 400 oC. Higher temperature of the treatment facilitates the Ag+ ion aggregation and the Agn clusters formation.13 Carbon monoxide is the probe molecule widely applied for the studies of the electron donor/acceptor properties of transition metal cations.14-16 A characteristic feature of CO bonded to unsaturated transition metal cation is that these species usually possess higher frequency than the CO in the gas phase (2143 cm–1). In contrast, the lower frequencies are observed for CO engaged into interaction with metals or transition metal cations of the lowest oxidation states. This phenomenon points to the vital competition between the relative extend of σ-donation from CO to the cation which can increase of C≡O bond strength thus its frequency, and π-back donation from cation to CO molecules resulting in the opposite effect: weakening of C≡O bond. Thus, the up- or downshift of metal cation monocarbonyl band in the respect to the position of gaseous CO resulting from the electron flow between probe and cation, can be used as a measure of the redox properties of Ag species. IR studies have shown that CO sorption in Ag-exchanged zeolites led to the creation of Ag+(CO) monocarbonyls band at ca. 2190 cm–1, which at low temperatures and in the presence of gaseous CO phase are easily converted into dicarbonyls species Ag+(CO)2 characterized by the 2195 and 2189 cm–1 bands. The carbonyls of Ago were found to appear at 2160 cm–1.


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There is no doubt that IR spectroscopy can be considered as the starting point for a complete characterization of acidic sites in zeolite. Sorption of probe molecules followed by IR spectroscopy can provide a direct insight into quantitative aspect of the speciation of Brønsted and Lewis sites. Pyridine sorption experiments are particularly valuable since the assignments of modes associated with both pyridinium ions PyH+ and coordination complexes at Lewis sites, have been well established. Many other probe molecules were dedicated to measure the concentration of acidic sites: ammonia17-20, pivalonitrile21, acetonitrile22, and others.23 However, to use IR spectroscopy quantitatively, a scaling factor, the absorption coefficient, is necessary. There are many articles that should be consulted for details with the regard to the values for molar absorption coefficients employed for the quantitative analysis of site in zeolite systems.24, 25 Undeniably, the determination of the value absorption coefficient of a well-defined band of probe-site adduct is a key problem in quantitative IR studies of molecules adsorbed on solid surfaces. The simplest thus the most desired is the situation when each molecule reacts with the adsorption site selectively. In such a case, the intensity of IR band increases linearly with the concentration of molecules adsorbed and the slope of the line is taken as the absorption coefficient value. The situation becomes more complicated if several kinds of the adsorption sites on the solid surface are able to bond the probe. Thus it is required to estimate the amount of probe molecule interacting with each kind of the sites separately. In all cases each molecule introduced into the cell is required to be adsorbed on active site; neither molecules in gas phase nor physisorbed ones can be detected. Accordingly, the sorption of probe requires the proper experimental conditions. Numerous studies have been devoted to the examination of the status of Ag species in zeolites and their interaction with reactant molecules.1-4, 12, 26 Nevertheless, the previous consideration possessed only qualitative character and they have not offered quantitative analysis of the speciation of silver sites in zeolites. Consequently, this work was aimed to deliver the fully quantitative tool for the investigation of silver species of the different type in zeolites of various topologies.



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Experimental

The AgZSM-5 sample was obtained by the standard ion exchange procedure from the parent ammonium form (Si/Al = 30, Zeolyst, Inc.). The mixture of 100 cm3 0.2 M AgNO3 and 1.5 g zeolite was stirred for 24 h at 70 oC. After the ion exchange procedure sample was washed with distilled water and dried at 100 oC. Zeolite NaX (Si/Al=1.3) and NaY (Si/Al=2.5) were purchased from Sigma Aldrich and Zeolyst, Inc., respectively. They were exchanged to ammonium forms by six fold treatment with 0.2 M NH4NO3 at 60 oC for 24 hours. The Na+/NH4+ exchange degree, confirmed by the ICP analysis equal to 60 % for zeolite X (Na33.5(NH4)50.0[(SiO2)108.5(AlO2)83.5]) and 80 % for zeolite Y (Na11(NH4)44[(SiO2)137(AlO2)55]). Zeolites NaNH4X and NaNH4Y were transformed to Ag-forms by the same ion exchange procedure that was previously adapted for zeolite ZSM-5. The resulting materials were characterized by ICP method, revealing the Ag content. The composition of these zeolites is given in Table 1. The ion exchange procedure, carried out in the light exposure resulted in the photoreduction of Ag(I) species and consequent formation of [Agn]x+ clusters.27 Photoreduced samples were denoted hereafter as AgZSM-5, AgX and AgY. In some experiments photoreduced Ago forms in AgZSM-5 were re-oxidised by the treatment in O2 atmosphere (60 Tr in gas phase) at 400 oC for 1 h. The oxidised samples were colourless. Otherwise, the reduction of Ag+ species metallic forms was performed in H2 atmosphere (60 Tr in gas phase) at 400 oC for 1 h. The resulting samples were denoted hereafter as AgZSM-5/ox and AgZSM-5/red, respectively. Table 1. The composition of zeolites (expressed by Si/Al and Ag/Al ratios) and the treatment applied to produce silver species of the different kinds. zeolite

