Critical Au Concentration for the Stabilization of Au–Cu Nanoparticles

Letter pubs.acs.org/JPCL

Critical Au Concentration for the Stabilization of Au−Cu Nanoparticles on Rutile against Dissociation under Oxygen A. Wilson,†,‡,§ R. Bernard,†,‡ Y. Borensztein,†,‡ B. Croset,†,‡ H. Cruguel,†,‡ A. Vlad,§ A. Coati,§ Y. Garreau,∥ and G. Prévot*,†,‡ †

Institut des NanoSciences de Paris, Université Paris 6, 4, place Jussieu, 75252 Paris CEDEX 05, France Institut des NanoSciences de Paris, UMR CNRS 7588, 4, place Jussieu, 75252 Paris CEDEX 05, France § Synchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin - BP 48 91192 Gif-sur-Yvette CEDEX, France ∥ Université Paris Diderot, Sorbonne-Paris-Cité, MPQ, UMR 7162 CNRS, Bâtiment Condorcet, Case 7021, 75205 Paris CEDEX 13, France ‡

S Supporting Information *

ABSTRACT: Controlling aging of catalysts is of crucial importance to preserve their properties, in particular for bimetallic nanoparticles (NPs) where reaction can modify the composition. Herein, we have studied the stability upon oxygen exposure of gold−copper NPs supported on rutile. We have used in situ scanning tunneling microscopy to follow the evolution of individual Au, Cu and Au−Cu NPs with various compositions grown on the TiO2(110) surface, during each step from their nucleation to their modification with oxygen. We demonstrated a direct relation between the stability of the nanoparticles and their Au concentration. Whereas pure Cu nanoparticles dissociate under O2, Au−Cu NPs containing at least 20% Au are stable. This is explained by a modification of the local density of states of Cu atoms upon alloying.

O

from oxidation in air.28 In the case of Au−Cu, Cu improves the stability toward sintering of Au particles.23,29−31 On the other hand, the role of Au on the stability of Cu NPs is not so clear. Whereas scanning tunnelling microscopy (STM) observations show that small Cu nanoparticles on TiO2(110) substrates dissociate upon exposure to low pressure oxygen and disappear progressively,32,33 experiments during ambient pressure CO oxidation of Au−Cu nanoparticles on silica show that Cu is oxidized and form CuOx patches around the particles.18 Such behavior has also been observed for Au−Cu catalysts on TiO2.29 These studies thus raised the question of the stability of Au−Cu NPs in oxidant atmosphere, which could depend on their composition. To shed light on this question, we have followed in situ with STM the oxidation of Cu and Au−Cu NPs supported on

xide supported metallic nanoparticles (NPs) are widely used for catalytic applications.1,2 Among them, Cu NPs are efficient for various reactions,3−9 especially oxidation reactions such as partial oxidation of methanol,10 propene epoxidation,9 selective oxidation of methane,11 or CO oxidation.12 Cu is also used together with other metals to form bimetallic nanocatalysts such as Cu−Pd,13,14 Cu−Pt,13 or Au−Cu.15 Due to synergetic effects between the two metals, bimetallic NPs display catalytic properties different from those of the pure metals.16,17 For example, Au−Cu NPs have been shown to display higher reactivity and selectivity than pure Au or Cu metals of similar characteristics for numerous reactions such as CO oxidation, preferential oxidation (PROX), propene epoxidation, oxidations of alcohols, electrochemical reduction of CO2, or photocatalysis.15,18−26 Moreover, the NPs stability during a reaction is often higher for alloys than for pure metals. For Pd−Rh NPs on α-Al2O3, it has been shown that Rh prevents Pd-containing particles from sintering and that Pd hinders diffusion of Rh3+ ions into the bulk of the support at high temperatures.27 Au has also been shown to protect Pd NPs © XXXX American Chemical Society

