Kinetic studies as well as morphological and physico-chemical surface studies have been carried out on the deactivation of a palladium carrier catalyst for vinyl acetate synthesis in the ethylene gas phase process. The experiments cover concentration and temperature ranges of technical interest at a total pressure of 900 kPa. The deactivation rate depends on the temperature and the oxygen and acetic acid concentrations. The decline in activity is best described by a rate law of second order relative to the activity number. This behaviour is attributed to the observed aggregation of finely dispersed palladium. It can be concluded that palladium(I1) acetate acts as a transport species in a chemically assisted sintering mechanism. At temperatures Ts438 K the more advantageous final texture of the catalyst will only be formed on reaction.
Table I . Isotope separation effects in column chromatography with "[28.2.2] polymer" ds function of temperature. y (maximum separation efficiency of column) = (R,>/R,)-I : I (isotope separation effect) = ( 1 -1. Table 1 contains the E values determined by an approximation method commonly employed in isotope chemistry [x]. They are in good agreement with the values determined in a static experiment at 20°C [~(~~C a / " C a ) = 0 . 5 5x and E ( " C~/~( ' C~) = 1.21 x 10-7. The value of E determined at 20 "C is by far the greatest known calcium isotope separation effect in an equilibrium step on a n ion exchanger; the E values obtained on sulfonic acid resin exchangers are up to two orders of magnitude smallerl'l. Only the isotopic separation reported for "Ca/ 40Ca in an extraction system with a crown ether (i.e. F = 0.4 x 10 -') Js comparable[31. On average c (4XCa/40Ca) is greater than ~(~'ca/'"Ca) by a factor of 1.8 (Table 1). This corresponds to the ratio of the relative mass difference (8/ 48): (4/44) = 1.83. If the logarithm of the K , values [eq. (l)] is plotted against the reciprocal absolute temperature, then all the values lie on a van't Hoff reaction isobar whose slope gives AH ("Ca/'('Ca) =46 J/mol and AH(4XCa/4uCa) = 87 J/ mol. The few A H values known so far for isotope exchange reactions on cation or anion exchangers in aqueous systems are smaller: AH(7Li/6Li) = -9.6 J/m~l["'~; AH(37C1/35C1)The separation results demonstrate for the first time that the chemical pre-enrichment of heavy calcium isotopes can be realized in amounts of interest on a technical scale. For example, use of the above mentioned solvent mixture in the elution makes isolation of the enriched material a practicable proposition; the linearity between isotope separation and amount of substance eluted (Fig. 1) enables isolation of larger amounts of enriched isotope.In a preliminary experiment, we have been able to isolate 30 mg of calcium in which 4XCa was enriched by 3.3% at 0 "C from 210 mg of calcium having a natural isotope abundancel"']. = -5.4 ~/ m 0 1 [~~1 .
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