J . Phys. Chem. 1984, 88, 2582-2587
2582
Spectroscopic Study of CO Adsorption on Coo-MgO Solid Solutions. 2. CO Adsorption on Isolated Cobalt Ions Located on Edges and Steps A. Zecchina,*+G . Spoto,t E. Borello,t and E. Giamellot Instituto di Chimica Fisica and Istituto di Chimica Generale ed Inorganica, Facoltri di Farmacia, Universitd di Torino, 101 25 Torino, Italy (Received: November 7, 1983)
The room-temperature surface chemistry of diluted MgO-Coo solid solutions toward CO is totally accounted for by the selective adsorption of CO on edges and steps whereby the MgZC,Cozt and 02-coordinately unsaturated ions primarily react as 02-Mg2+0and/or Oz-CoztO2-triplets to form CzO?- (ketenic-like) and [ (C02),CoCO]2- (carbenoid-carbonylic) species whose relative concentration depends upon the cobalt oxide mole fraction. Besides these species, also MgZ+-CO (a-bonded) and C O ~ + ( C O(u--a-bonded) )~ complexes are contemporarily detected. The simultaneous presence in fixed ratios of both CoZf, Mgz+ and (02-MgZf02-),(Oz-Co2+Oz-)reactive ions and groupings of ions in the same surface regions is considered as a "fingerprint" manifestation of the "edge chemistry" and has been interpretedon the basis of a simplified geometric (unrelaxed) model of the edge regions. The vibrational assignments of all the surface structures have been made on the basis of a complete set of 12C013C0isotopic substitution experiments.
Introduction In part 1 of this series we described in detail the interaction of CO with Mg2+and Co2+ions located in regular lattice positions of extended (100) faces. As these sites represent 96-98% of the total number present on the surface, the study of CO-solid solution (dilpted) interaction at 77 K has enabled us to investigate the adsorption properties of the major fraction of the surface. Other species were also detected: some of them (also present on pure MgO) monitor the residual activity of coordinately unsaturated oxygen ions in low coordination located on edge (step) positions; the others (A,-A3 triplet) are specifically associated with very reactive cobalt-containing centers presumably located on very exposed positions. In this paper the structure of these species will be discussed in detail on the basis of experiments (IR, UV-visnear-IR reflectance and volumetric) carried out at room temperature.
Figure 1. IR spectra of CO adsorbed at room temperature on a MC2.5 high-surface-area sample in the 21 50-1 lOO-cm-' range. Desorption experiment: (-) 6, (..-) 0.6, (---) 6 X and (---) 6 X lo-' kN m-z; outgassed 1 min; (-) outgassed 10 min. Inset: difference spectra in the 1550-1 150-cm-I range obtained by subtraction of the background (Le., the spectrum of the room-temperatureirreversible species). (-a*-)
Experimental Section All the experimental conditions are described in paper 1. The I3CO used in these experiments was from Prochem B.O.C. (91.8 atom %), The mixtures were prepared in a suitable vacuum manifold by mixing known volumes of I3CO and I2CO. Results IR Spectra of Adsorbed l2C0. Figure 1 shows the IR spectra of CO adsorbed on MC2.5 sample activated in vacuo at 1073 K. Similar spectra are obtained for MC5. Under 6 kN m-2 CO, the AI-A3 triplet is present on both samples, the overall relative intensity being in the order MC2.5 > MC5. Several other components are observed at 1970, 1845, 1790, 1742, 1655, -1635 (only on MC2.5), 1575-1565 (doublet), 1545, 1520, 1495, 1458, 1425,1380-1370 (complex), 1130, 1315 (observed only on MC2.5 for adsorption times not longer that 15 min), -1265 (sh), 1257, 1222 (only on MC5), 1195, and 1167 cm-I. Hereafter the bands at 1970, 1845, and 1790 cm-' (presumably associated with carbonyl groups) will be referred to as the B1-B3 triplet. For the MC2.5 sample the 1575-1565, 1545, 1380-1370-, and 1167-cm-* bands show higher relative intensity; on the other hand, for the MC5 sample (spectra not reported for the sake of brevity) the B1-B3 triplet and the 1742-, 1655-, and 1330-cm-I bands are definitely more intense. The bands at 1575-1565, 1545, 1380-1370, and 1167 cm-I are typical of CO adsorption on the MgO matrix' and their preferential presence on the more diluted samples is not unexpected. Volumetric determinations indicate that the total C O coverage is typically in the 0.014-0.017 range (expressed as the number of CO molecules adsorbed for Mg2+02'Istituto di Chimica Fisica. Instituto di Chimica Generale ed Inorganica, Facultl di Farmacia.
