The influence of potassium adatoms on the adsorption and desorption kinetics of hydrogen on the Pt(111) single-crystal surface has been studied by temperature-programmed desorption (TPD). We report measurements of the dissociative sticking coefficient for H? on Pt(111) over the entire range of coverages of H(a) and K(a) between zero and monolayer coverage. Small amounts of potassium strongly reduce the dissociative sticking coefficient for H2 on Pt(111) and eliminate the influence of a precursor in the adsorption kinetics. For example, when the K coverage, θK is 0.043 (defined relative to the Pt(111) surface atom density) the initial sticking coefficient of H2on Pt(111) at ISO K is reduced by a factor of 20. However, hydrogen adatoms on the K-modified surface are thermally stabilized compared to Pt(111). The peaks observed during H2 TPD occur at temperatures up to ISO K higher on the K-modified surface than for equivalent hydrogen coverages on K-free Pt(111). Comparisons with the Bi-modified surface demonstrate that the influence of K(a) exceeds local site blocking effects. We explain these observations in terms of a model in which K(a) modifies the electronic structure of the surface Pt atoms in nearby sites. This has two effects: (i) destabilization of the bonding of H2(a) to the surface and (ii) stabilization of the bonding of H(a) to the surface. The weaker interaction between H2(a) and the Pt(111) surface caused by K(a) decreases the role of a H2(a) precursor in the adsorption kinetics, lowers the initial dissociative sticking coefficient for H2, and causes an increase in the activation energy for dissociative adsorption of H2. While a slightly increased Pt-H bond strength due to K(a) could lead in principle to an enhancement of dissociative H2 adsorption, we find that the former effects on H2(a) dominate to strongly reduce the dissociative sticking coefficient for H2 on Pt(111) modified by K adatoms.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry