Surface properties of gas phase deposited mixed
earth alkaline metal oxides (*)
P. HOFMANNand E. KNO¨ZINGER
Institut für Physikalische Chemie, TU Wien - Getreidemarkt 9/156, A-1060 Wien, Austria (ricevuto il 28 Febbraio 1997; approvato l’8 Maggio 1997)
Summary. — On the basis of theoretical calculations a procedure was developed
which permits the production of highly reactive CaO surface states on MgO as support. Chemical Vapour Deposition (CVD) provides nanostructured particles of MgO/CaO mixed oxides. They are subjected to an appropriate thermal treatment, which initiates Ca2 1 segregation into the surface. This gives rise to a significant
thermal stability of the oxide particles and creates highly reactive surface centres which tend to chemisorb methane already at room temperature.
PACS 68.35 – Solid surfaces and solid-solid interfaces. PACS 82.65.Jv – Heterogeneous catalysis at surfaces. PACS 01.30.Cc – Conference proceedings.
1. – Introduction
For many years MgO has played an important role in the process of Oxidative Coupling of Methane (OCM). One of the strategies in the endeavour of raising the activity of the catalyst is to increase its basicity [1]. In principle, MgO should, therefore, be replaced by CaO, or even by SrO or BaO. In fact, the expected increase of catalytic activity is counterbalanced by two types of adverse trends in the series of earth alkaline oxides: the more basic oxides exhibit less thermal stability and preferentially form less basic carbonates [2].
The aim of the present investigation is to combine the properties of solid MgO (thermal stability) and CaO (higher basicity/reactivity) by mixing them. Theoretical calculations evidence segregation of Ca2 1 into the surface of solid MgO/CaO
mixtures [3, 4]. In addition, a significant reduction of the specific surface energy was predicted for MgO on doping the surface with Ca2 1[3, 4]. These findings promise the
feasibility of catalytically relevant and thermally stable CaO surfaces.
(*) Paper presented at the “First International Workshop on Reactivity of Oxide Materials. Theory and Experiment”, Como, 8, 9 November 1996.
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2. – Experimental
The earth alkaline oxides are prepared in a flow reactor system by the reaction of metal vapour with O2 in the gas phase. The apparatus essentially consists of two
concentric quartz tubes mounted inside a cylindrical furnace. The inner tube serves for the conduction of Ar gas which transports the two types of metal atoms being evaporated out of ceramic boats. At the downstream end of this tube the metal atoms meet with the reactant oxygen which flows in the space between the two tubes. The resulting metal oxide clusters and crystallites are deposited downstream in a stainless steel net. The whole apparatus is described in greater detail elsewhere [5].
The mean particle diameter is 10 nm. The structure analyses are based on X-ray powder diffractometry (Model XRD 3000TT, Seifert, Ahrensburg, Germany), whereas the BET method is applied in order to determine specific surface areas (adsorption gas: N2). Isolated OH and other surface probes permit the discrimination of different oxide
surface centers by FT-IR spectroscopy (Model IFS 113v, Bruker, Karlsruhe, Germany). The IR cell applied was described elsewhere [6]. The identification and character-ization of surface radical states is carried out by EPR spectroscopy (Model EMX 10/12, Bruker, Karlsruhe, Germany).
3. – Results and discussion
Earth alkaline metal oxides are known as catalysts which efficiently favour the activation of methane and related compounds. Their catalytic activity augments monotonously with the basicity of the oxides, i.e., from MgO to BaO. For one type of oxide it may be raised by increasing the number of low coordinated surface sites per unit mass. A non-equilibrium preparative technique which is particularly well suited for this purpose is Chemical Vapour Deposition (CVD) [7]. Thus a simple strategy for the production of the basic catalyst in question becomes apparent. It has, however, no chance to be put in practice, since—with the exception of MgO—the resulting nanostructured particles exhibit an absolutely unsatisfactory thermal stability (fig. 1): in a standard annealing procedure the CaO sample loses 80% of its BET specific surface area (for comparison: for the respective MgO sample the loss is 25%) and at the same time most of the reactive low coordinated surface centres by sintering. For oxides containing the heavier earth alkaline metals the situation is even less encouraging.
