() Hachioji Technical High School - Sen’nin-cho, Hachioji, Tokyo 193, Japan (ricevuto il 28 Febbraio 1997; approvato l’8 Maggio 1997)
Summary. — MgO is a typical base catalyst and its basicity has been thought to arise
from surface OLC2 2(LC: low coordination). On the other hand, the roles of MgLC2 1in LC state in catalysis and adsorption have been often underestimated. In this study simple gases such as H2, CH4, CO, and CO2were used as test molecules to examine the roles of both ions in adsorption, especially at low temperatures. Infrared and TPD spectroscopic observations and, partly, MO calculations for these adsorption systems indicated that MgLC2 1 plays rather more important roles than OLC2 2 in the adsorption of these gases. Dynamic behavior of adsorbed species at low temperatures is also found.
PACS 82.65.Jv – Heterogeneous catalysis at surfaces. PACS 01.30.Cc – Conference proceedings.
1. – Introduction
MgO is known to behave as a typical base catalyst in many catalytic reactions and its basicity has been believed to arise from surface OLC2 2ions (LC: low coordination) [1].
The presence of low coordinated ions such as 3C, 4C, and 5C sites on well outgassed MgO surface was already revealed experimentally [2-7]. However, many experimental and theoretical studies carried out for adsorption of hydrogen on this oxide surface have indicated that MgLC2 1 also plays an important role [6-17]. This fact suggests that
MgLC2 1 may play more important roles in adsorption and catalysis of other gases than
that so far considered.
In the present paper, after briefly reviewing the adsorption behavior of H-containing gases such as H2and CH4, the surface property of MgO for adsorption of non–H-containing
(*) Paper presented at the “First International Workshop on Reactivity of Oxide Materials. Theory and Experiment”, Como, 8, 9 November 1996.
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gases such as CO and CO2are studied mostly at and below room temperature by mainly
using IR and TPD techniques.
2. – Experimental
An MgO sample used was smoke-type supplied from the Catalysis Society of Japan as one of the reference catalysts with a specific surface area of 10–15 m2
Og. Pressed MgO disks were previously evacuated at 1123 K before each run. The same disks were repeatedly used in situ in a series of runs. Adsorption pressures used were below 1.3 kPa, and for CO2mostly below 133 Pa. TPD spectra were obtained at a heating rate
of 5 K min21using a quadrupole mass spectrometer as a detector. FTIR spectra at low
temperatures were measured in the presence of 260 Pa He in the cell to attain better thermal contact.
3. – Results and discussion
3.1. Adsorption of hydrogen-containing molecules. – Hydrogen is a gas so far most frequently studied by many workers to examine the adsorption behavior of MgO [6-17]. The results obtained from this adsorption system are very important to interpret the adsorption of other gases. Hence a typical TPD curve observed after H2adsorption at
77 K is shown in fig. 1, where various adsorbed species denoted by W2–W8appear and
are classified into three groups, W2 and W3, W4 and W5, and W6to W8, according to
their thermal stability and nature. These species gave IR absorption bands due to the stretching mode of both Mg-H and O-H bonds [9, 18], and an ESR signal of O22 was
formed upon subsequent admission of O2[19, 20]. Therefore all the adsorbed species of
H2observed in fig. 1 seem to be formed by heterolytic dissociation of H2into H2 and
H1on adsorption.
Fig 2. – TPD curve of adsorbed methane measured after adsorption of 1.3 kPa CH4 at 300 K followed by cooling down to 220 K.
A second example used as a H-containing gas is CH4, and its TPD curve observed
after adsorption below room temperature is shown in fig. 2. There appear three TPD species, M1 A, M1 B, and M1 C. The adsorption sites for these three species have been
proved to be completely identical to those for species W5 to W8 in H2 adsorption by
poisoning experiments [21]. Oxidation of the M1-group by O2gave both IR bands due to
O-CH3 stretching mode and ESR signals due to O22[22]. These facts clearly indicate
that CH4 is also heterolytically dissociated into CH32and H1 on MgO surface though
Stone et al. could not detect its presence [6].
In the adsorption of these H-containing molecules RH, the nearest pair of both MgLC2 1 and OLC2 2ions (MgLC2 1-OLC2 2) is required as an active site, where MgLC2 1 and OLC2 2act
as acidic and basic sites, respectively [23]. The molecule RH behaves as a Brönsted acid on this site upon adsorption and is heterolytically dissociated into R2and H1to form
both MgLC2 1-R2and OLC2 2-H1bonds as shown in fig. 3 [6, 18]. Our theoretical calculations
using a small cluster model also support heterolytically dissociated adsorption of H2
and CH4[14, 21]. The coordination number of both the ions must be low and difference
in it produces the several types of adsorbed species observed in figs. 1 and 2.
The coordination number required for each type of adsorbed species of H2 was
determined by considering the relationship between the occurrence of LC ions and the adsorptive property of MgO at various pretreatment temperatures [13]. Obtained results are listed in table I. The adsorption energy calculated from density functional
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TABLE I. – Coordination number of MgLC2 1-OLC2 2 pair site and theoretical adsorption energy for
hydrogen adsorption.
