Researchers produce ethanol from syngas in carbon nanotubes
Carbon nanotubes (CNTs) are increasingly recognized as promising materials for catalysis, either as catalysts themselves, as catalyst additives or as catalyst supports. Researchers in China now have used CNTs loaded with rhodium (Rh) nanoparticles as reactors to convert a gas mixture of carbon monoxide and hydrogen into ethanol. The gas, commonly known as syngas, can be obtained from the gasification of biomass. When liquefied, 'synthetic biofuels' are the result. The most common technique used to convert syngas into liquids (GTL) is the Fischer-Tropsch process. But now CNTs may offer another pathway.
The Chinese breakthrough appears to be the first example where the activity and selectivity of a metal-catalyzed gas-phase reaction benefits significantly from proceeding inside a nanosized CNT reaction vessel. Dr. Xinhe Bao, professor at the State Key Laboratory of Catalysis at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, and head of the institute's Nano and Interfacial Catalysis Group, and Dr. Xiulian Pan, published their findings [abstrac] in Nature Materials.
CNTs distinguish themselves from other carbon materials, such as activated carbon and carbon nanofibers, in that they have well graphitized graphene with semiconducting or metallic characteristics and a tubular morphology with well defined dimensions. Earlier theoretical studies have shown that the electron density is shifted from the inside to the outside of CNT channels, and that inside gas molecules exhibit a binding energy different from those outside of the nanotubes:
biofuels :: energy :: sustainability :: ethanol :: syngas :: biomass-to-liquids :: gas-to-liquids :: Fischer-Tropsch :: carbon nanotubes ::
"We were curious about what would happen if we combined these graphene tubes with metal nanoparticles, which have interesting redox and catalytic properties by themselves" Dr. Xinhe Bao told NanoWerk. "We previously found that the redox properties of iron and iron oxide particles are tunable via encapsulation within CNTs."
Bao found that, for instance, iron oxide particles within 4-8 nm wide nanotubes are auto-reduced at 600 degrees Celsius while the particles located on the outer surface of the nanotubes need 800 degrees Celsius. Furthermore, the auto-reduction temperature of inside particles decreases with the nanotube diameter. On the other hand, the oxidation of metallic iron nanoparticles is retarded inside nanotubes compared to those particles located on the outer surface of nanotubes. Both experiments indicate the modification of the redox properties of these particles inside CNTs and the stabilization of metallic Fe inside nanotubes.
The researchers suspected that the modification of the redox properties of metal particles inside CNTs is a general characteristic and that this could be exploited in catalysis.
Therefore, they introduced a promoted RhMn catalyst for syngas conversion into carbon nanotube channels. Syngas is a 1:2 mixture of CO and H2. This reaction is known to be very sensitive to the redox states of Rh and Mn. Oxygenates containing two carbon atoms such as ethanol, acetyldehyde and acetic acid were produced, and surprisingly, the yield over the CNT-encapsulated catalyst was extraordinarily high, clearly exceeding that of the very good silica-supported catalyst. Furthermore, catalysts with metal particles confined inside CNTs were also significantly more active than those with the metal particles dispersed on the outer surface of the nanotubes, even though the latter are more easily accessible.
The results of the Chinese scientists suggest a host-guest interaction between the confined metal particles and CNTs, which is different from that on the outside of the nanotubes. Other effects may also play a role, like the stringent size restriction of metal particles inside CNTs and the high affinity of hydrogen to the inner surfaces of opened CNTs as exemplified in their extraordinary hydrogen adsorption capacity. Pan says that she believes that other conversions could benefit in a similar way from taking place inside CNTs, in particular if they involve hydrogen. "We also anticipate that the study of the host-guest interaction within CNTs will attract greater attention as a result."
Bao points out that experimental study of the redox properties and the electronic host-guest interaction in these systems is still a challenge and might require refined characterization techniques. "Other effects may also play a role in these catalysts" he says, "like the stronger size restriction of metal particles inside CNTs and the high affinity of hydrogen to the inner surfaces of opened CNTs. The understanding and distinction between these contributions needs to be advanced by further experimental and theoretical studies. Besides, we are currently looking at new experimental characterization techniques which provide deeper insight into the nature of these confined systems."
