The visible way

By: David Bradley

The efficient conversion of sunlight to chemical energy has been until now the preserve of photosynthesising plants. However, a method for splitting water with visible light rather than ultraviolet radiation devised by Japanese researchers could be the key to unlocking artificial photosynthesis.
  Using the energy of sunlight to release chemical energy in the form of hydrogen for running fuel cells would provide a cheap and renewable source of energy for vehicles and both domestic and industrial electricity production.
 Hironori Arakawa of the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan, and colleagues there and at the Science University of Tokyo in Chiba have developed a new mixed catalyst that will extract energy from light of less than 400 nanometres and allow water to be split. Previously experimental methods for splitting water with electromagnetic radiation had relied on the higher energy of ultraviolet.
  The team had already demonstrated water splitting with ultraviolet radiation using a unique synergy effect between two different titania photocatalysts suspended in aqueous sodium iodide solution. In that system, hydrogen gas was evolved on a platinum titanate (the mineral anatase) catalyst while the iodide was oxidised to iodate. Oxygen was formed on titania (the mineral rutile) as the iodate shuttles back to iodide.
 Now, the team has achieved the same stoichiometric splitting of water into H2 and O2 under visible light rather than ultraviolet radiation to release hydrogen gas, which bodes well for the exploitation of sunlight. Non-stoichiometric splitting would be necessity involve an additional reagent to balance the equation.
  In the visible version of Hironori's process, the mixed catalyst is based on platinum tungstate and platinum strontium titanate (doped with Cr-Ta). The same iodine-based mediator again acts as a shuttle for the electrons involved in the water splitting.
  The team are yet to perfect the catalytic mix to make it produce the best water splitting efficiency under visible light. "Unfortunately the quantum efficiency is quite low," concedes Hironori, "we need to improve the value to 10% or so." He reckons photocatalytic water splitting technology will mature over the next twenty to thirty years. 

Chem. Commun.; DOI: 10.1039/b107673f

Stellar system stifles landfill stench

Anyone living near a landfill will be familiar with the awful smell of decomposing waste. But, those nasty niffs and disgusting gases could soon be history, thanks to EPSRC-supported research that is blasting apart the chemicals responsible using a plasma (see Sparks Fly below).

The miasma of noxious vapours rising from landfill sites the world over is responsible for some of the world's worst smells. From the rotten egg scent of sulphur compounds to the pungent pong of phosphorus-loaded vegetable matter. There are countless odorous volatiles produced by millions of tonnes of domestic waste that cloud the opinions of those who live and work nearby. The problem is a long-term one, 'The smells are not just produced whilst the landfill site is in operation but continue long after it is filled and closed, often for decades,' explains Christopher Whitehead of The University of Manchester.
  Whitehead who is supported by a “proof-of-concept” EPSRC grant - under its Waste and Pollution Management programme - thinks he has come up with the answer and Accentus plc (part of AEA Technology) is helping to commercialise the development.
  It takes only a tiny quantity of some of the smelliest gases - just a few molecules among billions to pique the olfactory membranes of the human nose. Indeed, the nose is far more sensitive to certain molecules than the best laboratory instrument. But, landfill odour is not just about offending our delicate noses. Many of the volatiles are linked with respiratory and environmental problems. If there were a simple way to get rid of the ethyl butanoate, the halogenated compounds and the malodorous sulphides, a breath of fresh air could sweep through the waste disposal industry.
  Whitehead is an expert in handling so-called low-temperature gas plasmas. Plasmas are sometimes described as the fourth state of matter, after solids, liquids and gases. They are like a gas but instead of being composed of neutral atoms or molecules, these are ionised so that all their negatively charged electrons are separate from the positive atomic nuclei. These electrons are accelerated to high energy in an electric field and when they collide with the atoms and molecules in the gas, they create excited states and free radicals that provide a reactive soup that can destroy the pollutants in the gas.
  Plasma is a highly reactive state that can tear molecules apart and it is this property that could be the end of smelly landfill gases. Pipe the odourous molecules from a landfill through a plasma and they will be converted into virtually harmless and odourless compounds, such as carbon dioxide and water before they are released to the atmosphere. The sulphur, admittedly, would be released as sulphur dioxide, toxic but significantly less odorous, but the small quantities present what Whitehead says is 'a negligible risk'.
  The Whitehead team has been investigating just how feasible it would be to use a plasma generated by a powerful electric discharge. The system would work at room temperature producing high-energy electrons, which decompose the gas into reactive radicals. There would be no costly to run heating element and because it operates at atmospheric pressure no reinforced valves and chambers would be needed. The plasma would simply be produced by a high frequency and high voltage electric discharge like the ones used to generate ozone for purifying drinking water.
  Whitehead's colleague analytical scientist Imtiaz Ahmad is measuring how well their plasma reactor can destroy odorous molecules. She uses online FTIR (Fourier-transform infrared spectroscopy) to measure the 'before and after' of mock landfill gas containing a single odour-producing compound. FTIR provides a characteristic fingerprint of molecules present. Gas chromatography-mass spectrometry then allows her to detect the products formed by the plasma. The tests are assessing how well five common odorous gases are removed from the mock landfill gas.
  Among the smelly compounds are halocarbons. 'Halogenated molecules are common components of many household products,' explains Whitehead, 'and will be found in domestic waste.' He points out that such materials 'may be potential greenhouse gases and represent a threat to the stratospheric ozone layer.'
  Whitehead emphasises that they will also be looking at the complicated reaction chemistry of the destruction of the smelly molecules. This will tell them whether other volatile compounds in real landfill gas will cause problems with by-products of the plasma process.
  'We presently have an EPSRC funded (Chemical Engineering) project to look at the incorporation of real-time computer control into plasma processing of waste gas streams,' Whitehead told Newsline. The team is also focusing on using plasma technology for the remediation of waste gas streams, diesel exhausts and air quality enhancement. 'We will be particularly involved in understanding and modelling the chemistry involved in plasma processing as an aid to developing new processes,' he adds.
  With European Union pressure on to cut down on landfill usage significantly in coming years, a fundamental state of matter found among the stars could offer a down to earth solution to the problem.

