Ethylene surface concentration

Figure 3.55a. Reaction energy diagram of ethylene hydrogenation on a Pd(lll) surface at low ethylene surface concentration .

The addition of ethylene to a CO-H flow on a Rh-CeO catalyst (fig. 4), which should enhance the surface concentration of C H groups increased the formation of propanol and propionaldehyde and decreased the ethanol and acetaldehyde production. 

The optimal distribution of silver catalyst in a-Al203 pellets is investigated experimentally for the ethylene epoxidation reaction network, using a novel single-pellet reactor. Previous theoretical work suggests that a Dirac-delta type distribution of the catalyst is optimal. This distribution is approximated in practice by a step-distribution of narrow width. The effect of the location and width of the active layer on the conversion of ethylene and the selectivity to ethylene oxide, for various ethylene feed concentrations and reaction temperatures, is discussed. The results clearly demonstrate that for optimum selectivity, the silver catalyst should be placed in a thin layer at the external surface of the pellet. 

A net adsorption value (corrected for any IRS prism contamination) which could be related to calibration curves for surface concentration was obtained from the ratio of the amide I (C=0 stretching) band at 1640 cm to a standard band for each substrate polymer. The standard bands used were the CH bending vibration at 1400 cm for poly (dimethyl siloxane) and the CF2 scissoring deformation at 1450 cm for fluorinated ethylene/propylene copolymer. Because of the amide I band in polyurethanes, a 4X expanded abscissa was used for the other two polymers, the IX scale was sufflcient. 

Figiure 2 shows the ethylene (C2H4) concentrations in the surface waters between Hawaii and Tahiti (Table II). Excluding the values obtained just outside Hawaii (3/21/74) and Tahiti (3/29/74), a broad general increase is apparent with a maximum average value of 4.0 X 10 ml/1 and a low average value of 2.4 X 10 ml/1. Plotted over... 

Cremer P, Su X, Shen Y, Somorjai GA (1996) The first measurement of an absolute surface concentration of reaction intermediates in ethylene hydrogenation. Catal Lett 40 143... 

Pre-adsorbed chloride reduced the amount of adsorbed ethylene and enhanced the adsorption of products.Assuming that chloride also reduces the surface concentration of atomic oxygen, then catalyst selectivity will rise since the partial oxidation reaction is first order on surface species while the complete oxidation is second order. 

The three steps postulated for the catalytic hydrogenation of ethylene indicate that the rate may be influenced by both adsorption and desorption (steps 1 and 3) and the surface reaction (step 2). Two extreme cases can be imagined that step 1 or step 3 is slow with respect to step 2 or that step 2 is relatively slow. In the first situation rates of adsorption or desorption are of interest, while in the second the surface concentration of the adsorbed species corresponding to equilibrium with respect to steps 1 and 3 is needed. In any case we should lilce to Tnow the n of sites on the catalyst surface, or at least the surface area of the catalyst. These questions require a study of adsorption. More is known about adsorption of gases, and this will be emphasized in the sections that follow. 

A model Phillips catalyst for ethylene polymerization has been prepared by spin coating of a Cr(III) precursor (Cr(acac)3) on a flat silicon wafer (100) covered by amorphous silica. The spin coating parameters were chosen in order to obtain a homogeneous film. The model catalyst was submitted to an activation process. The surface concentration of Cr decreased from about 0.8 to 0.4 Cr atom/nm as the temperature increased from 150 to 550°C. Direct information concerning the surface molecular species and the environment of Cr was provided by ToF-SIMS and XPS. At 350°C, the catalyst precursor was decomposed Cr species were in the form oxide and surface-anchored chromates. Upon final activation at 650°C for 6 h, Cr species were below the XPS detection limit however the model catalyst was active for ethylene polymerization at 160°C and 2 bar pressure. 

The dependence of partial surface concentrations of 02 and C2H4 on variations in (j> of the catalyst seems to explain the unsteady nature of reaction kinetics. It will be expected, moreover, that addition of acceptor impurities will raise the reaction order for oxygen and decrease it for ethylene. The zero reaction order with respect to oxygen in the formation of C2H40 and C02, as found by Temkin et al., may apparently be explained as follows. 

The presence of sulfur impurities in silver results in an increased ethylene oxide yield further increase in sulfur concentration would lead to poisoning of the catalyst. Roginskii et al. (167) found that the same was observed for silver samples modified with chlorine. The work function of sulfur-containing samples, as well as of those containing chlorine, will increase. The activity maximum is controlled by the ratio of ethylene to oxygen surface concentrations. The reaction order with respect to ethylene will decrease with increasing sulfur concentration, i.e., with tj>, while that for oxygen will build up. At a certain value the rate w1 = K01 Cq, C iHj will be maximum, when n = m. This is apparently the reason why silver is modified with sulfur. When chlorine is added to silver, no increase in the ethylene oxide yield is observed, as pure silver involves ion-chlorines in excess over the optimum amount. 

During ethylene hydrogenation over Pt(lll) the reaction intermediate appears to be weakly bound 7t-bonded ethylene which produces most of the ethane, while ethylidyne and di-o bonded ethylene are spectators during the catalytic process. The surface concentration of k-bonded ethylene is 4% of a monolayer during the turnover, which yields an absolute turnover rate 25 times higher than the turnover rate per platinum atom. 

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