Pd(100) Alloy Surface - ACS Publications

Article pubs.acs.org/JPCC

Structure of the Au/Pd(100) Alloy Surface Michael Garvey,† Jorge A. Boscoboinik,†,‡ Luke Burkholder,† Joshua Walker,† Craig Plaisance,§ Matthew Neurock,§ and Wilfred T. Tysoe*,† †

Department of Chemistry and Laboratory for Surface Studies, University of WisconsinMilwaukee, Milwaukee, Wisconsin 53211, United States ‡ Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany § Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904, United States ABSTRACT: The distribution of gold atoms on the surface of a Au/Pd(100) alloy with various gold coverages was explored using density functional theory (DFT) calculations and measurements of the low-energy electron diffraction (LEED) patterns. DFT calculation revealed the presence of first-, second-, and third-neighbor interactions. This contrasts the behavior of Au/Pd(111) alloys, where there were only nearestneighbor interactions between the surface gold and palladium atoms. The presence of longer-range interactions was confirmed by LEED, which showed c(2 × 2) structures for palladium coverages between 0.5 and ∼0.75 monolayers (ML) and a (3 × 3) pattern between 0.5 and ∼0.85 ML. The surface structure was simulated using first-, second-, and thirdneighbor interactions using Monte Carlo methods and was successfully able to reproduce the experimentally observed LEED patterns. The simulations were then used to calculate the variation in coverage of bridge-bonded carbon monoxide on the Au/ Pd(100) alloy as a function of alloy composition, which also agreed well with experiment.

1. INTRODUCTION Palladium−gold alloys are known to be active catalysts for a large number of reactions including CO oxidation,1−4 cyclotrimerization of acetylene to benzene,5,6 vinyl acetate monomer (VAM) synthesis,7−17 aerobic oxidation of alcohols to aldehydes or ketones,18−23 and the oxidation of hydrogen to hydrogen peroxide.24−38 Of particular importance for bimetallic systems that comprise reactive (palladium) and unreactive (gold) metals is the distribution of the components on the alloy surface to create active ensembles for a particular reaction. For example, the (100) face of gold−palladium alloy single crystals has been shown to be particularly active for VAM synthesis and has been rationalized by an ensemble effect14−16 where the active ensemble has been suggested to comprise two, isolated palladium atoms located on opposite corners of the surface unit cell that are surrounded by gold. Scanning-tunneling microscopy (STM) experiments of the surface of a Au/Pd(100) alloy single crystal have suggested that they occur with a greater frequency than would be expected for a purely random distribution.39 Analogous ensembles have been identified for acetylene cyclotrimerization,5 and density functional theory (DFT) calculations have suggested that isolated palladium atoms that are completely surrounded by gold are required to selectively hydrogenate dioxygen to hydrogen peroxide.40,41 The structures of Au/Pd(111) alloys have been investigated previously,42,43 and it has been found that the gold and palladium distribution on the surface of these alloys is not © 2012 American Chemical Society

completely random. Monte Carlo simulations, combined with DFT calculations, were used to model the variation in adsorbate coverage with alloy composition for such nonrandom systems.42,44 Carbon monoxide provides a particularly useful probe for the surface structures of alloys that comprise reactive and unreactive metals since the CO desorption temperatures and vibrational frequencies are very sensitive to the nature of the adsorption site.45−54 This strategy has been used to explore the surface structure of Au/Pd(100) alloys.55 CO desorbs from clean Pd(100) at 350 and 490 K and reflection−absorption infrared spectroscopy (RAIRS) reveals that CO occupies only bridge sites on this surface up to a coverage of 0.5 monolayers (ML), where it displays a c(2√2 × √2)R45° low-energy electron diffraction (LEED) pattern47,56,57 and desorbs at ∼490 K. CO stretching frequencies between ∼1895 and 1950 cm−1 are found as a function of gold coverage below 0.5 ML in accord with the assignment to adsorption on palladium−palladium bridge sites. Previous work for carbon monoxide on Au/Pd(100) alloys reveals that it adsorbs on palladium−palladium bridge sites at high palladium coverages, desorbing at ∼480 K at a gold coverage of ∼0.24 ML, and shifting slightly to a desorption Received: November 8, 2011 Revised: January 26, 2012 Published: January 31, 2012 4692

dx.doi.org/10.1021/jp2107445 | J. Phys. Chem. C 2012, 116, 4692−4697

The Journal of Physical Chemistry C

Article

Figure 1. (A) Structures of the Au/Pd(100) alloy surfaces used to calculate interaction energies and (B) a schematic depiction of the process used to obtain interaction energies. Yellow circles indicate gold surface atoms, and gray circles indicate palladium surface atoms.

