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Influence of α-alumina supports on oxygen binding to Pd, Ag, Pt, and Au
Chemical Physics Letters 484 (2010) 231–236
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Influence of a-alumina supports on oxygen binding to Pd, Ag, Pt, and Au Li Xiao 1, William F. Schneider * Department of Chemical and Biomolecular Engineering and Department of Chemistry and Biochemistry, University of Notre Dame, 182 Fitzpatrick Hall, Notre Dame, IN 46556, United States
a r t i c l e
i n f o
Article history: Received 21 August 2009 In final form 10 November 2009 Available online 14 November 2009
a b s t r a c t The influence of a metal oxide support on the reactivity of adsorbed metals is a key question in surface reactivity and heterogeneous catalysis. Density functional theory (DFT) calculations are used here to examine support effects on one diagnostic of reactivity, oxygen binding, for Pd, Pt, Ag, and Au atoms supported on two representations of an a-alumina surface. Supported metal atoms uniformly bind oxygen more strongly than unsupported ones, so that even supported Au exothermically binds O. Further, oxygen adsorption is enhanced on a hydroxylated support by spillover of H from support to M–O bond. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Practical heterogeneous catalysts are commonly composed of catalytic metals dispersed onto a high surface area metal oxide support, an approach that provides an inexpensive and effective way to maximize the accessibility of metal surface to reacting gas [1]. There are many examples in which the catalyst support significantly influences the reactivity of a supported metal. The nature of the metal–support interaction and its implications for reactivity are thus clearly of key importance in understanding, developing, and applying these materials [2]. These questions of support effects become particularly germane as one moves from nanoscale particles containing thousands of atoms to the most dispersed, few atom molecular clusters, where interactions with support are expected to play a particularly significant role in reactivity [3]. Catalysis occurs within a reactive environment, and there is a growing recognition of the importance of this environment in dictating the active form of a catalytic material. For instance, much attention and debate has surrounded the involvement of surface oxides formed in situ during catalytic oxidations over the nominally noble metals Pd [4,5], Ag [6–10], Pt [11,12], and even Au [13]. Similar issues emerge in understanding supported metal particles, although here the complexity of the materials greatly increases the challenges, as both metal and support are susceptible to modifications by the environment [14]. Much of this work has taken advantage of surface science techniques able to probe the chemical state of extended surfaces. Density functional theory (DFT) calculations have also featured prominently in characterizing the state of catalytic materials under reactive conditions, as
* Corresponding author. Fax: +1 574 631 8366. E-mail address: [email protected] (W.F. Schneider). 1 Current address: Accelrys Inc., 10188 Telesis Court, Suite 100, San Diego, CA 92121-4779, United States. 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.11.019
they provide a means of systematically probing the structure and stability of various states of a surface or particle [15–17]. Here, we report plane-wave, supercell DFT simulations designed to examine the effect of catalyst support on one probe of reactivity, oxygen affinity, for supported Pd, Ag, Pt, and Au metal atoms (M):
M=support þ 1=2O2 $ O—M=support
ð1Þ
We consider two different support models, a stoichiometric
a-alumina (0 0 0 1) surface and a gibbsite-like hydroxylated a-alumina (0 0 0 1) one, the former representative of alumina in a dry, water-free environment and the latter of alumina in a humid environment [18,19]. We find in general that the presence or absence of support has a significant impact on the M–O bond strength, reversing, for instance, the unfavorable energetics of reaction (1) when M@Au. The supported metal atoms other than Ag are found to have high affinity for oxygen, and this affinity is consistently enhanced by hydrogen spillover to the metal atom at the hydroxylated surface. The results illustrate the synergy between support and reactive environment in dictating the local structure and reactivity of supported metal atoms. 2. Computational details Plane-wave, supercell DFT calculations were carried with the VASP code [20–22]. The interaction of valence electrons with frozen cores were described using the projector augmented wave (PAW) method [23], and plane waves were included to an energy cutoff of 400 eV. The exchange and correlation energies were computed using the Perdew-Wang 91 form (PW91) of the generalized gradient approximation (GGA) [24]. A Gaussian smearing function with a width of 0.05 eV was applied to states near the Fermi level; converged occupancy numbers are integer for these insulating
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systems. Electronic energies were converged to 104 eV, and ionic relaxations were considered converged when the forces on the ions were less than 0.03 eV/ Å. Surface simulations are carried out using a 2 2 supercell of the a-Al2O3 (0 0 0 1) surface. The alumina slab contained 12 atomic layers, with the seven top-most ones allowed to relax from bulk locations. A vacuum region of 18 Å was applied in the c-direction to eliminate interactions between slab images. Overall supercell dimensions were 9.61 9.61 26.34 Å and contained 80 atoms. A 3 3 1 C-centered k-point mesh was used. The fully hydroxylated, gibbsite-like surface model had similar construction save the replacement of the four surface Al atoms with 12 hydrogen. A single metal atom deposited on the 2 2 surface results in a coverage of 1/12 geometrical monolayer (ML) (one metal atom per 12 surface oxygen atoms). Adsorbate atoms are placed off symmetry sites to avoid artificial constraints during geometry relaxations. Isolated atoms and molecules are simulated within 10 10 10 Å supercells.
