Microporous and Mesoporous Materials 162 (2012) 175–180

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Microporous and Mesoporous Materials

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Copper sites in zeolites - quantitative IR studies

K. Góra-Marek a,⇑, A.E. Palomares b, A. Glanowska a, K. Sadowska a, J. Datka a

a Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Polandb Instituto de Tecnología Química, Universidad Politécnica de Valencia, Camino de Vera s.n., 46022 Valencia, Spain

a r t i c l e i n f o

Article history:Received 7 May 2012Received in revised form 19 June 2012Accepted 20 June 2012Available online 28 June 2012

Keywords:IR spectroscopyCu-exchanged zeolitesCO sorption

1387-1811/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.micromeso.2012.06.029

⇑ Corresponding author. Tel.: +48 12 663 20 81; faxE-mail address: gorak@chemia.uj.edu.pl (K. Góra-M

a b s t r a c t

Quantitative IR studies were realized in order to determine the concentration of various kinds of Cu sitesin zeolites CuZSM-5, CuITQ-2, CuTNU-9, and CuY. These zeolites contained Cu in the form of Cu2+, Cu+ (inthe form of exchange cations Cu+

ex. and oxo-form Cu+ox) and metallic Cuo (in reduced samples). The

experimental conditions in which known amounts of CO (used as probe molecule) reacted with each kindof Cu sites in CuZSM-5 were elaborated and subsequently the extinction coefficients of IR band of CO inCun+(CO) adducts were established. Following values of the extinction coefficients of monocarbonylbands were obtained: 0.11 cm2/lmol for Cu2+(CO), 1.30 cm2/lmol for Cu+

ex(CO), 0.91 cm2/lmol forCu+

ox(CO), and 0.79 cm2/lmol for Cuo(CO). The procedure elaborated for CuZSM-5 was also applied forother zeolites. Consequently, the concentrations of Cu2+, Cu+ both in exchange positions and oxo-forms,as well as of Cuo were determined in CuZSM-5 treated at various conditions (reduced, oxidized and trea-ted with H2O) as well as in zeolites CuZSM-5, CuITQ-2, CuTNU-9, and CuY.

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1. Introduction

The most widely applied methods of the controlling of NOx emis-sions are the selective catalytic reduction by NH3 [1] or urea [2] andthe selective catalytic reduction of NOx with hydrocarbons in thepresence of oxygen [3–7]. Main attention has been paid to the sup-ported copper catalysts which have been widely studied as a conse-quence of their high catalytic activity in the SCR NOx and the lowcost of the copper. On the other hand, Cu-exchanged zeolites donot present enough hydrothermal stability for commercial applica-tion [6], so that many efforts are concentrated on Fe [8], Co [9,10],and Ni [11,12] cations hosted in zeolites because of their high activ-ity, even in the presence of large amounts of water. The Cu+ ions inzeolites were also found to activate multiple bonds in organic mol-ecules such as alkenes, benzene, and aldehydes [13,14]. The bondweakening was realized by p back donation of electrons from Cu+

(enhanced by the interaction with zeolite framework) to p⁄ anti-bonding orbital of molecule. In general, two types of transition me-tal ions (TMI) centers can be distinguished: isolated mononuclearTMI cations present in exchange positions, and oxo-like clustersMxOy of poorly defined stoichiometry. The concentration of bothisolated and clustered centers can be adjusted by the modificationof the zeolite type, its Si/Al, and the TMI concentration.

To attain information about the oxidation state of the coppersites, IR spectroscopy with the use of the probe molecules suchas CO and NO is usually applied. Many studies has been devoted

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: +48 12 634 05 15.arek).

to CO adsorption on copper-based catalysts but there are still manyproblems under debate, e.g. the discrimination between the vari-ous copper sites such as Cu2+, Cu+ and Cuo. It is well known thatCu+ cations in exchange positions, when interacted with carbonmonoxide, produce highly stable monocarbonyls adducts Cu+(CO)that appear at 2150–2160 cm�1 [15,16]. When the equilibriumCO pressure increases the Cu+(CO) monocarbonyls are transformedinto dicarbonyls Cu+(CO)2 and further into tricarbonyls Cu+(CO)3

when temperature reaches 170 K [15,16]. The IR bands of CObonded to bulk Cuo and Cu+ in the form of the oxide-species appearusually about 2130 cm�1 and they are characterized by low stabil-ity; they are decomposed by the evacuation at room temperature.The interaction of CO with Cu2+ species results in the formation ofCu2+(CO) adducts, that are unstable and they are detectable only at170 K after CO sorption at low temperature as a consequence of theelectrostatic nature of the interaction of CO with Cu2+ species[17,18].

