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Layer-by-layer catalysts
RESEARCH NEWS
Layer-by-layer catalysts POROUS MATERIALS Researchers from Michigan State University have developed a particularly convenient method for modifying alumina and polymeric membranes with nanoparticles by using layer-by-layer adsorption of polycations and citrate-stabilized Au colloids [Dotzauer et al., Nano Lett. (2006) 6, 2268]. The method creates highly accessible and well-separated nanoparticles, and enables control over the amount of colloid deposited. Metal nanoparticles make attractive catalysts because their large surface area to volume ratio enables efficient use of expensive metals. Frequently, however, catalytic activity is greatly reduced because the nanoparticles aggregate, leading to undesirable bulk-like properties being observed. In order to prevent aggregation, catalytic nanoparticles are immobilized on supports or in polymeric materials. Porous membranes can also be used, with the added benefit that they provide a geometry that allows for flow-through reactions. The group's approach uses deposition of polyanions on to the membrane, followed by consecutive deposition of polycations and Au colloid to immobilize the Au nanoparticles, without inhibiting access to catalytic sites. Measuring the efficiency of reduction of nitroaromatic compounds to their corresponding amino derivatives shows that the modified alumina membranes have a remarkable conversion ability and are able to catalyze >99% reduction of 4-nitrophenol to 4-aminophenol at high flow rates. The polyelectrolyte multilayer approach is an extremely versatile and convenient method for immobilizing accessible and catalytically active nanoparticles in a variety of porous supports including alumina.
Catherine Reinhold
The more holey the better POROUS MATERIALS
such large fractions of internal The potential applications for empty space. crystalline molecular sieves are Only tetrahedrally coordinated Al numerous, ranging from gas is present in ITQ-33, generating separation to ion exchange. In Brönsted acidity in the structure, particular, zeolites – a family which coupled with the extraof aluminosilicates, are widely large interconnected pores opens used as catalysts in hydrocarbon up a variety of new possibilities cracking. However, most zeolites for catalysis. For example, during are limited by their internal catalytic cracking of vacuum dimensions, which restrict access gasoil, ITQ-33 can produce more of larger molecules to the active diesel and less gasoline than sites within the pores. Researchers existing zeolites. at Valencia’s Institute of Chemical Zeolite ITQ-33, viewed along the large “Catalysis with larger molecules Technology in Spain have made 18-ring pore channels. (Courtesy of Avelino Corma.) will allow the expansion of the a significant breakthrough in field of crystalline molecular this area by synthesizing a new sieves to process heavier fractions of oil, future coalsilicogermanate zeolite, ITQ-33, with straight, largeand biomass-derived products, and fine chemical pore channels [Corma et al., Nature (2006) 443, 842]. products,” comments lead researcher Avelino Corma. This new zeolite has large, 12.2 Å diameter, circular To exploit this potential fully, further synthesis work openings along the c-axis, interconnected by smaller will be required to produce Ge-free Al-ITQ-33. It is channels (5.6 Å diameter) running perpendicular, that hoped that ITQ-33 will transform the field of zeolite create a structure with a very large pore volume. What catalysis, if its stability and economics can be further makes ITQ-33 interesting is that, until now, circular improved upon. pore openings bigger than 10 Å have only been Catherine Reinhold observed in two other systems, neither of which show
Screening scheme for electrocatalytic materials ENERGY GENERATION Theoretical materials science has long comparable to, or even better than, pure been searching for methods to design Pt, the archetypical HER catalyst. The BiPt functional materials based on electronicsurface alloy is particularly interesting with structure calculations. However, not only regard to the HER because of the contrast are the calculations computationally between its two constituent elements; pure demanding, but they require efficient Pt exhibits high catalytic activity, whereas and accurate methods for solving the pure Bi is not active at all. electronic-structure problem. Researchers A three-step procedure was developed from the Technical University of Denmark to synthesize this alloy, and its activity is have carried out a moderately large shown experimentally to be superior to that Combinatorial arrays of DFT calculations predicting the catalytic of Pt, confirming the prediction using DFT scale combinatorial screening process activity of hundreds of metal alloys to identify alloy catalyst materials calculations. for the HER. (Courtesy of Jens K. using density functional theory (DFT) “One of the long term goals of our work is to Nørskov.) calculations [Greeley et al., Nat. Mater. be able to use computational methods based (2006) 5, 909]. on a solution of the electronic-structure problem to help The high-throughput screening technique was used to screen for materials with interesting functional properties”, evaluate the electronic structure of over 700 binary says lead researcher, Jens K. Nørskov. surface alloys and estimate the stability of each alloy in The computational screening technique shows promise for use electrochemical environments to identify new electrocatalysts in catalyst searches and, as the accuracy and quality of kinetic for the hydrogen evolution reaction (HER). models and DFT calculations improves, it should become more The search identified a BiPt surface alloy as one of the most broadly used in the search for catalytic materials. promising candidate materials, with a predicted activity Catherine Reinhold
DECEMBER 2006 | VOLUME 9 | NUMBER 12
15
Layer-by-layer catalysts POROUS MATERIALS Researchers from Michigan State University have developed a particularly convenient method for modifying alumina and polymeric membranes with nanoparticles by using layer-by-layer adsorption of polycations and citrate-stabilized Au colloids [Dotzauer et al., Nano Lett. (2006) 6, 2268]. The method creates highly accessible and well-separated nanoparticles, and enables control over the amount of colloid deposited. Metal nanoparticles make attractive catalysts because their large surface area to volume ratio enables efficient use of expensive metals. Frequently, however, catalytic activity is greatly reduced because the nanoparticles aggregate, leading to undesirable bulk-like properties being observed. In order to prevent aggregation, catalytic nanoparticles are immobilized on supports or in polymeric materials. Porous membranes can also be used, with the added benefit that they provide a geometry that allows for flow-through reactions. The group's approach uses deposition of polyanions on to the membrane, followed by consecutive deposition of polycations and Au colloid to immobilize the Au nanoparticles, without inhibiting access to catalytic sites. Measuring the efficiency of reduction of nitroaromatic compounds to their corresponding amino derivatives shows that the modified alumina membranes have a remarkable conversion ability and are able to catalyze >99% reduction of 4-nitrophenol to 4-aminophenol at high flow rates. The polyelectrolyte multilayer approach is an extremely versatile and convenient method for immobilizing accessible and catalytically active nanoparticles in a variety of porous supports including alumina.