Received: April 16, 2015 Accepted: May 15, 2015

2050

DOI: 10.1021/acs.jpclett.5b00791 J. Phys. Chem. Lett. 2015, 6, 2050−2055

Letter

The Journal of Physical Chemistry Letters

that the change in height of the NPs must be attributed to the detachment of Cu atoms from the islands, a modification of the tunneling conductance of the islands being excluded. Grainy features were also visible after 1 h exposure to 10−7 mbar O2 and attributed to small two-dimensional clusters of Cu oxide.32,33 We also observe such grainy features (Figure 1b) after oxygen exposure under the above-mentioned conditions. Figure 1b also shows that Au and Au−Cu NPs are stable. The height comparisons for Au and Au−Cu NPs before and after O2 exposure show no significant evolution. The mean height of Au NPs is stable, whereas the mean height of Au−Cu NPs is 0.04 nm smaller. This clearly shows that Au−Cu NPs are much more stable than pure Cu NPs. From the images presented here, it is also possible to evidence possible differences in shape for the NPs as a function of their composition. The width versus height relation of NPs located on terraces and measured from profiles performed along the same direction (fast scan direction) is shown in Figure 1c before oxygen exposure. In STM experiments, the topographic measurement corresponds to the convolution of the true nanoparticle profile with the profile of the tungsten tip. Here, the very same tip is used to scan the different particles so that the convolution effects are the same for all nanoparticles. We do not observe any difference between the shape of Au, Cu, or Au40Cu60 nanoparticles. This indicates that Au, Au40Cu60, and Cu nanoparticles have, for these small sizes, a similar aspect ratio. The same results are obtained after O2 exposure, for the Au and Au−Cu nanoparticles. Unfortunately, it is not possible to compare the profiles before and after oxygen exposure due to expected tip changes during oxidation. In order to quantify the role of Au atoms on the relative stability of the bimetallic NPs, we have prepared Au−Cu NPs with different Au concentrations on the same substrate and investigated their evolution as a function of oxygen exposure. The composition of each nanoparticle was determined from the sequence of evaporation and a variation of the mean composition on the sample surface was obtained through a motion of the STM tip during Au evaporation (complete details in the SI). Figure 2a−e show the evolution of an area, where pure Cu NPs are grown and exposed to oxygen. Figure 2a shows the bare TiO2 surface. Due to the reduction state of the sample, a high density of surface vacancies is present at the surface together with a small density of defects that can be identified as TiOx clusters.36 During Au evaporation, the area shown in Figure 2a is nearly always masked by the STM tip. Only very few Au NPs have grown, as can be seen in Figure 2b. After Cu evaporation, the density of NP has increased (Figure 2c), which can be attributed to nucleation of pure Cu NPs. After 1 h exposure 10−7 mbar O2, these Cu NPs begin to shrink (Figure 2d). After 1 h exposure at 10−5 mbar, they have entirely disappeared (Figure 2e), as already observed in Figure 1. The only remaining NPs are those already visible in Figure 2b, and thus containing Au atoms. Figure 2f−j show a similar evolution for Au−Cu NPs. Starting from a bare surface (Figure 2f), Au is first evaporated (Figure 2g) then Cu (Figure 2h). On this area, the mean composition is Au20Cu80. The comparison of Figure 2g,h shows that the initial Au NPs have grown due to incorporation of Cu,34 and that a few additional NPs, which are presumably pure Cu, have also formed. Upon exposure to 10−7 mbar O2 (Figure 2i), no clear evolution is visible. After exposure at 10−5 mbar O2 (Figure 2j), the comparison with Figure 2g shows that pure Cu NPs have disappeared whereas bimetallic particles are still present.

TiO2(110). This study relies on two key points. First, we have individually measured the composition of the NPs, and second, we have simultaneously monitored four regions of the sample corresponding to Au, Cu, Au−Cu NPs, and a bare surface as a reference. We have followed the NPs from their synthesis to their exposure to low pressure of oxygen (up to 10−4 mbar), thus in the exact same imaging and experimental conditions for all compositions. For this purpose, we have used a sequential deposition procedure at room temperature, evaporating first Au then Cu, in order to grow bimetallic NPs.34 The composition of a NP is derived from the comparison of its height after Au deposition and after Cu deposition. Moreover, we have adopted a synthesis procedure derived from the one proposed by Kolmalov and Goodman,35 by keeping the STM tip in tunneling conditions above the TiO2 surface during evaporation. This creates two shadow cones, free of Au and Cu, respectively, and four regions on the sample (complete details are in the Supporting Information (SI)). Figure 1a shows an STM image of the surface of the TiO2(110) sample after evaporation of Au followed by

