Specmxhimica Ada. Vol. 49A. No. 9. pp. 12R9-129% 1993 0584-x539/93 s&M+ 0.00 Printed in Great Britain 0 I993 Pergamon Press Lid

Surface morphology and reactivity towards CO of MgO particles: F’TIR and HRTEM studies

SALVATORE COLUCCIA,* MARCELLO BARICCO, LEONARDO MARCHES,

GIANMARIO MARTRA and ADRIANO ZECCHINA

Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita di Torino, via P. Giuria 7, 10125 Torino, Italy

(Received 8 April 1992; accepted 23 June 1992)

Abstract-The morphology and adsorptive behaviours of two different MgO samples are compared. High resolution transmission electron micrographs indicate that MgO powders produced by decomposition of the hydroxide contain extremely rough particles, whereas nicely shaped and smooth cubelets are present in the MgO smoke sample. The proportion of surface sites exposed on (001) planes (five-coordinated) as compared with sites exposed on edges (four-coordinated) and comers (three-coordinated) is much larger on MgO smoke than on MgO ex-hydroxide. CO adsorption shows that at 300 K various families of polymeric anionic species are formed by interaction with basic sites in the lowest coordination (0:;) in corner positions. The relative population of the different species depends on the morphology of the particles. CO adsorption at 77 K produces bands due to CO adsorbed on acidic Mg $, Mg$ and Mg:: sites, whose relative intensities also depend on the morphology of the particles.

INTRODUCTION

MgO IS OFTEN adopted as a model system in studies of dispersed oxides because it can be easily obtained in the form of high surface area powders with microcrystals showing a simple cubic structure predominantly limited by (001) faces. Accordingly, it was determined by spectroscopic studies [l-4] that the sites exposed on the surface are cations and anions in low coordination states (Mg2+ Lc and 0:;) and, more specifically, five-coordinated on the (001) facelets (Mg$: and O$), four-coordinated on edges (MS and Ok,-) and three-coordinated on corners (Mgz and 0:;).

One further advantage of MgO as a model is that it can be produced in two forms differing considerably in the morphology of the microparticles: (a) MgO produced by decomposition of the hydroxide (MgO-h) [l-4] and (b) MgO smoke (MgO-s) produced by burning magnesium in air [5]. The relative populations of the three families of surface sites with different coordination (3C, 4C and SC) are significantly different on the surfaces of the two samples [5], so that the activities towards hydrogen of the various sites can be studied separately [6].

The comparison is now extended to the reactivity towards CO, a highly informative probe molecule for most dispersed systems. It is well known [7-111 that at room temperature (300 K) in case of high surface area MgO (a) only an exceedingly small fraction of surface sites (~0.5%) is able to adsorb CO and (b) surface basicity (that is the reactivity of 0:~ ions) is essentially revealed. CO, in these conditions, gives complex anionic polymeric species, which show IR bands at low frequencies. The effect of surface morphology on the distribution of such species produced by basic sites is considered here, comparing the behaviours of MgO smoke, MgO ex-hydroxide and sintered MgO samples.

However, preliminary studies [12] indicated that acidic sites (that is Mgs cations) can also be revealed on MgO by adsorption of CO at low temperature. In fact, CO coverage is very high at 77 K and, in the case of MgO ex-hydroxide, a complex absorption produced by the overlap of several bands is observed at high frequencies.

A similar absorption, though much simpler, is observed with MgO smoke, and such differences allow each IR component to be related to a specific family of acidic M&g sites.

* Author to whom correspondence should be addressed.

1289

1290 SALVATORE CoLuccr.4 et al.

EXPERIMENTAL

Materials

MgO ex-hydroxide (MgO-h) was prepared by thermal decomposition of the parent Mg(OH)r [l-S]. Prior to CO adsorption, the pellets were outgassed at 1123 K. The final surface area was 200m2g-r.

A sample underwent two progressive sintering treatments by heating for increasing times (1 h and 16 h) in the presence of 100 Torr O2 (1 Torr = 133 kNm-‘) at 1073 K; after the last sintering treatment the specific surface area was 30 * m g -‘. At each stage, prior to CO adsorption, the sample was outgassed at 1123 K.

MgO smoke (MgO-s) was prepared by burning magnesium in air [5]. The smoke was collected on a silica plate, carefully scraped and finally pelleted in a self-supporting disk suitable for IR analyses. A portion of the powder was used for high resolution transmission electron microscopy (HRTEM) .

High purity CO and O2 gases (Matheson) were used with no further purification except liquid nitrogen trapping.

