Infrared spectroscopic evidence for two ways of adsorbed CO coordination

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

3143

Infrared spectroscopic evidence for two ways of adsorbed CO coordination A.A.Tsyganenko '~, C.Otero Arean b, and E.Escalona Platero b alnstitute of Physics, St. Petersburg State University, 198904 St. Petersburg, Russia bDep.de Quimica, Universidad de las Islas Baleares, 07071, Palma de Mallorca, Spain By FTIR spectroscopy at variable temperatures it was shown that CO interacts with extraframework alkali or Ca 2§ cations in zeolites to form both M...CO and M..-OC species that are in a temperature dependent equilibrium. The C-bonded species have always higher adsorption energy and account for the strong high-frequency vCO band, while the O-bonded structures reveal themselves in a weak band shifted to lower wavenumbers with respect to the free CO molecule. The difference between the energies of C- and O-bonded species depends upon the cation, and to some extent on the zeolite framework. These CO reorientation energy values, as well as the frequency shifts are well reproduced by a simple electrostatic model of dipole-cation interaction. For CaY zeolite, where two CO molecules can be adsorbed on the same cation, the presence of O-bonded molecules accounts for additional frequency increase of the adjacent CO, that could be interpreted as adsorption-induced increase of Lewis acidity. 1. INTRODUCTION The catalytic properties of zeolites, besides the framework structure, depend on the nature of interaction between the adsorbed molecules and extra-framework cations (see, e.g. Refs. [ l, 2]. Electrostatic fields created by these cations and surrounding framework anions polarize the adsorbed molecules affecting their electron structure and reactivity. Interaction between CO and zeolites is of particular interest, since carbon monoxide is not only a reactant in a great deal of important catalytic processes, but also a widely used probe molecule for IR spectroscopic studies of surface properties of adsorbents and catalysts. Physical parameters of this molecule are well known from the experiment and quantumchemical calculations. This facilitates interpretation of spectroscopic data and enables one to obtain information on the strength and concentration of surface acidic sites of oxide adsorbents, lateral interaction between adsorbed molecules and to estimate the electric fields in zeolites from the IR spectra of adsorbed CO [3, 4]. Low-temperature IR spectra of CO adsorbed on alkali-metal-exchanged zeolites show a main absorption band shifted upward with respect to the value of a free CO molecule (2143 cm~). The magnitude of the shift depends on the alkali metal cation. Thus, for the M+-ZSM-5 zeolites this high-frequency (HF) band was observed at wavenumbers from 2157 cm"l to 2188 cm l [5, 6]. Both experimental and theoretical results show that this I-IF band corresponds to the fundamental C-O stretching mode of carbon monoxide perturbed by the electrostatic field created by cations and surrounding framework anions in M+-CO adducts [6-8]. Besides the HF band, a minor low frequency (LF) band was also noted in the 1R spectra of zeolites containing adsorbed CO [6, 9, 10]. This LF band, which appears below 2143 cm-l, is also cation-specific, displaying for the above series of M+-ZSM-5 zeolites greater frequency shifts for the systems with the higher position of the HF CO band.

