Stability and Reactivity of Graphene-Templated Nanoclusters - The

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Stability and Reactivity of Graphene-templated Nanoclusters Timm Gerber, Elin Grånäs, Ulrike A. Schröder, Patrick Stratmann, Karina Schulte, Jesper N. Andersen, Jan Knudsen, and Thomas Michely J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07828 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 26, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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.

Page 1 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Stability and Reactivity of Graphene-Templated Nanoclusters Timm Gerber,∗,†,k Elin Gr˚ anäs,‡,⊥ Ulrike A. Schröder,† Patrick Stratmann,† Karina Schulte,¶ Jesper N. Andersen,§,¶ Jan Knudsen,§,¶ and Thomas Michely† †II. Physikalisches Institut, Universität zu Köln, Germany ‡Division of Synchrotron Radiation Research, Lund University, Sweden ¶MAX-IV Laboratory, Lund University, Sweden §Division of Synchrotron Radiation Research, Lund University , Sweden kPresent address: Peter Gr¨ unberg Institut (PGI-6), Forschungszentrum J¨ ulich GmbH,Germany ⊥Present address: DESY NanoLab, Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany E-mail: [email protected] Phone: +49 (0) 2461 61 9369

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Exploiting the versatility of the graphene/Ir(111) template we performed a comparative study on arrays of various metallic nanoclusters. Using scanning tunneling microscopy and synchrotron-based high-resolution X-ray photoelectron spectroscopy, we investigated the size dependent stability of Pt, Ir, and Au nanoclusters against sintering upon gas exposure. These clusters display gross differences in their gas adsorption properties, as well as in their activity for the CO oxidation reaction.

Introduction The corrugated surface of graphene (Gr) supported by a lattice-mismatched metal substrate provides a periodical template for self-assembly of metal nanoclusters (NCs). Due to their narrow size distribution, equal spacing, high density, and the inertness of the substrate, such NC arrays on graphene are attractive as model systems for heterogeneous industrial catalysis, where metal particles dispersed on porous supports are widely used. 1–3 The atomic structure of such metal particles and how it relates to their interaction with gases is usually inaccessible due to the complexity of industrial catalysts. However, studies on model systems are able to relate catalytic reactivity to the size and shape of particles, as well as to their spatial distribution and environment. 4,5 Well-defined moiré patterns of graphene with metal substrates have been used to grow lattices of equally sized and spaced NCs of, for example, Pt, Ir, Pd, Re, Ru, and a variety of metallic and bimetallic NCs on graphene/Ru(0001) 6–13 , graphene/Rh(111) 14–17 , and Ir(111) 18–21 . Other metals, such as Fe and Au, do not form well-ordered NC lattices at room temperature. However, such metals can still be grown in well-ordered arrays at lower temperatures, or when seeded by deposition of a metal forming stable arrays at room temperature 19,20,22 . The perfection of the NC lattice strongly depends not only on the metal of the NCs, but also on the metal supporting the graphene film. 23,24 For the previously mentioned substrates, NC lattices on graphene on Ir(111) show exceptional order and narrow size 2 ACS Paragon Plus Environment

Page 2 of 33

Page 3 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


distribution that is tunable from roughly 4 to 130 atoms per cluster 18 , which makes them particularly attractive laboratories for fundamental studies of nanocatalysis. 25–27 Further, graphene on Ir(111) can be grown in a single orientation on a mm scale 28 , giving large areas for the NC arrays, making it an even more attractive support. 19 A prerequisite to such nanocatalysis studies is the investigation of the stability of the NC/support system at elevated temperatures and in the presence of reactants (i.e. under reaction conditions). Sintering, a decrease in particle number and corresponding increase of particle size, is a process that deactivates catalysts 29–31 . In previous work, we have shown that small Pt NCs on Gr/Ir(111) are very sensitive to CO adsorption and sinter through the process of Smoluchowski ripening 32 . NCs larger than a critical cluster size of 10 atoms were found to be immobile upon CO adsorption. Two other molecules that play an important role in catalysis and, thus, also as probe molecules in surface science are H2 and O2 . Together these three molecules cover different adsorption forms [non-dissociative (CO) and dissociative (O2 33,34 and H2 35,36 )], different adsorption sites (on-top and hollow 37–45 ), as well as different adsorption strengths 46,47 . In this article, we use a combination of scanning tunneling microscopy (STM) and synchrotron-based high-resolution X-ray photoelectron spectroscopy (XPS) to extend previous work on the stability of Pt NCs on graphene/Ir(111) upon CO exposure, and now also include O2 and H2 as probe molecules. We test the stability of Pt NCs with respect to different gases, as well as the stability of Ir and Au NCs upon CO exposure. Lastly, Pt and Au NCs are compared for their ability to catalyze the oxidation of CO. The unusual ability of Au NCs to oxidize CO 48 at temperatures as low as 120 - 140 K 49–51 is a particularly interesting example for nanocatalysis. Three decades after the initial observation by Haruta et al. the exact mechanisms are still a controversial issue. The size of the Au particles 52–55 , their charge state 50,51 , as well as their ability to adsorb oxygen 56 play a role. In addition, the relevance of the support is actively debated 49,57–61 . In order to investigate the properties of the Au NCs themselves, decoupled from the support, studies



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 33

with inert support materials are desired e. g. Au NCs on hexagonal boron nitride (h-BN) 62 or Au NCs supported by graphene, as employed in the present study.

Experimental Details The Ir(111) crystal was cleaned by cycles of Ar-ion sputtering at room temperature, followed by annealing in O2 at 1200 K (1·10−7 mbar, 600 s), and lastly annealing in ultra-high vacuum to above 1400 K. The temperature was measured with a chromel-alumel thermocouple spotwelded to the side of the crystal. A full monolayer (ML) of graphene was grown by room temperature ethylene (C2 H4 ) exposure until saturation at < 350 K followed by thermal decomposition at 1400 K, and subsequent exposure to 1 · 10−7 mbar C2 H4 for 1200 s at 1170 K. This results in a well-oriented, closed, defect-free graphene layer, as confirmed by low-energy electron diffraction (LEED) and XPS. A submonolayer graphene film with 0.2 ML coverage was grown by one cycle of ethylene exposure until saturation followed by thermal decomposition, as described above. NC growth on graphene/Ir(111) was conducted through evaporation of high-purity metal (Pt, Ir, Au) using a commercial E-beam evaporator, resulting in typical deposition rates of 3×10−2 ML/s. The evaporator was calibrated in the STM chamber by determining the fractional area of monolayer islands resulting from deposition onto clean Ir(111). Coverages are defined so that 1 ML corresponds to the surface atomic density of Ir(111). During metal deposition the sample was kept at 300 K, if nothing else is specified in the text. The pressure during evaporation remained in the low 10−10 mbar range. The average NC size sav is calculated from the deposited amount θ, the fraction n of moiré cells occupied by a NC (the filling factor), and the number Am = 87 of Ir atoms in one moiré unit cell via sav =

θ·Am . n

The XPS experiments were performed at beamline I311 63 at the MAX IV Laboratory on a system with a base pressure better than 1 · 10−10 mbar. All XP-spectra were measured in normal emission with photon energies of 190 eV for Pt 4f , Ir 4f and Au 4f , 390 eV for


Page 5 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C 1s, and 650 eV for O 1s, if not stated otherwise. The total energy resolution of the light and analyzer is better than 45, 60, and 200 meV for the respective core levels. Ir 4f spectra ˘ were fitted with a convolution of Doniach-Sunji´ c and Gaussian functions assuming a linear background. Regarding the analysis of the C 1s spectra: Upon cluster growth the main peak of the graphene shifts towards higher binding energy and a sp3 rehybridization shoulder appears 64 . The shift is assigned to doping by the clusters. In order to estimate the peak area of the rehybridization shoulder, a reference spectrum of the pristine graphene is fitted to the position of the main peak and the spectra are normalized to the peak intensity. The difference between the shifted C 1s spectrum of pristine graphene and that with clusters gives the intensity of the shoulder component. 32 STM measurements were carried out in a variable-temperature STM system in Cologne. STM imaging was conducted at room temperature. STM topographs were postprocessed using the WSxM software 65 . Tunneling resistances of ≈ 2 × 1010 Ω are used to avoid tip-NC interaction.