Figure 1. TiO2(110) surface under UHV (a) and after exposure to O2 during a few minutes at 10−5 mbar (b). Size 170 × 170 nm2. (c) Full width at half-maximum (fwhm) of the profiles of NPs along the x direction as a function of their height. Blue crosses: Au, red crosses: Cu, black circles: Au−Cu.

evaporation of Cu. The scanned area is located near the intersection of the two shadow cones, and the four regions are clearly visible. The left side of the image corresponds to the bare TiO2(110) surface, the top and bottom sides to Au and Cu NPs, respectively. The right side corresponds Au−Cu NPs with a mean composition of Au40Cu60. Figure 1b shows the same surface after O2 exposure to 10−6 mbar. Cu NPs have entirely disappeared, and the region initially covered by Cu resembles the bare surface. These results are in good agreement with the observations of Zhou et al.:32,33 for these ranges of pressure and NP size, Cu NPs supported on TiO 2 (110) shrink upon oxygen exposure, while their morphology (i.e., aspect ratio) is preserved. They have shown 2051

DOI: 10.1021/acs.jpclett.5b00791 J. Phys. Chem. Lett. 2015, 6, 2050−2055

Letter

The Journal of Physical Chemistry Letters

We have compared the height of the NPs after growth with their height after exposure to 10−5 mbar O2. A complete investigation has been performed for different compositions of Au−Cu NPs. Figure 3a shows the comparison for 275 NPs of

Figure 3. (a) Height of the Au−Cu NPs after 10−5 mbar O2 exposure as a function of their height after deposition. The color of the dots is related to the initial Au concentration through the color-scale bar. (b) Ratio of the final/initial height as a function of the composition. The green line corresponds to a lowering of the Cu contents up to 20% Au for NPs of Au concentration lower than 20%.

various composition (indicated by the dot color), and located within the TiO2 terraces. Green dots correspond to Cu NPs, red dots to Au NPs, and intermediate colors to Au−Cu NPs (see color bar). As expected, numerous NPs have totally disappeared (green dots in the bottom of the Figure, with final height equal to zero), and correspond to pure Cu NPs or NPs displaying a small Au concentration. Contrariwise, for a Au concentration greater than 40% (red dots), only small height variations occur. In Figure 3b, the ratio between the height of the NPs after and before oxygen exposure is shown as a function of their composition. Each point corresponds to a mean variation computed for a group of particles of similar composition. For Au concentration higher than 20%, the height remains almost unchanged. Below 20%, the height ratio falls down rapidly, but not to zero, indicating that a part of Cu atoms leaves the NPs. This evidences the fact that a very small Au amount has a huge effect on the stabilization of the NPs. In particular, a few percent of Au may be sufficient to enhance the stability of a Cu NP. For these small NPs, this corresponds to only one or two Au atoms. This is also evidenced by the fact that on regions where no Au has been evaporated, Cu NPs have fully disappeared after O2 exposure to 10−5 mbar, whereas on regions where Au has been evaporated, some NPs that were identified as pure Cu remain visible after a similar exposure. This could be explained by the fact that Au atoms have diffused and reached these NPs before or during O2 exposure, and have thus stabilized them. This phenomenon must, however, be limited since in these regions, a majority of NPs identified as pure Cu disappear during oxygen exposure. Dissociation of Cu NPs on TiO2(110) substrates is known to occur for oxygen pressure as low as 10−7 mbar.32,33 From STM observations, it has been proposed that Cu 3D particles dissociate into grainy features, which have approximately the height (0.2−0.3 nm) of 2D islands.32 Such grainy features are also visible after O2 exposure in the regions free of NPs, and could thus also be related to Ti interstitials that segregate from the near surface region to form TiOx clusters at the surface.36 Regardless of the mechanism of Cu dispersion on the surface, it appears from Figure 3 that above a minimum Au concentration

Figure 2. Evolution of the TiO2(110) surface as seen by STM during sequential evaporation of Au and Cu and O2 exposure. Each column corresponds to the same area of the sample (34 × 34 nm2). (a,f) Bare surface. (b,g) After Au evaporation. The area in panel b is in the shadow cone of the STM tip, and only a few Au NPs have nucleated (one is indicated with a black circle). (c,h) After Cu evaporation. On the Au-covered area, only a few pure Cu NPs have nucleated (one is indicated with a black circle). (d,i) After exposure to 10−7 mbar O2. (e,j) After exposure to 10−5 mbar O2. 2052

