Letter pubs.acs.org/JPCL
Hydrogen Dissociation, Spillover, and Desorption from CuSupported Co Nanoparticles Emily A. Lewis, Matthew D. Marcinkowski, Colin J. Murphy, Melissa L. Liriano, and E. Charles H. Sykes* Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States ABSTRACT: Co−Cu nanoparticles have recently been explored for Fischer−Tropsch synthesis (FTS) as a way to combine the long chain selectivity of Co with Cu’s activity for alcohol formation in order to synthesize oxygenated transportation fuels. Depending on particle size, hydrogen dissociation can be a rate-determining step in cobalt-catalyzed FTS. To understand the fundamentals of uptake and release of hydrogen from the Co/Cu bimetallic system, we prepared well-defined Co nanoparticles on Cu(111). We demonstrate that hydrogen spills over from dissociation sites on the Co nanoparticles to the Cu(111) surface via the Co−Cu interface and that desorption of H occurs at a temperature that is lower than from Co or Cu alone, which we attribute to the Co−Cu interface sites. From this data, we have constructed an energy landscape for the facile dissociation, spillover, and desorption of hydrogen on the Co−Cu bimetallic system. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
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to Pd(111)) and a weak binding energy (similar to Cu(111)).7 These single atom alloys are particularly relevant for catalyzing hydrogenation reactions in that they allow for the uptake of H on Cu(111) surfaces, which are not capable of H2 dissociation under ultrahigh vacuum (UHV) conditions but are active for hydrogenations in situ and allow for weak binding and low temperature desorption of hydrogenated products.11 It was further shown that the spilled-over H atoms could be trapped on the Cu surface by blocking the Pd dissociation/ recombination sites with CO molecules, allowing for H atoms to remain on the surface at temperatures beyond their normal desorption range.9 Interestingly, spillover of H from single Pd atoms to a Au(111) surface was not observed.8,10 Co is the preferred modern catalyst for Fischer−Tropsch synthesis (FTS), the reaction of syngas to alkanes, as it has a higher selectivity toward chain growth and a lower operating temperature than the other common FTS catalyst, Fe.12−14 Recently, a number of studies have investigated Co−Cu catalysts for FTS with the goal of coupling the long chain selectivity of Co with Cu’s activity for C2+ alcohol formation in order to synthesize oxygenated transportation fuels.15−21 These studies have shown that the addition of Cu to Co affects both the rate and selectivity of CO hydrogenation, resulting in a marked increase in alcohol production.19,21 It has also been demonstrated that the addition of Cu significantly enhances the reduction of Co in bimetallic nanoparticles.15,19−21 Importantly, under syngas or intentionally reducing/oxidizing environments, changes in the surface composition or restructuring of the Co− Cu nanoparticles were observed, resulting in segregation of the Co and Cu metals.15−19 Given this segregation, it is important to understand hydrogen uptake, spillover, and desorption in the
pillover of atoms, especially hydrogen, from one site to another is an important process that has broad applications in catalysis and hydrogen storage. In catalysis, spillover allows for the activation of a species at one site, after which it can diffuse to another site and react. In hydrogen storage applications, including the use of metal organic frameworks, it is often desirable to dissociate H2 at an active site and then spill it over to a storage site, leaving the active site open for continued H2 dissociation/recombination. However, although spillover processes are implicated in a wide variety of applications, they have proven difficult to examine.1 Goodman and co-workers performed some of the first experiments that overcame the challenges of studying spillover in a bimetallic alloy. Their early temperature-programmed desorption (TPD) work examining the diffusion of H from Ru to Cu2,3 demonstrated that H must spill over from Ru to Cu when Cu was deposited onto a H-covered Ru surface. Their H2 