Ambient Pressure STM

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In Situ Observation of Competitive CO and O Adsorption on the Pt(111) Surface with Near-Ambient Pressure STM Jeongjin Kim, Myung Cheol Noh, Won Hui Doh, and Jeong Young Park J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01672 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

In Situ Observation of Competitive CO and O2 Adsorption on the Pt(111) Surface with NearAmbient Pressure STM Jeongjin Kim†,‡, Myung Cheol Noh†,‡, Won Hui Doh‡, and Jeong Young Park*†,‡ †

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced

Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ‡

Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon

34141, Republic of Korea

ACS Paragon Plus Environment

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ABSTRACT

We investigated the competitive co-adsorption of CO and O2 molecules on a Pt model surface using a catalytic reactor integrated with a scanning tunneling microscope (STM) at elevated pressure. CO-poisoned incommensurate atom-resolved structures are observed on the terrace sites of the Pt(111) surface under gaseous mixtures of CO and O2. However, in situ surface measurements revealed that segmented local structures were influenced by the CO/O2 partial pressures in the catalytic reactor at a total pressure of a few Torr. This could be related to the expected formation of the theoretical oxygen precursor intermediates during dissociation of O2 on the surface before the chemical reaction. These findings provide a microscopic insight into the early steps of the catalytic reaction pathways on the Pt surface during CO oxidation in an industrial chemical reactor.


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INTRODUCTION Platinum (Pt) is widely used in a variety of catalysts because of its superior catalytic activity and durability for industrial chemical reactions.1,2 For instance, the removal of exhaust emissions, preferential oxidation of gas, hydrogen production, and electrochemical cell reactions are well-known representative catalytic reactions using Pt.3-8 However, all these reactions have a serious drawback in that adsorbed CO blocks the active sites on the Pt surface instead of improving the chemical reactivity. Since the pioneering work by Langmuir9 using Pt wire to define the mechanism for CO oxidation, CO poisoning10-12 has been a significant obstacle to overcome in catalysis, and intrinsic problems still complicate these chemical reactions. To understand the chemical interactions at the molecular level, the characteristic structures of CO on flat and stepped Pt surfaces have been extensively studied from low to high CO coverage, which are visualized using low-energy electron diffraction (LEED)13-15 and scanning tunneling microscopy (STM)16-23 techniques. From a molecular orbital view, partially filled d orbitals of the Pt atoms selectively affect the chemical bond strength between the carbon and oxygen atoms in the CO molecule via back donation of electrons,24 which creates concentrated pockets of distinctive adsorption structures as a function of CO coverage on the Pt surface. For example, the (√3 × √3)R30°–CO structure at θCO = 0.33 monolayer (ML) and the c(4 × 2)–CO structure at θCO = 0.50 ML on the Pt(111) surface18,19,25 show that the adsorption sites usually occupied by CO molecules are different than either the atop (on-top) or bridge sites. Moreover, the formation of incommensurate17 or commensurate (√19 × √19)R23.4°–13CO16,17 structures on the Pt(111) surface is also possible at saturation coverage (θCO = 0.68 ML). CO poisoning on the metallic Pt surface occurs spontaneously across a wide range of pressures and temperatures, while the formation of oxygen-induced Pt structures is relatively restricted within


