Influence of Step Geometry on the Reconstruction of Stepped

Catalysts 2017 7 (5), 134. Luca Vattuone , Letizia Savio , Mario Rocca. 2017,. Surface science under reaction conditions: CO oxidation on Pt and Pd mo...
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Influence of Step Geometry on the Reconstruction of Stepped Platinum Surfaces under Coadsorption of Ethylene and CO Zhongwei Zhu,†,§ Cédric Barroo,§,⊥ Leonid Lichtenstein,§ Baran Eren,§ Cheng Hao Wu,†,§ Baohua Mao,∥ Thierry Visart de Bocarmé,⊥ Zhi Liu,∥,# Norbert Kruse,⊥,⊗ Miquel Salmeron,*,§,‡ and Gabor A Somorjai*,†,§ †

Department of Chemistry and ‡Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States § Materials Sciences Division and ∥Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Chemical Physics of Materials, Faculty of Sciences, Université Libre de Bruxelles, Campus de la Plaine CP 243, 1050 Bruxelles, Belgium # School of Physical Science and Technology, ShanghaiTech University, Shanghai, 200031, China ⊗ Department of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99163, United States S Supporting Information *

ABSTRACT: We demonstrate the critical role of the specific atomic arrangement at step sites in the restructuring processes of low-coordinated surface atoms at high adsorbate coverage. By using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), we have investigated the reconstruction of Pt(332) (with (111)-oriented triangular steps) and Pt(557) surfaces (with (100)-oriented square steps) in the mixture of CO and C2H4 in the Torr pressure range at room temperature. CO creates Pt clusters at the step edges on both surfaces, although the clusters have different shapes and densities. A subsequent exposure to a similar partial pressure of C2H4 partially reverts the clusters on Pt(332). In contrast, the cluster structure is barely changed on Pt(557). These different reconstruction phenomena are attributed to the fact that the 3-fold (111)-step sites on Pt(332) allows for adsorption of ethylidynea strong adsorbate formed from ethylenethat does not form on the 4-fold (100)-step sites on Pt(557). SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis commonly separated by two types of steps. The first type, denoted as (111)-steps, is oriented along the [111̅] direction with triangular atomic arrangement, and the other type, denoted as (100)-steps, is along the [001] direction with square atomic arrangement.26 The two step geometries exhibit distinct chemical properties by virtue of their different electronic structures. For example, one-dimensional (1D) Rh oxide formed on stepped Rh crystals is continuous at (100)steps but defective at (111)-steps.27 Growth of 1D Rh oxide into a two-dimensional (2D) oxide layer is hindered at (100)steps but fast at (111)-steps below 250 °C, because the defectfree 1D oxide increases the oxygen diffusion barrier. Structural changes due to coadsorption of two or more reactants are essential in heterogeneous catalysis, because it is the competition between reactant species at the active sites that determines the reaction rate and selectivity. The coadsorption

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he heat of adsorption and the sticking probability of reactant molecules at low-coordinated catalyst surface sites, like atomic steps and kinks, are often higher than those on terrace sites.1−6 Chemical bonds in reactants are also more readily broken when reactants adsorb at steps and kinks.6−10 Although strong chemisorption sometimes impacts activity at low-coordinated sites,11 these sites are usually the real active sites in heterogeneous catalysis.1−3,7,8,12 The higher activity at steps has been documented in a variety of reactions at the solid−gas interfaces such as CO oxidation,12,13 hydrogen− deuterium exchange,14,15 metal oxide formation,16,17 and ammonia synthesis.18 Reactions at the solid−liquid interfaces and electrochemical reactions can also proceed rapidly at step sites.19,20 In addition to enhancing catalytic performance, lowcoordinated sites promote drastic restructuring of the catalysts in response to adsorbed reactant molecules.17,21−25 A large concentration of step sites can be controllably produced on surfaces with high Miller-indices, making these surfaces ideal model systems for studying surface reconstruction of real catalysts. In face-centered cubic (fcc) crystals, surfaces vicinal to (111)-terraces along the [11̅0] zone axis are © 2014 American Chemical Society

