Consecutive N2 Adsorption on CO Precovered NaY Zeolite: FTIR

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Consecutive N Adsorption on CO Precovered NaY Zeolite: FTIR Gives a Statistical Response of Surface Adsorbed Species. Olivier Cairon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp310559d • Publication Date (Web): 25 Nov 2012 Downloaded from http://pubs.acs.org on November 26, 2012

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The Journal of Physical Chemistry

Consecutive N2 Adsorption on CO precovered NaY Zeolite: FTIR gives a Statistical Response of Surface Adsorbed Species Olivier Cairon* * UFR Sciences & Techniques, BP 1155, 64013 Pau, France KEYWORDS: Adsorption · Adsorbed Species · NaY Zeolite · IR Spectroscopy · CO, N2 Probes. ABSTRACT We analyze IR spectra recorded after the consecutive adsorption at 77 K of N2 ensuing from CO precoverage as molecular gas-probes on the microporous NaY zeolite. The study encompasses N2 loading ensuing from both medium and low CO precoverages As a result, the corresponding IR spectra are thoroughly analyzed in connection with the adsorbed species that could be formed. Consequently, the formation conditions of polycarbonyls, polydinitrogen and other species having N2 and CO as mixed ligands are discussed. Close analysis of the IR spectra recorded for each situation reveals an undeniable correlation between the statistical IR responses of the asdetected adsorbed species. This breakthrough result contradicts the wide spread idea which states that each IR peak should correspond to one specific adsorbed specie. Instead, IR response accounts for a statistical distribution of the various IR positions connected with as-adsorbed species. Thus, depending on the respective contribution and amounts of the different surface

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adsorbed species, large sets of IR responses and positions of IR bands are possible. These new findings relating on a Statistical Infrared Response should provide useful advances for the understanding and assignments of some ambiguous IR spectra regarding surface adsorbed species.

1. INTRODUCTION Of the physicochemical techniques used for characterizing the interactions of molecules with solid surface adsorption sites, IR spectroscopy of adsorbed probes presents numerous advantages. Besides its highly sensitive response and great capacity to detect any modification in the chemical environment of the probed sites, FTIR investigations using gases as probes are well suited for assessing how gases undergo bonding with sites.1,2 This specific information accounts for the physicochemical properties of the probed site that IR spectra indirectly mirror. Nevertheless, FTIR investigations can rapidly become laborious when analyzing heterogeneous surfaces that have a large variety of adsorption sites or when considering the adsorption of multiple gases even on a homogeneous surface. IR spectra of the resulting adsorbed gas (also termed adsorbed species) rarely result in a set of single peaks with distinguishable shapes and positions. Instead, the results more often depict large and poorly resolved IR overlapping bands. Thus, establishing correlating information based on such poorly resolved IR spectra can rapidly become an insurmountable challenge. Consequently, as IR overlapping bands sometimes appear too complex to be properly solved and distinguished as individual peaks, the attributes of such surface adsorbed species become the subject of controversy in the literature.3-6 To illustrate the challenges that could be met through IR responses of adsorbed species, we focus on the analysis of IR spectra obtained for the consecutive adsorption at 77 K of N2 and CO


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on the microporous solid NaY zeolite. This material possesses Na cations as homogeneous specific adsorption sites. In part one, we provide background for our analysis by providing an overview of recent results regarding the individual adsorption of CO and N2 as weakly-bonding probes. In the second part, we analyze IR spectra of the gradual N2 adsorption ensuing from two sets of CO pre-coverage with different CO loadings (i.e low and medium).