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the same range of conditions because of the unfavorable dissociation step of molecular O2 in terms of energy.26-29 However, enhanced catalytic activity is facilitated by oxygen coverage because the oxygen chemically bound on the Pt surface is able to provide favorable reaction sites for the reactant molecules.30,31 The interplay between the competitive adsorption of these two important molecules should be determined to establish the chemical reaction pathways during chemical reactions. In the case of CO oxidation, it is generally known that the turnover rate is proportionally influenced by the O2 pressure and inversely influenced by the CO pressure when below the ignition temperature (i.e., per the Langmuir–Hinshelwood (LH) mechanism), as determined from reactivity measurements on Pt catalysts.32-35 Nevertheless, we lack clear evidence of the elementary reaction steps related to oxygen dissociation on the Pt surface, with the exception of several proposed approaches that only use theoretical calculations.36,37 In practical chemical reactors, the Pt catalyst surface has increased chemical potentials that are much higher and further from the ideal environments proposed in previous model mechanisms. Especially, adsorbate–adsorbate interactions are no longer negligible; rather, they actually affect the chemical reactions to such an extent as to reconstruct defect-ridden low-coordinated sites on transition metal surfaces at near-ambient pressure (NAP).38-40 Therefore, a comprehensive interpretation of molecular behavior is needed with respect to this increased chemical potential to understand the more “realistic” reaction conditions seen in industry. In this study, we report the direct observation of competitive molecular adsorption of CO and O2 on the Pt(111) surface under NAP conditions using STM at room temperature (RT). A large quantity of dissociative oxygen is observed at the step sites on the Pt(111) surface; however, the terrace sites do not show similar trends of oxidation at 1 Torr of O2. Although small amounts


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of dissociative molecular oxygen possibly remained on the terrace sites of the Pt(111) surface, we could not observe oxidized local PtOx structures at RT. Under the O2-rich environment, the terrace of the Pt(111) surface is covered by a layer of an incommensurate CO structure that also introduces small amounts of CO into the reactor (p[CO]/p[O2] = 0.01). The CO–CO repulsive interactions increase as the gas pressure ratio between CO and O2 increases up to 0.1, thus exhibiting an unexpected instability of the adsorbed CO molecules on the Pt(111) surface. We also observed transient CO layers between the disordered chain-like structure and the commensurate (√19 × √19)R23.4°–13CO structure under 10.5 Torr of mixed CO/O2 gas (1:3 ratio). Using fractal dimensional analysis of each NAP-STM image, we demonstrate that the segmented structural changes on the Pt(111) surface are periodic under these conditions. These results suggest that the poisoned Pt surface induces the formation of a CO-assisted intermediate for molecular O2 dissociation at elevated pressure. These direct in situ observations provide a mechanistic understanding of an early stage of the CO oxidation reaction.

EXPERIMENTAL METHODS A commercially available Pt(111) single crystal was purchased from MaTeck GmbH (Germany). The polished sample was prepared using a high-accuracy cutting angle (< 0.1º). Sample cleaning was performed in an ultra-high vacuum (UHV) chamber at a base pressure of 1 × 10−10 Torr such that the process was carried out using Ar+ ion-bombardment sputtering (measured sample current: 10 µA) for 20 min, followed by vacuum annealing at 1100 K for 5 min. The cycle was repeated until a completely clean surface was obtained without any


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contamination (e.g., carbon, oxygen, silicon), which was verified by STM measurements in UHV after each cycle. Topographic Pt(111) surface images were obtained with a small volume (~15 mL) of the catalytic reactor being integrated into an STM scanner (SPECS) in an UHV chamber at RT; we employed a chemically etched tungsten tip in constant current mode. The tunneling conditions for each STM measurement in the figure are denoted as Vs for the sample bias and as It for the tunneling current. The speed of the STM tip for each image during image scanning in constant current mode is shown in Table S1. For the NAP-STM measurements, high purity (99.999%) CO or O2 gas was introduced into the reactor using a precision leakage valve. Any potential carbon or water contamination was removed using a gas purifier (PALL Corp.); the cleanliness of the UHV chamber was confirmed using a mass spectrometer (SRS Corp.) before any gas was introduced into the reactor. From the residual gas analysis, any unintended carbon or oxygen impurities were below the detection limits of the instrument. In our microscopic analysis, surface contamination and gas impurities were strictly controlled to verify the effects of the adsorbate–adsorbate and adsorbate– Pt surface interactions as clearly as possible. Even a single Pt atom is an excellent catalyst,41 thus very stringent control of the vacuum and clean surface preparation are required during these experiments. For instance, segregated SiOx species from the subsurface of the Pt or residual carbon on the surface should be addressed.42,43 The gas pressure inside the reactor was monitored by a full-range gauge (Pfeiffer vacuum). The designs for the STM scanner, vacuum component configuration, and gas manifold for the NAP-STM system are described in detail elsewhere.23 The STM images and topological line profiles were examined using the WSxM 5.0 software package,44 which contains widely applicable utilities to verify measured surface structures. To


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quantitatively investigate the in situ NAP-STM images, the fractal dimension was computed numerically using the power spectrum density method.