desorption spectra showed a unique low-temperature feature at 190 K for the bimetallic system that was distinct from either Cu (310 K)4 or Ru (325 K)5 alone. They attributed this result to finely dispersed Cu in the Ru surface that was capable of forming multiple H−Cu bonds; however, no analysis was performed on the surface composition. Later TPD work by Goodman and co-workers demonstrated that H could also spill over from Ni to Cu when Cu was deposited onto H-covered Ni thin films and nanoparticles, but no new H desorption features due to the bimetallic alloy were observed.6 Previous studies in our lab furthered this work by demonstrating that it is possible to harness the spillover process by activating H2 dissociation at individual, isolated Pd atoms embedded in a Cu(111) surface and controllably spilling over the resulting H atoms to Cu sites where highly selective hydrogenation can occur.7−10 Similar to the Cu−Ru system, it was shown that the desorption behavior of the spilled-over H atoms is unique from either the Cu(111) or Pd(111) surfaces, as there is a low barrier to dissociation/recombination (similar © 2014 American Chemical Society
Received: August 8, 2014 Accepted: September 11, 2014 Published: September 11, 2014 3380
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Figure 1. STM images and schematic depicting the spillover of H atoms from Co nanoparticles to the Cu(111) support (θCo = 0.25 ML). (a) Frames from an STM time-lapse movie at 5 K in which H atoms can be seen diffusing on the Cu surface. Images taken ∼4 min apart. Scale bar = 4 nm. (b) STM image showing the high-density H-(1 × 1) structure of the H atoms (protrusions) on the Co nanoparticle surface, and the triangular defects that exist within the H overlayer. Image = 3 × 3 nm2. (c) STM image of H atom clusters (dark depressions) on the Cu(111) surface. Image = 5 × 5 nm2. (d) Schematic of Co nanoparticles on Cu(111) and the H spillover process.
Figure 2. H2 TPD spectra from the Cu(111),4 Co(0001),41 and Co/Cu(111) surfaces. Previously reported TPD spectra are replicated for a comparison of H2 desorption temperature. (a) H2 desorption from Cu(111) at saturation coverage, where hydrogen atoms were deposited onto the surface via an energetic H2 molecular beam (replicated from ref 4). (b) H2 desorption of a saturated Co(0001) single crystal surface and an intentionally roughened Co(0001) surface that was sputtered and not annealed (replicated from ref 41). (c) H2 desorption from ∼5 ML Co deposited on Cu(111) at a saturation deposition of H2 (100 Langmuir (L)). (d) TPD spectra of 100 L H2 deposited on a variety of Co coverages, as indicated in the figure (ML).
defined Co/Cu(111) surfaces with scanning tunneling microscope (STM) imaging and TPD. TPD enables us to quantify the uptake and desorption of H2 as a function of Co surface coverage, whereas STM imaging allows us to directly visualize the location of H atoms at Co and Cu sites as a function of coverage and temperature. These techniques in concert have enabled us to construct an energy landscape for the uptake, spillover, and release of hydrogen from the Co/Cu(111)
Co−Cu bimetallic, as H has been shown to be necessary for CO dissociation on Co.22−29 Additionally, H2 dissociation has recently been implicated as a rate-determining step in FTS on small Co nanoparticles,22 and as such, an understanding of the energetics of H2 dissociation on Co is vital to the development of these catalysts. To gain deeper insight into the interaction of H2 with the Co−Cu system, we investigated hydrogen adsorption on well3381
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Figure 3. STM images depicting the depopulation of Cu and Co sites on the Co/Cu(111) bimetallic (θCo = 0.25 ML) during H2 desorption induced by sequentially annealing the sample. The annealing temperature is given in the lower left corner of the images. The H coverage shown in (A) is the same as in Figure 1. The insets show high-resolution images of the H phase present on the surface of the Co nanoparticles as a function of annealing temperature: (A) H-(1 × 1) 1.0 ML phase, (C) 2H-(2 × 2) 0.5 ML phase. All imaging was performed at 5 K. Scale bars = 10 nm.