temperature of ∼470 K at a gold coverage of 0.24 ML.55 However, the integrated intensity of the CO desorption state from palladium−palladium bridge sites (between 490 and 470 K) decreases with increasing gold coverage and disappears altogether for gold coverages greater than ∼0.5 ML. The slight shift in desorption temperature with increasing gold coverage implies that ligand effects of gold on the heat of adsorption of CO on bridge sites are relatively small. This indicates that the attenuation of the ∼490−470 K CO desorption state with increasing gold coverage is due to a decrease in the coverage of palladium−palladium bridge sites on the surface. The complete absence of this desorption state at gold coverages of ∼0.5 ML and higher implies that there are no nearest-neighbor palladium (i.e., palladium−palladium bridge) sites at gold coverages greater than 0.5 ML. Geometrically a gold (or palladium) coverage of 0.5 ML that has no nearest-neighbor palladium sites can only be accommodated by the surface forming an ordered c(2 × 2) superstructure. The presence of such ordered surface structures would suggest that the interactions between gold and palladium on the (100) face of the alloy are much larger than for Au/Pd(111) surfaces. This implies that, in contrast to Au/ Pd(111) alloys, where no additional ordered LEED patterns are observed,42,43 they should be present for Au/Pd(100) alloys. Accordingly, the LEED patterns of Au/Pd(100) alloys were measured as a function of composition to test this prediction. The significant Pd−Pd, Pd−Au, and Au−Au interactions are identified by using DFT calculations. In the case of Au/

Pd(111), this revealed that only nearest-neighbor interactions were important.42,43 The relevant interaction energies are incorporated into a Monte Carlo theory model to calculate the distributions of gold and palladium in the alloy.

2. EXPERIMENTAL METHODS The equipment used for collecting LEED patterns has been described in detail elsewhere.58 Briefly, LEED measurements were carried out in a μ-metal-shielded ultrahigh vacuum chamber operating at a base pressure of 5 × 10−11 Torr containing a Pd(100) single crystal, which could be cooled to 80 K and resistively heated to 1200 K. The Pd(100) single crystal was cleaned using a standard procedure, and its cleanliness was monitored using Auger spectroscopy and temperature-programmed desorption (TPD) collected following oxygen adsorption.59 Gold was evaporated from a small alumina tube furnace60 that allowed controlled and reproducible evaporation rates to be achieved. To control the source temperature, a C-type thermocouple was placed into the gold pellet. The amount of gold deposited onto the surface was monitored using Auger spectroscopy from the peak-to-peak intensities of the AuNVV and PdMNN Auger features, and the monolayer coverage was gauged from breaks in the gold uptake signal.54 The gold−palladium alloy was formed by initially depositing four monolayers of gold onto the Pd(100) substrate and then annealing to various temperatures for a period of five 4693



Article

at the expense of breaking two gold−palladium bonds. The change in energy associated with this process corresponds to

