Structure Matters: Asymmetric CO Oxidation at Rh Steps with Different Atomic Packing

Abstract

Curved crystals are a simple but powerful approach to bridge the gap between single crystal surfaces and nanoparticle catalysts, by allowing a rational assessment of the role of active step sites in gas-surface reactions. Using a curved Rh(111) crystal, here, we investigate the effect of A-type (square geometry) and B-type (triangular geometry) atomic packing of steps on the catalytic CO oxidation on Rh at millibar pressures. Imaging the crystal during reaction ignition with laser-induced CO2 fluorescence demonstrates a two-step process, where B-steps ignite at lower temperature than A-steps. Such fundamental dissimilarity is explained in ambient pressure X-ray photoemission (AP-XPS) experiments, which reveal partial CO desorption and oxygen buildup only at B-steps. AP-XPS also proves that A-B step asymmetries extend to the active stage: at A-steps, low-active O–Rh–O trilayers buildup immediately after ignition, while highly active chemisorbed O is the dominant species on B-type steps. We conclude that B-steps are more efficient than A-steps for the CO oxidation.

A Polycrystalline Pd Surface Studied by Two-Dimensional Surface Optical Reflectance during CO Oxidation: Bridging the Materials Gap

“…In addition to this, studies have been performed on high-index stepped or kinked surfaces. Such studies have proven a clear correlation between the surface orientation and the catalytic properties of said surface. − …”

CO and O2 Interaction with Kinked Pt Surfaces

“…The combined use of curved surfaces with (NAP-)­XPS has successfully demonstrated its potential to straightforwardly assess the role of steps in the CO oxidation on Pd, Pt, and Rh vicinals − or Ag-oxidation, among other matters . The curved geometry allowed the identification of surface species at different reaction stages and an accurate determination of the ignition temperature across different facets.…”

“…The curved geometry allowed the identification of surface species at different reaction stages and an accurate determination of the ignition temperature across different facets. Surprisingly, species and ignition temperatures were found the same at A-type ({100}-oriented microfacets) and B-type ({111} microfacets) stepped Pt(111) surfaces, by contrast to the expected A/B asymmetries observed in Pd and Rh. , Yet the question arises whether such homogeneous and symmetric behavior in Pt occurs far beyond the (111) plane or features different step geometries in the vicinity of the Pt(111) surface, such as the more open kinked steps (Figure a). Their complexity is probably the reason why they have been poorly investigated with XPS in the CO oxidation context, even under UHV conditions. , Note that the UHV-XPS study is a very relevant step ,, since it allows to determine core-level shifts for the variety of chemical species that arise in the presence of steps, thereby providing accurate reference spectra for NAP-XPS, which generally exhibits lower resolution and poorer statistics.…”

“…Surprisingly, species and ignition temperatures were found the same at A-type ({100}-oriented microfacets) and B-type ({111} microfacets) stepped Pt(111) surfaces, 14 by contrast to the expected A/B asymmetries observed in Pd and Rh. 12 , 13 Yet the question arises whether such homogeneous and symmetric behavior in Pt occurs far beyond the (111) plane or features different step geometries in the vicinity of the Pt(111) surface, such as the more open kinked steps ( Figure 1 a). Their complexity is probably the reason why they have been poorly investigated with XPS in the CO oxidation context, even under UHV conditions.…”

A Method Probing High‐Temperature Oxidation Behavior of Crystalline Materials

“…Metals, alloys, ceramics, semiconductors, organometallics, and many other materials undergo oxidation, and many widely used metallic materials (including Cu, [1] Mo, W, C, [2] Nb, etc. [3,4] and their alloys [5,6] ) induce great waste and potential hazards. Inhibition of materials' oxidation through different approaches including addition of a protective layer to the material surface, eliminating oxygen and water from the environment, and applying antioxidants sufficiently to mitigate oxidation prolongs material life.…”