CO and O2 Interaction with Kinked Pt Surfaces

We investigate the chemical interaction of carbon monoxide (CO) and oxygen (O2) with kink atoms on steps of platinum crystal surfaces using a specially designed Pt curved sample. We aim at describing the fundamental stages of the CO oxidation reaction, i.e., CO-covered/poisoned stage and O-covered/active stage, at the poorly known kinked Pt facets by probing CO uptake/saturation and O2 saturation, respectively. Based on the systematic analysis that the curved surface allows, and using high-resolution X-ray photoemission, a diversity of terrace and step/kink species are straightforwardly identified and accurately quantified, defining a smooth structural and chemical variation across different crystal planes. In the CO-saturated case, we observe a preferential adsorption at step edges, where the CO coverage reaches a CO molecule per step Pt atom, significantly higher than their close-packed analogous steps with straight terrace termination. For the O-saturated surface, a significantly higher O coverage is observed in kinked planes compared to the Pt(111) surface. While the strong adsorption of CO at the kinked edges points toward a higher ignition temperature of the CO oxidation at kinks as compared to terraces, the large O coverage at steps may lead to an increased reactivity of kinked surfaces during the active stage of the CO oxidation.


LEED and Pt 4f core levels
Low-Energy Electron Diffraction (LEED) patterns of the curved sample were used to check the sample surface orientation and superstructure.The characteristic hexagonal diffraction pattern corresponding to the (111) fcc terraces was found close to the border of the crystal edge.Away from that point, the LEED pattern reveals a steady split of the LEED spots.This splitting results from the additional superstructure related to the ordered steps of the surface. 1 The splitting increases with the step density due to the narrowing of the terraces (and a shorter real lattice constant), [2][3][4] becoming very pronounced at the highly stepped (312) plane.In the Pt 4f region, peaks related to bulk and terrace Pt are easily detected in the (111) surface.At the kinked planes, the terrace contribution decreases in favor of step Pt atoms. 3 also checked the LEED after CO saturation.A sharp c(4 × 2) diffraction pattern was observed at the (111) plane, rapidly vanishing after electron beam exposure, revealing a strong sensitivity to the low-energy electron beam of the LEED apparatus.The kinked surfaces only featured the characteristic splitting of spots due to the increasing step density, hence we conclude that CO molecules do not arrange in a long-range ordered manner at the kinked vicinals and there is no step doubling.However, since the c(4 × 2) LEED pattern rapidly vanished at the (111) plane, another possibility is that we are disordering the adsorbate structure during the LEED measurement itself.The Pt 4f spectrum at the (111) plane features, in addition to terrace and bulk Pt atoms, peaks corresponding to CO adsorbed in top and bridge sites.Similarly as before, at the stepped surfaces a contribution from CO adsorbed at steps is also observed, while those of terrace species decrease.
For O 2 , the p(2×2) is neatly observed in the (111) plane, fading rapidly away as we move across the curved surface.In this case, the step splitting of the Pt vicinal substrate also vanishes, indicating that O adsorption causes a structural disruption of the step lattice, reflecting its higher interaction with the substrate.We acquired no Pt 4f spectra in this case.LEED diffraction images and Pt 4f XPS regions obtained after cleaning the sample (a, 300 K) and after dosing 10 L CO (c, dose at 300 K, measurement at 90 K) at the (111), ( 645) and (312) planes, respectively.In the LEED images, the increasing splitting distance of the spots as α increases is labelled as s. 4,5Positions of the [0,0] and [1,1] spots are also indicated.In the Pt 4f spectra, Pt B , Pt T and Pt S refer to bulk, terrace and step Pt atoms, while T T , T B and S T belong to CO molecules anchored in Terrace-Top, Terrace-Bridge and Step-Top sites.Differences at the stepped surfaces are better appreciated when subtracting the corresponding spectra at the (111) plane.As expected, terrace components (Pt T , T T , T B ) decrease at the stepped surfaces, in parallel to the growth of step-related peaks (Pt S , S T ).

