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FIG. 1 Schematic diagram of the chemical titration process of the carbonyl (a), hydroxyl (b) and carboxyl groups (c) on the carbon nanotube surface.
Fig. 2 Schematic diagram of the comparison of the activity of oxidative dehydrogenation of ethylbenzene catalyzed by carbon nanotubes and their titration derivatives (a); the dependence of the oxidative dehydrogenation activity of carbon nanotubes on the concentration of surface carbonyl groups (b).
The nano-carbon material exhibits advantages such as high reactivity, high selectivity of olefin products, and long catalytic activity retention time in the oxidative dehydrogenation reaction of alkanes. As a renewable and environmentally friendly catalyst, nano-carbon materials can replace traditional metals and their Oxide catalysts are directly applied to the catalytic conversion of alkanes and other related reactions. After rapid development in recent years, many breakthroughs have been made in the field of nanocarbon catalysis in the development and preparation of new catalysts and the establishment of novel catalytic reaction systems.
However, due to the lack of nano-carbon surface structure characterization methods, especially the lack of in-situ characterization methods, the researchers have addressed several core issues concerning the catalytic nature of nanocarbons, such as the type and number of catalytically active centers, and the measurement and comparison of the catalytic intrinsic catalytic activity of the catalysts. Inadequate understanding of such aspects, how to understand the nano-carbon catalytic process at the molecular and atomic scale has become one of the important topics in the related fields.
Since 2007, the research team headed by the researcher of the Department of Catalysis Materials of the National (Joint) Laboratory of Materials Science at the Chinese Academy of Sciences Institute of Metals, Su Dangsheng has been working on research in the field of non-metallic nanocarbon catalysis and has achieved a series of innovations. Research results (Science, 2008, 322, 73; Angew. Chem. Int. Ed., 2010, 49, 8640; ChemSusChem, 2010, 3, 169). In view of the qualitative and quantitative problems of nanocarbon catalyst active centers, Dr. Su Dangsheng and Dr. Wei Qi proposed a novel chemical titration method for quantifying the surface concentration of oxygen-containing functional groups on carbon nanotubes.
As shown in Figure 1, benzoquinone, benzoic anhydride, and 2-bromoacetophenone, etc., can selectively react quantitatively with ketone carbonyls, phenolic hydroxyls, and carboxyl groups on the surface of the carbon nanotubes, respectively, and derivatize the obtained carbon nanotubes. The structural analysis of the material can be used to calculate the concentration of these major oxygen-containing functional groups on the carbon nanotube surface. As shown in Fig. 2, through the comparison of the catalytic activity exhibited in the oxidative dehydrogenation reaction of ethylbenzene by several kinds of carbon nanotube derivatives obtained by passivating and removing the surface mainly containing oxygen functional groups, researchers for the first time directly demonstrated through chemical methods. Ketocarbonyl groups are the active sites for nanocarbon catalysis. This part of the work was published online in the form of a newsletter on November 8 at Angewandte Chemie International Edition (DOI: 10.1002/anie.201306825).
The innovation of the research is that the chemical titration method can directly give the absolute content of oxygen-containing functional groups on the carbon nanotube surface, avoiding the subjective experience of traditional temperature-programmed desorption or XPS analysis methods in peak separation, peak identification, and other processes. The error brought about has its unique advantages. At the same time, the TOF value of the oxidative dehydrogenation of alkane catalyzed by carbon nanotubes can be calculated from the carbonyl surface concentration obtained from the titration results. This value can reflect the intrinsic catalytic activity of the nanocarbon catalyst, and is an objective comparison of the activity of different catalytic materials and nanometers. The research on the mechanism and kinetics of carbon-catalyzed reaction provides important reference value.
The study was funded by the National Basic Research Program “973†Project (Grant No. 2011CBA00504) and the National Natural Science Foundation of China (Grant No. 21303226).
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