dissociation sites on the Co nanoparticles. Although we have previously reported that CO is able to destabilize H on Co nanoparticles and force it onto the Cu surface,32 the current result demonstrates that at catalytically relevant H coverages, H alone can diffuse to less stable, weaker binding sites on the Cu surface after all of the Co sites have been occupied. After establishing with STM imaging that H-atom spillover from Co to Cu is possible, we performed H2 TPD experiments on the Co/Cu(111) system to better understand the energetics of this process (Figure 2C and D). Three distinct H2 desorption peaks are observed: one at 330 K that agrees well with H2 desorption from the Co(0001) terrace,41−44 one at 235 K that corresponds to H2 desorption from a stepped Co(0001) surface,41 and a new, as yet unreported peak at temperatures 165−205 K that we postulate is due to Co−Cu interface sites. For comparison, H2 desorption spectra from Cu(111)4 and Co(0001)41 are shown (Figure 2A and B, respectively), and it is apparent that the new low temperature peak for the release of hydrogen is unique compared to either of the monometallic surfaces, which indicates the presence of a new low energy pathway for hydrogen uptake and release. Hydrogen desorbs from a Cu(111) surface in one peak centered around 310 K at saturation. On Co(0001), H2 desorption at saturation is higher in temperature (350 K), with the maximum coverage at 0.5 ML. van Helden et al. previously showed that this coverage could be increased to 0.75 ML by intentionally roughening the Co surface and were able to demonstrate a lower temperature desorption feature at 220 K, which they attributed to this higher-density hydrogen phase desorbing from stepped Co(0001).41 The new low temperature feature 165−205 K in our TPD spectra reveals that Co−Cu interfaces allow facile activation, spillover, and recombination sites for hydrogen. In addition, the fact that the most dense H-(1 × 1) phase can only be formed when Co nanoparticles are supported on Cu indicates that Co−Cu interface sites enable formation of this high coverage 1.0 ML H phase on Co. The onset of the low temperature desorption peak occurs at a Co coverage of 0.3 ML, which we attribute to the number of available Co−Cu interface sites at this coverage. This Co coverage is slightly higher than the Co coverage needed to observe H spillover in the low temperature STM experiments due to the higher background pressure of CO in the TPD chamber, as adsorption of CO tends to block H spillover.
bimetallic. Our results demonstrate that the Co−Cu interface offers a low temperature pathway for H atom spillover onto the Cu(111) surface and that spillover only begins once all of the Co sites are occupied. The high H coverage at which this pathway becomes active is relevant to the dense surface adsorbate coverages expected during catalysis, suggesting that this hydrogen spillover pathway could play an important role in the activity and selectivity of Co−Cu catalysts. To investigate the Co−Cu system with STM imaging and TPD, we deposited Co onto a Cu(111) surface, which yielded well-defined triangular Co nanoparticles whose chemical and physical properties have previously been characterized by our group and others.12,30−39 Briefly, the Co nanoparticles are two atomic layers above the Cu(111) surface, and based on the epitaxial nature of their growth, they orient in two directions dictated by the 6-fold symmetry of the underlying Cu(111) lattice. In our samples, the triangular nanoparticles appear to point up and down due to the nearly horizontal orientation of the Cu(111) substrate close-packed direction (Figures 1 and 3) and grow to be about 10−15 nm on a side when deposited at a surface coverage (θ) of ∼0.25 monolayers (ML). Note that as the Co nanoparticles are two layers high, this coverage corresponds to θ = 0.5 ML if the film grew in a layer-bylayer fashion. After dissociation of molecular H2 at Co sites, the resulting H atoms bind strongly to Co but weakly to Cu, which means that H atom spillover from Co to Cu is an activated process. Therefore, to explore the dissociative adsorption of H2 on the Co/Cu(111) system, it was necessary to deposit H2 at 80 K and then cool to 5 K for imaging, as individual H atoms diffuse too rapidly on Cu(111) at 80 K to be visible.40 Following this deposition procedure, H atoms were observed on the Cu surface, appearing in STM images as dark depressions on the terraces that exhibited the characteristic fast diffusion at 5 K due to quantum tunneling as seen in time-lapse STM images (Figure 1A).40 H atoms are also observed on the Co nanoparticles in their most dense H-(1 × 1) phase, as indicated by the presence of darker triangular features on their surfaces that arise from defects in the H overlayer packing (Figure 1A and B).30 The low temperature of H2 deposition (80 K) indicates that H2 activation is facile on these Co nanoparticles. Because Cu is not able to dissociatively adsorb H2 in UHV, all of the H atoms on the Cu surface must have spilled over from 3382
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Figure 4. Top: Energetics of the H spillover pathway from Co to Cu. Transitions originating at Co sites are drawn in blue, Co−Cu interface sites in purple, and Cu sites in red. All thermodynamic levels are given in eV per H atom. Bottom: Schematic illustrating the active sites for each step in the spillover process.