2. EXPERIMENTAL SECTION

Gradual N2 adsorption ensuing from the CO coverage was carried out at 77K for a selfsupporting thin pellet of fully sodium-exchanged Na56Y zeolite (Union Carbide, Si/Al ~ 2.5, anhydrous weight m = 3.61 mg). Both manometric measurements and IR spectra were simultaneously obtained in the IR cell. IR spectra of NaY zeolite were recorded with a resolution of 2 cm-1. The pellet of the NaY zeolites was first activated with a heating rate of 2 K min-1 up to 650 K and was held for 2 hours in a home-made designed cell connected to a vacuum adsorption system (P < 10-3 Pa). This activation totally dehydrated the Na56Y zeolite (no residual water was IR detected). After activation, the wafer was cooled down progressively to liquid nitrogen temperature (77 K). Introduction of CO to provide low and medium precoverages allowed the background spectrum of the NaY zeolite to be measured. Then, the N2 molecules were introduced stepwise from a control volume (2 cm3) at 77 K by successive additions. For each added dose, both the IR spectrum and the corresponding total equilibrium pressure were recorded thanks to a capacitive gauge (10-3 mbar or 0.1 Pa of precision). Note that it was checked that thermal contact was optimal and did not depend on the pressure we admitted in the cell. Consequently, the shifts of the IR bands we observed are not due to thermal effect.


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3. RESULTS AND DISCUSSION

3.1 Background results: adsorption of CO and N2 as individual probes. Cationic alkali-exchanged zeolites are synthetic microporous materials widely used as selective adsorbents, ion exchangers, and heterogeneous catalysts. Their use in the separation processes is based on gas/solid adsorption that combines both the affinity of cations for guest molecules and the zeolite’s gas-uptake capacity towards a given gas.7,8 FTIR spectroscopy of small-adsorbed probe molecules, like CO or N2, is usually performed to assess the performance of the zeolites in relation to a specific exchanged cation type and its affinity with a given gas.9,10 For example, CO or N2 adsorption at 77 K on the cationic NaY zeolite surveyed by means of FTIR provides molecular information on how these two gases undergo physical bonding with the exchangeable Na cations.11-24 This molecular understanding is based on the interpretation and assignments of the IR bands that are progressively displayed during the stepwise introduction of each gas. Recent studies that build on previous researches account for a progressive completion of the coordination vacancies of accessible cations that contain supercages (Na in SII positions) through physical adsorption of the gas (Scheme 1).20-24 Accordingly, as long as CO is considered mono, di- and tri-carbonyls could be formed as adsorbed species -Na(CO), Na(CO)2 and Na(CO)3, respectively. These carbonyls are IR evidenced through their respective vibrating average νCO positions ca 2171, 2163 (Figure 1) and 2150 cm-1.20-24 N2 bonding is based on the same physical adsorption mechanism because it involves the completion of the coordination vacancies of Na cations. The resulting mono- and di-N2 species


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(geminal or bis dinitrogen) are detected through their respective IR νN2 positions at 2336.8 and 2335.5 cm-1, respectively, Figure 2. Note that no tri-N2 has been detected.15,23

supercage CO CO

CO Na

+

O atoms Al or Si atoms β - cage

Scheme 1. Schematic representation of SII Na+ cation position in NaY Faujasite. Mono-, di- and tri-carbonyls as adsorbed species. N2 completion provides mono- and bis (or geminal) dinitrogen adsorbed species.

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2 A b s o rb a n c e

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0.2

1

2180

2160 Wavenumbers (cm-1)

2140

2120

Figure 1. FTIR spectra of gradual CO adsorption on NaY at 77 K. The three main steps for low (1), medium (2) and high (3) CO coverage. Comparison of spectra obtained for CO and N2 adsorbed species highlights the more resolved IR spectra in the case of N2 adsorption. In other words, the position of the bands of the adsorbed N2 and (N2)2 species, Figure 2, are better defined and invariant with surface coverage, contrary to the case of the CO-based bands, Figure 1. Instead, CO adsorption results in more complex


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spectra, especially when CO coverage (i.e. CO pressure) increases. As a result, unresolved overlapping-bands progressively shifting towards lowest wavenumbers are observed with the highest CO pressures. Nevertheless, the recent resolutions of that feature conclusively evidenced the existence of the three carbonyls species supported by both theoretical and quantitative assessments.20-22,24

2333.5 (118.7)

A b so rba n ce

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2336.8 (7.5)

0.01

(53.5) (27)

(4.6) (12.9)

(2.2)

2345

2340

2335 2330 W avenumbers (cm-1 )

2325

Figure 2. FTIR spectra of gradual N2 adsorption on NaY at 77 K. Spectra in solid line are for low N2 coverage. Spectra in dotted line are for higher coverage. In parentheses, some of N2 equilibrium pressures (in Pa) are reported. In the following part, we will consider two particular CO precoverages before the gradual admission of N2.

