Stable ordered structures of Sn on Pt(111) (maximum coverage
of 0.33 monolayers) were used as model catalysts to test the effect of
tin and potassium on the hydrogenation of ethylene at 300 K and dehydrogenation
of cyclohexane at 573 K and at pressures of 15 Torr
of hydrocarbon and 100 Torr of H2. Co-adsorption of tin and potassium
on Pt(111) resulted in the direct interaction between potassium,
tin, and platinum as verified by temperature-programmed
desorption of CO. Tin deposition yielded a maximum in the turnover
rate as a function of Sn coverage for ethylene hydrogenation and
cyclohexane dehydrogenation with maxima at about 0.2 monolayers
of tin and a turnover rate 75% higher than on clean Pt(111). This
enhancement was explained by a lower rate of deactivation as tin
was added. In contrast, the addition of potassium to Pt and Pt/Sn
produced only a monotonic decrease in cyclohexane dehydrogenation.
In an industrial system, where a higher tin coverage is used,
interaction of tin with potassium may form an effective site blocker
which could lower deactivation rates