RESULTS AND DISCUSSION Figure 1 shows the spherical atomistic models of adsorbed CO molecules on the Pt(111) surface as a function of CO coverage. The CO/Pt(111) structures of (√3 × √3)R30°–CO (θCO = 0.33 ML) and c(4 × 2)–CO (θCO = 0.50 ML) are shown in Figure 1a,b. These schematics show that the CO molecules are adsorbed on the atop sites of the Pt(111) surface. With increasing CO coverage, two structures coexist when additional adsorbed weakly-bound CO molecules (i.e., when the CO coverage is greater than 0.50 ML) prefer to adsorb on the bridge sites of the Pt(111) surface.19 When the CO/Pt(111) structure is almost of fully covered (θCO = 0.68 ML), the closely-packed CO molecules occupy both the atop and bridge sites of the Pt(111) surface (Figure 1c). Peculiar incommensurate and commensurate periodic structures are also reported on CO/Pt(111) at θCO = 0.5 ~ 0.68 ML,17,19 which clearly describes a relationship between the binding sites and coverage of CO molecules on the Pt(111) surface. Figure 2a shows the atom-resolved clean surface of hexagonal-packed Pt atom arrays (nearest-neighbor distance: 0.28 nm) taken at UHV and RT. At 1 Torr of CO, a moiré lattice pattern appears with a periodicity of ~1.3 nm on the Pt(111) surface, as shown in Figure 2b; the CO layer is well-matched to the commensurate (√19 × √19)R23.4°–13CO structure described in the literature.16,17 An embedded unit cell of the atomistic model is depicted in Figure 1c to aid in


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interpreting the detailed molecular configurations on the Pt(111) surface. The observed structure is quite stable at RT during the steady-state NAP-STM measurements at almost saturation coverage of CO (θCO ≈ 0.68 ML). From the Blyholder model,24 chemical bonds between the CO and Pt occur spontaneously because of electron back-donation; theoretical calculations show that this is an exothermic process. Thus, the adsorption of CO on the Pt surface is a thermodynamically favorable reaction pathway with a strong binding energy.15,45 Using highresolution STM images, the characteristic coverage-dependent structures of the adsorbed CO were revealed at liquid helium18,19 and room temperatures;16,17 the adsorbed CO has binding energies of −2.0 to −1.4 eV, depending on the specific site (i.e., either atop or bridge, respectively) on the clean Pt(111) surface.46-48 In other words, when considering CO–CO repulsive interactions, CO molecules prefer to occupy either the atop sites corresponding to the (√3 × √3)R30°–CO structure or bridge sites corresponding to the c(4 × 2)–CO structure. However, the molecular configuration for the (√19 × √19)R23.4°–13CO structure, illustrated in Figure 1c, occurs simultaneously at both the atop and bridge sites to reduce the surface energy such that the CO molecules are tilted towards each other. Site-selective adsorption has been explored using spectroscopic techniques.15,49,50 After evacuating the reaction cell, the CO pressure decreases to near UHV (≤ 1 × 10−8 Torr), at which point, the weakly-bound CO molecules at high coverage are easily desorbed from the Pt(111) surface and the demonstrated commensurate structure is no longer observed in the STM image (Figure 2c). In contrast to the clean Pt(111) surface (Figure 2a), we observed a different CO poisoned local topography on the Pt(111) surface (Figure 2c); the sparsely adsorbed CO molecules make it difficult to obtain an ordered structure of the CO/Pt(111) at RT.51


