Catalysing change

21 August 2014
Posted by Adam Drew.

Every biological chemical process in the universe would be impossibly slow without an enzyme, a biological catalyst, to facilitate it and speed things along. Even the seemingly simple processes of digestion and processing sugar would literally take millions of years without the enzymes present in our digestive tracts and cells. As well as naturally occurring catalysts, we also use them in industries. Numerous manufacturing processes use man-made catalysts on vast scales, even with our limited understanding of how they work. The industry is worth tens of billions of dollars worldwide and is indirectly responsible for hundreds of billions of dollars in commerce and sustaining billions of human lives. Everyday household products like margarine require catalysts to produce and, without the platinum based catalytic converters in cars, the streets would Catalytic Converterbe flooded with dangerous carbon monoxide and nitrogen oxide fumes. However, the mechanisms by which catalysts work are still being unraveled.

For a chemical reaction to take place, and successfully turn the reactants into products, it needs to go through a halfway point. This halfway point is an unstable chemical, known as the 'transition state', and represents the reacting molecules coming together into a single from, before they rearrange themselves into the final products. The speed of any reaction is controlled by how easily the transition state can be formed, which in turn depends on the speed and orientation the reacting molecules have upon collision. A catalyst can speed up a chemical reaction in two ways. First, by providing a new transition state which requires less energy to reach, or by stabilising the existing transition state, thus resulting in more molecular collisions, which then rearrange to form the product.

A chemical reaction

Most industrial catalysts consist of a solid metal sheet or mesh, to which reacting molecules can stick, or adsorb. The adsorption to the metal surface stretches and bends the bonds in reacting molecules, facilitating their rearrangement to the desired products. For example, the conversion of unsaturated fats to saturated fats via a process called hydrogenation uses a powdered nickel catalyst, to which hydrogen molecules bind, stretch and then detach into highly reactive hydrogen, speeding up their addition to the unsaturated fat molecules.

Industrially, catalysts are divided into two groups, heterogeneous: where catalyst and reaction are in two different phases, e.g. liquid and solid, and homogeneous: where the catalyst and reaction both occur in the same state, e.g. both are liquids. Heterogeneous catalysts occur typically in the solid state, usually as a fine mesh or powder in order to increase surface area the platinum/palladium mesh found in every day catalytic converters. Simpler reactions where chemical and shape specific selectivity are less important typically use heterogeneous catalysts for their ease of use: separation of the liquid or gaseous reactants from the solid catalyst is straightforward, allowing the catalyst to be reused in a cost effective way. Homogeneous catalysts are in the same state as the reacting mixture, usually liquid, and convey major operational advantages in the form of excellent control over the nature of the products' shape and chemical properties. The price for these advantages is a substantial increase in cost due to inherent difficulties in separating the catalytic species from the reactant/product mixture. There is also often high toxicity to both people and the environment associated with the transition metals generally used, such as nickel tetracarbonyl, one of the most toxic substances used in industry.

The long term security of a catalyst also depends heavily on its environmental impact and in the longer term, its effects on society at large. The Haber-Bosch catalyst has been credited as saving approximately 2.5 billion human lives fromFertiliser starvation by turning atmospheric nitrogen into a cheap and readily available source of fertiliser. Yet those 2.5 billion lives have themselves generated great disparity in wealth and resources, and placed global food supply in a precarious position. The vast increase in carbon dioxide emissions associated with such a population boom also cannot be ignored. Serious long term damage to ecosystems surrounding heavily fertilised agricultural land, due to problems such as algal bloom, has been attributed to this process. Not forgotten is the not entirely unreasonable accusation that the Haber-Bosch process extended the First World War by at least 2 years by providing a cheap and abundant source of fixed nitrogen for explosives as an alternative to the guano, a natural fertiliser made from animal droppings, monopoly previously held by Chile and Peru.

What the Haber-Bosch process can teach us is that industrial catalysts are an exceptionally powerful tool, but with unpredictable and potentially damaging consequences; hence the growing demand for catalysts contributing to a sustainable and environmentally sound economy. Generating power using artificial photosynthesis (how plants turn sunlight into energy)has been the subject of a great deal of research in recent decades and finding a suitable catalysis for the splitting of water using light (photolysis), in order to provide hydrogen for fuel cells, is a key part of the process. Much catalytic research has focused on dyed titanium dioxide (TiO2), a cheap and common metal oxide otherwise used as white paint. This otherwise unremarkable material was found to be superbly stable in the conditions occurring in photosynthesising cells, and shown to produce an electric current when shone with sunlight. Dyed TiO2 may also hold the key to creating self-cleaning materials, as exposure to sunlight can make the surface act like a detergent, and possibly reducing atmospheric pollution, as in some circumstances it can break down organic pollutants which deplete the ozone.

The future of catalysis appears promising. As our understanding of them grows, new designs for catalytic pathways may appear, potentially paving the way for a clean, sustainable chemical industry. However, as always, it is the way these developments are exploited which will decide who truly prospers from them.

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