Topics in Catalysis

, Volume 53, Issue 5, pp 384–392

Hydrogen on and in Selected Overlayer Near-Surface Alloys and the Effect of Subsurface Hydrogen on the Reactivity of Alloy Surfaces

Authors

  • Shampa Kandoi
    • Department of Chemical & Biological EngineeringUniversity of Wisconsin-Madison
    • UTC Power Corporation
  • Peter A. Ferrin
    • Department of Chemical & Biological EngineeringUniversity of Wisconsin-Madison
    • Department of Chemical & Biological EngineeringUniversity of Wisconsin-Madison
Original Paper

DOI: 10.1007/s11244-010-9444-5

Cite this article as:
Kandoi, S., Ferrin, P.A. & Mavrikakis, M. Top Catal (2010) 53: 384. doi:10.1007/s11244-010-9444-5

Abstract

The interaction of hydrogen with the close-packed facets of seventeen transition metals overlaid with 1 ML of five transition metals (Au, Ag, Cu, Pt, and Pd) has been studied using periodic self-consistent (GGA-PW91) density functional theory (DFT) calculations. For noble metal overlayers (Au, Ag, and Cu), hydrogen at the host-metal/overlayer interface (subsurface hydrogen) is more stable than subsurface hydrogen in the pure host. For certain Au and Ag overlayers, subsurface hydrogen is more stable than surface hydrogen in the same system. The presence of subsurface hydrogen was found to have a significant effect on the electronic structure of the overlayer, resulting in its modified surface reactivity.

Keywords

DFTHydrogenAlloys

1 Introduction

The interaction of hydrogen with transition metals has been widely studied for many industrial and environmental applications [14]. Recently, the potential migration to the so-called hydrogen economy has spurred the search for new catalytic materials for hydrogen-related reactions, specifically the production, storage/release, and use of hydrogen as a fuel [5, 6].

Towards this end, a new class of bimetallic alloys, called near-surface alloys (NSAs), have been shown to possess favorable properties for hydrogen-related reactions [7]. Bimetallic NSAs are defined as alloy systems that have a different composition at or near the surface than the bulk. These alloys often have different properties than either of their constituent metals or bulk alloys of such metals [810]. Au and Ag overlayers on Ir, for instance, bind hydrogen as weakly as the noble metals while still spontaneously dissociating hydrogen (H2) gas [1113]. Other NSAs show a lower barrier to hydrogen atom diffusion into the subsurface than either constituent metal, allowing for easier population of subsurface sites [14]. Still other NSAs show promise for CO tolerance while activating H2 gas, an ideal combination for the anode reaction of H2 fuel cells [15]. Some synthesized surface alloys show increased activity for hydrogenation reactions, specifically Pt and Pd on Ni [16], or possess enhanced coking resistance in such reactions (such as Au on Ni [17]). Other synthesized surface alloys are promising catalysts for the water–gas shift reaction [18], preferential oxidation of CO in the presence of H2 [19], and other chemical [20, 21] and electrochemical reactions [2227].

While specific NSAs have been synthesized and studied for individual reactions, a more fundamental understanding of the interaction of hydrogen with these alloys is desirable. For instance, previous work has shown that certain NSAs can trap hydrogen in their subsurface [13, 28, 29]. Beyond the possible hydrogen storage applications of such systems, subsurface and bulk hydrogen has been identified as a reaction intermediate for several catalytic reactions on monometallic catalysts, such as alkene hydrogenation on Pd [30, 31] and alkene hydrogenation and methanation on Ni [3235]. Additionally, subsurface hydrogen has been shown to affect the binding of other species on single crystals and nanoparticles [36]. Thus, a systematic understanding of the stability of subsurface hydrogen and its effect on the reactivity of NSAs would be helpful in establishing a better understanding of their catalytic behavior for hydrogen-related applications and in identifying alloys with promising catalytic properties.

In this work, we focus on overlayer NSAs, consisting of a single monolayer of solute metal over a bulk host. We have looked at five solutes (Au, Ag, Cu, Pt, and Pd) overlaid on the close-packed facet of 17 transition metal hosts. We calculate atomic hydrogen binding on these NSAs, both on the surface and in the subsurface. We have also investigated the effect of subsurface hydrogen on the electronic structure of the exposed surface. To look at the effect of subsurface hydrogen on the reactivity of certain NSAs, we use H2 dissociation as a probe reaction.