DOI: 10.1021/acs.jpclett.5b00791 J. Phys. Chem. Lett. 2015, 6, 2050−2055

Letter

The Journal of Physical Chemistry Letters cmin Au , Cu atoms stop leaving the NPs. Moreover, bimetallic NPs for which Au content cAu < cmin Au do not completely disappear. In fact, assuming that Au atoms are not mobile, the leaving of Cu atoms induces an increase of the Au concentration in the NPs. We have fitted the experimental values of Figure 3b with the assumption that NPs with Au concentration below cmin Au lose Cu min atoms until cAu reaches cmin Au . A good fit is obtained for cAu = 20% Au. Thus, 20% Au in the particles prevents Cu atoms from leaving the particles and diffusing on the TiO2 surface. The dissociation of metal nanoparticles on oxide support in the presence of reactants is generally interpreted as the result of a large Gibbs free energy of adsorption of reactants on the metal adatoms, and a lower energy barrier for an atom-reactant complex that leaves a particle.37 In our case, the enhanced stability of Au−Cu NPs may have either a thermodynamic or a kinetic origin. The stability of Cu atoms in a Au−Cu NP could be due to a higher cohesive energy of Cu atoms inside the alloy, through the negative free enthalpy of mixing ΔGmix. At room temperature, entropy can be neglected and the chemical potential can easily be derived from the enthalpy of mixing ΔHmix.38 In a first-order approximation and for a regular solution, ΔH0mix = xCu(1 − xCu)ΔH0mix, where ΔH0mix is constant and the contribution of the mixing enthalpy to the chemical potential of Cu atoms is Δμ = (1 − xCu)2ΔH0mix.38 For Au−Cu, ΔH0mix is not strictly constant. A minimum value ΔH0mix = −0.23 eV has been measured for xCu = 0.8.39 From these results, the variation of chemical potential Δμ expected for Cu in Au20Cu80 as compared to bulk Cu is of the order of 0.01 eV, and can thus be neglected. The stability of Cu atoms could also be favored by Au segregation at the surface that could occur at the surface of the NP.40 Such phenomenon has been shown for the surface of bulk alloys. In the case of Cu3Au, the (100) surface has a 50%− 50% composition that hinder O2 dissociation.41 In the case of NPs, experiments on Pt−Au and Rh−Au NPs supported on TiO2(110) have also shown that, under UHV, Au segregates to the surface of the NP.43,44 For Au−Pd NPs, Au segregation prevents CO oxidation in the 10−7 mbar pressure range.45 However, in the present case of Au−Cu/TiO2(110), our X-ray diffraction measurements did not evidence a significant Au segregation at the surface when Cu is deposited on Au NPs.34 Moreover, the surface composition in the presence of oxygen could be different from the composition under UHV. For Cu3Au(110), if the surface composition is also 50%−50% under UHV, exposition to 5.10−8 Torr of oxygen induces a Cu segregation and formation of Cu−O added rows.42 Cu segregation in the presence of oxygen is expected to occur also easily for NPs due to low-coordinated sites in the vertices and edges of the NPs. Such behavior has been observed for Rh−Pd NPs, where Pd is found at the surface under UHV and Rh under oxidizing environment.46 A more probable origin of the stability of the Au−Cu NPs is the modification of the electronic structure of Cu upon alloying, leading to a lower adsorption energy of oxygen molecules on the alloyed NPs. For example, for Ag−Cu nanoparticles, oxygen adsorption is weakened due to electron transfer from Cu to Ag atoms.47 More generally, it has been shown that chemisorption on transition metals is strongly correlated to the position of the d-band center in the local electronic density of states (LDOS).48 Chemisorption is stronger when the d-band is closer to the Fermi level. This position can be strongly modified upon alloying, in particular