Spectrometer and infrared cells

IR spectra were obtained by a Bruker IFS 48 instrument; the resolution was 4 cm-‘. The IR cell, suitable for spectra at 77 K, was permanently connected to a standard vacuum system (residual pressure: 10m6 Torr) and allowed all thermal pretreatments and adsorption-desorption experi- ments to be carried out in situ.

Electron microscopy

Electron micrographs were obtained by a Jeol2000 EX electron microscope equipped with a top entry stage; the MgO powder was suspended in n-heptane and then supported on a copper grid coated with a holey carbon film.

RESULTS

Electron micrographs

The overall shape of the particles in an MgO-h sample is shown in the low magnifica- tion photograph in Fig. 1. It can be recognized that thermal decomposition of the hydroxide produces MgO microcrystals in a plate-like relic structure characteristic of the

starting material [ 131. Details of the rough, highly irregular surface of such particles are illustrated in the high

magnification photograph in Fig. 2. Steep successions of steps and small terraces and, consequently, high concentration of edges and corners are characteristic features of MgO-h microcrystals.

On the other hand, fringes 2.09 8, apart (Fig. 2) parallel to the edges confirm that (001) planes are overwhelmingly exposed. Heavily terraced segments may simulate the presence of high index planes, but the actual surfaces are (001) faces, though, in most cases, of extremely reduced dimensions.

As already found for Co/MgO and Ni/MgO solid solutions [14], preliminary HRTEM results on sintered MgO-h samples show that transition towards larger particle sizes is accompanied by a drastic reduction of the population of steps and, consequently, of edges and corners.

By contrast, the microcrystals of MgO-s show a nicely shaped cubic habit with edge lengths mostly in the 45-70 nm range. Such overall regular morphology has alrady been well documented by low magnification electron micrographs [5,15,16].

The high magnification image of Fig. 3 confirms that such smoothness is maintained down to the level of a few angstroms [ 17-191. Nothing like the heavy successions of steps typical of MgO-h (Fig. 2) are present on MgO-s and, consequently, the proportion of sites on edges and corners as compared with sites on (001) planes must be much lower.

Surface morphology and reactivity towards CO of MgO particles 1291

Fig. 1. Low magnification electron micrograph of particles in MgO ex-hydroxide powders.

Most particles show fringes which are generated by the Bragg diffraction condition associated with a specific orientation relative to the incident electron beam. Fringes 2.09 A apart parallel to the edge and related to (001) planes and fringes 1.47 Bi apart due to (110) planes are seen in Fig. 3.

Infrared spectra of CO a&orbed at 300 K

In spite of the low coverages (only 0.5% of surface sites are involved), CO adsorption at room temperature on MgO-h produces an extremely complex spectrum which depends both on the pressure of CO and time of contact and is dominated by bands in the low frequency range.

SALVATORE Co~uccr~ etal.

1 ?igure 4a shows the spectrum obtained by adsorption of CO (40Torr) after co1 rtact at 300 K. Various families of related bands were identified previously [8,9] are : singled out in the figure by different letters. Figures 4b and c show the spectra of adz sorbed on the same MgO-h pellet after successive sintering treatments. Due to dec xeasing surface area, the overall intensity of the spectrum progressively decrease: par ising from curves a to c, but some bands are more severely affected than others

pai fiicular, those indicated with P are depleted preferentially, so that KD bands domii

Fig. 2. High resolution electron micrograph of MgO ex-hydroxide showing steep succession of steps and lattice fringes associated with (100) planes.

16h and co the

son 8. In late

Fig. 3. High resolution electron micrographs of MgO smoke particles. The two microcrystals show (100) and (110) lattice fringes.

Surface morphology and reactivity towards CO of MgO particles 1293

KD P

P KD

KD

.A_ d A -__----_--___~--~~~--___~ --._____h_

I 1 1 I I I I 002coo 1800 1600 l4co (_+ m30

Fig. 4. IR spectra of CO (40 Torr) adsorbed at 300 K on high surface area MgO-h (a), on progressively sintered MgO-h (b, c) and on MgO-s (d).

the spectrum of Fig. 4c. Notice that a weak band at ==22OOcm-’ in Fig. 4a-c is not related to any other component.

The trend observed upon sintering is enhanced further in the spectrum of CO adsorbed on an MgO-s sample (Fig. 4d). This spectrum is much simpler than those obtained with MgO-h, vestiges of KD bands being practically the only components still emerging from the background.

