1810
Langmuir 1990, 6, 1810-1812
Comments On the Use of Pyridine Adsorption as an Analytical Tool in Surface Chemistry Parry1 first proposed pyridine (py) as a suitable adsorbate to reveal various types of surface centers. The use of this test molecule for surface characterization of solid systems, especially oxidic ones, has become routine. Still, sometimes one gets the impression that the use of py as an analytical tool is not as useful (i.e., thorough and appropriate) as it could be. This was suggested by two papers, which appeared recently in this j o ~ r n a l . ~ * ~ The study by He& investigates the surface properties of ZrOz polymorphs by some of the usual procedures of surface chemistry including IR spectroscopy. Pyridine was adsorbed at ambient temperature and evacuated at 150 “C. He observed bands a t 1444 and 1606 cm-l and concluded that they “are diagnostic for py coordinatively bound to Lewis acid sites”. The bands reported by Hertl correspond to the 19b and 8a modes of strongly coordinated py, and the proposed interpretation is correct. If the aim of the interaction with py was the observation of all py/surface interactions (e.g., to check whether at the surface of ZrOz there is one or more families of acid sites), the interaction with py should have been investigated over a wider coverage range and in a wider activation temperature range. The observation of a py species with the 8a mode at 2 1595 cm-I which remained after evacuation at 150 “C is sufficient for the identification of a strongly chemisorbed species, coordinated a t (strong) Lewis centers. It does not address the possible existence of weaker adsorbing sites. The observation of weaker chemisorbed species, more sensitive to evacuation (e.g., see the case of some py species on aluminas4 and the case of CO on most non-d metal oxidesb), depends on the choice of coverage and/or activation conditions, and this possibility is most often ignored, as it is in the case of at least one of the two py papers considered here.2 To show this, we shall refer to Figure 1, which reports, in the 1620-1560-cm-’ range (the 8a-8b modes range), several spectra of py, most of which have been bandresolved. Curves I refer to the high-coverage ambient temperature adsorption of py (Pp, 8 Torr, 1Torr = 133.3 Pa) on monoclinic ZrOz vacuum activated at ambient temperature and curves I1 to py adsorbed in the same conditions on ZrOz vacuum activated a t 600 “C. It is evident that the ZrOz/ py system, observed under high py coverage, is somewhat more complex than reported by Hertl.2 The spectrum is made up of several components. Among them there is a nonspecific liquid-like physically adsorbed species, as expected in the experimental conditions adopted and as mentioned in most papers in which py adsorption is followed up to high coverages (e.g., see the work by Healy et al.,3the second py study considered here). (1) Parry, E.P. J. Catal. 1963,2,371. (2)Hertl, W. Langmuir 1989,5,96. (3) Healy, M. H.: Wieserman, L. F.; Arnett, E. M.: Wefers, K. Langmuir 1989,b,114. (4) Morterra. C.; Chiorino, A.: Ghiotti, G.: Garrone,E. J . Chem. SOC., Faraday Trans: 1 1979,75,271. (5) Morterra, C. J. Chem. SOC.,Faraday Trans. 1 1988,84,1617.
0743-7463/90/2406-1810$02.50/0
In order to be able to observe a t high coverage the spectral features of all py chemisorbed species, one must know the spectrum of the liquid-like (van der Waals) physically adsorbed species and subtract it. The spectra of liquid py and of liquid-like physically adsorbed py have been long known to be made up, in the 8a8b mode range, of several peaks (e.g., see refs 6 and 7), unlike what is claimed by Healy et al.,3 who state that “when physically adsorbed species are present on the surface, a band at 1578 cm-1 represents both the 8a and 8b modes of pyridine”. Curves I11 in Figure 1 show the spectrum of liquid py (top curve, bands a*, b*, c*, and sh*) and a spectrum for physically adsorbed py (lower curve, bands a, b, c, and sh), “constructed” from the resolved spectra in I and I1 by assuming the following: (i) all of the strong band b a t 1580.6 cm-l (the dominant feature in spectra I and 11) belongs to the physisorbed phase; (ii) the other spectral components of the (liquid-like) physically adsorbed phase possess, in respect of the band at 1580.6 cm-l, the same relative intensity they possess in the liquid phase. Spectral subtraction of the physically adsorbed phase from I and I1 by the use of the reconstructed spectrum in I11 (which allows for some minor differences of band position and band profile between the spectra of liquid py and liquid-like py) leads to curves IV and V, respectively. The latter show that, at high py coverage, the chemisorbed py phase, due to specific interactions at surface sites, is made up of three components. There is a H-bonded species (the relevant 8a mode, marked d in the figure, is at -1591 cm-’, and the 8b mode lies in the unresolved envelope of all 8b modes at ~ 1 5 7 cm-l), 5 and its importance logically declines with increasing activation temperature. There are also two Lewis coordinated species (the relevant 8a modes, marked e and f, respectively, are a t ~ 1 6 0 and 3 ~ 1 5 9 cm-l, 7 the former one corresponding to the py species reported in ref 2), and their surface concentration grows with increasing activation temperature, as expected of sites which form upon dehydration. A t least two arguments demonstrate that the Lewis coordinated species, whose 8a mode a t ~ 1 5 9 cm-l 7 (band f) virtually coincides with the (1 + 6a) mode of liquidlike py (band a), is real and not a computer artifact: (i) After a short py evacuation a t ambient temperature, which gets rid of the liquid-like phase so that no band subtraction is needed anymore t o observe chemisorbed species that resisted evacuation, ZrOz activated a t 600 “C still presents two Lewis coordinated species. This is seen (with some difficulty) in curves VI, as the 8a mode of the lower Lewis coordinated species (band f) is still observable after evacuation, though with a much lower intensity than in curves V. Note that on the sole basis of the spectra in VI it would be difficult to identify the presence of two families of Lewis centers, and that after py evacuation in more severe conditions, e.g., at 150 “ C as in the work of ref 2, it becomes impossible. (ii) If py adsorption is carried out on a ZrOn preparation surface poisoned with dosed amounts of sulfates, the (6) Kline, C.H., Jr.; Turkevich, J. J . Chem. Phys. 1944,12, 300. (7)Pichat, P.; Mathieu, M.-V.;Imelik, B. Bull. SOC.Chim. Fr. 1969, 8,2611.
