J . Phys. Chem. 1984, 88, 2587-2591
pure MgO-CO system will be published elsewhere. 1315-cm-' Peak. This transient peak is present in the initial stages of CO adsorption on both MgO and diluted M g O C O solid solutions; so it is primarily associated with the matrix. Its isotopic substitution pattern indicates that the responsible species contain only one carbon atom. Bl-B3 Triplet. This is a dominant feature in the spectra of high-concentration solid solutions and will be dealt with in part 3. Conclusions On the basis of a complete set of isotopic substitution exper-
2587
iments the presence of C O ~ + ( C O(carbonylic) )~ and [COCO(COz)z]2- (carbenoid-carbonylic) structures is demonstrated. These species are formed in fiied ratios in the same surface regions (edges and steps) by concerted interaction of CO with Co2+and 02-Coi+02-ions and groupings of ions. Besides these species, also Mg2+-C0 (&bonded) and (Cz0,)2- (ketenic-like) species formed in the cobalt-free portions of the edges and steps are detected. Acknowledgment. This paper has been carried out with the financial support of the Consiglio Nazional delle Ricerche, Progetto Chimica Fine e Secondaria. Registry No. CO, 630-08-0.
Spectroscopic Study of CO Adsorptlon on Coo-MgO Solid Solutions. 3. CO Adsorption on Aggregated Co2+ Ions A. Zecchina,* G. Spoto, E. Garrone, Istituto di Chimica Fisica, Universitd di Torino, 101 25 Torino, Italy
and A. Bossi Istituto G . Donegani Spa, 28100 Novara, Italy (Received: November 7, 1983)
Clustered Co2+ions present in large amounts at the surface of concentrated MgO-Coo solid solutions are readily reduced at room temperature by CO with formation of Co(CO).,- and carbonate-like species. The Co(CO),- species anchored to the surface are under a distorted C, symmetry. The entity of the distortion can be changed by coadsorbing on the surface lone-pair-containing molecules which, being able to "solvate" the surface MgZ+ions, efficiently shield their electric fields. The vibrational assignment of the CO(CO)~species has been made on the basis of a complete set of isotopic substitution experiments.
Introduction In parts 1 and 2 of this series we have described the interaction of CO with isolated Co2+ions located on both extended faces (part 1) and edges and steps (part 2). On more concentrated samples (MC5) the formation of irreversible carbonylic species (Bl-B4) was detected (together with carbonate-like groups) plausibly associated with nonisolated Coz+ ions. In this paper this problem is specifically taken into account by studying the effect of cobalt concentration in the whole 0-1 mole fraction range; moreover, the structure of the B species is elucidated on the basis of both isotopic substitution experiments and perturbation effects by other adsorbates. Experimental Section All the experimental conditions are the same as those described in paper 1. The I3CO used in these experiments was from Prochem B.O.C. (91.8 atom %). The mixtures were formed in a suitable vacuum manifold allowing well-known volumes of I3CO and I2CO of known pressure to mix for many hours. Results Effect of eo2+ Concentration on the IR Spectrum of Adsorbed CO (2200-1700 cm-' Range). In Figure 1 the IR spectra (2200-1700-~m-~range) of C O adsorbed at room temperature on MgQ (a), MC1 (b), MC5 (c), MClO (d), MC25 (e), and COO (f) are illustrated. One can see the following: (i) For MgO (-200 m2/g) a band pair at 2108 and 2095 cm-' is present which has 0022-365418412088-2587$01.50/0
already been assigned to dimeric ketenic-like C202,- species. (ii) For MC1 (-170 m2/g) the band pair typical of C O adsorbed on the MgO matrix is weakened, while the A1-A3 (2076-2018 cm-I) triplet (with high intensity) and the B1-B3 bands (1970-1790 cm-I) (with very low intensity) appear. (iii) For MC5 (-90 m2/g) the manifestations of the MgO matrix are severely weakened; at the same time the Al-A3 and B1-B3 bands undergo a strong intensity enhancement. (iv) For MClO (50-70 mZ/g) the B,-B, bands become very strong; on the other hand, the A, component is missing or very weak and the A2 one becomes definitely weaker and broader and slightly moves to higher frequencies (2034 cm-I). (v) For MC25 (40-50 m2/g) the Bl-B3 trend toward intensification is definitely established. On the other hand, the band at 2034 cm-I does not show any tendency to disappear (as expected on the basis of the diminution observed in the 5-10 atom 75 concentration range) but undergoes a frequency and intensity increment and further broadening; this fact suggests that around this position (-2040 crn-') a new component (B4) is present which grows with concentration in the same way as the BI-B3 triplet and that the original A, component is missing or weak. (vi) On passing to COO (- 15 m2/g), the spectrum of adsorbed C O remains qualitatively unchanged, the only difference being represented by the frequency maxima which are now at 2040, 1958, 1880, and 1805 (sh) cm-' and by the increased band broadness. The data reported in Figure 1 undoubtedly confirm that the AI-A3 and B1-B4 bands are associated with C O species adsorbed on isolated and clustered Co2+ ions, respectively. Effect of the Adsorption Time on the Spectrum of Absorbed CO. In Figure 2 the spectra in the 2200-1000-~m-~ range of CO
-
0 1984 American Chemical Society
2588 The Journal of Physical Chemistry, Vol. 88, No. 12, 1984
Zecchina et al.
