The use of vibrational spectroscopy in the surface
characterization of microcrystalline oxides (*)
A. ZECCHINA(**), D. SCARANO, P. GALLETTOand C. LAMBERTIDipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali Università di Torino - Via P. Giuria 7, 10125 Torino, Italy
(ricevuto il 28 Febbraio 1997; approvato l’8 Maggio 1997)
Summary. — A systematic investigation of the surface morphology and of the
vibrational properties of CO adsorbed on several simple oxides with regular crystalline habit and exposing thermodynamically stable and neutral faces, is briefly reviewed with the aim to elucidate the spectroscopic manifestations of CO adsorbed on well-defined crystallographic positions; the cases of MgO and a-Cr2O3 are then discussed in more details. In order to have a better insight on the role of surface defectivity in catalytic reactions, the activity towards CO of perfect, low-index faces and of more defective situations (like those associated with edges, steps, corners) is also discussed. Finally, the experimental C-O stretching frequencies are compared with those calculated on the basis of the local electric field strengths at the adsorption sites.
PACS 69.90 – Other topics in the structure, and nonelectronic properties of surfaces and interfaces; thin films and whiskers.
PACS 81.20.Ev – Powder processing: powder metallurgy, compaction, sintering, mechanical alloying and granulation.
PACS 01.30.Cc – Conference proceedings.
1. – Introduction
High surface metal oxides, widely employed as catalyst and catalytic supports, exhibit a highly disordered morphology because microcrystals are terminated by several crystallographic planes having a considerable concentration of steps, kinks, edges and other defects, which interrupt their bidimensional regularity. On this basis, it is evident that, although much needed, the accurate characterization of the structure
(*) Paper presented at the “First International Workshop on Reactivity of Oxide Materials. Theory and Experiment”, Como, 8, 9 November 1996.
of these complex and very reactive surfaces and of the species formed on them upon interaction with reactive gases is a task of considerable difficulty. This is made even more difficult by the fact that the most powerful characterization techniques typical of classical surface science, like the electron spectroscopies (UPS, XPS, EELS and corresponding angle-resolved spectroscopies), and structural techniques (grazing X-rays or electron diffraction, SEXAFS and X-rays standing waves) cannot be applied, due to the insulating and/or polycrystalline nature of most oxides. The lack of data on model oxide systems is particularly relevant when catalytic surfaces are considered, not only because the oxides possess catalytic properties on their own or participate as supports at many metal-catalyzed reactions, but also because it is becoming gradually
more and more clear that reactions which are universally considered as
metal-catalyzed reactions do actually occur on surfaces functionalized because of the presence of reactants and products. On the basis of these considerations, it is evident that the detailed knowledge of the surface properties of reference oxides is of vital importance because it can greatly help the understanding of the basic catalytic phenomena and mechanisms occurring at the surfaces, by acting as indispensable milestones for the full comprehension of the data obtained on dispersed materials. Well-defined surfaces can, in principle, be obtained in three different ways: i) by cutting macroscopic crystals along well-defined planes [1]; ii) by epitaxial growth of ordered oxide films on top of (macroscopic) single-crystal metal faces [2]; iii) by preparing severely sintered microcrystals, exhibiting well-defined morphology and exposing only a few and near defect-free extended faces [3-15].