The IR spectroscopy offers also the possibility of the quantita-tive determination of the concentration of various cationic sites[19,20] but the studies of the copper sites in this matter have beennot delivered up to now.

The aim of the present study was to create a tool for determina-tion the absolute values of the concentrations of various kinds ofcopper sites. The experimental conditions in which known amountof probe molecule (carbon monoxide) interacted with each type ofcopper sites were established. The values of the extinction coeffi-cients of the Cu2+(CO), Cu+

ex(CO), Cu+ox(CO), and Cuo(CO) monocar-

bonyls bands were determined. Subsequently, the concentration ofCu+ both in the exchange positions and in the oxo-forms as well as

176 K. Góra-Marek et al. / Microporous and Mesoporous Materials 162 (2012) 175–180

the concentration of various copper sites (Cu2+, Cu+ and Cuo) werecalculated and compared with the amount of Cu determined bychemical analysis.

Most of results obtained in this study concerns the effect ofreduction, oxidation and hydrolysis on CuZSM-5 zeolite. The appli-cation of the procedure of the quantitative IR studies of coppersites in CuZSM-5 was also tested for Cu-exchanged zeolites withdifferent topologies (CuITQ-2, CuTNU-9). Zeolites with 10- and12- membered ring channels, with the metal ion exchanged andwith a Si/Al ratio between 8 and 35 are reported as the adequatezeolite-based catalyst for the SCR of NO with propane [21]. Addi-tionally, zeolite CuY was also studied.

Fig. 1. The IR spectra of CO sorbed at room temperature in zeolite CuZSM-5. (a)Thermally treated at vacuum (820 K). (b) Treated with water at 670 K (followed byevacuation at the same temperature). (c) Treated with H2 at 1070 K (followed byevacuation at 820 K). (d) Treated with O2 at 820 K (followed by evacuation at thesame temperature) – CO sorption at 170 K.

2. Experimental

Zeolite CuZSM-5 was obtained from NaZSM-5 (Si/Al = 35, Zeo-lyst). FAU zeolite (Si/Al = 11.3) was a commercial zeolite obtainedfrom PQ Zeolites, ITQ-2 zeolite (Si/Al = 19.8) was prepared in ITQlaboratory [22], TNU-9 zeolite (Si/Al = 14.4) was synthesized using1,4-bis(N-methylpyrrolidinium)-butane and Na+ cations as thestructure-directing agents according to the procedure describedelsewhere [23]. The metal exchange in zeolites was done in anaqueous solution containing the appropriate concentration ofCu(CH3COO)2 to achieve the desired amount of metal on the zeolite,and with a solid/liquid ratio = 10 g � l�1. Previously to the ion ex-change, the zeolites were rinsed with a 0.04 M solution of NaNO3

in order to have the zeolites in the sodium form. The exchangewas carried out under stirring for 24 h, at room temperature. Afterthat, the zeolites were filtered and washed, and then calcined at550 �C for 3 h. The composition of the samples was analyzed byICP-O ES (Varian 715-ES) revealing the Cu/Al equal 0.47, 0.45,0.62, and 0.67 for zeolites CuY, CuZSM-5, CuITQ-2, and CuTNU-9,respectively.

The thin wafers of Cu-zeolite (5–10 mg/cm2) were activated inIR cell at 770 K at vacuum for 1 h. Thermal treatment of CuZSM-5in vacuum is associated to the autoreduction process when themajority Cu2+ species is reduced to Cu+ ions.

The IR spectra were recorded with a Bruker Tensor 27 spec-trometer (equipped with an MCT detector) with the spectral reso-lution 2 cm�1.

Carbon monoxide (PRAXAIR 9.5) was used as adsorbate.Zeolites used are marked according formula: zeolite type|Si/

Al|Cu/Al|treatment conditions.