Received: June 29, 2014 Accepted: July 18, 2014 Published: July 18, 2014 2626

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Figure 1. Structural changes of stepped Pt(332) and Pt(557) surfaces after introduction of CO and then C2H4. (a,b) STM images of clean Pt(332) and Pt(557) in ultrahigh vacuum (UHV). The insets illustrate their corresponding atomic models. (c,d) Formation of clusters on Pt(332) and Pt(557) under 0.5 Torr of CO. One cluster in each image is highlighted by a white ellipse as an example. (e) Addition of 0.5 Torr of C2H4 causes the disappearance of a substantial amount of clusters on Pt(332). A 570-pm-periodic pattern is visible along the cluster-free steps. (f) The cluster structure on Pt(557) does not change after addition of C2H4. (g) Histograms of the number of clusters on Pt(332) showing the decrease of the cluster concentration by ∼50% upon C2H4 exposure. (h) Histograms of the number of clusters on Pt(557) showing no significant changes upon C2H4 introduction.

Figure 1 shows the different restructuring processes on the two surfaces. In agreement with our prior studies,21 under 0.5 Torr of CO, Pt steps break into nanometer-sized clusters of roughly parallelograms on Pt(332) (Figure 1c) and roughly triangles on Pt(557) (Figure 1d).35 It is noteworthy that clusters form only in ∼40% of the step sites on Pt(332), whereas all the steps on Pt(557) break into clusters. More importantly, after subsequent addition of 0.5 Torr of C2H4 into the high-pressure cell, the density of clusters on Pt(332) decreases substantially (Figure 1e). In contrast, the cluster concentration does not change on Pt(557) (Figure 1f). To quantitatively analyze the changes in the cluster density, the number of Pt clusters was counted in 20 randomly selected (20 × 20 nm2) images under each condition. As shown by the histograms in Figure 1g, the cluster concentration on Pt(332) decreases by almost 50% with further introduction of C2H4. However, the cluster density barely changes on Pt(557), as shown in Figure 1h. Along the cluster-free step edges on Pt(332), a periodic pattern is observed with a distance of 570 pm between the maxima (Figure 1c and Figure S2), corresponding to twice the Pt−Pt distance. This periodicity can be attributed to CO that forms a c(4 × 2) pattern on Pt(111) terraces,36 and its coadsorption with ethylidyne that results in a (2 × 2) structure on Pt(111).37 The detailed structure on terraces is hard to resolve, since the terrace width (∼1.4 nm) is much smaller than the tip radius (>50 nm due to the Au coating). When the C2H4 partial pressure was further raised to 2.5 Torr, more Pt clusters disappear on the Pt(332) surface (Figure S3). The 570-pmperiodic structure remains at the step edges. In order to determine the adsorbate coverage and the chemical state of Pt surfaces, the same gas introduction procedures were performed on the same crystal samples in the AP-XPS apparatus. The C 1s and O 1s spectra are plotted in Figure 2 along with peak deconvolution. Under 0.5 Torr of CO, the spectra of both crystals contain three components from CO

of C2H4 and CO on Pt has previously been studied on Pt surfaces with low Miller-indices and on supported Pt nanoparticle catalysts.28−31 When C2H4 adsorbs first on Pt, CO can still adsorb by displacing π-bonded and di-σ-bonded ethylene, owing to the high mobility and the open unit cell of ethylene adsorbates. CO and ethylidyne (η ≡ C−CH3) coadsorb on the Pt(111) surface by forming a hexagonal pattern, in which CO occupies the top and bridge sites and ethylidyne resides on the 3-fold hollow sites.32 Preadsorbed CO, however, blocks the active sites for ethylene adsorption and hydrogenation. A pre-exposure of Pt(111) to 5 L CO completely impedes C2H4 adsorption, even under 40 Torr of C2H4.28 The influence of the sequence of gas addition on Pt surface structures is a result of adsorption kinetics. In this report, we demonstrate the significant influence of step geometry on reconstruction of stepped Pt surfaces Pt(332) and Pt(557)in the presence of CO−C2H4 gas mixtures in the Torr pressure range at room temperature. We studied the structural changes with two in situ techniques: highpressure scanning tunneling microscopy (HP-STM)33 and ambient-pressure X-ray spectroscopy (AP-XPS).34 Figure 1a,b shows that the steps on the clean Pt(332) and Pt(557) surfaces appear straight in STM images, with the average terrace widths of ∼1.4 nm on both surfaces. The insets illustrate the structures of the (111)- and (100)-steps as well as a surface unit cell (see Figure S1, Supporting Information, for detailed models). While the terraces of both surfaces are in the (111)-plane and six atoms wide, the Pt(332) surface has triangular (111)-steps, and the Pt(557) surface has square (100)-steps. The two surfaces are tilted by ∼10° relative to the (111) plane in opposite directions (+10° and −10°) along the [11̅0] zone axis. Ethylene Adsorption on Pt Surfaces Precovered by CO. Reconstruction of the two stepped Pt surfaces induced by the mixture of CO and C2H4 was studied by first introducing 0.5 Torr of CO and then 0.5 Torr of C2H4 at room temperature. 2627