0 1990 American Chemical Society
Comments
Langmuir, Vol. 6, No. 12, 1990 1811
rb
T
i 1620
T
yh t
1600
1580
i
1 5 6 0 1620 1600 YRVENUMBERS CM-1
-
1580
I 1560
i\\b*
A
I11
4 1620
WRVENUMBERS CH-1
C
1608 1580 YRVENUMBERS CM-I
I
1560
VI11 .I 1620
1600
1580
I 1560
YRVENUMBERS CM-1
Figure 1. Experimental and band-resolved IR spectra (absorbance vs wavenumbers) of pyridine in the 1620-1560-cm-1 range (the 8a-8b mode range). (I)8 Torr of py adsorbed, at ambient temperature, on ZrOz vacuum activated at ambient temperature. (11) 8 Torr of py adsorbed, at ambient temperature, on ZrOp vacuum activated at 600 "C. (The weak band in the central part of the spectrum is the difference between the experimental spectrum (top curve) and the four-band simulated spectrum. It is clearly made up of two components (whose origin should be clear on the basis of curves I and of the text): a residual component of the H-bonded py species, marked d in section V,and a weak component, marked sh in section 111, belonging to the liquid-like py species. (111) Top spectrum: liquid py. Lower spectrum: simulated spectrum of the liquid-like (physically adsorbed) py phase. (IV,V) The spectra in I and 11, after band subtraction of the liquid-like component. (VI)Spectrum of the sample shown in 11, after a 40-5py evacuation at room temperature. (VII)Spectra of py adsorbed, at ambient temperature, on sulfate-doped ZrOz: A, in equilibrium with 8 Torr of py; B, after band subtraction of the liquid-like component. (VIII)Spectra of py adsorbed, at ambient temperature, on sintered ZrOz: A, after admission of a very small dose of py (equilibriumPpr= 0); B, after admission of a larger dose of py (up to the first appearance of the spectral features of a physically adsorbed phase). spectrum of py changes somewhat (curves VII): there are
still two Lewis coordinated species, whose 8a modes (bands g and h) have nearly the same A q / 2 they had on pure ZrO2 and are shifted to higher wavenumbers by inductive effectx, from the sulfates8 The physically adsorbed py phase (bands a and b) is unaffected by sulfates, as expected of a nonspecific interaction, and there is no band superposition anymore at -1597 cm-l. Whenever feasible, band subtraction is a useful spectral manipulation to gain the maximum of information from the spectra of adsorbed species. In the case of py, band subtraction of the physically adsorbed phase allows the observation of weak, partly reversible, chemisorbed species. Moreover, it allows one to observe the spectrum of chemisorbed py over a wide range of surface coverages, up to high py pressures. This is quite important, because the bands of the modes of chemisorbed py (especially the (8) Beneitel, M.;Saw, 0.; Lavalley, J. C. Mater. Chem. Phys. 1987, 17, 249.