C 1s evolution during the CO desorption
The desorption of CO is the fundamental process that triggers the catalytic CO oxidation.
We investigate such phenomenon in Fig. S2 and S3.To this aim, the (111), ( 645) and (312) surfaces were CO-saturated at 300K with a 10 L dose, and subsequently heated at a constant heating rate of 5 K/min while continuously recording XPS.A photon energy of 650 eV was chosen to also probe the O 1s region.7][8][9] As seen in Fig. S2d,g, T B starts to desorb right after starting the heating ramp, and vanishes at around 390 K.However, T T remains rather stable until 320 K, then it starts to decrease, and completely desorbs at around 430 K, i.e., approximately 40 K higher than T B .Earlier desorption of CO anchored at T B positions rather than at T T sites is indeed expected. 6,8,10 the (645) surface (center column of Fig. S2), as in the (111) plane, the first species to desorb from the CO-covered surface is T B -CO.2][13] The evolution is similar in the CO-saturated (312) surface (right column of Fig. S2), where the main contribution is S T -CO, while the peak related to CO adsorbed at T T sites is remarkably smaller.In the (312) surface there is an additional feature initially attributed to CO anchored at T B sites, slightly shifted as compared to the (111) plane (285.75 and 285.93 eV, respectively), which was also observed during the uptake experiments.Nevertheless, upon heating the T T -CO peak starts to decrease prior to this 285.75eV feature.Additionally, this latter contribution remains almost constant with temperature and abruptly vanishes at 390 K.This reveals differences with CO molecules adsorbed at T B sites on large (111) terraces, as the latter would readily start to desorb rapidly after increasing the temperature above room temperature. 14The peak at 285.75 eV is that related to CO adsorbed at defects, 11 which can only be resolved in the (312) surface at this temperature (it is resolved in all surfaces at 90 K, see Fig. 3 in the text).CO anchored at both T T and defect sites completely desorb at 390 K. S T does   645) and (312) surfaces, respectively.The surfaces were exposed to 10 L CO in order to ensure saturation.d-f Selected fitted spectra at the (111), ( 645) and (312) planes at the beginning (300 K), middle (360 K) and end (440, 475 and 500 K, respectively) of each desorption experiment.g-i Coverage evolution as a function of temperature of individual CO species, extracted from the fit of all spectra shown in a-c).CO adsorption as Terrace-Top, Terrace-Bridge and Step-Top are denoted as T T , T B and S T , while graphitic carbon is denoted as "C".CO chemisorbed at defect sites at the (312) surface is colored in purple.CO adsorption at defects of the (312) surface is colored in purple.The experiments were carried out at a photon energy of 650 eV to also monitor the O 1s region, shown in the SI.so at 500 K, reflecting again a larger desorption temperature of CO adsorbed at steps as compared to terraces.
The three desorption experiments confirm the expected trend for desorption temperatures: T B → T T → S T , i.e., the reverse sequence with respect to the one observed during the uptakes discussed along with Fig. 2 of the main text.Impinging CO molecules adsorb first on kinks in the low coverage regime, and they will desorb from them only after the CO from the terraces has almost vanished.Accordingly, the bigger desorption temperature of CO adsorbed at steps is of relevance to the CO oxidation ignition in NAP experiments on these vicinal surfaces.Regarding graphitic carbon "C", it increases during the three desorption experiments.We also found that CO anchored at T B sites dissociates under more intense photon beams, pointing to beam-damage rather than to heating-induced CO cracking.

O 1s evolution during the CO desorption
The O 1s region was also recorded during the CO desorption experiments as shown in Fig. S3.No oxygen arising from the CO dissociation was detected in the O 1s region in any of the studied surfaces.We observed pronounced shifts at the (645) and ( 312

O 1s α-scans at low and high CO dose
The O 1s region was scanned at 650 eV after the exposure to 0.25 and 10 L CO at 300 K (Fig. S4).Contrary to the C 1s region, peaks from CO anchored at T T (532.6 eV) and S T (532.3 eV) sites are not resolved in the O 1s. 11 In addition, it is clear that the ratio between CO adsorbed at T B (531.0 eV) and T T sites (532.5 eV) is not 1, reflecting that the O 1s spectrum at the (111) plane suffers from PED effects. 16A closer look to the α-scans reveals a significant binding energy shift with α in the peak around 532.5 eV.This is expected, since at low α, the main contribution is T T -CO (532.6 eV), yet at the densely stepped surfaces S T -CO (532.3 eV) is the main species.
A substantial increment in the total intensity with α was observed.This effect is also attributed to enhanced O 1s emission due to PED effects and the curved shape of the crystals, and not to such large increase in the total coverage.The same intensity growth with α after CO saturation was observed for close-packed Pt vicinals, as described in Ref. 5 Binding Energy  645) and (312) planes.O K and O fcc allude to chemisorbed O at square sites at the kinks and at hollow fcc sites at steps and terraces, while 4O stands for 1D chains of the "4O" oxide.Refer to Fig. 4 for more details about the nature of these chemical species.
Experiments carried out at a photon energy of 650 eV.
Photoemission Intensity

Figure S1 :
Figure S1: LEED patterns and Pt 4f region of the clean and CO-covered surfaces.LEED diffraction images and Pt 4f XPS regions obtained after cleaning the sample (a, 300 K) and after dosing 10 L CO (c, dose at 300 K, measurement at 90 K) at the (111), (645) and (312) planes, respectively.In the LEED images, the increasing splitting distance of the spots as α increases is labelled as s.4,5 Positions of the [0,0] and[1,1] spots are also indicated.In the Pt 4f spectra, Pt B , Pt T and Pt S refer to bulk, terrace and step Pt atoms, while T T , T B and S T belong to CO molecules anchored in Terrace-Top, Terrace-Bridge andStep-Top sites.Differences at the stepped surfaces are better appreciated when subtracting the corresponding spectra at the (111) plane.As expected, terrace components (Pt T , T T , T B ) decrease at the stepped surfaces, in parallel to the growth of step-related peaks (Pt S , S T ).

(Figure S2 :
Figure S2: C 1s evolution during CO desorption experiments.a-c Photoemission intensity plots of the C 1s region during separate heating ramps in CO-saturated (111), (645) and (312) surfaces, respectively.The surfaces were exposed to 10 L CO in order to ensure saturation.d-f Selected fitted spectra at the (111), (645) and (312) planes at the beginning (300 K), middle (360 K) and end (440, 475 and 500 K, respectively) of each desorption experiment.g-i Coverage evolution as a function of temperature of individual CO species, extracted from the fit of all spectra shown in a-c).CO adsorption as Terrace-Top, Terrace-Bridge and Step-Top are denoted as T T , T B and S T , while graphitic carbon is denoted as "C".CO chemisorbed at defect sites at the (312) surface is colored in purple.CO adsorption at defects of the (312) surface is colored in purple.The experiments were carried out at a photon energy of 650 eV to also monitor the O 1s region, shown in the SI.