Results and Discussion Growth of Pt NCs As a prerequisite for designing and understanding gas adsorption experiments, the growth of Pt NCs on Ir(111) and their structure was investigated as a function of Pt coverage. Therefore, the amount of Pt deposited onto a full monolayer of Gr/Ir(111) was varied between 0.05 ML and 1 ML corresponding to an average NC size, sav , ranging from 5 to 174 atoms. Selected STM topographs from the Pt NC growth series are shown in Fig. 1a-c. As can be seen in Fig. 1a the NCs are initially confined to one moiré unit cell. After deposition of 0.75 ML Pt [Fig. 1b], very few elongated NCs can be observed which result from sintering of two neighboring NCs. Our STM results indicate that a coverage of 0.75 ML marks the onset of 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 6 of 33

(d) P t C ove rag e 0 .0 5 M L 0 .1 0 M L 0 .2 0 M L 0 .4 4 M L 0 .7 5 M L 1 .0 0 M L

1 .0 0 .8 0 .6 0 .4

(b)

0 .2 0 .0

n0

n1

n2

n3

n4

filling factors (e)

(c)

Figure 1: STM topographs of Pt NC arrays on Gr/Ir(111). The Pt coverages are (a) 0.10 ML, (b) 0.75 ML, (c) 1 ML. Image size is always 375 ˚ A × 375 ˚ A. (d) Histogram of NC heights, where ni is the fraction of moiré cells occupied by an i-layered cluster, and i = 0 corresponds to an empty moiré cell. (e) Fraction of occupied moiré cells, n, and average NC height, h, against deposited amount of Pt.


Page 7 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sintering for Pt NCs. At 1 ML Pt coverage, Fig. 1c, the majority of NCs has sintered and the resulting particles extend over several moiré cells. As the Pt NCs grow in a layer-bylayer mode 19,26 , the distinct height levels in the topographs correspond to one-, two- and three-layered NCs. Four-layered NCs are only observed occasionally and only for the highest coverages (0.75 and 1 ML). Fig. 1d shows a histogram of the NC heights for all Pt coverages. The average height of the NCs as well as the fraction of occupied moiré cells n is presented in Fig. 1e. Here, it can be seen that there are three regimes of NC growth: a nucleation regime up to ≈ 0.2ML coverage characterized by a steep increase of n and by the presence of onelayered NCs. For coverages between 0.20 ML and 0.75 ML, n increases only slowly from 0.8 to 1. In this growth regime the NCs evolve to almost exclusively two-layered forms. For higher coverages we observe NC sintering as indicated by a strong reduction of n and a noticeably increased number of three-layered NCs. Compared to the growth of Ir NCs deposited on Gr/Ir(111) at 350 K 18 , the Pt NCs tend to be flatter in the growth regime. They enlarge more laterally and less in height. Therefore, the onset of sintering occurs already at 0.75 ML for Pt NCs, unlike for Ir NCs which are confined to single moiré cells up to 1.5 ML. 18 The reason for the flatter shape of Pt particles is not obvious because their shape is influenced by several factors. The ratio of metal-carbon bond energy and metal cohesive energy is certainly important. While the former is similar for both metals (6.54 eV for Ir, 6.32 eV for Pt 66 ), the latter is larger for Ir (6.94 eV, 5.84 eV for Pt 67 ) supporting the tendency for Ir cluster to become more 3-dimensional. Surface free energy of the different cluster facets might also play a role, as flat clusters minimizes their fraction of (100) facets 26 . This effect would only matter, if the ratio of surface energies γ111 /γ100 were significantly different for Ir and Pt 68 . However, only subtle differences have been observed 69,70 . Lastly, as the growth temperatures were slightly different (350 K for Ir, 300 K for Pt), growth kinetics may also influence the shape of the particles.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Pt NCs Exposed to Gases After having characterized the Pt NCs as grown, we turn to studying NCs upon exposure to gases. Firstly, we analyze the reaction of the smallest clusters, created by deposition of 0.05 ML Pt, since small, weakly bound clusters can be expected to be most susceptible to changes in consequence of gas exposure. (a)

(b)

(c)

(d)

(e)

(f)

Figure 2: 0.05 ML Pt (a, c, e) before and (b, d, f) after gas exposure at the same sample location. (b) CO exposure (300 K, 10L, 1 · 10−8 mbar). (d) O2 exposure (300 K, 80 L, 1 · 10−7 mbar). (f) H2 exposure (90 K, 100 L, 1 · 10−7 mbar). Image size always 300 ˚ A × 300 ˚ A. Fig. 2 displays STM topographs of 0.05 ML Pt NC arrays on Gr/Ir(111) before (a, c, e) and after exposure to CO, O2 , and H2 at the same sample location (b, d, and f, respectively). The instability of small Pt NCs upon CO exposure is clearly seen in Figs. 2a and b. After exposure the NCs have drastically decreased in number and increased in size. In contrast, exposure to O2 , even for gas doses of the order of 100 L, does hardly induce NC mobility, as can be seen when comparing Fig. 2c and d. There is just a single event of a NC disappearance, indicated by a circle in Fig. 2c and d. However, upon oxygen adsorption about 1/3 of the 8 ACS Paragon Plus Environment

Page 8 of 33

Page 9 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NCs increase their apparent height. Based on the dosed amount of oxygen, we must assume that oxygen is adsorbed to all clusters, such that the apparent height increase cannot be due to O adsorption on only some clusters. As we invariably found that a cluster does not decompose into its constituents due to adsorption, but if at all moves as an entity, the absence of sintering implies shape transformations without mass transport between different clusters. We therefore propose here that some Pt clusters transform their shape to a more three-dimensional one, thereby increasing the amount of favorable adsorption sites at step edges. Similar shape changes were observed after CO exposure for larger, immobile Pt clusters after CO exposure (see ref. 32 and supporting information). H2 exposure was conducted at 90 K instead of 300 K because H desorption already starts at 200 K. 35,43 As obvious from a comparison of Fig. 2e and f, after H2 exposure at 90 K the NC lattice is unchanged except for NC height changes. We note that at 90 K NC mobility is reduced compared to 300 K, but not completely frozen. This was checked by exposing 0.05 ML Pt to CO at 90 K, which resulted in a reduction of n from 0.66 to 0.51 (-23%, see supporting information), compared to -45% at 300 K. The effect of CO at low temperature is thus smaller, but far from negligible. Contrarily, the Pt NCs are completely immobile upon H2 exposure at 90 K. The observed height changes are attributed to adsorption induced cluster shape transformations, as discussed above for O2 exposure. Thus, small Pt NCs are stable in the presence of O2 and H2 , unlike in the presence of CO where they are unstable. The destabilization upon CO adsorption is explained in terms of unbinding of the NC from the graphene substrate i.e. a weakening of the NC-graphene binding 32 . Apparently, this effect is much weaker for O2 and H2 adsorption. In order to further elucidate the behavior of Pt NCs exposed to gases we turned to XPS measurements of the Pt 4f core level. For these experiments a coverage of 0.20 ML is chosen as a compromise between cluster stability and a high fraction of cluster surface atoms. As discussed above, this coverage is at the threshold for cluster stability with a filling factor close to unity. Further, the prototypical cluster is two-layered with sav = 19 atoms,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

thus containing almost exclusively surface atoms which makes it perfect for adsorption and reactivity studies. In Fig. 3 the Pt 4f7/2 spectra of Pt NC arrays before (black) and after exposure to CO, O2 , and H2 are shown. As for the STM measurements, the CO and O2 exposures were performed at room temperature, while H2 was dosed at low temperature. The XPS measurements were performed at 140 K, compared to 90 K for STM. This temperature difference is not expected to influence the results because hydrogen desorption is absent for both temperatures. 35,43 . P t 4 f c le a n