3. Results and discussion 3.1. M/alumina surface models Fig. 1 shows top and side views of the dry and fully hydroxylated alumina surfaces. The dry surface exposes oxygen in the second (O(2)) and fifth (O(5)) subsurface layers. These oxygen are interconnected by top (Al(1)) and subsurface (Al(3) and Al(4)) ions. Pd, Pt, and Au atoms have previously been shown to prefer to bind atop single O(2) sites through a covalent mechanism; more readily oxidized Ag prefers a threefold hollow above O(5) [19]. On the fully hydroxylated surface three H atoms replace each Al(1), forming hydroxyls that orient parallel to and away from the surface plane [18,25]. The hydroxylated surface differs from the dry in exposing threefold hydroxyl sites above sixth-layer Al atoms, Al(6). These surface hydroxyls dynamically reorient at room temperature [19]. In the static simulations here we consider one representative realization of this surface, including one in-plane hydroxyl and two out-of-plane hydroxyl per Al removed. Metal atoms prefer to bind near in-plane hydroxyl to avoid the pendant hydrogen atoms and exhibit coordination preferences similar to the dry surface.
Hydroxylation uniformly decreases the binding energy of metal atoms to the oxide [19]. 3.2. O binding at M/dry alumina Before considering oxygen adsorption at the supported metal atoms, we first summarize the parent M–O bonding. The PW91calculated bond lengths and formations energies of the metal monoxides from metal atoms and O2 are shown at the top of Fig. 2. Calculated bond lengths increase in line with the decrease in bond energy and are in good agreement with available experimental results on the gas-phase diatomics [26–29]. Fig. 2 also illustrates the configurational and energetic consequences of oxygen adsorption on the dry alumina supported metal atoms. Oxygen adsorption is found to have no effect on the M atom site preferences: Pd, Pt, and Au maintain their preference for binding atop O(2) and Ag for the threefold O(5) site. As shown in Table 1, the PtO fragment is most strongly bound to the surface and evidences the strongest site preference. Al(1), Al(3), Al(4) and O(5) are not stable adsorption sites for PtO; PtO placed at these hollow sites relaxes spontaneously to the atop site. PdO and AuO exhibit weaker preferences for bonding atop O(2) with small differences between the two lowest energy sites. The binding of AgO atop O(2) is very similar to that on Al(4) site. Focusing on the most stable adsorption configurations (Fig. 2), the new M–O bond projects nearly vertically from the alumina plane, so that Pd, Pt, and Au are approximately twofold and Ag approximately fourfold coordinated. In the former cases the Al atom immediately adjacent to the adsorption site is seen to substantially relax out of the surface plane. We have previously shown that this relaxation arises from charge transfer from M to a vacant Al p orbital [19], and this mechanism operates to a reduced extent when M is partially oxidized by adsorbed O. The M–O distances are nearly unchanged from the unsupported metal atoms. The O(i)–M distances are, however, more sensitive to O adsorption. Pt and Pd relax slightly away from and towards O(2) in the presence of adsorbed O. Au undergoes the most dramatic relaxation, so that the O(2)–Au distance decreases from 2.4 Å in the absence of an O adsorbate to approximately 2.1 Å in its presence. Such differences could be a useful diagnostic of Au coordination on oxide supports. In fact, Au–O distances near 2.1 Å are characteristic of molecular Au adsorbed on oxides [3,30]. Lastly, oxygen adsorption causes Ag to move off the center of the O(5) site, closer to two surface oxygen. The pronounced downward relaxation Al(1) in supported Ag is lost when O is added to supported Ag. Fig. 2 illustrates the decomposition of M–surface and M–O bonding into a thermodynamic cycle. Three cases become evident when the energies of those steps are summarized in an energy diagram (Fig. 3). For Pt, both M adsorption and O reaction are exothermic and nearly (to within 0.2 eV) independent of one another; formation of adsorbed PtO is highly exothermic and favored. Pd and Au exhibit a significant synergistic effect, so that the M–O and M–surface bonds become stronger in the presence of each other. In fact, formation of Au–O from Au + 1/2 O2 switches from endothermic in the gas-phase to exothermic on adsorbed Au. Oxygen affinity is greater for supported than unsupported Pd and Au. Lastly, Ag also exhibits a synergistic effect, but the effect is unable to reverse the rather unfavorable energetics of the Ag–O bond. Supported or not, a Ag atom has no affinity for an oxygen atom. 3.3. O binding at M/hydroxylated alumina
Fig. 1. Top and side views of dry (left) and hydroxylated (right) a-alumina (0 0 0 1) surfaces and specific labels for the adsorption sites of the transition metals on the surfaces. Red circle is for oxygen, pink for aluminum, and white for hydrogen. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
As noted above, hydroxylation has no effect on M atom site preferences but does weaken M–support binding. We considered simple O adsorption at these M sites, and relaxed structures are shown in Fig. 4. Results parallel the dry surface: M–O bond lengths
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Fig. 2. PW91-calculated structures and energies of adsorbed M and MO on dry a-Al2O3 surface. Energies for Pd, Au, and Ag correspond to thermodynamic cycle illustrated for Pt.
Fig. 5 shows calculated energies and structures of the hydrogen spillover products. Hydrogen transfers from a surface OH adjacent to M to form a hydroxylated MOH fragment, and the metal atoms move to adopt new coordination environments: Pt and Pd prefer a bridge site with bonding to two surface oxygen ions, while Au remains atop a single O(2) and Ag remains in a higher coordination site. Distances between M and the surface O that was the source of H decrease while the distances between M and adsorbed OH increase relative to the non-spillover values. The energetics of O reaction and H spillover at the hydroxylated surface can again be summarized in a single energy diagram, shown in Fig. 6. Comparison with Fig. 3 shows that, in the absence of H spillover, the dry and hydroxylated surfaces behave quite similarly towards M adsorption and reaction with oxygen. Including H spillover has little effect on the Pt chemistry; in contrast, H transfer has a profound enhancing effect on the affinity of Pd, Ag, and Au towards oxygen. The apparent O adsorption energies increase 0.96 and 0.67 eV for supported Pd and Au, respectively, and for supported Ag, adsorption of O reverses from 0.4 eV endothermic (to AgO*) to more than 0.7 eV exothermic (to AgOH*).