2 Methods

The total energy calculation code DACAPO is used throughout this study [37, 38]. We have modeled the close-packed surfaces of 17 transition metals (the (111) facet of Au, Ag, Cu, Pt, Pd, Ni, Ir and Rh—face-centered cubic (fcc) metals, the (0001) facet of Os, Co, Ru, and Re—hexagonal close-packed (hcp) metals, and the (110) facet of Fe, W, Ta, Mo and V—body centered cubic (bcc) metals) using a slab consisting of four layers of metal atoms, periodically repeated in a 2 × 2 supercell geometry, with five layers of vacuum between any two successive metal slabs. The bottom two layers are fixed at the bulk lattice constant of the metal; the top two layers are allowed to relax. To model the NSA, the top layer of metal atoms is replaced by the desired overlayer metal atoms (Au, Ag, Cu, Pt or Pd). Ionic cores are modeled by Vanderbilt pseudopotentials [39] and the Kohn–Sham one-electron valence states are expanded on the basis of plane waves with a cutoff of 25 Ry. To achieve convergence of total energies, the surface Brillouin zone is sampled with varying k-point sets. For Pt and Pd overlayers on non-group 11 fcc and hcp metals, the surface Brillouin zone is sampled with 18 special Chadi-Cohen k-points [40]; for Pt and Pd overlayers on bcc metals, a 4 × 4 × 1 Monkhorst–Pack grid is used [41]. For group 11 substrates, as well as Au, Ag and Cu overlayers on fcc and hcp systems, 54 special Chadi-Cohen k-points are employed; bcc systems with Au, Ag and Cu overlayers utilize a 6 × 6 × 1 Monkhorst–Pack grid to sample the surface Brillouin zone. Adsorption is allowed on only one of the two exposed surfaces of the slab, and the electrostatic potential is adjusted accordingly [42, 43]. The exchange–correlation energy and potential are described by the generalized gradient approximation (GGA-PW91) [44]. Iterative diagonalization of the Kohn–Sham Hamiltonian, Fermi population of the Kohn–Sham states (kBT = 0.1 eV), and Pulay mixing of the resulting density allows for calculation of the ground state electron density. Total energies are extrapolated to kBT = 0 eV. Minimum energy paths for H2 dissociation on surfaces and the respective transition state energies are calculated using the climbing-image nudged-elastic-band method [45]. Transition state structures were verified by vibrational frequency analysis, as described previously [46].

3 Results and Discussion

3.1 Stability of Surface and Subsurface Hydrogen

3.1.1 Surface Hydrogen

Table 1 shows the binding energies and site preferences for surface and subsurface hydrogen on NSAs considered in this study. In general, surface hydrogen on Au overlayers is destabilized as compared to pure Au; however this trend is broken with several hosts (Ni, Cu, Fe, W, Mo, and V). Surface hydrogen on Ag overlayers is generally more stable than H on the surface of pure Ag, with only Ag*/Cu, Ag*/Pd, Ag*/Rh, Ag*/Ru, Ag*/Fe and Ag*/Co (where X*/Y denotes a monolayer of X on a Y substrate) at nearly the same or slightly weaker binding energies. Surface hydrogen on Cu overlayers is generally stabilized as compared to pure Cu. For Pt overlayers, the binding energy of surface hydrogen as compared to pure Pt varies by system, although it is generally destabilized: Pt*/Au, Pt*/Ag, Pt*/Pd, Pt*/W, and Pt*/Mo all have higher binding of surface hydrogen than pure Pt; the other NSAs have similar or lower H binding energies. For Pd*/Au and Pd*/Ag, the binding energy of surface hydrogen is larger than that of Pd; the other Pd overlayer NSAs show a decrease in H binding energy.
Table 1

Properties of surface and subsurface hydrogen adsorption for Au, Ag, Cu, Pt, and Pd overlayers on the close-packed surfaces of the host metals discussed in the text

System

Surface hydrogen

Subsurface hydrogen

Electronic structure

Degree of corrugation

Site preferencea

Binding energy (eV)

Site preferencea

Binding energy (eV)

BEHsub − BEHsurf (eV)b

Clean εd (eV)

εd with Hsub (eV)

Clean (Å)

With Hsub (Å)