C O + 0 .5 8 e V

O 2

+ 0 .3 6 e V

H 2

+ 0 .3 9 e V

7 3

7 2

7 1

7 0

6 9

B in d in g E n e r g y ( e V )

Figure 3: Pt 4f XP-spectra of 0.2 ML Pt on Gr/Ir(111) before (black) and after exposure to CO (green, 300 K, 10 L, 1 · 10−7 mbar), O2 (red, 300 K, 10 L, 1 · 10−7 mbar) and H2 (orange, 140 K, 1000 L, 5·10−6 mbar). The shift of the center of gravity of the peaks upon gas exposure is indicated. The clean particles show a relatively broad and asymmetric peak at ≈ 71 eV whose shape varies slightly from experiment to experiment. These peaks consist of several components, corresponding to Pt-atoms on different positions in the NC and with different coordination 64 . We note that the coordination number of the atoms crucially depends on the size of the clusters and thus on the Pt coverage. For the lowest coverages, almost all cluster atoms are highly under-coordinated. Thus, the differences in shapes of the clean Pt 4f7/2 peak can be attributed to small variations in the Pt coverage, or slight CO contamination from 10 ACS Paragon Plus Environment

Page 10 of 33

Page 11 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the background pressure since small amounts of CO unavoidably will adsorb during transfer from the preparation to the analysis chamber. Upon gas exposure the peaks shift to higher binding energies in all three cases. Analyzing the shift of the center of gravity for the different gas exposures we find that the shift for CO adsorption (+0.58 eV) is higher than for O2 (+0.36 eV) and H2 (+0.39 eV). In the following, we compare these shifts to those reported for adsorption on Pt single crystal surfaces. For Pt(111), Björneholm et al. 40 found Pt 4f shifts of +1.41 eV and +0.73 eV for CO adsorption in on-top and bridge geometry, respectively. The shift of +0.58 eV we observe matches better with the value for bridge adsorption. This is counter-intuitive, because it is established that CO prefers the on-top configuration 41 , and this configuration was actually confirmed by our C 1s and O 1s spectra 32 . Concerning O2 exposure Björneholm et al. find +0.62 eV for O adsorption in hollow sites. Our O 1s measurements (see below) indicate that this adsorption site is indeed occupied, but we observe a much smaller Pt 4f CLS of only +0.36 eV. Yet again, for H adsorption the observed CLS is smaller than those reported for H adsorption on Pt surfaces 71,72 . Thus, the Pt 4f CLS upon gas adsorption is always smaller for our NC as compared to Pt single crystal surfaces and seems not to be very suited for the correct determination of the adsorption site. Instead, we will rely on the information obtained from the C 1s and O 1s core spectra. After the investigation of the adsorbate-NC interaction we address the NC-graphene interaction. The binding of NCs to Gr/Ir(111) is understood as a cooperative mechanism involving both the graphene layer and the Ir substrate. Below the NC the graphene rehybridizes from a planar sp2 to a tetragonal sp3 -configuration, so that the carbon atoms alternately bind to the NC atoms and to the Ir(111) surface (compare insets in Fig. 4) 64,73 . Fig. 4a displays the Ir 4f and C 1s region for pristine Gr/Ir. In the Ir 4f region we observe two peaks which relate to bulk and surface Ir atoms, while the C 1s region consists of a single peak. Fig. 4b displays the same core levels for 0.75 ML Pt clusters. This coverage marks the upper limit of the growth regime i.e. it yields the largest clusters which are still



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 33

(a) Gr / Ir(111) C 1s

Ir 4f

sp2 surface

bulk

(b) Gr / Ir(111) +0.75 ML Pt

bulk

sp2 surface interface

sp3

(c) Gr / Ir(111) +0.75 ML Pt +CO bulk

sp2 surface interface

62

61

sp3

285 60 Binding Energy (eV)

284

Figure 4: Ir 4f7/2 and C 1s core level spectra for (a) the Ir(111) surface covered with 1 ML Gr, (b) after deposition of 0.75 ML Pt on Gr, and (c) after exposure to CO (300 K, 10 L, 1 · 10−7 mbar). Experimental data shown as dots, fits as solid red lines, and the filled curves represent fit components. Ir 4f : dark gray = Ir bulk, light gray = Ir surface, blue = Ir interface. C 1s: light gray = Csp2 , blue = Csp3 . The C 1s peak shape of clean Gr is fitted to the data in order to estimate the intensity of the Csp3 shoulder (see experimental section). The shift of the C 1s main peak due to doping by the clusters is a non-linear function of the amount of deposited material (see supporting information). For the particular coverage shown here, the shifts are close to zero. Schematic cross sectional ball model insets illustrate in (a) graphene adsorbed to Ir(111), in (b) changes in binding caused by Pt cluster adsorption, and in (c) weakening of cluster binding by CO adsorption. Ir surface atoms chemically bound to Gr and rehybridized C atoms are shaded in blue.


Page 13 of 33

regularly arranged and, thus, maximized the XPS signal of the NC-graphene interaction. Comparing a and b the twofold XPS signature of the binding mechanism is seen. In the Ir 4f region spectral weight is shifted from the Ir surface component to an Ir interface component, which we assign to Ir surface atoms binding to those carbon atoms in the graphene layer that are pinned down by the Pt NC (compare insets in Fig. 4) 21,73 . Simultaneously the C 1s peak develops a shoulder due to sp3 hybridized atoms (the so-called rehybridization shoulder 64 ). The intensity of both components (Irinterface and Csp3 ) is reduced upon CO adsorption indicating the weakened binding of the NCs to the graphene layer. R e d u c tio n o f S h o u ld e r In te n s ity ( % )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 0 0

C O O 2

8 0

H 2

6 0 4 0 2 0 0 0 .0

0 .2

0 .4 0 .6 P t C o v e ra g e (M L )

0 .8

Figure 5: Reduction of the shoulder intensity after CO (green squares, 300 K, 10 L, 1 · 10−7 mbar), O2 (red triangles, 300 K, 10 L, 1 · 10−7 mbar) and H2 exposure (orange diamond, 140 K, 1000 L, 5 · 10−6 mbar). Dashed lines are guides to the eye. Using the intensity of the rehybridization shoulder to track the degree of unbinding of the Pt NCs upon exposure to the three different gases we find that the influence of CO is remarkably different to that of O2 and H2 . As can be seen in Fig. 5, the shoulder intensity is reduced by 20% to 90% upon CO adsorption, depending on Pt coverage. We note that the effect of NC diffusion and coalescence upon CO exposure only contributes to the reduction of shoulder intensity for the smallest Pt coverages (< 0.2 ML), because higher coverages yield stable cluster lattices. While the reduction of shoulder intensity is at least 20% for CO exposure, its intensity is only reduced by less than 10% for O2 and H2 exposure. As Pt NCs of any size are stable upon O2 exposure the reduction does, in this case, not depend on Pt coverage. 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

To summarize the XPS results, we found that the effect of unbinding of the NCs from the graphene, as seen in the C 1s core level, is the strongest for CO adsorption and almost absent for O2 and H2 . Both observations agree fully with the STM measurements which show that only CO adsorption leads to diffusion and coalescence of small Pt NCs, whereas they are stable upon O2 and H2 exposure. The reason why CO adsorption weakens the binding of Pt clusters to the Gr substrate so much more than H or O adsorption is not evident. H is bound much weaker to Pt than CO and O, but the latter two display similar heats of adsorption on Pt(111) surfaces 74,75 . Also DFT calculations applied to this problem did not yield conclusive insight 32 . One obvious difference between CO and O is the preferred adsorption site. While the latter prefers hollow sites 41,76 , CO prefers one-fold coordinated atop sites 37,39 . As Pt also binds atop to the C atoms of the graphene layer, one might argue that CO and C compete for the same Pt electrons, whereby CO adsorption weakens the Pt-C bonds. However, as discussed in ref. 32 also this simple reasoning appears not to be consistent with the details of the CO bond formation to Pt. The strain state of the cluster might also play a role. Recently, it was shown that Pt NCs on Gr/Ir(111) are compressed when CO adsorbs 25 . This compressive strain on the clusters might influence the Pt-Gr bonds and, thus, the NC’s stability. Conceivably, different adsorbates yield different strain states, resulting in differences in cluster stability. However, further studies are necessary to correlate the strain state for different adsorbates with cluster stability.