*
0022-3654/84/2088-2582$01.50/0
and/or CozC02-pair), Le., slightly larger than on pure Mg0.I Moreover, 7 4 % is reversible at room tempeature. When the CO pressure is decreased, the AI-A3 triplet gradually disappears as well as the bands at 1520, 1495, 1458, 1425, 1265, 1257, 1195, and 1315 cm-' (present only on the MC2.5 and on pure MgO). As the behavior of the AI-A3 triplet upon desorption is quite complex, in Figure 2 an exploded view is reported. Another exploded view of the low-frequency rversible partners is reported in the inset of Figure 1 (actually the difference spectra are shown in order to minimize the disturbance caused by the simultaneous presence of irreversible species). From inspection of Figure 2, the following can be seen: (i) When the C O pressure decreased to 0.6 kN m-2, only the 2028-cm-I band is affected, showing that in this position a species (A2) characterized by a single carbonylic band is absorbing. (ii) At lower pressures the intensity of the 2076-cm-' band decreases also and there is a proportionality between the decrements of the 2076-cm-' band and the residue of the 2028-cm-' component (now shifted to -2030 cm-I). This indicates that they belong to a multicarbonylic species (A,) characterized by two carbonylic bands (2076 and -2030 cm-') with relative approximate intensity ratio 1:2. (iii) As the intensity of the 2076- and -2030-~m-~bands decreases, a new band grows at 2083 cm-l; an isosbestic point is observed at 2079 cm-'. (iv) kN m-2 also the 2083- and At pressures lower than 6 X 201 8-cm-l components decrease and completely disappear after 10- (the former) and 15-min (the latter) outgassing at room temperature.
-
(1) E. Guglielminotti, S. Coluccia, E. Garrone, L. Cerruti, and A. Zecchina, J . Chem. SOC.,Faraday Trans. 1, 15, 91 (1979).
0 1984 American Chemical Society
CO Adsorption on Coo-MgO Solid Solutions
The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 2583
cm-1.
a
Figure 3. (a) Reflectance spectra of a MC5 high-surface-areasample before (full line) and after CO adsorption (6 kN d)broken line) at room temperature. (b) Reflectance spectra of a MC5 high-surface-area sample after CO adsorption (6 kN m-*) (broken line) and after successive outgassing for 10 min (dotted line) at room temperature.
3
m
v
d 0
2 m
cm-'
xxx)
Figure 2. Exploded view of the CO bands in the 21 50-2000-cm-' range (MC2.5 sample): desorption experiments.