I
m
1
I
18co
16co
1
1000
1203
14CO
c ni'
Figure 2. Effect of the adsorption time on the spectrum of adsorbed CO at room temperature ( 6 kN m-2): after 10 min; (-) after 10 h. (am.)
Figure 1. IR spectra of CO adsorbed at room temperature (6 kN m-*) on MgO (a), MCl (b), MC5 (c), MClO (d), MC25 (e), and COO (f) (21 50-1 700-cm-' range).
adsorbed on a MC3.4 sample taken immediately after C O admission (6 kN m-2) and 10 h later are illustrated. The most relevant features are the following: (i) the carbonylic bands of the Al-A3,species reach immediately their maximum intensity, and then slightly decrease; (ii) the bands at 1970, 1845, 1790 cm-' (Bl-B3) strongly grow with time, so indicating that their formation process is activated in nature; (iii) the complex absorptions centered at 1655 and 1330 cm-I show an intensity increase which is roughly proportional to that observed for the B1-B3 bands. These results definitely confirm that the species which have been differentiated on the basis of their pressure and concentration dependence (A,-A,, Bl-B3) are formed in different processes (fast and slow, respectively). Isotopic Pattern of 1:l 12C&'3C0 Mixtures on MC5. In Figure 3 the spectra of lZCO, 13C0, and a 12CO-13C0(1:l) mixture adsorbed on a MC5 sample (under 6 kN m-2 CO) are compared. The results are essentially the same as those illustrated in part 2 (MC2.5). However, new data emerge more clearly. In particular, we note the following: (i) the splitting pattern of the B1-B3 I2CO triplet is definitely better established (a total of nine bands are present for the 1:l mixture at 1970, 1940, 1927, 1845, 1828, 1804,1790,1760, and 1750 cm-I, three of them being due to I2CO, three to "CO, and three to mixed '2CO-13C0 species; (ii) the isotopic pattern of the low-frequency component of the C032-species (doublet at 1330-1300 cm-') is now clearly seen. In Figure 4 the spectra of lZCO,I3CO, and 12CO-13C0(1 :1) irreversibly adsorbed on a MC5 sample (Le., obtained after prolonged outgassing at rmm temperature) are compared in order to better understand the isotopic pattern of the irreversible species without the disturbance caused by the presence of the reversible ones. The comparison allows the following conclusions to be achieved: (i) The B4 I2CO component (2040 cm-') also splits into a triplet (one due to I2CO (2040 cm-I), one to I3CO (1994 cm-I), and one to a mixed species (2025 cm-I); as a consequence, the B1-B4 ("CO) quartet as a whole originates a total number of 12
I
,' zm
mo
I E ~
I&U
1700
wc
rml
'wx
r
e
m
'ho
I
Figure 3. IR spectra of I2C0 (-), "CO (---), and 12CO-L3C0(1:l) adsorbed on a MC5 sample (6 kN m-2). (..e)
bands. (ii) All the previous conclusions concerning the carbonate-like species are totally confirmed. Effect of Pyridine (Py) and NH,on the B Species. In Figure 5a the effect of Py adsorption on the Bl-B4 bands is described. It can be seen that under the effect of increasing doses of Py the B1-B4 bands move gradually in frequency and that, at the highest coverages, the Bl-B3 peaks tend to collapse into one very strong absorption apparently centered at 1895 cm-', while the B4 component is shifted to lower frequency with decreased intensity. This effect is totally reversible and the original spectrum can be recovered by removing the adsorbed Py by outgassing at the beam temperature (-323 K) (actually the reported spectra refer to this part of the