minutes in ultrahigh vacuum to produce the desired Au/Pd atomic ratio.54

ΔEc + a − 2b = Ec + Ea − 2Eb

3. THEORETICAL METHODS

= ε1PdPd + ε1AuAu − 2ε1PdAu

3.1. Density Functional Theory Calculations. Firstprinciples, periodic density functional theory (DFT) calculations were performed using the projector augmented wave (PAW) method61,62 as implemented in the Vienna ab initio simulation package (VASP) code.63−65 The exchange and correlation energies were calculated using the PBE (Perdew, Burke, and Ernzerhof66) form of the generalized gradient approximation (GGA). The wave functions and electron density were converged to within 1 × 10−5 eV, whereas geometric structures were optimized until the forces on the atoms were less than 0.03 eV/Å. 3.2. Monte Carlo Theory Simulations. The simulations follow the model previously developed for fcc(111) bimetallic alloys.42,43 Briefly, a square 60 × 60 lattice was used to represent an fcc(100) surface. For each coverage, the gold and palladium atoms were first randomly distributed on the surface, and then, using the Kawasaki algorithm,67 the exchange of gold and palladium atoms between different sites on the surface was attempted 3.6 × 108 times, which was found to be sufficient to reach equilibrium in the distribution of surface ensembles. The simulations were carried out using dimensionless parameters, wi = εi/kT, where εi is the ith-neighbor interaction defined as ε1 for the first, ε2 for the second, and ε3 for the third nearestneighbor interaction energies, respectively. The probability of an exchange event is defined based on a Boltzmann distribution and depends on the change in number of first-, second-, and third-neighbor interactions and is given by

where Ea, Eb, and Ec are the energies of the surfaces depicted in 1 1 Figures 1(A)(a), (b), and (c), respectively, and εPdPd , εAuAu , and 1 εPdAu are the palladium−palladium, gold−gold, and palladium− gold first-neighbor interaction energies. Since this energy corresponds to a change in first-neighbor interactions, it is defined as the first-neighbor interaction parameter, ε1 = ΔEc+a−2b. Similarly, for second-and third-neighbor interactions, the parameters are defined as ε2 = ΔEd+a−2b and ε3 = ((ΔEe+a−2b)/2) since, for third-neighbors, the change in energy is divided by two because structure 1(e) has two-third neighbor interactions as a result of the periodic boundary conditions in the calculation. The resulting energies for the various structures in Figure 1(A) are summarized in Table 1 as well as the calculated values Table 1. Energies of Structures Depicted in Figure 1 along with the Resulting Interaction Energies low gold coverage

1+

1 2 3 eΔnPdPdw1+ΔnPdPdw2 +ΔnPdPdw3

high gold coverage

structure

energy/eV

structure

energy/eV

a b c d e ε1 ε2 ε3

−309.180451 −307.565411 −305.919588 −305.966412 −305.96216 0.052 −0.016 −0.006

f g h i j ε1 ε2 ε3

−282.682404 −284.389108 −286.043556 −286.101614 −286.083888 0.052 −0.006 0.006

for the interaction parameters, εi. This indicates that the second- and third-nearest neighbor interactions do have an effect on the energies. This behavior is in contrast to what was observed previously for Au/Pd(111) alloys.42,43 4.2. Low-Energy Electron Diffraction of Au/Pd(100) Alloys. The results of the DFT calculations presented in Section 4.1 as well as experimental evidence that no nearestneighbor (palladium−palladium bridge) sites exist for gold coverages ≥0.5 ML55 imply that the surface should exhibit order. This was explored by measuring the LEED patterns as a function of the composition of the Au/Pd(100) alloys. In addition to the anticipated c(2 × 2) pattern, (3 × 3) patterns were also observed; the range over which they appear along with some typical LEED images are shown in Figure 2. 4.3. Monte Carlo Simulations of Au/Pd(100) Surfaces. The effect of interaction between first-, second-, and thirdneighbors was simulated using Monte Carlo methods. On the basis of the results of the DFT calculations of the various alloys in Section 4.1, the first-neighbor interactions were taken to be repulsive and the second-neighbor interactions to be attractive. Both repulsive and attractive interactions were considered for third-neighbor interactions. The energies were varied to reproduce the experimentally observed variation in palladium−palladium bridge-site CO coverage as a function of alloy composition. The resulting calculated structures were Fourier transformed to simulate the LEED patterns. The best agreement with the experimental data was found with a firstneighbor interaction energy (ε1) of 5.95 kJ/mol, a second

1

P=

(2)

(1)

i where ΔnPdPd is the change in the number of palladium− palladium ith-neighbors associated with the exchange event. The matrix with only the position of palladium atoms was plotted with Rasmol68 to generate simulated images of the surface, which were Fourier transformed with SPIP image analysis software to yield the corresponding simulated diffraction pattern.