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The p(2 × 2)–O (θO = 0.25 ML) structure covered by atomic oxygen was obtained on the Pt(111) surface after flash annealing of the sample at 1 × 10−6 Torr of O2 in an UHV chamber. Figure 3a exhibits uniformly generated p(2 × 2)–O structures27,29 with several point defects on the wide terraces. The region of the p(2 × 2)–O structure indicated in Figure 3a has a periodicity of approximately 0.56 nm, thus the measured corrugation is about twice that of the theoretical nearest-neighbor distance for Pt atoms.19,52 A bright streak at the step edge shows the formation of irregular Pt oxide clusters, which are seen in the middle of the STM image, in contrast to the terrace sites on the Pt(111) surface. Height profile analysis (Figure 3b) clearly demonstrates that different shapes of Pt oxide are formed on the step edge. The red arrows in Figure 3b indicate irregular Pt clusters with corrugations separating them in the profile. The measured lateral size of each Pt oxide cluster is roughly 0.5 ~ 0.7 nm from the representative height profile in Figure 3a (dashed red line). This indicates that the degree of oxidation at the step edge is definitely different than that on the wide terrace of the Pt(111) surface even when surrounded by the same O2 chemical potential environment.28,53 An enlarged chemisorbed p(2 × 2)–O structure is displayed in the high-resolution STM image in Figure 3c. Dissociative oxygen adsorption on the clean Pt(111) surface is rather restricted at RT because thermal activation energy is needed to overcome the barrier for incipient O2 dissociation; however, molecular desorption inevitably occurs on the surface.29,37 Even though a small amount of dissociative oxygen remained on the surface, the O atoms spontaneously desorbed within a short time. Therefore, balancing the heat transfer between oxygen and the Pt surface is important for controlling the degree of oxidation on the Pt surface by the adsorbate‒adsorbate interaction,54 which follows the theoretically proposed chemical potential diagram within the greater framework of the LH mechanism. At temperatures below 110 K, molecular oxygen could be


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physically adsorbed on the Pt(111) surface by attractive Van der Waals interactions,55,56 however, the amount of apparent physisorption is extremely small on the terrace site because of the low sticking probability (s = 0.05 ~ 0.12).26 A small amount of adsorbed frozen molecular oxygen begins to slowly dissociate above 150 K, however, dissociative oxygen chemisorption is still low while the molecular O2 precursors are competitively desorbed from the Pt(111) surface as the temperature increases.57 Thus, oxygen coverage is limited for building any kind of oxide layer on the flat terrace at large scale. The p(2 × 2)–O structure seems to occur during relatively high oxygen coverage and has the highest catalytic activity,58,59 which was estimated using diverse spectroscopic techniques over the past few decades.43,60-63 It is also possible that higher oxygen coverage (i.e., above 0.25 ML) on the terrace sites of the Pt(111) surface occurs during evaporation of the oxygen source or ozone treatment at mild temperature (300 ~ 450 K),52,64,65 whereupon an oxide stripe (θO = 0.50 ML), surface oxide (θO = 1.00 ML), and finally α-PtO2 clusters (θO = 2.00) appear sequentially on the surface.66,67 Extensive investigations of the growth of surface Pt oxides have also discussed experimental and theoretical aspects suggesting that the coverage-dependent oxidation process could show various structural formations on the Pt(111) surface.68-71 However, these physical properties are dramatically altered as a function of the stepdensity on the Pt surface in which the edge of the steps plays a role as a defect site.22,23 As the number of defect sites increases, the sticking probability and bond strength between the oxygen and low-coordinated Pt atoms increase such that the correlation is similar to the case of CO adsorption on the Pt surface.53 When the reactor is filled with O2 molecules, Pt oxide cluster formation at the step edges is observed, whereas most of the wide terraces remain clean with no impurities, as shown in Figure 4a. The hexagonal-packed structures of the arrays of Pt atoms are also seen in the