experiment). The previous experiment has been carried out by dosing Py on presorbed CO; if the opposite is done, the result is absolutely the same. This fact implies that Py is able to selectively poison the sites where the Al-A3 species originate. It must be noticed that in the presence of Py the B species are formed in a much faster way than on the clean surface. In Figure 5b the results obtained with NH, instead of Py are reported: it can be seen that the results are very similar, so the conclusion can be safely reached that the reported phenomena are characteristic of the interaction of the B species with bases. Also in this case the rate of formation of the B species is faster than on the clean surface. Although less conclusive, the same effects are initially observed if water is dosed at high enough pressure to give molecularly adsorbed water; however, as HzO can deeply attack the
-
C O Adsorption on Coo-MgO Solid Solutions
The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 2589
OD
>in
&
>sa
&c
?&Cm.l
I&
3im
?am
lim
Figure 4. IR spectra of '*CO (-), I3CO (---), and 1zCO-'3C0 (1:l) adsorbed on a MC5 sample (and outgassed for 10 min). (sa-)
TI
can be explained by taking into consideration that clustered Co2+ are grouped into islands whose properties resemble those of bulky COO, which is an easily reducible oxide (like NiO).2 It is very likely that the initial attack is already occurring on 02-ions bonded to two adjacent Co2+ ions, because carbonate and Bl-B4 species are present in small amounts also on diluted samples. Let us examine more closely the hypothesis that the B1-B4 bands are associated with cobalt clusters whose nuclearity grows with the Co2+concentration, as already hypothesized in a previous paper.3 As the dimension of the cluster increases, it is expected that the relative amount of linear and bridged (edge and/or face bridged) carbonyl groups can change as found in homogeneous cluster carbonylic c o m p o ~ n d s . ~As a consequence it is expected that both the relative intensity and frequency of the carbonylic bands can change gradually with Cd+ concentration. On the other hand, in the whole 0-25% mole fraction range the relative intensities and the frequencies of the B1-B, quartet are rigorously constant. This fact does not rule out-the hypothesis of cluster formation; however, it is in contrast with the assumption that the cluster dimension grows with the cobalt loading. B Species Isotopic Pattern. In the Experimental Section we have concluded that the Bl-B, l 2 C 0 quartet gives 12 bands when a 1:l '2CO/13C0mixture is used (4 due to l2C0, 4 due to 13C0, species). The presence of 12 bands and 4 due to mixed 12CO-13C0 suggests that in the B species two different pairs of coupled CO oscillators are present in the same mononuclear and/or polynuclear structure. In fact, if we assume that the B4-B1 and Bz-B3 components are the coupled modes of two different co co
/co/
Figure 5. (a) Effect of Py on the spectrum of Bl-B4 bands. Pyridine pressures: 1.5 (-) and 0.15 (---) kN m-*; lower pressures obtained by outgassing for 1 and 15 (---) min. (b) Effect of NH3 on the spectrum of BI-B, bands. NH3 pressures: 6 (-), 0.6 (---), and 6 X kN m-2; outgassed for 1 min (-.-). (e..)
(as.)
solid solution leading to a gradual transformation of the whole solid (which is transformed back into the mixed hydroxide in 1 month), the experiment is not reported.