Si/Al

Ag/Al

formula

AgZSM-5 AgZSM-5/ox 30

0.53

Ag1.6(NH4)4.3[(SiO2)88.3(AlO2)3.1]

1.3 2.6

0.45 0.81

Ag38.0 Na18.0(NH4)27.5[(SiO2)108.5(AlO2)83.5] Ag43.0(NH4)12[(SiO2)137(AlO2)55]

AgZSM-5/red AgX AgY

pre-treatment light exposure at RT O2 atmosphere (60 Tr in gas phase) for 1 h at 400 oC then cooling to RT H2 atmosphere (60 Tr in gas o phase) for 1 h at 400 C then cooling to RT light exposure at RT

Prior to IR studies zeolites were pressed into thin self-supporting wafers and evacuated under a dynamical vacuum of p ∼10-5 Tr, inside a home-made quartz IR cell specially designed for in situ low and high-temperature treatments and for quantitative gas dosage. First, sample was evacuated at 4 ACS Paragon Plus Environment

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ambient temperature for 10 min, and then temperature was elevated to 450 oC and kept for 1 h. The presence of Ag+ and Ago species was confirmed by adsorption of carbon monoxide (Linde Gas 99.95%) performed at RT. IR spectra were recorded with the spectral resolution of 2 cm–1 on a Bruker Tensor 27 FTIR spectrometer equipped with a MCT detector. The X-ray diffraction (XRD) patterns of the samples were recorded with a D2 Phaser diffractometer (Bruker) using Cu Kα radiation (λ = 1.54060 Å, 30 kV, 10 mA). XRD studies were carried out for zeolites modified with silver to confirm the stability of zeolite structure both before and after the vacuum pre-treatment at 450 oC. The presence of reflections typical of the FAU and MFI structure in diffractograms of thermally treated silver zeolites as well as a high crystallinity of resulting zeolites (Table 2) indicates that thermal vacuum treatment disturbed neither zeolite ZSM-5 nor zeolites X and Y structures (Supporting Information, Fig. SI.1). Table 2. The crystallinity (% Cryst.) for parent and thermally treated zeolites calculated based on the total intensities of XRD peaks. parent zeolite

% Cryst. 100

thermally treated zeolite AgZSM-5

% Cryst.

AgZSM-5 AgX

100

AgX

85

AgY

100

AgY

95

95



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Results and discussion

3.1.1

Monocarbonyls in silver zeolites

The extraframework cations balancing the negative charge of the framework can occupy various positions and their population within those positions is depended on their ionic radii, coordination requirements and, finally, on the aluminum distribution in the framework. Interaction of carbon monoxide with silver sites in AgZSM-5/ox led to the appearance of a strong and symmetric band with the maximum settled at 2192 cm–1 (Fig. 1). In line with literature reports the band was assigned to Ag+(CO) monocarbonyls.28 Contrary to AgZSM-5/ox, in both faujasites, AgX and AgY, two bands of Ag+(CO) monocarbonyls were observed (Fig. 1) proving the presence of two Ag+ sites of different electron acceptor properties, the most probably resulting from the location of Ag+ in various cationic sites. The location of Ag+ cations in zeolites X and Y was examined by XRD analyses.27, 29, 30

Lamberti et al.27 reported that in dehydrated zeolite AgY the occupancy of sites SI in hexagonal

prisms and in cubooctahedra by Ag+ cations was predominant. Some cations were situated at SII positions, there was no Ag+ cations in SIII sites. Because of the structure restrictions the Ag+ ions in SI type sites are considered to be inaccessible to reactant molecules. Thus, the Ag+(CO) monocarbonyl band settled at 2175 cm–1 for AgY zeolite can originate from the Ag+ cations in SII sites. Furthermore, a small band at 2195 cm–1, due to the frequency nearly the same as for Ag+ in zeolite AgZSM-5, can be interpreted as the result of interaction of CO with Ag+ of lower number of oxygen atoms in vicinity, thus possibly in the SIII sites.

