Which Metals Can Be Used to Prepare Ozone Catalysts?
Ozone catalysis is widely used in air purification, industrial off-gas treatment and wastewater treatment. In a catalytic reaction the choice of catalyst strongly influences both the reaction rate and the selectivity of the products. So which metals and metal oxides show ozone catalytic activity, and what are their characteristics?
The Catalyst as a Porter
A catalyst is a porter: it changes the route, not the outcome. Strictly, a reaction proceeds only when the Gibbs free energy change is negative. A catalyst must therefore be able to bind and release species; modern catalysts usually bind at least two reactants and bring them together, as in coupling reactions and asymmetric hydrogenation. Some traditional catalysts need only bind or activate a single substrate. Metals as the main active centre appear in many forms: zero-valent metals, metal oxides with variable valence, and ionic metal complexes.
What Makes a Metal a Good Catalyst
Metals that work as catalysts generally need rich electronic properties and a large, easily deformed electron cloud, which favours contact with the reactants; a loose electron cloud also helps the products leave. Mechanistically, a catalyst must satisfy the following conditions.
1. Orbital matching. The d orbitals of Ni, Rh, Ir and similar metals can interact with the bonding orbital of hydrogen, allowing oxidative addition.
2. Suitable coordination number and geometry. Hydrogenation usually proceeds in a cis fashion, with the M-H unit and the C=C unit coplanar — four atoms in one plane. For ruthenium bisphosphine-bisamine and similar systems the arrangement is a six-membered ring plane. Binding both the substrate and hydrogen sometimes requires additional ligands, so centres of at least four-coordinate are needed; in asymmetric hydrogenation a defined three-dimensional geometry is also required, which a planar arrangement cannot provide, so at least five-coordinate centres are needed. Ni, Rh, Ru and Ir are typical qualifying metals, whereas linear Cu(I) and Au(I) are not suitable.
3. Practical requirements. For use as an industrial heterogeneous catalyst, high activity, low loss and resistance to poisoning are essential, together with adequate mechanical properties. Most metal, metal-oxide and metal-salt composites that can be used with ozone meet these requirements. Homogeneous catalysts are generally somewhat less active than heterogeneous ones; only Mn, Rh, Ru, Ti and some Fe catalysts achieve high activity, and they are mainly used for high-value, low-volume chiral compounds.
Why Transition Metals Dominate
Transition metals have an inherent electronic advantage and are regarded as the natural home of redox catalysis. The transition elements are those of the d block and ds block of the periodic table — groups IIIB to VIIB and group VIII for the d block, excluding the lanthanides and actinides, and groups IB and IIB for the ds block. The series includes the elements of groups 10 to 12 but not the f block, where elements 58–71 are the 4f inner transition elements and 90–103 the 5f series.
Because their valence d orbitals are incompletely filled, transition metals differ markedly from elements governed by the 18-electron rule. Many have numerous unpaired electrons — the manganese group, with its d5 configuration, is the extreme case — and these electrons are easily lost. Such metals therefore show variable valence, and some, iron for example, form several stable ions. Transition metals can reach oxidation states as high as +7 (manganese) and +8 (osmium); the former reflects the presence of single electrons, the latter a high, diffuse energy level. High oxidation states occur in metal acids or acyl species, such as vanadate and vanadyl ions.
Transition metals such as Fe, Co and Ni (+2, +3), Mn (+2 to +7), Pt, Pd and Ru have rich valence chemistry and readily form intermediates, which gives them good catalytic performance. The d shells of these cations easily lose or gain electrons, so the metals and their oxides have strong redox properties. Their semiconducting character gives them a particular advantage in electron transfer and energy conversion, which helps optimize reaction efficiency and raise catalyst activity. In Co, Ni, Mn and Ru oxides the inner valence orbitals of the metal ions can split with external orbitals, giving transition-metal catalysts more active sites and more reaction pathways, and therefore greater diversity and selectivity. Transition-metal oxide catalysts are consequently the most widely used in industrial catalysis.
Main-Group Elements: the Next Frontier
Although transition metals enjoy a natural advantage, main-group elements with d electrons are in principle also redox-active and may serve as redox catalytic centres. More importantly, their reactivity can likewise be tuned through ligand design to optimize yield and selectivity. The ability of main-group elements to take part in redox catalysis can therefore be expected to develop further, and may one day rival that of the transition metals.
Iodine, for example, has several stable valence states and can form labile covalent bonds with organic groups. Such bonds can be regarded to some extent as a special kind of coordination bond, which gives iodine redox catalytic potential.
Catalysis based on phosphorus redox couples of different valence is also developing. Researchers have designed a trivalent phosphine catalyst with a tridentate ligand in a T-shaped geometry, intended to mimic tridentate transition-metal phosphine complexes and activate covalent bonds efficiently in the region perpendicular to the ligand plane. Bismuth, too, can be built into catalytic cycles that change oxidation state under ligand control, in the same way as transition metals.
In summary, metal complex catalysts play a vital role in the modern chemical industry, especially in redox catalysis. Transition metals, with their distinctive electronic structure and rich valence chemistry, are the first choice for catalyst design. As chemical research advances, the potential of main-group elements is being explored and exploited; through skilful ligand design and optimization of reaction conditions, their performance is expected to improve further and in some areas to rival transition-metal catalysts. A better understanding of catalytic mechanisms and the emergence of new materials will make catalyst design and application more diverse, bringing further innovation to the chemical industry.