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After introduction of C2H4, a peak due to gas-phase C2H4 immediately appears at ∼286.0 eV in the C 1s spectra, accompanied by a small feature at ∼284.1 eV from ethylene adsorbates.40,41 The emergence of the ethylene adsorbate peak clearly evidences that C2H4 is able to chemisorb on stepped Pt surfaces, even when Pt surfaces are precovered by a dense layer of CO. The C2H4 coverage estimated from the C 1s peak areas is 0.08 ML on Pt(332) and 0.05 ML on Pt(557). Meanwhile, the CO coverage decreases to 0.80 ML on Pt(332) and 0.88 ML on Pt(557), implying that C2H4 adsorbs by displacing CO molecules. Upon adsorption, C2H4 converts to a variety of species, among which ethylidyne is the most stable at 3-fold sites.37 Ethylidyne could be present on the terraces of both crystals and at the steps of Pt(332). At 4-fold step sites, quad-σ acetylene was reported as the major adsorbate.42 The stronger adsorption of ethylidyne than quad-σ acetylene42 can account for the phenomenon that CO poisoning of ethylene adsorption on Pt(332) is less pronounced than on Pt(557). The drop in CO coverage on Pt(332) from 0.88 to 0.80 ML after addition of C2H4 raises the possibility that the decrease in the cluster concentration on Pt(332) is simply due to the lower CO coverage, because the strong CO−CO repulsion at high CO coverage is the driving factor for the formation of clusters.21 The Pt(332) surface structure at intermediate CO coverage and pressures, particularly around 0.80 ML CO, was examined to address this possibility. The CO coverage on Pt surfaces was first measured as a function of pure CO pressures up to 500 mTorr, as shown by the monotonically increasing trends in Figure 3a. At 7 mTorr of CO, which corresponds to 0.73 ML CO on Pt(332), clusters are observed under STM (Figure 3b) with an average number of 19.6 clusters in a (20 × 20 nm2) image (Figure 3c). The comparison between the histograms in Figure 3c and Figure 1g indicates that, even though the cluster concentration decreases with the CO coverage, the cluster concentration on Pt(332) under 7 mTorr of CO (0.73 ML, 19.6 clusters/image) is still higher than that under the gas mixture of CO and C2H4 (0.80 ML, 14.8 clusters/image). Ethylene adsorption is therefore essentially responsible for the decrease in the concentration of clusters on Pt(332). Another possible reason for the ethylene-induced decrease in the cluster concentration only on Pt(332) is the incomplete cluster coverage on Pt(332) in contrast with the complete coverage on Pt(557). A fraction of unreconstructed steps may be necessary for the initial ethylene adsorption to diminish the clusters. Guided by the trend of the CO coverage on Pt(557)

Figure 2. C 1s and O 1s spectra of Pt(332) and Pt(557) after adding CO and subsequently C2H4. In each panel, the spectra were taken in UHV (bottom), under 0.5 Torr of CO (second), after adding 0.5 Torr of C2H4 (third), and after gas evacuation (top). Blue lines denote CO species, and red lines denote ethylene species. C2H4 is able to adsorb on the stepped Pt surfaces even when a dense layer of CO has precovered Pt. Ethylene adsorbates remain on both Pt surfaces after gas evacuation, whereas CO partially desorbs.

molecules in the gas phase, at top sites, and at bridge sites.38,39 Although an extraordinarily weak feature appears at ∼284 eV in the C 1s spectra as a result of hydrocarbon molecules, this peak contributes to less than 0.01 ML of carbon atoms, indicating negligible hydrocarbon contamination. A high-binding energy shoulder grows in the Pt 4f spectra (Figure S4), corresponding to the low-coordinated Pt atoms at the cluster edges.21 The CO coverage is 0.88 ML on Pt(332) and 0.94 ML on Pt(557), as estimated from O 1s and Pt 4f peak areas (see Table S1 for all the coverage values along with top-to-bridge CO ratios).