Table 1 Binding energy (eV) of MO as a function of binding site on the dry a-alumina (0 0 0 1) surface. The preferred binding sites are highlighted.
PtO PdO AuO AgO
Al(1)
O(2)
Al(3)
Al(4)
O(5)
to to to to
–4.22 –1.98 –1.50 –0.61
to O(2) –1.60 –0.67 –0.33
to O(2) –1.90 –0.93 –0.62
to to to to
O(2) O(2) O(2) O(2)
O(2) O(2) O(2) O(2)
are similar, the M–O bond projects away from the surface, and coordination is approximately twofold about Pt, Pd and Au and distorted fourfold about Ag. The hydroxylated surface presents additions opportunities for reactions with adsorbed metal through the ‘spillover’ of surface H onto the adsorbate [31,32]. While with more active metal atoms, such as Co, this H transfer can be quite exothermic and even lead to liberation of H2 [33,34], we have show that such transfers are endothermic for Pd, Ag, and Au, and only slightly exothermic for Pt [19]. The presence of adsorbed O completely alters these trends.
1.00 0.50 0.00
AgO
-0.23
EB (eV)
-2.20
-1.99
-1.75 Pd* -0.63
PtO -2.00
-3.00
-0.78
Ag+1/2O2 0.75 +dry surface
-1.88
-1.35
-1.50
-2.50
0.38
-1.37
PdO
-0.50 -1.00
Au+1/2O2 +dry surface
Pt+1/2O2 Pd+1/2O2 +dry surface +dry surface
AuO
-1.05 0.43
Au* -0.72
AgO*
Ag* AuO*
PdO* Pt*
-2.02
-2.23
-3.50 -4.00 -4.50
PtO*
Fig. 3. PW91-calculated metal atom adsorption and oxygen reaction energies on dry a-alumina (0 0 0 1) surface. M* represents adsorbed metal and MO* adsorbed metal with O.
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Fig. 4. MO adsorbates at the fully hydroxylated a-alumina (0 0 0 1) surface.
Fig. 5. PW91-calculated structures and energies of products of hydrogen spillover onto adsorbed M and MO on hydroxylated a-Al2O3 surface. Energies for Pd, Au, and Ag correspond to thermodynamic cycle illustrated for Pt.
1.00 0.50
AgO Au+1/2O2 +wet surface
Pt+1/2O2 Pd+1/2O2 +wet surface +wet surface -0.23
-0.50
EB (eV)
-2.00
-1.87
-1.01 -1.99
Pd*
-1.86
-1.31
Au* -0.87
PdO*
Pt*
-1.15
H transfer PdOH*
0.37 Ag*
-1.14
AuO* -0.67
PtO
-2.15
-0.77
-0.96
-2.50 -3.00
Ag+1/2O2 0.75 +wet surface
AgO*
-0.62
-0.53 -1.50
0.38
PdO
0.00
-1.00
AuO
Htransfer AgOH*
H transfer AuOH*
-2.28
-3.50
PtO*
-4.00
-0.08 -4.50
H transfer PtOH*
Fig. 6. PW91-calculated metal atom adsorption, oxygen reaction, and H spillover energies on the hydroxylated a-alumina (0 0 0 1) surface. Indicates surface-bound species.
This tendency for H spillover to supported MO particles is related to the intrinsic MO–H bond strength, as illustrated in Fig. 7. H spillover can conceptually be decomposed into the energy to remove an H from the oxide surface, a process we calculate to cost
2.9 eV relative to H2, transfer of that hydrogen to the MO particle, an adsorption of the MOH particle to the surface defect site created by removing an H. Fig. 7 shows that the energy of the second step varies by a factor of 2.5 from the weakest case (PtO–H) to the
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PtO+1/2H2 +wet surface# 3.00
PdO+1/2H2 +wet surface#
AuO+1/2H2 +wet surface#
AgO+1/2H2 +wet surface#
2.50
-1.10 2.00
2.93
-2.43
-1.90 2.93
2.93
-2.76 2.93
PtOH
1.50
EB (eV)
1.00
PdOH 0.50
AuOH PtO +wet surface
0.00 -0.50
-2.15
PdO +wet surface
AuO +wet surface
-1.31
-1.87
-4.06
-1.00
-1.15 -3.04
-3.30
AgOH AgO +wet surface
PdO*
AgO*
-2.46
-1.50 -2.00 -2.50
PtO*
-0.08 H transfer PtOH*
-0.96
AuO*
-1.14
-0.67 H transfer PdOH*
H transfer AuOH*
H transfer AgOH*
Fig. 7. PW91-calculated metal oxide adsorption, metal hydroxide adsorption, hydrogen reaction, and H spillover energies on the hydroxylated a-alumina (0 0 0 1) surface. Indicates surface-bound species; and # indicates the hydroxylated surface with one hydrogen missing.
