Oxygen exchange between C18O2 and basic metal oxides (CaO, MgO, ZrO2 ZnO)

Spillover and Migration of Surface Species on Catalysts Can Li and Qin Xin, editors 9 1997 Elsevier Science B.V. All rights reserved.

Oxygen exchange between

C1802 and

277

basic metal oxides (CaO, MgO,

Zr02, ZnO) B. Bachiller-Baeza 1, P. Ferreira-Aparicio 1, A. Guerrero-Ruiz 2 and I. RodriguezRamos 1. Instituto de Catfilisis y Petroleoquimica. C.S.I.C. Campus Cantoblanco 28049. Madrid. Spain. 2 Departamento de Quimica Inorgfinica y T6cnica. Facultad de C iencias. UNED. 28040. Madrid. Spain.

A comparative Temperature Programmed Desorption study of the interaction of C~802 with the surface of different basic metal oxides (MgO, CaO, ZrO2, ZnO) has been carried out. Under our experimental conditions, C1802, C~80~60 and C~602 have been detected as desorption products by mass spectrometry. The isotopic distribution depends on the desorption temperature and at higher temperatures the oxygen exchange between chemisorbed C~802 and the solid surfaces is nearly total for all the studied oxides. Complementary information has been obtained from the infrared spectra of CO2 adsorbed at room temperature and after outgassing at increasing temperatures. It is found that the strength of the adsorption sites for carbon dioxide follows the order CaO > MgO > ZrO2 > ZnO. The extension of the oxygen exchange can be related with the surface structure of the metal oxide and with the type of C O 2 adsorbed species. The mechanism of oxygen exchange seems to be connected with the presence of bidentate or polydentate carbonates. The oxygen exchange reaction results from the migration of carbon dioxide on the oxide surface, which occurs at higher desorption temperatures. CaO shows a minimum oxygen exchange capacity.

1. INTRODUCTION Solid basic metal oxides and bifunctional acid-base oxides have been shown as efficient catalysts for various relevant industrial applications [1]. Although many of

This work was supported by CICYT of Spain under project MAT96-0859-C02-02.

278 these reactions are explained in terms of basic properties of catalytic surfaces, the number of characterization data concerning these active centers seems to be very few in contrast with the extensive research reported on the nature of acidic sites [2, 3]. Nevertheless, the investigation of basic properties and of acid-base pair sites would be interesting from the point of view of the design and development of an oxidic catalyst with improved selectivity. Methods such as Temperature Programmed Desorption (TPD) or Infrared Spectroscopy (IR) result very helpful to study the interaction of suitable probe molecules with the material surfaces [4] on which they are adsorbed. These methods provide information about the surface properties and adsorption strength of the active sites, and about possible reaction intermediates formed on the catalytic surfaces. In particular carbon dioxide chemisorption studied by TPD and IR can give a lot of information about the basic sites [5, 6]. Furthermore, oxygen isotopic exchange between C~sO2 and metal oxides can characterize reactive oxygen species present on the metal oxide surfaces at low concentrations [6, 7]. Active sites for oxygen isotopic exchange can play various roles in the control of the selectivity for many catalyzed reactions [8]. Thus, using a TPD technique coupled with a labelled molecule it is possible to gain information not only about the bond strength but also concerning the reactivity between adsorbed species and the catalytic surface. In the present work, we have carried out a comparative TPD study of the interaction of C~802 with the surface of different metal oxides" CaO, MgO, ZrO2 and ZnO. Both the effect of CO2 surface coverage and the isotopic distribution of evolved CxO2 species have been analyzed. The IR spectra of adsorbed CO2 have been also analyzed in order to obtain information about the multiplicity of the species originated on the surface and about their thermal stability.

2. E X P E R I M E N T A L

The metal oxide samples used in this work are commercially available" CaO from Fluka (SBET = 10.6m2/g), MgO from Fluka (SBET = 51 m2/g), ZrO2 from Harshaw Chemical Company (SBEx = 24.7 m2/g) and ZnO from B.D.H. (SBET = 3.6 m2/g). Surface areas of the samples were determined from the N2 adsorption isotherm at 77 K. These measurements were carried out using a automatic Micromeritics ASAP 2000 equipment. A molecular area of 0.162 nm 2 was assumed for each adsorbed nitrogen molecule. For TPD experiments, the samples were placed in a quartz adsorption vessel and pretreated at 873 K in oxygen to remove any residual carbonate from the surface and under vacuum at 773 or 873 K. After cooling to room temperature a known amount of C~aO2 (supplied by Isotec Inc., 95% isotopic purity) was admitted and adsorbed on the sample. The TPD was run at a heating rate of 10 K/min and the desorbed gases were analyzed by a quadrupole mass spectrometer (Balzers QMG 421 C). The ion current of the various products and the temperature of the sample were