Infrared spectra of CO adsorbed at 77 K

As the low frequency spectrum of CO adsorbed on MgO-h at 77 K has already been described [9], only the high frequency region (2070-2215 cm-‘) is considered in this report.

Figure 5A shows the spectra at decreasing coverages of CO adsorbed at 77 K on the high surface area MgO-h sample. Three main features are observed in the 2070-2120, 2120-2180 ranges and at ~2200 cm-‘.

Absorption at 2070-2120 cr.~-~. Bands in this region are also observed at room temperature. They have counterparts at low frequency in the family of bands indicated with KD in Fig. 4 associated to anionic species [9] and will not be considered further.

Absorption at 2120-218Ocm-‘. This is by far the dominant absorption in the low temperature spectrum and is strongly pressure dependent. At high coverage (Fig. 5A, a), a relatively sharp peak is observed at 2152cm-‘, together with a heavy shoulder at 2157 cm-‘. Upon decreasing the CO pressure, the intensity decreases and the complex structure of this absorption becomes more evident. The peak at 2152 cm-’ in Fig. 5A,a

1294 SALVATORE COLUCCIA et al.

0 A

I 02!5c

2200

3

s

z

a

2150 2100 Wavonumber, cm-l

0 B

I I

2150 2100 Wavonumber , cm-’

Fig. 5. IR spectra of CO adsorbed on MgO samples at 77 K. (A) Spectra of CO adsorbed on MgO-h under progressively reduced pressure from 10Torr (a) to 0.01 Torr (I) and after outgassing for 1 min at 77 K (m). (B) Comparison of the spectra of CO (10 Torr) adsorbed on

MgO-h (a, the same as a in A) and on MgO-s (b).

readily disappears and is already absent in Fig. 5A, c. Intermediate spectra between curves a and c, not reported for the sake of simplicity, show that this peak shifts to high frequency while its intensity decreases.

By the disappearance of the main peak, two components become evident at 2159 cm-’ and at 2148 cm-‘. The former is more intense than the latter in Fig. 5A,c and d, but its intensity rapidly declines and disappears (Fig. 5A, d-h) while its maximum shifts to higher frequency. The band at 2148 cm-’ is more resistant and is still observable, though very weak, in Fig. 5A, 1. Remaining vestiges of all bands in this region disappear upon outgassing briefly at 77 K (Fig. 5A,m).

Absorption at -2200 cm-‘. In contrast with the bands at 2120-2180 cm-‘, this band is also observed by adsorption of CO at 300 K. It can be noticed that it is not observed in the spectra at high CO coverages at 77 K (Fig. 5A, a); it appears with increasing intensity by lowering CO pressure in Fig. 5A, b-h and then starts to decline, but it is still present after outgassing at 77 K (Fig. 5A, m). Though this band is weak and relatively broad, some high frequency shift is observed from Fig. 5A, d to Fig. 5A, h.

The spectra of CO adsorbed at high coverage on the MgO-h and MgO-s samples are compared in Fig. 5B, showing that:

2070-2120 cm-‘: no absorptions are observed in the case of MgO-s in this range. 2120-2180cm-‘: this is also the dominant absorption for CO adsorbed on MgO-s, though much simpler in shape with a sharp peak emerging at 2148 cm-’ and practically no shoulder on the high frequency side. Such enhanced simplicity is confirmed by the spectra at decreasing coverages [19], which do not show the components observed in the case of MgO-h (Fig. 5A). 2200 cm-‘: this band is practically not observable on MgO-s at any coverage.

Surface morphology and reactivity towards CO of MgO particles

DISCUSSION

1295

High resolution transmission electron micrographs clearly show that the microcrystals in any MgO powder (Figs 2 and 3) exhibit an overall cubic habit limited by (001) faces, though such planes are of reduced extension on the small terraces of the extremely rough MgO-h material (Fig. 2). The high crystallinity of even the very high surface area sample is confirmed by the appearance of lattice fringes, clearly detectable in Fig. 2. Consequently, it is correct to consider three possible sets of surface sites which are five- coordinated (5C) on the (001) facelets, four-coordinated (4C) on edges and three- coordinated (3C) on corners [l-5]. The relative populations of such sites are strongly affected by the morphology of the microcrystals and, therefore, depend on the origin and pretreatment of the samples.

Due to the presence of steep successions of steps crossing each other (Fig. 2) the ex- hydroxide sample has the highest proportion of 4C and 3C sites [5,13], whereas the smoothness [17,18] of (001) faces on the relatively extended cubelets guarantees an overwhelming population of 5C sites on MgO-s [5,6].