Ir and Au NCs Exposed to CO In the previous section we have shown that Pt NCs interact the strongest with CO, compared to O2 and H2 . In this section we use CO to probe the stability of also Ir and Au NCs (see Fig. 6). Of these two materials, Ir is a platinum group metal with similar chemical properties to Pt and therefore the Ir NCs are expected to behave similarly. On the other hand, the 14 ACS Paragon Plus Environment

Page 14 of 33

Page 15 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


chemical properties of Au differ from those of the platinum group metals 77,78 and a quite different response of Au NCs to gas exposure is expected. Fig. 6 a and b show the same sample location before and after an array of 0.05 ML Ir NCs was exposed to 5 L CO at room temperature. Before CO exposure we observe a partially filled NC array (n = 0.74) as expected for the nucleation regime 18 . The STM images show that CO exposure causes substantial sintering of small Ir NCs, the filling factor n is reduced from 0.74 to 0.40. This is accompanied by a reduction of the rehybridization shoulder in the C 1s XP-spectra (-26% for 0.08 ML Ir, see Fig. 7a), signaling that unbinding takes place. For Pt NCs of a similar size n is reduced from 0.82 to 0.45, and the C 1s rehybridization shoulder by 62% (Fig. 7b). As expected, the overall response of Ir NCs to CO exposure is similar to that of Pt NCs, but less pronounced. (a) (c)

(b)

(c)

(d)

Figure 6: 0.05 ML Ir (a) before and (b) after CO exposure (300 K, 5L, 1 · 10−9 mbar), same sample location. 0.05 ML Au (c) before and (d) after CO exposure (90 K, 18L, 1·10−8 mbar), same sample location. Image size is always 300 ˚ A × 300 ˚ A. Turning to Au NCs we begin with noting that the growth behavior of Au NCs on graphene differs from that of Pt and Ir 19 : Au deposition at room temperature does not yield wellordered NC arrays due to low Au-C bond strength. Therefore, the deposition is performed at 90 K. However, also at this lower temperature the Au NC array is less well-ordered than those of Ir- and Pt NCs. This can be seen in Figure 6 c, which depicts 0.05 ML Au deposited 15 ACS Paragon Plus Environment


onto Gr/Ir(111) at 90 K. Fig. 6 d shows the same sample location as in c after exposure to 18 L CO at 90 K. At this temperature we expect CO to adsorb on the Au NCs, as it is well below the main CO desorption temperature for similarly sized Au NCs on h-BN 62 . Comparing the two images, it is obvious that the Au NCs are only slightly affected by CO exposure. We observe height changes indicating that also Au NCs are subject to shape transformations upon CO exposure. However, only a few sintering events occur (marked by circles). As discussed above, temperature has only little influence on the NCs’ behavior upon gas exposure. Thus, we conclude that, contrary to Pt NCs, Au NCs are close to being immobile upon CO exposure at 90 K. C 1 s 0 .0 8 M L Ir + C O

s p 2

Ir Ir+ C O

0 (c )

2 8 5 C 1 s 0 .1 M L A u + C O

(b ) 1

P t P t+ C O

2 8 5 (d ) 1

s p 2

A u

s p 2

0

2 8 4

A u + C O

C 1 s 0 .1 M L P t + C O

n o r m . In te n s ity ( a .u .)

(a ) 1

2 8 4

A u 4 f 0 .1 M L A u + C O

n o r m . In te n s ity ( a .u .)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 33

A u

A u -C O

A u -A u -C O 2 8 5 2 8 4 B in d in g E n e r g y ( e V )

0

8 6

8 5 8 4 B in d in g E n e r g y ( e V )

8 3

Figure 7: (a-c) C 1s spectra before (blue, yellow) and after CO exposure (green, 10L, 1 · 10−7 mbar, 300 K for Ir and Pt, 140 K for Au). Clean graphene reference spectra are shown in gray. (a) 0.08 ML Ir. (b) 0.1 ML Pt. (c) 0.1 ML Au. The C 1s spectra with NCs are shifted to correct for doping by the particles (see supp. info.). (d) Au 4f 7/2 corresponding to (c).

XPS measurements on 0.1 ML Au NCs show that the NC-induced rehybridization shoulder in C 1s has only half the intensity as comparable Pt- and Ir-coverages [see figure 7a to c]. The lower shoulder intensity implies that the interaction between the Au NCs and graphene 16 ACS Paragon Plus Environment

Page 17 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


is much weaker than that of Pt- and Ir NCs. This is in line with the STM observations of Au NCs on Gr/Ir(111), that shows both less well-ordered lattices and lower thermal stability 19 . The Au 4f spectrum of the 0.1 ML Au NCs in Fig. 7d shows a main peak located at 83.7 eV, with a small shoulder located at 1.2 eV higher binding energy. The position of the main component agrees with that of the Au(111) surface. 79 We tentatively assign the shoulder to a small amount of CO contamination, adsorbed from the background. Upon CO exposure, the Au 4f peak is split into two components: one is shifted by 0.2 eV compared to the main component of the clean NCs, and one is located at the same position as the previous shoulder i. e. 1.2 eV higher than the clean main peak. Weststrate et al. 79 found similar shifts in a study of Au NCs on CeO2 : the strongly shifted component was assigned to Au-atoms with an adsorbed CO-molecule (Au-CO), while the less shifted component was assigned to Au-atoms neighboring an Au-atom with CO adsorbed (Au-AuCO). By comparing the peak area of the Au-surface components before CO exposure to the CO-induced component after CO exposure Weststrate et al. find that for small Au NCs (< 0.5 ML) approximately 60% of the surface gold atoms have adsorbed CO on top. Comparing the peak areas of the surface and CO-induced components for the graphene supported Au NCs studied here yields, similar to Au NCs on CeO2 , that 55% of all Au atoms has an adsorbed CO on top. Further, the O 1s region also indicates the presence of adsorbed CO (see below). The Au 4f and O 1s core levels thus confirm the adsorption of CO, while the intensity of the C 1s rehybridization shoulder is not reduced, indicating that CO adsorption has a weak effect on the Au-Gr interaction. The XPS measurements thus imply that CO molecules are adsorbed onto the Au NCs, with more than half of the atoms being directly bound to CO. However, as seen in both XPS and STM, the adsorption does not destabilize the Au NCs. This finding is in stark contrast to Pt and Ir NCs where the NCs are destabilized upon CO exposure, as observed both by sintering events in STM and the reduced intensity of the C 1s shoulder in XPS. As mentioned above, the analogous behavior of the Pt and Ir NCs is not surprising



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

considering the similar chemical properties of the metals. As an example, let us compare the chemisorption of CO on the (111) surfaces of these metals: On both surfaces the CO molecule adsorbs preferentially in atop positions. 41,80 Also, the initial adsorption energy is almost the same: -1.43 eV/molecule on Pt(111) 81 and -1.55 eV/molecule on Ir(111) 80 , respectively. The interaction of CO with the Au(111) surface is rather different compared to Ir(111) and Pt(111): The adsorption energy is slightly positive (+0.15 eV/molecule 53 ) which means that CO does not adsorb on this surface except for step and kink sites. At kinks, the adsorption energy is -0.3 eV/molecule which is almost one order of magnitude smaller than for CO at kinks of a Pt surface (-2.1 eV/molecule 39 ). We therefore expect the CO molecule to interact much weaker with a Au NC than with an Ir- or Pt NC. Even though the Au NC is bound only weakly to the graphene layer, the interaction of CO with the particle is not strong enough to destabilize it.