However, despite their similar behavior upon desorption, the 2083- and 2018-cm-' bands do not belong to the same species because their intensity ratio is not constant (the 2018-cm-' Component is already present with its full intensity when the 2083-cm-' one is missing). In conclusion, as schematically shown in the inset, at -2028 cm-' two bands are superimposed, one due to a very reversible monocarbonylic species and the other (coupled with the 2076-cm-' one) associated with a polycarbonylic one. It must be noticed that the relative intensities of the A,-A3 bands are constant for all examined samples; this fact suggests that they belong to surface complexes which are formed in fixed ratios on the surface. By examining the whole set of spectra illustrated in Figures 1 and 2, one can derive the following conclusions: (i) The bands at 2028, 1495, 1425, 1265 (sh), and 1180 cm-' are readily depleted by lowering the C O pressure. As their relative intensities are constant, they must be assigned to a single species (Az). (ii) The existence of the isosbestic point indicates that the A, (2076 cm-l) species quantitatively changes into a new Al' species (2083 crn-') by C O abstraction. (iii) N o low-frequency bands show similar behavior; so the A1-Al' bands have no low-frequency partners. (iv) For longer outgassing times also the Al' species is desorbed. (v) The bands at 2018 (A,) and 1520,1458,1252, and 1195 cm-' show parallel behaviors upon room-temperature outgassing; so they belong to a single complex. The correlations are summarized as follows (cm-I): 2076,2030 (Al); 2083 (Al'); 2028, 1425-1495 (doublet), -1265-1180 (doublet) (Az); 2018, 1520-1458 (doublet), 1595-1252 (doublet) (A3). Both Az and A3 species are characterized by a high-frequency (carbonylic) band and by two pairs of low-frequency bands. The BI-BJ triplet, unaffected by room-temperature outgassing, is related to carbonylic species irreversibly bonded to the surface. The fraction of CO adsorbed reversibly has been estimated by means of secondary adsorption isotherms. On the basis of the assumption of a 1:l CO/Coz+ ratio,
it turns out that approximately 7-8% of surface Coz+ are able to form reversible species. The assumption of a 1:l CO/CoZe stoichiometry is highly oversimplified. In fact, it will be shown in the following that the reversible species contain three C O molecules per Co2+center. As a consequence, the fraction of Co2+ ions active in reversible adsorption is much lower (2.3-2.7%), so suggesting that only those ions which are in a very unusual coordinative state (i.e., on edges (steps) and/or corners, kinks, etc.) are involved. It is most noticeable that this figure is only slightly larger than that of the Coz+fraction present on the edges of perfect cubelets. The values found in this investigation are very similar to those found by Indovina et al.* for the concentration of the Co(CO), species which are reversible to room-temperature outgassing. After room-temperature desorption, a peak is observed at 2040 cm-' for both MC5 and, much weaker, also the MC2.5. We anticipate that it belongs to the BI-B3 family (vide infra); hereafter it will be labeled as B4. The irreversible 1655- and 1630-cm-' bands have intensity proportional to those of the BI-BI quartet. In part 3 of this series it will be shown that they are due to surface carbonates monitoring the occurrence of a surface reduction process with formation of the B species. W-Vis-Near-ZR Spectra of Adsorbed CO. Figure 3a shows the spectra of a MC5 sample before and after C O adsorption. Notice the following: (i) the main bands assigned to square-pyramidal Coz+ ion on the (100)faces are only marginally affected (an intensity erosion of a few percent is observed for the two peaks at 16230 and 7000 cm-' and a slightly greater one for the 13 800-cm-' peak6); (ii) the low-intensity band at 4000 cm-' is totally destroyed; (iii) an absorption increase is observed at -8500 cm-'; (iv) a strong and broad band develops with a maximum at 23 000 cm-'. By room-temperature desorption (Figure 3b) the following modifications are observed: (i) the 4000-cm-' band is totally recovered; (ii) the original intensity of the 8500-cm-' shoulder is restored; (iii) a small intensity recovery is observed at 13 800 cm-', suggesting the presence (in this position) of a transition associated with sites involved in the reversible process; (iv) the 23 000-cm-l band readily disappears; only a weak residue at 20000 cm-' is present after 10-min outgassing at room temperature; (v) the main peaks (square-pyramidal Coz+) do not recovery their original intensity, indicating that an irreversible modification of the surface (although involving a very small fraction of ions) has taken place. Isotopic Substitution Experiment. In Figure 4 the overall spectrum in the 2150-1100-~m-~range of a 1:l '3CO-12C0 mixture adsorbed at room temperature on a MgO-Coo solid solution (MC2.5) is illustrated in detail, while in the inset the difference spectra (in the 1550-1 150-cm-' range) between the lower solid curve and the upper curves are reported. This rep-
-
(2) V. Indovina, D. Cordischi, and M. Occhiuzzi, J . Chem. Soc., Faraday
Trans. 77, 811 (1981).
