Carbon, chemisorption

Carbon chemisorption on metals has been studied on the substrates Ni(100), Mo(100) and Zr(1000), Although studies of atomic C chemisorption are few, the resulting structures are diverse and are tabulated in table 7. 

The amount of methane chemisorbed may be estimated by the amount of hydrogen released, or better by TPO of the retained carbon . Chemisorption at high temperature leads to several distinguishable forms of carbon that differ in reactivity, but the composition of mono-carbon species CH formed at lower temperatures (573-723 K) can be assessed by the composition of deuteromethanes... 

Fig. 7.1 Several types of defects generated on carbon nanotube wall and silicon dioxide substrate after argon atom collision. Single vacancy (purple circle), kink (complex) defect (black circle), carbon chemisorption and doping on Si02 substrate (orange circle) [38]...

We consider first some experimental observations. In general, the initial heats of adsorption on metals tend to follow a common pattern, similar for such common adsorbates as hydrogen, nitrogen, ammonia, carbon monoxide, and ethylene. The usual order of decreasing Q values is Ta > W > Cr > Fe > Ni > Rh > Cu > Au a traditional illustration may be found in Refs. 81, 84, and 165. It appears, first, that transition metals are the most active ones in chemisorption and, second, that the activity correlates with the percent of d character in the metallic bond. What appears to be involved is the ability of a metal to use d orbitals in forming an adsorption bond. An old but still illustrative example is shown in Fig. XVIII-17, for the case of ethylene hydrogenation. 

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

Carbon dioxide cannot be recommended for routine determinations of specific surface on the other hand, it should be particularly suitable for the study of the polarity of surfaces in systems where chemisorption can be excluded from consideration. 

Regardless of method, desorption is never complete. Adsorbent capacity is always less following regeneration than it is on initial loading of adsorbent. Some adsorbable materials undergo chemisorption they chemically combine with the adsorbent. An example is the Reinluft process (52) for removing SO2 from flue gas on activated carbon. The SO2 is attached to the carbon as sulfuric acid. Desorption occurs only upon heating to 370°C a mixture of CO2, evolved from the chemically bound carbon, and SO2 are driven off. 

Collectors ndFrothers. Collectors play a critical role ia flotation (41). These are heteropolar organic molecules characterized by a polar functional group that has a high affinity for the desired mineral, and a hydrocarbon group, usually a simple 2—18 carbon atom hydrocarbon chain, that imparts hydrophobicity to the minerals surface after the molecule has adsorbed. Most collectors are weak acids or bases or their salts, and are either ionic or neutral. The mode of iateraction between the functional group and the mineral surface may iavolve a chemical reaction, for example, chemisorption, or a physical iateraction such as electrostatic attraction. 

CO oxidation catalysis is understood in depth because potential surface contaminants such as carbon or sulfur are burned off under reaction conditions and because the rate of CO oxidation is almost independent of pressure over a wide range. Thus ultrahigh vacuum surface science experiments could be done in conjunction with measurements of reaction kinetics (71). The results show that at very low surface coverages, both reactants are adsorbed randomly on the surface CO is adsorbed intact and O2 is dissociated and adsorbed atomically. When the coverage by CO is more than 1/3 of a monolayer, chemisorption of oxygen is blocked. When CO is adsorbed at somewhat less than a monolayer, oxygen is adsorbed, and the two are present in separate domains. The reaction that forms CO2 on the surface then takes place at the domain boundaries. 

Adsorption and Desorption Adsorbents may be used to recover solutes from supercritical fluid extracts for example, activated carbon and polymeric sorbents may be used to recover caffeine from CO9. This approach may be used to improve the selectivity of a supercritical fluid extraction process. SCF extraction may be used to regenerate adsorbents such as activated carbon and to remove contaminants from soil. In many cases the chemisorption is sufficiently strong that regeneration with CO9 is limited, even if the pure solute is quite soluble in CO9. In some cases a cosolvent can be added to the SCF to displace the sorbate from the sorbent. Another approach is to use water at elevated or even supercritical temperatures to facilitate desorption. Many of the principles for desorption are also relevant to extraction of substances from other substrates such as natural products and polymers. 