Stability of NCs at Atmospheric Pressure In the above sections we have seen that CO exposure has a strong impact on the stability of Ir- and Pt NCs, whereas exposure to gases like H2 and O2 is relatively harmless to the NC lattice. If clusters on graphene shall be used as a model system for heterogeneous catalysis, it is desired that they withstand gas exposure pressures that are realistic for relevant chemical reactions (i.e. p ≥ 1 bar). As such a test was not possible within the ultrahigh vacuum environment, the clusters were ex-situ exposed to air at atmospheric pressure. For this test we chose an array of large Ir NCs (θ = 0.45 ML, sav = 39 atoms, thus well above the critical cluster size) on a partially graphene-covered Ir surface (0.2 ML Gr). We emphasize that this experiment also tests the stability of the NC/Gr/Ir(111) system with respect to intercalation 82–84 . Fig. 8a shows the Ir NC array on a graphene flake which extends over two terraces of the Ir(111) substrate, separated by a monatomic step edge. Fig. 8b shows the sample after having been exposed to air at atmospheric pressure for a short time. Imaging was 18 ACS Paragon Plus Environment

Page 18 of 33

Page 19 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


012

032

042

Figure 8: 0.45 ML Ir (a) after growth, (b) after short exposure to air (1013 bar), and (c) after weeks exposed to air and cleaning with isopropyl alcohol. (a) and (c) are imaged in UHV, (b) in air. Image size is always 400 ˚ A × 400 ˚ A. performed under ambient conditions resulting in the poorer image quality compared to a, where imaging had been performed in ultra-high vacuum. Still the NC lattice is well visible, indicating that the NCs are stable when exposed to air even at atmospheric pressure. Fig. 8c shows the sample after having been kept under ambient conditions for several weeks before transferring it back to UHV conditions. Prior to transfer, the sample was also cleaned by rinsing it in isopropyl alcohol. Despite the poor image quality resulting from an adsorbate layer, the NC lattice is still recognizable. This proves that medium-sized Ir NCs are stable even upon long exposure to ambient conditions. Based on the reported similarities between Ir- and Pt NCs, we expect stability under ambient conditions also for Pt NCs.

CO Oxidation on Pt and Au NCs Lastly, we investigate CO oxidation on Pt- and Au NCs. The red curve in Fig. 9a shows the O 1s region after saturating 0.2 ML Pt NCs with 10 L molecular oxygen at 300 K. The O2 molecules adsorb dissociatively, giving rise to a component at 529.6 eV. 40 In the spectrum there is also a shoulder at 532.5 eV, indicating a small amount of adsorbed CO from contaminants in the background pressure. After exposure to CO (green), the O signal is reduced and the spectrum is now dominated by the CO component. The oxygen has likely reacted with CO to form CO2 , which then desorbed from the surface 85,86 . When the experiment was repeated with the NCs first saturated by CO, as depicted in Fig. 9 b, there is almost no change after O2 exposure (compare the green and blue spectra). Especially the intensity of the CO related peak is nearly unchanged. A tiny peak at 529.6 eV probably 19 ACS Paragon Plus Environment


indicates the adsorption of a small amount of O. Although, we believe the clusters to be close to fully saturated with CO after pre-dosage of 10 L CO, it is imaginable that occasionally O adsorbs in hollow sites in between the CO molecules pre-adsorbed on atop sites. From the intensity of the peak one must conclude that this is a very rare process. Thus, at 300 K the CO molecules block the adsorption sites for oxygen, so called CO-poisoning, and CO oxidation cannot occur. The absence of CO oxidation at room temperature is consistent with other studies of Pt NCs on Gr/Ir(111). 25 . (a ) O

0 .2 M L P t + O 2

1 s

N o r m a liz e d In te n s ity ( a .u .)

C O

+ C O

O

5 3 6

5 3 4

5 3 2

5 3 0

5 2 8

5 2 6


(b ) O

1 s

5 3 6

0 .2 M L P t + C O + O 2

C O


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

O

5 3 4

5 3 2

5 3 0

5 2 8

5 2 6


Figure 9: (a,b) O 1s of 0.2 ML Pt NCs exposed to CO and O2 (300 K, 10 L, 1 · 10−7 mbar). Black: clean particles. Red: exposed to O2 . Green: +CO. Blue: +O2 . The peaks at 532.5 eV and 529.6 eV corresponds to CO and O adsorption, respectively. 37,40 A similar experiment was performed with 0.2 ML Au NCs. Fig. 10 shows a series of Au 4f spectra. The spectrum does not change when the clean particles (yellow) are exposed to 10 L of O2 (red). Furthermore, no features were observed in the O 1s region before or after exposure (see inset of Fig. 10). Thus, O2 adsorption is absent on the Au particles. On the other hand, upon 10 L CO exposure (green) the spectrum changes: the main peak shifts and 20 ACS Paragon Plus Environment

Page 20 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 21 of 33

8 6

O

5 3 8

5 3 6 5 3 4 B E (e V )

A u 4 f

1 s

5 3 2 0 .2 M L A u + O 2

+ C O + O

8 5

8 4 8 3 B in d in g E n e r g y ( e V )

2

8 2

Figure 10: Au 4f of 0.2 ML Au NCs exposed to gases (140 K, 1 · 10−7 mbar). Yellow: clean particles. Red: exposed to 10 L O2 . Green: + 10 L CO. Blue: + 50 L O2 . Inset: Corresponding O 1s spectra. Experimental data shown as open circles, smoothed interpolation as lines. O 1s spectra recorded with a photon energy of 725 eV. a new component develops at higher binding energies (compare Fig. 6 (f)). As described in a previous section, both features signal CO adsorption (also indicated by the O 1s spectrum, see inset of Fig. 10). After additional exposure to 50 L O2 , the spectrum does not change further. In conclusion, adsorption of CO to saturation on the Pt or Au NCs prior to O2 exposure does not yield an oxidation reaction in either case. The crucial difference in the behavior of Pt and Au NCs is the fact that O2 dissociatively adsorbs on Pt NCs, but does not at all adsorb on Au NCs. Therefore, a subsequent CO exposure enables CO oxidation by Pt NCs, but not by Au NCs. Thus, under the employed conditions Au NCs supported by graphene are inactive for CO oxidation. This finding highlights the influence of the support on the NCs’ reactivity. Using experimental conditions (pressure, reaction temperature, cluster size) similar to those of our study, it was shown that Au NCs on MgO are active for CO oxidation 51 . Later, it was revealed that – in this particular system – charge transfer from the substrates F-centers into the deposited clusters plays a key role in promoting their chemical activity 50 . In our case, apparently, the graphene substrate does not provide such a mechanism. Further, the complete absence of oxygen adsorption on Au NCs on Gr indicates the absence of reactive sites at the clusters’



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

perimeter, which are often considered to be decisive 49,59 . This allows for the conclusion that the Au NCs on graphene are inactive for CO oxidation.