Discussion Concentration Dependence of the B,-B, Quartet and the Carbonate Bands. Unlike the Al-A3 bands, the B1-B4 ones are favored at the highest loadings (Figure 1). Hence, they are associated with Co2+ions in aggregated form. Moreover, at room temperature they are always accompanied by a band pair at 1655-1330 cm-' which are undoubtedly due to carbonate-like species (the presence of only one carbon atom in the moiety responsible for these bands will be demonstrated in the following). These oxidized species represent a clear monitor of a surface reduction process occurring at room temperature. This process is activated; in fact, it is not observed at all at 77 K, and is slow even at room temperature (Figure 2). Following this hypothesis, the B,-B4 bands should be due to the vibrations of carbonyl groups bonded to reduced cobalt centers. Moreover, as they originate from reduction of aggregated ions, a plausible hypothesis is that they are associated with CO adsorbed on cobalt clusters whose nuclearity depends upon the dimension of the original surface ionic aggregates. Before examining the last hypothesis, we stress that a reduction process is proved not only by the presence of carbonate ions (oxidized species) but also by the frequency values of the B1-B4 bands; in fact, they fall into a frequency region which is undoubtedly characteristic of CO stretching modes in zerovalent or negatively charged cobalt complexes.' The reason that clustered Co2+ions are more easily reduced than the isolated ones
moieties present in the same B entity, the correct number of bands is readily derived for the 1:l mixture (the 1:2:1 relative intensity of the species is also correctly understood). This preliminary assignment is confirmed by solving the two 2 X 2 secular determinants. In fact, on this basis the following K and i constants (in N m-l units) for the two dicarbonylic moieties (a and b) are obtained ( K = (4.0383 X 1 0 + ) ~ - ~ )K: , = 1625.42, i, = 56.74, Kb = 1332.22, ib = 40.314. Therefore, the frequencies of the carbonylic bands for the mixed species are readily obtained. It turns out that they coincide with the experimental ones within f l cm-l, with the exception of the 1828- and 1760-cm-' peaks, which differ from the calculated ones by -3.4 and +4.2 cm-', respectively. The agreement is very good and so the structure of the B species can be formulated as follows co co 'co
/
0 ' '
co
coo
/
'cob
where the two moieties can be associated with two different cobalt atoms in a dinuclear or polynuclear entity, or with a mononuclear one co co \
/
co/ c o \ c o having the formula CO(CO)~"(x = 0, 1-) with structure very distorted from the T d one. The K values obtained are very similar to those of the Co(C0)L anion in different solvents, and so a mononuclear C2, distorted structure is suspected. For the time being a more complete assignment is not possible and more experiments are needed, which will be discussed in the following. Effect of Basic Molecules on the A Species. In the presence of 10 and 100 torr of Py and NH3 (1 torr = 133 N m-2), re(1) P. s. Braterman, "Metal Carbonyl Spectra", Academic Press, London, 1975, and references therein. (2) E. Guglielminotti, L. Cerruti, and E. Borello, Gazz. Chim. Ital., 107,
503 (1977). (3) A. Chiorino, E. Garrone, G . Ghiotti, E. Guglielminotti, and A. Zec-
china in "Proceedings of the 7th International Congress on Catalysis, Tokyo", T. Seiyama and K. Tanabe, Eds., Elsevier, Amsterdam, 1981, p 136. (4) P. Chini, Inorg. Chim.Acta, Rev., 31 (1968).
2590
The Journal of Physical Chemistry, Vol. 88, No. 12, 1984
spectively, the A-type species are absent and the spectrum of the B ones is deeply modified. The inhibiting effect of bases on the A species formation can be explained in a simple way: Le., N H 3 and Py are preferentially adsorbed on reactive cobalt ions via strong a-bonds (some dissociation not being excluded), so preventing any further reaction with CO. The inhibiting effect of N H 3 and Py is also observed when they are doses on presorbed CO, so confirming that a displacement reaction is taking place. The displacing effect of Py and N H 3 is also well justified if the reversible nature of C O species of the A type (Al, A2, and A,) is taken into account. The effect of water is likely more complex because it is extensively dissociated on the surface, as demonstrated by the formation of the hydroxyl stretching bands (data not reported for sake of brevity). Effect of a-Donors on B Species. In the presence of both Py and NH3 the B species are readily formed with a high-intensity and altered spectrum. Two questions arise: i.e., (i) why do bases promote the formation of the B species; (ii) why does the spectrum of the B species depend upon the Py and/or ammonia coverage? As far as the first question is concerned, the answer is straightforward. In fact, from the discussion in paper 1 we have concluded that the adsorption of a molecule on a cationic site (MgZ+and/or Co2+)through a bond having prevailing u-donor