2584 The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 I
I
Wn,
11
Al Figure 4. Room-temperature spectra of a 1:l ' 2 C ~ 1 3 Cmixture 0 on a MC2.5 (high-surface-area)sample in the 21 5121- 100-cm-I range: desorption experiment. The pressures are the same as those reported in Figure 1. Inset: difference spectra in the 1550-1 150-cm-' range obtained by subtraction of the background (Le., the spectrum of the room-temperature irreversible species).
Figure 5. Room-temperature spectra of I2CO(-), "CO (-.-), and 1:l 'zCO-13C0(--) (6 kN m-2) adsorbed on a MC2.5 sample.
resentation is similar to that reported in Figure 1 and is particularly useful for correlating the room-temperature reversible bands in both the high-frequency (2100-1950 cm-') and the low-frequency (1 550-1 150 cm-l) regions without the disturbance of the irreversible ones (whose relative intensity is particularly large at low frequency). The following can be seen: (i) the A2 (l2C0) carbonylic peak (2028 cm-l) has a single low-frequency (13CO) partner at 1986 cm-' (A, = 42 cm-'); (ii) the A, (l2C0) carbonylic
Zecchina et ai. peak at 2018 cm-' has a (13CO)isotopic partner at -1973 cm-I (Ai, = -45 cm-I); (ii) the A'' ("CO) carbonylic peak shows an isotopic component at 2038 cm-' (A, = 45 cm-'); (iv) mediumintensity peaks and shoulders are observed at 2069,2058, -2008, and -1996 cm-', which (because of their low intensity and reversibility) originate from the Al species; in the figure these peaks are labeled with an asterisk; (v) the bands due to the B species show indications of isotopic splittings; however, their intensity is too low for a MC2.5 sample to allow any certain correlation; (vi) the high-frequency components of the "carbonate-like" species are slip into two components at 1655 (I2CO?- and 1618 (I3CO3") cm-'; (vii) the isotopic pattern of the low-frequency component of the carbonate-like species is obscured by the bands of the ketenic-like species (a quartet at 1380, 1365, 1350, and 1332 cm-') (see inset); (viii) one of the low-frequency partners of the A, species (1458 cm-l) splits into a doublet (1458 and 1425 cm-I); (ix) the narrow peak at 1315 cm-', observed also for pure MgO in the initial stages of the l2C0adsorption, gives a second I3CO component at 1285 cm-'. In Figure 5 the IR spectra at room temperature (6 kN m-2) of '*CO, "CO, and 12CO-13C0(1:l) adsorbed on a MC3.4 sample are compared. It can be seen that (i) except for a few low-intensity components at 2069,2058, -2008, and -1996 cm-' (and labeled with an asterisk) the spectrum in the high-frequency range of a 1:l mixture is essentially the superposition of the I3CO and l2C0 spectra, (ii) each of the B1-83 bands likely originates a triplet of components (this will be confirmed later on a more concentrated sample), and (iii) also the low-frequency part of the 12CO-13C0 (1:l) spectrum is apparently very similr to the superposition of the l 2 C 0 and I3CO spectra with the exception of the splitting of the ketenic-like bands which indeed originate a quartet (1380, 1365, 1350, and 1332 cm-I). The spectra of the reversible species (in both the high- and low-frequency ranges) are compared in greater detail in Figures 6 and 7 at two different coverages. In part b of both figures only the difference spectra are compared because they, being completely free from the contributions of the irreversible species, can be more easily compared with those obtained in the high-frequency range (2100-1950 cm-'). One can see the following: (i) each component observed at low frequency splits into two bands (l2C0 and 'TO); this means that the spectrum of the 1:l mixture is identical with the superposition of the pure ' T O and 13C0 ones; (ii) it is confirmed that the narrow component observed at 1315 cm-' only on a diluted sample (and due to species adsorbed on the MgO matrix) splits into two components at 1315 ( W O ) and 1285 (13CO) crn-'; (iii) the
0. I
Figure 6. Room-temperaturespectra of I*CO (-), I3CO(---),and 'zCO-'3C0 (1:l) 6 . s ) (6 kN m-z) in the 215C-1950- and 1550-1150-cm-' ranges (exploded view).