4. RESULTS 4.1. Density Functional Theory Calculations of Au/ Pd(100) Surfaces. To establish the range of interaction energies on Au/Pd(100) alloys, the energies of a series of alloy surface structures with (4 × 4) unit cells, shown in Figure 1(A), were calculated without surface relaxation. As described in the previous section, the energies of structures 1(A)((a)−(e)) are used to gauge the importance of first, second, and thirdneighbor interactions for low gold coverages, while 1(A)((f)− (j)) are used for high gold coverages. For these calculations, three layers of palladium atoms were included in addition to the surface layer. In a hypothetical process of making surfaces 1(c) and 1(a) from two units of surface 1(b), as depicted in Figure 1(B), a gold−gold bond and a palladium−palladium bond are formed 4694



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Figure 2. LEED patterns of Au/Pd(100) alloys for palladium coverages of 0.50, 0.76, and 0.87 ML. Shown also are the coverage ranges over which the c(2 × 2), (3 × 3), and (1 × 1) diffraction patterns appear.

Figure 3. Plot of the integrated CO desorption yields from the ∼450 K state due to CO adsorbed on bridge sites (red circle) and the integrated area from CO desorbing from lower-temperature states (blue triangle) as a function of palladium coverage in the Au/Pd(100) alloys. Shown also are the coverage ranges over which the c(2 × 2), (3 × 3), and (1 × 1) LEED patterns appear and the simulated variation in coverage using a first-neighbor interaction parameter ε1/kT of −2.40, a second-neighbor interaction parameter ε2/kT of 1.20, and a third-neighbor interaction parameter ε3/kT of −1.60.

nearest-neighbor interaction energy (ε2) of −2.97 kJ/mol, and a third-neighbor interaction (ε3) of 3.97 kJ/mol, under the assumption that the systems exist at thermodynamic equilibrium at 298 K. Since the simulations were carried out using dimensionless parameters, wi = (εi)/(kT), if the equilibrium is reached at a different temperature, the magnitudes of εi will change, but the ratios between first-, second-, and thirdneighbor interaction energies will remain the same. The calculated coverages of bridge sites as a function of palladium coverage are plotted in Figure 3 (left panel) along with the normalized integral of the CO TPD peak corresponding to bridge sites (at ∼480 K55). The resulting calculated surface structures for selected alloys are displayed in Figure 4, along with their simulated LEED patterns, where good agreement with experiment is found. It should be noted that a simple Fourier transform assumes a single scattering event, while electron diffraction involves multiple scattering of the electron.69 Thus, the experimental relative spot intensities are not likely to be well reproduced. Monte Carlo simulations, using the above interaction energies, were subsequently carried out to calculate the number of palladium atoms that are completely surrounded by gold atoms (without any nearest-neighbor palladium atoms), and the results are also plotted in Figure 3 (right panel). The result is compared with the sum of the desorption yields from states other than that assigned to CO adsorbed on palladium− palladium bridge sites (taken from the TPD data presented in

ref 55). The correlation implies that the low-temperature desorption states are associated with a surface palladium atom with exclusively gold nearest neighbors. This idea will be used as a basis for the DFT calculations to establish the nature of these CO adsorption sites. Note that the number of gold and palladium atoms in next-nearest-neighbor sites will vary with gold coverage.

5. DISCUSSION The range of interactions found on the (100) face of model gold−palladium alloys is distinctly different from that found on the (111) face. In the latter case, both DFT calculations and the lack of any surface order by LEED indicated that only nearestneighbor interactions were important,42,43 resulting in an almost random distribution of the surface gold and palladium atoms. In contrast, both DFT calculations and LEED experiments (Figure 2) confirm the existence of longer-range interactions on the (100) face. Monte Carlo simulations were carried out by adjusting the values of εi to reproduce the experimental diffraction patterns (Figure 4) and the experimentally observed variation in CO desorption yield from palladium−palladium bridge sites with alloy composition (Figure 3). These results show that the Au/Pd(100) alloy forms ordered structures and rationalize the absence of CO on palladium−palladium bridge sites for gold coverages greater than 0.5 ML. 4695



