Catalysts hydrocarbon adsorption

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... 

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. 

Hydrocarbon Trap System. The concept of a hydrocarbon trap or adsorber system is based on molecular sieve hydrocarbon adsorber systems. The temperatures at which hydrocarbon adsorption takes place exist in the auto engine exhaust system during the period of cold start of an automobile when the catalytic control system has not yet reached functional temperature. Zeolites have been reportedly useful for hydrocarbon adsorption (53,169). Zeolites desorb hydrocarbons at temperatures of 400°C, ie, once the catalytic control system is functional. Therefore, hydrocarbons adsorbed by the zeolite can also be desorbed then oxidized by a catalyst. Methods to accomplish cold start hydrocarbon adsorption, heatup of the main catalyst, and desorption have been identified. Some of these systems use exhaust pipe valves to divert the exhaust gases to the hydrocarbon trap for the low temperature portion, and by-pass the gases around the trap after the main catalyst has heated up. One device that uses a heat exchanger is shown in Figure 15 (44). The Si—Al ratio in the zeolite is important, and by lowering the alumina content, the zeolite is rendered more hydrophobic and more able to adsorb... 

Experimental Activation Energies 1 e Bond Energies of Atoms in the Reacting Molecules with the Catalyst Qak Adsorption Potentials q and the Heights of the Potential Barriers E kg. cat./mole on Chromias of Different Methods of Preparation-, the Subscripts Designate 1—Dehydrogenation of Hydrocarbons, II—Dehydrogenation of Alcohols and Acids, III—Dehydration of Alcohols ... 

Calorimetric Measurements of Hydrocarbon Adsorption at 300 K on Different Metal Catalysts... 

One important difference in activation energies, however, is apparent for Pi, which has been associated with "soft" coke probably adjacent to metal sites, where the activation energy of the cyclohexene-coked catalyst is ca. 15 kJ mok higher than for spent and 1-hexene coked catalysts. This difference may be related to the dynamics of hydrocarbon adsorption. For linear alkenes only one end of the chain may be attached to the catalyst surface, while for small cyclic alkenes, which have a more compact structure, the entire molecule may be adsorbed onto or electronically affected by the catalyst surface. That is, adsorbed cyclohexene which may lie... 

In the case of complex hydrocarbon molecules, the nucleophilic addition of oxygen may take place at different sites of the molecule. It will take place at a site which is made most electropositive by appropriate bonding of the molecule at the active center of the catalyst. When adsorption of the hydrocarbon molecule results in the formation of a radical, interaction between adsorbed molecules is favoured and dimerization or polymerization occurs. When the adsorbed species are negatively charged. 

From the partial reaction orders in the CjH -Oj reaction system and characterization by XPS and TPR on the catalysts, it was concluded that the alkaline addition to the Pd three-way catalyst weakened the adsorption strength of hydrocarbons on Pd. The addition of alkaline earth metal suppressed the hydrocarbon chemisorption on the Pd catalyst and therefore allowed the catalytic reaction to proceed smoothly. On the other hand, the addition of alkali metals, in particular K or Cs, caused such a strong oxygen adsorption on Pd that rejected the hydrocarbon adsorption and therefore suppressed the reaction. It was considered that the effect of the alkaline addition to the strength of adsorbed hydrocarbons on Pd was caused by the increase of electron density of Pd. 

On platinum catalysts, the kinetics of HC oxidation, NO reduction and NO oxidation are strongly dependant on the hydrocarbon adsorption strength. These mechanisms occur at higher temperature with olefins than with long chain alkanes. 

Differences in adsorption behaviour are observed for the investigated systems. No NO or NO2 desorption peaks are observed for Pt/SiC or Pt/ZSM-5, while a clear desorption of NO, with a maximum at 158°C, can be observed for Pt/Al203. The Pt/SiC system is also inert towards hydrocarbon adsorption, while Pt/ZSM-5 adsorbs a substantial amount of hydrocarbons. It can be observed that for all tested catalysts, the CO2 formation and the NOx reduction are closely correlated the maximum in NOx reduction is observed at almost complete hydrocarbon oxidation. 

There are two problems here, namely, NO inhibits hydrocarbon adsorption, thus the amount of N2O is large and second, surface can be poisoned by carbonaceous species. These two factors can be controlled by the proper selection or modification of the catalyst system. 

The rates of hydrocarbon adsorption on all oxidation catalysts (VA, Cu20, etc.) are so high as to make kinetic studies impossible. Characteristic equilibrium isotherms for ethylene sorption on magnesium chromite are shown in Fig. 1. This is a spurious equilibrium,... 

The catalyst surface is markedly heterogeneous both with respect to oxygen and to hydrocarbon adsorption. 

Some specific factors should be considered when using zeolites as catalysts in isobutane/olefin alkylation. The first is the strong hydrocarbon adsorption of zeolites, especially at low temperatures, which makes the actual concentration of the reactants in the zeolite quite high. In addition, and due to the high concentration of acid sites in the zeolite cavities, zeolite would behave as a liquid-acid catalyst but with a high solubility of reactants. Taking this into account it can be easily assumed that the Si/Al ratio can determine the nature of reaction products. In this way, it has been proposed that zeolites... 

COLD START HYDROCARBON EMISSIONS CONTROL VIA ADMIXING THREE WAY CONVERSION CATALYSTS WITH HEAT EXCHANGE AND HYDROCARBON ADSORPTION... 

Carbon nanofibers with controllable nanoporous structures can be prepared via different ways. An oldest method is the catalytic decomposition of certain hydrocarbons on small metal particles such as iron, cobalt, nickel, and some of their alloys [80]. The mechanism includes hydrocarbon adsorption on a metal surface, conversion of the adsorbed hydrocarbon to adsorbed surface carbon via surface reactions, subsequent segregation of surface carbon into the layers near the surface, diffusion of carbon through metal particles, and then precipitation on the rare side of the particle [81]. The size of the catalyst nanoparticles seems to be the determining factor for the diameter of the carbon nanostructures grown on it. Small nanoparticles catalyze this grown better than the big ones due to that exhibit pecu-... 

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