Physical adsorption—surface areas of any stable solids, e.g., oxides used as catalyst supports and carbon black Chemisorption—measurements of particle sizes of metal powders, and of supported metals in catalysts... 

In this article, we will discuss the use of physical adsorption to determine the total surface areas of finely divided powders or solids, e.g., clay, carbon black, silica, inorganic pigments, polymers, alumina, and so forth. The use of chemisorption is confined to the measurements of metal surface areas of finely divided metals, such as powders, evaporated metal films, and those found in supported metal catalysts. 

Previously studied possibilities for bromine storage systems are listed in Table 1. The widely known reduction of the Br2 vapor pressure by formation of adducts with various carbon materials results from strong chemisorption interactions and has Table 1. General possibilities for bromine storage... 

Activated carbon filters are employed primarily as RW contaminant removal systems for chlorine (by chemisorption) and various organics such as trihalomethanes (THMs), petroleum products, and pesticides (by adsorption). In addition, they act as physical filters and therefore incorporate sufficient freeboard in their designs to permit periodic backwashing. 

In the presence of mineral phases containing anions that would form sparingly soluble compounds (e.g. POt - and F for the lower oxidation states) an enhanced plutonium uptake due to chemisorption can be expected (57). For plutonium in the higher oxidation states the formation of anionic carbonate complexes would drastically reduce the sorption on e.g oxide and silicate surfaces. 

Figure 2.14. The molecular orbitals of gas phase carbon monoxide, (a) Energy diagram indicating how the molecular orbitals arise from the combination of atomic orbitals of carbon (C) and oxygen (O). Conventional arrows are used to indicate the spin orientations of electrons in the occupied orbitals. Asterisks denote antibonding molecular orbitals, (b) Spatial distributions of key orbitals involved in the chemisorption of carbon monoxide. Barring indicates empty orbitals.5 (c) Electronic configurations of CO and NO in vacuum as compared to the density of states of a Pt(lll) cluster.11 Reprinted from ref. 11 with permission from Elsevier Science.

The temperature regimes for the stability of intermediates is different for various transition metals. For example on Fe(lll) the adsorbed ethylene decomposes partially at 200 K, while the conversion to surface carbon is complete at 370 K. Similarly, on nickel faces molecular chemisorption of ethylene is restricted to temperatures below ambient. At temperatures between approximately 290 K and 450 K ethylene chemisorption on nickel... 

The mode of chemisorption of CO is a key-factor concerning selectivity to various products. Hydrocarbons can only be produced if the carbon-oxygen bond is broken, whereas this bond must stay intact for the formation of oxygenates. It is obvious that catalysts favoring the production of hydrocarbons must chemisorb carbon monoxide dissociatively (e.g. Fe) while those favoring the formation of oxygenates must be able to chemisorb carbon monoxide molecularly (e.g. Rh). 

Many molecules undergo partial oxidation on adsorption and many alkanes and alkenes are believed to yield an adsorbed CHO group on adsorption (Petrii, 1968). These processes usually lead to the complete oxidation of the organic molecule to carbon dioxide and few workers have attempted to halt the reaction at an intermediate stage. Hence, although there are undoubtedly possibilities for using dissociative chemisorption for synthetic reactions, this chapter will not consider these processes further. 

It is well established that sulfur compounds even in low parts per million concentrations in fuel gas are detrimental to MCFCs. The principal sulfur compound that has an adverse effect on cell performance is H2S. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Chemisorption on Ni surfaces occurs, which can block active electrochemical sites. The tolerance of MCFCs to sulfur compounds is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, and gas cleanup). Nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Moreover, oxidation of H2S in a combustion reaction, when recycling system is used, causes subsequent reaction with carbonate ions in the electrolyte [1]. Some researchers have tried to overcome this problem with additional device such as sulfur removal reactor. If the anode itself has a high tolerance to sulfur, the additional device is not required, hence, cutting the capital cost for MCFC plant. To enhance the anode performance on sulfur tolerance, ceria coating on anode is proposed. The main reason is that ceria can react with H2S [2,3] to protect Ni anode. 

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