with Au for which the d-center is far from the Fermi level. This is the so-called ligand effect. Such modification of the adsorption energy upon alloying has been, for example, evidenced for oxygen adsorption on the surface of various Au-based alloys. The absolute value of the adsorption energy of atomic and molecular oxygen on the surface of a transition metal decreases upon alloying with Au. Shifts of 0.8, 1.1, and 1.6 eV have been calculated for Pt, Pd, and Ni alloyed with Au, respectively, as compared to the values obtained for the pure metals.49 In the case of Au−Cu, the ligand effect has been used to explain that the dissociative sticking probability of O2 is much lower on Cu3Au(100) than on Cu(100).50 Strain also modifies the d-band center position. When a metal atom with a small lattice constant is put as an impurity or overlayer at the surface of a metal with a larger lattice constant, the embedding electron density and therefore the local dbandwidth are lowered, and the center of the d band is caused to shift up in order to preserve the degree of d-band filling.51 This is the case for Cu on Au(111).51 For a Cu atom inserted in a Au(111) surface, a shift of 0.55 eV of the d-band center of the LDOS on the Cu atom has been computed, and for a Cu pseudomorphic layer on Au(111), a shift of 0.70 eV is calculated.51 The corresponding binding energy of oxygen is thus enhanced by 0.68 eV.52 For nanoparticles for which stress relaxation is easier, strain effects should play a lower role, and the shift of the d band center should mainly be driven by ligand effects. Recent DFT calculations have shown that O2 adsorption was weaker on Au3Cu3 clusters as compared to Cu6 clusters. This is also in agreement with experiments showing that the rate of oxidation of AuCu NPs decreases with increasing Au content.53 Thus, one can conclude that alloying with Au atoms reduces the dissociative adsorption of oxygen on the NP, and increases the energy barrier to overcome for the formation of a Cu adatom-oxygen complex. This should explain why alloying with Au contributes to stabilize Cu NPs. In summary, we have observed by STM the growth and evolution upon oxygen exposure of Au, Cu and Au−Cu NPs. In these experiments, the same NPs have been followed during the whole process. This enabled us to measure the height evolution of the NPs as a function of their composition. By this way, we have demonstrated that, whereas Cu NPs dissociate upon oxygen exposure, Au−Cu NPs are stable if their Au concentration is higher than 20%. This stabilization most likely results from a lowering of the oxygen adsorption energy induced by a modification of the NP electronic structure.

■

ASSOCIATED CONTENT

S Supporting Information *

Experimental details, methods used for data acquisition and corrections, details of height measurements, and composition calculations. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpclett.5b00791.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2053