In this brief review we will discuss about the vibrational properties, obtained mainly by IR or Raman spectroscopies, of surface species formed on oxidic particles prepared following the third route, the morphology of which has been carefully determined by SEM, high-resolution TEM and/or atomic force microscopies. It is worth mentioning that, although not discussed here, also UV-Vis, EPR and NMR spectroscopies and microcalorimetry represent important investigation techniques for these materials. Let us stress that one relevant advantage of method iii) lies in the possibility (by using appropriate sintering procedures) of virtually preparing samples with any type of surface area varying from the values typical of the real catalysts to those more typical of single crystals. In this way the adsorptive properties of the whole range of highly disordered, very reactive and catalytically useful surfaces and of the reference low-index faces of the single crystals can be studied, so contributing to the establishment of a link between the results obtained on catalytic materials and the results obtained on single crystals by means of surface science methods. The progressive sintering route has been followed in our laboratory for many years and the IR spectra of simple molecules adsorbed on several simple oxidic systems have been interpreted; as an example we can mention: MgO [4], NiO [5], NiO-MgO [6], CoO-MgO [7], ZnO [8], ZnO-CoO [9], a-Cr2O3[10], a-, d- and g-Al2O3[11] and a-Fe2O3[12], MgAl2O4and other spinels [13], TiO2[14], ZrO2[14e,15], La2O3[16]. In two very recent contributions [17], using an atomistic approach, we have modeled the main faces of MgO, ZnO, a-Cr2O3 (and other solids) and computed the local electric field strengths at the adsorption sites. We have then compared the observed C-O stretching frequencies of CO adsorbed on them with the calculated ones obtained through the ED n relationship proposed by Pacchioni and Bagus [18]. This comparison has strongly validated the adopted method based on the combined use of morphology control and IR studies of adsorbed probe molecules. The general validity of the adopted ED n(CO) relationship has been previously successfully tested on CO adsorbed on alkali-metal exchanged zeolites [19].
2. – Results and discussion
In this brief review we shall discuss only case-studies concerning a cubic (MgO) and a prismatic (a-Cr2O3) oxide. For the sake of brevity, all information concerning the shape of microcrystals of both solids, as obtained with SEM and TEM, is omitted and the reader is referred to the quoted references for more information; the same holds for the experimental details of the spectroscopic measurements.
2.1. MgO: a case-study for oxides with rock salt structure. – The surface of MgO can be considered, to a first approximation, as a bidimensional array of cations (Mg2 1) and anions (O2 2), which can be thought as acids and bases of Lewis type, respectively. The formation, upon adsorption of CO, of weak Mg2 1RCO adducts (as documented by the abundant literature [4]) involving a bond with predominantly electrostatic character can thus be also viewed, on simple chemical grounds, in terms of an acid-base reaction where the carbon monoxide acts as a weak Lewis base. Its basic nature also explains why the interaction of CO with the basic O2 2
5 cusof the (001) faces can be neglected. Note how the same does not hold for the highly unsaturated and more active O2
-3 cusat the corners (vide infra point v)
)
. The IR spectra of increasing doses of CO adsorbed at 77 K on three different MgO samples of increasing crystallites perfection [4c, 4d, 6e] are reported in fig. 1a, 1b and 1c (ssa B200, 35 and 10 m2g21, respectively). The main features of the spectra illustrated in fig. 1 can be interpreted in terms of Mg2 1RCO adducts as detailed in the following: i) the weak adsorption at about 2203 cm21, clearly observed only on the high surface sample (fig. 1a), is due to Mg2 13 cusRCO adducts; ii) the absorption at 2170–2164 cm21 (fig. 1a and b) appears as a shoulder at high coverages and becomes a well-defined peak only for u K0 and is ascribed to CO adsorbed on Mg2 1
4 cus ions located on edges and steps (absent on smoke, see fig. 1c); iii) the dominant absorption at 2157–2148 cm21(for uKu
