Reduced Carbon Monoxide Saturation Coverage on Vicinal Palladium Surfaces: the Importance of the Adsorption Site

Abstract

Steps at metal surfaces may influence energetics and kinetics of catalytic reactions in unexpected ways. Here, we report a significant reduction of the CO saturation coverage in Pd vicinal surfaces, which in turn is relevant for the light-off of the CO oxidation reaction. The study is based on a systematic investigation of CO adsorption on vicinal Pd(111) surfaces making use of a curved Pd crystal. A combined X-ray Photoelectron Spectroscopy and DFT analysis allows us to demonstrate that an entire row of atomic sites under Pd steps remains free of CO upon saturation at 300 K, leading to a step-density-dependent reduction of CO coverage that correlates with the observed decrease of the light-off temperature during CO oxidation in vicinal Pd surfaces.

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

“…The activity of curved Pd crystals, which cover the region from the (553) orientation via (111) to the (322) orientation, has been investigated previously. ,, Blomberg et al show that the side with B-type steps becomes active before that with the A-type steps, which ignites before the (111) surface with no steps. This suggests that the (111) surface, which forms the thickest oxide layer (Figure c), is the least active.…”

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

“…The intensity of the T H -CO peak decreases from the (111) surface to the (223) and (553) planes, while that of T T -CO does the reverse. This is explained by considering a transition of some CO molecules from T H to T T sites, as T H -CO is known to become less favorable as the step density increases and the terraces narrow . Step-related CO contributions cannot be resolved in the O 1s region, hence the peaks are renamed as (S B + T H ) and (S T + T T ).…”

“…This is explained by considering a transition of some CO molecules from T H to T T sites, as T H -CO is known to become less favorable as the step density increases and the terraces narrow. 40 Step-related CO contributions cannot be resolved in the O 1s region, 39 hence the peaks are renamed as (S B + T H ) and (S T + T T ). The CO (g) emission peaks at 536.5 eV.…”

CO and O2 Interaction with Kinked Pt Surfaces

“…Their complexity is probably the reason why they have been poorly investigated with XPS in the CO oxidation context, even under UHV conditions. 17,18 Note that the UHV-XPS study is a very relevant step 8,10,11 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. CO and O 2 adsorption studies using UHV-XPS are abundant for Pt(111) and its A/B vicinal surfaces; hence, the resulting CO/Pt and O/Pt interfaces and adsorption sites are well established.…”

“…Their curved surfaces allow for a smooth transition in the crystal orientation, enabling a systematic comparison of various facets under identical reaction conditions. Moreover, with a proper selection of the crystal sector, a curved surface also offers a sophisticated but consistent means of analyzing vicinal surfaces, hence the effect of undercoordinated, step atoms in surface-catalyzed reactions. − …”