Conclusions We have shown that even the smallest Pt nanoclusters are stable against sintering in O2 and H2 , whereas they are destabilized upon CO exposure. We found that the NC-graphene interaction as monitored with the C 1s and Ir 4f core level is weakened by CO adsorption, but hardly affected by O2 and H2 , consistent with the observed NC stability. CO adsorption was also studied on small Ir and Au NCs: Ir NCs are destabilized very similar to Pt, whereas the Au particles, although only weakly bound to the graphene substrate, are stable when CO is adsorbed. Remarkably, large Ir NCs are found to be stable when exposed to air at ambient pressure. Pt NCs are active for CO oxidation as long as they are not poisoned by CO. Contrarily, Au NCs on graphene are not active for CO oxidation because oxygen neither adsorbs on the NCs nor on the graphene substrate. This differs from the behavior of Au NCs on oxide supports and, thus, demonstrates the strength of NCs supported by inert graphene for unambiguous studies on NC reactivity.

Acknowledgments Funding by the DFG (MI 581/17-2) and by the Swedish research council (2012- 3850) as well as support during the measurements by the MAX IV Laboratory personnel are gratefully acknowledged. We thank Andreas Stierle for a fruitful discussion.


Page 22 of 33

Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Supporting Information Available Pt NCs exposed to CO at room temperature, Pt NCs exposed to CO and H2 at low temperature, and the C 1s CLS upon cluster growth and upon CO exposure. This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Heiz, U., Landman, U., Eds. Nanocatalysis; Springer-Verlag: Berlin, 2007. (2) Freund, H. J. Preparation and Characterization of Model Catalysts: From Ultrahigh Vacuum to In Situ Conditions at the Atomic Dimension. J. Catal. 2003, 216, 223–235. (3) Schauermann, S.; Nilius, N.; Shaikhutdinov, S.; Freund, H.-J. Nanoparticles for Heterogeneous Catalysis: New Mechanistic Insights. Acc. Chem. Res. 2013, 46, 1673–1681. (4) Heiz, U.; Sanchez, A.; Abbet, S.; Schneider, W.-D. Catalytic Oxidation of Carbon Monoxide on Monodispersed Platinum Clusters: Each Atom Counts. J. Am. Chem. Soc. 1999, 121, 3214–3217. (5) Piccolo, L.; Henry, C. R. Reactivity of Metal Nanoclusters: Nitric Oxide Adsorption and CO+NO Reaction on Pd/MgO Model Catalysts. Appl. Surf. Sci. 2000, 162-163, 670–678. (6) Pan, Y.; Gao, M.; Huang, L.; Liu, F.; Gao, H.-J. Directed Self-Assembly of Monodispersed Platinum Nanoclusters on Graphene Moiré Template. Appl. Phys. Lett. 2009, 95, 093106. (7) Donner, K.; Jakob, P. Structural Properties and Site Specific Interactions of Pt with the Graphene/Ru(0001) Moiré Overlayer. J. Chem. Phys. 2009, 131, 164701.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(8) Zhou, Z.; Gao, F.; Goodman, D. W. Deposition of Metal Clusters on Single-Layer Graphene/Ru(0001): Factors that Govern Cluster Growth. Surf. Sci. 2010, 604, L31 – L38. (9) Sutter, E.; Albrecht, P.; Wang, B.; Bocquet, M.-L.; Wu, L.; Zhu, Y.; Sutter, P. Arrays of Ru Nanoclusters with Narrow Size Distribution Templated by Monolayer Graphene on Ru. Surf. Sci. 2011, 605, 1676 – 1684. (10) Engstfeld, A. K.; Hoster, H. E.; Behm, R. J.; Roelofs, L. D.; Liu, X.; Wang, C.-Z.; Han, Y.; Evans, J. W. Directed Assembly of Ru Nanoclusters on Ru(0001)-Supported Graphene: STM Studies and Atomistic Modeling. Phys. Rev. B 2012, 86, 085442. (11) Engstfeld, A. K.; Beckord, S.; Lorenz, C. D.; Behm, R. J. Growth of PtRu Clusters on Ru (0001)-Supported Monolayer Graphene Films. ChemPhysChem 2012, 13, 3313–3319. (12) Liao, Q.; Zhang, H.; Wu, K.; Li, H.; Bao, S.; He, P. Nucleation and Growth of Monodispersed Cobalt Nanoclusters on Graphene Moiré on Ru (0001). Nanotechnology 2011, 22, 125303. (13) Zhang, H.; Fu, Q.; Cui, Y.; Tan, D.; Bao, X. Fabrication of Metal Nanoclusters on Graphene Grown on Ru(0001). Chin. Sci. Bull. 2009, 54, 2446–2450. (14) Sicot, M.; Bouvron, S.; Zander, O.; R¨ udiger, U.; Dedkov, Y. S.; Fonin, M. Nucleation and Growth of Nickel Nanoclusters on Graphene Moiré on Rh(111). Appl. Phys. Lett. 2010, 96, 093115. (15) Gotterbarm, K.; Bronnbauer, C.; Bauer, U.; Papp, C.; Steinr¨ uck, H.-P. GrapheneSupported Pd Nanoclusters Probed by Carbon Monoxide Adsorption. J. Phys. Chem. C 2014, 118, 25097–25103.


Page 24 of 33

Page 25 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(16) Gotterbarm, K.; Steiner, C.; Bronnbauer, C.; Bauer, U.; Steinr¨ uck, H.-P.; Maier, S.; Papp, C. Graphene-Templated Growth of Pd Nanoclusters. J. Phys. Chem. C 2014, 118, 15934–15939. (17) Gotterbarm, K.; Spaeth, F.; Bauer, U.; Bronnbauer, C.; Steinr¨ uck, H.-P.; Papp, C. Reactivity of Graphene-Supported Pt Nanocluster Arrays. ACS Catal. 2015, 5, 2397– 2403. (18) N’Diaye, A. T.; Bleikamp, S.; Feibelman, P. J.; Michely, T. Two-Dimensional Ir Cluster Lattice on a Graphene Moiré on Ir(111). Phys. Rev. Lett. 2006, 97, 215501. (19) N’Diaye, A. T.; Gerber, T.; Busse, C.; Myslivecek, J.; Coraux, J.; Michely, T. A Versatile Fabrication Method for Cluster Superlattices. New J. Phys. 2009, 11, 103045. (20) Vo-Van, C.; Schumacher, S.; Coraux, J.; Sessi, V.; Fruchart, O.; Brookes, N. B.; Ohresser, P.; Michely, T. Magnetism of Cobalt Nanoclusters on Graphene on Iridium. Appl. Phys. Lett. 2011, 99, 142504. (21) Cavallin, A.; Pozzo, M.; Africh, C.; Baraldi, A.; Vesselli, E.; Dri, C.; Comelli, G.; Larciprete, R.; Lacovig, P.; Lizzit, S.; et al. Local Electronic Structure and Density of Edge and Facet Atoms at Rh Nanoclusters Self-Assembled on a Graphene Template. ACS Nano 2012, 6, 3034–3043. (22) Henry, C. R. 2D-Arrays of Nanoparticles as Model Catalysts. Catal. Lett. 2015, 145, 731–749. (23) Batzill, M. The Surf. Sci. of Graphene: Metal Interfaces, CVD Synthesis, Nanoribbons, Chemical Modifications, and Defects. Surf. Sci. Rep. 2012, 67, 83–115. (24) Liu, X.; Han, Y.; Evans, J. W.; Engstfeld, A. K.; Behm, R. J.; Tringides, M. C.; Hupalo, M.; Lin, H.-Q.; Huang, L.; Ho, K.-M.; et al. Growth Morphology and Properties of Metals on Graphene. Prog. Surf. Sci. 2015, 90, 397–443. 25 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