The original situation may be gradually modified in the case of MgO-h by progressive sintering; this promotes the increase of particle sizes and the reduction of the morpho- logical defects (steps and corners), so enlarging the (001) planes and increasing the presence of 5C sites.

These observations guide in correlating the adsorbed species to adsorbing sites and allow a deeper insight into both the basicity (associated with 0;~ anions) and the acidity (associated with MgG cations) exhibited by dispersed MgO samples.

Basic sites

The adsorption of CO at room temperature on MgO was unambiguously associated with the reactivity of basic O& sites exposed with the lowest coordination (3C) onto the surface of MgO mivrocrystals [7]. Some conclusions have been reached on the structure of such species, which are formed by CO polymerization. The assignment of the IR bands and the reaction mechanisms have been elucidated [8], though the proposals are subject to continuous revisions [9,10,20]:

(a) CO:- transient species are precursors for the trimeric species shown in Fig. 4 (KD bands);

(b) some of the trimeric species may evolve, by addition of CO, into more complex polymers which may have cyclic [8] or, more probably, linear structures (P bands) [9];

(c) trimeric and polymeric species further evolve with time, by fragmentation in oxidized (carbonate-like groups, 0 bands) and reduced counterparts [9]; these latter species are responsible for the bands marked by Q and Q’ in Fig. 4, but their structure has not yet been definitely identified.

The multiplicity of trimeric as well as polymer species [8, 91 clearly indicates the presence of heterogeneity, even in the very small set of sites in the lowest coordination on corner positions. In particular, only some of the trimers may add CO to produce larger polymers.

The results in Fig. 4 may give a hint for understanding the different behaviours of similar species. In fact, examining the relative intensities of the various families of bands in Fig. 4a-c, it can be noticed that those due to polymers (P) are more deeply affected by the sintering treatments becoming extremely weak in spectrum c. Evidently, the formation of larger polymers is less and less favoured as the microcrystals tend to become larger with a more regular morphology. This confirms that the largest polymeric species may only form in regions with enhanced irregularity, where a larger number of 3C sites are able to form two or more trimeric groups in nearby positions which, adding further CO molecules, may coalesce into larger entities [9].

These regions are bound to be preferentially depleted by any process which, like sintering, reduces the roughness of the material. It is most noticeable that the largest polymers are practically absent on the perfect MgO-s particles (Fig. 4d).

1296 SALVATORECOLLJC~IA~~ al.

Finally, it has to be noticed that in Fig. 4 only the tiny band at =22OOcm-’ can be attributed to CO molecules stabilized on positive ions by a-bonds [8,9, 121 or, possibly, by plain electrostatic interactions [21,22], which induce a high frequency shift of the CO stretching mode as compared with the free molecule (2143 cm-‘). Only the M& sites are able to adsorb CO at 300 K [8,9] and this is the only information which can be obtained on the acidic sites in such conditions.

Acidic sites

CO adsorption at 77 K produces very weak bands at low frequencies because most processes producing anionic species are activated [8,9]. Consequently, absorptions at frequencies higher than the stretching mode of the free CO molecule are the dominant features at low temperature. Such bands are neither correlated to each other nor to bands at lower frequencies, so that each of them is associated with a different CO adsorbed species.

CO stretching frequencies higher than 2143 cm-’ are typical of molecules adsorbed on positively charged sites with the carbon pointing towards the cation, the shift increasing as the strength of the positive field increases [12,22,23]. Though the positive centres are all Mgpc ions, different coordinations generate cations with different polarizing power. Indeed, the positive field associated with an M&b site on a plane, where it is surrounded by five next neighbours 02-, must be lower than the field of M& on an edge where the same charge (+2) is balanced by only four next neighbouring negative ions and, in turn, this must be lower than the field on Mgg on a corner position, surrounded by only three 02- anions [l-5].

The weak band at the highest frequency (2200 cm-‘) has already been assigned [8,9] to CO adsorbed on M&d . CO molecules adsorbed on M& and Mg:d are expected to absorb at progressively lower frequencies, and the two main bands observed at 2159 and 2152 cm-’ (Fig. 5A) respectively may well be associated with such species. Notice that only one band is observed in the case of MgO-s (Fig. 5B, b), assignable to CO adsorbed on the overwhelming 5C sites [19].

