Surface oxygen complexes carbon surfaces

Figure 5.5. Diagram of energy profile (dotted line) as molecular oxygen reacts with a surface of carbon to form surface oxygen complexes, carbon monoxide, and carbon dioxide. describes activation energies of intermediate stage reactions.

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. 

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 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 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. 

Untreated furnace blacks contain small amounts of chemically bound surface oxygen complexes (17, 35). The abundance of these may be diminished by devolatilization at 900—1000° C, or increased by air oxidation between 250 and 400° C. These treatments do not materially affect the combined hydrogen content of carbon black (36,37). Virtually all non-carbon constituents are removed by graphitization above 2000° C. 

In the case of 308-nm irradiation with fluences above the threshold of ablation, the most complex behavior is found (Fig. 20). Only minor changes took place between 1 and 10 pulses. These changes can be attributed to removal of oxygen and carbon surface contaminations. In addition, the largest roughening of the surface (Fig. 11a) is detected for 10 pulses, which can also influence the relative atomic ratios. After more than 10 pulses the atomic ratios approach, within the error of the experiment, the starting values. This indicates that no surface modification took place and the polymer is ablated layer by layer. 

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. 

Mn(salhd)Cl] [72a], resulted in low metal complex loadings, even in the presence of an oxygen-rich carbon surface, and therefore it was proposed that the unfunctionalized Mn(IIl) complex was also immobilized onto the oxidized ACs, not only by it-n interactions, but also through the acidic groups by ionic exchange. 

Each H30 ion occupies an area associated with that of a rr-electron pair (i.e. a benzenoid ring, 0.05224 nm ). For non-graphitizable microporous carbons, the EDA interaction is restricted by the porosity and access to surfaces. As a result, for such carbons, only a small fraction of each graphene layer (defective) can hold the EDA-complexed H30 ions. For carbons with small amounts of surface oxygen complexes, this fraetion increases with increasing graphene layer size and perfection of the structure within the graphene layer. 

Figure 4.32 shows the variation of concentrations of acidic and basic sites, indicating that a large decrease in acid sites is accompanied by an increase in the number of basic sites. About six acid sites are removed when one basic site is formed. These results are best interpreted by accepting that the acidic sites are associated with the surface oxygen complexes and the basic sites are best described as being concentrations of delocalized TT-electrons within the graphene layers of the carbon structures. 

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). 

Whereas Figure 4.52(a, b) show the effect of HTT of carbons on enthalpies of immersion, Figure 4.53 illustrates how the chemical composition of surface oxygen complexes, on the same carbons as used for Figure 5.52 a, b) and subsequently analyzed using a TPD system (to 1100 °C) to monitor CO2 and CO production and also measuring enthalpies of immersion in water, influences enthalpies of immersion into water, Rodrfguez-Reinoso et al (1997). 

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