Au

fcc

−2.18

tt

−1.55

0.63

−3.44

−3.49

0.00

0.23

Au*/Ag

fcc

−2.04

tt

−1.43

0.61

−3.63

−3.57

0.00

0.14

Au*/Cu

bridge

−2.19

tt

−1.89

0.30

−3.36

−2.97

1.24

1.24

Au*/Pt

fcc

−2.11

tt

−2.07

0.04

−3.61

−3.28

0.00

0.47

Au*/Pd

fcc

−2.04

tt

−2.24

−0.20

−3.59

−3.59

0.00

0.25

Au*/Ni

bridge

−2.56

th

−2.20

0.36

−3.19

−3.33

1.98

2.09

Au*/Ir

bridge

−1.99

tt

−2.14

−0.15

−4.11

−3.02

0.00

0.90

Au*/Rh

fcc

−1.89

tt

−2.21

−0.32

−4.13

−3.17

0.00

0.83

Au*/Co

fcc

−2.13

octa

−2.38

−0.25

−2.91

−3.27

2.10

1.84

Au*/Os

fcc

−1.90

tt

−2.14

−0.24

−4.24

−3.08

0.00

0.87

Au*/Ru

fcc

−1.88

tt

−2.09

−0.21

−4.18

−3.40

0.00

0.74

Au*/Fe

lb

−2.30

unhol

−2.46

−0.16

−3.11

−3.08

1.48

1.50

Au*/Re

bridge

−2.05

tt

−2.42

−0.37

−4.28

−3.21

0.00

0.89

Au*/W

hol

−2.20

unhol

−2.30

−0.10

−4.17

−3.18

0.00

0.77

Au*/Mo

hol

−2.21

unhol

−2.18

0.03

−3.99

−3.45

0.00

0.53

Au*/Ta

top

−2.12

unhol

−2.18

−0.06

−4.42

−4.01

0.00

0.26

Au*/V

lb

−2.33

unhol

−2.60

−0.27

−3.45

−2.97

1.07

1.31

Ag

fcc

−2.13

tt

−1.45

0.68

−4.15

−4.19

0.00

0.18

Ag*/Au

fcc

−2.21

tt

−1.51

0.70

−4.06

−4.00

0.00

0.26

Ag*/Cu

fcc

−2.01

tt

−2.09

−0.08

−3.93

−3.80

1.15

1.22

Ag*/Pt

fcc

−2.18

tt

−2.04

0.14

−3.96

−3.91

0.00

0.48

Ag*/Pd

fcc

−2.12

octa

−2.32

−0.20

−3.98

−3.99

0.00

0.00

Ag*/Ni

bridge

−2.32

tt

−2.37

−0.05

−3.99

−3.83

1.90

1.41

Ag*/Ir

fcc

−2.20

tt

−2.06

0.14

−4.33

−3.73

0.00

0.90

Ag*/Rh

fcc

−2.11

tt

−2.20

−0.09

−4.33

−3.89

0.00

0.76

Ag*/Co

bridge

−2.05

tt

−2.37

−0.32

−3.99

−3.83

1.86

1.40

Ag*/Os

fcc

−2.15

tt

−2.12

0.03

−4.47

−3.83

0.00

0.80

Ag*/Ru

fcc

−2.10

tt

−2.10

0.00

−4.42

−4.10

0.00

0.60

Ag*/Fe

lb

−1.81

uhol

−2.40

−0.59

−3.85

−3.80

1.39

1.44

Ag*/Re

bridge

−2.26

tt

−2.42

−0.16

−4.47

−3.85

0.00

0.79

Ag*/W

hol

−2.34

unhol

−2.36

−0.02

−4.38

−4.04

0.00

0.53

Ag*/Mo

hol

−2.25

unhol

−2.37

−0.12

−4.27

−4.12

0.00

0.38

Ag*/Ta

top

−2.25

unhol

−2.47

−0.22

−4.62

−4.26

0.00

0.35

Ag*/V

lb

−2.24

unhol

−2.72

−0.48

−4.24

−3.83

0.79

1.15

Cu

fcc

−2.45

octa

−2.05

0.40

−2.55

−2.57

0.00

0.07

Cu*/Au

fcc

−2.70

tt

−1.39

1.31

−2.10

−2.22

0.00

0.14

Cu*/Ag

fcc

−2.54

tt

−1.45

1.09

−2.12

−2.15

0.00

0.08

Cu*/Pt

fcc

−2.64

th

−2.29

0.35

−2.00

−2.03

0.00

0.03

Cu*/Pd

fcc

−2.58

th

−2.42

0.16

−1.84

−1.88

0.00

0.02

Cu*/Ni

fcc

−2.47

octa

−2.09

0.38

−2.63

−2.60

0.00

0.03

Cu*/Ir

fcc

−2.64

th

−2.24

0.40

−2.39

−2.33

0.00

0.03

Cu*/Rh

fcc

−2.58

th

−2.30

0.28

−2.27

−2.22

0.00

0.04

Cu*/Co

fcc

−2.50

octa

−2.10

0.40

−2.72

−2.67

0.00

0.03

Cu*/Os

fcc

−2.52

th

−2.26

0.26

−2.57

−2.52

0.00

0.03

Cu*/Ru

fcc

−2.51

th

−2.25

0.26

−2.43

−2.41

0.00

0.05

Cu*/Fe

hol

−2.52

unhol

−2.08

0.44

−2.78

−2.68

0.00

0.20

Cu*/Re

fcc

−2.64

octa

−2.24

0.40

−2.66

−2.64

0.00