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Figure 4. Depictions of the simulated surface structures by Monte Carlo theory and the Fourier transforms of the surface structures for palladium coverages of 0.5, 0.7, and 0.9 ML using a first-neighbor interaction parameter ε1/kT of −2.40, a second-neighbor interaction parameter ε2/kT of 1.20, and a third-neighbor interaction parameter ε3/kT of −1.60. The c(2 × 2), (3 × 3), and (1 × 1) unit cells are marked as dotted lines in the depictions for the coverages 0.5, 0.7, and 0.9 ML, respectively. Only the Pd atoms are shown for clarity. (2) Gao, F.; Wang, Y. L.; Goodman, D. W. J. Am. Chem. Soc. 2009, 131, 5734−5735. (3) Gao, F.; Wang, Y. L.; Goodman, D. W. J. Phys. Chem. C 2009, 113, 14993−15000. (4) Piednoir, A.; Languille, M. A.; Piccolo, L.; Valcarcel, A.; Aires, F. J. C. S.; Bertolini, J.-C. Catal. Lett. 2007, 114, 110−114. (5) Baddeley, C. J.; Tikhov, M.; Hardacre, C.; Lomas, J. R.; Lambert, R. M. J. Phys. Chem. 1996, 100, 2189−2194. (6) Baddeley, C. J.; Ormerod, R. M.; Stephenson, A. W.; Lambert, R. M. J. Phys. Chem. 1995, 99, 5146−5151. (7) U.S. Patent number 365888, 1967. (8) U.S. Patent number 08/670860, 1996. (9) Samanos, B.; Boutry, P.; Montarnal, R. J. Catal. 1971, 23, 19−30. (10) Moiseev, F. L.; Vargaftik, M. N. In Perspectives in Catalysis, Chemistry for the 21st Century; Thomas, J. M., Zamaraev, K. I., Eds.; Blackwell Scientific Publications: Oxford, 1992; p 91. (11) Stacchiola, D.; Calaza, F.; Burkholder, L.; Schwabacher, A. W.; Neurock, M.; Tysoe, W. T. Angew. Chem. 2005, 44, 4572−4574. (12) Stacchiola, D.; Calaza, F.; Burkholder, L.; Tysoe, W. T. J. Am. Chem. Soc. 2004, 126, 15384−15385. (13) Gao, F.; Wang, Y.; Calaza, F.; Stacchiola, D.; Tysoe, W. T. J. Mol. Catal. A: Chem. 2008, 281, 14−23. (14) Han, Y. F.; Kumar, D.; Sivadinarayana, C.; Clearfield, A.; Goodman, D. W. Catal. Lett. 2004, 94, 131−134. (15) Chen, M. S.; Kumar, D.; Yi, C. W.; Goodman, D. W. Science 2005, 310, 291−293. (16) Calaza, F.; Li, Z.; Gao, F.; Boscoboinik, J.; Tysoe, W. T. Surf. Sci. 2008, 602, 3523−3530. (17) Chen, M. S.; Luo, K.; Wei, T.; Yan, Z.; Kumar, D.; Yi., C. W.; Goodman, D. W. Catal. Today 2006, 117, 37−45. (18) Meenakshisundaram, S.; Nowicka, E.; Miedziak, P. J.; Brett, P. J.; Jenkins, R. L.; Dimitratos, N.; Taylor, S. H.; Knight, D. W.; Bethell, D.; Hutchings, G. J. Faraday Discuss. 2010, 145, 341−356.

6. CONCLUSIONS The distribution of gold and palladium atoms in a Au/Pd(100) alloy was modeled using DFT and explored experimentally by LEED. Both the calculations and diffraction results revealed that there are longer-range interactions on Au/Pd(100) alloys than for Au/Pd(111). These interactions were used in a Monte Carlo simulation of the surface structure that was able to successfully reproduce the experimentally observed diffraction patterns and variation in CO desorption yield with alloy composition. These results confirm previous assignments of the ∼480 K CO desorption state to CO adsorbed on bridge sites on adjacent palladium atoms that appear at low gold coverages.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (414) 229-5222. Fax: (414) 229-5036. E-mail: wtt@ uwm.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support of this work by the U.S. Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences, under grant number DE-FG0292ER14289.

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Pd(100) Alloy Surface - ACS Publications

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