Hydrocarbon Adsorption on High-index Faces

Hydrocarbon Adsorption on High Index Faces. Pt(S)-[9(l 11) x (100)] or Pt(S)-[4(111) X (100)] show carbonaceous deposits thus, in contrast to low-index faces, the stepped surfaces break C—H and C—C bonds even at 300 K and 10 —10 torr. Ordered carbonaceous residues appear to form on [9(100) x (100)] disordered on [7(111) x (310)], which has a high concentration of kinks in the steps. 

Pt displays a unique surface chemistry in that low co-ordination number sites on steps and kinks are responsible for bond-breaking processes. 

H2ID2 Exchange. Wachs and Madix have taken another look at Bernasek and Somorjai s data on H2/D2 exchange on stepped (111) Pt surfaces, where the latter authors claimed an enhancement in rate of 10 —10 times that on a smooth (111)surface. Their re-investigation was prompted by Lu and Rye s observationthat adsorption was only twenty times faster on stepped Pt. Wachs and Madix suggest a different mechanism which gives only a factor of twenty in the rates. 

The role of steps on Pt surfaces for adsorption, desorption, and reaction of hydrogen has also been investigated by Christmann and Ertl. Whereas before it had been thoughtthat at low temperatures H2 did not dissociate on the low-index (111) plane, this turns out to be untrue. Adsorption does occur with a low energy of 10 kcal mol The aim of this AES, LEED, ELS, and TPD paper d was to examine the influence of steps by using Pt(S)-[9(lll) x (111)] stepped surfaces. 

In the same context Cavalier and Chornet note that H2/D2 equilibration rates show a five-fold difference in reactivity for Fe in the forms polycrystalline, (110), (100), and (111). An exponential correlation between reactivity and atomic surface density was obtained. 

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