0.01

Cu*/W

hol

−2.66

tt

−1.72

0.94

−2.60

−2.39

0.00

0.33

Cu*/Mo

hol

−2.61

unhol

−2.30

0.31

−2.42

−2.43

0.00

0.15

Cu*/Ta

hol

−2.48

unhol

−2.45

0.03

−2.90

−2.83

0.00

0.18

Cu*/V

hol

−2.53

tt

−2.46

0.07

−2.94

−2.76

0.00

0.20

Pt

fcc

−2.72

tt

−2.03

0.69

−2.52

−2.49

0.00

0.31

Pt*/Au

fcc

−3.01

tt

−1.90

1.11

−1.95

−1.99

0.00

0.22

Pt*/Ag

fcc

−2.97

tt

−1.92

1.05

−1.96

−1.99

0.00

0.16

Pt*/Cu

hcp

−2.32

tt

−1.68

0.64

−3.20

−3.25

0.00

0.36

Pt*/Pd

fcc

−2.77

tt

−2.39

0.38

−2.37

−2.34

0.00

0.22

Pt*/Ni

bridge

−2.66

tt

−2.24

0.42

−3.02

−2.54

0.86

1.23

Pt*/Ir

top

−2.48

tt

−1.65

0.83

−3.09

−2.89

0.00

0.42

Pt*/Rh

top

−2.42

tt

−1.86

0.56

−3.01

−2.91

0.00

0.23

Pt*/Co

fcc

−2.62

tt

−2.25

0.37

−3.90

−2.83

0.00

0.78

Pt*/Os

top

−2.50

tt

−1.62

0.88

−3.10

−2.84

0.00

0.31

Pt*/Ru

top

−2.42

tt

−1.87

0.55

−2.98

−2.92

0.00

0.27

Pt*/Fe

top

−2.65

unhol

−1.95

0.70

−2.89

−2.47

0.74

0.98

Pt*/Re

lb

−2.57

octa

−1.74

0.83

−3.35

−3.16

0.00

0.01

Pt*/W

hol

−2.95

tt

−1.28

1.67

−3.20

−2.47

0.00

0.68

Pt*/Mo

lb

−3.00

tt

−1.50

1.50

−2.91

−2.59

0.00

0.49

Pt*/Ta

hol

−2.45

tt

−2.11

0.34

−2.96

−2.95

0.00

0.14

Pt*/V

hol

−1.97

unhol

−2.33

−0.36

−3.54

−3.56

0.04

0.10

Pd

fcc

−2.88

octa

−2.53

0.35

−1.83

−1.82

0.00

0.03

Pd*/Au

fcc

−2.98

tt

−1.87

1.11

−1.57

−1.59

0.00

0.18

Pd*/Ag

fcc

−2.98

tt

−1.93

1.05

−1.58

−1.60

0.00

0.16

Pd*/Cu

hcp

−2.52

oct

−1.96

0.56

−2.57

−2.59

0.00

0.06

Pd*/Pt

fcc

−2.81

th

−2.35

0.46

−1.97

−1.97

0.00

0.08

Pd*/Ni

fcc

−2.63

octa

−2.41

0.22

−2.18

−1.65

0.95

1.13

Pd*/Ir

fcc

−2.61

th

−1.88

0.73

−2.45

−2.43

0.00

0.06

Pd*/Rh

fcc

−2.60

th

−2.15

0.45

−2.35

−2.33

0.00

0.89

Pd*/Co

fcc

−2.55

octa

−2.30

0.25

−2.33

−1.65

0.89

1.11

Pd*/Os

fcc

−2.54

th

−1.93

0.61

−2.49

−2.40

0.00

0.01

Pd*/Ru

fcc

−2.61

th

−2.19

0.42

−2.36

−2.33

0.00

0.05

Pd*/Fe

hol

−2.65

unhol

−2.20

0.45

−2.03

−1.85

0.88

1.08

Pd*/Re

fcc

−2.55

octa

−2.04

0.51

−2.71

−2.61

0.00

0.00

Pd*/W

hol

−2.61

tt

−1.57

1.04

−2.76

−2.08

0.00

0.66

Pd*/Mo

hol

−2.83

unhol

−1.75

1.08

−2.46

−2.06

0.00

0.58

Pd*/Ta

hol

−2.35

unhol

−2.12

0.23

−2.67

−2.63

0.00

0.14

Pd*/V

hol

−2.23

tt

−2.25

−0.02

−2.97

−2.70

0.00

0.34

εd is the d-band center of the most reactive atom in the overlayer with respect to the Fermi level. Degree of corrugation is the largest absolute difference in the calculated z-coordinates of the overlayer atoms. Binding energies are calculated with respect to the clean slab and atomic gas-phase hydrogen. Nomenclature: X*/Y refers to a Y host overlaid by one ML of X

aSite preferences are: hol 3-fold hollow site, lb long-bridge site, tt tetrahedral under-top site, th tetrahedral under-hcp site, octa octahedral site, unhol under-hollow site. For more detailed description of these sites, see reference [13, 54]