Pan notes that, apart from the still considerable challenge of cost efficient, large-scale production of CNTs with precise diameter and chirality control, a further challenge pertaining to catalysis is the homogeneous dispersion of metal nanoparticles within the CNT channels, since this can strongly influences the activity of these catalysts.
Apart from applications in catalysis, such CNT encapsulates might also be interesting for composite materials which require a modulation of the electronic state, such as magnetic sensor or storage materials.
Image: Carbon nanotubes for ethanol production. Schematic diagram showing ethanol production from syngas inside Rh-loaded carbon nanotubes.The black spheres denote carbon atoms, which form the graphene layers of the carbon nanotubes. The streams in light orange and green entering the nanotubes indicate the gas mixture of CO and H2, respectively. The three stacks of small spheres in rose, blue, green and red inside the tubes represent catalyst particles that may comprise more than one component. The streams in light cyan tailing behind the catalyst particles along the axis of the nanotubes represent ethanol. Courtesy: NanoWerk / Nature Publishing Group.
More information:
Xiulian Pan, Zhongli Fan, Wei Chen, Yunjie Ding, Hongyuan Luo & Xinhe Bao, "Enhanced ethanol production inside carbon-nanotube reactors containing catalytic particles", Nature Materials, published online: 21 May 2007, doi:10.1038/nmat1916.
Wei Chen, Xiulian Pan, and Xinhe Bao, "Tuning of Redox Properties of Iron and Iron Oxides via Encapsulation within Carbon Nanotubes", J. Am. Chem. Soc.; 2007; 129(23) pp 7421 - 7426; (Article) DOI: 10.1021/ja0713072
NanoWerk: Ethanol production inside carbon nanotubes - June 8, 2007.
Article continues
The Chinese breakthrough appears to be the first example where the activity and selectivity of a metal-catalyzed gas-phase reaction benefits significantly from proceeding inside a nanosized CNT reaction vessel. Dr. Xinhe Bao, professor at the State Key Laboratory of Catalysis at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, and head of the institute's Nano and Interfacial Catalysis Group, and Dr. Xiulian Pan, published their findings [abstrac] in Nature Materials.
CNTs distinguish themselves from other carbon materials, such as activated carbon and carbon nanofibers, in that they have well graphitized graphene with semiconducting or metallic characteristics and a tubular morphology with well defined dimensions. Earlier theoretical studies have shown that the electron density is shifted from the inside to the outside of CNT channels, and that inside gas molecules exhibit a binding energy different from those outside of the nanotubes:
biofuels :: energy :: sustainability :: ethanol :: syngas :: biomass-to-liquids :: gas-to-liquids :: Fischer-Tropsch :: carbon nanotubes ::
"We were curious about what would happen if we combined these graphene tubes with metal nanoparticles, which have interesting redox and catalytic properties by themselves" Dr. Xinhe Bao told NanoWerk. "We previously found that the redox properties of iron and iron oxide particles are tunable via encapsulation within CNTs."
Bao found that, for instance, iron oxide particles within 4-8 nm wide nanotubes are auto-reduced at 600 degrees Celsius while the particles located on the outer surface of the nanotubes need 800 degrees Celsius. Furthermore, the auto-reduction temperature of inside particles decreases with the nanotube diameter. On the other hand, the oxidation of metallic iron nanoparticles is retarded inside nanotubes compared to those particles located on the outer surface of nanotubes. Both experiments indicate the modification of the redox properties of these particles inside CNTs and the stabilization of metallic Fe inside nanotubes.
The researchers suspected that the modification of the redox properties of metal particles inside CNTs is a general characteristic and that this could be exploited in catalysis.
