Promoters are the subject of great interest in catalyst research due to their remarkable influence on the activity, selectivity and stability of industrial catalysts. It is sometimes difficult to define precisely the function of the promoters that has not been elucidated [1, 2].
Ni and Co promoters in Mo/Al2O3 catalysts are well-known for their success in the hydrodesulphurization (HDS) of petroleum feedstock and coal liquefaction products [48]. The promoting role of both promoters was found to increase the Mo dispersion and reduction, in addition to the increase in H2 mobility, an intercalation effect with MoS2, a decrease in deactivation and an increase in surface segregation of mixed sulphide phases. For instance, when Co was introduced into Mo/Al2O3, various effects occurred. Free MoO3, as well as Al2(MoO4)3, was converted into CoMoO4, and the Co addition resulted in a decrease of the isolated Mo tetrahedral concentration and favoured the formation of the polymeric form.
Moreover, Topsøe et al. reported direct evidence of Co and Mo existing in the active form as a Co-Mo-S surface phase [49, 50].
The purpose of doping a semi conductive carrier in order to enhance the catalytic activity of supported metal catalysts has recently been applied in developing “three-way” catalysts [51].
The conductivity of semiconductors is generally low but can be considerably changed by either incorporating with other oxides or upon pre-treatments. Their crystal lattices tend to release or take up oxygen. Therefore, alloying the metals with semiconductors can increase or decrease the activity. This effect has some industrial relevance since can both accelerate desired reactions and suppress undesired reactions [52, 53]. For instance, the addition of Sn to Pd gives selective catalysts for the removal of acetylene from ethylene streams [54].
In the case of n-type semiconductors (e.g. ZnO, TiO2, CeO2, SnO2), a pre-treatment in a reducing atmosphere generates electron-donor levels (oxygen vacancies VO, metal under a lower oxidation state), which increases the free electron concentration [55-57].
The presence of electron-donor levels gives rise to electronic transitions, which may occur in the infrared region:
M(n−1)+ → Mn+ + e− (Eq-12)
VO → VO+ + e− (Eq-13)
VO+ → VO2+ + e− (Eq-14)
For instance, on heating or by reaction with reducing gases such as H2, CO for n-type semiconductors such as CeO2, the release of oxygen can be described as below:
CeO2 ↔ CeO2-x + ½ xO2(g) (Eq-15) Oox ↔ Vo + ½ O2(g) (Eq-16) Ce4+ + Vo ↔ Ce3+ + Vo+ (Eq-17) Where VO, VO+ and VO2+ are the neutral and ionized oxygen vacancies, respectively, OOx is the lattice oxygen [2].
On the other hand, the adsorption of oxygen on a nonstoichiometric oxide, containing oxygen vacancies VO, generates lattice oxygen OOx. At the same time, metal ions are oxidized at the surface and the conductivity is lowered for n-type semiconductors because the oxygen acts as an electron acceptor:
½ O2(g) + Vo ↔ Oox (Eq-18) Ce3+ + O2 ↔ Ce4+ + O2– (Eq-19) The adsorption of oxygen results eventually in complete coverage of the surface by O– or O 2-and the heat of the adsorption remains practically constant while the surface becomes saturated with oxygen and negatively polarized.
Considering the adsorption of hydrogen on n-type semiconductors, it has been shown that H2
mainly undergoes heterolytic dissociation [2, 58-60]:
M2+ + O2- + H2 → M+ H + OH– (Eq-20)
On heating, the hydroxyl ions are decomposed to water and anionic defects, and a corresponding number of metal cations are reduced to atoms. In this strong chemisorption, a free electron or positive hole from the lattice is involved in the chemisorptive bonding. This changes the electrical charge of the adsorption center, which can then transfer its charge to the adsorbed molecule. Thus, chemisorbed hydrogen acts as an electron donor and increases the conductivity of n-type semiconductors. Furthermore, the change in the electrical charge density on the surface can hinder the further adsorption of the same gas. A decrease in the heat of adsorption with increasing degree of coverage is then observed, and hence a deviation from Langmuir adsorption isotherm occurs [2, 61-65].