phase transition on many different Co coverages from submonolayer to >3 ML. As the H2 must leave the surface through the same pathway that it entered, H2 desorption from the Cu surface leads to the low temperature desorption peak observed in the TPD spectra at 165−205 K, corresponding to the Co−Cu interface sites, as opposed to direct desorption from Cu(111) sites, which would be expected ∼300 K. Using known values for hydrogen dissociation barriers and H-atom binding7,8,32,41 in combination with the experimental data described herein, we explored the energetics of hydrogen on the Co/Cu(111) system and constructed an energy landscape that illustrates our proposed pathway for H dissociation, adsorption, compression, spillover, and the reverse of these processes (Figure 4). The binding of H to Co−Cu mixed sites was calculated to be ∼0.4 eV by considering a weighted average of H atom binding to 1 Cu and 2 Co atoms (which corresponds to (111) steps at the Co−Cu interface), as H typically adsorbs on metals at 3-fold hollow sites.8 This is the strongest that H could bind to these mixed sites and still spill over onto the Cu sites at 80 K as observed experimentally; however, the binding energy for H to this site could be as low as 0.35 eV (2 Co and 2 Cu atoms at a (100) step), with a small barrier to spillover. The energy landscape in Figure 4 illustrates that compression of the 6H-(3 × 3) to the H-(1 × 1) via spillover of H atoms from Co step sites competes with desorption from this most dense phase, which explains why it is never observed in monometallic Co systems.41−43 The unique Co−Cu interface sites provide a necessary lower-barrier pathway that facilitates both the formation of the most dense H-(1 × 1) phase on our Co nanoparticles that has not be
In order to examine the desorption mechanism, uptake and annealing experiments were performed in the 5 K STM to obtain a deeper insight into the reverse spillover process. These experiments indicate that (1) the Cu(111) terraces are only populated after the H-(1 × 1) phase on the Co(0001) surface has been completely saturated and (2) that H on the Cu terraces is the first species to desorb. After the initial deposition of H2 on the Co/Cu(111) surface that saturated the Co sites and spilled H atoms over on the Cu(111) surface (Figure 3A), the system was sequentially annealed to higher temperatures to visualize the depopulation of surface hydrogen from the Cu and Co sites. The offset in temperature between the STM annealing and TPD ramp arises due to the relatively slow warming and cooling in the STM, leading to lower apparent desorption temperatures in the STM experiments. STM imaging revealed that after annealing to 120 K, all of the Cu-bound H desorbs from the surface leaving the dense H-(1 × 1) phase on the Co nanoparticles (Figure 3B). The H-(1 × 1) phase on the Co appears to be more defective after the anneal, indicating that some desorption of H2 from the Co has taken place. However, the fact that the H-(1 × 1) still remains on the nanoparticles demonstrates that the Cu terraces are the first sites to be depopulated during H2 release. Similarly, and in accordance with microscopic reversibility, our STM uptake experiments indicate that H spillover from Co and population of Cu occurs only after all of the Co sites are occupied with the highest density H-(1 × 1) 1.0 ML phase; thus, the desorption process is the reverse of this because Cu depopulates before Co. Further annealing the sample to 160 K desorbs more of the H2 from Co, resulting in the least dense 2H-(2 × 2) phase (θ = 0.5 ML) on the Co surface (Figure 3C). We have observed this 3383
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99.999%) onto the prepared Co/Cu(111) sample. All H2 depositions were carried out with the sample held at 80 K with liquid nitrogen, after which the sample was cooled to 5 K with liquid helium for STM imaging. Thermal annealing of the sample was performed by removing the sample from the cooled STM stage with a room temperature manipulator for a set period of time. Temperatures are given ±20 K. Etched W tips with tunneling conditions ≤0.05 V and ≤50 pA were used to image H atoms on Cu and between 0.01−0.05 V and 20−400 pA to image H on Co. TPD experiments were performed in a home-built chamber (P ∼ 1 × 10−10 mbar) containing a quadrupole mass spectrometer (Hiden) and at a heating rate of 1 K/sec. Cleaning of the Cu crystal and Co deposition was carried out to mimic the STM procedures. The Cu(111) crystal (MaTecK) was cleaned by cycles of Ar+ sputtering (1.5 kV, 15 μA) and annealing, and cobalt depositions were carried out using an EMF3 e-beam evaporator (Focus GmbH) on a room temperature Cu(111) sample with flux of ∼0.05 ML/min. Cobalt coverages were calibrated by CO TPD titration experiments. Hydrogen (Airgas, 99.999%) was deposited on the sample at 85 K via a collimated leak valve pointed at the sample. TPD data were baseline corrected, and curves were smoothed with a 10 point moving average.