Sparks fly

Plasmas are often associated with more dramatic situations than landfills. A comet's tail, the solar wind, an astronomical nebula, lightning, the aurora, even the core of the sun is in a plasma state. In fact, plasmas are the most common form of matter, making up more than 99% of the visible universe.

To make a plasma, one either has to heat a low-pressure gas very strongly - to more than 50 000 Celsius, some eight times hotter than the surface of the sun. At this temperature, the energy of the gas molecules or atoms is high enough that the electrons are released from the tight grip of the nucleus. But, heat isn't the only way to form a plasma, one can also use an electrical discharge - the gas in a fluorescent lamp forms a plasma when a high voltage is applied to it. In a non-thermal plasma, the electrons are at very high temperatures - 10,000 Celsius or more - the gas atoms and molecules in the gas remain at room temperature.

This article originally appeared in EPSRC Newsline

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A closed outlook on life

In the growing field of research into closed ecosystems, scientists hope to improve their understanding of what sustains life. This understanding will help in building life-support systems for our future journeys into space, and for solving environmental problems on Earth such as recycling wastewater, purifying air, and growing food crops in extreme conditions.
  According to Mark Nelson of the Institute of Ecotechnics, London, UK, closed ecological systems research grew from both the mesocosm work studying miniature ecosystems in the laboratory of researchers like University of Florida's Howard Odum and the closed experimental ecospheres (1-10 litre laboratory flasks) created in the 1960s by Clair Folsome at the University of Hawaii. The early Russian and American space programs too made simple bioregenerative closed systems that used algae to recycle air and water and could sustain human life for short periods. The field entered the popular imagination when the Biosphere 2 system, which included many natural ecosystems as well as the agricultural and human living quarters was built in Arizona.
  The meeting featured presentations from researchers in Germany, Russia, Japan and the US. Volker Bluem at the University of Bremen and Klaus Slenzka and colleagues at OHB-System GmbH, on the Technology Park at the University of Bremen for instance have developed aquatic organism life-support systems, which have undergone spaceflight experiments. Other papers included bioregenerative recycling of wastewater using highly biodiverse constructed wetlands, a spin-off technology from the Biosphere 2 project authored by Nelson, Odum and colleagues. The Russians continue their innovative work in the field, both in Moscow and at the Institute of Biophysics, Krasnoyarsk, Siberia, and several of their papers described computer modelling and investigations of simple microbial and plant systems.
  The Japanese team working on CEEF (Closed Ecological Experiment Facilities) also presented their first experimental data which includes understanding how higher crop plants and domestic animals might respond to environmental changes. Keiji Nitta and colleagues believe CEEF could help in predicting the effect of small temperature rises, perhaps due to global warming, on rice growth. Nitta has also investigated the potential impact on the carbon cycle of small amounts radioactive carbon dioxide that will likely be released into the air by the Rokkasho-Mura nuclear fuel reprocessing plant, which is currently under construction and could still be modified.
  The Russian team of Alexander Tikhomirov and Sofiya Ushakova at the Institute of Biophysics (Russian Academy of Science, Siberian Branch) meanwhile have examined how photosynthesis can be affected by different conditions using a bioregenerative life-support system. They have discovered that wheat plants kept in 0 Celsius air can tolerate prolonged periods of darkness much better.
  Life faces many problems, understanding how we can sustain it at the extremes - whether under threat of global warming, the harsh Siberian winter, or the depths of space could be one of the greatest scientific challenges of this century.
  Papers from the F4.4 Life Science session on Closed Ecological Systems:
Earth and Space Applications are published in [[LINK]]; a major session at the 33rd COSPAR general assembly in Warsaw, Poland.