However, when a metal is applied to an appropriate n-type semiconductor, its electron density increases [1, 2]. The general behaviour of some nonstoichiometric semiconductor oxides is summarized in Table 2.
Table 2. Behaviour of nonstoichiometric semiconductor oxides
n-type p-type
For instance TiO2, SnO2, CeO2 NiO, CoO, FeO Type of conductivity Electrons Positive holes
Addition of M21+ O oxides Lowers conductivity Increases conductivity Addition of M23+O3 oxides Increases conductivity Lowers conductivity Adsorption of O2, N2O Lowers conductivity Increases conductivity Adsorption of H2, CO Increases conductivity Lowers conductivity
The optical absorption of semiconducting oxides arises from five different phenomena: (i) intrinsic absorption, corresponding to transitions between (full) valence bands and (empty) conduction bands, which occur often in the UV–visible range and sometimes in the near-infrared (NIR) (for narrow gap semiconductors); (ii) transitions between valence bands, called intervalence transitions, only observed in p-type materials, which may appear in the NIR; (iii) free carrier absorption, arising from transitions within one band; (iv) transitions of an electron to or from a localized state; (v) lattice vibrational absorption. Their mid-infrared examination offers special difficulties due to mainly the transition types (iii) and (iv), which involve the absorbance due to free carriers and electron- or hole-donors, whose concentration depends on the semiconduction type, the surrounding atmosphere and the temperature [62].
These difficulties are seriously enhanced when the sample under study is a metal supported on an n-type semiconducting support, the reduction of the support is then greatly favoured by the metal, e.g. through activation in vacuum and spill over of hydrogen or CO. For example, this has been observed in the case of metals supported on ZnO and ceria [63-65].
Lanthanide ions of variable valence particularly Ce3+/4+ usually lead to nonstoichiometric CeO2-x. It has been reported earlier that the latter aspect and the defect structure on ceria was due to oxygen vacancies accompanied by triply and/or quadruply Ce interstitial to maintain the electrical neutrality. However, lately, oxygen vacancies have been finally affirmed as the prevailing defects neglecting the negligible effect of Ce interstitials in such a fluorite-structured oxide system [66-72]. The availability of these defect sites on the surface is probably related to their high bulk concentration. The oxygen anions (O2-) on ceria surface may be one, two or three coordinated to cerium cations. From what has been conferred for CeO2 in addition to its role as either oxygen storage and release or thermal stabilizer. It has been used either as a promoter or as a support for metal catalysts in many applications since
ceria has a beneficial effect for CO oxidation and NOx reduction under both stoichiometry and excess oxygen beside for CO/NO reaction, CO and CO2 hydrogenation. Since the catalytic oxidation of CO has acquired tremendous attraction lately particularly in connection with world-wide endeavors to curb the detrimental impacts of automotive emissions on the atmosphere. Although the detailed mechanism of the reactions mentioned above is still unknown, the researchers clearly assigned the promotional effect of the catalysts to the role of ceria in creating Ce3+/4+ redox couple [73-75]. However, due to the limited supply of precious metals and some impractical properties, an attention has been given to transition metals and their oxides as catalysts supported on CeO2 or doped with CeO2, since the ability of ceria to donate oxygen to supported metals is also a key feature in other catalytic reactions like for example, catalytic combustion and water gas shift reaction [76-78].
M. Mokhtar investigated the influence of ceria on Mo/TiO2 and found that the presence of ceria leads to increase the concentration of polymerized surface Mo oxide species, and rather initiated the formation of MoO3 over-layers. Additionally, the involvement of ceria, on the other hand, retarded the strong association rendered between Mo and Ti and thus stimulated the formation of discrete amounts of the corresponding oxides. More specifically, ceria was found to work as a mediator between Mo and Ti [79, 80].
Lucia and co-workers found that the presence of cerium in the Mo-Sn system increases the rate of ethanol dehydrogenation as well as the selectivity to acetic acid and acetaldehyde. In addition, it caused changes in the distribution of Mo species and in the textural properties, but mostly increasing the basicity of the catalyst [81].