strongest (AgO–H), in line with the net driving force for spillover. The MO–H bond energies are anticorrelated with the surface defect–MOH bond energies, but the latter variations are too small to offset the intrinsic MO–H bond energy effect. 4. Conclusions In this work, we report DFT simulations characterizing the affinity of supported late transition metal atoms for oxygen. In all case, the support is found to have a significant effect on oxygen affinity, and the effect is sensitive to support surface termination. Figs. 3 and 6 nicely summarize the key results. At a hydrogen-free alumina surface, characteristic of a hot and dry reaction environment, Pt atoms have a strong tendency to bind O, Pd and Au have weaker but still energetically favorable O affinities, and Ag resists addition of O when the source is gas-phase O2. At the fully hydroxylated surface, characteristic of an a-alumina in a humid environment, all four adsorbed metal atoms exothermically bind O from O2. The Pt chemistry is largely unchanged from dry to hydroxylated surface, but O binding is significantly enhanced on Pd, Au, and Ag at a hydroxylated surface due to the availability of an H spillover channel. Under all conditions, the affinity of metal atoms for oxygen is increased when surface-adsorbed over that in the gasphase. These results are obtained from only one type of oxide support, probe only M–O bond strength as a reactivity metric, and neglect the important role of kinetics in surface reactivity. Nonetheless, they illustrate the rather substantial and coupled effects that a support and reaction environment can have on the chemistry of atomically dispersed metals. Similar effects are expected for multi-atom metal clusters. Support effects are clearly not small at this scale, and the ability to tune the M–support interaction may provide a productive means of controlling catalytic activity.
Acknowledgments LX thanks Dr. Victor A. Ranea for his assistance with alumina surface models. This work was supported by the US Department of Energy-Office of Basic Energy Sciences, Grant DE-FG0206ER15839. References [1] C.H. Bartholomew, R.J. Farrauto, Fundamentals of Industrial Catalytic Processes, Wiley, Hoboken, 2006. [2] C.R. Henry, Surf. Sci. Rep. 31 (1998) 235. [3] J.C. Fierro-Gonzalez, S. Kuba, Y. Hao, J. Phys. Chem. B 110 (2006) 13326. [4] B.L.M. Hendriksen, S.C. Bobaru, Surf. Sci. 552 (2004) 229. [5] G.H. Zhu, J.Y. Han, D.Y. Zernlyanov, J. Phys. Chem. B 109 (2005) 2331. [6] X.C. Guo, J. Phys. Chem. B 107 (2003) 3105. [7] W.X. Li, C. Stampfl, M. Scheffler, Phys. Rev. Lett. 90 (2003) 256102. [8] M.L. Bocquet, P. Sautet, J. Cerda, C.I. Carlisle, M.J. Webb, J. Am. Chem. Soc. 125 (2003) 3119. [9] M.L. Bocquet, A. Michaelides, D. Loffreda, P. Sautet, A. Alavi, J. Am. Chem. Soc. 125 (2003) 5620. [10] A. Michaelides, M.L. Bocquet, P. Sautet, A. Alavi, Chem. Phys. Lett. 367 (2003) 344. [11] B.L.M. Hendriksen, J.W.M. Frenken, Phys. Rev. Lett. 89 (2002) 0461011. [12] M.S. Chen, Y. Cai, Z. Yan, K.K. Gath, S. Axnanda, Surf. Sci. 601 (2007) 5326. [13] S.Y. Quek, C.M. Friend, E. Kaxiras, Surf. Sci. 600 (2006) 3388. [14] H.-J. Freund, M. Bäumer, Adv. Catal. 45 (2000) 333. [15] K. Reuter, C. Stampfl, M. Scheffler, in: S. Yip (Ed.), Handbook of Materials Modeling: Fundamental Models and Methods, Springer, Berlin, 2005. [16] R.B. Getman, Y. Xu, J. Phys. Chem. C 112 (2008) 9559. [17] Y. Xu, W.A. Shelton, J. Phys. Chem. B 110 (2006) 16591. [18] V.A. Ranea, I. Carmichael, J. Phys. Chem. C 113 (2009) 2149. [19] L. Xiao, Surf. Sci. 602 (2008) 3445. [20] G. Kresse, Phys. Rev. B 54 (1996) 11169. [21] G. Kresse, Comput. Mater. Sci. 6 (1996) 15. [22] G. Kresse, Phys. Rev. B 47 (1993) 558. [23] G. Kresse, Phys. Rev. B 59 (1999) 1758. [24] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, Phys. Rev. B 46 (1992) 6671. [25] Z. Lodziana, J.K. Nørskov, P. Stoltze, J. Chem. Phys. 118 (2003) 11179. [26] W.D. Bare, A. Citra, G.V. Chertihin, J. Phys. Chem. A 103 (1999) 5456.
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[27] L.C. O’Brien, A.E. Oberlink, J. Phys. Chem. A 110 (2006) 11954. [28] T. Okabayashi, F. Koto, K. Tsukamoto, E. Yamazaki, Chem. Phys. Lett. 403 (2005) 223. [29] K.P. Huber, G. Herzberg, Molecular Spectra and Molecular Structure. 4: Constants of Diatomic Molecules, Van Nostrand Reinhold, New York, 1979. [30] J. Guzman, S. Kuba, J.C. Fierro-Gonzalez, Catal. Lett. 95 (2004) 77.
