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  • Issue 2  |
  • January 2010

Open-cage Fullerene-like Graphitic Carbons as Catalysts for Oxidative Dehydrogenation of Isobutane

A team of researchers at Oak Ridge National Laboratory has demonstrated that the open edges of fullerene-like features in carbon catalysts are the sites responsible for catalytic activity in oxidative dehydrogenation reactions. The result can be attributed to the use of synthetic carbon catalysts where the relative amount of the nanoscale structural features can be manipulated and correlated with catalytic performance.

The concept is based upon the utilization of a soft-template approach to synthesize high purity glassy carbon with enclosed fullerene-like cages of 2-3 nm and open mesopores of 7 nm. The open mesopores provide access to the fullerene-like cages, which could be opened and closed through heat treatments in air and inert gas at various temperatures as revealed by electron microscopy. Oxygenated sites were created by air treatment and thermally removed without affecting the openness of the cavities as revealed by both microscopy and desorption techniques. In this way it is possible to control both the nanoscale structure and the oxygen functionalities of the synthetic graphitic mesoporous carbons (GMCs). The GMCs showed obvious catalytic activities in the isobutane oxidative dehydrogenation (ODH) reaction when the fullerene-like cages were opened, regardless of the existence of the surface oxygenated functionalities. The GMC catalysts were deactivated after the fullerene-like cages were closed by thermal treatment.

A key problem in nanoscale catalysis is the identification of active sites. Correct identification of these sites is essential for learning how to manipulate material structure to achieve high catalytic yields of the desired products. Carbon-based materials have been widely used in catalysis, but the active sites are not well-understood due to

GMC samples

HR-TEM images of GMC samples. The red circles
indicate the ‘loop back structures’ whereas the blue
circles indicate the open edge sites of the graphitic
carbon. The scale bars represent 5 nm.

the complexity of the carbon structure. This problem is complicated by the fact that there are almost no appropriate spectroscopic means to characterize the surface functionalities. In the past, researchers have used activated carbon as a source of catalysts, but these have poorly defined structures and compositions. ORNL researchers have now used a new approach to successfully probe the open edges of graphitic carbon catalysts and the role of oxygen functionalities in the oxidative dehydrogenation production of isobutene, an important precursor chemical used to make many different polymers. The approach offers a unique method to manipulate the structure and chemical properties of carbon catalysts and, consequently, unravel and control the nature of active sites.

GMC samples

Supplementary information: Graphitic mesoporous carbons were prepared using a “soft-template synthesis”, which is a versatile approach for the synthesis of uniform mesoporous carbon materials with controlled pore size and periodicity. The image below shows the enclosed fullerene-like cavities with the size of 2-3 nm and the initial “loop-back” structures. These structures are characteristic of defects of the graphitic structures or small amount of the amorphous carbon and can be removed by gasification of the carbon atoms as shown in the sequence of micrographs. Reheating at high temperatures (2600 ºC) can reverse the process and cause the open fullerene-like cavities to close again.

Reaction-rate measurements convincingly demonstrate that the catalytic activity is related to the openness of the fullerene-like cavities and that the open edge plane is a prerequisite for the activity of glassy carbon in the ODH reactions. As shown in the figure, negligible activity was found on catalysts SC01 and SC04, which have no open-edge sites, while SC02 and SC03 containing open-edge sites presented high activity. No significant differences were found on catalysts SC02 and SC03. This fact indicates the initial oxygenated functional groups have no effect on the activity of the catalysts.

Chengdu Liang, Hong Xie, Viviane Schwartz, Jane Howe, Sheng Dai, and Steven H. Overbury,  J. Am. Chem. Soc., 2009, 131 (22), pp 7735–7741

This research at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Hong Xie acknowledges the ORNL postdoctoral Research Associates Program.