3144 Several authors attribute the LF band to the C-O stretching mode of M+-OC adducts [8, 9]. Indeed, formation of such o-bonded species should lead to a low-frequency shift of the C-O stretching vibration [ 11]. Ability of CO to form H-bonded complexes both via C and O atoms with hydrogen fluoride is well established [12]. O-bonded complex for such system is less stable and has lowered C-O stretcing frequency. For zeolites CO adsorption via C or O atoms could occur either on different sites, or both the structures coexist in thermodynamic equilibrium, like for the H-bonded complexes with hydrogen fluoride [ 12]. The aim of our study was to answer this question from the analysis of temperature dependence of IR spectra of CO adsorbed on Na-ZSM-5, Na-Y and CaY zeolites. Some results have already been reported [ 13]. Here we would like to complete and summarise the obtained data and to discuss possible consequences of these observations for catalysis. 2. EXPERIMENTAL Na-ZSM-5, Na-Y, and CaY zeolites were synthesized following standard methods [13]. For IR studies, thin self-supported wafers were prepared and activated in a dynamic vacuum for 2h at 680 inside the low-temperature IR cell described elsewhere [ 14]. For better thermal contact between the zeolite wafer and the cooled environment, about 0.5 Torr of helium was then admitted into the sample compartment. After recording the background spectrum at liquid nitrogen temperature, CO was dosed up to an equilibrium pressure of about 1 Torr. Then the cell was closed and IR spectra were registered at 77 and on gradual warming up of the cell after removal of liquid nitrogen. Spectra were taken with about 10 K intervals for the whole temperature range from 77 to 303 K for Na-zeolites and to 368 K for CaY. To register spectra at elevated temperatures, the coolant compartment was filled with hot water. Repeated cooling of the cell demonstrated the reversibility of the observed changes with temperature. Transmission IR spectra were recorded by means of a Bruker IFS66 FTIR spectrometer with 3 cm 1 resolution. The zeolite background spectrum, taken at 77K before CO admitting, was subtructed to obtain the pure spectra of adsorbed CO. Temperature was measured by means of a platinum resistance thermometer inserted in the coolant compartment with the estimated accuracy not worse than +_5 K. 3. RESULTS Fig. 1 shows some selected spectra for CO adsorbed on Na-Y. The HF and LF bands are observed at 2171 and 2122 cm "1, respectively. The HF band has distinct shoulders at 2156 and 2183 cm ~. Complex nature of the LF band is also observable in some of the spectra in Fig. 1. It should be expected, because each component of the HF band could be supposed to have its own LF maximum. At 77 K intensity of LF band is extremely low. However on raising the temperature, when the HF band starts to decrease, the LF band first exhibits intensity increase (curves 1-3). Then, at higher temperatures (spectra 5- 7) both bands decrease rapidly, but the ratio of integrated intensities of the LF and HF bands grows over the whole temperature range, from 77 to 303 K. The same temperature dependence was observed in the case of Na-ZSM5 zeolite, where the HF and LF bands appear at 2178 and 2112 cm q [13]. One more weak band centred at 2130 cm 1 was attributed to the ~3CO counterpart of the main HF band, since its position 13 exactly corresponds to that one expected from the reduced mass increase on C substitution. 13, For CO/Na-Y the most intense CO band has to overlap with the LF band, because the separation between HF and LF peaks practically coincides with the isotopic shift. For this reason, for the intensity measurements the ALF values for NaY were corrected by subtracting 1% (the natural abundance of the 13CO isotope) of the corresponding Ant values. - "

3145 .2

i

i

HF "

1

-1.5"

1.0

-2.0" 0.8

R2=0 9r6 SD=0.055

-2.5' 0.6

8 o

--~ -3.0.

R2=0.993 SD=0.050

C

0.4

-3.5 0.2 -4.0"

0.0 2220

2180 2140 Wavenumber, cm "1

2100

Fig. l FTIR spectra of CO adsorbed on NaY at 167(1), 177(2), 187(3), 197(4), 215(5), 225(6), and 233(7), K.

(1/T)10 a / K"1

Fig.2 Van't Hoff plots for CO/NaY (circles) and CO/Na-ZSM-5 (squares) systems.