character causes an increase of electron density on the ions located in their immediate vicinity. This effect is expected to depend largely upon the basicity of the molecule and so to be very great when Py and NH, are used. This electron density excess will be delocalized on both the cations (Mg2+ and/or Co2+) and the anions (02-). If Coz+ions are considered, their back-bonding ability toward a r-acceptor molecule (like CO) will be increased (as already observed and discussed). If the 02-ions are taken into consideration, the basicity is expected to be enhanced. If a (Co2+),(02-), cluster is considered, the contemporaneous increase of electron density on both Co2+and Oz- ions ultimately facilitates formation of the B species, which at the same time requires (vidg infra) basic oxygen ions (leading to CO?- groups) and efficient interaction with n-acceptor molecules. As already discussed, once formed the spectrum of the B species is strongly affected by the amount (coverage) of Py and N H 3 reversibly adsorbed on the surface. When the Py and NH3 coverage is high, the spectrum of the B species is mainly represented by a very strong triplet centered at 1895 cm-' and by a weak component at -2010 cm-I. The frequency of the stronger absorption is very near to that of the Co(C0); ion in the undistorted Td so a mononuclear structure is suspected. The normal C O stretching modes of the CO(CO)~;group with tetrahedral ( T d ) structure are of the Al (IR inactive) and Tz (IR active, 3-fold degenerate) type. If k and i are the force and interaction constants, then the two frequencies of the undistorted anion are as follows: k 3i (A1), IR inactive; k - i (T2), IR active. If the Me(C0)4group is distorted by an axial field (which causes a decrease in the symmetry from Td to C,"), then the T2mode splits into two components with frequencies k - i 26 and k - i - 6 (6: distortion parameter) and the Al one becomes IR active. For 6 values smaller than i, the IR spectrum of the axially distorted Me(C0)4 group will consist of a weak narrow band due to the totally symmetric AI mode with frequency near to k + 3i and a strong broad absorption centered at k - i (which is due to the overlap of two components). It is most noticeable that the CO(CO)~-ion in T H F ~ o l u t i o n (where, ~ ~ * ~ ~due to the action of the counterion, it is thought to be in an axially distorted C3, situation) shows an absorption at 2002 cm-' and a very strong one at 1888 cm-' which closely corresponds to the absorptions observed at the highest Py and N H 3 coverages. However, on the basis of a simple axial distortion, the presence of four bands (a narrow weak one and a strong triplet) cannot be explained. If a deeper distortion is introduced into the original Td symmetry, then a Cz, situation is obtained where the stretching inodes of the
-
+
+
-
(5) W. F. Edge11 and J. Lyford IV, J . Chem. Phys., 52, 4329 (1970). (6) W. F. Edge:Il, M. T. Yong,and N. Koizumi, J . Am. Chem. Soc., 87, 2563 (1965).
Zecchina et al.
U
+ +
complex are represented by rstr = 2Al B1 B2 and are all IR active. The frequencies of the four modes are given by the equations Ig-i+a)-K
A,
2i
(k+i-a)-K
Bl
K=k-i+6
B2
K=k-i-6
I (1)
where k and i are the same as with the undisturbed anion (and are available from the literature). In our situation, we are dealing species is present, it is in C2, with four bands, so if a CO(CO)~symmetry. The k and i values are intrinsic properties of the tetrahedral species while 6 is a measure of the distortion caused by the surface and so depends upon Py and NH3 coverage [ 6 6 ( 6 ) ] . At every 6, a set of four frequency values is obtained, so the k , i, and 6 can be readily calculated. It comes out that (i) the k and i values are fairly independent of 6 and are very similar to those of the undistorted anion, and (ii) the 6 values decrease when the Py and N H 3 coverage is increased. If the frequency values are plotted vs. 6, the points of Figure 6 are obtained. The bold lines are calculated on the basis of eq 1 using the k and i values determined for the free CO(CO)~anion.' One can see the following: (i) The experimental points reasonably fit the theoretical equations. (ii) On the clean surface the distortion parameter 6 is larger than i. (iii) The dotted lines reported in the figure represent the limiting curves for the totally symmetric vibration Kl = k + i + 6
K2 = k + i - 6 which are obtained from eq 1 when 6 >> i. (iv) On the clean surfaces the four modes are quite well described by the limiting equations K(A1), = k i + 6 K(Bl) = k - i + 6
+ K(A1)2= k + i - 6 K(B2) = k - i - 6 or, by calling k + 6 = K,, k - 6 = Kb K(Bl) = K, - i K(A1), = K, + i K(B2) = Kb - i K ( A , ) , = Kb + i
