Andy Benniston at Newcastle University, UK, explains how photocatalysts could provide the answer to the planet's energy crisis, and reduce CO₂ emissions while they're at it

The world is going through a major change as countries start to come to grips with the realisation that current sources of energy - coal, oil and gas - are not unlimited. Coupled with the expected growth in population, it is clear that our limited fossil fuels will not be able to meet future energy requirements.  And as global warming predictions kick in, the production of the greenhouse gas carbon dioxide (CO₂) from burning fossil fuels must be minimised on a global scale.  It is clear that solving these two problems is a priority, and will require not just a handful of nations taking up the challenge but a real world-wide effort. 

Although the picture painted may seem all 'doom and gloom', it should not be forgotten that the fuels we burn today come from carbon sources that were, in a simple sense, 'fixed' by nature many millions of years ago. The current situation is only the consequence that we are now using up these sources at an alarming rate and nature cannot keep up!  

So how can we solve the energy crisis and reduce CO₂ emissions at the same time?  The answer is either to find a fuel that does not produce CO₂, or to find a process that uses CO₂ in fuel production. If the two processes could be coupled together then the cycle set up would clearly be highly beneficial.  This may seem far-fetched, but this cycle is almost the same as that used by plants to covert water into oxygen that we breathe, and CO₂ into carbohydrates that we eat. We breathe out CO₂ completing the cycle. The energy source for all this to operate comes from the sun, which to all intent purposes affords the planet unlimited power. 

A major challenge for scientists is to mimic the process that plants successfully use to harness solar energy. We may think that this is a new idea, but back in the late 1970s and 1980s many groups tried to solve the problem, to differing levels of success. 

One area of research focussed on the splitting of water to form hydrogen, which is an ideal fuel since its combustion reforms water. In a related manner the oxidation of water to oxygen was also attempted, since by linking the two processes together the overall reaction would be splitting of water into its constituent elements.  Photochemical driven reactions were found that could carry out hydrogen and oxygen production separately, but coupling the two reactions together was never satisfactorily solved. 

If the above reactions are difficult to achieve then the controlled reduction of CO₂ is even more so, since several different products are feasible depending on the number of added electrons and protons.   One particularly interesting outcome is the six electron/proton product, methanol, which again is a fuel that can be burned to afford energy.  It can be speculated that an ideal scenario would be coupling of the water splitting reaction to the CO₂ reduction reaction.  The greenhouse gas would be converted to a fuel and sunlight would provide the energy source, thus solving all our problems in one go!

What this idyllic story lacks is the science behind how to achieve some of the reactions needed. The real challenge lies in finding photocatalysts that capture sunlight and drive the chemical transformations.  

There has been a major research effort to construct artificial model systems using the same type of building blocks used by nature for sunlight capture. One part of the work performed in this area uses porphyrin-based conjugates that incorporate metal-based poly(pyridyl) relays. Careful thought has gone into their design in an attempt to create photocatalysts capable of mimicking the light driven charge separation reaction used by plants to create fuel. 

But this is only part of the story as the synthesis of such catalysts is time consuming, and requires controlled build up of the final structure. Even after careful design and synthesis, detailed photophysical studies can often reveal the system does not behave as expected and it's back to the drawing board.  

There is a still a long way to go before an 'artificial leaf' is available and a new generation of scientists is needed to take up the challenge now.

Andy Benniston