As the extinction coefficient does not vary significantly in this range [12,23-251, the relative intensities support such an assignment. In fact, the 2152 cm-’ band associated with the abundant sites on (001) planes is the most intense, whereas the 2200 cm-’ band, associated with the much less numerous sites in corner positions, is the weakest. The reversibility also agrees with the assignment as: (a) species associated with the less acidic sites (5C) on planes and absorbing at 2152 cm-’ are the less stable and, consequently, the first to desorb (Fig. 5A, a-c); (b) species stabilized by 4C sites (band at 2159 cm-‘) are somewhat more stable (Fig. 5A, c-g); and (c) CO adsorbed on the most acidic sites on corners (3C) is not reversible at 77 K and is also present at 300 K.

Finally, the band at 2148cm-’ in the spectra of CO adsorbed on MgO-h must be commented on. At higher coverages this component is hidden by the dominant peak at 2152 cm-’ and starts to be clearly observable only after the absorption due to CO on Mg$z has faded away (Fig. 5A, c). The 2148 cm-’ band shows two apparently contrasting features: (a) it is at a frequency which is lower than that of CO adsorbed on M&c’, and this would suggest that the CO molecule is adsorbed on an even less acidic cation; (b) it is more resistant to desorption (Fig. 5a, c-h) and this would require a more acidic site. Moreover, it has to be noted that, in contrast with the other bands, its position does not depend on coverage. It may be concluded that CO species absorbing at 2148 cm-’ must have a different structure from those linearly adsorbed via the carbon atom on a single Mgi,: cation.

Similar absorptions, observed in the spectra of CO adsorbed on halides with rock-salt structure [25,26] and on Co/MgO and Ni/MgO solid solutions, were assigned to CO anchored both by the carbon and oxygen atoms to two cations at step defect sites. The interaction of the oxygen atom with a positive site lowers the stretching frequency and enhances the extinction coefficient [22,25-261. On the other hand, the stabilization by two sites, instead of one, justifies the somewhat higher stability.

Surface morphology and reactivity towards CO of MgO particles 1297

+ Mg*+

_ o*-

Scheme 1. CO adsorbed on MgO ex-hydroxide.

All the molecular CO species described in this section are represented in Scheme 1, together with the related stretching frequencies.

Shifts of CO stretching bands with coverage

It has been shown (Fig. SB) that the maximum of the band associated with CO adsorbed on the (001) facelets is at 2152 cm-’ in the case of MgO-h and at 2148 cm-’ in the case of MgO-s. This difference has to be ascribed to adsorbate-adsorbate interac- tions, which develop as the coverage tends to the maximum value [12, 19, 25-301. The balance of static and dynamic effects in the case of rock-salt oxides and halides [12, 25- 28, 301 is such that the band moves to lower frequency as the coverage increases. The overall entity of the shift depends on the number of coupled oscillators [19,28,29] and, therefore, on the extension of the planes which accommodate the adsorbed layers.

Since the (001) facelets on the microcubes of MgO-s are much more extended than those on the rough particles of MgO-h, the low frequency shift at maximum coverage in the former case is larger than in the latter. It may be noticed that, considering the successions of steps in Fig. 2, terraces of lo-2OA are very common features on the surface of MgO-h. Such microplanes (NO-4OOA’) may accommodate some 5-20 parallel coupled oscillators. In fact, 12 Mg2+ cations [31] are exposed per 100 8, and, due to steric hindrance, only one out of two may adsorb a CO molecule at maximum coverage [12,30]. By contrast, the sizes of most cubelets in the smoke are in the 450- 700 A range and the extension of the (001) planes in the 200-500 x 10’ A2 range, which leads to adlayers containing lo-20 X lo3 CO molecules. These figures must be assumed only as an indication of the fact that the populations of adlayers on MgO-h and on MgO-s differ by 2-3 orders of magnitude and may well justify differences of a few cm-’ in the position of the band [28].

Finally, the evolution of the intensity of the band due to CO adsorbed on corner sites (2200 cm-‘) may be commented on. It was shown that the band is observed at low and intermediate coverages (Fig. 5A, d-m) and fades away at the highest coverages (Fig. 5A, a-c). This might be connected with the fact that at the highest coverages the 3C sites may start adsorbing more than one CO molecule, tending towards a full octahedral coordina- tion, where each molecule is sensing a reduced electrostatic field and, hence, a reduced upward shift. Should this be so, at high coverage the differences among CO molecules adsorbed on sites originally different for the coordination may become smaller as more and more homogeneous adlayers are built up, and this allows extension of the coupling among oscillators.

Acknowledgemen&-The authors acknowledge financial support by CNR, MURST and ASP.

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