Figure 1. The bands of the Ag+(CO) monocarbonyls normalized to the same intensity (the height).

On the other hand, in dehydrated zeolites AgX the silver cations have been found in the SI, SII and in SIII positions. Again, the Ag+ ions in SI type sites were expected to be inaccessible to reactant molecules. Accordingly, the Ag+(CO) monocarbonyls bands at 2184 and 2170 cm–1 were assigned to CO molecules interacting with the Ag+ cations in SII and SIII sites. The lower frequency of the Ag+(CO) monocarbonyl bands in zeolite X then Y was correlated with electronegativity of framework ruled by 6 ACS Paragon Plus Environment

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lower Si/Al ratio. What is more, the heterogeneity of Ag+ cations in AgX and AgY within the SII and SIII sites can be related to the various degree of neutralization of Ag+ sites by framework oxygen and will be further discussed in Section 3.1.3. Additionally, for photoreduced AgZSM-5 zeolite the presence of silver in metallic form is evidenced by the weak band of monocarbonyls Ago(CO) at 2160 cm–1. It is worth mentioning that negligible intensity of this band is not absolute evidence for the minor number of metallic silver species on the surface of catalysts due to a greatly reduced value of the absorption coefficient of the Ago(CO) monocarbonyls band. This feature will be discussed in Section 3.1.4. For Ag-faujasites the Ago(CO) bands, if exist, are hardly detected due to overlapping by the band of monocarbonyls Ag+(CO).

+

Figure 2. The bands of the Ag (CO) monocarbonyls in zeolites photoreduced AgZSM-5 and reoxidized AgZSM+ 5/ox. Spectra were normalized to the same sample mass (A) and the same intensity of the Ag (CO) monocarbonyl band.

The changes of the speciation of silver forms, both in qualitative and quantitative manner were imposed by the treatment of zeolite AgZSM-5 in oxidative or reducing atmosphere. The contact of zeolite AgZSM-5 with oxygen at 400 oC for 1 h resulted in the presence of the Ag+(CO) monocarbonyls band at 2192 cm–1 of significant intensity (Fig. 2). Normalization of the 2192 cm–1 band for photoreduced and oxidized sample to the same intensity is an evidence that no other moieties than those represented by the 2192 cm–1 band are created by the oxidation of metallic Ago species. Furthermore, according to theoretical calculations performed by Casarin et al.31, interaction of CO with Ag2O would be appeared as a band at 2158 cm–1. Similarly, the bands at 2164 and 2161 cm–1 were observed on Ag/Al2O332,

33

and in case of Ag deposited on ceria-zirconia oxide32,

respectively. Thus, certainly not Ag2O but the Ag+ cations in the exchangeable positions are postulated to be formed in the oxidation of photoreduced AgZSM-5. Additionally, the analysis of the Si(OH)Al group band (Fig. 3) can provide further evidence for the different nature of species balancing zeolite framework after oxidation and reduction processes. The main species balancing negative framework in zeolite photoreduced zeolite AgZSM-5 are Ag+ and H+ ions. The presence of 7 ACS Paragon Plus Environment


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the later ones is manifested by the Si(OH)Al group band at 3616 cm–1. This new fraction of the Si(OH)Al groups was created in the reaction: Ag+ Si(O-)Al + 1/2 H2 = Ago + H+ Si(O-)Al. Migration of metallic species formed by autoreduction and their interaction with silver ions Ag+ to form clusters has been widely reported. Depending on the zeolite type and the extent of silver exchange various types of clusters of different composition and geometry Ag+-Ag0-Ag+ (linear) to Ag86+, Ag54+, and Ag128+ (“bulk phase”) have been reported.32-35

Figure 3. IR spectra registered at RT for zeolite photoreduced AgZSM-5 vacuum treated, oxidized, and reduced in the region of the hydroxyls vibration. Spectra normalized to the 10 mg mass sample.