Figure 3. (a) Variation of the CO coverage on Pt(332) (black squares) and on Pt(557) (red circles) as a function of the CO pressure. The horizontal dashed line marks the 0.80 ML CO on Pt(332) obtained after adding 0.5 Torr of C2H4 into 0.5 Torr of CO. (b) STM image of Pt(332) under 7 mTorr of pure CO at ∼0.73 ML coverage. (c) Histogram of the number of clusters in many images showing that the concentration of clusters on Pt(332) under 7 mTorr of CO is higher than that after adding 0.5 Torr of C2H4 into 0.5 Torr of CO. 2628

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periodicity of 570 pm (line profile in Figure S6). Given the C2H4 coverage of 0.52 ML on Pt(332) at 0.5 Torr (Table S1), this pattern likely arises from ethylidyne that is known to form a (2 × 1) structure on Pt(111) at 0.50 ML.37 Such periodic structure is not observed on Pt(557). Instead, bright spots of 800 pm in width and 80 pm in height decorate the step edges (Figure S7). These spots could originate from quad-σ acetylene, which was previously identified as the major adsorbate species at the 4-fold step sites.42 Further addition of 0.5 Torr of CO does not lead to significant structural changes on the two stepped Pt surfaces, as displayed in Figure 4c,d. In particular, Pt clusters are not formed even 4 h after CO introduction. The absence of Pt clusters is attributed to the fact that stepped Pt surfaces, when precovered by ethylene, accommodate only ∼0.4 ML CO (Table S1). This coverage is even less than 0.50 ML, where no clusters are formed by exposing the initially clean Pt crystals to 5 × 10−9 Torr of CO.21 CO−CO repulsive forces at such low coverage are probably too low to drive Pt diffusion for cluster formation. Consequently, the stepped structure mostly remains on both Pt surfaces regardless of the step orientation. Figure 5 shows the C 1s and O 1s spectra of the two stepped Pt surfaces recorded under 0.5 Torr of C2H4 and after subsequent addition of 0.5 Torr of CO. Under 0.5 Torr of C2H4, two peaks in C 1s spectra at ∼286.2 eV and ∼284.2 eV originate from gas-phase C2H4 and from ethylene adsorbates,40,41 respectively, in consistence with the assignment in

shown in Figure 3a, we performed experiments starting with 4 mTorr of CO, under which only ∼60% of the steps on Pt(557) break into clusters (Figure S5). After subsequent introduction of 5 mTorr of C2H4, no change in the cluster concentration could be observed, nor could any change be seen when raising the C2H4 partial pressure to 500 mTorr. Ethylene adsorption thus does not reduce the cluster concentration at (100)-steps, regardless of whether the (100)-steps completely break into clusters, which in turn underpin the role of different step configurations in determining distinct restructuring processes. While these results have provided some insight into surface dynamics, it is worth nothing that understanding the whole structural evolution process during reactions requires considering other possible reaction intermediates, particularly atomic carbon, oxygen, and hydrogen at real catalytic temperatures from reactant dissociation. After evacuating the XPS chamber to 10−8 Torr, the coverage of ethylene adsorbates increases from 0.08 to 0.10 ML on Pt(332) and from 0.05 to 0.07 ML on Pt(557). CO partially desorbs, leading to a decrease of CO coverage from 0.80 to 0.49 ML on Pt(332) and from 0.88 to 0.63 ML on Pt(557). The evolution of the shape of the O 1s spectra suggests that most bridge-CO desorbs on Pt(332), whereas on Pt(557) it is topCO that mainly desorbs. The partial CO desorption upon gas evacuation is due to the small differential heat of adsorption at high coverage.43 CO Adsorption on Pt Surfaces Precovered by Ethylene. The different structural changes of stepped Pt surfaces were also studied by reversing the sequence of CO and C2H4 exposures. Figure 4a,b shows that the stepped structure, along with step heights and terrace widths, is preserved on both stepped Pt surfaces under 0.5 Torr of C2H4. Short periodic patterns are resolved in part of the steps near the edges on Pt(332) with a