279 simultaneously collected in a personal computer. The IR spectra were recorded on a Nicolet 5 ZDX Fourier Transform Infrared Spectrometer, equipped with a MCT detector and with a resolution of 4 cm 1. Selfsupporting wafers of the samples with weight-to-surface ratios of about 10 mg • c m -2 were placed in a vacuum cell assembled with greaseless stopcocks and KBr windows. Pretreatments were carried out in the in-situ cell heated with a furnace at 773 K as described above for the TPD experiments. Then, an amount of CO2 (50 Torr) was introduced into the cell and the infrared spectra recorded after removing the gas phase. The CO2 desorption under vacuum was also investigated by increasing the temperature by 100 K steps. The time under vacuum treatment at each temperature was 15 min. The infrared spectra of the adsorbed species were obtained by subtracting the spectrum of the clean sample from the spectrum obtained after adsorption. All spectra were smoothed and the baseline corrected.

3. RESULTS AND DISCUSSION IR spectra of C O 2 adsorbed at room temperature on the surface of the various metal oxides are shown in Figure 1 (spectra a). In this Figure are also presented the spectra obtained after evacuation at increasing temperatures (spectra b-f). Table 1 lists the positions and general assignments of the bands. It is seen that CO2 adsorption on metal oxides leads to multiplicity of species. First, bands in the range 1215-1230 c m I can be assigned to bending vibration of bicarbonate species. The corresponding symmetric and antisymmetric stretching vibrations lie at: 1419, 1630 cm -~ for ZnO; 1428, 1625 cm -~ for Z r O 2 and 1383, 1650 cm -~ for MgO [9-11]. In the latter case the bands are masked by those of unidentate and bidentate carbonate modes. These bicarbonate species appear in all samples, except for CaO, and are related with some hydroxyl groups that remain on the oxide surface after outgassing at temperatures as high as 773 K. Examination of the IR spectra in the wavenumber range characteristic of hydroxyls (3000-4000 cm -~) shows that on CaO hydroxyls are removed after outgassing at 773 K. However, it has been proved by deuterium exchange reaction that even after outgassing MgO at 1273 K a measurable number of surface hydroxyls remain on the surface [12]. The bicarbonate species are adsorbed reversibly at room temperature and tend to disappear with time. Secondly, bands at 1345 and 1575 cm -t for ZnO [9]; 1324 and 1571 cm -~ for Z r O 2 [11, 13]; 1306 and 1653, and 1340 and 1682 cm ~ for MgO [2, 10, 14] can be assigned to symmetric and antisymmetric stretching vibrations of bidentate carbonates. Finally, there are bands corresponding to the symmetric and antisymmetric stretching vibration of unidentate carbonates: 1390 and 1515 cm ~ for ZnO; 1400 and 1500 cm -~ for Z r O 2 ; 1383 and 1576 cm -~ for MgO. The adsorption of CO2 on CaO is different from that on the rest of the metal oxides. Only bands at 1415, 1474, 1549 c m -1 appear which can be attributed to unidentate carbonates on the basis of the small spectral separation between the Vco modes. However, they can correspond to polydentate

280

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Figure 1. Infrared spectra of CO2 adsorbed on: A) ZnO, B) ZrO2, C) MgO and D) CaO. Samples outgassed at 773 K following a) complete CO2 chemisorption at 298 K and after heating in vacuum at: b) 373 K, c) 473 K, d) 573 K, e) 673 K or f) 773 K.

carbonates due to their strong thermal stability [15] (see Figure 1). Note that these species are the only species formed on CaO, independently of the amount of adsorbed CO2. This is in agreement with that previously reported by Fukuda et al. [14]. By elevating the temperature in 100 K steps spectral changes are observed in Figure 1. The species with less thermal stability are the bicarbonates which disappear between 373-473 K. Bands corresponding to bidentate and unidentate carbonates decreased also in this temperature range. The former decreasing more than those of unidentate carbonates. Furthermore on ZnO and ZrO2, new bands appear upon thermal treatment in vacuum. The new bands of the ZnO at 1470 and 1341 cm ~ can be assigned to polydentate carbonates [5, 9]. The species with polydentate structure, not very different from that of bulk species, show a strong resistance to thermal decomposition. Similar assignation is given for the new bands of ZrO2. On MgO and