observed on Co single crystals and enables H spillover to bare Cu sites. The schematic in Figure 4 shows the three active Co sites that contribute to the formation of the different H phases on the Co/Cu(111) system. First, H2 adsorbs dissociatively on the Co terraces forming a 2H-(2 × 2) structure with θ = 0.5 ML. After saturation of this phase, Co steps are necessary to allow compression of the 2H-(2 × 2) structure into the 6H-(3 × 3) structure.30,41 This could occur either by the Co steps acting as the active H2 dissociation sites or via spillover of H from Co terrace sites to the Co step sites, resulting in Co terrace site vacancies becoming available for H2 dissociation. Our experimental data indicates that in order to compress the 6H(3 × 3) to the H-(1 × 1) phase, Co−Cu interface sites must be present, which again either become active for H2 dissociation themselves or act as H reservoirs that allow for new vacancies to form on Co terraces. Finally, once the 1.0 ML H-(1 × 1) phase has been fully populated, H atom spillover to the Cu terraces becomes possible, which further creates vacancies through which more H2 can be dissociated on the Co nanoparticles. The desorption process proceeds first via depopulation of Cu sites and release of H2 at much lower temperature than would be expected from Cu or Co alone, indicating the unique energetic landscape of the Co−Cu bimetallic interface sites. In accordance with microscopic reversibility, after the Cu sites are depopulated, H2 desorbs from the Co nanoparticles. By exploring H2 uptake on a model catalytic system composed of Co nanoparticles deposited on Cu(111), we were able to characterize the spillover of H atoms from energetically preferred Co sites to a Cu support at catalytically relevant surface coverages of H. We show that this spillover to Cu, as well as the formation of a high-density H-(1 × 1) phase on Co, is facilitated by Co−Cu interface sites and that desorption from these sites occurs at a temperature that is lower than either the Cu or Co surfaces alone. Given this new low-temperature pathway for H2 uptake and release from Co− Cu interfaces, these results have ramifications for Co FTS catalysts, as it was recently demonstrated that H2 dissociation is an important step in the rate of FTS on small Co nanoparticles. Additionally, H spillover from Co (where carbon monoxide prefers to adsorb) could limit the interface between H and CO during FTS, which could pose kinetic limitations on their reaction. This effect could be even more pronounced for the bimetallic Co−Cu catalysts currently being explored for FTS, given that segregation of the two metals has been observed in multiple systems and that CO dissociation is known to occur only on Co sites, whereas our data indicate facile spillover of H atoms to Cu sites.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Division of Chemical Sciences, Office of Basic Energy Sciences, Condensed Phase and Interfacial Molecular Science Program, U.S. Department of Energy under Grant No. FG02-10ER16170 for support of this work.