DOI: 10.1021/acs.jpclett.5b00791 J. Phys. Chem. Lett. 2015, 6, 2050−2055

Letter

The Journal of Physical Chemistry Letters

■

over Au-Cu/TiO2 Alloy Catalysts. J. Mol. Catal.: Chem. 2007, 274, 159−168. (21) Llorca, J.; Dominguez, M.; Ledesma, C.; Chimentao, R. J.; Medina, F.; Sueiras, J.; Angurell, I.; Seco, M.; Rossell, O. Propene Epoxidation over TiO2-Supported Au−Cu Alloy Catalysts Prepared from Thiol-Capped Nanoparticles. J. Catal. 2008, 258, 187−198. (22) Della Pina, C.; Falletta, E.; Rossi, M. Highly Selective Oxidation of Benzyl Alcohol to Benzaldehyde Catalyzed by Bimetallic GoldCopper Catalyst. J. Catal. 2008, 260, 384−386. (23) Liu, X.; Wang, A.; Wang, X.; Mou, C.-Y.; Zhang, T. Au−Cu Alloy Nanoparticles Confined in SBA-15 as a Highly Efficient Catalyst for CO Oxidation. Chem. Commun. 2008, 27, 3187−3189. (24) Mozer, T. S.; Dziuba, D. A.; Vieira, C. T. P.; Passos, F. B. The Effect of Copper on the Selective Carbon Monoxide Oxidation over Alumina Supported Gold Catalysts. J. Power Sources 2009, 187, 209− 215. (25) Kim, D.; Resasco, J.; Yu, Y.; Asiri, A. M.; Yang, P. Synergistic Geometric and Electronic Effects for Electrochemical Reduction of Carbon Dioxide Using Gold−Copper Bimetallic Nanoparticles. Nat. Commun. 2014, 5, 4948. (26) Hai, Z.; El Kolli, N.; Chen, J.; Remita, H. Radiolytic Synthesis of Au−Cu Bimetallic Nanoparticles Supported on TiO2: Application in Photocatalysis. New J. Chem. 2014, 38, 5279−5286. (27) Vedyagin, A. A.; Volodin, A. M.; Stoyanovskii, V. O.; Kenzhin, R. M.; Slavinskaya, E. M.; Mishakov, I. V.; Plyusnin, P. E.; Shubin, Y. V. Stabilization of Active Sites in Alloyed Pd−Rh Catalysts on Γ-Al2O3 Support. Catal. Today 2014, 238, 80−86. (28) Ward, T.; Delannoy, L.; Hahn, R.; Kendell, S.; Pursell, C. J.; Louis, C.; Chandler, B. D. Effects of Pd on Catalysis by Au: CO Adsorption, CO Oxidation, and Cyclohexene Hydrogenation by Supported Au and Pd−Au Catalysts. ACS Catal. 2013, 3, 2644−2653. (29) Sandoval, A.; Louis, C.; Zanella, R. Improved Activity and Stability in CO Oxidation of Bimetallic Au-Cu/TiO2 Catalysts Prepared by Deposition-Precipitation with Urea. Appl. Catal., B 2013, 140, 363−377. (30) Liu, X.; Wang, A.; Zhang, T.; Su, D.-S.; Mou, C.-Y. Au−Cu Alloy Nanoparticles Supported on Silica Gel as Catalyst for CO Oxidation: Effects of Au/Cu Ratios. Catal. Today 2011, 160, 103−108. (31) Sangeetha, P.; Zhao, B.; Chen, Y.-W. Au/CuOx−TiO2 Catalysts for Preferential Oxidation of CO in Hydrogen Stream. Ind. Eng. Chem. Res. 2010, 49, 2096−2102. (32) Zhou, J.; Kang, Y. C.; Chen, D. A. Oxygen-Induced Dissociation of Cu Islands Supported on TiO2 (110). J. Phys. Chem. B 2003, 107, 6664−6667. (33) Zhou, J.; Kang, Y.; Ma, S.; Chen, D. Adsorbate-Induced Dissociation of Metal Clusters: TiO2(110)-Supported Cu and Ni Clusters Exposed to Oxygen Gas. Surf. Sci. 2004, 562, 113−127. (34) Wilson, A.; Bernard, R.; Vlad, A.; Borensztein, Y.; Coati, A.; Croset, B.; Garreau, Y.; Prévot, G. Epitaxial Growth of Bimetallic Au− Cu Nanoparticles on TiO2(110) Followed by Scanning Tunneling Microscopy and Grazing-Incidence X-ray Diffraction. Phys. Rev. B 2014, 90, 075416. (35) Kolmakov, A.; Goodman, D. W. In Situ Scanning Tunneling Microscopy of Oxide-Supported Metal Clusters: Nucleation, Growth, and Thermal Evolution of Individual Particles. Chem. Rec. 2002, 2, 446−457. (36) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53−229. (37) Ouyang, R.; Liu, J.-X.; Li, W.-X. Atomistic Theory of Ostwald Ripening and Disintegration of Supported Metal Particles under Reaction Conditions. J. Am. Chem. Soc. 2013, 135, 1760−1771. (38) Alloyeau, D.; Prévot, G.; Le Bouar, Y.; Oikawa, T.; Langlois, C.; Loiseau, A.; Ricolleau, C. Ostwald Ripening in Nanoalloys: When Thermodynamics Drives a Size-Dependent Particle Composition. Phys. Rev. Lett. 2010, 105, 255901. (39) Okamoto, H.; Chakrabarti, D. J.; Laughlin, D. E.; Massalski, T. B. The Au−Cu (Gold−Copper) System. J. Phase Equilib. 1987, 8, 454−474.

