Catalytic Properties

Chemical industries manufacture an enormous range of everyday and specialty products -- polymers, flavouring compounds, pharmaceutical drugs and many others -- but a large fraction of conventional manufacturing routes carry real adverse environmental costs, whether through energy consumption, waste generation, or hazardous by-products. Against this backdrop, catalyst-based manufacturing processes are strongly advantageous from an eco-friendly, green-chemistry standpoint: catalysed routes typically require substantially lower activation energy (and hence lower process energy input), they tend to minimise unwanted waste and by-product formation by directing the reaction more selectively toward the desired product, and they generally enhance the overall efficiency of converting starting reactants into the target products.

Many large-scale industrial processes exploit transition metals, or their compounds, as catalysts, precisely because of two structural features that distinguish transition metals from other elements: first, transition metals possess energetically accessible (low-lying, empty or partially filled) d orbitals that are able to accept electron density donated from an incoming reactant molecule; second, a transition-metal catalyst is able to form a temporary bond directly with a reactant molecule by using its own d electrons, holding that reactant molecule in a favourable geometry and electronic state for the desired transformation to proceed.

A classic illustrative example is the catalytic hydrogenation of an alkene using a nickel catalyst and hydrogen gas: the alkene substrate first bonds to an active catalytic site on the metal surface by donating electron density from its π (pi) bond into an empty d orbital of the catalyst. Simultaneously (or in a closely coupled step), the strong σ (sigma) bond within the adsorbed hydrogen molecule is broken, with each of the two resulting hydrogen atoms individually bonding to a d electron on an atom of the catalyst surface. These two now-separated, catalyst-bound hydrogen atoms then transfer, one after the other, onto the partially broken π bond of the adsorbed alkene, ultimately forming the fully saturated alkane product, which then desorbs from the catalyst surface, freeing that active site to repeat the cycle with a fresh alkene molecule.

In certain other catalytic processes, it is specifically the VARIABLE oxidation states of transition metals (rather than simple d-orbital coordination) that are exploited. The industrial manufacture of sulphuric acid from sulphur trioxide provides the textbook example: vanadium pentoxide (V₂O₅) is used as the catalyst to oxidise sulphur dioxide (SO₂) to sulphur trioxide (SO₃) in the Contact Process. During this catalytic cycle, the V₂O₅ catalyst is itself temporarily reduced -- from vanadium in the +5 state down to vanadium(IV) oxide, VO₂ -- as it donates an oxygen atom to the SO₂ substrate, and the catalyst is subsequently re-oxidised back to V₂O₅ by atmospheric oxygen, completing a catalytic cycle that relies squarely on vanadium's ability to shuttle between two accessible oxidation states. …