Oxygen Surface 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. 

Apparently we can assume that chemical activity of surface oxygen complexes in relation to solvent considerably increases in highly polar media due to creation of contact or solvate-divided ion pairs... 

In reality, it is believed that the oxidation of carbonaceous surfaces occurs through adsorption of oxygen, either immediately releasing a carbon monoxide or carbon dioxide molecule or forming a stable surface oxygen complex that may later desorb as CO or C02. Various multi-step reaction schemes have been formulated to describe this process, but the experimental and theoretical information available to-date has been insufficient to specify any surface oxidation mechanism and associated set of rate parameters with any degree of confidence. As an example, Mitchell [50] has proposed the following surface reaction mechanism ... 

The value of 0 is a function of the magnitude of the individual rate constants for the formation of the surface complex and its conversion to a desorbable product and the pressure of the reacting gas. If the product of the rate constant for the formation of the surface-oxygen complex and the pressure of the reacting gas is large compared with the rate constant for... 

Reaction temperature can also affect the order of a reaction. It is generally agreed that the rate constant for the conversion (or desorption) of the surface-oxygen complex has a higher activation energy than the rate constant for the formation of the complex. Therefore, a reaction which is zero order at low temperatures and a given pressure can become first order at the same pressure and a sufficiently high temperature. 

The writers have found in their laboratory that invariably after a certain burnoff (depending upon the reactor, temperature, and sample), a subsequent extended period of constant reaction rate, expressed in grams of carbon reacting per unit time, is attained. In this bumoff region, there obviously is equilibrium between the rate of formation of the surface-oxygen complex and its removal with a carbon atom. It is felt that this is the reaction rate most characteristic of a given temperature and should be used in kinetic calculations. In principle, Wicke (31) concurs with this reasoning and reports reactivity data only after the sample has attained a total surface area which is virtually constant. 

The usual methods of ASA evaluation (gravimetric and TPD) present some inherent problems due to the uncertainty regarding the total elimination of the physisorbed oxygen from the ASA calculation, or to the errors arising from the quantification of surface oxygen complexes evolved. The OCI method seems to be an interesting and alternative method for ASA determination, as it avoids the main potential source of errors that the usual methods present. 

The spectra of the annealed carbons (D—H, D—N) (Fig. 4) confirm the observation that heat treatment under vacuum or in ammonia diminishes the content of oxygen surface complexes, especially that of strongly acidic surface groups. There the band typical of carboxylic structures disappears and the bands of the remaining surface oxygen complexes are much reduced. 

Those surface oxygen complexes on Spheron 6 which thermally desorb as CO2 are thought to be responsible for the acidity of the carbon.34 Two types of acidic oxide have been observed. An oxide which acts as a very weak monobasic acid is decomposed at ca. 250 °C whereas a second oxide, which is a stronger dibasic acid, is decomposed at ca. 600 °C these two structures are unlike those found on graphite, which decompose at ca. 400 °C and have different acidic properties. 

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... 

Surface oxygen complexes also affect the electronic density of the graphene layers [16], which in turn affects the dispersion interactions between the carbon surface and the adsorptive molecules. For instance, carboxyl groups fixed at the edges of the graphene layers have the ability to withdraw electrons, whereas phenolic groups release them. Thus, Tamon and Okazaki [17] determined by... 

Introduction of surface oxygen complexes on the carbons negatively affected the TCE and MTBE adsorption from aqueous solution. However, the MTBE adsorption from cyclohexane solution was greatly increased (by a factor of about 5-6) for oxidized samples. This was explained by preferential MTBE adsorption on carboxyl and phenohc groups, which form H-bonds with the ether functionahty of MTBE. These H-bonds in aqueous solution would be preferentially formed with water molecules, producing water clusters that would reduce the accessibility to the rest of the carbon surface. 

The results of TCE adsorption from cyclohexane solution showed that the effect of the surface chemistry was neghgible, indicating that TCE was not preferentially adsorbed on the surface oxygen complexes. However, these complexes reduced TCE adsorption from the aqueous solution with respect to the nonoxidized carbon, due to the formation of water clusters, as in MTBE adsorption. 

All these results show the importance of the carbon surface chemistry and pore texture on the adsorption of nonelectrolytic organic solutes. Thus, for hydrophobic carbons, which generally have a low content of surface oxygen complexes, the adsorption of organic molecules is by dispersion and hydrophobic interactions, and the pores involved in the adsorption depend on the molecular size of the adsorptive. Conversely, when the adsorbent s content of surface oxygen complexes increases or its hydrophobicity decreases, there is a preferential adsorption of water on these complexes, which reduces the adsorption capacity of the adsorbent. 

More recently, Pereira and coworkers [44] found a close relationship between anionic dye adsorption and carbon surface basicity due to oxygen-free Lewis sites. Surface oxygen complexes of acid character inhibited anionic dye adsorption. However, the latter complexes had a positive effect on cationic dye adsorption. The removal of these groups by heat treatment in H2 at high temperature... 

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