[31] J. Conner, J.L. Falconer, J.L. Falconer, Chem. Rev. 95 (1995) 759. [32] A. Guerrero-Ruiz, I. Rodriguez-Ramos (Eds.), Studies in Surface Science and Catalysis, vol. 138: Spillover and Mobility of Species on Solid Surfaces, Elsevier Science, 2001. [33] S.A. Chambers, T. Droubay, D.R. Jennison, Science 297 (2002) 827. [34] D.R. Jennison, Surf. Sci. 544 (2003) L689.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Influence of a-alumina supports on oxygen binding to Pd, Ag, Pt, and Au Li Xiao 1, William F. Schneider * Department of Chemical and Biomolecular Engineering and Department of Chemistry and Biochemistry, University of Notre Dame, 182 Fitzpatrick Hall, Notre Dame, IN 46556, United States
a r t i c l e
i n f o
Article history: Received 21 August 2009 In final form 10 November 2009 Available online 14 November 2009
a b s t r a c t The influence of a metal oxide support on the reactivity of adsorbed metals is a key question in surface reactivity and heterogeneous catalysis. Density functional theory (DFT) calculations are used here to examine support effects on one diagnostic of reactivity, oxygen binding, for Pd, Pt, Ag, and Au atoms supported on two representations of an a-alumina surface. Supported metal atoms uniformly bind oxygen more strongly than unsupported ones, so that even supported Au exothermically binds O. Further, oxygen adsorption is enhanced on a hydroxylated support by spillover of H from support to M–O bond. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Practical heterogeneous catalysts are commonly composed of catalytic metals dispersed onto a high surface area metal oxide support, an approach that provides an inexpensive and effective way to maximize the accessibility of metal surface to reacting gas [1]. There are many examples in which the catalyst support significantly influences the reactivity of a supported metal. The nature of the metal–support interaction and its implications for reactivity are thus clearly of key importance in understanding, developing, and applying these materials [2]. These questions of support effects become particularly germane as one moves from nanoscale particles containing thousands of atoms to the most dispersed, few atom molecular clusters, where interactions with support are expected to play a particularly significant role in reactivity [3]. Catalysis occurs within a reactive environment, and there is a growing recognition of the importance of this environment in dictating the active form of a catalytic material. For instance, much attention and debate has surrounded the involvement of surface oxides formed in situ during catalytic oxidations over the nominally noble metals Pd [4,5], Ag [6–10], Pt [11,12], and even Au [13]. Similar issues emerge in understanding supported metal particles, although here the complexity of the materials greatly increases the challenges, as both metal and support are susceptible to modifications by the environment [14]. Much of this work has taken advantage of surface science techniques able to probe the chemical state of extended surfaces. Density functional theory (DFT) calculations have also featured prominently in characterizing the state of catalytic materials under reactive conditions, as
* Corresponding author. Fax: +1 574 631 8366. E-mail address: [email protected] (W.F. Schneider). 1 Current address: Accelrys Inc., 10188 Telesis Court, Suite 100, San Diego, CA 92121-4779, United States. 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.11.019
they provide a means of systematically probing the structure and stability of various states of a surface or particle [15–17]. Here, we report plane-wave, supercell DFT simulations designed to examine the effect of catalyst support on one probe of reactivity, oxygen affinity, for supported Pd, Ag, Pt, and Au metal atoms (M):
M=support þ 1=2O2 $ O—M=support
ð1Þ
We consider two different support models, a stoichiometric
a-alumina (0 0 0 1) surface and a gibbsite-like hydroxylated a-alumina (0 0 0 1) one, the former representative of alumina in a dry, water-free environment and the latter of alumina in a humid environment [18,19]. We find in general that the presence or absence of support has a significant impact on the M–O bond strength, reversing, for instance, the unfavorable energetics of reaction (1) when M@Au. The supported metal atoms other than Ag are found to have high affinity for oxygen, and this affinity is consistently enhanced by hydrogen spillover to the metal atom at the hydroxylated surface. The results illustrate the synergy between support and reactive environment in dictating the local structure and reactivity of supported metal atoms. 2. Computational details Plane-wave, supercell DFT calculations were carried with the VASP code [20–22]. The interaction of valence electrons with frozen cores were described using the projector augmented wave (PAW) method [23], and plane waves were included to an energy cutoff of 400 eV. The exchange and correlation energies were computed using the Perdew-Wang 91 form (PW91) of the generalized gradient approximation (GGA) [24]. A Gaussian smearing function with a width of 0.05 eV was applied to states near the Fermi level; converged occupancy numbers are integer for these insulating
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systems. Electronic energies were converged to 104 eV, and ionic relaxations were considered converged when the forces on the ions were less than 0.03 eV/ Å. Surface simulations are carried out using a 2 2 supercell of the a-Al2O3 (0 0 0 1) surface. The alumina slab contained 12 atomic layers, with the seven top-most ones allowed to relax from bulk locations. A vacuum region of 18 Å was applied in the c-direction to eliminate interactions between slab images. Overall supercell dimensions were 9.61 9.61 26.34 Å and contained 80 atoms. A 3 3 1 C-centered k-point mesh was used. The fully hydroxylated, gibbsite-like surface model had similar construction save the replacement of the four surface Al atoms with 12 hydrogen. A single metal atom deposited on the 2 2 surface results in a coverage of 1/12 geometrical monolayer (ML) (one metal atom per 12 surface oxygen atoms). Adsorbate atoms are placed off symmetry sites to avoid artificial constraints during geometry relaxations. Isolated atoms and molecules are simulated within 10 10 10 Å supercells.