REFERENCES

(1) Henry, C. R. Surface Studies of Supported Model Catalysts. Surf. Sci. Rep. 1998, 31, 235−325. (2) Gates, B. C. Supported Metal Clusters: Synthesis, Structure, and Catalysis. Chem. Rev. 1995, 95, 511−522. (3) Men, Y.; Gnaser, H.; Zapf, R.; Hessel, V.; Ziegler, C. Parallel Screening of Cu/CeO2/γ-Al2O3 Catalysts for Steam Reforming of Methanol in a 10-Channel Micro-structured Reactor. Catal. Commun. 2004, 5, 671−675. (4) Peppley, B. A.; Amphlett, J. C.; Kearns, L. M.; Mann, R. F. Methanol−Steam Reforming on Cu/ZnO/Al2O3. Part 1: The Reaction Network. Appl. Catal. Gen. 1999, 179, 21−29. (5) Peppley, B. A.; Amphlett, J. C.; Kearns, L. M.; Mann, R. F. Methanol−Steam Reforming on Cu/ZnO/Al2O3 Catalysts. Part 2. A Comprehensive Kinetic Model. Appl. Catal. Gen. 1999, 179, 31−49. (6) Wang, X.; Rodriguez, J. A.; Hanson, J. C.; Gamarra, D.; MartínezArias, A.; Fernández-García, M. In Situ Studies of the Active Sites for the Water Gas Shift Reaction over Cu−CeO2 Catalysts: Complex Interaction between Metallic Copper and Oxygen Vacancies of Ceria. J. Phys. Chem. B 2006, 110, 428−434. (7) Zhang, M.; Huang, Y.; Li, R.; Li, G.; Yu, Y. A DFT Study of Ethanol Adsorption and Dehydrogenation on Cu/Cr2O3 Catalyst. Catal. Lett. 2014, 144, 1978−1986. (8) Delannoy, L.; Thrimurthulu, G.; Reddy, P. S.; Méthivier, C.; Nelayah, J.; Reddy, B. M.; Ricolleau, C.; Louis, C. Selective Hydrogenation of Butadiene over TiO2 Supported Copper, Gold and Gold−Copper Catalysts Prepared by Deposition−Precipitation. Phys. Chem. Chem. Phys. 2014, 16, 26514−26527. (9) Vaughan, O. P. H.; Kyriakou, G.; Macleod, N.; Tikhov, M.; Lambert, R. M. Copper as a Selective Catalyst for the Epoxidation of Propene. J. Catal. 2005, 236, 401−404. (10) Navarro, R. M.; Peña, M. A.; Fierro, J. L. G. Production of Hydrogen by Partial Oxidation of Methanol over a Cu/ZnO/Al2O3 Catalyst: Influence of the Initial State of the Catalyst on the Start-up Behaviour of the Reformer. J. Catal. 2002, 212, 112−118. (11) An, D.; Zhang, Q.; Wang, Y. Copper Grafted on SBA-15 as Efficient Catalyst for the Selective Oxidation of Methane by Oxygen. Catal. Today 2010, 157, 143−148. (12) Tu, C.-H.; Wang, A.-Q.; Zheng, M.-Y.; Wang, X.-D.; Zhang, T. Factors Influencing the Catalytic Activity of SBA-15-Supported Copper Nanoparticles in CO Oxidation. Appl. Catal., A: Gen. 2006, 297, 40−47. (13) Toshima, N.; Wang, Y. Preparation and Catalysis of Novel Colloidal Dispersions of Copper/Noble Metal Bimetallic Clusters. Langmuir 1994, 10, 4574−4580. (14) Mazumder, V.; Chi, M.; Mankin, M. N.; Liu, Y.; Metin, O.; Sun, D.; More, K. L.; Sun, S. A Facile Synthesis of MPd (M = Co, Cu) Nanoparticles and Their Catalysis for Formic Acid Oxidation. Nano Lett. 2012, 12, 1102−1106. (15) Bracey, C. L.; Ellis, P. R.; Hutchings, G. J. Application of Copper−Gold Alloys in Catalysis: Current Status and Future Perspectives. Chem. Soc. Rev. 2009, 38, 2231−2243. (16) Ferrando, R.; Jellinek, J.; Johnston, R. L. Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles. Chem. Rev. 2008, 108, 845−910. (17) Cuenya, B. R. Synthesis and Catalytic Properties of Metal Nanoparticles: Size, Shape, Support, Composition, and Oxidation State Effects. Thin Solid Films 2010, 518, 3127−3150. (18) Liu, X.; Wang, A.; Li, L.; Zhang, T.; Mou, C.-Y.; Lee, J.-F. Structural Changes of Au−Cu Bimetallic Catalysts in CO Oxidation: In Situ XRD, EPR, XANES, and FT-IR Characterizations. J. Catal. 2011, 278, 288−296. (19) Bauer, J. C.; Toops, T. J.; Oyola, Y.; Parks, J. E., II; Dai, S.; Overbury, S. H. Catalytic Activity and Thermal Stability of Au−CuO/ SiO2 Catalysts for the Low Temperature Oxidation of CO in the Presence of Propylene and NO. Catal. Today 2014, 231, 15−21. (20) Chimentao, R. J.; Medina, F.; Fierro, J. L. G.; Llorca, J.; Sueiras, J. E.; Cesteros, Y.; Salagre, P. Propene Epoxidation by Nitrous Oxide 2054