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magnified inset image from a local terrace on the Pt(111) surface. The elevated O2 pressure has a “pressure gap” effect of almost ten times (at logarithmic scale) that at UHV, which has a surface energy difference of ~0.3 eV from the fundamental Gibbs–Helmholtz relation.72 In other words, the increased chemical potential created when the Pt surface is surrounded by reactant molecules offers an increased number of molecular collisions for both the adsorbate–adsorbate and adsorbate–solid interfaces.54 The difference in surface energy caused by the pressure gap is sufficient to cause adsorbate-induced surface restructuring in certain situations, as reported for Pt(557)22,23 and Cu(111).73 As a matter of fact, the terraces of the Pt(111) surface do not change during our NAP-STM measurements until the formation of Pt oxide clusters at the step edge. This means that most of the molecular oxygen acts as a “spectator” because of its low sticking probability compared with that of CO molecules in the same environment. Although oxygen chemisorption may occur on the surface at 1 Torr of O2, local domain structures of p(2 × 2)–O would be formed on roughened sites and not on the flat sites of the terrace on the Pt(111) surface. Previous NAP-XPS reports already exhibit a critical discrepancy between the formation of oxygen-induced structures on the flat (111) and stepped (557) Pt surfaces at 1 Torr of O2,53 as well as a discrepancy between the evolution of Pt oxides at 0.5 Torr of O2 and elevated temperatures.66 Therefore, as shown in Figure 4a, we confirm that large amounts of oxidation on the flat Pt(111) surface under increased oxygen pressure at RT are not observed, which is in agreement with the spectroscopic results in the literature. More evidence for the role of free oxygen molecules acting as spectators at 1 Torr of O2 is clearly suggested by direct in situ observation of the surface, as displayed in Figure 4b,c. After the CO gas molecules were carefully introduced into the reactor, disparate molecular behavior is observed because of the fast diffusion of CO molecules during the scanning measurement. When


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the relative partial pressure (p[CO]/p[O2]) in the reactor reached 0.01, adsorbed CO molecules were observed with rather unclear periodicity at steady state, as shown in Figure 4b, such that the layer is quite stable and maintains its structure even after several repeated STM measurements at the same location. Even though the bright array is not completely composed of the moiré pattern following the schematic model of the (√19 × √19)R23.4°–13CO structure, as illustrated in Figure 1c, the centers of most of the bright arrays match the suggested unit cell of the lattice shown in Figure 4b. This implies that each component of the observed pattern consists of atop- and bridgebonded CO molecules at a certain coverage (0.40 ML < θCO ≤ 0.60 ML) on the surface.17-19 In addition, the reported direct pressure-dependent observations of CO or CO/O2 mixtures at NAP and RT demonstrate the spontaneous formation of periodic patterns after exposing the Pt(111) surface to CO under the contamination level (i.e., 1 × 10−6 Torr of CO in the analysis chamber);17,74 theoretical calculations also support the experimental results by comparing the adsorption energy between the CO and O2, as mentioned above.31,47 However, the periodic pattern gradually collapsed as the relative pressure of the CO molecules increased. In Figure 4c, we observed suddenly disordered bright spots on the Pt(111) surface when the p[CO]/p[O2] ratio reached 0.1. As depicted in Figure 4b, this disarrayed structure does not match the suggested unit cell of the lattice, which implies that molecular rearrangement of the adsorbed CO molecules occurred on the surface. It is worth noting that this unexpected molecular behavior is related to active interactions between the “spectator” O2 and the “interloper” CO on the Pt(111) surface. Unfortunately, estimating co-adsorption coverage is not a simple task based purely on the topographic information in Figure 4c because the contrast in the measured images could also be influenced by the electronic structure of the surface. Technically, in the STM image, each point of the z-axis modulation as well as the scanning direction originate from the tunneling