3.2 N2 adsorption ensuing from the low CO precoverage. As depicted in Figure 3, the first situation accounts for low CO precoverage (PCO= 0.8 Pa) while the second corresponds to medium precoverage (PCO= 3.2 Pa). Spectra obtained after the gradual addition of N2 ensuing from the low CO precoverage are depicted in Figures 4-5 and in Figure 6 for the CO and N2 vibration region, respectively. In the CO region, Figure 4, constant additions of N2 result in three striking spectral evolutions denoted A, B and C.


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2164.5

Absorbance

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2173.5 0.1 b a

2190

2180

2170 2160 Wavenumbers (cm -1)

2150

2140

Figure 3. The two CO precoverages low (a) and medium (b) (in solid lines, 0.8 and 3.2 Pa of CO pressure, respectively) before N2 admittance. Intermediate CO situations are provided for reference (in dotted lines; see also Fig.2). For step A, Figure 4, early doses of N2 cause increases of the νCO band’s intensity that finally stacks progressively as total pressure in the cell attains c.a 3 Pa. Along the A step, the νCO band’s position evolves from 2172 cm-1 to 2173.5 cm-1 while the shape of the band remains almost symmetric (though a little shoulder starts to develop in the low wavenumbers at the end of the A step). Further N2 loading, B set, results in a gradual decrease of the 2172 cm-1 band that nonetheless remains limited, and finally, the νCO band attains a constant intensity at the end of step B. It is worth noting that this decrease is accompanied by a regular red-shift of the band towards low wavenumbers until the 2168 cm-1 position is reached. Throughout step B, total pressure evolves from c.a 6 Pa to 17 Pa and the above mentioned asymmetry develops only a little more. The C step highlights a more pronounced red-shift of the band from the 2168 cm-1 position to 2164.7 cm-1. All along, the intensity of this band remains almost constant; thus additions of N2 seem to affect only the position of the νCO band.


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2172

B, C

A

2164.7

Absorbance

2173.5 0.05

2180

2170

2160

2150

Wavenumbers (cm -1)

Figure 4. The νCO region after N2 additions ensuing from the low CO pre-coverage (PCO=0.8 Pa). The three A, B and C steps correspond to a total pressure of 6, 17 and 625 Pa, respectively. The above-mentioned features can also be analyzed using the low CO coverage spectrum as background. Such as-resulting difference spectra are depicted in Figure 5 and effectively illustrate the red-shift of the νCO band. 2164.5

B,C

Absorbance

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0.2

2173.5

2170

A(end)

2171.5

A 2180

2175

2170

2165

2160

2155

Wavenumbers (cm -1)

Figure 5. Difference spectra (low CO coverage spectrum as background) of N2 additions. A, B and C steps are those of Fig.4. Let us now consider the νN2 region, Figure 6, in connection with the above three sets A, B, C for low CO coverage.


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2333

Absorbance

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C

0.005

2336.8 B A

2340

2335

2330

W avenumbers (cm-1)

Figure 6. The νN2 region after N2 additions ensuing from the low CO pre-coverage. The three A, B and C steps are those of Fig.4 and Fig.5. For step A, early doses of N2 cause the gradual emergence of the 2336.8 cm-1 band that corresponds to mono-N2 as reported in Figure 2. At the end of the A step, intensity of the monoN2 band stacks while a shoulder around the 2333 cm-1 position develops. During the B set, this shoulder progressively expands to finally emerge as a clearly observable band along the C set. Note that the 2333 cm-1 position is very close to the 2333.5 cm-1 position of di-N2 as reported in Figure 2. The slight decrease of the mono-N2 band along the B and C sets should also be mentioned. To summarize, analysis of the N2 region accounts for the presence of mono- and diN2 as-adsorbed species. Note that compared to the adsorption of N2 as a single probe, Figure 2, co-adsorption with CO precoverage accounts for the limited formation of mono-N2 whereas diN2 formation rapidly takes place, Figure 6 (check relative intensities of mono- and di-N2 in the two situations). This illustrates well the pressure dependence of adsorbed species’ formation as previously reported.11-24 Let us now complete this information in connection with CO adsorbed species as depicted above through their IR signal. Comparison of spectra recorded for the A set, Figures 4 and 5,


