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Argonne team finds copper cluster catalyst effective for low-pressure conversion of CO2 to methanol with high activity

Researchers at Argonne National Laboratory have identified a new material to catalyze the conversion of CO2 via hydrogenation to methanol (CH3OH): size-selected Cu4 clusters—clusters of four copper atoms each, called tetramers—supported on Al2O3 thin films.

In a study published in the Journal of the American Chemical Society, the team measured catalytic activity under near-atmospheric reaction conditions with a low CO2 partial pressure, and investigated the oxidation state of the clusters using in situ grazing incidence X-ray absorption spectroscopy. Results indicated that size-selected Cu4 clusters are the most active low-pressure catalyst for catalytic conversion of CO2to methanol; Density functional theory calculations revealed that Cu4 clusters have a low activation barrier for the conversion. The results suggest, they concluded, that small copper clusters may be excellent and efficient catalysts for the recycling of released CO2.

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The copper tetramer catalyst consists of small clusters of four copper atoms (show in gold) each, supported on a thin film of aluminum oxide (red and blue). These catalysts work by binding to carbon dioxide molecules, orienting them in a way that is ideal for chemical reactions. The structure of the copper tetramer is such that most of its binding sites are open, which means it can attach more strongly to carbon dioxide and can better accelerate the conversion. (Image courtesy Larry Curtiss; click to view larger.) Click to enlarge.

The industrial process of methanol (CH3OH) synthesis from syngas (CO, CO2 and H2) is carried out at high pressures (10 to 100 bar) using a Cu/ZnO/Al2O3 catalyst. Due to increasing emission of CO2 from fossil fuel combustion and other anthropogenic activities, this catalytic system has also become the focus of interest for obtaining sustainable CH3OH by hydrogenation of captured CO2 (CO2 + 3H2 → CH3OH + H2O). Efforts have been made to modify and improve the industrial Cu/ZnO/Al2O3 catalyst. Nevertheless, the high pressure required for achieving a quality yield of CH3OH using these catalysts brings a great challenge for reducing the energy input and cost for this process. Also, effective catalysts are in need for alternative feed streams with lower CO2 concentrations. Thus, developing an effective low-pressure catalyst for CO2 reduction to CH3OH is highly attractive.

Recently, size-selected subnanometer transition metal clusters have received considerable attention in catalysis, because of their unique electronic and catalytic properties, which differ from bulk metal surfaces and larger nanoparticles. Although a number of computational and experimental studies have been focused on catalytic and electrocatalytic CO2 reduction to fuels on various metal clusters and larger nanoparticles, there is a paucity of research on CH3OH synthesis from CO2 and H2 on size-selected non-precious metal clusters. Previously, we have successfully synthesized subnanometer metal clusters with narrow size distributions on thin-film support materials (e.g., Al2O3 and Fe3O4), and these materials have shown great potential in the catalytic conversion of small molecules. Here, we report on Al2O3 supported Cu4 clusters as an effective catalyst for the reduction of CO2 to CH3OH at a low CO2 partial pressure (0.013 atm), with a higher activity than those of recently developed low-pressure catalysts.

—Liu et al.

These CO2 conversion catalysts work by binding to carbon dioxide molecules, orienting them in a way that is ideal for chemical reactions. The structure of the copper tetramer is such that most of its binding sites are open, which means it can attach more strongly to carbon dioxide and can better accelerate the conversion.

The turnover rate (TOR) of methanol formation from CO2 hydrogenation over supported Cu4 clusters—i.e., the yield of methanol formed per copper atom per second—peaked at ∼4 × 10–4 molecule·s–1·atom–1 at 225 °C. The rate of methanol production drops above 325 °C. The authors noted that the observed TOR for methanol synthesis is fairly high in comparison to the numbers in the literature.

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Turnover rate (TOR) of CO2 reduction to CH3OH over Al2O3-supported Cu4 clusters. Credit: ACS, Liu et al. Click to enlarge.

The current industrial process to reduce carbon dioxide to methanol uses a catalyst of copper, zinc oxide and aluminum oxide (Cu/ZnO/Al2O3). A number of its binding sites are occupied merely in holding the compound together, which limits how many atoms can catch and hold carbon dioxide.

With our catalyst, there is no inside. All four copper atoms are participating because with only a few of them in the cluster, they are all exposed and able to bind.

—Stefan Vajda, co-author

To compensate for a catalyst with fewer binding sites, the current method of reduction creates high-pressure conditions to facilitate stronger bonds with carbon dioxide molecules. But compressing gas into a high-pressure mixture takes a lot of energy. The benefit of enhanced binding is that the new catalyst requires lower pressure and less energy to produce the same amount of methanol.

We’re interested in finding new catalytic reactions that will be more efficient than the current catalysts, especially in terms of saving energy.

—Larry Curtiss, Argonne Distinguished Fellow and co-author

Copper tetramers could allow the capture and conversion of carbon dioxide on a larger scale—reducing an environmental threat and creating a useful product such as methanol that can be transported and burned for fuel.

Potential obstacles include instability and figuring out how to manufacture mass quantities. There’s a chance that copper tetramers may decompose when put to use in an industrial setting, so ensuring long-term durability is a critical step for future research, Curtiss said. And while the scientists needed only nanograms of the material for this study, that number would have to be multiplied massively for industrial purposes.

Meanwhile, the researchers are interested in searching for other catalysts that might even outperform their copper tetramer.

These catalysts can be varied in size, composition and support material, which results in a list of more than 2,000 potential combinations, Vajda said. To narrow the field, the scientists will use advanced calculations to make predictions, and then test the catalysts that seem most promising.

For this research, the team used the Center for Nanoscale Materials as well as beamline 12-ID-C of the Advanced Photon Source, both DOE Office of Science User Facilities.

Curtiss said the Advanced Photon Source allowed the scientists to observe ultralow loadings of their small clusters, down to a few nanograms, which was a critical piece of this investigation.

Co-authors of the paper in JACS also included researchers from the University of Freiburg and Yale University.

Resources

  • Cong Liu, Bing Yang, Eric Tyo, Soenke Seifert, Janae DeBartolo, Bernd von Issendorff, Peter Zapol, Stefan Vajda, and Larry A. Curtiss (2015) “Carbon Dioxide Conversion to Methanol over Size-Selected Cu4 Clusters at Low Pressures” Journal of the American Chemical Society 137 (27), 8676-8679 doi: 10.1021/jacs.5b03668

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