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REFERENCES
(1) Prins, R. Hydrogen Spillover. Facts and Fiction. Chem. Rev. 2012, 112, 2714−2738. (2) Goodman, D. W.; Yates, J. T.; Peden, C. H. F. The Reaction of Atomic Copper with Chemisorbed Hydrogen on Ruthenium. Surf. Sci. 1985, 164, 417−424. (3) Yates, J. T.; Peden, C. H. F.; Goodman, D. W. Copper Site Blocking of Hydrogen Chemisorption on Ruthenium. J. Catal. 1985, 94, 576−580. (4) Anger, G.; Winkler, A.; Rendulic, K. D. Adsorption and Desorption Kinetics in the Systems H2/Cu(111), H2/Cu(110) and H2/Cu(100). Surf. Sci. 1989, 220, 1−17. (5) Shimizu, H.; Christmann, K.; Ertl, G. Model Studies on Bimetallic Cu/Ru Catalysts: II. Adsorption of Hydrogen. J. Catal. 1980, 61, 412−429. (6) Yao, Y.; Goodman, D. W. Direct Evidence of Hydrogen Spillover from Ni to Cu on Ni−Cu Bimetallic Catalysts. J. Mol. Catal. A 2014, 383−4, 239−242. (7) Kyriakou, G.; Boucher, M. B.; Jewell, A. D.; Lewis, E. A.; Lawton, T. J.; Baber, A. E.; Tierney, H. L.; Flytzani-Stephanopoulos, M.; Sykes, E. C. H. Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations. Science 2012, 335, 1209−1212. (8) Tierney, H. L.; Baber, A. E.; Kitchin, J. R.; Sykes, E. C. H. Hydrogen Dissociation and Spillover on Individual Isolated Palladium Atoms. Phys. Rev. Lett. 2009, 103, 2461021−2461024. (9) Marcinkowski, M. D.; Jewell, A. D.; Stamatakis, M.; Boucher, M. B.; Lewis, E. A.; Murphy, C. J.; Kyriakou, G.; Sykes, E. C. H.
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EXPERIMENTAL METHODS STM experiments were carried out in an Omicron lowtemperature STM. The Cu(111) crystal (MaTecK) was first cleaned by cycles of sputtering with Ar+ (1 kV, 15 μA) and annealing to ∼1000 K in an isolated preparation chamber (P ≤ 1 × 10−10 mbar). After cleaning, the sample was transferred to the STM chamber (P ≤ 1 × 10−11 mbar) within 5 min, where it was cooled to cryogenic temperature within ∼1 h. Cobalt depositions were carried out on a room temperature Cu(111) sample in the preparation chamber with an electron beam evaporator (Focus GmbH). A flux of ∼0.05 ML/min was used to obtain Co surface coverages of 25−30%. High precision leak valves on the STM chamber enabled deposition of H2 (Airgas, 3384
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Controlling a Spillover Pathway with the Molecular Cork Effect. Nat. Mater. 2013, 12, 523528. (10) Baber, A. E.; Tierney, H. L.; Lawton, T. J.; Sykes, E. C. H. An Atomic-Scale View of Palladium Alloys and Their Ability to Dissociate Molecular Hydrogen. ChemCatChem 2011, 3, 607−614. (11) Boucher, M. B.; Zugic, B.; Cladaras, G.; Kammert, J.; Marcinkowski, M. D.; Lawton, T. J.; Sykes, E. C. H.; FlytzaniStephanopoulos, M. Single Atom Alloy Surface Analogs in Pd0.18Cu15 Nanoparticles for Selective Hydrogenation Reactions. Phys. Chem. Chem. Phys. 2013, 15, 12187−12196. (12) Lewis, E. A.; Jewell, A. D.; Kyriakou, G.; Sykes, E. C. H. Rediscovering Cobalt’s Surface Chemistry. Phys. Chem. Chem. Phys. 2012, 14, 7215−7224. (13) Wang, Z.-J.; Yan, Z.; Liu, C.