While I was a post-doctoral fellow at the University of Calgary, a guest speaker placed a small apparatus on an overhead projector and asked the audience to watch. Sure enough, in a minute or so, bubbles started to appear in the solution and collect in the head space of the equipment.
The speaker was making hydrogen from water using a photolytic pathway.
Specifically, he was employing a ruthenium porphyrin as the photosensitizer with acetate as a sacrificial electron donor - if my memory serves me correct.
Unfortunately, the consumption of acetate made the system untenable as a source of hydrogen.
Making electricity from sunlight is something physicists have been doing for years with solar cells. Making chemicals using sunlight or the light of an overhead projector is much more difficult.
Recent advances, though, are leading chemists to believe they will be able to create "artificial leaves" in the near future which will allow us to convert sunlight, water, and carbon dioxide into many different organic compounds.
The ultimate goal is to create a carbon fuel cycle which would see our continued consumption of small molecule hydrocarbons to generate the necessary energy to run our present economy. The carbon dioxide generated would be captured directly and fed into reactors which would turn the oxidized gas back into the reduced products. The latter reaction requires a lot of energy so the trick will be to do this with sunlight instead of through the burning of even more fossil fuels.
We can already achieve some of these reactions. Solar cells, such as those powering buildings and houses today, could be utilized to generate electrical energy which could electrolyze water resulting in hydrogen and oxygen.
As the electrical energy is already being generated by the solar cells, making hydrogen and oxygen might seem like an unnecessary step. However, hydrogen and oxygen can be stored and the fuel can be run at night when the sun isn't shining. The system is not tied to the vagaries of sunshine.
These sorts of systems already exist and are available for the home market.
The goal chemists are now trying to achieve is to take the process several step furthers and eliminate the solar cells by making small molecules directly within a solar-powered chemical reactor.
They are also seeking to make a wide variety of compounds such as formate, ethanol, isopropyl alcohol, ethylene and other organic compounds.
All of these compounds have been synthesized by demonstration systems with some systems operating with 10 per cent solar energy efficiency. (Natural photosynthesis is only one to two per cent efficient.) But none of these catalytic systems are more than bench scale curiosities.
Being able to eventually scale-up laboratory reactors and develop robust, affordable, efficient reactors would ultimately change the entire energy equation and our economy.
Consumers would have the freedom to fill up at home and be assured the carbon content they were emitting would be returned directly into fuel instead of increasing the carbon dioxide levels in the atmosphere.
Making hydrogen is relatively simple but reducing carbon dioxide is a major thermodynamic hurdle. Present catalysts can produce single carbon molecules such as methanol, formate and formaldehyde at reasonable scales but the compounds are a mishmash requiring separation to be useable.
Further, in order to run a present day reactor, the carbon dioxide must be purified and bubbled into the electrolyte. The cost of clean carbon dioxide must be factored into the economic analysis. Present catalytic systems can't simply take the carbon dioxide from air and reduce it to gasoline.
But the best photoelectrocatalysts, as they are called, are a trio of semiconductors capable of reducing carbon dioxide to methanol. Gallium phosphide, copper indium sulphide and cadmium telluride can all catalyze the reaction.
Unfortunately, the electrolyte for these reactions is acidified water containing pyridine. Isolating the methanol produced is difficult.
Ultimately, although methanol is an incredibly useful starting compound for generating a number of more complex molecules, including the constituents of gasoline, the purification step remains a problem.
Taking it one step further and reducing methanol to methane gas does clear up a few problems as the gas bubbles out of solution spontaneously. Like a leaf, chemists are working on the concurrent photoelectrosynthesis of both methanol and hydrogen with the hydrogen then being used in a dark reaction to reduce the methanol to the gaseous methane.
Shifting oxidized carbon back to reduced compounds, such as methanol and methane, is seen as one of the major challenges facing chemists today. With a little luck and a lot of persistence, the right catalyst might be found.
Chemists are optimistic we will be able to use sunlight, water, and carbon dioxide - artificial leaves - to feed our expanding energy needs while saving the environment from climate change.
