FTIR Spectroscopic detection of CO adsorption on the catalysts

As already cited, coordinatively unsaturated cations exposed on the surface of ionic oxides give rise to surface Lewis acid sites. Consequently, basic molecules can interact with these sites by forming a new coordination bond, so completing or increasing the overall coordination at the surface cation. The stronger are the polarizing power of the Lewis acidic cation (charge to ionic radius ratio) and the basic strength of the adsorbate, the stronger is the Lewis interaction. Upon this interaction, electrons flow from the basic molecules towards the catalyst surface. These electronic perturbations as well as the molecular symmetry lowering

arising from this contact are the causes of a vibrational perturbation of the adsorbate. In most cases, the vibrational perturbation only consists in shifts of the vibrational frequencies, the more pronounced, the stronger is the interaction, i.e. the greater is the Lewis strength of the surface site. Accordingly, the shift of the position of some very sensitive bands of the adsorbate upon adsorption can be taken as a measure of the Lewis acid strength of the surface sites and the decrease in frequency has been associated with increasing acid strength [175].

As the type of probe molecule chosen will influence the obtained characteristics of the probed solid and, hence, will also affect the structure–activity relationship derived, the choice of the appropriate molecule is very important. On the use of carbon and nitrogen monoxides as probes for the surface cationic centers it is evident that the carbon and nitrogen monoxides are very weak bases and largely used for the surface characterization of cationic centers on metal oxide surfaces [176-179].

CO implies a triple bond between C and O, according to the literature, the stretching frequency for the free molecule in the CO gas is measured at 2143 cm^-1 (see the roto-vibrational absorption band in Fig. 6). In principle, the electrons are distributed symmetrically between C and O atoms, so that the lower positive charge of the C nucleus with respect to O implies the formation of a dipole with the negative charge at the C atom in spite of the lower electronegativity of C with respect to the O nucleus. For this reason, the CO molecule tends to interact through the C end with cationic centers. This interaction is rather weak, usually completely reversible by outgassing at room temperature and should be studied at room or lower temperature (e.g. at liquid nitrogen, 77 K) [177].

According to theoretical calculations, the metal-CO interaction is a simple polarization, with no formation of a true coordinative σ-bond with the cationic center. This interaction tends to increase the CO bond order, so that the CO stretching frequency tends to increase upon it.

Accordingly, the experimental measure of the CO stretching frequency for CO interacting with surface cations can be taken as a measure of the polarizing power of the cation or, in other terms, of its Lewis acidity. However, when the cation or the metal atom contains, besides empty orbitals, also full or partly filled d-type orbitals, they can interact with the empty π*- type orbitals of CO, via a π-type electron backdonation from the metal to CO. This implies that these antibonding orbitals become partly filled so that the bond order and the CO stretching frequency are decreased by this last interaction [178-181]. In this case, the interaction can become very strong and very stable metal-carbonyl complexes (carbonates) can be formed. The experimental CO stretching frequency in this case is a complex function of the electron accepting power of the cation (Lewis acidity) and of its π-type electron

donating power. Accordingly, the CO stretching frequency of CO adsorbed on several metal cations is very informative on the oxidation state of the adsorbing ion. The associated process can be easily followed by IR spectroscopy [179-182].

In fact, acidic OH groups form very weak hydrogen bonded adducts with both CO and H2

probes and the shift of the stretching frequency induced by the perturbation is proportional to the charge present on the hydrogen (and hence indirectly upon its Brönsted acidity). On the contrary, basic OH groups do not show any tendency to interact with CO and H2 [183-187].

Yet, a few have been published on IR study of CO adsorption on Mo-containing catalysts in which the Mo loading was mostly 8-10 %. For instance, Zs. Németh undertook a low temperature IR study at -196°C of the interaction between Mo/Al2O3 (8 %) and CO within the extent of reduction from 500°C up to 900°C. A correlation was found between the extent of reduction and the increase of adsorbed CO species. Furthermore, the five bands detected (1991, 2025, 2052, 2159, 2205 cm^-1) were assigned to molybdenum having different valence states as the result of the higher reduction without the detection of adsorbed carbonate species.

Moreover, two bands at 1991 and 2025 cm^-1 assigned to metallic Mo⁰ were observable after reduction at 700°C [188, 189].

Fig. 6. FTIR roto-vibrational spectra of gaseous CO (left) and NO (right)

1.6. OBJECTIVES

Knowing that a lot of researchers prepared and studied Mo/Al2O3 catalysts with low Mo loadings (8-10 wt%), not taking into account the isoelectric point of the solid support and the calcination temperature and time, since as expected free MoO3 clusters could not be produced at Mo loading lower than 15 wt% on Al2O3 depending on the calcination temperature and time. For this reason, my first objective was to prepare MoO3-rich catalysts and thus how affects the final catalyst.

The second objective was to explore the influence of ceria and tin (as either promoters or supports) on molybdena, the changes with adsorption of molybdates in comparison with the Mo/Al2O3 catalyst since IR and thermal analysis studies regarding these systems are rather limited. So that the knowledge gained from the catalyst formation should be helpful in the characterization of the resulting catalyst.

A complementary approach, to help reducing such uncertainties in composition and bonding, is to study these catalysts, which are chosen to mimick the real catalysts as closely as possible in terms of overall chemical composition. Therefore, the third objective was to study these model catalysts by various characterization techniques. Each characterization technique has its limitations, thus it is dangerous to rely on one method. Whenever, various techniques were applied, not only to obtain supplemental information about the catalysts but also to serve mutual checks. These included the multipoint BET analysis method, X-ray diffraction (XRD), Thermal analysis (TG-DTA), Electron Spin Resonance (ESR) and Diffuse reflectance Fourier Transform Infrared (DRIFT) spectroscopy.

Additionally, their reduction characteristics with H2 and their activity towards CO adsorption and CH4 decomposition were also aimed to provide insight into their surface characteristics.

The overall objective was to seek correlations and differences between these catalysts to collate the results providing better understanding and contributing to the identification of the physicochemical characteristics of Mo-containing catalysts that could influence their catalytic behaviour in many reactions.

2. EXPERIMENTAL