After contacting of zeolite with oxygen the band of bridging hydroxyls was significantly reduced which implied the consumption of the Si(OH)Al groups in oxidation process and their substitution for other species positively charged, i.e. Ag+ cations. The Ag+ cations can be created according the scheme: 2 Ago + O2 + 4H+Si(O-)Al = 2 Ag+ Si(O-)Al + 2 H2O. All these considerations strongly support the hypothesis that the Ag+ cations in the exchange positions are the only species in zeolite AgZSM-5/ox. What is more, Reikert et al.36 found that the extent of oxidation of pre-reduced zeolite AgY could be controlled by experimental conditions and even fully reduced AgY could be completely reoxidized to achieve original zeolite. 3.1.2

Requirements for the selective reaction of CO with Ag+ and Ago species

The most demanding task in the quantitative IR experiments is the determination of the absorption coefficient of the band of adducts formed by the interaction of probe molecules with adsorption sites 8 ACS Paragon Plus Environment

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on solid surfaces. Generally, the selective reaction of probe molecule with a defined kind of adsorption site is required. The simplest and the most desired is the situation when each molecule introduced into IR cell reacts selectively with only one kind of adsorption site and the stoichiometry of this interaction is perfectly known. The composition of unit cell of studied zeolites (Table 1) specified that the Ag+ ions were not the extraframework species exclusively balancing both FAU and MFI framework. In both structure some protons, formed by NH4+ ions decomposition, neutralized the framework. Additionally, the Na+ ions were also found to be accommodated in zeolite AgX. Nevertheless, under the exact experimental conditions the presence of H+ and Na+ ions could not bother the sorption of carbon monoxide on Agexchanged zeolites and the requirement for the selective reaction of CO with Ag+ cations would be completely fulfilled. Carbon monoxide was proven to interact both with the Si(OH)Al groups and alkali cations at low temperature, usually ca. from -80 to -196 oC, due to its low basicity thus a low value of the heat of adsorption.37, 38 Molecule of CO hydrogen bonded to the Si(OH)Al sites gives the C≡O band at 2175-2170 cm–1. Simultaneously, the red shift of the Si(OH)Al band from original hydroxyl band position is observed.39, 40 Sorption of carbon monoxide sorption carried out at room temperature in AgZSM-5 (i.e. zeolite hosting both Ag+ and H+ cations) revealed neither the CO band in the 2175-2170 cm–1 frequency region nor the perturbation of the Si(OH)Al group band (Supporting Information, Fig. SI.2A). Such changes were triggered when sorption of CO was performed at -100 oC (Supporting Information, Fig. SI.2B). The appearance of the band of CO hydrogen bonded to bridging hydroxyls at 2175 cm–1 was accompanied by the downshift of the Si(OH)Al group band up to 305 cm– 1

. At low temperature monocarbonyls were easily converted into dicarbonyls species Ag+(CO)2

characterized by the 2195 and 2189 cm–1 bands. Additionally, the band of physisorbed CO and its vibration-rotation spectrum could be easily distinguished. Consequently, the possibility of the interaction of the Si(OH)Al sites with probe molecule at room temperature was definitely excluded. In zeolite AgX the Na+ ions were also found to be possible centres for CO sorption. The development of the 2178 cm–1 band assigned to CO molecules polarized by Na+ ions was reported when sorption of CO performed at -196 oC in zeolite NaZSM-5 of Si/Al = 35.37 The influence of framework charge was evidenced by the fact that in NaY the Na+(CO) band emerged in the 2175-2171 cm–1 region.38, 41, 42 In order confirm that the interaction of probe molecule with Na+ cations in exchange positions is not effective at room temperature, carbon monoxide was adsorbed in zeolite NaAgZSM-5. In this zeolite the bands of the Na+(CO) and Ag+(CO) are perfectly resolved, while in studied zeolite AgX (hosting also both Ag+ and Na+ ions) these bands tend to be overlapped. Sorption of carbon monoxide at room temperature in NaAgZSM-5 resulted in the development of the Ag+(CO) monocarbonyl band at



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2192 cm–1 and; no bands at 2178 cm–1 attributed to CO bonded to Na+ cations were present (Supporting Information, Fig. SI.3A). These latter ones are build up when sorption was done at -100 o

C (Supporting Information, Fig. SI.3B). The Na+(CO) band in the 2180-2170 cm–1 frequency region is

usually denoted as a high frequency band because it is upward shifted with respect to the 2143 cm–1 value of free CO molecule. Besides the high frequency band, a weaker low frequency band of the C≡O stretching mode of M+···OC adducts at frequencies below 2143 cm–1 was also reported for alkaline zeolites saturated with CO.43-46 When the high frequency band is detected at low temperatures only, the low frequency band, usually of a slight intensity, is observed also at room temperature. It should be definitely said that the sorption of CO in our studied zeolites did not revealed the presence of the M+···OC adducts at frequencies below 2143 cm–1. It can be definitely stated that in the experimental conditions elaborated for the determination of the Ag+(CO) band absorption coefficient, i.e. at room temperature, the only species being able to interact with probe molecule were Ag+ and Ago moieties; the possibility of the interaction of Na+ and H+ cations with carbon monoxide was totally eliminated. Additionally, the sorption of small doses of probe resulted in saturation of silver species with monocarbonyls; the dicarbonyls species were not detected. Consequently, the case of the sorption of small amounts of probe performed at room temperature on pre-treated zeolites AgZSM-5/ox and AgZSM-5/red fully obey the rule of the selective reaction of probe with well-defined adsorption site. 3.1.3

