New catalytic system for conversion of CO2 to methanol shows much higher activity than others now in use
1 August 2014
Scientists at the US Department of Energy’s (DOE) Brookhaven National Laboratory, with colleagues from the University of Seville (Spain) and Universidad Central de Venezuela, have discovered a new, highly active catalytic system for converting carbon dioxide to methanol.
The pure metals and bimetallic systems used for the chemical activation of CO2 usually have low catalytic activity; the new system exhibits significantly higher activity than other catalysts now in use. The new catalyst system converts CO2 to methanol more than a thousand times faster than plain copper particles, and almost 90 times faster than a common copper/zinc-oxide catalyst currently in industrial use.
In a paper in the journal Science, the team presents experimental and theoretical evidence for this completely different type of site for CO2 activation: a copper-ceria interface that is highly efficient for the synthesis of methanol.
The activation of CO2 and its hydrogenation to alcohols or other hydrocarbon compounds is an important approach to recycle the released CO2. This is a challenging task because of the difficulties associated with the chemical inertness of CO2. A recent study has identified the active site for the activation of CO2 and the synthesis of methanol on Cu/ZnO/Al2O3 industrial catalysts. The active site consists of Cu steps decorated with Zn atoms. Cu alone interacts very poorly with CO2 and alloying with Zn is necessary in order to bind the reactant better and accelerate its transformation into methanol.
Here we present experimental and theoretical evidence for a completely different type of site for CO2 activation: A Cu-ceria interface, which is highly active for the synthesis of methanol. The combination of metal and oxide centers in the Cu-ceria interface provides favorable reaction pathways for the CO2 → CH3OH conversion not seen over a Cu-Zn alloy.—Graciani et al.
Because CO2 is normally such a reluctant participant in chemical reactions, interacting weakly with most catalysts, it’s also rather difficult to study. These studies required the use of newly developed in-situ imaging and chemical “fingerprinting” techniques. These techniques allowed the scientists to peer into the dynamic evolution of a variety of catalysts as they operated in real time. The scientists also used computational modeling at the University of Seville and the Barcelona Supercomputing Center to provide a molecular description of the methanol synthesis mechanism.
The team was particularly interested in exploring a catalyst composed of copper and ceria (cerium-oxide) nanoparticles, sometimes also mixed with titania. The scientists’ previous studies with such metal-oxide nanoparticle catalysts demonstrated their exceptional reactivity in a variety of reactions. In those studies, the interfaces of the two types of nanoparticles turned out to be critical to the reactivity of the catalysts, with highly reactive sites forming at regions where the two phases meet.
|This movie of scanning tunneling microscopy images shows the copper-oxide component of a catalyst (red) as it is gradually reduced to metallic copper (yellow) in the presence of hydrogen gas to produce the active phase for the transformation of carbon dioxide to methanol.|
To explore the reactivity of such dual particle catalytic systems in converting CO2 to methanol, the scientists used spectroscopic techniques to investigate the interaction of CO2 with plain copper, plain cerium-oxide, and cerium-oxide/copper surfaces at a range of reaction temperatures and pressures. Chemical fingerprinting was combined with computational modeling to reveal the most probable progression of intermediates as the reaction from CO2 to methanol proceeded.
These studies revealed that the metal component of the catalysts alone could not carry out all the chemical steps necessary for the production of methanol. The most effective binding and activation of CO2 occurred at the interfaces between metal and oxide nanoparticles in the cerium-oxide/copper catalytic system.
The key active sites for the chemical transformations involved atoms from the copper and oxide phases, said Jesús Graciani, a chemist from the University of Seville and first author on the paper.
The researchers suggest that their study illustrates the substantial benefits that can be obtained by properly tuning the properties of a metal-oxide interface in catalysts for methanol synthesis.
It is a very interesting step, and appears to create a new strategy for the design of highly active catalysts for the synthesis of alcohols and related molecules.—rookhaven Lab Chemistry Department Chair Alex Harris
The work at Brookhaven Lab was supported by the DOE Office of Science. The studies performed at the University of Seville were funded by the Ministerio de Economía y Competitividad of Spain and the European Regional Development Fund. The Instituto de Tecnologia Venezolana para el Petroleo supported part of the work carried out at the Central University of Venezuela.
Jesús Graciani, Kumudu Mudiyanselage, Fang Xu, Ashleigh E. Baber, Jaime Evans, Sanjaya D. Senanayake, Darío J. Stacchiola, Ping Liu, Jan Hrbek, Javier Fernández Sanz, and José A. Rodriguez (2014) “Highly active copper-ceria and copper-ceria-titania catalysts for methanol synthesis from CO2” Science 345 (6196), 546-550 doi: 10.1126/science.1253057
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