Stannic oxide thin films are attractive for many applications due to their unique physical properties such as high electrical conductivity, high transparency in the visible part of spectrum, and high reflectivity in the IR region. In particular, tin oxide films are stable at high temperatures, have excellent resistance to strong acids and bases at room temperature, are resistant to mechanical wear, and have very good adhesion to many substrates [82-85]. Thus, transparent and electrically conductive stannic oxide films are widely used for a variety of applications. Briefly, these applications include: as electrodes in electroluminescent displays, imaging devices, protective coatings, antireflection coatings, gas and chemical sensors, transducers applications based on transparent conductors and other optoelectronic devices.
Furthermore, tin oxide films are more stable than the other transparent conducting oxide (TCO) films such as zinc oxide (ZnO). Moreover, they have a lower material cost. Recently, the synthesis of ultra fine tin oxide particles is of great technological and scientific interest
owing to their superior physical and chemical properties and their use as either catalysts for the oxidation of organic compounds or gas sensors [83-86].
As the electrical conductivity of SnO2 derived from the variable valence on the Sn atomic center is very sensitive to oxidative and reducing atmospheres, tin oxides as gas sensors detecting a trace amount of the gases have been applied to processes in chemical, heretical and fermentation industries to control the amount of the harmful wastes discharged from the plants, the explosion of the combustible gases and incomplete combustion, exhaust gases from automobiles [87-90]. However, molybdenum–tin thin films seems to be promising gas sensors. It has recently been stated that addition of MoO3 to SnO2 increases the sensor response to CO and NO2 [91-94].
All the above properties have led to intense research of SnO2 coatings over the past few decades. Currently, numerous techniques exist for the preparation of tin oxide films such as chemical vapor deposition, spray pyrolysis, sputtering, and sol–gel deposition [95-98].
Nevertheless, the ability of SnO2 to generate defects has been only recently shown to induce interesting performances for supported Pd catalysts, e.g. in deNOx reactions [99]. Conversely, tin dioxide has received limited attention in the catalysis field and the use of Mo-Sn oxides in selective oxidation appears to be unique industrial application [100-109].
On the other hand, Mo/SnO2 catalysts have been used for selective oxidation reactions due to their high activity. Niwa et al. [100] studying the methanol oxidation with several supported molybdenum catalysts found the following sequence of activity:
Mo/SnO2 > Mo/Fe2O3 > Mo/ZrO2 > Mo/TiO2 > Mo/Al2O3
Goncalves et al. [101] and Medeiros et al. [102] have shown that acetic acid can be obtained from ethanol oxidation in only one-step with high yield when Mo/SnO2 catalysts prepared by precipitation procedure are used.
Recently, Liu et al. [103] have shown that Mo/SnO2 catalysts are very active for the oxidation of dimethyl ether although they are more selective for formaldehyde than Mo/Al2O3 catalyst.
V. Lochar claimed that the activity of MoO3/SnO2 catalyst for methanol oxidation could be associated with its Brönsted and Lewis acidity as the result of the catalyst reduction [104].
Other catalytic activity results suggest the existence of synergy between the apparently pure phases of MoO3 and SnO2. Therefore, MoO3 in close contact with SnO2 has shown to be much more active and selective than the individual pure phases. The high dispersion of
molybdenum species on the highly reducible SnO2 support was suggested to be responsible for the exceptional activity of these catalysts [105-109].
However, the information available in the literature on the interpretation of infrared and Raman spectra of Mo/Sn and Mo/Ce compounds prepared on different surfaces is rather limited. In assigning vibrational spectra, some DFT (Discrete Fourier Transform) calculations or some vibrational spectroscopic data relating to Mo/Sn and Mo/Ce systems can be relied on [65, 80, 88, 91, 97, 104-109]. On the other hand, few papers can be found in the literature on IR and TG studies related to either Mo/Sn or Mo/Ce system in contrast to publications relating to noble metals with ceria and tin.
Anyhow, a lot of debates concerning the role played by ceria and tin oxides necessitate further studies in order to explore the influence of CeO2 and SnO2 on the structure and surface characteristics of molybdena for better understanding the nature, structure and the physico-chemical properties of these oxides, since the nature of the interactions between metal oxides and supports are often attributed to the complexity of these systems and differences in the preparation and experimental conditions adopted.