Activation energy ethylene hydrogenation

Both PtRu/MgO catalysts prepared from cluster precursor and organometallic mixture were active for ethylene hydrogenation. The apparent activation energy of the former catalyst obtained from the Arrhenius plot during -40 to -25°C was 5.2 kcal/mol and that of the latter catalyst obtained during -50 to -30°C was 6.0 kcal/mol. The catalytic activity in terms of turn over frequency (TOP) was calculated on the assumption that all metal particles were accessible for reactant gas. Lower TOP of catalyst prepared from cluster A at -40°C, 57.3 x lO" s" was observed probably due to Pt-Ru contribution compared to that prepared from acac precursors. 

In a number of experiments by Farnsworth and his colleagues, bombardment with low-energy argon ions has been used as part of a surface cleaning technique. The catalytic activity for ethylene hydrogenation on freshly bombarded samples of nickel was found to be about 100 times that of the same sample after annealing with platinum the ratio of bombarded to annealed was about 10 44). Pure copper had only a small catalytic activity before and after bombardment (45), and no activity could be detected in germanium (46). 

There is a similar opportunity for a threshold experiment with zinc oxide. There is evidence that the destruction of activity for ethylene hydrogenation is an effect of atom displacement, and this could be established by bombardment at a series of controlled energies. 

Table 4. Ag/Pt molar ratio and its influence on ethylene hydrogenation rates and apparent activation energy for nanoparticle encapsulated shape-controlled Pt nanoparticles [17].

However, due to the difficulties in calculating ion yields in SIMS, quantitation of the data is not very reliable, and their work was not conclusive. We have determined here that the reaction of chemisorbed ethylene to form ethylidyne is first order in ethylene coverage. A noticeable isotope effect was observed, with activation energies of 15.0 and 16.7 Kcal/mole for C H and 02 respectively. These values are smaller than those calculated from TDS, but the differences can be reconciled by including the recombination of hydrogen atoms on the surface in the interpretation of the thermal desorption experiments. 

Fig. 6. Computer simulation for hydrogen TDS from chemisorbed ethylene on Pt(lll) (a) First order process only, with activation energy - 15.0 Kcal/mole (dashed line) (b) same as (a) but including a hydrogen recombination step (solid line) (c) experimental data (crosses). See details in text.

We have measured the kinetics of ethylidyne formation from chemisorbed ethylene over Pt(lll) surfaces. The rates of reaction display a first order dependence on the ethylene coverage. There is an isotope effect, since the reaction for CjH is about twice as fast as for CjD. We obtain values for the activation energy of 15.0 and 16.7 Kcal/mole for the normal and deuterated ethylene, respectively. These values are lower than those obtained from TDS experiments, but the differences can be reconciled by taking into account the hydrogen recombination when analyzing the thermal desorption data. 

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. 

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