Catherine Reinhold
The more holey the better POROUS MATERIALS
such large fractions of internal The potential applications for empty space. crystalline molecular sieves are Only tetrahedrally coordinated Al numerous, ranging from gas is present in ITQ-33, generating separation to ion exchange. In Brönsted acidity in the structure, particular, zeolites – a family which coupled with the extraof aluminosilicates, are widely large interconnected pores opens used as catalysts in hydrocarbon up a variety of new possibilities cracking. However, most zeolites for catalysis. For example, during are limited by their internal catalytic cracking of vacuum dimensions, which restrict access gasoil, ITQ-33 can produce more of larger molecules to the active diesel and less gasoline than sites within the pores. Researchers existing zeolites. at Valencia’s Institute of Chemical Zeolite ITQ-33, viewed along the large “Catalysis with larger molecules Technology in Spain have made 18-ring pore channels. (Courtesy of Avelino Corma.) will allow the expansion of the a significant breakthrough in field of crystalline molecular this area by synthesizing a new sieves to process heavier fractions of oil, future coalsilicogermanate zeolite, ITQ-33, with straight, largeand biomass-derived products, and fine chemical pore channels [Corma et al., Nature (2006) 443, 842]. products,” comments lead researcher Avelino Corma. This new zeolite has large, 12.2 Å diameter, circular To exploit this potential fully, further synthesis work openings along the c-axis, interconnected by smaller will be required to produce Ge-free Al-ITQ-33. It is channels (5.6 Å diameter) running perpendicular, that hoped that ITQ-33 will transform the field of zeolite create a structure with a very large pore volume. What catalysis, if its stability and economics can be further makes ITQ-33 interesting is that, until now, circular improved upon. pore openings bigger than 10 Å have only been Catherine Reinhold observed in two other systems, neither of which show
Screening scheme for electrocatalytic materials ENERGY GENERATION Theoretical materials science has long comparable to, or even better than, pure been searching for methods to design Pt, the archetypical HER catalyst. The BiPt functional materials based on electronicsurface alloy is particularly interesting with structure calculations. However, not only regard to the HER because of the contrast are the calculations computationally between its two constituent elements; pure demanding, but they require efficient Pt exhibits high catalytic activity, whereas and accurate methods for solving the pure Bi is not active at all. electronic-structure problem. Researchers A three-step procedure was developed from the Technical University of Denmark to synthesize this alloy, and its activity is have carried out a moderately large shown experimentally to be superior to that Combinatorial arrays of DFT calculations predicting the catalytic of Pt, confirming the prediction using DFT scale combinatorial screening process activity of hundreds of metal alloys to identify alloy catalyst materials calculations. for the HER. (Courtesy of Jens K. using density functional theory (DFT) “One of the long term goals of our work is to Nørskov.) calculations [Greeley et al., Nat. Mater. be able to use computational methods based (2006) 5, 909]. on a solution of the electronic-structure problem to help The high-throughput screening technique was used to screen for materials with interesting functional properties”, evaluate the electronic structure of over 700 binary says lead researcher, Jens K. Nørskov. surface alloys and estimate the stability of each alloy in The computational screening technique shows promise for use electrochemical environments to identify new electrocatalysts in catalyst searches and, as the accuracy and quality of kinetic for the hydrogen evolution reaction (HER). models and DFT calculations improves, it should become more The search identified a BiPt surface alloy as one of the most broadly used in the search for catalytic materials. promising candidate materials, with a predicted activity Catherine Reinhold
DECEMBER 2006 | VOLUME 9 | NUMBER 12
15