Weighing Fractions of Monolayers: Application to the Adsorption and Catalytic Reactions ofH2, CO, andO2on Pt

The adsorption and catalytic reactions of ${\mathrm{H}}_2$, CO, and ${\mathrm{O}}_2$ on Pt were investigated with a quartz-crystal microbalance having a sensitivity of ∼ ${10}^{-10\mathrm{}}$ g/${\mathrm{cm}}^2$ (<${10}^{-2\mathrm{}}$ monolayers of oxygen). At zero coverage the sticking coefficients for ${\mathrm{H}}_2$, CO, and ${\mathrm{O}}_2$ were 0.62, 0.54, and 0.38, respectively. The catalytic oxidation reactions all exhibited Langmuir-Hinshelwood behavior. Detailed rate curves were measured for the ${\mathrm{H}}_2$-${\mathrm{O}}_2$ reaction. The maximum reaction probability was 0.32 per impinging ${\mathrm{H}}_2$.