Activated carbon surface oxygen complexes

It is found that the CNF-HT has not catalytic activity for ODP. After oxidation, all the three samples show hi ly catalytic performances, which are shown in Fig.3. CNF-HL has the longest induction period among the three samples, and it has relatively low activity and propene selectivity at the beginning of the test. During the induction periods, the carbon balance exceeds 105% and then fall into 100 5%, which implies the CNF structure is stable and the surface chemistry of CNF reaches a dynamic equilibrium eventually. These results indicate that the catalytic activity of ODP can be attributed to the existence of surface oxygen complexes which are produced by oxidation. The highest propene yield(lS.96%) is achieve on CNF-HL at a 52.97% propane conversion. 

Stoeckli, F., Moreno-Castilla, C., Carrasco-Marin, F., and L6pez-Ram6n, M.V. (2001). Distribution of surface oxygen complexes on activated carbons from immersion calorimetry, titration and temperature-programmed desorption techniques. Carbon, 39(14), 2235-7. 

Billinge, B.H.M. and Evans, M.G. (1984). The growth of surface oxygen complexes on the surface of activated carbon exposed to moist air and their effect on methyl iodide-131 retention. / Chimie Physique, Physico-Chimie Biologique, 81, 779-84. 

The variety of mechanisms that may be involved in the sorption process of metal ions onto activated carbon induces a great number of factors that control the adsorption the surface oxygen complex content, the pH of point of zero charge, the pore texture of carbon, the solution pH and its ionic strength, the adsorption temperature, the nature of the metal ion given by its speciation diagram, its solubility, and its size in adsorption conditions. The influence of these various conditions is detailed in Section 24.2.1.4. 

The surface chemistry of activated carbons essentially depends on their heteroatom content, mainly their surface oxygen complex content, which determines the charge of the surface, its hydrophobicity, and the electronic density of the graphene layers. Thus, when a solid such as a carbon material is immersed in an aqueous solution, it develops a surface charge that derives from the... 

Ahmed et al. [116] carried ont a detailed stndy with the objective of identifying the properties of activated carbons that are important for the SCR of NO they concluded that chemical properties such as surface oxides and mineral matter play a more important role than their physical properties, such as surface area and pore structure. In effect, they found that the catalyst activity correlated directly with the oxygen content of the carbon samples and inversely with their pH. These results indicate that the NO conversion is favored on more acidic carbons. They also reported that NO reduction by ammonia was negligible in the absence of oxygen. Indeed, it has been shown [117] that oxygen enhances the C-NO reaction through the formation of surface oxygen complexes, which are essential for the C-NO reaction to proceed. 

Singoredjo et al. [122] used activated carbons modified with nitrogen- and oxygen-containing compounds in the SCR of NO with NH3 at 385 to 550 K. Of several additives tested, glucosamine resulted in an outstanding increase in activity, ascribed by the authors to the formation of stable surface oxygen complexes. 

An activated carbon in contact with a salt solution is a two-phase systan consisting of a solid phase that is the activated carbon surface and a liquid phase that is the salt solution containing varying amounts of different ionic and molecular species and their complexes. The interface between the two phases acts as an electrical double layer and determines the adsorption processes. The adsorption capacity of an activated carbon for metal cations from the aqueous solutions generally depends on the physico-chemical characteristics of the carbon surface, which include surface area, pore size distribution, electro-kinetic properties, the chemistry of the carbon surface, and the nature of the metal ions in the solution. Activated carbons are invariably associated with acidic and basic carbon-oxygen surface groups. 

Several chemical groups of different stability on the carbon surfaces were recognized as carboxylics, phenols, lactones, carbonyls, quinones and hydroquinones. The TPD profiles were distinctly different, carbon for carbon, indicating differences in the stability of the carbon surface groups. The activated carbon, CAL, has the highest contents of surface oxygen complexes. 

The effects of oxygen-surface groups, on this graphite, on the enthalpies of immersion using benzene, water and methanol were also studied by Barton et al. (1972,1975). In addition, Rodriguez-Reinoso et al. (1997) studied carbons from olive stones, activated to 37 wt% bum-off in steam at 730 °C, and finally oxidized to several extents with nitric acid (6N) to place surface oxygen complexes on the surfaces. This series of carbons was then heated in the range 100-900 °C (10 samples in all). 

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