Therefore, they introduced a promoted RhMn catalyst for syngas conversion into carbon nanotube channels. Syngas is a 1:2 mixture of CO and H2. This reaction is known to be very sensitive to the redox states of Rh and Mn. Oxygenates containing two carbon atoms such as ethanol, acetyldehyde and acetic acid were produced, and surprisingly, the yield over the CNT-encapsulated catalyst was extraordinarily high, clearly exceeding that of the very good silica-supported catalyst. Furthermore, catalysts with metal particles confined inside CNTs were also significantly more active than those with the metal particles dispersed on the outer surface of the nanotubes, even though the latter are more easily accessible.
The results of the Chinese scientists suggest a host-guest interaction between the confined metal particles and CNTs, which is different from that on the outside of the nanotubes. Other effects may also play a role, like the stringent size restriction of metal particles inside CNTs and the high affinity of hydrogen to the inner surfaces of opened CNTs as exemplified in their extraordinary hydrogen adsorption capacity. Pan says that she believes that other conversions could benefit in a similar way from taking place inside CNTs, in particular if they involve hydrogen. "We also anticipate that the study of the host-guest interaction within CNTs will attract greater attention as a result."
Bao points out that experimental study of the redox properties and the electronic host-guest interaction in these systems is still a challenge and might require refined characterization techniques. "Other effects may also play a role in these catalysts" he says, "like the stronger size restriction of metal particles inside CNTs and the high affinity of hydrogen to the inner surfaces of opened CNTs. The understanding and distinction between these contributions needs to be advanced by further experimental and theoretical studies. Besides, we are currently looking at new experimental characterization techniques which provide deeper insight into the nature of these confined systems."
Pan notes that, apart from the still considerable challenge of cost efficient, large-scale production of CNTs with precise diameter and chirality control, a further challenge pertaining to catalysis is the homogeneous dispersion of metal nanoparticles within the CNT channels, since this can strongly influences the activity of these catalysts.
Apart from applications in catalysis, such CNT encapsulates might also be interesting for composite materials which require a modulation of the electronic state, such as magnetic sensor or storage materials.
Image: Carbon nanotubes for ethanol production. Schematic diagram showing ethanol production from syngas inside Rh-loaded carbon nanotubes.The black spheres denote carbon atoms, which form the graphene layers of the carbon nanotubes. The streams in light orange and green entering the nanotubes indicate the gas mixture of CO and H2, respectively. The three stacks of small spheres in rose, blue, green and red inside the tubes represent catalyst particles that may comprise more than one component. The streams in light cyan tailing behind the catalyst particles along the axis of the nanotubes represent ethanol. Courtesy: NanoWerk / Nature Publishing Group.
More information:
Xiulian Pan, Zhongli Fan, Wei Chen, Yunjie Ding, Hongyuan Luo & Xinhe Bao, "Enhanced ethanol production inside carbon-nanotube reactors containing catalytic particles", Nature Materials, published online: 21 May 2007, doi:10.1038/nmat1916.
Wei Chen, Xiulian Pan, and Xinhe Bao, "Tuning of Redox Properties of Iron and Iron Oxides via Encapsulation within Carbon Nanotubes", J. Am. Chem. Soc.; 2007; 129(23) pp 7421 - 7426; (Article) DOI: 10.1021/ja0713072
NanoWerk: Ethanol production inside carbon nanotubes - June 8, 2007.
Article continues
Friday, June 08, 2007
Scientists study impacts of industrial logging in Central Africa
Woods Hole Research Center scientists are now reporting how they used satellite imagery taken from 1976 to 2003 to study the development of industrial logging and road density in Central Africa so that scientists, conservation agencies and other organizations can better understand the trends and implications of such expansion. The work is profiled in the current issue of Science.
According to Nadine Laporte, an associate scientist at the Woods Hole Research Center and lead author of the work, "It has never been timelier to monitor forest degradation in Central Africa because there is still an opportunity to make a significant difference in reducing the amount of deforestation. The Democratic Republic of Congo contains most of the remaining forest and is the last frontier for logging expansion in Africa."