max) is assigned to CO adsorbed on the ions of the predominant (001) faces: the narrow FWHM (4.5 cm21, for u Ku
max) of this band in the most perfect sample (smoke), is remarkably small (fig. 1c) and is correlated with the high perfection of the crystals which minimize the inhomogeneous broadening effects; iv) the weak absorption at 2150–2146 cm21, distinctly observable on high surface area sample for u K0 only, is tentatively assigned to CO simultaneously bonded through both C and O ends to pairs of Mg2 1 at step sites [3d, e]; v) the weak absorptions at 2120–2070 cm21 (fig. 1a and b), absent on smoke (fig. 1c), are ascribed to the stretching vibrations of ketenic C˜C˜O groups in CnO22n11 species formed by
step-wise interaction of CO with unsaturated O2 23 cus located at the corners [3d, 3e, 4b] following the path: O2 21 CO K CO2 2
2 ; CO2 22 1 CO K C2O2 23 ; C2O2 23 1 CO K C3O2 24 . The formation of such polymeric species, which is faster and abundant at RT, is also demonstrated by the simultaneous appearance of a complex and not yet fully understood group of bands in the 1800–1000 cm21 range (see fig. 2 of ref. [3d]). The formation of these species is the direct consequence of the high basicity of O2 23 cusspecies. Coming back to the 2157–2148 cm21band, described in point iii) and associated with the predominant Mg21
5cus sites on (001) faces, it is evident from fig. 1a, 1b and, more clearly 1c, that this band progressively shifts from 2157 cm21(for u K0) to 2148 cm21(for u K umax). This effect has been explained in terms of gradual modifications of lateral interactions among CO oscillators in adsorbed layers. As already discussed by several authors [20] these adsorbate-adsorbate interactions can occur through space and/or solid. As the Mg2 1RCO interaction mainly causes a polarization of the CO molecule [21], without substantial electron transfer, the through-solid interaction
Fig. 1. – FT-IR spectra of CO adsorbed at 77 K for coverages ranging from u 41 (5.33 kPa) to
u K0 on: a) high surface area MgO sample; b) sintered MgO sample; the spectrum of
12CO/13CO (15/85) isotopic mixture at u 41 is also reported; c) MgO smoke (u41 f2.67 kPa). Part d): relaxed (001) surface of MgO as optimized using MARVIN code [22] together with the potential map generated by such reconstructed distribution of cations and anions: solid, dot-dashed and dashed lines represent the (100) cut (passing through Mg1, Mg2and Mg3cations) of the positive, null and negative equipotential surfaces, respectively, draft each 0.05 u.a. (1 u.a. A 27 V). Surface structure has been plotted using MOLDRAW code [23]. Parts a), b) and c) adapted from refs. [4c, 4d, 3d], respectively.
plays, in this case, a negligible role. As a consequence, the observed shift with u is mainly due to direct through-space interactions. In the dipole-dipole approximation, the frequency shift Dn is the sum of static and dynamic effects (Dn 4Dnst1 Dndyn), and the relative weights of the two contributions can be estimated by using the limit isotopic dilution method proposed by Hammeker et al. [20a]. In fact, the difference (at uKumax) between the stretching frequency of the Mg21R12CO adduct when pure12CO is used (2148 cm21) and when a diluted12
CO/13CO mixture is dosed (2144.5 cm21) gives
Fig. 2. – Representation of a monoatomic step on the (001) surface of MgO: a) unrelaxed; b) relaxed as optimized using MARVIN code [22]. A monoatomic step on the (001) surface of a cubic solid can be modelled by cutting the solid along the (10n) plane: as a result, the step will be repeated each n cells. Surface relaxations and corresponding electric field maps have been computed for n 41 to 6, in order to evaluate the perturbation on the local electric field generated at a Mg2 1
4 cus step site of adjacent steps. We have found that, in the center of mass of the CO molecule, this perturbation is negligible for n F2. These figures have been realized using the (104) cut.
TABLE I. – Comparison between the experimental (nexp) and the computed (nth) C-O stretching
frequencies of CO adsorbed on different faces and sites of MgO and a-Cr2O3. Computations on
both relaxed and unrelaxed surfaces have been reported.