Fig. 2 shows the van't Hoff plot of In(ALF/AHF) v e r s u s I/T for CO/Na-ZSM-5 and CO/NaY systems. One can see that for both the systems nice linear dependences take place. From the slopes of the corresponding lines, the values AH~ and 2.4 kJ m o l "1 w e r e derived for CO on Na-ZSM-5 and Na-Y zeolites, respectively. The straight lines intercept the vertical axis (1/T -- 0) in different points, at -0.26 for CO/Na-ZSM-5 and at -0.75 for CO/Na-Y. Spectrum of CO adsorbed on CaY zeolite (fig.3) is more complicated. At ambient or elevated temperatures the picture is close to that of Na + zeolites. Two predominating bands at 2197 and 2094 cm "] could be associated with the HF and LF forms of CO bound t o C a 2+ cations. At low temperatures the intensity of the 2197 cm 1 band reaches its maximum at about 240 K and one more band occurs at 2191 c m "1. The latter is the only intense CO band observed at 77 K when the 2197 cm 1 band is absent. It should be noted that below 260 K the intensities of these two bands change in opposite way: the increase of one is accompanied by the decrease of another, and an isobestic point could clearly be observed between the two maxima. The LF band at low temperatures is weak, while its maximum is shitted to 2099 cm 1. Besides this LF band, one more weak band can clearly be seen at 2211 cm ~. Its intensity is also negligible at 77 K and increases with growing temperature until most of CO is desorbed at elevated temperatures.

3146

77K ,.,

1.0-

221]

0-

":-,

............

;le!t !i'=" 'ii

.....

0r

i

.Q-

['

\

~

',

0

"\.

<

/~ " ~ .

~

I,, 1

_\\.

.. , . ~ 2 1 _ 0 0.0_ _______ . , . ~

2099 12094

"<

..................

2200

.,,. __.

21'50

_ _ <_-._.~

21'00 cm -1

Fig.3. IR spectrum of CO (about I ton') adsorbed at different temperatures on CaY zeolite pretreated at 680K. To show the weak bands, each curve is repeated expanded x20. 4. DISCUSSION

The observed in this work and reported in literature frequency values for the HF and LF bands for different zeolites as well as there shifts with respect to that of a gaseous CO (2143 cm l ) are summarized in table 1. Different positions of the main bands of CO adsorbed on the same cations in different frameworks, as follows from the comparison of Na or Ca forms of ZSM-5 and Y zeolites, illustrates the sensitivity of cation properties to the environment. Complex nature of the HF band for Na-zeolites suggests that Na § ions accessible to CO occupy different positions in zeolite framework. In Na-ZSM-5 the Na § sites are, evidently, more homogeneous than in Na-Y [ 15]. Table I System

v(HF)

v(LF)

Av(HF)

Av(LF)

Ref.

CO/Li-ZSM-5 CO/Na-ZSM-5 CO/K-ZSM-5 CO/Rb-ZSM-5 CO/Cs-ZSM-5 CO/H-ZSM-5 CO/Ca-ZSM-5 CO/Na-Y CO/Ca-Y

2188 2178 2166 2162 2157 2172 2176 2171 2198

2108 2112 2117 2119 2122 2115 2112 2122 2094

+45 +35 +23 + 19 +14 +29 +33 +28 +55

-35 -31 -26 -24 -21 -28 -31 -21 -49

6, 10 13 6, 10 6, 10 6, 10 10 16 Present work Present work

The observed by us temperature dependence of the ALF/AHFratio is precisely what could be expected for an equilibrium process between C-bonded and O-bonded species, as described by Equation (1), where Z stands for the zeolite framework. ZNa+-CO

K ~>

ZNa+-OC

( 1)

3147 The ratio of the O- and C-bonded species concentrations, measured as E;HFALF/E;LF AHF , should be equal to the equilibrium constant, K of the equation (1). The observed nearly perfect linear plot of In(ALF/AHF) versus 1/T infers the validity of van't Hoff equation (2), for constant AH~ and AS~ in the whole studied temperature range. In K = In(ALF/AnF)--In(eLF/eHF)= -AH~

(2)

