Hydrocarbon adsorption

Fig. 25. Hydrocarbon adsorption and release as a function of ORVR fill rate...

A vehicle fuel vapor control system must be designed to meet both driving and refueling emission level requirements. Due to the nature of hydrocarbon adsorption, this emission control is a continuous operation. 

The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. 

On some important side effects. When an alloy is brought into contact with a gas which shows a higher affinity for one of the alloy components, a gas induced segregation takes place. Such a process has been also found for the Pt/Cu alloys. The authors (34) established, that by hydrocarbon adsorption, Pt is preferentially covered by a carbon(aceous) layer, but under this layer the Pt concentration is higher than under vacuum (figure 5). However, this finding does not change principally the picture of the surface one had earlier. 

Most of the common 15 hazardous pesticides are chlorinated hydrocarbons. Adsorption can be an important process for most. All except DDT, endosulfan, and heptachlor resist hydrolysis, and most are also resistant to biodegradation. Kearney and Kaufman118 review conditions under which chlorinated pesticides are biodegraded. 

Jaynes WF, Boyd SA (1991b). Hydrophobicity of siloxane surfaces in smectites as revealed by aromatic hydrocarbon adsorption from water. Clays Clay Miner 39 428-436... 

Galvanostatic current-pulse method, 38 29, 33 Galvanostatic technique hydrocarbon adsorption, 30 256 surface coverage, 30 299-300 Gamma emission, 26 124, 125, 127 Garin-Gault mechanism, 30 20, 23 Gas chromatography -atomic emission detection... 

Hj Dj exchange on, 26 39-43 heteropolyanion-supported, 41 230-231 high MiUer index, 26 12-15,35,36 -H-USY zeoUte, 39 186-187 hydrocarbons adsorption, 38 229-230 reactions of cyclopropane, cyclohexane, and n-heptane, 26 51-53 structural effects, 30 25-26 hydrogen adsorption on, 23 15 hydrogenation, 30 281-282 olefins, in ethanol, 30 352-353 in hydrogenation reaction, 33 101 -iron alloys, 26 75 isomerization, 30 2-3 isotope, NMR properties, 33 213,274 kinetic oscillations, 37 220-228 ball models of densely packed surfaces, 37 221-222... 

Kulprathipanja, S. (2001) Process for monomethyl acyclic hydrocarbon adsorptive separation. U.S. Patent 5,252,127. 

Hydrocarbon Adsorption. Hydrocarbon adsorption experiments were carried out to determine the effect of the zeolite s acid sites on... 

Hydrocarbon adsorption experiments show significant differences between the nickel contaminated zeolitic and non-zeolitic particles at metals levels comparable to those of the catalytic experiments. Neither hexane nor 1-hexene showed any interaction with nickel on the low surface area, non-zeolitic particles (the unpromoted material of Table I) at temperatures up to 425 C. Additionally, no interaction between hexene and the nickel on the zeolitic particles was observed over the temperature range studied. However, the nickel on the zeolitic component did cause significant retention of hexane at temperatures as low as 200 C with generation of what appeared to be higher molecular weight products. No cracking products were observed. With the uncontaminated zeolitic particles, hexane retention only occurred at temperatures above 300°C. Thus, the lower temperature retention for the contaminated particles appears to be due to the presence of nickel. 

Our third hypothesis, i.e., that the activity enhancement involves the proximity of the zeolite s acid sites, appears to be consistent with the hydrocarbon adsorption experiments, but may also be due to differences in the nickel dispersion arising from surface area differences between the two types of particles. Clearly, the adsorption of hexane at lower temperature on the nickel contaminated zeolitic particles suggests a significantly altered environment from both the uncontaminated and the non-zeolitic materials. 

The authors wish to acknowledge the invaluable assistance of S. M. Kuznicki who conducted the hydrocarbon adsorption experiments, D. R. Anderson who ran and assisted in the interpretation of the TPR, and H. Furbeck who prepared the many samples tested. 

FIGURE 7.16 Effect of calcium exchange for sodium in zeolite A on hydrocarbon adsorption. Replacement of four sodium ions by two calcium ions permits easy diffusion of n-alkanes into the zeolite channels. 

Considerable progress has been made in accumulating information on the electronic structure of metals and alloys, on some aspects of the structure of hydrocarbon adsorption complexes, etc. Also, information on the relative importance of the electronic structure effects of alloying—as contrasted to the geometric, ensemble size effects—has grown appreciably. 

It is the di-cr species on Pt(lll) which is converted at higher temperature into ethylidyne. This conversion had also been investigated by infrared-visible sum-frequency generation (SFG) by Cremer et al. (371), a welcome first application of this new spectroscopic technique to hydrocarbon adsorption chemistry. They observed an absorption characteristic of an intermediate with a nCH3 band at 2957 cm-1 and suggested that this arises from an ethylidene M2CHCH3 (or possibly ethyl) species with its C-CH3 axis at an angle to the surface. It is very clear experimentally, as discussed in Part I, that ethylidyne on Pt(lll) is formed from di-cr-ethene and not directly from its 77-bonded isomer. 

Regarding the kinetics, the oxidation of o-xylene and o-tolualdehyde were compared for catalysts with different V/Ti ratios (Table 36). The ratio between partial and complete oxidation (X for o-xylene and Y for o-tolualdehyde) are influenced similarly, indicating that a change in the catalyst structure influences all the reaction steps. The oxidation of o-tolualdehyde in mixtures with o-xylene revealed that o-tolualdehyde reduces the o-xylene oxidation rate by a factor of about 2. The authors conclude that a redox model is inadequate and that hydrocarbon adsorption cannot be rate-determining. Adsorption of various products should be included, and equations of the Langmuir—Hinshelwood type are proposed. It should be noted that the observed inhibition is not necessarily caused by adsorption competition, but may also stem from different... 

The present-day literature contains many more spectra obtained from singlecrystal metal surfaces by VEELS than by RAIRS. However, the much higher resolution available from the more recently developed RAIRS technique and its capability of operating in the presence of a gas phase suggest that it will contribute increasingly important information in the hydrocarbon adsorption field. The three spectroscopic techniques discussed above are much the most important ones in this area, with transmission infrared spectroscopy as the predominantly useful one for work with finely divided samples. A few other vibrational spectroscopic techniques (25) have provided information on adsorbed hydrocarbons, but are at present of more limited or specialized applications. Their principal characteristics are more briefly summarized below. 

Overall perspectives of the results from ethene and the higher alkenes have been attempted in Sections VI.B.6 and VI.G. What has become clear, particularly in the context of hydrocarbon adsorption, is that the study of spectra on single-crystal surfaces is of great assistance in finding the correct interpretation of the more complex multispecies spectra obtained from finely divided metal catalysts. This has only become possible by the development of VEELS and RAIRS, the latter allied with the Fourier-transform methods that have also transformed the quality of the spectra from metal-particle catalysts obtained by transmission infrared spectroscopy. The use of RAIRS in turn has emphasized the general significance of the MSSR. 

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