Face Site nexp ( cm21) nth ( cm21)
relaxed nth ( cm21) unrelaxed MgO(001) MgO(001) MgO(001) aCr2O3( 011 – 2 ) aCr2O3( 2 – 116 ) aCr2O3( 2 – 116 ) aCr2O3( 112 – 0 ) M2 1 5 cus M2 1 4 cus M2 1 3 cus Cr3 1 5 cus Cr3 1 5 cus Cr3 1 4 cus Cr3 1 5 cus 2157 2170 2230 2181 2172 2179 2158 2162 2173 in work 2178 2171 2174 2163 2162 2175 2216 2178 2191 2213 2165
Dndyn4 13.5 6 0 . 5 cm21 and Dnst4 212.5 6 0.5 cm21 (see fig. 1b). From Dndyn a vibrational polarizability of an4 0.031 Å3 is evaluated, which is quite close to the value
of the unperturbed CO gas. This result further demonstrates that the perturbation induced by the surface is not associated with relevant electron transfer.
Figure 1d reports the relaxed (001) surface of MgO as optimized using MARVIN code [22] together with the potential map generated by such distribution of cations and anions; its gradient gives the corresponding electric-field map. The knowledge of the electric-field modulus computed in the center of mass of the adsorbed CO molecule directly gives, through the Bagus-Pacchioni relationship [18], the predicted theoretical value of the C—O stretching frequency (nth). More details on the methodology adopted in these simulations can be found in refs. [17]. It is worth noticing that the effect of surface relaxation is negligible for regular sites but becomes quite relevant for step and corner sites. In fact, the surface relaxation of both anions and cations is hardly appreciable from the ions distribution (lower part of fig. 1d) and can just be observed in the potential map (upper part of fig. 1d), where the projection of the V 40 surfaces are no more lines parallel to the [001] direction passing through the average point of each Mg2 1
5 cus-O2 25 cussegment but are bent curves joining two of those adjacent average points (see dot-dashed curves). This effect is due to the greater sinking of Mg2 1
5 cuscations with respect of O2 2
5 cusanions in the (001) relaxation process of MgO; as a consequence of this, all the space above the V 40 surfaces lies in a negative potential. On the contrary, the surface relaxation of a step implies a considerable reorganization of both anions and cations positions, as can be directly appreciated from the comparison between unrelaxed and relaxed structures (fig. 2a and 2b, respectively). Table I reports the comparison between experimental and simulated C—O stretching frequencies of carbon monoxide adsorbed on regular Mg2 1
5 cus, sites and on steps (Mg2 14 cus) and corners ( Mg2 1
3 cus) of the (001) surface. The close correspondence between the experimental and the calculated frequencies demonstrates on one side that the assignments are correct and, on the other side, that the surface modelling obtained with force-fields methods [17] gives reasonable results.
2.2. a-Cr2O3: a case system for prismatic oxides. – Following the same approach described for MgO, the adsorption properties of CO on a-Cr2O3 have been studied on
Fig. 3. – FT-IR spectra of CO adsorbed at 77 K on progressively sintered a-Cr2O3. CO coverages ranging from u 4umax (5.33 kPa) to u K0: a) high surface area sample; b) sample sintered at 1173 K b); c) sample sintered at 1273 K. In part b) the spectrum of 12CO/13CO (15/85) isotopic mixture at u41 is also reported. Part d): the (011–2 ) surface of a-Cr2O3(the equipotential surfaces have been cut on plane passing through cations Cr1and Cr2and parallel to the direction [ 011
– 2 ],
i.e. perpendicular to the surface), see comments of part d) of fig. 1. Parts a), b) and c) adapted from
samples with microcrystals dimensions ranging from 20–40 nm (ssa B40–70 m2g-1), up to 500–800 nm (ssa E1 m2g21), see fig. 3 from a) to c). Samples morphology was determined by HRTEM and SEM, and modeled using computer graphics methods [10d,10e]. The shape of the particles can be realistically represented by polyhedra exposing predominantly ( 011–2 ) and to a lesser extent ( 2–116 ) and ( 112–0 ) faces, as schematically reported in fig. 5 of ref. [17a]. In all samples the rhombohedral ( 112–0 ) faces are predominant and exhibit equivalent Cr3 15 cusions; the less abundant ( 2