(25) Franz, D.; Blanc, N.; Coraux, J.; Renaud, G.; Runte, S.; Gerber, T.; Busse, C.; Michely, T.; Feibelman, P. J.; Hejral, U.; et al. Atomic Structure of Pt Nanoclusters Supported by Graphene/Ir(111) and Reversible Transformation under CO Exposure. Phys. Rev. B 2016, 93, 045426. (26) Franz, D.; Runte, S.; Busse, C.; Schumacher, S.; Gerber, T.; Michely, T.; Mantilla, M.; Kilic, V.; Zegenhagen, J.; Stierle, A. Atomic Structure and Crystalline Order of Graphene-Supported Ir Nanoparticle Lattices. Phys. Rev. Lett. 2013, 110, 065503. (27) Billinge, S. Nanoparticle Structures Served up on a Tray. Nature 2013, 495, 453–454. (28) Van Gastel, R.; N’Diaye, A.; Wall, D.; Coraux, J.; Busse, C.; Buckanie, N.; Meyer zu Heringdorf, F.-J.; Horn von Hoegen, M.; Michely, T.; Poelsema, B. Selecting a Single Orientation for Millimeter Sized Graphene Sheets. Appl. Phys. Lett. 2009, 95, 121901. (29) Campbell, C. T.; Parker, S. C.; Starr, D. E. The Effect of Size-Dependent Nanoparticle Energetics on Catalyst Sintering. Science 2002, 298, 811–814. (30) Bartholomew, C. H. Mechanisms of Catalyst Deactivation. Appl. Catal., A 2001, 212, 17–60. (31) Sehested, J.; Gelten, J. A.; Remediakis, I. N.; Bengaard, H.; Nørskov, J. K. Sintering of Nickel Steam-Reforming Catalysts: Effects of Temperature and Steam and Hydrogen Pressures. J. Catal. 2004, 223, 432–443. (32) Gerber, T.; Knudsen, J.; Feibelman, P. J.; Gr˚ anäs, E.; Stratmann, P.; Schulte, K.; Andersen, J. N.; Michely, T. CO-Induced Smoluchowski Ripening of Pt Cluster Arrays on the Graphene/Ir(111) Moiré. ACS Nano 2013, 7, 2020–2031. (33) Matsushima, T. Dissociation of Oxygen Admolecules on Rh(111), Pt(111) and Pd(111) Surfaces at Low Temperatures. Surf. Sci. Lett. 1985, 157, 297–318.


Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34) Rettner, C. T.; Mullins, C. B. Dynamics of the Chemisorption of O2 on Pt(111): Dissociation via Direct Population of a Molecularly Chemisorbed Precursor at High Incidence Kinetic Energy. J. Chem. Phys. 1991, 94, 1626–1635. (35) Christmann, K.; Ertl, G.; Pignet, T. Adsorption of Hydrogen on a Pt(111) Surface. Surf. Sci. 1976, 54, 365–392. (36) Badescu, S. C.; Salo, P.; Ala-Nissila, T.; Ying, S. C.; Jacobi, K.; Wang, Y.; Bed¨ urftig, K.; Ertl, G. Energetics and Vibrational States for Hydrogen on Pt(111). Phys. Rev. Lett 2002, 88, 136101. (37) Tränkenschuh, B.; Fritsche, N.; Fuhrmann, T.; Papp, C.; Zhu, J. F.; Denecke, R.; Steinr¨ uck, H.-P. A Site-Selective In Situ Study of CO Adsorption and Desorption on Pt(355). J. Chem. Phys. 2006, 124, 074712. (38) Tränkenschuh, B.; Papp, C.; Fuhrmann, T.; Denecke, R.; Steinr¨ uck, H.-P. The Dissimilar Twins-A Comparative, Site-Selective In Situ Study of CO Adsorption and Desorption on Pt(322) and Pt(355). Surf. Sci. 2007, 601, 1108–1117. (39) Hammer, B.; Nielsen, O. H.; Nørskov, J. K. Structure Sensitivity in Adsorption: CO Interaction with Stepped and Reconstructed Pt Surfaces. Catal. Lett. 1997, 46, 31–35. (40) Björneholm, O.; Nilsson, A.; Tillborg, H.; Bennich, P.; Sandell, A.; Hernnäs, B.; Puglia, C.; M˚ artensson, N. Overlayer Structure from Adsorbate and Substrate Core Level Binding Energy Shifts: CO, CCH3 and O on Pt(111). Surf. Sci. 1994, 315, L983–L989. (41) Steininger, H.; Lehwald, S.; Ibach, H. Adsorption of Oxygen on Pt(111). Surf. Sci. 1982, 123, 1–17. (42) Wang, J. G.; Li, W. X.; Borg, M.; Gustafson, J.; Mikkelsen, A.; Pedersen, T. M.;



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Lundgren, E.; Weissenrieder, J.; Klikovits, J.; Schmid, M.; et al. One-Dimensional PtO2 at Pt Steps: Formation and Reaction with CO. Phys. Rev. Lett. 2005, 95, 256102. (43) Badan, C.; Koper, M. T. M.; Juurlink, L. How Well Does Pt(211) Represent Pt[n(111)x(100)] Surfaces in Adsorption/Desorption? J. Phys. Chem. C 2015, 119, 13551–13560. (44) Lee, J.; Cowin, J. P.; Wharton, L. He Diffraction from Clean Pt(111) and (1 x 1)H/Pt(111) Surface. Surf. Sci. 1983, 130, 1–28. (45) Liang, Z.; Yang, H. J.; Oh, J.; Jung, J.; Kim, T.; Trenary, M. Atomic-Scale Dynamics of Surface-Catalyzed Hydrogenation/Dehydrogenation: NH on Pt(111). ACS Nano 2015, 9, 8303–8311. (46) Morgan, A. E.; Somorjai, G. A. Low Energy Electron Diffraction Studies of Gas Adsorption on the Platinum (100) Single Crystal Surface. Surf. Sci. 1968, 12, 405–425. (47) Nilsson, V.; Van den Bossche, M.; Hellman, A.; Grönbeck, H. Trends in Adsorbate Induced Core Level Shifts. Surf. Sci. 2015, 640, 59–64. (48) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Novel Gold Catalysts for the Oxidation of Carbon Monoxide at a Temperature far below 0◦ C. Chem. Lett. 1987, 16, 405–408. (49) Green, I. X.; Tang, W.; Neurock, M.; Yates, J. T. Spectroscopic Observation of Dual Catalytic Sites During Oxidation of CO on a Au/TiO2 Catalyst. Science 2011, 333, 736–739. (50) Yoon, B.; Häkkinen, H.; Landman, U.; Wörz, A. S.; Antonietti, J.-M.; Abbet, S.; Judai, K.; Heiz, U. Charging Effects on Bonding and Catalyzed Oxidation of CO on Au8 Clusters on MgO. Science 2005, 307, 403–407.