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accounts for the increase of the 2173.5 cm-1 νCO band. It should correspond to the main formation of mono-CO as its position is full within the expected wavenumbers range of monoCO positions with pressure dependence.21 In parallel, N2 adsorption results on mono-N2 as adsorbed species. One striking question is how CO adsorbed species can be formed when CO is no longer being introduced into the IR cell? To answer, it is useful to remember that adsorbed species’ formation reflects thermodynamic equilibrium reactions. Mono-CO and mono-N2 adsorbed species result from the two following equilibrium reactions: Na+ + COg ↔ Na+(CO)ads and Na+ + (N2)g ↔ Na+( N2)ads It is obvious that these two equilibrium reactions have dependence with the partial pressure of their respective gas. Therefore, the quantity of carbonyl should only depend on the CO partial pressure and not on the total pressure that farther gradually increases after N2 admission. In other words, the carbonyls’ formation should stop regardless of further additions of N2. The limited enhancement of carbonyls we observed for low CO precoverage (Fig. 4) and the progressive mono- to dicarbonyls conversion we observed for medium CO precoverage (Fig. 7) would both attest to an increase of the CO partial pressure, thus provoking the displacement of the CO equilibriums. We think a simple thermodynamic feature explains such an increase of the CO partial pressure along the N2 admittance (even though no additional CO is introduced in the cell). Indeed, while N2 is progressively introduced, the volume of each supercage containing the SII Na sites where adsorption occurs could be progressively diminished by the steric N2 adsorbed species that are gradually formed. As a consequence, the initial partial pressure of CO in the gas phase corresponding to the initial equilibrium set of adsorbed CO (before N2 is introduced) should increase in the supercage in parallel with the progressive N2 adsorption thanks to the progressive diminishing of the free volume of the supercage. The as-resulting local enhancement of the CO partial pressure should thus displace the initial CO equilibrium that takes place on the


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supercage and would give rise to various further carbonyls sets according to the new increased CO pressure reached in the cell. Unfortunately, only the total pressure was measured during our experiments and CO partial pressures were not assessed apart. However, according to the arguments we developed above, the increase of the total pressure should indirectly indicate CO pressure enhancement in parallel with N2 admission. As a result, spectra in Figure 4 would depict a situation where monocarbonyls developed farther ahead during the A set (from the initial CO pressure of 0.8 Pa). At the end of the A set, the total pressure in the cell is about 4 Pa. Therefore, and again according to the above arguments, CO partial pressure should be high enough to make the conversion of mono- to dicarbonyls possible as observed along the B and C sets. Note that when CO is adsorbed alone, this conversion occurs near 1.5-2 Pa of CO pressure. 20-24 Moreover, this conversion is indeed well highlighted in Figures 4 and 5 for the later B and C sets. From a spectral result, IR bands of monocarbonyls progressively disappear whereas the conversion of mono- to dicarbonyls results in a series of CO bands spanning from 2172 cm-1 to 2164.5 cm-1. This feature clearly indicates that none of these IR bands (whatever the position) correspond to a specific adsorbed species. Rather, this series depicts a statistical IR response of the progressive mono- to dicarbonyls conversion. This argument is fully consistent particularly at the end of the C set when the 2164.5 cm-1 position is reached. Indeed, previous recent studies have reported that dicarbonyls are more likely to vibrate at the 2163 cm-1 position.20-24 The slightly higher position we observe in Figure 4 and 5 at 2164.5 cm-1 and the constant intensity of the corresponding band would indicate that mono- to dicarbonyls conversion finally stops at the end of the C set after substantial stacking. It would also relate to the stack of the CO partial pressure that has attained its maximum value at the end of the C set (total pressure is 625 Pa) thus explaining that no further CO conversion is possible. As a result, the 2164.5 cm-1 position corresponds to an