–
116 ) faces expose both Cr3 1
5 cus and Cr3 14 cus ions and are thus more unsaturated and less stable; finally, the prismatic ( 112–0 ) faces expose Cr3 1
5 cus strongly shielded by the surrounding oxygen ions. By comparing the spectra of CO adsorbed on increasing sintered samples (fig. 3a-c) we can ascribe the narrow band at 2181 cm21 (u K0) and at 2167 cm21 (u K umax) to Cr3 15 cusQ Q Q CO adducts on (011
–
2) faces. The higher FWHM (12 cm21 at u K0 and 9 cm21 at u Ku
max, see fig 3a) observed on the unsintered sample (on samples sintered at 1273 K the FWHM is 6 cm21at u K0 and 1.5 cm21at u Ku
max, see fig. 3c) is due to inhomogeneous broadening effects associated with the smaller extension of the faces. Two shoulders observed at 2172 and 2179 cm21 in fig. 3a for the unsintered sample, become two distinct and narrow bands on more sintered samples (see fig. 3b, c) and are assigned to CO adsorbed on Cr3 1
5 cusand Cr3 14 cusions on ( 2 –
116 ) faces. Finally, the band at 2158 cm21, clearly visible in fig. 3c, is ascribed to Cr3 1
5 cusQ Q Q CO adducts on (112 – 0) prismatic faces. Similarly to the MgO/CO system, all these components undergo a progressive red-shift upon increasing the coverage, from u K0 to uKumax; this shift is due to the progressive building up of dipole-dipole interactions. The hypsochromic shift of the C-O stretching frequency, with respect to the 2143 cm21 value of the unperturbed molecule, indicates that the Stark effect is predominant and thus that the Cr3 1Q Q Q CO interaction has mainly electrostatic character [3d, 3e, 18, 21d-f, h]. However the presence of a weak overlap interactions of s and d 2 p type is not excluded [3d, 3e, 21a] in the present case in view of the transition character of the adsorbing cation. The last hypothesis is in agreement with the results obtained with the method of isotopic mixture [20a], which gives Dndyn4 113.56 0.5 cm21 and Dn
st4 227.5 6 0.5 cm21 (fig. 3b) and thus av4 0.1007 Å3. The latter
value is significantly higher than that of CO gas and of CO adsorbed on other ionic solids with similar structure and without d-electrons (e.g., ZnO [8]). This result demonstrates that d-p contributions associated with the transition nature of Cr3 1 are also present.
In fig. 3d the relaxed structure of the (011–2) surface of a-Cr2O3 is reported together with the potential map generated by such distribution of cations and anions (analogous pictures of other faces are omitted for brevity). On this basis the electric field at the center of mass of the CO adsorbed on cationic sites has been evaluated. Table I reports the comparison between experimental and simulated C-O stretching frequencies of carbon monoxide adsorbed on Cr3 1 ions located on different faces and sites. The effect of the surface relaxation on nth is particularly important for ( 2
– 116 ) face.
3. – Conclusion
From MgO and a-Cr2O3 case-studies, the importance of sintered oxides of well-defined morphology as model solids for vibrational studies of adsorbed probes has been clearly evidenced. The considerably small FWHM of C—O stretching bands, observed
at these surfaces for u Kumax, is a direct consequence of the high regularity of the exposed faces. These studies have facilitated the understanding of the nature of the Mx 1Q Q Q CO bond (being Mx 1the surface cation) and have determined the relative role of
electrostatic and overlap forces of s and/or d-p type. These studies have also allowed the investigation on the reactivity of surface defects (steps, edges, corners), which are particularly abundant on dispersed systems such as those encountered in high surface area catalytic materials. Finally, it is demonstrated that an appropriate simulation of relaxed surfaces can be made by means of force-field methods. The subsequent computation of the electric field map above the surface and of the shift of C—O stretching frequency induced by the associated Stark effect has proved to be extremely helpful in confirming the assignments made by the combined use of morphology study and IR spectroscopy.
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