3. Results and discussion 3.1. M/alumina surface models Fig. 1 shows top and side views of the dry and fully hydroxylated alumina surfaces. The dry surface exposes oxygen in the second (O(2)) and fifth (O(5)) subsurface layers. These oxygen are interconnected by top (Al(1)) and subsurface (Al(3) and Al(4)) ions. Pd, Pt, and Au atoms have previously been shown to prefer to bind atop single O(2) sites through a covalent mechanism; more readily oxidized Ag prefers a threefold hollow above O(5) [19]. On the fully hydroxylated surface three H atoms replace each Al(1), forming hydroxyls that orient parallel to and away from the surface plane [18,25]. The hydroxylated surface differs from the dry in exposing threefold hydroxyl sites above sixth-layer Al atoms, Al(6). These surface hydroxyls dynamically reorient at room temperature [19]. In the static simulations here we consider one representative realization of this surface, including one in-plane hydroxyl and two out-of-plane hydroxyl per Al removed. Metal atoms prefer to bind near in-plane hydroxyl to avoid the pendant hydrogen atoms and exhibit coordination preferences similar to the dry surface.
Hydroxylation uniformly decreases the binding energy of metal atoms to the oxide [19]. 3.2. O binding at M/dry alumina Before considering oxygen adsorption at the supported metal atoms, we first summarize the parent M–O bonding. The PW91calculated bond lengths and formations energies of the metal monoxides from metal atoms and O2 are shown at the top of Fig. 2. Calculated bond lengths increase in line with the decrease in bond energy and are in good agreement with available experimental results on the gas-phase diatomics [26–29]. Fig. 2 also illustrates the configurational and energetic consequences of oxygen adsorption on the dry alumina supported metal atoms. Oxygen adsorption is found to have no effect on the M atom site preferences: Pd, Pt, and Au maintain their preference for binding atop O(2) and Ag for the threefold O(5) site. As shown in Table 1, the PtO fragment is most strongly bound to the surface and evidences the strongest site preference. Al(1), Al(3), Al(4) and O(5) are not stable adsorption sites for PtO; PtO placed at these hollow sites relaxes spontaneously to the atop site. PdO and AuO exhibit weaker preferences for bonding atop O(2) with small differences between the two lowest energy sites. The binding of AgO atop O(2) is very similar to that on Al(4) site. Focusing on the most stable adsorption configurations (Fig. 2), the new M–O bond projects nearly vertically from the alumina plane, so that Pd, Pt, and Au are approximately twofold and Ag approximately fourfold coordinated. In the former cases the Al atom immediately adjacent to the adsorption site is seen to substantially relax out of the surface plane. We have previously shown that this relaxation arises from charge transfer from M to a vacant Al p orbital [19], and this mechanism operates to a reduced extent when M is partially oxidized by adsorbed O. The M–O distances are nearly unchanged from the unsupported metal atoms. The O(i)–M distances are, however, more sensitive to O adsorption. Pt and Pd relax slightly away from and towards O(2) in the presence of adsorbed O. Au undergoes the most dramatic relaxation, so that the O(2)–Au distance decreases from 2.4 Å in the absence of an O adsorbate to approximately 2.1 Å in its presence. Such differences could be a useful diagnostic of Au coordination on oxide supports. In fact, Au–O distances near 2.1 Å are characteristic of molecular Au adsorbed on oxides [3,30]. Lastly, oxygen adsorption causes Ag to move off the center of the O(5) site, closer to two surface oxygen. The pronounced downward relaxation Al(1) in supported Ag is lost when O is added to supported Ag. Fig. 2 illustrates the decomposition of M–surface and M–O bonding into a thermodynamic cycle. Three cases become evident when the energies of those steps are summarized in an energy diagram (Fig. 3). For Pt, both M adsorption and O reaction are exothermic and nearly (to within 0.2 eV) independent of one another; formation of adsorbed PtO is highly exothermic and favored. Pd and Au exhibit a significant synergistic effect, so that the M–O and M–surface bonds become stronger in the presence of each other. In fact, formation of Au–O from Au + 1/2 O2 switches from endothermic in the gas-phase to exothermic on adsorbed Au. Oxygen affinity is greater for supported than unsupported Pd and Au. Lastly, Ag also exhibits a synergistic effect, but the effect is unable to reverse the rather unfavorable energetics of the Ag–O bond. Supported or not, a Ag atom has no affinity for an oxygen atom. 3.3. O binding at M/hydroxylated alumina
Fig. 1. Top and side views of dry (left) and hydroxylated (right) a-alumina (0 0 0 1) surfaces and specific labels for the adsorption sites of the transition metals on the surfaces. Red circle is for oxygen, pink for aluminum, and white for hydrogen. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
As noted above, hydroxylation has no effect on M atom site preferences but does weaken M–support binding. We considered simple O adsorption at these M sites, and relaxed structures are shown in Fig. 4. Results parallel the dry surface: M–O bond lengths
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Fig. 2. PW91-calculated structures and energies of adsorbed M and MO on dry a-Al2O3 surface. Energies for Pd, Au, and Ag correspond to thermodynamic cycle illustrated for Pt.
Fig. 5 shows calculated energies and structures of the hydrogen spillover products. Hydrogen transfers from a surface OH adjacent to M to form a hydroxylated MOH fragment, and the metal atoms move to adopt new coordination environments: Pt and Pd prefer a bridge site with bonding to two surface oxygen ions, while Au remains atop a single O(2) and Ag remains in a higher coordination site. Distances between M and the surface O that was the source of H decrease while the distances between M and adsorbed OH increase relative to the non-spillover values. The energetics of O reaction and H spillover at the hydroxylated surface can again be summarized in a single energy diagram, shown in Fig. 6. Comparison with Fig. 3 shows that, in the absence of H spillover, the dry and hydroxylated surfaces behave quite similarly towards M adsorption and reaction with oxygen. Including H spillover has little effect on the Pt chemistry; in contrast, H transfer has a profound enhancing effect on the affinity of Pd, Ag, and Au towards oxygen. The apparent O adsorption energies increase 0.96 and 0.67 eV for supported Pd and Au, respectively, and for supported Ag, adsorption of O reverses from 0.4 eV endothermic (to AgO*) to more than 0.7 eV exothermic (to AgOH*).