-J.; Goodman, D. W. Surface Science Studies on Cobalt Fischer−Tropsch Catalysts. ChemCatChem 2011, 3, 551−559. (14) Beaumont, S. K. Recent Developments in the Application of Nanomaterials to Understanding Molecular Level Processes in Cobalt Catalysed Fischer−Tropsch Synthesis. Phys. Chem. Chem. Phys. 2014, 16, 5034−5043. (15) Alayoglu, S.; Beaumont, S. K.; Melaet, G.; Lindeman, A. E.; Musselwhite, N.; Brooks, C. J.; Marcus, M. A.; Guo, J.; Liu, Z.; Kruse, N.; et al. Surface Composition Changes of Redox Stabilized Bimetallic CoCu Nanoparticles Supported on Silica under H 2 and O 2 Atmospheres and during Reaction between CO2 and H2: In Situ Xray Spectroscopic Characterization. J. Phys. Chem. C 2013, 117, 21803−21809. (16) Beaumont, S. K.; Alayoglu, S.; Pushkarev, V. V.; Liu, Z.; Kruse, N.; Somorjai, G. A. Exploring Surface Science and Restructuring in Reactive Atmospheres of Colloidally Prepared Bimetallic CuNi and CuCo Nanoparticles on SiO2 In Situ Using Ambient Pressure X-ray Photoelectron Spectroscopy. Faraday Discuss. 2013, 162, 31−44. (17) Mo, X.; Tsai, Y.-T.; Gao, J.; Mao, D.; Goodwin, J. G. Effect of Component Interaction on the Activity of Co/CuZnO for CO Hydrogenation. J. Catal. 2012, 285, 208−215. (18) Carenco, S.; Tuxen, A.; Chintapalli, M.; Pach, E.; Escudero, C.; Ewers, T. D.; Jiang, P.; Borondics, F.; Thornton, G.; Alivisatos, A. P.; et al. Dealloying of Cobalt from CuCo Nanoparticles under Syngas Exposure. J. Phys. Chem. C 2013, 117, 6259−6266. (19) Wang, J.; Chernavskii, P. A.; Khodakov, A. Y.; Wang, Y. Structure and Catalytic Performance of Alumina-Supported CopperCobalt Catalysts for Carbon Monoxide Hydrogenation. J. Catal. 2012, 286, 51−61. (20) Mouaddib, N.; Perrichon, V.; Martin, G. A. Characterization of Copper-Cobalt Catalysts for Alcohol Synthesis from Syngas. Appl. Catal., A 1994, 118, 63−72. (21) Smith, M. L.; Kumar, N.; Spivey, J. J. CO Adsorption Behavior of Cu/SiO2, Co/SiO2, and CuCo/SiO2 Catalysts Studied by In Situ DRIFTS. J. Phys. Chem. C 2012, 116, 7931−7939. (22) Tuxen, A.; Carenco, S.; Chintapalli, M.; Chuang, C.-H.; Escudero, C.; Pach, E.; Jiang, P.; Borondics, F.; Beberwyck, B.; Alivisatos, A. P.; et al. Size-Dependent Dissociation of Carbon Monoxide on Cobalt Nanoparticles. J. Am. Chem. Soc. 2013, 135, 2273−2278. (23) Ojeda, M.; Li, A.; Nabar, R.; Nilekar, A. U.; Mavrikakis, M.; Iglesia, E. Kinetically Relevant Steps and H2/D2 Isotope Effects in Fischer−Tropsch Synthesis on Fe and Co Catalysts. J. Phys. Chem. C 2010, 114, 19761−19770. (24) Ojeda, M.; Nabar, R.; Nilekar, A. U.; Ishikawa, A.; Mavrikakis, M.; Iglesia, E. CO Activation Pathways and the Mechanism of Fischer−Tropsch Synthesis. J. Catal. 2010, 272, 287−297. (25) Huo, C.-F.; Li, Y.-W.; Wang, J.; Jiao, H. Formation of CHx Species from CO Dissociation on Double-Stepped Co(0001): Exploring Fischer−Tropsch Mechanism. J. Phys. Chem. C 2008, 112, 14108−14116. (26) Frennet, A.; de Bocarmé, T. V.; Bastin, J.-M.; Kruse, N. Mechanism and Kinetics of the Catalytic CO−H2 Reaction: An Approach by Chemical Transients and Surface Relaxation Spectroscopy. J. Phys. Chem. B 2005, 109, 2350−2359.