The Central-African forests are the second largest expanse of rainforest on the planet, after the Amazon. Countries that host this vast ecosystem must be encouraged to preserve it in the name of biodiversity, but also to limit greenhouse gas emissions from deforestation. Mechanisms to help conservation efforts by compensating forest-rich nations ('compensated reduction') are being developed. Certainly in the context of biofuels such mechanisms must be implemented urgently. There is enough potential land in Central-Africa to produce biofuel feedstocks without cutting down trees (earlier post), but if the countries involved are not compensated for keeping their forests intact, the risk exists that vast swathes of rainforest are turned into lucrative biofuel plantations. Monitoring the impact of logging operations and screening deforestation rates is of prime importance for such mechanisms to succeed. The Woods Hole Research Center is actively contributing to the development of 'avoided deforestation' schemes.
The researchers mapped nearly 52,000 km of logging roads within the forested region, which includes Cameroon, Central African Republic, Equatorial Guinea, Gabon, Republic of Congo, and Democratic Republic of Congo (map, click to enlarge). Prior to this work, there were few reliable data sets available to monitor both legal and illegal logging:
energy :: sustainability :: climate change :: greenhouse gas emissions :: rainforest :: deforestation :: logging :: biodiversity :: Central-Africa ::
This study provides the first synoptic view of industrial logging in Central Africa, enabling conservation agencies, government agencies, scientists, industry officials, and others to better gauge how the expansion of logging is impacting the forest and its inhabitants, and how better planning might mitigate damage.
Jared Stabach, a research assistant at the Center and second author, comments, "Roads provide access, and this research provides clear evidence that the rainforests of Central Africa are not as remote as they once were - a bad thing for many of the species that call it home."
Monitoring the expansion of logging in last dense humid forest of Central Africa is not only important for biodiversity conservation but also for climatic change. Industrial logging in Central Africa is the most extensive land use with more than 30 percent of the forest under logging concession and the clearing of these forests could significantly increase carbon emissions.
Co-author Scott Goetz, a senior scientist at the Center, notes that the combination of increasing population, economic development and climatic change means that "Africa is poised for irreversible change, so it is important to help African countries with tools to monitor what is happening to their forests."
Dr. Laporte adds, "This work helps to provide key data to local scientists, allowing them the tools needed to work with policy makers to help manage their forests, and in the process reduce biodiversity loss and carbon emissions from deforestation."
Dr. Laporte is a biologist whose research centers on the applications of satellite imagery to tropical forest ecosystems, including vegetation mapping, land-use change, and deforestation causes and consequences. She has been involved in numerous environmental projects in Africa over the past 20 years, working with in-country scientists, foresters, and international conservation organizations to develop integrated forest monitoring systems and promote forest conservation. She received her doctorate in tropical biogeography from l'Université Paul Sabatier in Toulouse, France.
Mr. Stabach works in the Geographic Information Systems (GIS) and Remote Sensing Laboratory on the Center's Africa program, monitoring changes and threats to the rainforests and threatened species throughout the Central Africa region. His master's research focused on the use of remote sensing technologies to identify Matschie's tree kangaroo habitat in Papua New Guinea. He received his B.S. from Providence College and his M.S. from the University of Rhode Island.
Dr. Goetz works on the application of satellite imagery to analyses of environmental change, including monitoring and modeling links between land use change, forest productivity, biodiversity, climate, and human health. Before joining the Center, he was on the faculty at the University of Maryland for seven years, where he maintains an adjunct associate professor appointment, and was a research scientist at NASA's Goddard Space Flight Center. He received his Ph.D. from the University of Maryland.
Image: Logging concessions and road distribution in Central Africa: Cameroon (1), Central African Republic (2), Equatorial Guinea (3), Gabon (4), Republic of Congo (5), Democratic Republic of Congo (6). Courtesy: Woods Hole Research Center.
More information:
Nadine T. Laporte, Jared A. Stabach, Robert Grosch, Tiffany S. Lin, Scott J. Goetz, "Expansion of Industrial Logging in Central Africa" - Science, 8 June 2007: Vol. 316. no. 5830, p. 1451, DOI: 10.1126/science.1141057
Woods Hole Research Center: Woods Hole Research Center Partnering with The Goldman Sachs Center for Environmental Markets to Develop Project on the Valuation of Avoided Deforestation - Sept. 22, 2006.
Article continues
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