bThe difference in stability of surface and subsurface hydrogen. Positive values indicate surface hydrogen is more stable

The difference in surface binding energies between pure metals and overlayers of the same metal on a foreign substrate has been attributed to the change in the electronic structure of the overlayer metal in the presence of other metals. According to the theory proposed by Nørskov et al. [37, 4749], the sharing of the electrons from the relatively full d-bands of the noble metals, Pt and Pd overlayers with the underlying relatively emptier d-band of the host transition metals would lead to a general lowering of the d-band center and therefore a weakening of the metal-hydrogen bond on these surfaces. This explains the weakening of surface hydrogen binding on most Pt and Pd overlayer NSAs. This theory can also explain the stronger binding of surface H on Pt*/Au, Pt*/Ag, Pd*/Au and Pd*/Ag NSAs, where the host metal pushes the d-band center of the overlayer closer to the Fermi level.

In addition to this interaction effect on the electronic structure of the metals, there is a second, geometric, effect. The overlayers calculated here are pseudomorphic; the overlying metal is either compressed or expanded, depending on the relative lattice constants of the pure substrate and overlayer metals. Strain has been shown to have an effect on the electronic structure [4951] of the overlayer metal and binding energy of adsorbates on those surfaces [52, 53]. Expansive strain alone generally leads to a stronger binding of adsorbates; compressive strain leads to weaker binding [51]. Because of this strain, Cu overlayers (which have a large expansive strain on all 4d and 5d group metals) show a stronger binding of surface hydrogen than pure Cu, despite the full Cu d-band. Thus, in the case of Cu, the geometric effect dominates.

The combination of both geometric and electronic effects would predict that for all Ag and Au overlayers, the NSA will have weaker hydrogen binding than pure Au and Ag. All of the metals studied have a smaller lattice constant than Au or Ag, thus a pseudomorphic overlayer of Au and Ag will have considerable compressive strain. Many of the systems calculated in this study show that Ag and Au overlayers indeed have weaker binding than the pure Ag and Au hosts. However, several systems have anomalous binding of hydrogen on certain Ag and Au overlayers. The relatively high binding energy of hydrogen (up to ca. 0.2 eV larger than on pure Ag and 0.4 eV larger than on pure Au) on certain overlayers is striking. In some of these cases, notably Au*/Ni, Au*/Cu, and Ag*/Ni, the corrugated nature of the NSA surface may partially relieve the compressive strain, allowing for stronger H bonding to under-coordinated surface metal atoms. In the case of bcc host metals, the difference in geometry between the (111) facet of the pure overlayer metal and the (110) facet of bcc metals may also play a role in changing the hydrogen binding characteristics.

