Based on the interconnected network of repeating units of metal-oxygen bonds that make up an oxide, the catalytic properties differ depending on the redox properties of the active metal ion. The change in catalytic properties is brought about by changes in bond length, strength, and subsequently bond energy in metal-oxygen bond. Also, the nature of the metal ion plays a crucial role in tuning catalytic properties. The oxygen mobility on the surface of the oxide is another factor that determines the activity of the oxide catalysts [22, 23]. The availability of oxygen vacancies on the oxide surfaces is also believed to trigger different catalytic behavior in many oxides [24, 25]. This phenomenon has been well documented for ceria [26, 27].
Amongst the many applications of both binary and ternary metal oxides, catalysis has been an interesting one [17]. Metal oxides are capable of catalyzing a wide range of reactions such as oxidation [20, 28-31], reduction [32], and ammoxidation [33, 34]. However, based on the limited scope of this work, emphasis will be placed on oxidation reactions. The choice of oxidation reactions is solely based on the expected synergistic effect that the oxygen-rich metal oxides and the active nanostructures could possess.
1.2.1. Oxidation reactions
Oxidation reactions are an important class of reactions both in academia and industrially [35]. Academically their importance derives from the complex scientific challenge they pose to scientists trying to study their properties. Industrially, oxidations continue to take center stage in the production of large scale fine chemicals and pharmaceuticals. The production of these chemicals via the oxidation processes is an old process which dates back decades. The value of oxidation products back in the 1980’s was estimated to be between US$ 20-40 billion per annum and the value kept on increasing with time [36]. Though there is a tremendous profit generated in the production of the above chemicals, the task of producing them is a difficult one. There are many drawbacks such as the use of environmentally unfriendly reactants and solvents, and most importantly, the over-oxidation of the desired partial oxidation products. Thus, ways to design catalysts that are efficient in environmentally friendly reaction conditions have been the subject of research for a long time. These environmentally friendly conditions include the use of peroxide-free oxidants and chlorine free catalysts. Thus, the use of molecular oxygen from air has been utilized as a cost effective and a greener oxidant. The catalysts themselves should be synthesized with cost effective methods and be fully characterized to study the surface properties.
1.2.1.1. Types of oxidation catalysts
Industrially, the production of oxygenates using the oxidation route employs a variety of catalysts such as coordination compounds and metal oxides, to mention a few. Both sets of catalytic systems mentioned had been extensively used in many oxidation reactions and have had their successes and drawbacks.
1.2.1.1.1. Coordination complexes
The palladium-based coordination compounds have been successfully used in the Wacker-Hoeschst process for the production of acetaldehyde [37, 38]. In the Wacker-Hoeschst process, the chloride is the ligand of choice and the process is precisely homogeneously catalyzed.
The catalytic coordination compounds are not limited to palladium complexes, vanadium metal can actively catalyze the oxidation of olefins. Oxovanadium(IV) complexes with different ligand systems successfully oxidized styrene to benzaldehyde [39]. These type of catalysts have drawbacks. The presence of chlorine is considered as a catalyst poison and affects the catalytic activity. Also the stability of these catalytic systems is questionable.
1.2.1.1.2. Metal oxides
Metal oxides have been utilized as catalysts for oxidation reactions [28, 40]. However, studies to uncover their surface properties have proved to be a complex task. This leads to poorly drawn kinetic trends between catalyst structures and reaction mechanisms [35]. The complexity in understanding the oxide catalyst structures also leads to the difficult task of tuning product selectivity in many oxidation reactions. However, attempts to study oxidation mechanisms using oxides have been made and progress to date is promising. One example is the use of bismuth molybdate-based catalysts in the oxidation of alkanes [23].
1.2.2. Established mechanisms in metal oxide catalyzed reactions
1.2.2.1. Oxidative dehydrogenation
This oxidation process is initiated by dissociation of adsorbed oxygen on the surface of the oxide [5]. Direct participation of dissociated reactive oxygen species in the oxidation of the substrate is assumed [41]. This type of mechanism is often referred to as oxygen assisted dehydrogenation. Furthermore, for oxidation to take place on the surface of the catalytic metal
oxide, both reactants must be activated on the surface. The oxidative dehydrogenation is often associated with the Langmuir-Hinshelwood mechanism. However, in some instances, the involvement of the lattice oxygen in the oxidation process, which is latter replenished by surface-dissociated oxygen from the oxidant of choice, makes this mechanism to be viewed in terms of the Mars-van Krevelen model. Figure 1.4 shows the oxidative dehydrogenation mechanism for the formation of propanal.
Figure 1.4. Oxidative dehydrogenation mechanism illustrating oxygen insertion to hydrocarbons.
1.2.2.2. Dehydrogenation
Unlike the oxidative dehydrogenation, dehydrogenation assumes hydrogen transfer by the substrate onto the catalyst surface [5]. The substitution of oxygen as an oxidant by hydrogen acceptor molecules confirmed the hydrogen transfer mechanism [42]. The role of the oxygen is the oxidation of the byproduct, hydrogen, to form water. Similar to the oxidative dehydrogenation, dehydrogenation also requires adsorption of the substrate on the oxide surface. Figure 1.5 shows dehydrogenation of methanol on oxide catalyst.
Figure 1.5. Illustration of the dehydrogenation mechanism for methanol conversion to formaldehyde
Due to the requirement of surface adsorption of the substrate in both mechanisms, reactivity on the oxide surface has been the subject of extensive research. There are kinetic models that have been developed to study the surface reactions. There are three models that have been studied extensively, both in liquid- and gas-phase reactions. They are (1) The Eley-Rideal, (2) Langmuir-Hinshelwood, and (3) the Mars-van Krevelen models. They all differ in kinetic interpretation and they are all mathematical approximations. However, when supported by precise experimental and theoretical evidence, they become useful in the design and application of novel metal oxide catalysts.
1.3. Introduction to transition metal nanoparticles and their application as heterogeneous