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intermediate statistical IR response of mono- to dicarbonyls conversion and attests of an interrupted conversion due to the lack of the required CO pressure to attain its accomplishment. Let us now examine the B and C sets together with the two CO and N2 regions. On the one hand, the gradual decrease of the mono-CO band accounts for the disappearance of mono-CO. Moreover, the negative part in Figure 5 amplifies along the C set while, in parallel, a positive band emerges with a continuous shift towards low wavenumbers until this band attains the 2164.7 cm-1 position. These features correspond to the progressive formation of di-CO.21,24 However, hypothesis of mixed di-species (having CO and N2 as mixed ligands) should also be considered. Let us remember that mixed di-species would result in a blue-shift and in a red-shift compared to the positions of di-CO and di-N2, respectively. Indeed, compared to their respective non-mixed di-species, the blue-shift should be due to a lowering of the binding force when N2 binds a mono-CO whereas red-shift, as an opposite trend, should account for strengthening when CO binds a mono-N2. The 2164.7 cm-1 position of this line, compared to the expected 2164-2163 cm-1 of di-CO, might reveal the formation of a few mixed ligands di-species. Examination of the νN2 region, Figure 6, provides additional conclusive insights. Indeed, the progressive emergence of the 2333 cm-1 band is very close to the 2333.5 cm-1 position of di-N2 as depicted in Figure 2 and accounts for the main formation of di-N2 over the course of the B and C sets. This slight shift (0.5 cm-1) compared to the di-N2 position might indicate the formation of a small number of mixed di-species. Nevertheless, it has been demonstrated that positions of the νCO band strongly depend on the CO filling (i.e pressure).21,24. As a result, the 2164.5 cm-1 position of the νCO band could also be due to the presence of some mono-CO as the existence of mixed di-species remains hardly detectable in the N2 region, Figure 6. All in all, mixed-di-species, if any, remain few. To conclude, the A, B and C sets account for the formation of carbonyls and dinitrogen asadsorbed species mainly composed of mono- (A set) and non-mixed di-species (B, C sets).


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Indeed, the presence of even few mixed di-species is hardly conclusive as no singular band is observed and the following equilibrium reactions predominate along the A, B and C sets for low CO precoverage: Na+ + COg ↔ Na+(CO)ads and Na+ + (N2)g ↔ Na+( N2)ads Na+(CO)ads + COg ↔ Na+(CO)2,ads and Na+(N2)ads + (N2)g ↔ Na+( N2)2,ads Based on the arguments we developed above, N2 admission clearly influences CO equilibrium sets. The reverse is also true: CO equilibrium sets influence N2 adsorption. Indeed, close inspection of the N2 region (Figure 6) indicates that the relative proportions of mono-N2 and diN2 dramatically change with the presence of preadsorbed CO compared to the situation where N2 is solely adsorbed (Figure 2). In that sense, the equilibrium sets where both N2 and CO are adsorbed are closely linked, although mixed-ligand adsorbed species remain hardly detectable. Instead, relative amounts of N2 or CO adsorbed species are dramatically modified compared to the situation where each probe is adsorbed alone. This raises the question of whether or not N2/CO co-adsorption is a repulsive process. In other words, the assertion ‘any site covered by one species will not be useable by the other’ is judiciously questionable. However, this would imply that 1) whatever the basic strength of CO and N2, these two probes only compete according to the sequence of their adsorption, 2) following this idea, CO adsorption on precovered N2/NaY conduces to the parallel observations we made for CO precovered NaY. It is more likely that at the end of the C set, Figure 4, CO remains adsorbed because of its higher basic strength. However, it would be of great interest to further survey experiments based on CO admission ensuing from N2 precoverage.