Adsorbed species
Desorption temperature (K)
Coordination number Adsorption energy by MO (kcal mol21) MgLC2 1 OLC2 2 W2and W3 W4and W5 W6–W8 230–260 300–360 420–610 3C 4C 3C 4C 3C 3C 10.5 4.3 23.5
(DF) method using an Mg6O6 cluster model is also given in this table, which indicates
that the presence of 3C ions is essential and no 5C ions can participate in adsorption [15, 24]. Local density of states calculated for bare surface of this cluster indicates that a change in coordination number from 3C to 4C causes a more pronounced shift of surface level for MgLC2 1 than for OLC2 2. These characteristics well
agree with the experimental facts mentioned above.
3.2. Adsorption of carbon monoxide. – Carbon monoxide can behave as either an acid or base molecule according to the surface properties of the oxide used. Magnesium oxide has acidic and basic sites on the surface and hence the adsorptive behavior of CO on it becomes rather complex. Therefore the total scheme of this adsorption system is still under debate though there have been many studies on this adsorption system since a report of Smart et al. [25-30]. Most of these works were carried out by using a single analytical method, mostly IR spectroscopy, but not using plural measuring techniques.
Fig. 4. – TPD curve of adsorbed carbon monoxide measured after two-step CO adsorption. First, 1.3 kPa CO at 210 K followed by evacuation at 273 K; second, 6.5 Pa CO at 120 K.
Fig. 5. – IR spectrum of adsorbed carbon monoxide measured after adsorption of 1.3 kPa CO at 300 K.
Using a combination of two techniques, e.g. IR and TPD, as used in this study should be preferred to investigate this complex adsorption system.
Five adsorbed species K0, K18 , K2, K3, and K4appear in a TPD curve shown in fig. 4
which was observed after two-stage adsorption of CO below room temperature in order to produce all the five groups of species with comparable intensity. The peak appearing at the lowest temperature region seems to consist of a single species. However, an IR spectrum measured at low temperature clearly indicated the presence of two species at this temperature region [31] and hence this TPD peak consists of two adsorbed species of K0and K18.
The IR spectrum measured at room temperature in the presence of gaseous CO is shown in fig. 5 where species K2and K3are present as main species. Species K4is also
stable at room temperature, but its IR bands are not clear in this figure because of TABLEII. – Adsorbed species of carbon monoxide.
Species Structure Desorption temperature
(K) IR band (cm21) (typical case) K0 K81 K2 K3 K4 linear monomer on MgLC2 1 linear monomer on OLC2 2 chained tetramer on OLC2 2 linear trimer on OLC2 2 unknown (oligomer) E 230 E 230 ca. 400 ca. 550 ca. 700 2165 1475 1615, 1595, 1570 1475, 1265, 1180 2090, ca. 1550, 1375, 1320, 1170 1450, 1375, 1105
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their weakness in intensity. Desorbing temperatures and IR absorption bands of all the species observed in this study are summarized in table II. As seen from IR bands, these five adsorption states are completely different from each other, which is different from the adsorption behavior in RH-type molecules where all the species formed are basically identical in structure except for the coordination number of the adsorption site. Table II also shows structures of adsorbed species which have been determined on the basis of a) IR absorption bands, b) tracer study using C18O and13CO, c) analysis of mutual transformation between adsorbed species, and d) analysis of desorbing molecules in TPD, etc. [32]. Among the five species, K0and K18 are monomers desorbing
below room temperature. Species K0is a linear-type CO monomer where C is linked to
surface MgLC2 1, and several subspecies are formed depending on its coordination
number. On the other hand, a linear-type monomer adsorbed on OLC2 2(species K18) is too
unstable to be observable in IR spectra as an independent species; their bands were observable only when species K1 is paired with K0 at the nearest MgLC2 1 site with a
specific structure at low temperatures [31]. Density functional calculations also lead to the conclusion that species K18 is usually unstable even on a 3C-site while species K0is
stable [33].
Species K2to K4 are stable at room temperature and, among them, K2 and K3are
oligomer species adsorbed on OLC2 2to which a C atom of a terminal CO in the oligomer is
linked. Species K2is a chained-type tetramer where the fourth CO molecule in the top
position is also linked to MgLC2 1located a little apart from the active site OLC2 2and species
K3 is a linear-type trimer with a ketenic group. Species K3 seems to correspond to
“Species KD” found by Zecchina et al. [28] and “Species V” found by Babaeva et al. [29]. In the present study species K3is proved to consist of three CO molecules. Species K4
may be also an oligomer on the basis of the number of IR bands, but its details are unknown.
