Carbonaceous surface oxides

The surface oxide groups on carbon play a major role in its surface properties for example, the wettability in aqueous electrolytes, work function, and pH in water are strongly affected by the presence of surface groups on the carbonaceous material. Typically, the wettability of carbon... 

Real reasons due to (a) the occurance of very fast (and therefore in most cases diffusion controlled) catalytic reactions on the electrode surface, (b) Formation of non-conducting carbonaceous or oxidic layers on the catalyst electrode surface. 

There is a third real reason for deviations from Eq. (5.18) in the case that a non-conductive insulating product layer is built via a catalytic reaction on the catalyst electrode surface (e.g. an insulating carbonaceous or oxidic layer). This is manifest by the fact that C2H4 oxidation under fuel-rich conditions has been found to cause deviations from Eq. (5.18) while H2 oxidation does not. A non-conducting layer can store electric charge and thus the basic Eq. 5.29 (which is equivalent to Eq. (5.18)) breaks down. 

This XPS investigation of small iron Fischer-Tropsch catalysts before and after the pretreatment and exposure to synthesis gas has yielded the following information. Relatively mild reduction conditions (350 C, 2 atm, Hg) are sufficient to totally reduce surface oxide on iron to metallic iron. Upon exposure to synthesis gas, the metallic iron surface is converted to iron carbide. During this transformation, the catalytic response of the material increases and finally reaches steady state after the surface is fully carbided. The addition of a potassium promoter appears to accelerate the carbidation of the material and steady state reactivity is achieved somewhat earlier. In addition, the potassium promoter causes a build up on carbonaceous material on the surface of the catalysts which is best characterized as polymethylene. 

Similarly, the emission of soot from many practical devices, as well as from flames, is determined by the rate of oxidation of these carbonaceous particles as they pass through a flame zone and into the post-combustion gases. As mentioned in the previous chapter, the soot that penetrates the reaction zone of a co-annular diffusion flame normally bums if the temperatures remain above 1300K. This soot combustion process takes place by surface oxidation. 

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 ... 

Carbon surfaces of various types have been the subject of most studies. However, other types of surfaces, including alumina, fly ash, dust, MgO, V2Os, Fe203, and Mn02, have also been shown to oxidize S02 and/or remove it from the gas phase (Hulett et al., 1972 Judeikis et al., 1978 Liberti et al., 1978 Barbaray et al., 1977, 1978 Halstead et al., 1990). As expected, the rate of removal depends on the nature of the particular surface, the presence of copollutants such as N02, and, as in the case of carbonaceous surfaces, the relative humidity. The increase with increasing water vapor suggests that oxidation of the S02 may occur in a thin film of water on the surface of the solid. 

Langley LA, Fairbrother DH. (2007) Effect of wet chemical treatments on the distribution of surface oxides on carbonaceous materials. Carbon 45 47-54. 

The objective of this paper is to demonstrate that unique surface heterogeneity of sludge derived materials is a factor governing t " performance as desulfurization adsorbents. Complex chemical nature of inorganic matrix provides active centers for adsorption/oxidation. On the other hand, carbonaceous matter, is responsible for existence of pores of small diameters where oxidation process occurs and where catalysts can be highly dispersed. All of these provide active centers and space for storage of the surface oxidation products. 

Based on the above information, the following sequence has been proposed (172,173, 181,182 for the formation of carbonaceous surface intermediates during alkane oxidation on platinum ... 

Under most conditions, oxygen release into rhizosphere should be adequate to oxidize excessive levels of reduced compounds in order for wetland plants to survive soil anaerobiosis. Release of oxygen into the rhizosphere is demonstrated by the observation of oxidation of Fe + to Fe + and precipitation on the root surface, oxidation of carbonaceous compounds, and nitrification of ammonium nitrogen. A detailed discussion on the fate of oxygen in the rhizosphere is presented in Chapter 6. 

Most recently, Dedryvere and Edstrom et al. conducted the extensive research work on the interfacial mechanisms of Si anode, as shown in Fig. 5.35, including reaction of surface oxide, Li-Si alloying process, and passivation layer formation by XPS [102]. To reveal more depth information of SEI ingredients, they conducted a thorough nondestructive depth-resolved analysis by nsing both soft X-rays (100-800 eV) and hard X-rays (2,000-7,000 eV) from two different synchrotron facilities compared with in-house XPS (1,487 eV). The formation of SEI starts from 0.5 V vs. Li/LP. At the end of discharge (0.01 V vs. Li/LP), a thick SEI layer has formed, and carbon black (284 eV, black shadows) can be observed only at 2,3(K) and 6,900 eV. New carbonaceous species have been identified at the surface, C-0... 

L.A. Langley, D.E. Villanueva, D.H. Fairbrother, Quantification of surface oxides on carbonaceous materials, Chem. Mater. 18 (2006) 169—178. 

The HPLP apparatus has been used to study the hydrogenation of carbon monoxide on iron and rhodium polycrystalline specimens as well as on sit e crystals and various rhodium compounds." Postreaction surface analysis revealed the presence of a catalytically active carbonaceous layer on all the samples investigated. In addition, precise control of the rhodium surface oxidation state by oxygen pretreatment in the UHV chamber was found to have a marked effect on the product distribution of the rhodium catalyzed reaction. 

C is retained in the graphite and associated carbon deposits primarily as elemental carbon on surfaces, some oxygen will be associated with it. " C produced in reactions other than from C (J.e. from or 0) is likely only to be present in surface oxide and in the carbonaceous deposits (C/H/O compounds of high molecular weight), which are potentially more mobile in a turbulent flow or leaching situation, but the total activity in this form will be relatively small when safe storage commences. 

Li and co-workers used carbonaceous polysaccharide microspheres as hard templates. The surface of a template was functionalized with -OH and C=0 groups, which can bind with metal cations through the coordination of electrostatic interactions. After calcination, the adsorbed metal species formed oxide hollow spheres, while the carbonaceous spheres oxidized into carbon oxide species. 

The surface oxide groups on carbon play a major role in its surface properties for example, the wettability in aqueous electrolytes, work function, and pH in water are strongly affected by the presence of surface groups on the carbonaceous material. Typically, the wettability of carbon blacks increases as the concentration of surface oxides increases [16]. The pH of an aqueous slurry of carbon decreases as the volatile or oxygen content of the carbon increases [17]. The work function of carbon blacks shows a minimum at a pH near 6 [18]. 

Activated carbons are made by first preparing a carbonaceous char with low surface area followed by controlled oxidation in air, carbon dioxide, or steam. The pore-size distributions of the resulting products are highly dependent on both the raw materials and the conditions used in their manufacture, as maybe seen in Figure 7 (42). 

The weld was riddled with mildly undercut, gaping pits. Attack was confined to fused and heat-affected zones, with a pronounced lateral or circumferential propagation (as in Fig. 6.10). The resulting perforation at the external surface was quite small. Pits were filled with deposits, friable oxides, and other corrosion products. Black plugs embedded in material filling the gaping pit contained high concentrations of iron sulfide. Bulk deposits contained about 90% iron oxide. Carbonaceous material was not detected. 

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