Absorption coefficient of the Ag+(CO) band

As mentioned above the sorption of relatively small amounts of CO performed at room temperature on oxidized zeolite AgZSM-5/ox fulfils the requirement for the selective reaction of CO with Ag+ cations. The spectra in Figure 4 were recorded upon the sorption of the small measured doses of CO at RT in zeolites AgZSM-5/ox, AgY and AgX. For zeolite Ag/ZSM-5 hosting exclusively the Ag+ cations in the exchange positions at low loading carbon monoxide reacted selectively with exchangeable Ag+ what was manifested by the development of the 2192 cm–1 monocarbonyl Ag+(CO) band as the only species. No bands at 2040-2060 cm–1 were visible, which implied the absence of metallic silver species in the zeolite AgZSM-5/ox. Neither the spectrum of gaseous CO nor the bands of dicarbonyls Ag+(CO)2 were detected. Consequently, the rule of the selective reaction of CO with Ag+ sites was fulfilled. Thus, the integral intensities (Fig. 5A) of the Ag+(CO) bands at 2192 cm–1 in the spectra recorded upon the sorption of the first CO doses in AgZSM-5/ox, were measured and plotted against the concentration of CO sorbed. The points attained in four independent experiments were


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represented by the same linear dependence (Fig. 5). The slope of the line taken as the integral absorption coefficient of the 2192 cm–1 band was equal to 6.98 ± 0.01 cm/µmol. The value of the absorption coefficient attained from the calibration line based on the heights of the Ag+(CO) bands was found to be equal 0.50 ± 0.01 cm2/µmol (Fig. 5B). In another series of experiments, the doses of CO were sorbed at room temperature in zeolites AgY and AgX (Fig. 4). The intensities of the monocarbonyl bands Ag+(CO) increased linearly with the amount of CO sorbed. The spectrum of gaseous CO or the bands of dicarbonyls Ag+(CO)2 were also not detected. The sum of integral intensity of the Ag+(CO) bands at 2175 cm–1, 2194 cm–1 and 2170 cm–1, 2184 cm–1 for respective AgY and AgX zeolites, were plotted against the concentration of CO sorbed. The points obtained for zeolites AgY and AgX zeolites fitted the same line obtained for CO sorption on zeolite AgZSM-5/ox (Fig. 5). Consequently the value of the absorption coefficient of the 2192 cm–1 band is independent from the structure’s type of zeolite regardless the various CO stretching frequencies (2192 cm–1 for AgZSM-5, 2194 and 2175 cm–1 for AgY, as well as 2184 and 2170 cm–1 for AgX) of the Ag+(CO) monocarbonyls. The heterogeneity of Ag+ cations in AgX and AgY within the SII and SIII sites is well seen in the spectra collected after sorption of CO doses: the Ag+(CO) band is downshifted with the CO amount adsorbed. The extent of an alteration of the electron-acceptor properties within the same Sn position is smaller, nevertheless the sites of a higher electron affinity react with CO in the first order. Again, this heterogeneity within the same Sn can be related to the various degree of neutralization of Ag+ sites by framework oxygen. A description of the bonding of molecules to transition metal (Cu+, Ag+, Ago, ect.) consist of two synergic processes: donation and back-donation. The donation is the transfer of electrons from the filled π-orbital (π-donation, e.g. alkenes) or lone electron pair orbital (σ-donation, e.g. CO, N2) of the ligand into an empty orbital of the metal, while the back donation releases the electrons from the nd or nf orbital of the metal into the empty π*-antibonding orbital of the ligated molecule. The positive charge of cations hosted in zeolite is strongly neutralized by electronegative framework, thus the back donation effect is particularly enhanced. The cations in zeolites, e.g. Cu+, were found to activate multiple bonds in both organic molecules (alkenes, benzene, aldehydes) as well as inorganic ones (CO, N2), while Ag+ sites were not so effective. The extent of the weakening of the bond in ligated CO molecule is reflected in the downshift of the respective monocarbonyl bands.



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Figure 4. The spectra recorded at RT upon the sorption of CO doses in AgZSM-5/ox, AgY, and AgX and used for the determination of the absorption coefficient of the Ag+(CO) band.