3.1.2 Subsurface Hydrogen

While the binding of hydrogen to the surface of these overlayer NSAs is affected as compared with the pure metal, there is also an important difference in the binding of hydrogen in the subsurface of NSAs as compared with pure metals. For Au overlayer NSAs, hydrogen in the interfacial region is strongly stabilized (0.34–1.05 eV) as compared with hydrogen at the subsurface of pure Au (Au*/Ag is an exception to this rule). Ag overlayers follow the same pattern, with stabilization of subsurface hydrogen between 0.59 and 1.27 eV as compared with subsurface hydrogen in pure Ag (Ag*/Au is also an exception, showing only a small stabilization). Cu overlayer NSAs also generally show stabilization as compared with hydrogen in the subsurface of pure Cu, but the effect is more modest. Pt overlayers on Pd, Ni, Co, Ta and V have higher interfacial hydrogen binding than subsurface hydrogen binding in pure Pt, while the other NSAs show lower subsurface hydrogen binding energies. Subsurface H is destabilized at the Pd-metal interface as compared with subsurface hydrogen in pure Pd in all cases.

The increased stability of subsurface H combined with the decrease in stability for surface H allows some NSAs to have more stable hydrogen in their subsurface than on their surface. This occurs for many NSAs with Au and Ag overlayers, as well as Pt*/V and Pd*/V. Many other NSAs have a relatively small difference between the energetics of surface and subsurface H (see Table 1). This is in contrast with pure metals, where surface hydrogen is at least 0.30 eV more stable than subsurface hydrogen on all metals [14, 54]. On these NSAs, one can expect that subsurface H can be a significant portion of the total H content under certain conditions. Additionally, on several NSA systems studied, the stability of subsurface H is higher than that of gas-phase H2 (1/2 bond energy of H2 = −2.27 eV). Among the pure metals, this is only the case for Pd [14, 54]. This observation can have significant ramifications for catalytic reactions, as subsurface H in pure metals has been implicated as a reaction intermediate in certain hydrogenation reactions [34, 55, 56]. The relative stabilization of this reaction intermediate may have an effect on the activity of alloy catalysts, and in particular core–shell nanoparticles [7, 19], towards these reactions.

3.2 Electronic Structure of NSAs With and Without Subsurface Hydrogen

As illustrated in Fig. 1, there is a large difference between the electronic structure of a pure metal and the same metal as an overlayer. Three factors play a role in determining the final surface electronic structure: (1) degree of strain imposed on the overlayer by the substrate, (2) electronic coupling between the overlayer and the support metal, and (3) corrugation of the overlayer both in vacuum and in the presence of adsorbed H. Isolating the effect of each factor is not straightforward. We will hereby list the observations derived from our calculations, without an attempt to deconvolute these effects.
https://static-content.springer.com/image/art%3A10.1007%2Fs11244-010-9444-5/MediaObjects/11244_2010_9444_Fig1_HTML.gif
Fig. 1

d-band center of NSAs with and without ¼ ML subsurface H. The d-band center shown is that of the surface metal atom with a d-band center closest to the Fermi level. X axis refers to substrate; horizontal rows are labeled by the overlaid metal. Circles and solid line refer to surface without hydrogen. Squares and dotted line refer to surfaces with ¼ ML subsurface H. The d-band center is referenced to the Fermi level

For Au overlayers, the d-band center of pure Au is closer to the Fermi level than that of Au on any uncorrugated slab. For 4d and 5d substrates, the d-band center generally moves further from the Fermi level as one goes to the left on the periodic table. For Ag overlayers, there is a slight upshift in d-band center for Ag in Ag*/Pd, Ag*/Pt, and Ag*/Au NSAs as compared with pure Ag, but for uncorrugated NSAs, it follows the same general trend as Au overlayers, with a downshift in d-band center across the periodic table, with an anomalous upshift in the cases of Mo and W substrates. Clean Cu overlayers over 4d and 5d elements have upshifted d-band centers as compared with pure Cu. However, the trend across the periodic table on Cu is the same as on Ag; there is an upshift in the d-band center for Cu in Cu*/Pt and Cu*/Pd followed by a downshift as one moves to the left on the periodic table.

