Hydrogen Dissociation Dynamics on Precovered Pd Surfaces: Langmuir is Still Right

A recent scanning tunneling microscopy study by Mitsui et al. [Nature (London) 422, 705 (2003)] challenged the well-accepted picture based on early studies of Langmuir that an ensemble of at least two empty, catalytically active sites is required for the dissociative adsorption; instead, aggregates of three or more vacancies should be necessary. We have performed the first ab initio molecular dynamics study of the adsorption dynamics on a precovered surface providing detailed insights into the coverage dependence of the adsorption probability. The simulations show that there is no need to refine the Langmuirian picture: A dimer vacancy is still sufficient to dissociate hydrogen provided the kinetic energy of the molecules is large enough to overcome the relatively small adsorption barrier. In addition, we elucidate further aspects of the dissociation dynamics at precovered surfaces.

Figure 1

Dissociative adsorption probability of hydrogen on clean (A) Pd(111) and (B) Pd(100) calculated by ab initio molecular dynamics simulations. The results are compared to experimental molecular beam data by Gostein and Sitz [20] for nonrotating molecules (rotational quantum state $j=0$) and by Beutl et al. [29] for molecules with a thermal distribution of rotational states on Pd(111) and by Rendulic et al. [22] and by Rettner and Auerbach [23] on Pd(100). For Pd(111), further theoretical ab initio results by Di Cesare et al. [12] obtained by classical trajectory calculations for a fixed substrate are included.

Figure 2

(a) Dissociative adsorption probability of hydrogen on hydrogen-covered Pd(100) and Pd(111) as a function of coverage for an initial kinetic energy of 0.1 eV. 2 V, $3V_H$ and $3V_T$ denote the dimer vacancy and the trimer vacancy centered around a Pd hollow and a Pd top site, respectively. (b) Energy distribution upon the dissociative adsorption in a hydrogen dimer vacancy on Pd(100) along a typical trajectory.

Figure 3

Snapshots of three trajectories within a ($3\times 3$) surface periodicity illustrating details of the adsorption dynamics at a hydrogen-covered Pd(100) surface with a dimer vacancy.