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probability at that moment, and not from a mechanical discrepancy between the STM tip and the Pt(111) surface in the Cartesian coordinate system.75,76 Thus, some sites containing an oxygen atom on the surface could appear as dark spots because the electronic structure was depleted to near the Fermi level during the measurement.77,78 Figure 5a,b shows the representative morphological difference between the disordered structure and the commensurate structure under 10.5 Torr of mixed CO/O2 gas (1:3 ratio) at RT. The disordered structure has chain-like honeycomb patterns with a coarsely formed local periodicity, that are fairly similar to the NAP-STM image in Figure 4c. Surprisingly, the atomresolved geometrical configurations are observed to be reversible during the NAP-STM measurements at near-ambient pressure such that the structural instability continued across a series of observations that included diverse metastable structures, as shown in Figure 6. Figure 6 displays selected in situ STM images of the atom-resolved intermediate structures at 10.5 Torr of mixed CO/O2 gas (1:3 ratio). The observed metastable structures are seen as chaotic arrangements that do not match the reported geometries for CO or O2 molecular adsorption on the Pt(111) surface. In the time-lapse images, any measurement drift was carefully minimized for local in situ analysis to allow for a precise comparison of the results from each geometry. The periodic bright spots on the CO-poisoned surface in Figure 6a vanish instantly in the few remaining separated domains, as shown in Figure 6b. The initial high CO coverage decreased significantly during the interaction of the adsorbate molecules in the reactor. In Figure 6c, a zig-zag structure consisting of elongated bright spots appears on the surface, but the metastable shapes are partially disconnected in about two hundred seconds, as shown in Figure 6d. However, Figure 6e exhibits linked ring structures that appear again instead of the vague assembly whose structure is slightly rearranged in Figure 6f with a more complex geometry than


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in former NAP-STM images. In Figure 6g,h, we cannot find any ordered structures or periodic domains; the distinctive features of each image are definitely neither the (√3 × √3)R30° nor c(4 × 2) structures of a CO layer on the Pt(111) surface. To quantify the systematic analysis, we employed fractal dimensional analysis to derive a geometrical interpretation from the measured morphology of each image. Fractal refers to the mathematical self-similarity of the measured geometric form observed on objects.79 The calculated fractal dimension is attributed to a finite set of segmented unit complexities that resemble themselves at different observable scales. The reasonable dependence of the cluster growth parameter could be proposed in a variety of systems by the relationship between fractal and dendritic island formation, as suggested by examples of aggregate silver clusters on Pt(111),80 polymorphic antimony nanostructures on graphite,81 and submonolayer C60 islands82 on alkali halides. The formation of a branching regular moiré structure or chaotic irregular pattern could be quantified as a scaling fractal dimension in the analysis.83,84 Because the moiré pattern contains fractal features, we could intuitively expect the STM image with the moiré lattice pattern to exhibit a high fractal dimension.85 Therefore, we think the information from fractal dimensional analysis provides useful information about the superposition of periodic features that are an essential feature of CO/O2 on the Pt(111) surfaces. The computed fractal dimensional values from all the STM images are plotted against time in Figure 6i, which illustrates the randomized oscillations in a narrow time scale (~100 sec), but they also show the periodic picture of a curve fitted to a sine function (T: 1067 sec). This phenomenon is definitely different than the fast diffusion of molecules on a surface at the time scale of the motion of small molecules (10−12 ~ 10−6 sec),86 which is too fast to be directly detected by the scanning tip (10−4 ~ 10−3 sec/pixel) in our experimental recording of the STM images at atomic-scale resolution. On the other hand, the molecular arrangements that create a