Non-noble metal overlayers show a similar pattern to noble metal overlayers. Both Pt and Pd overlayers on Au and Ag have a higher d-band center than the respective pure metals. However, moving left on the periodic table for substrate metals tends to lower the d-band center of both Pd and Pt overlayers, for NSAs with only minor corrugation. There is a slight rise in d-band center in some of the overlayers on bcc metals on the far left of the d-group (Pt*/Ta, Pd*/Ta, Pt*/Mo, Pd*/Mo and Pt*/W) as compared with the same overlayer metal on certain hcp metals (Re, Ru). Even in these cases, however, the d-band center is much lower than that of pure Pd or Pt.

As shown in Fig. 1 and Table 1, the presence of subsurface H changes the electronic structure of the overlayer metals significantly. For Au overlayers, on most substrates (Au, Pd, Ni and Co are exceptions), the addition of ¼ ML subsurface H leads to an upshift in the d-band center of the surface Au atoms. Because of this change, the d-band center of several Au-NSAs containing subsurface H is closer to the Fermi level than pure Au. The addition of subsurface H to Au overlayers corrugates the metal surface whereas in its absence, only the 3d-substrate-Au pairs are corrugated. The effect of the change in d-band center due to the presence of subsurface H is reflected in the binding energy of surface hydrogen, when subsurface H is present. Figure 2 shows the d-band center versus binding energy for certain Au overlayers (Au, Au*/Ag, Au*/Pt, Au*/Pd, Au*/Ir, and Au*/Rh) with and without subsurface H. The d-band center for these Au overlayers is a good predictor of the binding energy of hydrogen on these surfaces, even for the somewhat corrugated subsurface-H-modified surfaces. The stabilization of surface H in the presence of subsurface H is significant, especially for those alloys (such as Au*/Ir) where interfacial H is more stable than surface H [13]. For Ag overlayers, the addition of ¼ ML hydrogen in the subsurface has a similar effect as that for Au overlayers, although of a smaller magnitude. For Au overlayers, the change in d-band center due to ¼ ML of subsurface H ranges between −0.36 eV and +1.16 eV; for Ag overlayers, the change in d-band center is between −0.04 and +0.64 eV on the substrates studied. For Ag overlayers, the change is almost always (see Table 1 and Fig. 1 for details) to move the d-band center closer to the Fermi level. As with Au, the presence of subsurface H corrugates nearly every surface (except Ag*/Pd) even though in the absence of hydrogen, substrates of 4d and 5d metals are not corrugated.
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Fig. 2

Binding energy of ¼ ML surface H (BEH) versus the d-band center of selected Au NSAs (Au, Au*/Ag, Au*/Pt, Au*/Pd, Au*/Ir and Au*/Rh) with and without ¼ ML subsurface H. Open circles refer to NSAs with no subsurface H; filled circles contain subsurface H. Binding energy is calculated with respect to a clean slab and atomic gas-phase hydrogen. For slabs containing subsurface hydrogen, the binding energy is with respect to the clean slab with ¼ ML H in the most energetically favorable subsurface configuration and a second atomic hydrogen in the gas phase. Line through the points is the best fit line, and is drawn only as a guide to the eye

The presence of subsurface H in Cu overlayers has an even smaller effect on the d-band center of the overlayer—the range of the change is only between −0.12 eV and +0.22 eV. As with both Ag and Au overlayers, however, the effect is generally to shift the d-band center toward the Fermi level (Cu, Cu*/Au and Cu*/Pt are (111) surfaces that are exceptions to this rule). In the case of Cu, the addition of subsurface H has only a modest effect on the corrugation of the surface, especially when Cu ML is on top of fcc(111) and hcp(0001) host-metal facets.

The presence of subsurface H in NSAs with Pt overlayers has an effect on the d-band center that is slightly smaller but similar to that of Au overlayers. For Pt overlayers on noble metals, the presence of subsurface H slightly decreases the d-band center—other substrate metals (except Pt*/V) show an increase in d-band center in the presence of subsurface H. As with the other NSAs studied, Pt overlayers show some corrugation in the presence of subsurface hydrogen, although the effect in Pt overlayers is less pronounced than Au and Ag overlayers for most substrate metals. Pd overlayers have similar shifts in electronic structures as Pt overlayers, though somewhat smaller in magnitude. In the case of Pd overlaid on noble metals, subsurface H moves the d-band center away from the Fermi level; on other metals, it generally has the opposite effect. Generally, the corrugation as a result of subsurface H is smaller on Pd overlayers than on Pt or Au overlayers, especially with hcp(0001) and fcc(111) facets of the host metal, although in a few cases (notably Pd*/Rh) this is not the case.