Table 1 Binding energy (eV) of MO as a function of binding site on the dry a-alumina (0 0 0 1) surface. The preferred binding sites are highlighted.
PtO PdO AuO AgO
Al(1)
O(2)
Al(3)
Al(4)
O(5)
to to to to
–4.22 –1.98 –1.50 –0.61
to O(2) –1.60 –0.67 –0.33
to O(2) –1.90 –0.93 –0.62
to to to to
O(2) O(2) O(2) O(2)
O(2) O(2) O(2) O(2)
are similar, the M–O bond projects away from the surface, and coordination is approximately twofold about Pt, Pd and Au and distorted fourfold about Ag. The hydroxylated surface presents additions opportunities for reactions with adsorbed metal through the ‘spillover’ of surface H onto the adsorbate [31,32]. While with more active metal atoms, such as Co, this H transfer can be quite exothermic and even lead to liberation of H2 [33,34], we have show that such transfers are endothermic for Pd, Ag, and Au, and only slightly exothermic for Pt [19]. The presence of adsorbed O completely alters these trends.
1.00 0.50 0.00
AgO
-0.23
EB (eV)
-2.20
-1.99
-1.75 Pd* -0.63
PtO -2.00
-3.00
-0.78
Ag+1/2O2 0.75 +dry surface
-1.88
-1.35
-1.50
-2.50
0.38
-1.37
PdO
-0.50 -1.00
Au+1/2O2 +dry surface
Pt+1/2O2 Pd+1/2O2 +dry surface +dry surface
AuO
-1.05 0.43
Au* -0.72
AgO*
Ag* AuO*
PdO* Pt*
-2.02
-2.23
-3.50 -4.00 -4.50
PtO*
Fig. 3. PW91-calculated metal atom adsorption and oxygen reaction energies on dry a-alumina (0 0 0 1) surface. M* represents adsorbed metal and MO* adsorbed metal with O.
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Fig. 4. MO adsorbates at the fully hydroxylated a-alumina (0 0 0 1) surface.
Fig. 5. PW91-calculated structures and energies of products of hydrogen spillover onto adsorbed M and MO on hydroxylated a-Al2O3 surface. Energies for Pd, Au, and Ag correspond to thermodynamic cycle illustrated for Pt.
1.00 0.50
AgO Au+1/2O2 +wet surface
Pt+1/2O2 Pd+1/2O2 +wet surface +wet surface -0.23
-0.50
EB (eV)
-2.00
-1.87
-1.01 -1.99
Pd*
-1.86
-1.31
Au* -0.87
PdO*
Pt*
-1.15
H transfer PdOH*
0.37 Ag*
-1.14
AuO* -0.67
PtO
-2.15
-0.77
-0.96
-2.50 -3.00
Ag+1/2O2 0.75 +wet surface
AgO*
-0.62
-0.53 -1.50
0.38
PdO
0.00
-1.00
AuO
Htransfer AgOH*
H transfer AuOH*
-2.28
-3.50
PtO*
-4.00
-0.08 -4.50
H transfer PtOH*
Fig. 6. PW91-calculated metal atom adsorption, oxygen reaction, and H spillover energies on the hydroxylated a-alumina (0 0 0 1) surface. Indicates surface-bound species.
This tendency for H spillover to supported MO particles is related to the intrinsic MO–H bond strength, as illustrated in Fig. 7. H spillover can conceptually be decomposed into the energy to remove an H from the oxide surface, a process we calculate to cost
2.9 eV relative to H2, transfer of that hydrogen to the MO particle, an adsorption of the MOH particle to the surface defect site created by removing an H. Fig. 7 shows that the energy of the second step varies by a factor of 2.5 from the weakest case (PtO–H) to the
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PtO+1/2H2 +wet surface# 3.00
PdO+1/2H2 +wet surface#
AuO+1/2H2 +wet surface#
AgO+1/2H2 +wet surface#
2.50
-1.10 2.00
2.93
-2.43
-1.90 2.93
2.93
-2.76 2.93
PtOH
1.50
EB (eV)
1.00
PdOH 0.50
AuOH PtO +wet surface
0.00 -0.50
-2.15
PdO +wet surface
AuO +wet surface
-1.31
-1.87
-4.06
-1.00
-1.15 -3.04
-3.30
AgOH AgO +wet surface
PdO*
AgO*
-2.46
-1.50 -2.00 -2.50
PtO*
-0.08 H transfer PtOH*
-0.96
AuO*
-1.14
-0.67 H transfer PdOH*
H transfer AuOH*
H transfer AgOH*
Fig. 7. PW91-calculated metal oxide adsorption, metal hydroxide adsorption, hydrogen reaction, and H spillover energies on the hydroxylated a-alumina (0 0 0 1) surface. Indicates surface-bound species; and # indicates the hydroxylated surface with one hydrogen missing.
