The active phase of Pd during methane oxidation is a long-standing puzzle, which, if solved, could provide routes for design of improved catalysts. Here, density functional theory and in situ surface X-ray diffraction are used to identify and characterize atomic sites yielding high methane conversion. Calculations are performed for methane dissociation over a range of Pd and PdOx surfaces and reveal facile dissociation on either under-coordinated Pd sites in PdO(101) or metallic surfaces. The experiments show unambiguously that high methane conversion requires sufficiently thick PdO(101) films or metallic Pd, in full agreement with the calculations. The established link between high activity and atomic structure enables rational design of improved catalysts.
Shedding light on light-off: Photoemission electron microscopy, DFT, and microkinetic modeling were used to examine the local kinetics in the CO oxidation on individual grains of a polycrystalline sample. It is demonstrated that catalytic ignition (“light-off”) occurs easier on Pd(hkl) domains than on corresponding Pt(hkl) domains. The isothermal determination of kinetic transitions, commonly used in surface science, is fully consistent with the isobaric reactivity monitoring applied in technical catalysis.
Understanding
how specific atom sites on metal surfaces lower the
energy barrier for chemical reactions is vital in catalysis. Studies
on simplified model systems have shown that atoms arranged as steps
on the surface play an important role in catalytic reactions, but
a direct comparison of how the light-off temperature is affected by
the atom orientation on the step has not yet been possible due to
methodological constraints. Here we report in situ spatially resolved
measurements of the CO2 production over a cylindrical-shaped
Pd catalyst and show that the light-off temperature at different parts
of the crystal depends on the step orientation of the two types of
steps (named A and B). Our finding is supported by density functional
theory calculations, revealing that the steps, in contrast to what
has been previously reported in the literature, are not directly involved
in the reaction onset but have the role of releasing stress.
Metals are commonly oxidized under ambient conditions. Although bulk oxidation has received considerable attention, far less is known about oxidation at the subnanometer scale. This is unfortunate, as metal particles used in heterogeneous catalysis typically range from subnanometer to some nanometers. Here, density functional theory calculations are used to explore oxidation of gas-phase transition metal clusters in the range from the dimer to the dodecamer. Comparisons with the corresponding bulk systems uncover that the decomposition temperature of stoichiometrically oxidized clusters may be lower than for the bulk. Despite pronounced variations in ground state geometries, oxidized clusters closely mimic energetic trends across the periodic table valid for bulk systems.
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