3. Results and disscusion

3.1. The interaction of CO with Cu sites in CuZSM-5 zeolites

The spectra of CO interacting with Cu+ sites in the exchangepositions and in the oxide forms (Cu+

ex and Cu+ox) as well as with

Cu2+ and Cuo in zeolites CuZSM-5 thermally treated at vacuum,treated with water at 670 K, reduced and oxidized are presentedin Fig. 1.

Interaction of CO with copper sites in zeolite thermally treatedat vacuum at 820 K results in the appearance of a strong band at2157 cm�1 of Cu+

ex(CO) at low CO loadings whereas much weakerband at 2130 cm�1 of Cu+

ox(CO) appears when CO is sorbed inhigher amounts (Fig. 1, spectrum a). It suggests that almost all cop-per ions are in the exchange positions and the Cu+ in oxide form ismuch less populated.

Treatment of zeolite with water at 670 K results in the transfor-mation of some exchangeable cations Cu+

ex into Cu+ox in oxo-

forms. It is well seen in the spectrum of CO sorbed in water treatedzeolite (Fig. 1, spectrum b) exhibiting the increase of Cu+

ox(CO)

band accompanied by the Cu+ex(CO) band decrease (in comparison

with zeolite thermally treated at vacuum - spectrum a).Treatment of CuZSM-5 zeolite with H2 at 1070 K results in the

reduction of Cu+ to metallic Cuo. The spectrum of CO sorbed atroom temperature (Fig. 1, spectrum c) reveals a distinct decreaseof the Cu+

ex(CO) band at 2157 cm�1 and appearance of the bandof the Cuo(CO) monocarbonyls at 2124 cm�1.

Reaction of CuZSM-5 zeolite with oxygen at 820 K leads to oxi-dation of some Cu+ in exchange positions to Cu2+ species. It is man-ifested by the decrease of the band of Cu+

ex(CO) and the appearanceof a weak band at 2206 cm�1 of the Cu2+(CO) monocarbonyls in thespectrum of CO sorbed (Fig. 1, spectrum d). Vibrational frequencies(in cm�1) of copper carbonyl species are presented in Table 1.

3.2. Extinction coefficients of CO interacting with Cu+, Cu2+ and Cuo

species

The key problem in the determination of extinction coefficientof IR band of CO interacting with Cu species of different nature isthe elaboration of the appropriate experimental conditions. Theknown amounts of CO are demanded to react with Cu sites forwhich the extinction coefficient of Cun+(CO) band is to be deter-mined. The simplest and the most desired is the situation whenonly one kind of Cu sites is able to bond CO molecules. On the otherhand, when two (or more) kinds of the adsorption sites are presentit is necessary to know the amount of CO interacting with all kindsof sites.

In our experiments, at relatively low carbon monoxide loadings,all CO molecules introduced into the cell interacted with coppersites in zeolite; the vibration–rotation spectrum of gaseous COwas not detected. Only such spectra (presented in Figs. 2–5) weretaken for the determination of the extinction coefficients.

The sorption of small doses of CO in zeolite CuZSM-5 thermallytreated at vacuum at 830 K obeys the rule of the selective reactionof probe molecule with one type of the adsorption sites i.e. Cu+

ex.As mentioned above, this is the simplest and the most desirable sit-uation. Selective reaction of CO with Cu+

ex is manifested by thedevelopment of only one band of the Cu+

ex(CO) at 2157 cm�1

(Fig. 2A). The intensity of this band increased linearly with theamount of CO sorbed (Fig. 2B) and the slope of this line was taken

Table 1Observed C„O stretching modes of copper carbonyl species on zeolites.

Frequency/cm�1 Type of adduct Stability

2206 Cu2+(CO) Monocarbonyls of Cu(II) oxo-species Stable at low temperatures (170 K)2177 and 2150 Cu+

ex(CO)2 Dicarbonyls of Cu(I) in the exchange positions Stable at RT but easy transforming into Cu+ex(CO) by evacuation

2157 Cu+ex(CO) Monocarbonyls of Cu(I) in the exchange positions Stable to 670 K

2130 Cu+oxo(CO) Monocarbonyls of Cu(I) oxo-species Stable at room temperature

2124 Cuo(CO) Monocarbonyls of metallic copper Stable at RT, easily decomposing by evacuation at RT

A

B

Fig. 2. (A) The spectra recorded upon the sorption of doses CO in CuZSM-5thermally treated at vacuum at 820 K. (B) The intensity of Cu+

ex(CO) band at2157 cm�1 as function of the concentration of CO sorbed.