A higher frequency of the Ag+(CO) monocarbonyls band compared with the Cu+(CO) band (Table 3) is reasonable due to two principal effects: (i) the electrostatic interaction and (ii) σ-donation from the 5σ orbital of CO molecule to d (Cu+) or f (Ag+) orbitals of cation. It is expected that the electrostatic effect will be lesser for Ag+ than Cu+ because of the larger ionic radius of the Ag+ cation. In fact, the ionic radius of silver cation drops between the values typical of Na+ and K+, which bonding CO molecule give the bands in 2178 and 2166 cm–1, respectively. Nevertheless, the influence of the electrostatic effect is not enough to explain both a higher frequency of the Ag+(CO) band, when 12 ACS Paragon Plus Environment

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comparing with Cu+(CO). A vital factor is σ-donation from the orbital of CO to f orbitals of Ag+ cation. For the d10 configuration ions (both Ag+ and Cu+) the polarization energy is reinforced, and this effect increases with atomic number. A strong polarization of 5σ orbital in CO molecule by Ag+ cation results in the higher contribution of σ donations and, thus, a higher frequency of the Ag+(CO) monocarbonyl bands is observed. The third effect, the π-back donation process, attributed to the transfer of d or f electrons from Cu+ or Ag+ cations to antibonding π* orbital of CO and responsible for the weakening of the C≡O bond, is negligible for Ag+ cations.

Figure 5. The plots of the calibration lines, based on the area (A) and the height (B) of the 2192 cm–1 Ag+(CO) band, used in determination of the Ag+ cation concentrations (AgZSM-5/ox – blue diamonds, AgX – green triangles, AgY – red squares). Table 3. The band position (in cm–1) and the values of absorption coefficients determined from the integral intensity (in cm/µmol) and from the height of the Ag+(CO) and Cu+(CO) monocarbonyl bands (in cm2/µmol). Cation Band position Absorption coefficient –1 [cm ] from the integral intensity from intensity (the height of band) [cm/µmol] [cm2/µmol] Ag+ 2192 6.98 ± 0.01 0.50 ± 0.01 Cu+ 2157 16.50 ± 0.03* 1.30 ± 0.0247 –1 + *obtained from the integral intensity of the 2157 cm Cu (CO) band according the procedure described in the Supporting Information.



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Absorption coefficient of the Ago(CO) band

The Ag+(CO) carbonyls were found to be stable species at room temperature due to the strong synergism between a σ-bond and π-back donation. The analysis of the evolution of the integrated CO bands vs. time, in both oxygen atmosphere and vacuum, evidenced that the intensity of the 2192 cm– 1

Ag+(CO) band was well-preserved for 48 h pointing to a high stability of the Ag+(CO) monocarbonyl

species. In the same way, Bartolomeu et al.32 demonstrated a higher stability of oxidized Ag+ species present on zeolite HZSM-5 than of those present on Ag/Al2O3 and Ag/ceria-zirconia oxide support in CO atmosphere. In opposite, between metallic silver Ag0 and CO a weak bond of a mainly π-character is formed. Thus, the surface Ag0(CO) complexes are detected predominantly at low temperature.48 The estimation of the absorption coefficient of the 2157 cm−1 band assigned to Ago(CO) monocarbonyls was performed at RT on the H2-reduced zeolites AgZSM-5/red. Small, measured doses of carbon monoxide were sorbed in reduced Ag/ZSM-5/red until the presence of gas phase was not detected. In spite of high reduction temperature and the excess of reducing agent some Ag+ species persisted to reduction. The interaction of CO with Ago metallic species simultaneously produced two bands of monocarbonyls: the Ago(CO) (2157 cm–1) as well as the Ag+(CO) (2198 cm–1) (Fig. 6).

Figure 6. The spectra recorded at RT upon the sorption of CO doses in AgZSM-5/red and next used for the determination of the absorption coefficient of the Ago(CO) band.