+ AS~

Quantum chemical calculations of the enthalpy difference at absolute zero All ~ corresponding to the isomerization process between C-bonded and O-bonded species described by Equation (1), led to the values of 10.9 and 5.6 kJ mol "1 for the bare Na + cation [8] and the model [HAI(OH)3]'Na§ cluster [17], respectively. The experimentally determined values of AH~ or 2.4 kJ mo1-1, are even lower, showing that the zeolite framework leads to further diminishing of the energy difference between C- and O-bonded adducts. Since the computational estimate for AS ~ is two orders of magnitude smaller than the observed value [8], it can be assumed that the observed vertical displacement of straight lines in the van't Hoff plot is due to different values of the extinction coefficients of the two species. Then it follows that for CO/Na-ZSM-5 system ~LF/eHF=0.8, while for CO/Na-Y the corresponding ratio is eLF/eHF= 0.5. Temperature dependence of the spectrum of CO/CaY can be explained on a supposition that two CO molecules could be bound to the same Ca 2§ cation, and that this Ca(CO)2 species accounts for only one, the 2191 cm 1 band in the stretching CO region. Then the presence of isobestic point between the 2197 and 2191 cm -1 bands at lowered temperatures reflects the equilibrium between Ca-CO and Ca(CO)2 species, while at elevated temperatures the former monomeric adsorption predominates. If we adopt CO reorientation with the formation of Obonded species, then, taking into account negligible probability of Ca(OCh formation, the overall scheme will be as follows: +CO Ca < > Ca-CO

+co co < > Ca / \ CO

+co

Ca-OC t

CO

"- Ca / OC

O-bonded monomers Ca-OC could be associated with the 2094 cm~ band, observed at elevated temperatures when the corresponding Ca-CO band is the most intense. Then the 2099 cm ~ band should be attributed to O-bonded molecules in Ca(CO)OC dimeric structures, while the observed simultaneously 2211 cm 1 band can be assigned to the adjacent C-bonded molecule. The increased CO frequency due to the lateral interaction is not unexpected. While adsorption of second CO molecule via C atom on the Ca-CO fragment leads to a lowered band position of the newly adsorbed molecule, that testify for the weakening of the cation electric field, CO addition to the Ca-OC group with opposite direction of the dipole attached to the cation, on the contrary, results in the frequency increase. Thus, CO adsorption via oxygen on Ca 2§ cation results in the increase of its electric field, and adsorption of the next CO molecule results in a band at the position typical of strong Lewis sites. It should be noted that for CO interaction with cations a simple electrostatic model [ 11, 13] could be used where the frequency shitt Av of adsorbed CO with respect to a free molecule gives a measure of the electric field E. The latter could be obtained from the expression

3148 Av = ksx E

(3)

where ksx = 4.29 x 10.9 r ~ V"~ m is the vibrational Stark constant for CO. Then the energy difference AE between the two orientations of the CO dipole (0,1098 D) in this field can easily be calculated. From the Av(HF) for CO on Na-Y, Na-ZSM-5 systems such estimate gives 2.6 and 3.6 kJ mol ~, respectively, close enough to the experimental AH~ values. 5. CONCLUSIONS Using FTIR spectroscopy at variable temperatures, two coordination modes of carbon monoxide in zeolites which account for the HF and LF bands, were found to be due to M--.CO and M..-OC adducts with the same extraframework cations M, that co-exist in a temperature-dependent equilibrium Frequency shifts for the C-O stretching mode and the energy difference between these two structures could be described by a simple electrostatic model We believe that the observed effect takes place not only in zeolites and the existence of two different configurations is anticipated for other adsorbed heteroatomic molecules Thermally excited less stable O-bonded CO species should have enhanced reactivity and coordination via oxygen could be an important way of CO activation in different catalytic processes For CaY zeolite, two CO molecules can be adsorbed on the same cation, then the presence of O-bonded molecules results in the increase of the cation electric field, detected by additional high-frequency CO band This phenomenon could be interpreted as thermally activated creation of adsorption-induced Lewis sites that also can participate in catalytic processes on zeolites REFERENCES

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