A more promising approach is suggested by theoretical calculations [3, 4] related to binary solid mixtures of earth alkaline oxides, e.g., MgO/CaO. They predict a decrease of specific surface energy of MgO (100) and (110) planes, if they are doped by larger cations, e.g., Ca2 1. Generalizing these results, we should expect that a surface coverage
of MgO with CaO should stabilize the respective particles. Thus CaO surfaces may become available which are even more stable than those of comparable MgO particles. The calculations also predict a considerable tendency of larger metal cations isolated in the bulk MgO phase to segregate into the surface [3, 4]. For CaO in MgO this is in agreement with the existence of a large miscibility gap in the phase diagram of MgO/CaO between the mole fractions XCa4 0.02 and XCa4 0.98 at 1600 7C [8]. Both the
phase diagram and the theoretical results related to segregation recommend a feasible method of preparation of CaO surfaces supported by MgO: CVD as non-equilibrium technique permits to create relevant non-equilibrium mixtures of CaO/MgO. The resulting nanostructured particles lose significantly less BET specific surface area
Fig. 1. – Influence of the annealing temperature (duration: 20 min) of nanostructured earth alkaline metal oxides on their specific surface area (BET, N2).
than the respective pure MgO and CaO samples during thermal treatment (20 min) at increasing temperature up to 900 7C (fig. 1). Whether the segregation providing the CaO surface film has already taken place during the CVD production or later on during the stability test presented in fig. 1 is an open, but essentially irrelevant question.
Fig. 2. – Dependence of the MgO lattice constants on the mole fraction of CaO in MgO (thermal treatment: 900 7C for 20 min).
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Fig. 3. – IR spectra of surface OH groups on nanostructured earth alkaline metal oxides. Before annealing a thermal treatment at 600 7C (20 min) was applied. Annealing (not applied to the CaO sample) means thermal treatment at 900 7C (3h for MgO, 82h for Ca0.11Mg0.89O).
The X-ray powder diffractogram of the above-mentioned Ca0.20Mg0.80O sample
annealed during 20 min at 900 7C does not exhibit any contribution of a CaO phase to coherent scattering. The MgO lattice constant relates, however, linearly to the CaO content up to the mole fraction XCa4 0.1 (fig. 2). Without any doubt not a stable
MgO/CaO mixture (XCaE 0.02), but rather a metastable one is in agreement with
Vegard’s rule. Thus roughly half of the originally 20 mol% CaO has segregated into the surface of the nanostructured oxide particles. For fundamental reasons both fractions of CaO, the randomly distributed species in the bulk and those forming the surface coverage, do not contribute to coherent scattering.
It is generally accepted that chemisorption of H2O on highly dispersed MgO and
subsequent thermal treatment at about 600 7C creates isolated IR active surface OH groups [9]. The distribution of reactive surface centres on nanostructured oxide particles may, therefore, be monitored by FT-IR spectroscopy using surface OH groups as probes. Thus the influence of annealing at 900 7C can be demonstrated for different samples (fig. 3). Pure MgO loses some specific surface area and—consequently—a fraction of the reactive (low coordinated) surface centres. This is evidenced by a significant reduction of integral absorbance in the OH stretching region. For the MgO/CaO mixture (Ca0.11Mg0.89O) the loss of surface centres related
to MgO is roughly compensated by the gain of those related to CaO (see difference curve) which has segregated into the surface during the annealing procedure. In the
Fig. 4. – EPR spectra of pure MgO and MgO surface doped with CaO. The nanostructured oxide samples were previously subjected to a thermal treatment at 900 7C (20 min).