However, the intensity of Ag+(CO) monocarbonyl band is negligible, thus the amount of CO consumed by the Ag+(CO) adducts was also trivial. The amount of CO left was considered as interacting with metallic Ago species. The linear dependence of the intensity of 2157 cm–1 Ago(CO) band against the amount of CO interacting with Ago was presented in Figure 7. The slope of the line was taken as the 2157 cm–1 Ago(CO) band integral absorption coefficient giving the value equal 0.24 ±


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0.01 cm/µmol (Fig. 7A). Similarly, the value of the absorption coefficient attained by the plotting of the intensities the Ago(CO) bands against the concentration of CO sorbed (Fig. 7B) was found to be equal 0.02 ± 0.002 cm2/µmol. The 0.02 ± 0.002 cm2/µmol value of the absorption coefficient was derived from the calibration line based on the heights of the Ago(CO) bands. It should be underlined that the value of the absorption coefficient for the Ag+(CO) band is 29-fold higher than for the Ago(CO) band. Moreover, the Ago(CO) band is downshifted in the comparison to the Ag+(CO) band in line with the literature data evidencing a lower frequency of CO bonded to isolated monovalent Cu+ cations (2157 cm–1) than for zerovalent Cuo (2124 cm–1).47 Both lower frequency and considerably low value of the absorption coefficient of the Ago(CO) band evidenced that π back donation effect realized by the transfer of d electrons from zerovalent Ago to antibonding π* orbital of CO is more effective for metallic species than for exchangeable Ag+ cations. Such predominant π back donation effect is responsible for vital weakening of the C≡O bond.

Figure 7. The plots the calibration lines, based on the area (A) and the height (B) of the 2157 cm–1 Ago(CO) band in AgZSM-5/red, used in determination of the Ago concentration.

3.2

Concentration of Ag+ and Ago species

To determine the concentration of the silver species in studied zeolites the doses of carbon monoxide was sorbed until saturation of both Ag+ and Ago sites, i.e. the maximum intensities of the



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monocarbonyl Ag+(CO), Ago(CO) bands were achieved (Fig. 8). The concentrations of respective sites were calculated from the maximum intensities and their absorption coefficients (Table 4). 3.2.1

Zeolite ZSM-5

The values of concentration of silver sites (Ag+ and Ago) in zeolites AgZSM-5 photoreduced, treated with hydrogen (AgZSM-5/red) and oxidized (AgZSM-5/ox) were compared with the Ag concentration determined by chemical analysis (Table 4). In zeolite AgZSM-5/ox thermally treated with oxygen no metallic Ago species were detected and the greatest amount of silver was hosted in the form of exchangeable Ag+ cations (1.53 Ag+/u.c). The latter value corresponded to the value from chemical analysis (1.60 Ag/u.c.). This is an important point validating the accuracy of our experimental procedure for the quantification of silver sites in Ag-exchanged zeolites. Similar procedures of the quantitative IR studies were also developed for copper47 cobalt49, and nickel species50 in zeolites MFI, FER, and FAU type.

+

o

Figure 8. The spectra recorded at RT representing the maximum intensities of the Ag (CO) (A) and Ag (CO) (B) bands in studied zeolites. They were recorded upon the saturation with CO of all cationic Ag+ and metallic Ago species in each Ag-zeolite. Spectra normalized to the 10 mg mass sample. Table 4. Concentration of Ag+ and Ago species in studied zeolites attained from IR quantitative experiments and the total concentration of Ag derived from chemical analysis. zeolite AgZSM-5/ox AgZSM-5 AgZSM-5/red AgX AgY AgZSM-5

IR studies Ag/u.c. + o Ag Ag 1.53 0 0.55 0.71 0.01 0.57 7.07 0 4.55 0 0.55 0.71

+

Ag + Ag 1.53 1.26 0.58 7.07 4.55 1.26

o

Chemical analysis Ag/u.c 1.60 38.0 43.0 1.60

According to the data presented in the Table 4, the concentration of the Ag+ cations in photoreduced AgZSM-5 was lowered (0.55 Ag+/u.c.) what is accompanied by the appearance of considerable 16 ACS Paragon Plus Environment

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amount of metallic species (0.71 Ag+/u.c.). The silver metallic clusters can evolve both from photoreduction and autoreducion process taking place under vacuum treatment at high temperature. The overall stoichiometry of autoreduction can be represented as34: 2Ag+ Si(O-)Al = 2Ag0 + 1/2O2 + Si+(…)Al, where the Si+(…)Al represents a fragment of zeolite framework with a missing oxygen link, i.e. with a Lewis acidic site. Treatment of zeolite with H2 resulted in the reduction of the rest of Ag+ cations to metallic moieties: the Ag+ species nearly vanished (2192 cm–1). Reduction of Ag+ cations was not accompanied by the increase of the amount of metallic species; their concentration decreased to 0.57 Ag+/u.c.. This phenomenon can be explained by the agglomeration of metallic species induced by the high temperature of reduction with hydrogen: the Ag0 species tend to interact with other Ag0 and Ag+ species and aggregate. Finally, owing to less severe reduction conditions the dispersion of metallic species in photoreduced sample is more assessed. Agglomeration of metallic species is also responsible for the discrepancy between the amount of Ag determined by IR spectroscopy and chemical analysis. It pointed to lower dispersion of metallic silver species: 68% of Ago was detected by CO in photoreduced sample, while double fold lower amount (36%) of Ago is available for CO molecule; the rest is hidden inside bulk phase. 3.2.2

