Ethylene desorption

The simultaneous appearance of ethane with the low temperature ethylene desorption peak suggests that the low temperature ethylene peak may correspond to the desorption of a molecularly adsorbed ethylene species that can also undergo hydrogenation and subsequently desorb as ethane. If this is the case, the effective activation energies for desorption and hydrogenation of the molecular ethylene species are approximately equal. 

We note finally that in view of the apparent fragmentation of parent species, it seems somewhat surprising that the low temperature ethylene desorption peak is not accompanied by a partial decline in the intensity of the C2H2 ion. In fact, the intensity of this ion appears to increase slightly in this region. We suggest that this is due to the formation of additional acetylenic complexes by decomposition of adsorbed ethylene upon heating. 

One of the earliest studies of the reaction of C2H4 with D2, in which a full mass spectrometric analysis of the products was performed, used a nickel wire as catalyst [115,116]. Some typical results are shown in Fig. 11. These results showed that ethylene exchange was rapid and the deutero-ethylenes are probably formed in a stepwise process in which only one deuterium atom is introduced during each residence of the ethylene molecule on the surface, that is there is a high probability of ethylene desorption from the surface. From Fig. 11(a) it can also be seen that the major initial products are ethane-d0 and ethane-d,. This is consistent with a mechanism in which hydrogen transfer occurs by the reaction... 

Fig. 12. Variation of the percentage chance of ethylene desorption (p ) with temperature for various alumina-supported metals [105].

Figure 7 shows the CTL response and partial pressure of the desorbed gases during heating (1 °C/s) from the y-AkOs catalyst which pre-adsorbed ethanol vapor at room temperature. In Fig. 7a, the mass spectrometer measurements show the desorptions of water (m/z 18), the physisorbed ethanol (m/z 46), diethylether (m/z 59), and ethylene (m/z 25) for the catalyst heated in Ar, but the CTL emission is not observed. In Fig. 7b, the desorptions of ethanol (80 °C), diethylether (230 °C) and ethylene (250 °C) are observed above the same temperature as in Ar, in the course of heating in a mixed gas of 21% O2 and 79% Ar. The desorption of ethylene in the atmosphere containing oxygen, however, begins to decrease at a lower temperature (320 °C) in Fig. 7b than the peak temperature (340 °C) of ethylene desorption in Fig. 7a. Desorption peaks of CO2 (m/z 44) and water appear at 340 °C, and the CTL peak is observed at the same temperature in Fig. 7c.

Figure 4. Typical results of ZLC method. Ethylene desorption from MHF5.9 sample.

Figure 4 shows the experimental results obtained with this technique for the MH5.9F sample at three different temperatures. C indieates the measured concentration during ethylene desorption at time t, and Co indicates the inlet ethylene concentration. 

FIGURE 7.15 The ethylene desorption from an unpromoted Ag/a-Al203 catalyst containing the rO state only m/z = 26, ethylene m/z = 32, O2. 

FIGURE 7.16 The ethylene desorption difference spectrum between an unoxidised and i-0 oxidised Ag/a-Al203 catalyst. 

ETHYLENE DESORPTION FROM AN UNOXIDISED AND OXIDISED Cs-PROMOTED Ag/ct-AljOj CATALYST... 

The desorption spectrum of ethylene from the imoxidised Cs-promoted Ag/a-Al203 catalyst is identical to that shovm in Fig. 7.13 for ethylene desorption from an unoxidised, unpromoted Ag/a-Al20s catalyst. The ethylene desorption spectrum from an oxidised Cs/Ag/a-Al203 catalyst is identical to that from an... 

FIGURE 7.17 The ethylene desorption spectrum from an oxidised Ag/a-A Oj catalyst containing the a2"0 state only m/z — 26, ethylene m/z — 32, O2. 

FIGURE 7.18 The difference spectrum between ethylene desorption profiles from an unoxidised and an oxidised Ag/a-Al203 catalyst containing the a2-0 state only. 

Figure 7.19 is the ethylene desorption spectrum from an oxidised Cl-promoted Cs/Ag/a-Al203 catalyst. It shows one peak at 193 K (—80 °C). No peak is seen at this temperature from an unoxidised Cl-promoted Cs/Ag/a-Al203 catalyst, indicating that the process of chlorinating that catalyst in the pilot plant has contaminated the a-AhOs-... 

The 193 K ethylene peak derives from ethylene desorption from the Cl-weakened Ag-O bond. Its desorption activation energy is 53 kj/mol its coverage is 3 X 10 molecule/cm and its activation energy to adsorption is 12 kj/mol. 

Table XIII contains two distributions calculated by the method described in Section II, C, and the parameters employed are given beneath the table. These show that the chance of ethyl reversal is high (95-99%) and that the failure of deuterated ethylenes to appear extensively in the gas phase is due to the very low chance of ethylene desorption (1-2%). The initial appearance of multiply exchanged ethylenes is thereby explained, and the increase in the relative rate of olefin exchange exchange with rising temperature is due to the increasing ease of olefin desorption. The values of q and s suggest that ethyl radicals are an important source of atoms for forming both ethyl radicals and ethane.

Table XIX presents a selection of the results obtained in a study of the reaction of ethylene with deuterium over rhodium-alumina (31), together with some calculated distributions obtained by the method previously employed. The proportion of deuterated ethylenes in the initial products rises from 30% at —18° to 75% at 110°. In contrast to the behavior of palladium, ethane-dj is the major ethane throughout and hydrogen exchange is significant at all but the lowest temperature studied. The parameters used in the calculations attribute the greatest effect of temperature to the variation of the chance of ethylene desorption, which rises from 25% at —18° to 62% at 110°. The effect of temperature on the chance of alkyl reversal is relatively small. Another resjject in which the reaction over rhodium differs from that over palladium is that the chance of acquisition of deuterium in the hydrogenation steps is higher, and indeed it appears that, as with iridium, molecular deuterium may be substantially responsible for the conversion of ethyl radicals to ethane. E — E, is 3 kcal mole and E, — E, is 4.5 kcal mole. The reaction is first-order in hydrogen and zero in ethylene.

In addition to ethylene desorption in a large temperature domain, acetaldehyde was clearly observed evidenced by its m/e 15, 29 and 44 (Table 3). The formation of acetaldehyde from... 

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