which are the equations of two different pairs of coupled C O oscillators, characterized by two different force constants K, and Kb. From all these considerations it comes out that (i) the B1-B4 bands belong to the CO(CO)~-mononuclear species, (ii) the substitution pattern is completelyjustified, (iii) on the clean surface the CO(CO)~-is in a C, situation because of the interaction with the surface, and (iv) if molecules able to "solvate" the surface cations (Mg2') are presorbed on the surface, the distortion decreases and at the highest coverages the CO(CO)~become more symmetric and probably partially free. It is worth mentioning that similar effects have been observed in solution, where the Co(CO),- spectrum is strongly influenced by the nature of the ~ o l v e n t . ' , ~In - ~solvents able to efficiently solvate the positive counterion, the spectrum is near to that expected for the T d symmetry of a Co(CO), species; in solvents where contact pair formation is not prevented, four bands are observed, as in our case. The shift of the bands is fairly continuous with Py and N H 3 coverage changes, and this fact is apparently difficult to understand. In fact, the CO(CO)~anion should be primarily in contact
C O Adsorption on Coo-MgO Solid Solutions
/ ,
The Journal of Physical Chemistry, Vol. 88. No. 12, 1984 2591 undergo only a very small decrement, while B bands continuously grow at room temperature, also when no more change is observed for the A ones. These facts indicate that (i) only a very limited fraction of the A species (a few percent) are transformed into the B ones, and (ii) the major fraction is formed independently in a process occurring on different surface regions. Effect of Co2+on the MgO Chemistry. From inspection of Figure 1 it is deduced that starting from MClO solid solutions the ketenic-like species are missing or very weak. The same happens for the A,-A3 ones. This fact can only be explained by assuming that in MClO solid solutions no (or very few) O2-MgO2and isolated 02-Co2+02-groupings are present on the edges. The total disappearance of such groupings is only expected for higher Co2+contents, where Co2+clusters become the major structure. So we conclude that a Co2+enrichment occurs on the edges, which is caused by the preferential tendency of the Co2+ ions to occupy low coordinate sites (in agreement with the known stability of tetrahedral and distorted tetrahedral complexes).
, ,
Conclusion
Figure 6. Frequency of the B1-B4 bands (expressed in terms of K and i units) vs. the distortion parameter (K = 4.0383 X lo4 cm-*) (in N m-I units).
with a few positive counterions of the surface and so discontinuous effects are expected corresponding to the selective step-by-step solvatation of the nearest counterions. We think that the explanation of the continuous shift is related to the long-range nature of the Coulombic forces, which causes the Co(C0)c entity to sense solvation effects occurring also several spacings away. Formation Mechanism of the B Species. As they are formed only on islands of clustered Co2+ ions the mechanism is lOC0
-
+ 3 0 ” + 2CoZ+
~ C O ( C O )+ ~ -2C032-
(2)
In the B species the cobalt is in the lowest oxidation state (1-), while in the AZ-A3 ones it is nearly zerovalent. So the question arises whether the Az-A3 species are intermediates for the B species formation. The answer is given by examination of the intensity dependence of the associated bands. It comes out that the A bands reach immediately their maximum intensity and then
The microcrystalline MgO-Coo solid solutions can be prepared in the form of cubelets where 98-96% of the surface ions are located on the (100) faces and 2-4% on edges (steps). C O adsorption on extended (100)faces of diluted solid solutions occurs on both Mg2+and Co2+ions with formation of a- and 6-a-bonded carbonyls. C O groups adsorbed on the Mg2+ ions extensively interact via dipoledipole coupling; the C O groups on isolated Co2+ ions are decoupled from the C O oscillators on Mg2+ions (which are the major fraction); however, a mutual interaction (chemical in nature) is observed which occurs via an inductive effect through the solid. CO adsorption on edges (steps) gives an entirely different chemistry: in fact, C O primarily reacts with 02-Mg2+02-and/or 02-Co2+02-triplets to form ketenic-like and carbonylic-carbenoid entities, respectively. The presence of a Mg2+ and Co2+ only chemistry (leading to both Mg2+-C0 and C O ~ + - ( C O adducts) )~ is also evident. The contemporaneous presence in fixed ratios of both Co2+, Mg2+ and 02-Co2+02-,02-Mg2+02-reactive species is considered as a typical (fingerprint) manifestation of “edge chemistry” and has been interpreted on the basis of a simplified (unrelaxed) model of the edges (steps). Clustered Co2+are readily reduced by CO at room temperature with formation of Co(C0); species and carbonate-like groups. The CO(CO)~-species are under a distorted C2, symmetry. The entity of the distortion can be changed by coadsorbing on the surface lone-pair-containing molecules which, being able to solvate the surface Mg2+ ions, efficiently shield their electric fields. Acknowledgment. This paper has been supported by the Consiglio Nazionale delle Ricerche, Progetto Chimica Fine e Secondaria. R&hy NO. Co(CO)L, 14971-27-8; CO, 630-08-0; NH,, 7664-41-7; Py, 110-86-1.