A

B

Fig. 3. (A) The spectra recorded upon the sorption of doses CO in CuZSM-5 treatedwith water at 670 K (followed by evacuation at the same temperature). (B) Theintensity of Cu+

ox(CO) band at 2130 cm�1 as function of the concentration of COinteracting with Cu+

ox.

K. Góra-Marek et al. / Microporous and Mesoporous Materials 162 (2012) 175–180 177

as the extinction coefficient of the 2157 cm�1 Cu+ex(CO) band. The

value 1.30 cm2/lmol was obtained. It should be noted that similarvalue of the extinction coefficient (1.10 cm2/lmol) was obtained inour earlier study for Ni+(CO) band [20].

The determination of the extinction coefficient of the Cu+ox(CO)

at 2130 cm�1 is more demanding. As the amount of the Cu+ox is

very low in CuZSM-5 thermally treated at vacuum (low intensityof the 2130 cm�1 band - Fig. 1, spectrum a), hence the amount ofthe Cu+

ox species was enhanced by the hydrolysis (treatment withwater at 670 K – Fig. 1, spectrum b). Carbon monoxide moleculessorbed in such prepared zeolite reacted at the same time withCu+

ex and Cu+ox – the bands at 2157 and 2130 cm�1 developed

simultaneously with increasing amounts of CO (Fig. 3A). Theamounts of CO engaged into interaction with Cu+

ex were calculatedfrom the intensity of the 2157 cm�1 band and the extinction coef-ficient (1.30 cm2/lmol) determined as described in previous para-graph. The amounts of CO bonded to Cu+

ox was estimated as thedifference between the total amount of CO introduced into the celland the amount of carbon monoxide interacting with Cu+

ex. TheCu+

ox(CO) band intensity at 2130 cm�1 was represented in Fig. 3Bas a function of CO (lmol per cm2) interacting with Cuox. Theintensity of the 2130 cm�1 band was found to increase linearlywith the amount of CO forming Cu+

ox(CO) adducts (Fig. 3B). Theslope of this line was taken as the extinction coefficient of theCu+

ox(CO) band. The value 0.91 cm2/lmol was obtained.

The experiments of CO sorption in CuZSM-5 zeolite reduced bythe treatment with H2 at 1070 K were also carried out. Moleculesof CO introduced into the cell reacted simultaneously only withCu+

ex and the Cu+ex(CO) band at 2157 cm�1 appeared. At higher

loadings, molecules of CO are bonded to metallic copper and theband of Cuo(CO) at 2124 cm�1 is developed (Fig. 4A). This indicatesthat Cu+

ex sites bond CO stronger than Cuo ones. The amount of COinteracting with Cuo was calculated as the difference between thetotal amount of CO sorbed and the amount of CO engaged into theinteraction with Cu+

ex. The 2124 cm�1 band intensity was repre-sented in Fig. 4B as a function of CO (lmol per cm2) interacting withCuo. Linear dependence of the intensity of the Cuo(CO) band on theamount of CO in Cuo(CO) adducts (Fig. 4B) was obtained and theslope of this line was taken as extinction coefficient of the2124 cm�1 Cuo(CO) band. The value 0.79 cm2/lmol was obtained.

The extinction coefficient of CO interacting with divalent Cu2+

species in zeolite CuZSM-5 enriched with Cu2+ ions by the treat-ment of CuZSM-5 with O2 at 820 K was also determined. Moleculesof CO sorbed at 170 K in such oxidized zeolite reacted simulta-neously with both Cu+

ex, which survived oxidation, and Cu2+. TheCu+

ex(CO) band at 2157 cm�1 appeared together with very weekone at 2206 cm�1 of the Cu2+(CO) monocarbonyls (Fig. 5A). As inprevious cases, the amounts of CO reacting with Cu+

ex were calcu-lated from the intensity of the Cu+

ex(CO) band at 2157 cm�1 and itsextinction coefficient. Consequently, the amount of CO bonded to

A

B

Fig. 4. (A) The spectra recorded upon the sorption of doses CO in CuZSM-5 treatedwith H2 at 1070 K (followed by evacuation at 820 K). (B) The intensity of Cuo(CO)band at 2124 cm�1 as function of the concentration of CO interacting with Cuo.