C O Adsorption on Coo-MgO Solid Solutions
The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 2585
I
I 2103
2050
Figure 7. Room-temperature spectra of l2C0(-),
XIX
cm-'
"CO (--), and 12CO-'3C0(1:l)
carbonylic components of the A, species at 2076 and -2030 cm-l originate a complicated isotopic pattern (at least four new bands at 2069, 2058, -2008, and 1996 cm-') showing that a polycarbonylic structure is involved; (iv) the A' and A, species contain only a single CO oscillator because the spectrum in the carbonylc region is the superposition of the pure l2C0 and I3CO spectra (Figure 7).
-
1403
ICM, (-.a)
(6
K4X
1xT)
kN t K 2 ) adsorbed on a MC2.5 sample.
pattern is so more complex and is more similar to a ligand exchange reaction than to a plain ligand abstraction. A plausible mechanism is represented in eq 2, where the removal of two C O
?
...'I.,
Discussion A I Species (2076-2030 cm-'). The A , species have mononuclear character, because their intensity is maximum for the most diluted samples. They are associated with two IR bands at 2076 ligands is accompanied by the simultaneous insertion of an and -2030 cm-' with a 1:2 intensity ratio. The isotopic l2C0equivalent number of surface ligands (Oz-). This scheme explains 13C0 pattern (six bands) is consistent only with a tricarbonylic (i) the absence of dicarbonylic species as intermediates and (ii) ~ t r u c t u r e . ~If the exact 2032-cm-' figure is assumed for the the frequency increase, because the insertion of the 0,- ligands frequency of the second component, it is possible to calculate (by enhances the positive character of the Co2+ion. Following these solving the secular e q ~ a t i o n the ) ~ force and interaction constants considerations it is inferred that the Co2+ ions originating the K and i. On this basis, the frequencies of the four isotopic species Al'-Al complexes are slightly mobile on the surface. C O * + ( ~ ~ C O ) , ,C O ~ + ( ~ ~ C O C) ~O ,~ + ( ~ ~ C O ) ( ~ ~ C O ) ,A,, and A , Species. They have mononuclear character (because C O ~ + ( ' ~ C O ) ( ' ~ Ccan O )be ~ readily obtained. It turns out that they are favored at the lowest loadings) and contain only one CO the calculated values agree well with the observed ones (within (carbonyl) group. The CO stretching frequencies of these com& 1 cm-'). The agreement is very good and supports the asplexes are very similar to those expected for zerovalent (neutral) signment. The existence of only two bands in the 13C0spectrum cobalt carbonyls. For instance, the observed values are very near with 1:2 intensity is a clear indication that the complex has a C3, to those associated with the l 2 C 0 stretching in 12COCoo(13CO)3 symmetry; as a consequence the structure is complexes isolated in cryogenic mat rice^.^ Unlike the A,-A,' species, the A2-A3 ones are also characterized by two pairs of co co co low-frequency bands. These bands (having very unusual frequency values) should involve the asymmetric and symmetric stretching x=0 ' of carbon-oxygen bonds isharacterized by bond orders intermediate I between one and two as in carboxylate-type units. Indeed, the X existence of only one carbon atom in these moieties is confirmed where X is a surface site (02-) and where a C, axis exists along by isotopic substitution experiments; in fact, the spectrum of the the Co2'X bond. Upon C O removal the tricarbonylic species is '2CO-13C0 (1:l) mixture is equivalent to the sum of pure l 2 C 0 transformed into a monocarbonylic A,' one following the (reand 13C0contributions. The observed frequencies do not comversible) mechanism pletely fit the known values for monodentate, bidentate, and polydentate carboxylate^,^ so the hypothesis is made that they have carbenoid structure. In fact, this structure is preferred when a carboxylate unit is directly bonded through a Me-C bond to a Mechanism 1 is, as such, oversimplified. In fact, due to the smaller transition-metal atom.6-8 Moreover, because both species have number of C O ligands attached to the Co2+ center in the AI' structure, the CO frequency is expected at definitely lower values , and Co(C0) (as found, for instance, in the C O ( C O ) ~Co(CO),, (4) L. A. Hanland, H. Huber, E. P. Kunding, B. R. McGarvey, and G . F. series synthetized in cryogenic mat rice^).^ The decarbonylation Ozin, J . Am. Chem. Sac., 1'7, 7054 (1975).