modes of species AI) undergo frequency shifts with coverage, which ought to be understood. For instance, in the AlzOa/py system, dealt with in ref 3 and shown here in Figure 2, an increase of py coverage brings about a progressive red shift of the 8a modes of Lewis coordinated species. In this respect, Healy et aL3 state that "the decrease in frequency (with py pressure) is indicative of weaker complexes being formed on the solid. The most active or energetic sites are filled first and are characterized by higher frequencies". It is our opinion that their observation is correct, as a lower frequency of the 8a mode does indeed correspond to a weaker complex, but their interpretation is not correct. In fact, if there were the stepwise formation of new weaker and weaker py surface complexes, one should observe, rather than a spectral shift, a gradual broadening toward lower wavenumbers of the 8a band(s), and this is not the case. With coverage, all of the 8a mode of coordinated py shifts downward without appreciable broadening, meaning
1812 Langmuir, Vol. 6,No. 12, 1990
Comments Figure 2 presents the interaction of py with an aluminum oxide system which is gradually transformed, upon thermal activation in vacuo, from the hydroxide phase (gibbsite) to transition spinel phases (6 and 7 or 8-A1203)10and eventually to the corundum phase ( c Y - A ~ ~ O The ~ ) spectra . are relative to py adsorption a t high coverage (A), after band subtraction of the liquid-like component (B)and after short py evacuation a t room temperature (C). It is confirmed also that for the alumina/py system holds the frequency-coverage correlation of the 8a mode(s) (of symmetry species AI) discussed above. The correlation is particularly evident, as expected, in the case of the strong Lewis coordinated py species a t high (e.g., see the dotted segment in curves I, 11,and IV) for which the charge release is more pronounced. The spectra of Figure 2 also show that the spectral region around 1595 cm-' of the alumina/py system is quite complex and contains more than one (rather labile) chemisorbed species. One such species, observed as long as there are abundant surface hydroxyls, is due to py interacting by H-bonding, whereas another component of variable position, equally quite reversible and thus best observed at high coverages, must be ascribed to a weak coordinated species. A more detailed description and assignment of the bands is not pertinent here. What is indeed pertinent to this discussion is the conclusion that, in the case of systems as complex as those dealt with in refs 2 and 3, much more information can be gained by the use of py adsorption if one takes into account some of the procedures outlined here, concerning py coverage, activation temperature, spectral resolution, and band subtraction.
vB * ,L
1620 1680 1560 URVENUMBERS Ctf-1
1560
Figure 2. IR spectra (absorbance vs wavenumbers) in the 8a8b mode range of py adsorbed at ambient temperature on some aluminum oxides: (A) in equilibrium with 4 Torr of py; (B) after band subtraction of the liquid-like component; (C) after a 30-5 evacuation at room temperature. Dotted segments: as spectra C, with no ordinate displacement. (I) Al(OH)3(Gibbsite),vacuum activated at ambient temperature. (11)Gibbsite, vacuum activated (decomposed) at 400 O C . (111) Gibbsite, vacuum activated (decomposed) at 600 O C . (IV) O-Al203, vacuum activated at 600 "C. (V) a-A120a,vacuum activated at 600 O C . that proceeding adsorption does not reveal an intrinsic heterogeneity, Le., the presence of some sites of lower energy, but a coverage-induced heterogeneity of all the sites. The extent of charge release from the N lone-pair orbital to the adsorbing site, which is actually measured by the blue shift of the 8a mode of the py complex in respect of that of free py, is affected, through inductive effects, by the overall surface concentration of charge-releasing and/ or charge-withdrawing species. As a consequence, the 8a mode of all the Lewis surface complexes shifts downwards with py coverage and shifts upwards in the presence of acceptors, e.g., sulfates. This effect is not peculiar of the A1203 system but is observed with all nonconducting systems (see curves VI11 in Figure 1, relative to a very low py coverage (A) and a higher py coverage (B)on ZrO2) and is exhibited by all surface interactions of the Lewis acid/base type (e.g., see the Ti02/COS and Zr02/COB systems).
Experimental Section All samples, either in the form of self-supporting disks or of thin-layer depositionson a pure Si plate, were activated in vacuo (P5 Torr) at the temperatures reported in the legends and were equilibrated with 4 Torr of py after cooling to ambient temperature. From the spectra run in the presence of 8 Torr of py, the spectrum of the vapor phase was subtracted, although with the optical path of our cells (2mm) ita contribution was negligible whenever the absorbance of the spectrum due to adsorbed species was 20.1 au. All spectra (256scans) were run at resolution 4 cm-I on a Bruker 113v FTIR spectrometer equipped with DTGS detector and are reported with no smoothing. Spectral simulations were performed by using an iterative Pascal program by Bruker (Simband), by fixing only the number of spectral components and the minimum accuracy desired. Materials. ZrOp: 95% monoclinic (1.4% Hf), surface area (sa) = 86 m2g-l. ZrO2 sulfate doped: same ZrO2, with 200 rmol gl SO,2-, sa = 90 m2g-l. Sintered ZrOz: ZrO2 treated at 1OOO O C , sa = 10 m2g-l. Al(OH)a: gibbsite, sa = 10 m2g-'. B-AI&: sa = 32 m2g-l. a-Al203: sa = 12 m2 gl. C. Morterra' and G. Cerrato Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universitd di Torino, via P. Giuria 7, 1-10125 Torino, Italy Received July 20, 1989 In Final Form: May 22, 1990 Registry No. Si, 7440-21-3; Zr02, 1314-23-4; Hf, 7440-58-6; Sod2-, 14808-79-8;Al(OH)3, 21645-51-2; A1203, 1344-28-1; pyridine, 110-86-1. (9) Morterra, C.; Aschieri, R.; Bolis, V.; Borello, E. In Structure and Reactivity of Surfaces; Elsevier: Amsterdam, 1989; p 703. (10) Lippens, B. C.; Steggerda,J. J. In Physical and Chemical Aspects of Adsorbents and Catalysts;Linsen, B. G.,Ed.;Academic Press: London, 1970; p 190.