281

Table 1 Wavenumber (cm -~) and assignment of the bands resulting from CO2 adsorption on the different metal oxides. ZnO

ZrO

MgO

CaO

1390 1515

1400 1500

1396 1568

1415 1474 1540

1345 1575

1324 1571

1306, 1340 1653, 1682

1227 1419 1630

1220 1428 1625

1215 1405 1645

Unidentate O M ~ O ~ C

/ \ O

Bidentate O M C ~ O Bicarbonate M~O~C

OmH

CaO, desorption of species by heating under vacuumdid not modify the wavenumber of the remaining bands showing that the corresponding adsorption sites were thermally stable. It is also important to note for each metal oxide, the temperature at which the complete elimination of adsorbed species is achieved. The thermal stability might be subsequently related with the results of the TPD experiments. This latter is essential for the interpretation of the TPD results, since a given surface species can be associated with its oxygen exchange reactivity [7]. Figures 2 and 3 show for the different metal oxides the TPD profiles of each type of isotopically labelled carbon dioxide. In the former the amount of C~802 adsorbed was 2.5/~mol/m 2 while in Figure 3 the surface coverage is 0.25/zmol/m 2. The profiles corresponding to MgO and ZnO are similar to those reported by other authors [6,7,16]. It appears that the strength of basic sites, intimately related with the temperature at which CO2 is desorbed, follows the order: CaO > MgO > ZrO2 > ZnO. Also, for a given desorption temperature the degree of oxygen exchange measured as the (C1602-+-C180160)/(C1602--j-CI8016Od-C1802) ratio is: ZnO > ZrO2 > MgO > CaO. These differences in oxygen exchange can be explained by the different type and thermal stability of the carbonate species present over each metal oxide surface.

282

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Figure 2. TPD profiles of C~aO2 adsorbed on various metal oxides" A) ZnO, B) ZrO2, C) MgO and D) CaO with a surface coverage of 2.5 #mo/m 2. Signals from mass spectrometer . . . . . . C~802, C180160, """ C1602, and total CO2.

It is noteworthy that depending on the surface coverage different CO2-TPD profiles are obtained. From Figure 1, it is deduced that these differences can be assigned to the type of adsorbed species which exhibit different thermal stability. Furthermore, it can be observed that in the whole range of desorption temperatures the oxygen exchange between C~802 and the oxide surface is very extensive. On the other hand, at higher desorption temperatures, where the exchange reaction is controlled by the CO2 migration process over the metal oxide surface, the fraction of desorbed labelled carbon dioxide seems to depend on the initial ratio between ~80 and ~60 atoms in the experimental system: [~80 (C1802 gas)/160 (surface of M16Ox) ]. Following some authors, C~80~60 formation can be explained as due to a single adsorption-desorption of bidentate carbonate species [6,11,16]. However, to explain the isotopic distribution observed at higher desorption temperatures a multiple oxygen exchange between CO2

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Figure 3. TPD profiles of C1802 adsorbed on various metal oxides: A) ZnO, B) ZrO2, and C) MgO with a surface coverage of 0.25 #mo/m 2. Signals from mass spectrometer: . . . . . CX802, C~80160,.... C~602, and total CO2.

and the lattice oxygen of the oxide has to occur. Repetitive adsorption-desorption of CO2 is excluded because the partial pressure of CO2 after admission of 0.25 #mol.cm ~ was negligible. Therefore, it can be proposed that this oxygen exchange results from adsorbed CO2 migrating on the metal oxide without leaving the surface [11]. The migration seem to be minimum for CaO, this indicating an inhibited mobility of carbonates over this surface probably due to a stronger adsorption. Alternatively, the particular behaviour of CaO surface could be explained as consequence of a lower migration facility of the oxygen ions from the bulk to the surface. Thus the information obtained by the oxygen exchange can provide a helpful picture not only concerning the type of surface species originated by interaction with CO2 but also about the oxygen ions able to react during a redox process.

4. CONCLUSIONS The C~802 TPD and IR studies allow to establish that the distinct facility of oxygen exchange with a metal oxide surface may be attributed to differences in the structure of the CO2 adsorbed species at a given temperature for each oxide. It appears that oxygen exchange can be related to bidentate and polydentate carbonates. A minimum oxygen exchange ability for the reaction between ClsO2 and the surface

284 oxygen ions has been found for CaO.

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