The formation mechanism and structure of species K2 ad K3, which have been
determined experimentally as described above, are illustrated in fig. 6. These oligomer formation is summarized as follows. a) The first step is the K0-monomer formation on
the acidic MgLC2 1site with no participation of OLC2 2; this process is very fast even at low
temperatures and adsorption equilibrium between gas phase and species K0 is easily
established. b) A second CO molecule is adsorbed on OLC2 2at the nearest lattice position
of K0 to form a very unstable K18-K0 pair species (precursor of oligomers), and this
surfaces, many papers on CO2adsorption have been reported since a pioneering work
by Evans [34-36]. These works have been mostly performed above room temperature by using IR spectroscopy and have demonstrated that bidentate-type carbonates are present as main species. More recent works in which mainly TPD method was applied, have indicated the occurrence of oxygen isotope exchanges between adsorbed species and surface O2 2[37-39]. However, dynamic behaviors of adsorbed species, especially at low temperatures, are still open to question.
A TPD curve measured after a mall amount of CO2adsorption at a low temperature
is shown in fig. 7, which indicates the presence of five types of adsorbed species, peak-0 to peak-4. Among them Peak-2 species is always very weak in intensity and no further mention is made hereafter. An IR spectrum measured after the CO2 admittance at a
low temperature is shown in fig. 8 where the band positions of each species, mainly determined from thermal stability measurements and isotopic study, are also indicated. These results, in addition to their structure determined from observed IR bands [34, 40], theoretical models [39], and isotopic study, are summarized in table III. No oligomerization occurs in CO2adsorption different from CO adsorption.
Fig. 7. – TPD curve of adsorbed carbon dioxide measured after adsorption of 27 Pa CO2 at 77 K.
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Fig. 8. – IR spectrum of adsorbed carbon dioxide measured after adsorption of 6.5 Pa CO2 at 180 K.
Usually CO2is considered to be an acidic molecule. In Peak-0 species, however, CO2
can be adsorbed as a basic molecule though this species is least thermally stable among the observed ones. Each O atom of a CO2molecule behaves as a base and makes a weak
bond with an acidic site MgLC2 1to produce linear Peak-0 species.
All the other adsorbed species are carbonate-type, CO32 2. Peak-1 species with a
unidentate structure is stable only below room temperature and formed by the bond formation between an acidic C atom of CO2and a basic OLC2 2ion as shown in fig. 9. Since
its desorption temperature is higher than that of Peak-0 species, it seems that CO2
species bound with OLC2 2is more stable than that with MgLC2 1, which is different from the
behavior observed in adsorption of other gases such as CO described above. This difference seems to exclusively come from very high stability of resultantly formed CO32 2ion.
Peak-3 and 4 species are bidentate type CO32 2 formed as major species in CO2
adsorption. The difference between the two species may originate from the coordination number of the site since their IR absorption bands appear at similar positions. Bidentate species is thought to be formed from the unidentate type as shown TABLEIII. – Adsorbed species of carbon dioxide.
Species Structure Desorption temperature
(K) IR band (cm21) (typical case) peak-0 peak-1 peak-2 peak-3 peak-4 linear CO2on MgLC2 1 unidentate CO32 2 (unknown) bidentate CO32 2(type b) bidentate CO32 2(type a) 190 230 300 390 ca. 800 2355 1274, 1711 951, 1274, 1626 1022, 1326, 1660
species, Peaks-3 and 4. Among carbonate species, bidentate-type which has two bonds of C-OLC2 2 and O-MgLC2 1 with surface is more stable than unidentate-type with only one
bond of C-OLC2 2. Especially Peak-4 species can remain on the surface until ca. 800 K.
Dynamic behavior of these adsorbed species at low temperatures was mainly investigated by using isotopic techniques. One of the most important results obtained is the substitution of adsorbed species with a gaseous molecule. For example, a part of Peak-4 species is easily substituted by CO2 in the gas phase below room temperature
though this species can desorb from the surface in the high temperature range around 800 K in vacuo. This kind of substitution between adsorbed species and gaseous CO2is
not characteristic of Peak-4 species, and other species can be substituted rather more easily.
Adsorbed species desorbs directly from the site where they have been adsorbed as the sample temperature is increased. However a part of them begins to diffuse on the surface prior to their desorption even below room temperature. During this surface diffusion, the diffusing species can be subject to three dynamic interactions. A first case is an isotope exchange with a surface ion or another adsorbed species, which occurs through the bond rupture in the diffusing molecule. A second case is substitution of molecule between the diffusing species and another adsorbed species, which is not accompanied by the bond rupture in the diffusing molecule. Thirdly, the diffusing molecule can be readsorbed on an unoccupied active site as a more stable species. These phenomena are rather widely observable for adsorbed species of CO2on
MgO.
4. – Conclusion
A hydrogen-containing molecule is adsorbed on an MgLC2 1-OLC2 2 pair site with
heterolytic dissociation and adsorption of CO and CO2 also requires the presence of
both MgLC2 1and OLC2 2surface ions. The coordination number of MgLC2 1seems to have more
pronounced effect on these adsorptions than that of OLC2 2, though the adsorption is
influenced by coordination number of both the surface ions.
Thermal stability of adsorbed species formed upon adsorption of CO and CO2
ranges widely. The most stable species can desorb only above 700–800 K, but even these species can diffuse on the surface and behave dynamically below room temperature.
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