Novel catalysts look set to clean up
14 January 2011
Researchers from the University of Manchester have developed a simple, one-step process for the biosynthesis of magnetic nanocatalysts that can be reused and recovered more successfully than their conventional counterparts. Their recently patented approach could have widespread commercial use, from the production of pharmaceuticals to the remediation of contaminated groundwater.
Gold, platinum and palladium make highly effective catalysts. But they are expensive, which, when combined with difficulties in recovering these precious metals from spent catalysts, has limited their commercial use. In a recent study funded by BBSRC, scientists in the School of Earth, Atmospheric and Environmental Sciences at the University of Manchester have harnessed the powers of a bacterium to create a biomagnetic support for palladium nanoparticles that allows these precious metals to be easily reused, recovered and recycled.
False coloured TEM image of a bionanocrystal with magnetite (red) and palladium (blue).
© The University of Manchester.
Although only discovered in 1987, Geobacter is proving to be an organism that can perform fascinating transformations of metals, with potential applications in many technological areas ranging from energy production in microbial fuel cells, to bioremediation and chemical synthesis. Somewhat of a reclusive microorganism, living underground in oxygen-depleted environments, it has an unusual metabolism (changing the electron state of metals to create energy), which, has a dramatic impact on the chemistry of the subsurface. One of the potentially useful tricks that the microbe performs is the conversion of iron oxide grains (essentially rust) to the highly magnetic and commercially useful mineral magnetite.
Back in the lab, Professor Jon Lloyd and colleagues have spent the past 10 years working to optimise production of magnetite from Geobacter sulfurrreducens. The team experimented with growth rates, magnetic properties and organic coatings to make a suitable biomagnetite support for a range of applications, including as a support for palladium nanoparticles. This multidisciplinary work was conducted with mineralogists in Manchester and the magnetic spectroscopy group of Professor Gerrit van der Laan at the Diamond Light Source in Oxfordshire. Also involved were catalysis groups at Cardiff University and the University of Birmingham and the Unit of Functional Bionanomaterials, led by Professor Lynne Macaskie, also at the University of Birmingham. This BBSRC-funded research programme has resulted in a range of nanoscale engineered materials with highly unusual and useful properties, including the magnetically recoverable palladium catalyst.
So what's the attraction?
"Conventional chemical approaches to catalyst production tend to involve complicated procedures to produce the nanoparticles and functionalise the material surface," explains Lloyd. We found that our bacterial culture prime coats the surface of the magnetite crystal for palladium adsorption and precipitation, without the need for further processing."
"The structure of the biomagnetite support keeps the palladium dispersed in the solution and prevents the particles from sticking together and losing vital surface area, which would otherwise lead to loss of activity - a common problem with conventional colloidal particles," Lloyd adds.
What resulted was a one-step process and a highly functional catalyst, whose rates of reaction were equal to or superior to those obtained with a commercial colloidal palladium catalyst. But the real advantage of the magnetite-based catalyst was that it could be readily recovered at the end of the reaction by simply decanting the solution from the reaction vessel while retaining the solid catalyst by applying a magnetic field to the base of the flask.
In comparative experiments, Lloyd's team showed that the colloidal catalyst significantly lost activity over successive runs. By the second run most of the catalyst had been lost during the recovery step, and the remaining particles had aggregated, reducing conversion rates substantially. The palladium-coated biomagnetite, on the other hand, retained much higher conversion rates over four successive cycles.
But the advantages of the Manchester team's biotechnological approach do not end there. As the growth of the bacteria takes place at ambient temperatures and pressures and uses inexpensive feedstocks, it offers a low-cost, environmentally friendly alternative to manufacturing catalysts. A process that Lloyd and his collaborators believe should be applicable to other metal catalysts such as gold and platinum at a time when the costs of precious metals are continuing to rise.
"We found that our bacterial culture prime coats the surface of the magnetite crystal for palladium adsorption and precipitation, without the need for further processing."
"By harnessing a natural process the simplicity of the production of our catalyst, together with the excellent activity in our experiments, make this novel material applicable to a range of industrial processes, and provides a green chemistry route for nanocatalyst production," says Dr Vicky Coker, a member of the Manchester research team.
And if the cost of the catalysts can be reduced, so can the cost of the products, which could open the door to a wider range of applications.
Lloyd's team have recently looked at the remediation of water contaminated with the industrial solvent trichloroethylene (TCE) and also problematic toxic heavy metals.
"We have achieved what seem to be the fastest kinetics ever observed in the remediation of TCE, and also very promising results for other organics and redox active toxic metals and radionuclides," says Lloyd. The team are also developing a suite of magnetic materials for other applications, including cancer treatment and the bioremediation of nuclear facilities.
Microbial engineering of nanoheterostructures: biological synthesis of a magnetically recoverable palladium nanocatalyst. doi:10.1021/nn9017944
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