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metastable structure on the solid surface need a relatively longer time than that of the motion of the physically adsorbed molecules. Therefore, we believe that the observed atomic-scale movement can be attributed to the formation of intermediate species that promote thermodynamically favored chemical reactions by the interplay of adsorbate–adsorbate and adsorbate–solid interactions at the gas/solid interface. From theoretical calculations, extreme CO–CO or O–CO repulsive interactions possibly lead to unstable molecular overlapping on the Pt surface that could make an unexpected chemical reaction pathway.47,87 With the increased chemical potentials of CO and O2 in the reactor under ambient pressure, each molecule tends to avoid unstable energy states when possible, which would create a driving force for initiating surface rearrangements at the interface.88 From direct observation, the overall molecular behavior shows a different trend than that previously reported in elegant studies that were performed at UHV and low temperature using modern surface science techniques. These studies identified an ideal fundamental relationship between the adsorbate and the Pt surface at active sites, in accordance to the LH mechanism, but the physicochemical environment has a large gap compared with the conventional operating conditions of chemical reactors in the real world. Extrapolation of conventional mechanistic concepts from refined ideal conditions cannot always be applied to realistic reactor environments, thus resulting in a large disparity between the kinetics for industrial chemical reactions as a function of relative partial pressure. For example, Langmuir’s proposed elementary steps for CO oxidation are insufficient when dealing with kinetic analysis studies because most steps for intermediate species formation are neglected when deriving the reaction rates.9,74,89 However, the reactivity measurements during CO oxidation have clearly shown that the turnover rate and activation energy barrier of Pt catalysts can be attributed to CO coverage on the surface because


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the active sites for oxygen dissociation are blocked.89,90 Therefore, the desorption process of the blocked CO molecules is needed for O2 dissociation on the surface such that the apparent activation energy barrier is naturally influenced by the desorption enthalpy of the CO molecules; however, this trend varies gradually from the proposed kinetic model as the total pressure increases. Thus, the formation of intermediates should be considered elementary steps for CO oxidation because the reactive species could react with neighboring chemisorbed species on the Pt surface, while spontaneous decomposition causes CO2 evolution. The theoretically suggested elementary steps of O2 precursor-mediated states in O2 dissociation36 or CO-assisted dissociative oxygen chemisorption48 are more persuasive when pointing out arguments for the estimated kinetics gap at elevated pressures. Dissociative oxygen adsorbed on specific sites of the Pt(111) surface are denoted as hollow, atop, or bridge, corresponding to their coverage because of delectron hindrance between neighboring Pt atoms.91 Otherwise, O2 molecules could be placed on the edge or near a specific precursor by modifying the potential energy via various configurations at the initial physisorption states.92 However, the Pt surface is already covered by chemisorbed CO at practical reactor conditions for CO oxidation, which is a critical obstacle for having site-specific precursor-mediated conditions for O2 dissociation. From ab initio model calculations on Pt(111), the clean surface has −1.23 eV of exothermic energy, whereas the COpoisoned surface (θCO = 0.44 ML) has +0.06 eV of endothermic dissociated atomic oxygen adsorption energy.48 The dissociation of oxygen seems to be a difficult process as the CO coverage on the Pt surface increases, but previous studies do not discuss in depth the influence of adsorbate–adsorbate interactions under active chemical reaction environments. Although the computational calculations mentioned here have not described the sophisticated CO and O2 free molecule interactions in certain circumstances, their molecular behavior should contribute to the


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initial reaction steps for CO oxidation in a reactor. Furthermore, a proposed mechanism for the formation of the super-oxo O2− intermediate on the Pt surface demonstrates more rationalized elementary steps that elucidate direct O2 activation for effective CO2 evolution and chemisorbed oxygen in the reaction pathway.35 Consequently, our direct observations at NAP could give significant insight into the connection between the theoretically expected intermediate formation and the catalytically active reaction pathway for CO oxidation.

CONCLUSION We performed in situ STM to investigate the behavior of adsorbate-mediated molecular structures on the Pt(111) surface under NAP conditions at RT. CO poisoning occurs spontaneously on the terrace of the surface in the presence of 1 Torr of O2 after adding CO gas into the reactor; characteristic molecular structures form as the relative partial pressure of CO and O2 changes. This implies that molecular O2 adsorption and the subsequent dissociation processes on the CO-poisoned Pt surface can be influenced by competitive CO and O2 adsorption at NAP. Under 10.5 Torr of mixed CO/O2 gas (1:3 ratio), dynamically changing adsorbed CO layer structures are clearly observed on the terrace of the Pt(111) surface. A disordered chainlike structure and the commensurate (√19 × √19)R23.4°–13CO structure are observed in turn at 300 K. Fractal dimensional analysis of the in situ NAP-STM measurements confirms that the changes to the intermediate structures are of a periodic nature with a time cycle of 1067 sec. This phenomenon is related to the reported theoretical calculations used to explore possible chemical mechanisms for CO oxidation on the Pt catalyst surface. The correlation between molecule-


