Adsorption and reaction of bicyclic hydrocarbons at Pt(1 1 1) and Sn/Pt(1 1 1) surface alloys: trans-decahydronaphthalene (C 10H 18) and bicyclohexane (C 12H 22)

Abstract

Adsorption and desorption of trans-decahydronaphthalene (C 10H 18) and bicyclohexane (C 12H 22) can be used to probe important aspects of non-specific dehydrogenation leading to surface carbon accumulation and establish better estimates of activation energies for C–H bond cleavage at Pt–Sn alloys. This chemistry was studied on Pt(1 1 1) and the (2 × 2)-Sn/Pt(1 1 1) and (√3 × √3) R30°-Sn/Pt(1 1 1) surface alloys by using temperature programmed desorption (TPD) mass spectroscopy and Auger electron spectroscopy (AES). These hydrocarbons are reactive on Pt(1 1 1) surfaces and fully dehydrogenate at low coverages to produce H 2 and surface carbon during TPD. At monolayer coverage, 87% of adsorbed C 10H 18 and 75% C 12H 22 on Pt(1 1 1) desorb with activation energies of 70 and 75 kJ/mol, respectively. Decomposition of C 10H 18 is totally inhibited during TPD on these Sn/Pt(1 1 1) alloys and decomposition of C 12H 22 is reduced to 10% of the monolayer coverage on the (2 × 2)-Sn/Pt(1 1 1) alloy and totally inhibited on the (√3 × √3) R30°-Sn/Pt(1 1 1) alloy. C 10H 18 and C 12H 22 are more weakly chemsorbed on these two alloys compared to Pt(1 1 1) and these molecules desorb in narrow peaks characteristic of each surface with activation energies of 65 and 73 kJ/mol on the (2 × 2) alloy and 60 and 70 kJ/mol on the (√3 × √3) R30°-Sn/Pt(1 1 1) alloy, respectively. Alloyed Sn has little influence on the monolayer saturation coverage of these two molecules, and this is decreased only slightly on these two Sn/Pt(1 1 1) alloys. The use of these two probe molecules enables an improved estimate of the activation energy barriers E* to break aliphatic C–H bonds in alkanes on Sn/Pt alloys; E* = 65–73 kJ/mol on the (2 × 2)-Sn/Pt(1 1 1) alloy and E* ⩾ 70 kJ/mol on the (√3 × √3) R30°-Sn/Pt(1 1 1) alloy.