Figure 5. C 1s and O 1s spectra of Pt(332) and Pt(557) under C2H4 and after addition of CO. The spectra in each panel were recorded under UHV (bottom), under 0.5 Torr of C2H4 (middle), and after further introduction of 0.5 Torr of CO (top). Blue and red lines denote CO and ethylene species, respectively. Stepped Pt surfaces precovered by C2H4 can admit ∼0.4 ML CO, which only displaces less than 0.04 ML of ethylene adsorbates.

Figure 4. STM images of Pt(332) and Pt(557) recorded under 0.5 Torr of C2H4 and after subsequent addition of 0.5 Torr of CO. (a) Periodic patterns are observed along the steps on Pt(332). (b) Bright spots are present at step edges on Pt(557) without clear periodicity. (c,d) Further introduction of CO does not lead to any changes in the stepped structure, especially no cluster formation on either Pt surface. 2629

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grants. C.B. and T.V.d.B. thank the Wallonia-Brussels Federation (Action de Recherches Concertées No. AUWB 2010-2015/ULB15). L.L. acknowledges the support from the Humboldt Foundation of Germany. B.E. acknowledges support from the Early Postdoctoral Mobility fellowship of the Swiss National Research Funds. B.M. and Z.L. thank the support of National Natural Science Foundation of China under Contract No. 11227902. We acknowledge Prof. Jeong Young Park for his help on coating STM tips with Au.

Figure 2. The absence of O 1s peaks under 0.5 Torr of C2H4 indicates that the crystals are free from CO contamination from background gases. After introducing 0.5 Torr of CO, top-CO and bridge-CO peaks grow in the O 1s spectra at ∼532.8 and ∼531.3 eV. In addition, the gas-phase C2H4 peak downshifts by 0.5 eV as a result of the sample work function increase.44 Charge transfer from Pt atoms to CO adsorbates through πback-donation, which was corroborated by the red-shift of the C−O stretch in infrared spectroscopy,45 leads to the increase in the Pt work function.38 Replacement of ethylene adsorbates by CO is minimal, since the ethylene adsorbate peaks at ∼284.2 eV retain ∼90% of their initial intensity on both surfaces. Most CO occupies top sites on Pt(332) precovered by ethylene, whereas bridge sites are preferred on Pt(557). In conclusion, we have investigated the influence of step geometry on the reconstruction of Pt surfaces that have the same terrace orientation and width but different step geometries. The two model catalyst surfaces, Pt(332) and Pt(557), were studied under mixtures of CO and C2H4 in the Torr pressure range at room temperature. We observed that the concentration of clusters induced by 0.5 Torr of CO decreases by ∼50% on Pt(332) upon addition of 0.5 Torr of C2H4. In contrast, ethylene adsorption barely changes the cluster concentration on the Pt(557) surface. This phenomenon is neither due to the small decrease in CO coverage upon ethylene adsorption nor the initial incomplete cluster coverage, but rather a consequence of the peculiar structural (electronic and chemical) properties of the different step orientations of the two surfaces. When 0.5 Torr of C2H4 is introduced first, a periodic structure due to ethylidyne is visible along the step edges on Pt(332) but not on Pt(557). The step heights and terrace widths are preserved on both surfaces. Subsequent introduction of 0.5 Torr of CO does not create Pt clusters in the time scale of several hours because of the low CO coverage.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental methods, ball models of Pt(332) and Pt(557) surfaces, a table summarizing the coverage and top-tobridge CO ratio values, and additional STM images and topographic profiles. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.S.). *E-mail: [email protected] (G.A.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. C.B. gratefully thanks the Fonds de la Recherche Scientifique (F.R.S.-FNRS) for the financial support (Ph.D. grant), as well as Wallonie Bruxelles International WBI (SUB/2011/17971) and the Fédération Wallonie-Bruxelles (BV12-15) for the travel 2630

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The Journal of Physical Chemistry Letters

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