Zeolites of the different topology

Concentration of silver sites in zeolites AgZSM-5, AgX, AgY was presented in the Table 4. In Agfaujasite type of zeolites the only kind of species detectable by carbon monoxide were bare Ag+ cations in exchange positions. However, Sayah et al.51 reported the high instability of part of the ionic silver species supported on porous oxides. Auto-reduction and/or chemically induced reduction processes have been reported to easily take place in zeolite, even at room temperature. Especially, in zeolites AgNaX29,

51

and NaAgA52,

53

a part of silver was reduced as soon as dehydration started,

leading to a covering of the surface of the zeolite crystals by small nanoparticles in which Ag oxide and reduced Ago can coexist. The

109

Ag solid state NMR studies have been also shown that the

reduction occurs during dehydration of Ag+ exchanged faujasite under flowing O2 at 360 °C followed by evacuation at 400 °C. Thus, it was proposed that framework distortions and the high thermal mobility of Ag+ species contribute to the autoreduction of silver upon water removal.54 Nevertheless, in studied zeolites AgX and AgY the majority of silver species, both cations and metal clusters, can be hidden inside hexagonal prisms and/or cubooctahedra, thus the concentration of silver moieties, found in quantitative IR studies, was substantially lower when comparing with value 17 ACS Paragon Plus Environment


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gained from chemical analysis. It is due to strong affinity of the Ag+ cations for site SI, SI’ and SII. Additionally, the Ag+ cations do not always respect the rule stating that sites SI and SI’ cannot be occupied simultaneously in the same hexagonal prism and this phenomenon could be explain by weak Ag+···Ag+ interaction and the formation of Ag clusters (Ag3+ ; Ag32+; Ag33+). The metallic species Ag0, if appears, are able to migrate and aggregate on the zeolite surface. This migration is enabled by the negative charge density of framework. Reduced species Ag0 also tend to interact with other Ag0 and Ag+ moieties and aggregate. Taking into account a strong affinity of silver to sites in prisms and sodalite cages, the Ag0 species formed in zeolites X and Y are trapped inside of hexagonal prisms and the size of the prisms also brings length constraints on the size of these metal particles (much less than 40 Å). Despite the fact that the zeolites AgZSM-5, AgX, AgY were photoreduced (grey and dark grey colour of samples) the metallic species were detected only for AgZSM-5 zeolite. Taking into account that the Ago species tend to agglomerate and the absorption coefficient of the Ago(CO) band was of the small value, the metallic sites even being present in photoreduced samples were represented by the band of very low or no detectable intensity. Interestingly, the re-oxidation with O2 at 400 oC, which was previously applied for zeolite AgZSM-5, was not efficient for zeolites AgX and AgY. Even though the Ago species have been oxidized at high temperature (the sample colour changed from dark grey to white), their auto-reduction started at 200 oC. In comparison with native photoreduced AgX and AgY any additional Ag+ species were not detected at room temperature; the speciation of silver species remains the nearly unchanged.


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4

CONCLUSIONS

A procedure based on IR studies of CO molecules adsorbed on silver exchanged zeolites offered a new quantitative tool for the speciation analysis of silver in zeolites. The values of the absorption coefficients of IR bands of CO interacting with exchangeable cations (Ag+) and metallic species (Ago) were attained, and subsequently the concentrations of silver species of different kinds were calculated in zeolites AgZSM-5, AgX and AgY. The values of the absorption coefficients are valid for zeolites of FAU and MFI topologies thus they are supposed to be used for zeolite of other structures.

ACKNOWLEDGMENT This work was financed by Grant No. 2013/09/B/ST5/00066 from the National Science Centre, Poland. SUPPORTING INFORMATION AVAILABLE X-ray diffraction (XRD) patterns of the samples. IR spectra of recorded after the sorption of CO at room temperature and at -100 oC in zeolite AgZSM5 hosting Ag+ and H+ cations. IR spectra recorded after the sorption of CO doses in zeolite NaAgZSM-5 hosting Na+ and Ag+ cations. The procedure description according to the value of absorption coefficient of the 2157 cm–1 Cu+(CO) band was obtained from its integral intensity (in cm/µmol). This material is available free of charge via the Internet at http://pubs.acs.org.



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