The effect of the change in d-band center due to the presence of subsurface H is reflected in the binding energy of surface hydrogen, when subsurface H is present. Figure 2 shows the d-band center versus binding energy for certain Au overlayers (Au, Au*/Ag, Au*/Pt, Au*/Pd, Au*/Ir, and Au*/Rh) with and without subsurface H. As shown in Fig. 1 and Table 1, of the five overlayers studied, Au overlayers have the largest change in the d-band center in the presence of subsurface H; thus, its effect on the surface H binding energy can be most easily captured with these systems. As mentioned previously, the change in d-band center in the presence of subsurface H leads to a stabilization of surface H. This subsurface-H induced shift in the thermochemistry of surface H adsorption may induce the enhanced population of the Au surface H state after a partial filling of the interfacial (subsurface) region.

3.3 H2 Dissociation on Au Overlayers

In order to better highlight the effect of subsurface H on the reactivity of NSAs, we examined dissociative adsorption of H2 on Au overlayers as a probe reaction. This reaction is spontaneous on most pure non-noble metals; however, on noble metals, it is an activated process [7]. Figure 3 shows the activation energy barrier calculated for H2 dissociation as a function of the d-band center for Au overlayers on six fcc metals (Au, Ag, Pt, Pd, Ir, and Rh) both with ¼ ML and without subsurface H. As the d-band center of the surface Au moves toward the Fermi level, the activation energy barrier decreases. In the NSAs studied, the presence of subsurface H facilitates further surface hydrogen adsorption. Thus, beyond a possible direct participation of subsurface H in catalytic hydrogenation reactions as an intermediate, subsurface H has a second effect on the reactivity of the catalytic surface—to enhance the reactivity of the catalyst surface, through electronic structure modification.
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Fig. 3

Activation energy barrier for H2 dissociation versus d-band center on selected Au overlayers. Open diamonds refer to NSAs with no subsurface H; filled diamonds refer to NSAs containing ¼ ML of subsurface H. Line through the points is the best fit line, and is drawn only as a guide to the eye

4 Conclusions

Using periodic self-consistent density functional theory (PW91-GGA), the adsorption of atomic hydrogen has been studied on the close-packed facets of 17 transition metals with Au, Ag, Cu, Pd and Pt overlayers. While on pure metals, surface hydrogen is more stable than subsurface hydrogen, on many of these overlayer NSAs, subsurface hydrogen (that is, hydrogen at the metal–metal interface) is more stable than surface hydrogen, by as much as 0.37 eV, at ¼ ML coverage.

The change in adsorption energy of hydrogen on overlayers versus pure metals can be understood through the combined effects of: (i) the electronic interaction between the overlayer and host metal, (ii) the strain on the overlayer metal induced by the host metal, and (iii) the surface corrugation. On Au and Pt overlayers, these effects work to generally decrease the adsorption energy of surface H, while on Cu, the strain effect dominates and tends to increase the adsorption energy as compared with pure Cu.

Subsurface hydrogen has a remarkable effect on the electronic structure and reactivity of certain NSAs studied. With the exception of some group VIII and noble metal substrates, the effect of subsurface hydrogen is to move the d-band center of the overlayer toward the Fermi level, leading to enhanced reactivity, with the largest effects seen in Au and Pt overlayers. In particular, subsurface hydrogen was shown to have a significant effect on the reactivity of Au overlayers towards H2 dissociation and the binding of H to the surface of the overlayer.

Acknowledgments

All three authors have greatly benefited from interactions with Prof. Jens K. Nørskov over the past several years and wish to congratulate him on his 2009 ACS Gabor A. Somorjai award for Creative Research in Catalysis. Financial support by the DOE-BES, Chemical Sciences Division, is greatly appreciated. Research was performed in part using supercomputing resources at the following institutions: (1) EMSL, a national scientific user facility located at Pacific Northwest National Laboratory; (2) the National Center for Computational Sciences (NCCS) at Oak Ridge National Laboratory; (3) the Center for Nanoscale Materials (CNM) at Argonne National Laboratory; and (4) the National Energy Research Scientific Computing Center (NERSC). EMSL is sponsored by the US Department of Energy’s Office of Biological and Environmental Research. NCCS, CNM, and NERSC are supported by the Office of Science of the US Department of Energy under Contract No. DE-AC05-00OR22725, DE-AC02-06CH11357, and DE-AC02-05CH11231, respectively.

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© Springer Science+Business Media, LLC 2010