\&/
(3) P. S. Braterman, 'Metal Carbonyl Spectra", Academic Press, London, 1975.
(5) G . B. Deacon and R. .J. Phillips, Coord. Chem. Reu., 33,227 (1980). ( 6 ) F. A. Cotton, Prog. Inorg. Chem., 16, 487 (1972). (7) E. 0. Fisher, Pure Appl. Chem., 24, 353 (1970). (8) E. 0. Fisher, Pure Appl. Chem., 24, 407 (1970).
2586 The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 two different low-frequency band pairs, the hypothesis is made that two slightly different carbenoid (carboxylate) units are present in each mononuclear species, which are formed by CO attack on two 02-ions adjacent to a cobalt ion. The overall reaction scheme is
co (3)
A reactive 02-Coz+02-cluster is expected to be present on edges (steps); in fact, in these situations the ions are in 4-coordination and consequently will show enhanced reactivity. The presence of carbenoid units is also demonstrated by (i) the quasineutral character of the cobalt center (due to the well-documented ability of the carbenoid units to transfer electronic charge on the metallic centers attached to them)bs and (ii) the presence of the reversible band at 23 000 cm-' in the reflectance spectrum, which is typical of these s t r ~ c t u r e s . ~ ~ ~ Formation Mechanism of AI-A3 Species. Ai, AZ,and A3 species involve only 2.5-2.7% of the total amount of surface cobalt ions, close to the fraction of cobalt ions located on edges and steps. It is so inferred that the Coz+ ions on edges (steps) interact with CO to give Co2+(C0), and [ C O C O ( C O ~ ) ~species. ]~In order to understand this fact, it is worth analyzing first the "edge chemistry" of CO on pure MgO. The ions of a row located on an edge (step) can be represented (in a first approximation, Le., leaving out relaxation effects) as in (4). They primarily react (4 I
o=
0
:
Mg"
X
oxoxoxoxoxoxoxoxoxoxoxo with CO in groups of three ( 1Mg2+,202-) following mechanism 5 as already demonstrated in ref 1. In consequence of the reaction
9
C
II ,c
\
0- Mg2* 0'-
(5)
CO; Mg2'O-
of all possible Oz-Mg2+0" triplets of ions of the row, an equivalent number of Mgz+ ions (characterized by the absence of reactive anions in adjacent positions becomes available (as shown in ( 6 ) ) ;
P
(61
F
x :Mg2' ~
x
~
x
(
o
x
o
)
x
(
o
x
~
x
~
'\
=C%MfO-
which shows the typical (cationic-only) chemistry of Mg2+-exchanged zeolites where the Mg2+ions react with CO giving strong ~ - a d d u c t s . ~ JIndeed, ~ for both pure MgO and diluted solid solutions the band at 2200 cm-' is observed which monitors the presence of such ions. Following the previous hypothesis both u-adducts and ketenic-like species are the necessary products of the CO attach on edges. When isolated Co2+ions are inserted into the original row, the situation becomes as represented in (7). (7) X
= Mg2'
0
c$+
0x0.0x0x0.0x0x0x0x0.oxo
The basic edge chemistry of the matrix will be consequently modified, because of the appearance of the specific reactivity of the Oz-Coz+02-groupings. The edges (steps) of the solid solution microcrystals will be primarily covered by ketenic-like and carbenoid-carbonylic units, the relative amount of the two species depending upon the cobalt concentration (see (8)): However, in the same way as on pure MgO the formation of ketenic-like species necessarily causes the appearance of the Mgz+only adducts; (9) C. L. Angel1 and P. S. Shafter, J . Phys. Chem., 70, 1413 (1966). (10) Y. A. Lokhov and A. A. Davidov, Kinet. Katal., 21, 1093 (1981).