(27) van Helden, P.; van den Berg, J.-A.; Ciobîcă, I. M. HydrogenAssisted CO Dissociation on the Co(211) Stepped Surface. Catal. Sci. Technol. 2012, 2, 491−494. (28) Inderwildi, O. R.; Jenkins, S. J.; King, D. A. Fischer−Tropsch Mechanism Revisited: Alternative Pathways for the Production of Higher Hydrocarbons from Synthesis Gas. J. Phys. Chem. C 2008, 112, 1305−1307. (29) Inderwildi, O. R.; King, D. A.; Jenkins, S. J. Fischer−Tropsch Synthesis of Liquid Fuels: Learning Lessons from Homogeneous Catalysis. Phys. Chem. Chem. Phys. 2009, 11, 11110−11112. (30) Lewis, E. A.; Le, D.; Murphy, C. J.; Jewell, A. D.; Mattera, M. F. G.; Liriano, M. L.; Rahman, T. S.; Sykes, E. C. H. Dissociative Hydrogen Adsorption on Close-Packed Cobalt Nanoparticle Surfaces. J. Phys. Chem. C 2012, 116, 25868−25873. (31) Lewis, E. A.; Le, D.; Jewell, A. D.; Murphy, C. J.; Rahman, T. S.; Sykes, E. C. H. Segregation of Fischer−Tropsch Reactants on Cobalt Nanoparticle Surfaces. Chem. Commun. 2014, 50, 6537−6539. (32) Lewis, E. A.; Le, D.; Jewell, A. D.; Murphy, C. J.; Rahaman, T. S.; Sykes, E. C. H. Visualization of Compression and Spillover in a Coadsorbed System: Syngas on Cobalt Nanoparticles. ACS Nano 2013, 7, 4384−4392. (33) de la Figuera, J.; Prieto, J. E.; Ocal, C.; Miranda, R. Scanning Tunneling Microscopy Study of the Growth of Cobalt on Cu(111). Phys. Rev. B 1993, 47, 13043−13046. (34) Negulyaev, N. N.; Stepanyuk, V. S.; Bruno, P.; Diekhöner, L.; Wahl, P.; Kern, K. Bilayer Growth of Nanoscale Co Islands on Cu(111). Phys. Rev. B 2008, 77, 1254371−1254377. (35) Pedersen, M. Ø.; Bönicke, I. A.; Lægsgaard, E.; Stensgaard, I.; Ruban, A.; Nørskov, J. K.; Besenbacher, F. Growth of Co on Cu(111): Subsurface Growth of Trilayer Co Islands. Surf. Sci. 1997, 387, 86− 101. (36) Pietzsch, O.; Kubetzka, A.; Bode, M.; Wiesendanger, R. SpinPolarized Scanning Tunneling Spectroscopy of Nanoscale Cobalt Islands on Cu(111). Phys. Rev. Lett. 2004, 92, 0572021−0572024. (37) Sicot, M.; Kurnosikov, O.; Swagten, H. J. M.; Koopmans, B. Hydrogen Superstructures on Co Nanoislands and Cu(111). Surf. Sci. 2008, 602, 3667−3673. (38) Rastei, M. V.; Heinrich, B.; Limot, L.; Ignatiev, P. A.; Stepanyuk, V. S.; Bruno, P.; Bucher, J. P. Size-Dependent Surface States of Strained Cobalt Nanoislands on Cu(111). Phys. Rev. Lett. 2007, 99, 2461021−2461024. (39) Oka, H.; Ignatiev, P. A.; Wedekind, S.; Rodary, G.; Niebergall, L.; Stepanyuk, V. S.; Sander, D.; Kirschner, J. Spin-Dependent Quantum Interference within a Single Magnetic Nanostructure. Science 2010, 327, 843−846. (40) Jewell, A. D.; Peng, G.; Mattera, M. F. G.; Lewis, E. A.; Murphy, C. J.; Kyriakou, G.; Mavrikakis, M.; Sykes, E. C. H. Quantum Tunneling Enabled Self-Assembly of Hydrogen Atoms on Cu(111). ACS Nano 2012, 6, 10115−10121. (41) van Helden, P.; van den Berg, J.-A.; Westrate, C. J. Hydrogen Adsorption on Co Surfaces: A Density Functional Theory and Temperature Programmed Desorption Study. ACS Catal. 2012, 2, 1097−1107. (42) Bridge, M. E.; Comrie, C. M.; Lambert, R. M. Hydrogen Chemisorption and the Carbon Monoxide-Hydrogen Interaction on Cobalt (0001). J. Catal. 1979, 58, 28−33. (43) Habermehl-Cwirzen, K. M. E.; Kauraala, K.; Lahtinen, J. Hydrogen on Cobalt: The Effects of Carbon Monoxide and Sulphur Additives on the D2/Co(0001) System. Phys. Scr. 2004, T108, 28−32. (44) Xu, L.; Ma, Y.; Zhang, Y.; Chen, B.; Wu, Z.; Jiang, Z.; Huang, W. Surface Chemistry of C2H4, CO, and H2 on Clean and Graphite Carbon-Modified Co(0001) Surfaces. J. Phys. Chem. C 2011, 115, 3416−3424.
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