Thermal Chemistry of N$_{2}$, CO$_{2}$, and CH$_{4}$ on Cesiated Pt(111)

Promoting transition metal surfaces with alkali metals is known to drastically reduce the surface work function. We have investigated the effect of Cs promotion on the thermal chemistry of CH$_{4}$, N$_{2}$ and CO$_{2}$ using temperature programmed desorption (TPD). At low Cs coverage the CH$_{4 }$TPD peak broadens and shifts to higher temperature. As the Cs coverage increases further three CH$_{4}$ peaks develop: one at higher temperature than on a bare Pt surface, and two at lower temperatures. With Cs coverage of one saturation monolayer (\textit{$\theta $}$_{sat}$ = 0.41 ML) or more only the lowest temperature peak remains. CH$_{4}$ sticking also varies as a function of Cs coverage. N$_{2}$ displays similar behavior to CH$_{4}$ in response to coadsorption of Cs, but does not stick to Cs multilayers. CO$_{2}$ exhibits very unusual behavior as a function of Cs coverage. At low Cs coverage the physisorbed CO$_{2}$ peak broadens, then splits into a doublet, with one peak at higher temperature than the ``normal'' CO$_{2}$ peak, and the other at lower temperature. With increasing Cs coverage CO$_{2}$ begins to dissociate, evidenced by high temperature recombinative desorption and the presence of chemisorbed CO. As the Cs coverage approaches \textit{$\theta $}$_{sat}$ the physisorbed peak disappears entirely, and only dissociative chemisorption is seen.