Zecchina et al.
on diluted solid solutions an analogous Co2+only chemistry is induced, whereby the cobalt ions react with CO without the participation of 02-ions in adjacent positions. Following this hypothesis, the Co2+ (CO), and the [(CO)CO(CO,),]~-surface complexes represent the precise analogues of the Mg2+(CO)and (C02-)Mg2+ (Cz02)2-ones typical of the "edge" chemistry of the matrix. The only difference is represented by the polycarbonylic nature of the C O ~ + ( C Oadducts )~ (in agreement with the availability of low-energy d orbitals in the transition-metal ion) and by the contemporary presence of both the oxidized (carbenoid C 0 2 ) and reduced (Co) moieties in the same complex (in contrast with (C032-)Mg2+(CzOz)2-where the (C03)2-and (C202)2-species are separate entities). The merit of this explanation lies in the following facts: (i) the basic chemistry of edges in pure MgO and diluted MgO-Coo solid solutions is explained by the same scheme; (ii) the gradual decline of the typical edge chemistry of the pure matrix when Co2+ ions are inserted is justifid, (iii) the fingerprint nature of the A,-A, triplet (constancy of the relative intensity of the components of the triplet) on diluted solid solutions is explained, because it originates in a concerted way in the same surface regions; (iv) the absence of important effects on the reflectance spectrum of Coz+ions (square pyramidal) emerging on the (100) facelets is totally understood. If the previous scheme is able to explain the main features of the chemistry of the Coz+ ions located on the edges, it fails to explain the presence of two types of [CO-CO(CO~)~]~(Al, A,) complexes. This fact is not surprising because (i) also on pure MgO two types of ketenic-like species are formed at room temperature' and (ii) the previous scheme is based on the assumptoin that the ions of the edge are in an unrelaxed situation (which is an unrealistic statement)." We think that only on the basis of a relaxed model can all the minor features (two types of similar Az-A3 complexes, the local C3, symmetry of the Al species, and their behavior upon decarbonylation) be explained with success. d-d Spectrum of Co2+ Ions on Edges (Steps). When the reversible species are formed, the small peak at 4000 cm-' is selectively destroyed with appearance of the broad 23 000-cm-' absorption (associated with the carbenoid-carboxylate units) and with the intensitication of the 8500-cm-I shoulder. Upon roomtemperature desorption the 23 000- and 8500-~rn-~ components tend to disappear while the 4000-cm-' one is restored. It is concluded that the 4000-cm-' peak is a d-d transition of CoZf ions located on edges (steps), in agreement with its intensity which is -2 orders of magnitude lower than that of the square-pyramidal species present on the common {loo)faces. In the Experimental Section we have observed a modest intensity gain at 13 600 upon CO desorption; as a consequence a second transition is present in this position. A further hypothesis on the local geometry of ligands would not be justified as distorted tetrahedral,I2 trigo~~al-bipyramidal,'~ and ~quare-planar'~ structures would all absorb at these frequencies. Isotopic Pattern ofthe Ketenic-like Species. The highest mode falls in a frequency range (-2100 crn-') where also surface cobalt carbonyls have strong tails, so its isotopic splitting cannot be described in detail. Vice versa the behavior of the lowest frquency mode at 1380 cm-' (which is the dominant feature for samples contacted with CO for a period of time less than 10 min) can be easily observed. In fact, a quartet appears in the presence of a 1:l mixture, indicating that these species contain more than one carbon atom. A more detailed investigation carried out on the (11) M. R. Welton-Cook and W. Berndt, J . Phys. C, 15, 569 (1981). (12) F. Mason in "Structure and Bonding", Vol. 39, J. D. Dunitz et al., Eds., Springer-Verlag, West Berlin, 1980, pp 43-81. (13) W. Abel, R. A. N. McLean, and S. P. Tyfield, J . Mol. Spectrosc., 30, 29 (1969). (14) C. Dahl, C. Wilhelm Schkpfer, and A. von Zalewsky, "Structure and Bonding", Vol. 36, J. Dunitz et al., Eds., Springer-Verlag, West Berlin, 1979, pp 129-7 1.
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 range of CO CO. In Figure 2 the spectra in the 2200-1000-~m-~
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0 1984 American Chemical Society
