Temperature-programmed surface ethylene

Temperature-Programmed Surface Reaction (TPSR) Experiments at 800 Torr. Pretreated and preoxidized silver exhibited no reactivity toward an ethylene/argon mixture at reaction temperatures (443 - 543 K) and atmospheric pressures (750-800 torr). The desorption spectrum of a pretreated sample showed no evidence of oxygen desorption when the sample was heated in vacuo to 673 K. These... 

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

P. Berlowitz, C. Megiris, J.B. Butt, and H.H. Kung, Temperature-Programmed Desorption Study of Ethylene on a Clean, a H-Covered, and an O-Covered Pt( 111) Surface, Langmuir 1, 206-212 (1985). 

Figure 2.16 Temperature programmed reaction between O atoms and ethylene adsorbed on Rh(l 11). The majority of the adsorbed ethylene decomposes in several steps to H and C atoms, which react with the adsorbed O atoms to form H2, H20, CO and C02. Because there is insufficient oxygen, the surface still contains carbon at the end of the experiment (adapted from [36],...

A kinetic description of these reactions is difficult to give, due to the complicated decomposition pathways of the hydrocarbons on noble metal surfaces. The temperature programmed reaction between adsorbed ethylene and NO on rhodium in Fig. 5.16 illustrates some of the many reactions that may occur [58]. As seen before, the NO molecule starts to dissociate aroimd room temperature. Ethylene decomposes in several steps at different temperatures as evidenced by the release of H2 and H2O. The formation of CO and some CO2 between 500 and 600 K is well above the respective desorption temperatures of these gases, and suggests that the C-C bond of the hydrocarbon breaks in this temperature range and limits the rate of the oxidation on rhodium surfaces. Formation of HCN is observed as well. Note that a large reservoir of surface CN species forms at temperatures of 500 K and remains on the surface until 700-800 K, where it decomposes and is followed by the instantaneous desorption of N2. 

Fig. 5.16. Temperature programmed reaction spectroscopy of ethylene and NO coadsorbed on a rhodium surface reveals the many reactions that are possible. Note the formation of CO and CN species on the surface as visible in SIMS, and the formation of HCN as a gas phase product. The stability of the CN species is the reason that desorption of N2 occurs at very high temperatures only...

The ethylene temperature-programmed reaction spectrum of an oxygen-covered Cs/Ag/a-Al203 catalyst produced a peak for the co-evolution of EO and CO2 at 373 K (100 °C) with a selectivity to EO of 44%. The Cs had had no effect on the kinetics of desorption of oxygen from Ag(lll) nor on the amount of oxygen desorbing from that surface, nor on its selectivity in its oxidising ethylene to EO (Eig. 7.10). 

Thermal desorption of CO, Auger electron spectroscopy, and temperature programmed oxidation all show that the carbon layer 1) is Immobile below 550 K 2) forms a more densely packed surface phase at temperatures of 550-1150 K and 3) dissolves into the bulk at 1350 K. SIMS measurements of isotope mixing in the ions confirm formation of dense-phase (graphitic) islands after heating the carbon layer to 923 K. SIMS spectra also demonstrate that at 520 K, CO dissociates on Ru(OOl). The oxygen-free carbon layer that forms behaves similarly to the carbon from ethylene. Both SIMS and thermal desorption results show no positive interaction between adsorbed CO and D but significant attraction between and C formed by CO dissociation. 

Temperature Programmed Oxidation. These measurements characterize both the amount and chemical nature of the carbon on the surface. After a surface is exposed to ethylene and pretreated as desired, it receives a 6 L dose of O2 at 323 K. The TPO spectrum is the CO desorption signal at a 6 K/sec programming rate. CO2 accounts for less than 1% of the oxidation, so the CO signal accounts for essentially all of the carbon removed. O2 dosing is repeated until no further CO is evolved during heating. SIMS results show that all carbon has been removed from the surface at the TPO end point. 

The reactions of acetaldehyde and ethylene have been investigated on the surfaces of UO2 and UO3 by temperature programmed desorption (TPD). On UO2 two molecules of acetaldehyde undergo reductive coupling to C4 olefins. This is due to the fluorite structure of UO2, which can accommodate large numbers of excess oxygen, up to U02 25- The... 

Figure 6.14. (a) Temperature-programmed desorption of hydrogen from the thermal decomposition of chemisorbed ethylene on Pt(l 11) [31]. (b) Proposed surface reaction mechanisms to account for the sequential decomposition [32, 33]. Asterisk denote data taken from reference [331 daggers denote data taken from reference [32]. 

The following procedure was used (10) ethylene was introduced into the reactor at various temperatures and pressures and allowed to remain in contact with the catalyst for various periods of time. To remove the gas phase and physically adsorbed ethylene, the catalyst was evacuated for 10 minutes at the reaction temperature and was then cooled to room temperature. (A constant amount of ethylene remains on the surface after evacuation for 10 to 30 minutes at room temperature and butene, adsorbed on the same sites, is not removed by evacuating at 80° in excess of 120 minutes (4). The highest reaction temperature used was 75°). The temperature-programmed desorption was carried out in the usual manner after the stream of helium had been diverted through the reactor. The desorbed gas was condensed in a... 

---