Two-Dimensional Oxide on Pd(111)

Abstract

The oxidation of Pd(111) leads to an incommensurate surface oxide, which was studied by the use of scanning tunneling microscopy, surface x-ray diffraction, high resolution core level spectroscopy, and density functional calculations. A combination of these methods reveals a two-dimensional structure having no resemblance to bulk oxides of Pd. Our study also demonstrates how the atomic arrangement of a nontrivial incommensurate surface can be solved by molecular dynamics in a case where experimental techniques alone give no solution.

Nanometer and Subnanometer Thin Oxide Films at Surfaces of Late Transition Metals

“…11 is not only a kinetic precursor, but represents the stable phase for an intermediate range of ∆µ O . 33 This is therefore similar to the reported findings at Ag(111), of a p(4 × 4) surface oxide that is thermodynamically stable over a wide range of O chemical potentials, bounded on the one end by the stability range of a low coverage adsorbate phase and on the other end by the lowstability Ag 2 O bulk oxide. 23,39 Both the "( √ 6 × √ 6)" surface oxide on Pd(111) and the p(4 × 4) surface oxide on Ag(111) seem to have a highly complex, O-metal-O trilayered atomic structure that does not resemble the corresponding bulk oxides, cf.…”

“…This may result in commensurable or incommensurable overlayers, and both types have already been observed experimentally on the late 4d basal surfaces. 31,32,33,34 In addition, strain may be easier relaxed at more open sites like steps, pointing again at the relevance of the latter for the oxidation process. Apart from the strain energy, another influential factor is the thermodynamic driving force to form an oxide.…”

“…Recent experimental studies seem to support this view, i.e., they indicate that thin oxidic structures, now mostly coined surface oxides, start to form roughly at coverages Θ c . 30,31,32,33,34 For the "true" bulk oxide a higher oxygen concentra- tion might be required, but for oxide nucleation and for the formation of oxidic islands such increased oxygen content is only needed locally. In this respect it is remarkable that although for the on-surface adsorption at the late 4d TM basal surface the adatom-adatom interaction is repulsive (see the discussion of Fig.…”

Oxide formation at the surface of late 4d transition metals: insights from first-principles atomistic thermodynamics

“…While traditionally such films were conceived as closely-related thin versions of the corresponding bulk oxides, recent atomic scale characterizations of initial few-atom thick oxide overlayers especially on Pd and Ag surfaces revealed structures that had only little resemblance to their bulk counterparts, and/or were to a large degree influenced by a strong coupling to the underlying metal substrate 14,30,31 . Due to this coupling and structures particularly suited for layered configurations, one may expect the stability range for such surface oxides to exceed that of the hitherto discussed bulk oxides.…”

Oxidation State and Symmetry of Magnesia-Supported Pd13OxNanocatalysts Influence Activation Barriers of CO Oxidation

“…Its second adlayer Pd 4 O 4 unit has already been found by one of us for magnesia-supported Pd 9 nanooxides. 12 Interestingly, it represents the basic building block of a Pd(111) surface oxide 5 and differs significantly from the √5 × √5R27°surface oxide on Pd(100) whose stability has been predicted by Rogal et is of the same order of magnitude as the desorption energy and because the prefactor in the Arrhenius expressions of the reaction rates is usually higher for desorption than for dissociation, 33 we consequently assume that the fourth dimer is likely to desorb from the cluster before dissociation.…”