3.3 N2 adsorption ensuing from the medium CO precoverage.


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Let us now analyze spectra obtained for medium CO precoverage. For this task, spectrum (b) in Figure 3 is used as background. Difference spectra are reported in Figure 7 for the νCO region. 2160

2162 E 2160 A bsorbance

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0.1

D 2163

2170 2180

2170

2160 Wavenumbers (cm -1)

2150

2140

Figure 7. Difference spectra (medium CO coverage spectrumas background, 3.2Pa of CO pressure) after N2 additions. At the end of the D and E sets, total pressure is 96 and 1050 Pa, respectively. Two striking sets, D and E, can be highlighted. The D set accounts for the decrease of monoCO (negative part c.a 2170 cm-1) while di-CO are progressively formed (positive band from 2163 to 2160 cm-1). Note that spectrum b in Figure 3 already included the formation of mono, di and tri-CO as well outlined by recent qualitative and quantitative assessments.24 Further N2 addition, Figure 7, results in mono to di-CO conversion and the tail around the 2160 cm-1 position is more likely due to some tri-CO.20,21,24 The set E points out the end of mono to di-CO conversion as outlined by the negative part that remains constant whatever the additional N2 doses may be. Moreover, the tail around 2160 cm-1 progressively disappears and the band stabilizes around 2162 cm-1. This feature accounts well for desorption of tri-CO obtained for the highest N2 doses. Comparison with the νN2 region, Figure 8, completes the analyses. Along the D set, only very few N2 molecules are adsorbed as indicated by the very low intensity of the νN2 band. This


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confirms that adsorbed species are mainly those resulting from the mono to di-CO conversion (CO in the gas phase being readsorbed as discussed in the aforementioned A, B, and C sets). Instead, N2 adsorption starts significantly only for the E set in parallel with desorption of tri-CO. This would indicate that di-N2 adsorbed species that are formed in this way are more stable than tricarbonyls. 2332.3

Absorbance

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0.002

2331.3

E

D

2340

2335

2330

2325

W avenumbers (cm-1)

Figure 8. The νN2 region after N2 additions ensuing from the medium CO pre-coverage. See also Fig. 7. Taking into account both this deduction and the position of N2 adsorbed species (c.a 2332.3 cm-1) we conclude that mainly di-N2 is formed along the E set. As exposed and discussed above for sets A, B, and C, the formation of mixed di-species, if any, seems to be negligible for sets D and E. To conclude, N2 adsorption ensuing from medium CO precoverage results mainly in the formation of di-CO and di- N2 as well as some tri-CO.

4. CONCLUSION To conclusively summarize, N2 adsorption ensuing from CO precoverage of the microporous NaY zeolite relates to the formation of adsorbed species having either CO or N2 as homogeneous ligands. Throughout the experimental sets we considered, no mixed-ligand species having


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specific IR signature have been detected. Rather, the adsorbed species we observed after N2 loading ensuing from the two particular CO precoverages we considered, more likely display according to their relative stability as indicated by the following rank: mono-CO> mono-N2> diCO> di-N2> tri-CO. As a result, IR spectra of the corresponding situations mirror various distributions of such surface species and result in a statistical Infrared response. Consequently, positions detected on IR spectra strongly depend, on the one hand, on the nature and the relative quantities of adsorbed species that are formed. In other words, each of the various positions of IR overlapping bands that display throughout the sets we considered can not automatically be related to specific adsorbed species. Rather, IR overlapping bands encompass a statistical IR response and positions relate to the respective quantities of the different adsorbed species in connection with their progressive formation. On the other hand, because of physical parameters like partial pressure that influences the corresponding N2 and CO equilibrium reactions, N2 admission clearly influences the initial CO precoverage equilibrium sets. The reverse is also true as the presence of CO preadsorbed species farther affects N2 adsorption. These insights and particularly the statistical IR response should be further considered before any assignment of IR bands especially when different surface species are expected or when solid surfaces having heterogeneous adsorption sites are probed. In such cases, positions of bands should be carefully assessed in connection with the aforementioned findings before any conclusive assignment regarding adsorbed species is proposed.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Consecutive N2 Adsorption on CO Precovered NaY Zeolite: FTIR

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