DOI: 10.1021/acs.jpclett.5b00791 J. Phys. Chem. Lett. 2015, 6, 2050−2055

Letter

The Journal of Physical Chemistry Letters (40) Christensen, A.; Ruban, A. V.; Stoltze, P.; Jacobsen, K. W.; Skriver, H. L.; Nørskov, J. K.; Besenbacher, F. Phase Diagrams for Surface Alloys. Phys. Rev. B 1997, 56, 5822. (41) Niehus, H.; Achete, C. Surface Structure Investigation of Nitrogen and Oxygen on Cu3Au(100). Surf. Sci. 1993, 289, 19−29. (42) Morgenstern, K.; Niehus, H.; Comsa, G. Reconstruction of the Oxygen Covered Cu3Au(110) Surface Investigated by Low Energy Ion Back Scattering and LEED. Surf. Sci. 1995, 338, 1−10. (43) Ó vári, L.; Bugyi, L.; Majzik, Z.; Berkó, A.; Kiss, J. Surface Structure and Composition of Au−Rh Bimetallic Nanoclusters on TiO2(110): A LEIS and STM Study. J. Phys. Chem. C 2008, 112, 18011−18016. (44) Tenney, S. A.; Ratliff, J. S.; Roberts, C. C.; He, W.; Ammal, S. C.; Heyden, A.; Chen, D. A. Adsorbate-Induced Changes in the Surface Composition of Bimetallic Clusters: Pt−Au on TiO2 (110). J. Phys. Chem. C 2010, 114, 21652−21663. (45) Gao, F.; Wang, Y.; Goodman, D. W. Reaction Kinetics and Polarization-Modulation Infrared Reflection Absorption Spectroscopy (PM-IRAS) Investigation of CO Oxidation over Supported Pd−Au Alloy Catalysts. J. Phys. Chem. C 2010, 114, 4036−4043. (46) Tao, F.; Grass, M. E.; Zhang, Y.; Butcher, D. R.; Renzas, J. R.; Liu, Z.; Chung, J. Y.; Mun, B. S.; Salmeron, M.; Somorjai, G. A. Reaction-Driven Restructuring of Rh−Pd and Pt−Pd Core−Shell Nanoparticles. Science 2008, 322, 932−934. (47) Kim, N. R.; Shin, K.; Jung, I.; Shim, M.; Lee, H. M. Ag−Cu Bimetallic Nanoparticles with Enhanced Resistance to Oxidation: A Combined Experimental and Theoretical Study. J. Phys. Chem. C 2014, 118, 26324−26331. (48) Hammer, B.; Norskov, J. K. Theoretical Surface Science and Catalysis - Calculations and Concepts. In Impact of Surface Science on Catalysis; Advances in Catalysis Book Series; Gates, B. C., Knozinger, H., Eds.; Elsevier Academic Press Inc: San Diego, 2000; Vol 45. (49) Dhouib, A.; Guesmi, H. DFT Study of the M Segregation on MAu Alloys (M = Ni, Pd, Pt) in Presence of Adsorbed Oxygen O and O2. Chem. Phys. Lett. 2012, 521, 98−103. (50) Okada, M.; Moritani, K.; Fukuyama, T.; Mizutani, H.; Yoshigoe, A.; Teraoka, Y.; Kasai, T. Comparative Study of Oxidation on Cu and Cu3Au Surfaces with a Hyperthermal O2 Molecular Beam. Surf. Sci. 2006, 600, 4228−4232. (51) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Norskov, J. K. Surface Electronic Structure and Reactivity of Transition and Noble Metals. J. Mol. Catal.: Chem. 1997, 115, 421−429. (52) Greeley, J.; Nørskov, J. K. A General Scheme for the Estimation of Oxygen Binding Energies on Binary Transition Metal Surface Alloys. Surf. Sci. 2005, 592, 104−111. (53) Xu, Z.; Lai, E.; Shao-Horn, Y.; Hamad-Schifferli, K. Compositional Dependence of the Stability of AuCu Alloy Nanoparticles. Chem. Commun. 2012, 48, 5626.

2055

DOI: 10.1021/acs.jpclett.5b00791 J. Phys. Chem. Lett. 2015, 6, 2050−2055