Page 28 of 33

Page 29 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(51) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W.-D.; Häkkinen, H.; Barnett, R.; Landman, U. When Gold is Not Noble: Nanoscale Gold Catalysts. J. Phys. Chem. A 1999, 103, 9573–9578. (52) Valden, M.; Lai, X.; Goodman, D. W. Onset of Catalytic Activity of Gold Clusters on Titania with the Appearance of Nonmetallic Properties. Science 1998, 281, 1647–1650. (53) Lopez, N.; Janssens, T.; Clausen, B.; Xu, Y.; Mavrikakis, M.; Bligaard, T.; Nørskov, J. On the Origin of the Catalytic Activity of Gold Nanoparticles for Low-Temperature CO Oxidation. J. Catal. 2004, 223, 232–235. (54) Lee, S.; Fan, C.; Wu, T.; Anderson, S. L. CO Oxidation on Aun /TiO2 Catalysts Produced by Size-Selected Cluster Deposition. J. Am. Chem. Soc. 2004, 126, 5682–5683. (55) Herzing, A. A.; Kiely, C. J.; Carley, A. F.; Landon, P.; Hutchings, G. J. Identification of Active Gold Nanoclusters on Iron Oxide Supports for CO Oxidation. Science 2008, 321, 1331–1335. (56) Lee, S.; Fan, C.; Wu, T.; Anderson, S. L. Cluster Size Effects on CO Oxidation Activity, Adsorbate Affinity, and Temporal Behavior of Model Aun /TiO2 Catalysts. J. Chem. Phys. 2005, 123, 124710. (57) Saavedra, J.; Doan, H. A.; Pursell, C. J.; Grabow, L. C.; Chandler, B. D. The Critical Role of Water at the Gold-Titania Interface in Catalytic CO Oxidation. Science 2014, 345, 1599–1602. (58) Molina, L. M.; Hammer, B. Some Recent Theoretical Advances in the Understanding of the Catalytic Activity of Au. Appl. Catal., A 2005, 291, 21–31. (59) Molina, L. M.; Hammer, B. Active Role of Oxide Support during CO Oxidation at Au/MgO. Phys. Rev. Lett. 2003, 90, 206102.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(60) Schubert, M. M.; Hackenberg, S.; van Veen, A. C.; Muhler, M.; Plzak, V.; Behm, R. CO Oxidation over Supported Gold Catalysts—“Inert” and “Active” Support Materials and Their Role for the Oxygen Supply during Reaction. J. Catal. 2001, 197, 113 – 122. (61) Haruta, M. Size- and Support-Dependency in the Catalysis of Gold. Catal. Today 1997, 36, 153–166. (62) McKee, W. C.; Patterson, M. C.; Huang, D.; Frick, J. R.; Kurtz, R. L.; Sprunger, P. T.; Liu, L.; Xu, Y. CO Adsorption on Au Nanoparticles Grown on Hexagonal Boron Nitride/Rh(111). J. Phys. Chem. C 2016, 120, 10909–10918. (63) Nyholm, R.; Andersen, J. N.; Johansson, U.; Jensen, B. N.; Lindau, I. Beamline I311 at MAX-LAB: a VUV/Soft X-ray Undulator Beamline for High Resolution Electron Spectroscopy. Nucl. Instrum. Methods Phys. Res., Sect. A 2001, 467-468, Part 1, 520– 524. (64) Knudsen, J.; Feibelman, P. J.; Gerber, T.; Gr˚ anäs, E.; Schulte, K.; Stratmann, P.; Andersen, J. N.; Michely, T. Clusters Binding to the Graphene Moiré on Ir(111): Xray Photoemission Compared to Density Functional Calculations. Phys. Rev. B 2012, 85, 035407. (65) Horcas, I.; Fernández, R.; Gómez-Rodr´ıguez, J. M.; Colchero, J.; Gómez-Herrero, J.; Baro, A. M. WSXM: A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (66) Simoes, J. A. M.; Beauchamp, J. L. Transition Metal-Hydrogen and Metal-Carbon Bond Strengths: The Keys to Catalysis. Chem. Rev. 1990, 90, 629–688. (67) Kittel, C. Einf¨ uhrung in die Festkörperphysik - 14. Auflage; Oldenbourg: M¨ unchen, 2006.


Page 30 of 33

Page 31 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(68) Nolte, P.; Stierle, A.; Jin-Phillipp, N.; Kasper, N.; Schulli, T.; Dosch, H. Shape Changes of Supported Rh Nanoparticles During Oxidation and Reduction Cycles. Science 2008, 321, 1654–1658. (69) Rost, M. J.; Michely, T.; Comsa, G. Comment on “Self-Diffusion and Dynamic Behavior of Atoms at Step Edges on Iridium Surfaces”. Phys. Rev. B 1998, 57, 1992. (70) Michely, T.; Comsa, G. Temperature Dependence of the Sputtering Morphology of Pt (111). Surf. Sci. 1991, 256, 217–226. (71) D¨ uckers, K.; Prince, K. C.; Bonzel, H. P.; Cháb, V.; Horn, K. Adsorption-Induced Surface Core-Level Shifts of Pt(110). Phys. Rev. B 1987, 36, 6292–6301. (72) Janin, E.; Björkqvist, M.; Grehk, T.; Göthelid, M.; Pradier, C.-M.; Karlsson, U. O.; Rosengren, A. Hydrogen Adsorption on the Pt (111)(3× 3) R30-Sn Surface Alloy Studied by High Resolution Core Level Photoelectron Spectroscopy. Appl. Surf. Sci. 1996, 99, 371–378. (73) Feibelman, P. J. Pinning of Graphene to Ir(111) by Flat Ir Dots. Phys. Rev. B 2008, 77, 165419. (74) Yeo, Y. Y.; Vattuone, L.; King, D. A. Calorimetric Heats for CO and Oxygen Adsorption and for the Catalytic CO Oxidation Reaction on Pt(111). J. Chem. Phys. 1997, 106, 392–401. (75) Brown, W.; Kose, R.; King, D. Femtomole Adsorption Calorimetry on Single-Crystal Surfaces. Chem. Rev. 1998, 98, 797–832. (76) Feibelman, P. J.; Esch, S.; Michely, T. O Binding Sites on Stepped Pt (111) Surfaces. Phys. Rev. Lett. 1996, 77, 2257. (77) Hammer, B.; Nørskov, J. K. Why Gold is the Noblest of All the Metals. Nature 2005, 376, 238–240. 31 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(78) Hammer, B.; Morikawa, Y.; Nørskov, J. K. CO Chemisorption at Metal Surfaces and Overlayers. Phys. Rev. Lett. 1996, 76, 2141–2144. (79) Weststrate, C. J.; Resta, A.; Westerstrom, R.; Lundgren, E.; Mikkelsen, A.; Andersen, J. N. CO Adsorption on a Au/CeO2 (111) Model Catalyst. J. Phys. Chem. C 2008, 112, 6900–6906. (80) Lauterbach, J.; Boyle, R. W.; Schick, M.; Mitchell, W. J.; Meng, B.; Weinberg, W. H. The Adsorption of CO on Ir(111) Investigated with FT-IRAS. Surf. Sci. 1996, 350, 32–44. (81) Ertl, G.; Neumann, M.; Streit, K. Chemisorption of CO on the Pt(111) Surface. Surf. Sci. 1977, 64, 393 – 410. (82) Gr˚ anäs, E.; Knudsen, J.; Schröder, U. A.; Gerber, T.; Busse, C.; Arman, M. A.; Schulte, K.; Andersen, J. N.; Michely, T. Oxygen Intercalation under Graphene on Ir(111): Energetics, Kinetics, and the Role of Graphene Edges. ACS Nano 2012, 6, 9951–9963. (83) Gr˚ anäs, E.; Andersen, M.; Arman, M. A.; Gerber, T.; Hammer, B.; Schnadt, J.; Andersen, J. N.; Michely, T.; Knudsen, J. CO Intercalation of Graphene on Ir (111) in the Millibar Regime. J. Phys. Chem. C 2013, 117, 16438–16447. (84) Gr˚ anäs, E.; Gerber, T.; Schröder, U. A.; Schulte, K.; Andersen, J. N.; Knudsen, J.; Michely, T. Hydrogen Intercalation under Graphene on Ir(111). Surf. Sci. 2016, 651, 57–61. (85) Langmuir, I. The Mechanism of the Catalytic Action of Platinum in the Reactions 2CO + O2 = 2CO2 and 2H2 + O2 = 2H2 O. Trans. Faraday Soc. 1922, 17, 621–654. (86) Engel, T.; Ertl, G. Elementary Steps in the Catalytic Oxidation of Carbon Monoxide on Platinum Metals. Adv. Catal. 1979, 28, 1–78. 32 ACS Paragon Plus Environment

Page 32 of 33

Page 33 of 33

ap he n

Pt

e

Graphical TOC Entry

Gr

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Au

Stability and Reactivity of Graphene-Templated Nanoclusters - The

Recommend Documents