strongest (AgO–H), in line with the net driving force for spillover. The MO–H bond energies are anticorrelated with the surface defect–MOH bond energies, but the latter variations are too small to offset the intrinsic MO–H bond energy effect. 4. Conclusions In this work, we report DFT simulations characterizing the affinity of supported late transition metal atoms for oxygen. In all case, the support is found to have a significant effect on oxygen affinity, and the effect is sensitive to support surface termination. Figs. 3 and 6 nicely summarize the key results. At a hydrogen-free alumina surface, characteristic of a hot and dry reaction environment, Pt atoms have a strong tendency to bind O, Pd and Au have weaker but still energetically favorable O affinities, and Ag resists addition of O when the source is gas-phase O2. At the fully hydroxylated surface, characteristic of an a-alumina in a humid environment, all four adsorbed metal atoms exothermically bind O from O2. The Pt chemistry is largely unchanged from dry to hydroxylated surface, but O binding is significantly enhanced on Pd, Au, and Ag at a hydroxylated surface due to the availability of an H spillover channel. Under all conditions, the affinity of metal atoms for oxygen is increased when surface-adsorbed over that in the gasphase. These results are obtained from only one type of oxide support, probe only M–O bond strength as a reactivity metric, and neglect the important role of kinetics in surface reactivity. Nonetheless, they illustrate the rather substantial and coupled effects that a support and reaction environment can have on the chemistry of atomically dispersed metals. Similar effects are expected for multi-atom metal clusters. Support effects are clearly not small at this scale, and the ability to tune the M–support interaction may provide a productive means of controlling catalytic activity.
Acknowledgments LX thanks Dr. Victor A. Ranea for his assistance with alumina surface models. This work was supported by the US Department of Energy-Office of Basic Energy Sciences, Grant DE-FG0206ER15839. References [1] C.H. Bartholomew, R.J. Farrauto, Fundamentals of Industrial Catalytic Processes, Wiley, Hoboken, 2006. [2] C.R. Henry, Surf. Sci. Rep. 31 (1998) 235. [3] J.C. Fierro-Gonzalez, S. Kuba, Y. Hao, J. Phys. Chem. B 110 (2006) 13326. [4] B.L.M. Hendriksen, S.C. Bobaru, Surf. Sci. 552 (2004) 229. [5] G.H. Zhu, J.Y. Han, D.Y. Zernlyanov, J. Phys. Chem. B 109 (2005) 2331. [6] X.C. Guo, J. Phys. Chem. B 107 (2003) 3105. [7] W.X. Li, C. Stampfl, M. Scheffler, Phys. Rev. Lett. 90 (2003) 256102. [8] M.L. Bocquet, P. Sautet, J. Cerda, C.I. Carlisle, M.J. Webb, J. Am. Chem. Soc. 125 (2003) 3119. [9] M.L. Bocquet, A. Michaelides, D. Loffreda, P. Sautet, A. Alavi, J. Am. Chem. Soc. 125 (2003) 5620. [10] A. Michaelides, M.L. Bocquet, P. Sautet, A. Alavi, Chem. Phys. Lett. 367 (2003) 344. [11] B.L.M. Hendriksen, J.W.M. Frenken, Phys. Rev. Lett. 89 (2002) 0461011. [12] M.S. Chen, Y. Cai, Z. Yan, K.K. Gath, S. Axnanda, Surf. Sci. 601 (2007) 5326. [13] S.Y. Quek, C.M. Friend, E. Kaxiras, Surf. Sci. 600 (2006) 3388. [14] H.-J. Freund, M. Bäumer, Adv. Catal. 45 (2000) 333. [15] K. Reuter, C. Stampfl, M. Scheffler, in: S. Yip (Ed.), Handbook of Materials Modeling: Fundamental Models and Methods, Springer, Berlin, 2005. [16] R.B. Getman, Y. Xu, J. Phys. Chem. C 112 (2008) 9559. [17] Y. Xu, W.A. Shelton, J. Phys. Chem. B 110 (2006) 16591. [18] V.A. Ranea, I. Carmichael, J. Phys. Chem. C 113 (2009) 2149. [19] L. Xiao, Surf. Sci. 602 (2008) 3445. [20] G. Kresse, Phys. Rev. B 54 (1996) 11169. [21] G. Kresse, Comput. Mater. Sci. 6 (1996) 15. [22] G. Kresse, Phys. Rev. B 47 (1993) 558. [23] G. Kresse, Phys. Rev. B 59 (1999) 1758. [24] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, Phys. Rev. B 46 (1992) 6671. [25] Z. Lodziana, J.K. Nørskov, P. Stoltze, J. Chem. Phys. 118 (2003) 11179. [26] W.D. Bare, A. Citra, G.V. Chertihin, J. Phys. Chem. A 103 (1999) 5456.
236
L. Xiao, W.F. Schneider / Chemical Physics Letters 484 (2010) 231–236
[27] L.C. O’Brien, A.E. Oberlink, J. Phys. Chem. A 110 (2006) 11954. [28] T. Okabayashi, F. Koto, K. Tsukamoto, E. Yamazaki, Chem. Phys. Lett. 403 (2005) 223. [29] K.P. Huber, G. Herzberg, Molecular Spectra and Molecular Structure. 4: Constants of Diatomic Molecules, Van Nostrand Reinhold, New York, 1979. [30] J. Guzman, S. Kuba, J.C. Fierro-Gonzalez, Catal. Lett. 95 (2004) 77.
[31] J. Conner, J.L. Falconer, J.L. Falconer, Chem. Rev. 95 (1995) 759. [32] A. Guerrero-Ruiz, I. Rodriguez-Ramos (Eds.), Studies in Surface Science and Catalysis, vol. 138: Spillover and Mobility of Species on Solid Surfaces, Elsevier Science, 2001. [33] S.A. Chambers, T. Droubay, D.R. Jennison, Science 297 (2002) 827. [34] D.R. Jennison, Surf. Sci. 544 (2003) L689.