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mediated precursor states in O2 dissociation and adsorbate–adsorbate interaction effects is needed to specifically understand the reaction under realistic conditions. Our experimental findings could contribute to clarifying reasonable catalytic reaction pathways for CO oxidation on the Pt catalyst surface at the molecular level.


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Figure 1. Spherical atomistic model structures of adsorbed CO molecules on the Pt(111) surface as a function of CO coverage. (a) (√3 × √3)R30°–CO (θCO = 0.33 ML) structure, (b) c(4 × 2)–CO (θCO = 0.50 ML) structure, and (c) commensurate (√19 × √19)R23.4°–13CO (θCO = 0.68 ML) structure depicted with a unit cell (blue), Pt (gray) atoms, and CO (red) molecules.


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Figure 2. STM images of the clean Pt(111) surface (a) at UHV [Vs = 0.23 V; It = 0.21 nA], (b) at 1 Torr of CO [Vs = 0.63 V; It = 0.21 nA], and (c) after evacuation ( ≤ 1 × 10−8 Torr) of the reaction cell [Vs = 0.22 V; It = 0.20 nA].


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Figure 3. (a) STM image of the p(2 × 2)–O (θO = 0.25 ML) structure on the Pt(111) surface at UHV after flash annealing at 600 K and at 1.0 × 10−6 Torr of O2 [Vs = 0.46 V; It = 0.21 nA]. (b) A representative height profile along the dashed red line in (a) with arrows indicating the corrugations from Pt oxide cluster formation at the step site on the Pt(111) surface. (c) An enlarged STM image that was acquired from a terrace site in (a) showing the atom-resolved p(2 × 2)–O structure [Vs = –0.22 V; It = 0.21 nA].


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Figure 4. (a) In situ NAP-STM images on the Pt(111) surface at 1 Torr of O2 [Vs = 0.94 V; It = 0.24 nA]. Pt oxide clusters are only formed on the step sites of the Pt(111) surface along the dotted white lines. The magnified inset in (a) confirms at the atomic level that dissociative oxygen is not adsorbed on the terrace [Vs = 0.24 V; It = 0.20 nA]. (b) After the introduction of 10 mTorr of CO gas into the reactor, bright spots with a relatively periodic structure appear on the surface [Vs = 0.33 V; It = 0.21 nA]. (c) Further introduction of 90 mTorr of CO causes the formation of a disordered structure [Vs = 0.39 V; It = 0.21 nA].


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Figure 5. The formation of (a) a disordered chain-like structure and (b) commensurate (√19 × √19)R23.4°–13CO structure on the terrace site of the Pt(111) surface under 10.5 Torr of mixed CO/O2 gas (1:3 ratio). Both of the atom-resolved structures are observed to be reversible from the in situ NAP-STM measurements at 300 K [Vs = 0.97 V; It = 0.21 nA].


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Figure 6. (a–h) In situ NAP-STM images of the terrace site of the Pt(111) surface under 10.5 Torr of mixed CO/O2 gas (1:3 ratio) [Vs = 0.67 V; It = 0.16 nA]. The NAP-STM images show structural changes at atomic scale as time elapses. (i) Plot of the fractal dimensional analysis results from each NAP-STM image, which creates a picture of the periodic function (T = 1067 sec).


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AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] (J.Y.P.).; Tel.: +82-42-350 1713; Fax: +82-42-3501710; Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by Institute for Basic Science (IBS) [IBS-R004-A2-2017-a00].

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Ambient Pressure STM

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