Fig. 5. – IR spectra of CH4 chemisorbed on pure MgO surfaces doped with CaO. The nanostructured oxide samples were previously subjected to a thermal treatment at 900 7C (20 min). The band at 3017 cm21 exhibiting rotational fine structure originates from gaseous
P.HOFMANNandE.KNO¨ZINGER 1692
case of pure CaO the corresponding IR spectrum of the annealed sample is not available because of strong sintering (see above).
Some of the previously discussed surface OH groups may also be created by the reaction of H2 with the respective surface centres at room temperature [10]. Most
likely, the reaction mechanism involves surface radical anions. This has, however, so far not been evidenced experimentally by EPR spectroscopy. On the other hand, the presence of surface radical states on appropriately calcinated MgO obtained by precipitation has been reported repeatedly in the past [11-13]. We observed a characteristic EPR signal for the CaO-doped MgO prepared by CVD (fig. 4). The g value (2.0063) differs considerably from that of similarly treated pure CVD MgO (2.0036, fig. 4) and from those reported in the literature [11, 12].
Only recently FT-IR spectroscopy has been applied in order to study the chemisorption of methane on two types of oxide samples (fig. 5) similar to those previously subjected to EPR measurements (fig. 4). Without any doubt the one with CaO in the surface exhibits not only the higher spin density, but also—already at room temperature—a reactivity towards methane which is by more than an order of magnitude higher than that related to pure MgO (fig. 5). The resulting bands centred around 2810 and 2740 cm21 are in the range where aldehyde-like CH groups absorb.
On the other hand there is no significant IR spectroscopic evidence for carbonyl groups. The structure of the respective surface complex is, therefore, still under discussion.
4. – Conclusions
Chemical vapour deposition turned out to be a valuable tool for the production of nanostructured particles of pure earth alkaline metal oxides and their binary mixtures. The latter ones exhibit a suprisingly high thermal stability. Owing to the concentration and basicity of their low coordinated surface centres, these materials appear to be promising candidates for Oxidative Coupling of Methane and methane activation in general.
* * *
Financial support from the Austrian Fonds zur Förderung der wissenschaftlichen Forschung (FWF:Nr. P11542-CHE), from the Max-Buchner-Forschungsstiftung (Nr 1829), and from the Hochschuljubiläumsstiftung der Stadt Wien (Nr. H-00027/95) is gratefully acknowledged. Thanks are also due to Dr. P. SUCH(Bruker) for cooperation
related to EPR measurements.
R E F E R E N C E S
[1] RUCKENSTEINE. and KHANA. Z., J. Catal., 141 (1993) 628. [2] MAITRAA. M., Appl. Catal. A, 114 (1994) 65.
[3] COLBOURNE. A., Surf. Sci. Rep., 15 (1992) 281.
[4] TASKERP. W., COLBOURNE. A. and MACKRODTW. C., J. Am. Ceram. Soc., 68 (1985) 74. [5] BECKERA., BENFERS., HOFMANNP., JACOBK.-H. und KNO¨ZINGERE., Ber. Bunsenges. Phys.
Chem., 99 (1995) 1328.
[7] HOFMANN P., JACOBK. H. and KNO¨ZINGERE., Ber. Bunsenges. Phys. Chem., 97 (1993) 316. [8] DOMANR. C., BARRJ. B., MCNALLYR. N. and ALPERA. M., J. Am. Ceram. Soc., 63 (1964)
313.
[9] KNO¨ZINGERE., JACOBK.-H., SINGHS. and HOFMANNP., Surf. Sci., 290 (1993) 388.
[10] KNO¨ZINGER E., JACOB K.H. and HOFMANN P., J. Chem. Soc. Faraday Trans., 89 (1993) 1101.
[11] LUNSFORDJ. H. and JAYNEJ. P., J. Phys. Chem., 70 (1966) 3464.
[12] CORDISCHID., INDOVINA V. and OCCHIUZZIM., J. Chem. Soc. Faraday Trans. 1, 74 (1978) 456.
[13] GIAMELLOE., MURPHYD., RAVERAL., COLUCCIAS. and ZECCHINAA., J. Chem. Soc. Faraday Trans., 90 (1994) 3167.