A

B

Fig. 5. (A) The spectra recorded upon the sorption of doses CO in CuZSM-5 treatedwith O2 at 820 K (followed by evacuation at 820 K) – CO was sorbed at 170 K. (B)The intensity of Cu2+(CO) band at 2206 cm�1 as function of the concentration of COinteracting with Cu2+.

178 K. Góra-Marek et al. / Microporous and Mesoporous Materials 162 (2012) 175–180

Cu2+ was estimated as the difference between the total amount ofCO introduced into the cell and the amount of CO interacting withexchangeable Cu+. The intensity of the 2206 cm�1 band was plot-ted as function of the amount of CO interacting with Cu2+ sites(Fig. 5B). The dependence was linear and the slope was taken asthe extinction coefficient of Cu2+(CO) band. The value 0.11 cm2/lmol was obtained. It should be noted that the value of the extinc-tion coefficient of Cu2+(CO) band is similar to the values of theextinction coefficients of the bands of CO interacting with otherdivalent ions of transition metals both in the form of the exchange-able cations and oxide forms, e.g. 0.12 cm2/lmol for Co2+

ex(CO)[19], 0.13 cm2/lmol for Co2+

ox(CO) [19], and 0.14 cm2/lmol forNi2+

ex(CO) complexes [20] hosted in ZSM-5.Our results [19,20,24] as well as the results of other authors

[25–28] evidenced that the frequency of CO bonded to isolatedmonovalent Cu+ and Ni+ cations (2157 and 2109 cm�1, respec-tively) was distinctly lower than for divalent Cu2+ (2206 cm�1),Ni2+ (2220 and 2212 cm�1), and Co2+ (2210 and 2206 cm�1) cat-ions. It evidences that p back donation of d electrons from cation(enhanced by the interaction of cation with zeolite’ framework)to antibonding p⁄ orbital of CO (weakening of C„O bond) is moreeffective for monovalent cations than for divalent ones. Therefore,the strong p back donation effect and the synergism between rand p back donation [17,18] is believed to be responsible fortenfold increase of the extinction coefficients of IR bands of COinteracting with monovalent Cu+ and Ni+.

3.3. Concentration of Cu sites in diverse zeolites after differenttreatments

To determine the concentration of the Cu2+, Cu+ex, Cu+

ox and Cuo

sites in CuZSM-5, CuITQ-2, CuTNU-9, and in CuY, the doses of car-bon monoxide CO was sorbed until maximum intensities of theCu2+(CO), Cu+

ex(CO), Cu+ox(CO) and Cuo(CO) monocarbonyl bands

were achieved. The concentration of Cu sites was calculated fromthese maximum intensities and their extinction coefficients.

3.3.1. Concentration of Cu sites in CuZSM-5 zeolite treated with H2, O2

and H2OThe values of concentration of Cu sites (Cu2+, Cu+

ex, Cu+ox, and of

Cuo) in zeolite CuZSM-5 thermally treated at vacuum, treated withH2 at 770 K and at 1070 K, as well as with O2 at 820 K and H2O at620 K are presented in Table 2. The concentration of Cu determinedby chemical analysis is presented as well.

In CuZSM-5 zeolite thermally treated at vacuum most of copperis in the form of exchangeable Cu+ cations (Cu+

ex(CO) band at2157 cm�1). Only small amount of copper exists in the oxide form(Cu+

ox(CO) at 2130 cm�1) and in the Cu2+ form (Cu2+(CO) band at2206 cm�1). Total concentration of all Cu sites (1.06Cu/u.c.) deter-mined in our quantitative IR studies corresponds to the value re-ceived from chemical analysis (1.19Cu/u.c.). This is an importantpoint validating the accuracy of our experimental procedure forthe quantification of different copper sites in Cu-exchanged zeo-lites. The procedure of the quantitative IR studies was also offeredfor cobalt and nickel cations hosted in ZSM-5, FER, and Y [19,20].

According to the data presented in the Table 2 the treatment ofzeolite with H2 at 770 K resulted in the reduction of Cu2+ to Cu+:the Cu2+ species vanished whereas the concentration of the Cu+

in the exchange positions increased a little. Reduction of theexchangeable Cu+ to metallic Cuo did not occur at 770 K. However,the reduction of Cu+ to metallic Cuo took place when zeolite wastreated with H2 at 1070 K: the concentration of Cu+ decreased sub-stantially and Cuo metallic species (IR band of Cuo(CO) at2124 cm�1) were formed. The amount of Cuo (0.43Cuo/u.c.)

