Oxygen on Pd(100): Order, reconstruction, and desorption

Abstract

We have investigated the temperature and coverage-dependent transformations of the ordered structures of oxygen on Pd(100). The four ordered structures are the chemisorbed c(2×2) and p(2×2) lattices, and the reconstructed p(5×5) and ((5)1/2×(5)1/2)R27° lattices. We present evidence that the p(5×5) reconstruction forms in an activated stepfrom c(2×2) regions. The onset of p(5×5) formation is associated with an increase in oxygen sticking coefficient. In thermal desorption, there are three states which can be correlated directly with the structure of the adsorbed phase during desorption: At lowest coverage, the α state shows the traits of second-order kinetics and is due to desorption from a disordered adlayer. At higher coverage, the β state appears and is due to desorption from a layer with c(2×2) order. There is a lower barrier to desorption in the β state than in the α state because of the repulsive second-nearest-neighbor interactions in the c(2×2). At highest coverage, the sharp and narrow γ state emerges. This is accompanied by decomposition of the ((5)1/2×(5)1/2)R27° reconstruction, in which cooperative stabilization of the reconstruction by oxygen atoms effectively creates strong quasiattractive oxygen–oxygen interactions. There are interesting similarities between the oxygen-stabilized reconstructions of Pd(100) and the initial stages of oxidation of Ni(100), as well as the oxygen-stabilized reconstructions of Pt(100). The data are obtained from low-energy electron diffraction coupled with a computer-interfaced Video camera, Auger electron spectroscopy, and thermal desorption spectroscopy.

Atomistic lattice-gas modeling of CO oxidation on Pd(100): Temperature-programed spectroscopy and steady-state behavior

“…In summary, due to these various factors, our modeling presumably somewhat underestimates the physical sticking coefficient. Figure 9 shows results of TPD simulations for different oxygen exposures at 400 K. Consistently, experimental TPD spectra 45 show that the dominant peak shifts from above 800 K to around 750 K as oxygen exposure increases. An additional shoulder or peak around 650 K starts to emerge when the initial coverage of oxygen is above 0.25 ML.…”

“…Figure 10 shows a snapshot of adlayer configuration at 620 K during TPD. We do not reproduce the very sharp peak for observed oxygen exposure over 30 L. 45,46 However, this sharp peak is due to the ͑ ͱ 5 ϫ ͱ 5͒R27°phase mentioned above 47 which is not incorporated into our modeling.…”

Model studies in heterogeneous catalysis at the microscopic level: from the structure and composition of surfaces to reaction kinetics

“…The interaction of oxygen with Pd surfaces has been controversially discussed in the literature for more than thirty years (see e.g. [76][77][78][79][80][81][82][83] ).…”

Chemical diffusion of CO in mixed CO+O adlayers and reaction-front propagation in CO oxidation on Pd(100)

“…19,22 Of some relevance for this study is the fea- ture that CO͑ads͒ tends to exhibit c͑2 ͱ 2 ϫ ͱ 2͒R45°ordering for CO coverages approaching 0.5 ML ͑monolayer͒, 23 and that O͑ads͒ tends to exhibit p͑2 ϫ 2͒ ordering for O coverages around 0.25 ML. 24 See also Appendix A and Fig. 4 for illustration of the ordered structures.…”