Hydrocarbon product selectivity coverage

Fig. 6 Hydrocarbon product selectivity as a function of TiOx coverage on a Rh foil (12). The indicated TiOx coverages should be multiplied by a factor of 3.3 to account for the corrections recently reported by Williams et al. (9).

During induction, catalyst activity and selectivities to aromatics and propene increase steadily. Improvement of catalyst performance is due to increase in Ga dispersion and formation of dispersed Ga species (Gao) which are efficient for the heterolytic recombinative release of hydrogen [18,191. The Ga/H-MFI catalyst then reaches its optimal aromatisation performance (stabilisation). Ci to C3 hydrocarbons productions are at their lowest. The gallium dispersion and the chemical distribution of Ga are optimum and balance the acid function of the zeolite. Reversible deactivation during induction and stabilisation of the catalyst is due to site coverage and limited pore blockage by coke deposition. 

Titania promotion also alters the selctivity of Rh, as may be seen in Fig. 6. The methane content of the hydrocarbon product falls from a value of 94% when no titiania is present to nearly 60% for a titania coverage of 0.20 ML [0.66 ML on the corrected scale (9) ]. Ethylene and propylene are the predominant higher hydrocarbon species, comprising roughly 34 mol% of the total hydrocarbon product. At higher titania coverages, the selectivities return to values more charateristic of clean Rh. 

It has been reported that the concentration of proton and adsorbed hydrogen can be controlled by adjusting the anodic and cathodic bias in the pulsed method [7]. The hydrogen adsorbed on the electrode surface seems to interrupt the reaction for the electrochemical reduaion of COj. The CO2 coverage on the electrode surface may be increased by the elimination of adsorbed hydrogen during anodic period. In the subsequent cathodic period, the electron transfer to CO2 was promoted, yielding CO2 radical anions. The selectivity of products for the electrochemical reduction of CO2 was determined in association with electrode material and CO2 radical anion [10,11]. CO is intermediate species in the reaction process of hydrocarbonization [8]. 

When the temperature increases, the equilibrium shifts in the direction of higher dissociation pressure of the oxide and the surface becomes more and more populated with electrophilic oxygen species. When used as catalysts in oxidation of hydrocarbons, such oxides may show high selectivity to partial oxidation products at low temperatures, wherein the surface coverage with transient electrophilic oxygen forms is low. Under such conditions the conversion is very low. On raising the temperature the selectivity to partial oxidation products rapidly decreases, whereas the conversion in total oxidation increases, becoming the predominant reaction... 

Supported metal oxides are currently being used in a large number of industrial applications. The oxidation of alkanes is a very interesting field, however, only until recently very little attention has been paid to the oxidation of ethane, the second most abundant paraffin (1). The production of ethylene or acetaldehyde from this feed stock is a challenging option. Vanadium oxide is an important element in the formulation of catalysts for selective cataljdic reactions (e. g. oxidation of o-xylene, 1-3, butadiene, methanol, CO, ammoxidation of hydrocarbons, selective catalytic reduction of NO and the partial oxidation of methane) (2-4). Many of the reactions involving vanadium oxide focus on the selective oxidation of hydrocarbons, and some studies have also examined the oxidation of ethane over vanadium oxide based catalysts (5-7) or reviewed the activity of vanadium oxide for the oxidation of lower alkanes (1). Our work focuses on determining the relevance of the specific oxide support and of the surface vanadia coverage on the nature and activity of the supported vanadia species for the oxidation of ethane. 

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