Table 2The values of extinction coefficient of the CO bands in adducts: Cu2+(CO), Cu+

ex(CO), Cu+ox(CO), Cuo(CO). The concentrations (per unit cell) of Cu2+, Cu+ both in exchange positions

and oxo-forms, as well as of Cuo determined in IR studies in CuZSM-5 zeolite treated in various conditions and Cu determined by chemical analysis.

Extinction coefficient (cm2/lmol)

Cu2+(CO) Cu+ex(CO) Cu+

ox(CO) Cuo(CO)0.11 1.30 0.91 0.79Concentration of Cu sites determined by IR studies Chem. analysis

Cu2+ Cu+ex Cu+

ox Cuo RCu/u.c. Cu/u.c.

CuZSM-5/35/0.45 0.11 0.9 0.05 0 1.06 1.19CuZSM-5/35/0.45/[770/H2] 0 0.95 0.05 0 1.00 1.19CuZSM-5/35/0.45/[1070/H2] 0 0.1 0 0.43 0.53 1.19CuZSM-5/35/0.45/[O2] 0.76 0.35 0.06 0 1.17 1.19CuZSM-5/35/0.45/[H2O] 0 0.13 0.85 0 0.98 1.19

K. Góra-Marek et al. / Microporous and Mesoporous Materials 162 (2012) 175–180 179

produced was smaller than the amount of the Cu2+ and Cu+ lost(0.96Cu/u.c.), hence the metallic Cu clusters are supposed to beformed. In such clusters some Cu atoms are hidden inside clusterbeing inaccessible to CO (or reactant) molecules. The treatmentof CuZSM-5 zeolite with H2 at 1070 K results also in the increaseof the band of the acidic Si(OH)Al groups (3610 cm�1) – Fig. 6,spectrum b. The additional amount of the bridging hydroxyls issupposed to be formed in the reaction: 2Cu+ + H2 = 2Cuo + 2H+.The zeolite framework which was neutralized by Cu+ before thereduction is now neutralized by protons producing acidic Si(OH)Algroups.

The treatment of CuZSM-5 zeolite with O2 at 820 K results in theoxidation of Cu+ to Cu2+. The amount of the Cu2+ species formed(0.65Cu2+/u.c.) is comparable to the amount of Cu+ lost in the oxida-tion process (0.55Cu+/u.c.). The intensity of the Si(OH)Al band(Fig. 6, spectrum c) remains unchanged suggesting that the globalelectrical charge of the Cu species did not change and monovalentCu+ was oxidized to oxygen containing species such as Cu2+O2�Cu2+

(or similar).

Fig. 6. The spectra of OH groups in CuZSM-5 thermally treated at vacuum (820 K)(a), reduced with H2 at 1070 K (followed by evacuation at 820 K) (b), oxidized withO2 at 820 K (followed by evacuation at the same temperature) (c), treated with H2Oat 670 K (followed by evacuation at the same temperature) (d).

Table 3The concentrations of Cu2+, Cu+ both in exchange positions and oxo-forms, as well as of Cuchemical analysis.

Si/Al Cu/Al Concentration of Cu sit

Cu2+ Cu

CuZSM-5 35 0.45 0.11 0.9CuITQ-2 19.8 0.62 0.35 1.3CuTNU-9 14.4 0.67 1.05 6.3CuY 11.3 0.47 0.35 1.3

The treatment of CuZSM-5 zeolite with water at 620 K trans-formed the exchangeable Cu+ into Cu+ in the oxide form (Cu+

ox).The amount of Cu+

ox formed by hydrolysis (0.80Cu+ox/u.c.) is the

same as the amount of Cu+ex lost (0.77Cu+

exch./u.c.). As in othercases, total concentration of all copper species determined in quan-titative IR studies (0.98Cu/u.c.) was comparable to the concentra-tion of Cu received from the chemical analysis (1.19Cu/u.c.).Transformation of the exchangeable Cu+ into electrically neutraloxide species in the hydrolysis process is also related to the forma-tion of protons neutralizing zeolite framework. This is manifestedby the increase of IR band of acidic Si(OH)Al groups (Fig. 6, spec-trum d).

3.3.2. Concentration of Cu sites in Cu-zeolites with different topologyConcentration of copper sites (Cu2+, Cu+

ex and Cu+ox) in zeolites

CuZSM-5, CuITQ-2, CuTNU-9 and CuY is presented in the Table 3. Inall kinds of zeolites most of copper is in the form of bare Cu+ cat-ions in exchange positions. The amount of Cu2+ is relatively smalland reaches 10–31% of total Cu content. The nature of the Cu2+ spe-cies is not clear. The Cu2+ is usually believed to exist not as bareexchangeable cation but in the oxygen containing species suchCu2+O2�Cu2+ or similar. What is interesting, the smallest amountof the Cu2+ (10%) species is observed for CuZSM-5 of the highestSi/Al (Si/Al = 35), i.e. of the lowest density of AlO4

� tetrahedra. Itsuggests that long distances between AlO4

� tetrahedral in ZSM-5structure do not assure the stabilization even divalent charge ofCu2+O2�Cu2+ species. Zeolites of lower Si/Al (Si/Al = 11.3–19.8) i.e.of higher density of AlO4

� tetrahedra contain more Cu2+ species(21–31%).

In most of zeolites all copper takes the form of the exchangeablecations (Cu+

ex). The contribution of Cu in the oxide form (Cu+ox) was

more significant in CuITQ-2. The ITQ-2 is a laminar zeolite formedby sheets of 2.5 nm and with a high external surface area(386 m2/g). This external surface area is much higher than that ofthe other zeolites tested (e.g. 70 m2/g for ZSM-5). The copper sitesare more stabilized in a conventional zeolite framework that doesnot facilitate their mobility. On the contrary, if the copper speciesare in the external surface, their mobility is favored and the forma-tion of copper oxide is more feasible. Therefore, the presence of cop-per oxide in ITQ-2 of highly developed external surface is favored.

o determined in IR studies in zeolites with different topologies and Cu determined by

es by IR studies Chem. analysis

+ex Cu+

ox RCu/u.c. Cu/u.c.

0 0.05 1.06 1.190 0.15 1.80 2.205 0 7.40 8.400 0 1.65 7.30

180 K. Góra-Marek et al. / Microporous and Mesoporous Materials 162 (2012) 175–180

Total concentration of all copper forms determined in our IRstudies compares to the concentration of Cu determined by chem-ical analysis validating the accuracy of our experimental procedure.Additionally, this agreement points also to the independence theextinction coefficient value on the type of zeolite. Therefore, thevalues of the extinction coefficient of the bands Cu2+(CO), Cu+

ex(CO),and Cu+

ox(CO) determined for CuZSM-5 zeolite can be applied forthe determination of the concentration of copper sites in othertypes of zeolites.

Only in zeolite CuY, in which most of Cu is hidden inside hexag-onal prisms and/or cubooctahedra, the concentration of coppersites found in quantitative IR studies was substantially smallerthan that determined by chemical analysis.

4. Conclusions

1. A procedure based in IR studies of CO molecules adsorbed oncopper exchanged zeolites that allow the quantification of thevarious copper sites present in was established.

2. The extinction coefficients of IR bands of CO interacting withCu+

ex cations in exchange positions, Cu+ox in the form of oxide

forms, metallic Cuo and with Cu2+ were determined. Subse-quently, the concentrations of all forms of copper were estab-lished in CuZSM-5 zeolites treated at various conditions aswell as in zeolites of different types (CuITQ-2, CuTNU-9, andCuY).

3. Total concentration of all copper sites determined in our quan-titative IR studies was nearly the same as the concentration ofCu determined by chemical analysis. This is an important pointconfirming the accuracy of our experimental procedure ofquantitative IR studies.

4. The influence of reduction of zeolite CuZSM-5 with H2, oxida-tion with O2 and treatment with H2O on the concentration ofCu+

ex, Cu+ox., Cu2+ and Cuo was followed.

Acknowledgment

This research project was supported by Grant NO. 2011/01/B/ST5/00915 from the National Science Centre, Poland. A.E. Palo-

mares thanks the Spanish Government (projects MAT2009-14528-C02-01, and CONSOLIDER INGENIO 2010) for financialsupport.

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